The successful exploitation of sediments by benthic invertebrates is affected by numerous variables, such as grain size distribution, food availability, predator abundance, competition, and the amount of toxicants. Because the species-specific requirements for these variables are often known incompletely, it remains difficult to identify the causes for the presence/absence of individual species. This study aimed to explain the persistence of benthic invertebrates in polluted sediments. The specific responses of two species, the mayfly Ephoron virgo and the midge Chironomus riparius, to environmental variables were investigated using bioassays and it was evaluated how these responses determine their persistence in polluted sediments.

The responses of both species to sediments with different levels of food quality and toxicants were assessed in sediment bioassays. Survival and growth of E. virgo were severely affected by the sediment-bound toxicants, independent of the food quality in the sediments. In contrast, high growth rates of C. riparius were observed in highly nutritive sediments, in spite of the toxicants present. To discern the effects of food quality and toxicants on the midge C. riparius, the larvae were exposed to sediment spiked with different combinations of artificial food and copper. The advantages of increased food availability prevailed against the potential adverse effects of copper. Elevated food levels suppressed the negative effects of copper on survival, and resulted in higher growth rates, until copper concentrations reached a critical threshold.

Hence, the question remains whether habitat selection of opportunistic chironomids under natural conditions is also steered by food quality in the sediments. Therefore, the preference of C. riparius larvae for particular sediments was assessed in a choice experiment. The larvae showed a clear preference for sediments with higher food quality, which overruled the avoidance of sediments with higher toxicant concentrations..

Finally, it was investigated if the responses of C. riparius to food quality and toxicants match with the persistence of chironomids in the field. In sediments with high chironomid densities, high contaminant levels were observed. Laboratory bioassays confirmed that the toxicants in these sediments had no effect on survival and growth rates, although higher incidences of mentum deformities were observed. The high growth rates in in situ enclosures in two of the three contaminated sediments indicated that chironomids were able to exploit contaminated sediments. Yet, the positive relation between growth rate of C. riparius and nutritional value of the sediments in the laboratory bioassays suggested that higher densities of chironomids might have been present in sediments with high food quality. However, in these sediments with high food quality (and low levels of contaminants) chironomids may have suffered from competition or predation by taxa that were not able to persist at the contaminated sites.

The different responses of the two species indicated that the long-term persistence of benthic invertebrates under specific conditions is bound to a set of characteristics and is unlikely to be determined by single traits. Yet, it was demonstrated that the pollution level in some floodplain lake sediments of the River Rhine alone is already prohibitive for benthic insects with the sensitivity of E. virgo. In most cases, however, a combination of conditions determines the persistence of benthic invertebrates in polluted sediments. These combined effects may cause limiting conditions to indirectly promote certain benthic invertebrate species, such as C. riparius. Even though sediment pollution drives this species close to intoxication, the high availability of food, caused by the exclusion of less tolerant species, enables them to persist very well.