A comunidade fitoplanctônica do reservatório de Itupararanga (Votorantim-SP) e a problemática das cianobactérias e cianotoxinas
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Data
2022-04-04
Autores
Machado, Leila Dos Santos [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
Reservatórios tropicais, em geral, são suscetíveis ao processo de eutrofização artificial, devido aos seus múltiplos usos. Além disso, as mudanças climáticas influenciam os regimes de precipitação, afetando a hidrologia destes ecossistemas, alterando vazão, tempo de retenção, promovendo estratificação da coluna da água, favorecendo o aporte de efluentes e processos de lixiviação. Assim, a produtividade primária é diretamente influenciada, devido a estas alterações na composição da comunidade fitoplanctônica, as quais estão relacionadas à sensibilidade e tolerâncias ambientais que as espécies de fitoplâncton possuem. Uma forma de analisar a resposta destes organismos a alterações ambientais é por meio da aplicação dos grupos funcionais, que consiste em agrupar espécies com as mesmas características ecofisiológicas, e que respondem de maneira similar às alterações das condições ambientais. Em ambientes eutrofizados, é comum que cianobactérias sejam dominantes, devido às suas adaptações ecofisiológicas relatadas em diversos estudos no Brasil e no mundo. O reservatório de Itupararanga, responsável pelo abastecimento de grande parte da cidade de Sorocaba e região, foi avaliado como eutrófico e supereutrófico em estudos recentes. No presente estudo, dentre os organismos fitoplanctônicos analisados, foi observada a dominância da cianobactéria Raphidiopsis raciborskii. No período chuvoso, porém, a biomassa de R. racibosrkii diminuiu drasticamente, sugerindo sensibilidade da espécie à estratificação térmica da coluna da água, que ocorreu neste período. Contudo, quando houve a diminuição da biomassa de R. raciborskii, foi observada alternância na ocorrência de outras espécies potencialmente tóxicas como Aphanizomenon gracile e Dolichospermum spp. Esta resposta foi mais evidente na região fluvial do reservatório, a qual é a mais afetada pelo aporte de efluentes domésticos. A presença de cianotoxinas (saxitoxinas e microcistinas) foi analisada, de modo que as concentrações de saxitoxina foram relativamente maiores em comparação com as microcistinas, porém ambas estiveram abaixo dos limites determinados pela legislação. Não foi observada correlação significativa entre os teores de saxitoxina e a biomassa de R. raciborskii, o que sugere que possa haver alternância entre o desenvolvimento de cepas tóxicas e não tóxicas no reservatório, ou ainda, que a saxitoxina seja liberada em condições de estresse, já que os maiores teores foram observados no período chuvoso, quando a biomassa de R. raciborskii foi menor. Ainda que a saxitoxina tenha sido encontrada em baixas concentrações, esta toxina foi identificada em todas as cinco campanhas realizadas entre os anos de 2016 e 2017, evidenciando sua permanência constante no reservatório. Neste sentido, ensaios ecotoxicológicos foram elaborados com a finalidade de avaliar a toxicidade da saxitoxina em uma espécie nativa de quironomídeo, utilizando as concentrações próximas às ambientais identificadas para o reservatório. A hipótese inicial do estudo foi confirmada, ainda que as concentrações de saxitoxina testadas estejam dentro dos parâmetros estabelecidos pela legislação, a exposição crônica à baixos teores de saxitoxina foi capaz de causar danos aos organismos.
Tropical reservoirs, in general, are susceptible to the process of artificial eutrophication, due to their multiple uses. In addition, climate change influences precipitation regimes, affecting the hydrology of these ecosystems, changing flow, retention time, promoting stratification of the water column, favoring the increase of effluents and leaching processes. Thus, the primary productivity is directly influenced, due changes in the composition of the phytoplankton community related to the sensitivity and environmental tolerances of phytoplankton species. The analysis of the response of these organisms to environmental changes can be through the application of functional groups, which consists of grouping species with the same ecophysiological characteristics, which respond in a similar way to changes in environmental conditions. The dominance of cyanobacteria in eutrophic environments is common, due to their ecophysiological adaptations reported in several studies in Brazil and worldwide. The Itupararanga reservoir is responsible for supplying much of the city of Sorocaba and region, however, it has been assessed as eutrophic and super eutrophic in recent studies. In addition, in the present study, the dominance of the cyanobacterium Raphidiopsis raciborskii was observed among the analyzed phytoplanktonic organisms and the biomass values were higher in the dry season. However, in the rainy season the R. raciborskii biomass decreases significantly, suggesting sensitivity of the species to the thermal stratification of the water column that occurred during this period. On the other hand, when there was a decrease in the biomass of R. raciborskii, alternation was observed in the occurrence of other potentially toxic species such as Aphanizomenon gracile and Dolichospermum spp. This response was more evident in the fluvial region of the reservoir, where the discharge of domestic effluents is intense. The presence of cyanotoxins (saxitoxins and microcystins) was analyzed and the concentrations of saxitoxin were relatively higher compared to microcystins, but both were below the limits determined by legislation. There was no significant correlation between the levels of saxitoxin and the biomass of R. raciborskii, which suggests that there may be an alternation between the development of toxic and non-toxic strains in the reservoir, or even that the saxitoxin is released under stress conditions, since the highest levels were observed in the rainy season, when the biomass of R. raciborskii was lower. Although saxitoxin was found in low concentrations, this toxin was identified in all five campaigns carried out between the years 2016 and 2017, showing its constant permanence in the reservoir. Thus, ecotoxicological tests were developed with the objective of evaluating the toxicity of saxitoxin in a native species of chironomid, using the environmental concentrations identified in the Itupararanga reservoir. The initial hypothesis of the study was confirmed, although the concentrations of saxitoxin tested are within the parameters established by the legislation, the chronic exposure to low levels of saxitoxin was capable of causing damage to the organisms.
Tropical reservoirs, in general, are susceptible to the process of artificial eutrophication, due to their multiple uses. In addition, climate change influences precipitation regimes, affecting the hydrology of these ecosystems, changing flow, retention time, promoting stratification of the water column, favoring the increase of effluents and leaching processes. Thus, the primary productivity is directly influenced, due changes in the composition of the phytoplankton community related to the sensitivity and environmental tolerances of phytoplankton species. The analysis of the response of these organisms to environmental changes can be through the application of functional groups, which consists of grouping species with the same ecophysiological characteristics, which respond in a similar way to changes in environmental conditions. The dominance of cyanobacteria in eutrophic environments is common, due to their ecophysiological adaptations reported in several studies in Brazil and worldwide. The Itupararanga reservoir is responsible for supplying much of the city of Sorocaba and region, however, it has been assessed as eutrophic and super eutrophic in recent studies. In addition, in the present study, the dominance of the cyanobacterium Raphidiopsis raciborskii was observed among the analyzed phytoplanktonic organisms and the biomass values were higher in the dry season. However, in the rainy season the R. raciborskii biomass decreases significantly, suggesting sensitivity of the species to the thermal stratification of the water column that occurred during this period. On the other hand, when there was a decrease in the biomass of R. raciborskii, alternation was observed in the occurrence of other potentially toxic species such as Aphanizomenon gracile and Dolichospermum spp. This response was more evident in the fluvial region of the reservoir, where the discharge of domestic effluents is intense. The presence of cyanotoxins (saxitoxins and microcystins) was analyzed and the concentrations of saxitoxin were relatively higher compared to microcystins, but both were below the limits determined by legislation. There was no significant correlation between the levels of saxitoxin and the biomass of R. raciborskii, which suggests that there may be an alternation between the development of toxic and non-toxic strains in the reservoir, or even that the saxitoxin is released under stress conditions, since the highest levels were observed in the rainy season, when the biomass of R. raciborskii was lower. Although saxitoxin was found in low concentrations, this toxin was identified in all five campaigns carried out between the years 2016 and 2017, showing its constant permanence in the reservoir. Thus, ecotoxicological tests were developed with the objective of evaluating the toxicity of saxitoxin in a native species of chironomid, using the environmental concentrations identified in the Itupararanga reservoir. The initial hypothesis of the study was confirmed, although the concentrations of saxitoxin tested are within the parameters established by the legislation, the chronic exposure to low levels of saxitoxin was capable of causing damage to the organisms.
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Palavras-chave
Limnologia, Eutrofização, Cianobactéria, Cianotoxinas, Limnology DLC, Eutrophication, Cyanobacterial toxins