Ecological assessment of water quality in freshwater wetlands based on the effect of environmental heterogeneity on phytoplankton communities in Northeast China.

Phytoplankton community characterized by strong vitality response to environmental change in freshwater ecosystems. This study aims to evaluate the effectiveness of using phytoplankton diversity as a water quality indicator in wetlands, and find out the main environmental variables affecting the distribution of phytoplankton. From 2020 to 2021, we examined phytoplankton assemblages and water environmental variables in spring, summer, and autumn at eight sampling sites from Hulanhe Wetland, Northeast (NE) China. The results showed that Bacillariophyta was the dominant species. Phytoplankton composition and abundance differed among sampling sites in each season; the abundance in summer (613.71 × 104 ind. L-1) was higher than that in autumn and spring. The water quality assessment of the trophic state index (TSI) based on the four physicochemical indicators was compared with phytoplankton diversity indices, which indicated that the phytoplankton community was stable, and these two indices were significantly lower in summer than in spring and autumn. According to redundancy analysis (RDA), total phosphorus (TP) and nitrogen (TN) were the main environmental variables affecting the distribution of phytoplankton. Temperature and dissolved oxygen (DO) changes also played a role, and their impact on the community was discussed. This work can provide relevant scientific references on the usefulness of phytoplankton diversity structure in assessing water quality in cold regions, in which the succession can be significantly affected by nutrients and temperatures.

Spatio-temporal dynamics of phytoplankton community structure in the coastal waters of the Southern Beibu Gulf.

This study was conducted during October 2021 (autumn) and April 2022 (spring) to explore the phytoplankton community structure, their distribution characteristics, and the influence of environmental factors in the coastal waters of the Southern Beibu Gulf. The 15 sampling sites were grouped based on the difference in offshore distance to analyze the temporal and spatial differences in community structure and environmental driving in the investigated sea area of the coastal waters of the Southern Beibu Gulf. Permutational multivariate analysis of variance was conducted on the sample data in time and space, revealing that there is no significant difference in space (p > 0.05), but there is significant difference in time (p < 0.05). Notably, water pressure, pH, chemical oxygen demand, nitrite, and labile phosphate were higher in autumn, while total ammonia nitrogen, dissolved oxygen, and suspended solids were significantly higher in spring. Additionally, the study identified 87 phytoplankton species belonging to 6 phyla, dominating by Bacillariophyta, followed by Dinophyta and Cyanophyta. The phytoplankton density, Shannon Weiner's diversity index (H'), Pielou's evenness index (J), and Margalef's richness index (D) ranged from 84.88 to 4675.33 cells L-1, 0.56 to 2.58, 0.26 to 0.89, and 1.21 to 3.64, respectively. Permutational multivariate analysis of variance showed non-significant spatial differences in phytoplankton composition (p > 0.05) but seasonal differences (p < 0.05). Furthermore, canonical correspondence analysis (CCA) identified pH, dissolved oxygen, suspended solids, chemical oxygen demand, nitrite, and labile phosphate as key environmental factors influencing the phytoplankton community structure (p < 0.05). In this study, the dynamic changes of phytoplankton community structure and environmental factors in the southern coastal waters of Beibu Gulf were analyzed in detail from two aspects of time and space. The key environmental factors to protect the ecological environment in the southern coastal area of Beibu Gulf were found out. It provides a reference method and theoretical basis for the management and protection of Beibu Gulf and other tropical marine environment.

Phaeocystis globosa and diatom blooms promote distinct bacterial communities and associations in a coastal ecosystem.

Phytoplankton and bacteria form the foundation of marine food webs. While most studies on phytoplankton bloom influence on bacteria dynamics focus on diatom-dominated blooms due to their global ecological significance, it is unclear if similar patterns extend to other species that compete with diatoms like Phaeocystis spp. This study aimed to contribute to the understanding of associations between phytoplankton and bacteria in a temperate ecosystem. For this, we studied the dynamics of phytoplankton and bacteria, combining 16S metabarcoding, microscopy, and flow cytometry over 4 years (282 samples). Phytoplankton and bacterial communities were studied throughout the year, particularly during contrasting phytoplankton blooms dominated by the Haptophyte Phaeocystis globosa or diatoms. We applied extended local similarity analysis (eLSA) to construct networks during blooming and non-blooming periods. Overall, the importance of seasonal and species-specific interactions between phytoplankton and bacteria is highlighted. In winter, mixed diatom communities were interconnected with bacteria, indicating a synergistic degradation of diverse phytoplankton-derived substrates. In spring, despite the intensity variations of P. globosa blooms, the composition of bacterial communities remained consistent over several years, suggesting establishing a stable-state environment for bacterial communities. Specific associations between monospecific diatom blooms and bacteria were evidenced in summer.

Effects of environmental variability on phytoplankton structure, diversity and biomass at the Brazil-Malvinas Confluence (BMC).

The Brazil-Malvinas Confluence (BMC) is a significant biological frontier where distinct currents meet, fostering optimal conditions for phytoplankton development. In this study we tested the hypothesis that eddys promote an increase in phytoplankton biomass at the Brazil-Malvinas Confluence (BMC), altering species diversity. Phytoplankton were collected with Niskin bottles and nutrient concentrations assessed at two depths (Surface and Deep Chlorophyll Maximum Layer - DCML) in areas outside and under the influence of Cold-Core (CCE) and Warm-Core (WCE) Eddies. Environmental variables were determined in situ using a CTD profiler. Four regions were separated based on environmental variables and phytoplankton species, namely, the Brazil Current (BC), Malvinas Current (MC), CCE, and WCE. Species diversity was higher in the eddies. The conditions of the WCE were different from those of the CCE, with low temperature and salinity and high cell density values in the latter. The phylum Bacillariophyta was predominant in terms of species richness in all regions and was responsible for the higher cell density in the MC, while dinoflagellates were dominant in the BC and eddies. Therefore, eddy activity alters the structure, diversity and biomass of the phytoplankton community in the BMC.

Climate change triggered planktonic cyanobacterial blooms in a regulated temperate river.

Harmful algae blooms are a rare phenomenon in rivers but seem to increase with climate change and river regulation. To understand the controlling factors of cyanobacteria blooms that occurred between 2017 and 2020 over long stretches (> 250 km) of the regulated Moselle River in Western Europe, we measured physico-chemical and biological variables and compared those with a long-term dataset (1997-2016). Cyanobacteria (Microcystis) dominated the phytoplankton community in the late summers of 2017-2020 (cyano-period) with up to 110 µg Chlorophyll-a/L, but had not been observed in the river in the previous 20 years. From June to September, the average discharge in the Moselle was reduced to 69-76% and water temperature was 0.9-1.8 °C higher compared to the reference period. Nitrogen (N), phosphorus (P) and silica (Si) declined since 1997, albeit total nutrient concentrations remained above limiting conditions in the study period. Cyanobacterial blooms correlated best with low discharge, high water temperature and low nitrate. We conclude that the recent cyanobacteria blooms have been caused by dry and warm weather resulting in low flow conditions and warm water temperature in the regulated Moselle. Under current climate projections, the Moselle may serve as an example for the future of regulated temperate rivers.

Salinity decline promotes growth and harmful blooms of a toxic alga by diverting carbon flow.

Global climate change intensifies the water cycle and makes freshest waters become fresher and vice-versa. But how this change impacts phytoplankton in coastal, particularly harmful algal blooms (HABs), remains poorly understood. Here, we monitored a coastal bay for a decade and found a significant correlation between salinity decline and the increase of Karenia mikimotoi blooms. To examine the physiological linkage between salinity decreases and K. mikimotoi blooms, we compare chemical, physiological and multi-omic profiles of this species in laboratory cultures under high (33) and low (25) salinities. Under low salinity, photosynthetic efficiency and capacity as well as growth rate and cellular protein content were significantly higher than that under high salinity. More strikingly, the omics data show that low salinity activated the glyoxylate shunt to bypass the decarboxylation reaction in the tricarboxylic acid cycle, hence redirecting carbon from CO2 release to biosynthesis. Furthermore, the enhanced glyoxylate cycle could promote hydrogen peroxide metabolism, consistent with the detected decrease in reactive oxygen species. These findings suggest that salinity declines can reprogram metabolism to enhance cell proliferation, thus promoting bloom formation in HAB species like K. mikimotoi, which has important ecological implications for future climate-driven salinity declines in the coastal ocean with respect to HAB outbreaks.

Phytoplankton-chytrid-zooplankton dynamics via a reaction-diffusion-advection mycoloop model.

Mycoloop is an important aquatic food web composed of phytoplankton, chytrids (one dominant group of parasites in aquatic ecosystems), and zooplankton. Chytrids infect phytoplankton and fragment them for easy consumption by zooplankton. The free-living chytrid zoospores are also a food resource for zooplankton. A dynamic reaction-diffusion-advection mycoloop model is proposed to describe the Phytoplankton-chytrid-zooplankton interactions in a poorly mixed aquatic environment. We analyze the dynamics of the mycoloop model to obtain dissipativity, steady state solutions, and persistence. We rigorously derive several critical thresholds for phytoplankton or zooplankton invasion and chytrid transmission among phytoplankton. Numerical diagrams show that varying ecological factors affect the formation and breakup of the mycoloop, and zooplankton can inhibit chytrid transmission among phytoplankton. Furthermore, this study suggests that mycoloop may either control or cause phytoplankton blooms.

Seasonal patterns and bloom dynamics of phytoplankton based on satellite-derived chlorophyll-a in the eastern yellow sea.

Satellite-derived chlorophyll-a concentration (Chl-a) is essential for assessing environmental conditions, yet its application in the optically complex waters of the eastern Yellow Sea (EYS) is challenged. This study refines the Chl-a algorithm for the EYS employing a switching approach based on normalized water-leaving radiance at 555 nm wavelength according to turbidity conditions to investigate phytoplankton bloom patterns in the EYS. The refined Chl-a algorithm (EYS algorithm) outperforms prior algorithms, exhibiting a strong alignment with in situ Chl-a. Employing the EYS algorithm, seasonal and bloom patterns of Chl-a are detailed for the offshore and nearshore EYS areas. Distinct seasonal Chl-a patterns and factors influencing bloom initiation differed between the areas, and the peak Chl-a during the bloom period from 2018 to 2020 was significantly lower than the average year in both areas. Specifically, bimodal and unimodal peak patterns in Chl-a were observed in the offshore and nearshore areas, respectively. By investigating the relationships between environmental factors and bloom parameters, we identified that major controlling factors governing bloom initiation were mixed layer depth (MLD) and suspended particulate matter (SPM) in the offshore and nearshore areas, respectively. Additionally, this study proposed that the recent decrease in the peak Chl-a might be caused by rapid environmental changes such as the warming trend of sea surface temperature (SST) and the limitation of nutrients. For example, external forcing, phytoplankton growth, and nutrient dynamics can change due to increased SST and limitation of nutrients, which can lead to a decrease in Chl-a. This study contributes to understanding phytoplankton dynamics in the EYS, highlighting the importance of region-specific considerations in comprehending Chl-a patterns and bloom dynamics.

The recent disappearance of a persistent Planktothrix bloom: Characterization of a regime shift in the phytoplankton of Sandusky Bay (USA).

Sandusky Bay is the drowned mouth of the Sandusky River in the southwestern portion of Lake Erie. The bay is a popular recreation location and a regional source for drinking water. Like the western basin of Lake Erie, Sandusky Bay is known for being host to summer cyanobacterial harmful algal blooms (cHABs) year after year, fueled by runoff from the predominantly agricultural watershed and internal loading of legacy nutrients (primarily phosphorus). Since at least 2003, Sandusky Bay has harbored a microcystin-producing bloom of Planktothrix agardhii, a species of filamentous cyanobacteria that thrives in low light conditions. Long-term sampling (2003-2018) of Sandusky Bay revealed regular Planktothrix-dominated blooms during the summer months, but in recent years (2019-2022), 16S rRNA gene community profiling revealed that Planktothrix has largely disappeared. From 2017-2022, microcystin decreased well below the World Health Organization (WHO) guidelines. Spring TN:TP ratios increased in years following dam removal, yet there were no statistically significant shifts in other physicochemical variables, such as water temperature and water clarity. With the exception of the high bloom of Planktothrix in 2018, there was no statistical difference in chlorophyll during all other years. Concurrent with the disappearance of Planktothrix, Cyanobium spp. have become the dominant cyanobacterial group. The appearance of other potential toxigenic genera (i.e., Aphanizomenon, Dolichospermum, Cylindrospermopsis) may motivate monitoring of new toxins of concern in Sandusky Bay. Here, we document the regime shift in the cyanobacterial community and propose evidence supporting the hypothesis that the decline in the Planktothrix bloom was linked to the removal of an upstream dam on the Sandusky River.

Physical structure of the environment contributes to the development of diversity of microalgal assemblages.

Aquatic macrophytes form a three dimensional complex structure in the littoral zones of lakes, with many physical, chemical and biological gradients and interactions. This special habitat harbours a unique microalgal assemblage called metaphyton, that differs both from the phytoplankton of the pelagial and from the benthic assemblages whose elements are tightly attached to the substrates. Since metaphytic assemblages significantly contribute to the diversity of lakes' phytoplankton, it is crucial to understand and disentangle those mechanisms that ensure their development. Therefore, we focused on the question of how a single solid physical structure contribute to maintaining metaphytic assemblages. Using a laboratory experiment we studied the floristic and functional differences of microalgal assemblages in microcosms that simulated the conditions that an open water, a complex natural macrophyte stand (Utricularia vulgaris L.), or an artificial substrate (cotton wool) provide for them. We inoculated the systems with a species rich (> 326 species) microalgal assemblage collected from a eutrophic oxbow lake, and studied the diversity, trait and functional group composition of the assemblages in a 24 day long experimental period. We found that both natural and artificial substrates ensured higher species richness than the open water environment. Functional richness in the open water environment was lower than in the aquaria containing natural macrophyte stand but higher than in which cotton wool was placed. This means that the artificial physical structure enhanced functional redundancy of the resident functional groups. Elongation measures of microalgal assemblages showed the highest variation in the microcosms that simulated the open water environment. Our results suggest that assembly of metaphytic algal communities is not a random process, instead a deterministic one driven by the niche characteristics of the complex three dimensional structure created by the stands of aquatic macrophytes.

Detecting Margalefidinium polykrikoides through high-frequency imagery: Example of a bloom formation, environmental conditions, and phytoplankton community composition changes.

In August 2018, the harmful algae species Margalefidinium polykrikoides bloomed to levels previously unobserved in the open waters of Narragansett Bay, Rhode Island, in a transient but intense bloom. Detected by an Imaging FlowCytobot providing hourly data, it is characterized by a time span of less than a week and patchiness with sub-daily oscillations in concentration. The highest concentrations are recorded at lower salinity and higher temperature, suggesting the bloom may have developed in the upper bay and was transported south. The proportion of chains increased during the height of the bloom, and many of the images contained 4-cells per chain. The development of the bloom was favored by optimal temperature and salinity conditions as well as increased nitrogen coincident with greater precipitation and river flow. The period preceding bloom formation also saw a sharp decrease in the dominating large chain-forming diatom Eucampia sp. and highly abundant Skeletonema spp., thus reducing competition over resources for the slow-growing M. polykrikoides. The height of the bloom was reached during the lowest tidal range of the month when the turbulence and water displacement were lower. This time series highlights an out-of-the-ordinary bloom's environmental and biological conditions and the importance of frequent sampling during known favorable conditions.

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To explore the temporal and spatial variations in phytoplankton community in small estuaries, we collected surface water samples from Yongjiang River estuary during wet, normal, and dry seasons and determined the main driving factors of phytoplankton community. A total of 358 species belonging to nine phyla and 123 genera were identified in all seasons. During wet, normal, and dry seasons, species number was 276, 154 and 151, and the abundance was (170.45±225.43)×103, (51.92±30.28)×103 and (31.65±12.79)×103 cells·L-1, respectively. Diatoms dominated the phytoplankton community, and the main dominant species were Cyclotella meneghiniana, Skeletonema costatum, and Paralia sulcata. Shannon diversity and Pielou evenness indices decreased from inside mouth to outside mouth in wet season, but there was no obvious spatial difference in normal season or dry season. Results of non-metric multidimensional scaling analysis and analysis of similarities showed that phytoplankton community composition differed significantly among different regions (inside, at and outside mouth) and different seasons. In wet season, phytoplankton abundance was significantly positively correlated with temperature, dissolved inorganic nitrogen, and dissolved reactive phosphorus, but significantly negatively correlated with salinity. In normal season, phytoplankton abundance was significantly negatively correlated with temperature. In dry season, it was not significantly correlated with environmental factors. Results of redundancy analysis showed that temperature, salinity, ammonium and dissolved reactive phosphorus explained the variations in phytoplankton community by 19.5%, 11.9%, 9.4% and 8.2%, respectively. These results revealed high dominance of diatoms and the main driving factors (temperature, salinity and nutrients) of phytoplankton community in Yongjiang River estuary.

Atlas of phytoplankton phenology indices in selected Eastern Mediterranean marine ecosystems.

Phytoplankton is a fundamental component of marine food webs and play a crucial role in marine ecosystem functioning. The phenology (timing of growth) of these microscopic algae is an important ecological indicator that can be utilized to observe its seasonal dynamics, and assess its response to environmental perturbations. Ocean colour remote sensing is currently the only means of obtaining synoptic estimates of chlorophyll-a (a proxy of phytoplankton biomass) at high temporal and spatial resolution, enabling the calculation of phenology metrics. However, ocean colour observations have acknowledged weaknesses compromising its reliability, while the scarcity of long-term in situ data has impeded the validation of satellite-derived phenology estimates. To address this issue, we compared one of the longest available in situ time series (20 years) of chlorophyll-a concentrations in the Eastern Mediterranean Sea (EMS), along with concurrent remotely-sensed observations. The comparison revealed a marked coherence between the two datasets, indicating the capability of satellite-based measurements in accurately capturing the phytoplankton seasonality and phenology metrics (i.e., timing of initiation, duration, peak and termination) in the studied area. Furthermore, by studying and validating these metrics we constructed a satellite-derived phytoplankton phenology atlas, reporting in detail the seasonal patterns in several sub-regions in coastal and open seas over the EMS. The open waters host higher concentrations from late October to April, with maximum levels recorded during February and lowest during the summer period. The phytoplankton growth over the Northern Aegean Sea appeared to initiate at least a month later than the rest of the EMS (initiating in late November and terminating in late May). The coastal waters and enclosed gulfs (such as Amvrakikos and Maliakos), exhibit a distinct seasonal pattern with consistently higher levels of chlorophyll-a and prolonged growth period compared to the open seas. The proposed phenology atlas represents a useful resource for monitoring phytoplankton growth periods in the EMS, supporting water quality management practices, while enhancing our current comprehension on the relationships between phytoplankton biomass and higher trophic levels (as a food source).

Land use change and coastal water darkening drive synchronous dynamics in phytoplankton and fish phenology on centennial timescales.

At high latitudes, the suitable window for timing reproductive events is particularly narrow, promoting tight synchrony between trophic levels. Climate change may disrupt this synchrony due to diverging responses to temperature between, for example, the early life stages of higher trophic levels and their food resources. Evidence for this is equivocal, and the role of compensatory mechanisms is poorly understood. Here, we show how a combination of ocean warming and coastal water darkening drive long-term changes in phytoplankton spring bloom timing in Lofoten Norway, and how spawning time of Northeast Arctic cod responds in synchrony. Spring bloom timing was derived from hydrographical observations dating back to 1936, while cod spawning time was estimated from weekly fisheries catch and roe landing data since 1877. Our results suggest that land use change and freshwater run-off causing coastal water darkening has gradually delayed the spring bloom up to the late 1980s after which ocean warming has caused it to advance. The cod appear to track phytoplankton dynamics by timing gonadal development and spawning to maximize overlap between offspring hatch date and predicted resource availability. This finding emphasises the importance of land-ocean coupling for coastal ecosystem functioning, and the potential for fish to adapt through phenotypic plasticity.

Modelling phytoplankton-virus interactions: phytoplankton blooms and lytic virus transmission.

A dynamic reaction-diffusion model of four variables is proposed to describe the spread of lytic viruses among phytoplankton in a poorly mixed aquatic environment. The basic ecological reproductive index for phytoplankton invasion and the basic reproduction number for virus transmission are derived to characterize the phytoplankton growth and virus transmission dynamics. The theoretical and numerical results from the model show that the spread of lytic viruses effectively controls phytoplankton blooms. This validates the observations and experimental results of Emiliana huxleyi-lytic virus interactions. The studies also indicate that the lytic virus transmission cannot occur in a low-light or oligotrophic aquatic environment.

Bloom dynamics under the effects of periodic driving forces.

Phytoplankton bloom received considerable attention for many decades. Different approaches have been used to explain the bloom phenomena. In this paper, we study a Nutrient-Phytoplankton-Zooplankton (NPZ) model consisting of a periodic driving force in the growth rate of phytoplankton due to solar radiation and analyse the dynamics of the corresponding autonomous and non-autonomous systems in different parametric regions. Then we introduce a novel aspect to extend the model by incorporating another periodic driving force into the growth term of the phytoplankton due to sea surface temperature (SST), a key point of innovation. Temperature dependency of the maximum growth rate (µmax) of the phytoplankton is modelled by the well-known Q10 formulation: [Formula: see text] , where µ0 is maximum growth at 0oC. Stability conditions for all three equilibrium points are expressed in terms of the new parameter ρ2, which appears due to the incorporation of periodic driving forces. System dynamics is explored through a detailed bifurcation analysis, both mathematically and numerically, with respect to the light and temperature dependent phytoplankton growth response. Bloom phenomenon is explained by the saddle point bloom mechanism even when the co-existing equilibrium point does not exist for some values of ρ2. Solar radiation and SST are modelled using sinusoidal functions constructed from satellite data. Our results of the proposed model describe the initiation of the phytoplankton bloom better than an existing model for the region 25-35° W, 40-45° N of the North Atlantic Ocean. An improvement of 14 days (approximately) is observed in the bloom initiation time. The rate of change method (ROC) is applied to predict the bloom initiation.

Long-term declines in chlorophyll a and variable phenology revealed by a 60-year estuarine plankton time series.

Long-term ecological time series provide a unique perspective on the emergent properties of ecosystems. In aquatic systems, phytoplankton form the base of the food web and their biomass, measured as the concentration of the photosynthetic pigment chlorophyll a (chl a), is an indicator of ecosystem quality. We analyzed temporal trends in chl a from the Long-Term Plankton Time Series in Narragansett Bay, Rhode Island, USA, a temperate estuary experiencing long-term warming and changing anthropogenic nutrient inputs. Dynamic linear models were used to impute and model environmental variables (1959 to 2019) and chl a concentrations (1968 to 2019). A long-term chl a decrease was observed with an average decline in the cumulative annual chl a concentration of 49% and a marked decline of 57% in winter-spring bloom magnitude. The long-term decline in chl a concentration was directly and indirectly associated with multiple environmental factors that are impacted by climate change (e.g., warming temperatures, water column stratification, reduced nutrient concentrations) indicating the importance of accounting for regional climate change effects in ecosystem-based management. Analysis of seasonal phenology revealed that the winter-spring bloom occurred earlier, at a rate of 4.9 ± 2.8 d decade-1. Finally, the high degree of temporal variation in phytoplankton biomass observed in Narragansett Bay appears common among estuaries, coasts, and open oceans. The commonality among these marine ecosystems highlights the need to maintain a robust set of phytoplankton time series in the coming decades to improve signal-to-noise ratios and identify trends in these highly variable environments.

Influences of lower pH on phytoplankton growth in alkaline lakes after water transfer: Insights from a coupled hydrodynamic-algal ecological model and experimental analysis.

Alkaline lakes with high pH and unique ecological communities often face water-level drawdown and ecological degradation problems due to climatic and hydrologic factors. Water transfer is becoming a popular method for solving these problems. However, a high pH is often considered the key to maintaining the stability of alkaliphilic algal communities, and a lower pH induced by water transfer from a neutral-pH river may threaten ecosystems in alkaline lakes. To explore the response characteristics of phytoplankton in alkaline lakes to pH changes, we conducted cultivation experiments on one species of dominant Cyanobacteria and one species of dominant Chlorophyta from alkaline lakes under different pH conditions. Subsequently, we constructed a coupled hydrodynamic and algal mathematical model considering the effect of pH to predict the dynamic changes in phytoplankton in a typical alkaline lake under water-transfer conditions. Both species are basophilic, and pH has a "low-inhibition and high-promotion" effect on their growth. A lower pH is detrimental to cyanobacterial growth and competitiveness, which may cause Cyanobacteria to lose their dominance in weakly alkaline environments with a pH < 8.5; additionally, water transfer causes a decrease in the total biomass and proportion of Cyanobacteria in Lake Chenghai, with decreases induced by pH changes accounting for 13.4% and 70.1%, respectively. The decrease in pH is the main reason for the decrease in dominance of Cyanobacteria after water transfer. These results provide a basic summary of the effects of pH changes on the algal growth in alkaline lakes and are a useful for formulating ecological water-transfer strategies for alkaline lakes.

A two-timescale model of plankton-oxygen dynamics predicts formation of oxygen minimum zones and global anoxia.

Decline of the dissolved oxygen in the ocean is a growing concern, as it may eventually lead to global anoxia, an elevated mortality of marine fauna and even a mass extinction. Deoxygenation of the ocean often results in the formation of oxygen minimum zones (OMZ): large domains where the abundance of oxygen is much lower than that in the surrounding ocean environment. Factors and processes resulting in the OMZ formation remain controversial. We consider a conceptual model of coupled plankton-oxygen dynamics that, apart from the plankton growth and the oxygen production by phytoplankton, also accounts for the difference in the timescales for phyto- and zooplankton (making it a "slow-fast system") and for the implicit effect of upper trophic levels resulting in density dependent (nonlinear) zooplankton mortality. The model is investigated using a combination of analytical techniques and numerical simulations. The slow-fast system is decomposed into its slow and fast subsystems. The critical manifold of the slow-fast system and its stability is then studied by analyzing the bifurcation structure of the fast subsystem. We obtain the canard cycles of the slow-fast system for a range of parameter values. However, the system does not allow for persistent relaxation oscillations; instead, the blowup of the canard cycle results in plankton extinction and oxygen depletion. For the spatially explicit model, the earlier works in this direction did not take into account the density dependent mortality rate of the zooplankton, and thus could exhibit Turing pattern. However, the inclusion of the density dependent mortality into the system can lead to stationary Turing patterns. The dynamics of the system is then studied near the Turing bifurcation threshold. We further consider the effect of the self-movement of the zooplankton along with the turbulent mixing. We show that an initial non-uniform perturbation can lead to the formation of an OMZ, which then grows in size and spreads over space. For a sufficiently large timescale separation, the spread of the OMZ can result in global anoxia.