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Abstract Immune responses can be energetically expensive and subject to trade-offs. Prior work on the freshwater zooplankton, Ceriodaphnia cornuta, demonstrated an association between eye size and infection, leading to questions about whether investment in eyes trades off against investment in immunity. We used the crustacean host, Daphnia dentifera, and its fungal parasite, Metschnikowia bicuspidata, to investigate the relationships between eye size, parasite resistance and infection. In the field, we found a negative correlation between size-corrected eye area (SCEA) and Metschnikowia infection, suggesting that either SCEA decreases infection (thereby indicating resistance) or that infection decreases SCEA. Controlled laboratory experiments reinforced the latter result: exposure to the fungal parasite decreased a host’s SCEA, regardless of the parasite dose or host genotype. We also uncovered significant plasticity in this trait—both host age and resource level increased SCEA. Identifying causality in physiological correlations is challenging. Our results suggest that negative associations between parasitism and energetically-expensive traits can arise through plasticity. 

Abstract Plankton form the foundation of marine food webs, playing fundamental roles in mediating trophic transfer and the movement of organic matter. Increasing ocean temperatures have been documented to drive evolution of plankton, resulting in changes to metabolic traits that can affect trophic transfer. Despite this, there are few direct tests of the effects of such evolution on predator–prey interactions. Here, we used two thermally adapted strains of the marine mixotroph (organism that combines both heterotrophy and autotrophy to obtain energy) Ochromonas as prey and the generalist dinoflagellate predator Oxyrrhis marina to quantify how evolved traits of mixotrophs to hot and cold temperatures affects trophic transfer. Evolution to hot temperatures reduced the overall ingestion rates of both mixotroph strains, consequently weakening predator–prey interactions. We found variability in prey palatability and predator performance with prey thermal adaptation and between strains. Further, we quantified how ambient temperature affects predator grazing on mixotrophs thermally adapted to the same conditions. Increasing ambient temperatures led to increased ingestion rates but declines in clearance rates. Our results for individual, pairwise trophic interactions show how climate change can alter the dynamics of planktonic food webs with implications for carbon cycling in upper ocean ecosystems. 

Abstract Zooplankton play an integral role as indicators of water quality in freshwater ecosystems, but exhibit substantial variability in their density and community composition over space and time. This variability in zooplankton community structure may be driven by multiple factors, including taxon-specific migration behavior in response to environmental conditions. Many studies have highlighted substantial variability in zooplankton communities across spatial and temporal scales, but the relative importance of space vs. time in structuring zooplankton community dynamics is less understood. In this study, we quantified spatial (a littoral vs. a pelagic site) and temporal (hours to years) variability in zooplankton community structure in a eutrophic reservoir in southwestern Virginia, USA. We found that zooplankton community structure was more variable among sampling dates over 3 years than among sites or hours of the day, which was associated with differences in water temperature, chlorophyll a, and nutrient concentrations. Additionally, we observed high variability in zooplankton migration behavior, though a slightly greater magnitude of DHM vs. DVM during each sampling date, likely due to changing environmental conditions. Ultimately, our work underscores the need to continually integrate spatial and temporal monitoring to understand patterns of zooplankton community structure and behavior in freshwater ecosystems.