Compositional variability of settling organic particles in non-tidal coastal and shelf enviroments

Abstract: The coupling between the composition of settling organic particles and near-bed dynamics was studied in non-tidal coastal and shelf environments. Field investigations were made in Skagerrak and Limfjorden between 1990 and 1995. Three upper shelf positions (50-60 m) and one deeper position (350 m) were studied in Skagerrak whereas Limfjorden represents a shallow coastal lagoon. The settling particles were collected using time-series sediment traps and also current measurements and wave-action estimates were made. The collected particles were analysed on carbon, nitrogen and amino acids. In Limfjorden a significant relationship was found between increased shear velocity and decreased proportion of amino acid nitrogen relative to total nitrogen. A significant correlation between shear velocity and vertical flux was only noticed in Skagerrak in May 1992 when settling diatoms changed the benthic fluff resulting in more easily resuspended particles at the sediment surface due to low density. Seasonal differences in the amino acid composition coupled to the phytoplankton cycle were noticed both at the shelf and shelf slope. Multivariate analyses of amino acid composition were used to reveal information on changing composition in the settling organic matter. At an upper shelf position in Skagerrak a period influenced by fresh phytoplankton material could be differentiated from settling of more degraded matter. In Limfjorden suspended and sinking material showed different composition of amino acids during both spring and summer.