The effect of groundwater seepage on nutrient delivery and seagrass distribution in the northeastern Gulf of Mexico

A hypothesis was tested to determine if a relationship exists between rates of submarine groundwater discharge and the distribution of seagrass beds i n the coastal, nearshore northeastern Gulf of Mexico. As determined by nonp arametric statistics, four of seven seagrass beds in the northeastern Gulf of Mexico had significantly greater submarine groundwater discharge compare d with adjacent sandy areas, but the remainder exhibited the opposite relat ionship. We were thus unable to verify if a relationship exists between sub marine groundwater discharge and the distribution of seagrass beds in the n earshore sites selected. A second objective of this study was to determine the amount of nitrogen and phosphorus delivered to nearshore areas by subma rine groundwater discharge. We considered new nutrient inputs to be those d elivered to surface waters by the upward flux of fresh water. This upward f lux of water encounters saline porewaters in the surficial sediments and th ese porewaters contain recycled nutrients; actual nutrient flux from the se diment to overlying waters includes both new and recycled nutrients. New in puts of nitrogen to overlying surface waters for one 10-km section of coast line, calculated by multiplying groundwater nutrient concentrations from fr eshwater wells by measured seepage rates, were on the order of 1,100 +/- 19 0 mol N d(-1). New and recycled nitrogen fluxes, calculated by multiplying surficial porewater concentrations by measured seepage rates, yielded fluxe s of 3,600 +/- 1,000 mol N d(-1). Soluble reactive phosphate values were 15 0 +/- 40 mol P d(-1) using freshwater well concentrations and 130 +/- 3.0 m ol P d(-1) using porewater concentrations. These values are comparable to t he average nutrient delivery of a small, local river.