Biol. Bull. 207: 173. (October 2004)
© 2004 Marine Biological Laboratory
Estimating Groundwater-Derived Nitrogen Flux Into a Coastal Embayment: Salt Pond, Cape Cod, Massachusetts
Kayla Halloran1,
Matt Charette2,
Paul Henderson2,
Kevin Kroeger2,
Lindsey Ryckman3,
John Crusius3 and
Dirk Koopmans3
1 Bucknell University, Lewisburg, Pennsylvania
2 Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
3 United States Geological Survey, Woods Hole, Massachusetts
Submarine groundwater discharge (SGD) often contributes significant fluxes of nutrients to coastal waters and is an important vehicle for nitrogen transport. Salt Pond, a small eutrophic coastal embayment in Cape Cod, Massachusetts, is connected by a single outlet to the Nauset Marsh System and receives elevated anthropogenic nitrogen inputs associated with human development. A series of seepage meter deployments found that SGD was not related to tidal stage, suggesting that the groundwater hydraulic head is above the sea level at all times. The average SGD rate from seepage meters of 12 cm d–1 was comparable to a similar concurrent experiment, which employed radon as a tracer of SGD (4 cm d–1). Groundwater sampling indicated that nitrate was transported conservatively through the aquifer, suggesting that little denitrification occurs during the mixing of fresh and salty groundwater. Using the seepage meter flow rates and average total dissolved nitrogen (TDN) concentrations in the groundwater, the SGD-derived TDN flux to the pond was 11.5 mmol m–2 day–1 (67% as nitrate, 30% as dissolved organic nitrogen, 3% as ammonium). Surface water sampling in the tidally flushed channel over a tidal cycle revealed that the Nauset Marsh is a net source of TDN to Salt Pond (12.7 mmol m–2 day–1). Thus, Salt Pond is a net nitrogen sink, with a total of 24.2 mmol m–2 day–1 being retained by the system, most likely in the form of algal biomass and denitrification in the fine-grained organic rich sediments that characterize the deep basin.
