Environmental organic carbon availability determines metabolic and consumption rates in freshwater isopods

In forested freshwater ecosystems, benthic detritivorous invertebrates have been recognized as important agents of organic carbon (OC) cycling. In comparison with these detrital‐rich ecosystems, OC processing by invertebrates in heterotrophic groundwater ecosystems characterized by low OC availability has been poorly documented. The availability of OC should significantly influence the basal metabolism and the feeding rates of aquatic invertebrates, thus affecting how quickly these organisms are able to process OC sources. Therefore, obligate‐groundwater species living in OC‐poor ecosystems should process OC sources at lower rates than surface‐dwelling species to satisfy their energy requirements. Despite their implications for groundwater ecosystem functioning, conclusive tests of these predictions are lacking. We tested these two predictions by evaluating how the environmental availability of three main OC sources (coarse and fine particulate organic matter, and sedimentary biofilm) influences the basal metabolic and OC consumption rates by four species of freshwater asellid isopods. The four isopod taxa used consist of two surface‐dwelling and two obligate‐groundwater species inhabiting environments characterized by profoundly asymmetric OC availabilities. As expected, obligate‐groundwater asellids exhibited reduced basal metabolic rates in comparison with surface‐dwelling asellids. This result was associated with a positive correlation between basal metabolic rates of isopods and the availability of OC in their environments. The processing rate of each OC source was significantly and positively correlated with the basal metabolic rate of isopod species. Finally, obligate‐groundwater asellids exhibited reduced metabolic and associated OC processing rates to cope with resource scarcity in groundwater ecosystems. Nevertheless, when OC consumption rates were expressed relative to the quantity of OC in the environment of each species, obligate‐groundwater asellids processed at least the same percentages of OC available in their environments as surface‐dwelling asellids. This finding suggests that obligate‐groundwater isopods could contribute significantly to carbon cycling in their ecosystems.