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Particulate Organic Matter in Permeable Marine Sands—Dynamics in Time and Depth

In a North Sea intertidal sandflat sediment, depth profiles of dissolved and particulate compounds were measured and evaluated with respect to transport and degradation of particulate organic matter (POM). Bimonthly from July 1997 to July 1998, the amount was measured of fine-grained (<70 μm) material in the permeable sand matrix, its N, Corgand chlorophyll content as well as porewater DIC, NO−x and NH+4 concentrations. Depth profiles of fine particle concentrations indicated hydrodynamic influence down to 4–8 cm below the sediment surface. Worst-case calculations on the macrofaunal contribution to particle transport resulted in a biodiffusion coefficient of DB≤1·85×10−6cm2 s−1, corresponding to less than 50% of total transport. Chl and POC contents and DIN concentrations exhibited summer/autumn and winter/spring characteristics in their profiles, revealing the seasonal importance of early diagenesis and advective transport, respectively. Areal inventories of POC, PN and Chl indicated the dominance of degradation and hydrodynamic removal of organic material during autumn/winter and fresh POM input throughout spring and summer. In the upper 5 cm, seasonal variation in the particle, POC, PN and Chl concentrations of the sediment was 1·4–5·3 times as large as below. Calculations based on POC loss or short-term DIC accumulation yielded estimates of annual carbon turnover rates ranging between 55 and 123 g C m−2 yr−1. Possible implications of POM dynamics on the role of permeable sands in the marine carbon cycle are discussed.