Unveiling the link between Raphidiopsis raciborskii blooms and saxitoxin levels: Evaluating water quality in tropical reservoirs, Brazil.

The proliferation of Raphidiopsis raciborskii blooms has sparked concerns regarding potential human exposure to heightened saxitoxins (STXs) levels. Thus, comprehending how environmental elements drive the proliferation of this STXs-producing species can aid in predicting human exposure risks. This study aimed to explore the link between cyanobacteria R. raciborskii, STXs cyanotoxins, and environmental factors in 37 public supply reservoirs in the tropical region and assess potential health hazards these toxins pose in the reservoir waters. A Structural Equation Model was used to assess the impact of environmental factors (water volume and physical and chemical variables) on R. raciborskii biomass and STXs levels. Furthermore, the potential risk of STXs exposure from consuming untreated reservoir water was evaluated. Lastly, the cumulative distribution function (CDF) of STXs across the reservoirs was computed. Our findings revealed a correlation between R. raciborskii biomass and STXs concentrations. Total phosphorus emerged as a critical environmental factor positively influencing species biomass and indirectly affecting STXs levels. pH significantly influenced STXs concentrations, indicating different factors influencing R. raciborskii biomass and STXs. Significantly, for the first time, the risk of STXs exposure was gauged using the risk quotient (HQ) for untreated water consumption from public supply reservoirs in Brazil's semi-arid region. Although the exposure risks were generally low to moderate, the CDF underscored the risk of chronic exposure due to low toxin concentrations in over 90% of samples. These outcomes emphasize the potential expansion of R. raciborskii in tropical settings due to increased phosphorus, amplifying waterborne STXs levels and associated intoxication risks. Thus, this study reinforces the importance of nutrient control, particularly phosphorus regulation, as a mitigation strategy against R. raciborskii blooms and reducing STXs intoxication hazards.

The role of bioturbation triggered by benthic macroinvertebrates in the effectiveness of the Floc & Lock technique in mitigating eutrophication.

Anthropogenic activities have led to excessive loading of phosphorus and nitrogen into water bodies, leading to eutrophication and promoting the growth of cyanobacteria, posing a threat to the health of humans and aquatic animals. Techniques such as Floc & Lock have been developed to mitigate eutrophication by reducing phosphorus concentrations in water and preventing algal blooms. However, little attention has been given to the impact of phosphorus resuspension by sediment-associated organisms such as benthic macroinvertebrates, on the effectiveness of this technique. Here, we experimentally evaluated whether the presence of snails Melanoides tuberculata (Müller, 1774) and larvae of Chironomus sancticaroli (Strixino and Strixino, 1981) affects the efficiency of the Floc & Lock technique. Snails and chironomid larvae are benthic macroinvertebrates commonly found in high abundance in eutrophic reservoirs. Specifically, we tested the hypotheses that (i) the presence of benthic macroinvertebrates reduces the efficiency of coagulants and clays in removing phosphorus and algal biomass from the water column, and (ii) this effect is species-dependent, as some organisms such as the snails, revolve the substrate and resuspend sedimented particles, while other ones, such as chironomid larvae, aid in the removal of phosphorus from the water column by depositing them in the sediment. Our findings revealed that the impact of benthic macroinvertebrates on the effectiveness of the Floc & Lock technique is species-dependent. Chironomid larvae positively influenced the efficiency of the technique by aiding in the removal of total phosphorus, soluble reactive phosphorus, and algal biomass from the water column, depositing them in the sediment. In contrast, the presence of snails had the opposite effect, resulting in increased phosphorus concentration and algal biomass in the water. Surprisingly, the snails consumed the flocs formed by the coagulant and clay within a short time interval of 72 h, raising concerns due to the presence of toxic cyanobacterial biomass in these flocs. Our study emphasizes the importance of considering benthic macroinvertebrates and their impact on the effectiveness of eutrophication management techniques.

Impacts of the Floc and Sink technique on the phytoplankton community: A morpho-functional approach in eutrophic reservoir water.

The Floc and Sink technique promotes, through the application of coagulants and clays, the removal of phosphorus and algal biomass from the water column by flocculation and sedimentation. Although it is a promising technique for mitigating harmful cyanobacterial blooms, little is known about the impacts on other phytoplankton species as well as it is not known how species with mechanisms of resistance to sedimentation respond to the application of these products. In this study, a laboratory experiment was carried out with water from a eutrophic reservoir to assess the impact of applying aluminum-based coagulants, aluminum sulfate and polyaluminium chloride, and chitosan, alone and combined with lanthanum modified bentonite and natural bentonite on the phytoplankton community, in a functional approach based on morphology (Morphology-Based Functional Groups - MBFG, Kruk et al., 2010 and Reynolds et al., 2014), with an emphasis on the characteristics that provide resistance to sedimentation. We tested two hypotheses: phytoplankton species with adaptive mechanisms that provide buoyancy to cells are more resistant to the removal from the water column by coagulants and clays; and coagulants based on metals and modified clays are more efficient in sedimentation of microalgal cells compared to natural products, regardless of the presence of an adaptive mechanism of resistance to sedimentation. Our results showed that aluminum sulfate and polyaluminium chloride alone or combined with lanthanum modified bentonite and natural bentonite effectively sedimented the cells, regardless of the presence of buoyancy mechanisms. In contrast, the natural coagulant chitosan alone or combined with lanthanum modified bentonite and natural bentonite removed only those species that were small-celled or small colonial without specialized structures or with the presence of spines, arms and siliceous exoskeleton. In the case of species with aerotopes and flagella, the removal was not effective and still caused an increase in algal biomass due to the formation of suspended cell aggregates on the surface of the water column of the experimental units. Therefore, we concluded that the Floc and Sink technique has an impact on the phytoplankton community because it removes from the water column species that are not the target of coagulants and clays, but that are considered important sources of energy in freshwater trophic webs. This result differs according to the type of product used as well as it is related to the morphological adaptations that favor the buoyancy of cells in the water column. Species with aerotopes and flagella are more resistant to sedimentation and may make the use of products applied in the Floc and Sink technique unfeasible. Nevertheless, these results represent only an immediate effect of the technique on the phytoplankton community, thus requiring a longer time scale evaluation to determine the algae that can effectively recover water column. Therefore, we also emphasize that algae of nutritional value can recover over time and make the long-term application of the technique acceptable.

Removal efficiency of phosphorus, cyanobacteria and cyanotoxins by the "flock & sink" mitigation technique in semi-arid eutrophic waters.

Geoengineering techniques have been used to control phosphorus and cyanobacteria in lakes promising greater and quicker chemical and ecological recovery. Techniques that use coagulants and clays to remove particulates and dissolved phosphorus from the water column have received great. In this study, bench-scale "flock & sink" assays were carried out to evaluate the efficiency of the coagulants aluminium sulphate (SUL), polyaluminium chloride (PAC) and chitosan (CHI), alone and combined with natural bentonite clays (BEN) and lanthanum-modified bentonite (LMB), to remove of phosphorus from a eutrophic reservoir in a semi-arid region of Brazil. In addition, the study seeks to assess the effects on the cyanobacteria density and the intra- and extracellular concentrations of cyanotoxins after the application of these geoengineering materials. The SUL and PAC coagulants effectively reduced the total phosphorus (TP), reactive soluble phosphorus (SRP), turbidity, chlorophyll-a, cyanobacteria density and intracellular microcystin, whereas CHI showed a low removal efficiency. Lanthanum-modified bentonite proved to be more effective than BEN; however, the application of the coagulants only was sufficient to successfully remove phosphorus and cyanobacteria from the water column. In addition, the efficiency of the "flock & sink" technique in cell removal varied among the cyanobacteria species. Small colonial species such as Aphanocapsa delicatissima, Merismopedia glauca and Merismopedia tenuissima were removed regardless of the treatment used, including those with CHI and BEN. As for the filamentous cyanobacteria, Cylindrospermopsis raciborskii, Geitlerinema amphibium, Planktothrix agardhii and Pseudanabaena catenata, removal was achieved only using PAC, SUL and LMB alone or when combined. The intracellular concentrations of saxitoxin and cylindrospermopsin and the extracellular fraction of these cyanotoxins and of microcystin were not influenced by the application of coagulants and clays. This indicates that cell lysis did not occur with the addition of the geoengineering materials. These results demonstrate that the "flock & sink" technique could be used for restoration of eutrophic waters.