Cyanobacteria In a climate change scenario: a new approach

Abstract

Cyanobacteria are knownto produce a wide variety of bioactive, toxic secondary metabolites generally described as hepatotoxins, neurotoxins, cytotoxins, or dermatoxins. In Brazil, the monitoring of the frequency and concentration of cyanotoxins has intensified after the death of 65 patients in a hemodialysis clinic in Caruaru in the state of Pernambuco due to microcystin exposure. The primary objective of this study was to use multivariate statistics that incorporated environmental parameters (both biotic and abiotic) to forecast blooms of cyanobacteria and their toxic secondary metabolites in 20 drinking water reservoirs managed by the Water Treatment Company of Ceará (CAGECE) in the semi-arid region of Ceará, Brazil, as also develop an index for the study of cyanobacteria (Icyano)taking into account the main monitored parameters for the control of cyanobacteria in order to provide summary information.Across fouryears (January 2013 to January 2017), 114 different phytoplankton taxa were identified, including 24 cyanobacterialspecies. In general, Ceará reservoirs were dominated by cyanobacteria due to eutrophication but also because of the dry and warm climate found throughout the region. Interestingly, specific cyanobacterial taxa were influenced by different biotic and abiotic factors. For example, total nitrogen was related to saxitoxin-producing taxa, especially Cylindrospermopsis raciborskii, while temperature, irradiation and transparency (measured as Secchi depth) were associated with microcystin-producing taxa, such as Microcystis aeruginosa. Planktothrix agardhiiabundance was related to total phosphorus concentration and rainfall. Considering that climate forecasts predict higher evaporation and temperatures in the semi-arid Ceará region and that such changes will likely magnify droughts and water scarcity as well as promote toxic cyanobacterial blooms in reservoirs in the future, understanding the factors associated with algal blooms dominated by specific taxa is paramount for water resource management.In the proposedindex model, temperature, evaporation, sunlight, electrical conductivity,and phosphorus show Wi (weight of each parameter) = 0.073, 0.061, 0.032, 0.039 and 0.042, respectively. Chlorophyll (Wi= 0.202),total nitrogen(Wi = 0.191),and cyanobacteria concentration (W = 0.214) were the three most important parameters to the index composition, representing approximately60% of the Wi composition. All reservoirs classified as goodIcyanoused direct filtration water treatment technology,whilemany of the medium Icyanoclassified reservoirs (AMR, EQ, PM, PU, SD, SN and T) used a pretreatment unit followed by a direct filtration unit. Additionally, PS(Bad)reservoir also has beentwo treatment units (pretreatment + filtration direct), and the reservoir FQ(Very bad)presented the complete cycle technology.It can be seen that as Icyano increases, water treatment plants switchfrom theiroriginal conformation (direct filtration) to using apre-treatmentto provide raw water to the quality required by current Brazilian legislation.The parameters that most contributed to the explanation of the proposed model were temperature, evaporation, sunlight, electrical conductivity, chlorophyll,total nitrogen, total phosphorus, Secchi depth,andconcentration of cyanobacteria. A high ratio of N:Pwas observed andmay best explainthe dominance ofcyanobacteria inthestudiedreservoirs, however, it cannot be considered as the only factor favoring the dominance ofcyanobacteria.It was possible to set up an index that relates water quality to the risk of cyanobacteria, and comparing the index with the water treatment technology and auxiliary in the prioritizationof the investments in water treatment plants.