The influence of environmental variables on planktonic biodiversity is widely known. However, the absence of information about the cyanobacterial community in tropical estuarine regions motivated this work, whose objective was to investigate the spatio-temporal variation of cyanobacterial density related to physicochemical factors in a Brazilian Amazonian estuary. For the qualitative and quantitative study of cyanobacteria and physicochemical variables, samples were collected in April/July/2009 and April/August/2010. We identified 31 species of the orders Chroococcales, Oscillatoriales and Nostocales. Species of the genera Aphanocapsa, Dolichospermum, Komvophoron, Microcystis, Pseudanabaena and Merismopedia were frequent and abundant throughout the study period. Some of the found genera have already been described as potential toxin producers. The dynamics of cyanobacteria were temporal, the highest densities occurred in 2010 (average= 1080.86 ± 702.86 cells.mL-1) mainly influenced by the high values of pH, temperature, electrical conductivity, total dissolved solids, ammonium nitrogen which led cyanobacteria to present different responses in terms of richness, density and diversity between the years.

Marine phycotoxins are organic compounds synthesized by some species of microalgae, which accumulate in the tissues of filter-feeder organisms such as bivalve mollusks. These toxins can cause acute intoxication episodes in humans, a severe threat to aquaculture and fisheries. In the State of Pará, Brazil, oyster farming has community, artisanal and sustainable bases, using mangroves as cultivation environment and seed banks. In small-scale production, there are often no established methods of safeguarding the health of consumers elevating the potential risks of shellfish poisoning outbreaks. Our study evaluated the presence of phycotoxins in oysters cultivated in five municipalities in the region of the Atlantic Amazon (Pará, Brazil) assessing the quality of the final product. We further evaluated the microalgae, water quality, and the spatio-temporal variation of physicochemical factors in the same area. Diatoms dominated the microalgae composition, followed by dinoflagellates, some of which are reported to be potentially toxic and producers of paralytic shellfish toxins. For the first time, we describe the occurrence of the potentially toxic dinoflagellate Ostreopsis sp. in the Amazon region. Furthermore, for the first time, toxins were detected in oyster farming in the northeast of the State of Pará, namely GTX2,3, STX, and dc-STX nevertheless, with nontoxic values. The identified toxins represent a potential threat to shellfish consumers.

The records of blooms and cyanotoxins in the Brazilian Legal Amazon are scarce and do not represent the reality observed in recent years when there was an increase in notifications and studies carried out in this region. In this article, we carefully analyzed 48 studies to identify the locations where the blooms occurred, the most frequent species, and the tests performed to detect cyanotoxins within the territory of the Brazilian Legal Amazon. The analyzed studies identified approximately 145 taxa of cyanobacteria, and the most frequent species belong to the genera Microcystis and Planktothrix. The most frequently detected cyanotoxin was microcystin, and, in some locations, even with a low cell density, there was the production of toxins. In most cases, Microcystis and Dolichospermum were the major genera associated with blooms and toxin production. The state of Pará had the highest number of records of toxin-producing cyanobacteria, including records of seasonal events, while Acre had no records. This work contributes to our knowledge of the geographic distribution and occurrence of cyanobacteria and cyanotoxins in the Brazilian Amazon and proposes new strategies for improving the monitoring of cyanotoxins in the Amazon region.

Cyanobacteria are photosynthetic prokaryotic organisms that in aquatic environments can compose the phytoplankton and phytobenthos, being important in the primary production of these ecosystems. The objective of this work was to identify the main morphological variations of the cells, filaments and thallus of the cyanobacteria Nostoc sp. LBALBR-2 submitted to ultraviolet radiation (UV-C) in microcosm systems. The cyanobacteria Nostoc sp. was subjected to four treatments: 1- control treatment in nutrient medium; 2- treatment in nutrient medium with exposure to UV-C radiation; 3- control treatment in medium without nutrients and 4- treatment in medium without nutrients with exposure to UV-C radiation. Optical density and chlorophyll-a analyses were performed to determine the growth of Nostoc populations and microscopic analyses to characterize the morphological development of the species. Lack of nutrients generated short trichomes with terminal heterocytes, scattered solitary akinetes and appearance of hormogonia. On nutrient medium the trichomes of Nostoc sp. showed predominantly the vegetative serial form. Cultures exposed to UV-C produced anomalous cells, thick mucilage, fragmented trichomes and hormogonia. It is concluded that Nostoc sp. LBALBR-2 grew well under eutrophication conditions and although it showed cell deformation it was resistant to UV-C radiation.

The study investigated the use of phytoplankton as an environmental assessment tool in a supply reservoir in the Brazilian Amazon, analyzing how macrophyte colonization influences ecological structure. Three scenarios were established with different levels of floating macrophyte cover: scenario 1 (~90% visibility of the water surface, in December/2013), scenario 2 (~50% of the water surface covered with macrophytes, in March/2014), scenario 3 (~80% of the water surface covered with macrophytes, in September/2014). Phytoplankton, chlorophyll-a and limnological data were collected at three points and three depths in the reservoir. Multivariate analyses revealed significant differences in physico-chemical and biological factors between the scenarios, influenced by seasonality and the colonization of macrophytes. In scenario 1, the reservoir was mesotrophic, deep and transparent, with high salinity and total dissolved solids, dominated by planktonic species. In scenario 2, eutrophication was present, with a higher concentration of ammoniacal nitrogen due to the rains, and the presence of planktonic and benthic species associated with the microhabitats formed by the macrophytes. In scenario 3, shallow and dominated by macrophytes, benthic and shade-adapted species were prevalent, reflecting high turbidity and the presence of high iron. The Species Indication Value approach proved effective (71.2%), highlighting the influence of turbidity, potassium, magnesium, salinity and precipitation on phytoplankton structure. The study recommends the continued use of phytoplankton as a bioindicator in future research, strengthening its role in the ecological assessment of Amazonian aquatic environments.

Ecological indices are used in the study of phytoplankton because they are related to their productivity and can describe the stresses caused by anthropogenic impacts on the aquatic environment. The aim of this study was to evaluate the phytoplankton structure and the application of ecological indices in the monitoring of water quality in public supply reservoirs in Belém (Pará). Monthly sampling occurred in four stations in the Bologna and Água Preta reservoirs from January to October/2019, to determine the abiotic and biotic factors and the calculation of ecological indices. Turbidity, transparency, precipitation and dissolved oxygen were the main abiotic variables that differentiated the reservoirs, evidencing the estuarine, seasonal and anthropogenic influence on species distribution. A total of 187 phytoplankton species were identified with representation of diatoms. In January, Água Preta reservoir presented lower diversity (2.14 ± 0.58 nats.ind-1), equitability (0.57 ± 0.14) and higher dominance (0.25 ± 0.19) with the species Trachelomonas volvocina in higher density, contrary to June that presented higher diversity and equitability and no species dominance. The Bolonha reservoir showed lower richness (8.3 ± 1.9), diversity (1.2 ± 0.5 nats.ind-1) and equitability (0.32 ± 0.12) with dominance of the species Gonyostomum sp. (0.55 ± 0.19). In this sense, the Água Preta reservoir was considered diverse with species well distributed in the environment, while the Bolonha reservoir showed low diversity and dominance of a phytoplankton species. Ecological indices describe well the spatio-temporal variation of phytoplankton in these reservoirs and are a good approach in monitoring environmental quality.