Chapter Five - Increased Stream Productivity with Warming Supports Higher Trophic Levels

Metabolic theory predicts that warming will increase the energetic demands of organisms, with especially strong effects on larger individuals. Mean individual body size should therefore decline, which also implies a loss of biomass at higher trophic levels. If resources are plentiful and easily assimilated, however, the required to persist in warmer environments may be attained, leading to faster growth rates and an overall increase in the biomass of apex predators. Here, we investigated the response of different trophic groups to increasing temperature in a system of geothermal streams in Iceland, exposed to a temperature gradient of 5-21 degrees C. These streams provide an ideal natural experiment for isolating the effects of warming in multispecies systems, as they have broadly similar geographical and physicochemical features. The macrophyte cover increased significantly with increasing stream temperature, suggesting a greater resource pool for macroinvertebrates (either through direct grazing or feeding on epiphytes). This was reflected by a greater number of generations in 1 year among macroinvertebrates: species in the coldest streams were either uni- or bivoltine, while those in the warmer streams were mostly bivoltine or multivoltine. Differences in phenology were also seen among streams, with emergence of adults limited mostly to the summer months in the colder streams, but occurring year-round in the warmer streams. Macroinvertebrates also grew faster with increasing temperature, contributing to greater population biomass and secondary production in the warmer streams. This increase in prey availability likely produced more favourable conditions for top predators in the warmer streams, leading to an increasing biomass of brown trout with increasing temperature. These findings suggest that warming does not necessarily favour the small in aquatic ecosystems, with high-resource availability, faster reproductive and growth rates and greater production all contributing to meet the high-metabolic demands of apex predators in warmer environments.