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Fertilization of an oligotrophic coastal montane lake using high N:P ratio fertilizers : effects on phytoplankton and zooplankton community structure Werring, John


Comparisons of the response of phytoplankton communities to different methods of nutrient addition used in past whole-lake fertilization studies, suggested that it might be possible to generate predictable changes in the phytoplankton and zooplankton communities of lakes by controlling the frequency, magnitude and N:P ratio of the autochthonous nutrient load. Results obtained by several workers in separate, but related, laboratory and small scale enclosure experiments supported this belief. The evidence strongly suggested that N:P ratios are important in determining the outcome of competitive interactions between algal taxa, with green algae outcompeting blue-green algae at high N:P ratios, while the frequency of nutrient supply is important in determining the size structure of an algal community. There is also evidence that suggests that competitive relationships between zooplankton species are sensitive to changes in the species and size composition of the phytoplankton community. Large cladocerans perform poorly when large algae are present in abundance. This thesis examines the effects of small, frequent doses of high N:P ratio fertilizers on the littoral and limnetic phytoplankton and zooplankton communities in a small oligotrophic coastal montane lake in the Coastal Range Mountains of British Columbia. The experiments were carried out in-situ in eight large polyethylene enclosures, each measuring 150 m² in area. The enclosures were fertilized three times each week from June to October, 1982 with small doses of NH₄NO₃ and (NH₄)₂HPO₄ at an N:P atomic ratio of 40:1. Phosphorous was thought to be the nutrient limiting phytoplankton growth in this lake but the results suggest that nitrogen may be more important in this regard. The role of Sphagnum mats surrounding the lake in the control of nitrogen supply to the lake is discussed. In general, effects of fertilization were more pronounced in the limnetic than the littoral zone. Phytoplankton standing crop increased 4-fold and 7-fold in littoral and limnetic fertilized enclosures, respectively, over controls. Cyanophyceae and Chrysophyceae, which dominated the algal biomass in the lake and controls were almost completely eliminated from all fertilized enclosures by mid-summer and replaced by Chlorophyceae. More than 80% of the algal biomass in fertilized enclosures consisted of algae less than 20 um in size, whereas less than 20% of the biomass in controls and the lake fell into this size range. Changes in the algal community led to significant changes in the species composition, structure and biomass of the zooplankton community. With the exceptions of Daphnia rosea and Holopedium gibberum all herbivorous zooplankton in all enclosures followed the pattern of species succession that occurred in the lake but at higher densities. Fecundity and body size of all herbivorous zooplankton increased following fertilization, but Daphnia rosea capitalized on the enhanced algal biomass and became the community dominant over the whole summer in fertilized enclosures while in controls and the lake, it ceased to be dominant by late July. Holopedium disappeared from all enclosures by the end of July. Fish predation and competitive interactions with Daphnia probably led to their decline. Fish predation was generally selective for Hoiopedium and D. rosea in control and fertilized enclosures and its role in the disappearance of Holopedium and in structuring the zooplankton community in both, is discussed. Adult cutthroat trout (Salmo clarki clarki) in fertilized enclosures altered their diet in favour of Daphnia. Fish in control enclosures and the lake fed almost exclusively on terrestrial insects. Planted juvenile (fry) cutthroat trout did not survive in any of the enclosures. Avian predation appeared to be responsible for their disappearance suggesting that predation is more important than food to the survival of juvenile fish in lakes. My results suggest that the phytoplankton and zooplankton communities in freshwaters can be manipulated in a predictable manner based on a control of the pattern of nutrient supply. This knowledge can be of great benefit to the management lakes for both fish production and ameliorating the effects of eutrophication.

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