griselda chaparro

In dynamic systems, many species use dormancy as a strategy to survive harsh periods and disperse through time, a fundamental aspect scarcely explored in metacommunities studies. We compared spatial patterns of resting egg bank and active zooplankton diversity across scales (rotifers and cladocerans), explored relationships between diversity and environmental and spatial factors and compared their species composition in riverine floodplains of the Danube River. The egg bank was similarly or even more heterogeneous than the active communities, and signals of community homogenization detected for active communities were not observed for the egg bank. The strong relationships detected between rotifers egg bank diversity and local hydrological parameters suggest that floods would not weaken diversity–environment relationships, as observed for the active communities. These results imply that spatial patterns of resting and active stages are affected by different processes and that the eg...

Environmental heterogeneity is an essential quality of ecosystems as it has important implications in community structure. Macrophytes are a
main component of environmental heterogeneity in floodplain lakes, and their dynamics are highly influenced by water level changes. In this
context, we analysed at different spatial scales the relationship among hydrological variations, environmental heterogeneity associated to
macrophytes and zooplankton regional diversity (γ diversity) in a South American floodplain lake adjacent to the Paraná River, and we also
compared the local zooplankton diversity (α diversity) among the different environments that comprised the lake heterogeneity. At very low
waters, the environmental heterogeneity was reduced as the lake was mainly limited to open water areas with low zooplankton diversity. At
high waters, the profuse vegetation development (emergent and free-floating), in mixed or homogeneous patches, determined a higher
lake environmental heterogeneity with enhanced regional zooplankton diversity; littoral species increased over limnetic ones. Zooplankton
α diversity was higher in environments with free-floating macrophytes than in those without these plants. The structural complexity in the
water column provided by plant roots would be closely related to the enhanced diversity found under free-floating mats. This study
contributes to the knowledge on the effects of strong water level variations on environmental heterogeneity, which is strongly associated to
macrophytes and on zooplankton diversity, and highlights the role of free-floating plants as diversity hosts and ‘key structures’ in floodplain lakes.

Macrophytes play an important role in structuring
communities in aquatic ecosystems due to their influence on
ecological processes and attributes of biological aquatic
assemblages. Freshwater macroinvertebrates comprise a functionally
and taxonomically diverse group in shallow lakes,
which serve as food for fish, amphibians, and water birds, and
are involved in the breakdown of organic matter and nutrients.
Here, we investigated macroinvertebrate assemblages associated
with small and medium-sized free-floating plants (FFP)
by describing their structure, analyzing functional aspects
(considering functional feeding groups and habits), and determining
how much of the variation in fauna composition is
explained by environmental factors, mainly FFP. Differences
in structure, functional feeding groups and habits of macroinvertebrate
assemblages were associated with different compositions
and percentages of cover of FFP. Gradients of
richness and diversity of macroinvertebrates were positively
related to the complexity of FFP mats, which was associated
with the structure of roots and leaves.

We examined the zooplankton abundance and composition of Laguna Grande, a floodplain wetland of
the Lower Paraná Basin (Argentina), during an extraordinary drought–flood cycle that affected both
the environment and the biological conditions of the lake. Low waters were characterised by remarkably
high conductivities and pH values, and high phytoplankton and bacterioplankton abundances with
cyanobacterial blooms, while high waters showed opposite features. In relation to zooplankton, the
mean abundances of all the taxonomic groups (rotifers, cladocerans, copepods, ciliates, and heterotrophic
nanoflagellates) were slightly higher at low waters. Major changes were observed in the specific composition
of metazooplankton: the euryhaline species assemblage that dominated in the dry warm period was
replaced by several oligohaline littoral and planktonic species characteristic of the Paraná River Basin,
when the water level rose. Mean species richness values at high waters doubled those of low waters and
were directly correlated to water depth. Most of the rotifers of the genus Brachionus and the cladoceran
Moina micrura switched from parthenogenetic to sexual reproduction during low waters, as a response to
a harsh environment and crowding. We suggest that the main changes in the environmental conditions
in this eutrophic floodplain lake are driven by the hydrology, which regulates the zooplankton succession.
The herein described shifts in the zooplankton structure and dynamics of Laguna Grande over an
extraordinary drought–flood cycle contribute to the understanding of the processes that might occur
under the scenarios predicted by climate change models.

In this study, we analyse the spatial
distribution of cyanobacterial summer blooms in a
large subtropical reservoir located in the Uruguay
River, from 2007 to 2011; these extraordinary algal
growth events are mainly represented by scum-forming
and nitrogen-fixing eco-strategists of the Dolichospermum
and Microcystis genera. The use of the
eco-strategists approach, based on ecophysiological
work and field observations, allowed us to explain the
differences in the distribution pattern and temporal
dynamics of both cyanobacterial complexes. Spatial
differences were produced due to much higher and
fluctuating cyanobacterial abundances at the right
margin of the reservoir and at the littoral areas closer
to the dam. Satellite imagery (LANDSAT 5 TM)
clearly depicted the stronger algal development in the
reservoir arms and in the section closer to the dam. The
Microcystis spp. complex achieved higher density
than the Dolichospermum spp. complex. We hypothesise
that the hydrological cycle explains the interannual
fluctuations of the intensity and frequency of
cyanobacterial blooms, and that spatial differences in
cyanobacterial presence between the reservoir arms,
its margins and the main channel is mainly a response
to morphometrical and hydrological characteristics.

This 10-year field data study explores the relevance
of water level fluctuations in driving the shift from a
free-floating plant (FFP) to a phytoplankton dominated state
in a shallow floodplain lake from the Lower Parana´ River.
The multi-year natural flood pulse pattern in the Lower Parana
´ River drove the ecosystem regime from a FFP-dominant
state during very high waters (1998–1999) to absolute phytoplankton
prevalence with blooms of nitrogen fixing
Cyanobacteria during extreme low waters (2008–2009).
Satellite images support the observed changes over the
decade and show the decrease of the surface lake area covered
by FFP as well as the modification of the spectral firm in
open waters, which documents the significant increases in
phytoplankton chlorophyll a concentrations. We discuss the
possibility that, despite a slow eutrophication in these highly
vegetated systems, water level changes and not nutrients
account for the shift from a floating macrophyte community
to phytoplankton dominance. Cyclic shifts may occur in
response to the seasonal floodpulse, but more strongly, as
indicated by our results, in association to the extreme drought
and flood events related to the El Nin˜o Southern Oscillation,
which is linked to discharge anomalies in the Parana´ River.

Dense mats of free floating plants (FFP)
often produce severe underwater light attenuation and
strong oxygen depletion in the water column. In this
study, we experimentally assessed the zooplankton
response to artificial shading using field mesocosms.
During 30 days, we simulated three different light
scenarios by mimicking the persistence, absence, and
fluctuation of FFP typically encountered in vegetated
shallow subtropical lakes. We used dark meshes to
simulate the abiotic effects engineered by FFP. Both in
the permanently covered and fluctuating situations,
anoxia impaired zooplankton development. Anoxia
constituted a major driving force in shaping the
zooplankton response, whereas the feeding resource
availability (phytoplankton) seemed to play a minor
role; no top down effect on phytoplankton occurred in
anoxic situations. In the fluctuating cover regime
(periodic darkness and anoxia), the temporal variation
of nanophytoplankton was not affected by zooplankton;
once again oxygen availability seemed the main
force shaping the zooplankton dynamics. Either
periodical or permanent shading, associated to anoxic
conditions, impaired the success of small herbivores.
Large herbivores and microphytoplankton were
negatively affected only under persistent shade and
anoxia. In contrast, when neither light nor oxygen
limitation occurred, such as in the scenario without
shading, top-down control occurred. This study highlights
the importance that the oxygen dynamics driven
by the presence of FFP exert on the structure and
dynamics of zooplankton assemblages and on the top
down cascading effects on phytoplankton in warm
temperate or subtropical shallow lakes.

SUMMARY
1. There is an increasing concern to understand the role of free-floating plants (FFP) on the
functioning and structure of shallow lakes, particularly the mechanisms by which their
dominance is self-stabilizing and how they may outcompete phytoplankton.
2. In a field experiment with mesocosms, we simulated three commonly encountered
scenarios in warm temperate shallow lakes: FFP dominance, FFP fluctuation and FFP
absence. We explored the effects of several key processes, triggered by FFP dynamics, on
the composition, diversity and production of phytoplankton, and on the physicochemical
conditions.
3. The effects of persistent floating mats on light interception triggered a complex response
by the phytoplankton: species were lost and biomass was low, yet high diversity and
productivity potential were maintained. A contrasting phytoplankton response characterized
mesocosms lacking FFP, where light was sufficient but nitrogen was limiting.
Fluctuating FFP cover brought periodic shifts between these two limiting resources for the
phytoplankton, which most probably explain the maintenance of richness, diversity and
production in these shallow lakes.
4. These results support the recently proposed framework of alternative stable states,
providing experimental field evidence of the mechanisms resulting from the shifts
between floating plants and phytoplankton dominance.