Diet composition, selectivity, food niche width and food niche overlap of early life stages of fishes were studied in the Barther Bodden system and in the Weser and Elbe estuaries during spring and summer from 1987 to 1991 and from 1993 to 1994. Larvae of perch (Perca fluviatilis) and roach (Rutilus rutilus) dominated in the Barther Bodden. Herring (Clupea harengus) and perch were the dominant species in the Barther Strom. Smelt (Osmerus eperlanus) was the most common species in the Elbe estuary. Sprat (Sprattus sprattus) was of most importance in the Weser estuary. The dominant prey of fish larvae were copepodids, nauplii and eggs of Eurytemora afinis. The highest food niche width was observed for nine-spined stickleback (Pungitius pungitius), ruffe (Gymnocephalus cernuus), three-spined stickleback (Gasterosteus aculeatus) and perch in the Barther Bodden. Negative selectivity indices indicated that populations of rotifers were scarcely influenced by predation by fish larvae. In contrast, E. affinis was preferred by early life stages of fish. Positive relationships were estimated between maximum prey lengths and predator lengths of different fish species. Key species with regard to niche overlap were nine-spined stickleback, roach, three-spined stickleback and perch in the Barther Bodden, perch and herring in the Barther Strom, herring and three-spined stickleback in the Elbe estuary, and sprat and common goby (Pomatoschisrus microps) in the Weser estuary. High geographical overlap between fish species occurred in the Elbe estuary and Barther Bodden, whereas high diet overlap was estimated for both the Weser estuary and Barther Strom.

In June 1992, an extensive investigation programme was carried out in the turbidity zone of the Elbe river. Special attention was paid to salinity and suspended particulate matter (SPM) and their influence on bacteria, phytoplankton, zooplankton, mysids and fish. SPM was separated into three fractions of different settling velocities. Mean settling velocity (Ws) was 0.05 cm s -1. The major part of SPM belonged to the slow settling fraction (w s <0.02 cm s-l).Bacterial exoenzymatic activity showed a positive correlation with SPM and chlorophyll-a content, and also to total dissolved free amino acids.Phytoplankton biomass reached maximum values of 5.7 pg chlorophyll-a 1-1 at ebb tide. Chlorophyll-a correlated negatively with salinity, indicating riverine input of phytoplankton. A positive correlation was found between chlorophyll-a and dissolved oxygen.Abundance of zooplankton species and their developmental stages varied over the tidal cycles; abundance of cirriped larvae and copepodite stages of the dominant species Eurytemora affinis (Copepoda, Crustacea) was positively correlated with salinity. Individual filtering rates (IFR) of Eurytemora affinis were negatively affected by the SPM content of the water. Maximum IFR for adults was 7.2 ml h -1. Community grazing reached maximum rates of 30.3 ml 1-1 h -1 (Le. 72.7% d-l).The dominant mysid Neomysis integer showed maximum abundance at night, possibly resulting from diel vertical migration. Abundance of N. integerwas positively correlated with SPM content.The fish community, consisting of 17 fish species, was characterised by high densities of smelt (Osmerus eperlanus). A positive correlation was found between salinity and abundance of typical marine fish species, such as sprat (Sprattus sprattus). Similar temporal variation of abundance of smelt, sprat, and Eurytemora copepodites indicates processes of habitat preference of these planktivorous fish in relation to optimal food supply.SPM was the factor controlling both distribution of organisms and the turnover of nutrients. Salinity only was important for the distribution of organisms.

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