Eutrophication Research Articles

Cross-watershed leakage of agricultural nutrient runoff

Agricultural nutrient runoff has been a major contributor to hypoxia in many downstream coastal ecosystems. Although programs have been designed to reduce nutrient loading in individual coastal waters, cross watershed interdependencies of nutrient runoff have not been quantified due to a lack of suitable modeling tools. Cross-watershed pollution leakage can occur when nutrient runoff moves from more to less regulated regions. We illustrate the use of an integrated assessment model IAM that combines economic and process-based biophysical tools to quantify Nitrogen loading leakage across three major US watersheds. We also assess losses in consumer and producer surplus from decreased commodity supply and higher prices when nutrient delivery to select coastal ecosystems is restricted. Reducing agricultural N loading in the Gulf of Mexico by 45% (a) increases loading in the Chesapeake Bay and Western Lake Erie by 4.2% and 5.5%, respectively, and (b) results in annual surplus losses of $7.1 and $6.95 billion with and without restrictions on leakage to the Chesapeake Bay and Lake Erie, respectively.

Improving Turf and Soil Health, Reducing Energy Use and Assessing Tick Populations by Mulching Leaves in Place

Mulching leaves in place was studied for 4 years in Westchester County, New York State, USA. Soil testing showed an advantage in long-term mulch mowing for available water capacity and soil proteins with no detrimental effects on the health of the soil. There were no differences in the concentration of macro or micronutrients in the leaf and duff material between treatments or in the concentration of those nutrients being added or removed from the soil. Lawns that had been mulch mowed showed significantly higher pest and traffic tolerance than those where leaves had been removed. Drought stress tolerance remained the same on sites where mulch mowing had occurred, but decreased where leaves had been removed. Mowing practices accounted for less than one percent of the variation in tick numbers, indicating that other variables, not measured had a larger impact. The amount of fuel used for community pick-up of leaves was 4 and 33 times as much as mulch mowing for the two treatment groups. Based on the results of this 3-year study, mulch mowing of leaves provides some soil health benefits, many turf quality benefits, does not improve the habitat for pests and uses less fuel than community pick-up. This study showed that mulch mowing of leaves (i.e., leaving leaves on the lawn) does provide some benefits to soil health and turf quality, without having an adverse effect on nutrient load or tick habitat.

Study on quality of treated wastewater for urban agriculture use in Addis Ababa, Ethiopia

Non-conventional water supplied from Bole Lemi Industrial Park (BLIP) in Addis Ababa, Ethiopia is treated to remove contaminants, but never has been analyzed for its quality for urban agriculture (UA) use. The objective of this study is to analyze the quality of treated wastewater (TWW) using treated wastewater quality index (TWWQI). Treated wastewater samples were taken at the influent and effluent of the wastewater treatment plant located within BLIP. The physico-chemical properties of the water samples from BLIP were analyzed at the third-party laboratory. The result showed that the aggregate TWWQI value falls under the category of very poor water for urban irrigation agriculture (UIA) use. This very poor water quality grade attributes 85 % to the presence of heavy metals, 4 % to nutrients load, 8 % to saline condition contributors and 4 % to miscellaneous contaminants. Discrete analyses of the indices for heavy metals, nutrient loads, saline condition contributors and miscellaneous contaminants shifted the water quality from very poor to very polluted, excellent, poor and good for UIA use respectively. Though the aggregate TWWQI is of very poor category for UIA use, the treatment plants of BLIP exhibited contaminants removal efficiencies of between 30 and 100 % with aggregate removal efficiency of 58 %. Results also revealed that there were contaminants that exhibited higher effluent values than influent after treatment indicating the failure of BLIP’s treatment plants to remove these contaminants. In conclusion, treated wastewater from BLIP, falls under very poor water category for use in irrigation agriculture where Cr+3, Cl− and TDS have contributed most in that order. Therefore, BLIP should upgrade and optimize its treatment plants to increase the removal efficiency for the respective contaminants. Moreover, BLIP should enhance the capacity of the experts through training and continually monitor the quality of the water to protect the environment and ensure its contribution to the building of a resilient city.

Diel and seasonal mixing patterns and water quality dynamics in a multipurpose tropical semiarid reservoir.

Eutrophication has become a recurrent concern in reservoirs worldwide. This problem is intensified in tropical semiarid regions, where the reservoirs have high seasonal and annual variability of water level and volume. Therefore, an extensive understanding of the diel variation of water quality key-parameters can help improve management of such reservoirs. This study focuses on Castanhão reservoir with the largest multipurpose dam in the Brazilian semiarid. Its main water uses are irrigation, fish farming, and human supply. The reservoir faced a decline in water quality due to a prolonged drought period. While previous research has predominantly emphasized the seasonal dynamics of thermal and chemical stratification, our investigation provides diel assessments of multiple water quality parameters, including nutrient concentrations and phytoplankton abundance. Our primary objective is to compare seasonal and diel variations in stratification and nutrient distribution within the reservoir. Key findings reveal a diel cycle of thermal stratification, primarily during dry season, driven by higher wind speeds. This is corroborated by a significant negative correlation between wind speed and the relative water column stability index. In contrast, during the rainy season, the reservoir experiences continuous thermal stratification due to inflowing water being warmer than the reservoir's water temperature. Notably, a significant negative correlation between total phosphorus and chlorophyll-a, along with a two-fold increase of this nutrient throughout the day during the rainy season, underscores the influence of the phytoplankton community dynamics on the diel nutrient variation. Chemical stratification of dissolved oxygen occurred during dry and rainy seasons, indicating that even during the dry season, where there is no significant inflow, the internal nutrient loading can also significantly impact the water quality of a reservoir. This study advances the understanding of diel water quality dynamics in tropical semiarid reservoirs, shedding light on both climatic and anthropogenic influences on water resources.

Remote sensing insights for sustainable development: Water quality and landscape dynamics in Mirik Lake, Darjeeling District, West Bengal, India

The study employs remote sensing and GIS techniques to assess the water quality dynamics of Mirik Lake, located in the Darjeeling Himalayas, West Bengal, India. To analyse the impact of land use and land cover (LULC) changes on the water quality of Mirik Lake from 1993 to 2023. Landsat imagery spanning from 1993 to 2023 was used to detect significant alterations in LULC patterns. Remote sensing and GIS techniques were utilised to analyse the data, focusing on changes in LULC and their implications for water quality. The results indicate a steady increase in total phosphorus (TP), total nitrogen (TN), and Biological Oxygen Demand (BOD) levels, attributed to anthropogenic activities such as urbanisation and tourism development. LULC change analysis highlights the expanding built-up areas and agricultural lands surrounding the lake, contributing to nutrient loading and organic pollution. The spatial distribution of pollution categories underscores the influence of tourist infrastructure on water quality degradation. Integrated watershed management and sustainable development strategies are recommended to mitigate anthropogenic impacts and preserve the ecological integrity of Mirik Lake.

Study on Spatio-temporal Distribution of Chlorophyll-a on Pelagic Catch Productivity in Muara Bendera, West Java, Indonesia

Graphical Abstract Highlight Research The trend of chlorophyll-a in the pelagic fish catchment area of the Muara Bendera has increased. Trend of chlorophyll-a values around the waters of the Citarum and Cisadane River Estuaries increased. Chlorophyll-a and pelagic CPUE have a negative relationship. The waters around the mouth of the Citarum River are mesotrophic - eutrophic waters Abstract Chlorophyll-a has been considered an indicator of pelagic fish abundance in waters. Although a high nutrient load causes eutrophication, leading to fish mortality, global-scale climate anomalies will also influence the oceanographic conditions of the seas. This study aimed to investigate spatial patterns and trends of chlorophyll-a, the relationship between chlorophyll-a and pelagic fish catch productivity, and the effect of ENSO and IOD on pelagic fish catch productivity. The chlorophyll-a data were obtained from the SeaWiFS and Terra-Aqua MODIS time-series datasets of ocean color satellites. The results indicated that Jakarta Bay had the highest chlorophyll-A concentration. The Chlorophyll-A concentrations declined as the distance between the estuary and the coast grew. From 1997 to 2021, the regional pattern of increasing chlorophyll-a concentrations in the Citarum and Cisdane estuaries was continuous. Since 2001, the concentration of chlorophyll-a in Jakarta Bay has declined, whereas it has begun to climb in the seas of the North Seribu Islands. The increasing chlorophyll-A trend in the Seribu Islands is attributed to rising nitrate levels induced by human-driven coral degradation. Between 1997 and 2021, the concentration of chlorophyll-a in the Muara Bendera fishing region grew by +0.013 mg/m3/year, or 1.43 percent each year. The correlation between chlorophyll-a and pelagic fish catch productivity was -0.13. ENSO does not affect the productivity of pelagic catches in this region. However, it was discovered that IOD reduced the productivity of low-category pelagic catches.

Tissue-Specific Tolerance to High-Temperature and Nutrient-Poor Conditions in a Canopy-Forming Macroalga, Surviving at an Ocean Warming Hotspot.

Most canopy-forming macroalgae have disappeared from temperate reefs in southern Japan, one of the ocean warming hotspots, but Sargassum nipponicum is surviving in this region. As this species' annual shoots emerge from holdfasts during summer, both plant components may be highly tolerant to warm and nutrient-poor conditions in this season. The present study examined the effects of temperature and nutrient conditions on holdfast growth, shoot emergence from holdfasts, and shoot growth in S. nipponicum samples collected in Tanegashima Island, southern Japan. The summer temperature in this region (30 °C) allowed holdfast growth and shoot emergence but inhibited shoot growth. Nutrient-poor conditions had limited effects on the first two parameters but suppressed shoot growth. These results suggested that during warm summers and under nutrient-poor conditions in southern Japan, shoots can emerge from S. nipponicum holdfasts but cannot further grow. Additionally, nutrient loading from a nearby river was higher at the only site dominated by S. nipponicum, than at the other sites where this species was absent on Tanegashima Island. This was observed especially between autumn and winter, implying that such a nutrient-rich environment may contribute to shoot growth in S. nipponicum and to the persistence of its population in the area.

Key drivers of hypoxia revealed by time-series data in the coastal waters of Muping, China

Coastal hypoxia (low dissolved oxygen in seawater) is a cumulative result of many physical and biochemical processes. However, it is often difficult to determine the key drivers of hypoxia due to the lack of frequent observational oceanographic and meteorological data. In this study, high-frequency time-series observational data of dissolved oxygen (DO) and related parameters in the coastal waters of Muping, China, were used to analyze the temporal pattern of hypoxia and its key drivers. Two complete cycles with the formation and destruction of hypoxia were captured over the observational period. Persistent thermal stratification, high winds and phytoplankton blooms are identified as key drivers of hypoxia in this region. Hypoxia largely occurs due to persistent thermal stratification in summer, and hypoxia can be noticeably relieved when strong wind mixing weakens thermal stratification. Furthermore, we found that northerly high winds are more efficient at eroding stratification than southerly winds and thus have a greater ability to relieve hypoxia. This study revealed an episodic hypoxic event driven by a phytoplankton bloom that was probably triggered by terrestrial nutrient loading, confirming the causal relationship between phytoplankton blooms and hypoxia. In addition, we found that the lag time between nutrient loading, phytoplankton blooms and hypoxia can be as short as one week. This study could help better understand the development of hypoxia and forecast phytoplankton and hypoxia, which are beneficial for aquaculture in this region.

Seagrass recovery trajectories and recovery potential in relation to nutrient reduction

Abstract Seagrass recovery has been reported across the globe where previously eutrophied waters have become less nutrient‐rich. In the European Wadden Sea, different recovery trajectories were found after riverine nutrient loads decreased, namely full, temporary and no recovery. We compiled intertidal seagrass presence (Zostera noltei and Z. marina) and eutrophication data for 1930–2020, to relate the seagrass trajectories and regional eutrophication differences to riverine nutrient loads, and inferred prospects for seagrass recovery. Seagrass fully recovered in the less eutrophic North Frisian region. The recovery trajectory was tightly coupled to riverine nutrient load reduction. Relative seagrass area (meadow area/region area) dropped from 10% prior to eutrophication to 2% during the eutrophication peak, increased to 7% during the nutrient reduction period and subsequently expanded to 13%. Colonization of marginal habitats was observed, indicating propagule spillover from neighbouring meadows. The more eutrophic southern regions showed no or only temporary seagrass recovery. Prospects for (limited) recovery are good in only two out of four southern regions, provided that riverine nutrient loads are further reduced by ~40% (reference: 2010–2017). Without this reduction, seagrasses may only temporarily recover and will remain vulnerable to erratic disturbances like macroalgae accumulation or storms. Historical evidence and application of habitat suitability models suggest that the potential relative seagrass area in the southern regions is low: less than 0.2% in the western Dutch region and maximum 2.4% in the Ems‐Jade region. Synthesis and applications. Within a large seascape (15,000 km2) the least eutrophicated region showed seagrass recovery upon nutrient reduction. We translated the critical riverine nutrient loads for this recovery, via regional eutrophication indicators, to loads that may enable a sustained recovery in the other regions. This technique is applicable in other complex systems, provided sufficient historical data are available. Propagule spillover exerts a positive feedback at metapopulation scale leading to acceleration of recovery. Occupied and potential seagrass habitat (e.g. assessed by the maximum recorded area in the past) are thus important landscape selection criteria for restoration, particularly when eutrophication is not yet sufficiently reduced.

Small water body significantly contributes to nitrous oxide emissions in China's aquaculture

Aquaculture systems are expected to act as potential hotspots for nitrous oxide (N2O) emissions, largely attributed to substantial nutrient loading from aquafeed applications. However, the specific patterns and contributions of N2O fluxes from these systems to the global emissions inventory are not well characterized due to limited data. This study investigates the patterns of N2O flux across 127 freshwater systems in China to elucidate the role of aquaculture ponds and lakes/reservoirs in landscape N2O emission. Our findings show that the average N2O flux from aquaculture ponds was 3.63 times higher (28.73 μg N2O m−2 h−1) than that from non-aquaculture ponds. Additionally, the average N2O flux from aquaculture lakes/reservoirs (15.65 μg N2O m−2 h−1) increased 3.05 times compared to non-aquaculture lakes/reservoirs. The transition from non-aquaculture to aquaculture practices has resulted in a net annual increase of 7589 ± 2409 Mg N2O emissions in China's freshwater systems from 2003 to 2022, equivalent to 20% of total N2O emissions from China's inland water. Particularly, the robust negative regression relationship between N2O emission intensity and water area suggests that small ponds are hotspots of N2O emissions, a result of both elevated nutrient concentrations and more vigorous biogeochemical cycles. This indicates that N2O emissions from smaller aquaculture ponds are larger per unit area compared to equivalent larger water bodies. Our findings highlight that N2O emissions from aquaculture systems can not be proxied by those from natural water bodies, especially small water bodies exhibiting significant but largely unquantified N2O emissions. In the context of the rapid global expansion of aquaculture, this underscores the critical need to integrate aquaculture into global assessments of inland water N2O emissions to advance towards a low-carbon future.

The underestimated role of manganese in modulating the nutrient structure in a eutrophic seasonally-stratified reservoir

Accumulation and subsequent release of nutrients have great potential to trigger algal blooms in lakes and reservoirs. We conducted high vertical resolution (2 m interval) monitoring at ∼monthly intervals over a year for hydrological parameters, Chl-a, ammonium (NH4+), nitrate (NO3−) and different species of phosphorus (P) and manganese (Mn) in a 40-meter-deep subtropical reservoir (Shanmei Reservoir) in Fujian, southern China. In this seasonally stratified reservoir featured with high nutrient loading, the consistent trend in the ratio of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorus (DIP) between the euphotic zone and the hypolimnion, coupled with its mirrored correlation with Chl-a concentration indicates that upward flux from the hypolimnion affects phytoplankton growth in the euphotic zone. The monthly variation of the depth-integrated multiple species of N and P indicates that during the stratification period in the hypoxic hypolimnion, approximately 80% of the DIP is removed, leading to a remarkable decoupling phenomenon between NH4+ and DIP. This process effectively increases the ratio of DIN to DIP in the hypolimnion, thereby significantly reducing the potential of algal blooms caused by the upward flux. A robust positive linear correlation between iron-manganese bound phosphorus (CBD-P) and particulate Mn was observed during stratification period implying that DIP was scavenged by sediment-released Mn throughout the water column. Vertical profiles during stratification showed that upward diffusion of Mn2+ facilitated the formation of Mn oxide zones near the oxycline. The most significant decrease in DIP inventory occurs when Mn oxide zones migrate either upwards from the bottom or downwards from the oxycline, indicating that the migration of Mn oxides on the vertical profile is a key factor in the decoupling of NH4+and DIP.Our findings underscore the importance of Mn cycling as an underappreciated DIP self-immobilization process in the water column of reservoirs characterized by high nutrient loading. Furthermore, we propose that denitrification and Mn cycling establish a consecutive feedback mechanism, preventing excessive nutrient accumulation in low oxygen bottom water. In the context of global changes, we anticipate a heightened prominence of this feedback mechanism.

Importance of internal and external nutrient loading to the primary productivity of Lake Tanganyika

A coupled hydrodynamic-ecosystem model (GOTM-FABM-ERGOM) was applied to test the hypothesis that primary production in the upper mixed layers of Lake Tanganyika is primarily controlled by internal nutrient inputs. The model was calibrated (data: May 2015–April 2016) and validated (data: May 2016–April 2017) against monthly field data of water temperature, dissolved oxygen, nutrients (nitrate, ammonium, phosphate) and chlorophyll a collected from Kigoma Bay in the northern part of the lake. Data of nutrients and discharge from the rivers (Ruzizi and Malagarasi) and atmospheric dry and wet deposition were derived from the literature. The model generally showed good agreement with the observed data for water temperature, dissolved oxygen and nutrients during the calibration and validation periods. The model satisfactorily reproduced the lake’s seasonal dynamics (dry and wet seasons) induced by the lake’s hydrodynamic processes. We found that both internal and external sources contribute importantly to total nutrient loading in the lake. Our results indicate that nutrient supply from rivers into Lake Tanganyika is more important than previously known. However, we call for further studies to investigate the contribution of other sources of regenerated nutrients (e.g. N2-fixation) to the overall primary productivity of Lake Tanganyika.

Ecology of Elodea canadensis Michx. and Elodea nuttallii (Planch.) H. St. John-Insights from National Water Monitoring in Croatia.

Elodea canadensis Michx. (common waterweed) and Elodea nuttallii (Planch.) H. St. John (Nuttall's waterweed), two invasive aquatic plants from North America, have coexisted in European water bodies since the early 20th century. New localities for both species in Croatia continued to be discovered during a study that ran from 2016 to 2023 as a part of the annual implementation of Water Framework Directive monitoring that covered the entire territory of Croatia (786 sampling points in total). Based on these data, the distribution and ecology of both species were analysed. Elodea canadensis was found at 30 sampling points, mostly in rivers, and E. nuttallii at 15 sampling points, mostly in artificial canals. Nearly three-quarters (72.5%) of all elodea sampling points were in the Pannonian Ecoregion. Elodea canadensis was discovered for the first time in the Continental-Dinaric and Mediterranean-Dinaric Subecoregions. To study the ecology of the species, for each sampling point, vegetation relevés were performed and monthly measurements of physico-chemical parameters were collected. The most common accompanying species for both elodeas are presented, and the difference in species assemblages between the sites with E. canadensis and E. nuttallii was confirmed with the ANOSIM test. Furthermore, Indicator Species Analysis revealed eight species characteristic of E. canadensis sites and eleven species characteristic of E. nuttallii sites. Fitting multivariate models (CCA and NPMR) to species abundance revealed the ecological reaction of E. canadensis and E. nuttallii to environmental descriptors. The most strongly contributing environmental descriptors that influence the distribution of both Elodea species are biochemical oxygen demand, electrical conductivity and total phosphorus. In Croatia, the replacement of E. canadensis with E. nuttallii was observed in several water bodies with high nutrient loads.

Rewetting effects on nitrogen cycling and nutrient export from coastal peatlands to the Baltic Sea

Coastal nutrient loads from point sources such as rivers are mostly well-monitored. This is not the case for diffuse nutrient inputs from coastal catchments unconnected to rivers, despite the potential for high inputs due to intensive land use. The German Baltic Sea coastline consists of numerous peatlands that have been diked and drained. However, some of the dikes have been removed in order to re-establish the hydrological connection to the Baltic Sea, restore local biodiversity, and promote natural CO2 uptake. Since these peatlands were used for agriculture, their rewetting may release accumulated nutrients, leading to nutrient export into the Baltic Sea and intensified coastal eutrophication. Data on these potential nutrient exports are mostly lacking. Therefore, this study investigated nutrient exports from two former agricultural, coastal peatlands: Drammendorfer Wiesen, rewetted in 2019, and Karrendorfer Wiesen, rewetted in 1993. Nutrients (NO3–, NO2–, NH4+, PO43–), nitrous oxide (N2O), particulate organic matter (POM, comprising POC and PON; δ13C-POC), chlorophyll-a, and nitrification rates were analyzed in surface water and porewater sampled weekly to monthly in 2019 and 2020 to compare the effects of different time scales after rewetting on nutrient cycling and potential exports. NH4+, NO2−, and PO43− concentrations were higher in the porewater than in the overlying water at both sites, while nutrient concentrations were generally higher at the recently rewetted Drammendorfer Wiesen than at the Karrendorfer Wiesen. NO3− concentrations in porewater, however, were lower than in the overlying water, indicating NO3− retention within the peat, likely due to denitrification. Nitrification rates and N2O concentrations were generally low, except for a high N2O peak immediately after rewetting. These results suggest that denitrification was the dominant process of N2O production at the study sites. Both peatlands exported nutrients to their adjacent bays of the Baltic Sea; however, N exports were 75% lower in the longer-rewetted peatland. Compared to major Baltic Sea rivers, both sites exported larger area-normalized nutrient loads. Our study highlights the need to monitor the impact of rewetting measures over time to obtain accurate estimates of nutrient exports, better assess negative effects on coastal waters, and to improve peatland management.

Spatial and Temporal Variation of C, N, and S Stable Isotopes and Seagrass Coverage Related to Eutrophication Stress in Zostera marina

Zostera marina is an ecologically valuable species that has been declining due to anthropogenic environmental stressors. In this study, spatial and temporal indicators of eelgrass stress, such as coverage and biomass, were compared with the isotopic composition of C, N, and S to understand the mechanism(s) of plant stress. Eelgrass samples were collected in June, July, and August of 2020 at five stations along an estuary spatial gradient in the southern Gulf of St. Lawrence to measure above- and below-ground biomass and tissue isotopes in eelgrass leaves and roots/rhizomes. Eelgrass biomass was lowest at the innermost sampling station, which coincided with eutrophication-induced hypoxia relative to outer sampling stations. δ13C levels at the upstream station were depleted compared to downstream stations. Comparatively, δ15N and δ34S findings were not correlated with plant biomass. Thus, sulfide intrusion was not a major stressor for eelgrass in this estuary. Between the years 2014 and 2020, eelgrass coverage was found to have increased, which coincided with high and low recorded external nutrient loads from the Wheatley River, respectively. Ultimately, these findings indicate that isotopic composition and biomass can be useful in assessing the health of eelgrass in temperate estuaries.

Nutrient and organic substances emissions from diffuse sources to the rivers of Ukrainian Carpathians

Formulation of the problem. The transition from territorial-administrative to basin-based water resources management in Ukraine requires an assessment of the load of water bodies with pollutants from diffuse and point sources. Among the various components of the chemical composition of water entering water bodies, organic matter and nutrients are the most important. The Danube is the second largest river in Europe, with a basin covering the territories of 19 countries and is an important transportation waterway. Within Ukraine, the Danube is divided into the Lower Danube sub-catchment and the Tisza, Prut, and Siret River basins within the Carpathian region. The scientific results presented in this paper were obtained during research within the framework of state budgetary research works of the UHMI, the implementation of which will contribute to the further development of knowledge in the field of hydrometeorology. The results presented in the publication are important for supplementing information when writing the Danube River Basin Management Plan, which is being developed in accordance with Ukraine's obligations under the Association Agreement with the European Union. Aim of the study. To calculate the supply of nutrients and organic matter from diffuse sources to the rivers of the Danube basin within the Ukrainian Carpathians. Methods. Monitoring data of organic substances and nutrients provided by the Danube Hydrometeorological Observatory (the state surface water monitoring network of the State Emergency Service of Ukraine) for 2018 were used for calculations. To assess the load of water bodies by diffuse sources, a conceptual scheme was developed that allowed to take into account the main pathways of substances supply, i.e precipitation; water runoff from arable land, forests, meadows and pastures, built-up areas, rock outcrops, as well as from the population of rural regions without sewage systems. Scientific novelty. For the first time: the load of nutrients and organic matter in the Danube rivers within the Ukrainian Carpathians by the sources of their income was evaluated; it was found that agricultural land is the main source of emissions of inorganic nitrogen and phosphorus compounds. Practical value. The analysis of the diffuse load by nutrients and organic matter in the Danube rivers can be used as an important part of the River Basin Management Plan. The results of the calculations can also be used to develop measures to achieve certain environmental objectives. Results. The main source of organic matter in the water of rivers of the Carpathian region ere agricultural enterprises locating within rural settlements that are not equipped with sewage systems. Surface water of Prut and Tisza River basins is the most polluted by organic matter. By source, the nutrient emissions are distributed as follows. For the Tisza and Prut rivers, more than 50% of the total nitrogen compounds emissions come from agricultural land, while for the Siret River the source of nitrogen compounds are forests (46% of the total nitrogen emissions). Accordingly, the total phosphorus runoff for the Tisza and Prut rivers was distributed as follows: the dominant share (up to 45%) comes from agricultural land, the load caused by population not connected to the sewerage systems is 36%. For the Siret River, agricultural land and forested areas are of equal shares (33%) among the sources of phosphorus compounds.

Urbanization of a Subtropical Island (Okinawa, Japan) Alters Physicochemical Characteristics and Disrupts Microbial Community Dynamics in Nearshore Ecosystems

Subtropical and tropical islands are undergoing rapid urbanization as the human population expands globally. Urbanization disrupts coastal ecosystems through several pathways—including the replacement of natural habitats with concrete structures that increase runoff pollution—but it remains difficult to isolate and characterize specific impacts of urbanization on marine ecosystems. The historical gradient in urbanization on the subtropical island of Okinawa, Japan, sets up a natural laboratory to study urbanization effects on nearshore ecosystems. Physicochemical parameters and bacterial community composition were assessed every 2 weeks for 1 year at two nearshore sites adjacent to watersheds with > 70% urban land use and two nearshore sites adjacent to watersheds with > 70% rural land use. Urbanization increased freshwater input and nutrient loading—indicated by decreased salinity and elevated nitrate + nitrite, ammonium, and phosphate at urban sites—despite the urban sites being more open to flushing due to land reclamation projects filling in the coral lagoon. Urbanization significantly altered microbial community composition by increasing diversity through the addition of fecal indicator and pathogenic bacteria—eight orders of bacteria were only detected in urban samples, whereas only Verrucomicrobiales was unique to rural samples. The change in microbial community composition at urban sites persisted throughout the seasonal cycle, suggesting a regime change or sustained disturbance. The altered physicochemical conditions and microbial communities at urban sites could degrade nearby coral reefs and their ecosystem services, highlighting the importance of coastal land management in marine conservation efforts.

Perspectives on total phosphorus response in rivers: Examining the influence of rainfall extremes and post-dry rainfall

Eutrophication is a significant environmental problem caused by nutrient loads from both point and non-point sources. Weather variables, particularly precipitation, affect the concentration of nutrients in water bodies, particularly those from non-point sources, in two contrasting ways. Heavy precipitation causes surface runoff which transports pollutants to rivers and increases nutrient concentration. Conversely, increased river flow can dilute the concentration, lowering it. This study investigates the impact of extreme precipitation, prolonged precipitation, and precipitation after a dry period on the total phosphorus concentration in the Moehne and Erft rivers in Germany, given the projected increase in frequency of extreme precipitation events and long drought periods due to climate change. The study comprises two parts: selecting extreme weather days from 2001 to 2021 and comparing observed Total Phosphorus concentrations with estimated concentrations derived from Generalized Additive Models and linear regression based on the discharge-concentration relationship. Changes in river TP concentration in response to continuous precipitation and precipitation after a dry period were also studied. Our results showed that during wet extreme and post-dry period rainfall events, TP concentration consistently surpassed expected values, underscoring the profound influence of intense rainfall on nutrient mobilization. However, we observed the impact of continuous rainfall to be non-unidirectional. Our work is distinguished by three key innovations: 1) addressing limitations in studying the effects of extreme weather on water quality due to limited temporal resolution, 2) incorporating both linear and non-linear modeling approaches for discharge-concentration relationships, and 3) performing a comprehensive analysis of temporal and spatial patterns of Total Phosphorus concentrations in response to varying rainfall patterns.

High-frequency monitoring during rainstorm events reveals nitrogen sources and transport in a rural catchment

Rural areas lacking essential sewage treatment facilities and collection systems often experience eutrophication due to elevated nutrient loads. Understanding nitrogen (N) sources and transport mechanisms in rural catchments is crucial for improving water quality and mitigating downstream export loads, particularly during storm events. To further elucidate the sources, pathways, and transport mechanisms of N from a rural catchment with intensive agricultural activities during storm events, we conducted an analysis of 21 events through continuous sampling over two rainy seasons in a small rural catchment from the lower reaches of the Yangtze River. The results revealed that ammonia-N (NH4+-N) and nitrate-N (NO3−-N) exhibited distinct behaviors during rainstorm events, with NO3−-N accounting for the primary nitrogen loss, its load being approximately forty times greater than that of NH4+-N. Through examinations of the concentration-discharge (c-Q) relationships, the findings revealed that, particularly in prolonged rainstorms, NH4+-N exhibited source limited pattern (b = −0.13, P < 0.01), while NO3−-N displayed transport limited pattern (b = −0.21, P < 0.01). The figure-eight hysteresis pattern was prevalent for both NH4+-N and NO3−-N (38.1% and 52.0%, respectively), arising from intricate interactions among diverse sources and pathways. For NO3−-N, the hysteresis pattern shifted from clockwise under short-duration rainstorms to counter-clockwise under long-duration rainstorms, whereas hysteresis remained consistently clockwise for NH4+-N. The hysteresis analysis further suggests that the duration of rainstorms modifies hydrological connectivity, thereby influencing the transport processes of N. These insights provide valuable information for the development of targeted management strategies to reduce storm nutrient export in rural catchments.

Under pressure: assessment of chemical stress on restored river sections using effect‐based methods

Aquatic ecosystems are affected by multiple stressors, including hydrological and morphological degradation, high nutrient loading, and chemical pollution. To improve freshwater habitats, hydromorphological restorations have been increasingly implemented. However, follow‐up assessments often show little to no improvement in ecological status, even years after restoration measures have been implemented. The success of restoration projects can be compromised by other stressors, such as insufficient water and sediment quality, which often receive less attention compared to nonchemical stressors. In this study, the impact of chemical stress on the outcome of five river restorations was evaluated ecologically, chemically, and ecotoxicologically. Overall, the habitat structure was considerably improved through the restoration measures, whereas the species communities did not show a consistent trend toward an improved ecological status. Effect‐based methods were used for an integrative assessment of the exposure to chemical mixtures in water and sediment samples of restored stream sections. Differences in toxicity between restored and non‐restored sections were found but did not show a consistent trend among the applied assays. In contrast, the chemical analysis showed that the sections of the same stream were similar in their chemical composition, and differences within a stream were primarily due to sediment contamination. The results of this study suggest that chemical pollution is a relevant factor preventing the success of restoration measures and, ultimately, the improvement of the ecological status of rivers. They also demonstrate the applicability of EBMs in water quality monitoring to detect mixture toxicity in streams and link chemical and ecological assessment.