A review of land–sea coupling by groundwater discharge of nitrogen to New England estuaries: Mechanisms and effects

Hydrologists have long been concerned with the interface of groundwater flow into estuaries, but not until the end of the last century did other disciplines realize the major role played by groundwater transport of nutrients to estuaries. Mass balance and stable isotopic data suggest that land-derived NO3, NH4, and dissolved organic N do enter estuaries in amounts likely to affect the function of the receiving ecosystem. Because of increasing human occupancy of the coastal zone, the nutrient loads borne by groundwater have increased in recent decades, in spite of substantial interception of nutrients within the land and aquifer components of watersheds. Groundwater-borne nutrient loads have increased the N content of receiving estuaries, increased phytoplankton and macroalgal production and biomass, decreased the area of seagrasses, and created a cascade of associated ecological changes. This linkage between land use and eutrophication of estuaries occurs in spite of mechanisms, including uptake of land-derived N by riparian vegetation and fringing wetlands, ‘‘unloading’’ by rapid water removal, and direct N inputs to estuaries, that tend to uncouple the effects of land use on receiving estuaries. It can be expected that as human activity on coastal watersheds continues to increase, the role of groundwater-borne nutrients to the receiving estuary will also increase. 2006 Elsevier Ltd. All rights reserved. 0883-2927/$ see front matter 2006 Elsevier Ltd. All rights reserved doi:10.1016/j.apgeochem.2006.09.002 * Corresponding author. Tel.: +1 508 289 7515; fax: +1 508 289 7949. E-mail address: [email protected] (I. Valiela). 1 Present address: The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543, USA. 2 Present address: US Geological Survey, Woods Hole Science Center, Woods Hole, MAA 02543, USA. 3 Present address: P.O. Box 3814, Hailey, ID 83333, USA. 4 Present address: Save The Bay, Narragansett Bay, Providence, RI 02908, USA. 5 Present address: Dauphin Island Sea Lab, 101 Bienville Blvd., Dauphin Island, AL 36528, USA. 1. Background Awareness of the importance of transport of materials by freshwater flow through rivers and streams to receiving coastal waters has existed as long as the history of natural science, but knowledge about the role of subsurface flows has a more recent history. Interest in the interaction of groundwater and coastal waters emerged early. Key features of flow through aquifers were identified in pioneering work by Darcy (1856), and by Baden Ghijben (1888–1889) and Herzberg (1901), who interpreted