The composition and concentration of dissolved organic matter (DOM) in freshwater ecosystems are influenced by the abundance and diversity of phytoplankton, which are, in turn, affected by nutrient availability. The interactions among DOM, phytoplankton, and nutrients are not fully understood, particularly regarding dissolved Si (DSi), a required nutrient for diatoms but generally not for other major phytoplankton groups. In this study, we sought to characterize the relationships among the optical properties of DOM, phytoplankton abundance (as chlorophyll a) and functional group relative biovolume, and nutrient concentrations (dissolved NO3-, soluble reactive P, dissolved organic C, and DSi) in a large reservoir in south-central Indiana, USA, a site representative of dimictic, mesotrophic reservoirs in temperate climates. We found that DOM composition varied throughout the summer growing season, and the variation was correlated to changes in phytoplankton abundance. We also found strong relationships between diatom biovolume, DSi concentrations, and the protein fractions of DOM, which indicates that the availability of DSi may influence the composition of DOM, likely mediated through the productivity of diatoms. This proposed relationship suggests changes in DSi availability could indirectly elicit responses in the composition, processing, and fate of DOM in freshwater systems. Describing these relationships in a reservoir is of particular interest given that dams drive changes in water retention time, light availability, and biogeochemical processes that affect DOM production and transformations. This novel coupling of C and Si cycling highlights the need to better understand controls on the sources and fates of C in reservoirs, particularly given the increase in dam construction across much of the world.
