Implications of water column ammonium uptake and regeneration for the nitrogen budget in temperate, eutrophic Missisquoi Bay, Lake Champlain (Canada/USA)

Assessing ammonium (NH4 (+)) availability in aquatic systems requires accurate concentration and turnover rate data. Water column NH4 (+) regeneration, potential NH4 (+) uptake, and nitrogen (N) fixation rates were measured in Missisquoi Bay, Lake Champlain, to help constrain internal N dynamics affecting phytoplankton community structure and cyanobacteria (Cy) blooms. Cyanobacteria dominated phytoplankton biomass during occasional summer bloom periods, but low or undetectable N-2 fixation rates and low heterocyte abundances suggested that N fixing cyanobacteria did not rely on atmospheric N-2. Light/dark incubations revealed that photosynthetic and dark NH4 (+) uptake generally were balanced, highlighting the importance of bacterial uptake. Our results suggest that phytoplankton were not controlled by nutrients from the bottom-up; rather, water column N dynamics responded to phytoplankton patterns. Basin-scale water column NH4 (+) regeneration rates were about 700,000 mol N day(-1) (9.8 t N day(-1)), which is almost twice the estimated N load from tributaries, and suggests a primary role for water column N regeneration in supporting primary production. Comparisons of basin-scale NH4 (+) regeneration and demand imply that primary production is not sustained fully by combined water column regeneration and tributary N inputs; thus, future research should constrain additional sources and sinks affecting N balance in this and other aquatic systems.