Aspects of carbon and nitrogen cycling in a shallow marine environment

Studies on the water column of a semi-enclosed bay (Roskeeda Bay, Co. Galway) showed a close association between photosynthesis and respiration. Gross annual production of both the water column and the sediment was lower than the total annual decomposition. Sediment traps showed evidence of constant resuspension of fine sediment surface material, which frustrated the direct measurement of the transfer of particulate organic material from water to sediment. Fluxes of inorganic nitrogen compounds across the sediment-water boundary were measured by using an in situ bell jar technique. Mean rates of release of nitrate and nitrite were insignificant compared with the release of ammonia, which was equivalent to 20% of the phytoplankton nitrogen requirement over the summer. An approximate diffusion coefficient of 4.5 × 10^-5 cm²·s^-1 was estimated for ammonia release from the sediment. Ammonia release was strongly correlated with oxygen uptake by the sediment and less strongly with temperature. During the productive season nitrogen supply to the water column from the biological recycling of organic material in both water and sediment was in excess of phytoplankton requirements. As standing levels of inorganic nitrogen did not increase significantly over this period, it is suggested that the intertidal macrophyte community acts as both a sink of inorganic nitrogen and a source of organic carbon for the system. A tentative annual carbon budget is presented.