Abstract
This study assesses the quantitative impact of parasitic chytrids on the planktonic food web of two contrasting freshwater lakes during different algal bloom situations. Carbon-based food web models were used to investigate the effects of chytrids during the spring diatom bloom in Lake Pavin (oligo-mesotrophic) and the autumn cyanobacteria bloom in Lake Aydat (eutrophic). Linear inverse modeling was employed to estimate undetermined flows in both lakes. The Monte Carlo Markov chain linear inverse modeling procedure provided estimates of the ranges of model-derived fluxes. Model results confirm recent theories on the impact of parasites on food web function through grazers and recyclers. During blooms of “inedible” algae (unexploited by planktonic herbivores), the epidemic growth of chytrids channeled 19–20% of the primary production in both lakes through the production of grazer exploitable zoospores. The parasitic throughput represented 50% and 57% of the zooplankton diet, respectively, in the oligo-mesotrophic and in the eutrophic lakes. Parasites also affected ecological network properties such as longer carbon path lengths and loop strength, and contributed to increase the stability of the aquatic food web, notably in the oligo-mesotrophic Lake Pavin.
Highlights
Parasites are known to be ubiquitous in their environments
Since we have demonstrated in a previous work (Grami et al, 2011) that parasites drive an increase in species richness, trophic level, connectance, and trophic chain length of the food web, we wanted to establish the effects of parasites on the ecosystem properties linked to stability during monospecific algal proliferations
Due to blooms of large algae in Lake Pavin (Synedra sp., Melosira sp., Cyclotella sp., and Asterionella sp.) and filamentous cyanobacteria in Lake Aydat (Anabaena sp. and Oscillatoria sp.) the major contribution to the gross primary production was provided by microphytoplankton (75% and 89% in Lake Pavin and Aydat, respectively)
Summary
Parasites are known to be ubiquitous in their environments. they have been considered as important forcing factors for ecological processes (Hudson et al, 2006), they have only recently been included in food web studies. Microscopic observations provided evidence for the presence of both forms in freshwater ecosystems (Rasconi et al, 2009; Jobard et al, 2010) These parasites mostly affect primary producers (Canter, 1950; Sparrow, 1960), food web dynamics (Mccallum et al, 2004; SimeNgando, 2012) and ecological processes (Hudson et al, 2006). Recent studies have suggested a role for fungal parasites in destroying large filamentous phytoplankton, which are considered important for seasonal pelagic succession (Rasconi et al, 2012; Gerphagnon et al, 2013) This finding raises the hypothesis that parasites may play important roles during monospecific blooms of inedible algae because they can release dissolved substrates for microbial processes through host destruction, and provide energetic particles as zoospores for higher trophic levels (Kagami et al, 2007; Grami et al, 2011)
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