Nutrient Load Reduction Research Articles

Temperature, redox, and amendments alter wetland soil inorganic phosphorus retention dynamics in a Laurentian Great Lakes priority watershed

Surface water phosphorus loading must be reduced to improve water quality and decrease harmful algal blooms. Many wetlands have a natural capacity to retain inorganic reactive PO43− via soil sorption. However, soil PO43− retention capacity is finite and may be limited by soil legacy phosphorus effects in agricultural and urban areas. This study evaluated soil PO43− retention in soils from a wetland constructed on former agricultural land in the Lake Erie, Maumee River watershed targeted for nutrient load reduction. Soil PO43− sorption isotherms were evaluated under cool (10 °C), warm (22°), aerobic, and anaerobic treatments to determine changes in PO43− retention due to environmental conditions and estimate seasonal changes in PO43− sorption. The soils displayed a strong capacity for PO43− retention by sorption. However, results indicate that cooler temperatures and anaerobic conditions decreased PO43− sorption and lowered retention rates at PO43− concentrations observed in the region. Soil amendment experiments investigated opportunities to increase PO43− retention because many soils display elevated phosphorus concentrations due to historic land use, limiting their ability to adsorb additional PO43−. Amendments increased PO43− retention capacity compared to unamended soils in the presence of high PO43− concentrations, suggesting soil PO43− retention can be improved in areas where natural storage capacity has been exhausted. Results from this study can inform natural resources managers in the Laurentian Great Lakes and elsewhere when identifying potential nutrient reduction wetland locations and assist with developing operational guidelines to optimize PO43− retention and water quality improvements using wetlands.

Opportunities for combining data of Estonian and Russian monitoring to reflect on water quality in large transboundary Lake Peipsi

Lake Peipsi, one of the world’s largest lakes, is shared between Estonia and Russia. The water quality in different parts of the lake has so far been assessed independently. Here we explore opportunities for combining data of Estonian and Russian monitoring. For that, we 1) analysed the compatibility of data for some water quality variables; 2) estimated the potential effects of the differences in sampling frequency; 3) provided a few regression models to calculate the missing data for months not sampled by the Russian side. Data of the concurrent Estonian and Russian sampling indicated a good compatibility. Estonian data analysis suggested that water quality assessment results are sensitive to sampling frequency. For example, total phosphorus (TP) in the largest basin showed a long-term decreasing trend in three month data that disappeared when data for other months were added. Disregarding some months may lead to under- or overestimation of certain factors with no consistency in the response of different basins. Hence, data of the whole ice-free period are recommended for an adequate water quality assessment. Furthermore, we demonstrated that monthly values of the water quality variables of the same year are autocorrelated. Based on this, we filled the gaps in the long-term data and compiled a dataset for the whole lake that enables its most comprehensive use in water quality assessment and management. Long-term data revealed no water quality improvement of Lake Peipsi. Further reduction of the external nutrient load is needed. Eutrophication is sustained by high internal phosphorus load.

Hydrological disconnection from the Yangtze River triggered rapid environmental degradation in a riverine lake

Interactive River-Flood Ecosystem (RFE) is an important feature characterizing the dynamics of the hydrological connectivity between lake and river worldwide. Any disruption in the RFE can impose significant effects on the functioning of floodplain ecosystems. However, due to the lack of long-term monitoring data, the understanding of the process and mechanism behind limnological responses to the changes in hydrological alterations and nutrient loading in floodplain lakes is restricted. The middle and lower reaches of the Yangtze river floodplain lake system of China is one of the significant food bowls of the nation, and makes important contribution to ecosystem services including drinking water and food to the society for generations. The interactive RFE of the Yangtze floodplain lakes is profoundly disrupted by anthropogenic activities over the recent decades. However, the response of such changes in river-lake connectivity is not comprehensively investigated yet. Here, we studied Cehu Lake in the lower reaches of the Yangtze floodplain system to examine the long-term environmental changes with altered river-lake connectivity and increased nutrient loading using high-resolution paleolimnological multi-proxy analyses including geochemistry, grain size and diatom assemblages of a 210Pb/137Cs dated core. Our results suggest that Cehu Lake have experienced two pronounced shifts: immediately after the damming in Yangtze river (1964) completely ceasing the lake-river connectivity, and in 1994. The disruption in the hydrological connectivity in 1964 led to an abrupt decrease in the abundance of Aulacoseira granulata, and an increase in the organic matter (loss-on-ignition), epiphytic and benthic diatoms, while the disruption in 1994 led to increased total phosphorus (TP), lead (Pb), dry mass accumulation rate, and a decline in Fe/Mn, and consequently enhanced lake eutrophication with emergence of enriched nutrient tolerant diatoms (Cyclotella meneghiniana, Cyclostephanos tholiformis). The redundancy analysis (RDA) showed increased TP loads in lake sediment after damming in the Yangtze river. The condition stimulated macrophyte growth with, but altered diatom flora. Reconnecting Cehu lake with Yangtze river together with reducing nutrient loading to the lake from the incoming river may be an effective management practice to relieve the ecological stress.

Nearshore fish assemblage dynamics in southern Lake Michigan: 1984–2016

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.

Impact of plant species and intense nutrient loading on CH4 and N2O fluxes from small inland waters: An experimental approach

Severe eutrophication threatens freshwater systems around the world. The application of aquatic buffer zones with plants, for example around agricultural lands, can increase nutrient retention and thereby reduce nutrient loading to downstream systems. However, not much is known about greenhouse gas (GHG) emissions from these buffer zones and how they are affected by nutrient loading and the dominant plant species. Here, using a full-factorial mesocosm experiment with different nutrient loadings (20–4000 mg N and 1–200 mg P m-2 d-1) and plant types (e.g. submerged and free-floating species), we show that emissions of methane (CH4) and nitrous oxide (N2O) were strongly related to nutrient loading, leading to total greenhouse gas emissions up to 177.84 g CO2-eq m-2 d-1. Overall, total GHG emission (as the sum of CH4 ebullition and diffusive water-atmosphere emission of CH4 and N2O in CO2 equivalents) was not significantly affected by plant species. CH4 ebullition was significantly lower in experimental units with submerged plants that rooted in the sediment as compared to non-rooted plants, possibly related to rhizosphere CH4 oxidation fueled by radial oxygen loss or plant-mediated transport that limits the build-up of gaseous CH4 in the sediment. We conclude that aquatic buffer zones that experience intense nutrient loading (e.g. due to release of sewage or agricultural fertilizer) can be GHG emission hotspots and recommend careful consideration of environmental conditions (e.g. the organic carbon content), expected nutrient loadings, and alternatives, prior to their construction.

Decadal Response of Mesozooplankton Assemblages to Nutrient Load Reductions in Laizhou Bay, China

Reducing nutrient loads from catchments is an important strategy to mitigate coastal eutrophication. The decadal responses of ecosystems to nutrient load reductions can help us understand the effectiveness of environmental restoration. Here, the cascading trophic interactions of mesozooplankton and phytoplankton in Laizhou Bay were analyzed to examine the effects of nutrient load reductions from rivers during 2004–2014. The results showed that the diversity of the mesozooplankton assemblages and the proportion of higher trophic species increased after nutrient load reductions, and mesozooplankton abundance showed an obviously decreasing annual trend. A significantly positive correlation between mesozooplankton and phytoplankton abundance was identified for August. Mesozooplankton abundance was mainly influenced by phytoplankton abundance in August, but by microzooplankton prey or jellyfish predators in May when phytoplankton abundance was low. This result indicates that the recovery of mesozooplankton assemblages in Laizhou Bay is a slow process after nutrient load reductions and might have cascading trophic interactions with phytoplankton recovery.

Can top-down effects of planktivorous fish removal be used to mitigate cyanobacterial blooms in large subtropical highland lakes?

Removal of planktivorous fish is used extensively in northern temperate lakes to reduce phytoplankton abundance via enhanced zooplankton grazing. However, whether this method would work also in large subtropical highland lakes to alleviate cyanobacterial blooms is unknown. We conducted a one-year pilot in situ experiment where we removed a substantial biomass of fish in a fenced-in area, followed by a 3-year whole-lake experiment where the dominant fish species (Japanese smelt) was removed in Lake Erhai in southwest China. The fencing experiments showed that between July and November, when the biomass of the removed stock reached 4 g/m2, the zooplankton biomass inside the fence increased significantly compared to a control fence. In the full-lake experiment, we found that sustained removal of Japanese smelt led to an increase in the biomass of cladocerans (Daphnia spp. but especially of Bosmina spp.) and a significant decrease in the biomass of Cyanobacteria and Chlorophyta. Additionally, a marked increase in the ratio of zooplankton to phytoplankton biomass, as well as an increase in the body size of cladocerans, emphasising the importance of enhanced top-down control for mitigating cyanobacterial blooms following extensive fish removal. Our results reveal that removal of small fish (here Japanese smelt) can lead to a reduction of the phytoplankton and cyanobacteria biomass through a trophic cascade in highland deep subtropical lakes. Thus fish removal may be a feasible additional restoration tool to external nutrient loading reduction in such lakes.

Preliminary assessment of water quality post-the first phase of the development plans in Lake Burullus, Deltaic coast of the Mediterranean Sea, Egypt

Lake Burullus is the second largest lake in Egypt and a Ramsar site located north of the Nile Delta. The lake is connected to the Mediterranean Sea through the Burullus inlet and the River Nile through Brimbal Canal. Lake Burullus receives a large amount of agricultural drainage water (about 97% of its water balance), which affects its water quality. The national plan, launched in 2017, aimed to improve the condition, water quality, and fish production of Lake Burullus. The current study provides an effective protocol to monitor the water quality of Lake Burullus post the first phase of the national plan. Aglydopting the proposed protocol would enable researchers and managers to evaluate the effectiveness of the development plans in resolving the environmental problem facing Lake Burullus and similar lakes. Intensive sampling was carried out in December 2019 and October 2020 to collect ecological parameters. The results showed a general improvement in the lake water indicated by the phytoplankton composition and diversity. Compared to previous research, the current study recorded a lower number of species belonging to Chlorophyceae and Cyanophyceae and new records of marine species belonging to Dinophyceae. The changes in phytoplankton community structure were mainly due to the deepening of the seawater inlet which has resulted in a better exchange between seawater and the lake and a subsequent increase in salinity. Based on TSIChl-a, the lake has shifted from being predominantly hypereutrophic to eutrophic. Nutrient-load reduction strategies for Lake Burullus may result in measurable changes in trophic state parameters.

Eutrophication evolution trajectory influenced by human activities and climate in the shallow Lake Gehu, China

Lake eutrophication has become a global environmental pollution problem. Analysis of the eutrophication recovery trajectory of lakes can help to accurately implement regulations and reduce the cost of management. The degradation and recovery trajectories of Lake Gehu were firstly analyzed based on 34-years monitoring data of water quality. A dynamic correlation analysis of the relationship between chlorophyll-a (Chla) and the total phosphorus (TP) (total nitrogen [TN]) at an annual scale was conducted using the filtering trajectory method (FTM). The result showed that the recovery trajectory of Lake Gehu was nonlinear, and showed a short lag time in Chla decrease. The regime shift from a clear-water state to a turbid-water state occurred in Lake Gehu when the TP threshold was 120 μg/L. Combined the filtering trajectory diagram with a principal component analysis, the results revealed that the adverse climate change did not completely offset the positive effect of nutrient load reduction, which required further attention and had significance for eutrophication control. According to the dynamic changes in the algae production efficiency of total phosphorus (ETP), the result showed that the sensitivity of lake eutrophication increased and the TP carrying concentration reduced by 40%-60%. The water quality goal for controlling algae was determined as TP = 40 μg/L (Chla = 10 μg/L), which was 20% stricter than the national standard (TP = 50 μg/L). In conclusion, aquatic vegetation restoration and biological manipulation can be effective alternatives for controlling eutrophication in addition to nutrient load reduction. The eutrophication control target and adaptive strategies were determined according to the recovery trajectory of Lake Gehu in this study. The ETP has reference significance for the management of shallow lakes.

Effectiveness of best management practices for non-point source agricultural water pollution control with changing climate – Lithuania’s case

This study aimed at the evaluation of the change in the effectiveness of Best Management Practices (BMPs) for agricultural pollution control in terms of reduction of nutrient loads to surface water bodies in Lithuania in the context of climate change (CC). The SWAT model was used by applying two CC representative concentration pathways (RCP4.5 and RCP8.5), three time horizons (historical – 2000–2019, mid-century – 2040–2059, and end-century – 2080–2099), and five BMPs (arable land conversion to grasslands, reduced fertilization, no-till farming, catch-crops, and stubble fields throughout winter). The most effective (with the greatest nutrient reduction) BMPs include grasslands, stubble fields, and winter cover crops. No-plough farming and reduced fertilization BMPs appear to be the least effective ones. CC would increase the potential of reduced fertilization and conversion to grasslands BMPs up to 179% and 17%, respectively for total nitrogen (TN) reduction. Regarding total phosphorus (TP), the reduction potential would increase 226% for fertilization reduction, and remain approximate for grasslands (at median −4%). Due to winter reduction of loads, no-plough stubble fields, and winter cover crops would decrease by as much as − 50%, − 12% and − 26%, respectively for TN, and − 30%, − 11%, and − 17%, respectively for TP, compared to the historical period. The greatest change would be recorded for the most extreme CC scenario (RCP8.5) and the longest time horizon (end-century). The effectiveness of BMPs (except for no-plough) for TN load reduction would increase more, and on average would become greater in agriculture dominated areas (of central-northern Lithuania) than elsewhere, although the maximum reduction values would occur in western (and in some cases eastern) Lithuania (except for reduced fertilization). This suggests the importance of the application of the measures in both areas. BMPs (except for grasslands) are expected to have relatively little effect on the overall crop yield.

Exploring the type and strength of nonlinearity in water quality responses to nutrient loading reduction in shallow eutrophic water bodies: Insights from a large number of numerical simulations

Reducing the load of nutrients is essential to improve water quality while water quality may not respond to the load reduction in a linear way. Despite nonlinear water quality responses being widely mentioned by studies, there is a lack of comprehensive assessment on the extent and type of nonlinear responses considering the seasonal changes. This study aimed to measure the strength of nonlinearity of theoretically possible water quality responses and explore their potential types in shallow eutrophic water bodies. Hereto, we generated 14,710 numerical water body cases that describe the water quality processes using the Environmental Fluid Dynamics Code (EFDC) and applied eight load reduction scenarios on each water body case. Inflows are simplified from Lake Dianchi. The climate conditions consider three cases: Lake Dianchi, Wissahickon Creek, and Famosa Slough. We then developed a nonlinearity strength indicator to quantify the strength and frequency of nonlinear water quality responses. Based on the quantification of nonlinearity, we clustered all the samples of water quality responses using K-Means, an unsupervised Machine Learning algorithm, to find the potential types of nonlinear water quality responses for TN (total nitrogen), TP (total phosphorus), and Chla (chlorophyll a). Results show linear or near-linear response types account for 90%, 69%, and 20% of TN, TP, and Chla samples respectively. TP and Chla could perform more types of nonlinearity. Representative nonlinear water quality responses include disproportional improvement, peak change (disappear, move forwards or afterward), and seasonal deterioration of TN after load reduction. This study would contribute to the current understanding of nonlinear water quality responses to load reduction and provide a basis to study under which conditions the nonlinear responses may emerge.

Biogeochemical functioning of the Baltic Sea

Abstract. Location, specific topography, and hydrographic setting together with climate change and strong anthropogenic pressure are the main factors shaping the biogeochemical functioning and thus also the ecological status of the Baltic Sea. The recent decades have brought significant changes in the Baltic Sea. First, the rising nutrient loads from land in the second half of the 20th century led to eutrophication and spreading of hypoxic and anoxic areas, for which permanent stratification of the water column and limited ventilation of deep-water layers made favourable conditions. Since the 1980s the nutrient loads to the Baltic Sea have been continuously decreasing. This, however, has so far not resulted in significant improvements in oxygen availability in the deep regions, which has revealed a slow response time of the system to the reduction of the land-derived nutrient loads. Responsible for that is the low burial efficiency of phosphorus at anoxic conditions and its remobilization from sediments when conditions change from oxic to anoxic. This results in a stoichiometric excess of phosphorus available for organic-matter production, which promotes the growth of N2-fixing cyanobacteria and in turn supports eutrophication. This assessment reviews the available and published knowledge on the biogeochemical functioning of the Baltic Sea. In its content, the paper covers the aspects related to changes in carbon, nitrogen, and phosphorus (C, N, and P) external loads, their transformations in the coastal zone, changes in organic-matter production (eutrophication) and remineralization (oxygen availability), and the role of sediments in burial and turnover of C, N, and P. In addition to that, this paper focuses also on changes in the marine CO2 system, the structure and functioning of the microbial community, and the role of contaminants for biogeochemical processes. This comprehensive assessment allowed also for identifying knowledge gaps and future research needs in the field of marine biogeochemistry in the Baltic Sea.

Alarming waterweeds proliferation in the Vembanad Lake System might significantly increase water loss through transpiration.

Eichhornia crassipes and Monochoria vaginalis are waterweeds, and their uncontrolled proliferation in fresh and brackish water habitats is a serious ecological problem in many parts of the world. These weeds are quite common in the Vembanad Lake System (VLS), India's second-largest Ramsar wetland. During the non-monsoon season, the Thannermukkom saltwater barrage divides the VLS into two zones: saline water downstream and freshwater upstream. The field sampling of the current study was carried out in the upstream zone of the VLS during the Pre-Monsoon (March 2017). Fresh Eichhornia and Monochoria samples were collected, transported to the lab, and experiments were conducted under natural light conditions to determine how much extra water they transpire into the atmosphere. The results showed that the water loss in experimental tanks with Eichhornia (evapotranspiration) is roughly twice that in control tanks without them (only evaporation). Monochoria transpires fairly more water to the atmosphere than Eichhornia. These results reveal that the proliferation of waterweeds has a significant adverse effect in conserving water in all freshwater bodies infested with them. The current study also points out that the expansion of waterweeds has the potential to worsen drought conditions as they cause excess water loss into the atmosphere and a faster drying up of freshwater reservoirs. Two possible approaches for managing the waterweeds in the VLS include reducing nutrient loading upstream and more frequent opening of the Thannermukkom saltwater barrage to allow saltwater intrusion, which could inhibit the growing waterweeds.

Decreased Internal Phosphorus Loading From Eutrophic Sediment After Artificial Light Supplement: Preliminary Evidence From a Microcosm Experiment

Nutrient loading reduction is an essential approach for controlling eutrophication. In addition to external nutrient loading, internal phosphorus (P) loading is usually considered one of the most important factors determining nutrient levels in water. The underwater light climate plays a pivotal role. However, few studies have been reported on the effect of light intensity on P release, and the existing studies have not drawn a definitive conclusion on whether underwater light has a positive or negative effect on P release. To confirm the effect of underwater light on internal P release, a 1-month microcosm experiment was carried out (18 November–15 December 2020) under three light intensities. The P release flux (FP) was significantly higher in the control (no light) group than in the low light group during the first 2 weeks (p = 0.03). No difference among treatments occurred for accumulative FP in the past 4 weeks, although it tended to be higher in the control than in both the low and high light groups (p > 0.05). Spearman rank correlations showed that photosynthetic photon flux density (PPFD) was positively correlated with DO, pH, phytoplankton chlorophyll a (Chl aPhyt) and benthic algae chlorophyll a (Chl aBent). DO was positively correlated with Chl aPhyt and Chl aBent. The results indicate that light may promote the growth of phytoplankton and benthic algae, both of which may increase DO and pH. DO inhibits P release from sediment and pH promotes P release from sediment. Lake-scale studies are needed to fully evaluate its effect under natural conditions, as many other factors (e.g., wind or fish disturbance) in natural lakes could regulate P release from sediment.

Socioeconomic effects of a bottom-up multifunctional land consolidation project

Taking agricultural land out of production or shifting from crop production to permanent grassland have recently been proposed in Denmark as general measures for contributing to greenhouse gas (GHG) reductions. One of the particular features of such measures is the creation of co-benefits, as taking agricultural land out of intensive production may affect a range of ecosystem services and economic goods, such as reduced nutrient loads and emissions of GHGs, improved biodiversity, and improved recreational opportunities. In this paper, we present results from an ex ante socioeconomic Cost-Benefit Analysis (CBA) with the purpose of assessing whether the expected benefits of a bottom-up local development plan for multifunctional land use in the catchment of Lake Glenstrup in Denmark will outweigh the expected costs. After quantifying the wide range of impacts, we apply benefit transfer to value them within the CBA framework. Among the primary effects are reduced emission of GHGs, reduced leaching of nutrients, increased recreational options, and opportunity costs from agriculture. The results indicate that the initiatives could lead to a net social gain of 1.4 mil. €. We perform a sensitivity analysis which shows that the choice of the spatial extent of the recreational effects has a large impact on the results, which range from 0.4 to 5.2 mil. €. Also, the shadow price of reduced GHG emissions play a major role. Finally, a distribution analysis shows that especially farmers bear the majority of costs, whereas other stakeholders enjoy most of the benefits.

Temperature and precipitation dominates millennium changes of eukaryotic algal communities in Lake Yamzhog Yumco, Southern Tibetan Plateau

Despite significant climate change on the Tibetan Plateau, the historical succession trend and underlying driving mechanism of aquatic ecosystem in alpine lake remain unclear. In this study, palaeolimnological analysis and high-throughput sequencing of sedimentary DNA were used to investigate environmental changes, primary productivity, and eukaryotic algal community succession over the past millennium in Lake Yamzhog Yumco of the southern Tibetan Plateau. Lake primary productivity significantly increased after ~1850 CE and algal community succession occurred in three stages including the Medieval Warm Periods (approximately 1000–1250 CE), the Little Ice Age (1250–1850 CE), and the Current Warm Period (1850–2020 CE). Moreover, succession was synchronous with inferred climate changes. Partial least square path modeling indicated that climate factors affected primary productivity and eukaryotic algal community structure by affecting nutrient loading. The results suggest that glacier melting and permafrost degradation caused by climate warming, combined with increased precipitation, may be the major driving factors of nutrient concentration increases, phytoplankton biomass increases, and shifts in community composition. Considering the expected trends of future climate change and continuous warming, the restoration of vegetation cover and reduction of non-point source nutrient loading in the Tibetan Plateau is urgently needed to mitigate climate change impacts on alpine lake aquatic ecosystems.

Inter-model comparison of simulated Gulf of Mexico hypoxia in response to reduced nutrient loads: Effects of phytoplankton and organic matter parameterization

Complex simulation models are a valuable tool to inform nutrient management decisions aimed at reducing hypoxia in the northern Gulf of Mexico, yet simulated hypoxia response to reduced nutrients varies greatly between models. We compared two biogeochemical models driven by the same hydrodynamics, the Coastal Generalized Ecosystem Model (CGEM) and Gulf of Mexico Dissolved Oxygen Model (GoMDOM), to investigate how they differ in simulating hypoxia and their response to reduced nutrients. Different phytoplankton nutrient kinetics produced 2–3 times more hypoxic area and volume on the western shelf in CGEM compared to GoMDOM. Reductions in hypoxic area were greatest in the western shelf, comprising 72% (∼4,200 km2) of the total shelfwide hypoxia response. The range of hypoxia responses from multiple models suggests a 60% load reduction may result in a 33% reduction in hypoxic area, leaving an annual hypoxic area of ∼9,000 km2 based on the latest 5-yr average (13,928 km2).

Towards climate-robust water quality management: Testing the efficacy of different eutrophication control measures during a heatwave in an urban canal

Harmful algal blooms are symptomatic of eutrophication and lead to deterioration of water quality and ecosystem services. Extreme climatic events could enhance eutrophication resulting in more severe nuisance algal blooms, while they also may hamper current restoration efforts aimed to reduce nutrient loads. Evaluation of restoration measures on their efficacy under climate change is essential for effective water management. We conducted a two-month mesocosm experiment in a hypertrophic urban canal focussing on the reduction of sediment phosphorus (P)-release. We tested the efficacy of four interventions, measuring phytoplankton biomass, nutrients in water and sediment. The measures included sediment dredging, water column aeration and application of P-sorbents (lanthanum-modified bentonite - Phoslock® and iron-lime sludge, a by-product from drinking water production). An extreme heatwave (with the highest daily maximum air temperature up to 40.7 °C) was recorded in the middle of our experiment. This extreme heatwave was used for the evaluation of heatwave-induced impacts. Dredging and lanthanum modified bentonite exhibited the largest efficacy in reducing phytoplankton and cyanobacteria biomass and improving water clarity, followed by iron-lime sludge, whereas aeration did not show an effect. The heatwave negatively impacted all four measures, with increased nutrient releases and consequently increased phytoplankton biomass and decreased water clarity compared to the pre-heatwave phase. We propose a conceptual model suggesting that the heatwave locks nutrients within the biological P loop, which is the exchange between labile P and organic P, while the P fraction in the chemical P loop will be decreased. As a consequence, the efficacy of chemical agents targeting P-reduction by chemical binding will be hampered by heatwaves. Our study indicates that current restoration measures might be challenged in a future with more frequent and intense heatwaves.

Seasonal Dynamics of Algal Net Primary Production in Response to Phosphorus Input in a Mesotrophic Subtropical Plateau Lake, Southwestern China

A comprehensive 3-dimensional hydrodynamic and eutrophication model, the environmental fluid dynamics code model (EFDC) with three functional phytoplankton groups, was applied to simulate the algal dynamics in a mesotrophic P-limited subtropical plateau lake, Lake Erhai, Southwestern China. Field investigations revealed the seasonal patterns in external total phosphorus (TP) input and TP concentration, as well as the composition of the phytoplankton community. The model was calibrated to reproduce qualitative features and the succession of phytoplankton communities, and the net primary production was calculated. The modeled daily net primary production (NPP) ranged between −16.89 and 15.12 mg C/m2/d and exhibited significant seasonal variation. The competition for phosphorus and temperature was identified as the primary governing factor of NPP by analyzing the parameter sensitivity and limitation factors of the lake. The simulation of four nutrient loading reduction scenarios suggested high phytoplankton biomass and NPP sensitivity to the external TP reduction. A significant positive correlation was found among NPP, total phytoplankton biomass and TP concentration. Overall, this work offers an alternative approach to estimating lake NPP, which has the potential to improve sustainable lake management.

Biogeochemical states, rates, and exchanges exhibit linear responses to large nutrient load reductions in a shallow, eutrophic urban estuary

AbstractWe examined responses of water‐column conditions, sediment‐water fluxes, ecosystem metabolism, and nutrient export in the Back River estuary during the past three decades following multiple phases of nutrient load reductions from a large wastewater treatment plant. Total nitrogen (TN) loads from the treatment plant declined from 7000 kg N d−1 in the mid‐1980s to 1500 kg N d−1 following the implementation of enhanced nutrient removal in late 2017. Total phosphorus (TP) loads declined by ~90% from peaks in the mid‐1980s–1990s and have been stable ever since. In response, TN and TP concentrations measured since 1985 show declines that generally mirror that of loads from the treatment plant, and box model computations suggest significant reductions in nutrient export to adjacent Chesapeake Bay. As a consequence, water‐column chlorophyll a (Chl a) concentrations have declined modestly over the record, despite inter‐annual variability. This reduction in Chl a coincided with a reduced frequency of nitrogen and phosphorus concentrations that would saturate phytoplankton growth, as well as reductions in ecosystem gross primary production and respiration derived from high‐frequency oxygen time‐series. Sediment‐water fluxes of dissolved nitrogen, phosphorus, and oxygen, as well as associated sediment concentrations of nitrogen, phosphorus, and carbon also declined over the record. These temporal patterns were reproduced in a 35‐yr model simulation that suggests a relatively rapid response to reduced organic matter deposition changes. Finally, the recycling of ammonium for a given TN load declined substantially, consistent with high observed rates of denitrification, indicating that well‐mixed estuaries can recover relatively rapidly in response to nutrient remediation.