Iron enrichment from hypoxic hypolimnion supports the blooming of Raphidiopsis raciborskii in a tropical reservoir

Control of cyanobacterial blooms with iron addition can favor stress-tolerant toxic species

The control of internal phosphorus (P) load by in-lake measures has been the subject of decades of research. Although iron (Fe) is effective in precipitating P, it has been less tested due to its redox sensitivity. The effectiveness of Fe in controlling P availability and sinking cyanobacterial blooms contrasts to its function as a nutrient for phytoplankton growth. Both roles of Fe were tested in enclosures placed for 36 days in a shallow lake with a perennial cyanobacterial bloom, and in laboratory experiments with Fe-deficient Raphidiopsis raciborskii. Based on total P (TP) of lake water, we applied two doses of FeCl3, corresponding to 30:1 (16 mg Fe L-1) and 90:1 (47 mg Fe L-1) (Fe:P molar), to cause P precipitation, and flocculation and sinking of cyanobacterial populations. Three enclosures per treatment and three without FeCl3 additions (control) were used. The 90:1 treatment sank the main cyanobacterial biomass (50-fold) dominated by Planktothrix agardhii, with a significant decrease in turbidity, chlorophyll a and TP, without lasting decrease in pH, and achieved mesotrophic-like conditions. However, signs of recovery of R. raciborskii were detected between days 3 and 14. In laboratory experiments, Fe-deficient R. raciborskii MVCC19 grown under nitrate availability (+N) and N2-fixation (-N) were exposed to five FeCl3 concentrations from 0 to 17.9 (10:1) mg Fe L-1 (Fe:P molar). A remarkable tolerance to high Fe was found at concentrations 7-fold higher than culture medium and decreased under -N. Also R. raciborskii stood low Fe levels in +N and exhibited higher Fe requirements under N2-fixation. The increase in trichome length suggests resistance to the stressor, with shorter trichomes in -N. Therefore, effective management of R. raciborskii requires additional control of N in lakes. Our results point out the double role of Fe applications in which stress-tolerant species may become dominant under the reoligotrophication scenario.

Geosmin Events Associated with Dolichospermum circinale Abundance Promoted by Nitrogen Supply in a Chinese Large Tropical Eutrophic Reservoir

Taste and odor (T/O) compounds are a global threat in drinking water, mainly produced by cyanobacteria in freshwater environments. Temperature plays a crucial role in regulating geosmin dynamics in temperate and subtropical lakes, while its influence may be lower in tropical waters. To better understand the factors affecting geosmin occurrence in tropical waters, a dataset from a field investigation conducted in a large tropical reservoir was analyzed. The water temperature varied between 16 °C and 32 °C, with geosmin concentration ranging from below the detection limit (3 ng/L) to as high as 856 ng/L. Elevated geosmin levels exceeding > 10 ng/L were observed over the whole year except for in September, suggesting that the annual temperature was suitable for geosmin production. Among the diverse cyanobacteria, Dolichospermum circinale was identified as the main producer of geosmin in the reservoir, both by correlation analysis and cells' geosmin measurements. Geosmin concentration was also significantly related to the abundance of D. circinale. None of the environmental variables (temperature, pH, transparency and nutrients) were significantly directly correlated with geosmin concentration. But the high total nitrogen significantly explained the increase in D. circinale abundance associated with geosmin elevation. Our results suggest that nutrients, particularly nitrogen, directly affected the competitive advantage and abundance of key geosmin producers and thus modified geosmin levels in this tropical reservoir. Our study thus hints at the possible management of the geosmin problem through nutrient reduction in tropical reservoirs.

Current status and prognosis of Raphidiopsis raciborskii distribution in Bulgaria as part of the southeastern region of Europe

The cyanobacterial species Raphidiopsis raciborskii (Wołoszyńska) Aguilera et al. has a high invasiveness potential, which in less than a century leads to its cosmopolitan spread. In the temperate climate of Europe, R. raciborskii has been reported in many countries, but there is still a lack of detailed information about the current status of its distribution in lakes of Bulgaria, as a part of the southeastern range of its spread in Europe. We investigated the distribution of the species using data on the phytoplankton of 122 lakes surveyed during 13-years period (2009-2022). The species was found in 33 lakes (up to 324 m asl), and 14 new localities were registered during the studied period. The results reveal that the number of lakes with the presence of R. raciborskii (27 % of all research lakes) and its contribution to the total phytoplankton biomass, has increased significantly over the last decade. The species has successfully adapted and dominated the phytoplankton in 9 lakes, forming a bloom in 8 of them. The dominant position of R. raciborskii causes loss of species and functional diversity of phytoplankton and displaces the native bloom-forming cyanobacteria. Lakes with and without the species were compared based on the available data on bioclimatic and local environmental variables. Statistically significant differences were established with respect to water transparency, conductivity, maximum depth and maximum air temperature in the warmest month. Species distribution models (SDMs) were used to identify lakes in high risk of future invasion by R. raciborskii. The results of the SDMs implementation confirmed the high maximum air temperature and low water transparency to be important predictors of the occurrence of R. raciborskii in freshwater lakes in Bulgaria. In the areas with high summer temperatures the most suitable for R. raciborskii development were found to be shallow polymictic or medium deep lakes with small surface area and low water transparency. In areas with a suitable climate, the large, deep reservoirs with high transparency as well as macrophyte dominated lakes have a low probability of occurrence of R. raciborskii. Future colonization of lakes above 500 m asl (but most likely below 700 m asl) is also possible, especially in the conditions of global warming. SDMs account for climatic and biogeographic differences of lakes and could help in elucidating the underlying factors that control the occurrence and adaptation of R. raciborskii in a given area.

Spatial and Temporal Characteristics of Phytoplankton Communities in Drinking Water Source Reservoirs in Shenzhen, China

Phytoplankton diversity and community characteristics are closely associated with aquatic environmental factors. Understanding these dynamics can provide insights into the ecological health of water bodies. We investigate the spatial and temporal characteristics of phytoplankton communities in 27 drinking water source reservoirs in Shenzhen, China. As a method, we collected samples during the dry season in 2021 and the wet season in 2022, analyzed the alpha and beta diversities of phytoplankton communities, and correlated these with the environmental factors. The results reveal that Cyanobacteria dominate the phytoplankton communities in the Shenzhen reservoirs. Phytoplankton diversity is greater during the dry season. The algal composition varies spatially, and the phytoplankton diversity tends to decrease with increasing eutrophication. A co-occurrence network analysis indicates denser and stronger correlations among phytoplankton nodes during the wet season than dry season. Reservoirs with moderate eutrophication levels exhibit denser nodes and stronger correlations compared to those with low or high eutrophication levels. The chemical oxygen demand, water temperature, pH, and total nitrogen are identified as key influencers of the phytoplankton community structure. Our results contribute to the enhanced understanding of the spatial and temporal dynamics of phytoplankton communities in reservoirs in South China and provides insights into the management and conservation of these drinking water reservoirs.