TEMPERATURE EFFECTS ON NITROGEN CYCLING IN COASTAL AQUIFERS

Coastal unconfined aquifers represent an important transition zone between terrestrial groundwater systems and nearshore coastal waters. Biogeochemical reactions in these systems can have a marked impact on groundwater derived nutrient and contaminant loads to coastal ecosystems via submarine groundwater discharge (SGD). Microbial-mediated chemical transformations are sensitive to temperature, thus chemical fluxes from coastal aquifers to coastal and estuarine ecosystems may vary spatially and temporally with changes to groundwater temperature. In this study we used a variable-density groundwater flow and reactive transport model with a global groundwater temperature dataset to investigate the effects of groundwater and surface water temperature on nitrogen cycling and nitrate removal efficiencies in coastal aquifers. Model results show that nitrate removal efficiency increased from 5% to 93% as fresh groundwater and seawater temperature increased uniformly from 5 to 35 °C. Denitrification rates increased by an order of magnitude across the range of temperature values tested. Aerobic respiration rates also increased by an order of magnitude over the same temperature range, leading to anoxic conditions near the fresh groundwater discharge zone. Rapid DOC oxidation at higher temperatures also led to elevated ammonium concentrations along discharging flow paths. The results demonstrate the role of temperature on nitrate removal in coastal aquifers and have implications for seasonal and spatial variability of nutrient inputs to coastal waters.