Photosynthesis and nitrogen fixation during cyanobacteria blooms in an oligohaline and tidal freshwater estuary

Rates of photosynthesis and nitrogen fixation were measured along with species composition and plankton biomass during cyanobacteria blooms in the upper Sassafras River, a poorly buffered tidal freshwater tributary of Chesapeake Bay, USA. The mixed species composition of the cyanobacteria consortium appears to allow these species to take advantage of temporal variations in dissolved inorganic carbon (DIC) and dissolved oxygen (DO) to fix carbon (C) and nitrogen (N) under conditions limiting to most phytoplankton. At the early stage of the bloom, diazotrophic (nitrogen fixing) cyanobacteria species became dominant, likely as a consequence of nitrogen limitation; both rates of N-2-fixer biomass-normalized N-2 fixation and chlorophyll a-based photosynthesis increased with irradiance during this bloom stage. When the bloom reached peak biomass, photosynthetic carbon uptake and oxygen production resulted in DIC depletion and oxygen oversaturation. The constraint of oxygen on nitrogenase activity and concomitant C limitation on photosynthesis caused inhibition of N-2 fixation in the light during bloom peaks. N-2 fixation in the dark period accounted for similar to 40% of daily N-2 fixation when pH and DO were extremely high, and cyanobacteria species known to fix N-2 in the dark were dominant in the consortium at these times. The presence of cyanobacteria species with different environmental tolerances resulted in bloom persistence under limiting conditions for most eukaryotic phytoplankton. N-2 fixation by cyanobacteria consortia is an important component of the nitrogen cycle in some oligohaline and tidal freshwater estuaries.