Combined influence of parasites and temperature on nutrient excretion rates and body stoichiometry of a freshwater fish

Abstract Animal‐mediated nutrient cycling is a key component of zoogeochemistry and is impacted by a variety of factors including external drivers such as temperature. In aquatic environments, warming water temperatures exacerbated by climate change may increase nutrient release by animals. However, there is much variability in the rate of predicted increases in nutrient release due to warming. Climate change is also expected to influence parasite transmission, likely affecting the health of host animals, with potential implications for feedbacks on ecosystems. Yet, ecologists often ignore the influence of parasites when conducting research on animal‐mediated nutrient cycling at individual, population and ecosystem levels. Thus, there is a need for studies that examine how warming, parasites and their interaction affect nutrient cycling of potential hosts. We measured element excretion rates and body elemental composition of a common freshwater fish, pumpkinseed sunfish ( Lepomis gibbosus ), across a range of natural parasite intensity and at three experimental temperatures (20, 25 and 30°C), simulating actual and projected future climate scenarios. After acclimating fish to experimental temperatures, we quantified their excretion rates (NH 4 + , PO 4 − and DOC) as well as the C, N and P concentrations in their bodies, in an effort to examine the singular and potentially additive and interactive effects of these dual stressors (elevated temperature and parasite infection). Excretion rates varied among temperature treatments with C, N and P each responding uniquely. C and P excretion rates increased with temperature but decreased with parasite intensity; furthermore, these factors acted interactively, with stronger parasite effects at higher temperatures. C and N concentrations in fish bodies (per unit dry mass) declined significantly with parasite intensity, as did body N:P ratio, indicating poor fish health. Our results suggest that these dual stressors related to climate change will likely impact elements sequestered by animals and rates at which they release elements to ecosystems, thereby potentially affecting zoogeochemical cycles.