Exploring connections between seagrass ecosystem services and meadow hydrodynamics

Abstract

Meadows of aquatic vegetation, such as seagrass, modify the flow of water and transport of sediment in the environment. The hydrodynamic drag generated by a seagrass meadow contributes to the numerous ecosystem services it provides, which includes quiescent habitat for other species, wave damping, water quality enhancement, and carbon sequestration. This thesis reports on a series of studies using physical experiments, simulations, and field measurements to relate the interactions between seagrass, waves, currents, and sediment to two ecosystem services, wave dissipation and carbon sequestration.

First, laboratory studies and simulations were used to explore how plants interact with waves and currents with the goal of predicting wave dissipation and turbulence generation. The flexibility of a plant is critical in defining its interactions with the environment. Seagrass plants deflect under currents, which streamlines the plants and reduces the parts of the plants directly experiencing the flow, and sway under waves, which reduces the relative motion between the plants and the flow. These responses, known as reconfiguration, reduce the drag seagrass plants experience compared to rigid plant of the same length. Laboratory flume and numerical experiments showed that the relative magnitudes of current and wave velocities determine the influence of reconfiguration on drag, and therefore on seagrass-induced wave attenuation and turbulence. For more flexible leaves, defined as having a ratio of drag force to restoring force due to stiffness greater than 100, drag reduction due to current-induced deflection competes against drag augmentation due to lower relative motion, such that enhancing current speeds reduces wave energy dissipation only when the current velocity is less than one-third of the maximum wave velocity. For stiffer leaves, drag augmentation dominates drag reduction so that adding a current enhances wave energy dissipation. Meanwhile, the measured effects of reconfiguration on plant-generated turbulence were used to propose a hybrid analytical model for predicting the turbulence to account for the relative contributions of waves and currents.

Second, field experiments were performed in three Massachusetts, USA seagrass meadows to relate spatial patterns in hydrodynamics with spatial patterns in sediment organic carbon. Lower velocities were expected to reduce sediment mobility and thus enhance the deposition and retention of sediment carbon. At a wave-dominated continuous meadow, results showed decreasing sediment carbon accretion rates with increasing wave velocities, which could be predicted by accounting for seagrass-induced wave damping and wave shoaling. However, at a current-dominated lagoonal continuous meadow, sediment carbon increased with increasing tidal velocities. The spatial reduction in sediment carbon at the latter site was attributed to spatial diminishment of sediment supply with increasing distance into the meadow, away from the lagoon inlet. Lastly, in a patchy current-dominated meadow the spatial variability in sediment carbon stocks did not correlate with the spatial distribution of patches. One vegetated patch showed substantially higher sediment carbon than the rest of the meadow, which was attributed to the recent persistence of the specific patch. In addition, preliminary results for a field study comparing different methods of estimating net ecosystem carbon exchange in a seagrass meadow are also presented.