Mediterranean streams are characterised by seasonal droughts, the frequency and intensity of which vary spatially and are expected to increase with global change. We studied the potential effects of drought and climate change on the fish assemblage and its trophic ecology in a Mediterranean stream by comparing an intermittent tributary with two more permanent neighbouring reaches. Although the three sites were dominated by the same two fish species, Mediterranean barbel (Barbus meridionalis) and chub (Squalius laietanus), the intermittent tributary had a lower overall fish density and fewer eel (Anguilla anguilla). The intermittent tributary had macroinvertebrates with lower density, smaller taxa and higher diversity. Fish in the intermittent tributary had significantly lower biomasses in their gut contents (adjusted for fish length) and more negative electivities than those in the permanent reaches, as well as significantly lower taxonomic diversity. These results indicate that there was reduced resource availability in the intermittent tributary, which resulted in significantly lower condition and gonadal weight (adjusted for length) of barbel and chub. The data obtained in this Mediterranean stream support the observation that reduced water flow may affect fish at both individual and assemblage levelsThis study was financially supported by projects CGL2007-65549/BOS, CGL2008-05618-C02-02, CGL2009-12877-C02-01 and Consolider-Ingenio 2010 CSD2009-00065 of the Spanish Ministry of Science. Catalan Water Agency, the Government of Catalonia and the Barcelona Provincial Council (‘Observatori de la Tordera’
As a result of climate change, streams are warming and their runoff has been decreasing in most temperate areas. These changes can affect consumers directly by increasing their metabolic rates and modifying their physiology and indirectly by changing the quality of the resources on which organisms depend. In this study, a common stream detritivore (Echinogammarus berilloni Catta) was reared at two temperatures (15 and 20°C) and fed Populus nigra L. leaves that had been conditioned either in an intermittent or permanent reach to evaluate the effects of resource quality and increased temperatures on detritivore performance, stoichiometry and nutrient cycling. The lower quality (i.e., lower protein, soluble carbohydrates and higher C:P and N:P ratios) of leaves conditioned in pools resulted in compensatory feeding and lower nutrient retention capacity by E. berilloni. This effect was especially marked for phosphorus, which was unexpected based on predictions of ecological stoichiometry. When individuals were fed pool-conditioned leaves at warmer temperatures, their growth rates were higher, but consumers exhibited less efficient assimilation and higher mortality. Furthermore, the shifts to lower C:P ratios and higher lipid concentrations in shredder body tissues suggest that structural molecules such as phospholipids are preserved over other energetic C-rich macromolecules such as carbohydrates. These effects on consumer physiology and metabolism were further translated into feces and excreta nutrient ratios. Overall, our results show that the effects of reduced leaf quality on detritivore nutrient retention were more severe at higher temperatures because the shredders were not able to offset their increased metabolism with increased consumption or more efficient digestion when fed pool-conditioned leaves. Consequently, the synergistic effects of impaired food quality and increased temperatures might not only affect the physiology and survival of detritivores but also extend to other trophic compartments through detritivore-mediated nutrient cycling.
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.
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