3.1 Spatial chl-a distribution and effect of nutrient additions on growth
The different locations of XXB during summer showed strong spatial variabilities in the chl-a distributions as demonstrated by the control across the three points (Fig. 1); XX01 (19.41 ± 3.29 µgL− 1), XX06 (38.98 ± 4.04 µgL− 1), and XX09 (183.02 ± 28.31 µgL− 1). For XX01 and XX06 in day 2 (Fig. 1a), all nutrients addition involving N enrichment led to biomass production which differed significantly with the control (p < 0.05). However, in XX09, N-alone addition resulted in maximum growth response (248.74 ± 14.97 µgL− 1) which was almost 2 times the control (183.02 ± 28.31 µgL− 1) and the P-alone addition (180.19 ± 18.19 µgL− 1). The growth response (257.78 ± 12.54 µgL−1) obtained from N&P simultaneous addition showed no significant difference with the N-alone addition but significantly differed with both control and P addition. This trend in growth responses was consistent through day 4, especially in the point XX09. The growth response observed in P-alone addition (176.05 ± 5.45 µgL− 1) did not differ significantly with the control (191.15 ± 17.50 µgL− 1) throughout the incubation (Fig. 1b). The result of the growth rate (Fig. 1c) showed that P enrichments across the studied points; XX01 (-0.03 ± 0.01 µgL− 1d− 1), XX06 (-0.01 ± 0.02 µgL− 1d− 1), and XX09 (-0.06 ± 0.02 µgL− 1d− 1) did not influence primary productivity as the consistent negative rate of growth just like the control characterized all P enrichments. Conversely, N enrichments led to positive growth rate suggesting significant stimulation on the biomass production in summer induced by N dynamics in the XXB. The maximum growth rates (0.11 ± 0.02 µgL− 1d− 1) for N alone and (0.14 ± 0.03 µgL− 1d− 1) for NP contemporaneous enrichments were reported in XX06. The nutrient response pattern strongly revealed N-limitation indicating that N addition controlled the bulk of the growth in summer. This was, especially true for XX09 wherein N enrichment induced maximum productivity. The N-limited conditions in summer have been reported e.g., the large but shallow hyper-eutrophic Lake Taihu, China (Paerl et al., 2011) and XXB (Li et al., 2020). Under the productive period usually summer, N-limitation in the aquatic system is predominantly observed (Domingues et al., 2011; Yahdjian et al., 2011). Liikanen and Martikainen, (2003) and Rangel et al., (2012) have also indicated that high temperature is associated with ammonification and exacerbates nitrate reductions (denitrification) processes with greater Fe-bound phosphorus releases from the chemical weathering of rocks. Although points; XX01 and XX06 also demonstrated potentially N limited conditions, the magnitude of N limitation in the mainstream (XX09) was more prominent throughout the incubation. The large row-crop agriculture dominant in XX01 and XX06 over XX09 could have contributed to the considerable N replenishment in these points (Zhou et al., 2012). For example, a high concentration of DIN in the mouth of XXB and high PO4−P towards the upstream has been previously reported (Ye et al., 2007). It was further observed that P-alone addition led to slight repression on growth from 191.15 ± 17.50 µgL− 1 in the control to 176.05 ± 5.45 µgL− 1 in the P-alone addition as incubation lasted through day 4. This suggested P replete condition in XXB during summer and excess P supply in summer would not influence primary productivity. Another possible reason could be the fact that N- fixation usually does not seem to be able to fully satisfy N limitations in rivers and streams when P is present over N (Dodds and Smith, 2016) indicating that excess of P further reduces N-availability with possible repression on growth of the high N-dependent primary producers. Again, a recent study reported repression on the algal growth, Chlorella regularis by a large excess of P (250 mg L− 1) under an N- limited condition (Fu et al., 2019). The significant biomass production in XX09 could be attributed to the water column nutrient the concentrations (TN = 0.8 mgL− 1, TP = 0.07 mgL− 1) in the mainstream of the Three Gorges Reservoir (Li et al., 202; Nwankwegu et al., 2020) which have been considered far beyond the internationally recognized eutrophication thresholds (Eitzmann et al., 2009; Urquhart et al., 2017). Water with high TN and TP is flushed into the XXB tributary during the backwater phenomenon in peak rise seasons (Nwankwegu et al., 2019). Although, the spatial distribution of chl-a in Lake Poyang, the largest freshwater lake in Yangtze River, China apart from nutrient fluxes has been linked to the shade index (Wu et al., 2014). The Yangtze River Basin, particularly the XXB reaches is an open system with no significant shade index throughout the year (Nwankwegu et al., 2020b). This indicates that the nutrient variations induced by the TGR operations which significantly affect downstream N and P stoichiometry could be the major factor determining the spatial distribution of biomass as chlorophyll-a. The spatial chl-a distribution has also been attributed to the variations in the limiting factors induced by the concurrent hydrological modifications (Mangoni et al., 2017) and the interchanging hydraulic control due to damming (Nwankwegu et al., 2019). The effect of different nitrogen forms; Nitrate-nitrogen and ammonium-nitrogen, as well as phosphorus in separate combinations with Fe was further evaluated (Fig. 2). It indicated that growth responses varied considerably. The nutrient additions involving; NO3 + Fe and NH4 + Fe revealed no significant variation under 2 d incubation but differed significantly (p < 0.05) with the growth response obtained through P + Fe addition under the same condition and time. However, as incubation lasted through day 4, a growth response which significantly differed with NH4 + Fe was observed in the NO3 + Fe combined enrichment. This confirms the metabolic association between NO3 and Fe over NH4 and Fe in the aquatic ecosystem functioning (Nwankwegu et al., 2020b). The net growth rates reported in the combined enrichment NO3−/Fe (0.139 ± 0.009 µgL− 1d− 1) and P/Fe (0.141 ± 0.03 µgL− 1d− 1) supported the preference of the combinations in promoting growth than the NH4+/Fe affirming the key role of iron in the nitrate uptake by the majority of phytoplankton (Nwankwegu et al., 2019). This substantially corroborated recent reported that iron requirement is greatly influenced by the nitrogen sources and phytoplankton growing on NH4+ have low Fe requirement than those on NO3− (Nwankwegu et al., 2020b). The possible reason could be attributed to the fact that the former is directly incorporated into amino acids while the later has to be converted to NH4 (Berman-Frank et al., 2007; Schoffman et al., 2016) prior to assimilation thus the high energetic investment would only select a dominance of a few taxa, particularly the diatoms (Nwankwegu et al., 2019) which demonstrate obligate nitrate uptake. However, maximum growths, (45.25 ± 5.33 µgL− 1) in day 2 and (70.84 ± 3.26 µgL− 1) in day 4 were reported in P + Fe contemporaneous addition. A previous study also reported the enhancement of cyanobacterial bloom biomass (CBB) through phosphorus enrichment in the eutrophic Lake Taihu, China (Chen et al., 2014). Besides, absolute cyanobacterial dominance is usually induced at low N:P ratio (Li et al., 2020; Nwankwegu et al., 2019). The cyanobacterial biomass shows great response to changes in concentration of P in both annual and monthly ranges (Ding et al., 2018). Further, Sondergarrd et al (2011) previously reported strong relationship of biomass production as Chl-a with TP over TN in the late summer bloom.
3.2 Seasonal nutrient dynamics
The nutrients concentrations (Fig. 3) across the four seasons showed strong variations. While the seasonal variations in TN did not show a significant difference among seasons, the seasonal variations in the TP revealed a significant difference (p < 0.05) among seasons with the least TP concentrations observed during autumn (0.16 ± 0.11 µgL− 1) and spring (0.10 ± 0.00 µgL− 1) suggesting the limitation by P in both autumn and spring. This depicts that stricter N or N&P reductions would be crucial for lasting bloom control, especially during autumn and spring in XXB. Although, studies had indicated that all nutrient depletion may not essentially define limitation as phytoplankton often build up their intracellular nutrient pools via luxury consumption (Li et al., 2020; Domingues et al., 2017) for utilization when nutrient gradient tends low in the ecosystem. The maximum concentrations in TP were observed during summer (0.93 ± 0.13 µgL− 1) and winter (0.70 ± 0.092 µgL− 1). The seasonal dynamics in the TN/TP ratio were; 12.37 ± 1.98 in autumn, 16.81 ± 2.43 in winter, 20.37 ± 0.19 in spring, and 6.44 ± 1.91 during summer. This indicates that the dynamics in the TN/TP stoichiometry is principally controlled by seasonal variability in the TP concentrations. Considering that Ding et al., (2018) had stated that TN/TP < 9 characterizes N limitation while TN/TP > 22.6 defines p-limitation, the TN/TP in the present study thus revealed strong N limitation in summer and P limitation in spring (Fig. 3a). However, the reliability of TN/TP ratios (stoichiometry) as an index for nutrient limitation has been previously questioned (Lv et al., 2011). The maximum concentration of NO3 (2.98 ± 0.28 µgL− 1) and the least concentration in NH4 (0.28 ± 0.09 µgL− 1) was reported in spring indicating a nitrate replete and ammonium deficit condition during spring bloom while the minimum concentration of NO3 (0.81 ± 0.02 µgL− 1) was observed during summer. This further explains why in Fig. 2 above, Fe addition with NO3 led to significant biomass production. The maximum concentration in NH4 (1.08 ± 0.07 µgL− 1) was reported in winter probably reflecting the total absence of ammonium-dependent phytoplankton groups e.g., the Cyanophyta in winter (Zhou et al., 2012). The dissolved N (DTN) and P (DTP) also showed strong fluctuations across seasons. The maximum DTN concentration (1.58 ± 0.09 µgL− 1) was obtained in autumn and least in spring (Fig. 3b). It was, however, not surprising to observe a maximum concentration in DTP of 0.09 ± 0.01 µgL− 1 during summer. Statistics revealed that the annual fluctuations in the DTP differed significantly (p < 0.05) across seasons and the significantly elevated DTP in summer could be attributed to the high-temperature dependent P release from legacy stores (Li et al., 2020). Based on the DTN and DTP seasonal variations, it would be sufficient to understand why N-limitation characterizes summer bloom. The seasonal N limitation for harmful algal bloom has been attributed to the internal P loading from sediment (Ding et al., 2018). The high TP concentration in Lake Erie has been linked to the internal phosphorus loading from the profundal sediment (Nürnberg et al., 2019). All forms of N are soluble thus no precipitation mechanism is available for N immobilization (Paerl et al., 2016) and an ecosystem N dynamic is greatly influenced by the phytoplankton uptakes (Nwankwegu et al., 2020a) in addition to natural processes of nitrification and denitrification. Although, significant N-cycling processes are often not possible under microcosm conditions (Domingues et al., 2011). The atmospheric deposition, run-off, and decomposition, which are directly dependent on temperature, pH, and rainfall are the few strategies that ensure ecosystem N replacement. However, the Microcystis spp which are exceptionally harmful, disrupt food web and generate hypoxia often dominate the summer bloom causing the populations of the N2-fixers including the Aphanizomenon, Nostoc, Anabaena, and Cylindrospermopsis to be laid off the system (Li et al., 2020; Paerl et al., 2011). In this case, the protracted N utilization without replenishment and an increasing P supply from sediment and the geogenic resources e.g., phosphorus rocks can exacerbate a critical N-limitation in freshwaters. On the whole, the spatial nutrient response pattern during summer strongly reveals N-limitation indicating that N enrichment would greatly control growth while prolonging primary productivity in the entire system.
3.3. Seasonal dynamics in biomass production as chl-a
The phytoplankton biomass (Fig. 4) considerably varied across seasons. Each season showed peculiar responses on growth induced by the different nutrient enrichments. In autumn, nutrient additions involving N, P, and in combinations revealed biomasses as chl-a in N (48.47 ± 6.05 µgL− 1), P (59.76 ± 6.73 µgL− 1), and N + P (68.40 ± 5.73 µgL− 1). This indicated that all the nutrient enrichment stimulated growth responses which differed significantly with the ambient chl-a concentrations both in the initial and control conditions. Although nutrient promotions on growth by N and P separate additions during autumn showed no significant difference (p > 0.05), the nutrient response adequately showed P-limited than N limited growth condition. However, the XXB nutrient limitation pattern slightly differed from the previous study by Paerl et al., (2011) in a shallow eutrophic freshwater, Lake Taihu where P limitation was reported in winter-spring but it is important to recall that these two freshwater systems have different ecosystem behaviours both in depth, hydrodynamics, nutrient fluxes, and phytoplankton species structure. For example, a consistent P-limitation of phytoplankton growth in eight (8) deep (mean depth ranges 40 m to 107 m) Brazilian tropical hydroelectric reservoirs have been previously reported (Rangel et al., 2012). Again, the nutrient limitation is related to ecosystem local factors including Secchi depth, land use, hydrology, and catchment characteristics (Rangel et al., 2012). In this consideration, Lake Taihu (mean depth = 1.9 m) is a very shallow hyper-eutrophic system (Paerl et al., 2011; Xu et al., 2015). Based on the European Union Water Framework Directive stipulation, freshwater systems with average depths (mean depth > 15 m) is deep, systems with an average depth between 3 and 15 m are shallow while very shallow system show average < 3 m deep (Zou et al., 2020: Phillips et al., 2008), XXB with average mean depth = 39.6 m (Nwankwegu et al., 2020) is, therefore, a typical deep system. Consequently, the natural P-input into XXB associated with the estuarine/watershed upwelling would be essentially negligible, unlike Lake Taihu often characterized by large turbulent burst and sediment resuspension events (Wei et al., 2020; Li et al., 2018) that induce a constant internal P release (Ding et al., 2012). Internal nutrient loading is a potential process regulating phosphorus dynamics contributing up to 86% P reduction from legacy stores, phytoplankton, chlorophyll-a, and cyanobacterial blooms (Radbourne et al., 2019). Qin et al., (2020) recently showed that while P limitation predominates deep lakes and reservoirs, N limitation predominates shallows lakes and reservoirs. This indicates that a deviant nutrient limitation pattern is possible and largely dependent on the water depth. In the same study, the authors argued therein, that the biogeochemical mechanisms associated with water depth essentially control the nutrient dynamics in the freshwater systems. It further demonstrated that in shallow systems usually characterized by mixing depth > maximum depth, sediment exchanges and water column influences are dynamic thus often exacerbate potential N loss (denitrification) and enormous P release from the legacy store through precipitation, leading to low N:P ratio and consequently elicits N limitation. Conversely, in the deep systems, which are characterized by mixing depth < mean depth, the hypolimnion boundary receive minimal turbulent/perturbation seasonally while the maximum hydrodynamic actions are concentrated on the epilimnion. The retarded N loss with the increased P loss often through sedimentation and immobilization directly triggers the elevated N:P ratio causing P limitation to prevail. The release of the high level of remobilized P from sediment which significantly encourages maximal primary productivity has been recently reported in the deep (max depth = 31 m; mean depth = 13.6 m) Rostherne Mere, Cheshire, U.K. (Radbourne et al.., 2019). A significant positive correlation (r = 0.74, p < 0.01) between Secchi depth and chlorophyll-a (chl-a) has been previously reported in the deep (mean depth = 28 m) Çaygören Reservoir, Turkey (Celik, 2012). This invariably suggests that light availability which is critical to chlorophyll-a dynamics decreases with depth in the eutrophic freshwaters. The N&P simultaneous addition in autumn led to maximum growth with P potentially stimulating optimal growth than N in the combinations. It thus reveals that in the light of the nutrient structure, P-alone rather than the traditional association of N&P could drive to a logaritmic biomass production in XXB during autumn.
In winter, all the nutrient additions including N, P and their contemporaneous additions resulted in growth responses which did not differ (p > 0.05) with the control. It was evident to deduce that nutrient limitation did not characterize winter bloom in XXB indicating that the limitation could be attributed to other factors including the dramatically reduced photosynthetically active radiation (PAR) and water temperature. Consequently, ANOVA post hoc analysis revealed significant variations (p < 0.05) in the winter temperature (9.31 ± 1.13°C) relative the other seasons coupled with high dissolved oxygen (DO) of 11.39 ± 2.0 mgL− 1 in winter as shown in Table 1. A positive response of the ecosystem to the light enrichment particularly in winter has been previously reported (Domingues et al., 2017). Again, Tomasky et al., (1999) while studying nutrient limitation of phytoplankton growth in Childs River, an estuary in Waquoit Bay, MA, USA reported no response to nutrient additions during the colder months. The study thus attributed the restriction on the phytoplankton growth during months other than May-Aug growth to the constantly changing factors, such as light, temperature or a physiological mechanism. Ye et al., (2007) also reported temporary disappearance of bloom following a drop of temperature and precipitation in XXB. The present study reported similar ambient physicochemical parameter especially during spring (Table 1) as that of the previous study by Ye et al., (2007) in XXB 13 years ago with the mean values of water quality indices as; DO (mean = 11.81 mg/L), pH (mean = 8.64 mgL− 1), and temperature (mean = 14.94◦C). The protracted stability in the climatic variables, particularly temperature suggests that the XXB eutrophic status over the last 13 years are controlled by the direct impact of human activities while the climate change only contributes to the indirect effect.
In spring, a similar nutrient limitation characteristic as autumn was observed but the magnitude of biomass production in both seasons significantly differed (p < 0.05). In summer, nutrient limitation shifted to N as N-alone addition caused a growth response which did not differ significantly with N&P combined addition but differed significantly with both P-alone addition and the control. The N limitation reported in summer corroborated previous studies in the different freshwater systems (Paerl et al., 2011; Xu et al; 2015). In Waquoit Bay, USA, the accumulation of phytoplankton biomass in brackish and saline water was limited by the supply of nitrate during warm months (Tomasky et al., 1999).
The effect of different nutrient combinations with the essential micro-nutrients including Fe (both alone and in combinations), Si, Mn, Zn, and copper showed strong variability to seasons (Fig. 5). The response on growth stimulation by Fe was significant in both spring and summer. Across the seasons, the trace metals interaction in concert with N and P only significantly promoted growth (56.73 ± 5.18 µgL− 1) in autumn while the culture involving the trace metals combination without N and P additions caused significant growth inhibition rather than stimulation in all seasons. The growth responses in Fe enrichments in spring and summer indicated strong Fe limitations while the Si enrichment in autumn and summer revealed potential scenarios of Si co-limitations as growth responses in systems involving Si enrichments led to biomass production as chl-a which differed significantly with the control (p < 0.05). The multiple trace metal enrichments only demonstrated strong positive growth stimulation in autumn with only a sight growth stimulation in summer. Based on the response of Fe enrichment in summer, it can be deduced that the presence of other trace metal (Mn, Zn, and Cu) significantly preempted the effective Fe activity in promoting growth in the Fe + Mn + Zn + Cu combined enrichments. On the whole, it can, therefore, be concluded that while the multiple trace metals are essential for driving primary productivity in autumn, it potentially caused inhibition of growth and repressed Fe activity on growth in the other seasons. Similarly, a recent study by Huang et al., (2020) reported an initial growth inhibition which was eventually followed by an exponential resumption of the growth towards the end of incubation by multiple trace metal enrichments during a bioassay experiment during autumn in XXB. The significant growth enhancement by Fe in summer could be attributed to the dramatically low ferrous concentration which was below its detectable limit as revealed by FAAS analysis. Conversely, our recent study reported significant Fe distribution of 0.62 ± 0.03 mgL− 1 in autumn, 0.06 ± 0.02 mgL− 1 in winter, and 0.004 ± 0.01 mgL− 1 in spring (Nwankwegu et al., 2020b). This confirms that XXB shows strong seasonal variability in Fe distribution. A significant yield in chl-a concentration (82.70 ± 4.01 µgL− 1) was observed in NPSi combined enrichment in autumn. The NPSi again relatively caused maximum growth stimulation (9.89 ± 1.65 µgL− 1) in winter indicating that Si was crucial for the phytoplankton species structure in both seasons. In spring, maximum growth responses were reported in both NPSi (15.76 ± 1.47 µgL− 1) and Fe –alone (16.02 ± 0.21 µgL− 1) cultures. In summer, significant growth stimulations were demonstrated in Si alone (91.50 ± 7.66 µgL− 1) and Fe- alone (99.92 ± 6.35 µgL− 1) additions. Generally, XXB shows a potentially eutrophic-hypereutrophic system across following the magnitudes of chl-a based on trophic state stipulations (Celik, 2012; OECD, 1982).
3.4. Seasonal taxonomic dynamics
Significant seasonal variabilities in the population of each taxonomic group were observed (Fig. 6). Five principal phytoplankton taxa notably; Cyanophyta, Baccilliariophyta, Chlorophyta, Cryptophyta, and Pyrrophyta were identified in XXB although, in autumn, the community structure included a few populations of the Euglenophyta and Xanthophyta. The pattern of taxonomic dominance showed strong sensitivity to variations in season. Each season, therefore, selected the dominance of one or more taxa but not all at the same time except in autumn where all the taxonomic groups were represented in the largest amount relative to other seasons. This indicates that autumn condition supports a wide range of phytoplankton taxa. During autumn, the community structure revealed significant cell densities in both the Cyanophyta (3.8×107 cellsL− 1) and Chlorophyta (3.3×107 cellsL− 1). A relatively lower cell densities were observed in Bacilliariophyta (1.00×107 cellsL− 1), Cryptophyta (3.10×106 cellsL− 1), and Pyrrophyta (2.5×106 cellsL− 1). In winter, significant cell density was observed in the Bacilliarophyt (1.01×107 cellsL− 1) with significant losses in cell densities observed in Cyanophyta (8.57×104 cellsL− 1), Chlorophyta (1.3×105 cellsL− 1), and Cryptophyta (1.04×105 cellsL− 1) while the Pyrrophyta were exclusively dominated. In spring, the Cryptophyta (1.23×107 cellsL− 1) demonstrated maximum cell density. Decreases in cell densities were reported in Bacilliariophyta (5.40×106 cellsL− 1), Chlorophyta (2.40×106 cellsL− 1), and the Pyrrophyta (1.2×104 cellsL− 1) with a total extinction of the Cyanophyta. In summer, an absolute dominance by the Cyanophyta (9.01×107 cellsL− 1) was reported. The Chlorophyta (1.19×107 cellsL− 1) was the next in terms of population strength indicating a slight similarity with the autumn bloom although the compositional turnover between the seasons varied considerably. The community structure in summer showed significantly low cell densities in the Pyrrophyta (5.85×106 cellsL− 1) and Bacilliariophyta (1.13×106 cellsL− 1) with the total elimination of the Cryptophyta. The seasonal population dynamics could be attributed to the characteristic seasonal fluctuations in the nutrient concentrations, as well as the high taxonomic variability. The phytoplankton community structure is essentially controlled by the frequency in nutrient rate and supply ratios (Nwankwegu et al., 2019). The nutrient concentration dynamics including TN, TP, and the nitrogen oxidation state (NO3 & NH4) fluxes greatly affect phytoplankton community structure in the aquatic system (Harris et al., 2017). The total loss of the Cyanophyta and high Cryptophyta in spring could be linked to the significantly low concentration of NH4 while the nitrate replete condition prevailed. Previous studies have reported the preferential uptake of NH4 by the Cyanophyta during bloom (Xu et al., 2013; Xu et al., 2015; Schoffman et al., 2016). The Cyanophyta show high competitiveness for NH4 utilization while the diatoms demonstrate high competitiveness for NO3 uptake (Harris et al., 2017). The significantly high population turnover in the Bacilliarophyta in winter confirms the absolute tolerance of diatoms to cold spells and the high mixing regimes that characterize winter bloom in XXB following the backwater intrusion from the mainstream TGR during the peak rise season. Absolute ecosystem control by the Bacilliariophyta, dominated by the Cyclotella in 15 subtropical, urban shallow lakes in Wuhan China has been previously reported (Lv et al., 2011). It has also been demonstrated that the Bacilliarophyta e.g., diatoms prefer the system fast and well mixed while the Cyanophyta prefer it slow, hot and significantly stratified thus dominate summer blooms (Nwankwegu et al., 2019). Again, the diatoms possess the ability to thrive under low light and temperature thus constitute the dominant species in both lentic and lotic systems during the winter bloom (Paerl et al., 2016).
3.5. Species transitions across seasons
Species/genera dominance varied strongly across seasons (Table 2). The compositional turnover among the taxonomic groups revealed that variability in the seasons played a significant role in the seasonal species transitions. There is no stability in the species compositional pattern except in the taxon, the Cryptophyta where a stable genera/species distribution dominated (d) by Chroomonas acuta prevailed during autumn, winter, and spring with a sharp species decline in summer where single dominance (d) of species, Cryptomonas erosa was reported. The significant prevalence of the Cryptophyta dominated by the Chroomonas acuta (D) was reported in spring. In the Bacilliariophyta, the genera, Cyclotella dominated (d) across seasons except for summer where only two species of the group; Synedra acus and Cocconeis sp survived with species dominance (d) controlled by the later. While no members of the Cyanophyta was observed in winter, the Cyclotella was the dominant (D) genera in the entire community structure. The Chlorophyta showed maximum genera/species composition, particularly in autumn and winter with regime shift in the intra-species dominance (d) which alternated among the members of the taxon across the seasons from Scenedesmus spp in both autumn and spring to Ulothrix sp in winter, and the Actinastrum fluviatile during summer. In the Cyanophyta, species dominance was controlled by the Microcystis spp both within the taxon (d) and among community structure (D) in autumn and summer. The phytoplankton community structure in both winter and spring did not include any member of the Cyanophyta. The possible reasons could be the significantly low temperature in winter (Paerl et al., 2016) and the ammonium-nitrogen (NH4) deficit condition in spring which the Cyanophyta preferentially utilize (Li et al., 2020; Xu et al., 2013). Although the previous study by Cardoso et al., (2012) attributed the change in phytoplankton community to the local ecosystem factors, the seasonal variability and nutrient dynamics are the principal factors that strongly control the phytoplankton community shift in XXB. The loss of the several species of other taxa during summer could be linked to the absolute dominance of the toxic, hypoxia generating, and food web disrupting Microcystis spp (Nwankwegu et al., 201; Paerl et al., 2011). The cyanobacteria can swim along the water column dominating the epilimnion and hypolimnion (Li et al., 2020; Nwankwegu et al., 2019). The development of other phytoplankton taxa including the eukaryotic organisms in the photic zone is preempted by a significant turbidity and hypoxia associated with cyanobacterial dominance under intense bloom (Sanseverino and Conduto, 2016). The maximum dominance of Microcystis spp in summer could be attributed to the significantly low TN/TP. The low TN: TP (N- limitation) usually result in cyanobacterial dominance in the eutrophic freshwaters (Harris et al., 2017).