Climate-induced changes in headwater streams: Effects of warming and drought on resource-consumer trophic interactions

Changes in temperature and precipitation due to climate change are expected to increase stream water temperature and reduce their discharge. Consequently, more streams might face temporality, experiencing flow intermittency or complete streambed drying. The general aim of this thesis was to determine how climate-induced changes in stream hydrology (drought) and water temperature (warming) influence resource- consumer trophic interactions. Our results showed that although highly reliant on nutrient-poor allochthonous organic matter, most consumers in a forested Mediterranean stream complemented their diet with nutrient-rich algal resources when algae became more abundant in the stream; that is in the drying phase and following flow fragmentation, if light was not limiting. We also observed an abrupt decrease in consumers’ body tissue C:P ratios at flow disruption and for some taxa upon rewetting as well. However, these changes were not related to shifts in their resource stoichiometry, indicating they were rheostatic. Both resource conditioning in pools and streambed drying lowered the quality of basal resources whereas warmer stream temperatures increased it. This resulted in lower consumption rates for the shredder Stenophylax and the herbivore Physella accuta when fed on terrestrial conditioned resources; the proper also experiencing lower growth rates. The consumption rates of the shredders Sericostoma vittatum were higher at warmer temperatures due to their higher metabolic rates and other temperature mediated factors such as changes in leaf quality. Higher lipid storage and earlier pupation of S. vittatum was also observed at warmer temperatures. Finally, our results showed that the assimilation of phosphorus was constrained when shredder Echinogammarus berilloni was fed pool-conditioned leaves, which were P-depleted. At 15ºC, E. berilloni compensated this lower nutrient retention through compensatory feeding. However, this compensation was not enough at 20ºC, resulting in significantly lower phosphorus assimilation efficiency when the temperature and leaf quality were altered simultaneously. Under these conditions, growth rates were higher and changes in body tissue composition and increased mortality occurred, indicating synergistic effects of impaired food quality due to conditioning in pools and increased water temperatures. These effects on shredder physiology and metabolism altered nutrient ratios in faeces and excreta and consumer egestion and excretion rates. In conclusion, our study shows that both drought and warming affect the quality of benthic resources and that change in stream flow influences their relative abundance. These effects, in addition to direct effects of temperature on invertebrate metabolism, affected consumer feeding behaviour, nutrient assimilation efficiencies, growth and pupation. However, consumer body stoichiometry was hardly ever affected by the quality of resources, and the observed changes mostly responded to changes in consumer physiology. Finally, the simultaneous effects of increased metabolism due to warmer stream temperatures and lower resource quality due to stream flow fragmentation resulted in synergistic effects on stream detritivores physiology and fitness. Overall, these results suggest potential effects on stream food webs and ecosystem functioning through changes in leaf litter decomposition, detritivore- mediated nutrient cycling and secondary production. However, the extend and direction of these effects might be difficult to predict due to the different mechanisms among the species to cope with environmental changes and the synergistic effects of multiple climate-derived stressors