The Beach Bioreactor: Unraveling the Anomaly of Intensive Remineralization above a Deep Oxycline

High-energy beaches receive high inputs of organic matter from seawater infiltration, fueling intensive oxygen (O₂) consumption rates in the upper sand layer, which depend on seasonal variations in temperature and organic matter supply and can exceed 10 times the average subtidal rates during the growing season. Despite the intensive rates, recent studies have found deep O₂ penetration underneath, extending down several meters. To investigate this anomaly, we applied a reactive transport model to simulate O₂ supply and consumption for winter and summer conditions. Well-known conditions at a high-energy beach on Spiekeroog Island, Germany, were used to define topography, hydraulic conductivity, tide, and wave amplitudes, and the model was built using measured O₂ distribution and O₂ consumption rates. The tide-resolving model was capable of simulating the periodic tidal desaturation of the surface layer. We found that the aeration with atmospheric O₂ during desaturation is a significant O₂ source, contributing up to 30-60% of total O₂ consumption. Meanwhile, seawater O₂ quickly bypasses the upper reactive layer, extending the oxycline to depths of 11-16 m in the summer and winter, respectively. This mechanism mitigates the seasonal imprint into deeper layers yet promotes intensive aerobic OC remineralization of up to 0.7 gC m^-2 d^-1.