Ammonium uptake by seagrass communities: effects of oscillatory versus unidirectional flow

Nutrient dynamics of aquatic communities are dependent on the flux of nutrients to organisms within the communities. Flux is dependent on water column concentration and hydrodynamic factors that affect both advection of nutrients through the community and rates of diffusion at the surfaces of organisms. In this study, we measured rates of ammonium uptake for a seagrass community under various hydrodynamic conditions and determined the effects of water velocity and oscillatory flow on uptake rates. Experiments were conducted using a portable flume deployed in natural Thalassia testudinum communities. Uptake rate constants ranged from 9.9 to 25.4 × 10–5 m s–1 and were ~1.5 times higher in oscillatory flow than in unidirectional flow. Uptake rate constants were positively dependent on both water velocity and turbulent energy in the water column. These results demonstrate the importance of hydrodynamics on biogeochemical cycling in seagrass beds and provide evidence of the efficacy of merging research on hydrodynamics and biogeochemistry in understanding nutrient processes in complex nearshore communities.