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Effects of hypolimnetic aeration on functional components of the lake ecosystem Ashley, Kenneth Ian 1981

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EFFECTS OF HYPOLIMNETIC AERATION ON FUNCTIONAL COMPONENTS OF THE LAKE ECOSYSTEM by IAN ASHLEY Of B r i t i s h Columbia, 1976 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Zoology and Institute of Animal Resource Ecology) We accept t h i s thesis as conforming to the required standard KENNETH B.Sc, The University THE UNIVERSITY OF BRITISH COLUMBIA March 20, 1981 ©Kenneth Ian Ashley, 1981 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I further agree that permission for extensive copying of t h i s thesis for s c h o l a r l y purposes may be granted by the head of my department or by his or her representatives. It i s understood that copying or p u b l i c a t i o n of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of The University of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 n i r _ £ 10 /"7Q \ 11 ABSTRACT Effe c t s Of Hypolimnetic Aeration On Functional Components Of The Lake Ecosystem by Kenneth Ian Ashley The whole-lake experimental approach was used to examine the effect of hypolimnetic aeration on several key components of the lake ecosystem. These included c i r c u l a t i o n and decomposition processes, major nutrient, ion and-pH interactions as well as phytoplankton and zooplankton populations. A small (3.9 ha, Z max.=9.0 m) naturally eutrophic lake was divided into experimental and control halves by a p l a s t i c curtain, and a hypolimnetic aerator i n s t a l l e d in the experimental half and operated from A p r i l 1978 to March 1979. Hypolimnetic aeration had no ef f e c t on thermal s t r a t i f i c a t i o n during the ice-free season but c i r c u l a t e d the entire experimental half under ice cover. Aeration increased hypolimnetic t u r b i d i t y but did not affect epilimnetic transparency. Hypolimnetic oxygen levels were increased along with a ten f o l d r i s e in oxygen consumption. Hypolimnetic ammonia level s were reduced and s u f f i c i e n t oxygen was added for n i t r i f i c a t i o n to occur. Internal phosphorous loading and hypolimnetic orthophosphate lev e l s were also reduced however aerobic P regeneration, increased. Aeration vented accumulated CO^ from the hypolimnion and decreased i t s calcium, magnesium, bicarbonate and orthophosphate content via calcium carbonate co p r e c i p i t a t i o n . Phytoplankton abundance and species composition (averaged over the water column) were not affected by hypolimnetic aeration. The zooplankton community exhibited similar v e r t i c a l d i s t r i b u t i o n on both halves of the lake however greater numbers were found on the experimental half a f t e r several months aeration. Management implications of hypolimnetic aeration are also discussed. iv TABLE OF CONTENTS Abstract i i L i s t of Tables v i i L i s t of Figures v i i i Acknowledgements x Introduction 1 History of Lake Aeration 7 D e s t r a t i f i c a t i o n 7 Hypolimnetic Aeration 9 Study Area , 11 Lake History 11 Climate and Watershed 12 Lake Description and Morphometry 12 Materials and Methods 16 Aeration System 16 Curtain 18 Operation 19 Sampling 19 Physical 20 Chemical 20 Phytoplankton 21 Zooplankton 22 Aerator 22 S t a t i s t i c s 23 Results 24 C i r c u l a t i o n Processes 24 V Temperature 25 Transparency 28 Decomposition Processes 30 Oxygen 30 Total Organic Carbon 33 Major Nutrients 35 Nitrogen 35 Phosphorus- 40 N:P Ratios 45 Major Ions 47 Total A l k a l i n i t y 47 Calcium and Magnesium 48 Manganese 50 pH Interactions 52 Phytoplankton 53 Biomass 53 Composition • 55 Zooplankton 59 Discussion 66 C i r c u l a t i o n 67 Decomposition 70 Major Nutrients 77 Nitrogen 77 Phosphorus 81 N:P Ratios 84 Major Ions 86 pH 90 Phytoplankton 92 v i Zooplankton 94 Management Implications and Suggestions 99 Summary and Conclusions 101 Literature Cited 104 Appendix 116 v i i LIST OF TABLES Table 1. Morphometric Features Of Black Lake 15 Table 2. Environmental Laboratory Water Chemistry Methods .116 Table 3. L i s t Of Personal Communications 118 Table 4. L i s t Of F Values For Water Quality Parameters ....119 Table 5. L i s t Of F Values For Zooplankton . . 120 v i i i LIST OF FIGURES Figure 1. Black Lake showing depth contours, compressor s i t e , curtain position, aerator location and sampling sit e s 13 Figure 2. A schematic diagram of the Black Lake hypolimnetic aerator 17 Figure 3. Temperature isopleths for experimental (west) and control (east) sides 26 Figure 4. Secchi and 1% transmission depths for experimental (west)' and control (east) sides 29 Figure 5. Oxygen isopleths for experimental (west) and control (east) sides 31 Figure 6. Total oxygen content in experimental (west) and control (east) sides 32 Figure 7. Total organic carbon isopleths for experimental (west) and control (east) sides 34 Figure 8. Ammonia nitrogen isopleths for experimental (west) and control (east) sides 36 Figure 9. Nitrate nitrogen isopleths for experimental (west) and control (east) sides 38 Figure 10. Total organic nitrogen isopleths for experimental (west) and control (east) sides 40 Figure 11. Orthophosphate phosphorus isopleths for experimental (west) and control (east) sides 41 Figure 12. Hypolimnetic dissolved organic phosphorus content in experimental (west) and control (east) sides 43 Figure 13. Total phosphorus content in experimental (west) and control (east) sides 45 Figure 14. Whole lake N:P rat i o s in experimental (west) and control (east) sides 46 Figure 15. Total a l k a l i n i t y isopleths for experimental (west) and control (east) sides 48 Figure 16. Dissolved calcium isopleths for experimental (west) and control (east) sides 49 Figure 17. Dissolved manganese isopleths for experimental (west) and control (east) sides 51 Figure 18. Hypolimnetic pH levels in the experimental (west) and control (east) sides 53 Figure 19. Chlorophyll a isopleths for experimental (west) and control (east) sides 54 Figure 20. Phytoplankton composition in experimental (west) and control (east) sides 57 Figure 21. Total zooplankton, Daphnia pulex and Keratella quadrata (numbers/m2) in the experimental (west) and control (east) sides 60 Figure 22. Cyclops bicuspidatus n a u p l i i , copepodites and adults (numbers/m2) in the experimental (west) and control (east) sides 63 Figure 23. Diaptomus leptopus n a u p l i i , copepodites and adults (numbers/m2) in the experimental (west) and control (east) sides 65 X ACKNOWLEDGEMENTS A project of this magnitude could not have been possible without the assistance of several i n d i v i d u a l s . I am especially grateful to F.A. Ashley and G.T. "Ozone" Sutherland whose mechanical i n t u i t i o n and construction s k i l l s ensured the successful construction and i n s t a l l a t i o n of experimental equipment. C.J. B u l l , D. McKay, D. Smith and K. Tsumura (Fish and W i l d l i f e Branch) were helpful on a continuous basis. I would l i k e to thank Dr. W.E. N e i l l and Dr. Adrienne Peacock for introducing me to experimental limnology. Edgar Guindon and G.J. Steer provided valuable assistance with their knowledge of computer programing. Special thanks are due to Dr. K.J. H a l l , Dr. T.G. Northcote and Dr. A.F. Tautz who offered timely suggestions throughout my thesis program and greatly assisted in improving the manuscript. F i n a l l y , I would l i k e to express my gratitude to the students (past and present) and faculty of the Institute of Animal Resource Ecology for providing a stimulating and thoroughly entertaining research environment. This study was supported by the Fisheries Research Section of the B r i t i s h Columbia Fish and W i l d l i f e Branch (Ministry of Environment) and an NRC grant (67-3454) to Dr. T.G. Northcote. 1 INTRODUCTION Excessive f e r t i l i z a t i o n of natural waters is one of the most serious water quality problems in the world today (Dunst et a l . , 1974; NAS, 1969). Cultural eutrophication is caused by excessive addition of nutrients such as phosphorus and nitrogen to lakes, streams, r i v e r s , estuaries and coastal waters (Wetzel, 1975). In lakes, these additions result in increased aquatic plant growth, undesireable changes in species composition, oxygen depletions, f i s h k i l l s and decreased water quality for domestic, recreational and i n d u s t r i a l use (Lee, 1970a). Following the limiting-nutrient controversey of the late 1960's, attention in the 1970's focused on reducing nutrient inputs and r e h a b i l i t a t i n g c u l t u r a l l y eutrophied lakes. Certain lakes recovered from excessive nutrient loading after nutrient diversion eg. Lake Washington (Edmondson, 1972), however nearby Lake Sammamish did not respond s i m i l a r l y (Rock, 1974). Lake Trummen in Sweden i s another lake in which the eutrophic status remained unchanged following nutrient diversion (Bjork et a l . , 1972). Lakes of thi s type were s u f f i c i e n t l y eutrophic to maintain their present state via internal nutrient recycling long after external nutrient sources were removed. As a re s u l t , the f i e l d of lake restoration came into existence as limnologists and engineers attempted to develop methods for restoring eutrophic lakes. Lake restoration refers to "... the .manipulation of a lake ecosystem to eff e c t an in-lake improvement in degraded, or undesirable conditions" (Dunst et a l . , 1974). A r t i f i c i a l aeration i s one technique used in 2 restoring eutrophic lakes (Lorenzen and Fast, 1977). A r t i f i c i a l aeration reoxygenates depleted hypolimnetic waters and tec h n i c a l l y creates oligotrophic oxygen conditions in eutrophic lakes. However, as i s often the case with new technology, a r t i f i c i a l aeration as a lake restoration technique was i n i t i a l l y applied with l i t t l e understanding of i t s ecological impact (eg. Patriarche, 1961). Shapiro (1978) stated "Lake restoration i s not a science yet. It is s t i l l in need of research.... If only 5% of the moneys allocated for doing were to be diverted to understanding, the returns would be substantial." Fast (1975) eloquently suggested "...lake medicine is s t i l l in the f i f t h t e e n t h century. Lake doctors with their bags of toxicants, dredges, coagulants and other devices are s t i l l in the medical equivalent of the bloodletting stage." Clearly, the f i e l d of lake restoration requires increased ecological awareness. The purpose of this experiment was to examine functional components of the lake ecosystem from a process oriented viewpoint using the . whole lake perturbation approach. S p e c i f i c a l l y i t s objectives were to investigate p a r t i c u l a r components of the lake ecosystem which I f e l t , after extensive l i t e r a t u r e review, were poorly understood in terms of their response to hypolimnetic aeration. These component areas were as follows: 1. C i r c u l a t i o n Processes. The hypolimnion of s t r a t i f i e d lakes c i r c u l a t e s slowly in r e l a t i o n to the epilimnion, and 3 v e r t i c a l exchange c o e f f i c i e n t s between the two zones are very low (Hesslein and Quay, 1973). Hypolimnetic aeration accelerates c i r c u l a t i o n currents within the hypolimnion and may increase the v e r t i c a l transfer of substances across the thermocline. This could have serious consequences i f essential nutrients were delivered to the epilimnion during the c r i t i c a l summer s t r a t i f i c a t i o n period. In addition, the effects of c i r c u l a t i o n currents on sediment-water exchange mechanisms are lacking. Physical mixing is often the rate c o n t r o l l i n g step in chemically mediated exchange reactions and may be the overriding factor c o n t r o l l i n g a l l sediment-water exchange reactions (Lee, 1970b). These questions must be answered before hypolimnetic aeration can be recommended as a lake restoration technique. 2. Decomposition Processes. Decomposition of organic material reduces hypolimnetic oxygen concentrations as thermal s t r a t i f i c a t i o n - and ice cover preclude oxygen renewal during most of the year (Fast, 1973). In eutrophic lakes t h i s may lead to complete anaerobiosis below the thermocline and hypolimnetic aeration i s often recommended as a remedial treatment. However theo r e t i c a l ( F i l l o s , 1976) and experimental evidence (Smith et a l . , 1975) suggests hypolimnetic aeration stimulates a 3-4 f o l d increase in hypolimnetic oxygen consumption which further decreases oxygen l e v e l s . I believe hypolimnetic aeration should result in a short-term increase in hypolimnetic oxygen consumption as accumulated organic debris i s oxidized, eventually leading to a long-term decline in oxygen consumption. In addition, side e f f e c t s of hypolimnetic aeration 4 (eg. hypolimnetic warming and benthic recolonization) may influence hypolimnetic oxygen consumption. 3. Major Nutrients. One of the most int r i g u i n g areas of limnological research involves nutrient regeneration under aerobic and anaerobic conditions. Mortimer's (1941-42) theory of P release is well documented in the l i t e r a t u r e , however recent evidence indicates P release is minimal regardless of oxygen tension (Schindler et a l . , 1980). Hypolimnetic aeration, by oxygenating the hypolimnion, should reduce P release. However most hypolimnetic aeration experiments have been unable to support t h i s contention due to poor experimental design (Fast, 1971; Garrel et a l . , 1977; and Smith et a l . , 1975). Nitrogen release during aerobic/anaerobic conditions also influences a lake's nutrient budget. Nitrogen may be lost via d e n i t r i f i c a t i o n (Chen et a l . , 1973), gained through N f i x a t i o n (Home, 1979) or transformed biochemically (Brezonik et a l . , 1969). The influence of hypolimnetic aeration on the nitrogen cycle and subsequent N/P ratios i s largely unknown. The " s p l i t -lake" experimental approach should eliminate design problems and provide insight into nutrient behaviour during hypolimnetic aeration. 4. Major Ions. Major ion behaviour (Ca + 2,Mg + 2,HCO^") is a sensitive indicator of lake metabolism, esp e c i a l l y with respect to ion exchange processes at the sediment-water interface. Preliminary evidence suggests C a + 2 p r e c i p i t a t i o n occurs during hypolimnetic aeration (Fast, 1971). I believe t h i s could reduce phosphorus concentrations via carbonate coprec.ipitation. Sediment release of metals under reducing conditions is a 5 well known phenomenon (Wetzel, 1975) and reduced metals are toxic to aquatic l i f e (LaBounty and King, 1977). Hypolimnetic aeration should oxidize reduced metals, thus improving water quality for a variety of aquatic organisms. In addition, oxidation and reduction of Mn and Fe compounds may be involved in P p r e c i p i t a t i o n which should occur during hypolimnetic aeration (Fast, 1975; Mortimer, 1971). 5. pH Interactions.. pH i s an important variable influencing numerous chemical and b i o l o g i c a l reactions within the lake ecosystem. However, pH response to hypolimnetic aeration has received l i t t l e attention. Fast (1971) observed increased pH l e v e l s following hypolimnetic aeration of a small Michigan lake. If aeration consistantly increases pH t h i s should affect lake metabolism by increasing ammonia t o x i c i t y (Trussel, 1972), decreasing hydrogen s u l f i d e t o x i c i t y (Smith and Oseid, 1975) and p r e c i p i t a t i n g major ions (bicarbonate, calcium and magnesium). This aspect of hypolimnetic aeration requires further investigation. 6. Phytoplankton. The e f f e c t of hypolimnetic aeration on the phytoplankton community i s largely unknown. Bernhardt (1967) documented physical r e d i s t r i b u t i o n of hypolimnetic algae by aeration currents, however few studies have focused on epilimnetic algae. Fast's (1971) results are questionable as leaks in the aeration tower stimulated dense a l g a l blooms. Generating hypotheses about a l g a l response is d i f f i c u l t due to the number of impinging variables, many of which are poorly understood. These include iron a v a i l a b i l i t y (Murphy et al.-, 1980), nutrient release (Fast, 1975), pH s h i f t s (Shapiro, 1978) 6 and t u r b i d i t y changes (Fast, 1971). I would not expect hypolimnetic aeration to exert an immediate ef f e c t on the phytoplankton community as c i r c u l a t i o n currents are generally confined to the hypolimnion. Long-term changes in species composition ( i e . fewer blue-greens) should occur after several c i r c u l a t i o n periods as phosphorus levels are gradually reduced and zooplankton numbers increase. 7. Zooplankton. The zooplankton community has also received l i t t l e attention in hypolimnetic aeration experiments due to i t s r e l a t i v e i s o l a t i o n from the i n i t i a l objective of r a i s i n g oxygen le v e l s . Hypolimnetic aeration, by virtue of i t s a b i l i t y to modify the physical, chemical and b i o l o g i c a l environment, has the potential to a l t e r zooplankton d i s t r i b u t i o n , abundance and species composition. Shapiro (1978) demonstrated that pH s h i f t s could affect zooplankton by changing the p a l a t a b i l i t y of their food source. K i t c h e l l and K i t c h e l l (1980) elegantly demonstrated how oxygen s t r a t i f i c a t i o n can modify zooplankton community structure. I would expect hypolimnetic aeration to s i g n i f i c a n t l y increase zooplankton v e r t i c a l d i s t r i b u t i o n and population size by increasing oxygen l e v e l s , venting toxic gases (H^S, NH^) and oxidizing reduced metals (Fe + 2,Mn + 2) in the hypolimnion. The whole-lake experimental approach was selected for t h i s research project. Experimental manipulation of small lakes provides • a r e a l i s t i c setting for experimental investigation while avoiding extrapolation errors of laboratory/enclosure experiments and economic/logistic problems associated with manipulating large lakes. Several strategies are currently available within the realm of whole-lake experiments. Among 7 these, the " s p l i t - l a k e " design, similar to Schindler's Lake 226 project (Schindler, 1974) was chosen for this experiment as i t allows simultaneous experimental and control treatments within a single lake basin. HISTORY OF LAKE AERATION A r t i f i c i a l aeration of lakes refers to the process whereby a i r or oxygen is injected to increase dissolved oxygen conditions and c i r c u l a t e the water. There are a variety of lake aeration techniques, commonly divided into two groups: d e s t r a t i f i c a t i o n and hypolimnetic aeration. D e s t r a t i f i c a t i o n D e s t r a t i f i c a t i o n is the most widely used procedure for aerating thermally s t r a t i f i e d lakes. This technique increases dissolved oxygen in bottom waters by reducing thermal gradients and homogenizing the entire water mass (Dunst et a l . , 1974). D e s t r a t i f i c a t i o n was f i r s t used in 1919 (Scott and Foley, 1919), 8 and is usually accomplished by mixing cold anoxic hypolimnetic water with warmer epilimnetic water. The bottom water absorbs oxygen and heat before sinking to a new equilibrium depth. The entire lake can usually be c i r c u l a t e d from a single s i t e and eventually becomes isothermal (Fast and St. Amant, 1971). The most common destrati,fication method involves compressed a i r injection through perforated pipes located near the lake bottom (Fast, 1968). This is the most e f f i c i e n t d e s t r a t i f i c a t i o n technique as r i s i n g bubbles l i f t bottom waters to the surface where turbulent mixing is induced. Variations of t h i s method include the Aero-Hydraulic Cannon. This is a low-head, high-volume po s i t i v e displacement pump which operates via periodic ejection of large a i r bubbles (Toetz et a l . , 1972). Mechanical pumping has also been employed in several d e s t r a t i f i c a t i o n projects. Hooper et a l . (1952) d e s t r a t i f i e d a small Michigan lake by pumping hypolimnetic water to the surface, and Summerfelt et a l . (1976) d e s t r a t i f i e d a eutrophic reservoir by pumping epilimnetic water to the bottom via a x i a l flow pumps. Ridley et a l . (1966) employed angled jets of water to d e s t r a t i f y a large above ground reservoir. The p r i n c i p l e disadvantage of d e s t r a t i f i c a t i o n is greatly increased heat budgets. Thermal gradients are reduced and the entire water mass usually approachs normal surface temperatures. This i s a serious problem during warm summer months as i t eliminates cold water f i s h habitats and increases sediment oxygen demand. In addition, c i r c u l a t i o n currents may transport nutrients to the photic zone and stimulate phytoplankton production. 9 Hypolimnetic Aeration Hypolimnetic aeration is the second generation of lake aeration devices. This technique enables one to aerate anoxic hypolimnetic water while maintaining thermal s t r a t i f i c a t i o n . This procedure minimizes heat budget increases and can maintain cold water f i s h e r i e s in warm climates. Hypolimnetic aeration was f i r s t used in Switzerland in the late 1940's (Mercier and Perret, 1949). They aerated the hypolimnion of Lake Bret by mechanically pumping hypolimnetic water to a shore based splash basin where i t was aerated and allowed to return by gravity flow through a pipe to the hypolimnion. The next s i g n i f i c a n t development in hypolimnetic aeration occurred in 1967 (Bernhardt, 1967). In this system hypolimnetic water was a i r l i f t e d to the surface and oxygenated water returned to the hypolimnion via return tubes. This p r i n c i p l e has been incorporated into most modern hypolimnetic aeration systems. Hypolimnetic aeration systems using compressed a i r inje c t i o n are categorized as p a r t i a l and f u l l a i r l i f t designs. P a r t i a l l i f t devices are those systems where the air/water mixture does not upwell to the lake surface, whereas in f u l l a i r l i f t designs i t reaches the surface. Several advanced methods of hypolimnetic aeration have been recently developed. These include down-flow a i r i n j e c t i o n (DAI), which uses a mechanical water pump to force an air/water mixture into the hypolimnion (Speece, 1970). The advantage of thi s system i s increased oxygen transfer e f f i c i e n c y , however nitrogen gas may also reach supersaturation l e v e l s . 10 Side stream pumping systems (SSPS) operate by in j e c t i n g pure oxygen into water mechanically pumped from the hypolimnion. Oxygenated water i s then returned to the hypolimnion through high pressure discharge l i n e s . This system i s e f f i c i e n t at ra i s i n g dissolved oxygen l e v e l s , however i t i s expensive and less e f f e c t i v e at reducing hydrogen s u l f i d e and ammonia levels than p a r t i a l or f u l l l i f t designs. Deep oxygen bubble injec t i o n (DOBI; Speece, 1971) involves pure oxygen in j e c t i o n at great depths in the hypolimnion. Theoretically, most bubbles w i l l dissolve before reaching the thermocline and w i l l not d e s t r a t i f y the lake. This system has not been tested f u l l - s c a l e in a lake. Downflow bubble contact aeration (DBCA; Speece, 1971) is e s s e n t i a l l y a DAI device using pure oxygen. The aeration system used in this experiment was a f u l l a i r l i f t design . chosen for i t s high oxygen transfer c a p a b i l i t y and energy e f f i c i e n c y (Lorenzen and Fast, 1977). 11 STUDY AREA  Lake History Black Lake l i e s at an elevation of 750 m near the d i v i s i o n between Keremeos Creek and the Marron Valley in the Southern Interior Plateau limnological region of B r i t i s h Columbia (Northcote and Larkin, 1956). Black Lake originated as part of nearby Yellow Lake, whose basin was cut into volcanic rock by a major meltwater outflow which drained the Kaledon tongue of the main Okanagan ice lobe (Nasmith, 1962). Following deglaciation, approximately 8900 years B.P., Black Lake became isolated from Yellow Lake by gradual erosion and a l l u v i a l deposits from Yellow Lake Creek. The area was surveyed in 1907 by p r o v i n c i a l engineers investigating potential storage reservoir s i t e s for the town of Keremeos, 19 km southwest of Yellow Lake. Their report indicated a v e s t i g i a l stream channel connected the two lakes (J.E. F a r r e l , Water Rights Branch, pers. comm.). However, in 1947 the present highway was constructed and the channel between Black and Yellow Lake f i l l e d with several meters of roadbed material . The lakes are now connected by a single 0.76 m x 22 m culvert which drains Black Lake at high water l e v e l s . The future of Black Lake i s in doubt as the Ministry of Transportation and Highways plan to p a r t i a l l y f i l l i t during future highway improvement work (J.H Makeiv, Dept. of Highways, pers. comm.). 12 CIimate and Watershed The Southern Interior Plateau of B r i t i s h Columbia has a mild, continental climate with low annual p r e c i p i t a t i o n . In Keremeos, average annual r a i n f a l l i s 25 cm/yr with a June maximum and a March-April minimum. Mean annual snowfall of 60 cm occurs c h i e f l y in December and January. Temperatures in July-August average 22 C and may reach 41 C, while temperatures in January average -3.3 C, occasionally dropping to -30 C (Climate of B r i t i s h Columbia, 1976). The Black Lake watershed is south facing, low in elevation (1524 m max.) with poor water retention capacity (J. Botham, Water Rights Branch, pers. comm.). Although i t s size (1532 ha) appears large in re l a t i o n to Black Lake's surface area (3.95 ha), very l i t t l e surface runoff actually reaches the lake via one small creek (Yellow Lake Creek). Mean annual runoff i s estimated' at 268,000 m3, however up to 75% i s lost via evaporation (D.E. Reksten, Water Investigations Branch, pers. comm.). Yellow Lake Creek flowed from A p r i l to June 1978, peaking in mid-May at 3-5 m 3/niin. Drainage basin lithology i s porous Tertiary volcanic rock, mainly basalt and andesite (Learning, 1973), forming r o l l i n g ponderosa pine-sagebrush uplands c h a r a c t e r i s t i c of B r i t i s h Columbia's dry i n t e r i o r zone (Lyons, 1952). Lake Description and Morphometry ' Black Lake (Figure 1) i s a naturally eutrophic dimictic lake with marked thermal s t r a t i f i c a t i o n in summer, inverse 14 s t r a t i f i c a t i o n in winter and h i s t o r i c a l l y low levels of dissolved oxygen (Halsey and MacDonald, 1971). Aquatic vegetation is confined to the shallow west and southeast ends of the lake, consisting mainly of Ceratophyllum sp. and Potamogeton sp. (J. Pinder-Moss, U.B.C. Herbarium Curator, pers. comm.). The north, east and south shorelines are barren of t e r r e s t r i a l vegetation and composed mainly of broken rock from natural c l i f f erosion and highway construction. A small grove of mixed deciduous-coniferous trees i s located at the western end. Algal blooms usually reduced transparency and Secchi disk readings averaged (over the ice-free season) 3.7 m. Sediments were loosely compacted in shallow water and deep water samples were highly organic and gelatinous with a strong H^S odour. Fish were not present in Black Lake i n i t i a l l y , however some rainbow trout ( Salmo qairdneri ) and brook trout ( Salvelinus  f o n t i n a l i s ) entered the lake during high water in the spring of 1978 through the drainage culvert to Yellow Lake. Following their discovery in Black Lake, a g i l l net (5.8 m x 12.8 m; 5 cm, 3.8 cm and 2.5 cm diagonal mesh) was set on each side of the curtain for 48 hours during each sampling t r i p . A t o t a l of 96 trout were caught on the west (experimental) side and 18 on the east (control) side. Ambystoma tigrinum melanostictum was present on both sides of the curtain throughout the experiment. Important morphometric features of Black Lake are l i s t e d in Table 1. The contour map was based on echo-sounder transects applied to an a e r i a l photograph of the lake outline. 15 TABLE 1 Morphometric Features Of Black Lake 1. Location-Lat. 49 20' 30" Long. 119 44' 5" 2. Elevation-750 m 3. Area-3.95 ha 4. Volume-178,543 m3 5. Max. Depth-9.0 m 6. Mean depth-4.52 m 7. Shoreline development-1.32 8. Shoreline length-927 m 9. Max. Length and orientation-363 m, NW-SE 10. Max. Width and orientation-134 m, N-S 11. Max. l e v e l change (spring-fall)-0.73 m 12. Drainage area-1532 ha 13. Ice off-March 31,1978 14. Ice on-November 14, 1978 15. Max. ice thickness -0.38 m Whole Lake Experimental Control z Area (m 2) Strata Vol.(m 3 ) Area Vol. Area Vol. 0 39456 0-1 37464 20204 19047 19252 18417 1 35506 1-2 32850 17912 15965 17594 16885 2 30279 2-3 27652 14094 13261 16185 14391 3 25114 3-4 23206 12445 11407 12669 11799 4 21350 4-5 19662 10400 9512 10950 10150 5. 18020 5-6 16041 • 8650 7791 9370 8250 6 14141 6-7 11967 6962 5988 7179 5979 7 9920 7-8 6975 5065 3602 4855 3373 8 4399 8-9 2726 2316 1398 2083 1328 9 1348 651 697 Total 178543 87971 90572 16 MATERIALS AND METHODS Aeration System The hypolimnetic aeration system used in thi s study was based on systems described by Bernhardt and Wilhelms (1975), Hess (1975), and Smith et a l . (1975) (Figure 2) ". The aerator consisted of an insulated open box (2.4 m x 1.2 m x 0.9 m) constructed of 19 mm plywood and 5 cm .x 10 cm framing. Styrofoam f i l l e d pontoons (0.3 m x 0.3 m x 2.4 m) were attached to both sides of the box and provided 360 kg of posit i v e buoyancy. Two 0.76 m c i r c u l a r holes were cut in the floor through which 0.76 m x 7.3 m galvanized steel pipes were f i t t e d . An a i r dif f u s o r was i n s t a l l e d 0.3 m inside the bottom of the intake pipe. The diff u s o r consisted of four 0.38 m iron pipes (3.81 cm ID) connected to a common center, and d r i l l e d with ten 1.5 mm a i r release holes per arm. The outlet pipe was f i t t e d with a 45 degree elbow to prevent r e c i r c u l a t i o n of aerated water. The unit was b u i l t in Vancouver, disassembled and trucked to Black Lake, then reassembled in a horizontal position on the ice surface. Chain saws were used to remove ice from around the aerator, which was then lowered into i t s normal v e r t i c a l operating p o s i t i o n . The entire unit weighed 453 kg and was free f l o a t i n g . After ice-off the aerator was towed into position above the 9 m contour and securely anchored in place. The aerator floated 0.6 m above water at rest, the lake bottom being 1.4 m below the lower end of the outlet tube. A concrete pad. (3 m x 3 m. x 0.3 m) was poured at the lake's west end, 18 m from the shoreline and a plywood shed erected to 1 7 WASTE AIR COMPRESSED AIR W A TER OU T L E T WATER INLET F I G U R E 2. A SCHEMATIC DIAGRAM OF THE B L A C K L A K E H Y P O L I M N E T I C AERATOR. 18 house the compressor and provide working space. An e l e c t r i c a l contractor i n s t a l l e d fuse panels and transformers for 230 volt three phase operation. A new 7.5 kw rotary vane compressor (Hydrovane SR 4000 rated 1.13 m3/min. @ 7.0 kg/cm2) was purchased and i n s t a l l e d in the shed. An automatic drain o i l / a i r f i l t e r , regulator and valves were connected to the compressor thus allowing precise volume and pressure regulation of o i l - f r e e a i r . A weighted a i r l i n e (106 m x 1.9 cm I.D.) f i t t e d with one-way, valves was l a i d out on the ice surface connecting the compressor and aerator and allowed to sink into position at ice-o f f . Curtain The lake was divided into approximately two equal sections by a p l a s t i c curtain (Figure 1 and Table 1) designed by the author and manufactured in Vancouver (False Creek Industries Ltd.). The main section (103 m x 10.4 m) was composed of woven pol y o l e f i n (Dupont Fabrene Type P) which transmits 80-85% of v i s i b l e l i g h t (W.R. Eadie, DuPont Ltd., pers. comm.). A double c o l l a r (103 m x 0.3 m) of u l t r a - v i o l e t resistant black woven pol y o l e f i n (Dupont Fabrene Type TM) was attached to the top of the main section. This p a r t i c u l a r design was intended to minimize shadow formation, r e s i s t u l t r a - v i o l e t degradation and prevent surface punctures. A rope stretched across the ice surface marked the curtain i n s t a l l a t i o n position and the entire distance (103 m) was cut open by chain saws. The curtain was unfolded along the s l o t , 113 kg of lead rope attached to i t s 19 lower edge and a styrofoam g i l l n e t f l o a t l i n e strung through the black surface c o l l a r . The curtain was then pushed into place and sunk into p o s i t i o n . Rocks were p i l e d on the near-shore area, to ensure a snug f i t and SCUBA observation confirmed the lower edge was well sealed into the sediment. The fl o a t i o n c o l l a r floated 10 cm above the water surface and minimized surface water exchange. Operat ion The west end of the lake contained the aerator and was designated as the experimental side while the east end served as the simultaneous control side. The compressor and experimental period started A p r i l 11, 1978, 11 days after i c e - o f f , and ran continuously u n t i l March 6, 1979, a period of 329 days. The entire system was i n s t a l l e d in ten 3-4 day t r i p s to Penticton s t a r t i n g July 20, 1977 and f i n i s h i n g March 31, 1978. Sampling A l l samples were coll e c t e d from permanent sampling stations located near the center of the lake approximately 20 meters apart on either side of the curtain (Figure 1). Maximum station depth ranged from 8.27 m to 9.0 m during the experimental period. The west (experimental) station was situated at right angles to the aerator outlet tube to avoid sampling d i r e c t l y in the plume of aerated water. Replicate oxygen-temperature p r o f i l e s taken prior to the experiment indicated the two central stations were representative s i t e s . After ice formation c i r c u l a r 20 holes were cut above each sample s i t e and 100 l i t r e weighted rubber p a i l s inserted into each opening. This procedure allowed quick entry to the lake during sub-zero temperatures and minimized water disturbance when sampling. Samples were taken every two weeks from A p r i l to October 1978 and at three week intervals from November 1978 through March 1979. Black Lake was sampled twenty times during the experiment, each f i e l d t r i p averaging three days in duration. In addition to regular data c o l l e c t i o n , routine compressor maintenance, aerator adjustments and log clearings were carried out on each t r i p . Samples were usually c o l l e c t e d between 1000 and 1400 hrs. Physical Physical data were c o l l e c t e d at 1 m depth i n t e r v a l s . Temperature series were taken with a thermistor (YSI Model 54 ARC). Transparency was measured with a standard 20 cm Secchi disc using a glass bottom viewing box. Percent l i g h t transmission was obtained with a Beckman EV3 Enviroeye l i g h t meter. Chemical Dissolved oxygen was measured at one meter inter v a l s with-an oxygen meter (YSI 54 ARC). Two replicate Winkler t i t r a t i o n s (Azide modification) were used to c a l i b r a t e the meter during each sampling period. A l l other chemical parameters were c o l l e c t e d at two meter intervals with a e l e c t r i c bilge pump (Jabsco @ 26 l i t r e s / m i n . ) . 21 Samples were poured into polyethylene bottles, stored at 0 C in light-proof containers and transported to the laboratory within 24 hours. The Environmental Laboratory (Water Resources Service, Ministry of the Environment, 3650 Wesbrook UBC) performed a l l analyses according to their methods manual (Water Resource Service, 1976). A brief description of each method is given in the appendix (Table 2). Chlorophyll a Chlorophyll a samples were taken at 2 meter intervals and stored in light-proof coolers p r i o r to f i l t e r i n g . Two replicate sample volumes were vacuum f i l t e r e d at 2/3 atmosphere, preserved with magnesium carbonate solution, stored at -73 C in dark bottles containing a s i l i c a desiccant and delivered to the Environmental Lab within 48 hours. Chlorophyll a and phaeophyton a concentrations were determined c o l o r i m e t r i c a l l y after extraction in 90% acetone (Strickland and Parsons, 1968). Phytoplankton Phytoplankton were c o l l e c t e d at 2 meter inter v a l s with a 3 l i t r e Van Dorn water bottle, combined into one sample and preserved with Lugol's solution (Lind, 1979). For analysis, 100 ml subsamples were pipetted into graduated cylinders and allowed to s e t t l e overnight. The supernatant l i q u i d was then decanted and the remaining 25 mis rinsed into sedimentation chambers and allowed to r e s e t t l e for 24 hours. Plankton were counted at 400 x using an inverted microscope with a 300 u diameter f i e l d of view, and results were expressed as numbers of c e l l s / m l . 22 Plankton genera were partitioned into four major phyla with the remaining genera lumped into one group. Due to the q u a l i t a t i v e nature of the phytoplankton data only general trends were noted and no quantitative analysis was undertaken. Plankton i d e n t i f i c a t i o n and counting was performed by Mr. Al Redenback at U.B.C. Zooplankton Zooplankton were coll e c t e d at one meter intervals with a 27 l i t r e Schindler-Patalas trap (Schindler, 1969) using 84 u Nytex mesh. During the f i r s t f i v e sampling t r i p s one trap set per meter was used, after which two replicate sets (54 l i t r e s total) were combined for each depth i n t e r v a l . Samples were rinsed into p l a s t i c v i a l s and preserved with 4% formaldehyde/sucrose solution (Haney and H a l l , 1973). Plankton were subsampled with a 1 ml Stempel pipette into 10 ml Sedgwick-Rafter c e l l s and counted under a dissecting microscope (Wild M5) at 25 x. Subsample size was adjusted to obtain at least 100 counts for any given species up to a maximum of 10 mis per subsample, after which the entire sample was counted. Sample counts were expressed as numbers/m2 and tabulated on a computer program designed for the experiment (G.J. Steer, S.F.U. Grad..Student, pers. comm.). Aerator Water flow rates were measured with a recently cal i b r a t e d flowmeter (General Oceanics No. 2035) situated 2 meters below the surface in the downflow tube. Air flow was calculated from 23 nomograms and tables for various a i r pressures and o r i f i c e diameters (Atlas Copco, 1978). Oxygen was measured in the down flow tube at a depth of f i v e meters with a Winkler cali b r a t e d oxygen meter (YSI 54 ARC). S t a t i s t i c s The s t a t i s t i c a l test used to analyse Black Lake experimental data is a two-way analysis of variance program written by N.E. Gilbert of U.B.C. Conceptually, the program creates two-way tables using depths as the row variable and sampling dates as the column variable. By subtracting the experimental (west) data from i t s duplicate control (east) data the program produces a single two-way table based on the differences between experimental and control sides. If both sides were similar the net result should be near zero and show no s i g n i f i c a n t differences. If however, there are real differences between sides the program analyses the magnitude of the differences, and by using the appropriate F test one can either accept the n u l l hypothesis or reject i t and attribute the observed results to the experimental manipulation. A l l tests were performed at the 1% l e v e l of s i g n i f i c a n c e . A complete l i s t of F values for water chemistry parameters and zooplankton i s given in the appendix (Table 4 and Table 5). This p a r t i c u l a r method of analysis was recommended by N.E. G i l b e r t and Dr. P.A. Larkin and i s outlined in further d e t a i l by G i l b e r t (1972). 24 RESULTS Ci r c u l a t i o n Processes The compressor was started on A p r i l 11, 1978 immediately after pre-aeration data were c o l l e c t e d from both sides of the lake. Air bubbles r i s i n g up the inflow tube acted as an a i r - l i f t pump and generated a large volume-low velocity water flow. The air-water plume rose 10 cm above the water surface in the separator box while degassing, then flowed across the box towards the downflow tube. Aerated water then entered the downflow tube and discharged back into the hypolimnion. A strong odour of H^S was immediately released from upwelling water and could be detected several meters away from the separator box. No H^S odour was evident during the next sampling period, however a c h a r a c t e r i s t i c musty odour persisted for the remainder of the experiment. Water in the aerator remained clear during i n i t i a l startup and regular operation indicating sediment disruption was not occurring. The aerator was examined under several water flow regimes to determine an e f f i c i e n t operational setting. The f i n a l setting chosen for the experiment was 10.67 m3/min which generated a dai l y flow rate of 15365 m3/day. Theoretical hypolimnetic c i r c u l a t i o n time (hypolimnion volume=18779 m3) at th i s setting was 1.2 days, and 5.7 days were required to c i r c u l a t e the entire experimental side (87971 m3) under ice cover. The compressor was intended to run continuously from A p r i l 11, 1978 to March 6, 1979 however e l e c t r i c a l problems developed in early September. As a re s u l t , the compressor operated for 25 only three days in the period from September 5, 1978 to October 23, 1978. The separator box froze into the ice during November 1978 and the entire system functioned normally throughout the winter season. A small ring of ice formed around the inside of the separator box but thi s was kept to a minimum by the rapidly moving air-water current. Temperature Black Lake experienced balmy weather conditions during the 1978 spring ice melt. Consequently the entire ice cover disappeared within three days and the lake quickly s t r a t i f i e d without f u l l y c i r c u l a t i n g . Aeration started 11 days after i c e -off when temperatures ranged from 8.5 C (surface) to 4.0 C (bottom). In spite of t h i s small temperature gradient, the aerated side s t r a t i f i e d normally while the aerator c i r c u l a t e d i t s lowermost four meters. Thermal s t r a t i f i c a t i o n was maintained throughout the summer in both experimental and control portions of the lake (Figure 3). Maximum surface temperature (22 C) was reached on August 1 and maximum bottom temperatures of 12.6 C (experimental) and 11.0 C (control) occurred on August 29 and September 10 respectively. The most pronounced temperature effect of hypolimnetic aeration was c i r c u l a t i o n of the bottom four meters and creation of an isothermal hypolimnion. The temperature d i f f e r e n t i a l between five and nine meters was 0.5 C or less throughout the entire year and the new' hypolimnion assumed temperature c h a r a c t e r i s t i c s of the fi v e to six meter stratum. This resulted 2 6 F i g . 3 Temperature isopleths f o r experimental (west) and control (east) sides. ' 27 from mixing and d i l u t i o n of smaller cold volumes (7-8m, 8-9m) with larger cool strata (5-6m, 6-7m). Heat content on the aerated side increased 956 x 10' cal o r i e s from A p r i l 11 to August 1, at an average rate of 8.5 x 10' calories/day. Control side heat gain was similar, absorbing 968 xlO 5 c a l o r i e s over the same period and averaging 8.6 x 10' calories/day. Both sides d e s t r a t i f i e d simultaneously in mid October and experienced vigorous wind driven autumnal c i r c u l a t i o n u n t i l ice formation in mid November. A small (5 m x 10 m) area near the shore d i r e c t l y in l i n e with the aerator's outflow tube remained ice-free throughout most of the winter. Aerated water discharging from the outflow tube contained s u f f i c i e n t momentum to flow across the lake, deflect up the steep shoreline, penetrate inverse s t r a t i f i c a t i o n and melt several centimeters of ice cover. The aerator created a small open water area near i t s e l f however one could safely work nearby as i t remained firmly frozen in the ice. Hypolimnetic aeration under ice cover c i r c u l a t e d the entire experimental side (except for m i c r o s t r a t i f i c a t i o n at 0-1 m) as the weak inverse s t r a t i f i c a t i o n could not r e s i s t mixing currents generated by the aerator. The control side maintained inverse s t r a t i f i c a t i o n throughout winter. Control side heat content decreased 1323 x 1.0' c a l o r i e s from August 1 to i t s lowest point on March 6, averaging 6.1 x 10' calories/day. The aerated side l o s t 1315 x 10' c a l o r i e s from August 1 to i t s . coldest point on February 13, cooling more rapidly at 6.7 x 10' calories/day. Temperature results were s i g n i f i c a n t (2 way ANOVA, p < .01) 28 as the experimental hypolimnion was warmer during spring and summer, and cooler through f a l l and winter than the control hypolimnion. Transparency Secchi disk transparency was similar (< 1 m difference) on both sides throughout the experiment except for three periods in mid July, mid August and January when control values were greater (Figure 4). Minimum readings of 1.6 m (aerated) and 1.0 m (control) were obtained on A p r i l 25, coinciding with the vernal phytoplankton bloom. Secchi depths gradually, increased with the onset of summer reaching maximum values of 5.4 m (aerated) and 7.0 m (control) on.July 18. Transparency remained high through July and August as chlorophyll values were at a seasonal minimum. Secchi depths began decreasing in late August in conjunction with the f a l l phytoplankton bloom and remained near 3 m through f a l l c i r c u l a t i o n and winter ice cover. Values of 1% incident surface l i g h t followed a pattern similar to Secchi depths (Figure 4). Low values of 2.5 m on both sides were recorded during the spring bloom, gradually increasing to maximum readings of 5.2 m (aerated) and 6.4 m (control) in late July to early August. Control values were c l e a r l y greater (>1 m) during late July and August. 1% depths began declining in late August as the f a l l bloom developed and continued downward through f a l l c i r c u l a t i o n into winter ice cover. Minimum 1% depths of 2.0 m were recorded on both sides in late January-early February when ice cover reached i t s maximum thickness. BLACK LAKE DATA 1978 -79 SECCH1 DEPTH WEST + SECCHI DEPTH EAST X BLRCK LAKE DATA 1978 -79 1/ TRANSMISSION DEPTH WEST + 1/ TRANSMISSION DEPTH EAST X F i g . 4 Secchi and 1% transmission depths f o r experimental (west) and control (east) sides. 30 Decomposition Processes Oxygen The aerator increased dissolved oxygen by an average of 0.7 mg/1 on each cycle through the system. Water flow was constant at 15365 m3/day, therefore a t o t a l of 10.76 kg O^/day was added to the experimental hypolimnion (5-9 m). As a re s u l t , hypolimnetic oxygen concentrations increased from 0.2 to 2.0-2.5 mg/1 after 13 days aeration while control values remained at 0.1-0.3 mg/1 (Figure 5). Hypolimnetic oxygen levels remained below saturation a l l summer however near bottom values (9m) in the aerated portion were consistently higher (2.7 mg/1 av.) than in the control portion (0.2 mg/1 av.). Surface values were similar on both sides and averaged 8.5 mg/1 from May u n t i l August. Oxygen s t r a t i f i c a t i o n was disrupted by f a l l c i r c u l a t i o n and by October 24 surface and bottom values d i f f e r e d by less than 0.8 mg/1. F a l l c i r c u l a t i o n was vigorous and breaking waves were observed over the whole lake surface for the f i r s t time. Consequently, oxygen content on both sides doubled (Figure 6) within three weeks (October 24-November 14). Black Lake froze completely on November 14 and entered winter with oxygen levels of 7.2 (9 m) to 7.6 (0 m) mg/1 (experimental) and 5.5 (9 m) to 7.1 (0 m) mg/1 (control). Aeration continued throughout winter c i r c u l a t i n g the entire experimental side under ice cover. Experimental oxygen levels remained above 4.9 mg/1 a l l winter and by March 6 the lowest recorded value was 6.0 mg/1. In contrast, control oxygen 3 1 F i g . 5 Oxygen isopleths for experimental (west) and control (east) sides. 3 2 BLRCK LAKE DflTfl 1 978 - 79 TOTAL OXYGEN WEST + TOTAL OXYGEN EAST X F i g . 6 Total oxygen content i n experimental (west) and con t r o l (east) sides. 33 concentrations declined markedly during winter and less than 1.0 mg/1 was present in the lowermost strata (8-9 m) from January 23 to March 6. Hypolimnetic aeration stimulated oxygen consumption as experimental side hypolimnetic oxygen depletion rates were considerably higher (0.39-0.78 mg/l/day) than control side values (0.03-0.09 mg/l/day). Summer and winter oxygen depletion rates were variable however winter values (0.39-0.61 mg/l/day experimental; 0.04-0.06 mg/l/day control) were generally lower than summer values (0.46-0.78 mg/l/day experimental; 0.03-0.09 mg/l/day c o n t r o l ) . As would be expected, oxygen results were s i g n i f i c a n t (2 way ANOVA, p < .01) as the experimental side was consistently higher in dissolved oxygen. Total Organic Carbon TOC l e v e l s ranged from 8-14 mg/1 (experimental) and 7-16 mg/1 (control) in early A p r i l . Concentrations remained high in late A p r i l and May as the spring phytoplankton bloom reached i t s maximum (Figure 7). TOC then declined during the summer months when phytoplankton biomass was at i t s seasonal minimum. Surface values (0 m) averaged 8.7 mg/1 on both sides during summer, however hypolimnetic values (9 m) were higher on the experimental side (9.3 mg/1 av. experimental;6.7 mg/1 av. control) as c i r c u l a t i o n currents reduced d e t r i t a l sedimentation rates. TOC increased again t.o 12-18 mg/1 in September as the autumnal bloom developed and remained high' through f a l l overturn. TOC declined s l i g h t l y after ice formation as 3 4 3LRCK-LRKE-WEST-ORGAN IC -CRRBQN-MG/L "APRIL MAY JUNE JULY AUG AUG SEPT OCT DEC FEB APRIL 1 9 7 8 SAMPLING DATE " 1979 ! B LRCK - LRKE -ER5T -GRGRNIC -CRRBON-MG/L ° ~ \ V 1 V 0 MO 600 I ^ y \ \ Vl<KJ APRIL MAY JUNE JULY AUG AUG SEPT OCT DEC FEB APRIL 1 9 7 8 SAMPLING DATE ' 1979 F i g . 7 Total organic carbon isopleths for experimental (west) and control (east) sides. 35 parti c u l a t e • matter settled from the water column and allochthonous inputs were reduced. Despite higher hypolimnetic TOC levels on the aerated side, TOC values were not s i g n i f i c a n t l y (p > .01) influenced by hypolimnetic aeration. Major Nutrients (Note:Nitrogen and phosphorus data are expressed in terms of the amount of N and P they contain.) Nitrogen The immediate effect of experimental aeration was a marked reduction in hypolimnetic ammonia l e v e l s . Hypolimnetic lev e l s on the experimental side decreased from 3900 ug/1 to 25-54 ug/1 after just 13 days aeration whereas bottom concentrations on the control side remained at 2100-3090 ug/1 (Figure 8). Hypolimnetic (9 m) ammonia levels gradually increased during summer however experimental values (300 ug/1 av.) remained considerably lower than control values (1741 ug/1 av.). Surface concentrations of NH -N were similar on both sides during summer, averaging 33 ug/1 (aerated) and 36 ug/1 (control). Ammonia s t r a t i f i c a t i o n was eventually disrupted by f a l l c i r c u l a t i o n and levels declined and converged during three weeks of vigorous mixing. Ammonia level s on both sides were evenly d i s t r i b u t e d beneath ice cover and changed l i t t l e during winter. Ammonia results were s i g n i f i c a n t (2 way ANOVA, p < .01) as aeration reduced experimental side hypolimnetic ammonia le v e l s during spring and summer months. 3 6 BLRCK -LRKE -WE5T -N ITRQGEN : RMMONIR-;JG/L ' APRIL MAY JUNE JULY AUG . AUG SEPT OCT DEC FEB 1978 SAMPLING DATE BLRCK - LRKE -ER5T -N ITRGGEN : RfiMQN I R-jJG/L APRIL MAY JUNE JULY AUG AUG SEPT OCT DEC FEB APRIL 1978 SAMPLING DATE " ' 1979 F i g . 8 Ammonia nitrogen isopleths for experimental (west) and control (east) sides -, 37 Bacterial n i t r i f i c a t i o n was also influenced by hypolimnetic aeration. Nitrate f i r s t appeared in measureable concentrations (> 0.02 mg/1) on June 13. Experimental side hypolimnetic n i t r a t e then rose during summer, increasing from 0.05 mg/1 (June 13) to 0.12 mg/1 (August 29) (Figure 9). However, due to low oxygen l e v e l s , n i t r i f i c a t i o n did not occur in the control hypolimnion. As a result, control side bottom levels remained low and 9 m n i t r a t e samples were undetectable u n t i l October 24 (Figure 9). Surface n i t r a t e was consistantly low a l l summer, ranging from 0.02 mg/1 to 0.04 mg/1 (experimental) and 0.02 to 0.08 mg/1 (control). Nitrate increased considerably during f a l l c i r c u l a t i o n as increased oxygen levels stimulated n i t r i f i c a t i o n on both sides of the lake. A well defined n i t r i f i c a t i o n sequence was then observed. I n i t i a l l y NH^-N decreased and NO^-N appeared as an intermediate product. N i t r i t e s t a b i l i z e d at 0.014-0.016 mg/1 a f t e r 4 weeks f a l l c i r c u l a t i o n , then began declining and eventually decreased below detection levels (0.005 mg/1) in late January. Nitrate was homogeneously d i s t r i b u t e d at 0.33-0.34 mg/1 on both sides at ice formation and increased slowly as winter progressed. Maximum values of 0.50 mg/1 (experimental) and 0.48 mg/1 (control) were recorded on March 6 after nearly four months of . i c e cover. Nitrate l e v e l s were s i g n i f i c a n t l y (2 way ANOVA, p < .01) higher in the aerated hypolimnion during summer. N i t r i t e data was not s t a t i s t i c a l l y analyzed as 70 % of the samples were blank (< 0.005 mg/1). Total organic nitrogen (TON) results were variable and related to seasonal trends in phytoplankton abundance. Pre-3 8 BL.RCK-LRKE-WE5T-N I TROGEN : N I TRRTE-MG/L .020 t . , , , "APRIL MAY JUNE JULY AUG AUG SEPT OCT DEC FEB APRIL 1 9 7 8 SAMPLING DATE ' 1979 BLRCK - LRKE - ER5T -N ITROGEN :N ITRRTE -MG / L APRIL MAY . JUNE JULY AUG AUG SEPT OCT DEC FEB APRIL " 7 8 , SAMPLING DATE ' 1979 F i g . 9 Nitrate nitrogen isopleths for experimental (west) and contr o l (east) sides. 39 aeration values of TON ranged from 0.96 mg/1 (0 m) to 1.22 mg/1 (2 m experimental) and 0.53 mg/1 (8 m) to 1.18 mg/1 (6 m con t r o l ) . After two weeks aeration experimental bottom (8 m) values declined to 0.98 mg/1, then averaged 0.75 mg/1 during summer (Figure 10). Control bottom values increased to 0.90 mg/1 on A p r i l 24 and averaged only 0.63 mg/1 through summer as quiescent conditions allowed sedimentation of det r i t u s (Figure 10). Surface values of TON were higher on the control side, averaging 0.57 mg/1 (May 9-August 29) while experimental side values averaged 0.50 mg/1. This difference may result from stream water d i l u t i n g TON values on the experimental side during i n i t i a l stages of the experiment. TON increased s l i g h t l y on both sides at f a l l c i r c u l a t i o n , then decreased during winter. TON values were s i g n i f i c a n t l y (2 way ANOVA, p < .01) di f f e r e n t between sides as experimental values were higher in the hypolimnion and lower in the epilimnion during summer months. Phosphorus Hypolimnetic aeration pre c i p i t a t e d orthophosphate (PO^-P) ions in the experimental hypolimnion. Prior to aeration, hypolimnetic PO^-P values reached 1100 ug/1 (experimental) and 995 ug/1 (control). After two weeks aeration experimental 8 and 9 m PO4-P declined to 321-322 ug/1, while control 8 and 9 m PO^-P remained at 735-955 ug/1 (Figure 11). Hypolimnetic (9 m) values on the experimental side averaged 405 ug/1 during summer, however PO^-P increased from 344 ug/1 (May 9) to 480 ug/1 (August 15) due to aerobic P release and sedimentation of epilimnetic P compounds. Control side 9 m 4 0 BLACK-LAKE-WEST-NITROGEN:ORGAN IC -MG/L JUNE JULY AUG AUG SEPT OCT DEC FEB SAMPLING DATE 1979 BLRCK - LHKE -EA5T -N ITROGEN:ORGANIC -MG/L JUNE JULY AUG AUG SEPT OCT DEC FEB APRIL SAMPLING DATE ' 1979 F i g . 10 Total organic nitrogen isopleths f o r experimental (west) and control (east) sides. 41 BLRCK-LRKE-WE5T-0RTH0PH05PH0RU5-AIG/L APRIL MAY JUNE JULY AUG AUG SEPT OCT DEC FEB APRIL " 7 8 SAMPLING DATE ' 1979 BLRCK -LRKE -ERST -0RTH0PH05PH0RU5 - / JG /L 1978 SAMPLING DATE ' 1979 F i g . 11 Orthophosphate phosphorus isopleths for experimental (west) and control (east) sides. 42 values were much-higher (682 ug/1 av.) and increased from 625 ug/1 (May 9) to 809 ug/1 (August 29) as anoxic conditions stimulated anaerobic P release. Unusually high epilimnetic PO^-P levels occurred in Black Lake. Both sides averaged 284 ug/1 during summer despite an aerobic hypolimnion on one side and an anoxic hypolimnion on the other. This large r e l a t i v e l y constant background l e v e l of phosphorus and unusually low chlorophyll levels (see phytoplankton results) suggests most PO^-P i s in excess of b i o l o g i c a l requirements and would not be markedly affected by seasonal changes in a l g a l a c t i v i t y . Orthophosphate s t r a t i f i c a t i o n was eliminated during f a l l c i r c u l a t i o n and became isochemical at 360 ug/1 on October 24. PO^-P levels remained near 350 ug/1 on both sides during winter. However, control side 9 m PO^-P began increasing again in late January as oxygen levels declined below 0.5 mg/1 and anaerobic release occurred. Orthophosphate leve l s were s i g n i f i c a n t l y lower (2 way ANOVA, p < .01) in the experimental hypolimnion throughout most of the year. Experimental aeration increased the concentration of dissolved organic phosphate (DOP) in the aerated hypolimnion (Figure 12). This occurred as aeration currents reduced d e t r i t a l sedimentation rates by a c t i v e l y c i r c u l a t i n g the hypolimnion (theoretical hypolimnetic c i r c u l a t i o n time=1.2 days). This allowed more time for decomposition of sedimenting d e t r i t a l material which i s reflected in higher hypolimnetic oxygen consumption rates on the experimental side. Particulate phosphate results were also influenced by the 43 BLACK LAKE DATA 1978 -79 HYPOLIMNETIC OOP WEST + HYPOLIMNETIC DOP EAST X S A M P L I N G DATE F i g . 12 Hypolimnetic dissolved organic phosphorus content i n experimental (west) and control (east) sides. 44 experimental treatment. Epilimnetic values followed seasonal trends in phytoplankton abundance and were generally higher in the control side in spring and in the experimental side in f a l l . The reason for thi s i s not cle a r , however stream d i l u t i o n may be involved. Hypolimnetic values r e f l e c t e d d e t r i t a l sedimentation. Midsummer values were generally higher on the aerated side as c i r c u l a t i o n currents delayed sedimentation rates. Both DOP and parti c u l a t e phosphate were s i g n i f i c a n t l y (2 way ANOVA, p < .01) higher in the experimental hypolimnion. Although aeration increased aerobic P regeneration, t o t a l P remained lower on the experimental side (Figure 13). N:P- Ratios Total Nttotal P ratios (weight:weight) on both sides of Black Lake were remarkably constant throughout the entire experiment. Whole lake, epilimnetic (0-5 m) and hypolimnetic (5-9 m) rati o s were examined on both experimental and control sides and a l l ra t i o s were between 1.6-3.8 (Figure 14). Early spring N:P ratios were the highest, reaching 3.8 (experimental) and 3.6 (control) in A p r i l 1978. Ratios then declined to their lowest values (1.6 experimental;1.7 control) during summer months. N:P ratio s started r i s i n g again in early f a l l and gradually increased to 3.1 (experimental) and 3.0 (control) in March 1979. 45 BLACK LAKE DATA 1978-79 TOTAL P WEST' + TOTAL P EAST X APRIL MAY 1978 JUNE JULY AUG SEPT FEB MARCH 1979 S A M P L I N G DRTE F i g . 13 Total phosphorus content i n experimental (west) and control (east) sides. 46 BLACK LAKE DATA 1978-79 N:P RATIO WEST + N:P RATIO EAST X O CE 3.06 2 .72 4 Q_ 2 ..38 + 2 .04 APRIL MAY JUNE JULY AUG AUG SEPT OCT NOV DEC JAN 1978 FEB MARCH 1979 S A M P L I N G DATE F i g . 14 Whole lake N:P r a t i o s i n experimental (west) and control (east) sides. 47 Major Ions Total A l k a l i n i t y Hypolimnetic aeration profoundly influenced major ion exchange reactions at the sediment-water interface. For example, hypolimnetic a l k a l i n i t y (as CaCO^) decreased from 233-261 mg/1 to 213 mg/1 after just 13 days aeration. Control hypolimnion values were near 256 mg/1 during the same period. Experimental hypolimnion (9 m) levels remained much lower a l l summer, averaging 199 mg/1 as compared to 241 mg/1 in the control hypolimnion (Figure 15). Epilimnetic a l k a l i n i t y was not affected by the experimental treatment as both sides averaged 197 mg/1 through summer. F a l l c i r c u l a t i o n eliminated a l k a l i n i t y s t r a t i f i c a t i o n and both sides maintained similar levels throughout winter. Calcium and Magnesium Hypolimnetic calcium lev e l s also decreased during aeration, thus supporting the p r e c i p i t a t i o n theory outlined in the introduction. Experimental hypolimnetic (9 m) calcium declined from 49 mg/1 to 43 mg/1 following aeration whereas control values increased from 49 mg/1 to 51 mg/1 (Figure 16). This effect persisted a l l summer (43 mg/1 av. experimental;58 mg/1 av. co n t r o l ) , eventually disappearing at f a l l c i r c u l a t i o n . Epilimnetic calcium was not affected and both sides averaged 42 mg/1 during the same period. Winter values were similar on both sides. Magnesium ions responded in an analogous F i g . 15 Total a l k a l i n i t y isopleths f or experimental (west) and control (east) sides. B L H C K - L f l K E - W E S T - C f l L C I U M - M G / L APRIL MAY JUNE JULY AUG AUG S E P T O C T DEC FEB APRIL 1978 SAMPLING DATE 1 9 7 9 B L R C K - L A K E - E R S T - C R L C I U M - M G / L APRIL MAY JUNE JULY AUG AUG S E P T O C T DEC FEB APRIL 1978 SAMPLING DATE 1 9 7 9 F i g . 16 Dissolved calcium isopleths f o r experimental (west) and control (east) sides. 50 fashion i e . lower hypolimnetic l e v e l s during summer (13 mg/1 av. experimental;18 mg/1 av. co n t r o l ) , similar epilimnetic lev e l s (11.6 mg/1 av.) and equivalent concentrations on both sides through winter. Manganese Hypolimnetic manganese levels in Black Lake were 20-60 f o l d higher than epilimnetic l e v e l s . Low oxygen conditions in the hypolimnion reduced the oxidized barrier at the sediment-water interface and large quantities of reduced (Mn + 2) were released. As was expected, the effect of hypolimnetic aeration on manganese d i s t r i b u t i o n was s t r i k i n g . Experimental side dissolved manganese (< 0.45 u) was reduced below detection l i m i t s (0.02 mg/1) within 2 weeks (Figure 17). Particulate manganese (> 0.45 u) then b r i e f l y appeared, sedimented and remained absent for the remainder of the summer. Dissolved manganese in the control hypolimnion remained high (0.88-1.04 mg/1) during this period (Figure 17) however particulate manganese was also detected. This e f f e c t persisted a l l summer as hypolimnetic (9 m) levels on the experimental side were an order of magnitude lower (0.20 mg/1 av.) than control values (1.15 mg/1 av.). Surface values were similar on both sides throughout summer and were usually below 0.02 mg/1. Autumnal cooling and f a l l c i r c u l a t i o n disrupted dissolved manganese s t r a t i f i c a t i o n and the oxidation-sedimentation sequence was repeated. Manganese was homogeneously dis t r i b u t e d at 0.04-0.05 mg/1 by October 24, and remained at this l e v e l during winter as aerobic conditions were present on both sides. A l l major ions examined ( a l k a l i n i t y , calcium, 51 BL f l CK -LRKE -WEST-DI550LVED-MRNGRNE5E -MG/L "APRIL MAY JUNE JULY AUG AUG SEPT OCT DEC FEB APRIL " 7 8 SAMPLING DATE - " 7 9 BLRCK-LRKE -E f l ST -D ISSOLVED-MRNGf lNESE -MG/L '.'78 SAMPLING DATE ' " 7 9 F i g . 17 . Dissolved manganese isop l e t h s f o r experimental (west) and control (east) sides. 52 magnesium and manganese) were s i g n i f i c a n t l y lower (2 way ANOVA, p < .01) in the experimental hypolimnion. Iron concentrations were usually below detection lev e l s (0.1 mg/1). pH Interactions The hypolimnion of Black Lake was anoxic prior to experimental aeration. As a res u l t , i t s pH levels were depressed (7.7-7.8) in r e l a t i o n to normal surface values (8.0-8.1). Hypolimnetic aeration s i g n i f i c a n t l y increased (2 way ANOVA, p < .01) pH levels on the experimental side. Epilimnetic values were not affected however hypolimnetic levels were generally 0.1-0.4 pH units higher than corresponding control side values (Figure 18). F a l l c i r c u l a t i o n minimized pH differences between sides and homogeneous levels persisted throughout the winter. Phytoplankton Biomass (chlorophyll a) The spring phytoplankton bloom was well under way when sampling started in early A p r i l 1978. As a re s u l t , A p r i l chlorophyll values were among the highest recorded during the experiment. Control chlorophyll reached a maximum of 54 ug/1 and a 43 ug/1 maximum was recorded on the experimental side. After the spring bloom, chlorophyll on both sides declined to low levels (Figure 19). Average surface values during summer months (May-August) were 4.2 ug/1 (experimental) and 4.5 ug/1 (control). Bottom values (9 m) were also low averaging 5.5 ug/1 53 BLACK LAKE DATA 1978-79 HYPOLIMNETIC (9 M) P H WEST + HYPOLIMNETIC (9 M) P H EAST X F i g . 18 Hypolimnetic pH l e v e l s i n the experimental (west) and control (east) sides. F i g . 19 Chlorophyll a isopleths for experimental (west) and control (east) sides. . 55 (experimental) and 4.5 ug/1 (control). The f a l l phytoplankton bloom occurred in September and started two weeks e a r l i e r on the experimental side. As a re s u l t , f a l l chlorophyll concentrations were higher on the experimental side. Chlorphyll then declined to low levels (4-8 ug/1) with the onset of f a l l c i r c u l a t i o n and remained low u n t i l ice formation. A large under-ice bloom (55 ug/1 max. experimental; 85 ug/1 max. control) occurred in December-January and was generally confined to the surface (0-2 m) layers. Chlorophyll concentrations declined after the winter bloom and remained below 10 ug/1 for the duration of the experiment. The seasonal concentrations of chlorophyll were s i g n i f i c a n t l y (2 way ANOVA, p < .01) d i f f e r e n t as control side values were higher during the spring bloom and lower during the f a l l bloom. Phaeophytin was detected during the spring bloom and remained below detection leve l s for the remainder of the year. Composition Cyanophyta Blue-green algae in Black Lake were represented by several taxa: c o l o n i a l Merismopedium sp., Aphanizomenon sp. and a small (2-4 u dia.) unidentified coccoid form which dominated numerically. Merismopedium sp. and the coccoid were present throughout the entire year whereas Aphanizomenon sp. appeared only during the f a l l bloom. In addition, Anabaena sp. b r i e f l y appeared in August samples. Blue-greens appeared in large numbers in late spring (4000-56 6000 cells/ml) before declining to low levels (1760 cells/ml) in June and July (Figure 20). Their abundance increased again in early autumn and reached.a f a l l peak on October 10 at 6775 cells/ml (experimental) and 5135 cells / m l (control). Blue-green numbers then declined steadily for the remainder of the experiment. The seasonal occurrence of blue-greens was similar on both sides. Cryptophyta Chroomonas sp. and Cryptomonas sp. were the two Cryptophyta i d e n t i f i e d in Black Lake samples. Cryptophyta were most abundant during spring and f a l l months and less prevalent through summer and winter. The seasonal abundance of Cryptophyta was similar on both sides (Figure 20). The spring bloom peaked on May 9 (experimental) and A p r i l 24 (control), then diminished over summer as Cryptomonas sp. declined to undetectable l e v e l s . After August 15 both sides increased due to reappearance of Cryptomonas sp. and enhancement of existing Chroomonas sp. The f a l l bloom peaked on November 14 (experimental) and December 5 (control) at 376 and 340 c e l l s / m l . Total numbers of Cryptophyta then dwindled as winter progressed. Chlorophyta Two genera of Chlorophyta, Chlamydomonas sp. and Schroderia sp., were i d e n t i f i e d in Black Lake samples. Chlamydomonas sp . was i n i t i a l l y present in samples but declined to low levels BLACK LAKE PHYTOPLANKTON CYHNOPHYTA WEST-CELLS/ML + CYHNOPHYTA EAST-CELLS/ML X BLACK LAKE PHYTOPLANKTON CIILOROPHYTH UFiHT- CF.LLS/Ml. + CHLOROPHYTA EHST-CELLS/ML X 58 after a few weeks aeration (Figure 20). Schroderia sp. then appeared and became the dominant Chlorophycean. The seasonal abundance of Chlorophyta fluctuated throughout the summer months. Experimental side Chlorophyta dipped to undetectable leve l s in late August while control side numbers remained at 90 c e l l s / m l . Both sides increased to 360 cells/ml in early October as Chlamydomonas sp. reappeared in the water column. The f a l l bloom persisted u n t i l December 5, then gradually dwindled over the remaining winter months. Both sides exhibited similar seasonal trends with the exception of a late December-January bloom on the control side. Baciliariophyceae Bacillariophyta in Black Lake were represented by the orders Centrales and Pennales. A small spring pulse of diatoms developed on the control side and peaked on May 9, however a spring pulse was not observed on the experimental side (Figure 20). Experimental side diatoms eventually appeared on June 13. The abundance of diatoms fluctuated during the f a l l bloom, however both sides converged at 54 cells/ml on October 10. The winter pulse of diatoms peaked on January 23 (experimental) and November 14 (control) at 409 and 161 c e l l s / m l . Diatoms remained abundant for the duration of the experiment. In general, experimental side diatoms were more numerous through summer, f a l l and winter months whereas control side diatoms dominated in spring. 59 Zooplankton The limnetic macrozooplankton community in Black Lake was quite simple, consisting of a calanoid copepod, a cyclopoid copepod, one Daphnia species and a single species of r o t i f e r . Zooplankton data was analysed for both v e r t i c a l and seasonal differences in d i s t r i b u t i o n and abundance. The v e r t i c a l d i s t r i b u t i o n of zooplankton was not s i g n i f i c a n t l y d i f f e r e n t (2 way ANOVA, p > .01) between control and experimental sides for any species, therefore zooplankton abundance was converted to an areal basis and expressed as no./m2. Total Zooplankton Total zooplankton ( Daphnia pulex , Keratella quadrata , Cyclops bicuspidatus and Diaptomus• leptopus ) numbers were s t a t i s t i c a l l y d i f f e r e n t (2 way ANOVA, p < .01) between sides. This was primarily due to 2-4 fo l d greater numbers on the control side during the spring bloom (Figure 21). Total numbers were similar during summer, however experimental side numbers were generally higher during f a l l and winter months. I w i l l now examine the zooplankton community i n d i v i d u a l l y to determine which species were responsible for the aforementioned differences. Daphnia pulex The seasonal abundance of Daphnia pulex was not s i g n i f i c a n t l y (2 way ANOVA, p > .01) influenced by hypolimnetic aeration as nearly i d e n t i c a l seasonal trends occurred on both 60 17000000 LJJ UJ O LO 8710.000 BLACK LAKE ZOOPLANKTON TOTAL ZOOOLflMKTOH VEST + T0FHL ZOOPLANKTON EAST X SLACK LAKE ZOOPLANKTON • DAPHNIA WEST-NO./SO.M. + DAPHNIA EAST-NO./SO.M. X CK 251000 LjJ LiJ 125000 APRIL MAY 1978 JUNE JULY. AUG AUG SEPT OCT NOV DEC BLACK LAKE ZOOPLANKTON KERATELLfl WEST-NO./SO.M. + KERHTELLA EAST-NO./SO.M. X JAN FEB MARCH 1979 . 7iaooo ; UJ ! F i g . 21 Total zooplankton, Daphnia pulex and Keratella quadvata (numbers/m2) i n the experimental (west) and control (east) sides. 61 sides of the lake (Figure 2 1 ) . Daphnia were present in low numbers (< 600 m2) at the start of the experiment and f i r s t appeared in appreciable numbers ( 113 ,800/m 2 experimental; 80,500/m 2 control) on May 9 . The population then rapidly expanded and remained above 60 ,000/m 2 throughout the summer and early f a l l months. Maximum abundance occurred during mid June to mid August with peak numbers of 313 ,800/m 2 (experimental) and 262 ,200/m 2 (control). Daphnia numbers increased b r i e f l y during the f i r s t two weeks of f a l l c i r c u l a t i o n , however by mid November Daphnia numbers had declined considerably and continued decreasing throughout the winter months. • Keratella quadrata The seasonal abundance of Keratella quadrata was s t a t i s t i c a l l y higher (2 way ANOVA, p < . 0 1 ) on the experimental side, p a r t i c u l a r l y during f a l l and winter months. Keratella was i n i t i a l l y detected in late A p r i l - e a r l y May but remained scarce for several weeks. Rotifer abundance then increased above 10,000/m 2 on June 13 (Figure 2 1 ) . Control side r o t i f e r s were generally more abundant than their experimental side counterparts throughout the summer and early f a l l months. As f a l l c i r c u l a t i o n commenced Keratella abundance increased 2 -3 f o l d over late September values and continued increasing throughout f a l l and winter. Experimental side r o t i f e r s were now more numerous and remained higher for the rest of the experiment. The fa l l - w i n t e r population peaked on January' 23 at 898 ,300/m 2 (experimental) and 745 ,700/m 2 (control), then started 62 dec l i n i n g . Control side r o t i f e r s decreased more rapidly as only 81,400/mJ were present on March 6 as compared to 560,300/m2 on the experimental side. Cyclops bicuspidatus The population of Cyclops bicuspidatus inhabiting Black Lake during the experimental period was multivoltine, exhibiting a large spring pulse and a smaller f a l l peak in abundance. The majority of Cyclops overwintered in the form of planktonic late stage copepodites. Each developmental stage investigated ( n a u p l i i , copepodites and adults) was similar in v e r t i c a l d i s t r i b u t i o n on both sides, however each stage was s i g n i f i c a n t l y (2 way ANOVA, p < .01) d i f f e r e n t in terms of seasonal abundance. In general, each stage was more abundant on the control side during the spring months and similar on both sides through summer (Figure 22). This large spring population was responsible for the 2-4 f o l d difference observed in spring t o t a l zooplankton numbers (Figure 21). The experimental side was s l i g h l t y more populated in f a l l and both sides were variable during winter months. The development of Cyclops bicuspidatus from nauplii to copepodites to adults was well defined and e a s i l y followed through both spring and f a l l generations. Diaptomus leptopus Diaptomus leptopus was the only limnetic calanoid copepod present in Black Lake during the experimental period. In contrast to Cyclops bicuspidatus, Diaptomus leptopus was 63 5L"K LAKE Z^PLRNKFON CYCLOPS NflfJPLIJ W - .5r -N0 . /50 .M. CYCLOPS NAUPLII i.ASr-NO./SQ.M. o ^~A) 4,2 3 0.000 APRIL MAY JUNE 1978 AUG AUG SEPT OCT FEB MARCH 1 1979 BLACK LAKE ZOOPLANKTON CYCLOPS COPEPODITES 1-5 WEST-NO.NO./SO.METER + CTCLOPS COPEPODITES 1-5 ERST-NO.NO./SQ.METER X APRIL MAY 1978 JUNE JULY. AUG AUG SEPT FES MARCH 1979 SLACK LAKE ZOOPLANKTON CYCLOPS ADULTS WEST-NO./SO.M. + CYCLOPS P.DULTS EAST-NO . /SO . M. X APRIL MAY 1978 JUNE JULY AUG AUG SEPT JAN FEB MARCH 1979 F i g . 22 Cyclops bicuspidatus n a u p l i i , copepodites and adults (numbers/m2) i n the experimental (west) and control (east) sides. 64 univoltine and produced a brief midsummer generation of nauplii and copepodites (Figure 23). Adult Diaptbmus were present in the water column from June 1978 to March, 1979. Diaptomus overwintered in the form of resting eggs produced during f a l l c i r c u l a t i o n . Copepodite stages of Diaptomus leptopus were not influenced by hypolimnetic aeration, however nauplii and adults showed s i g n i f i c a n t (2 way ANOVA, p < .01) differences in seasonal abundance. In general, control side nauplii were more abundant in early summer and similar for the rest of the experiment. Experimental side adults were more abundant during late summer to f a l l months and variable at other times. 65 BLACK LAKE ZOOPLfmfOX DIAPTOMUS -NAUPLII WESf- N 3 . /30 METER 4-DIAPTOMUS NAUPLII EAST- NO./30.METER X 3^800 APHIL MAY 19/8 AUG AUG SEPT OCT NOV DEC BLACK LAKE ZOOPLANKTON DIAPTOMUS ADULT WEST-NO./SO.M. + DIAPTOMUS ADULT EAST-NO./SO.M. X /.—l—xz L* FEB MARCH 1979. APRIL MAY JUNE JULY AUG AUG SEPT OCT NOV DEC JAN FEB MARCH F i g . 23 Diaptomus leptopus n a u p l i i , copepodites and adults (nurabers/m2) i n the experimental (west) and control (east) sides. 66 DISCUSSION The hypolimnetic aeration of Black Lake was a success from both a technical viewpoint of experimental design and a theo r e t i c a l standpoint of simultaneously examining several functional components of the lake ecosystem. However, despite considerable planning and forethought, certain events beyond my control influenced the outcome of t h i s experiment. F i r s t l y , Black Lake was in a state of temporary meromixis prior to and during the f i r s t half of the experiment. Pre-aeration data (K.I. Ashley, unpub.) indicated the hypolimnion was anoxic during summer, f a l l and winter of 1977-78, and that Black Lake did not f u l l y c i r c u l a t e in f a l l 1977 and spring 1978. However, Black Lake experienced several weeks vigorous c i r c u l a t i o n in f a l l 1978 which minimized physical-chemical differences between sides-for the remainder of the experiment. Therefore, the influence of hypolimnetic aeration was most noticeable during spring and summer of 1978, espe c i a l l y in the lower (8+9 m) portions of the hypolimnion. Secondly, the creek which flowed into Black Lake (Yellow Lake Creek) was dry in 1977 and no h i s t o r i c a l records existed for previous flow periods. Unfortunately the creek flowed from A p r i l to June in 1978. Creek water entered the experimental side of the lake, mixed with lake water and flowed around the curtain into the control side. The lake l e v e l rose 0.75 m before lake water reached the outflow culvert which drained from the experimental side into Yellow Lake (Figure 1). This flushing of the experimental side during spring 1978 was probably 67 responsible for lower zooplankton numbers and epilimnetic organic water chemistry measurements (eg. chlorophyll a, organic N) on the experimental side during spring 1978. Ci r c u l a t i o n C i r c u l a t i o n currents generated by the aerator had no effect on the formation and maintenance of thermal s t r a t i f i c a t i o n throughout spring, summer and early f a l l months. Heat content increased at similar rates on both sides, indicating minimal disturbance from the aerator. Maximum surface temperature (22 C) occurred on the same date (August 1) on both sides and maximum bottom temperature on the experimental side (12.6 C) was only 1.6 C warmer than the control side (11.0 C). This resulted from mixing within the hypolimnion and heat transfer across the inflow and outflow tubes. This indicates normal density s t r a t i f i c a t i o n is s u f f i c i e n t to is o l a t e an a c t i v e l y c i r c u l a t i n g hypolimnion (theoretical c i r c u l a t i o n time=1.2 days) from the epilimnion with no danger of d e s t r a t i f i c a t i o n or thermocline erosion. Both sides d e s t r a t i f i e d at approximately the same time (October 10). The aerator continued operating during f a l l c i r c u l a t i o n and did not delay ice formation as both sides froze over in mid November. Total heat loss to the atmosphere was similar on both sides however the experimental side depletion rate was higher due to c i r c u l a t i o n of freezing water immediately beneath the ice surface and exposure of lake water to sub-zero temperatures in the separation box. The aerator ci r c u l a t e d the entire lake under ice cover. 68 This is a common feature of winter hypolimnetic aeration as inverse s t r a t i f i c a t i o n i s too weak to r e s i s t mixing currents (Wirth et a l . , 1975). Winter aeration did not weaken the ice surface and minimized the open water hazard usually associated with such a c t i v i t i e s . Secchi disk and 1% transmission depths were similar on both sides of the lake when Secchi and 1% depths were less than 5 m. This indicates hypolimnetic c i r c u l a t i o n did not increase the v e r t i c a l transfer of substances across the thermocline. Previous hypolimnetic aeration experiments have decreased surface transparency when nutrient r i c h hypolimnetic water leaked through the aerator walls into the epilimnion and stimulated dense algal blooms (Fast et a l . , 1973). The Black Lake aerator was watertight and no leakage occurred. A noticeable (>1 m) difference occurred when Secchi and 1% transmission depths exceeded 5 meters. Experimental depths were r e s t r i c t e d to approximately 5 meters (max. Secchi=5.4 m, max. 1%=5.2 m) whereas control depths were not r e s t r i c t e d (max. Secchi=7.0 m, max. 1%=6.4 m) (Figure 4). I believe this difference was due to increased t u r b i d i t y in the experimental hypolimnion (5-9 m) which was experiencing active c i r c u l a t i o n by the aeration system ( c i r c u l a t i o n time=1.2 days). Hypolimnetic aeration increased t u r b i d i t y in the experimental side because c i r c u l a t i o n currents reduced d e t r i t a l sedimentation rates. Temperature data support t h i s conclusion as hypolimnion temperatures d i f f e r e d by 0.5 C or less throughout the year indicating a well mixed hypolimnion. Fast (1971) decreased the transparency of an oligotrophic lake as 69 c i r c u l a t i o n currents kept detrit u s in suspension, and Ridley et a l . (1966) observed decreased sedimentation of s i l t and organic debris in an English reservoir due to a r t i f i c i a l c i r c u l a t i o n currents. Increased hypolimnetic t u r b i d i t y may be a common feature of hypolimnetic aeration. This should not degrade epilimnetic transparency as c i r c u l a t i o n currents are usually confined to the hypolimnion during thermal s t r a t i f i c a t i o n . Under these circumstances the function of the hypolimnion changes from a passive s e t t l i n g zone to an a c t i v e l y c i r c u l a t i n g decomposition zone not unlike a "dark epilimnion." D e t r i t a l material entering the hypolimnion is kept in suspension much longer than usual and increased oxygen levels ensures more complete decomposition of sedimenting material. One would expect increased oxygen consumption rates and higher leve l s of decomposition products (eg. dissolved organic P and organic N) to appear in a ci r c u l a t e d hypolimnion. Analysis of oxygen consumption and nutrient levels (see decomposition and nutrients) in Black Lake confirm t h i s hypothesis. Continued suspension of de t r i t u s should reduce organic loading to the sediments (Hargrave, 1975) and eventually decrease sediment oxygen demand. This i s an important step in the restoration of c u l t u r a l l y eutrophic lakes. 70 Decomposition Experimental aeration s i g n i f i c a n t l y increased hypolimnetic oxygen concentrations and maintained aerobic conditions at the sediment-water interface throughout the year. However, hypolimnetic oxygen levels fluctuated within a narrow range (1.6-3.7 mg/1) a l l summer despite addition of 10.76 kg O^/day by the aerator. I believe hypolimnetic aeration modified Black Lake decomposition processes in two ways. F i r s t l y , c i r c u l a t i o n currents generated by the aerator stimulated sediment oxygen demand. The water column component of whole-lake oxygen consumption i s generally less than the sediment component due to the accumulation of sedimented organic material and bacteria at the sediment-water interface (Wetzel, 1975; Mathias and Barica, 1980). Sediment oxygen consumption therefore results from b i o l o g i c a l respiration and chemical oxygen demands of reduced substances emanating from deeper anaerobic layers ( F i l l o s , 1976). Sediment oxygen uptake rates exhibit asymptotic responses and appear largely independent of oxygen concentrations in overlying waters when oxygen leve l s exceed 2-3 mg/1. However, when oxygen declines to threshold lev e l s of 2-3 mg/1, oxygen uptake decreases as eddy d i f f u s i o n i s unable to overcome oxygen gradients at the sediment surface and the rate of oxygen supply becomes l i m i t i n g (Hargrave, 1969; Mathias and Barica, 1980). A r t i f i c i a l mixing eliminates concentration gradients and markedly increases oxygen uptake at low dissolved oxygen levels (Hargrave, 1969). Hypolimnetic aeration continually replenished the sediment-water interface 71 with oxygen and prevented anaerobic conditions from developing, thereby increasing sediment oxygen consumption. Secondly, continued suspension of sedimenting organic material increased the water column component of hypolimnetic oxygen consumption. Once sedimenting material leaves the epilimnion of a shallow lake such as Black Lake (Z max.=9 m), i t travels a r e l a t i v e l y short distance in the hypolimnion before reaching the lake bottom. As a re s u l t , the majority of hypolimnetic decomposition occurs at the sediment surface. Hypolimnetic aeration increases the " e f f e c t i v e depth" of the hypolimnion by a c t i v e l y c i r c u l a t i n g d e t r i t u s and reducing s e t t l i n g rates. Sedimenting organic matter then experiences more complete oxidation in the water column and the water column assumes greater importance as a decomposition zone in re l a t i o n to the sediment surface. This process i s analogous to increased depth of mixing in larger lakes which results in greater water column mineralization of organic material (Hargrave, 1973). During experimental aeration, hypolimnetic respiration was not r e s t r i c t e d by low oxygen levels and sediment-oriented decomposition, and consumed oxygen at i t s maximum temperature-dependent rate. A mass balance analysis of hypolimnetic (5-9 m) oxygen confirms t h i s assumption as experimental side consumption rates were an order of magnitude higher '(0.39-0.78 mg/l/day) than control side values (0.03-0.09 mg/l/day). As the aerator supplied more oxygen, the sediments and water column consumed more oxygen. The aerator was physically unable to exceed the additional oxygen demand consequently the system o s c i l l a t e d about a steady-state condition (1.6-3.7) for the summer period. 72 Literature threshold oxygen leve l s (2-3 mg/1) ( F i l l o s , 1976; Mathias and Barica, 1980) are suprisingly close to the observed range in the aerated hypolimnion which suggests oxygen supply and demand were closely matched. Oxygen leve l s increased at f a l l c i r c u l a t i o n as the entire lake was vigorously mixed, and experimental side winter oxygen levels usually ranged between 6-7 mg/1. Temperature strongly influences seasonal variations in sediment oxygen uptake (Hargrave, 1969), hence the observed r i s e in winter oxygen was related to decreased benthic and water column consumption. Winter oxygen depletion rates were generally lower than summer rates. Graneli (1978) reported sediment Q10 values of 2.0-3.0 for the i n t e r v a l 5-10 C. This approximates observed differences in experimental side oxygen and temperature values between summer and winter, and indicates that despite higher winter oxygen l e v e l s , supply and demand remained closely balanced. Total organic carbon (TOC) results were not s i g n i f i c a n t l y d i f f e r e n t between sides. Fast (1971) also reported l i t t l e change in organic carbon during hypolimnetic aeration. This result is not suprising as the majority of TOC i s dissolved organic carbon (DOC) which i s primarily refractory organic compounds resistant to microbial degredation (Wetzel and Otsuki, 1974). In order to accurately assess the impact of hypolimnetic aeration on TOC i t would be necessary to fractionate TOC into i t s dissolved and pa r t i c u l a t e components and c o l l e c t samples at shorter intervals to d e l i n i a t e possible changes in the more l a b i l e dissolved organic f r a c t i o n s . The aeration system was incapable of meeting increased 73 oxygen demands in the experimental hypolimnion as dissolved oxygen increased by a mere 0.7 mg/1 on each cycle through the system. Oxygen increases per cycle usually range from 2.3 mg/1 (Smith et a l . , 1975) to saturation values (Fast, 1971). The reason for such low values is twofold. The main factor was the shallow depth of Black Lake. Previous studies on hypolimnetic aeration have r e l i e d on high hydrostatic pressure as the driving force for oxygen transfer. The co-current mode of bubble-water transport in the inflow tube becomes progressively less e f f i c i e n t at oxygen transfer in shallow depths. Declining hydrostatic head i s mainly responsible for this drop,, however decreasing oxygen content of r i s i n g a i r bubbles also contributes to poor transfer e f f i c i e n c y in shallow water (Speece et a l . , 1974) . Smith et a l . (1975) reported dissolved oxygen increased' from 0 to 2.3 mg/1 i n the lower 6 m of their Mirror Lake (Zm=13 m) aeration tube, and observed no further increase in the remaining 6 m r i s e to the surface. Bernhardt (1967) also discovered most oxygen transfer occurred in the lower 20 m of the Wahnbach Reservoir (Zm=44 m) aerator, with no increase during the remainder of the ascent. This phenomenon was further substantiated by Hess (1976) during hypolimnetic aeration of shallow (Zm=6 m) Spruce Knob Lake. Hess (1976) re a l i z e d transfer e f f i c i e n c y from r i s i n g a i r bubbles was low and switched to mechanical aeration in an e f f o r t to improve oxygen tranfer. Therefore hypolimnetic aeration in shallow lakes (< 10 m) i s handicapped by low hydrostatic pressure and oxygen transfer e f f i c i e n c i e s . The Black Lake aerator floated 7.3 m below the 74 surface, and next to Spruce Knob Lake i s the shallowest lake to have undergone hypolimnetic aeration. Incorrect bubble size also reduced oxygen increase per cycle. The influence of bubble size on oxygen transfer i s related to the mechanics of oxygen d i f f u s i o n at the bubble-water interface and e f f i c i e n c y of a i r - l i f t pumps. The oxygen transfer process occurs in three stages (Eckenfelder and Ford, 1968). I n i t i a l l y , oxygen molecules from the gas phase are rapidly transported to the l i q u i d surface, resulting in saturation conditions at the interface. This l i q u i d interface or fi l m i s at least three molecules thick and i s composed of water molecules oriented with their negative ends facing the gas phase. In the second phase, oxygen molecules pass through t h i s f i l m by molecular d i f f u s i o n . In the t h i r d stage, oxygen i s mixed into the water body by d i f f u s i o n and convection currents. At very low mixing levels the rate of oxygen absorption i s controlled by molecular d i f f u s i o n through the undisturbed l i q u i d film.- As turbulence l e v e l s increase, the surface fi l m i s disrupted and renewal of the film becomes responsible for transferring oxygen to the l i q u i d (Eckenfelder, 1969). Therefore, to maximize transfer e f f i c i e n c y bubble v e l o c i t y should be high in rel a t i o n to water v e l o c i t y i e . large bubbles (Suschka, 1971). Unfortunately, large bubbles and higher v e l o c i t i e s decrease air-bubble contact time in the aerator and reduce a i r - l i f t pump e f f i c i e n c y by increasing s l i p v e l o c i t y between bubbles and l i q u i d in the inflow tube (Andeen, 1974). Small bubbles (< 0.5mm), in addition to having lower r i s e v e l o c i t i e s , • provide a higher surface area to volume r a t i o which 75 enhances oxygen exchange (Andersen and Hurd, 1971). However small bubbles require small d i f f u s o r o r i f i c e s which can lead to clogging problems, and at very small bubble diameters the energy required just to overcome surface tension and form bubbles may severely reduce aerator e f f i c i e n c y (Smith et a l . , 1975). Consequently, bubble size in the diameter range of 2.0 to 2.5 mm i s recommended for hypolimnetic aeration (Speece et a l . , 1974). Bubble size at Black Lake, although d i f f i c u l t to measure, was estimated by eye at 5-20 mm in diameter. This large size contributed to poor oxygen transfer e f f i c i e n c y however since bubbles tend to r i s e in "clouds" and coalesce during ascent i t may be very d i f f i c u l t to accurately regulate bubble size. I suggest further research in t h i s f i e l d is necessary before firm conclusions can be reached regarding bubble size and transfer e f f i c i e n c i e s in hypolimnetic aerators. Aside from obvious benefits of increased hypolimnetic oxygen leve l s (eg. oxidation of reduced substances, elimination of f i s h k i l l s ) these results suggest several conclusions. F i r s t l y , sediment oxygen uptake rates r e f l e c t several years accumulation of organic material, and drastic changes in lake productivity would not cause immediate declines in benthic consumption (Graneli, 1978). Consequently, hypolimnetic aeration would not s i g n i f i c a n t l y reduce sediment oxygen demand after a few months operation, i t would increase i t . Therefore several years may be required to oxidize accumulated organic material before noticeable declines in oxygen consumption appear. Circumstantial evidence from the Yellow Lake aeration project (Halsey and MacDonald, 1971) supports t h i s hypothesis as 76 increased oxygen levels are f i n a l l y appearing after 11 years intermittent aeration (C.J. B u l l , Fish and W i l d l i f e Branch, pers. comm.). The Black Lake experiment operated for only 329 days, therefore s i g n i f i c a n t declines in sediment oxygen demand were un l i k e l y . Secondly, when oxygen concentrations are below threshold levels physical mixing and oxygen input associated with a r t i f i c i a l aeration w i l l hasten the onset of anoxia by stimulating sediment and water column oxygen demand. Smith et a l . (1975) also observed a 3-4x increase in oxygen consumption during hypolimnetic aeration. This explains the catastrophic declines in oxygen levels often associated with l a t e - s t a r t i n g aeration projects (Patriarche, 1961; Seaberg, 1966; Wirth et a l . , 1975). F i n a l l y , benthic recolonization following successful hypolimnetic aeration may increase sediment oxygen demand. Benthic macroinvertebrates increase the surface area over which oxygen transport and reactions occur (Lee, 1970) and c i r c u l a t e substantial quantities of water (Brinkhurst, 1972). This aspect of hypolimnetic aeration requires further investigation. 77 Major Nutrients Nitrogen The nitrogen cycle is a complex dynamic system primarily mediated by microbial reactions. Hypolimnetic aeration, by modifying the physical and chemical environment, profoundly influenced several components of this b i o l o g i c a l l y mediated nutrient cycle. For example, hypolimnetic ammonia levels decreased dramatically after just two weeks aeration. This decline was a result of several interacting processes. An unknown amount of NH^-N escaped d i r e c t l y to the atmosphere when hypolimnetic water c i r c u l a t e d through the separation box. Elevated hypolimnetic pH levels encountered during aeration f a c i l i t a t e d this reaction by displacing the ammonium ion (NH^*)-ammonia (NH^) e q u i l i b r i a towards the gaseous form (NH^) which i s more e a s i l y v o l a t i l i z e d during vigorous bubbling (Stratton, 1969). Secondly, increased hypolimnetic oxygen l e v e l s enabled n i t r i f y i n g bacteria to oxidize NH^-N to NO_^ -N and NOj-N thereby reducing NH^-N levels (Brezonik et a l . , 1969). Nitrate was not i n i t i a l l y detected in the water column however since the spring bloom was under way any available n i t r a t e would be rapidly u t i l i z e d by phytoplankton. In addition, the presence of dissolved oxygen at the lake bottom oxidized the sediment surface and reduced NH^-N release (Graetz et a l . , 1973). Ammonia gradually increased in the experimental hypolimnion during summer as higher temperatures and increased b i o l o g i c a l production accelerated 78 b a c t e r i a l ammonification of sedimented organic material. Hypolimnetic pH levels also decreased s l i g h t l y which reduced ammonia venting via the aerator. F a l l c i r c u l a t i o n reduced NH -N levels on both sides as dissolved oxygen oxidized the sediment surface and stimulated b a c t e r i a l n i t r i f i c a t i o n . The n i t r i f i c a t i o n process was also influenced by hypolimnetic aeration. In th i s reaction NH-j-N i s progressively oxidized to NOa"N and NO^-N by Nitrosomas and Nitrobacter bacteria. The overall n i t r i f i c a t i o n reaction: NH^+ + 2 0 — r N 0 3 " + H^ O + 2H + proceeds normally above 0.3 mg/1 0^, and requires two moles of oxygen for the oxidation of each mole of NH^+ (Wetzel, 1975). Surface n i t r a t e on both sides was reduced below detectable lev e l s (< 0.02 mg/1) during the f i r s t weeks of aeration by intense photosynthetic uptake. However, hypolimnetic n i t r a t e on the experimental side did not appear despite s u f f i c i e n t oxygen for n i t r i f i c a t i o n and a dramatic decline in NH^-N. This suggests most NH^-N present on A p r i l 11 was liberated d i r e c t l y to the atmosphere and l i t t l e converted to NO^-N or NO^-N. Hypolimnetic NO^-N eventually appeared on June 13 and gradually accumulated during the remaining summer months. This i n i t i a l delay may represent the time required for n i t r i f y i n g bacteria to colonize new substrate given cool temperatures and a previous history of anoxic conditions. Control side hypolimnetic n i t r a t e (8 m + 9 m) remained low a l l summer as i n s u f f i c i e n t oxygen was present for n i t r i f y i n g bacteria to e x i s t . F a l l c i r c u l a t i o n introduced substantial quantities of oxygen to both sides and a c l e a r l y defined n i t r i f i c a t i o n 79 sequence was observed. I n i t i a l l y , NH^-N levels declined and NO^-N appeared. After several weeks c i r c u l a t i o n NO^-N was replaced by NO3-N which then accumulated throughout the winter. Experimental evidence suggests NO?-N w i l l not continue accumulating under aerobic conditions but actually decrease over time as NO^-N diffuses into lake sediments (which are anoxic beneath the s u r f i c i a l layer) and los t via d e n i t r i f i c a t i o n (Chen et a l . , 1972). Experimental n i t r a t e remained constant in late winter however control (9 m) nit r a t e began decreasing in January when oxygen declined below 0.5 mg/1. Oxidized nitrogen ions (NO^-N and NO^-N) trapped in the anoxic control hypolimnion underwent b a c t e r i a l d e n i t r i f i c a t i o n . Many fa c u l t a t i v e anaerobic bacteria (eg. Pseudomonas and Achromobacter ) are capable of d e n i t r i f i c a t i o n and this reaction i s p a r t i c u l a r l y prevalent in the anoxic hypolimnia of eutrophic lakes where large quantities of oxidizable organic material have accumulated. The response of t o t a l organic nitrogen (TON) to hypolimnetic aeration was confounded by stream inflow during spring months. Surface (0 m) values on the experimental side were noticeably lower on May 9, coinciding with the peak inflow of Yellow Lake Creek (K.I. Ashley, unpub. data). After the runoff period epilimnetic TON values on both sides followed seasonal fluctuations in phytoplankton biomass. Hypolimnetic TON was generally higher on the experimental side as the aerobic hypolimnion stimulated decomposition and reduced s e t t l i n g rates of sedimenting organic material. In summary, hypolimnetic aeration influenced several 80 reactions within the nitrogen cycle. Sedimenting organics in the experimental hypolimnion were rapidly ammonified and oxidized to n i t r i t e and n i t r a t e , which then became available for a l g a l uptake and d e n i t r i f i c a t i o n . This i s an important link in the nitrogen cycle as NH^ -N tends to be l o s t from solution by sorption onto particulate material or v o l a t i l i z a t i o n at high pH values (Brezonik, 1973). Conversion into NO^-N results in a more, stable form. The implications of these results are wide-ranging. Un-ionized ammonia (NH^) is highly toxic to most aquatic organisms and becomes increasingly so at low oxygen levels (Merkens and Downing, 1957). Hypolimnetic aeration greatly reduced hypolimnetic ammonia concentrations while increasing dissolved oxygen l e v e l s . This process w i l l improve habitat conditions within the hypolimnion and benefit the aquatic community. Although the concentration of toxic un-ionized ammonia (NHjj ) increases with pH (Trussel, 1972), the o v e r a l l decline in t o t a l NH 3 -N ensured un-ionized ammonia le v e l s did not exceed recommended chronic exposure levels for f i s h (EIFAC, 1970). The presence of increased NO-^ -N in the experimental hypolimnion may enhance oxidation of bottom sediments. Ripl (1976) reported a novel technique for "in s i t u " oxidation of nutrient r i c h organic sediments via n i t r a t e i n j e c t i o n and subsequent d e n i t r i f i c a t i o n : 2N03- + 3CH 0 ? + 3CCy+ 3H^O. Anderson (1976) also discovered water column addition of n i t r a t e enhanced organic matter degradation in lake sediments through d e n i t r i f i c a t i o n . Therefore hypolimnetic aeration, by adding 81 oxygen and stimulating n i t r i f i c a t i o n , should oxidize the sediment surface in Black Lake. Nitrogen could be removed from eutrophic lakes by alternating periods of hypolimnetic aeration and r e s t r a t i f i c a t i o n to effect a n i t r i f i c a t i o n - d e n i t r i f i c a t i o n sequence. This procedure has been proposed for N removal in sewage treatment plants (Brezonik, 1973) and could result in s i g n i f i c a n t N removal from lakes. Chen et a l . (1979) examined nitrogen transformations in simulated lake sediment-water systems and concluded n i t r i f i c a t i o n - d e n i t r i f i c a t i o n reactions may result in s i g n i f i c a n t N losses from the system. Phosphorus Phosphorus reactions at the sediment-water interface are an integral component of the phosphorus cycle. Their d i r e c t i o n and net result are d i r e c t l y regulated by the presence or absence of dissolved oxygen. Hypolimnetic aeration increased oxygen at the sediment-water interface and s i g n i f i c a n t l y influenced the phosphorus cycle in Black Lake. The immediate effect of experimental aeration was a large ~ decline in hypolimnetic PO^-P. The exact mechanism by which PO^-P levels were reduced is uncertain because iron l e v e l s appeared too low (< 0.1 mg/1, K.I. Ashley, unpub. data) to account for the observed decline. Oxidized manganese compounds do not co-pr e c i p i t a t e with PO^-P (Hutchinson, 1957) and despite their association with Fe complexes i t is unlikely that Mn p r e c i p i t a t i o n contributes s i g n i f i c a n t l y to PO^-P accumulation in lake sediments (Syers et a l . , 1973). I believe most PO^-P 82 coprecipitated with calcium carbonate compounds formed during the i n i t i a l stages of aeration (see major ions discussion). Phosphorus has been shown to p r e c i p i t a t e with calcium carbonate (Otsuki and Wetzel, 1972). and the pH r i s e in the experimental hypolimnion may have been s u f f i c i e n t to cause t h i s reaction. In addition, phosphorus may have adsorbed onto pa r t i c u l a t e d e t r i t u s c i r c u l a t i n g in the hypolimnion. Experimental hypolimnetic PCXrP increased slowly during summer despite aerobic conditions at the sediment-water interface. The work of Mortimer (1941-42) i s often c i t e d as evidence that orthophosphate release from lake sediments i s at a minimum under aerobic conditions. The oxidized microzone at the sediment surface forms an e f f i c i e n t barrier for manganese and iron as well as PO4-P which i s adsorbed onto and complexed with f e r r i c oxides, hydroxides, apatites and carbonates (Wetzel, 1975). This s u p e r f i c i a l oxidized layer prevents phosphorus movement out of the sediments while simultaneously scavenging P O L I - P from the water column (Mortimer, 1971). However, Lee (1970b) indicates appreciable POu/-P release occurs under aerobic conditions. Actual regeneration rates probably l i e somewhere between the two extremes as exemplified by Burns and Ross (1972). They discovered 25% P regeneration from the aerobic hypolimnion of Lake Erie and suggest most P G v ^ P produced by organic decomposition i s in close proximity to precipitated f e r r i c hydroxides contained in the oxidized microzone. This would readily lead to formation of insoluble f e r r i c hydroxy-phosphate complexes and explain the low but constant rate of PO^-P release under aerobic conditions. 83 Hypolimnetic aeration increased aerobic P release by reducing sedimentation rates of organic material and increasing oxygen l e v e l s . This allowed more complete decomposition in the water column which was r e l a t i v e l y free of f e r r i c hydroxides. As a r e s u l t , hypolimnetic PO^-P, dissolved organic P and p a r t i c u l a t e P increased despite aerobic conditions. Higher sediment temperatures and organic loading during summer would also stimulate aerobic PO.J-P release. Although pH i s a c o n t r o l l i n g factor in sediment water exchange reactions, pH s h i f t s encountered during hypolimnetic aeration were above the point of minimum P f i x a t i o n and should not result in extra PCv-P release (Nur and Bates, 1979). Control POL<--P levels exhibited marked inverse clinograde p r o f i l e s in accordance with the theory of PO^-P release under anoxic conditions (Morimer, 1941-42). Fer r i c hydroxy-phosphate complexes in the sediments were reduced and ferrous iron, divalent manganese and PO^-P released into the hypolimnion. The oxygen concentration at which anoxic PO 4-P regeneration occurred was between 0.2 and 0.5 mg/1. Literature values range from 0.6 mg/1 (Burns and Ross, 1972) to 4 mg/1 (Bernhardt and Wilhelms, 1975). These differences may be explained by the various sediment • types involved as Lee (1970b) discovered POu~P release was highly dependent on sediment c h a r a c t e r i s t i c s and varied considerably depending on sample locat i o n . Orthophosphate s t r a t i f i c a t i o n was eliminated during f a l l c i r c u l a t i o n , however iron and calcium carbonate levels were i n s u f f i c i e n t to cause complete PO^-P p r e c i p i t a t i o n . Consequently PO^-P le v e l s on both sides converged and remained near 350 ug/1 84 a l l winter. In summary, the c l a s s i c model of iron-phosphorus release and p r e c i p i t a t i o n operated in Black Lake. Unfortunately Fe levels were low and P levels unusually high. As a res u l t , considerable PO^-P remained in the water column despite i n i t i a l p r e c i p i t a t i o n reactions and aerobic conditions. Hypolimnetic aeration increased aerobic P release by stimulating decomposition. Organic detritus was quickly decomposed and P recycled throughout the hypolimnion on a continuous basis, rather than resuspended at spring and f a l l c i r c u l a t i o n as on the control side. Although aeration increased aerobic P release, t o t a l P on the experimental side remained lower than on the control side. This indicates hypolimnetic aeration reduced internal phosphorus loading which i s the primary goal of most lake restoration projects. It i s clear however, that the c l a s s i c limnological model should not be accepted per se for every lake. Aerobic PO -P release does occur and some lakes do not operate according to the ferric-phosphate model (Lee et a l . , 1977), especially those in which PO -P is bound to humic substances rather than redox-sensitive minerals. N:P Ratios Hypolimnetic aeration had l i t t l e or no effect on N:P rat i o s in Black Lake. This was a disappointing result as I predicted P coprecipitation would increase the N:P ra t i o and i n h i b i t nuisance blooms of blue-green algae (Schindler, 1977). Unusually high P levels were p a r t i a l l y responsible for thi s result which 85 minimized the effect of P p r e c i p i t a t i o n . In lakes with lower concentrations of P, hypolimnetic aeration should increase N:P ratio s by p r e c i p i t a t i n g P, preventing P release from the sediments and converting nitrogen into a more stable form (NOj-• N). This process would require several annual c i r c u l a t i o n periods and should eventually s h i f t a l g a l composition away from blue-green forms. 86 Major Ions The temporal and s p a t i a l d i s t r i b u t i o n of major ions i s regulated by a variety of factors including oxygen tension and hydrogen ion a c t i v i t y . Experimental aeration increased dissolved oxygen levels and vented hypolimnetic gases, thereby i n i t i a t i n g a series of reactions involving both dynamic and conservative major ions. Experimental aeration removed accumulated carbon dioxide from the hypolimnion. Although escaping gas was not analysed for CO^ content, i t s removal was inferred from several sources. F i r s t l y , aeration of water for CO^ removal i s common pratice in water treatment engineering (Sawyer and McCarty, 1978). Secondly, vigorous bubbling within the aerator was e f f e c t i v e in venting hypolimnetic gases as evidenced by the rapid removal of accumulated HZS and NH5. F i n a l l y , the c a l c u l a t i o n method of Rainwater and Thatcher (1969) indicates experimental hypolimnetic CO^ decreased from 10.2 to 4.2 mg/1 in the f i r s t two weeks of aeration while control (9 m) CO^ increased from 7.8 to 12.5 mg/1. Hypolimnetic pH lev e l s increased 0.1-0.4 pH units in response to CO_z. removal (see pH discussion). As a re s u l t , C a + 2 , Mg + 2 and HCO^" precipitated as carbonates. CaC0 3 p r e c i p i t a t i o n in natural waters can be represented by two reactions (Otsuki and Wetzel, 1974): 1. C a + 2 + 2HCCV — > CaC05 + CO^ + H 0 2. Ca + 2 + C0 5" 2 CaC0 3 Reaction 1, in conjunction with CCK removal, increases pH 87 values while reaction 2 decreases pH. Since bottom pH values increased during aeration, reaction 1 i s the equation explaining CaCOj p r e c i p i t a t i o n . Magnesium ions are more conservative than calcium ions in terms of chemical r e a c t i v i t y and b i o l o g i c a l requirements in natural lakes systems (Wetzel, 1975). Despite t h i s conservative nature, magnesium levels decreased considerably in response to hypolimnetic aeration. This is an unusual result since magnesium compounds are more soluble than calcium compounds and pre c i p i t a t e only at high pH (>10) levels (Wetzel, 1975). However, Fast et a l . (1973) observed a similar decline during hypolimnetic aeration of a small hardwater lake. Therefore I suggest magnesium precipitated in conjunction with CaCO^, possibly as CaMg(CO^)2 . These conclusions are supported by several observations. S o l u b i l i t y product calculations indicate both CaCO and CaMg(C0j)2 p r e c i p i t a t i o n could occur under these conditions. Secondly, dissolved calcium, magnesium and bicarbonate levels in the experimental hypolimnion declined s i g n i f i c a n t l y after just 13 days aeration and remained lower throughout thermal s t r a t i f i c a t i o n . F i n a l l y , analysis of sediment samples (n=2, upper 10 cm) taken A p r i l 15, 1979 at 8 m revealed calcium concentrations of 31.3 mg/gr (dry wt.) on the control side and 43.0 mg/gr (dry wt.) on the aerated side. After the i n i t i a l p r e c i p i t a t i o n reactions aerobic conditions in the experimental hypolimnion maintained lower bicarbonate, calcium and magnesium level s by preventing anaerobic release of bicarbonates and diss o l u t i o n of prec i p i t a t e d CaCO,. 88 In summary, major ion reactions i n i t i a t e d by hypolimnetic aeration are analogous to epilimnetic reactions caused by intense algal a c t i v i t y i e . increased pH, decreased CO^ and calcium levels and CaCO^ p r e c i p i t a t i o n . Other than Fast (1971) the author i s unaware of any hypolimnetic aeration study reporting similar r e s u l t s , the implications of which are int r i g u i n g . Wetzel (1975) suggested CaCO ^  p r e c i p i t a t i o n i s a major sink for inorganic and organic carbon and stated "Elevation of the pH of water containing t y p i c a l concentrations of calcium should lead to apatite formation." Otsuki and Wetzel (1972) reported coprecipitation of phosphate ions with CaCOj as pH lev e l s increased, and indicated t h i s may function as a population control mechanism for primary producers. Therefore, hypolimnetic aeration could reduce phosphate levels by copr e c i p i t a t i o n . Experimental aeration reduced hypolimnetic orthophosphate lev e l s in Black Lake and i t appears calcium-phosphate coprecipitation was responsible for t h i s result (see phosphate discussion). The effect of hypolimnetic aeration on manganese d i s t r i b u t i o n was s t r i k i n g . Soluble (< 0.45 u) manganese on the experimental side was reduced to undetectable levels (< 0.02 mg/1) within two weeks as Mn + Z was oxidized to part i c u l a t e MnO^ by the presence of hypolimnetic oxygen. Experimental side -particulate manganese (MnO^) then gradually sedimented and remained absent for the duration of the summer. Aerobic conditions in the aerated hypolimnion oxidized the sediment surface and prevented large accumulations of Mn + 2. In contrast, the control hypolimnion remained anoxic and 89 high concentrations of reduced manganese were present a l l summer. When oxygen concentrations declined, a sequence of conversions occurred and manganese changed from i t s oxidized to reduced (Mn + 2) divalent state. The oxidized barrier at the sediment-water interface then disappeared and manganese was released from the sediments and accumulated in the anoxic hypolimnion. Manganous manganese (Mn + 2) i s toxic to aquatic l i f e and creates serious problems in public water supplies (LaBounty and King, 1977) . At f a l l c i r c u l a t i o n the entire procedure was repeated as reduced Mn + 2 was oxidized to MnC^ , which then precipitated out of solution. Divalent manganese levels remained low on the control side during winter as s u f f i c i e n t oxygen was introduced at f a l l c i r c u l a t i o n to prevent reoccurrence of most reducing conditions. Brezonik et a l . (1969) reported oxidation of soluble Mn + 2 to p a r t i c u l a t e manganese oxides and i t s subsequent p r e c i p i t a t i o n during d e s t r a t i f i c a t i o n of Cox Hollow Lake, Wisconsin. Previous hypolimnetic aeration experiments have also reported lower manganese concentrations following aeration (eg. Bernhardt, 1974). Oxidation of reduced iron and manganese compounds at the sediment-water interface should minimize nutrient regeneration from redox-sensitive sediments (Mortimer, 1971). Although phosphate ions do not pr e c i p i t a t e with oxidized manganese compounds (Hutchinson, 1957), oxidized iron complexes with orthophosphate (eg. FePO<{) and could be an important factor in reducing hypolimnetic phosphorus concentrations in i r o n - r i c h lakes undergoing hypolimnetic aeration. Divalent manganese has 90 caused major f i s h k i l l s in hatcheries (Ingols, 1975) and hypolimnetic aeration of the source lake would prevent t h i s problem. pH As mentioned in the results and major ions discussion, hypolimnetic pH levels increased 0.1 to 0.4 units in response to experimental aeration. The aeration process vented accumulated COj7 from the hypolimnion, and since CO^ i s an a c i d i c gas, i t s removal increased pH according to: C0 7 + H,0^-~ H CO., HCO " + H +(Sawyer and McCarty, 1978). This s h i f t was responsible for a number of important reactions. The influence of increased hypolimnetic pH on calcium, magnesium, bicarbonate and phosphate levels has already been discussed (see major ions and major nutrients). Needless to say, thi s reaction s i g n i f i c a n t l y decreased the concentration of several ions in the experimental hypolimnion. Also discussed was the influence of pH on ammonia speciation which displaced the ammonium ion (NH^+)-ammonia (NH5) e q u i l i b r i a towards the gaseous form (NH^) which was more e a s i l y vented by the aerator (see major nutrients). The e f f e c t of higher hypolimnetic pH levels on hydrogen 91 s u l f i d e t o x i c i t y was less obvious. Hydrogen sulf i d e odour was detected on the experimental side during the f i r s t sampling t r i p , however on the remaining t r i p s i t s odour was replaced by a musty scent. In contrast, control 8 + 9 m samples contained H^S odour throughout spring, summer and early f a l l . Although H S concentrations were always below detectable levels (1 mg/1), i t s extreme t o x i c i t y warrants further discussion. Smith and Oseid (1975) reported acute t o x i c i t y l e v e l s of 0.025 mg/1 H S for j u v i n i l e brook trout ( Salvelinus f o n t i n a l i s ) and discovered long-term impairment of growth and reproduction at 0.009 mg/1. Hydrogen s u l f i d e occurs in lake water in three forms: undissociated H^S, hydrosulfide ions (HS") and s u l f i d e ions (S" z) (Hutchinson, 1957). In the normal pH range of lakes only H^S and HS" are present. H S is toxic and emits a strong odour whereas HS" is nontoxic and odourless. At pH 7, H^S and HS" are present in equal amounts, however at pH 8 HS" accounts for over 90% of the t o t a l hydrogen s u l f i d e (WHO, 1978). The elimination of H^S odour was a result of at least three processes. F i r s t l y , pH levels increased during aeration. This shifted the hydrogen s u l f i d e r e l a t i o n s h i p towards HS" which assisted in reducing the concentration of un-ionized H^S to a nontoxic and odourless l e v e l . Secondly, an unknown amount of un-ionized" H S was purged d i r e c t l y into the atmosphere by vigorous bubbling within the aerator as evidenced by i t s strong odour about the separator box. F i n a l l y , some un-ionized H^S was oxidized to SO^"2 within the hypolimnion by reacting with dissolved oxygen indroduced by the aerator: 2HiS + 50^ 2SOH" 2 +^ H^ O 9 2 Unfortunately no data exist to confirm these processes at Black Lake. Nonetheless, these reactions reduced the concentration of un-ionized H^S and would apply to any lake experiencing an anoxic hypolimnion. This i s an important step in the restoration of eutrophic lakes since H^S removal is ne.cessary prior to recolonization of aerobic organisms. Although phytoplankton species composition (Shapiro, 1978) and zooplankton feeding rates (Kring and O'Brien, 1976a) are influenced by pH s h i f t s , aeration induced s h i f t s in Black Lake were too small to cause any e f f e c t s . Phytoplankton The phytoplankton community in Black Lake was characterized by seasonal blooms of Chlorphyta, Cryptophyta and diatoms superimposed against a r e l a t i v e l y large background population of blue-green algae. Slight differences did exist between control and experimental sides, however these were generally short-lived and the o v e r a l l pattern of seasonal abundance was similar except for the December-January period. During t h i s period, diatom abundance was up to ten fold higher on the experimental side and Chlorophyta numbers were up to four ' f o l d greater on the control portion. I believe this difference was related to aeration currents c i r c u l a t i n g the 93 experimental side under ice cover. Diatoms would be favoured in th i s environment as they often occur naturally in turbulent conditions such as spring and f a l l c i r c u l a t i o n . Chlorophyta would be favoured on the control side as quiescent conditions would allow them to s t r a t i f y in the shallow photic zone immediately beneath the ice cover (Wetzel, 1975). Black Lake chlorophyll a values were high during spring, f a l l and midwinter however midsummer level s resembled oligotrophic values despite large reserves of orthophosphate (Schindler, 1974). Nitrate levels were generally less than 0.02 mg/1, therefore I expected large blooms of N f i x i n g blue-green algae to occur. Their absence and low numbers of other species suggests midsummer phytoplankton in Black Lake were r e s t r i c t e d by some form of micronutrient. Murphy et a l . (1976) and Goldman (1966) have both demonstrated the importance of micronutrients in stimulating phytoplankton growth and N f i x a t i o n in blue-green algae. Molybdenum levels were not determined however iron was consistantly below 0.1 mg/1 during summer months. This may explain the absence of large blooms of N f i x i n g algae. Epilimnetic (0-4 m) chlorophyll a values were lower on the experimental side in early May. I believe t h i s was a result of stream flushing and d i l u t i o n by Yellow Lake Creek which peaked in mid-May at 3-5 m3/min. Algal composition data was less affected by stream flushing as i t was averaged over several depth i n t e r v a l s . Chlorophyll a data also indicated the f a l l phytoplankton bloom started two weeks e a r l i e r and was larger on the aerated side. This result was probably due to micronutrients which had accumulated in the experimental hypolimnion throughout 94 the summer. At f a l l turnover these nutrients were mixed throughout the water column and stimulated an e a r l i e r f a l l bloom. Murphy et a l . (1980) discovered higher iron values on the aerated side of Black Lake which lends support to t h i s hypothesis. Long-term effects of hypolimnetic aeration on the phytoplankton community should become more apparent after several annual c i r c u l a t i o n periods. It i s d i f f i c u l t to predict exactly which changes w i l l occur as the micronutrients which limited a l g a l growth were not i d e n t i f i e d . I suspect one of the few ways to s i g n i f i c a n t l y a l t e r the phytoplankton community in Black Lake would be via t o t a l d e s t r a t i f i c a t i o n or iron and nitrogen additions (eg. Barica et a l . , 1980). Zooplankton The zooplankton community in Black Lake did not respond as expected to the experimental treatment. The reasons for this are twofold. F i r s t l y , the 5 to 7 m strata in the control hypolimnion was aerobic throughout the entire experiment. This reduced the eff e c t of hypolimnetic aeration on zooplankton v e r t i c a l d i s t r i b u t i o n as the control hypolimnion was already p a r t i a l l y aerobic. Therefore, as far as the zooplankton were concerned, the two sides of Black Lake were r e l a t i v e l y similar hence the 95 s i m i l a r i t y in v e r t i c a l distribution« Secondly, stream flushing in spring 1978 reduced j u v i n i l e Cyclops numbers on the experimental side. Yellow Lake Creek inflow peaked in early May which coincided -with the maximum difference between Cyclops numbers on the control and experimental sides. This theory is supported by organic N, chlorophyll a, dissolved organic P and particulate P data which were also unexpectedly lower on the experimental side at this time. Therefore, zooplankton data must be interpreted with these confounding factors in mind. The seasonal abundance of Daphnia pulex was not influenced by hypolimnetic aeration. This i s an interesting result since low oxygen levels are known to reduce Daphnia f i l t e r i n g and respiration rates (Heisey and Porter, 1977) and even cause mass die - o f f s (Nicholls et a l . , 1980). One possible explanation i s long-term exposure to low oxygen l e v e l s . Kring and O'Brien (1976b) observed low oxygen levels (1-3 mg/1) i n i t i a l l y depressed f i l t e r i n g rates in Daphnia pulex, however prolonged exposure (8-12 hrs) stimulated haemoglobin production and enabled Daphnia to resume i t s i n i t i a l high f i l t e r i n g rates. Many Daphnia c o l l e c t e d during spring and summer months were noticeably red • stained, possibly representing haemoglobin synthesized in response to low oxygen l e v e l s . This would allow Daphnia to remain in the control hypolimnion and explain the s i m i l a r i t y in v e r t i c a l and seasonal d i s t r i b u t i o n . The population of Keratella quadrata inhabiting Black Lake exhibited a late autumn-winter maximum, thus conforming to a cold stenothermal type of seasonal d i s t r i b u t i o n (Hutchinson, 96 1967). The seasonal abundance was similar on both sides through spring, summer and early f a l l however late f a l l and winter numbers were considerably higher on the experimental side. Low oxygen leve l s in the control hypolimnion (8+9 m) did not influence r o t i f e r v e r t i c a l d i s t r i b u t i o n , and t h i s response may r e f l e c t behavioural or metabolic adaptations to low oxygen conditions (Ruttner-Kolisko, 1975). Seasonal population changes in r o t i f e r s are poorly understood and quite variable (Wetzel, 1975). Planktonic r o t i f e r s feed mainly on sedimenting seston, and I believe c i r c u l a t i o n currents generated by the aerator enhanced the aerated side food supply by decreasing s e t t l i n g rates of seston during the c r i t i c a l winter period when autochthonous production was low and allochthonous inputs n e g l i g i b l e . In addition, long-term exposure to higher oxygen leve l s may improve some aspect of r o t i f e r growth or reproductive biology thus explaining higher experimental r o t i f e r numbers during f a l l and winter months. The v e r t i c a l d i s t r i b u t i o n of Cyclops bicuspidatus was similar on both sides despite hypolimnetic (8+9 m) oxygen depletion in the control portion. This result was not e n t i r e l y unexpected as Cyclops sp. are capable of b r i e f l y undergoing anaerobic metabolism (Chaston, 1969). Cyclops may have migrated v e r t i c a l l y during part of the day to escape low oxygen conditions. Cyclops nauplii and copepodites were more abundant on the control side during spring and on the experimental side during f a l l . I believe spring differences in abundance were a result of stream flushing on the experimental side. This reduced j u v i n i l e 97 Cyclops numbers however other species were not affected because they were not present at t h i s time (eg. Diaptomus ) or were in their adult form and able to r e s i s t flushing currents (eg. Daphnia ). F a l l differences were due to an increased food supply on the experimental side res u l t i n g from an e a r l i e r and larger f a l l bloom. In addition, long-term exposure to low oxygen levels reduces Cyclops bicuspidatus abundance through adult mortality and diapause of copepodite stages (Heberger and Reynolds, 1977). Higher experimental side oxygen leve l s may have reduced the number of Cyclops m o r t a l i t i e s and diapausing copepodites. As a r e s u l t , more Cyclops were present in the water column during late summer-fall despite i n i t i a l l y higher control side numbers. Unfortunately both sides experienced vigorous wind-driven c i r c u l a t i o n in October which minimized side to side differences for the remainder of the experiment. The seasonal d i s t r i b u t i o n of Diaptomus leptopus was s i g n i f i c a n t l y influenced by the experimental treatment. Control nau p l i i were more abundant in early summer and experimental adults more numerous in late summer-fall. Copepodite stages, while not s i g n i f i c a n t l y d i f f e r e n t , were s l i g h t l y more abundant on the aerated side. I believe seasonal differences between sides were related to resting egg development and adult survival under aerobic/anaerobic conditions. Brewer (1964) worked extensively with Diaptomus stagnalis resting eggs and concluded anoxic conditions, were a necessary stimulus for successful hatching of resting eggs. Diaptomus leptopus in Black Lake overwintered as 98 resting eggs deposited during f a l l c i r c u l a t i o n of the previous year. Hypolimnetic aeration of the experimental side may have delayed hatching of resting eggs by reducing their exposure to low oxygen conditions. This would explain i n i t i a l l y higher control numbers. However, once experimental resting eggs hatched higher oxygen levels may have enhanced their survival as low oxygen levels are toxic to Diaptomus sp. (Cooley, 1971). F a l l c i r c u l a t i o n increased oxygen on both sides and minimized any remaining side to side differences. On an o v e r a l l basis the zooplankton community was not greatly altered by one year of hypolimnetic aeration. However, th i s does not eliminate the p o s s i b i l i t y of long-term changes which could occur after several years. If the control side reverted to i t s former state of complete anoxia below the thermocline, these changes would be accelerated. Most l i t e r a t u r e surveyed (Chaston, 1969; Cooley, 1971; Heberger and Reynolds, 1977; Heisey and Porter, 1977) indicates limnetic zooplankton cannot tolerate anoxia or the toxic products (H^S, Mn + 2) which accumulate in an anoxic hypolimnia for extended periods of time. Therefore, after several years I would expect zooplankton on the aerated side to s i g n i f i c a n t l y expand their v e r t i c a l range and experience fewer anoxia related m o r t a l i t i e s than their control side counterparts. The introduction of planktivorous f i s h (which usually follows successful aeration) would intensify these differences by providing experimental zooplankton with a predation-free refuge (Shapiro, 1978). Increased l i g h t transmission may offset t h i s advantage in certain lakes (eg. K i t c h e l l and K i t c h e l l , 99 1980) however th i s would not occur in Black Lake due to increased hypolimnetic t u r b i d i t y . This demonstrates the importance of oxygen s t r a t i f i c a t i o n in structuring the limnetic macrozooplankton community. Management Implications and Suggest ions P r a c t i c a l applications often evolve from t h e o r e t i c a l research. This project was no exception and several management implications and suggestions arose during the course of the experiment. Hypolimnetic aeration is e s p e c i a l l y useful during winter months as i t maintains the ice surface and minimizes the open water hazard usually associated with such a c t i v i t i e s . Lakes are becoming increasingly important as winter outdoor recreation s i t e s and hypolimnetic aeration allows the lake manager to preserve the ice surface and retain multiple use options for the lake. Many lakes are surrounded by houses which often obtain their water d i r e c t l y from the lake. Hypolimnetic aeration, in addition to removing objectionable odours (H S), .decreases hypolimnetic hardness by p r e c i p i t a t i n g divalent metallic cations. The decline in hardness s h i f t e d Black Lake bottom water 100 from very hard (181-300 mg/1) to hard (121-180 mg/1) (Lind, 1979). This s h i f t would benefit water consumers by reducing soap consumption and pipe scaling problems. Lake restoration projects are often p a r t i a l l y funded by surrounding home owners, and benefits of this type would encourage their support. Aeration stimulates oxygen consumption when oxygen concentrations are below threshold l e v e l s . Therefore timing of aeration projects becomes . an important factor and aeration projects should begin well before oxygen leve l s decline to c r i t i c a l concentrations. In addition, sediment oxygen demand increases markedly with temperature. Hypolimnetic aerators which cause sediment warming (eg. Fast, 1971) may actually reduce oxygen l e v e l s . Hypolimnetic aerators should be designed to minimize hypolimnetic warming, and d e s t r a t i f i c a t i o n of shallow lakes during summer months may of questionable value (eg. Leach and Harlin, 1970; Nicholls et a l . , 1980). F i n a l l y , a long term commitment to lake restoration i s necessary and a great deal of research remains to be done. Lakes did not become excessively eutrophic overnight, and hypolimnetic aeration should not be expected to cure them in s t a n t l y . 101 SUMMARY AND CONCLUSIONS 1. Hypolimnetic aeration had no effect on the formation and maintenance of thermal s t r a t i f i c a t i o n throughout the ice-free season as normal density s t r a t i f i c a t i o n isolated the epilimnion from the a c t i v e l y c i r c u l a t i n g hypolimnion. 2 . Inverse s t r a t i f i c a t i o n was too weak to r e s i s t mixing currents and hypolimnetic aeration c i r c u l a t e d the entire experimental side under ice cover. 3. Winter aeration did not weaken the ice surface and minimized the open water hazard usually associated with such a c t i v i t i e s . 4. Hypolimnetic c i r c u l a t i o n did not increase the v e r t i c a l transfer of substances across the thermocline and epilimnetic transparency was unaffected. 5 . Hypolimnetic aeration thoroughly mixed the experimental hypolimnion and increased t u r b i d i t y levels by reducing d e t r i t a l sedimentation rates. 6. Hypolimnetic aeration changed the function of the hypolimnion from a passive c o l l e c t i n g zone to an act i v e l y c i r c u l a t i n g decomposition zone. 7. Continued suspension of det r i t u s should reduce organic loading to the sediments and eventually decrease sediment oxygen demand. 8 . Hypolimnetic aeration s i g n i f i c a n t l y increased hypolimnetic oxygen concentrations and maintained aerobic conditions at the sediment-water interface throughout the year. 9. Experimental aeration . increased hypolimnetic oxygen 102 consumption by stimulating sediment oxygen demand and enhancing the water column component of hypolimnetic oxygen consumption. 10. Hypolimnetic oxygen saturation was not achieved due to high oxygen demand, shallow depths and incorrect bubble size in the aerator. 11. A r t i f i c i a l aeration at low oxygen leve l s w i l l hasten the onset of anoxia by stimulating sediment and water column oxygen demand. 12. Aeration reduced hypolimnetic ammonia concentrations by a variety of mechanisms and increased n i t r a t e levels by supplying s u f f i c i e n t oxygen for b a c t e r i a l n i t r i f i c a t i o n . 13. Lower hypolimnetic ammonia level s would benefit the aquatic community while increased n i t r a t e l e v e l s may enhance sediment oxidation and allow nitrogen removal through n i t r i f i c a t i o n / d e n i t r i f i c a t i o n sequences. 14. Hypolimnetic aeration reduced orthophosphate levels via calcium carbonate coprecipitat ion. 15. Aeration increased aerobic P release by enhancing water column decomposition of organic material but reduced t o t a l internal phosphorous loading by maintaining aerobic conditions at the sediment-water interface. 16. Experimental aeration vented accumulated carbon dioxide from the hypolimnion and reduced hypolimnetic calcium, magnesium and bicarbonate levels through calcium carbonate p r e c i p i t a t i o n . 17. Reduced manganese (Mn + 2) in the experimental hypolimnion was oxidized and precipitated in accordance with Mortimer's observations. 18. Experimental aeration increased hypolimnetic pH levels 103 by CO^ removal, which i n i t i a t e d general ion p r e c i p i t a t i o n , s h i f t e d ammonia • speciation towards the gaseous form (NH^) and reduced H S t o x i c i t y . 19. Hypolimnetic aeration exerted a minimal effect on phytoplankton abundance and species composition due to unusually high orthophosphate l e v e l s , an apparent shortage of micronutrients and confinement of aeration currents to the hypolimnion during thermal s t r a t i f i c a t i o n . 20. Stream flushing on the experimental side and a p a r t i a l l y aerobic hypolimnion on the control side confounded zooplankton response to hypolimnetic aeration. 21. Daphnia pulex was not influenced by the experimental treatment due to long-term adaptation to low oxygen l e v e l s . 22. The winter population of Keratella quadrata was larger on the experimental side as aeration currents enhanced i t s food supply. 23. Cyclops bicuspidatus and Diaptomus leptopus were generally more abundant after several months aeration as higher oxygen leve l s increased adult and juvenile s u r v i v a l . 24. Management implications of hypolimnetic aeration include retention of multiple use options during winter, increased p o t a b i l i t y of hypolimnetic water, timing suggestions for aeration, projects and modifications to minimize hypolimnetic warming. 104 LITERATURE CITED Andeen, G.B. 1974. Bubble pumps. Compressed Air Magazine. 79:16-19. Andersen D.R. and M. Hurd. 1971. Study of a complete mixing activated sludge system. J. Water Pollution Control Federation 43(3):422-432. Andersen, J.M. 1977. Importance of the d e n i t r i f i c a t i o n process for the degredation of organic matter in lake sediments. pp. 357-362 in H.L. Golterman (ed.) Interactions Between Sediments and Fresh Water. D.W. Junk. The Hague. Atlas Copco Manual. Third ed. 1978. Stockholm, Sweden. H. Kling and J. Gibson. Barica, J 1980. Bernhardt 1967. Bernhardt 1974. Experimental manipulation of al g a l bloom composition by nitrogen addition. Can. Journ. Fish. Aq. S c i . 37(7):1175-1183. H. Aeration of Wahnbach Reservoir without changing the temperature p r o f i l e . J. Amer. Water Works Assoc. 59:943-964. H. Ten years experience of reservoir aeration. Progress in Water Technology 7(3-4 ): 483-495. Bernhardt, H. and A. Wilhelms. 1975. Hypolimnetic aeration as a means of c o n t r o l l i n g redox processes on the bottom of a eutrophic reservoir. Verh." Int. Verein. Limnol. 19:1957-1959. Bjork, S., L. Bengtsson, H. Berggren, G. Cronberg, G. D i g e r f e l t , S. Fleischer, C. Gelin, G. Lindmark, N. Maimer, F. Plejmark, W. Ripl and P.O. Swanberg. 1972. Ecosystem studies in connection with the restoration of lakes. Verh. Int. Verein. Limnol. 18:379-387. 105 Brezonik, 1969, Brezonik 1973 P.L., J.J. Delfino and G. Fred Lee. Chemistry of N and Mn in Cox Hollow Lake, Wise., following d e s t r a t i f i c a t i o n . A.S.C.E. J. Sanitary Engineering Div. 95:929-940. P.L. Nitrogen sources and cycl i n g in natural waters, E.P.A. 660/3-73-002. U.S. Brewer, R.H. 1964. The phenology of Diaptomus stagnalis (Copepoda:Calanoida) The development and the hatching of the egg stage. Physiol. Zool. 37(l):l-20. Brinkhurst, R.O. 1972. Burns N.M. 1972. The role of sludge worms in eutrophication. Ecol. Res. Series. U.S. E.P.A., Washington, D.C. and R.C. Ross. Oxygen-nutrient relationships within the central basin of Lake E r i e . pp.193-250 in H.L. Allen and J.R. Kramer (eds.) Nutrients in Natural Waters. Interscience, J. Wiley and Sons, Ontario. Chaston, ] 1969. Chen R.L. , 1972. Chen, R.L, 1979. Anaerobiosis in Cyclops varicans 14(2):298-300. Limnol. Ocean D.R. Keeney, D.A. Graetz and A.S. Holding. D e n i t r i f i c a t i o n and n i t r a t e reduction in Wisconsin lake sediments. J. Environ. Qual. 1:158-162. , D.R. Kenney and L.J. Sikora. Effects of hypolimnetic aeration on nitrogen transformations in simulated lake water-sediment systems. J. Environ. Qual. 8(3) : 429-433. Climate of B r i t i s h Columbia p r e c i p i t a t i o n and sunshine. 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Chemical exchanges between sediments and water in the Great Lakes-Speculations on probable mechanisms. Limnol. Ocean. 16(2): 387-404. Murphy, T.P., D.R.S. Lean and C. Nalewajko. 1976. Blue-green algae:Their excretion of iron-selective chelators enables them to dominate other algae. Science 192:900-902. Murphy, T.P., K. Hall and I. Yesaki. 1980. Iron requirements of blue-green algae in a naturally eutrophic lake. Abstract from 1980 SIL, Japan. NAS (National Academy of Sciences). 1969. Eutrophication: Causes, Consequences and Correctives. -Washington, D.C. 661 pp. Nasmith, H. 1962. Late g l a c i a l history and s u r f i c i a l deposits of the Okanagan Valley, B r i t i s h Columbia. B.C. Dept. Mines and Petroleum Resources. B u l l . No. 46. 46 pp. N i c h o l l s , K.H., W. Kennedy and C. Hammett. 1980. A f i s h - k i l l in Heart Lake, Ontario, associated with the collapse of a massive population of Ceratium  hirundinella (Dinophyceae) . 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Control of thermal s t r a t i f i c a t i o n on Thames Valley Reservoirs. Proc. Soc. Water Treatment Exam. 15:225-244. Biochemical oxidation of polluted lake sediments with n i t r a t e : A new lake restoration method. Ambio 5(3):132-135. Problems in restoring a mesotrophic lake using nutrient diversion. Paper presented at 37 th annual meeting of Amer. Soc. Of Limnol. Ocean. June 23-28, 1975. Ruttner-Kolisk'o, A. 1975. The v e r t i c a l d i s t r i b u t i o n of plankton r o t i f e r s in a small alpine lake with a sharp oxygen depletion (Lunzer Obersee). Verh. Int. Verein. Limnol. 19:1286-113 1294. Sawyer, C.N. and P.L. McCarty. 1978. Chemistry for Environmental Engineering. Third ed. McGraw-Hill Co. New York. 532 pp. Schindler, D.W. 1969. Two useful devices for v e r t i c a l plankton and water sampling. J. Fish. Res. Bd. Can. 26(7):1948-1955. Schindler, D.W. 1974. Eutrophication and recovery in experimental lakes: Implications for lake management. Science 184:897-898. 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Limnol. 19:1960-1970. 116 4.5, APPENDIX TABLE 2 Environmental Laboratory Water Chemistry Methods General Ions - u n f i l t e r e d , unpreserved, 3.785 l i t r e b o t t l e . 1. Alkalinity:Total-potentiometric t i t r a t i o n , pH endpoint 2. Calcium-atomic absorption. Direct aspiration, 211 nm. 3. Magnesium-atomic absorption. Direct aspiration, 285 nm. 4. Nitrogen:Ammonia-automated colorimetric; orthotolidine, 420 nm. 5. Nitrogen:Nitrate-cadmium reduction plus d i a z o t i z a t i o n . 6. Nitrogen:Nitrite-automated colorimetric; d i a z o t i z a t i o n , 520 nm. 7. Nitrogen:Organic-calculation, TKN-NH3=ON. 8. Nitrogen:Kjeldahl-acid digestion plus Nesslerization. 9. pH:pH meter. 10. Phosphorus:Orthophosphate-filtration (0.45 u M i l l i p o r e ) , automated colorimetric; ascorbic acid reduction, 885 nm. 11. Phosphorus:Total phosphate-automated colorimetric, digestion plus ascorbic acid reduction, 885 nm. 12. Phosphorus:Total dissolved phosphate-filtration (0.45 u M i l l i p o r e ) , automated colorimetric, digestion plus ascorbic acid reduction, 885 nm. 13. Phosphorous:Dissolved organic phosphate-calculation, TDP-OP=DOP. 14. Phosphorous:Particulate phosphate-calculation, TP-TDP=PP. Dissolved Metals - f i e l d pressure f i l t r a t i o n (0.45 u Mil l i p o r e ) preserved with 2 ml cone. HNO , in acid washed 500 ml bottles. 117 15. Iron:Dissolved-atomic absorption, d i r e c t aspiration, 248 nm. 16. Manganese:Dissolved-atomic absorbtion, direct a s p i r a t i o n , 279 nm. U n f i l t e r e d Metals -preserved with 2ml cone. HNO , acid washed 500 ml bottles. 17. Iron:Total-acid digestion, atomic absorption, 248 nm. 18. Manganese:Total-atomic absorption, di r e c t aspiration, 279 nm. Carbon - u n f i l t e r e d , unpreserved, 250 ml bottles. 19. CarbonrTotal organic-infrared analyzer. Sulfide - u n f i l t e r e d , preserved with 1 ml 2N zinc acetate, 500 ml bottl e . 20. Sulfide:Total-iodometric t i t r a t i o n . 118 TABLE 3 L i s t of Personal Communications 1. J.A. Botham. Technician. Water Rights Branch (Kelowna), Ministry of Environment, Province of B r i t i s h Columbia. 2. W.R. Eadie. Technical Service Supervisor, Woven Polyolefins D i v i s i o n , DuPont of Canada Ltd., Ontario. 3. J.E. F a r r e l l , P.Eng. Regional Engineer, Water Rights Branch (Kelowna), Ministry of Environment, Province of B r i t i s h Columbia. 4. J.H. Makiev. Design and Survey Branch, Project Engineer, (Penticton), Ministry of Transportation and Highways, Province of B r i t i s h Columbia. 5. J. Pinder-Moss. Herbarium Curator, Department of Botany, University of B r i t i s h Columbia. 6. D. E. Reksten, P.Eng. Senior Hydraulic Engineer, Hydrology Di v i s i o n , Environmental and Engineering Service, Water Investigations Branch, Ministry of Environment, Province of B r i t i s h Columbia. 7. G.J. Steer. Graduate Student. Simon Fraser University, Burnaby, B r i t i s h Columbia. 8. C.J. B u l l . Regional Fis h e r i e s B i o l o g i s t , Fish and W i l d l i f e Branch (Penticton), Ministry of Environment, Province of B r i t i s h Columbia. 119 TABLE 4 L i s t Of F Values For Water Quality Parameters row 5 d.f./90 d.f. 1%=3.23, columns 18 d.f./90 d.f. 1%=2.15 Parameter Row Column A l k a l i n i t y 13 .72 3 .37 Calc ium 11 .03 2 .26 Carbon:TOC 1 .19 1 .07 Chlorophyll a 0 .25 2 .59 Magnesium 12 .15 3 .38 Manganese:Di ss. 27 .79 1 .58 Manganese:Part. 3 .11 1 .44 Nitrogen:NH3 30 .54 1 .72 Nitrogen:N03 18 .44 2 .11 Nitrogen:TON 0 .89 2 .37 Oxygen 11 .60 2 .56 Phosphorus:Ortho. 25 .16 1 .51 Phosphorus:DOP 6 .15 0 .80 Phosphorus:PP 1 .85 2 .40 pH 7 .87 3 .34 Temperature 5 .08 3 .74 120 TABLE 5 L i s t Of F Values For Zooplankton row 9 d.f./162 d.f. 1%=2.52, columns 18 d.f./162 d.f. 1%=2.05 Parameter Row Column Cyclops:Naupli i 0.96 2.20 Cyclops:Cl-C5 1.81 11.8 Cyclops:Adults 0.72 4.01 Daphnia 1.35 1.29 Diaptomus:Naupli i 1.02 3.20 Diaptomus:CI-C5 1.40 1.6 Diaptomus:Adults 0.69 3.02 Keratella 0.81 2.79 Total zooplankton 2.10 10.5 

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