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Phosphous limited community dynamics of steam benthic algae and insects Quamme, D. L. 1994

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Phosphorus limited community dynamics of stream benthic algae andinsectsbyDarcie L. QuainmeB.Sc., The University of British Columbia, 1988A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF SCIENCEinTHE FACULTY OF GRADUATE STUDIESDepartment of ZoologyWe accept this thesis as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIAMarch 1994© Darcie L. Quamme, 1994In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.Department of JO -1--L.The University of British ColumbiaVancouver, CanadaDate I&CC 3O/4DE.6 (2188)IIABSTRACTThe relationship between external soluble phosphorus (P) concentration and the abundanceand taxonomic composition of stream insects was determined in streamside artificial troughs.The response of peak algal biomass (PB) to target P concentrations of 0, 0.5, 2.5, 5, 10, 50j.g P 1 was also monitored. A log-linear function of P concentration was used toapproximate PB and insect abundance. PB measured as chlorophyll a increased with Pconcentration linearly to 7.4 mg m2 at 2.5 jg P 11 and reached an asymptote at9.2 mg m2 (2.7X the controls). Adult baetid mayflies showed a significant increase innumber after 23 days of P addition; this effect was maintained over 9 weeks of treatment.Numbers of benthic baetids, nemourid and perlodid stoneflies and hydroptilid tricopteranssampled at the end of the experiment significantly increased with P concentration. Adult andbenthic insects of these taxa exhibited similar rapid increases in abundance from 0 -2.5 g P. 1.1 and all showed signs of saturation at approximately 1.5 - 3 times the controls atconcentrations greater than 2.5 P l1. Increased abundances of insects resulted fromgreater food availability. There was no detectable difference in the numbers of large Baetidnymphs drifting from the troughs with increasing P concentration. Increased survival ofBaetidae nymphs with increasing P level was thought to account for higher numbers of adultand benthic baetids observed with increasing P concentration. Graphical comparisonsbetween the control and treated troughs showed that they were similar in taxonomiccomposition of insects. Insect taxonomic richness did not change with increasing Pconcentration. These findings are important to fisheries researchers who are assessing thepotential of stream fertilization as a technique to enhance salmonid populations in nutrientdeficient streams.H’TABLE OF CONTENTS1.0 . . . . . . . . . .... .. . .. . .. . . . . . . . . . . ...... ii2.0 LIST OF I’4B1FS .......... iv3.0 I_4ST OF IGiTRFS v4.0 ix5.0 16.0 OF SITE . 77.0 . 108.0 ......................................... 229.0 IISDLJSSION .. ... 5410. 7011. APPENDL’C 1 77lv2.0 LIST OF TABLESTable 1. Mean inorganic phosphorus concentrations (jtg P 1’) measured for eachtarget treatment ... 23Table 2. Mean inorganic nitrogen concentrations (g N 11) measured for eachtarget treatjnent..................... ....... . 24Table 3. Percent composition of the algal cell number by taxa of the artificialstream troughs for each target phosphorus concentration (ug P11).Percentages are based on algae collected from trough gravel atthe end of the experiment (July 8) .................... 31Table 4. Linear regression analyses relating total numbers (y) of variousinsect taxa collected as adults from the emergence traps totarget phosphorus concentration. Sample size (n) = 15 in all cases.Ordered with respect to decreasing p value.... . . . . . . . . . .34Table 5. Mean number of adult insects immigrating and emigrating pertrough day by taxon. Sample size (n) = 3. Standard error is given inparentheses . ..38Table 6. Linear regression analyses relating the numbers (y) of variousinsect taxa collected from the trough benthos at the end of theexperiment to target phosphorus concentration. Sample size (n) = 13in all cases. Ordered with respect to decreasing p value.49V3.0 LIST OF FIGURESFigure 1. Keogh River watershed with the location of the artificial streamtroughs 9Figure 2. Photo of mesocosm apparatus. The experimental treatments of thetroughs from left to right were 0.5, 0 + no insect immigration, 0,0, 2.5, 5, 10, 2.5, 5, 10, 0.5, 2.5, 5, 0, 0.5, 10, 50 jg• .. . . • •.. . • • . . •..• .. . . 13Figure 3. Mean algal biomass accumulation measured as chlorophyll afrom styrofoam plates. Sample size (n) = 3 except at 50 ugP11 where n = 1. Nutrient treatments started May 18.(3.1) First set of styrofoam plates, placed in troughs from April -May 31.(3.2) Second set of styrofoam plates, placed in troughs from May31 - July 8(3.3) Algal biomass accumulation (chlorophyll a) on the second setof styrofoam plates. The grazer excluded trough with nophosphorus addition (n = 1) compared to the means of thecontrol troughs (n = 3) 26Figure 4. Peak biomass on styrofoam plates measured as chlorophyll a(mgm2)against target phosphorus concentration. Linearregression of the data fitted by least squares. Regression lineabove (all data) is y = 4.05 + 419 log(P). r2 = 0.67. p < 0.001.Regression line (not shown) for treatments 0 - 10 ug Pt’ only isy = 5.09 * log(P) + 3.70. r2 = 0.69 p < 0.001 .... 27Figure 5. Density (cellsinm2)of algal cells collected from the trough gravel atthe end of the experiment (July 8) against target phosphorusconcentration. Linear regressions of the data were fitted by leastsquares.(5.1) Total cell numbers. Regression line above (for all data) islog(y) = 3.56 + -0.95 * log(P). r2 = 0.44. p = .003. Regression linefor treatments 0 - 10 ug Ph1 only is log(y) = -0.73 * log(P) + 3.48.= 0.21. p = 0.05.(5.2) Achnanthes mimutissima. Regression line above (for all data)is y = 316.41 + -200.19 * log(P). r2 = 0.48. p = .001. Regressionline for treatments 0 - 10 ug Pt’ only isy = -221.96 * log(P) + 324.83. r2 = 0.41. p = 0.005.(5.3) Oscilk#oria sp. Regression line above (for all data) islog(y) = 3.36 + -1.39 * log(P). r2 = 0.45. p = .002. Regression linefor treatments 0 - 10 ug P1’ only is log(y) = -1.29 * log(P) + 3.32.= 0.29. p = 0.02 ..... 30viFigure 6. (6.1) Daily minimum and maximum stream temperatures measured atthe mesocosm head tank as a function of time.(6.2) Numbers of adult insects caught in emergence traps between May19 and July 4 by target phosphorus concentration (ugPt’). In eachcase, the mean number of adult insects is plotted per date. Sample size(n) = 3 except at 50 ug P11 where n = 1....... ... 33Figure 7. Percent composition of the predominant adult insect taxa collectedfrom emergence traps of the artificial stream troughs at twophosphorus concentrations. Percentages are based on total insectscollected from May 19 to July 4 and result from the mean of threereplicate troughsFigure 8. Number of adult Baetidae caught in emergence traps between May19 and July 4 by target phosphorus concentrations (ug Pt’). Ineach case, the mean number of adult insects is plotted per date.Sample size (n) = 3 except at 50 ug P1’ where n = 1..... 37Figure 9. Number of adult Baetidae captured at the end of the experiment asa function of target phosphorus concentration (ug P1’). Linearregression lines of the data were fitted by least squares.(9.1) Numbers caught in emergence traps on July 4. Regression lineabove (all data) is y = 29.19 + 33.46 * log(P). r2 = 0.34.p = 0.011. Regression line (not shown) for treatments 0 - 10 ug P1’only is y = 23.16 * log(P) + 49.03. r2 = 0.51 p = 0.002(9.2) Numbers caught in drift nets on July 1.Regression line above (for all data) is y = 11.32 + 21.43 * log(P).= 0.69. p < .001. Regression line for treatments 0 - 10 ug P1’only is y = 20.47 * log(P) + 10.27. r2 = 0.51 p = 0.001..................39Figure 10. Contribution of Ephemeroptera to total biomass of adults collectedin emergence traps on June 27 by target phosphorus concentration(ug P1—’) . ‘14)Figure 11. Adult Ephemeroptera biomass (total dry weight of all individuals)collected on June 27 as a function of target phosphorusconcentration (ug P1-’). Linear regression line of the data fitted byleast squares. Variances were not stabffized by transformation.Regression line above (all data) is y = 8.193 + 14.241 * log(P).= 0.48. p = 0.002.Regression line (not shown) for treatments 0 - 10 ug Pt’ only isy = 11.76 * log(P) + 9.15. r2 = 0.25 p = 0.03 41Figure 12. Percent composition of Ephemeroptera collected as adults at the endof the experiment on June 27 by target phosphorus concentration(ug Pt’) 42vuFigure 13. Numbers of adult insects by target phosphorus concentrationcollected in emergence traps. All insects were captured betweenMay 19 and July 4. In each case, the mean number of adult insectsnumbers is plotted per date. Sample size (n) = 3 except at50 ug Pt’ where n = 1.(13.1) Chironomidae. (13.2) Trichoptera. (13.3) Plecoptera... . . . .. . . . . . . . . . . . . . . 45Figure 14. Number of insects collected from the benthos at the end of theexperiment on July 8 as a function of target phosphorusconcentration (ug P1’). Linear regression lines of the data werefitted by least squares. See Table 7 for the results of regressionanalyses carried out on 0 - 10 ug Pt’ treatments for the same data.(14.1) Baetidae. y = 152.06 + 89.96 * log(P). r2 = 0.443. p = .006.(14.2) Nemouridae. y = 117.92 + 93.02 * log(P). r2 = 0.421. p =0.008(14.3) Perloclidae. y = 9.41 + 13.31 * log(P). r2 = 0.402. p = 0.01(14.4) Hydroptilidae. y = 13.95 + 10.26 * log(P). r’ = 0.16. p = 0.08Figure 15. Dominant insect families collected from the benthos of the artificialstream troughs on July 8 as a function of phosphorus concentration.Percentages are based on the mean of three replicate troughs.50Figure 16. (16.1) Daily immigration rates of large Baetidae nymphs to theartificial stream troughs over time. Immigration rates are based on24 hour drift samples taken at the trough inflow.(16.2) Daily emigration rates of mature Baetidae nymphs from theartificial stream troughs over time by target phosphorusconcentration (ug P1-’). Emigration rates are based on 3 day driftsamples collected from the trough outflow. In each case, the meannumber of Baetidae nymphs is plotted per date with the associatederror bars. Sample size (n) = 3, for all points. 53Figure 17. Relative peak algal biomass (PB: PB(max)), chlorophyll a, as afunction of phosphorus treatment (ug P1’). PB(max) is PB at50 ug Pt’. A. modified from Bothwell (1989). B. Present study.Sample size (n) = 3 at each treatment level except at 50 ug Pt’where ii = 1 57VillFigure 18. Number of adult Baetidae captured in emergence traps per weekfrom July 8 - August 22 as a function of target phosphorusconcentration (ug P1-’). Linear regression lines of the data werefitted by least squares. Regression line above (all data) islog4y) = 1.41 + 0.49 * log(P). r2 = 0.74. p < 0.001.Regression line (not shown) for treatments 0 - 10 ug Pt’only is log(y) = 1.41+ 0.48 * log(P). r2 = 0.62. p = .002. . . . . . . ... . 1 • • • . . . . . . . .. . . .. . . . .76Figure 19. Number of insects collected from the benthos at the end of thesecond experiment on August 22 as a function of target phosphorusconcentration (ug Pt’). Linear regression lines of the data werefitted by least squares.(19.1) Baetidae. y = 2088.56 + 1701.56 * log(P). r2 = 0.45. p .003.(19.2) Nemouridae. y = 339.45 + 252.29 * log(P). r2 = 0.37. p = .008(19.3) Perlodidae. log(y) = 1.07 + 0.52 * log(P). r2 = 0.29. p = .02• • • 11••• • • I.e...... •••• • • • • •• I.e...... .77lx4.0 ACKNOWLEDGEMENTSI would like to thank my advisory committee which included Dr. M.L. Bothwell, Dr.T.G. Northcote, Dr. J.D. MePhail, Dr. G.G.E. Scudder, Mr. P.A. Slaney and Dr. C.J.Walters for advice and constructive comments on my thesis. I, also, appreciate the helpfulreviews provided by Dr. N.T. Johnston and Dr. H.A. Quamme. The following people wereimportant in the support of the project; Mr. K.I. Ashley, Mr. A.D. Martin, Mr. C.J.Perrin, Mr. B.O. Rublee, Mr. B.R. Ward and Mr. K.W. Wilcox. Also, Dr. J.H. Mundieand Dr. J.R. Post provided valuable suggestions on experimental design and techniques.This work was supported by an NSERC Operating Grant to Dr. T.G. Northcote and fundingfrom the British Columbia Ministry of Environment, Fisheries Branch and the NechakoFisheries Conservation Program. Valuable technical assistance was provided by Mark Beere,Jeff Burrows, Ray Carriere, Vera Bredow, Chris Giroux, Franzika Gross, Lindy Looy,Heather Quamme, Stephan Schug, Ryan Slaney, Katharine Staiger-Williams, Lynnel Steinke,Michelle Venne and the staff of the North Vancouver Island Salmonid Enhancement Society.Thanks to T.W. Chamberlin and the staff of the B.C. Ministry of Environment, Fish andWildlife Branch, Skeena Region Headquarters for work space and support provided inSmithers, British Columbia.1Phosphorus limited community dynamics of stream benthic algae andinsects5.0 INTRODUCTIONThe relative importance of predators and resources in controlling the abundance or biomassof aquatic organisms has been long studied in lakes and ponds (e.g. Hrbacek et al. 1961,Arruda 1979, Shapiro 1979, Shapiro and Wright 1984, Carpenter et al. 1985, 1987, Ranta etal. 1987, Perrson et al. 1988). About half of the explained variation in productivity amonglakes is thought to result from nutrient supply, turnover time of the water and vertical mixing(Schindler 1978, Schindler et al. 1978, Carpenter and Kitchell 1987, 1988). The remainderresults from the effects of predation “cascading” through to lower trophic levels (Carpenter etal. 1985, Carpenter and Kitchell 1988).In contrast, experimental assessments of bottom-up (resource limitation) and top-down(predation) trophic interactions in riverine and stream ecosystems are relatively few (Deeganand Peterson 1992, Power 1984b, Peterson et al. 1993a, Peterson er al. 1985, Feminella etal. 1989, Johnston et al. 1990). The extent to which bottom-up nutrient control in streamspropagates throughout the food web has been poorly studied.Recently, studies have shown that the biomass of insects (Hart and Robinson 1990,Mundie et al. 1991) and insectivorous fish (Slaney and Ward 1993, Peterson et al. 1990)2may be enhanced by addition of inorganic phosphorus and nitrogen through increases inautotrophic production.Some periphytic algae are thought to be high quality food for many aquatic insect grazers(Lamberti and Moore 1984). Nitrogen and phosphorus augmentation can enhance arealbiomass of benthic algae (Stockner and Shortreed 1978, Elwood er a!. 1981, Grimm andFisher 1986, Perrin et al. 1987, Bothwell 1989, Lohman et a!. 1991) and microbial growthin streams (Peterson et a!. 1985, Hullar and Vestal 1989). The effect of external phosphorusconcentration on algal biomass (periphytic diatoms) has been well described by Bothwell(1989).The abundance or biomass of insect algal grazers in streams may be set by the availabilityof periphytic algae. It is though that the stream invertebrates are food limited (Richardson1993). Evidence to support this hypothesis comes from correlative studies betweenmacroinvertebrate densities and their food supply (Egglishaw 1964, McKay and KaIf 1969,Hawkins and Sedell 1981, Drake 1984, Flecker 1984, Murphy et al. 1986, Richardson andMcKay 1991), invertebrate density manipulations (Lamberti et al. 1987, McAuliffe 1984a, b,Hart 1985, 87, Richardson 1993) and food supply manipulations.The extent to which nutrient levels affect different elements of the periphytic communityand translate into population effects for aquatic insects has rarely been determined.Information concerning insect response to nutrient enhancement in terms of abundance,composition, and timing of response is limited (Mundie et al. 1991). The relationship3between ambient nuthent concentration and the insect abundance, biomass and compositionhas never been quantified experimentally in streams over a broad range of nutrient levels.Previous fertilization experiments have usually involved a single treatment at a high levelof nutrients in either artificial stream troughs or whole rivers. Mundie et a!. (1991)assessed the insect community response to nutrient enrichment for one level of phosphorus(10 ug P11) in fish-excluded flow-through troughs and found increases in total insectnumbers mainly due to higher numbers of Chironomidae. Hart and Robinson (1990)followed the effect of phosphorus addition at one high level (60 ug Pl1) on two species ofgrazing caddisflies, Leucotrichia pictipes and Psychomyia flavida, in wooden flumes wherefish, crayfish and predatory insects were present. They found that the larvae of both insectspecies had higher individual mass, developmental rates, and population densities inphosphorus treated flumes.Three whole river fertilization studies (Elwood et al. 1981, Johnston et a!. 1990, Petersonet a!. 1993a) have resulted in greater abundance or biomass at higher trophic levels includinginvertebrates and/or fish. Increased densities of the snail, Goniobasis clavaeformis, werefound in an experimentally enriched reach of Walker Branch, a Tennessee woodland stream(Elwood et a!. 1981). Johnston et al. (1990) and Deegan and Peterson (1992) found thatfertilization resulted in greater fish growth. Increases in sizes of fish have been attributed toincreased algal productivity and higher insect abundances, a major component of fish diet.On the Keogh River, in British Columbia, mean weights of juvenile steelhead trout and4coho salmon increased with fertilization (Johnston et al. 1990). A preliminary study, by thesame researchers, suggested that the upper Keogh River produced 3-5 times the total insectbiomass in fertilized sections than untreated sections; however, the effects of nutrientadditions could not be separated from location effects. The authors suggested that increasesin fish biomass may have resulted from greater insect abundance with nutrient treatment.Their proposition was supported by previous studies in which lotic salmonid abundance(Slaney and Northcote 1974, Mason 1976, Wilzbach et al. 1986) and size (Mason 1976,Wilzbach et a!. 1986) had been shown to increase with food availability. Salmonid standingstocks (Murphy et al. 1981, Bowiby and Roff 1986) and growth rates (Warren et al. 1964,Wilzbach et al. 1986) are often correlated with biomass of benthic prey. Researchers havealso found that fish growth (Gibson and Haedrich 1988) or standing crop (Hoyer andCanfield 1991) is correlated with nutrient levels measured in stream surveys.Whole river fertilization increased the size of both young of the year and adults of Arcticgrayling on the Kupuruk River in Alaska (Peterson et al. 1993a). Neutral lipid storage inadult grayling was increased in treated areas compared to controls (Deegan and Peterson1992). The abundance of the total number of insects was not strongly affected byphosphorus additions but the relative importance of different species shifted over time.There were increases in numbers of Baetis lapponicus and Brachycentrus americanus whileProsimulium martini declined in abundance (Peterson et al. 1993a). The growth rates of thefour dominant large insect species increased in response to treatment. By examining thenitrogen and carbon stable isotope ratios of Kuparuk River food web components, Peterson etal. (1993a) traced algal production stimulated by fertilization through to tissues of insect5ivorus grayling.It has been suggested that fertilization may be used as a technique to enhance streamrearing salmonid populations that are food limited (Johnston et al. 1990). Whole riverfertilization has been shown to increase smolt size and may improve marine survival ofstream reared coho and steelhead (Johnston et a!. 1990 and Slaney and Ward 1993).Increased adult returns have been reported for coho salmon (Hager and Nobel 1976) andsteethead trout (Ward and Slaney 1988) with greater smolt size.Despite the body of knowledge described above, few studies of streams have examined theeffect of nutrient concentration on the abundance or biomass of higher trophic levels under auniform set of environmental conditions. The objectives of this study were to quantativelyestablish the relationships between: (1) phosphorus concentration and the areal biomass ofperiphytic algae, and (2) phosphorus concentration and the abundance and composition ofstream insects. Uniform physical conditions were established by carrying out experiments ina stream mesocosm, 17 artificial stream troughs located on the Keogh River. This river nearPort Hardy, British Columbia, was selected as a study site because it is a nutrient limitedcoastal stream. Previous research involving a whole river fertilization experiment conductedon the Keogh River between 1983 and 1986 (Johnston et al. 1990), provided backgroundexperimental data. Experiments were simplified by excluding insectivorous fish from themesocosm.Previous work has shown that phosphorus is very low (< 1 g P. l’) in the Keogh Riverand suggested that phosphorus may be the primary limiting nutrient to productivity (Perrin et6al. 1987). Target nutrient concentrations for this study (0.5 - 50 g P 1.1) were selected toenhance growth rates of cells within the algal mat that might be phosphorus limited due toslow diffusion of phosphorus to the underlying cells (Bothwell 1989). Nitrogen was added ata constant ratio to ensure that it did not become limiting.The establishment of the above relationships is important to fisheries researchers who areassessing the potential for fertilization of nutrient deficient coastal streams as a technique toenhance salmonid populations in British Columbia (Johnston et al. 1990, Slaney and Ward1993). This study will also provide information for aquatic resource managers concernedwith riverine eutrophication.76.0 DESCRIPTION OF STUDY SITEThe experiment was carried out on the Keogh River (127’25’ W by 50°35’N), a thirdorder coastal stream on northeastern Vancouver Island, near the town of Port Hardy insouthwestern British Columbia. The experimental mesocosm was located 31 km upstream ofthe river mouth, approximately 1 km downstream from Keogh Lake outlet (see Figure 1).The river is 32 km long with a 130 km2 watershed in the Coastal Western Hemlock forest.Western hemlock (Tsuga heterophylla), red cedar (Thuja plicata), and spruce (Piceasitchensis) dominate mature forests. Approximately 45% of the watershed has been loggedduring the last 30 years. The headwaters of the Keogh River were logged to thestreambank about 20-25 years ago including the area where the experiment was located. Inlogged areas the major riparian species are red alder (Alnus rubra), salal (Gaultheriashallon), willow (Salix spp.), and sedges and grasses (Johnson et al. 1990) and to a lesserextent coniferous species. Detailed descriptions of the Keogh River are given in Ward andSlaney (1979), Slaney et al. (1986), Perrin et a!. (1987), and Johnston et a!. (1990).The wetted mean width of the river at mean summer flow is 8.1 m (range 5-15 m). Meanannual discharge is 5.3 m3s1 with a minimum in mid-summer of 0.1 m3s1 and an estimatedwinter maximum flow of 254m3s’. Mean summer flows in the upper-river (28 km from themouth) and near the mouth (3 km) are about 0.5 m3s’ and 1.6m3s’, respectively (valuesfrom Johnston et a!. 1990). In 1991, at this experimental site, the wetted width ranged from6.0 - 7.0 m and the flows ranged from 0.14 - 0.39 m3s’ between May 10 and8July 8.Ambient nutrient concentrations of the Keogh River in spring to summer are very low.Johnston et a!. (1990) determined soluble reactive phosphorus (SRP) levels to be < 1 gtotal dissolved phosphorus, 5 g l’; nitrate nitrogen, < 15 g 11; and total ammonia,< 5 g 1 . Nutrient concentrations have been found to elevate with declining flowsthrough the summer in the upper river (Perrin et al. 1987); inorganic dissolved phosphorusincreased slightly while nitrate nitrogen increased up to 4-fold. Mean pH is 6.9, totalalkalinity is 7.0 mg l’ and total dissolved solids are 30 mg i (Johnston et a!. 1990).Mean pH at the study site was 6.9; mean true colour, 19; mean turbidity, 1.3; and meantotal alkalinity, 9.0 mg l’ at the mesocosm head tank, based on five collection dates (1991;May 29, June 4, 10, 18, 25) at a mean flow of 0.24m3s1. The extremes of watertemperatures at the study site were 12.5 - 21 °C over the course of the experiment.9JJ- CI—F.— —--, —.‘ 4— “)O‘ MIsy Lake1’o 123 45ksi \ e—Scai.0 Connor• Lak.C MuirL_t27/Varcve( oQ. -%Iad Pott )EXPERIMENTAL )SITEIiLake /0 3Ok OG- L7 IRI’1E.Figure 1. Keogh River watershed with the location of the artificial streamtroughs.107.0 METhODS7.1 Description of mesocosm apparatusThe mesocosm consisted of seventeen flow-through troughs (each 1.52 m x 0.20 m x 0.20m) assembled at the stream side. The troughs were fabricated from clear plexiglass. Waterand biota from Keogh River were delivered to the mesocosm by gravity through a 300 mlong (15 cm diameter) plastic pipeline fitted to a head tank. Water flow from the head tankto each of the troughs was adjusted to approximately 1 l.51 by the use of rotating polyvinylchloride (PVC) standpipes and flexible nylon tubing similar to Mundie et al. (1991). Highrates of hydraulic flushing within the troughs (approximately 9 seconds residence time)ensured that trough water was similar in quality to river water and that added nutrientsremained constant over the trough length. Water flows in the troughs were checked andadjusted at least 3 times per week. Excess water, maintained in order to adjust water flow tothe troughs, overflowed from slots at the back of the head tank and a valve off the mainpipeline.Each trough was filled with gravel to a depth of 7 cm over an area of 0.30 m2. Numbersof lotic insect larvae have been found to be highest in the upper 10 cm of stream substrates(William and Hynes 1979). The design of the artificial streams in this experiment did notallow for vertical and horizontal migration of crawling insects to and from the troughs.Seventeen percent of trough sand and gravel passed through a seive size of size 7.9 mm;forty-three percent was 7.9 - 19.1 mm, and forty percent was 19.1 - 50.8 mm (by mass).Four baskets (100 cm2 each) filled with gravel were placed at the downstream end of the11gravel for purposes of subsampling benthic insects. Water in the troughs covered the gravelto a depth of 3.0 cm (standard error (SE) = 0.3, n = 17). Surface velocities of the troughsaveraged 21.3 cms’ (SE = 0.5, n = 17). Results of simple linear regression analysesdemonstrated that there was no systematic variation in surface velocity across the mesocosm(p = 0.34, r2 = 0.00, t6 = -0.98). The trough environment was designed to simulatethe characteristics of a stream riffle including a fast current, coarse substrates, some sandcontent and a low accumulation of organic matter. Water depths in the troughs were similarto only the shallowest areas of the natural stream riffles where water depths ranged 5 - 14cm and water velocities averaged 27.3 cms1 (for six sites located near the mesocosm).The bottom of the trough, downstream of the gravel, was fitted with a sheet (320 cm2) ofopen cell Styrofoam DB (Snowfoam Products Ltd., El Monte, California) that provided asubstrate for periphytic algae that could be easily monitored throughout the study.Artificial substrates such as styrofoam are commonly used to reduce periphyton variability.In a recent study by Morrin and Cattaneo (1992), chlorophyll estimates were found to besignificantly less variable on artificial than on natural substrates. However, other estimatesof biomass in this study (including ash-free dry mass, dry mass, density and biovolume) didnot confirm this finding. I used chlorophyll a per area as a means to estimate algal biomasson the styrofoam substrates. Styrofoam substrates selectively promote the colonization ofperiphytic diatom communities (for example Bothwell 1989). The mesocosm site waspartially shaded by a vascular plant canopy. All troughs received full shade, partial shadeand full sunlight during the course of a day as shadows moved across the mesocosm.12A funnel trap for emerging adult insects covered the length and completely sealed the topof each trough (Mundie et a!. 1991). Openings on the side walls of the traps were fittedwith 355 m mesh Nitex netting to allow air movement but, prevent the escape of capturedinsects. All adult insects were funneled into a drop trap containing 50% ethanol plus severaldrops of liquid soap to act as a surfactant to reduce surface tension in order to trap insectsmore easily.Nitex drift nets (100 m mesh) were used to monitor insect immigration to and emigrationand from the troughs by filtering the inflowing or outfiowing stream water. The inflow netsprevented insect immigration and colonization of the troughs one day a week during theexperimental period (described in Methods).13Figure 2. The mesocosm apparatus. The experimental treatments of thetroughs from left to right were 0.5, 0 + no insect immigration, 0,0, 2.5, 5, 10, 2.5, 5, 10, 0.5, 2.5, 5, 0, 0.5, 10, 50 g P11.147.2 Nutrient treatments and physical measurementsThis experiment involved six target levels of total phosphorus; 0, 0.5, 2.5, 5, 10,and 50 g P 11. Each treatment was replicated 3 times except for the 50 g 1 treatmentwhich was unreplicated. Treatments were assigned to the troughs at random so as not tocorrespond with possible effects resulting from horizontal gradients across the troughs.Nitrogen was added to troughs at a constant ratio of 2:1, N:P (atomic weight) or 0.9:1(wtlwt). Blooms of inedible blue-green algae can occur at low N:P ratios in lakes (Schindler1975, Tilman 1977). Previous whole river fertilization of the Keogh River at an N:P ratio of1:1 (wt./wt.) in 1984 did not result in a bloom of blue-green algae (monitored usingstyrofoam substrates). A similar N:P ratio was chosen for the present experiment.Background levels of dissolved inorganic nitrogen (< 20 g N• 1.1) on the Keogh Riverwere thought to be high enough to permit maximum algae cellular growth rates under naturalconditions. A bioassay conducted on the Nechako River (Perrin 1989) examined the biomasslevels of periphytic algae as a function of nitrogen addition (0 - 100 g N• l’) at surplusphosphorus concentrations. In Pen-ins’s experiment sixty to seventy percent of the maximumbiomass response measured was achieved at a N concentration of 10 g 1’.The source of nutrients (supplied by Coast - Agri, Abbotsford, British Columbia) used inthe present study was a liquid agricultural fertilizer blend of 32-0-0 (50% urea, 25%ammonium and 25% nitrate by mass) and 10-34-0 (10% ammonium and 34% totalphosphorus asP205). Total phosphorus was 100% water soluble and available as 25-35%ortho-phosphate and 65-75% 4-12 chain polyphosphates. The ammonium polyphosphate15fertilizer (10-34-0) was also comprised of micronutrients. These included Boron (0.01 % Bby mass), Calcium (0.07% CaO), Copper (0.0005% CuO), Iron (0.56% Fe203), Magnesium(0.5% MgO), Manganese (0.015 MgO), Potassium (0.12% K20), Sulfate (1.8% SO4) andZinc (0.10% ZnO). This fertilizer blend was used because it has been shown to be costefficient and easily applied to whole rivers in large scale studies (Slaney and Ward 1993).Beginning April 27, 1991 water from the stream was run through the troughs for 21 daysto allow colonization by stream biota. Nutrient additions began May 18 and ran until July 8,1991. A second experiment was carried out from July 8 - August 22. It was thought thatpolyphosphates in the agricultural fertilizer might not have been biologically available overthe length of the troughs. Nutrient levels in the second experiment were the same as the firstexperiment but fertilizer types were switched from the agricultural fertilizer to readilyavailable reagent grade nutrients in an attempt to achieve a more dramatic increase in algalbiomass with increasing phosphorus concentration. In the second experiment, Totaldissolved phosphorus was available as Soluble orthophosphate. The second experiment was,however, confounded by the first because gravel and insects from the first experiment werenot replaced. Styrofoam substrates for algal biomass accrual were replaced. Results of thesecond experiment are not reported in detail here.Fertilizer was dripped into the head of the artificial stream troughs via microbore tubingusing a Technicon auto-analyzer pump. Rate of nutrient delivery was adjusted by alteringmicrobore tubing size and stock concentration. Phosphate concentrations in troughs werecalculated from known dilutions of standard phosphorus and nitrogen solutions. A plexiglassbaffle at the head of each trough created water turbulence and ensured mixing of stream16water and nutrient additions.Samples for water chemistry were taken at weekly intervals from all troughs in order tocheck computed nutrient additions. Because of limited sensitivity of analytical methods forphosphorus, only higher levels of phosphorus additions (2 - 50 g P. 1’) could be accuratelyverified. Water samples were collected and analysed within 24 hours. Nitrogen andphosphorus analyses were carried out by Zenon laboratories, Bumaby, British Columbia on aTechnicon AA2 auto-analyzer. Analyses of NO3 + NO2 -N, N02-N, Total P, Soluble TotalP and Soluble ortho-P were performed according to modified procedures of Taras et al.(1971) described in McQuaker (1976). Analyses of NH4-Nwere performed according tomodified procedures of Greenberg (1980) described in McQuaker (1989). The limit ofdetection was 3 g P 1’ for Total P, Soluble Total P and Soluble ortho-P; the limit for lowlevel Soluble ortho-P was 1 g P 1’. The detection limit for NH4-N and N02-Nwas5 g N 1’; low level N02-Nwas 1 g N and NO3 + NO2 -N and N03-Nwas20 g Nl’.Water temperature was measured with a continuously recording Ryan model J-90submersible thermograph, placed in the mesocosm head tank. The instrument was calibratedweekly. No difference was found between stream temperature and water running through themesocosm.177.3 Algal communityAlgal biomass was sampled by removing two periphyton cores (6.2 cm2), weekly, fromstyrofoam sheets in each trough. The time-course of algal biomass accrual as measured bychlorophyll a was monitored on styrofoam plates from April 2 - May 31. These plates werereplaced when algae the sloughed from styrofoam, and new plates were monitored from May31 - July 8.Chlorophyll a analyses followed the procedure of Parsons et a!. (1984). The styrofoamcores were frozen at -20 °C until later (3-5 months) chlorophyll a flourometric determination.Algal cells were disrupted in 10-15 mL of 90% acetone (at 0°C) using a Potter-Elvehjemtissue grinder (3 minutes) and a sonication bath (5 minutes). The homogenate was incubatedin a dark refrigerator (3°C) for approximately 20 hours and then centrifuged for 5 minutes atsetting 5 in a Damon IEC clinical centrifuge to remove solids. Chlorophyll a from each corewas measured separately using a Turner Designs model 10 flourometer. A correction wasmade for phaeophytin (Parsons et al. 1984). Duplicate cores from each trough wereaveraged for each date. Bothwell (1983) found that chlorophyll a levels extracted fromindividual cores (5.0 cm2) of algae (n = 5) adhering to styrofoam had a coefficient ofvariation of 10.9%. In my study, duplicate cores from each trough had coefficients ofvariation that ranged from 2 - 55% (mean = 14%) across the seventeen troughs (determinedbefore treatment initiation, May 17).The peak algal biomass (PB) found on the styrofoam substrate over time was used to18describe the relationship between P concentration and areal algal biomass (Bothwell 1989).PB was estimated by averaging chlorophyll a values observed on the second set of styrofoamsheets (May 31 - July 8) for the final two collection dates (July 3 and 8). Peak chlorophyll alevels on the first set of styrofoam sheets occurred after only eight days of treatmentapplication and were thus not used to estimate PB.Extraction and quantification techniques of chlorophyll a used in this study were comparedto those used previously (Johnston et a!. 1990 and Perrin et a!. 1987). Four cores ofperiphyton were sampled per sheet of styrofoam with various levels of accrued periphytonbiomass from the Slocan River near Nelson, B.C. Eighteen styrofoam sheets were sampledin total. Chlorophyll a on two of the cores of each sheet was measured using techniques ofthis study described above and two were analyzed using previous techniques and were carriedout by Zenon laboratories.Periphyton adhering to gravel was sampled on July 8 and fixed in Lugol’s solution fortaxonomic identification and cell counts. Two stones per trough were dropped directly inLugol’s solution of a standard volume of 500 ml; later, algae was scraped from both stonesand pooled. Quantitative cell counts were made at 500X magnification in Utermohlchambers from subsample volumes of 25 ml. A minimum of 100 individuals of thepredominant species and at least 500 cells in total were counted. The areas of the rockswere calculated with a planimeter after obtaining an impression of the surface withaluminium foil.197.4 Insect communityAdult insects were collected weekly from emergence traps and preserved in 90% ethanol.Drift nets that collected emigrating insects from the outflow of the troughs were emptiedtwice a week (3 day collection periods). Once a week, drift nets were placed on the inflowof the troughs for 24 hours to assess the immigration of insects to each trough. One pipefrom the head tank was monitored for immigrating insects three times a week (two 3 day andone 24 hour collection period). As a result drift nets excluded insects from one troughfollowing the 21 day colonization period described above.Benthic insects from two of the baskets in each trough (200 cm2) were sampled at the endof the experiment, July 8 . Drift nets (100 jm mesh) were simultaneously placed on inflowand outflow of each trough while baskets full of gravel were removed from the trough.Insects were released from the gravel by gently brushing and rinsing the stones. Releasedinsects were captured in the outflow net. Drifting and benthic insects were preserved in 5%formalin.Benthic insects were also sampled directly from the stream using a Hess sampler of 0.05m2 with a 100 m mesh net. The substratum was sampled to a depth of approximately 10cm during base flow conditions, in water depths of 7 - 15 cm. Three samples were takenfrom three different riffles.Samples were hand-picked at lox magnification. Sorting efficiencies were >90% forbenthic and drifting insects determined by repeated picking. All adult insects were identified20and enumerated. Drifting and benthic insects were sieved through nested screens. Benthicinsects greater than 650 m were identified and enumerated. Generally, drifting insectsgreater than 1000 m were enumerated and identified. All sizes of drifting insects samplednear the beginning of the experiment (May 31 and June 3) were examined.Insect taxonomy follows Merritt and Cummins (1984). Invertebrates were identified atleast to the family level and occasionally to genus. Generally, insect taxa were not sub-sampled. However, a gridded petri dish was used to sub-sample drifting Baetidae. Varianceto mean ratio tests (x2 tests) for agreement with a Poisson series were performed accordingto Elliott (1977).Results (below) show that Ephemeroptera (mayflies) was the only order collected tocontain adult taxa that increased in number with increasing phosphorus treatment. Thus theeffect of treatment on Ephemeroptera adult body length and total biomass was assessed forinsects collected near the end of the experiment (June 27). Body length measurements weremade using a dissecting scope at lox magnification and a Summasketch Model MMIIIdigitizer calibrated with a stage micrometer. To make biomass measurements, insects wereremoved from ethanol, oven dried for approximately 20 hours at 60°C, and weighed on aCahn/Ventron 21 automatic electrobalance.217.5 StatisticsAll linear regression analyses were completed using Microsoft Excel version 4.0 by leastsquares fitting methods. In all tests, the critical level of significance was a = 0.05. Whenthe relationship between the variables was visibly non-linear, I did a log transformation ofthe x axis that linearized the data prior to the analyses (Zar 1984). A log transformation ofthe y axis was made occasionally in order to stabilize the variance. Assumptions of theregression were validated before and after transformation by graphical examination ofresiduals.All regressions were initially completed on replicated treatments only (0 - 10 jg P 1.1) totest for significance. These results are presented in tables and text. If the above regressionswere significant I carried out regression analyses of the same data, but also included the highunreplicated treatment 50 g P 1 and displayed the results graphically.228.0 RESULTS8.1 Nutrient treatments and Algal Community ResponseIncreases in phosphorus addition to troughs led to measured increases in Total P (Table 1).Measured Total phosphorus above background levels was approximately three times lowerthan target levels of phosphorus for treatments of 2.5-10 g P. 1*1. Measured nutrients wereelevated up to 8.3 and 2.6-3.75 times the controls for Total P and Dissolved InorganicNitrogen, respectively (Table 1 and 2). Measured N:Total P ratios ranged from 1.5 - 5 inexperimental and control troughs.There was no significant difference among troughs in chlorophyll a levels beforetreatments began (measured from cores collected on May 17, 45 days after initial placementof the first set of styrofoam plates in the troughs, p = 0.66, r2 = -0.06, t05)j4 = -0.44, fortreatment troughs 0-10 tg P. 1.1). There was no systematic effect of trough location onchlorophyll a levels across the mesocosm for the same date (p = 0.40, r2 = -0.02, t05 =-0.86). A significant increase in chlorophyll a with increasing phosphorus concentration wasobserved after only 6 days (May 24) of treatment (p = .001, r2 = 0.54, t05)14 = 4.05)(Figure 3.1). However, I was not able to follow the effects of phosphorus treatment on theaccrual of algal biomass on these styrofoam plates over time because algae began sloughingoff the plates.23Mean inorganic phosphorus concentrations (jig P 1’) measuredfor each target treatment.Target Measured Soluble SolubleTotal P Total P2 Total P2 ortho-P2Addition mean(SE) (range) (range)0 4.6(0.6) <3-3 <i0.5 4.6(0.4) <3-3 <i32.5 5.1(0.1) <3 <35.0 6.3(0.2) 4-6 <310.0 9.4(0.4) 5-6 <3-450.0 38.3 32 321Mean of three replicate troughs. Each replicate is an average value of totalP measured on June 18,25 and July 2,91. Standard errors are given inbrackets.2Ranges of three replicate troughs measured on July 2,91. Values aretypical of those measured weekly from May 22 to July 2.3Values from low level nutrient analyses.Table 1.24Mean inorganic nitrogen concentrations (jig N 1-’)measured for each target treatmentTarget MeasuredDissolved DissolvedInorganic InorganicNitrogen NitrogenAddition0 16.2320.5 14_2322.3 <25-254.5 <25-269.0 <25-2945.1 601Measured total N (range) is based on adding the ranges ofsoluble NH4-N and NO3 + NO2 -N. Ranges are based onwater samples from three replicate troughs measured onJuly 2, 91. Values are typical of those measured weekly fromMay 22 to July 2.2Values from low level nutrient analyses.Table 2.25Twenty-one to twenty-eight days (June 21 - 28) after placing a second set of styrofoamsheets in the troughs, there was an exponential increase in chlorophyll a for all nutrienttreatments (Figure 3.2). One trough from which insects were excluded did not followthis pattern (Figure 3.3). Algal biomass built up quickly on the styrofoam sheets in thistrough and remained high but fluctuated widely with time. Sloughing of the algae fromthe styrofoam was noted and may have accounted for some of the variability that wasobserved. Measurements of chlorophyll a from the insect exclusion trough were greaterthan control troughs over time for four of five sampling dates.A significant relationship was observed between algal biomass and the log ofphosphorus concentration (Figure 4). Peak algal biomass increased rapidly withphosphorus concentrations of 0 - 2.5 g P. 1.1 but showed diminishing returns at 2.5 -10 g P1.Chlorophyll a levels determined from styrofoam cores of this experiment were lowerthan previous studies of phosphorus concentration on periphytic areal biomass (Bothwell1989, Perrin et al. 1987, Mundie et al. 1991). A check on the extraction andquantification techniques for chlorophyll a used here showed that this study had onaverage higher measurements than Zenon laboratories (mean from this study = 33.4 mgm2, mean from Zenon = 19.7 mg m2, paired-sample Student’s t test, t05(2)(17 = 6.74,p < 0.05). Both laboratories showed similar relative trends in chlorophyll a levels.Thus, low chlorophyll a levels observed in this study did not result from amethodological artifact.263.13.214 12---c-. 5—*-— tonu:ens’”:__44 46 48 50 52 54 10 15 20 25 30 35 40DAYS FROM PLACEMENT OF DAYS FROM PLACEMENT OFSTYROFOAM IN TROUGHS STYROFOAM (N TROUGHS3.310—0——— grazers present• grazers excluded8.1-JOOtU2010 15 20 25 30 35 40DAYS FROM PLACEMENT OFSTYROFOAM IN TROUGHSFigure 3. Mean algal biomass accumulation measured as chlorophyll afrom styrofoam plates. Sample size (n) = 3 except at 50 ugP11 where n = 1. Nutrient treatments started May 18.(3.1) First set of styrofoam plates, placed in troughs from April -May 31.(3.2) Second set of styrofoam plates, placed in troughs from May31 - July 8.(3.3) Algal biomass accumulation (chlorophyll a) on the second setof styrofoam plates. The grazer excluded trough with nophosphorus addition (n = 1) compared to the means of thecontrol troughs (n = 3).271210 . .2‘_‘50TARGET PHOSPHORUS CONCENTRATION (ug P/I)Figure 4. Peak biomass on styrofoam plates measured as chlorophyll a(mgm2)against target phosphorus concentration. Linearregression of the data fitted by least squares. Regression lineabove (all data) is y = 4.05 + 4.19 log(P). r2 = 0.67. p < 0.001.Regression line (not shown) for treatments 0 - 10 ug P11 only isy = 5.09 * log(P) + 3.70. r2 = 0.69 p < 0.00128The density of total cells, Ocillatoria sp., Achnanthes minutissima significantlydeclined with increasing phosphorus concentration (Figure 5). Regression analyses ofSynedra ulna, and Eunotia sp. showed no significant changes in number with increasingphosphorus concentration (p = 0.57, r2 = -0.05, t05)14 = -0.57 and p = 0.72, r2 = -0.07, t0514 = -0.37, respectively for treatments 0-10 g P 1). Low numbers of otheralgal taxa did not permit similar analyses.Algal cell composition sampled from the trough gravel substrate showed thatcyanophytes, especially Oscillaroria sp., predominated at lower phosphorus treatments,while at higher concentrations Achnanthes mimutissima comprised a greater percentage ofthe total cell number (Table 4). There was no difference in the number of algal taxa withincreasing nutrient additions (p = 0.26, r2 = 0.03, to5a)13 = -1.17, for treatments 0-10 g P 1’).The density of total algal cells (9726 cells mm2), Achnanthes mimutissima (884 cellsmm-2), and Oscillatoria sp. (5581 cells mm-2) in the insect excluded trough were 4.5, 2.4and 4.2 times greater than the controls, respectively. In the controls, mean total celldensity was 2178 cells mm2 (SE = 728), mean density of Achnanthes mimutissima was365 (SE = 88) and mean density of Oscillatoria sp was 1314 cells mm2 (SE = 429).Visual observations of algae biomass suggested that removal of insects from the troughhad a dramatic effect of increasing the algal standing crop. The composition of algalcells in the insect excluded trough was generally similar to control troughs (Table 4),except that there was an increase in the percent composition of one chiorophyte genus,Oedogonium sp., 24% of total cells, in the insect excluded trough, compared to 0.1 -1.4% of total cells in control and treatment troughs.29305.1 5.210000600E .E • Ee1000Cr50°‘U 400a, a,_I— .-I-J 300‘U . UicJ0:100‘U‘Ua,_z0•1000 200zIv0 .5 2.5 5 10 50 0 .5 2.5 5 10 50TARGET PHOSPHORUS CONCE nON (ug P4) TARGET PHOSPHORUS CONCENTRATiON (ug P11)5.310000E• 1000•‘Ua.a,-J1000U..10 •z1•0.5 2.5 5 10 50TARGET PHOSPHORUS CONCENTRATION (ug P4)Figure 5. Density (cells-mm2)of algal cells collected from the trough gravel atthe end of the experiment (July 8) against target phosphorusconcentration. Linear regressions of the data were fitted by leastsquares.(5.1) Total cell numbers. Regression line above (for all data) islog(y) = 3.56 + -0.95 * log(P). r2 = 0.44. p = .003. Regressionline for treatments 0 - 10 ug P1’ only islog(y) = -0.73 * logP) + 3.48. r2 = 0.21. p = 0.05.(5.2) Achnan,thes mimutissima. Regression line above (for all data)is y = 316.41 + -200.19 * log(P). r2 = 0.48. p = .001.Regression line for treatments 0 - 10 ug P11 only isy = -221.96 * log(P) + 324.83. r2 = 0.41. p = 0.005.(5.3) Oscilla.toria sp. Regression line above (for all data) islog(y) = 3.36 + -1.39 * log(P). r2 = 0.45. p = .002.Regression line for treatments 0 - 10 ug P1’ only islog(y) = -1.29 * log(P) + 3.32. r2 = 0.29. p = 0.02.31Table 4. Percent composition of the algal cell number by taxa of the artificialstream troughs for each target phosphorus concentration (ug P11).Percentages are based on algae collected from trough gravel at theend of the experiment (July 8).0* 0” 0.5” 2.5” 50b 10” 50*Taxon + noimigrafionCyanophytaOscillatoria sp. 57.4 60.3 83.3 71.5 74.1 38.1 16.4Lyngbya sp. 5.7 2.0 3.5 7.6 5.5 6.9 0.0Other 0.0 4.9 0.0 0.0 2.0 11.4 0.0ChrysophytaBacfflariophyceaeAchnanthes 9.1 16.8 8.4 7.7 9.3 26.2 42.7mimutissimaEunotiasp. 0.3 0.6 0.3 1.1 0.7 3.1 7.3Synedra ulna 0.2 1.2 0.4 1.5 1.8 1.3 8.2Tabellariafenestra 1.4 2.2 0.0 4.3 1.5 2.6 5.5Other 1.8 2.8 1.3 3.5 1.1 5.5 3.5ChlorophytaOedogoniumsp. 24.0 0.1 1.4 1.3 0.8 1.1 0.0Other 0.1 9.2 1.2 1.4 3.1 3.8 16.4a.percent composition from one unreplicated troughbpercent composition based on pooled counts from 3 replicatetroughs328.2 Insect Community Response8.2.1 Adult insectsThe timing pattern of total adult insect emergence was not affected by phosphorusconcentration (Figure 6.2). Five to six weeks after the treatment initiation, the total adultinsects reached peak numbers for all phosphorus concentrations. High numbers of emergingadults coincided with high stream temperatures (Figure 6). Total adult insect abundancecollected at the end of the experiment (July 4) in emergence traps increased with phosphorusconcentration; the relationship was nearly significant (y = 85.68*log(P) + 184.4, p = 0.08,= 0.15, t)13 = 1.89, power (1-fl) = 0.71, for 0-10 g P1’). At 50 g P1’(unreplicated), total insect abundance appeared to increase after 17 days of fertilization andreached approximately three times the level of the controls after 47 days (Figure 6.2). Adultinsects were also collected in drift nets placed on the outflow of the troughs. The totalabundance of adults collected in drift nets near the experiment end (June 24) increased withincreasing phosphorus concentration. The relationship was nearly significant (y =67.66*log(P) + 113.8, p = 0.07, r2 = 0.20, tOS()13 = 2.00, for treatments 0-10 g P1”).The regression statistics for the total number of adult insects collected in emergence trapsover the entire experintent are given in Table 4.The adult insect taxa (emergent insects traps) were comprised largely of Baetidae,Chironomidae, Simuliidae, and Trichoptera (see Figure 7). Adult Baetidae caught inemergence traps showed a significant increase in number to nutrient treatment (y =14.4*log(P) + 10.79, p = 0.01, r = 0.34, t05)14 = 2.93, for treatments 0-50 g P1’)336.122Ui 20• Ma,amumUingnum1210 17 23 30 40 476.24501I,—o.— OugP/1010 17 23 30 40 47DAYS FROM START OFNUTRIENT ADDITIONSFigure 6. (6.1) Daily minimum and maximum stream temperatures measured atthe mesocosm head tank as a function of time.(6.2) Numbers of adult insects caught in emergence traps between May19 and July 4 by target phosphorus concentration (ugPt’). In eachcase, the mean number of adult insects is plotted perdate. Sample size (n) = 3 except at 50 ug P11 where n = 1.34Linear regression analyses relating total numbers (y) of variousinsect taxa collected as adults from the emergence traps totarget phosphorus concentration. Sample stze (n) = 15 in all cases.Ordered with respect to decreasing p value.Insect Regression’ p r2Family Equation(Order)Baetidae y = 18.95 * log(P) + 0.004 3.39 0.42(Ephemeroptera) 15.130.08 1.86 0.15Total insects y = 40.0 * log(P) +101.09(Miscellaneous y = 2.21 * log(P) + 2.31 0.10 2.92 0.13Tricoptera)Chironomidae y = 19.07 * log(P) + 0.22 1.28 0.04(Diptera) 62.08(Miscellaneous Plecoptera) y = 0.08 * P + 1.92 0.29 1.11 0.02Simuliidae y = -0.28 * P + 11.90 0.34 -0.99 0.00(Diptera)(Miscellaneous y = 0.40 * log(P) + 2.87 0.62 0.51 -0.06Diptera)Leptophlebiidae y = 0.03 * log(P) + 0.69 0.40 -0.06(Ephemeroptera) 2.00Heptageneiidae y = -0.02 * log(P) + 1.93 0.73 -0.35 -0.07(Ephemeroptera)(Miscellaneous Coleoptera) y = log(P) + 0.63 0.96 -0.05 -0.08(1) Regression analyses are based total taxon counts collected in emergence traps overtime divided by the number of weeks for phosphorus treatments of 0-10 g P1’ only.Regression analysis was determined to be significant at the “table-wide” a level using thesequential Bonferroni technique (Rice 1988). Taxa without this subscript next to the p valuewere nonsignificant at the “table-wide” a level.Table 4.35Figure 7.00z0(j)coOow•Cl)0.0—iD0u-zo0xI.Percent coit-osition of the predominant adult insect taxa collectedfrom emergence traps of the artificial stream troughs at twophosphorus concentrations. Percentages are based on total insectscollected froi May 19 to July 4 and result from the mean of threereplicate troughs.10075.50250III other insectsQTrichoptera0 SimulidaeChironomidaeBaetidae• Misc. Ephemeroptera0 10TARGET PHOSPHORUSCONCENTRATION (ug P/I)36after 23 days (June 10) (Figure 8). The magnitude of this response increased from May 28- June 27, over the first 40 days of the experiment. The mean number of adult Baetidaeimmigrating to troughs in the drift was very low and averaged (for example) 1.3 per day (seeTable 5). Adult Baetidae were captured both in emergence traps and drift nets. Numbers ofadult Baetidae caught in both emergence traps and drift nets at the end of the experimentshowed similar positive responses to increasing nutrient concentration (Figure 9). Numbersof baetid adults increased rapidly with increasing phosphorus concentrations of 0 -2.5 g P 1.1 but showed diminishing returns at 2.5 - 10 jg P 1. The regression statisticsfor the total number of baetid adults collected in emergence traps per week over the courseof the experiment are given in Table 4.Numerically, adult Ephemeroptera comprised 20.3 - 35.1 % of total adult insectscollected in emergence traps near at the end of the experiment (June 27). However, biomassmeasurements of the same samples indicate that Ephemeroptera contributed to a largerpercentage (46.7 - 71.7) of the total insect dry mass (Figure 10). The effect of phosphorusconcentration on total adult Ephemeroptera biomass was significant (Figure 11), but wasnonsignificant for the biomass of other adult insects (taxa pooled) (p = 0.72, r2 -0.07,= 0.35, for treatments 0 - 10 g P’lj. The biomass of total ephemeropteran adultsincreased rapidly with increasing phosphorus concentrations of 0 - 2.5 g P. 1’ but showedsaturation at 2.5 - 10 g P 1’. Treatment responses of ephemeropteran numbers andbiomass, largely, resulted from positive increases of the genus Baetis (Family Baetidae)(Figure 12). Average mass per individual for adult Ephemeroptera did not changesignificantly with increasing phosphorus concentration (p = 0.44, r2 = -0.03,37150pI’ICioo I—0-—— 00 0 I A. “ —a--— sj .1I I 2.5UI /I___5LL1 -A 1050 ——••*—— 50<UILU<co0’0 10 17 23 30 40 47DAYS FROM START OFNUTRIENT ADDITIONSFigure 8. Number of adult Baetidae caught in emergence traps between May19 and July 4 by target phosphorus concentrations (ug Pt’). Ineach case, the mean number of adult insects is plotted per date.Sample size (ii) = 3 except at 50 ug P1’ where n = 1.38Table 5. Mean number of adult insects immigrating and emigratingper trough-day’ by taxon. Sample size (n) = 3. Standard error isgiven in parentheses.Mean number of Mean number of Mean number ofTaxon adults drifting1 to adults emigrating2 adults emigrating2troughs day’ from control from 10 ug Pt’(Standard error) troughs-day’ troughs-day’(SE) (SE)Baetidae 1.3 (0.3) 14.4 (0.6) 33(3.0)Chironomidae 57.0 (11.9) 37.9 (5.2) 39.6 (8.7)Trichoptera 0.33 (0.33) 1.6 (0.6) 2.1 (0.8)Simuliidae 24 (3.5) 4.6 (1.4) 3.0 (1.0)‘Immigration rates are based on 24 hour drift samples collected at inflow of troughs onJuly 3.2Emigration rates are based on total numbers of adults trapped per day in emergence traps(10 day sample collected June 27) and drift nets (3 day sample collected June 24) for eachtrough.399.112Z.u ic•_TARGET PHOSPHORUS CONCENTRATCN (ugfl)Figure 9. Number of adult Baetidae captured at the end of the experimentas a function of target phosphorus concentration (ug Pt’). Linearregression lin of the data were fitted by least squares..(9.1) Numbers caught in emergence traps on 3uly 4. Regressionline above (all data) is y = 29.19 + 33.46 * log(P). rZ = 0.34.p = 0.011. Regression line (not shown) for treatments0 - 10 ug P11 only is y = 23.16 S Iog(P) + 49.03.r=0.51 p =0.002(9.2) Numbers caught in drift nets on July 1.Regression line above (for all data) is y = 11.32 ± 21.43 * Iog(P).r2 = 0.69. p < .001. Regression line for treatments 0 - 10 ug Pt’only is y = 20.47 * log(P) ± 10.27. r2 = 0.51 p = 0.00140other risectsEphemeropteraContribution of Ephemeroptera to total biomass of adults collectedin emergence traps on June 27 by target phGsphorus concentration(ug P1’).0-4D—% 0000>-0C-)Figure 10.TA3ET PHOSPHORUSCONCENTRATION (ug P/I)414O. .Cl)cx<30DOz •.JLUDI-.uJujLL. Lii0ODO,LL 10Lo0- .,,,,, I0 .5. 2.5 5 10 50TARGET PHOSPHORUS CONCENTRATION (ug P/I)Figure 11. Adult Ephemeroptera biomass (total dry weight of all individuals)collected on June 27 as a function of target phosphorusconcentration (ug P1’). Linear regression line of the data fitted byleast squares. Variances were not stabilized by transformation.Regression line above (all data) is y = 8.193 ÷ 14.241 * log(P).r2 = 0.48. p = 0.002.Regression line (not shown) for treatments 0 - 10 ug P1’ only isy 11.76 * log(P) + 9.15. r2 = 0.25 p = 0.03.42LUL100ZWZOcLxI- LU0LU=I-Figure 12. Percent composition of Ephemeroptera collected as adults at theend of the experiment on June 27 by target phosphorusconcentration (ug PT’).100- IIHIIIIIIIIIIIIIIII—III HeptageneidaeParaleptophiebia sp.• Baetiso 10PHOSPHORUS CONCENTRATION(ug P/I)43t05(2)13 = -0.80, for treatments 0 - 10 pg P.1-i). Adult mayflies (collected near experimentend on July 4) exhibited no difference in mean body length per trough as a result oftreatment (p = 0.49, r2 = -0.04, t()13 = 0.71, for treatments 0 - 10 g Plj. Adult bodylengths ranged from 2.04 to 6.85 mm.In two taxa, Chironomidae and Tricoptera, adults captured in emergence traps increased innumber only at the highest nutrient concentration of 50 g P1’ (unreplicated trough).Increasing phosphorus concentration increased adult Chironomidae and Trichoptera numbersbut the effect was nonsignificant from 0-10 jg P-1’ (see Table 4 for regression statistics).However, at 50 g Pl the numbers of adult Chironomidae collected at the end of theexperiment (July 4) were three times the controls (Figure 13). The number of adultTrichoptera captured from the 50 g P1-1 trough was eight times that of the control troughsat peak emergence (June 27) (Figure 13). The numbers of adult Chironomidae collected indrift nets on trough outflows near the experiment end (June 24) showed no significantresponse to increasing phosphorus concentration (p = 0.92, r2 = -0.08, t05(2)13 = -0.10, fortreatments 0-10 g P14).The mean number of adult Plecoptera emerging per week declined over the course of theexperiment to less than one per week by July 4 for all treatments (Figure 13). No significanttreatment effects were observed on the total number of Plecoptera captured from May 19 toJuly 4 (see Table 4). Other major insect taxa collected in the emergence traps (Simuliidae,miscellaneous Diptera; Leptophlebiidae; Heptageniidae; and Coleoptera) showed nosignificant response to fertilization Table 4). The relationship of increasing phosphorusconcentration on the numbers of adults of taxa other than Baetidae and Chironomidae44collected in the drift near the end of the experiment (June 24) could not be assessed becauseof low numbers.A descriptive comparison was made between number of adults drifting into the troughs perday and those emigrating from the troughs per day for 0 and 10 P-i4 treatments. Thetotal number of adults immigrating per day to the troughs was based on a 24 hour driftsample. The total number of adults emigrating from the troughs per day was estimated byadding the daily catch of adults of the emergence trap (sampled over 10 days) to those in thedrift nets (sampled over 3 days). Greater than 91 % and 79% of the baetid and trichopteranadults trapped per day, respectively, originated from trough gravel for these treatments(Table 5). None of the simuliid nor chironomid adults trapped per day originated fromtrough gravel (Table 5). Comparisons for other taxa were not made because of lownumbers.4513.1 13.2250 80E -- .EgI200 ‘III.’I SI SaILdISO40 ‘‘U100ZO4Z 2O50 uacJI-010 17 23 30 40 *7 0 0 10 17 23 30 40 47DAYS FROM START OF DAYS FROM START OFNUTRIENT ADOmONS NUTRIENT ADDITIONS13.3121‘-. loj& I•Ce I3.- . —0—-a—0-— .3___SSo:.-uJc.10 17 23 30 40 3DAYS FROM START OFNUTRIENT ADDITIONSFigure 13. Numbers of adult insects by target phosphorus concentrationcoUected in emergence traps. All insects were captured betweenMay 19 and July 4. In each case, the mean number of adult insectsnumbers is plotted per date. Sample size (n) = 3 except at50 ug Pit where n = 1.(13.1) Chironomidae. (13.2) Trichoptera. (13.3) Plecoptera.468.2.2 Benthic insectsIncreasing phosphorus concentration resulted in an increase in total benthic insects countssampled at the end of the experiment. The relationship was neariy significant (y =349.85*log(P) + 1000.55, p = 0.06, r2 = 0.20, t05(2)11 = 2.05, power (1 - = 0.67, fortreatments 0 - 10 ug P1-1).Numbers of benthic Baetidae, Hydroptilidae, Nemouridae, and Perlodidae showed asignificant increase with increasing nutrient levels (Figure 14 and Table 6). Numbers of allfour of these insect taxa showed rapid increases at low phosphorus concentrations of 0 -2.5 g P. t’ and showed signs of saturation at 2.5 - 10 g P. l’. Other major insect taxacollected from the benthos showed no significant response to fertilization (Table 6).The number of taxa per sample (richness) of the trough benthos was not influenced bytreatment (p = 0.80, r2 = -0.08, t)j2 = 0.25). A comparison of the number of taxa persample in the control troughs to the natural stream showed no significant difference(p > 0.10, t05 = 1.52).The control trough benthos was comprised of four main families including Chironomidae,Philopotamidae, Baetidae, and Nemouridae which made up 82.6 - 99.1 percent of the totalinsects. While the benthos of shallow natural riffles near the mesocosm were comprised ofBaetidae, Chironomidae, Chloroperlidae, Elmidae and Leptophlebidae which made up 89.2percent of the total insects (Figure 15). Although the percent composition of the controltroughs and stream benthos appears different, the same taxa were at least present in bothtrough and stream benthos.474814.1 13.2400 300aa LuLuI-Lii30O_—.Go200 • OJ,Li. Lii 100OLuo 100zz0• 5 S 10 50 .52.5 5 10 50TARGET PH0SPH0P1.S CONCENTRATiON (uq P11)TARGET PHOSPHORUS CONCENTRATION (u P1)14.414.34050 ECaaLiiI—uJC,Lii 40 30hii.D 30 ...2OCU -‘0’:. .ZLLJ10 -I 101CC,z.5 2.5 s 10 50.0 .5 2.5 5 10TARGET PHOSPHCRS CONCENTRATION (ug P/I)TARGET PHOSPHORUS cONcENTRATIcH : P11)Figure 14. Number of insects collected from the benthos at the end of theexperiment on July 8 as a function of target phosphorusconcentration (ug Pt’). Linear regression lines of the data werefitted by least squares. See Table 7 for the results of regressionanalyses carried out on 0 - 10 ug P11 treatments for the same data.(14.1) Baeticlae. y = 152.06 + 89.96 * log(P).r2=0.443. p=.0O6.(14.2) Nemouridae. y=1l7.92 + 93.02 * log(P). =0.421. p0.0O8(14.3) Perlodidae. y=94l + 13.31 * log(P). r=0.402. pO.0l(14.4) Hydroptilidae. y=l3.95 + 10.26 * 10gW). rz0.16. p=0.0849Linear regression analyses relating the numbers (y) ofvarious insect taxa collected from the trough benthos collected atthe end of the experiment to target phosphorus concentration.Sample size (n) = 14 in all cases. Ordered with respect toincreasing p value.Insect Regression(2)TrophicFamily quationW t0•5(2),12 Relationship(Order)Perlodidae y = 16.29 - log(P) + 16.29 0.02 2.77 0.34 predators(Plecoptera)Nemouridae y = 108.54 * log(P) + 112.25 0.02 2.72 0.33 shredders,(Plecoptera) collectorsHydroptilidae y = 16.91 * log(P) + 14.01 0.02 2.58 0.30 scrapers,(Tricoptera) piercers,collectorsBaetidae y = 93.25 log(P) + 150.86 0.03 2.49 0.29 scrapers,(Ephemeroptera) collectorsPolycentropodidae y = 3.56 *log(P) + 5.40 0.07 1.96 0.18 collectors,(Tricoptera) some predatorsPhilopotamidae y = 70.45 * log(P) + 206.72 0.09 1.84 0.15 collectors(Tricoptera)Miscellaneous y = 0.21 * P + 4.68 0.15 1.54 0.09 scrapers,Ephemeroptera collectorsElmidae y = 0.25 * p + 5.35 0.26 1.18 0.03 collectors(ColeopteraChloroperlidae y = -0.26 * P + 4.82 0.31 -1.06 0.01 predators(Plecoptera)Hydropsychidae y = -0.47 * P + 14.01 0.39 -0.88 -0.02 collectors,(Tricoptera) some predatorsChironomidae y = 4.17 * P + 453.82 0.56 0.60 -0.05 various(Diptera)Miscellaneous y = 0.32 * P + 24.03 0.79 0.27 -0.08 variousDiptera“i Regression analyses included phosphorus treatments of 0-10 g Pl only.None of the regression analyses tested were significant at a “table-wide”a level accordingto the sequential Bonferroni technique (Rice 1988).Table 6.50__1000z075C1)(1)LU 50Ocnow00z0><F-250othe insectsEmidaePhilopotamidaeChironomdaeLeptophlebidaeBaetidaeChoroperlidaeNemouriciaeFigure 15. Dominant insect families collected from the benthos of the artificialstream troughs on July 8 as a function of phosphorus concentration.Percentages are based on the mean of three replicate troughs.0 ugPII 10TROUGH STREAM518.2.4 Drifting immaturesThe total number of insect immatures captured in drift nets at the trough inflows was nothighly variable (mean = 108.7, standard error = 1.29, n = 3) on June 21. Thecomposition of immigrating immatures on this date was largely made up of Baetidae(42.3%), Chironomidae (15.9%), Simuliidae (10%), Trichoptera (10%), and Leptophlebiidae(5%). Perlodidae and Nemouridae had very low immigration rates and comprised only .02and .01 percent, respectively, of the total insects collected on June 21.The effect of phosphorus concentration on emigration rates of drifting large Baetidaenymphs (those retained on a 1 mm sieve) was, generally, not significant (for dates June 3, 6,13, and July 1, 1991). The total number of emigrants pooled over time showed anonsignificant relationship with increasing phosphorus concentration (y = 21.30 * P + 723,p = 0.13, r2 = 0.11, t.)j2 = 1.59, (1 -j3) = 0.77, fortreatmentsof0- 10gPi’).However, on June 24, emigration rates showed a significant positive response to increasingphosphorus concentration (y = 109.27*log(P) + 160.56, p = 0.01, r2 = 0.32, t.05(2)13 =2.74). Daily immigration rates and daily emigration rates of Baetidae nymphs over time aredepicted in Figure 16.Immigrating nymphs less than 1 mm in size comprised 50% of total drifting Baetidae (meanper day = 32.3, standard error = 11.0, n = 3 ) enumerated on May 31, 1991. EmigratingBaetidae nymphs less than 1 mm in size showed no significant response to nutrient treatmentas of June 3 (p = 0.76, r2 = -0.07, to5()l3 = -0.31, for treatments of 0 - 10 g Pt’).Immigration and emigration rates of Hydroptilidae, Nemouridae and Perlodidae were notassessed because of very low numbers in the drift.5253(16.1) Daily frnmigration rates of large Baetidae nymphs to theartificial stream troughs over time. Trnmigration rates are based on24 hour drift samples taken at the trough inflow.(16.2) Daily emigration rates of mature Baetidae nymphs from theartificial stream troughs over time by target phosphorusconcentration (ug P1’). Emigration rates are based on 3 day driftsamples collected from the trough outflow. In each case, the meannumber of Baetidae nymphs is plotted per date with the associatederror bars. Sample size (n) = 3, for all points.16.115010050016.215010050LUZ).OzS2j:uJ)-u.uJI.<C’,0>-<LU13 28 34 41——-‘—— 10 ug P/I—°—-•- 00- 15 23 26 37 43DAYS FROM START OFNUTRIENT ADDITIONSFigure 16.549.0 DISCUSSIONThis is the first study to describe the relationship between insect abundance andcomposition and nutrient concentration in a freshwater stream. Results showed a significantincrease in peak algal biomass with increasing phosphorus concentration measured fromStyrofoam substrates. PB increased with phosphorus concentration linearly up to 2.5g P 1’ after which a diminishing response was observed. In contrast, a significant declinein total algal cell density with higher phosphorus concentrations was observed on troughgravel. The family Baetidae was the only taxon collected as adults to show a significantincrease with phosphorus addition. Adult baetids comprised 47 - 72 % of total insectbiomass at the end of the experiment. Numbers of benthic baetids, nemourid and perlodidstoneflies and hydroptilid tricopterans sampled at the end of the experiment significantlyincreased with phosphorus concentration. Adult and benthic insects of these ta.xa exhibitedsimilar rapid increases in abundance from 0 - 2.5 g P t’ and showed signs of saturation at1.5 - 3 times the controls at concentrations greater than 2.5 g P 11. Removal ofimmigrating insects from one trough resulted in increased in algal biomass on styrofoamplates and higher cell numbers on trough gravel compared to controls. There was nodetectable difference in the numbers of large baetid nymphs emigrating from the troughs withincreasing phosphorus concentration. Graphical comparisons between the control and treatedtroughs showed that they were similar in taxonomic composition of insects. Insect taxonomicrichness did not change with increasing phosphorus concentration.559.1 Effect of Phosphorus Additions on PeriphytonPeak algal biomass collected from Styrofoam substrates and plotted as a function oftarget phosphorus concentrations showed saturation kinetics similar to that found by Bothwell(1989). About 70% of the maximum chlorophyll a value was achieved at low P additions(between 0.5 and 2.5 g Pl’) similar to Bothwell’s results (Figure 17). Peak biomassincreased with phosphorus concentration linearly to 7.4 mg m2 at 2.5 ug P 11 and reachedan asymptote at 9.2 mg m2 (2.7X the controls). Areal biomass may have increased from2.5 - 10 ug P1’ but at a lower rate; this trend is unclear, however, because of the lowresponse seen in the 5.0 ug P1’ replicates.The levels of chlorophyll a from the styrofoam substrates were more than 40 times lowerthan those reported by Bothwell (1989) and 3 - 6 times lower than those described by(Mundie et al. 1991). Grazing, in my study, had considerable impact on the algal biomasslevels measured from the styrofoam plates. The removal of insects from one trough resultedin up to a 7 fold increase in chlorophyll a levels on the styrofoam plates compared to thecontrols. It is possible that high grazing rates may have accounted for the low chlorophyll alevels observed compared to previously reported values (Bothwell 1989, Mundie et al. 1991,Stockner and Shortreed 1978). The mean density of insects in the control (47,350insects m2) and the 10 g P 1’ treated (66,950 insects m) trough gravel of this study were2.1 and 1.7 fold higher, respectively, than the mean density of invertebrates at the sametreatments in a similar artificial trough study by Mundie et al. (1991). In my study, meaninsect densities were based on numbers of animals retained on a 630 m mesh sieve while inMundie et al. (1991) insects were sampled with a much smaller 50 m net. As a result the56relative difference in mean insect density between the two studies is underestimated.Numbers of insects grazing directly on artificial substrates used to collect algal biomass werenot quantified in the present experiment nor in previous work.Many researchers have shown that grazers can significantly reduce algal biomass instreams (Lamberti and Resh 1983, McAuliffe 1984, Jacoby 1987, Hart and Robinson 1990,Scrigeour et al. 1991). High macroinvertebrate abundances have been associated with lowerthan expected algal biomass levels in two studies of point source nutrient enrichment in aBritish Columbian River (Bothwell et al. 1992) and New Zealand streams (Welch et al.1992). Recently, Bothwell (pers. comm. Dr. M.L. Bothwell, 1993) observed lowerchlorophyll a levels in experimental stream troughs in the Thompson River, British Columbiacompared to past observations (Bothwell 1989) and has hypothesized that lower levels aredue to increased grazing pressure.The discrepancy between low values of algal biomass reported in my study and highervalues in past work may have also resulted from differing physical conditions such as currentvelocity, flow stability, light levels, sediment scouring, within the troughs (Bothwell 1993).Mean water velocity in the troughs of the present study (21.3 cms’) was lower than introughs of Stockener and Shortreed (1978) (40 cms’)and Bothwell (1989) (50 cms’) butmuch higher than Mundie et al. (1991) (5.3 cmsj. Increased turbulence and water velocitygenerally has a positive effect on the metabolism and nutrient uptake of attached algae; it canresult in higher biomass accrual (Bothwell 1993). Variations in mean water velocity between570 5 10 15 20 25 30 35 40 45 50 55PHOSPHORUS CONCENTRA11ON (g P/I)Figure 17. Relative peak algal biomass (PB: PB(max)), chlorophyll a, as afunction of phosphorus treatment (ug P11). PB(max) is PB at50 ug Pr’. A. modified from Bothwell (1989). B. Present study.Sample size (n) = 3 at each treatment level except at 50 ug P1’where n = 1.58the studies did not consistently explain differences in algae biomass. Comparisons of flowstability, light level, and sediment scouring could not be made among the above studiesbecause these variables were not consistently monitored.Finally, low chlorophyll a levels might be expected if long chain polyphosphates(comprising 65 - 75 % of total dissolved phosphorus added) were not biologically availableover the length of the mesocosm (pers. comm. Dr. M.L. Bothwell). However, in a secondexperiment (see Methods) where I switched to a readily available form of phosphorus(orthophosphate) but maintained the same treatment levels, low levels of algal biomass werestill observed. Low levels of algal standing crop observed in my study may have resultedfrom the interaction of several of physical or chemical factors as well as invertebrate grazing.Background dissolved nitrogen levels of the Keogh river were thought to be high enoughto saturate cellular growth rates. However, underlying cells of the periphyton mat may havebeen nitrogen limited if nitrogen diffusion within the mat was slow (similar to phosphoruslimited growth kinetics described by Bothwell 1989). Pen-in (1989) showed periphyticbiomass increased logarithmically from 0 - 10 ug N 1, but continued to increase linearlyfrom 10 - 100 ug N i’ at surplus phosphorus. However, thick algal mats did not developwithin my troughs instead there was a thin film that was approximately 3 mm thick withsome longer filaments. Hydraulic retention times of the troughs were short; this assured thatnutrient levels remained constant in the bulk water. Thus, the nitrogen requirements of cellswithin the algal mat were thought to have been satisfied (Bothwell 1993).The total number of algal cells sampled from gravel within the troughs significantly59declined with increasing phosphorus concentration. This decline may have resulted fromincreased grazing pressure (increased numbers of Baetidae) which was positively correlatedwith phosphorus concentration in the troughs. There was a 4.5 fold increase in total cellnumbers collected from the grazer excluded trough compared to the controls. This alsosuggests that the stimulatory effect of phosphorus on total cell number was offset byincreases in grazing pressure indicating that in fact fertilization can result in successfulpropagation of increased production up the food chain. Styrofoam substrates provide habitatfor only small instars of chironomid and simuliid insects (pers. comm. Dr. M.L. Bothwell,1993). Higher grazing pressure on gravel versus Styrofoam likely accounts for differences inalgal accumulation response observed on the two substrates. Algal standing crop andherbivore grazing rates on Styrofoam substrates may not correspond to standing crop andgrazing rates on gravel (Aloi 1990). I did not assess the effect of phosphorus concentrationon the biomass of algae attached to trough gravels.Diatoms and cyanophytes made up over 89.6% of the total cell numbers collected from thetrough gravel. Diatoms are high quality food for grazing insects (Lamberti and Moore1984). Cyanophytes and filamentous algae are generally thought to be less readily digestedby insect grazers (Lamberti and Moore 1984). However, Mundie et al. (1991) found thatfilamentous algae made up 88% of insect gut contents and suggested that cyanophytes mayalso be an important food item for some insect grazers. They showed that an increase intotal insect number occurred with increased phosphorus concentration despite an increase inthe percentage of cyanophytes making up the algae in their artificial stream troughs.609.2 The effect of phosphorus additions on the abundance of the insect communityThe families of insects that showed the greatest increases in density with nutrientenrichment were Baetidae, Nemouridae, Perlodidae and Hydroptilidae. Densities of all ofthese taxa showed signs of saturation at high phosphorus levels. Densities initially increasedrapidly at phosphorus concentrations of 0 - 2.5 ug P. 1 and reached asymptotes atconcentrations of 2.5 - 10 ug P 1’.Positive responses of Baetidae density to increasing phosphorus concentration wereobserved for adults in the emergence traps and drift nets (Figure 9) and for nymphs collectedfrom the benthos (Figure 14). Numbers of Baetidae immigrating into troughs were similar(Figure 16.1). Thus, either emigration or mortality rates must have been lower in fertilizedtroughs to account for increased numbers. Lower emigration rates of Baetidae nymphs fromthe troughs were not found over the course of the experiment. As a result, aggregation ofBaetidae nymphs in the enriched troughs was not predominately responsible for the increasednumber of individuals found with treatment. However, I cannot separate whetheraggregation and increased survival or increased survival alone account for the highernumbers of Baetidae found in treated troughs because I was not able to estimate whatproportion of the outfiowing drift originated from the trough benthos. Increased survival ofBaetidae nymphs must partly account for the higher numbers of adults and benthic nymphs.Higher survivals can be attributed to increased quality and/or amounts of available food.The periphytic community, including attached algae, is high quality food for Baetis nymphs(Chapman and Demory 1963, Kohier 1985). Bacteria, a good source of protein for streaminsects (Lamberti and Moore 1984), may have also contributed to higher grazer survival.61Bacterial productivity may have been enhanced by phosphorus additions in my experiment(Elwood et al. 1981, Hersey et al. 1988, Peterson et al. 1993) but was not monitored.Emigration rates for small baetids (those passing through a 1 mm seive) were assessedonly for one date, early in the experiment. Thus, I may have missed an aggregativeresponse of immature baetids by undersampling. However, in order for aggregating baetidsof less than one mm to account for a significant adult response 24 days (June 10) aftertreatments began (Figure 8), high aggregations rates might be expected very early in theexperiment. Small baetids would require time to develop into adults. For example, Baetisnymphs that hatched in July in an Ontario river took three and a half months to develop intoadults (Corkum and Pointing 1979). Thus, even if the number of sampling days is low, onemight expect to find depressed emigration rates of small baetids early in the experiment ifthese individuals were to account for the significant increase in adults later in the experiment.Drift is thought to be a means by which algal feeding stream invertebrates move betweenpatches of high and low quality food . Drift rates of invertebrates have been shown todecrease with increasing density of periphyton in artificial stream troughs (Hildebrande1974). Mature Baetis tricaudatus will actively drift when food abundance is low (Kohier1985) in laboratory streams. Substrates with high periphyton densities have been shown tobe more rapidly colonized by grazers compared to substrates with low periphyton densities(McAuliffe 1983). In contrast to these studies, my data agree with Mundie et al. (1991)who found no evidence of depressed insect drift with increased phosphorus enrichment andhigher algal biomass in artificial stream troughs. The stream side experimental mesocosm62approach that was used by myself and Mundie et al. (1991) incorporated greater complexitycompared to previous experiments. Our experiments were conducted over a longer timeframe that allowed for natural colonization, instar growth, development and variable survivalrates. Aggregation of insects in patches of high quality food have generally been observed instudies where insect survival rates were constant and growth and development were notpermitted (Hildebrande 1974, McAuliffe 1983, Kohler 1985).Numbers of baetid mayflies emerging from the troughs peaked at the end of theexperiment. The synchrony of phosphorus enrichment, algal response and nymphaldevelopment probably resulted in strong positive responses of Baetidae to treatment observedin emergence traps, drift nets and benthos. A number of short term food enrichment studiesconducted in artificial streams have found that insects with short generation times may beable to exploit increased food abundance (Mundie et a!. 1991, Richardson 1991). Themidge, Brilla retfinis, increased more than ten times in density in detritus enriched troughscompared to control troughs (Richardson 1991). Mundie et a!. (1991) found that phosphorusenrichment favored small chironomids (1.76-2.25 mm) such as Corynonera,Thienemanniella, Synorthocladius, and Cladotanytarsus. Baetid mayflies have a “fastseasonal life history type”. For example, species of Baetis typically have two generationsper year. Generation time in general is highly variable and temperature dependent (Edmundset a!. 1976).The benthic nymphs of two plecopteran families, Perlodidae and Nemouridae, also showeda positive response to enrichment. Plecopteran emergence for all nutrient levels was highest63when treatments were initiated and declined to very low levels by the end of the experiment.Plecopteran emergence patterns were not synchronized with nutrient treatments. It is notsurprising, then, that a positive response of adult plecopteran numbers was not observed withphosphorus treatment. Also, there may be a lag in stonefly response time to nutrienttreatment if their numbers depend on interactions mediated through prey (Begon et al. 1990)and/or other indirect effects of fertilization.Nemourids depend on organic matter as a food resource. The family Nemouridae is madeup of species that are either collector-gathers feeding on fine particulate organic matter(FPOM) or shredder-detrivores that feed on coarse particulate organic matter (CPOM)(Merritt and Cummins 1984). It may be that phosphorus treatment directly enhancednemourid food abundance by increasing the abundance of dead and decaying algae availableto these nymphs. Alternatively, increases in the numbers of grazers correlated withincreasing phosphorus may have facilitated an increase in abundance of FPOM. It iscommonly thought that insects make finer particles of detritus available to collectors throughtheir feeding and defecation activities (Merritt et al. 1984). Increased fecal material fromgrazers as a food resource indirectly may have benefitted nemourids in treated troughs.Finally, nemourids may have benefirted from increases in algal or bacterial productivity withphosphorus enrichment if they fed directly on the periphytic community. Collector-gatherers can often be opportunistic in their choice of food (Lamberti and Moore 1984).In this study, I observed a “bottom-up response” of perlodid predators to increasingnutrient concentration. Increased biomass at the consumer trophic level may have resulted inincreased available prey for perlodid stoneflies. Only recently has nutrient augmentation in64rivers been shown to increase biomass at higher trophic levels such as insectivorous fish(Johnston et al. 1991, Peterson et at. 1993). Previous to this study, predatory insectresponses to experimental nutrient enhancement have rarely been demonstrated. Johnston etat. (1990) described increases in stonefly populations with whole river fertilization.Predatory stoneflies have been shown to depress baetid mayfly populations by displacingthem to refuge areas (Cooper et at. 1993). However, the importance of “top-down” controlby insect predators on consumers was not assessed in this experiment; such control wouldlikely involve longer time scales than were practical to to study with this mesocosm.Benthic nymphs of one family of Trichoptera, Hydroptilidae, increased in abundance withincreasing phosphorus concentration. Hydroptilids are piercer-herbivores, scrapers orcollector gatherers (Merritt and Cummins 1984). Their numbers may have been enhanced byincreased food abundance through direct fertilization effects on periphyton biomass orindirect effects on the availability of FPOM. The mechanism by which plecopteran ortricopteran numbers increased was not examined. They may have aggregated in treatedtroughs where food resources were more abundant, and/or the survival rates of nymphs mayhave been higher in treated troughs.Adult Chironomidae and Trichoptera showed increased densities only at 50 ug Pl’. Thissuggests (based on one unreplicated treatment) that that abundances of certain insect taxa mayincrease only at higher phosphorus levels.The ability to detect a positive effect of fertilization on numbers of adult insects producedin the troughs depended on the magnitude of the effect relative to the number and variability65of adults drifting into the troughs. Data from Table 5 suggest that possible small effects offertilization on Chironomidae and Simuliidae adults may be undetectable because the numbersof adults drifting into troughs was high relative to numbers emigrating (in outflow drift andemergence traps) from troughs. Numbers of adult Baetidae, Chironomidae, Trichoptera andSimuliidae drifting into the troughs per day were estimated from one 24 hour sample whilenumbers emigrating from troughs per day were estimated from three day (drift nets) andweekly catches (emergence traps). A twenty-four immigration rate may not be a goodestimate of a three day or weekly emigration rate if drift is variable through time. However,the trends described in Table 6 were maintained on other dates in the second experiment thatI conducted in July and August (see Methods).An alternative hypothesis to food limitation for why insect numbers increased withincreasing phosphorus concentration is that increasing algal mat thickness may haveimproved insect survival. An increased algal mat may have provided improved habitat,reduced competitive interference among nymphs, or reduced predation rates (Richardson1989). This is unlikely, however, since I did not see any consistent increase in the size ofthe algal mat with increasing phosphorus concentrations. Total algal cell numbers per areascraped from gravel showed no increase with greater phosphorus concentration. Also, algalbiomass was higher in the trough where insects were excluded indicating that grazers werefeeding on periphytic algae.The percent composition of benthic insect taxa was different between the troughs and thestream. This may have resulted from differences in physical characteristics between the theartificial stream troughs and natural riffles or if colonization rates of the artificial substrates66by insect groups was not proportional to their abundance in the river. There was nodifference between the troughs and the stream in the kinds of insect taxa present andrichness.Increases in the densities of insects (baetid adults and nymphs, nemourid and perlodidnymphs) with higher phosphorus treatment observed in this experiment were maintained in asecond experiment that I carried out in the same troughs in July and August (Appendix 1,Figures 18 and 19).9.3 Effect of nutrient enrichment on ephemeropteran size, biomass and timing ofemergenceThe quantity and quality of food resources may significantly alter aquatic insect life historycharacteristics (Sweeney 1984). Insect growth (ie. Colbo and Porter 1979, Collins 1980,Richardson 1991) has been shown to increase with greater food availability. Measurementsthat I made of adult ephemeropteran lengths and mass per individual showed no detectableincrease with phosphorus concentration. However, I did not examine the sizes and massesby species nor by sex. Also, individual mass loss is probable because adults were preservedand stored in 90% ethanol but shrinkage in total length is unlikely to have occurred(Giberson and Galloway 1985). In contrast to my findings, Baetis showed a growthresponse in all four years of a whole river fertilization study on the Kupuruk River, Alaska(Peterson et a!. 1993a). Development time may also be affected by food quantity. Delayedemergence has been observed for aquatic insects on suboptimal diets (Anderson andCummins 1979, reviewed in Sweeney 1984) or under severe food reduction (Danks 1978,Colbo and Porter 1979). No change in emergence timing was observed with phosphorus67concentration in this study for any taxa of insect. Brittain (1976) found that providingunlimited quantities of food compared to field conditions did not change insect emergencetiming. Increasing food supported denser, but not faster growing ephemeropteranpopulations, in my experiment and in others by Hawkins (1986) and Mundie et al. (1991).If food per capita was similar amongst treatments, it may account for denserephemeropteran populations with higher phosphorus treatment but no detectable change inlength and mass or timing of emergence.9.4 Extrapolation of Mesocosm Results to Whole River FertilizationThe mesocosm approach used in this study is powerful because it allowed for experimentalcomparisons with replication and appropriate controls. Whole stream manipulations aredifficult, and whole stream replication is not always possible.The stream-side mesocosm in which I conducted these experiments was an open flow-through system. Exchange of nutrients and organic material, immigration and emigration ofstream organisms, and physical conditions such as temperature appeared to mimic naturalconditions. Some of the disadvantages to using these types of artificial stream troughs arethat there is reduced physical and biological heterogeneity, variation in natural dischargevariation is usually damped, and the immigration and emigration of organisms that crawlalong the benthos is eliminated. In my experiment, fish predators that may have impactedinsect populations were excluded from the troughs. In addition, most studies conducted inmesocosms are short term in nature. Short term studies of community responses, such as thepresent one, may not be adequate to predict long term ecosystem responses. Peterson et al.(1993a) were not able to predict an Alaskan river’s community response to a four year68fertilization treatments from short term studies (days to weeks). Unanticipated long terminteractions included: 1. a switch from bottom up nutrient control of algal biomass in thefirst two years to top down grazer control in the last two years; 2. a decline in blackflyabundance with fertilization in years two and three.To manage a river for enhanced sports or commercial fish production through fertilization,it will be important to understand long term, interacting responses. For instance, it may beimportant to know if a baetid mayfly response such as that observed in my experiments couldbe maintained and translated into improved adult salmon returns in a long term whole riverfertilization study. Baetid mayflies were a very important component of both steelhead andcoho salmon fry diet on the Keogh River in 1984-85 (Johnston et al. 1990).We may not be able to predict long term complex types of interactions from a short termmesocosm study. However, experiments such as this one can be powerful tools if they areintegrated with closely related field research and mathematical modelling (Mclntire 1993).My study provides important information on the potential of at least three different trophicgroups to respond to increases in nutrient concentrations. It also suggests that particularinsect groups may increase numerically even at relatively low levels of nutrient additions. Itis the first study to describe the relationship between insect abundance and composition inrelation to nutrient concentration in a freshwater stream.In the future, food web tracers (Hall 1993, Peterson et a!. 1993a, Peterson et a!. 1993b,Shepard and Waddill 1976) could be used with studies of streamside artificial troughs thatincorporate natural invertebrate colonization to determine whether abundance changes (of69adults or benthos) with treatment result from differences in aggregation or survival rates. 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New Jersey.p.718.7711. APPENDIX 1200LULULULU 150<C’)-JLUC)u-z 100 .OWLUL .50 .z ..C.)0-0 .5 2.5 5 10 50TARGET PHOSPHORUS CONCENTRATION (ug PR)Figure 18. Number of adult Baetidae captured in emergence traps per weekfrom July 8 - August 22 as a function of target phosphorusconcentration (ug P1’). Linear regression lines of the data werefitted by least squares. Regression line above (all data) islog(y) = 1.41 ± 0.49 * log(P). r2 = 0.74. p < 0.001.Regression line (not shown) for treatments 0 - 10 ug P1’only is log(s) = 1.41+ 0.48 * log(P). r2 = 0.62 p = 0.0027319.119.25000• : 6004000O-z goci_g.I).• •uo •2000 • 4000(3U •a0 0.5 2.5 5 10 0 0.5 2.5 5 fOTARGET PHOSPHORUS CONCENTRATTCM (ug p TARGET PHOSPHORUS CONCENTRATION (ug ?i1)19.3U)09u-Q.t &uJuJDOZC31c 0.5 2.5 5 10TARGET PHOSPHORUS CONCENTRATION (ugI’Figure 19. Number of insects collected from the benthos at the end of thesecond experiment on August 22 as a function of target phosphorusconcentration (ug P1’). Linear regression lines of the data werefitted by least squares.(19.1) Baetidae. y = 2088.56 + 1701.56 * logcP).r2=0.45. p=.003(19.2) Nemouridae. y = 339.45 + 252.29 * log(P. =0.37. p.O08(19.3) Perlodidae. log(y) = 1.07 + 0.52 * Iog(F). r2 = 0.29. p.O2


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