337(Gerphagnon et al. 2013b ), thereby enhancing the contribu-
tion of fungal parasites to the bloom decline. This so-called
“mechanistic fragmentation” of cyanobacterial fi laments
(Gerphagnon et al. 2013b ) can decrease the resistance of
hosts to grazing, which could further accelerate the decline
of cyanobacterial blooms. Thus, these results extend the role
of chytrid zoospores which are known to upgrade the bio-
chemical diet of zooplankton, establishing zoosporic fungi
as potential key players in the food web dynamics, as inferred
from intensive fungal studies in the Lakes of the French
Massif Central during the past 10 years (Rasconi et al. 2009 ;
Jobard et al. 2010a ; Sime-Ngando 2012 ).
20.5 Trophic Dynamics and PEG-Model
Application to Chytrid-Phytoplankton
Pairing
We recently presented an extensive seasonal study of chytrid-
phytoplankton trophodynamics in the oligotrophic Lake
Pavin compared to the eutrophic Lake Aydat (Rasconi et al.
2012 ), based on our CFW staining protocol of infective spo-
rangia and phenotypic identifi cation (Rasconi et al. 2009 ).
We were able to identify up to 15 different chytrid species on
diverse host populations, with specifi c biovolume ranging
from 7 to 72 μm^3 sporangium −1. Seasonal abundances
increased from 0.0005 − 0.4 × 10^6 sporangia l −1 in Lake
Pavin to 0.0 − 32 × 10^6 sporangia l −1 in Lake Aydat. In both
lake systems, sporangium abundances peaked with the devel-
opment of preferred diatom hosts in spring (Pavin) and cya-
nobacteria in autumn (Aydat), the autumn peak being higher
in eutro- compared to oligotrophic conditions, when a mono-
specifi c bloom of the fi lamentous cyanobacterium Anaebaena
occurred.
All 15 identifi ed species were monocentric (i.e. with one
center of growth and development) and eucarpic (using part
of the thallus for the fruit-body formation and with a special-
ized rhizoidal system ), typical of the class Chytridiomycetes ,
and belonged to two orders: the Rhizophidiales which con-
tained one genus ( Rhizophidium ), and the Chytridiales which
contained three genera ( Chytridium , Zygorhizidium , and
Rhizosiphon ). The species of Rhizophidium spp. infected a
wide diversity of hosts, including both large size (e.g. the
Chlorophyta Staurastrum spp. and the diatoms Asterionella
formosa , Synedra spp. and Fragilaria crotonensis ) and small
size algae (e.g. the diatom Cyclotella spp. and the Chlorophyta
Chodatella ciliata and Ankistrodesmus convolutus ). The spe-
cies Chytridium spp. infected the chlorophyte Oocystis sp,
the diatom F. crotonensis and the cyanobacterium Microcystis
sp, while the species of Zygorhizidium infected the diatoms
Melosira spp. The genus Rhizosiphon comprised two species
that are specifi c parasites of vegetative cells and akinetes ( R.
crassum ) and of akinetes alone ( R. akinetum ). These two
species correspond to different niches offered by the fi la-
mentous cyanobacterium host Anabaena macrospora in the
productive Lake Aydat (Gerphagnon et al. 2013b ). Although
almost all chytrid species were observed in all lakes ranging
from oligo- to eutrophic conditions, the seasonal fungal
community composition was largely dominated by species
of the genus Rhizophidium (90 % of total sporangium abun-
dance) in oligotrohic conditions (Pavin), and of the genera
Rhizophidium (56 %), Zygorizidium (22 %), Chytridium
(19 %), and Rhizosiphon (14 %) in eutrophic conditions
(Aydat) (Rasconi et al. 2012 ).
The community structure of natural chytrids is intimately
linked to the availability of hosts (Ibelings et al. 2004 ).
However, there is still no study assessing the fungal species
successions in natural environments. Based on the seasonal
study in Lakes Pavin and Aydat (Rasconi et al. 2012 ), we
proposed a general empirical model on chytrid seasonality
and trophodynamics (i.e. with their hosts) based on the theo-
retical PEG (i.e. Plankton Ecology Group) model of sea-
sonal succession of planktonic events in freshwaters
(Sommer et al. 1986 ). During winter, the development and
activities of both chytrid parasites and their phytoplanktonic
hosts were at their lowest levels, because of low temperature,
freezing or ice-cover. Between late winter and spring, the
environmental conditions, primarily the increase in water
temperature and in mixing-derived nutrient availability,
favor the development of host communities, with the domi-
nance of k-strategists (e.g. large diatoms) towards spring.
As a consequence, the probability of host – parasite contact
increases, and led to higher chytrid infectivity, and the pro-
duction of large amount of zoospores. Enhanced infection
prevalence subsequently limits the development of large dia-
toms and induces their decline, liberating niches for a diver-
sifi ed phytoplankton community of small-size r-strategists.
The abundance of chytrid sporangia reaches their lowest
level, while the availability of food (i.e. small phytoplankton
and fungal zoospores) benefi ts the development of grazers
and the establishment of a typical clear-water phase at the
end of spring. During the summer months, favorable envi-
ronmental conditions, together with a high grazing pressure,
allow the development of a diversifi ed and complex plankton
community. Small edible hosts are inhibited by the grazing
pressure, while the availability of large size hosts favors the
proliferation of different chytrid species towards the end of
the summer. From there, oligotrophic lakes (Pavin) signifi -
cantly diverge from productive waters (Aydat). In oligotro-
phic situations, autumnal overturn promotes species
coexistence and phytoplankton diversity leads to the associa-
tion of different chytrid species that parasitize chlorophytes
and diatoms, but with general low infection prevalence due
to a balanced host – parasite growth. In eutrophic lakes,
nutrient conditions and persistent stratifi cation favor the
bloom of fi lamentous cyanobacteria from the end of the sum-20 Chytrid Parasites of Phytoplankton in Lake Pavin