Effects of heavy metal-mediated intraspecific variation in leaf litter on the feeding preferences of stream detritivores.

Plant litter inputs from terrestrial ecosystems are indispensable resources for stream ecosystems. Heavy metal pollution in the environment may indirectly affect the food webs of streams by changing the traits of leaf litter. In the present study, willow leaf litter was collected in polluted and non-polluted sites (natural willow), and leaf litter was produced in the lab by exposing willow saplings to different concentrations of heavy metals in water (cultivated willow). The collected willow leaf litter was used for feeding preference experiments with stream detritivores (shrimps and snails). Metal pollution significantly decreased the lignin concentration and toughness of litter and increased Zn and Cd concentrations. Both detritivores preferred to consume metal-enriched litter, with their consumption rates of this litter being significantly higher than those of non-enriched litter. The toughness of the willow litter was the key factor determining the feeding preferences of shrimps and snails. The detritivores that consumed metal-enriched leaf litter contained more Zn and Cd in their bodies than those that consumed non-enriched litter. The Zn and Cd concentrations in shrimp faeces were higher for shrimps that consumed metal-enriched litter than for those that consumed non-enriched litter. The heavy metal concentrations and chemical oxygen demand (COD) of the water following litter consumption were significantly higher for the metal-enriched litter than for the non-enriched litter, resulting in decreased water quality in the former context. The specific resource allocation patterns that result from heavy metal pollution in the environment will have ecological consequences.