Effects of copper, food quality and exposure history on aquatic insect emergence: Insights from a multigeneration study

Emergent aquatic insects serve as high-quality prey for terrestrial consumers. However, persistent contaminants, such as metals, can affect aquatic insect larvae development, potentially inducing a selection process favoring more tolerant individuals and populations. Whether this adaptation ultimately influences the quantity and quality of subsidy to terrestrial riparian ecosystems remains unclear. Here, we investigated the capacity of Chironomus riparius (Diptera: Chironomidae) populations to adapt to the essential metal copper (Cu). First, we assessed the initial sensitivity of C. riparius towards environmentally relevant Cu sediment contents (0-400 mg/kg dw) in combination with two basal food sources (Spirulina, low quality; TetraMin, high quality). After about six months of chronic exposure to Cu (100 mg/kg dw) across multiple overlapping generations, sensitivity of both naive and pre-exposed populations was re-assessed. Copper exposure mainly shaped the emergence quantity of C. riparius, resulting in dose-dependent decreases in adult emergence at 200 mg/kg dw and higher (up to about 90 % at 400 mg/kg dw). In contrast, food quality primarily determined emergence quality, approximated by the chironomid fatty acid profile. Unexpectedly, pre-exposed populations exhibited limited signs of adaptation. While emergence was higher in pre-exposed compared to naive populations (reduction in females by about 65 % and 85 %, respectively) at 300 mg/kg dw, the overall effect of exposure history on subsidy quantity and quality appeared to be superimposed by the Cu sediment contents used in the sensitivity tests and by food quality. While these findings require verification through extended chronic exposure phases, they imply sustained alterations of aquatic-terrestrial linkages.