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Located in the northern South China Sea, the Pearl River Estuary (PRE) is one of the most important estuaries in China and is surrounded by several megacities. Hypoxia mainly occurs in the bottom waters of the PRE during summer and is more prominent near the Humen outlet and the subestuary outside Modaomen and Jitimen. A well-validated three-dimensional (3-D) coupled physical-biogeochemical model was used to explore the changes in dissolved oxygen (DO) dynamics and hypoxic conditions in the PRE over an intra-annual cycle and elucidate the processes controlling the generation, development, and dissipation of hypoxia. In summer, oxygen consumption due to the high-intensity sediment oxygen demand (SOD) intensified by large inputs of riverine particulate organic carbon (POC) exceeded the DO supplemented by vertical diffusion (largely inhibited by strong fresh water–induced stratification), thus leading to a significant decrease in DO in the bottom waters; in other seasons, these DO source and sink terms were nearly balanced so that the bottom DO concentrations were maintained at higher levels. Moreover, the lag analysis shows that there is an approximately 2-month lag between riverine POC and SOD. By comparison, the low-oxygen area near Humen has a controlling mechanism distinct from that of the subestuary outside Modaomen and Jitimen. Specifically, the timely and sufficient oxygen supplement brought by vertical diffusion can replenish the bottom DO consumed by SOD in this region, and the formation and development of low-oxygen conditions (DO ≤ 4 mg L−1) is mainly affected by riverine low-oxygen inflows. In summary, our study clarified that the terrestrial organic pollutant input and low-oxygen water from the upper reaches have an important impact on the DO in different areas of the PRE, especially the low-oxygen area near Humen, which is controlled by the water quality of the upper reaches. This conclusion is of great significance for regional environmental management.

The summertime low-oxygen conditions in the Pearl River Estuary (PRE) have experienced a significant spatial expansion associated with notable deoxygenation in recent decades. Nevertheless, there is still a lack of quantitative data on the long-term trends and interannual variabilities in oxygen conditions in the PRE as well as on the driving factors. Therefore, the long-term deoxygenation in a subregion of the PRE (the coastal waters off Hong Kong) was comprehensively investigated in this study using monthly observations during 1994–2018. To evaluate the changes in scope and intensity of oxygen conditions, an indicator (defined as the low-oxygen index, LOI) that integrates several metrics related to low-oxygen conditions was introduced as the result of a principal component analysis (PCA). Moreover, primary physical and biogeochemical factors controlling the interannual variabilities and long-term trends in oxygen conditions were discerned, and their relative contributions were quantified by multiple regression analysis. Results showed that the regression models explained over 60 % of the interannual variations in LOI. Both the wind speeds and concentrations of dissolved inorganic nitrogen (DIN) played a significant role in determining the interannual variations (by 39 % and 49 %, respectively) and long-term trends (by 39 % and 56 %, respectively) in LOI. Due to the increasing nutrient loads and alterations in physical conditions (e.g., the long-term decreasing trend in wind speeds), coastal eutrophication was exaggerated and massive marine-sourced organic matter was subsequently produced, thereby resulting in an expansion of intensified low-oxygen conditions. The deteriorating eutrophication has also driven a shift in the dominant source of organic matter from terrestrial inputs to in situ primary production, which has probably led to an earlier onset of hypoxia in summer. In summary, the Hong Kong waters have undergone considerable deterioration of low-oxygen conditions driven by substantial changes in anthropogenic eutrophication and external physical factors.

The summertime low-oxygen conditions in the Pearl River Estuary (PRE) have experienced a significant expansion in spatial extent associated with notable deoxygenation in recent decades. Nevertheless, there is still a lack of quantitative understanding of the long-term trends and interannual variabilities in oxygen conditions in the PRE as well as the driving factors, which was comprehensively investigated in this study using monthly observations in the eastern PRE during 1994–2018. To evaluate the changes in scope and intensity of oxygen conditions, an indicator (defined as the Low-oxygen Index, LOI) that integrates several metrics related to low-oxygen conditions was introduced through the principal component analysis (PCA). Moreover, primary physical and biogeochemical factors controlling the interannual variabilities and long-term trends in oxygen conditions were discerned, and their relative contributions were quantified by the multiple regression analysis. Results showed that the regression models explained over 60 % of the interannual variations in LOI. Both the wind speeds and concentrations of dissolved inorganic nitrogen (DIN) played a significant role in determining the interannual variations (by 39 % and 49 %, respectively) and long-term trends (by 39 % and 56 %, respectively) in LOI. Due to the increasing nutrient loads and alterations in physical conditions (e.g. the long-term decreasing trend in wind speeds), coastal eutrophication was exaggerated and massive marine-sourced organic matter was subsequently produced, thereby resulting in an expansion of intensified low-oxygen conditions. It has also driven a shift in the dominant source of organic matter from terrestrial inputs to in situ primary production, which has probably led to an earlier onset of hypoxia in summer. In summary, the eastern PRE has undergone considerable deterioration of low-oxygen conditions in the context of substantial changes in anthropogenic eutrophication and external physical factors.

The influence of overlying hydrodynamics on the exchange behaviour and fluxes of heavy metals at the sediment-water interface (SWI) is poorly understood. In the study, metals exchange behaviour and exchange rate at the SWI under resuspended and undisturbed scenario were investigated The results showed that dissolved Cr, Cu, Zn, and Pb concentrations increased rapidly to attain maximum values between 0.3 and 0.5 N·m

Climate change poses a challenge to the management of marine ecosystems and fisheries. Estuarine ecosystems in particular are exposed to a broad range of environmental changes caused by the effects of climate change both on land and in the ocean, and such ecosystems have also had a long history of human disturbance from over-exploitation and habitat changes. In this study, we examine the effects of climate change and fishing on the Pearl River Estuary (PRE) ecosystem using Ecopath with Ecosim. Our results show that changes in net primary production and ocean warming are the dominant climatic factors impacting biomass and fisheries productivity in the PRE. Additionally, physiological changes of fishes and invertebrates that are induced by climate change were projected to be modified by trophic interactions. Overall, our study suggests that the combined effects of climate change and fishing will reduce the potential fisheries catches in the PRE. Reducing fishing efforts can reduce the impacts of climate change on selected functional groups; however, some prey fishes are expected to experience higher predation mortality and consequently decreases in biomass under low fishing intensity scenarios. Thus, our study highlights the non-linearity of the responses of estuarine ecosystems when climate change interacts with other human stressors. In this study, the whole-ecosystem model (Ecopath with Ecosim) is used to examine the effects of climate change and fishing on a highly developed estuarine ecosystem (Pearl River Estuary, PRE) in the subtropical western Pacific. The oceans variables are extracted from the global earth system model (GFDL ESM2M), including changes in sea surface temperature (SST), hydrogen ion concentration (pH), dissolved oxygen (DO) concentration and net primary production (NPP) under the two scenarios RCP2.6 and RCP8.5. We developed a EwE model of the PRE ecosystem and simulated the effects of changing ocean conditions under alternative climate change scenarios as well as three fishing scenarios on the biodiversity and fisheries productivity of the PRE.

A three-dimensional unstructured-grid Finite Volume Community Ocean Model was applied to investigate the principal dispersal patterns and intraseasonal variability of the Pearl River plume in summer (June–August) during 1999–2010. The favorable forcing conditions and underlying dynamics for the formation of plume patterns were also explored. The model was validated against a suite of water level, salinity and temperature observations collected during the simulation periods. The simulated results agree well with the observed data, indicating that the model reasonably reproduces the main hydrodynamic processes occurring in the Pearl River estuary. Using Self-Organizing Maps, we extracted eight typical dispersal patterns of the Pearl River plume from the multiyear simulated daily surface salinity fields and estimated the occurrence frequency of each pattern. The eight plume patterns were further categorized into five types based on the morphologies and extension distances of each pattern, namely, west alongshore spreading, fettered spreading, offshore bulge spreading, bidirectional spreading and east offshore spreading; these five types account for 26.3%, 9.8%, 11.5%, 29.1% and 23.3% of the total occurrence frequency, respectively. The pattern that is characterized by bidirectional spreading with a strong eastward extension has the largest occurrence frequency among the eight patterns. Through an analysis of the best matching units derived from Self-Organizing Maps and the corresponding river discharge and wind conditions, we find that both wind and river discharge can significantly affect the Pearl River plume patterns. The plume orientation is mainly modulated by the wind, while the plume area and eastward extension distance are mainly affected by the river discharge. Generally, the favorable forcing conditions and underlying dynamics are different for each plume pattern, and various combinations of river discharge and wind can generate different dynamics to form different plume patterns. The climatological surface salinity field in summer shows a bidirectional structure with the plume extending westward attaching to the coast and eastward detaching from the coast. The areas and eastward extension distances of the Pearl River plume gradually decrease from June to August, affected by the intraseasonal changes in river discharge and wind. The statistical results of the daily plume patterns for June–August show that east offshore spreading, bidirectional spreading and west alongshore spreading are the primary types occurring in June, July and August, respectively. Furthermore, the occurrence frequencies of the west alongshore spreading and offshore bulge spreading increase, while the occurrence frequency of the east offshore spreading decreases from June to August.

The Pearl River estuary (PRE) frequently experiences low-oxygen conditions in summer, with large extents of low-oxygen events and a long-term deoxygenation trend being reported recently. In this study, we provide a synthesis of the spatiotemporal patterns and incidence of different low-oxygen levels in the PRE based on the in-situ observations collected from 1976 to 2017, and aim to elucidate the underlying mechanisms of low-oxygen conditions and their changes over the past 4 decades. The long-term observations show that the oxygen content in the PRE had significant temporal variability and spatial heterogeneity. Low-oxygen conditions occurred mostly in the bottom waters of 5–30 meters during summer and early autumn, with locations and severity varying substantially among years. Coastal waters from the southwest of Lantau Island to the northeast of Wanshan Islands were identified as the hotspot area prone to subsurface low-oxygen conditions due to the combined effects of comparatively deep topography, proper residence time and stability of the water column, and enhanced oxygen depletion related to high phytoplankton biomass. In addition, the low-oxygen waters, either directly imported from the upstream reaches or generated locally and further transported with the estuarine circulation, also had considerable impacts on the oxygen levels in the estuary. As for early autumn, marked low-oxygen conditions were present both in the surface and bottom waters. A large area affected by low oxygen (~ 4,450 km2) was found in September 2006, where the low-oxygen conditions were comparable to the most severe ones observed in summer and formed by distinct mechanisms. Our analysis also reveals an apparent expansion of the summertime low-oxygen conditions at the bottom of the PRE since the years around 2000, coincident with the major environment changes in the Pearl River region. Overall, the PRE seems to be undergoing a transition from a system characterized by episodic, small-scale hypoxic events to a system with seasonal, estuary-wide hypoxic conditions. Although exacerbated eutrophication associated with anthropogenic nutrient inputs was generally considered the primary cause for the deterioration of low-oxygen conditions in the PRE, the sharp decline in sediment load may play an important role as well via increasing water transparency and thereby supporting higher and broader phytoplankton biomass in the estuary.

Many estuaries have undergone severe saltwater intrusion in addition to simultaneously experiencing serious heavy metal pollution. To explore the effect of water density stratification associated with saltwater intrusion on the behaviour of heavy metals (Cr, Co, Ni, Cu, Zn, As, Pb, and Cd) in water and sediments, a field survey was conducted in a typical estuary (Modaomen). The content, distribution, and mobility of heavy metals were investigated, as well as the influence of environmental factors on their future. The results showed that Modaomen estuary was characterised by a notable variation in salinity along the estuary, presenting total freshwater upstream, high salinity stratification water in the mouth, and saltwater offshore. Dissolved metals presented a prominent gradient vertically, with 1.2-2.1 times higher in bottom water than in surface water and the highest contents in the highly-stratified bottom water. Elevated salinity and restricted mixing induced by water stratification were likely the causes of this outcome. The distribution of heavy metals in sediments was greatly governed by grain size, Fe/Mn (hydr)oxides, total organic carbon, salinity, and dissolved oxygen. Comprehensive evaluation, combined with total contents and chemical fractions of heavy metals, indicated that internal release from sediments contributed a considerable part to the higher levels of heavy metals in bottom water, particularly for Zn and Pb, which was fully consistent with their status in water body, and elevated salinity and lack of oxygen were likely the primary driving factors. During the phase-partition processes between bottom water and sediments, partitioning coefficients were markedly lower in the highly stratified zone, implying that saltwater intrusion facilitated the mobility and repartitioning processes of metals. Because of increased levels and toxicity of heavy metals in water and extended residence time during saltwater intrusion, the potential damage to the estuarine ecosystem should receive more attention.

A validated hydrodynamic-biogeochemical model was applied to investigate the effects of physical forcing (i.e., river discharge, winds, and tides) on the summertime dissolved oxygen (DO) dynamics and hypoxia (DO &lt, 3 mg L&minus, 1) in the Pearl River estuary (PRE), based on a suite of model sensitivity experiments. Compared with the base model run in 2006 (a wet year), the simulated hypoxic area in the moderate year (with 75% of river discharge of the base run) and the dry year scenario (with 50% of river discharge of the base run) was reduced by ~30% and ~60%, respectively. This is because under the lower river discharge levels, less particulate organic matter was delivered to the estuary that subsequently alleviated the oxygen demand at the water&ndash, sediment interface, and in the meantime, the water stratification strength was decreased, which facilitated the vertical diffusion of DO. Regarding the effect of winds, the highly varying and intermittent strong winds had a significant impact on the replenishment of bottom DO by disrupting water stratification and thus inhibiting the development of hypoxia. Sensitivity experiments showed that the hypoxic area and volume were both remarkably increased in the low wind scenario (with a bottom hypoxic zone extending from the Modaomen sub-estuary to the western shoal in Lingdingyang Bay), whereas hypoxia was almost absent in the strong wind scenario. The DO budget indicated that winds altered the bottom DO mostly by affecting the DO flux due to vertical diffusion and horizontal advection, and had a limited influence on the DO consumption processes. Moreover, the DO concentration exhibited remarkable fluctuations over the spring-neap tidal cycles due to the significant differences in vertical diffusion. The results of a tide-sensitivity experiment indicated that without tide forcing, most of the shallow areas (average water depth &lt, 5 m) in the PRE experienced severe and persistent hypoxia. The tides mainly enhanced mixing in the shallow areas, which led to higher vertical diffusion and enhanced replenishment of bottom DO.

Human activities associated with estuaries have already affected estuarine carbon cycles and flows. Especially with rapid development and urbanization, widespread wastewater treatment plants in watersheds have altered the amounts and proportions of riverine nutrients and organic carbon, which has strongly impacted carbon dynamics and budgets in coastal oceans. In this study, a well-validated carbon cycle model was used to reveal the response of carbon dynamics to reasonable estimated disturbance to riverine inputs (i.e., nutrients, dissolved organic carbon (DOC) and particulate organic carbon (POC)) in the Pearl River Estuary (PRE). Scenario results demonstrate the distinct response patterns and spatial variability of carbon dynamics. Specifically, with phytoplankton blooms and biological production closely linked to riverine nutrients, riverine nutrient alterations mainly impact photosynthesis in the PRE, especially in the lower part of the PRE. As DOC fuels bacterial respiration in the water column, heterotrophic respiration is particularly sensitive to riverine DOC disturbances. POC not only relates to light penetration in turbid estuarine waters, which impacts photosynthesis, but also undergoes deposition, which affects the biochemical reactions in the sediment. Thus, perturbations to riverine POC input led to significant changes in both biological production and sediment dissolved inorganic carbon (DIC) release. Multiple linear regression results reveal that both net community production and sediment DIC release flux modulate the variations of air-sea CO2 flux in the PRE, which highlights the roles of terrestrial POC input and deposition in a shallow estuary. When delivered into the PRE, a high proportion of POC tends to be deposited and contribute to sediment DIC release or carbon sequestration, while a significant fraction of riverine DOC is transported offshore to shelf regions. This study underscores the impacts of intensive human activities on carbon cycle system in coastal oceans and the importance of terrestrial organic carbon reduction and nutrient control in upstream basins.

The subsurface chlorophyll maximum (SCM) is widespread in the oligotrophic ocean and significantly contributes to primary production. One reason for the SCM formation is believed to be the rapid export of phytoplankton from surface layers, which might be caused by aggregation, faster sinking rates under nutrient limitation, or the formation of a resting stage. In this study, these three processes were included in a biological model to investigate their contributions to subsurface chlorophyll. To further identify their individual effects on SCM formation, four modeling experiments were carried out. Three used a simple approach with either (a) density-dependent aggregation, (b) accelerated sinking rate of phytoplankton, or (c) a resting stage. The other experiment combined all three approaches (a–c). A set of observations in the South China Sea was used to optimize the four experiments and compare their abilities to replicate observed values. The results of the experiments with the resting stage showed the best fit to the field observations. All experiments were able to capture major features of the chlorophyll field (e.g. surface bloom and SCM). The experiment with accelerated sinking rate failed to reproduce the observed profile of particulate organic carbon. The experiment with only aggregation predicted lower chlorophyll concentrations in summer than those measured in the field, while experiments with the resting stage reproduced more accurate chlorophyll concentrations. Formulas including the resting stage more successfully captured the timing of phytoplankton export than did those including aggregation and accelerated sinking rate. The processes of aggregation and accelerated sinking rate made small contributions to the SCM formation in the last experiment. Overall, these results show that introducing the resting stage improves SCM simulations of the South China Sea. The results of the experiment with only the resting stage showed that the resting cells shift rapidly from the surface water, encounter better nutrient conditions in nutricline layers, partially germinate into vegetative cells, become shade-adapted, and increase subsurface chlorophyll concentrations during summer, which contributes to SCM formation.

The identification and quantification of pharmaceutical and personal care products (PPCPs) in aquatic ecosystems is critical to further studies and elucidation of their fate as well as the potential threats to aquatic ecology and human health. This study used mass balances to analyse the sources, transformation, and transport of PPCPs in rivers based on the population and consumption habits of residents, the removal level of sewage treatment, the persistence and partitioning mechanisms of PPCPs, hydrological conditions, and other natural factors. Our results suggested that in an urbanized river of Guangzhou City, China, the daily consumption of PPCPs was the main reason for the variety of species and concentrations of PPCPs. Through the determination of PPCPs in the river water samples and a central composite design (CCD) methodology, the dominant elimination mechanisms of caffeine and carbamazepine from river water were photolysis and biodegradation, but that of triclosan was sorption rather than biodegradation. The mass data of 3 PPCPs were estimated and corroborated using the measured data to evaluate the accuracy of the mass balance. Finally, caffeine, carbamazepine and triclosan discharged from the Shijing River into the Pearl River accounted for 97.81%, 99.52%, and 28.00%, respectively, of the total mass of these three compounds in the surface water of Shijing River. The results suggest that photolysis are the main process of natural attenuation for selected PPCPs in surface waters of river systems, and the transfer processes of PPCPs is mainly attributed to riverine advection. In addition, the low concentration of dissolved oxygen inhibited the degradation of PPCPs in the surface water of Shijing River.

The aim of this study is to investigate the distribution, ecological risk and quantitative source apportionment of seven heavy metals in surface sediments along a partially mixed estuary (Modaomen, China). The study area was divided into three regions based on salt-freshwater mixing: a tidal river channel, weakly mixed zone and strongly mixed zone. The concentrations of Cu, Zn, Pb, Cd, Cr and Ni were significantly higher in the weakly mixed zone, while a higher Co content was found in the tidal river channel. From enrichment factor (EF) analyses, Cd was moderate to extreme severely enriched at most stations, and other heavy metals were minor enrichments. Adverse effects on aquatic biota may occur occasionally for all seven heavy metals based on the sediment quality guidelines (SQGs). The correlations between Al and heavy metals suggested that metals in sediments were mainly attributed to anthropogenic sources. Then Positive matrix factorization (PMF) and Geostatistic method were used to quantify the heavy metal sources and determine impacted regions. A mixed source from the inner Lingding Bay and West River contributed approximately 50% of the Cu, Zn, Pb, Cr and Ni. The electroplating industry accounted for about 57% of the Cd content, and the major contaminated area was concentrated at the confluence of the West River and its tributaries. The alloy industry contributed approximately 65% of the Co, most of which was near an industrial park.

We examined the occurrence and fate of pharmaceuticals and personal care products in surface water by combining laboratory experiments with numerical simulations. The degradation processes of two typical PPCPs (triclosan and carbamazepine) collected from the Shahe Stream were studied. Hydrolysis, biodegradation, and photolysis were the three major routes of triclosan (TCS) and carbamazepine (CBZ) degradation. A central composite design method was used to investigate the effects of related natural parameters (including pH, dissolved oxygen, salinity, temperature, light intensity, and humic acid) on the TCS and CBZ degradation processes in the laboratory. Our results showed that the main degradation pathway of CBZ and TCS was direct photolysis during the daytime and that the maximal biodegradation rates of CBZ and TCS occurred at 22 °C when the optimum temperature function was used. Based on our experimental results, the observed degradation of CBZ and TCS followed pseudo-first-order kinetics, and the degradation kinetic equations under the influence of multiple natural parameters were established with estimated average degradation rate constants of 1.2452E−7 s−1 and 3.1260E−5 s−1 for CBZ and TCS, respectively. The degradation rate constants were incorporated into a one-dimensional, simply integrated hydrodynamic and water quality model. The proposed numerical model was applied to depict the transportation and transformation of CBZ and TCS in surface water and was validated by observational data from the Shahe Stream. The results showed that our model reproduced the observed patterns of CBZ and TCS concentrations reasonably, with slight overestimations compared to the observed data; the relative errors between the simulated and the observed concentrations were 5.85%–6.82% for CBZ and −156.85%–−7.18% for TCS. According to our simulation, the spatial distribution of TCS in surface water was determined by biochemical degradation processes that were most affected by temperature under natural conditions; in contrast, the distribution of CBZ was largely controlled by diffusion.

In summer, the Pearl River Estuary experiences hypoxia, largely driven by the high input of freshwater with low dissolved oxygen (DO) and abundant nutrients and particulate organic carbon from the Pearl River network. In this study, we used a well-validated coupled physical-biogeochemical model to study the response of hypoxia and oxygen dynamics to variations of anthropogenic inputs (i.e. DO, nutrients, and particulate organic carbon). Model results showed that hypoxia in the Pearl River Estuary was confined to the shelf off the Modaomen sub-estuary with a hypoxic area of ~ 200 km2 mainly due to the combined effect of re-aeration and sediment oxygen demand. Numerical experiments suggested that hypoxia in the Pearl River Estuary was most sensitive to riverine inputs of particulate organic carbon , followed by DO concentrations and nutrients. Specifically, a 50 % decrease (increase) in riverine input of particulate organic carbon led to a 47 % decrease (64 % increase) in hypoxic area, with the sediment oxygen demand and water column production being the two most important processes contributing to the changes in DO concentration and hypoxic extent. Changes in the riverine inputs of DO and nutrients had little impact on the simulated hypoxia because of the buffering effects of re-aeration, i.e. the re-aeration compensated the changes in surface apparent oxygen utilization (AOU) associated with river-induced variations of oxygen source and sink processes. The Pearl River Estuary features shallow waters (with averaged depth of 10 m) where oxygen provided by the re-aeration could penetrate to bottom waters via vertical diffusion that largely offset the changes in DO contributed by other oxygen source and sink processes. This study highlights the importance of re-aeration in determining the hypoxic extent and the buffering effects of re-aeration in reducing hypoxia variability in shallow estuary.

Conservation tillage and fertilization based on soil test have been promoted by the Chinese government in recent years. Their impacts on non-point source pollution were simulated using the Soil and Water Assessment Tool (SWAT) for the Liuxi River watershed, the only one local drinking-water-supply river basin of Guangzhou. The model was calibrated through comparing model outputs with observations to ensure reliable hydrologic, crop yield, and water quality simulations. The model results indicated that, 5% of total nitrogen (TN) load and 12% of total phosphorus (TP) load could be reduced at the watershed outlet by implementing the conservation tillage system of “rice parachute transplanting with no-tillage and straw mulching” for paddy rice fields which account for 9% of the watershed area. It is valuable to develop conservation tillage systems suitable for other types of agricultural land in southern China. When modeling the impacts of various fertilization levels derived from recommended fertilizer doses for medium fertility soil of the Pearl River Delta region where the watershed is located, the total crop yield was estimated to decrease by only 2% for up to 24% and 28% reduction in TN and TP loads at the watershed scale, respectively. The crop yields versus nutrient losses relationship simulated by SWAT indicated that great effects in TN and TP loads reduction could be achieved at the expense of minimal impacts on crop yield through optimal fertilization. The modeling approach presented in this study can be a useful tool for estimating the effects of policy instruments and pollution control measures.

[1] Every summer, a large area (15,000 km2 on average) over the Texas–Louisiana shelf in the northern Gulf of Mexico turns hypoxic due to decay of organic matter that is primarily derived from nutrient inputs from the Mississippi/Atchafalaya River System. Interannual variability in the size of the hypoxic zone is large. The 2008 Action Plan put forth by the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, an alliance of multiple state and federal agencies and tribes, calls for a reduction of the size of the hypoxic zone through nutrient management in the watershed. Comprehensive models help build mechanistic understanding of the processes underlying hypoxia formation and variability and are thus indispensable tools for devising efficient nutrient reduction strategies and for building reasonable expectations as to what responses can be expected for a given nutrient reduction. Here we present such a model, evaluate its hypoxia simulations against monitoring observations, and assess the sensitivity of the hypoxia simulations to model resolution, variations in sediment oxygen consumption, and choice of physical horizontal boundary conditions. We find that hypoxia simulations on the shelf are very sensitive to the parameterization of sediment oxygen consumption, a result of the fact that hypoxic conditions are restricted to a relatively thin layer above the bottom over most of the shelf. We show that the strength of vertical stratification is an important predictor of dissolved oxygen concentration in bottom waters and that modification of physical horizontal boundary conditions can have a large effect on hypoxia simulations because it can affect stratification strength.

The continental shelf of the northern Gulf of Mexico receives high dissolved inorganic nitrogen and phosphorus loads from the Mississippi and Atchafalaya rivers. The nutrient load results in high primary production in the river plumes and contributes to the development of hypoxia on the Texas-Louisiana shelf in summer. While phytoplankton growth is considered to be typically nitrogen-limited, phosphorus limitation has been observed in this region during periods of peak river discharge in spring and early summer. Here we investigate the presence, spatio-temporal distribution and implications of phosphorus limitation in the plume region using a circulation model of the northern Gulf of Mexico coupled to a multi-nutrient ecosystem model. Results from a 7 yr simulation (2001–2007) compare well with available observations and suggest that phosphorus limitation develops every year between the Mississippi and Atchafalaya deltas. Model simulations show that phosphorus limitation results in a delay and westward shift of a fraction of river-stimulated primary production. The consequence is a reduced flux of particulate organic matter to the sediment near the Mississippi delta, but enhanced fluxes westward in the Atchafalaya and far-field regions. Two discharge scenarios with altered river phosphate concentrations (±50 %) reveal a significant variation (±40 % in July) in the spatial extent of phosphorus limitation with changes in phosphate load.

A multivariate sequential data assimilation approach, the Localized Ensemble Kalman Filter (LEnKF), was used to assimilate daily satellite observations of ocean chlorophyll into a three-dimensional physical–biological model of the Middle Atlantic Bight (MAB) for the year 2006. Covariance localization was applied to make the EnKF analysis more effective by removing spurious long-range correlations in the ensemble approximation of the model's covariance. The model is based on the Regional Ocean Modeling System (ROMS) and coupled to a biological nitrogen cycle model, which includes seven state variables: chlorophyll, phytoplankton, nitrate, ammonium, small and large detrital nitrogen, and zooplankton. An ensemble of 20 model simulations, generated by perturbing the biological parameters according to assumed probability distributions, was used. Model fields of chlorophyll, phytoplankton, nitrate and zooplankton were updated at all vertical layers during LEnKF analysis steps, based on their cross-correlations with surface chlorophyll (the observed variable). The performance of the LEnKF scheme, its influence on the model's predictive skill and on surface particulate organic matter concentrations and primary production are investigated. Estimates of surface chlorophyll and particulate organic carbon are improved in the data-assimilative simulation when compared to one without any assimilation, as is the model's predictive skill.

The South China Sea is an oligotrophic marginal sea located in the tropical-subtropical Northwestern Pacific Ocean. Under the influences of monsoon winds, both the physical and biogeochemical processes exhibit distinct seasonal variability in the upper waters. In order to study the seasonal variations of surface phytoplankton, a one-dimensional coupled physical-biogeochemical model was developed and applied to the deep basins of the Northern South China Sea, away from the coastal upwelling regions. Forced under real-time surface monsoon winds and heat flux, the model reproduced the mixed layer depth, sea surface temperature and surface chlorophyll-a compared with satellite observations and previous reported values. In seasonal mean, the mixed layer depth was highest in winter (similar to 61.62 m) and lowest in spring (similar to 12.07 m). The sea surface temperature was lowest (similar to 25.05 degrees C) in winter and highest (similar to 29.20 degrees C) in summer. Furthermore, conspicuous phytoplankton blooms occurred in winter with the highest chlorophyll- a concentration up to similar to 0.21 mg m(-3). In other seasons, the concentration remained relatively low, especially in summer (similar to 0.05 mg m(-3)). The spatial distributions of phytoplankton were closely related with patterns of surface nutrient availability, as well as mixed layer depth and sea surface temperature. These relationships indicate that surface phytoplankton primary production was mainly controlled by nutrient availability, which was dominated by vertical turbulent diffusion in the deep basin of the northern Sea which is away from the coastal upwelling regions. Overall, our model results indicated that the seasonal variability of surface phytoplankton was modulated by coupled effects of physical and biogeochemical processes in the Northern South China Sea.

A coupled physical and sediment transport model was used to study the mass flux budgets of water and suspended sediments in the Pearl River Delta (PRD). The coupled model incorporates the Pearl River network, the Pearl River Estuary (PRE) and adjacent coastal waters in one overall modeling system. The results indicate that the river network and the PRE both have pronounced temporal and spatial variability in water and sediment fluxes, in hydrodynamic features and in sediment depositional patterns. In the river network, the riverine fluxes of water and suspended sediments are dominated by the West River, and those that are exported to the PRE (defined as the estuarine fluxes) are primarily contributed by Modaomen. The river outlets are highly responsive to the main tributaries in terms of water and sediment fluxes, revealing a close coupling between the upstream and the downstream boundaries. Most of the annual riverine and estuarine fluxes occur in the wet season, approximately 74% of the water flux and riverine and estuarine fluxes of suspended sediments of 94% and 87%, respectively. Although the water and sediment transport is dominated by river discharge, the tides are also an important factor, especially in regulating the structures of seasonal deposits in the river network (deposition in the wet season and erosion in the dry season). In the PRE, various types of physical forcing, including river discharge, monsoon winds, tides, coastal currents and the gravitational circulation associated with a density gradient, operate in concert to control the water and sediment transport in the estuary. Most of the oceanic fluxes of water and suspended sediments entering the South China Sea take place in the dry season and are primarily conveyed by strong western coastal currents. The PRE is a sedimentary system characterized by intricate depositional structures in space and time. Several depositional patterns and the associated driving mechanisms were identified. A fan-shaped deposition zone, the most intense deposition belt in the PRE, was found in the outer Modaomen Bay, where hypoxia has been reported. This work provides a basis for subsequent water quality applications in the PRD, including studies of hypoxia, eutrophication and maximum turbidity.

Over recent years, accelerated anthropogenic nutrient discharges have exerted great pressure on the water quality management in the Pearl River Delta (PRD), China. There is a concern about the eutrophication processes and hypoxia in this region. A better understanding of the origins and transport of nutrients is required before accurate prediction of impacts of nutrients on eutrophication and hypoxia in the PRD can be anticipated. Therefore a coupled physical–biological model is developed to simulate the fluxes and transformations of nutrients in the PRD. The coupled model combines a one-dimensional model for the river network (called the RNPRD) and a three-dimensional model for the Pearl River Estuary (PRE), which are both physical–biological models. The model is calibrated and validated to different sets of field data. The model results of water surface elevation, discharges, salinity, suspended sediment and water quality variables are in reasonable agreement with the observational data, suggesting that the model is robust enough to capture the physical and biogeochemical dynamics in the PRD. Also, the fluxes and transformations of carbonaceous biochemical oxygen demand (CBOD), ammonia nitrogen (NH3), nitrate plus nitrite nitrogen (NO23) and inorganic phosphorus (IP) in July 1999 (wet season) are explored and discussed. Results show that the RNPRD act as a source for NO23, but a sink for CBOD, NH3 and IP that consumes 50%, 37% and 11% of their external loads, respectively. The riverine fluxes of nutrients exported from the RNPRD to the PRE are generally controlled by high river discharge and significantly contributed by upstream inputs. The riverine fluxes are the largest inputs of nutrients to the PRE. The PRE also behaves as a source for NO23, but a sink for CBOD, NH3 and IP that consumes 90%, 80% and 16% of their external loads, respectively. The estuarine fluxes of nutrients exported from the PRE to the South China Sea are significantly contributed by the external and internal sources of nutrients in the PRE. In the RNPRD, the transformations of CBOD, NH3 (also NO23) and IP are dominated by carbonaceous oxidation, nitrification and deposition, respectively. Regarding the PRE, carbonaceous oxidation, nitrification and phytoplankton uptake are identified as the dominant processes with respect to CBOD, NH3 (also NO23) and IP. Unlike the RNPRD, the phytoplankton dynamics and internal sources of nutrients play an important role in the nutrient budgets in the PRE. Also, seasonal variations of the nutrient budgets in the PRD are discussed. Model results indicate that the dry season and wet season have a similar feature in terms of transformations of nutrients, but show significant seasonal variations in terms of nutrient fluxes. At the same time, the PRE is compared to the Changjiang and Mississippi Rivers with regard to differences in nutrient inputs between these similar river-dominated systems.