Purpose The development of Sediment Quality Guidelines (SQGs) is one of the remaining challenges for a better protection of aquatic biodiversity and in particular sediment dwelling organisms. So far, sediment quality assessment in Flanders was based on a comparison of chemical concentrations to the geometric mean of the concentrations at 12 reference sites. The study described in this paper addressed the need for more science-based guidelines. The developed guidelines are already incorporated into Flemish legislation. Materials and methods Based on a large sediment monitoring database, containing physico-chemical properties, concentrations of chemicals, macrobenthic community assemblages and ecotoxicological data, Sediment Effect Concentrations (SECs) were calculated as basis for the SQGs. The derived SECs were based on ecological effects, namely Lowest and Severe Effect Levels (LEL/SEL), as well as ecotoxicological endpoints, namely Threshold and Probable Effect Levels (TEL/PEL). The average values of the ecological and ecotoxicological SECs were used to distinguish five sediment quality classes.
Results and discussionThe ecological values were in general less stringent than the ecotoxicological values. However, the Lowest Effect Levels (95% of the benthic taxa can be present under this level) and Threshold Effect Levels (no toxic effect is expected under this level) did not differ significantly. Probable Effect Levels (concentrations above this level will certainly result in toxic effects) were generally lower than the Severe Effect Levels (above this level only 5% or less of the taxa are present). The SECs calculated in this study enabled us to correctly identify 87.9% of the sediments as toxic. The development of SQGs based on a combination of the LEL/SEL and TEL/PEL methods enabled us to underpin these SQGs based on field observations and will improve the assessment of sediment quality based on chemical parameters. Although sediments typically contain complex mixtures of contaminants, only a limited number of these contaminants will be measured. Additional application of bioassays for the overall sediment quality assessment is therefore recommended. Conclusions This study describes the development of SQGs in Flanders, which are based on ecological and ecotoxicological data derived from a TRIAD monitoring network. The combination of the LEL and PEL resulted in SQGs that were recently incorporated in Flemish legislation and for which the respective pore water concentrations were in the same order of magnitude as the Annual Average
The phosphate quality standards in the lowland rivers of Flanders (northern Belgium) are exceeded in over 80% of the sampling sites. The factors affecting the molybdate reactive P (MRP) in these waters were analyzed using the data of the past decade (>200 000 observations). The average MRP concentration in summer exceeds that winter by factor 3. This seasonal trend is opposite to that of the dissolved oxygen (DO) and nitrate concentrations. The negative correlations between MRP and DO is marked (r = -0.89). The MRP concentrations are geographically unrelated to erosion sensitive areas, to point-source P-emissions or to riverbed sediment P concentration. Instead, MRP concentrations significantly increase with increasing sediment P/Fe concentration ratio (p < 0.01). Laboratory static sediment-water incubations with different DO and temperature treatments confirmed suspected mechanisms: at low DO in water (<4 mg L), reductive dissolution of ferric Fe oxides was associated with mobilization of P to the water column from sediments with a molar P/Fe ratio >0.4. In contrast, no such release was found from sediments with lower P/Fe irrespective of temperature and DO treatments. This study suggests that internal loading of the legacy P in the sediments explains the MRP concentrations which are most pronounced at low DO concentrations and in regions where the P/Fe ratio in sediment is large.
First-stage larvae of the midge Chironomus riparius were exposed in small enclosures at 19 sites located in three different river basins in Flanders (Belgium). Sediments were sampled and sieved at 200 microm at all exposure sites. A layer of approximately 2 cm of sediment was placed in each cage and 100 midge larvae were added. Cages were placed in watercourses where resident midge larvae were present. Accumulation of Cd, Cr, Cu, Pb, Ni, and Zn was determined after four weeks of exposure when larvae had reached the fourth stage. Comparing metal levels between caged and resident larvae revealed no significant differences. A significant correlation between metal levels in caged and resident larvae was found when all sites were considered. However, such correlation was low (r2 = 0.28) for Pb. The highly significant r2 values found for Cu and Ni probably were due to only one site. Metal levels in tissue were related to levels in water and sediment, taking into account some sediment characteristics (particle size distribution and organic carbon) and oxygen level in the water. To determine the relative importance of these different sediment factors contributing to the variation in metal accumulation by the chironomids, nonlinear regression models were constructed. With the models used, 56.1, 32.2, and 57.4% of the variation for Cd, Pb, and Zn, respectively, could be described. None and 26.9% of the variation could be described for Cu and Ni, respectively. Among the environmental factors, organic carbon and oxygen levels in water were important in describing the accumulation of metals.
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