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In riverine floodplains, the hydrological connectivity of the aquatic habitats with the main channel is paramount for aquatic metacommunities, as it influences organism dispersal and the local environmental conditions. We assessed phytoplankton, rotifer and microcrustacean metacommunity structures and species richness along an empirical gradient of hydrological connectivity of the aquatic habitats with the main channel and compared them between high and low water levels of the main river. In addition to exploring the relative influence of hydrological connectivity, local environmental and spatial factors on metacommunity composition and the relationship between species richness and environmental and hydrological connectivity variables during both hydrological conditions. During high water levels, when high nutrient and suspended solid concentrations occurred in sites with higher hydrological connectivity and sites with opposite features and high macrophyte cover occurred in sites with lower connectivity, metacommunities showed Quasi-Nested structures (species-poor sites were subsets of species-richer sites). During low water levels, the environmental variation of the water bodies was not associated to the connectivity gradient and metacommunities showed Clementsian or Quasi-Clementsian structures: discrete groups of species replaced each other along the floodplain. Spatial factors generally were the most important for community composition; during high water levels the flood contributed to the spatial effect by transporting organisms among neighbor water sections, and during low waters it indicated dispersal limitation and/or unmeasured variables that are spatially structured. Species richness was not associated to the gradient of hydrological connectivity, no matter the hydrological condition, and showed stronger association with local factors during low than during high water levels, indicating a better match between organisms and the environment in the absence of recent floods. Our results show that the discharge regime of the river influences plankton metacommunities and their relationship with local environmental factors. This highlights the relevance of natural river water level fluctuations for aquatic communities in the present context of increasing river flow regulation worldwide. [ABSTRACT FROM AUTHOR]

Strobel Plateau hosts more than 1,500 shallow lakes displaying different water regimes, which constitute the habitat for many species. Although the region is naturally fishless, many of the lakes were stocked with trout, bearing uncertainty about the possible effects on the ecosystem. The main objective of this study was to analyze the characteristics of planktonic autotrophic communities of lakes differing in regime (phytoplankton turbid, clear vegetated, and clear unvegetated) and presence/absence of fish. During late spring and summer, 14 water bodies were sampled in 2011 and 2013 considering different regimes and presence/absence of fish. Besides limnological variables, the autotrophic communities, from pico to microplankton, were also analyzed. Differences in physical and chemical characteristics observed among the lakes corresponded to their current regime and the presence/absence of trouts. Autotrophic picoplankton and phytoplankton > 20 µm abundances differed among lake types being highest in fish-stocked lakes. Although the three type of lakes presented phycoerythrin-rich picocyanobacteria and picoeukaryotes, only fish-stocked lakes hosted phycocyanin-rich picocyanobacteria. Moreover, fish-stocked lakes were dominated by cyanobacteria, while chlorophytes abounded in fishless systems. Evidences that lake regime and fish occurrence influence planktonic autotrophic communities of Strobel plateau is here provided, highlighting the intra- and interannual dynamism of the aquatic systems. [ABSTRACT FROM AUTHOR]

Heterocytous Cyanobacteria show high trait variation at the cellular, organismal, and population levels. Members of this group can produce specialized cells such as akinetes and heterocytes that influence their ecology, including bloom development and population survival. This study characterizes patterns of variation in the traits of these species, including the traits of specialized cells, to expand our ecological knowledge and predictive capacity for this group. We compiled and synthesized morphological and physiological traits of planktic heterocytous Cyanobacteria from the published literature and experiments, and assessed trait distributions, trait relationships, and their similarities among species. Although the volumes of akinetes and heterocytes were positively related to that of vegetative cells, the shape of cells differed in ways that may reflect their function, and the position of heterocytes within filaments may relate to growth rate. Maximum growth rates differed significantly among genera, yet surprisingly did not correlate with cell volume. Also, despite the high energetic cost of N fixation in low N conditions, our results suggest that growth rate seems unrelated to nitrogen availability. The degree of trait variation within heterocytous Cyanobacteria, which suggests the existence of three functionally distinct subgroups, may offer new insights into which taxa dominate bloom assemblages under different conditions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We investigated nine Pampean shallow lakes (Argentina) across a moisture gradient (mean annual precipitation 1,000-500 mm), under the framework of a network project of lake monitoring whose main objective was to analyse the responses of ecosystems to different stressors associated with human activities. We postulate that species bloom in different shallow lakes will be determined mainly by local optical features, nutrients and salinity. We analysed the phytoplankton structure during the warm season 2012-2013, identifying those species and functional groups that proliferate in the lakes. Most lakes showed high phytoplankton biomass, except a saline lake located in the driest part of the gradient and an inorganic turbid lake. In some lakes, cyanobacteria blooms occurred in summer, persisting until autumn. Small colonial cyanobacteria and small fast-growing chlorophytes were abundant in most turbid enriched and anthropogenically impacted systems. Lowest species and functional diversity occurred in the saline lake. Multivariate analysis indicated that the phytoplankton structure was mainly defined by conductivity, transparency and variables associated with the trophic state, also reflecting the importance of the geographical location. Our results evidenced the vulnerability of most shallow lakes in the region to water level fluctuations, eutrophication and canalization. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In order to assess the effects of the introduced bivalve Limnoperna fortunei on water-column properties of Salto Grande reservoir, experiments were conducted using six 400 L mesocosms: 2 with 100 mussels, 2 with 300 mussels, and 2 controls (without mussels). At 0, 1, 2, 3, 7, 14, 21, 28, and 35 days we measured nutrient and chlorophyll a concentrations, counted and identified the phytoplankton, and estimated the density, size, and number of cells of the colonies of Microcystis spp. Cumulative periphyton growth and total accumulated sediments were assessed in all enclosures at the end of the experiment. Throughout the experiment, in the controls ammonia and phosphates dropped to near zero, whereas in the mesocosms with L. fortunei they increased two- to tenfold. Nitrates decreased in all mesocosms. In the presence of the mussel, chlorophyll a and algal cells dropped until day 3 increasing thereafter, whereas in the controls they increased from day 0. Periphyton growth and sediment accumulation were significantly higher in the mesocosms with mussels that in the controls. Cell density, proportion of colonial cells and colony size of Microcystis spp. increased in all enclosures, but these increases were dramatically (and very significantly) higher in enclosures with 100 and, especially, with 300 mussels, than in the controls. Our results indicate that L. fortunei modifies nutrient concentrations and proportions, and promotes aggregation of solitary Microcystis spp. cells into colonies; both these effects can favor blooms of this often noxious cyanobacteria. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Phosphorus has been traditionally regarded as the controlling nutrient for phytoplankton growth, however, N-limitation is likely to occur in several environments. For example, nitrogen is considered the main nutrient limiting phytoplankton in floodplain lakes of the Paraná River basin. However, N2-fixing heterocystous cyanobacteria (N2-cyano) are usually absent in these water bodies. The low pH values frequently found may limit the development of these algae. We hypothesise that long-term lake isolation allows conspicuous phytoplankton growth during summer, resulting in high photosynthetic rates and pH. This scenario combined with low DIN (dissolved inorganic nitrogen) would favour the development of N2-cyano. Phytoplankton composition was studied during 16 months in a vegetated and isolated floodplain lake in the Paraná basin. Additionally, pH was artificially increased in in situ mesocosms to test effects on phytoplankton structure. Lake phytoplankton was dominated by flagellates (cryptophytes and euglenophytes) and small coccoid algae (chlorophytes and colonial cyanobacteria). Algal biomass was highest during warm periods. Although pH increased up to 8.8 during the second summer period, N2-cyano remained rare, most likely because of the high DIN concentration recorded. The alkalophilic diatom Cyclotella meneghiniana dominated and was positively correlated with pH. Conversely, PO concentrations in the mesocosm experiment were high and DIN remained relatively low. pH enhancement in the treated mesocosms (avg. pH = 8.2) promoted the development of N2-cyano ( Anabaena spp.) and C. meneghiniana, which after 1 month of incubation accounted together for 50% of the biomass in contrast to less than 1.6% in control containers. Our results support the hypothesis that during the warm season and under low DIN concentration, high pH favours N2-cyano growth in these lakes. We provide new evidence supporting the idea that even under optimal nutrient conditions, N2-cyano do not thrive unless other requirements are satisfied. [ABSTRACT FROM AUTHOR]

The interplay of the geochemistry of the lower stretch of the Luján River is analyzed with its natural geomorphology (basin features), local hydrometeorology (tidal regime and “sudestada” events) and the impact of polluted tributaries. Major ions, dissolved heavy metals and limnological variables were analyzed using multivariate techniques. The water quality of the mainstream of the Luján River, flowing through urbanized and industrialized areas is strongly and positively influenced by the input of the Paraná River through deltaic watercourses and is negatively impacted after receiving the discharge from polluted watercourses. The longitudinal spatial variations evidenced major discontinuities in the Lower Luján Basin, showing clearly the riverine and the deltaic water influenced zones. Seasonal variations were also marked and were either temperature driven or associated with the estuarine cycle and the “sudestada”. [ABSTRACT FROM AUTHOR]

This study analyses the influence of the underwater light climate on the morphometric characteristics of the phytoplankton at the population and community levels. The differences in light conditions across the floodplain were mainly defined by the patchiness of floating macrophytes and humic acids concentration. A morphometric response at the community level to the underwater PAR was registered. Sites with strong light constraints were characterised by non-flagellated organisms or with a small proportion of unicellular flagellates. Short organisms (<10 μm) with a unit volume of less than 1,000 μm3 and a high surface:volume ratio (S/V >2) were the morphotypes related to poorly illuminated environments. Moreover, the organisms showed forms more slender under these limiting conditions. This pattern was different to that registered in well-illuminated sites where longer and larger organisms, with a smaller S/V and frequently flagellated, coexisted with the previously mentioned organisms. The autotrophic picoplankton, the smallest phytoplankton fraction, revealed lower abundances at sites with higher humic substances. Short term morphological changes were additionally studied for the dominant species by means of mesocosm experiments simulating different light climates. Intraspecific morphological plasticity was observed with respect to the filament length and the vacuolization of cells. [ABSTRACT FROM AUTHOR]

This study analyzes the phytoplankton structure and dynamics, as well as the main limnological characteristics in Los Coipos Lake, located in the Reserva Ecológica Costanera Sur (Buenos Aires, Argentina). Phytoplankton generic composition was studied before (April-June 1998), during (July 1998-December 1999), and after (January-October 2003) pumping from the Puelche aquifer waters, characterized by extremely high conductivity values. The capacity of natural recovery from salt contamination was analyzed. The 1998-1999 parameters were obtained from a technical database (Aguas Argentinas water supply company), while two-monthly samples were collected and analyzed between January and October 2003. Decrease and replacement of genera was related to the conductivity increase; recovery of the number of genera was recorded once the input ceased. Chlorophyceae and Cyanophyceae were responsible for the greatest phyto-plankton densities. During ground-water input, four high phytoplankton density events were recorded. One hundred fifty-three infrageneric phytoplanktonic and tichoplanktonic taxa were registered during 2003, together with high total phytoplankton densities. Phytoplankton structure and dynamics did not seem to be regulated by dissolved salt content in the post-pumping period. [ABSTRACT FROM AUTHOR]

This study deals with the variation in the algal assemblages across the transversal dimension of a wetland of the Lower Paraná River from October 1998 to September 1999. The relationship between the algal composition and the physico-chemical variables is analyzed by means of Canonical Correspondence Analysis. The abundant floating macrophytes generated a severe reduction in light penetration in the relictual oxbow lakes (ROLs), almost anoxic conditions and high values of P, N and K. Such characteristics accounted for their very particular algal flora dominated by Cyanobacteria and several diatoms, species probably mixotrophic regarding their capacity to grow in darkness and tolerate very low oxygen contents. In the shallow lake, the phytoplankton comprised many small autotrophic green algae, accompanied by many flagellates of the classes Cryptophyceae, Euglenophyceae and Dinophyceae. Our results indicate that the macrophyte cover was probably the stirring factor in the selection of algal species along the transitional zone comprising a floodplain shallow lake and several ROLs. [ABSTRACT FROM AUTHOR]

This study was conducted in a wetland on the Lower Paraná River (Argentina). Algal assemblages from a permanent shallow lake, three relict oxbow lakes (ROL) and an isolated small pond were sampled fortnightly during the spring-summer time of 1998–99. The algal assemblages strongly differed among the permanently vegetated ROLs and the shallow lake and small pond, both temporarily covered by floating macrophytes. Anoxic conditions and poor light penetration have been pointed out in previous studies to be the leading factors in the selection of algal species in this wetland. Two or three Cyanobacteria species and some large pennate diatoms dominated the algal biomass in the ROLs, whereas only the former taxa contributed significantly to density. Assemblages were characterised by their low diversity and species richness; the highest SIMI values between consecutive samplings were registered in the ROLs. Even though some characteristics of the Z and S2 Reynolds associations describe the ROLs assemblages, they do not strictly match with them. Changes in algal density and biomass were more pronounced in the shallow lake. An alternative dominance of Cryptomonas spp., Peridinium spp., Euglena spp., Monoraphidium spp., Oscillatoriales and Chroococcales occurred, with a consequent replacement of the Y, Lo, W1X1,S2, K, and F Reynolds associations. Variation in the macrophytic cover of the small pond accounted for the fluctuations in its algal composition. When the cover was complete, the assemblages resembled those of the ROLs, whereas when its surface was free of plants, they were more similar to those of the shallow lake. Steady state assemblages related to the persistence of a dense macrophyte cover may be found in wetlands in environments with constant and extreme ecological conditions such as those recorded in the ROLs. In contrast, `unpredictable' assemblages can be expected in shallow lakes with fluctuating conditions. [ABSTRACT FROM AUTHOR]

Due to an unfortunate turn of events, the first- and surnames of all authors were transposed in the original publication. The original article has been corrected and the proper representation of the authors' names and their affiliation is also listed here. [ABSTRACT FROM AUTHOR]

A correction to the article "Which environmental factors trigger the dominance of phytoplankton species across a moisture gradient of shallow lakes?" that was published in the previous issue is presented.