Genera Dolichospermum Research Articles

First Data on Cyanotoxins and Genes of Their Biosynthesis in the Phytoplankton of the Mesotrophic Lake Pleshcheyevo (Russia) During the Bloom Formation of Cyanobacterium <i>Gloeotrichia echinulata</i>

The article presents for the first time the data on cyanobacterial toxins and the genes of their biosynthesis in the phytoplankton of the mesotrophic Lake Pleshcheyevo Yaroslavl Region, during the period of cyanobacterium Gloeotrichia echinulata (Smith et Sowebry) Richter – bloom. In the phytoplankton of the lake, the presence of hepatotoxins microcystins was recorded using chromatography – mass spectrometry; in DNA isolated from plankton, the mcyE gene for the biosynthesis of these cyanotoxins was detected using PCR. During the study period, other types of cyanotoxins (cylindrospermopsin, anatoxin – a, saxitoxins) and the presence of genes for their synthesis in the phytoplankton were not identified. In thirty colonies of G. echinulata isolated from the lake, the microcystin biosynthesis genes mcyA and mcyE were absent, which is consistent with their inability to produce cyanotoxin. Using molecular methods, the potential ability to biosynthesize microcystins in Microcystis aeruginosa and species of the genus Dolichospermum inhabiting in the lake was demonstrated. The paper discusses the toxicity of Gloeotrichia echinulata and the need for further long-term monitoring of toxigenic cyanobacteria in Lake Pleshcheyevo.

First Data on Cyanotoxins and Genes of Their Biosynthesis in the Phytoplankton of the Mesotrophic Lake Pleshcheyevo (Russia) during the Bloom Formation of Cyanobacterium Gloeotrichia echinulata

Data on cyanobacterial toxins and the genes of their biosynthesis in the phytoplankton of the mesotrophic Lake Pleshcheyevo (Yaroslavl oblast) during the blooming period of cyanobacterium Gloeotrichia echinulata (Smith et Sowebry) Richter are given for the first time. In the phytoplankton of the lake, the presence of hepatotoxins of microcystins (MC) was recorded using chromatography–mass spectrometry; in DNA isolated from plankton, the mcyE gene for the biosynthesis of these cyanotoxins was detected using PCR. No other types of cyanotoxins (cylindrospermopsin, anatoxin-a, and saxitoxins) or the presence of genes for their synthesis in the phytoplankton are identified during the study period. MC biosynthesis genes mcyA and mcyE were absent in thirty colonies of Gloeotrichia echinulata isolated from the lake, which is consistent with their inability to produce cyanotoxin. Using molecular methods, the potential ability to biosynthesize MC in Microcystis aeruginosa and species of the genus Dolichospermum inhabiting in the lake is demonstrated. This paper discusses the toxicity of Gloeotrichia echinulata and the need for the further long-term monitoring of toxigenic cyanobacteria in Lake Pleshcheyevo.

Taxonomic and functional metagenomic assessment of a Dolichospermum bloom in a large and deep lake south of the Alps.

Untargeted genetic approaches can be used to explore the high metabolic versatility of cyanobacteria. In this context, a comprehensive metagenomic shotgun analysis was performed on a population of Dolichospermum lemmermannii collected during a surface bloom in Lake Garda in the summer of 2020. Using a phylogenomic approach, the almost complete metagenome-assembled genome obtained from the analysis allowed to clarify the taxonomic position of the species within the genus Dolichospermum and contributed to frame the taxonomy of this genus within the ADA group (Anabaena/Dolichospermum/Aphanizomenon). In addition to common functional traits represented in the central metabolism of photosynthetic cyanobacteria, the genome annotation uncovered some distinctive and adaptive traits that helped define the factors that promote and maintain bloom-forming heterocytous nitrogen-fixing Nostocales in oligotrophic lakes. In addition, genetic clusters were identified that potentially encode several secondary metabolites that were previously unknown in the populations evolving in the southern Alpine Lake district. These included geosmin, anabaenopetins, and other bioactive compounds. The results expanded the knowledge of the distinctive competitive traits that drive algal blooms and provided guidance for more targeted analyses of cyanobacterial metabolites with implications for human health and water resource use.

Elevated CO2 significantly increases N2 fixation, growth rates, and alters microcystin, anatoxin, and saxitoxin cell quotas in strains of the bloom-forming cyanobacteria, Dolichospermum

The effect of rising CO2 levels on cyanobacterial harmful algal blooms (CHABs) is an emerging concern, particularly within eutrophic ecosystems. While elevated pCO2 has been associated with enhanced growth rates of some cyanobacteria, few studies have explored the effect of CO2 and nitrogen availability on diazotrophic (N2-fixing) cyanobacteria that produce cyanotoxins. Here, the effects of elevated CO2 and fixed nitrogen (NO3−) availability on the growth rates, toxin production, and N2 fixation of microcystin, saxitoxin, and anatoxin-a - producing strains of the genus Dolichospermum were quantified. Growth rates of all Dolichospermum spp. were significantly increased by CO2 or both CO2 and NO3− with rates being highest in treatments with the highest levels of CO2 and NO3−for all strains. While NO3− suppressed N2 fixation, diazotrophy significantly increased when NO3−-enriched Dolichospermum spp. were supplied with higher CO2 compared to cultures grown under lower CO2 levels. This suggests that diazotrophy will play an increasingly important role in N cycling in CO2-enriched, eutrophic lentic systems. NO3− significantly increased quotas of the N-rich cyanotoxins, microcystin and saxitoxin, at ambient and enriched CO2 levels, respectively. In contrast, elevated CO2 significantly decreased cell quotas of microcystin and saxitoxin, but significantly increased cell quotas of the N-poor cyanotoxin, anatoxin. N2 fixation was significantly negatively and positively correlated with quotas of N-rich and N-poor cyanotoxins, respectively. Findings suggest cellular quotas of N-rich toxins (microcystin and saxitoxin) may be significantly reduced, or cellular quotas of N-poor toxins (anatoxin) may be significantly enhanced, under elevated CO2 conditions during diazotrophic cyanobacterial blooms. Finally, in the future, ecosystems that experience combinations of excessive N loading and CO2 enrichment may become more prone to toxic blooms of Dolichospermum.

Characterization of Potential Threats from Cyanobacterial Toxins in Lake Victoria Embayments and during Water Treatment.

Africa’s water needs are often supported by eutrophic water bodies dominated by cyanobacteria posing health threats to riparian populations from cyanotoxins, and Lake Victoria is no exception. In two embayments of the lake (Murchison Bay and Napoleon Gulf), cyanobacterial surveys were conducted to characterize the dynamics of cyanotoxins in lake water and water treatment plants. Forty-six cyanobacterial taxa were recorded, and out of these, fourteen were considered potentially toxigenic (i.e., from the genera Dolichospermum, Microcystis, Oscillatoria, Pseudanabaena and Raphidiopsis). A higher concentration (ranging from 5 to 10 µg MC-LR equiv. L−1) of microcystins (MC) was detected in Murchison Bay compared to Napoleon Gulf, with a declining gradient from the inshore (max. 15 µg MC-LR equiv. L−1) to the open lake. In Murchison Bay, an increase in Microcystis sp. biovolume and MC was observed over the last two decades. Despite high cell densities of toxigenic Microcystis and high MC concentrations, the water treatment plant in Murchison Bay efficiently removed the cyanobacterial biomass, intracellular and dissolved MC to below the lifetime guideline value for exposure via drinking water (<1.0 µg MC-LR equiv. L−1). Thus, the potential health threats stem from the consumption of untreated water and recreational activities along the shores of the lake embayments. MC concentrations were predicted from Microcystis cell numbers regulated by environmental factors, such as solar radiation, wind speed in the N–S direction and turbidity. Thus, an early warning through microscopical counting of Microcystis cell numbers is proposed to better manage health risks from toxigenic cyanobacteria in Lake Victoria.

Interannual variability of water quality conditions in the Nyanza Gulf of Lake Victoria, Kenya

The Nyanza Gulf of Lake Victoria is considered eutrophic with degrading water quality, as shown by harmful algal blooms formed by cyanobacteria of the genus Microcystis. The dilution of gulf water occurs mainly through the wide Rusinga Channel, whereas the much narrower and shallower Mbita Channel was reopened in May 2017. Specific conductivity and phytoplankton biovolume composition were monitored monthly from July 2017 to July 2018 at three stations and compared with records during 2008 – 2009, when higher eutrophication was observed, in the west gulf (WG, closely located to the Mbita Channel), the mid gulf (central part of the gulf), and the east gulf (EG, close to Kisumu City). As previously observed, conductivity showed a distinct increase from the WG to the EG but compared with records 2008–2009 it had decreased. Water level fluctuations, resulting from wind possibly causing water exchange with the main basin, were related to the decrease in conductivity at the EG. When all three stations were compared, the environmental variable transparency was positively related to most phytoplankton genera, whereas the genera Dolichospermum, Microcystis, and Aulacoseira were related to eutrophication. Compared with the period 2008–2009, the proportion of Microcystis decreased, while the proportion of Aulacoseira increased. Compared with the period 2008–2009, the observed dilution at the EG was primarily related to the overall increased water level in the main basin. The observed dilutional influence might have led to a decline in nuisance phytoplankton and overall improved water quality conditions.

&lt;p&gt;&lt;strong&gt;True branching and phenotypic plasticity in the planktonic cyanobacterium &lt;em&gt;Dolichospermum brachiatum sp. nov.&lt;/em&gt; (Nostocales, Aphanizomenonaceae), from south-eastern Australia&lt;/strong&gt;&lt;/p&gt;

Phenotypic plasticity in the cyanobacteria provides adaptive strategies to cope with changes in prevailing environmental conditions. Plasticity may manifest in morphological, physiological, or behavioural responses. Determining if variable phenotypes are truly novel expressions evolved via adaptive genetic change, rather than intrinsic characteristics within the spectrum of intraspecific diversity has rarely been studied. Poorly characterised or unrecognised intraspecific variability is as problematic as cryptic diversity in terms of imposing limitations on the use of morphology-based identifications to recognise algal diversity. We isolated strains from a population of filamentous cyanobacteria from the plankton of a temperate lake in Western Victoria, Australia putatively identified as the cylindrospermopsin producer Chrysosporum ovalisporum which displayed aberrant vegetative and apical cells and akinete morphology, and T-type true branching. In culture, strains grown in phosphorus-free media showed morphology and true branching consistent with the wild types suggesting that these features are a phenotypic response to P-limitation. Results of the phylogenetic analyses of these strains using the 16S-23S rRNA, and the secondary structure of the ITS region of the 16S-23S rRNA operon confirmed that the isolated strains were a novel species, here designated Dolichospermum brachiatum. This is the first record of true branching in the genus Dolichospermum and only the second observation from the Aphanizomenonaceae. These findings support our current understanding that true branching in cyanobacteria is a polyphyletic trait, having arisen several times in the evolution of this Phylum.

Trace metal and nitrogen concentrations differentially affect bloom forming cyanobacteria of the genus Dolichospermum

The role of nitrogen and phosphorus in promoting cyanobacterial blooms has been researched for many decades. In contrast, much less is known about the effect of trace metals on regulating cyanobacterial bloom formation. This is particularity relevant for nitrogen-fixing species, as some trace metals act as co-factors in nitrogenase and nitrite reductase enzymes. In the present study, we assessed the response of two nitrogen-fixing species, Dolichospermum lemmermannii and Dolichospermum planctonicum, to copper (Cu), cobalt (Co), molybdenum (Mo), iron (Fe) and nitrogen (N) using a culture-based approach. Response surface modelling showed growth rates in D. planctonicum were positively influenced by increased Co and Fe (p < 0.05), whereas Fe (p < 0.001) and Mo (p < 0.05) increased growth rates in D. lemmermannii. Heterocyst frequency in D. planctonicum was associated with nitrogen (negative response, p < 0.001) and all trace metals tested (positive; p < 0.001 or < 0.05) except Mo. Nitrogen had a strong negative effect (p < 0.001), and Fe a positive (p < 0.01) impact on heterocyst frequency in D. lemmermannii. These results indicate that in addition to regulating nitrogen and phosphorus inputs, management efforts should consider minimising trace metals from entering waterbodies.

THE BLOOM AND TOXICITY OF CYANOBACTERIA IN LAKE SEVAN

Seasonal hydrochemical, hydrophysical, and phytoplankton studies were conducted in a littoral zone of Lake Sevan in 2019. It is known that in recent years widespread algal blooms were usually started from the littoral zone of Big Sevan, and then spread towards the pelagic zone of the lake. The bloom was mainly caused by species belonging to the genus Dolichospermum (previously Anabaena). Our study has revealed that the physical and chemical properties of water were changed as a result of algal bloom. Particularly, the transparency of water and concentration of dissolved oxygen decreased, while the concentrations of ammonium, nitrite and phosphate ions increased. For the first time, the presence of the toxins microcystin and anatoxin-a was recorded in the lake.

Protected Freshwater Ecosystem with Incessant Cyanobacterial Blooming Awaiting a Resolution

For 50 years persistent cyanobacterial blooms have been observed in Lake Ludoš (Serbia), a wetland area of international significance listed as a Ramsar site. Cyanobacteria and cyanotoxins can affect many organisms, including valuable flora and fauna, such as rare and endangered bird species living or visiting the lake. The aim was to carry out monitoring, estimate the current status of the lake, and discuss potential resolutions. Results obtained showed: (a) the poor chemical state of the lake; (b) the presence of potentially toxic (genera Dolichospermum, Microcystis, Planktothrix, Chroococcus, Oscillatoria, Woronichinia and dominant species Limnothrix redekei and Pseudanabaena limnetica) and invasive cyanobacterial species Raphidiopsis raciborskii; (c) the detection of microcystin (MC) and saxitoxin (STX) coding genes in biomass samples; (d) the detection of several microcystin variants (MC-LR, MC-dmLR, MC-RR, MC-dmRR, MC-LF) in water samples; (e) histopathological alterations in fish liver, kidney and gills. The potential health risk to all organisms in the ecosystem and the ecosystem itself is thus still real and present. Although there is still no resolution in sight, urgent remediation measures are needed to alleviate the incessant cyanobacterial problem in Lake Ludoš to break this ecosystem out of the perpetual state of limbo in which it has been trapped for quite some time.

Climate variability promotes unprecedented cyanobacterial blooms in a remote, oligotrophic Ontario lake: evidence from paleolimnology

Dickson Lake, located in Algonquin Provincial Park, Ontario, is a remote, oligotrophic lake, where cyanobacterial blooms of the genus Dolichospermum (Ralfs ex Bornet & Flahault) P. Wacklin, L. Hoffmann and J. Komarek were reported for the first time in the fall of 2014, and subsequently in the late spring of 2015. To investigate the potential environmental triggers of these bloom events, we assessed long-term trends in water quality using a multi-proxy paleolimnological approach, examining sedimentary diatoms, chironomids, cladocerans, spectrally inferred chlorophyll a, and cyanobacterial akinetes preserved in a 210Pb-dated sediment core. Assemblage changes were modest in all biological proxies. A subtle increase in the abundance of warm-water chironomid taxa (Topt > 15 °C) commences in the year ~ 2000, with further increases in the most recent years of the sediment record (~ 2013–2015). End-of-summer volume-weighted hypolimnetic oxygen concentrations (CI-VWHO), inferred from chironomid remains, reveal a decline in oxygen concentrations over the last two decades coincident with the highest levels of sedimentary chlorophyll a and cyanobacterial akinetes in the sediment record. These paleolimnological findings corroborate observed reports of the onset of cyanobacterial blooms in Dickson Lake in late 2014 and are consistent with increasingly favourable bloom-forming conditions over the past few decades that are related to warmer air temperatures, sharp declines in wind speed, and a lengthening of the ice-free season by 2 weeks since 1975. It is plausible that late ice-out and a quick onset to stratification in 2014 may have resulted in incomplete spring mixing, early onset of hypolimnetic anoxia, and increased internal nutrient loading, that, occurring during a period when climate conditions were particularly ideal for cyanobacterial proliferation, may have fueled the unprecedented algal blooms in this remote lake. Collectively, the factors causing algal blooms in remote lakes such as Dickson Lake are not yet fully understood, and it is worrisome that with continued warming the triggering conditions may become a more common feature of Algonquin Park and other minimally impacted Boreal Shield lakes in the coming years.

Multi-year study of phytoplankton population in the Dnistrovskiy estuary in summer (2003-2018)

Purpose. Comparison of multi-year data on species composition, biodiversity and quantitative variables of phytoplankton that developed in the Dnistrovskiy Estuary every summer. Particular attention was focused on the development of potentially dangerous (toxic) species, which can deteriorate quality of water and threaten hydrobionts’ existence. Methods. The data used in the phytoplankton study have been collected from 22 stations in the Dnistrovskiy Estuary (456 samples). Phytoplankton sampling and analyses were carried out using standard methodologies. Nageotta и Sedgewick cameras (0,05 and 1 ml) were applied to count cell number. The samples were preserved using formalin. HUND-H600 and OLIMPUS-BH2 microscopes were used for sample analyses. Classification of microalgae and Cyanobacteria was done according to the standards of the international database. Results. The total number of species registered during 16 years exceeded 445 (13 taxa) out of which 54 are generally classified as potentially harmful species (genera Dolichospermum, Aphanizomenon, Microcystis, Cyclotella, Pseudo-nitzschia, Peridinium, Prorocentrum etc.). Minimum record of species number within 4 main taxa was in 2004 and 2016. Maximum number of species of Chlorophyta (75) was observed in 2012, and maximum of Bacillariophyta (53), Dinophyta (19) and Cyanobacteria (33) was in 2014. Northern and middle part of the estuary had the highest species diversity (Shannon index = 4.6 – 4.4 bit∙cells-1) i.e. the areas where flora of various geneses mixed. Over the years, number of species and α-diversity of phytoplankton tend to increase in the north and in the centre of the estuary. The range of changes of maximum values of microalgae biomass during 16-year period was from 2.5 g·m–3 (2010) to 231.2 g·m–3 (2018). Algal blooms were recorded with Bacillariophyceae and Cyanobacteria as dominants. Conclusions. There is a certain 12-year periodicity in gradual changes in the number of species and Shannon index within the main phytoplankton taxa. Recent years’ trends toward increase in the total biomass of microalgae and Cyanobacteria have been traced as well, the consequence of such an increase being deterioration of water quality which has downgraded to "polluted" or "extremely polluted" in those regions (as per national standards).

Cyanobacterial bloom in the world largest freshwater lake Baikal

Lake Baikal is a UNESCO World Heritage Site and holds 20% of the world’s freshwater reserves. On July 26, 2016, a cyanobacterial bloom of a green colour a few kilometers in size with a bad odor was discovered by local people in the Barguzinsky Bay on the eastern shore of Lake Baikal. Our study showed very high concentration of chlorophyll a (41.7 g/m3) in the sample of bloom. We found that the bloom was dominated by a nitrogen-fixing heterocystous cyanobacteria of the genus Dolichospermum. The mass accumulation of cyanobacteria in the lake water with an extremely high chlorophyll a concentration can be explained by a combination of several factors: the discharge of biologicaly-available nutrients, including phosphorus, into the water of Lake Baikal; low wind speed and weak water mixing; buoyant cyanobacterial cells on the lake surface, which drifted towards the eastern coast, where the maximum concentration of chlorophyll a was recorded. In the center of the Barguzinsky Bay and in the open part of Lake Baikal, according to satellite data, the chlorophyll a concentration is several orders of magnitude lower than at the shoreline.

Variations of microcystins in freshwater ecosystems

Abstract Koreivienė J., Belous O., Kasperovičienė J., 2012: Variations of microcystins in freshwater ecosystems [Mikrocystinai gėlavandenėse ekosistemose]. - Bot. Lith., 19(2): 139-148 Increased frequency, severity of harmful algae blooms and their extent worldwide have become a global challenge due to the production of toxins that are released to the water. Cyanotoxins are detected in 25-75% of blooms. Hazardous hepatotoxin-microcystin potential producers, spatial and temporal variations of toxins as well as their variations depending on environmental variables are discussed in this overview. The most common species among microcystin producers belong to the genera Dolichospermum and Microcystis. Variations of the amount of microcystins detected through the bloom are associated with the dominant cyanobacteria species or its genotype. The abundance of toxic cyanobacteria genotype and cyanotoxin values increase with the rise of water temperature and nutrient concentrations in the freshwaters. On the seasonal basis, cell-bound microcystin concentrations increase with bloom development, whereas extracellular cyanotoxin values rise with the senescing of bloom after cyanobacterial cell lysis.

Morphological and molecular studies of Sphaerospermopsis torques-reginae (Cyanobacteria, Nostocales) from South American water blooms

Werner V.R., Laughinghouse IV H.D., Fiore M.F., Sant'Anna C.L., Hoff C., Santos K.R.d.S., Neuhaus E.B., Molica R.J.R., Honda R.Y. and Echenique R.O. 2012. Morphological and molecular studies of Sphaerospermopsis torques-reginae (Cyanobacteria, Nostocales) from South American water blooms. Phycologia 51: 228–238. DOI: 10.2216/11-32.1Sphaerospermopsis torques-reginae (Komárek) Werner, Laughinghouse IV, Fiore & Sant'Anna comb. nov. was originally described as Anabaena torques-reginae Komárek from planktonic populations of Cuban eutrophic environments, characterized by twisted trichomes with spherical akinetes adjacent to the heterocytes. Recently, using molecular analyses, all planktonic Anabaena Bory ex Bornet & Flahault morphospecies were transferred into the genus Dolichospermum (Ralfs ex Bornet & Flahault) Wacklin et al., including Dolichospermum torques-reginae (Komárek) Wacklin et al. However, by a polyphasic characterization of strains of Anabaena reniformis Lemmermann and Aphanizomenon aphanizomenoides (Forti) Horecká & Komárek (=Anabaena aphanizomenoides Forti), these planktonic species were reclassified into Sphaerospermopsis Zapomělová et al. Our study's main objective was to characterize morphologically and molecularly cyanobacterial populations identified as Dolichospermum torques-reginae, observed in different aquatic ecosystems in South America. The 16S rRNA gene of two Dolichospermum torques-reginae strains (ITEP-024 and ITEP-026) was sequenced and phylogenetically analyzed for the first time. The morphological and phylogenetic analyses demonstrated the affiliation of the studied populations with the genus Sphaerospermopsis and, consequently, were denominated as Sphaerospermopsis torques-reginae. Furthermore, geographic distribution, ecology, and toxicity of the species are discussed. It was observed in different aquatic environments, natural and artificial, tropical and subtropical in Brazil, temperate in Argentina, and tropical in Colombia, suggesting a wide distribution in South America. It normally occurred in dense freshwater blooms, although it was also found in water with low salinity. Sphaerospermopsis torques-reginae toxic blooms have been reported in tropical water bodies in northeastern Brazil.

POLYPHASIC CHARACTERIZATION OF DOLICHOSPERMUM SPP. AND SPHAEROSPERMOPSIS SPP. (NOSTOCALES, CYANOBACTERIA): MORPHOLOGY, 16S rRNA GENE SEQUENCES AND FATTY ACID AND SECONDARY METABOLITE PROFILES(1).

The genera Dolichospermum (Ralfs ex Bornet et Flahault) Wacklin, L. Hoffm. et Komárek and Sphaerospermopsis Zapomělová, Jezberová, Hrouzek, Hisem, K. Řeháková et Komárk.-Legn. represent a highly diversified group of planktonic cyanobacteria that have been recently separated from the traditional genus Anabaena Bory ex Bornet et Flahault. In this study, morphological diversity, phylogeny of the 16S rRNA gene, production of fatty acids, and secondary metabolite profiles were evaluated in 33 strains of 14 morphospecies isolated from the Czech Republic. Clustering of the strains based on 16S rRNA gene sequences corresponded to wider groups of species in terms of morphology. The overall secondary metabolite and fatty acid profiles, however, were not correlated to each other and neither were they correlated to the 16S rRNA phylogeny nor the morphology of the strains. Nevertheless, a minor part of the detected secondary metabolites (19% of all compounds) was present only in close relatives and can be thus considered as autapomorphic features.

Planktic Cyanobacteria in the Lower Uruguay River, South America.

The Uruguay River is the second most important river in the Río de la Plata Basin. Taxonomical composition, abundance and distribution of cyanobacteria collected at nine sampling stations in the Lower Uruguay River (Uruguay) were analyzed seasonally from 2006 to 2009. A total of 24 taxa were identified, including 13 Chroococcales, 4 Oscillatoriales and 7 Nostocales species. The genera Dolichospermum and Microcystis presented the highest number of species among the planktic water bloom-forming cyanobacteria. The highest densities of cyanobacteria were recorded in summer during a bloom, with 6.2×106 cells.ml-1, and the most abundant species were Microcystis aeruginosa and Dolichospermum cf. pseudocompactum. In this case, the toxicity analyses by HPLC did not indicate the presence of microcystin-LR. Phytoplankton growth in the Uruguay River was found not to be nutrient-limited. The high correlation of cyanobacteria densities with nitrogen and phosphorous compounds is directly related to changes in flow. Cyanobacteria densities increased with summer high temperatures in low flow conditions. The ANOSIM analysis showed no significant differences between zones and sampling sites, but there were temporal significant differences in relation to seasonal samplings. Radiocystis fernandoi Komárek et Komarková-Legnerová was recorded for the first time in Uruguay and Dolichospermum cf. pseudocompactum (Watanabe) Wacklin et al. was recorded for the first time in South America.