Oyster aquaculture effects on submersed aquatic vegetation habitat are inconsistent across space and time

The aim of this study was to assess how oyster aquaculture affects estuarine submersed aquatic vegetation (SAV) habitat. We conducted seasonal field campaigns to measure physical and biogeochemical SAV habitat parameters in 2 oyster farms containing bottom cages. We found that the cages attenuated currents and wave-induced shear stress, which could potentially facilitate SAV propagule recruitment in the farms in areas not shaded by cages. Effects on water column properties appeared to be a function of variations in hydrodynamic mixing. For example, periods of substantial chlorophyll a drawdown (up to ~30 µg l -1 ) were associated with lower current velocities, suggesting that farm-scale filtration exceeded water renewal rates when tidal currents were slower. We also observed spatial gradients in the daily light integral, with increases of up to ~5 mol m -2 d -1 in the farms when chlorophyll a was reduced. However, it is unclear whether increased light would facilitate SAV growth because the effect was inconsistent. Sediment porewater sulfide and nutrient concentrations were elevated in the farms, possibly due to reduced porewater advection resulting from wave and current attenuation. At the highest observed porewater concentrations, SAV growth could be impeded due to sulfide or ammonium toxicity. In general, although the potential for positive hydrodynamic effects on SAV is clear, farm effects on biogeochemical habitat parameters were less consistent and probably related to variability in the local hydrodynamic regime.