Accelerated eutrophication and toxicity in tropical reservoir water and sediments: an ecotoxicological approach

Increased chlorophyll-a concentration in Barra Bonita reservoir during extreme drought periods

Climate projections models indicate that longer periods of droughts are expected within the next 100 years in various parts of South America. To understand the effects of longer periods of droughts on aquatic environments, we investigated the response of chlorophyll-a (Chl-a) concentration to recent severe drought events in the Barra Bonita Hydroelectric Reservoir (BBHR) in São Paulo State, Brazil. We used satellite imagery to estimate the Chl-a concentration from 2014 to 2020 using the Slope Index (NRMSE of 18.92% and bias of -0.20 mg m^-3). Ancillary data such as precipitation, water level and air temperature from the same period were also used. Drought events were identified using the standardized precipitation index (SPI). In addition, we computed the probability of future drought events. Two periods showed extremely dry conditions: 1) January-February (2014) and 2) April-May (2020). Both periods were characterized by a recurrence probability of 1in every 50 years. The highest correlation was observed between Chl-a concentration and SPI (-0.97) in 2014, while Chl-a had had the highest correlation with water level (-0.59) in 2020. These results provide new insights into the influence of extreme drought events on the Chl-a concentration in the BBHR and their relationship with other climate variables and reservoir water levels. Drought events imply less rainfall, higher temperatures, and atmospheric dryness, and these factors affect evaporation and the water levels in the reservoir.

Research Characteristics on Cyanotoxins in Inland Water: Insights from Bibliometrics

Eutrophication is a long-standing ecological and environmental problem, and the severity of harmful algal blooms continues to increase, causing large economic losses globally. One of the most important hazards created by harmful algal blooms is the production of cyanotoxins. This study aimed to analyze the characteristics and development trends of cyanotoxin research through bibliometric analysis. A total of 3265 publications from 1990 to 2020 on cyanotoxins were retrieved from the Science Citation Index (SCI) Expanded database, Web of Science. Over the past 30 years, most research has been concentrated in China (21.4%) and the USA (21.3%). Throughout the study period, microcystin was the focus of the research, accounting for 86% of the total number of publications. A word frequency analysis revealed that as people became more aware of drinking water safety and the construction of large-scale water conservation facilities, “reservoirs” and “rivers” became hot words for researchers, while “lakes” have always been important research objects. Nonmetric multidimensional scaling (NMDS) analysis of studies from the five countries with the largest numbers of publications showed that Chinese researchers typically associate eutrophication with Microcystis, while research subjects in other countries are more extensive and balanced. The development of cyanotoxin research around the world is not even, and we need to push for more research on major lakes that are outside of North America, Europe and China.

An abiotic typology and reference conditions for nutrients and chlorophyll-a in subtropical reservoirs (São Paulo State, Brazil)

The establishment of reference conditions and typology are two important steps in water resources management. The reference conditions enable the determination of how impacted an ecosystem is, while the typology facilitates the implementation of management procedures. A study of subtropical reservoirs in São Paulo State (Brazil) was performed to obtain (1) an abiotic typology, using principal component analysis (PCA) and cluster analysis (Ward's method with Euclidean distances), and (2) reference conditions for total phosphorus (TP), total nitrogen (TN), and chlorophyll-a (Chl-a) in the dry and rainy seasons for one of the established typologies. Two main statistical techniques were used: the lake population distribution approach and the trisection method. PCA identified an environmental gradient in the east-west direction, with reservoirs at higher latitudes and with larger dimensions in the west, and smaller reservoirs in eastern areas with higher altitude and higher average annual rainfall. The PCA and cluster analysis indicated that there were four main types of reservoirs. The nutrient criteria techniques, obtained for 13 type I rainy-east reservoirs, were not significantly different (t test, p < 0.05). Although the methods resulted in similar reference conditions, one-way ANOVA indicated significant differences between the seasons for Chl-a, which levels were slightly higher in the rainy season. As far as we know, this is the first study to simultaneously provide an abiotic typology and reference conditions for chlorophyll-a and nutrients in different seasons, considering subtropical reservoirs in South America. This investigation makes an important contribution to the monitoring and management of subtropical reservoirs, and the promotion of dialog between the scientific community and managers, aiming at ensuring the sustainability of water bodies.

Effects and control of metal nutrients and species on Microcystis aeruginosa growth and bloom

The effects and control of typical metal nutrients, copper, iron, and zinc, on the growth and bloom of Microcystis aeruginosa were investigated with a series of flask-shaking tests. The optimal concentrations of copper, iron, and zinc for algal growth were 0.001, 3-12, and 0.05 mg/L, respectively. The order of toxicity to the alga was Cu > Zn > Fe. The effects of the species, for a trace metal at the same concentrations, on the growth of M. aeruginosa were relatively remarkable. Ionic and complexation species induced more algal growth than the carbonate and sulfide-bound species. Changes in copper concentration and iron species were adopted to adjust and control the bloom of M. aeruginosa. Increases in copper concentrations significantly suppressed the M. aeruginosa bloom. The growth rate of M. aeruginosa slowed significantly when ionic iron was replaced with sulfide-bound iron, and the control of bloom was remarkable. PRACTITIONER POINTS: Using trace metal nutrient species and concentration to regulate and control algal growth and bloom may pave another way for the management of cyanobacterial bloom.