During seawater circulation in permeable intertidal sands, organic matter degradation alters the composition of percolating fluids and remineralization products discharge into surficial waters. Concurrently, coastal seawater nutrient and organic matter composition change seasonally due to variations in pelagic productivity. To assess seasonal changes in organic matter degradation in the intertidal zone of a high energy beach (Spiekeroog Island, southern North Sea, Germany), we analyzed shallow pore waters for major redox constituents (oxygen (O2), manganese (Mn), iron (Fe)) and inorganic nitrogen species (nitrite (NO2-), nitrate (NO3-), ammonium (NH4+)) in March, August, and October. Surface water samples from a local time series station were used to monitor seasonal changes in pelagic productivity. O2 and NO3- were the dominating pore water constituents in March and October, whereas dissolved Mn and Fe were more widely distributed in August. Seasonal changes in seawater temperature as well as organic matter and nitrate supply by seawater were assumed to affect microbial rates and respective pathways. Pore water and seawater variability led to seasonally changing constituent effluxes to surface waters. Mn, Fe, and NH4+ effluxes exhibited their minimum in March (3; 2; 7 mmol d-1 per meter shoreline, respectively) and reached their maximum in August (41; 159; 99 mmol d-1 per meter shoreline, respectively). Furthermore, the intertidal sands switched from being a net dissolved inorganic nitrogen (DIN) sink in March (-62 mmol d-1 per meter shoreline) to a net source in August (99 mmol d-1 per meter shoreline). In conclusion, we demonstrated the necessity of seasonal flux evaluations.

Supplement to: Ahrens, Janis; Beck, Melanie; Marchant, Hannah K; Ahmerkamp, Soeren; Schnetger, Bernhard; Brumsack, Hans-Jürgen (2020): Seasonality of Organic Matter Degradation regulates Nutrient and Metal Net Fluxes in a High Energy Sandy Beach. Journal of Geophysical Research: Biogeosciences