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Notwithstanding the continuing advancement of our understanding of the broader ecosystem functioning in Lake Ontario, emerging evidence suggests that there are fundamental knowledge gaps to accurately describe the relationship between exogenous phosphorus (P) loading and in-lake total phosphorus (TP) concentrations. The whole-lake load appears to frequently exceed the Great Lakes Water Quality Agreement (GLWQA) interim target of 7000 metric tonnes·year−1, although the offshore-water P concentrations are consistently below the GLWQA interim substance concentration objective of 10 µg TP·L–1. Contrasting ecological conditions prevail in different locations of Lake Ontario, owing to the significant urban footprint along the shoreline but also the capacity of dreissenid mussels to sequester P in the littoral zone. Specifically, low ambient P levels threaten fisheries productivity in offshore waters, while nuisance benthic algae (predominantly Cladophora) and toxin-producing cyanobacteria blooms affect the water quality in the nearshore zone. The present study offers a technical analysis of the recent and ongoing modelling work that has been conducted in Lake Ontario and can be potentially used to address the multitude of ecosystem management challenges. Our aim is to provide an overview of all the major models developed by identifying their fundamental assumptions, structural attributes, and general consistency against empirical knowledge derived from the system. The existing modelling work has opted for parsimonious representations of the lower food web coupled with granular grid configurations to effectively link hydrodynamic processes and mass transport between nearshore and offshore waters. The establishment of comprehensive ecophysiological modules that will recreate the mechanisms underlying the interplay among bioavailable phosphorus, planktonic dynamics, dreissenid mussels, and Cladophora is a critical undertaking to reproduce the water quality conditions in the nearshore zone of Lake Ontario. Striving to integrate the lower food web with fisheries and ecosystem-service modelling, we also offer a technical analysis of knowledge gaps and monitoring-assessment objectives that should be addressed to ensure that ecosystem processes of management interest are adequately measured and the local modelling enterprise is focused on suitable performance indicators. [ABSTRACT FROM AUTHOR]

Nitrogen (N) is a limiting nutrient for primary productivity in most terrestrial ecosystems, but whether N limitation is strengthening or weakening remains controversial because both N sources and sinks are increasing in magnitude globally. Temperate marshes are exposed to greater amounts of external N inputs than most terrestrial ecosystems and more than in preindustrial times owing to their position downstream of major sources of human‐derived N runoff along river mouths and estuaries. Simultaneously, ecosystem N demand may also be increasing owing to other global changes such as rising atmospheric [CO2]. Here, we used interannual variability in external drivers and variables related to exogenous supply of N, along with detailed assessments of plant growth and porewater biogeochemistry, to assess the severity of N‐limitation, and to determine its causes, in a 14‐year N‐addition × elevated CO2 experiment. We found substantial interannual variability in porewater [N], plant growth, and experimental N effects on plant growth, but the magnitude of N pools through time varied independently of the strength of N limitation. Sea level, and secondarily salinity, related closely to interannual variability in growth of the dominant plant functional groups which drove patterns in N limitation and in porewater [N]. Experimental exposure of plants to elevated CO2 and years with high flooding strengthened N limitation for the sedge. Abiotic variables controlled plant growth, which determined the strength of N limitation for each plant species and for ecosystem productivity as a whole. We conclude that in this ecosystem, which has an open N cycle and where N inputs are likely greater than in preindustrial times, plant N demand has increased more than supply. [ABSTRACT FROM AUTHOR]

In 2019, phytoplankton and environmental parameters were analysed monthly during the growing season from April to September at 26 sampling sites in the Danube and 10 additional sampling sites in the main tributaries as part of the Joint Danube Survey 4, organised by the ICPDR. Our results showed that both phytoplankton biomass and composition follow the River Continuum Concept on free-flowing sections, but also responds to hydromorphological changes where the largest dam Iron Gate represents the largest interruption of the river and the phytoplankton continuum. Besides longitudinal interruption, water residence time was the most important factor for phytoplankton composition, while nutrients were less relevant. The low phytoplankton biomass and its composition in the Danube support the oligotrophication trend, but this one-year study could not confirm it with certainty. Phytoplankton is the most important autotrophic component in the Upper and Middle Danube, where environmental conditions do not support the optimal growth of other river flora. The predominant FGs coda were A, C and D as a typical potamoplankton component, while the codon TB occurs throughout the Danube but is more prominent in the Upper reach and other river sections with higher discharge events. [ABSTRACT FROM AUTHOR]

Human impacts on lakes can lead to nutrient release, increased algal productivity and consequently low oxygen concentrations in deepwater environments. Here we report the unexpected and contrasting finding of a pronounced shift to chironomid and invertebrate assemblages suggesting oligotrophication and oxygen increase in Lac de Champex, a Swiss mountain lake, due to human activities during the Mediaeval period. Chironomid assemblages in the lower part of the lake sediment record show changes that agree with known climatic shifts during the Lateglacial period and early Holocene and a progressively stronger influence of peat development, and possibly humic conditions, from ca. 8500 calibrated 14C years BP (cal. BP) onwards. This resulted in low chironomid influx and assemblages with high abundances of Procladius and Chaoborus ca. 2000–3000 cal. BP. These assemblages resemble those found in relatively nutrient-rich, oxygen-poor lakes and may have developed due to natural, long-term development to more nutrient-rich and possibly humic conditions, and from the Bronze Age (ca. 4000 cal. BP) onwards, due to human activities such as pasturing. Ca. 1000 cal. BP, an abrupt increase in sediment accumulation, chironomid influx and distinct changes in the chironomid assemblage composition were observed, coinciding with evidence for local human activities from pollen data. A major transition in chironomid and invertebrate assemblages followed ca. 500 cal. BP with the disappearance of Chaoborus, a decrease in Procladius, and the increase or appearance of several chironomid taxa, including several rheophilous groups. We conclude that Late-Holocene human impact led to re-oligotrophication and increased oxygen availability for invertebrates in the lake. This reversal of the long-term Holocene trend was possibly promoted by increased erosion from the Mediaeval period onwards and from ca. 500 cal. BP likely by the construction of an irrigation canal (bisse), leading to inflow of clear, cool water which changed the lake's hydrological characteristics. [ABSTRACT FROM AUTHOR]

Understanding the drivers and consequences of global environmental change is crucial to inform predictions of effects on ecosystems. We used the mammal community of Białowieża Forest, the last lowland near‐primeval forest in temperate Europe, as a sentinel of global change. We analyzed changes in stable carbon (δ13C) and nitrogen (δ15N) isotope values of hair in 687 specimens from 50 mammal species across seven decades (1946–2011). We classified mammals into four taxonomic‐dietary groups (herbivores, carnivores, insectivores, and bats). We found a significant negative trend in hair δ15N for the mammal community, particularly strong for herbivores. This trend is consistent with temporal patterns in nitrogen deposition from (15N depleted) industrial fertilizers and fossil fuel emissions. It is also in line with global‐scale declines in δ15N reported in forests and other unfertilized, non‐urban terrestrial ecosystems and with local decreases in N foliar concentrations. The global depletion of 13C content in atmospheric CO2 due to fossil fuel burning (Suess effect) was detected in all groups. After correcting for this effect, the hair δ13C trend became non‐significant for both community and groups, except for bats, which showed a strong decline in δ13C. This could be related to an increase in the relative abundance of freshwater insects taken by bats or increased use of methane‐derived carbon in food webs used by bats. This work is the first broad‐scale and long‐term mammal isotope ecology study in a near‐primeval forest in temperate Europe. Mammal communities from natural forests represent a unique benchmark in global change research; investigating their isotopic temporal variation can help identify patterns and early detections of ecosystem changes and provide more comprehensive and integrative assessments than single species approaches. [ABSTRACT FROM AUTHOR]

Until 2006, the Lipno Reservoir had the largest catches of pikeperch in the Czech Republic, but since 2007 it has partially collapsed. This collapse was preceded by oligotrophication, when phosphorus concentrations declined by 25%. Conservation measures from 2009 to 2015 partially improved the population from 2015, but pikeperch biomass did not reach previous levels. Unsuccessful spawning and low survival of pikeperch fry were suspected as possible reasons. Pelagic trawling for pikeperch younger juveniles in June and older juveniles in August indicated that extreme mortality in the first year of life, rather than limited spawning, was the reason for low pikeperch recruitment. Large interannual differences in pikeperch density were observed in both spring and summer, and the growth was generally low. Pikeperch fry density changed over a trophic gradient with the highest densities in the eutrophic upper reservoir parts, while differences between depths were much less pronounced due to the lack of temperature and oxygen depth stratification in summer. Oligotrophication of the reservoir in recent years is probably partly responsible for the high mortality and slow growth of pikeperch fry that are still zooplanktivorous, resulting in a lower probability of surviving their first winter. Given the changing conditions in the reservoir, measures should be taken to strictly conserve populations to maintain self‐sustaining pikeperch populations in the Lipno Reservoir. [ABSTRACT FROM AUTHOR]

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The Adriatic Sea hosts diverse marine ecosystems, characterized by rich biodiversity and unique ecological dynamics. Its intricate coastal habitats and open waters support a range of species and contribute to the region's ecological and economic significance. Unraveling the consequences of the ongoing climate changes on this delicate environment is essential to ensure the future safeguard of this basin. To tackle this problem, we developed a biogeochemical model for the entire basin, with a horizontal resolution of about 2 km and 120 vertical levels, forced by the projections of atmosphere, hydrology and ocean circulation between 1992 and 2050, under emission scenario RCP8.5. The changes projected between 2031–2050 and 1992–2011 were evaluated on ecoregions characterized by different trophic conditions, identified using a k-medoid classification technique. The results point toward a generalized oligotrophication of the basin, especially intense in the northern estuarine areas, driven by a substantial decrease in river discharge projected for the rivers of the Po Plain. This scenario of unproductive and declining resources, together with the ongoing warming, salinization, and acidification of marine waters, cast doubt on the long-term resilience of the Northern Adriatic food web structure, which has evolved to thrive in high trophic conditions. The outcome of this study provides the stakeholders with a tool to understand how potential long-term decreases in the regimes of the Northern Adriatic Rivers could affect the marine ecosystem and its goods and services in the future.

Human-driven nutrient inputs into aquatic ecosystems must be managed to preserve biodiversity and to ensure that valued fishery and water quality services are not compromised by hypoxia and harmful algal blooms. Aiming for nutrient inputs that achieve an intermediate level of ecosystem productivity is expected to provide both high fish yield and good water quality. However, we argue that such an intermediate "optimum" may not exist for many aquatic ecosystems that support multiple fisheries with differing tolerances to eutrophication and that must provide multiple water quality services. We further support this argument with an empirical case study of nearly a century (1915-2011) of change in the productivity of Lake Erie and its lake whitefish (Coregonus clupeaformis), walleye (Sander vitreus), and yellow perch (Perca flavescens) fisheries. We discuss and show how the harvest of each fishery has been historically maximized at different levels of ecosystem productivity. Additionally, we examine how anticipated management efforts to improve water quality by reducing nutrient inputs (i.e., oligotrophication) may favor certain fisheries over others, resulting in no single optimal range of nutrient inputs that achieves all valued fishery and water quality objectives. Our synthesis and case study illustrate how the need to balance multiple services in aquatic ecosystems can create a wicked management problem with inevitable trade-offs. To navigate these trade-offs, we recommend the use of ecosystem-based management approaches, which can help decision makers identify and resolve complex trade-offs by facilitating cooperative research and communication among water quality regulators, fisheries managers, and end users. [ABSTRACT FROM AUTHOR]

Food web theory predicts that one of the consequences of oligotrophication is the decline in fisheries following the establishment of nutrient-deficient conditions and low biogenic production. However, emerging evidence suggests that more complex trophic interactions, induced by plankton composition, dietary diversification, temporal and spatial heterogeneity, variations in fish age and size structure can lead to ecosystem responses that deviate from theoretical predictions; especially in the shallow littoral zone. In this study, two end-to-end modelling strategies are used to characterize potential food web structural shifts and overall ecosystem productivity in response to oligotrophication. Our modelling strategy draws upon empirical and theoretical evidence from the Bay of Quinte, a semi-enclosed embayment in northeastern Lake Ontario that has been subjected to phosphorus (P) abatement strategies in the 1970s. We first show that the role of food abundance has been a primary factor shaping fish production, but the fish community gradually responded to the reduced availability of pelagic staples of their diet by displaying greater reliance upon alternative sources, including benthic and other autochthonous (or even allochthonous) food items. Our analysis further suggests that degree of diet overlap between competing trophic guilds can significantly modulate realized biomass levels. The composition of the autotrophic community is another major confounding factor that can induce significant variations from the predicted food web productivity patterns in response to oligotrophication. Overall, our analysis suggests that the proposed reduction of the ambient total phosphorus levels may not necessarily trigger a significant decline in fish biomass in the Bay of Quinte. [ABSTRACT FROM AUTHOR]

Trends and ecological consequences of phosphorus (P) decline and increasing nitrogen (N) to phosphorus (N:P) ratios in rivers and estuaries are reviewed and discussed. Results suggest that re‐oligotrophication is a dominant trend in rivers and estuaries of high‐income countries in the last two–three decades, while in low‐income countries widespread eutrophication occurs. The decline in P is well documented in hundreds of rivers of United States and the European Union, but the biotic response of rivers and estuaries besides phytoplankton decline such as trends in phytoplankton composition, changes in primary production, ecosystem shifts, cascading effects, changes in ecosystem metabolism, etc., have not been sufficiently monitored and investigated, neither the effects of N:P imbalance. N:P imbalance has significant ecological effects that need to be further investigated. There is a growing number of cases in which phytoplankton biomass have been shown to decrease due to re‐oligotrophication, but the potential regime shift from phytoplankton to macrophyte dominance described in shallow lakes has been documented only in a few rivers and estuaries yet. The main reasons why regime shifts are rarely described in rivers and estuaries are, from one hand the scarcity of data on macrophyte cover trends, and from the other hand physical factors such as peak flows or high turbidity that could prevent a general spread of submerged macrophytes as observed in shallow lakes. Moreover, re‐oligotrophication effects on rivers may be different compared to lakes (e.g., lower dominance of macrophytes) or estuaries (e.g., limitation of primary production by N instead of P) or may be dependent on river/estuary type. We conclude that river and estuary re‐oligotrophication effects are complex, diverse and still little known, and in some cases are equivalent to those described in shallow lakes, but the regime shift is more likely to occur in mid to high‐order rivers and shallow estuaries. [ABSTRACT FROM AUTHOR]

We evaluated the effects of a periphyton bioreactor on phytoplankton by experimentally simulating oligotrophication in a shallow eutrophic system. The experiment had two 50% diluted treatments with and without a periphyton bioreactor. Sampling was performed on days 6, 9, 12, 15, and 20 of the experimental period. The periphyton bioreactor accumulated biomass (chlorophyll-a, AFDM) and TP during the experimental period. Despite the biomass and TP loss due to periphyton detachment from the substrate after community reaching the algal biomass peak, the gains exceeded the losses, and the net rate was positive for all attributes in the bioreactor. Based on the average, our findings suggest that periphyton bioreactors negatively affected the phytoplankton total biovolume. Cyanobacteria were the most abundant phytoplankton group. However, the periphyton bioreactor caused the biomass loss of the Raphidiopsis raciborskii in phytoplankton. Our results suggest that bioreactor influenced the phytoplankton structure, reducing cyanobacterial biomass, especially Raphidiopsis raciborskii. However, the bioreactor did not reflect a significant increase in the epipelon biomass during the experimental period. We conclude that the periphyton bioreactor has the potential to assist in the maintenance of restored shallow lakes and reservoirs, especially in controlling phytoplankton growth. [ABSTRACT FROM AUTHOR]

Climate change and reduction in nutrient loads have significant effects on primary production and phytoplankton growth dynamics. Since in the last few decades in many regions, nutrients in lakes were reduced simultaneously as the climate changed. Yet, it remains unclear which of the two has impacted primary production the most. In this study, we couple the General Ocean Turbulence Model with the Ecological Regional Ocean Model to disentangle the effects of climate change and reoligotrophication on primary production (PP) in Lake Geneva, Switzerland–France. We apply a data assimilation method to calibrate the model with the observations from the past (1981–1990) and validate it against the in situ data from the present decade (2011–2019). Both decades represent different climate conditions and trophic states of the lake. We show that the model is skilful to reproduce assimilated and unassimilated observations from both periods. According to our results, the effect of reoligotrophication on PP is marginally higher than that of warming, leading to a net decrease in primary production by 10% from the past to the present. The areal phosphorus supply in Lake Geneva, in spite of a decrease by ∼ 70%, is still characteristic of a meso-to-eutrophic ecosystem. This points towards an incomplete reoligotrophication of the lake. The effects of future climate change on winter mixing and PP dynamics have also been studied. Although there would be a significant reduction in deep mixing, the autotrophic production in Lake Geneva is expected to increase by ∼ 20% by the end of twenty-first century, largely due to stimulation in biomass build-up of temperature-dependent algae (e.g. dinoflagellates and cyanobacteria). Considering our results to represent other large temperate lakes with similar trophic status and water residence time as Lake Geneva, future climate scenarios are expected to bring back symptoms of eutrophication. [ABSTRACT FROM AUTHOR]

In the Seto Inland Sea, the largest semi-enclosed sea in Japan, the most dominant diatom in the past, Skeletonema spp., has been replaced by another diatom Chaetoceros spp. since the 1980s, and this shift is often explained as the result of oligotrophication. Based on previous observations of a shift from Skeletonema spp. to Chaetoceros spp. under prolonged sunny conditions, the recent increase in solar insolation over the last 30 years might have also accelerated the replacement of Skeletonema by Chaetoceros, especially during the summer when nutrient levels are relatively low and solar insolation is high. In our experiments, culture strains of Skeletonema costatum and Chaetoceros lorenzianus under severely nitrogen-limited conditions exhibited less non-photochemical quenching (NPQ) under prolonged exposure (1 h) to high light (800 µmol-photons m-2 s-1) and a decrease in photochemical quenching (qP) which was especially notable in S. costatum. Conversely, marked increases in NPQ were observed under severely phosphorus-limited conditions, even under short time exposure (30 s) to high light, even though the increase in NPQ could not relieve the decrease in qP, which was more apparent in S. costatum. These trends in NPQ and qP were attributed to the limited nutrients because replenishment of the nutrients led to a decrease in NPQ and an increase in qP. Interestingly, this recovery was faster in C. lorenzianus than S. costatum. The results showed that phosphorus depletion caused severe photoinhibition especially in S. costatum, irrespective of active NPQ induction. Further, given the severe phosphorus-limited conditions in the Seto Inland Sea for an extended period, we conducted competition experiments using continuous coculture of both species to simulate the typical summer environment where severe phosphorus limitation and high light occur. The results showed that the shift from S. costatum to C. lorenzianus was accelerated by continuous exposure to high light, which could explain the recent shift in the dominant species in the summer in the study area.

Periphyton is a key primary producer in shallow lakes, sensitive to global warming and changes in nutrient balances. Reduced nitrogen availability due to accelerated denitrification at higher temperatures or in response to reduced N loadings aimed to reduce the eutrophication may affect periphyton biomass and composition, to compensate for the low N availability (e.g. promoting N2-fixing). We analysed periphyton responses to N decline in 12 eutrophic shallow lake mesocosms during one year of low N compared to high N, under three temperature scenarios: ambient, A2 IPCC scenario and A2 increased by 50%. We used two submerged macrophytes (Potamogeton crispus and Elodea canadensis) and artificial imitations of these as substrates for periphyton growth. Nitrogen decline increased periphyton biomass and induced compositional changes irrespective of season, plant type, and temperature. Periphyton biomass was negatively associated to phytoplankton and positively to plant complexity. Warmer scenarios negatively affected periphyton exclusively at high N loadings. Low N conditions were associated with lower periphyton taxonomic richness, lower N2-fixing cyanobacteria biovolume and increased biovolume of large-sized chlorophytes and non-N2-fixing cyanobacteria. Our results suggest that low N conditions promoted periphyton due to a more efficient use of nutrients and improved light conditions resulting from lower phytoplankton biomass and contrasting effects of temperature. [ABSTRACT FROM AUTHOR]

An effective strategy for utilizing anaerobic digestates is required to promote biomass power generation. We developed an anaerobic digestate tablet using liquid dairy cattle manure derived from a small mesophilic anaerobic digester installed on a dairy farm. Anaerobic digestate tablets are intended for use in the fertilization of oligotrophic coastal seas to promote primary production. The purpose of this study was to evaluate (1) the dissolution behavior of nutrients from anaerobic digestate tablets and (2) the effect of the application of anaerobic digestate tablets on the growth of benthic microalgae using a culture experiment. Batch experiments were conducted to investigate the dissolution behavior of the nutrients. Cumulative amounts of dissolved inorganic nitrogen and phosphate in the anaerobic digestate tablet ranged from 110 to 28.9 μg g−1 after 28 days. The dissolved inorganic nitrogen in the anaerobic digestate tablet was mainly ammonium nitrogen and accounted for 92.4–96.9%, which is advantageous for the growth of microalgae. The growth curve of the benthic microalga Nitzchia longissima was monitored using f/2 medium added to the anaerobic digestate tablet. The growth of Nitzchia longissima was two orders of magnitude greater than that of the positive control. The enhanced growth of Nitzchia longissima by the anaerobic digestate tablet was considered a concomitant effect of moderate dissolution of ammonium nitrogen and high affinity for benthic microalgae. In conclusion, the anaerobic digestate tablets prepared in this study have the advantage of supplying nitrogen to benthic microalgae. This study proposes a new method for utilizing anaerobic digestates. [Display omitted] • Anaerobic digestate tablet was prepared from liquid dairy cattle manure digestate. • Inorganic nitrogen and phosphate were dissolved in an anaerobic digestate tablet. • The dissolved inorganic nitrogen was mainly ammonium. • Nitzchia growth was two orders of magnitude higher than that of positive control. • We proposed utilization of anaerobic digestate for fertilization of coastal seas. [ABSTRACT FROM AUTHOR]

Human-driven nutrient inputs into aquatic ecosystems must be managed to preserve biodiversity and to ensure that valued fishery and water quality services are not compromised by hypoxia and harmful algal blooms. Aiming for nutrient inputs that achieve an intermediate level of ecosystem productivity is expected to provide both high fish yield and good water quality. However, we argue that such an intermediate “optimum” may not exist for many aquatic ecosystems that support multiple fisheries with differing tolerances to eutrophication and that must provide multiple water quality services. We further support this argument with an empirical case study of nearly a century (1915–2011) of change in the productivity of Lake Erie and its lake whitefish ( Coregonus clupeaformis ), walleye ( Sander vitreus ), and yellow perch ( Perca flavescens ) fisheries. We discuss and show how the harvest of each fishery has been historically maximized at different levels of ecosystem productivity. Additionally, we examine how anticipated management efforts to improve water quality by reducing nutrient inputs (i.e., oligotrophication) may favor certain fisheries over others, resulting in no single optimal range of nutrient inputs that achieves all valued fishery and water quality objectives. Our synthesis and case study illustrate how the need to balance multiple services in aquatic ecosystems can create a wicked management problem with inevitable trade-offs. To navigate these trade-offs, we recommend the use of ecosystem-based management approaches, which can help decision makers identify and resolve complex trade-offs by facilitating cooperative research and communication among water quality regulators, fisheries managers, and end users.

Many coastal ecosystems, including those of the Laurentian Great Lakes, suffer from various natural and anthropogenic stressors. Given that multiple stressors often concomitantly impact ecosystems, it may be difficult to disentangle which stressors are most influential. Upper trophic level communities, such as fish assemblages, can provide insights to the influence of diverse stressors as they may integrate cumulative effects over the long-term and also reflect responses of lower trophic levels. We used multivariate analyses and assemblage indices to investigate long-term (1984–2016) patterns in a nearshore fish assemblage indexed via annual trawling in the Indiana waters of southern Lake Michigan. Based on observations from other regions of the Great Lakes, we expected that oligotrophication, due to reduced nutrient loading and filtering by invasive mussels, would have a strong influence on the fish assemblage. However, we were unsure if the very nearshore fish assemblage would track observed decreased production patterns in offshore Lake Michigan or if observed increased primary production in the very nearshore would affect the fish assemblage. Consistent with the former expectation, overall abundance and richness of the assemblage declined over time. However, contrary to observations in other regions there was no overall evidence of species tolerant to more eutrophic conditions being replaced by more sensitive species. Moreover, there was limited evidence of the fish assemblage shifting towards species more tolerant of warm water, as might be expected with climate change. While increased numbers of invasive species added species to the system, overall species richness and native species richness declined. [ABSTRACT FROM AUTHOR]

The restoration of eutrophic lakes requires the identification of phosphorus objectives, i.e., the phosphorus reductions needed to achieve desired water quality goals. Due to inherent uncertainty, phosphorus objectives need periodic revision as the restoration progresses. We used monitoring data from a deep southern Alpine lake (Lake Lugano, Switzerland and Italy) to assess restoration progress and revise the current phosphorus objective of 30 mg m-3. Because one basin of the lake is meromictic (North basin) and the other is holomictic (South basin), restoration focussed on the mixolimnion for the North basin and the entire water column for the South basin. Time series analyses indicated that, thanks to restoration, phosphorus concentrations in the lake declined to values compliant with the objective (~20-30 mg m-3). In contrast, little progress was observed towards achieving the main water quality goals (chlorophyll a ≤4 mg m-3, primary production ≤150 g C m-2 year-1 and oxygen concentrations ≥4 mg L-1). Using predictive models, we estimated that achieving these goals requires a phosphorus objective of <10 mg m-3, which would bring the lake back to the original oligotrophic state. The concentration of <10 mg m-3 is lower than the objectives predicted for other (mainly northern) deep Alpine lakes. The apparent sensitivity of Lake Lugano, which we attribute to unfavorable hydrodynamic conditions common in lakes south of the Alps (weak mixing and long stratification), calls for particularly attentive phosphorus management. [ABSTRACT FROM AUTHOR]

Prior to the appearance of Dreissena , pronounced benthic nepheloid layers (BNL) near the bottom characterized by elevated levels of both turbidity and total phosphorus (TP), were a consistent and extensive feature of the offshore, stratified waters of all the Laurentian Great Lakes, except Lake Superior. In recent (2010–2019) years, the BNL has disappeared from all areas except for central Lake Erie, where only a small decrease in bottom turbidity has occurred. All stratified regions which exhibited a pre- Dreissena BNL, including central Lake Erie, experienced substantial post- Dreissena reductions in near-bottom TP, although the forms of phosphorus (particulate or soluble) responsible for these reductions have varied from lake to lake. Notably, the arrival of Dreissena at offshore sites was not accompanied by an increase in soluble phosphorus. Initiation of changes in the BNL almost invariably preceded appearance of Dreissena in the offshore, suggesting both that dreissenid impacts on the reductions in the BNL were largely remote, and by extension that the source of the BNL was also at least in part remote. Previous researchers' estimates of the importance of the benthic pool of phosphorus to offshore water column concentration suggest that the post-invasion reductions in bottom phosphorus during the stratified season could be contributing to the offshore oligotrophication of the lakes. [ABSTRACT FROM AUTHOR]

Freshwater chemistry across the circumpolar region was characterised using a pan‐Arctic data set from 1,032 lake and 482 river stations. Temporal trends were estimated for Early (1970–1985), Middle (1986–2000), and Late (2001–2015) periods. Spatial patterns were assessed using data collected since 2001.Alkalinity, pH, conductivity, sulfate, chloride, sodium, calcium, and magnesium (major ions) were generally higher in the northern‐most Arctic regions than in the Near Arctic (southern‐most) region. In particular, spatial patterns in pH, alkalinity, calcium, and magnesium appeared to reflect underlying geology, with more alkaline waters in the High Arctic and Sub Arctic, where sedimentary bedrock dominated.Carbon and nutrients displayed latitudinal trends, with lower levels of dissolved organic carbon (DOC), total nitrogen, and (to a lesser extent) total phosphorus (TP) in the High and Low Arctic than at lower latitudes. Significantly higher nutrient levels were observed in systems impacted by permafrost thaw slumps.Bulk temporal trends indicated that TP was higher during the Late period in the High Arctic, whereas it was lower in the Near Arctic. In contrast, DOC and total nitrogen were both lower during the Late period in the High Arctic sites. Major ion concentrations were higher in the Near, Sub, and Low Arctic during the Late period, but the opposite bulk trend was found in the High Arctic.Significant pan‐Arctic temporal trends were detected for all variables, with the most prevalent being negative TP trends in the Near and Sub Arctic, and positive trends in the High and Low Arctic (mean trends ranged from +0.57%/year in the High/Low Arctic to −2.2%/year in the Near Arctic), indicating widespread nutrient enrichment at higher latitudes and oligotrophication at lower latitudes.The divergent P trends across regions may be explained by changes in deposition and climate, causing decreased catchment transport of P in the south (e.g. increased soil binding and trapping in terrestrial vegetation) and increased P availability in the north (deepening of the active layer of the permafrost and soil/sediment sloughing). Other changes in concentrations of major ions and DOC were consistent with projected effects of ongoing climate change. Given the ongoing warming across the Arctic, these region‐specific changes are likely to have even greater effects on Arctic water quality, biota, ecosystem function and services, and human well‐being in the future. [ABSTRACT FROM AUTHOR]

In 1970–2010, during the period of spring circulation and summer stagnation, hydrochemical studies were conducted in Lake Ińsko (Western Pomeranian Lake Region, Poland) with determination of the lake susceptibility to degradation and trophic changes. Also, the effect of the catchment area on the water quality in this waterbody was assessed. The waters of the study lake were characterised by low static index, which is an additional indicator of low dynamics of water masses, and low susceptibility to degradation. In spite of this, significant changes in the lake quality and trophy were observed. The hydrochemical parameters defining water quality of the study lake continued to improve. In the 70’s, the water quality was at the border of class II and III, while in 2006 and 2010 it reached the level characteristic for class I waters. Moreover, in the 70’s and 80’s of the previous century, Lake Ińsko Duże was a mesotrophic lake. Then, an increase in the lake trophy was observed, resulting in signs of eutrophy. At the end of the 90’s and in the first decade of the 21st century, the study lake returned to the state of mesotrophy. No restoration works were undertaken in Lake Ińsko in the study period. The improvement in water quality, called oligotrophication, resulted most probably from the lake reaction to changes in the soil use in the catchment area, since fewer phosphorus and nitrogen compounds flow into the lake, and also from the regulation of the wastewater management in the town of Ińsko. [ABSTRACT FROM AUTHOR]

The construction of four large reservoirs for hydroelectric power generation in the main channel of the Paraná River and many others in the tributaries has eliminated important wetlands, except over an area of ∼2000 km2 between the reservoirs of the UHE Engenheiro Sérgio Motta (Porto Primavera) and the Itaipu. The up-river reservoirs created large changes in the river's hydrology and in its loads of suspended, bed and dissolved material. A detailed assessment of the impact of these changes requires a description and classification of the floodplain's different subunits, called macrohabitats, according to hydrological, hydromorphological, and vegetational parameters. The classification system presented herein is hierarchical and is embedded in the classifications of Brazilian wetlands. The system differentiates the Paraná River floodplain into 6 functional units, 13 subclasses, and 23 macrohabitats, which are presented in this paper. Changes in hydrology have reduced the flood amplitude, frequency and permanence, and thus confined the macrohabitats to an area currently subjected to a regular flood pulse of ∼3 m rather than the previous ∼6 m. The gradual terrestrialization of macrohabitats at the upper end of the flood gradient is expected, accompanied by a loss of macrohabitats at its lower end because of long-term flooding. The retention of sediments and dissolved nutrients in the reservoirs will lead to the oligotrophication of aquatic macrohabitats, the degradation of near-channel hydromorphic structures and their vegetation, and a general reduction of overall primary production in the floodplain, which may extend also to higher trophic levels. [ABSTRACT FROM AUTHOR]

Climate warming can restructure lake food webs if trophic levels differ in their thermal responses, but evidence for these changes and their underlying mechanisms remain scarce in nature. Here we document how warming lake temperatures by up to 2°C, rather than changes in trophic state or fishing effort, have restructured the pelagic food web of a large European lake (Lake Maggiore, Italy). Our approach exploited abundance and biomass data collected weekly to yearly across five trophic levels from 1981 to 2008. Temperature generally had stronger effects on taxa than changes in fish predation or trophic state mediated through primary productivity. Consequently, we found that, as the lake warmed, the food web shifted in numerical abundance towards predators occupying middle trophic positions. Of these taxa, the spiny water flea (Bythotrephes longimanus) most prospered. Bythotrephes strongly limited abundances of the keystone grazer Daphnia, strengthening top‐down structuring of the food web. Warmer temperatures partly restructured the food web by advancing peak Bythotrephes densities by approximately 60 days and extending periods of positive population growth by three times. Nonetheless, our results suggested that advances in the timing and size of peak Bythotrephes densities could not outpace changes in the timing and size of peak densities in their Daphnia prey. Our results provide rare evidence from nature as to how long‐term warming can favour higher trophic levels, with the potential to strengthen top‐down control of food webs. [ABSTRACT FROM AUTHOR]

Over the next century, many mid and high latitude temperate ecosystems are projected to experience rising growing season temperatures and increased frequency of soil freeze/thaw cycles (FTCs) due to a reduction in the depth and duration of the winter snowpack. We conducted a manipulative field experiment in a northern hardwood forest at the Hubbard Brook Experimental Forest in New Hampshire to determine the interactive effects of climate change across seasons on rates of net N mineralization, foliar N, and natural abundance foliar 15N (δ15N) in red maple (Acer rubrum) trees. We warmed soils 5 °C above ambient temperatures and induced winter FTCs to simulate projected changes over the next century. Net N mineralization was dominated by ammonification and increased with warmer soil temperatures, but was not affected by soil FTCs in the previous winter. Similarly, warming led to increased foliar N concentrations and δ15N, with no effect of soil FTCs. Together, our results show that growing season soil warming increases soil N availability and N uptake by trees, which may offset the previously observed negative effects of a smaller snowpack and more frequent soil freezing on N cycling. We conclude that soil warming in the growing season may counteract the trend of reduced soil N availability relative to plant N demand (i.e. N oligotrophication) observed in northern hardwood forests. This research demonstrates that climate change across seasons affects N cycling in northern hardwood forests in ways that would have not been apparent from examining one season alone. [ABSTRACT FROM AUTHOR]

SUMMARY We investigated the monthly variations of estimated in‐situ uptake and growth‐related demand for phosphorus in Sargassum macrocarpum during a growth period from December 2017 to April 2018, a maturation period from May to August 2018, and the second‐growth period from September 2018 to January 2019. Phosphate uptake was relatively low during maturation and high during growth, and coincided with changes in external phosphate levels. The demand for phosphorus was at its highest in March 2018, which corresponded with the highest growth rate and tissue phosphorus levels. Thereafter, the demand was relatively constant between April 2018 and January 2019. Consequently, the uptake rate exceeded demand from December 2017 to February 2018 and December 2018 to January 2019, suggesting that phosphorus accumulated in tissues. Additionally, the uptake rates between March 2018 and November 2018 were 0.1–0.8 times lower than the demand, probably resulting in the effective use of stored phosphorus to sustain growth during this phosphate‐depleted period. The variations between estimated uptake and demands for P found in S. macrocarpum indicate that P dynamics vary with life stage and are implicitly linked to the season. [ABSTRACT FROM AUTHOR]

Understanding and explaining the structure of communities in response to environmental gradients is a central goal in ecology. Trait‐based approaches are promising but yet rarely applied to understand community dynamics in response to changing environmental conditions.Here, we investigate seasonal succession patterns of functional traits in phytoplankton communities and how nutrient reductions (oligotrophication) alter these patterns. We used phytoplankton data from 40 years of observation from the Rappbode Reservoir (Germany), which underwent a strong shift in trophic conditions, and translated taxonomic composition into functional traits by assigning trait values compiled from the literature.All studied traits (morphological, behavioural and physiological traits) responded to changing environmental conditions and showed consistent, reoccurring seasonal developments. The seasonal succession of phytoplankton communities was shaped by a trade‐off between small‐celled, fast‐growing species that are able to rapidly incorporate existing resources (r‐strategists) and large‐celled species with more complex and efficient mechanisms to exploit scarce mineral nutrients or acquire previously unexploited nutrient pools (k‐strategists). In summer, when nutrients were scarce, the k‐strategy was prevailing (important traits: phosphate affinity, nitrogen fixation, motility and mixotrophy). During the rest of the year, nutrients and turbulence were high and r‐strategists dominated (important traits: maximum growth rate and light affinity).A comparison between eutrophic and oligotrophic years revealed that the main features of functional trait succession were largely preserved, but intra‐annual fluctuations from spring to summer were stronger during eutrophic years. Nutrient reductions mainly affected functional traits and biomass in spring, while in summer the functional community composition changed little.Synthesis. This study provides for the first time a quantitatively supported functional template for trait‐based succession patterns in lakes under different nutrient conditions. By translating taxonomic composition into trait information, we demonstrate that the quantification of functional characteristics enables ecological interpretation of observed community dynamics and provides not only a testable template but also a powerful tool towards a more mechanistic understanding. The quantification of functional traits further improves the predictability of community shifts in response to changing environmental conditions and thus opens new perspectives for predictive limnology using lake ecosystem models. [ABSTRACT FROM AUTHOR]

Nutrient loading and climate change affect coastal ecosystems worldwide. Unravelling the combined effects of these pressures on benthic macrofauna is essential for understanding the future functioning of coastal ecosystems, as it is an important component linking the benthic and pelagic realms. In this study, we extended an existing model of benthic macrofauna coupled with a physical–biogeochemical model of the Baltic Sea to study the combined effects of changing nutrient loads and climate on biomass and metabolism of benthic macrofauna historically and in scenarios for the future. Based on a statistical comparison with a large validation dataset of measured biomasses, the model showed good or reasonable performance across the different basins and depth strata in the model area. In scenarios with decreasing nutrient loads according to the Baltic Sea Action Plan but also with continued recent loads (mean loads 2012–2014), overall macrofaunal biomass and carbon processing were projected to decrease significantly by the end of the century despite improved oxygen conditions at the seafloor. Climate change led to intensified pelagic recycling of primary production and reduced export of particulate organic carbon to the seafloor with negative effects on macrofaunal biomass. In the high nutrient load scenario, representing the highest recorded historical loads, climate change counteracted the effects of increased productivity leading to a hyperbolic response: biomass and carbon processing increased up to mid‐21st century but then decreased, giving almost no net change by the end of the 21st century compared to present. The study shows that benthic responses to environmental change are nonlinear and partly decoupled from pelagic responses and indicates that benthic–pelagic coupling might be weaker in a warmer and less eutrophic sea. [ABSTRACT FROM AUTHOR]

Naroch lakes present a system of three interconnected water bodies of different trophic state, many years monitoring of which allowed to establish and reveal a number of fundamental patterns of functioning of aquatic ecosystems. Over the past 60 years, several stages in the evolution of the structure and functional organization of the ecosystem are clearly traced. Within the framework of the work the following periods were considered: 1) anthropogenic eutrophication (1978-1983); 2) oligotrophication (from 1984 to 1990 for Lake Naroch and from 1984 to 1990 for Lakes Myastro and Batorino) and 3) the period of bentification (since 1991 for Lake Naroch and since 1990 for Lakes Myastro and Batorino to the present). The values of the most part of parameters that are taken into account when assessing the trophic state of ecosystems (phosphorus, chlorophyll, phytoplankton, seston) are significantly reduced at bentification period compared to that of anthropogenic eutrophication. In the same time gross primary production of plankton in Lake Naroch actually did not change. One of the important reasons for this may be an increase in the depth of trophogenic layer and as a result an increase in its volume in regard to the entire volume of the lake. Production of submerged and emergent macrophytes in Lake Naroch during the period of bentification increased in 2 and 4 times respectively, compared with the period of eutrophication. The level of production of periphyton communities has not changed significantly. Based on the dynamics of the main hydroecological parameters of the Naroch lakes in different periods of their evolution, the uncertainty in assessing such a key characteristic of the ecosystem as its trophic state is evident, which can differ significantly depending on indicators that are used as the basis for its assessment in a specific waterbody

Coastal nutrient management often aims to reduce nutrients to avoid eutrophication; however, nutrient supply (i.e., enrichment) is an emerging issue in the post-eutrophication era when certain areas face oligotrophication. For example, part of Japan's Seto Inland Sea (SIS) faces oligotrophication, causing fish catch decline and bleaching of cultured nori (Piropia yezoensis). Hyogo Prefecture has implemented a nutrient management plan that includes five nutrient supply measures. However, concerns regarding public support for the plan existed because the key measure utilizes factories and sewage treatment plants (FSTP) to supply nutrients that may be associated with the negative preconception of "polluting the coastal sea." Successful management needs public understanding and support for the measure. Therefore, understanding public perception and contriving measures to gain public support is a critical management issue. Using five online questionnaire surveys in a before–after information design with 3200 residents in Hyogo Prefecture, this study revealed public perception and acceptance of the measures with a focus on nutrient supply from FSTP and discussed management implications. The respondents were unfamiliar with the plan and measures. Contrary to the concerns, the nutrient supply from FSTP was positively perceived and accepted, although it was the least favored in all aspects. A lack of knowledge and misunderstandings led to negative comments, and information provision effectively improved the affective evaluation of the five measures. The respondents with a stronger connection to the SIS tended to have a better knowledge of the oligotrophication of the SIS. Trust in responsible actors varied. Five management implications (i.e., overall support for nutrient supply measures, raising awareness of problems, education, trust building, and strengthening the connectedness of people with nature) were discussed to improve public support for the plan and measures. [Display omitted] • Factories and sewage treatment plant (FSTP) nutrient supply least favored. • The FSTP nutrient supply was the least favored but still gained public support. • Lack of knowledge led to negative associations with nutrient supply from FSTP. • Dissemination of information is effective for gaining public support. • Human–sea connections can leverage gain in public support. [ABSTRACT FROM AUTHOR]

Stimulated by excess levels of phosphorus, the attached, filamentous green alga Cladophora grows to nuisance proportions in Lake Michigan, one of the Laurentian Great Lakes. While nearshore waters impacted by local sources of the nutrient continue to support nuisance conditions, offshore waters have undergone oligotrophication in response to reductions in phosphorus loading and benthification of phosphorus cycling by invasive dreissenid mussels. A concept termed the Dual Challenge recognizes that implementation of more stringent phosphorus-loading objectives (to control Cladophora in the nearshore) stands in conflict with a foreseen need to mitigate oligotrophication in the offshore (to sustain a healthy fishery). Attention to this nearshore–offshore dynamic calls into play the role of cross-margin phosphorus transport in mediating both endmembers of the conflict. We applied a biophysical model simulating soluble reactive (SRP) and particulate (PP) phosphorus, mussel biokinetics, and cross-margin mass transport in addressing the Dual Challenge. Pre- and post-dreissenid monitoring results suggest that a reduction in offshore PP levels (food web nutrition) in excess of 40% (2.4 to 1.4 mgP·m−3) has driven oligotrophication and attendant food web dysfunction. Yet, in the absence of local sources, model-predicted nearshore SRP levels remain at or below those required to prevent nuisance growth. These findings indicate that there is a margin of ~1 mgP·m−3 over which offshore PP levels could be increased (to the benefit of the food web and the fishery) without hindering efforts to reduce nuisance algal growth through local source control.

Rapid warming and sea-ice loss in the Arctic Ocean are among the most profound climatic changes to have occurred in recent decades on Earth. Arctic Ocean biological production appears that it may be increasing as a result, but the consequences for nutrient concentrations are unknown. We have assembled a collection of historical field data showing that average concentrations of the macronutrients nitrate and phosphate have decreased by 79% and 29%, respectively, in surface waters of the western Arctic Ocean basin over the past three decades. The field observations and results from numerical ocean simulations suggest that this long-term trend toward more oligotrophic (nutrient-poor) conditions is driven primarily by the compound effects of sea-ice loss: a reduced resupply of nutrients from subsurface waters (due to fresh water addition and stronger upper-ocean stratification) coincident with increased biological consumption of nutrients (due to the greater availability of light needed for photosynthesis).

Fish kills are widespread visible events perceived by the civil society as a major cause for concern about water quality and ecosystem health. Investigations conducted so far have linked fish kills to multiple factors, but extensive studies examining quantitatively the likelihood of fish kills are missing. In the present study, factors related to fish kills in 67 Mediterranean reservoirs located in the Iberian Peninsula were investigated. Moreover, the variation in the likelihood of fish kills with the foreseen consequences of anthropogenically driven climate change upon temperature and precipitation was assessed. During the 23-year period studied (1995–2017) the number of fish kills per reservoir varied from 0 to 3 and happened mostly in Southern reservoirs and during the warmest months, particularly in June. Twelve explanatory variables showed significant differences between reservoirs with and without fish die-offs. Reservoir depth, surface area oxygen and chlorophyll a concentrations were the variables that, together, best discriminated between reservoirs with and without fish kills, explaining 67.2% of the total variation in the occurrence of fish die-offs. The variables retained in the mortality model explained unique parts of that variation, but a considerable amount of the explained variation was shared by all the variables. The number of fish kill events appears to be rising during the examined period and the projection made indicated an increase in the likelihood of fish kills towards the end of the 21st century, particularly for the RCP8.5 emission scenario, with the prevalence of fish kills reaching 0.865 in Southern reservoirs. In the future, oligotrophication is likely the only strategy to consistently reduce fish kills in Mediterranean reservoirs. Image 1 • Factors related to fish kills in Mediterranean reservoirs were investigated. • Number of fish kills happened mostly in the warmest months. • Four variables best discriminated between reservoirs with and without fish kills. • Number of fish kills increased from 1995 to 2017. • The likelihood of fish kill events is projected to increase in the future. [ABSTRACT FROM AUTHOR]

This study presents the first occurrence and recent distribution pattern of the river nerite (Theodoxus fluviatilis L., 1758) in Lake Balaton. The first appearance of this Prosobranch snail was recorded along the western shore of the Tihany peninsula situated on the middle section of the lake, at 21st of October 2013 during a routine macrozoobenthos sampling. Occurrence of the species was also recorded along the eastern shoreline of the peninsula in 2015, and it became highly abundant in both areas in the following years. Results of a comprehensive survey conducted in the autumn of 2018 in the littoral region of the lake revealed that the river nerite has colonized almost the entire shoreline of Lake Balaton, only the westernmost and easternmost areas were still devoid of this invader, therefore it seems that its spread has not been finished yet. The river nerite appeared in extremely high abundances (up to 4-5000 individuals/m2) in the middle area of the lake, therefore it is likely that this species will have a significant effect on the biota of Lake Balaton, especially in the littoral zone of the lake. [ABSTRACT FROM AUTHOR]

While total phosphorus (TP) is a critical determinant of freshwater ecosystem productivity, multiple stressors can induce shifts in energy pathways, with profound implications for ecosystem and fishery restoration. The Bay of Quinte (Lake Ontario, Canada) is a Great Lakes nearshore ecosystem that has been historically subjected to a variety of environmental perturbations: cultural eutrophication, low dissolved oxygen, reduced fisheries, climatic extremes, phosphorus (P) abatement, and aquatic invasive species. We used the Bay of Quinte to study how trophic state alterations affect fish assemblages in Great Lakes nearshore environments by examining the response of fish biomass to TP concentration variability in the presence of multiple stressors.Our analysis is based on a 42‐year (1972–2013) dataset from the Bay of Quinte for water quality through the food web to fishes. We employed a series of statistical tools that can offer insights into the structural changes induced by the events examined. We first used dynamic linear modelling to detect temporal trends in fish biomass, while accounting for year‐to‐year TP variability over three spatial segments of the bay. We then developed piecewise regression models to assess the extent to which specific ecological events induced distinct shifts in the fish assemblage. Multiple regression modelling was used to quantify the relative importance of TP, zooplankton, and surface water temperature on fish biomass.Based on gillnets, there were consistent fish biomass changes across the bay with increased biomass before P control (1972–1977), declines after P control followed by the establishment of a steady state or modest increase (1978–1994), and a declining trajectory during the recent period (1995–2013). Even when accounting for the role of water temperature and zooplankton, TP still had a significant effect on fish biomass. However, the strength and nature of the relationship varied among fish groups, and overall, the effect of TP on fish biomass has weakened in recent years.Our models show that fish biomass in the Bay of Quinte is shaped by the year‐to‐year TP variability. However, the relationship between P and fish abundance has been modulated by various ecological events with the consequence that the Bay of Quinte fish assemblage has changed and the food web now produces less fish biomass per unit of TP. A projected reduction of mean ambient TP levels from 30 to 25 μg/L, is expected to induce a 24% decline in total fish biomass, and further shift the fish assemblage with the biomass of planktivores and walleye declining by >60% and 30%, respectively.Recreational fishing provides important economic benefits in the Bay of Quinte, through tourism and other local business operations. Recognising the economic importance of fishing, our analysis provides critical insights regarding the on‐going management efforts to reduce external nutrient loadings (point and non‐point sources, urban storm water) and further lower ambient TP levels and primary productivity. The potential implications of already low nutrient concentrations for fish productivity represent a major challenge in effectively balancing water quality with fisheries management in the area. [ABSTRACT FROM AUTHOR]

Abstract French Mediterranean coastal lagoons have been subject to huge inputs of urban nutrients for decades leading to the eutrophication of these vulnerable ecosystems. In response to new environmental regulations, some of the lagoons have recently been the subject of large-scale management actions targeting the waste water treatment systems located on their watersheds. While the eutrophication of coastal ecosystems is well described, recovery trajectories have only recently been studied. To assess the rapidity and the extent of the effect of the remediation actions, we analysed data from a 14-year time series resulting from the monitoring of nutrients, biomass and the abundance of phytoplankton in the water column of French Mediterranean coastal lagoons covering the whole anthropogenic eutrophication gradient. Following a 50% to 80% reduction in total phosphorus (TP) and total nitrogen (TN) urban loadings from the watershed of hypertrophic and eutrophic ecosystems, the integrative parameters chlorophyll a , TN and TP, provide evidence for a rapid response (1 to 3 years) and for an almost complete recovery, suggesting no hysteresis for the eutrophic lagoon. However, our findings also show that recovery patterns depend on the eutrophication status before remediation and may include feedback responses. The different responses revealed by our results should help stakeholders prioritise remediation actions and identify appropriate restoration goals, especially in light of the targets of the Water Framework Directive (WFD). Highlights • Significant reductions in water nutrient concentrations due to decreasing urban nutrient loading. • Evidence for rapid recovery (1–3 y) of hypertrophic & eutrophic lagoons found in the water column. • Almost complete recovery observed for the eutrophic lagoon, highlighting no hysteresis. • The recovery patterns depend on the eutrophication status before remediation. • Feedback responses suggest ecosystem regimes can become unstable. [ABSTRACT FROM AUTHOR]

Over the past decade, nitrogen (N) loads to Narragansett Bay have decreased by more than 50%. These reductions were, in large part, the direct result of multiple wastewater treatment facility upgrades to tertiary treatment, a process which employs N removal. Here, we document ecosystem response to the N reductions and assess how the distribution of sewage N in Narragansett Bay has changed from before, during, and shortly after the upgrades. While others have observed clear responses when data were considered annually, our seasonal and regional comparisons of pre- and post-tertiary treatment dissolved inorganic nitrogen (DIN) concentrations and Secchi depth data, from bay-wide surveys conducted periodically from the early 1970s through 2016, resulted in only a few subtle differences. Thus, we sought to use stable isotope data to assess how sewage N is incorporated into the ecology of the Bay and how its distribution may have changed after the upgrades. The nitrogen (δ15N) and carbon (δ13C) stable isotope measurements of particulate matter served as a proxy for phytoplankton, while macroalgae served as short-term integrators of water column bio-available N, and hard clams (Mercenaria mercenaria) as integrators of water column production. In contrast to other estuarine stable isotope studies that have observed an increased influence of isotopically lower marine N when sewage N is reduced, the opposite has occurred in Narragansett Bay. The tertiary treatment upgrades have increased the effluent δ15N values by at least 2‰. The plants and animals throughout Narragansett Bay have similarly increased by 1-2‰, on average. In contrast, the δ13C values measured in particulate matter and hard clams have declined by about the same amount. The δ15N results indicated that, even after the N reductions, sewage N still plays an important role in supporting primary and secondary production throughout the bay. However, the δ13C suggests that overall net production in Narragansett Bay has decreased. In the 5 years after the major wastewater treatment facilities came on-line for nutrient removal, oligotrophication has begun but sewage remains the dominant source of N to Narragansett Bay. [ABSTRACT FROM AUTHOR]

Abstract: While there is a lot of data on interactive effects of eutrophication and warming, to date, we lack data to generate reliable predictions concerning possible effects of nutrient decrease and temperature increase on community composition and functional responses. In recent years, a wide‐ranging trend of nutrient decrease (re‐oligotrophication) was reported for freshwater systems. Small lakes and ponds, in particular, show rapid responses to anthropogenic pressures and became model systems to investigate single as well as synergistic effects of warming and fertilization in situ and in experiments. Therefore, we set up an experiment to investigate the single as well as the interactive effects of nutrient reduction and gradual temperature increase on a natural freshwater phytoplankton community, using an experimental indoor mesocosm setup. Biomass production initially increased with warming but decreased with nutrient depletion. If nutrient supply was constant, biomass increased further, especially under warming conditions. Under low nutrient supply, we found a sharp transition from initially positive effects of warming to negative effects when resources became scarce. Warming reduced phytoplankton richness and evenness, whereas nutrient reduction at ambient temperature had positive effects on diversity. Our results indicate that temperature effects on freshwater systems will be altered by nutrient availability. These interactive effects of energy increase and resource decrease have major impacts on biodiversity and ecosystem function and thus need to be considered in environmental management plans. [ABSTRACT FROM AUTHOR]

Abstract: Didymosphenia geminata is a stalk‐forming diatom capable of creating thick benthic mats in low‐nutrient streams. There are two hypotheses to explain the rapid worldwide increase in occurrence of nuisance D. geminata mats: (a) Cells are spread among rivers and across broad ecoregions through natural and anthropogenic vectors, or (2) pre‐existing D. geminata populations are forming mats in response to changing environmental conditions within the habitat. Low phosphorus (P) concentrations are a major trigger for stalk production by D. geminata cells. Although the environmental change hypothesis is gaining support among researchers, long‐term data sets demonstrating declining P concentrations prior to reported mat formation have been essentially absent from the literature. Here, we present long‐term datasets for two case studies for which long‐term P and D. geminata data coincide: the Matapedia River in Eastern Canada and the Kootenai River below Libby Dam in Montana, United States. Both rivers had declining P levels over time. However, there was a 2‐ and 20‐year lag time, respectively, between mat development and reaching the previously proposed average 2 μg/L soluble reactive P threshold for development. Although the Matapedia River provides some support of the environmental change hypothesis, the Kootenai River data set suggests other environmental factors may play a role in mat development. The data presented do not rule out the environmental change hypothesis but do suggest there may be conditions in addition to low P that must be met for mats to form and the environmental change hypothesis can likely be refined to include more parameters to better understand and mitigate the influence of mats. [ABSTRACT FROM AUTHOR]

Reservoirs have a wide variety of uses that have led to frequent conflicts over ecological conservation and contamination, especially as land management has intensified. Oligotrophication must be implemented in numerous tropical reservoirs that experience advanced eutrophication to maintain aquatic ecosystem functions. To quantify impacts on ecosystem functions and to develop an adaptive management policy, multiple studies have been conducted on the Itaparica Reservoir, São Francisco River, in the semi-arid north-eastern region of Brazil. Here, we add to that existing body of knowledge through investigating how nutrient accumulation is affected by water exchange between the main river flow and Icó-Mandantes Bay. Operational water-level fluctuations in the reservoir create large desiccated littoral areas that release high amounts of nutrients when they are rewetted. In particular, water-level variation promotes proliferation of Egeria densa, a noxious weed, thus elevating trophic levels of the Itaparica Reservoir and Icó-Mandantes Bay. Analysis with a P efficiency model determined 25 μg P L−1 to be the critical concentration and further indicated that the critical load in both bodies of water have been exceeded. Moreover, intensive fish aquaculture using net cages has led to further overtaxing of the reservoir. We conclude that an effective ecological reservoir management policy must involve oligotrophication, harvesting of noxious water weeds for use as soil amendment in agriculture or biogas production, “blue” aquaculture, and limiting hydroelectric power production based on current water availability. [ABSTRACT FROM AUTHOR]

We examined the changes in the concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) in the surface waters of the Yellow Sea (YS), the southern sea (SS) of Korea, and the East/Japan Sea (ES) from 1995 to 2021. These marginal seas neighboring the Korean Peninsula maintained nutrient concentrations approximately an order of magnitude higher than those in the Kuroshio waters, indicating extraordinarily large terrestrial source inputs. Generally, the DIN concentration in the YS increased gradually due to the accumulation of terrestrial inputs, while the nutrient concentrations in the ES declined gradually mainly due to enhanced water stratification. The SS showed the maximum DIN concentrations around 2005, associated with freshwater influence. In Korean coastal waters within ~10 km from the coastline, nutrient concentrations declined sharply during this period due to decreased terrestrial nutrient inputs. The rapid changes in the nutrient levels in these seas may significantly alter biological production. • Nutrient changes in the neighboring seas of Korea from 1995 to 2021 are studied. • In the Yellow Sea, DIN have increased mainly due to accumulated terrestrial inputs. • The southern sea of Korea showed distinct DIN increase during 2003–2005. • In the East/Japan Sea, the nutrients have declined gradually due to stratification. • The coastal seas of Korea showed rapid nutrient declines from 1995 to 2021. [ABSTRACT FROM AUTHOR]