Abstract. Submarine groundwater discharge (SGWD) and associated nutrient fluxes at Hasaki Beach along the Kashima coast in Japan were investigated through field measurements and numerical simulations. The field data indicate that (1) groundwater has higher concentrations of land-derived nutrients than seawater and river water; (2) microbial activity near the shoreline is likely to induce mineralization, reduction, and oxidation of nitrogen and phosphorus; and (3) in the portion of the aquifer underlying coastal forest, nitrate is appreciably reduced. A numerical model incorporating effects of water table and tidal fluctuations was developed to evaluate SGWD. Assuming that the aquifer consists of homogeneous sandy soil, the simulation results show that the nutrient flux due to SGWD is less than that of Tone River discharge. The nutrient flux via groundwater seepage is considered to be a minor component of primary productivity in the surf zone.
IntroductionCoastal marine ecosystems are affected by dissolved nutrient inputs from circulating offshore water, river runoff, and groundwater seepage. The regional nutrient budget also includes atmospheric deposition, fertilizer application, wastewater treatment plant discharge, livestock waste, and remineralization of organic matter in sediment. Among these sources the effects of groundwater on marine environments are not as well known as those of river water and offshore water. Groundwater containing inorganic nutrients can have a significant influence on coastal ecosystems, especially when nutrient concentrations are high or relative contribution of submarine groundwater discharge (SGWD) is large. Owing to nutrient leaching from surface-applied fertilizers, groundwater usually has a higher concentration of inorganic nutrients than does seawater. Therefore even low rates of SGWD often need to be accounted for in the nutrient budget for a coastal ecosystem.Ecologists have not paid much attention to groundwater as a source of inorganic nutrients for coastal marine ecosystems. Kohout [1964] and Kohout and Kolipinski [1967] were the first investigators to study the importance of fresh groundwater seepage into the sea. Their studies were conducted along the shore of Biscayne Bay, Florida, and showed a definite relationship between biological zonation and groundwater discharge into the bay. Johannes [1980] Bight might amount to a flux equivalent to 40% of the total flow entering the sea from adjacent rivers. We are unsure whether the figure of 40% is precise; however, the uncomplicated calculations might lead to overestimates of SGWD, as discussed by Younger [1996]. This paper reports comprehensive and spatially detailed assessment of submarine groundwater discharge, and the associated nutrient transport, at Hasaki Beach facing the Kashima Sea in Japan. Additionally, in order to calculate the nutrient fluxes we develop a numerical model including the effects of water table and tidal fluctuations to evaluate the velocity field in the coastal aquifer.
Study SiteHasaki Beach,...
