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The presence of dioxin-like compounds in sediments from harbors and reference sites along the Dutch coast was investigated using the dioxin receptor-chemically activated luciferase gene expression (DR-CALUX) bioassay. The DR-CALUX response varied between 0.2 and 136 ng/kg dry weight expressed in units of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxic equivalency quotients (TEQ). The highest CALUX-TEQ values (> 50 ng TEQ/kg dry wt) were found in sediments from the center of the Port of Rotterdam and from the North Sea canal near Amsterdam. The DR-CALUX activity of harbor sediments was classified by setting guideline values. None of the 257 harbor sediment samples investigated exceeded the maximum value of 1,000 ng TEQ/ kg, while 94% of the samples fail the target value of 2 ng TEQ/kg. Threshold values (25 and 50 ng TEQ/kg) are intended as pass/ fail criteria for offshore disposal of dredged material and were exceeded in 12 and 3% of the samples, respectively. DR-CALUX response did not always match with contamination of polychlorinated biphenyls (PCBs) as determined in bulk sediments (sigma7-PCB ranging from < 1 to 456 microg/kg dry wt). Concentrations of planar PCBs, polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated-dibenzofuranes (PCDFs), polybrominated biphenyls (PBBs), and polybrominated-diphenylethers (PBDEs) were determined in 20 selected sediment extracts. On average, PCDD/Fs explained 50% of the DR-CALUX activity and planar PCBs 6%. Further research is needed to elucidate the unexplained part of the DR-CALUX response. The DR-CALUX is a highly sensitive and reproducible bioassay that can usefully complement standard PCB analysis, improving the hazard assessment of the disposal of dredged material in the North Sea.

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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).

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