A short-term laboratory and in situ sediment assay based on the postexposure feeding of the estuarine isopod Cyathura carinata.

In situ assays based on feeding depression have been proposed as sublethal assays able to assess immediate contaminant effects on key ecosystem functions, long before effects on life-history traits can be detected. The in situ peculiarities provide more realistic exposure scenarios than laboratory-controlled conditions, which is particularly relevant for estuarine ecosystems where environmental conditions are highly variable. In this context, we developed a short-term cost-effective in situ assay based on the postexposure feeding of the estuarine species, Cyathura carinata, through the following steps: i) develop a methodology to quantify feeding under laboratory conditions; ii) quantify the sensitivity of the postexposure feeding response under laboratory conditions; iii) deploy the in situ assay at various field sites covering different environmental variables in a reference estuary and degrees of contamination in a contaminated estuary to, respectively, derive a model to predict postexposure feeding rates across sites varying in environmental variables and evaluate the potential of this in situ tool to assess sediment contamination in estuaries. A quantity of 100 defrosted nauplii of Artemia franciscana per isopod given during a short period of 30 min in darkness was adequate to allow estimating precise postexposure feeding rates. Laboratory toxicity tests showed the high sensitivity of postexposure feeding to Cd as a sublethal endpoint for C. carinata (7-fold lower than the corresponding 48-h LC50). However, this species appears to be able to face high concentrations of non-essential metals as Cd, with 96-h LC50 values (37 mg Cd/L) higher than values reported for other isopods, and among the highest concentrations reported for saltwater invertebrates. Results of in situ deployments at reference sites indicated that sediment with size fraction between 63-125 µm (very fine sand) influences feeding rates. The in situ assay across contaminated sites was able to discriminate between unpolluted and polluted estuaries, and also to detect degrees of toxicity among sampling sites within an estuary, after statistically adjusting feeding rates on the basis of the proportion of the sediment very fine sand influencing baseline feeding rates. The in situ postexposure feeding assay with C. carinata was found to be, therefore, a potential useful cost-effective tool for estuarine sediment toxicity assessments.