Influence of cyanobacteria blooms on sediment biogeochemistry and nutrient fluxes

To explore the influence of cyanobacteria blooms on internal nutrient loading and sediment biogeochemistry, we conducted a 3 yr (2008-2010) field investigation in the upper Sassafras River, a shallow tidal freshwater tributary of Chesapeake Bay, Maryland. During 2010 a dense diazotrophic cyanobacteria bloom was accompanied by abnormally high pH (9.0-10.5) and supersaturated oxygen throughout the summer. With the persistence of high pH in the water column and organic matter inputs from sedimented phytoplankton detritus, flux rates of NH4+ and soluble reactive phosphate were significantly higher during the summer of the bloom year than the same period in nonbloom years. Although low summer N-2-N flux rates generally coincided with reduced water-column NO3- concentrations, the efficiency of nitrification was constrained by elevated pH that converted NH4+ to more toxic NH3. Denitrification was subsequently limited by nitrate supply, elevated pH, and the deepening of the oxic layer in the sediment. During blooms, we observed increases in bioavailable nutrient release from the sediment, coupled with decreases in inorganic N:P flux ratios and inhibition of N-2 loss from the ecosystem. These changes in sediment nutrient recycling created positive feedbacks that may have fueled the prolonged blooms and selectively fostered growth of diazotrophic cyanobacteria.