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Laguna del Sauce is a eutrophic shallow lake and the second drinking water source in Uruguay. This ecosystem suffers recurrent blooms of potentially toxic nitrogen-fixing cyanobacteria, mostly during summertime. Here we analyze environmental drivers and potential impacts of blooms of the toxic cyanobacteria Raphidiopsis raciborskii in Laguna del Sauce from 2003 to 2020, and assess its effects on governance and biomonitoring. We registered three blooms of R. raciborskii, in 2004, 2015, and 2020, the last two expressed saxitoxin (lack of toxicity information for 2004). The 2015 bloom occurred in autumn, reaching an absolute biovolume of 104 mm3 l−1, and had high saxitoxin concentrations (up to 9.8 μg l−1) which continued to be detected even after the bloom collapsed. This was the first saxitoxin register in this ecosystem and drastically affected the drinking water supply. After this massive bloom, a monitoring program was established which continues up to date. The 2020 bloom also occurred in autumn and lasted until winter; it was of lower magnitude (up to 6.3 mm3 l−1) and had lower saxitoxin concentration (up to 0.9 μg l−1) compared to the 2015 bloom. Blooms of R. raciborskii were associated with a combination of persistent low water level, low water color, and high water transparency, conditions that rarely coincide in this ecosystem. Hence, the programs which started after the 2015 bloom, allowed better characterizing the environmental drivers of R. raciborskii blooms in Laguna del Sauce. Furthermore, these implied adapting the ecosystem management and water treatment strategies, and strengthening of academia—stakeholders' interactions. [ABSTRACT FROM AUTHOR] more...

Heterocytous cyanobacteria are able to fix nitrogen (in heterocytes) and to produce dormant cells (akinetes). Heterocyte and akinete shape, size, and relative position have taxonomical relevance and possibly ecological value too. We collected-from literature and nature-and compared morphological data on vegetative cells, heterocytes, and akinetes across four genera taxonomically separated from Anabaena. In average, heterocyte size doubled that of vegetative cells-probably because of extra cell wall deposition. Heterocyte morphology was remarkably similar across genera, both in size and shape (spherical). The latter may decrease oxygen diffusion from adjoining vegetative cells. Akinetes were huge (one order of magnitude bigger) compared to vegetative cells, probably because of its massive genome replication, extra deposition of wall layers, allocation of storage and number of vegetative cells fused during akinete differentiation. Akinete shape was mostly cylindrical, or oval, but rarely spherical. In line with molecular data, we found morphological differences between Anabaena (non-aerotopated, soil or benthic) and Dolichospermum (aerotopated, planktonic), including vegetative cell size, and akinete size, shape, and relative position to the heterocyte. Differences may relate to adaptations to their contrasting environments (benthic versus planktic). Further research is needed to generalize our results to other heterocytous genera. [ABSTRACT FROM AUTHOR] more...

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] more...

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] more...

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] more...

Functional characteristics play an important role in shaping phytoplankton distributions. This approach can help to explain the success of tropical taxa in temperate ecosystems. We used functional groups (FG) and functional traits (FT) to assess the phytoplankton distribution in five temperate lakes exposed to tropical taxa dispersal. We ran redundancy analyses to assess the relationship between each functional classification and the environment. Both approaches showed similar variance of the phytoplankton biomass and responded to similar environmental variables. The FG approach separated lakes by trophic status (total phosphorus: TP), while the FT approach reflected the lakes' hydromorphology (conductivity, depth, mixing regime). The traits of motility and mixotrophy coincided with low mixing and high light. Tropical taxa were dominant in two lakes, rare in one and absent in two lakes. When dominant, tropical taxa exerted an overwhelming effect on phytoplankton biomass and community composition, and contributed to foam occurrences. The success of tropical taxa in only two lakes seems related to the interplay of their traits (accessory pigments, morphology) and the prevailing environmental filters (low light, deep mixing, and intermediate TP). Their rarity or absence in the other lakes evidences how traits can increase fitness in one environment but not in other. [ABSTRACT FROM AUTHOR] more...

The eco-physiological responses of three nitrogen-fixing cyanobacteria (N-fixing cyanobacteria), Aphanizomenon gracile, Anabaena minderi, and Ana. torques-reginae, to light were assessed under nutrient saturation. The N-fixing cyanobacteria were isolated into monocultures from a natural bloom in a shallow colored lake and their growth irradiance parameters and pigment composition were assessed. The different ecological traits related to light use (μmax, α, Ik) suggest that these N-fixing cyanobacteria are well adapted to low light conditions at sufficient nutrients, yet interspecific differences were observed. Aphanizomenon gracile and Anabaena minderi had high relative growth rates at low irradiances (ca. 70% of those in high light), low half saturation constant for light-limited growth ( Ik < 9.09 μmol photon m−2 s−1) and high efficiency ( α < 0.11 day−1 μmol photon−1 m2 s). Conversely, Ana. torques-reginae showed poorer light competitiveness: low relative growth rates at low irradiances (ca. 40% of those in high light), low α (0.009 day−1 μmol photon−1 m2 s) and higher Ik (35.5 μmol photon m−2 s−1). Final densities in Aphanizomenon gracile and Anabaena minderi reached bloom densities at irradiances above 30 μmol photon m−2 s−1 with different hierarchy depending on irradiance, whereas Ana. torques-reginae never achieved bloom densities. All species had very low densities at irradiances ≤17 μmol photon m−2 s−1, thus no N-fixing blooms would be expected at these irradiances. Also, under prolonged darkness and at lowest irradiance (0 and 3 μmol photon m−2 s−1) akinetes were degraded, suggesting that in ecosystems with permanently dark sediments, the prevalence of N-fixing cyanobacteria should not be favored. All species displayed peaks of phycocyanin, but no phycoeritrin, probably due to the prevailing red light in the ecosystem from which they were isolated. [ABSTRACT FROM AUTHOR] more...