Spatially explicit feedbacks between seagrass meadows, sediment, and light: habitat suitability for seagrass growth

In shallow costal bays in which nutrient loading and riverine inputs are low, turbidity, and the consequent light environment are controlled by resuspension of bed sediments due to wind-waves and tidal currents. When sediment resuspension is high, resultant low light environments can limit benthic primary productivity. However, both currents and waves are affected by the presence of benthic plants such as seagrass. Moreover, vegetated areas influence a larger area than they footprint, including an adjacent downstream area. This region, while barren of seagrass, exhibits reduced shear stresses until boundary layer reformation occurs. We develop a simplistic approach to explore how the patchy structure of seagrass meadows may affect sediment resuspension and the consequent light environment under tidal and wind-wave forcing due to the presence of this sheltered region. In order to investigate the effect of patch modified hydrodynamics we generated heterogeneous vegetation covers comprised of a mosaic of randomly distributed patches with exponentially distributed radii. These heterogenous landscapes were generated from two perspectives. First, circular patches were randomly placed on a bare landscape; with each circular patch assumed to be a seagrass meadow. Second, starting from a landscape assumed to be homogenous meadow with constant shoot density, circular gaps were randomly generated. Each gap was considered a disturbance that completely removed all seagrass from within the circular region. Fractional cover of vegetation on the landscape was used to facilitate comparisons across landscape realizations and perspectives. Each landscape realization was then used to partition wave and current shear stresses on the landscape. Total suspended sediment and light attenuation characteristics were then calculated and averaged over the landscape to examine how both fractional cover and mean water depth affect the bulk sediment and light environment from the meadow or gap scattering scenarios. The results indicate that an effective fractional cover, which incorporates the sheltering area, has important controls on the distributions of shear stress, suspended sediment, light environment, and consequent seagrass habitat suitability across the landscape. As fractional cover increases, the effective fractional cover increases dramatically. In contrast, given a homogenous meadow landscape, the effective fractional cover diminishes slowly when disturbances remove fractional cover on that landscape. This hysteresis in effective fractional cover has significant implications regarding both initial establishment of seagrass meadows as well as the resilience of contiguous seagrass landscapes to disturbances that remove fractional cover.