Study System
We used the well-studied host-parasite model system (Ebert et al. 2016)D. magna and its bacterial parasite P. ramosa to experimentally investigate the role that temperature may play in driving seasonal host-parasite dynamics. D. magna are small planktonic crustaceans broadly distributed throughout the northern hemisphere (Bekker et al. 2018, Fields et al. 2018, Bourgeois et al. 2021) and are keystone species within aquatic food webs (Lampert 2011). D. magna reproduces via cyclic parthenogenesis, producing clonal (asexual) daughters, but may reproduce sexually in poor environmental conditions. Sexual reproduction results in resting stages (ephippia) that can survive freezing and drought and will hatch when conditions improve.Daphnia spp. Metabolism, grazing, development rates and reproductive output are accelerated by warming temperatures, up to around 25 oC (Burns 1969, Kirk et al. 2018), and this increase in metabolic rate may also increase their encounters with parasites and development of infection (Kirk et al. 2018, Kirk et al. 2019, Kirk et al. 2020). P. ramosa infection follows a stepwise process (Ebert et al. 2016) whereby 1) Daphnia spp. are exposed to spores by filtering contaminated water or mud; 2) spores become “activated” when in contact with Daphnia spp. (Duneau et al. 2011); 3) spores attach to, then 4) penetrate the gut wall; and 5) grow and reproduce within the host. During parasite growth inside the host,Daphnia spp. stop reproducing (parasitic castration), grow larger (gigantism) and turn dark red/orange in colour, making it easy to observe whether an individual is infected without destructive sampling and to track changes in population prevalence over time. The interactions of D. magna and P. ramosa are known to be highly polymorphic within populations and the two antagonists co-evolve (Decaestecker et al. 2007, Auld et al. 2016). A matching-allele-infection matrix has been observed (Bento et al. 2017), which may influence epidemic dynamics as the proportion of individuals susceptible to the dominant strain is likely to change.
The local food web in the Aegelsee is relatively simple, includingDaphnia pulex and D. curvirostris in addition to D. magna and P. ramosa , and small insect predators such asChaoborus larvae and chorixid water-bugs. Only D. magnabecomes infected with P. ramosa in this pond. The seasonal monitoring includes assessment of prevalence of P. ramosa andD. magna population dynamics and estimates of the proportions ofDaphnia “resistotypes”, i.e. the genetically determined ability of the D. magna to resist specific genotypes of the parasite. Finally, this pond undergoes a massive disturbance in late September/early October when hot, ammonium-rich, condensation water from a nearby sugar refinery is released into it, killing off all planktonicD. magna and other invertebrates in the pond, but not the resting stages of the host or parasite in the sediment.