Deep oxygen penetration drives nitrification in intertidal beach sands

We discovered a hotspot of elevated nitrate concentration (median = 431 μM) in shallow beach pore water that extended across the entire length of two barrier islands in the southeastern United States of America. We investigated this feature by surveying groundwater geochemistry, measuring fluctuations in in situ dissolved oxygen (DO) concentrations, modeling groundwater flow, and quantifying nitrification rates. Nitrification of groundwater ammonium was the only possible nitrate source, with a measured potential rate of 0.84 μmol m⁻² h⁻¹. However, the observed nitrate concentrations were far in excess of the predicted maximum achievable by aerobic nitrification assuming a 2 : 1 ratio of O : N and around 200 μM DO in air-saturated seawater. Groundwater DO concentrations within the hotspot (65 cm depth) were consistently 20–50 μM. The nitrate hotspot was located at the top of the water table beneath dry, undersaturated sand that allowed the penetration of air and the dissolution of excess oxygen into the pore fluids. The total dissolved nitrogen concentration of the hotspot was higher than anywhere else on the island, indicating nitrogen accumulation within the hotspot, most likely via ammonium adsorption. Vertical dispersion was the dominant pathway for nitrate loss from the hotspot. This nitrate was consumed in underlying anoxic sand, coupling microbial pathways of nitrogen oxidation and reduction and removing bioavailable nitrogen from the beach ecosystem.