Functional Effects of Parasites on Food Web Properties during the Spring Diatom Bloom in Lake Pavin: A Linear Inverse Modeling Analysis

Abstract

This study is the first assessment of the quantitative impact of parasitic chytrids on a planktonic food web. We used a carbon-based food web model of Lake Pavin (Massif Central, France) to investigate the effects of chytrids during the spring diatom bloom by developing models with and without chytrids. Linear inverse modelling procedures were employed to estimate undetermined flows in the lake. The Monte Carlo Markov chain linear inverse modelling procedure provided estimates of the ranges of model-derived fluxes. Model results support recent theories on the probable impact of parasites on food web function. In the lake, during spring, when ‘inedible’ algae (unexploited by planktonic herbivores) were the dominant primary producers, the epidemic growth of chytrids significantly reduced the sedimentation loss of algal carbon to the detritus pool through the production of grazer-exploitable zoospores. We also review some theories about the potential influence of parasites on ecological network properties and argue that parasitism contributes to longer carbon path lengths, higher levels of activity and specialization, and lower recycling. Considering the “structural asymmetry” hypothesis as a stabilizing pattern, chytrids should contribute to the stability of aquatic food webs.

Enumeration of Parasitic Chytrid Zoospores in the Columbia River via Quantitative PCR

“…However, during blooms of large, inedible phytoplankton in French lakes, parasitism by chytrids and the subsequent production of zoospores was estimated to shunt ϳ20% of the total primary production into a form edible to zooplankton (18). Additionally, models that included parasitic chytrids showed an increase in carbon transfer efficiency and higher grazing efficiency than in a system lacking parasitic chytrids (41). Our analysis suggests that chytrid zoospores in the Columbia River were present at concentrations sufficiently high to support the growth of small zooplankton during periods when the phytoplankton community is dominated by large, inedible diatom species (i.e., in middle to late spring).…”

Mineralogical Diversity in Lake Pavin: Connections with Water Column Chemistry and Biomineralization Processes

“…This peak has been shown to exhibit a variable intensity depending on the season and year: it is sharper in spring and early summer and has been shown to be related to algal blooms [50,51]. Diatom frustules were predominant at these depths for the five sampling campaigns and contributed to massive precipitation of silica in the lake.…”

A Double Staining Method Using SYTOX Green and Calcofluor White for Studying Fungal Parasites of Phytoplankton

“…Thus, the ability to access the production of zoospores of each chytrid species could provide interesting data for quantifying the carbon input of each species in a mixed fungal community. This could be very useful for authors trying to model the effects of parasitism in freshwater ecosystems (2,4,46,47).…”

“…However, it has been recognized that zoospores are an important source of carbon in freshwater ecosystems and act as a real link between inedible host algae and higher trophic levels (4,8). It is therefore surprising to find that the accurate quantification of chytrid fecundity (i.e., number of zoospores per sporangium), a key parameter in chytrid life cycles, is often omitted in field studies.…”

“…Chytrids, like some other parasites, are completely dependent on their host for their nourishment and their development, which results in the death of their host (3). Previous studies focused on the impact of fungal parasitism on phytoplankton dynamics and have highlighted the fact that fungal parasitism has had a real impact on the decline of several eukaryotic phytoplankton species, showing that fungal parasitism is potentially implicated in the phytoplankton succession (4)(5)(6)(7)(8)(9)(10).…”