In intertidal areas, large amounts of organic matter in the form of kelp are regularly deposited on the beach. Mineralization of this organic matter leads to the release of many intermediates and end products into? the sediments. A wide variety of aerobic and anaerobic microorganisms control the mineralization process. The high input of reduced matter into the local ecosystem is apparent from elevated levels of sulfide and the abundance of white filamentous bacteria in the nearby surfaces? Or sediments?. The input of nutrients into this normally oligotrophic environment may subsequently also stimulate primary production of the microphytobenthos. We investigated the influence of kelp deposits on in situ concentrations of CO, H2, Fe(II), sulfide, CH4 and various nutrients on a beach on Helgoland. Our preliminary results show an enrichment of CO and H2 below kelp deposits. Exposure to O2 seemed essential for CO and H2 release during the process of kelp degradation, as was observed during incubation experiments. The most pronounced influence of kelp deposits was observed for Fe(II), with higher Fe(II) concentrations below kelp deposits, although large heterogeneity exists. Sulfide and CH4 were found in significant concentrations within sediments between kelp deposits and the adjacent sea. Remarkably, their levels were lower directly? below the kelp, whereas Fe(II) showed the opposite trend. We aim to define the processes responsible for the high sulfide and CH4 concentrations within the sediments, with a focus on the role of the high CO and H2 levels for sulfate reduction and methanogenesis. Through in situ measurements we will study the effects of variable oxygenation on the release of these compounds. We also aim to study and model the hydrology of the beach to assess the transport modes of the intermediates and nutrients through the permeable sands. We will furthermore determine the consequences of the nutrient input for the local microphytobenthos.