Effects of increased water temperature on leaf litter quality and detritivore performance: a whole‐reach manipulative experiment

Abstract

Summary We compared leaf litter conditioning in two artificial stream channels that were connected to a second‐order stream (both in‐ and outflow). The water in one channel had the same temperature as the stream and in the other channel was warmed (3 °C above ambient stream water temperature). In addition to measuring leaf decomposition by microbes, we ran feeding trials in the artificial streams and the laboratory that addressed the feeding preferences, resource acquisition and performance of a dominant detritivore. Oak leaves were conditioned for three weeks in both channels and then offered during the following five weeks to Sericostoma vittatum larvae kept in cages in the same channels. Additionally, leaves conditioned in both conditions were simultaneously offered to the larvae in a feeding preference laboratory trial. Leaves conditioned in the channel with elevated temperature had reduced toughness, fewer phenols and lower carbon : nitrogen ratios. However, individuals did not prefer these leaves over those conditioned at ambient stream temperature. We observed lower carbon : nitrogen imbalances and increased consumption rates in the stream channel at warmer temperature but these were not translated into higher growth rates. Higher metabolic rates of the S. vittatum individuals and other temperature‐mediated factors such as changes in leaf quality could explain the increased consumption. We observed higher lipid storage and earlier pupation of S. vittatum at warmer temperatures, suggesting that individuals allocated resources to metamorphosis and reproduction, rather than to somatic growth. We argue that microbial and invertebrate‐mediated organic matter cycling will be faster in streams under future warming scenarios. However, these systems will likely become less efficient in carbon retention via effects on larval performance (growth and developmental time) and timing of prey availability to the stream and coupled riparian food webs.