Warming and nutrient-depleted food: Two difficult challenges faced simultaneously by an aquatic shredder

Global warming and changes in nutrient supply can alter the functioning of headwater stream ecosystems, where leaf-shredding invertebrates are pivotal for the transfer of nutrients through the food web. However, little is known about how warming and nutrient-depleted leaf litter interact and potentially exacerbate the already large elemental imbalance between shredders and their food. We hypothesized that: 1) shredders cannot increase their feeding rate to compensate for the effects of both factors simultaneously, and 2) shredders will increase assimilation efficiency of limiting nutrients, especially N, by decreasing excretion, increasing nutrient bioavailability in the gut, or both by adaptations of digestive enzymes. We performed a microcosms experiment with a caddisfly species subjected to 3 temperatures and 2 diets that differed in their N and P content. Overall, our results suggest that the combination of increasing temperatures and decreasing diet nutrient content lowered the capacity of larvae to acquire nutrients, either by decreasing nutrient assimilation, increasing nutrient excretion, or both. However, the trypsin enzyme maintained functionality under the range of temperatures we tested (5-15 degrees C), which likely allows larvae to ensure N availability. Moreover, we detected reduced excretion and improved apparent assimilation of N relative to P. However, larval survival still decreased significantly at the highest temperature (15 degrees C) when larvae were fed the low-nutrient diet. These results indicate that regulation mechanisms for reducing elemental constraints from poor food quality are less effective at higher temperatures. If these effects also occur in other detritivores, nutrient recycling and ecosystem function in headwater streams might be compromised as the climate warms.