To characterize the large amounts of organic matter in the Ems-Dollard estuary in terms of suitability as a food source for heterotrophic organisms, decomposition experiments were carried out. Water samples were taken at 6 weeks intervals from May 1979 to August 1980 and incubated for 4 weeks in the dark at field temperature. During incubation, changes in the concentration of dissolved and particulate organic matter, oxygen and inorganic nitrogen components were followed. The labile part of the investigated component was defined as the fraction that was decomposed during incubation. In the outer part of the estuary the labile fractions of particulate organic carbon (POC) and amino acids (PAC) were high in summer which could be ascribed to the local phytoplankton bloom. In the inner part the labile fractions of POC and PAC were much lower. The labile were low though in the outer part higher percentages were observed during phytoplankton bloom. No significant decomposition of dissolved and particulate carbohydrates were observed. The labile fractions of all components investigated were smallest in the middle part of the estuary which was ascribed to the low local primary production and the long time needed for allochtonous organic matter to reach that part of the estuary. Large amounts of organic matter originating from waste water, were decomposed in the innermost part of the estuary, and had no effect on the labile fraction of organic matter in the other parts of the estuary. Ammonification was frequently observed in the outer part, nitrification generally at all stations. The relation between the low labile fractions of organic matter in the inner part, and the poor growth of filter-feeding macrofauna in that part of the estuary was discussed.
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