Stability and Resilience of Seagrasses in Shallow Coastal Bays

Shallow coastal lagoons are environments where a dynamic equilibrium exists between water quality and seagrass cover. Dense seagrass canopies limit the resuspension of bed sediments thereby creating a clearer water column and a positive feedback for seagrass growth. Positive feedbacks are often associated with the existence of bistable dynamics in ecosystems. For example, a bare and a seagrass covered sediment bed could be both stable states of the system. This study develops: 1) a one-dimensional hydrodynamic model of vegetation-sediment-water flow interactions and uses it to investigate the strengths of positive feedbacks between seagrass cover, stabilization of bed sediments, turbidity of the water column, and the existence of a favorable light environment for seagrasses. 2) couples the hydrodynamic model to a seagrass vegetation growth model to examine the effect of dynamically-varying seasonal and interannual seagrass density on sediment resuspension, water column turbidity, and the subsequent light environment on hourly time steps and then run over decadal time scales. 3) a landscape scale model to investigate the effects of multiple meadows on the bulk sediment light environment and consequent emergence of bistability. The models were applied to Hog Island Bay, a shallow coastal bay on the eastern shore of Virginia. The effects of temperature, eutrophication, stresses associated with climate change, and bed grain size on bistability of seagrass ecosystems in the bay are explored. The results indicate that the positive feedback that exists due to the seagrass modified hydrodynamics is strong enough to induce bistable dynamics in limited depth range. This range is dependent on both stochastic environmental drivers (irradiance, water temperature, water depth, wind waves, tidal currents, bed grain size distribution and water column iii eutrophication), within meadow seagrass morphology (Shoot density, leaf density, above and belowground biomass, and leaf length), as well as seagrass effective fractional cover on the landscape. Within this bistable range, seagrass have limited resilience, such that a disturbance or sequences of disturbances in space and/or time, can push the system past some critical threshold resulting in collapse to a stable bare sediment system. Full collapse can take up to decades. Both flickering, and slowing down are seen as a leading indicators of this collapse.

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