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).