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Effects of nutrient and DOC gradients on vertical phytoplankton structure in lakes.

Maria Lorena, Longhi*,1, Beisner, Beatrix1, 1 Université du Québec à Montréal, Montréal, Québec, Canada

ABSTRACT- The staining by high coloured dissolved organic carbon concentrations and the occurrence of algal blooms in eutrophic lakes has been shown to reduce light penetration, and thus decrease epilimnion depth. It remains unclear, however, how gradients of lake trophic status and dissolved organic carbon (DOC) concentration affect the thermal structure of the water column, and, whether different patterns of habitat structure translate into differences in phytoplankton vertical distribution. To answer these questions, phytoplankton biomass and major taxonomic groups were studied with a bbe FluoroProbe in eighteen lakes of the Eastern Townships of Quebec, Canada. Phytoplankton vertical distribution was related to environmental variables including temperature, total phosphorus (TP) and DOC. Trophic status of lakes (level of TP) ranged from oligotrophic to hypereutrophic, while DOC concentrations ranged from 2.20 to 8.85 mg L-1. Preliminary results suggest that increased TP and DOC concentrations are associated with shallower thermocline depths, and consequently to changes in the depth and magnitude of the chlorophyll a maximum. Moreover, levels of TP and DOC in lakes influence the coefficient of vertical variation of the major taxonomic groups. Overall, our results demonstrate strong differences in patterns of phytoplankton structure across gradients of TP and DOC in lakes.

Key words: phytoplankton distribution, phytoplankton diversity, DOC, TP

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