COASTAL AND SUBMARINE GROUNDWATER DISCHARGE: NITROGEN BIOGEOCHEMISTRY AND ROLE IN EUTROPHICATION

Submarine groundwater discharge (SGD) is commonly an important pathway for increasing land-derived nitrogen (N) and phosphorus (P) loads that drive estuarine eutrophication. In addition to fresh groundwater, SGD includes substantial quantities of saline groundwater, carrying nutrients with a marine sediment source, and brackish groundwater produced by mixing of fresh and saline groundwater. There is therefore a need to understand processes controlling: a) nutrient loads carried by fresh groundwater; b) nutrient loads carried by saline groundwater; and c) biogeochemical transformations in discharge zones that alter the nutrient load carried to sea. Clear separation of water types and nutrient sources within SGD zones is critical because nutrient sources, concentrations and discharge rates of marine and terrestrial groundwater commonly differ. Increasing nutrient loads from groundwater discharge increase biomass of phytoplankton, macroalgae and periphyton in estuaries, and drive declines in seagrass coverage due to resulting light limitation. The total amount of N and P entering estuaries is important in driving eutrophication, but so are the chemical species compositions (reduced vs. oxidized; organic vs. inorganic), as well as the relative stoichiometries of N and P and other nutrients. Phytoplankton vary in their ability to take up different forms of nutrients, resulting in competitive advantages depending on the relative availability of nutrient forms. Since groundwater delivers distinct nutrient loads to the estuary depending on whether they are chemically reducing (carrying ammonium, organic N, and high P concentrations), or oxidizing (carrying primarily nitrate, less organic N, and little P), the impact of nutrient loads from these different sources would be expected to differentially stimulate phytoplankton. Although N concentrations and loads in fresh groundwater are largely controlled by watershed land uses, there is evidence that biogeochemical conditions and N speciation may be more influenced by watershed geological setting than by land uses. In this review of the role of coastal groundwater in driving estuarine eutrophication, we will draw on our own published and unpublished data, as well as the published works of others to synthesize the current state of knowledge.