Water Quality In Estuaries Research Articles

Quantifying the Potential Water Filtration Capacity of a Constructed Shellfish Reef in a Temperate Hypereutrophic Estuary

Shellfish reefs have been lost from bays and estuaries globally, including in the Swan-Canning Estuary in Western Australia. As part of a national program to restore the ecosystem services that such reefs once provided and return this habitat from near extinction, the mussel Mytilus galloprovincialis was selected for a large-scale shellfish reef construction project in this estuary. To assess the potential filtration capacity of the reef, estuary seston quality, mussel feeding behavior, and valve gape activity were quantified in the laboratory and field during winter and summer. In general, estuary water contained high total particulate concentrations (7.9–8.7 mg L−1). Standard clearance rates were greater in winter (1.9 L h−1; 17 °C) than in summer (1.3 L h−1; 25 °C), the latter producing extremely low absorption efficiencies (37%). Mussel valves remained open ~97% and ~50% of the time in winter and summer, respectively. They often displayed erratic behavior in summer, possibly due to elevated temperatures and the toxic microalgae Alexandrium spp. Despite numerous stressors, the reef, at capacity, was estimated to filter 35% of the total volume of the estuary over winter, incorporating 42.7 t of organic matter into mussel tissue. The reefs would thus make a substantial contribution to improving estuary water quality.

Water quality and health risk assessments of illegal gold mining-impacted estuaries in Ghana

This study evaluated the water quality, pollution, and health risks associated with illegal gold mining activities in two estuaries (Pra and Ankobra) in the Southwestern region of Ghana. The Canadian Council of Ministers of the Environment Water Quality Index (CCME-WQI) used for the evaluation indicated that both estuaries were in the marginal water quality category. Nemerow's Pollution Index (NPI) evaluation showed that turbidity (from siltation), PO 4 , Pb, Cu, and Fe contributed to the pollution of the two estuaries. The health risks index of Target Hazard Quotient (THQ) and Total Hazard Index (HI) evaluated on shellfish ( Macrobrachium ) from the estuaries were above USEPA recommended threshold; therefore, there is a likelihood of human health risks impacts on the coastal dwellers surrounding the estuaries. Overall, the study buttresses the need to take urgent strategic actions to control the direct impacts of pollution on the estuarine systems and the surrounding coastal communities. • Pollution footprints observed in two estuaries of Southwestern region of Ghana • The pollution index revealed turbidity, phosphate, lead, copper and iron as main contributing drivers of the estuaries. • There is a high likelihood of human health risks impacts on surrounding coastal dwellers. • Urgent action needed to control pollution in order to safeguard the lives of the communities

Effects of swell waves on the dynamics of the estuarine turbidity maximum in an idealized convergent partially mixed estuary

The dynamics of estuarine sediments plays a key role in the morphological evolution, water quality, and ecosystem health of estuaries. Estuarine turbidity maxima (ETMs) are ubiquitous in estuaries worldwide and have drawn enormous attention from researchers. Here, we use a coupled ocean–atmosphere–wave–sediment transport modeling system to examine the effects of swell waves on the location, extent, and suspended sediment concentration (SSC) of ETMs in a convergent partially mixed estuary. Our results reveal that swell waves influence the residual circulation, mixing, and bottom stress in the estuary. The wave–current interaction increases the bottom stress in the estuary but reduces mixing in the lower to middle reaches of the estuary, owing to the enhanced landward salt transport, and the increased stratification. The reduced mixing enhances the estuarine circulation in the middle reach of the estuary. These changes cause a landward shift of the primary ETM and an increase in the SSC in the lower estuary. Moreover, the Coriolis force changes the ETM distribution and influences the wave effects. Overall, the findings of this study further elucidate ETM dynamics in an estuary. • Swell wave advects more saline water in the estuary and increases stratification. • Wave-current interaction increases the bottom stress in the lower estuary. • Swell wave causes a landward shift of the primary estuarine turbidity maxima (ETM).

Rainfall induced water and nutrient fluxes at a mangrove estuary

Deep understanding of the nutrient exchange between a mangrove estuary and the surrounding coastal region is important for preserving nearshore biodiversity and sustaining appropriate human ecotourism. This study investigated the effect of tidal changes on the water quality and nutrient fluxes of a small mangrove estuary in Japan. Temporal changes in the water temperature, salinity, dissolved oxygen, turbidity, and nutrient concentrations (NO3, NH4, and TN) were measured. Nutrient fluxes were calculated using data collected from the Fukido Estuary in 2016 and compared against data collected in 2007. The results showed that NO3, NH4, and TN exhibited a negative correlation with salinity in 2016, but not in 2007. The water and TN net fluxes changed with rainfall. The findings of this study provide useful insights into the nutrient fluxes in a mangrove estuary and facilitate the evaluation of tidal fluctuations and weather-induced effects on nutrient fluxes.

The coastal macro-vortices dynamics in Hong Kong waters and its impact on water quality

Large-scale macro-vortices could be formed in the lee or around natural obstacles such as islands and headlands during the interaction between geographical features and coastal circulations induced by tides and winds. Tidal and wind-induced vortices play a fundamental role in transport processes of sediments and nutrients and the effects are then reflected on water quality of estuaries and coastal areas. In the present study, the coastal hydrodynamics of the Pearl River Estuary and Hong Kong waters was analyzed in detail using a high-resolution model based on the Finite-Volume Coastal Ocean Model (FVCOM), forced by tides and seasonal winds. The results showed that the local dynamics was strongly affected by the continuous shedding of macro-vortices. A classification of these coherent structures was suggested based on their typical evolution to distinguish between tidal vortices and wake vortices using four non-dimensional parameters. The spatial distribution, the seasonality and the probability density functions of their geometrical characteristics were presented and discussed in detail. The results suggested that tidal currents played a dominant role in most of the Pearl River Estuary and the Hong Kong waters, generating for the entire year a high and constant number of macro-vortices that occupied most of the north channels of Hong Kong. Finally, we linked the vortices distribution and their typical features with several important water quality parameters, suggesting possible explanations of the observed trends.

Human health risk assessment of heavy metal and pathogenic contamination in surface water of the Punnakayal estuary, South India

Variation in levels of toxic heavy metals in river system during the COVID-19 pandemic lockdown might potentially assist in development of a public health risk mitigation system associated with the water consumption. The water quality of Punnakayal estuary in the Thamirabarani River system from the south India, a vital source of water for drinking and domestic purposes, industrial usage, and irrigation was assessed here. A comparitive assessment of physico-chemical variables (pH, EC, TDS, DO, BOD, turbidity and NO3), microbiological parameters (total coliform bacteria, fecal coliform bacteria, fecal streptococci and escherichia coli) and toxic metals (As, Cr, Fe, Cu, Zn, Cd, and Pb) suggested a decrease of 20% in the contaminant ratio during the lockdown period in comparison to the pre-lockdown period. The Health risk assessment models (HQ, HI, and TCR) highlighted carcinogenic and non-carcinogenic hazards for both children and adults through the ingestion and dermal adsorption exposures. The HI values for both As and Cr exceeded the acceptable limit (>1) during the lockdown period, but the potential risk for children and adults remained low in compaisio with the pre-lockdown period. Our results suggested that the Thamirabarani River system remained hostile to human health even during the lockdown period, and it requires regular monitoring through a volunteer water quality committee with private and government participations.

Effects of Biophysical Processes on Diel-Cycling Hypoxia in a Subtropical Estuary

In shallow estuaries, fluctuations in bottom dissolved oxygen (DO) at diel (24 h) timescales are commonly attributed to cycles of net production and respiration. However, bottom DO can also be modulated by physical processes, such as tides and wind, that vary at or near diel timescales. Here, we examine processes affecting spatiotemporal variations in diel-cycling DO in Escambia Bay, a shallow estuary along the Gulf of Mexico. We collected continuous water quality measurements in the upper and middle reaches of the Bay following relatively high (> 850 m3 s-1) and low (< 175 m3 s-1) springtime freshwater discharge. Variations in diel-cycling amplitude over time were estimated using the continuous wavelet transform, and correlations between DO and biophysical processes at diel timescales were examined using wavelet coherence. Our results reveal that freshwater discharge modulated inter-annual variations in the spatial extent and duration of summertime hypoxia through its effect on vertical density stratification. In the absence of strong stratification (> 15 kg m-3), vertical mixing by tropic tides and sea breeze enhanced diel fluctuations in deeper areas near the channel, while in shallower areas the largest fluctuations were associated with irradiance. Our findings suggest that processes affecting diel-cycling DO in the bottom layer can vary over a relatively short spatial extent less than 2 km and with relatively small changes in bottom elevation of 1 m or less. Implications for water quality monitoring were illustrated by subsampling DO timeseries, which demonstrates how low-frequency measurements may misrepresent water quality in estuaries where diel-cycling DO is common. In these systems, adequate assessment of hypoxia and its aquatic life impacts requires continuous measurements that capture the variation in DO at diel timescales.

Collaborative Environmental Approach for Development of the Lower Laguna Madre Estuary Program Strategic Plan in South Texas

The National Estuary Program (NEP) is a promising eco-system based approach to improve the water quality and ecological integrity of estuaries of national importance in the United States. Due to population growth and concomitant development pressure in South Texas, the future of the Lower Laguna Madre (LLM) without an estuary program is problematic. The development of a management plan for the LLM will enable the region to develop local solutions to local problems. The fundamental purpose of this research was to develop a strategic plan for the foundation of the Laguna Madre Estuary Program of the Gulf Coast of Texas. The comprehensive plan provided local communities with information to restore water quality, conversing habitat, and protecting coastal resources along the Gulf coast. The strategic plan was organized along the lines of the NEP program focused on the three most important foundational elements to establish a NEP for the LLM. The three primary Thrust Areas are as follows: (1) the national significance of the Laguna Madre estuary system, (2) the needs and goals for a proposed program, and finally (3) the plan for the sustainability and support to operate and maintain such a NEP. The outcomes of the strategic plan can be used as a model by the decision-makers to promote community resilience and establish integrated local water quality and ecosystem management plans for their respective communities and jurisdictions. Ultimately, the main objective of this project is to assess the ability to integrate science and public policy development for the common good.

Restoration of Estuary Water Quality in the Petitenget Beach Area with the Use of Hydromacrophyte Plants in Badung Regency Bali

Tourism activities around the coast are growing rapidly, especially in the Petitenget Coastal area. Waste management by tourism actors around the area causes the contamination of the Petitenget Beach estuary. To overcome this, it is necessary to restore estuary water, one way is by utilizing the potency of the fragrant root plant as a phytoremediation agent. The purpose of this community empowerment program is to examine how effective the vetiver plant is as a phytoremediation agent for Petitenget Beach estuary water. This research was started by collecting water samples from the petitenget estuary and then conducting water quality research and testing water samples on Wangi Root plants using an artificial wetland system which was carried out for one month. From the research that has been done, it is known that the vetiver plant is quite effective in reducing the levels of chemical pollutants such as pH, BOD and COD, but this plant cannot reduce the microbiological levels contained in the water.

Spatial Distributions and Model Selections of Commercial Estuarine Fish (Sciaenidae) Populations Related to Water Quality, Chl-a, and AML in Musi River mouth, South Sumatra

Estuary and river mouth are essential habitats for many commercial estuarine fishes, including the Sciaenidae family. While recently, estuaries have been threatened by anthropogenic marine litter (AML) transported from nearby land and river. An important type of AML is plastic litter since it takes a long degradation time. In the South Sumatra Province, Indonesia, one of the vital estuaries is the Musi estuary. This paper aims to map the spatial distributions of two Sciaenids, including Panna microdon and Otolithoides pama, and Sciaenid’s environmental covariates, including water quality, chlorophyll a, and plastic litters in Musi estuary and model the correlations of Sciaenids with their covariates. The maps were developed using GIS, and the model was validated using AIC methods. The data were collected from 3 river mouths in the west, central, and east of the Musi estuary. The data showed that the populations of both Sciaenids were higher in the east river mouth rather than in the west. Sciaenid populations were positively correlated with high salinity, DO, chlorophyll a, moderate transparency, and low temperature. A high load of AML’s frequency (7.54 items/m2) and weights (36.8 gram/m2) has reduced both Sciaenid populations in the central river mouth of the estuary. In contrast, low AML loads in the east have correlated with high Sciaenid populations. Model selection based on AIC values shows the best model for P.microdon retained an effect of AML weight with AIC values of 22.591 and 28.321 for O. pama. This concludes that the weight of plastic litter in estuary water was the main limiting factor for Sciaenid populations in Musi.

Social–environmental analysis of estuary water quality in a populous urban area

Asia has been experiencing rapid industrialization, urbanization, and economic growth in recent decades. Taiwan was one of the 4 Asian dragons, regions that experienced rapid industrialization and exceptionally high growth rates between the early 1960s and 1990s, but at a high cost to the environment, and thus, it was heavily polluted. Estuaries are highly dynamic and diverse ecosystems that provide multiple ecosystem services that maintain marine ecosystem health and benefit humankind. However, estuaries and the ecosystem services they provide are rapidly degrading due to increasing pressures and changes, especially those in populous, urban areas. Social–environmental analysis integrates scientific information and social activities and thereby provides a comprehensive understanding for the multiparty, joint decision-making processes necessary for successful, sustainable management. In this study, 60 years of economic data and 26 years of water quality data are examined using social–environmental frameworks, the driver-pressure-state-impact-response framework, and the systems approach framework to analyze the management of water quality for an estuary in a populous urban area, the Tamsui River estuary, in Taiwan. Potential societal responses and management measures are identified that can be implemented to reduce human activities, diminish pressure, ameliorate water quality, and enhance the state of the estuarine systems in the Tamsui River and its estuary. The recommended societal responses are increased education, the establishment of community-based river rangers, wetland and mangrove conservation, the development of a circular economy, the implementation of governance measures, and improvements in monitoring and assessments. Improvement of the water quality in the Tamsui River estuary increases the hedonic value of property for people who live near the riverside. Currently, the number of tourists and tourism-based businesses have increased. Nevertheless, improvements in water quality in the Tamsui River estuary bring well-being and benefits that could be further enhanced to increase the cost/benefit relation of the management measures.

Effect of Mississippi River discharge and local hydrological variables on salinity of nearby estuaries using a machine learning algorithm

Estuarine water salinity is important to coastal ecosystems and the economy. To examine the potential effect of the Mississippi River freshwater discharge on water salinity of other estuaries, we used the machine learning Cubist algorithm to model salinity of three estuaries (Apalachicola Bay, Weeks Bay, and Grand Bay) of the northern Gulf of Mexico (GoM). The models were trained — both with and without the Mississippi River discharge as input — to examine its effect on the salinity of other estuaries along with other variables including wind, discharge, and water depth. By including the Mississippi discharge, the Root Mean Squared Error (RMSE) of the model training sets decreased by 0.81, 1.06 and 1.73 for the Apalachicola Bay, Weeks Bay, and Grand Bay, respectively, while the decrease of the model testing sets was 0.20, 0.64, and 0.84, respectively, using the practical salinity scale. The rank of predictor importance of the Mississippi discharge increased as the modeled estuary proximity increased to the Mississippi outlet. The results showed that the Mississippi River discharge can affect the water quality of other estuaries along the northern GoM, although the variation of the salinity explained by this flow was smaller than that explained by local river flow (except for the Grand Bay where there is no local river flow) and local wind.

Traceability and Emission Reduction of Dissolved Inorganic Nitrogen in Minjiang Estuary, China.

The accumulation of dissolved inorganic nitrogen (DIN) in estuaries has become a global environmental problem. A two-dimensional, hydrodynamic water quality model was constructed in this study to investigate the sources of DIN pollution in the Minjiang Estuary. The concentration response field between the stream input and DIN in the estuary was established by using the surveyed source data of the study area. A sharing coefficient method was used to calculate the contribution percentage of each outfall to derive and propose a reasonable nitrogen reduction plan. The results showed that the input of land-based nitrogen into the Minjiang River contributed more than half of the DIN in the near-shore sea; the middle and upper reaches of the Minjiang River largely influenced the estuary area (38.57%). Conversely, the estuary and the coastline accounted for a smaller proportion of only 5.24%, indicating that an integrated DIN reduction should be implemented in the estuary area of the whole river basin. The model calculations showed that the reduction results, after remediation according to the current national standards for wastewater discharge in rivers, were not satisfactory. Thus, a new scheme is proposed in this paper—the total nitrogen (TN) input from land-based sources into the Minjiang Estuary and from the Shuikou Dam to the Min’an section should be reduced to below 31.64%; simultaneously, the DIN concentration discharged from the Shuikou Dam should be controlled and maintained below 0.5 mg·L−1 (TN = 0.8 mg·L−1). These results will provide guidelines for developing strategies for the improvement of DIN and water quality in similar estuaries.

Can shrimp farming wastewater negatively affect water quality and zooplankton community structure of a Neotropical estuary? A case study during a productive cycle of Litopenaeus vannamei

Aquaculture wastewater can influence various communities’ structures in both marine and freshwater environments. This study describes the zooplankton community structure and environmental variables in both Passos River estuary (Northeast Atlantic, Brazil) and commercial shrimp farming ponds associated with this estuary. Samples for water quality analysis and quantification and identification of zooplankton were taken during a shrimp culture cycle (July to September 2014) from eight sites (two across the main channel, downstream and upstream, and six shrimp farming ponds). The main water quality parameters showed difference between the shrimp farming ponds and the Passos River estuary – higher levels of total phosphorus and chlorophyll-a were observed in the shrimp ponds. The zooplankton structure at different environments was dominated by the copepods (53.3 to 83.7%) and rotifers (9.2 to 35.5%) but no significant difference was observed among the individual densities. Meanwhile, nutrients availability was a key for high plankton densities in shrimp ponds. These findings have suggested that wastewater from shrimp farming did not influence the zooplankton community on a Neotropical estuary on a short-time scale.

Changes in Nutrient Concentrations in Shenzhen Bay Detected Using Landsat Imagery between 1988 and 2020

Shenzhen Bay (SZB), situated between Shenzhen and Hong Kong, is a typical bay system. The water quality of the bay is notably affected by domestic and industrial discharge. Rivers and various types of drainage outlets carry terrestrial pollutants into SZB, resulting in elevated concentrations of nitrogen and phosphorous as well as relatively poor water quality. For over 200 years, Hong Kong has practiced oyster farming within brackish estuarine waters. Oyster farming is a type of mariculture which includes oyster breeding in oyster rafts. Remote sensing is a monitoring technique characterized by large spatial coverage, high traceability, and low cost, making it advantageous over conventional point-based and ship-borne monitoring methods. In this study, remote-sensing models were established using machine-learning algorithms to retrieve key water-quality factors (dissolved inorganic nitrogen (DIN) and orthophosphate-phosphorous (PO4_P) concentrations, CDIN and CPO4_P, respectively) from long-term time-series data acquired by the Landsat satellites. (1) Spatially, the water quality in Inner SZB was worse than that in Outer SZB. (2) The water quality temporarily deteriorated between the end of the 20th century and the beginning of the 21st century; then it gradually improved in the late 2000s. (3) Monitoring the water quality in an oyster-farming area revealed that oyster farming did not adversely affect the water quality. (4) The result of monitoring the water quality in river estuaries in SZB shows that water quality was mainly affected by river input.

Effects of flood-associated stressors on growth and survival of early life stage oysters (Crassostrea virginica)

Oyster reefs provide essential ecosystem services but are severely degraded worldwide. Extreme flooding events, which can be intensified by water management decisions, reduce water quality in estuaries and further threaten oyster populations. Restoration and conservation of oysters is dependent on the success of early oyster life stages. This study examined the effect of water quality stressors associated with flooding events on the growth and survival of larval and juvenile oysters (Crassostrea virginica). In 96-h assays, we exposed D-stage larvae to a range of dissolved oxygen, microcystin-LR, pH, and salinity concentrations. These conditions were selected based on water quality data from the Mississippi Sound during a 2019 freshwater flooding event caused by the Bonnet Carré Spillway opening. There was no negative effect of microcystin-LR or pH on early veligers at the concentrations tested, but low salinity significantly reduced shell growth, and hypoxia (< 2 mg L−1 O2) decreased both larval growth and survival. Post-metamorphosis juvenile oysters were exposed to the same water quality stressors for 24 days in the lab. Low DO, pH, and salinity treatments reduced juvenile change in wet weight and shell growth rates, but had no effects on survival. These laboratory-exposed juveniles were subsequently deployed into the field to assess the ability of juveniles to recover from short-term exposure to simulated flooding-associated stressors. After deployment to natural conditions in the Mississippi Sound, juvenile oysters were able to compensate for reduced growth during the lab exposure, even though survival was reduced for juveniles previously exposed to low pH during the first two weeks in the field. In general, early oyster life stages were relatively tolerant of the duration and stressor concentrations tested, but negative sublethal impacts of flood-associated stressors must be considered in the face of increasing frequency and duration of flooding events due to climate change.

Effective Management of Closed Hypereutrophic Estuaries Requires Catchment-Scale Interventions

Increased nutrient loading associated with rapid population growth is the leading cause of deteriorating water quality in urbanized estuaries globally. Small estuaries are particularly sensitive to changes when connection with the marine environment is restricted, or lost, because of high water retention. The temporarily closed Hartenbos Estuary (South Africa) is an example of how such pressures can culminate in a severely degraded ecosystem. Wastewater treatment work (WWTW) discharges introduce substantial volumes of freshwater (8,000 m3 d–1) and nutrient loads (38 kg DIN d–1 and 22 kg DIP d–1) into this estuary. This constant inflow has necessitated frequent artificial breaching (inducing alternating states) of the estuary mouth to prevent flooding of low-lying developments and, occasionally, to mitigate against extreme events such as fish kills and sewage spills. This study investigated the efficacy of artificial mouth breaching practices in eliciting responses in selected abiotic and biotic parameters. Microalgal (phytoplankton and benthic diatoms), benthic macrofauna and fish community dynamics were assessed in response to mouth state and water quality conditions using a seasonal monitoring programme. The hypereutrophic nature of the Hartenbos Estuary was highlighted by persistent high-biomass phytoplankton accumulations (&amp;gt;100 μg Chl-a l–1), extreme dissolved oxygen conditions (0.4–20.5 mg O2 l–1) and the predominance of harmful algal bloom (HAB) events comprising Nannochloropsis sp. and Heterosigma akashiwo. Artificial breaching of the mouth facilitated limited tidal exchange and occurred approximately bimonthly once water levels exceeded 1.9 m above mean sea level (MSL). Current pressures and management interventions have culminated in an ecosystem void of natural fluctuations and instead characterised by low diversity and shifts between undesirable states. This is highlighted by the near year-round dominance of only a few opportunistic species/groups tolerant of adverse conditions (e.g., Nannochloropsis sp., Halamphora coffeiformis, oligochaetes, estuarine round herring Gilchristella aestuaria, and southern mullet Chelon richardsonii). Therefore, catchment-scale interventions such as the diversion of WWTW discharges and restoration of hydrodynamic variability are management priorities for improving the health and biodiversity of small, closed microtidal systems such as the Hartenbos Estuary.

High-Frequency Concurrent Measurements in Watershed and Impaired Estuary Reveal Coupled DOC and Decoupled Nitrate Dynamics

Rapid changes in land use, pollution inputs, and climate are altering the quantity, timing, and form of materials delivered from watersheds to estuaries. To better characterize these alterations simultaneous measurements of biogeochemical conditions in watersheds and estuaries over a range of times scales are needed. We examined the strength of watershed-estuarine biogeochemical coupling using in situ measurements of nitrate, terrestrial dissolved organic carbon (DOC), and chloride collected over a 7-month period in a nitrogen-impaired estuary in northeastern US. The watershed exerted strong control over concentrations of terrestrially derived DOC in the estuary, attributable to relative homogeneity of watershed sources from forested land combined with relatively conservative behavior in estuarine waters. Estuarine nitrate patterns were more complex, suggesting the influence of heterogeneous watershed distribution of non-point and point sources and high reactivity of nitrate in the estuary. Understanding estuarine biogeochemical patterns will be advanced through greater use of simultaneous sub-hourly measurements of inflows, salinity, and water quality in estuaries and their upstream watersheds.

How Effective Are Existing Phosphorus Management Strategies in Mitigating Surface Water Quality Problems in the U.S.?

Phosphorus is an essential component of modern agriculture. Long-term land application of phosphorous-enriched fertilizers and animal manure leads to phosphorus accumulation in soil that may become susceptible to mobilization via erosion, surface runoff and subsurface leaching. Globally, highly water-soluble phosphorus fertilizers used in agriculture have contributed to eutrophication and hypoxia in surface waters. This paper provides an overview of the literature relevant to the advances in phosphorous management strategies and surface water quality problems in the U.S. Over the past several decades, significant advances have been made to control phosphorus discharge into surface water bodies of the U.S. However, the current use of phosphorus remains inefficient at various stages of its life cycle, and phosphorus continues to remain a widespread problem in many water bodies, including the Gulf of Mexico and Lake Erie. In particular, the Midwestern Corn Belt region of the U.S. is a hotspot of phosphorous fertilization that has resulted in a net positive soil phosphorous balance. The runoff of phosphorous has resulted in dense blooms of toxic, odor-causing phytoplankton that deteriorate water quality. In the past, considerable attention was focused on improving the water quality of freshwater bodies and estuaries by reducing inputs of phosphorus alone. However, new research suggests that strategies controlling the two main nutrients, phosphorus and nitrogen, are more effective in the management of eutrophication. There is no specific solution to solving phosphorus pollution of water resources; however, sustainable management of phosphorus requires an integrated approach combining at least a reduction in consumption levels, source management, more specific regime-based nutrient criteria, routine soil fertility evaluation and recommendations, transport management, as well as the development of extensive phosphorus recovery and recycling programs.

Influence of Riverbed Incision and Hydrological Evolution on Water Quality and Water Age Based on Numerical Simulation: A Case Study of the Minjiang Estuary

In recent years, problems such as water quality deterioration, saltwater invasion, and low oxygen have appeared in estuaries all over the world. The Minjiang River in Fujian, as a typical tidal estuary area, is facing these thorny problems. In this paper, the effects of topography and hydrologic evolution on the water age and water quality of the lower reaches of the Minjiang River were simulated by building a hydrodynamic and water quality model. The results show that: (1) It was found that the riverbed incision of the lower reaches of the Minjiang River led to the overall decline of river water level, the increase of river volume, and the increase of downstream water age, which eventually led to the decrease of dissolved oxygen (DO) and the deterioration of water quality in the downstream from Shuikou to Baiyantan. However, the decline of topography led to the increase of tidal volume in the estuary, the enhancement of the dilution effect of oxygen-rich water bodies in the open sea, and the increase of DO in the lower reaches of Baiyantan. (2) Under no tidal action, the concentration of pollutants in the water of the North Channel increased, the DO decreased, and the DO decreased from Baiyantan to the offshore water. After the enhancement of tidal action, the dilution of oxygen-enriched water from the offshore water increased, and the DO increased. (3) The hydrological and water quality characteristics of the upper part of the lower reaches of the Minjiang River were mainly controlled by topography, runoff, and pollutant discharge, which were more affected by the tidal current transport operation and pollutant discharge near the open sea. In recent decades, the deterioration of water quality and the aggravation of saltwater intrusion in the Minjiang River were closely related to the serious topographic downcutting. The results provide a scientific basis for revealing the deterioration of estuary water quality and long-term management of the estuary.