Oxygen Metabolism in Restored Eelgrass (Zostera Marina L.) Meadows Measured by Eddy Correlation

Primary productivity in seagrass meadows is an important factor to consider in coastal systems as it can support economically important industries such as finfish and shellfish fisheries. Seagrass meadows are significant to global carbon budgets and represent an important carbon sink through biomass and burial. They are declining worldwide, and restoration can return this important ecosystem to coastal environments. Therefore, studying the drivers of seagrass productivity and how net ecosystem metabolism (NEM) changes with colonization is warranted to understand the effects of restoration on the ecosystem services provided by seagrass meadows. This thesis addresses the effects of seagrass restoration on oxygen metabolism using the eddy correlation technique (EC). Aquatic EC provides high temporal resolution measurements of NEM that integrates over a large spatial scale and does not alter the local hydrodynamics or light environment. As aquatic EC is relatively new, I investigated the capabilities of this method to incorporate heterogeneous fluxes using a 3-dimensional numerical model to provide guidance for measurements in patchy seagrass meadows. The results showed that by following certain empirical guidelines, heterogeneous fluxes are well-integrated, and eddy correlation measurements reflect ecosystem-scale fluxes. Using EC, I found that light, flow, and temperature were the major ecosystem-scale drivers of oxygen metabolism in seagrass meadows. From seasonal measurements, I found feedback processes occurring at multiple timescales – hourly, daily, and seasonal. Metabolism in both light and dark increased with seagrass colonization, suggesting that seagrass meadows are hotspots of both autotrophic and heterotrophic production. From ii high temporal resolution data, annual NEM was modeled based on available light for every hour throughout the year and showed young seagrass meadows were autotrophic while older meadows were heterotrophic. The annual NEM combined with trajectories of seagrass shoot density since 2007 indicate that young meadows were becoming denser and accumulating biomass while older meadows may be maintaining a critical biomass. Although the older meadow was heterotrophic on an annual scale, it was not different from the unvegetated sediments. While this restoration has increased the magnitude of productivity and provided other important ecosystem services, it has not altered the net rates of benthic metabolism in the lagoons.

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