Climate change and recovery from acid deposition has caused observable long-term increases in colored dissolved organic carbon (DOC) concentrations in many lakes. As global temperatures continue to rise, shorter durations of ice cover in dimictic lakes coupled with this “lake browning” phenomenon may promote a shift to monomictic regimes in higher latitude lakes. Subtropical, monomictic Lake Annie (Highlands County, FL) may serve as an important analog for these unprecedented changes. The lake is 80% groundwater fed, and experiences natural interannual fluctuations in DOC concentration (4-16 mg L-1). The primary objective of this study is to describe the relationship between DOC and other drivers to phytoplankton species richness, diversity, community assemblage, and succession. Multivariate community and time series analyses were conducted to elucidate potential drivers of phytoplankton succession from a fourteen-year dataset of monthly meteorological, hydrological, physicochemical, and phytoplankton composition data.
Results/Conclusions
Results suggest an inverse relationship between monthly species richness and DOC concentration following periods of high groundwater elevation (average 40.23±0.42 m AMSL). In years of low groundwater elevation (average 38.98±0.32 m AMSL), there is a decoupling of the relationship. Following periods of high groundwater elevation, diatoms were the dominant group, comprising a maximum 95% (mean 34%) of the community. In years of low groundwater elevation diatoms became rare, comprising an average 1% of the community. Silica scaled Synurophyceae and mixotrophic algae (i.e. Dinobryon spp.) became more abundant during this period. The coupling and decoupling of DOC and species richness indicate that the phytoplankton community in this lake is highly responsive to allochthonous inputs and associated physicochemical changes in the lake following periods of high groundwater elevation. During periods of low groundwater elevation, external variables such as terrestrial DOC have reduced influence on lake chemistry and biology. The phytoplankton community is likely regulated by internal physicochemical and biological processes such as priming effects, predation, and reduced water column stability.