Sixty years of artificial lake heating impact Daphnia fitness but not parasite resistance

Abstract The impacts of climate change are expected to be complex and severe, with rising temperatures potentially exacerbating the prevalence of infectious diseases. Aquatic ectotherms, such as Daphnia , are particularly vulnerable to temperature fluctuations, which can significantly impact their fitness and disease resistance. Using a unique system of lakes, some of which have been artificially heated for over 60 years, we investigated how elevated temperatures have affected fitness and parasite resistance in Daphnia longispina . Individuals from heated lakes as well as from nearby control lakes were experimentally exposed to the allopatric fungal parasite Metschnikowia biscuspidata at two temperatures (18 °C and 22 °C). We found that Daphnia exhibited high resistance to Metschnikowia , regardless of lake of origin or experimental temperature. While parasite exposure did not negatively affect Daphnia fitness, temperature had a significant impact. Specifically, Daphnia from heated lakes exhibited reduced fitness when maintained at 18 °C. Our findings highlight the importance of thermal history in shaping host fitness and suggest that parasite dynamics may be influenced more by local adaptation than by current temperature alone.