Seasonal Changes in Subsurface Hydrology Influence Nutrient Cycling in a Salt Marsh

Excess nutrient inputs to coastal waterways have caused widespread eutrophication in nearshore environments, negatively impacting nearby water quality and aquatic ecosystems. Salt marshes exist at the terrestrial-marine interface between watersheds and the ocean, serving as hot spots for nutrient cycling by receiving and processing both tidal and freshwater inputs. While this can influence overall estuarine surface water quality, there remains a critical gap in our understanding of the biotic and abiotic controls on the spatial and temporal variability of subsurface nutrient cycling in salt marshes. To understand how biotic and abiotic conditions impact porewater nutrient concentrations within salt marshes, we collected and processed monthly surface and porewater samples for a suite of water quality parameters including nutrients, salinity, and dissolved oxygen along a salt marsh transect at the Elkhorn Slough National Estuarine Research Reserve (ESNERR). To characterize environmental conditions, we collected high frequency water level measurements from sensors installed in nested piezometers, monthly remotely sensed observations of vegetation activity, and subsurface soil properties and composition characterization through lab analyses and a geophysical survey. We found that spatial differences in soil composition between marsh positions and seasonal changes in hydrologic mixing and vegetation activity create spatiotemporal patterns in porewater nutrient concentrations in salt marshes. Specifically, seasonal precipitation and deep subsurface water levels across the transect influenced porewater salinity, with lower porewater salinity and earlier vegetation activation in upper marsh positions during the spring season leading to changes in nutrient concentrations. Nuclear magnetic resonance surveys of the subsurface indicate a separation between shallow and deep subsurface flow paths along the transect, which may influence hydrologic cycling in the marsh subsurface. Together, our results suggest terrestrially derived subsurface freshwater inputs, vegetation activity, and subsurface architecture play a critical role in nutrient cycling in salt marshes.