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Water quality standards for copper are usually stated in total element concentrations. It is known, however, that a major part of the copper can be bound in complexes that are biologically not available. Natural organic matter, such as humic and fulvic acids, are strong complexing agents that may affect the bioavailable copper (Cu2+) concentration. The aim of this study was to quantify the relation between the concentration of dissolved natural organic matter and free Cu2+ in surface waters, and the biological effect, as measured in a standardized ecotoxicological test (48 h-median effective concentration [EC50] Daphnia magna, mobility). Six typical Dutch surface waters and an artificial water, ranging from 0.1 to 22 mg/L dissolved organic carbon (DOC), were collected and analyzed quarterly. Chemical speciation modeling was used as supporting evidence to assess bioavailability. The results show clear evidence of a linear relation between the concentration of dissolved organic carbon (in milligrams DOC/L) and the ecotoxicological effect (as effect concentration, EC50, expressed as micrograms Cu/L): 48-h EC50 (Daphnia, mobility) = 17.2 x DOC + 30.2 (r2 = 0.80, n = 22). Except for a brook with atypical water quality characteristics, no differences were observed among water type or season. When ultraviolet (UV)-absorption (380 nm) was used to characterize the dissolved organic carbon, a linear correlation was found as well. The importance of the free copper concentration was demonstrated by speciation calculations: In humic-rich waters the free Cu2+ concentration was estimated at approximately 10(-11) M, whereas in medium to low dissolved organic carbon waters the [Cu2+] was approximately 10(-10) M. Speciation calculations performed for copper concentrations at the effective concentration level (where the biological effect is considered the same) resulted in very similar free copper concentrations (approximately 10(-8) M Cu) in these surface waters with different characteristics. These observations consistently show that the presence of organic matter decreases the bioavailability, uptake, and ecotoxicity of copper in the aquatic environment. It demonstrates that the DOC content must be included in site-specific environmental risk assessment for trace metals (at least for copper). It is the quantification of the effects described that allows policy makers to review the criteria for copper in surface waters.

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Four laboratories were compared to ascertain the reproducibility of test methods for five bioassays: the ten day whole sediment bioassay with the amphipod Corophium volutator, the fourteen day whole sediment bioassay with adult sea urchins Echinocardium cordatum, the Microtox solid phase bioassay with the bacterium Vibrio fischeri, the oyster larvae sediment elutriate bioassay using Crassostrea gigas and the sediment pore water bioassay with the rotifer Brachionus plicatilis. The bioassays were all conducted according to the standard operating procedures of the National Institute for Coastal and Marine Management/RIKZ and carried out with one control sediment and three moderately contaminated dredged materials from the Netherlands. Reference toxicant tests were also performed for every bioassay, to assess the condition of the test species. Reproducibility and inter-laboratory variability were evaluated by calculating coefficients of variation for the sediment bioassays and considering the ability of each laboratory to achieve the test acceptability criteria and to identify a number of confounding factors. The bioassays involving the amphipods and sea urchins had an acceptable inter-laboratory variability, with average coefficients of variation of 20% or less. The Microtox solid phase bioassay showed high reproducibility and the least variability among laboratories, with average coefficients of variation of 12%. In contrast, the results for percent net response in the oyster larvae bioassay were very variable and poorly reproducible: in three of the four sediments the coefficients of variation exceeded 100%. Two laboratories did not meet the test acceptability criterion for oyster embryo development in the control sediment. Survival in the rotifer bioassay also varied greatly among laboratories with average coefficients of variation of 48%. We conclude that the main sources of inter-laboratory variability were 1) individual differences in the skill and experience of laboratory technicians (for the amphipod, oyster larvae and rotifer bioassays); 2) the use of different batches of test organisms (sea urchin bioassay) and 3) the use of different dilution water (oyster larvae bioassay).