Study system
We studied seven host species of Daphnia (D. dentifera ,D. retrocurva , D. dubia , D. parvula , D. pulicaria , D. ambigua , and D. mendotae ) and one closely related species of Ceriodaphnia (C. dubia; hereafter all hosts are collectively referred to as “Daphnia ” for simplicity). These species occur at varying densities in our study lakes. Daphnia are small planktonic crustaceans which live in freshwater lakes, feeding on phytoplankton and serving as prey to small fish and invertebrate predators (Tessier and Woodruff 2002). Many parasite species infect Daphnia , and here we focused on seven of the most commonly occurring species in our study sites. Our parasite analysis included: one fungus: Metschnikowia bicuspidata ; two microsporidians: Larsonnia obtusa and Gurleya vavrai ; two bacteria: Pasteuria ramosa and Spirobacillus cienkowskii ; and two oomycetes: Blastulidium paedophthorum and an unknown oomycete (which grows hyphae throughout the host’s body cavity, referred to as ‘Spider’ in the figures). Hosts were diagnosed based on characteristic symptoms of infection (Green 1974; Ebert 2005; Duffy et al. 2015). We excluded two common gut parasites of Daphnia (the ichthyosporean Caullerya mesnili and the microsporidianOrdospora pajunii ), because they were misclassified as the same species for part of the sampling period.
We studied host and parasite communities in 15 lakes in Southeast Michigan, US over three years (2014-2016). We sampled lakes roughly every two weeks from mid-July to mid-November each year (usually 9 total sampling events), a time period when most parasite epidemics in these lakes occur (Gowler et al. 2021). In addition to our normal sampling efforts, we sampled four of the study sites every three days during 2016 (this additional effort was due to another concurrently running project (Rogalski et al. 2021)). During each sampling event, we collected three replicate whole-water-column vertical tows from the bottom of the lake up through the surface with a 153 μm Wisconsin plankton net, pooling samples from three different locations in the deep basin of each lake for each replicate. For one replicate sample, we visually diagnosed parasite infections in live hosts under a dissection microscope at 10x magnification (or under a compound microscope at 20 to 40x magnification for early-stage infections). As Daphnia are mostly transparent, many parasite infections are visibly detectable with this method. We randomly subsampled the collected hosts, surveying at least 200 individuals of each host species for possible parasite infections or surveying all individuals of a given species when fewer than 200 of that host species were present. Overall, we analyzed 272,351 Daphniaindividuals for infections during this study. We preserved the other two replicate samples in 90% ethanol; at a later date, we randomly subsampled and counted one replicate to estimate the density of each host species. Density was calculated as the number of hosts throughout the water column for a given surface area of the lake (number of hosts per m2).