Climate change is the result of interconnected phenomena and processes that are altering the Earth's climate over time. The El Niño and La Niña phenomena, independent or when aligned with conditions in the Tropical Atlantic, are the most significant, as they are directly related to extreme droughts, floods and high rainfall in the Amazon region.

The year 2023 was marked by an extreme drought that had profound implications for the environmental and social dynamics of the region.

Among the consequences of this climate event, in addition to the mortality of dolphins (Inia geoffrensis and Sotalia fluviatilis) in lakes Tefé and Coari, a specific algae bloom was observed in these places. Some studies have linked the bloom of photosynthetic organisms to climate change. Algal blooms have the potential to cause environmental damage (e.g. eutrophication, oxygen depletion, fish kills and toxin production). There are practically no records of blooms in natural systems with toxic potential, with the exception of an intense proliferation of the cyanobacteria Anabaena sp. and Microcystis sp. in the Tapajós River in the state of Pará. To date, there is no information available on the bloom of euglenas at Amazônia, especially Euglena sanguinea (Ehrenberg). In this context, in the face of the extreme drought that occurred in 2023, Aqua Viridi recorded the unprecedented appearance of the Euglena sanguinea bloom in two black water lakes in the Amazon region.

Lake Tefé, located on the right bank of the middle Solimões River, is a black water lake and is about 60 km long. Lake Coari is a black water lake and receives water from the Coari River, a tributary on the left bank of the Solimões River, and its length is estimated at around 530 km. The samples were collected above point 1 of Lake Tefé in October 2023. On Lake Coari, the samples were collected at the center point of the channel in November 2023. Qualitative samples were collected by means of horizontal trawls, using a phytoplankton net with a mesh size of 20 µm. Quantitative samples were collected using a 5 L graduated container. In the laboratory, qualitative analyses were carried out using successive slides under an optical microscope . To measure the size of the E. sanguinea cells, 10 cells of each shape (elongated and round/oval) were selected from each lake, using an optical microscope with a micrometric eyepiece (10 x objective). For the quantitative analyses, the number of cells was counted using a Sedgewick Rafter camera.

A red-colored bloom was observed in both lakes studied. This red stain refers to the predominance of the freshwater microalga Euglena sanguinea. E. sanguinea cells took on elongated and round shapes in green and red colors. The elongated cells had an average size of 85.4 ± 10.6 µm and the round cells 38.1 ± 4.98 µm in Lake Tefé. In Lake Coari, elongated cells had an average size of 64.9 ± 4.3 µm and round cells 48.4 ± 5.5 µm. The density of E. sanguinea in Lake Tefé was 64666 cells/mL and 42000 cells/mL in Lake Coari. The composition of the phytoplankton in Lake Tefé was made up of five classes: Zygnemaphyceae (7 taxa), Bacilariophyceae (4 taxa), Chlorophyceae (3 taxa), Cyanophyceae (3 taxa) and Euglenophyceae (1 taxon).

The phenomenon observed can be partly explained by the fact that E. sanguinea forms a bloom (reddish in color) on the surface of the water when temperatures exceed 25ºC, as occurred in both lakes studied where temperatures were recorded at over 39ºC. The reddish color is due to the presence of granules (hematochromes) that protect the chlorophyll of cells exposed to intense solar radiation. The presence of other species in the phytoplankton community in Lake Tefé during the extreme drought in 2023 highlights the need to monitor the dynamics of these organisms, considering their abundance and composition, in relation to water level variation. This is essential to facilitate future comparisons in extreme events.

This observation and identification of the bloom during the 2023 drought in the Amazon took place through our biologists Renan Gomes, Raíze Mendes and Maiby Glorize, who voluntarily traveled to the Tefé region during the most alarming period. With the support of biologist Fabiane Almeida, Aqua Viridi has been contributing so that actions can be identified and addressed to mitigate the phenomenon in the Amazon region.