Nutrient export to a reef dominated estuary through submarine groundwater discharge
Spalt, Nicholas A.
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Oysters effectively remove nitrogen, the limiting nutrient in most coastal environments. It is important to identify areas favorable to oysters to offset nutrient loading that may lead to decreased ecosystem services and harmful algal blooms. Accordingly, water and solute fluxes from submarine groundwater discharge (SGD), an often overlooked component in nutrient budgets, were examined in this study across a variety of depositional environments (i.e. paleovalley, interfluve, paleovalley margin, and oyster reef). Oyster reefs have been shown to be oriented along permeable interfluve sediments near palleovalley margins, therefore this study examined the contribution of paleovalley-associated SGD to coastal environments and their potential ability to support the health and productivity of oyster reefs. A novel combination approach of radioactive geochemical tracers, stable isotopes, and petrophysical techniques was applied to Copano Bay, a reef-dominated estuary, in the semi-arid area of South Texas. The results of this study indicate that low gradient watersheds, characteristic of small surface water exports, can have large groundwater-surface water interaction components that introduce or reintroduce groundwater into the water column. Based on good agreement between petrophysical and geochemical analyses, two mechanisms that drive pore fluid transport in a secondary bay with minimal tidal influence were identified: turbulent exchange and pumping based on bathymetric relief change. Furthermore, this study found that under dry conditions, SGD could be responsible for 13-57 times that of riverine nitrogen input to Copano Bay, estimates comparable to other areas similar in nature. Through PCA analysis on time series data, groundwater was determined to be the principal component during dry periods while freshwater inflow explained the variance in geochemical parameters following a rain event. Thus, paleovalley margins appear to be an effective setting for the health of an oyster reef as they are adjacent to surface water inflows and provide a pathway for groundwater-surface water interaction. Also, adjacency to surficial estuarine mud may supply additional nutrients through porewater evacuation, which may enhance primary production, and ultimately nourishment for the reef.