Eutrophication and persistent pollutants are two of the main environmental problems in European marine and freshwater ecosystems. As they tend to co-occur, interactive processes between eutrophication and contaminants are suggested, that may lead to environmental effects that cannot be predicted from each process alone. In order to predict the consequences of remedial measures (changing the input of organic matter, nutrients and contaminants) it is important to understand mechanisms that alter the bioavailability and fate of contaminants. The environmental risks will depend on the speciation of contaminants and their association to media and matter and by that means affect exposure. Furthermore, the risks will depend on the mobility of the substances and their pathways in food chains. In 1995, the Swedish Environmental Protection Agency initiated a 5-year research program Interactions between EUtrophication and CONtaminants (EUCON). A background document was prepared listing a number of relevant questions and hypotheses. On the basis of this document a program was launched, addressing the problems related to the interaction between eutrophication and contaminants (persistent organic compounds and trace metals) in the marine environment, with focus on the Baltic Sea, and in lakes. This paper summarizes the state-of-the-art, hypotheses and highlights from the research program with emphasis on the implications and applications of the results.

Comparative temporal trend studies of ΣDDT and PCB on the Baltic pollution, using matrices from Baltic biota and dated lamina from sediment cores in anoxic sediment areas of the Baltic (no bioturbation) have been carried out. Chemical analyses of consecutive laminas of the cores representing individual years were compared to long-term temporal trend monitoring of Baltic biota based on annual sampling and analyses. Earlier studies of sediment cores from the Baltic have often indicated increasing concentrations of halogenated organic compounds in recent time whereas monitoring of Baltic biota showed decreasing concentrations from the 1970s and onwards. In the present study two sediment cores from the northwestern Baltic proper indicate a concentration (dry weight basis) peak in the same time period as for biota, but the decrease during recent decades is slower than for biota. In lamina representing the period 1940–1950 remarkably high concentrations of both PCB and ΣDDT were found. Interestingly also a high proportion of nonmetabolized DDT was found in these old lamina, whereas lamina representing 1960–1970 showed a low portion of nonmetabolized DDT and high portions of the metabolites DDE and DDD during the period 1970–1998. The results are discussed in relation to resuspension of sediments, diffusion and interstitial transport of halogenated organic compounds in the sediment cores. The results indicate the importance of resuspension of sediment and that carbon normalized data are inappropriate for interpretation of temporal trends. The results also show that we have no appropriate model to interpret the pollution of the Baltic by halogenated organic compounds, retrospectively, by using lamina from anoxic sediment cores, but that we urgently need more knowledge on the important retention mechanisms in sediment.

Several studies show that there are interactive processes between eutrophication and uptake of persistent organic pollutants (POPs) in aquatic biota. The main concept is that the increased biomass and production in aquatic ecosystems, due to excess discharge of nutrients, causes a chain event that results in reduced uptake of POPs in primary producers (phytoplankton). This effect is then transferred to consumers at different trophic levels. The chain event may work in an indirect way by increasing the sedimentation of organic matter and, thereby, increasing the downward flux of pollutants to the bottoms, where they are caught in the organic sediment matrix. The chain event may also work in a direct way; the uptake of POPs in the fast growing phytoplankton is decreased as a result of “growth-dilution”, and lower amounts of pollutants are transported in the food-web. The effect seen may also be a result of changed food-web structures that differ considerably from nutrient-poor to nutrient-rich ecosystems. In this paper, we use the existing knowledge of POPs behavior in aquatic systems of different nutrient status, to discuss possible interactive processes of eutrophication and contaminants in the Baltic Sea. The Baltic is known to be highly contaminated with POPs and exposed to ongoing eutrophication. Results from investigations in lakes are compared to laboratory and field studies of the Baltic Sea.

Concentrations of PCBs in water (dissolved), suspended particles (SPM), sediment trap material, and sediments in the Baltic Sea are discussed in relation to the dynamics of organic carbon and lipids as the sorbing matrices. Both carbon- and lipid-normalized concentrations of PCBs in SPM increase during the sedimentation giving rise to an increase in observed log K_oc with more that one order of magnitude for most of the studied PCBs. It appears that hydrophobic organic compounds (HOCs) are retained within the settling particles while the organic carbon and the lipids are degraded, rather than there being a continuous uptake of PCBs during settling. However, the explanation for the high concentration maintained in the sediments over long time periods, must be an increasing capability of the organic matrix to sorb HOCs during the ageing process. The results are of importance for current models of the fate of HOCs in the environment and imply that the use of a single equation to estimate equilibrium concentrations of HOCs in SPM may significantly under-estimate concentrations in settling particles.

Trace metal dynamics were studied in lakes to illustrate the direct and indirect effects of eutrophication on the turnover of metals. The decreasing atmospheric load of Pb was well reflected by the Pb-profile in dated sediment cores. The trend was, however, biased by resuspension which at times dominated the flux of particles to the sediments. Since sulfides played an important role for the sequestering of metals in the sediments, resuspension, and water circulation, would facilitate the dispersal of pollutants and the solubilization of metal sulfides by sulfide oxidation. Significant effects of “seston dilution” was demonstrated as decreased Pb and Hg levels in the eutrophic lake sediments. However, the bioavailability in fish for those metals did not decrease, as it did for Cd and Tl. In the eutrophic lake, higher concentrations of Hg in fish muscle were accompanied by increasing net production of methyl-Hg in hypolimnetic waters. Higher fluxes of metals in the eutrophic lake may partly be explained by a larger biomass and partly by different geology and pH. Eutrophication decreased the residence time of Cu, Ni, and Tl, probably partly due to sulfide fixation, and decreased the concentration of Cd and Mo in water, due to a large biomass and an efficient scavenging of metals.

Interactions between eutrophication and organic pollutants (PCB, DDTs and γ-HCH) were investigated during 1996-1997 in two lakes, one eutrophic (Lake E) and one oligotrophic (Lake O) but with otherwise similar morphometry and water chemistry. Soil samples from the watersheds indicated similar atmospheric exposure of the lakes. No biomass dilution was observed in Lake E compared to Lake O. Lake E had higher levels of PCB based on organic carbon and lipid mass in sedimenting matter (1.5×), surface sediment (1.4×) and perch (Perca fluviatilis) (2.4×), and also higher settling fluxes of particulate matter and PCB measured in sediment traps and in surface sediment. Phytoplankton was responsible for most of the PCB transport to the sediment in Lake E. Higher DDD/DDE ratios seemed to correlate to a high load of p,p′-DDT more than to that of anaerobic conditions. A high DDD/DDE ratio was recorded in the deep sediment core from Lake O together with high levels of sDDT and was also highest in the sediment traps during periods with, probably, the highest runoff of parental DDT from the watershed.

The expanding occurrence of laminated surficial sediments in the Baltic proper has increased the possibilities to assess inter-annual variation in sediment accumulation over the past four decades. During the same period of time, wind speeds and wind directions in the area have been recorded continuously on Gotska Sandön. When studying the fine structure in three distinctly laminated sediment cores, selected from 55 cores collected within a 39 km² area, annual dry matter accumulation was found to correlate positively with the frequency of high wind speeds, reaching an optimum at gale force ≥ 14 m s⁻¹. This confirms that resuspended sediments contribute signifi-cantly to the sediment accumulation, with a mean rate of 2 mm yr⁻¹ for 25 surficially laminated cores collected at depths of 115–145 m. Decadal changes in both storm frequency and sediment deposition were observed, which may have implications not only for the turnover of sediments and associated nutrients and contaminants, but also for interpretations using sediment cores as historical archives. Horizontal comparison of dated sediment layers in all cores revealed a record of substantial changes in deep-water conditions. Recent recolonization of large areas by benthic fauna, within the depth of 102–125 m, suggests that storm events in the early 1990s are responsible for substantially improved oxygen conditions. Although, bioturbation of laminated sediments, with a depth range of 0.5–5.5 cm in our samples, creates a false picture of an earlier improvement.

In order to examine the possible interactions of eutrophication and hydrophobic organic contaminants, data for polychlorinated biphenyl (PCB) concentrations (C_PCB) in three species of bivalves and in sediments were examined. These were correlated to total organic carbon content of sediments (TOC_sed), which was assumed to represent the trophic status of the sampling sites. Almost all C_PCB) on dry weight basis in the bivalves and in sediments were examined. There were correlated to total organic carbon content of sediments (TOC_sed), Which was assumed to represent the trophic status of the sampling sites. Almost all C_PCB on dry weight basis in the bivalves and in the sediments were positively linearly correlated to TOC_sed. Regarding the sediments, this simply reflects the high affinity of these compounds for TOC, but for the bivalves the correlation wan not explained. In the bivalves, the difference in PCB homologue concentration was positively linearly correlated to TOC_sed. To understand these field data, a box model was used to simulate the fate of PCB in oligotrophic, mesotrophic, eutrophic, and hypertrophic marine environments. The calculated results qualitatively predicted the change in C_PCB for the sediment correctly. For increased trophic status of the water compartment, on a scale from oligo- to hypertrophic, the model predicted decreasing fugacities for the PCBs, and thus could not support the observation of increased bivalve C_PCB seen in the field data study. It was suggested that the PCB pattern change in the bivalves could be influenced by algae-growth dynamics in the watercolumn. This would selectively intensify the washout of the ligher chlorinated homologues from the water and, thereby, decrease their concentrations in the bivalves.

Effects of inputs of organic matter were studied on bioavailability and cycling of hydrophobic organic contaminants (HOCs) in benthic ecosystems of the Baltic and Kattegat Seas. In laboratory experiments, effects of microalgae additions were studied on the bioaccumulation of HOCs (PCBs and PAHs) by the blue mussel Mytilus edulis, the brittle star Amphiura filiformis, and the polychaete Nereis diversicolor. Contrary to the equilibrium partitioning theory, bioaccumulation was proportional to the concentrations of algae and organic carbon. This was attributed to the high nutritional quality of the algal organic carbon and suggests that feeding rather than equilibrium partitioning governed bioaccumulation in these species. In the field, annual mass fluxes of PCBs in blue mussels and in brittle stars were estimated, as well as contaminant transfer to higher trophic levels. Our results suggest that: i) Eutrophication processes may contribute to increase HOC accumulation in benthic species. ii) Temporal variation in the quantity and quality of organic carbon needs to be considered when assessing contamination of benthic systems. iii) Macrofaunal feeding activities are important for the benthic-pelagic coupling of HOCs. iv) Bioturbation enhances the release of HOCs from sediment to overlying water.

In 1993, a large-scale investigation of polychlorinated biphenyls (PCBs) in Baltic Sea sediments was performed in cooperation between the Baltic states. sPCB (IUPAC # 52, 101, 118, 105, 153, 138, 180) in surficial sediments increased from north (mean 213 ng g⁻¹ C) to south (mean 382 ng g⁻¹ C), indicating atmospheric influence. In contrast to decreasing PCB trends in pelagic biota, the sPCB in dated sediment cores from the Baltic proper showed increased or constant concentrations during recent decades. Down-core congener patterns showed no changes during the 20th century. After re-calculation of sPCB deposition, assuming interannually variable dry matter deposition, the sPCB sediment deposition trend matches the observed sPCB concentration trend in Baltic biota although the reductions in recent decades are less for the sediments. A calculation of sediment burial of sPCB indicated that laminated Baltic proper sediments are efficient traps, and a mass balance showed that the sPCB retention time in the water mass is less than one year.

Temporal trend studies of PCBs and DDTs in two annually laminated sediment cores from the offshore NW Baltic proper and the Stockholm archipelago demonstrate that studies of laminated sediments are useful tools for detecting time trends of HOCs. The concentration trends of PCBs and DDTs, on a dry weight basis, from the two cores show quite large similarities to trends reported in pelagic biota from the NW Baltic proper, although the reduction in concentrations from the 1970s and onwards is approximately half of the reductions seen in biota. The sediment accumulation trend for these substances was not coupled to the concentration trend in biota but is closely linked to climate-related erosion/resuspension dynamics. The gross accumulation rate in the archipelago area proved to be 8 times higher compared to that of the offshore NW Baltic proper (885 and 108 g m⁻² water area yr⁻¹, respectively). Since PCB and DDT concentrations were similar in offshore and coastal areas, and the burial rates of PCB and DDT were closely linked to gross accumulation rates, the sediment burial of contaminants in relation to water area was severalfold higher in the archipelago compared to the offshore area. The high burial of sPCB and sDDT in the archipelago indicated an import of contaminants from the open sea to the archipelago. Although eutrophication-induced scavenging of contaminants may have occurred in the Baltic Sea, natural processes have obscured evidence of this.

PCBs associate with organic matter in the aquatic environment. Sediment of different bottom types at different water depths may have organic matter with different properties. Also the different bottoms can be either oxic/bioturbated or anoxic/laminated, depending on water depth. To investigate if these circumstances would influence the distribution of PCBs in sediment, samples were collected from different bottom types at different water depths in two areas in the Stockholm Archipelago in the Baltic Sea. The bottom types were classified according to their water content. It was concluded that the concentration of PCBs on an organic carbon basis is the same for the different bottom types and the two areas. The possible differences in properties of the organic matter in different sediments is of minor importance, as are the influences of bioturbation and redox conditions. Instead, it is the properties of the PCB congeners which determine their distribution in the sediments.

A systematic investigation of PCB in coastal sediments of the Baltic Sea was initiated in 1996 by retrieving dated subsurface layers (1992–1994) from 88 sites in 19 sheltered archipelago areas, where laminated sediments were found to be common. PCBs7 (HELCOM's standard sum of 7 congeners, ≈1/6 of total PCB) was quantified, as well as the abundance and isotopic composition of nutrients (N, C), in order to trace effects of eutrophication and potential influences on the turnover of PCB. In most of the areas, the median of PCBs7/C was within 70 to 120 ng g⁻¹ C. This is similar to recent values in regional offshore sediments, and thus consistent with a rapid exchange between coastal and open waters. Elevated PCBs7 levels up to around 1000 ng g⁻¹ C were found within 50 km from Stockholm. Here, but not in other areas, PCBs7/C increased linearly with the degree of eutrophication. This was evident from the isotopic composition of sediment N and C, reflecting local eutrophication as well as its link to freshwater input. In contrast, concentrations or ratios of nutrients in sediments were poor indicators of eutrophication. Our results demonstrate that the Stockholm watershed still is a source of both PCB and excess nutrients to the Baltic Sea, but do not support any enrichment or dilution of PCBs7/C ratios caused by eutrophication.