Particle-specific sorption/desorption properties determine test compound fate and bioavailability in toxicity tests with Chironomus riparius--high-resolution studies with lindane.

We studied the sorption (batch equilibrium experiments) and desorption (consecutively harsher supercritical fluid extractions) of lindane to different types of sediment and food particles, as well as larval uptake in standardized peat-based artificial sediment toxicity tests with the midge Chironomus riparius. Lindane sorption to organic particles was fast and efficient, reaching 98+/-0.1 and 97+/-0.1% of added compound in 48 h for peat and Tetraphyll(R), respectively, and 77+/-0.2% in whole sediment. Sorption to inorganic particles, that is, sand and kaolin clay, was much lower, 9.6+/-1.3% and 8.3+/-0.8%, respectively. Supercritical fluid extractions showed that most of the lindane sorbed to organic particles and sediment was loosely bound, as only 9 to 14% remained associated with particles after weak and intermediate extractions strengths. Larval uptake of dissolved lindane was 4.9+/-0.71 and 10.8+/-1.2 microg/g wet weight in 22 and 68 microg/L treatments, respectively, and four to five times higher than that of particle-associated lindane, ranging 1.0+/-0.15 to 2.7+/-0.21 microg/g in the above treatments. Surprisingly, larval uptake of lindane was similar from refractory peat and the more labile Tetraphyll particles. Despite an efficient larval uptake of dissolved lindane, sorption/desorption of lindane to/from Tetraphyll particles will facilitate digestive uptake in toxicity tests, particularly in spiked-water scenarios where food particles may act as vectors. Our results show that the exposure scenario is an important determinant for the behavior and bioavailability of test compounds in standardized toxicity tests.

Sublethal and sex-specific cypermethrin effects in toxicity tests with the midge Chironomus riparius Meigen.

We quantified sublethal and sex-specific cypermethrin effects in experiments (29 days) with the midge Chironomus riparius at different levels of sediment organic matter content (0, 5, and 20%). We found highly significant effects of cypermethrin concentrations and sediment type on emergence, mean development rate, and adult size. For example, emergence/survival rates were 70-100% below 0.8 µg/l and unaffected by organic matter content. At 3.2 µg/l, however, no larvae survived in sediment without organic matter, but survival successively increased to 26 ± 11% in sediment with 20% organic matter. Mean development rates were always higher for males than for females, and significant differences between sexes occurred consistently in controls and in treatments with our lowest concentration of 0.05 µg/l. Sex-specific differences in mean development rate decreased across the cypermethrin gradient, suggesting that male development was affected more than that of females at similar concentrations. We also found an increase in adult size across the concentration gradient in sediments lacking organic matter and suggest an increased feeding activity due to sublethal toxic stress as a probable causal mechanism. We speculate that the observed sex-specific effects on development rates and adult size can have strong repercussions on emergence timing and fecundity, respectively.

Deltamethrin toxicity to the midge Chironomus riparius Meigen-effects of exposure scenario and sediment quality.

We investigated how exposure scenario and sediment characteristics affected deltamethrin bioavailability and toxicity to Chironomus riparius. We designed whole-sediment experiments, including separate spiked-water and spiked-sediment experiments and using both a peat-based artificial and a natural sediment. Deltamethrin was highly toxic to larvae in artificial sediments, with LC(50)-values (28 d) of 11 microg/kg for sediment-exposures and 16 pg/L for water-exposures. In contrast, deltamethrin-induced mortality was absent in experiments with natural sediment, both in spiked water and in spiked sediment. This was attributed to the higher organic matter content of the natural sediment, 12.5+/-0.05%, compared with that in the artificial sediments, 4.1-4.8%, resulting in an efficient and fast sorption and lower bioavailability. Deltamethrin degradation was absent in artificial sediment, while in natural sediment, 50% degraded during 10 days of test-vessel acclimation. Despite a faster degradation, the highest measured concentration in spiked-natural sediment was more than seven times higher than the LC(50)-value obtained in tests with spiked-artificial sediment, indicating that low bioavailability, and not compound degradation, was the main reason for the observed lack of deltamethrin toxicity.

Stable isotopes and fatty acids reveal that Chironomus riparius feeds selectively on added food in standardized toxicity tests.

During long-term standardized toxicity tests with Chironomus riparius, food additions are a prerequisite for normal development and to avoid false-positive results. Consequently, larvae may selectively feed on added food rather than on contaminated sediment, which may confound toxicity test results. We designed a feeding study and estimated the degree of feeding on different food resources by using stable isotope and fatty acid (FA) analyses. In one treatment, larvae were offered both artificial sediment (peat, kaolin clay, sand, and calcium carbonate) and added food (TetraPhyll), whereas larvae in the two other treatments had access to either one of these potential food items. The highest biomass and survival were found among larvae with access to both artificial sediment and TetraPhyll. Two-source mixing models revealed that larval Chironomus that were offered both TetraPhyll and artificial sediment obtained 94 +/- 6.9% of their carbon and 90 +/- 4.3% of their nitrogen from added TetraPhyll. Larvae with access to only sediment had lower delta13C and delta15N (-23.34 +/- 0.56 per thousand and 0.33 +/- 0.52 per thousand) than those that were offered both sediment and TetraPhyll (-20.95 +/- 0.13 per thousand and 7.45 +/- 0.36 per thousand) or only TetraPhyll (-20.17 +/- 0.20 per thousand and 7.82 +/- 0.15 per thousand). In addition, FA composition of larvae that were offered both artificial sediment and TetraPhyll closely resembled that of those fed exclusively TetraPhyll. These results show that larval Chironumus strongly prefer added food, rather than artificial sediment in long-term toxicity tests. This preferential feeding behavior affects exposure pathways and ultimately toxicity test results.