Combined effects of salinity and infectious disease on Daphnia dentifera at multiple scales

Abstract

Abstract Temperate freshwater ecosystems are currently being salinised through anthropogenic activities. These increases in freshwater salinity can impact individuals, populations, and species interactions. We studied the effects of salinity on freshwater host–parasite interactions at multiple scales using the zooplankton host, Daphnia dentifera, and a fungal parasite (Metschnikowia bicuspidata). We conducted one experiment at the individual‐level to quantify the effects of salinity on infection prevalence and another to understand the combined population‐level effects of salinity and parasitism. In our first experiment, we found that the effects of salinity on infection prevalence varied by host genotype; increased salinity reduced infection prevalence in one genotype but had no effect on infection prevalence in another. In our second experiment, infection prevalence was lower when NaCl was added to the microcosms compared to the control (no added salt) treatments. We also found a significant parasite × salinity interaction on D. dentifera density in our second experiment, where the parasite only reduced host densities in our control treatment, probably due to the reduced infection prevalence as salinity increased. This study demonstrates that salinity can influence infection prevalence in freshwater hosts and that host population density may respond to the combined effects of salinisation and parasitism in a non‐additive manner.