Interactive effects of physical and biogeochemical feedback processes in a large submersed plant bed

Submersed plants are sensitive to nutrient loading because excess algal growth creates light-limiting conditions. However, submersed plant beds can also modify nutrient cycling through feedback loops whereby algal growth is limited and plant growth is enhanced. Whereas most studies on the effect of submersed aquatic vegetation (SAV) beds on nutrient cycling concentrate on either biogeochemical or physical controlling mechanisms, we use a holistic approach that analyzes how these processes interact. We measured a suite of physical and biological processes in a large SAV bed and developed a simple, 1-dimensional reactive transport model to investigate the mechanisms underlying SAV bed effects on nutrient cycling. We observed lower water column concentrations of dissolved inorganic nitrogen and phosphorus (DIN and DIP) inside relative to outside the SAV bed during the summer. Sediment denitrification (mean N2-N flux in August = 46 μmol m−2 h−1) and plant nutrient assimilation (August rates =385 μmol N and 25 μmol P m−2 h−1) were mechanisms of nutrient removal. We also found that the physical structure of the bed decreased advection and tidal dispersion, resulting in increased water residence time that enhanced biologically mediated nutrient loss. These processes create conditions that enable SAV to outcompete other primary producers, as water column nutrient concentrations were low enough to limit algal growth and associated light attenuation, while sediment pore water concentrations were sufficient to satisfy SAV nutrient demand. These findings suggest that interactions between physical and biological feedback processes in SAV beds can play a key role in structuring shallow aquatic ecosystems.