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Impoundments play a vital role as nutrient sinks, capable of retaining and exporting nitrogen (N) and phosphorus (P) at different rates. The imbalance in N and P stoichiometry relative to phytoplankton demand often determines the limiting nutrient of phytoplankton biomass in these systems. This critical factor has a substantial impact on the management of eutrophication, encompassing the formulation of nutrient control strategies and the setting of regulatory thresholds. Nonetheless, research remains relatively limited on phytoplankton limiting factors and nutrient stoichiometry interactions in subtropical impoundment reservoirs. This study fills a critical gap in the current research by providing a comprehensive assessment of the influences of N and P on phytoplankton biomass in Lake Qiandaohu, China. Through field monitoring, nutrient addition experiments, and novel constraint line regression model, we provide new insights into the nutrient-phytoplankton dynamics within the lake. Both bioassay experiments and statistics indicated primarily P-limitation in Lake Qiandaohu owing to its dam-induced deep-water conditions, characterized by a nearly 1:1 linear relationship between chlorophyll a (Chla) and total P (TP) concentrations. This underscores the pivotal role of P management plays in controlling algal blooms. Utilizing the constraint line equation that relates TP to Chla, we have proposed TP thresholds designed to keep Chla within the specified target ranges, specifically below 10, 12, 20, 24, 40, and 60 μg/L. Furthermore, leveraging Vollenweider's models with these TP concentration thresholds, we established TP loading targets that accommodate a range of hydrological conditions, from normal to wet and dry years. Furthermore, both nutrient addition experiments and constraint line regression model indicates potential N and P co-limitation in specific regions, particularly the riverine zone, where the unsettled particulate matter results in relatively lower N:P ratios. To address this, we introduces TN thresholds and suggests localized control measures, including the use of floating macrophytes beds, as effective alternatives. Considering the uniform nutrient management policy currently applied across Chinese lakes and reservoirs, which may lead to under- or over-protection for individual water bodies, our research provides a flexible cost-effective eutrophication management framework tailored for the China's subtropical region., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier Ltd.)

Water quality of eutrophic lakes is threatened by harmful cyanobacterial blooms, which are favored by summer heatwaves and expected to intensify with global warming. Societal demands on surface water for drinking, irrigation and recreation are also highest in summer, especially during dry and warm conditions. Here, we analyzed trends in online searches to investigate how public awareness of cyanobacterial blooms is impacted by temperature in nine different countries over almost twenty years. Our findings reveal large seasonal and interannual variation, with more online searches for harmful cyanobacteria in temperate regions during hot summers. Online searches and media attention increased even more steeply with temperature than the incidence of cyanobacterial blooms, presumably because lakes attract more people during warm weather. Overall, our study indicates that warmer summers not only increase cyanobacterial bloom incidence, but also lead to a pronounced increase of the public awareness of toxic cyanobacterial blooms., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Algal blooms (ABs) in inland lakes have caused adverse ecological effects, and health impairment of animals and humans. We used archived Landsat images to examine ABs in lakes (>1 km 2 ) around the globe over a 37-year time span (1982-2018). Out of the 176032 lakes with area >1 km 2 detected globally, 863 were impacted by ABs, 708 had sufficiently long records to define a trend, and 66% exhibited increasing trends in frequency ratio (FRQR, ratio of the number of ABs events observed in a year in a given lake to the number of available Landsat images for that lake) or area ratio (AR, ratio of annual maximum area covered by ABs observed in a lake to the surface area of that lake), while 34% showed a decreasing trend. Across North America, an intensification of ABs severity was observed for FRQR (p < .01) and AR (p < .01) before 1999, followed by a decrease in ABs FRQR (p < .01) and AR (p < .05) after the 2000s. The strongest intensification of ABs was observed in Asia, followed by South America, Africa, and Europe. No clear trend was detected for the Oceania. Across climatic zones, the contributions of anthropogenic factors to ABs intensification (16.5% for fertilizer, 19.4% for gross domestic product, and 18.7% for population) were slightly stronger than climatic drivers (10.1% for temperature, 11.7% for wind speed, 16.8% for pressure, and for 11.6% for rainfall). Collectively, these divergent trends indicate that consideration of anthropogenic factors as well as climate change should be at the forefront of management policies aimed at reducing the severity and frequency of ABs in inland waters., (© 2022 John Wiley & Sons Ltd.)

Harmful cyanobacterial blooms (CyanoHABs) are a rapidly proliferating global problem, threatening the use and sustainability of our freshwater resources. In recent decades, the United States, China, and other developed and developing countries threatened by CyanoHAB expansion have established collaborative efforts aimed at mitigating and managing this environmental and human health problem. However, an escalating negative political climate and restrictive policies on scientific exchange threaten these efforts. In this Perspective, I point to progress that has been made to counter the CyanoHAB problem on U.S.-Chinese fronts through our collaborations, which have been mutually beneficial from research and academic perspectives. Much like global efforts now needed to control pandemics, we are all "in the same boat" when to comes to countering the threat CyanoHABs pose for drinkable, swimmable, and fishable freshwater supplies and human health., (© 2020 Phycological Society of America.)

Many studies have focused on the interactive effects of temperature increases due to global warming and nutrient enrichment on phytoplankton communities. Recently, non-temperature effects of climate change (e.g., decreases in wind speed and increases in solar radiation) on large lakes have received increasing attention. To evaluate the relative contributions of both temperature and non-temperature effects on phytoplankton communities in a large eutrophic subtropical shallow lake, we analyzed long-term monitoring data from Lake Taihu, China from 1997 to 2016. Results showed that Lake Taihu's spring phytoplankton biovolume and composition changed dramatically over this time frame, with a change in dominant species. Stepwise multiple linear regression models indicated that spring phytoplankton biovolume was strongly influenced by total phosphorus (TP), light condition, wind speed and total nitrogen (TN) (radj2 = 0.8, p < 0.01). Partial redundancy analysis (pRDA) showed that light condition accounted for the greatest variation of phytoplankton community composition, followed by TP and wind speed, as well as the interactions between TP and wind speed. Our study points to the additional importance of non-temperature effects of climate change on phytoplankton community dynamics in Lake Taihu., Competing Interests: The authors have declared that no competing interests exist.

A severe cyanobacterial bloom in the mainstem of a large Chinese river was first reported from China. The Qiantang River is the longest river in the Zhejiang province, southeast China. It provides drinking water supply to ~ 16 million people, including Hangzhou city. Fifteen sites along the Qiantang River (including upper, middle (Fuchunjiang Reservoir), and lower reaches and tributaries) were sampled between August 13 and September 9, 2016 to conduct a preliminary examination of the outbreak of Microcystis blooms. Laboratory investigation revealed that Microcystis spp. are dominant in the Fuchunjiang Reservoir (an overflow reservoir on the mainstem of the Qiantang River) with an extremely high cell density of 2.3 × 10 8  cells/L, leading to a severe bloom in the mainstem of the Qiantang River. Investigations of the meteorological, hydrological, and nutrient characteristics associated with the bloom indicated that extremely dry (6.8 mm rainfall from August 13 to September 9, 2016) and hot (32 consecutive days of temperatures > 30 °C from July 20 to August 31, 2016) weather might be the key factors triggering the bloom. Additionally, the extremely low flow of the tributary, Lanjiang River (142 ± 56 m 3 /s from August 13 to September 9), and its high nutrient background, favored the bloom. While nutrient reductions are important, the most immediate and effective management approach might be to implement appropriate minimal flow conditions to mitigate the blooms.

Cyanobacteria can form dense and sometimes toxic blooms in freshwater and marine environments, which threaten ecosystem functioning and degrade water quality for recreation, drinking water, fisheries and human health. Here, we review evidence indicating that cyanobacterial blooms are increasing in frequency, magnitude and duration globally. We highlight species traits and environmental conditions that enable cyanobacteria to thrive and explain why eutrophication and climate change catalyse the global expansion of cyanobacterial blooms. Finally, we discuss management strategies, including nutrient load reductions, changes in hydrodynamics and chemical and biological controls, that can help to prevent or mitigate the proliferation of cyanobacterial blooms.

Elevated atmospheric nitrogen (N) deposition has significantly influenced aquatic ecosystems, especially with regard to their N budgets and phytoplankton growth potentials. Compared to a considerable number of studies on oligotrophic lakes and oceanic waters, little evidence for the importance of N deposition has been generated for eutrophic lakes, even though emphasis has been placed on reducing external N inputs to control eutrophication in these lakes. Our high-resolution observations of atmospheric depositions and riverine inputs of biologically reactive N species into eutrophic Lake Dianchi (the sixth largest freshwater lake in China) shed new light onto the contribution of N deposition to total N loads. Annual N deposition accounted for 15.7% to 16.6% of total N loads under variable precipitation conditions, 2-fold higher than previous estimates (7.6%) for the Lake Dianchi. The proportion of N deposition to total N loads further increased to 27-48% in May and June when toxic blooms of the ubiquitous non-N 2 fixing cyanobacteria Microcystis spp. are initiated and proliferate. Our observations reveal that reduced N (59%) contributes a greater amount than oxidized N to total N deposition, reaching 56-83% from late spring to summer. Progress toward mitigating eutrophication in Lake Dianchi and other bloom-impacted eutrophic lakes will be difficult without reductions in ammonia emissions and subsequent N deposition.

Phytoplankton (eutrophication, biogeochemical) models are important tools for ecosystem research and management, but they generally have not been updated to include modern biology. Here, we present a dynamic, mechanistic, molecular-level (i.e. gene, transcript, protein, metabolite) model of Anabaena - nitrogen interaction. The model was developed using the pattern-oriented approach to model definition and parameterization of complex agent-based models. It simulates individual filaments, each with individual cells, each with genes that are expressed to yield transcripts and proteins. Cells metabolize various forms of N, grow and divide, and differentiate heterocysts when fixed N is depleted. The model is informed by observations from 269 laboratory experiments from 55 papers published from 1942 to 2014. Within this database, we identified 331 emerging patterns, and, excluding inconsistencies in observations, the model reproduces 94% of them. To explore a practical application, we used the model to simulate nutrient reduction scenarios for a hypothetical lake. For a 50% N only loading reduction, the model predicts that N fixation increases, but this fixed N does not compensate for the loading reduction, and the chlorophyll a concentration decreases substantially (by 33%). When N is reduced along with P, the model predicts an additional 8% reduction (compared to P only)., (© 2016 Society for Applied Microbiology and John Wiley & Sons Ltd.)

Mitigating the global expansion of cyanobacterial harmful blooms (CyanoHABs) is a major challenge facing researchers and resource managers. A variety of traditional (e.g., nutrient load reduction) and experimental (e.g., artificial mixing and flushing, omnivorous fish removal) approaches have been used to reduce bloom occurrences. Managers now face the additional effects of climate change on watershed hydrologic and nutrient loading dynamics, lake and estuary temperature, mixing regime, internal nutrient dynamics, and other factors. Those changes favor CyanoHABs over other phytoplankton and could influence the efficacy of control measures. Virtually all mitigation strategies are influenced by climate changes, which may require setting new nutrient input reduction targets and establishing nutrient-bloom thresholds for impacted waters. Physical-forcing mitigation techniques, such as flushing and artificial mixing, will need adjustments to deal with the ramifications of climate change. Here, we examine the suite of current mitigation strategies and the potential options for adapting and optimizing them in a world facing increasing human population pressure and climate change., (Copyright © 2015 Elsevier B.V. All rights reserved.)

Excessive anthropogenic nitrogen (N) and phosphorus (P) inputs have caused an alarming increase in harmful cyanobacterial blooms, threatening sustainability of lakes and reservoirs worldwide. Hypertrophic Lake Taihu, China's third largest freshwater lake, typifies this predicament, with toxic blooms of the non-N2 fixing cyanobacteria Microcystis spp. dominating from spring through fall. Previous studies indicate N and P reductions are needed to reduce bloom magnitude and duration. However, N reductions may encourage replacement of non-N2 fixing with N2 fixing cyanobacteria. This potentially counterproductive scenario was evaluated using replicate, large (1000 L), in-lake mesocosms during summer bloom periods. N+P additions led to maximum phytoplankton production. Phosphorus enrichment, which promoted N limitation, resulted in increases in N2 fixing taxa (Anabaena spp.), but it did not lead to significant replacement of non-N2 fixing with N2 fixing cyanobacteria, and N2 fixation rates remained ecologically insignificant. Furthermore, P enrichment failed to increase phytoplankton production relative to controls, indicating that N was the most limiting nutrient throughout this period. We propose that Microcystis spp. and other non-N2 fixing genera can maintain dominance in this shallow, highly turbid, nutrient-enriched lake by outcompeting N2 fixing taxa for existing sources of N and P stored and cycled in the lake. To bring Taihu and other hypertrophic systems below the bloom threshold, both N and P reductions will be needed until the legacy of high N and P loading and sediment nutrient storage in these systems is depleted. At that point, a more exclusive focus on P reductions may be feasible.

The present paper utilizes a Bayesian Belief Network (BBN) approach to intuitively present and quantify our current understanding of the complex physical, chemical, and biological processes that lead to eutrophication in an estuarine ecosystem (New River Estuary, North Carolina, USA). The model is further used to explore the effects of plausible future climatic and nutrient pollution management scenarios on water quality indicators. The BBN, through visualizing the structure of the network, facilitates knowledge communication with managers/stakeholders who might not be experts in the underlying scientific disciplines. Moreover, the developed structure of the BBN is transferable to other comparable estuaries. The BBN nodes are discretized exploring a new approach called moment matching method. The conditional probability tables of the variables are driven by a large dataset (four years). Our results show interaction among various predictors and their impact on water quality indicators. The synergistic effects caution future management actions., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Concentrations of phosphorus and nitrogen in surface waters are being regulated in the United States and European Union. Human activity has raised the concentrations of these nutrients, leading to eutrophication of inland waters, which causes nuisance growth of algae and other aquatic plants. Control of phosphorus often has had the highest priority because of its presumed leading role in limiting development of aquatic plant biomass. Experimental evidence shows, however, that nitrogen is equally likely to limit growth of algae and aquatic plants in inland waters, and that additions of both nutrients cause substantially more algal growth than either added alone. A dual control strategy for N and P will reduce transport of anthropogenic nitrogen through drainage networks to aquatic ecosystems that may be nitrogen limited. Control of total phosphorus in effluents is feasible and is increasingly being required by regulations. The control strategy for nitrogen in effluents is more difficult, but could be made more feasible by recognition that a substantial portion of dissolved organic nitrogen is not bioavailable; regulation should focus on bioavailable N (nitrate, ammonium, and some dissolved organic nitrogen) rather than total N. Regulation of both N and P also is essential for nonpoint sources.

Harmful (toxic, food web altering, hypoxia generating) cyanobacterial algal blooms (CyanoHABs) are proliferating world-wide due to anthropogenic nutrient enrichment, and they represent a serious threat to the use and sustainability of our freshwater resources. Traditionally, phosphorus (P) input reductions have been prescribed to control CyanoHABs, because P limitation is widespread and some CyanoHABs can fix atmospheric nitrogen (N(2)) to satisfy their nitrogen (N) requirements. However, eutrophying systems are increasingly plagued with non N(2) fixing CyanoHABs that are N and P co-limited or even N limited. In many of these systems N loads are increasing faster than P loads. Therefore N and P input constraints are likely needed for long-term CyanoHAB control in such systems. Climatic changes, specifically warming, increased vertical stratification, salinization, and intensification of storms and droughts play additional, interactive roles in modulating CyanoHAB frequency, intensity, geographic distribution and duration. In addition to having to consider reductions in N and P inputs, water quality managers are in dire need of effective tools to break the synergy between nutrient loading and hydrologic regimes made more favorable for CyanoHABs by climate change. The more promising of these tools make affected waters less hospitable for CyanoHABs by 1) altering the hydrology to enhance vertical mixing and/or flushing and 2) decreasing nutrient fluxes from organic rich sediments by physically removing the sediments or capping sediments with clay. Effective future CyanoHAB management approaches must incorporate both N and P loading dynamics within the context of altered thermal and hydrologic regimes associated with climate change., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Eutrophication is a serious water quality problem in estuaries receiving increasing anthropogenic nutrient loads. Managers undertaking nutrient-reduction strategies aimed at controlling estuarine eutrophication are faced with the challenge that upstream freshwater segments often are phosphorus (P)-limited, whereas more saline downstream segments are nitrogen (N)-limited. Management also must consider climatic (hydrologic) variability, which affects nutrient delivery and processing. The interactive effects of selective nutrient input reductions and climatic perturbations were examined in the Neuse River Estuary (NRE), North Carolina, a shallow estuary with more than a 30-year history of accelerated nutrient loading and water quality decline. The NRE also has experienced a recent increase in Atlantic hurricanes and record flooding, which has affected hydrology and nutrient loadings. The authors examined the water quality consequences of selective nutrient (P but not N) reductions in the 1980s, followed by N reductions in the 1990s and an increase in hurricane frequency since the mid-1990s. Selective P reductions decreased upstream phytoplankton blooms, but increased downstream phytoplankton biomass. Storms modified these trends. In particular, upstream annual N and P concentrations have decreased during the elevated hurricane period. Increased flushing and scouring from storms and flooding appear to have enhanced nutrient retention capabilities of the NRE watershed. From a management perspective, one cannot rely on largely unpredictable changes in storm frequency and intensity to negate anthropogenic nutrient enrichment and eutrophication. To control eutrophication along the hydrologically variable freshwater-marine continuum, N and P reductions should be applied adaptively to reflect point-source-dominated drought and non-point-source-dominated flood conditions.

In estuaries, phosphorus (P) and nitrogen (N) inputs generally control freshwater and saltwater primary production, respectively. Improved wastewater P removal and a P-detergent ban in the late 1980s decreased P loading to the nutrient over-enriched Neuse River Estuary, NC, without a contemporaneous reduction in N loading. This led to a decrease in upstream freshwater phytoplankton production and a reduction in nuisance algal blooms. While this nutrient management approach appeared to be effective in reducing the symptoms of freshwater eutrophication, it may have also diminished the upstream algal N filter, promoting N enrichment, relative to P enrichment, and eutrophication of the more saline downstream N-limited waters. Recent N controls implemented by the State of North Carolina should help address the problem. These findings underscore the need for watershed- and basin-scale, dual nutrient (N and P) reduction strategies that consider the entire freshwater--marine continuum as well as hydrologic variability (e.g., hurricanes, floods, droughts) when formulating long-term controls of estuarine eutrophication.

The impacts of growing coastal pollution and habitat alteration accompanying human encroachment are of great concern at the microbial level, where much of the ocean's primary production and biogeochemical cycling takes place. Coastal ecosystems are also under the influence of natural perturbations such as major storwns and flooding. Distinguishing the impacts of natural and human stressors is essential for understanding environmentally-induced change in microbial diversity and function. The objective of this paper is to discuss the applications and merits of recently developed molecular, ecophysiological and analytical indicators and their utility in examining anthropogenic and climatic impacts on the structure and function of coastal microbial communities. The nitrogen-limited Neuse River Estuary and Pamlico Sound, North Carolina are used as examples of ecosystems experiencing both anthropogenic (i.e., accelerating eutrophication) and climatic stress (increasing frequencies of tropical storms and hurricanes). Additional examples are derived from a coastal monitoring site (LEO) on the Atlantic coast of New Jersey and Galveston Bay, on the Gulf of Mexico. In order to assess structure, function, and trophic state of these and other coastal ecosystems, molecular (DNA and RNA-based) characterizations of the microbial taxa involved in carbon, nitrogen and other nutrient transformations can be combined with diagnostic pigment-based indicators of primary producer groups. Application of these methods can reveal process-level microbial community responses to environmental variability over a range of scales. Experimental approaches combined with strategic monitoring utilizing these methods will facilitate: (a) understanding organismal and community responses to environmental change, and (b) synthesizing these responses in the context of ecosystem models that integrate physical, chemical and biotic variability with environmental controls.

In early August 2014, the municipality of Toledo, OH (USA) issued a 'do not drink' advisory on their water supply directly affecting over 400,000 residential customers and hundreds of businesses (Wilson, 2014). This order was attributable to levels of microcystin, a potent liver toxin, which rose to 2.5μgL-1 in finished drinking water. The Toledo crisis afforded an opportunity to bring together scientists from around the world to share ideas regarding factors that contribute to bloom formation and toxigenicity, bloom and toxin detection as well as prevention and remediation of bloom events. These discussions took place at an NSF- and NOAA-sponsored workshop at Bowling Green State University on April 13 and 14, 2015. In all, more than 100 attendees from six countries and 15 US states gathered together to share their perspectives. The purpose of this review is to present the consensus summary of these issues that emerged from discussions at the Workshop. As additional reports in this special issue provide detailed reviews on many major CHAB species, this paper focuses on the general themes common to all blooms, such as bloom detection, modeling, nutrient loading, and strategies to reduce nutrients.

Climate warming in combination with nutrient enrichment can greatly promote phytoplankton proliferation and blooms in eutrophic waters. Lake Taihu, China, is a large, shallow and eutrophic system. Since 2007, this lake has experienced extensive nutrient input reductions aimed at controlling cyanobacterial blooms. However, intense cyanobacterial blooms have persisted through 2017 with a record‐setting bloom occurring in May 2017. Causal analysis suggested that this bloom was sygenerically driven by high external loading from flooding in 2016 in the Taihu catchment and a notable warmer winter during 2016/2017. High precipitation during 2016 was associated with a strong 2015/2016 El Niño in combination with the joint effects of Atlantic Multi‐decadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO), while persistent warmth during 2016/2017 was strongly related to warm phases of AMO and PDO. The 2017 blooms elevated water column pH and led to dissolved oxygen depletion near the sediment, both of which mobilized phosphorus from the sediment to overlying water, further promoting cyanobacterial blooms. Our finding indicates that regional climate anomalies exacerbated eutrophication via a positive feedback mechanism, by intensifying internal nutrient cycling and aggravating cyanobacterial blooms. In light of global expansion of eutrophication and blooms, especially in large, shallow and eutrophic lakes, these regional effects of climate anomalies are nested within larger scale global warming predicted to continue in the foreseeable future. Plain Language Summary: Climate warming in combination with eutrophication can greatly promote algal blooms (algal scum aggregate at water surface) in eutrophic waters. But we know little of resultant response of algal bloom occurrence. Utilizing Lake Taihu, China, we examined how a large, shallow eutrophic system responds to extreme climate anomalies such as winter warmth and heavy rainfall. Intense algal blooms have persisted through 2017, after a decadal intensive effluent control. Causal analysis suggested that this bloom was synergistically driven by high external loading from flooding in 2016 and a notable warmer winter during 2016/2017, which were associated with a strong 2015/2016 El Niño, and joint effects of Atlantic Multi‐decadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO), whilst persistent warmth during 2016/2017 was strongly related to warm phases of AMO and PDO. Intense blooms led to water column pH increase and dissolved oxygen depletion near the bottom, both of which mobilized phosphorus from the sediment to overlying water and promoting algal blooms. Our finding indicates that regional climate anomalies exacerbated eutrophication via a positive feedback mechanism, by intensifying internal nutrient cycling and aggravating algal blooms. Key Points: High rainfall in 2016 and warm winter in 2016/2017 in Lake Taihu was caused by a strong 2015/2016 El Niño and Atlantic Multi‐decadal Oscillation and Pacific Decadal OscillationWarm winter in combination with high precipitation greatly promote phytoplankton proliferation and intense blooms in 2017 in Lake TaihuIntense blooms led to high pH and dissolved oxygen depletion and further mobilized phosphorus from the sediment to promote blooms [ABSTRACT FROM AUTHOR]

Shallow lakes can shift between stable states as a result of anthropogenic or natural drivers. Four common stable states differ in dominant groups of primary producers: submerged, floating, or emergent macrophytes or phytoplankton. Shifts in primary producer dominance affect key supporting, provisioning, regulating, and cultural ecosystem services supplied by lakes. However, links between states and services are often neglected or unknown in lake management, resulting in conflicts and additional costs.Here, we identify major shallow lake ecosystem services and their links to Sustainable Development Goals (SDGs), compare service provisioning among the four ecosystem states and discuss potential trade‐offs.We identified 39 ecosystem services potentially provided by shallow lakes. Submerged macrophytes facilitate most of the supporting (86%) and cultural (63%) services, emergent macrophytes facilitate most regulating services (60%), and both emergent and floating macrophytes facilitate most provisioning services (63%). Phytoplankton dominance supports fewer ecosystem services, and contributes most to provisioning services (42%).The shallow lake ecosystem services we identified could be linked to 10 different SDGs, notably zero hunger (SDG 2), clean water and sanitation (SDG 6), sustainable cities and communities (SDG 11), and climate action (SDG13).We highlighted several trade‐offs (1) among ecosystem services, (2) within ecosystem services, and (3) between ecosystem services across ecosystems. These trade‐offs can have significant ecological and economic consequences that may be prevented by early identification in water quality management.In conclusion, common stable states in shallow lakes provide a different and diverse set of ecosystem services with numerous links to the majority of SDGs. Conserving and restoring ecosystem states should account for potential trade‐offs between ecosystem services and preserving the natural value of shallow lakes. [ABSTRACT FROM AUTHOR]

Cyanobacterial harmful algal blooms (CyanoHABs) are an increasingly common feature of large, eutrophic lakes. Non-N2-fixing CyanoHABs (e.g., Microcystis) appear to be proliferating relative to N2-fixing CyanoHABs in systems receiving increasing nutrient loads. This shift reflects increasing external nitrogen (N) inputs, and a > 50-year legacy of excessive phosphorus (P) and N loading. Phosphorus is effectively retained in legacy-impacted systems, while N may be retained or lost to the atmosphere in gaseous forms (e.g., N2, NH3, N2O). Biological control on N inputs versus outputs, or the balance between N2 fixation versus denitrification, favors the latter, especially in lakes undergoing accelerating eutrophication, although denitrification removal efficiency is inhibited by increasing external N loads. Phytoplankton in eutrophic lakes have become more responsive to N inputs relative to P, despite sustained increases in N loading. From a nutrient management perspective, this suggests a need to change the freshwater nutrient limitation and input reduction paradigms; a shift from an exclusive focus on P limitation to a dual N and P co-limitation and management strategy. The recent proliferation of toxic non-N2-fixing CyanoHABs, and ever-increasing N and P legacy stores, argues for such a strategy if we are to mitigate eutrophication and CyanoHAB expansion globally. [ABSTRACT FROM AUTHOR]

Phosphorus (P) is a vital nutrient for algal growth. Aside from soluble reactive P (SRP), organic P (OP) is used by algae via alkaline phosphatase (AP) hydrolysis, which can play an important role in supplying P. Enzymatically-hydrolysable OP (EHP) can potentially be used as an indicator of bioavailability of P other than SRP in natural waters. We investigated the ecological significance of alkaline phosphatase activity (APA), EHP concentration and P turnover time in the inflowing rivers of Lake Taihu (Taihu) during three hydrologic periods. Results indicated high SRP concentration and low SRP demand by algal suppressed APA in the inflowing rivers, the highest proportion of OP mineralization rate (v) to the maximum reaction velocity of AP (Vmax) is only 14.7%. P turnover time of the inflowing rivers was generally from 3 to 7 days and in exceptional cases, it could exceed 10 days. The high EHP reserve and the sufficient AP for OP mineralization render the rivers a significant source of utilizable OP, further exacerbating eutrophication of Taihu. [ABSTRACT FROM AUTHOR]

The concept of lakes "evolving" phosphorus (P) limitation has persisted in limnology despite limited direct evidence. Here, we developed a simple model to broadly characterize nitrogen (N) surpluses and deficits, relative to P, in lakes and compared the magnitude of this imbalance to estimates of N gains and losses through biological N transformations. The model suggested that approximately half of oligotrophic lakes in the U.S.A. had a stoichiometric N deficit, but 72–89% of eutrophic and hypereutrophic lakes, respectively, had a similar N deficit. Although reactive N appeared to accumulate in the most oligotrophic lakes, net denitrification perpetuated the N deficit in more productive lakes. Productive lakes exported reactive N via biological N transformations regardless of their N deficit. The lack of N accumulation through N fixation underscores the need for a modern eutrophication management approach focused on reducing total external nutrient loads, including both N and P. [ABSTRACT FROM AUTHOR]

Reservoirs are essential for the wellbeing of human societies, but can also be subject to negative ecological impacts. The Three Gorges Reservoir (TGR) in the upper Yangtze River is remarkable for its size and engineering; however, its effects on water quality are poorly understood. To the best of our knowledge, this study is the first to describe long-term (1992–2016) monitoring. It showed that the chemical oxygen demand (COD, via the potassium permanganate index) and total phosphorus (TP) have decreased 40.9% ± 9.9% and 22.2% ± 9.7% respectively in the TGR mainstream between impoundment in June 2003 and 2016, while total nitrogen (TN) and ammonium (NH 4 -N) have increased 1.3% ± 2.4% and 8.2% ± 2.6%. In addition, phytoplankton biomass has increased by a factor of 2.7 (1.1–4.8) over pre-impoundment levels in the mainstream, and tributary algal blooms have increased in frequency since 2004. The reductions in COD and TP were caused primarily by decreases in water flow speed, which lead to sediment settlement. The anti-seasonal operation pattern and water volume increased TGR may also increases the dilution capacity. TN and ammonium are less affected by sediment deposition and have increased slightly under intensified human activities. Decreased water flow speeds and nutrient enrichment have promoted increases in algal biomass, leading to blooms in tributary backwater zones. In situ experiments indicate that phytoplankton growth in the TGR is phosphorus limited during all seasons. Therefore, controlling phosphorus will reduce the short-term eutrophication potential in the reservoir. However, concurrent control of nitrogen and phosphorus inputs are necessary in the long term. [ABSTRACT FROM AUTHOR]

A long‐standing debate in ecology deals with the role of nitrogen and phosphorus in management and restoration of aquatic ecosystems. It has been argued that nutrient reduction strategies to combat blooms of phytoplankton or floating plants should solely focus on phosphorus (P). The underlying argument is that reducing nitrogen (N) inputs is ineffective because N2‐fixing species will compensate for N deficits, thus perpetuating P limitation of primary production. A mechanistic understanding of this principle is, however, incomplete. Here, we use resource competition theory, a complex dynamic ecosystem model and a 32‐year field data set on eutrophic, floating‐plant dominated ecosystems to show that the growth of non‐N2‐fixing species can become N limited under high P and low N inputs, even in the presence of N2 fixing species. N2‐fixers typically require higher P concentrations than non‐N2‐fixers to persist. Hence, the N2 fixers cannot deplete the P concentration enough for the non‐N2‐fixing community to become P limited because they would be outcompeted. These findings provide a testable mechanistic basis for the need to consider the reduction of both N and P inputs to most effectively restore nutrient over‐enriched aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Abstract Primary production of vast regions of the world's estuarine, coastal and pelagic ocean waters is limited by availability of fixed nitrogen; this despite the fact that a highly diverse suite of microorganisms potentially capable of fixing N 2 (eubacteria and cyanobacteria), inhabit these waters. Theoretically, diazotrophs should supply the N needs to balance the N required to support primary production, assuming other key limiting nutrients, phosphorus and iron, are available and energy requirements are met. In practice however, N 2 fixation often does not meet ecosystem-scale N demands, even when these nutrients are replete. The problem lies with the fact that optimal rates of N 2 fixation are often controlled by additional environmental factors, including light and organic matter availability, turbulence, and high levels of dissolved oxygen which can suppress this process in N-deplete surface waters. In addition, rates of N loss via denitrification and anammox can exceed N 2 fixation and external N inputs on annual scales in coastal and pelagic waters, including those experiencing eutrophication. This creates a situation where chronic N limitation persists, even in the presence of anthropogenic nitrogen enrichment. Many aquatic ecosystems exhibit a perpetual "hunger" for fixed N to support primary and higher levels of production and this is likely to continue over forseeable biological and geologic timescales. [ABSTRACT FROM AUTHOR]

Abstract: Harmful algal blooms (HABs) are a global problem, exacerbated by rising temperatures, cultural eutrophication, urbanization, and agricultural development. During these HABs, phytoplankton consumption of CO2 may result in conditions of C limitation, where algal taxa best adapted for these conditions will be at a competitive advantage. Many cyanobacteria are capable of alleviating CO2 limitation by a variety of strategies, including the active assimilation of HCO 3 −. In this study, we utilized a high‐resolution, month‐long time series of stable C isotopes and high‐performance liquid chromatograph‐based algal taxonomy in the hypereutrophic Lake Taihu, China, to investigate whether cyanobacteria are indeed advantaged by CO2 limiting conditions. We employed a model of phytoplankton C acquisition to support the inferences derived from direct measurements. Diurnal cycles of production and respiration caused δ13CDIC to vary between −4‰ and −9‰, while δ13CPOC varied between −29.6‰ and −19.6‰. Measured and modeled phytoplankton fractionation of DIC were positively correlated with pCO2 and negatively correlated with cyanobacterial abundance, suggesting that CO2 limitation preferentially favored increased cyanobacterial biomass, relative to other taxa. We propose that the ability of many cyanobacteria to access otherwise limiting pools of inorganic C is intrinsically linked with their capacity to cope with CO2 limiting conditions, and may be a key factor in their dominance during HABs. [ABSTRACT FROM AUTHOR]

Nitrogen availability and form are important controls on estuarine phytoplankton growth. This study experimentally determined the influence of urea and nitrate additions on phytoplankton growth throughout the growing season (March 2012, June 2011, August 2011) in a temperate, eutrophied estuary (Neuse River Estuary, North Carolina, USA). Photopigments (chlorophyll a and diagnostic photopigments: peridinin, fucoxanthin, alloxanthin, zeaxanthin, chlorophyll b) and microscopy-based cell counts were used as indicators of phytoplankton growth. In March, the phytoplankton community was dominated by Gyrodinium instriatum and only fucoxanthin-based growth rates were stimulated by nitrogen addition. The limited response to nitrogen suggests other factors may control phytoplankton growth and community composition in early spring. In June, inorganic nitrogen concentrations were low and stimulatory effects of both nitrogen forms were observed for chlorophyll a- and diagnostic photopigment-based growth rates. In contrast, cell counts showed that only cryptophyte and dinoflagellate (Heterocapsa rotundata) growth were stimulated. Responses of other photopigments may have been due to an increase in pigment per cell or growth of plankton too small to be counted with the microscopic methods used. Despite high nitrate concentrations in August, growth rates were elevated in response to urea and/or nitrate addition for all photopigments except peridinin. However, this response was not observed in cell counts, again suggesting that pigment-based growth responses may not always be indicative of a true community and/or taxa-specific growth response. This highlights the need to employ targeted microscopy-based cell enumeration concurrent with pigment-based technology to facilitate a more complete understanding of phytoplankton dynamics in estuarine systems. These results are consistent with previous studies showing the seasonal importance of nitrogen availability in estuaries, and also reflect taxa-specific responses nitrogen availability. Finally, this study demonstrates that under nitrogen-limiting conditions, the phytoplankton community and its various taxa are capable of using both urea and nitrate to support growth. [ABSTRACT FROM AUTHOR]