Influence of environmental factors on the vertical distribution of phytoplankton in Lacamas Lake, WA
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Urbanization in watersheds has led to nutrient enrichment (eutrophication) and dissolved oxygen depletion (hypoxia) in many freshwater systems. These conditions impact species diversity, availability of habitat, and organism behavior within these systems. Lacamas Lake is a managed reservoir in Camas, WA that experiences seasonal stratification, hypoxia in bottom waters, and is highly eutrophic, sometimes resulting in harmful algal blooms. Lacamas Lake also undergoes a drawdown for dam maintenance purposes each autumn. To better understand the impacts of hypoxia and management actions on the phytoplankton community in Lacamas, we pursued three research questions: 1) How is phytoplankton biomass vertically distributed in relation to dissolved oxygen levels? 2) Are there differences between day and night vertical distributions of phytoplankton? 3) How does phytoplankton vertical distribution vary before and after lake drawdown. 4) What is the relationship between phytoplankton size and vertical distribution? During August (pre-drawdown) and October (post-drawdown) 2015, phytoplankton biomass was measured at six depths from surface to bottom and a weighted mean depth was calculated for each sampling time. We found that phytoplankton biomass was consistently concentrated above the hypoxic zone, indicating these organisms were avoiding the low oxygen water. There was a significant difference in vertical distribution between day and night in one size fraction, as well as a significant difference in the vertical distribution between pre- and post-drawdown. These results highlight the need for strategies that manage run-off flowing into the watersheds in urban areas and further research into the ecological implications of the annual drawdown at Lacamas Lake.