Food web and contaminants in Gunneklevfjorden, Porsgrunn, Norway

Summary

Food web structure and contaminants in the fauna have been investigated in Gunneklevfjorden in 2013. Analyses of mercury and chlorinated contaminants in zooplankton, benthic invertebrates and fish have been carried out. Mapping of the food web was done by analysis of stable isotopes of C and N. In addition, the sediment habitat quality was investigated with a sediment profile camera (SPI). Also, mapping of the subsea vegetation area in the southern part of the fjord was carried out in an adjacent activity. The investigation of fauna gave the impression of greater diversity and abundance of organisms within the vegetation area. However, the number of species in the fjord was found to be relatively low, both of benthos and fish. It was recorded 5 fish species, with Perch dominating in the catches. The food web in the fjord is based both on a pelagic and a littoral starting point, and fish in general seems to be based on a combination diet. It is a relatively simple food web, which range over 6-7 trophic levels. Top predators are mainly Perch, Pike and Eel. Habitat quality in the sediment is considered to be less good, and for many of the surveyed stations, it was not possible to do an analysis of habitat quality based on the sediment profile photography (SPI). This was due to the fact that the sediment was so soft that the sediment profile camera penetrated into the sea bottom. There seem to be a correlation between concentration of mercury within benthic organisms and the concentrations found in the sediments. Concentrations measured in the benthic organisms must, however, be interpreted with caution because there is uncertainty about the influence of their intestinal content, which may contain sediment particles. Concentrations of mercury in fish are below the recommended values for consumption (0.5 µg/g wet weight) for the majority of the individuals in the material, except for some Perch, most Eels and the one Flounder caught. Mercury concentrations in this material is found to be lower than in a previous study but in the same order of magnitude as in a survey of fresh-water fish from lakes in Telemark county, Norway. Comparison with previous surveys or surveys from the lakes, however, must be done with care. It is important to take into consideration the species, trophic level, age, length and time of year. In addition, water chemistry can affect the availability of mercury. Concentrations of dioxins and furans were below the EU's limit for fish and fisheries goods (4 ng TE/kg wet weight) for all species of fish except for Eels. Concentrations of HCB and OCS in fish were lower than on previous surveys. Bioaccumulation and biomagnification are shown for both mercury and chlorinated compounds.