Abstract Seawater infiltration into the permeable sands of beach aquifers creates a high input of biogeochemical reactants driven by tides and waves. The upper sand layer acts as a filter, retaining particulate organic matter (POM), which is degraded by bacteria under predominantly oxic conditions. The seasonal variation of seawater POM and oxygen (O 2 ) entering the infiltration zone, combined with the POM filtration efficiency of the highly morphodynamic upper layer, determines the organic matter turnover and subsequent redox gradients along porewater flowpaths. We investigated these effects by quantifying the seasonal O 2 consumption rates directly from the incubations of sediments taken along a transect in the seawater infiltration zone at Spiekeroog Beach, Germany. We carried out a two‐monthly year‐long sampling campaign with high spatial resolution measurements down to 1 m depth. In summer, O 2 consumption rates of up to 106 μM hr −1  were found in the first decimeters with a significant decline over depth, indicating efficient retention of reactive POM in the surface layer. Seasonal variation in organic carbon of the sand's suspendable particulates indicates rapid turnover and little storage. In winter, rates decreased significantly to below 11 μM hr −1 . Integrated over the investigated oxic layer, the estimated carbon mineralization varies between 15 (winter) and 143 (summer) mmol C m −2  d −1 with a yearly average of 73 mmol C m −2  d −1 . The yearly CO 2 production of 35 kg per meter shoreline characterizes the beach as a high‐throughout system with rapid OM remineralization in the retention layer, especially in summer, but with little OM storage. Plain Language Summary On the coast, sandy beaches act as a filter for seawater. Seawater seeps into the sand, and organic particles such as algae remains are trapped between the sand grains. Bacteria live on the surface of the sand grains and use the oxygen dissolved in seawater to break down the organic material. We investigated how seasonal changes from spring to winter, and the particle filtration affects the oxygen consumption by bacteria. Our study found that oxygen consumption follows the availability of fresh algal material during the year and is greatest in summer, when temperatures are high and algal blooms are intense. This cycle determines when and where in the sediment bacteria are active and controls the breakdown of the organic algae material to carbon dioxide (CO 2 ). We found that overall bacteria in the beach sands degrade a high amount of organic matter compared to other sandy seabeds. Our findings therefore give valuable information on bacterial activity present in beach sands, which impacts the breakdown of organic material and determines the quality of the beach groundwater that ultimately flows back into the ocean. These processes impact the health of coastal ecosystems and have consequences for human activities, such as fisheries or recreation. Key Points Seasonal variations in organic matter inputs and temperature regulate oxygen consumption in intertidal beach aquifers Retention of fresh particulate organic matter in the sand body in summer leads to a highly reactive top layer in the beach infiltration zone Seasonal variations of organic carbon content in the fine fraction of the sediments indicate rapid organic matter turnover and low storage