Drivers shaping episodic and gradual changes in phytoplankton community succession: Taxonomic versus functional groups

Interannual succession of phytoplankton community in a canyon-shaped drinking water reservoir during the initial impoundment period: Taxonomic versus functional groups

During the initial impoundment period of a canyon-shaped reservoir, the water body fluctuated violently regarding water level, hydrological condition, and thermal stratification. These variations may alter the structure of phytoplankton community, resulting in algal blooms and seriously threatening the ecological security of the reservoir. It is of great significance to understand the continuous changes of phytoplankton in the initial impoundment period for the protection of reservoir water quality. Therefore, a two-year in-situ monitoring study was conducted on water quality and phytoplankton in a representative canyon-shaped reservoir named Sanhekou and the interannual changes of phytoplankton community and its response to environmental changes during the initial impoundment period were discussed at taxonomic versus functional classification levels. The results showed that the total nitrogen and permanganate index levels were relatively high in the first year due to rapid water storage and heavy rainfall input, and the more stable hydrological conditions in the second year promoted the increase of algae density and the transformation of community, and the proportion of cyanobacteria increased significantly. The succession order of phytoplankton in the first year of the initial impoundment period was Chlorophyta-Bacillariophyta-Chlorophyta, or J/F/X1-P/MP/W1-A/X1/MP, respectively. And the succession order in the second year was Cyanobacteria/Chlorophyta-Bacillariophyta-Chlorophyta, or LM/G/P-P/A/X1-X1/J/G. Water temperature, relative water column stability, mixing depth, and pH were crucial factors affecting phytoplankton community succession. This study revealed the interannual succession law and driving factors of phytoplankton in the initial impoundment period and provided an important reference for the operation management and ecological protection of canyon-shaped reservoirs.

Effects of artificial mixing on phytoplankton in a warm stratified drinking water reservoir: Characterization, mechanism, and implication

To improve water quality, water-lifting aerators (WLAs) are usually installed in reservoirs for artificial mixing. In this study, using taxonomic groups methods of phytoplankton, the characteristics and mechanisms of their response to artificial mixing were investigated during a two-year monitoring period in Xikeng Reservoir (XKR) in southern China, and compared the differences in response in the context of higher and lower phytoplankton abundance. The results showed that artificial mixing caused a significant decrease in phytoplankton abundance in the surface water column, but a short-term increase followed by a decrease in phytoplankton in the middle and bottom, and ultimately a homogenization of vertical phytoplankton with complete mixing of the water column. The phytoplankton showed a shift from Cyanobacteria to Bacillariophyta in artificial mixing process, while morphological characteristics of phytoplankton shifted from the larger Volume (V) to the smaller V at the same time. Besides, artificial mixing may be more likely to result in structural variation in lower phytoplankton abundance than in higher abundance. The mixing depth (Zmix) and light availability (Zeu/Zmix; the euphotic depth (Zeu)) were the key drivers of phytoplankton succession caused by artificial mixing in XKR, rather than water temperature (WT) and nutrients. This study also provided a successful example of effective control of phytoplankton overgrowth in a reservoir under higher WT and nutrient conditions, which had important implications for ecological managers and researchers in reservoirs or lakes.

Phytoplankton response to artificial and natural mixing in a warm stratified drinking water reservoir: A simultaneous comparative study

Artificial mixing is usually one of the means to improve water quality in reservoirs or lakes. In this study, XiKeng Reservoir (XKR), which was equipped with water-lifting aerators (WLAs) capable of artificial mixing, was selected in southern China. Using two phytoplankton classification methods (taxonomic and functional groups), high-frequency monitoring was synchronized for six months in the old and new reservoir areas to investigate the characteristics and mechanisms of phytoplankton response to artificial mixing and natural mixing, and to compare the differences between the two mixing processes. The results showed that both artificial and natural mixing significantly decreased phytoplankton abundance in the surface water layer. The phytoplankton abundance in the vertical dimension became homogenized with complete water column mixing. During the artificial mixing process, the phytoplankton shifted from Cyanobacteria to Bacillariophyta, and the functional groups shifted from M, SN, and S1 to P and D. The taxonomic groups shifted in line with artificial mixing during the natural mixing process, while the functional groups shifted from SN and S1 to P. While the mixing depth (Zmix), light availability (Zeu/Zmix; the euphotic depth (Zeu)), and relative water column stability (RWCS) were the main drivers of phytoplankton change due to artificial mixing at XKR, the natural mixing process was driven by RWCS and water temperature (WT). This study also provided a successful example of effective control of phytoplankton overgrowth in a warmly stratified drinking water reservoir, which will be valuable to water quality and ecological managers.

Spatiotemporal Distribution of Phytoplankton Functional Groups in Baihua Reservoir: Implications for Ecosystem Management

Functional groups are an effective method for assessing water quality. From January 2020 to December 2023, the phytoplankton and environmental variables at five sites in Baihua Reservoir (BHR) were collected once a month. The succession rate (SR) and the average variation degree (AVD) of the functional groups were determined, and the corresponding driving factors were analyzed by using the Random Forest model, hierarchical partitioning, and Mantel test. A total of 95 phytoplankton species belonging to 7 taxonomic categories were identified, which can be divided into 27 functional groups and 8 dominant functional groups (B, D, L0, P, S1, W1, W2, Y). B, L0, and Y occupied dominant positions in spatiotemporal succession, indicating that the water body was in a mesotrophic to eutrophication state. Water temperature, total nitrogen, and transparency were the key factors driving the functional groups' succession. Total nitrogen, total phosphorus, permanganate index, and dissolved oxygen were significantly positively correlated with AVD (n = 230; p < 0.01). SR not only directly positively affected AVD (n = 230; p < 0.05) but also indirectly affected AVD by affecting physicochemical factors. Understanding the relationship between the succession, stability, and environmental factors of functional groups is of great significance for algae management and the prevention of water bloom.

Applicability of plant-clay mineral composite for rapid algae removal from eutrophic freshwaters at the laboratory and field scales

The global issue of water source eutrophication is exacerbated by increasing industrialization and urbanization, posing significant challenges for clean water management. Although strategies such as nutrient management and biomanipulation are employed, these methods often take longer to demonstrate effectiveness and indirectly work on algal blooms. This has led to the evaluation of eco-friendly technologies such as plant-mineral composites (PMCs) for faster and targeted control of algal proliferation and organic pollution. This study assessed the suitability of PMCs for rapid improvement of eutrophic water quality (focusing on algal control) and optimized their application methods at laboratory and field scales. Laboratory experiments were conducted to identify the critical factors influencing removal activity (RA), considering variables such as water temperature and light intensity. Field trials in reservoirs and a water treatment plant (WTP) explored the controlling factors influencing the RAs for various pollutants. Optimal conditions for maximizing PMC efficacy were determined using response surface methodology (RSM) and generalized linear models. RSM highlighted water temperature as a key factor influencing chlorophyll a RA in a unimodal manner, while demonstrating PMC's effectiveness across varying concentrations, depths, and pH levels. Results from the WTP emphasized the high PMC efficacy in humic matter-rich environments, and those from reservoirs consistently demonstrated PMC's effectiveness regardless of ambient water quality factors such as nutrient and conductivity levels. Comparative analyses indicated distinct PMC impact on algae-associated parameters, emphasizing its potential as an innovative solution for utilizing plant allelopathy and mineral adsorption for efficient algal bloom control and water quality enhancement.

Response of the algal-bacterial community to thermal stratification succession in a deep-water reservoir: Community structure, co-assembly patterns, and functional groups

Thermal stratification in lakes and reservoirs may intensify and become more persistent with global warming. Periodic thermal stratification is a naturally occurring phenomenon that indicates a transition in aquatic ecosystem homeostasis, which could lead to the deterioration of water quality and impaired aquatic communities. However, the responses of communities and associated nutrient cycling processes to periodic thermal stratification are still poorly understood. This study delved into the changes in water quality, algal-bacterial communities, and functional diversity influenced by thermal stratification succession, and their relationship with nutrient cycling. The results indicated that the apparent community dynamics were driven by environmental factors, with ammonium (NH4+) and nitrate (NO3--N) being the most important factors that influenced the algal and bacterial community structure, respectively. Ecological niche widths were narrower during thermal stratification, exacerbating the antagonism of the communities, and stochastic processes dominated community assembly. Then, the complexities of the co-occurrence network decreased with succession. Algal community assembly became more deterministic, while bacterial assembly became more stochastic. Moreover, the roles of algal-bacterial multidiversity in nutrient cycling differed: bacterial diversity enhanced nutrient cycling, whereas algal diversity had the opposite effect. These findings broadened our understanding of microbial ecological mechanisms to environmental change and provided valuable ecological knowledge for securing water supplies in drinking water reservoirs.

Response of phytoplankton community to dissolved organic matter composition and lake trophic state

Human activities, intensified urbanization and climate changes altered source and quantity of dissolved organic matter (DOM), complicating its interaction with phytoplankton in aquatic ecosystems. However, relationship between DOM and phytoplankton in urban lakes strongly disturbed by human activities was still unclear. Thus, a whole-year sampling campaign was conducted in the Tangxun Lake, China's largest urban lake, to reveal the interaction between DOM and phytoplankton. Results indicated that trophic state in the Tangxun Lake varied from mesotrophic to moderately eutrophic. Parallel factor analysis method combined with excitation-emission matrix fluorescence spectroscopy revealed that DOM in the Tangxun Lake consisted of three components, two protein-like components (C1, C3), and one humic-like component (C2). Protein-like components occupied 80% ± 11% of total CDOM pool, mainly due to urbanization driving DOM to be more protein-like, less humic-like. Besides, DOM in the Tangxun Lake was mainly autochthonous input and more recently formed. Furthermore, a total of 129 phytoplankton species were identified, belonging to 78 genera and 7 phyla. Tangxun Lake's phytoplankton community structure was dominated by the Chlorophyta-Bacillariophyta-Cyanophyta type. The temporal succession of phytoplankton varied significantly. It was found that the abundance of Cryptophyta and Cyanophyta were predominant in the mesotrophic state, while Cyanophyta and Bacillariophyta were prevailing in the eutrophic and middle-eutrophic states. As for the interaction between DOM and phytoplankton, results demonstrated that phytoplankton biomass was significantly positively correlated with a (254), a proxy of DOM abundance. Moreover, phytoplankton abundance and biomass significantly positively correlated with autochthonous and freshly released DOM, indicating that the more autochthonous and freshly released DOM, the higher phytoplankton abundance and biomass. Overall, this study provides profound environmental implications for aquatic ecosystem management, especially those strongly affected by human activities.

A functional-group-based perspective on the response of marine phytoplankton to mesoscale eddies

This study analyzed the response of marine phytoplankton to environmental changes induced by mesoscale warm eddies through the lens of functional groups, highlighting the complex interactions within the ecosystem. It was found that warm eddies significantly affected phytoplankton distribution, with cell abundance in the center being only 75.60 cells/L, compared to 1095.00 cells/L in the periphery. Vertical transport within warm eddies altered light conditions, affecting photophilic diatoms more, while increased temperatures favored the growth of warm-water dinoflagellates. This study also emphasized that ocean currents were significant factors, showing correlations with various functional groups and playing a key role in material transport and phytoplankton distribution. Additionally, the distinct responses of different functional groups to temperature and salinity underscored their unique adaptations to environmental changes. In periods without warm eddies, phytoplankton primarily congregated in shallower water layers.

Phytoplankton functional groups as indicators of environmental changes in weir and non-weir sections of the lower Nakdong River, Republic of Korea

The Nakdong River underwent water impoundment after eight weirs were constructed as part of South Korea's Four Major River Restoration Project from 2009 to 2012. In this study, we aimed to confirm whether the assemblage of phytoplankton based on phytoplankton functional groups (PFGs), could indicate environmental changes in the weir section (WS) and non-weir section (NWS) of the lower Nakdong River after the construction of the weir. Thus, we examined the relationships between PFGs and gradients in environmental drivers, such as physicochemical, meteorological, and hydrological variables. Environmental gradients were observed between the WS and NWS in dissolved oxygen (DO), electric conductivity (EC), biochemical oxygen demand (BOD), chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), dissolved total nitrogen (DTN), dissolved total phosphorus (DTP), ammonia nitrogen (NH3-N), nitrate nitrogen (NO3-N), and phosphorus (PO4-P), which were relatively higher in the WS. Seventeen PFGs were identified (A, B, C, D, E, F, G, H1, J, LM, LO, MP, P, T, W1, X1, and X2). Additionally, the LM and P groups, preferring an enriched lentic system more than other groups, were found to be the dominant PFGs that led the succession of assemblages. Traditional nutrients (N, P) and organic pollutants (BOD, COD) primarily affected the autochthonous growth of the most dominant PFGs in the WS as HRT (hydraulic retention time) increased. Furthermore, the hydrological variables associated with meteorological conditions have a synergistic effect on the composition of the major PFGs and chemical and physical variables in the WS. In other words, the WS may be a new source of inoculum that primarily determines the occurrence and maintenance of phytoplankton in the immediate downstream region (NWS). In particular, group LM (mainly potentially toxic Microcystis) developing in the upper weir impoundment is transported downstream, resulting in a high inoculation effect on further growth in the NWS during the summer monsoon season.

Spatio-temporal dynamics of phytoplankton in a diversion reservoir and the major influencing factors: taxonomic versus functional groups classification

Identifying factors affecting phytoplankton dynamics is crucial to the management of aquatic ecosystems. A lot of scholars have conducted intensive studies on phytoplankton in lake or reservoirs, but not many studies have been conducted on diversion reservoirs. To explore the seasonal and spatial variation of phytoplankton communities and their relationship with environmental factors in the context of water diversion, a case study was carried out at XiKeng (XK) reservoir in South China. In this study, month-by-month water samples and phytoplankton were collected from this reservoir from December, 2021, to July, 2022. The results showed that the phytoplankton community was characterized by significant spatial and temporal variations. There were significant differences in phytoplankton abundance and structure in the reservoirs in terms of time. The abundance of phytoplankton cells and the proportion of Cyanobacteria in the reservoir showed a trend of increasing from autumn to spring and then decreasing from spring to summer, while the functional group evolved from S1 in autumn to SN in spring and summer. The abundance of phytoplankton was influenced by the dynamic water division and the characteristics of the reservoir itself, resulting in a spatial distribution characteristic of AIII > AII > AI. Water temperature (WT) and nutrients were the key factors driving the changes in phytoplankton abundance and community structure in the reservoir. These findings will deepen our understanding of the spatial and temporal dynamics of phytoplankton community structure in diversion reservoirs and provide a basis for freshwater water ecological management strategies.

Stringent Response of Cyanobacteria and Other Bacterioplankton during Different Stages of a Harmful Cyanobacterial Bloom

We conducted a field study to investigate the role of stringent response in cyanobacteria and coexisting bacterioplankton during nutrient-deprived periods at various stages of bloom in a freshwater lake (Utah Lake) for the first time. Using metagenomics and metatranscriptomics analyses, we examined the cyanobacterial ecology and expression of important functional genes related to stringent response, N and P metabolism, and regulation. Our findings mark a significant advancement in understanding the mechanisms by which toxic cyanobacteria survive and proliferate during nitrogen (N) and phosphorus (P) limitations. We successfully identified and analyzed the metagenome-assembled genomes (MAGs) of the dominant bloom-forming cyanobacteria, namely, Dolichospermum circinale, Aphanizomenon flos-aquae UKL13-PB, Planktothrix agardhii, and Microcystis aeruginosa. By mapping RNA-seq data to the coding sequences of the MAGs, we observed that these four prevalent cyanobacteria species activated multiple functions to adapt to the depletion of inorganic nutrients. During and after the blooms, the four dominant cyanobacteria species expressed high levels of transcripts related to toxin production, such as microcystins (mcy), anatoxins (ana), and cylindrospermopsins (cyr). Additionally, genes associated with polyphosphate (poly-P) storage and the stringent response alarmone (p)ppGpp synthesis/hydrolysis, including ppk, relA, and spoT, were highly activated in both cyanobacteria and bacterioplankton. Under N deficiency, the main N pathways shifted from denitrification and dissimilatory nitrate reduction in bacterioplankton toward N2-fixing and assimilatory nitrate reduction in certain cyanobacteria with a corresponding shift in the community composition. P deprivation triggered a stringent response mediated by spoT-dependent (p)ppGpp accumulation and activation of the Pho regulon in both cyanobacteria and bacterioplankton, facilitating inorganic and organic P uptake. The dominant cyanobacterial MAGs exhibited the presence of multiple alkaline phosphatase (APase) transcripts (e.g., phoA in Dolichospermum, phoX in Planktothrix, and Microcystis), suggesting their ability to synthesize and release APase enzymes to convert ambient organic P into bioavailable forms. Conversely, transcripts associated with bacterioplankton-dominated pathways like denitrification were low and did not align with the occurrence of intense cyanoHABs. The strong correlations observed among N, P, stringent response metabolisms and the succession of blooms caused by dominant cyanobacterial species provide evidence that the stringent response, induced by nutrient limitation, may activate unique N and P functions in toxin-producing cyanobacteria, thereby sustaining cyanoHABs.

Environmental heterogeneity affecting spatial distribution of phytoplankton community structure and functional groups in a large eutrophic lake, Lake Chaohu, China

The growth and development of phytoplankton are influenced by physico-chemical parameters, which can also affect the spatial distribution of phytoplankton community structure. However, it is unclear whether environmental heterogeneity caused by multiple physico-chemical factors can affect the spatial distribution of phytoplankton and its functional groups. In this study, we investigated the seasonal variation and spatial distribution of phytoplankton community structure and its relationships with environmental factors in Lake Chaohu from August 2020 to July 2021. We recorded a total of 190 species from 8 phyla, which were divided into 30 functional groups, including 13 dominating functional groups. The average annual phytoplankton density and biomass were (5.46 ± 7.17) × 10⁷ cells/L and 4.80 ± 4.61 mg/L, respectively. The density and biomass of phytoplankton were higher in summer ((14.64 ± 20.34) × 10⁷ cells/L, 10.61 ± 13.16 mg/L) and autumn ((6.79 ± 3.97) × 10⁷ cells/L, 5.57 ± 2.40 mg/L), with the M and H2 of dominant functional groups. The dominant functional groups were N, C, D, J, MP, H2, and M in spring, whereas functional groups C, N, T, and Y dominated in winter. The distribution of phytoplankton community structure and dominant functional groups exhibited significant spatial heterogeneity in the lake, which was consistent with the environmental heterogeneity of the lake and could be classified into four locations. Location I had higher phytoplankton density and biomass than the other three locations. Additionally, dominant functional groups M, C, and H2 were present throughout the lake, and all 13 dominant functional groups were observed in Location II. Our findings suggest that environmental heterogeneity is a key factor influencing the spatial distribution of phytoplankton functional groups in Lake Chaohu.

Basin-specific pollution and impoundment effects on greenhouse gas distributions in three rivers and estuaries

Large uncertainties exist regarding the combined effects of pollution and impoundment on riverine greenhouse gas (GHG) emissions. It has also been debated whether river eutrophication can transform downstream estuaries into carbon sinks. To assess human impacts on the riverine and estuarine distributions of CO₂, CH₄, and N₂O, two source-to-estuary surveys along three impounded rivers in Korea were combined with multiple samplings at five or six estuarine sites. The basin-wide surveys revealed predominant pollution effects generating localized hotspots of riverine GHGs along metropolitan areas. The localized pollution effect was pronounced in the lower Han River and estuary adjacent to Seoul, while the highest GHG levels in the upper Yeongsan traversing Gwangju were not carried over into the faraway estuary. CH₄ levels were elevated across the eutrophic middle Nakdong reaches regulated by eight cascade weirs in contrast to undersaturated CO₂ indicating enhanced phytoplankton production. The levels of all three GHGs tended to be higher in the Han estuary across seasons. Higher summer-time δ¹³C-CH₄ values at some Nakdong and Yeongsan estuarine sites implied that temperature-enhanced CH₄ production may have been dampened by increased CH₄ oxidation. Our results suggest that the location and magnitude of pollution sources and impoundments control basin-specific longitudinal GHG distributions and estuarine carryover effects, warning against simple generalizations of eutrophic rivers and estuaries as carbon sinks.

Controlling spring Dinoflagellate blooms in a stratified drinking water reservoir via artificial mixing: Effects, mechanisms, and operational thresholds

Water-lifting aerators (WLAs) are often applied in stratified reservoirs to activate artificial mixing to inhibit harmful algal blooms (HABs). Here, the effects, mechanisms, and operational thresholds of spring Dinoflagellate control via artificial mixing were studied using a combination of taxonomic and functional groups and boundary line models. Algal cell density at two sampling sites (i.e., S1 and S2) decreased to below 1.0 × 10⁶ cells/L (corresponding chlorophyll-a content under 10 μg/L) during artificial mixing, with a Dinoflagellate removal efficiency of 97.1 % at S1 and 95.5 % at S2, respectively. The succession patterns of main phytoplankton taxonomic and functional groups changed greatly during artificial mixing at the sites: from Dinoflagellate and motile Chlorophyta to Bacillariophyta from groups A/L_O/P to A, respectively. Water temperature (WT), light availability (Z_eu/Z_mix), and mixing depth (Z_mix) were more effective factors influencing phytoplankton dynamics at a short-term scale, followed by total phosphorus (TP). A decrease in surface WT and Z_eu/Z_mix, and increase in Z_mix alongside the improvement of TP levels, which were induced by WLAs, drove the Dinoflagellate bloom control by a shift of phytoplankton structure from large, motile, and low surface to volume ratio (S/V) to small, immobile, and high S/V algae. The operational threshold values of WT, Z_eu/Z_mix, Z_mix and TP concentration for the suppression of Dinoflagellate growth using mixing systems are recommended as 9.6 °C, 0.17, 11.5 m, and 0.020 mg/L, respectively, based on a boundary line analysis. This work can help improve the cognition of mechanisms controlling HABs using mixing and aeration techniques in reservoirs.

Will a heavy sediment load affect responses of phytoplankton functional groups to aquatic environmental changes in different water body types?

Sediment, as a natural component of rivers, directly affects the abundance and function of phytoplankton by altering water physicochemical properties. Despite mounting evidence for the sensitivity of phytoplankton to environmental factors, the responses of phytoplankton functional groups to complex environmental changes in rivers with a heavy sediment load are still poorly understood. Herein, the effectiveness of phytoplankton functional groups was evaluated as an indicator of aquatic environmental changes in a heavily sediment-laden river. Samples were collected from 44 sites (22 free-flowing river sections and 22 man-made reservoir sections) with a mean annual sediment concentration of 4.69 kg m^-3 in the Yellow River, China. A total of 31 phytoplankton functional groups were classified during spring (April-May) and autumn (September-October) in 2019. Groups C, MP, and D, which are well adapted to strong water disturbances and turbid habitats, showed distinct advantages over other groups. Despite no significant differences in many environmental variables between the river and reservoir sections, these variables (especially nitrogen nutrients) had remarkable effects on the phytoplankton community structure. The phytoplankton functional groups were sensitive to environmental changes even under sediment interference, although geo-climatic variables also exhibited non-trivial effects. The mean niche breadth of the abundant taxa (river: 11.16; reservoir: 7.93) was higher than that of the rare taxa (river: 5.64; reservoir: 4.86) in different water bodies. Thus, growth and diffusion of the abundant taxa played paramount roles in maintaining ecosystem stability. The results indicate that, in a large-scale sediment-laden river, phytoplankton functional groups can effectively indicate changes in the aquatic environment of either a free-flowing river or a man-made reservoir.

Impact of River-Reservoir Hybrid System on Zooplankton Community and River Connectivity

Anthropogenic connectivity regulation in rivers, such as via weirs and dams, affects the plankton community. We hypothesized that the longitudinal similarity of the zooplankton community in a river could change in a river–reservoir hybrid system (RRHS). The impact of weir construction on zooplankton communities in terms of species diversity, abundance, and community structure was examined biweekly at six sites on the Nakdong River for 14 years (before construction: 2002–2008; after construction: 2012–2018). We checked time-series alignment using a dynamic time-warping method between longitudinal survey sites. After RRHS, the zooplankton community showed an increasing number of species. However, RRHS decreased the longitudinal similarity in terms of number of zooplankton species and population density. Our results demonstrate the negative effect of lateral infrastructures on zooplankton populations due to river fragmentation and habitat alteration.

Controlling phytoplankton blooms in a canyon-shaped drinking water reservoir via artificial and induced natural mixing: Taxonomic versus functional groups

Water-lifting aerators (WLAs) were often deployed in reservoirs to achieve artificial mixing (WLA activation) and induced (natural) mixing (early occurrence of complete natural mixing after WLA deactivation) for water quality improvement. Here, the mechanisms controlling phytoplankton growth via artificial and induced mixing were explored using a combination of taxonomic and functional classifications based on two-year monitoring data (i.e., non-operational and operational years of the WLAs). Artificial mixing resulted in a decrease of 99.2 % in phytoplankton cell density compared to that of the non-operational year, which continuously diminished to (3.06 ± 0.59) × 10⁶ cells/L during induced mixing. The succession of phytoplankton structure in taxonomic and functional classification levels changed from Cyanobacteria to Chlorophyta and Bacillariophyta, from groups F, J, H1, and L_M to A and X1, respectively, by comparison of the non-operational and operational years. Decreases in surface water temperature, total phosphorus concentration, and light availability, and increases in mixing depth via artificial and induced mixing were responsible for phytoplankton control, especially for cyanobacterial blooms, depending on a shift in phytoplankton composition from large or colonial, low surface to volume (S/V) to small, high S/V genera. Artificial and induced mixing also improved the trophic/ecological status of the reservoir, from "hyper-eutrophic and bad level" to "light-eutrophic and excellent level", based on an assessment of the trophic level index (TLI) and phytoplankton assemblage (Q) index. This study demonstrates that the suitable combination of artificial and induced mixing plays a crucial role in the maintenance and extension of healthy ecosystems in reservoirs.

Cyanobacterial community succession and associated cyanotoxin production in hypereutrophic and eutrophic freshwaters

Cyanobacterial harmful algal blooms (cyanoHABs) in freshwater bodies are mainly attributed to excess loading of nutrients [nitrogen (N) and phosphorus (P)]. This study provides a comprehensive review of how the existing nutrient (i.e., N and P) conditions and microbial ecological factors affect cyanobacterial community succession and cyanotoxin production in freshwaters. Different eutrophic scenarios (i.e., hypereutrophic vs. eutrophic conditions) in the presence of (i) high levels of N and P, (ii) a relatively high level of P but a low level of N, and (iii) a relatively high level of N but a low level of P, are discussed in association with cyanobacterial community succession and cyanotoxin production. The seasonal cyanobacterial community succession is mostly regulated by temperature in hypereutrophic freshwaters, where both temperature and nitrogen fixation play a critical role in eutrophic freshwaters. While the early cyanoHAB mitigation strategies focus on reducing P from water bodies, many more studies show that both N and P have a profound contribution to cyanobacterial blooms and toxin production. The availability of N often shapes the structure of the cyanobacterial community (e.g., the relative abundance of N₂-fixing and non-N₂-fixing cyanobacterial genera) and is positively linked to the levels of microcystin. Ecological aspects of cyanotoxin production and release, related functional genes, and corresponding nutrient and environmental conditions are also elucidated. Research perspectives on cyanoHABs and cyanobacterial community succession are discussed and presented with respect to the following: (i) role of internal nutrients and their species, (ii) P- and N-based control vs. solely P-based control of cyanoHABs, and (iii) molecular investigations and prediction of cyanotoxin production.

Phytoplankton community structural reshaping as response to the thermal effect of cooling water discharged from power plant

The increase of water temperature caused by the thermal effect of cooling water discharged from power plants has become a major environmental problem, especially its influence on phytoplankton community. The change of water temperature usually reshapes the structure of phytoplankton community. A research combining phytoplankton community and thermal discharge of power plants was conducted to identify the potential influences. Results indicated the average annual water temperature of the reservoir increased gradually by 5-11 °C because of the thermal discharge. Through annual diversity analysis, 139 species or taxa from 6 phyla (i.e., Bacillariophyta, Chlorophyta, Cyanobacteria, Euglenophyta, Dinoflagellata, and Cryptophyta) were found in different sampling sites, among which Bacillariophyta was the dominant community. Preliminary experimental results revealed the increasing temperature completely reshaped the phytoplankton community structure, especially during the cold season, and this was confirmed by the results of redundancy analysis. In addition, lots of thermophilic genera (i.e., Synedra, Nitzschia, and Navicula) were detected at sampling station 1 (Spt1) and sampling station 2 (Spt2) where the effect of thermal discharge was the most obvious. The increase in biomass and cell count of Bacillariophyta was the result of thermal effect, especially in cold season. Besides, consequences also revealed some environmental parameters (i.e., dissolved oxygen concentration, chlorophyll a concentration, and transparency) were affected by the thermal discharge. Chlorophyll a concentration exhibited a slow rising trend while dissolved oxygen concentration and transparency gradually decreased.

Phytoplankton Community Response to Nutrients, Temperatures, and a Heat Wave in Shallow Lakes: An Experimental Approach

Phytoplankton usually responds directly and fast to environmental fluctuations, making them useful indicators of lake ecosystem changes caused by various stressors. Here, we examined the phytoplankton community composition before, during, and after a simulated 1-month heat wave in a mesocosm facility in Silkeborg, Denmark. The experiment was conducted over three contrasting temperature scenarios (ambient (A0), Intergovernmental Panel on Climate Change A2 scenario (circa +3 °C, A2) and A2+ %50 (circa +4.5 °C, A2+)) crossed with two nutrient levels (low (LN) and high (HN)) with four replicates. The facility includes 24 mesocosms mimicking shallow lakes, which at the time of our experiment had run without interruption for 11 years. The 1-month heat wave effect was simulated by increasing the temperature by 5 °C (1 July to 1 August) in A2 and A2+, while A0 was not additionally heated. Throughout the study, HN treatments were mostly dominated by Cyanobacteria, whereas LN treatments were richer in genera and mostly dominated by Chlorophyta. Linear mixed model analyses revealed that high nutrient conditions were the most important structuring factor, which, regardless of temperature treatments and heat waves, increased total phytoplankton, Chlorophyta, Bacillariophyta, and Cyanobacteria biomasses and decreased genus richness and the grazing pressure of zooplankton. The effect of temperature was, however, modest. The effect of warming on the phytoplankton community was not significant before the heat wave, yet during the heat wave it became significant, especially in LN-A2+, and negative interaction effects between nutrient and A2+ warming were recorded. These warming effects continued after the heat wave, as also evidenced by Co-inertia analyses. In contrast to the prevailing theory stating that more diverse ecosystems would be more stable, HN were less affected by the heat wave disturbance, most likely because the dominant phytoplankton group cyanobacteria is adapted to high nutrient conditions and also benefits from increased temperature. We did not find any significant change in phytoplankton size diversity, but size evenness decreased in HN as a result of an increase in the smallest and largest size classes simultaneously. We conclude that the phytoplankton community was most strongly affected by the nutrient level, but less sensitive to changes in both temperature treatments and the heat wave simulation in these systems, which have been adapted for a long time to different temperatures. Moreover, the temperature and heat wave effects were observed mostly in LN systems, indicating that the sensitivity of phytoplankton community structure to high temperatures is dependent on nutrient availability.