Submarine Groundwater Discharge as a Potential Hidden Pathway for Eelgrass Decline in San Juan County

Abstract

Understanding recent eelgrass declines in San Juan County is of vital importance, as these eelgrass communities represent incredible ecological and economic value. The alarming magnitude of these losses and the unlikelihood of natural recolonization are additional reasons to continue exploring factors that may have contributed to decline. This project addressed one of the least studied vehicles for coastal pollution as a potential contributor to eelgrass loss: submarine groundwater discharge (SGD). SGD can introduce potent pollutants such as nutrients and herbicides directly to nearshore ecosystems. The goal of this project was to determine whether SGD is affecting eelgrass health in San Juan Island National Historical Park sites by adding excess nitrogen or phosphorous to waters at these sites. While no phosphate loading was detected, elevated nitrogen concentrations were observed both in seawater samples, at approximately 1 mg/L, as well as in SGD samples at 1-2 mg/L nitrogen. Roche Harbor, on the north side of the island showed the highest concentrations at 2.1 mg/L. Considering the harmful effects of nitrogen loading on eelgrass, it seems that SGD may have a negative effect on local eelgrass habitats.