Interactions between eutrophication and contaminants. I. Principles, experimental design and synthesis

In marine environments influenced by human activities, the sea-bed is a major recipient of both contaminants and organic matter. Deposition and recycling of contaminants from bottom sediments may be affected by the eutrophication status of the area. Contaminants and eutrophication factors may also interact to affect bioaccumulation as well as the growth, health and reproduction of benthic organisms. Consequently, from an environmental management point of view, these interactions should be considered when decisions are made about input of contaminants and organic matter to the aquatic environment. In order to elucidate interaction patterns between eutrophication and contaminants, radio-labelled contaminants were applied in a large scale experiment performed at Marine Research Station Solbergstrand, situated by the Oslofjord, Norway. The continous flow through experiment was performed in glass aquaria containing three sediment-dwelling species (Abra alba, Nereis diversicolor and Amphiura filiformis) placed in marine clay sediments. Blue mussels (Mytilus edulis) were kept in separate downstream aquaria and exposed to the out-flowing water from the sediment aquaria. In 12 contaminated aquaria the sediment was spiked with 203Hg (0.23 mg Hg kg−1 dry sediment), 109Cd (5 mg Cd kg−1) and 14C-benzo[a]pyrene (1 mg BaP kg−1). In 12 organically enriched aquaria, phytoplankton (2 mg C kg−1 dry sediment) was initially mixed into the sediment and two months later another batch corresponding to 20 g C m−2 was added to the aquaria. Finally, the concentration of oxygen was continuously lowered from 7–9 mg O2l−1 in 12 well-oxygenated aquaria to 2.4–3.5 mg O2l−1 in 12 low-oxic aquaria by purging the source water with nitrogen. Allowing three replicate aquaria for each treatment, all combinations of the two levels (high/low) of the three factors (contaminants, oxygen availability and carbon load) could be tested in a factorial design using 24 aquaria. Sulphide ion activies were continuously recorded by AgAgS electrodes in the sediments and overlying water, and redox potentials were measured by Pt-electrodes inserted in the sediment. There were only minor changes in redox and sulphide potentials during the experimental period. Also, mortalities of the test species were negligible in all aquaria. After an experimental period of three months, the concentration of contaminants was quantified in sediments, porewater and biota using beta/gamma emission from the isotopes. Effects of the treatments on sediment-dwelling invertebrates were assessed using growth (Nereis, Abra and Amphiura), arm regeneration (Amphiura) and biomarker responses. In this paper (paper I) the principles and hypotheses for studying interactions between organic loading and contaminants are outlined as well as the experimental design. The results from the experiment are presented in three separate papers (papers II–IV) elsewhere in this volume. A short synthesis is presented in this paper, comparing the conclusions from the individual studies.