Phytoplankton taxonomic compositional shifts across nutrient and light gradients in temperate lakes

Nutrient and light availability, and their balance, can modify community composition and structure in pelagic communities. Previous studies have demonstrated contradictory findings about whether total phosphorus (TP) concentrations alone or the ratio of total nitrogen (TN) to TP concentrations (TN:TP) drive Cyanobacteria dominance in freshwater ecosystems, and influences of light availability are often overlooked. Here, we analyzed a 12 year, 137 lake database to test paradigms of phytoplankton compositional patterns across nutrient (TN, TP, TN:TP) and light availability gradients in an agricultural region. We hypothesized that (1) TN:TP ratios would better predict phytoplankton compositional shifts than TP concentrations alone, (2) Cyanobacteria relative abundance would increase at low TN:TP ratios, and (3) Cyanobacteria biomass fluctuations would be the primary driver of light climate. We found that TN:TP ratios better described phytoplankton compositional patterns than TP concentrations, with Cyanobacteria proportions decreasing with increasing TN:TP while other taxa increased. Contrary to expectations, Cyanobacteria always dominated community composition (≥80% biomass), regardless of TP concentrations. Despite these patterns, N-fixing Cyanobacteria proportions were not correlated to TN:TP, suggesting that shifts toward N-fixation were not solely driving phytoplankton compositional patterns. Although Cyanobacteria biomass decreased with increasing light availability, inorganic particles explained more variance in light than total phytoplankton biomass, suggesting that buoyancyregulating Cyanobacteria may gain an initial competitive advantage in light acquisition before bloom development in turbid systems. These findings suggest that Cyanobacteria strongly influence pelagic community dynamics in nutrientenriched lakes, and their ability to manipulate light and nutrient environments enable their persistent dominance across large environmental gradients.