Inés O’Farrell

Water level as the main driver of the alternation between a free-floating plant and a phytoplankton dominated state: a long-term study in a floodplain lake

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 Paraná River. The multi-year natural flood pulse pattern in the Lower Paraná 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 Niño Southern Oscillation, which is linked to discharge anomalies in the Paraná River.

Regime shifts between free-floating plants and phytoplankton: a review

Field studies evidence shifts between phytoplankton and free-floating plant regimes; yet, it is unclear what drives these shifts and if they are critical transitions (alternative stable states). In this review, we synthesized field and experimental data on free-floating plants (of varying size and phylogenies) and phytoplankton regimes, to assess the effects of these producers on the environment. Nutrient-rich environments promote free-floating plants dominance—regardless of life form—which causes dark and anoxic environments, and nutrient release from sediments. This reinforces free-floating plants dominance, but controls phytoplankton biomass by strong shading (despite high nutrients and low grazing). Phytoplankton dominance renders turbid and oxygen-rich (when producing) environments. We also searched for case studies of regime shifts for free-floating plants and phytoplankton dominance. Most studies showed that when free-floating plants dominance was interrupted, phytoplankton biomass (usually Cyanobacteria) rose steeply. Likewise, when phytoplankton-dominated, the development of dense mats of free-floating plants covers usually controlled phytoplankton. Field evidence that suggests critical transitions include abrupt regime transitions in time and space; yet, evidence including indoor controlled experiments and mathematical models is needed for conclusive evidence of alternative stable states to be drawn.

Bloom forming cyanobacterial complexes co-occurring in a subtropical large reservoir: validation of dominant eco-strategies

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 inter-annual 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.

Zooplankton response to shading effects of free-floating plants in shallow warm temperate lakes: a field mesocosm experiment

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.

Temporal and Spatial Patterns of Macroinvertebrates Associated with Small and Medium-Sized Free-Floating Plants

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.

Seasonal-dependence in the responses of biological communities to flood pulses in warm temperate floodplain lakes: implications for the “alternative stable states” model

In floodplains located in temperate regions, seasonal variations in temperature affect biological communities and these effects may overlap with those of the flood regime. In this study we explored if and how timing (with regard to temperature seasonality) influences the responses of planktonic and free-floating plants communities to floods in a warm temperate floodplain lake and assessed its relevance for determining state shifts. We took samples of zooplankton, phytoplankton, picoplankton, heterotrophic nanoflagellates and free-floating macrophytes at four sites of the lake characterized by the presence-absence of emergent or free-floating macrophytes along a 2-year period with marked hydrological fluctuations associated to river flood dynamics. We performed ANOVA tests to compare the responses of these communities to floods in cold and warm seasons and among sites. Planktonic communities developed high abundances in response to floods that occurred in the cold season, while the growth of free-floating macrophytes was impaired by low winter temperatures. Spring and summer floods favored profuse colonization by free-floating plants and limited the development of planktonic communities. The prolonged absence of floods during warm periods caused environmental conditions that favored Cyanobacteria growth, leading to a “low turbid waters” regime. The occurrence of floods early in the warm season caused phytoplankton dilution and promoted free-floating plant colonization and a shift towards a “high clear waters” state. Zooplankton:phytoplankton biomass ratio was very low during floods in warm seasons, thus zooplankton grazing on phytoplankton seemed to play a minor role in the maintenance of the clear regime.

Long-term study of bloom-forming cyanobacteria in a highly fluctuating vegetated floodplain lake: a morpho-functional approach

This study portrays the frequency and intensity of bloom-forming cyanobacteria in relation to highly changing environmental conditions in a floodplain lake from the Chaco-Pampean region and identifies the factors promoting the growth of different species along a 15-year period marked by important drought and flood periods. Phytoplankton in Laguna Grande was increasingly dominated by cyanobacteria represented by eight bloom-forming species bearing different eco-physiological traits. The bloom development showed strong inter-annual variability in correspondence with the depth of the lake that was affected by local rainfall and the ENSO regional phenomenon. The water level of the lake strongly influenced the species composition, favoring non-heterocystous filaments in deep turbid waters and nitrogen fixing in very short well lit, nitrogen-depleted water columns. Species morphology, dimensions and occurrence of akinetes in the filaments also varied in relation to water level. We suggest that the akinete pool of the sediments played a key role on the occurrence of recurrent Anabaenopsis cf. elenkinii and Sphaerospermopsis cf. aphanizonemoides blooms.

Hydrology driven factors might weaken fish predation effects on zooplankton structure in a vegetated warm temperate floodplain lake

Fish predation on microcrustaceans is of key importance for dominance of small zooplankters in warm shallow lakes, yet its role in floodplain environments remains poorly explored. We studied seasonal and spatial variations of the abundance, diet composition and feeding selectivity of small omnivorous–planktivorous fish in relation to zooplankton at different habitats in a floodplain lake of the Lower Paraná River. Fish catches were very high in spring and summer and scarce in autumn and winter, and their distribution varied among habitats in response to changes in refuge provision and oxygen concentration. Fish diet comprised zooplankton, phytoplankton and detritus, except in summer when Cyanobacteria prevailed; in general, cyclopoid copepods and cladocerans were positively selected. Macrozooplankton biomass was the highest in spring when calanoid copepods dominated, probably because their fast swimming velocity enabled their escape from predators. Lower macrozooplankton biomass in summer with low waters, was likely caused by harmful effects of Cyanobacteria or elevated salinity, while washing-out or dilution processes may explain macrozooplankton scarcity during cold seasons under low fish predation pressure. Our results indicate that in floodplain lakes, factors affected by or related to hydrology (salinity, Cyanobacteria development, dilution and washing-out processes) may have stronger influence than predation on zooplankton structure.

Drivers shaping phytoplankton diversity and composition in a humid Pampean floodplain lake (Natural Reserve)

We aimed to identify the driver/s of phytoplankton diversity (gamma, beta, alpha) and community composition in a Pampean floodplain wetland (Otamendi), using species and functional groups. We performed a seasonal regional survey (2004–2006, phytoplankton regime) across the different aquatic systems in the wetland. Gamma diversity was 254 species. Beta diversity was 2.53 in late spring, 2.49 in winter, and was lowest in summer (2.05) when the wetland was over flooded. Alpha diversity (mean richness) ranged between 29 and 50 species. Multiple regressions showed that phytoplankton alpha diversity (richness, Shannon–Wiener, evenness, and Simpson diversity index) responded to dissolved inorganic nitrogen, suspended solids, light attenuation, and pH. Nutrients also explained well the distribution of phytoplankton functional groups. Dissolved inorganic nitrogen concentrations were usually limiting in the shallow lakes, whereas phosphate concentration was always high. Light attenuation was mostly caused by high suspended solids and high humic substances. We also re-analyzed data from a survey performed during a free-floating plant regime (1998–2000); the dense macrophyte cover attenuates most incoming light into the water column. Compared with the phytoplankton regime, the free-floating plant regime had markedly lower chlorophyll, lower alpha, higher beta (high heterogeneity among habitats with and without macrophytes), and similar gamma diversity.

Colonization and Succession of Zooplankton After a Drought: Influence of Hydrology and Free-Floating Plant Dynamics in a Floodplain Lake

We analyzed zooplankton colonization and succession following a temporary drought in a vegetated lake from a floodplain wetland of the Lower Paraná River (Argentina). We took samples of zooplankton and phytoplankton at four habitats of the lake characterized by the presence-absence of emergent or free-floating macrophytes along a 18-month period after the drought. Microzooplankton dominated along the study and macrozooplankton showed seasonal increments in cold seasons. Temporal patterns of broad taxonomic zooplankton groups were associated to hydrology-driven factors, like changes in water depth, conductivity and edible phytoplankton and seasonal changes and not related with free-floating plants. Nevertheless, spatial and temporal patterns of species composition were closely linked to the dynamics of free-floating plants. A clear shift from pelagic to littoral zooplankton taxa was registered when free-floating plants colonized. This study expands the knowledge on the composition and dynamics of zooplankton from floodplain lakes after the disturbance caused by a severe drought. Our results highlight the influence of water level fluctuations as drivers of zooplankton abundance and the joint effect of seasonal changes and free-floating plant dynamics on zooplankton species composition and richness.