Influence of environmental variables on saxitoxin yields by Cylindrospermopsis raciborskii in a mesotrophic subtropical reservoir.

Saxitoxins are a class of toxins produced by at least two groups of evolutionarily distant organisms (cyanobacteria and dinoflagellates). While the toxicity of these toxins is relatively well characterized, to date little is known about their drivers and ecological functions, especially in lower latitude tropical and subtropical freshwater ecosystems. In the present study, we aimed to obtain a better understanding of the main drivers of saxitoxin concentrations in aquatic environments. We investigated the relationships among saxitoxin concentrations in a mesotrophic subtropical reservoir dominated by the cyanobacteria Cylindrospermopsis raciborskii with physical, chemical and biological water variables. The highest saxitoxin concentrations were 0.20 µg·L-1, which occurred in the samples with the highest densities of C. raciborskii (maximum of 4.3 × 104 org·mL-1) and the highest concentration of dissolved nutrients (nitrate from 0.2 to 0.8 µg·L-1, ortophosphate from 0.3 to 8.5 µg·L-1). These correlations were confirmed by statistical analyses. However, the highest saxitoxin relative concentrations (per trichome) were associated with lower C. raciborskii densities, suggesting that saxitoxin production or the selection of saxitoxin-producing strains was associated with the adaptation of this species to conditions of stress. Our results indicate that C. raciborskii toxin yields vary depending on the enrichment conditions having potential implications for reservoir management.

The contribution of volunteer-based monitoring data to the assessment of harmful phytoplankton blooms in Brazilian urban streams.

Urban streams are vulnerable to a range of impacts, leading to the impairment of ecosystem services. However, studies on phytoplankton growth in tropical lotic systems are still limited. Citizen science approaches use trained volunteers to collect environmental data. We combined data on urban streams collected by volunteers with data obtained by professional scientists to identify potential drivers of phytoplankton community and determine thresholds for Cyanobacteria development. We combined datasets (n=117) on water quality and environmental observations in 64 Brazilian urban streams with paired data on phytoplankton. Sampling activities encompassed dry (July 2013 and July 2015) and warm (February and November 2014) seasons. Volunteers quantified phosphate (PO43-), nitrate (NO3-) and turbidity in each stream using colorimetric and optical methods and recorded environmental conditions in the immediate surroundings of the sites through visual observations. We used non-parametric statistics to identify correlations among nutrients, turbidity and phytoplankton. We also looked for thresholds with respect to high Cyanobacteria abundance (>50,000cells/mL). The streams were characterized by relatively high nutrient concentrations (PO43-: 0.11mg/L; NO3-: 2.6mg/L) and turbidity (49 NTU). Phytoplankton densities reached 189,000cells/mL, mainly potentially toxic Cyanobacteria species. Moderate but significant (p<0.05) correlations were observed between phytoplankton density and turbidity (ρ=0.338, Spearman) and PO43- (ρ=0.292), but not with NO3-. Other important variables (river flow, temperature and light) were not assessed. Volunteers' observations covaried with phytoplankton density (p<0.05, Kruskal-Wallis), positively with increasing number of pollution sources and negatively with presence of vegetation in the riparian zone. Our results indicate that thresholds for PO43- (0.11mg/L) can be used to separate systems with high Cyanobacteria density. The number of pollution sources provided a good indicator of waterbodies with potential cyanobacteria problems. Our findings reinforced the need for nutrient abatement and restoration of local streams and highlighted the benefits of volunteer-based monitoring to support decision-making.

Trophic State Evolution and Nutrient Trapping Capacity in a Transboundary Subtropical Reservoir: A 25-Year Study.

Artificial reservoirs have been used for drinking water supply, other human activities, flood control and pollution abatement worldwide, providing overall benefits to downstream water quality. Most reservoirs in Brazil were built during the 1970s, but their long-term patterns of trophic status, water chemistry, and nutrient removal are still not very well characterized. We aimed to evaluate water quality time series (1985-2010) data from the riverine and lacustrine zones of the transboundary Itaipu Reservoir (Brazil/Paraguay). We examined total phosphorus and nitrogen, chlorophyll a concentrations, water transparency, and phytoplankton density to look for spatial and temporal trends and correlations with trophic state evolution and nutrient retention. There was significant temporal and spatial water quality variation (P < 0.01, ANCOVA). The results indicated that the water quality and structure of the reservoir were mainly affected by one internal force (hydrodynamics) and one external force (upstream cascading reservoirs). Nutrient and chlorophyll a concentrations tended to be lower in the lacustrine zone and decreased over the 25-year timeframe. Reservoir operational features seemed to be limiting primary production and phytoplankton development, which exhibited a maximum density of 6050  org/mL. The relatively small nutrient concentrations in the riverine zone were probably related to the effect of the cascade reservoirs upstream of Itaipu and led to relatively low removal percentages. Our study suggested that water quality problems may be more pronounced immediately after the filling phase of the artificial reservoirs, associated with the initial decomposition of drowned vegetation at the very beginning of reservoir operation.