Riverine Nutrient Fluxes Research Articles

Climate change dominates the interannual variation of carbon export efficiency in each season in the Northern Gulf of Mexico during 1998–2011

Carbon export efficiency is a key indicator of the capacity of biological pump, but the controlling mechanism of the efficiency remains unclear. Our findings revealed that interannual variations in seasonal carbon export efficiency are determined by direct factors including riverine nutrient fluxes, stratification, residence time. These direct factors are finally attributed to two indirect factors (human activities and climate change). We quantified the absolute contributions of direct and indirect factors to carbon export efficiency. The results showed that the carbon export efficiency in the northern Gulf of Mexico in spring (summer; autumn; winter) was driven by human activities, which accounted for an absolute contribution of 16.02% (7.20%; 4.00%; 8.49%, respectively) through riverine nutrient fluxes, and by climate change, which accounted for an absolute contribution of 33.51% (21.43%; 25.73%; 15.80%, respectively) through stratification and water residence time. Moreover, carbon export efficiency could be predicted by MEI of 8 months earlier.

Historic changes in nutrient fluxes from the Yangtze River to the sea: Recent response to catchment regulation and potential linkage to maritime red tides

Riverine nutrient fluxes are of great implications for marine ecosystems. However, there is a lack of knowledge on the source-to-sink process of nutrient fluxes, particularly the total phosphorus (TP) and total nitrogen (TN), in the Yangtze River and its adjacent sea. Thus, this study provides historic changes in riverine nutrient fluxes to the sea over the past ∼70 years, and tries to figure out their response to catchment-related factors and potential ecological influence on the sea during the past two decades. It is found that the process of the nutrient flux to the sea over the past ∼70 years can be divided into three stages as a low-level stage before 1980, a rapid developing stage after reform and opening-up, and a final stage under government regulation. Based on our detailed measurements in the recent two decades (2004–2020), the increasing trends of the riverine nutrient fluxes have slowed down, and the TP and TN fluxes averaged 0.09 ± 0.02 and 1.61 ± 0.23 mt/yr, respectively 12.2 % and 4.8 % smaller than the previous five-year (1999–2003) averages. The seasonal characteristics of the nutrient fluxes primarily reflected the controlling effect of the Yangtze River water discharge, and the overall interannual characteristics of the nutrient fluxes were related to the fertilizer use in the catchment. Over the past two decades, the annual frequency and area of red tides occurring in the East China Sea averaged ∼38 red tides and 5120 km2, respectively. The major red tides (single area >100 km2) mainly occurred between March and September, and peaked in May (50.0 % and 57.9 % of the total frequency and area, respectively). The intensity of red tides had a relatively decent reaction to the nutrient fluxes in May, but also exhibited some uncertainties affected by some other important influencing factors. This study supplies a more continuous, accurate and detailed field-measured dataset of TP and TN fluxes from the Yangtze River to the ocean, and assists in exploring the potential linkage between the maritime red tides and riverine nutrient fluxes.

Biogeochemistry-ecosystem-social interactions on the Chinese continental margins

Chinese continental margins are experiencing remarkable environmental changes driven by anthropogenic activities and climate change. As an important habitat and sea-based fish farming resource in China, the Bohai was selected as a case study to understand how ecosystems and social interactions are influenced by multi-stressors. The Bohai ecosystem has been considerably modified. The Bohai coastline has been significantly changed (e.g., total sea area decreased and morphology changed) by sea reclamation and riverine sediment transport related to agriculture and erosion. Therefore, the strict reclamation policy and “Grain-for-Green” program have been issued to protect the coastline. Nutrient concentrations and composition have been changed by fertilizer application, wastewater discharge, and variations in seasonal patterns of riverine nutrient fluxes. Hence, pollution control and prevention are necessary. More seriously, fishing resources have been altered, as a result of environmental changes and overfishing. Therefore, a summer fishing ban and stock enhancement should be vital besides environmental improvement. This study can help to predict and mitigate impacts on global continental margins that are experiencing similar environmental stress.

Seasonal and Spatial Variability of Dissolved Nutrients in the Yenisei River

The accelerated rates of warming in high latitudes lead to permafrost degradation, enhance nutrient cycling and intensify the transport of terrestrial materials to the Arctic rivers. The quantitative estimation of riverine nutrient flux on seasonal and spatial scales is important to clarify the ongoing changes in land–ocean connectivity in the Arctic domain. This study is focused on a multiyear (2015–2021) analysis of concentrations of dissolved nutrients in the Yenisei River. Applying stationary water sampling, we studied seasonal variations in concentrations of phosphate, nitrate, nitrite and ammonia ions in the Yenisei River in the upper (56.0° N), middle (60.9° N) and lower (67.4° N) sections of the river. The waters of the upper river section demonstrated significant and steady nutrient enrichment throughout the hydrological year, reflecting the influence of the Krasnoyarsk reservoir. The downstream reaches of the Yenisei River had more apparent seasonal patterns of nutrient concentrations. Particularly, winter-season nutrient levels in the middle and lower river sections were the highest during the hydrological year and close to the upper section. At snowmelt, and especially the summer–fall seasons, all inorganic nutrient concentrations dropped dramatically after the inflow of the Angara River. On the other hand, the peak nitrite content observed during the early spring flood was most pronounced in the lower section of the river basin, reflecting the specific characteristics of the nitrogen cycle in permafrost soils. The spring flood plays the major role in the annual nutrient fluxes, except for nitrates, for which the maximum occurred in the winter season. The summer–fall season, despite its duration and considerable water runoff, demonstrated the lowest fluxes of dissolved inorganic phosphorus and nitrogen in comparison to other periods of the hydrological year, suggesting strong biological uptake and chemostasis.

Differential Response of Nutrients to Seasonal Hydrological Changes and a Rain Event in a Subtropical Watershed, Southeast China

A large amount of terrestrial nutrients are discharged into the ocean through rivers. However, the impact of seasonal hydrological variations on riverine nutrient concentrations and fluxes remains unclear, especially in the medium-sized subtropical rivers that are highly influenced by human activities. In this study, we investigated the monthly changes in nutrient concentrations (soluble reactive phosphorus, SRP; dissolved silicate, DSi; and dissolved inorganic nitrogen, DIN) in the North Stream (NS) and West Stream (WS) of the Jiulong River (JLR). The results show that the concentrations of SRP and DSi in the NS and the WS displayed a similar seasonal variability, which was different from the pattern of DIN. Hydrological conditions, chemical fertilizer loss and biogeochemical processes are responsible for the seasonal changes in the nutrients in the two streams, especially during extreme rain events. Nutrient concentrations in the NS exhibited a clockwise trajectory along with river discharge during rain events, while a reverse pattern in the WS was observed since it experienced a moderately long rain event. Different rainfall features between the two main tributaries resulted in the majority of nutrients being exported at the start of the rain event in the NS and the end of rain event in the WS. Indeed, the annual high flow (Q/Qm > 3) accounts for ~17.3% of the annual nutrient flux in the JLR even though this period spans only ~4.0% of a year, which suggests the importance of rain events on nutrient export in these subtropical rivers. Although the annual fluxes of DIN and SRP in the JLR were smaller than many rivers worldwide, higher areal yields of DIN and SRP were observed, indicating that the JLR is highly influenced by human activities. Our study systematically evaluated the response of nutrient concentrations to hydrological changes in two tributaries of the JLR, which is useful in better understanding the nutrient dynamics in medium-sized subtropical rivers.

Evaluating Uncertainties in Reconstructing the Pre-eutrophic State of the North Sea

The North Sea is affected by eutrophication problems despite the decreasing riverine nutrient fluxes since the late 1980s. Formally, assessment of the eutrophication state of European marine environments is based on their historical state. Model estimates are increasingly used to support monitoring data that often do not encompass such pre-eutrophic conditions. However, various sources of uncertainties emerge when producing these estimates. In this study, we systematically quantify various sources of uncertainties in terms of variability, and assess their importance for the North Sea. For the reconstruction of the historical state, we use two coupled physical-biogeochemical model systems: ECOHAM on a 20-km grid for the European shelf and GPM on a high-resolution (1.5–4.5 km) grid for the Southern North Sea. To gain insights into the impacts due to the uncertainty in riverine loadings, we consider the historical nutrient inputs from two alternative watershed-models (MONERIS and E-HYPE). Overall, the modeled historic state based on E-HYPE shows higher nutrient concentrations compared to the state based on MONERIS, especially in the coastal regions. Assessing the degree of methodological uncertainties by an inter-comparison of different sources and against natural variabilities provides insight into the reliability of the model-based reconstruction of the historical state. We find that in regions influenced by freshwater from major rivers uncertainties owed to riverine loading scenarios exceed the natural sources of variability. For the offshore regions, natural sources of variability dominate over those caused by model- and scenario-related uncertainties. These findings are expected to assist decision makers and researchers in gaining insight into the degree of confidence in evaluating the model results, and prioritizing the need for refinement of models and scenarios for the production of reliable projections.

Driving mechanisms of organic carbon burial in the Early Cretaceous South Atlantic Cape Basin (DSDP Site 361)

Abstract. Extensive black shale deposits formed in the Early Cretaceous South Atlantic, supporting the notion that this emerging ocean basin was a globally important site of organic carbon burial. The magnitude of organic carbon burial in marine basins is known to be controlled by various tectonic, oceanographic, hydrological, and climatic processes acting on different temporal and spatial scales, the nature and relative importance of which are poorly understood for the young South Atlantic. Here we present new bulk and molecular geochemical data from an Aptian–Albian sediment record recovered from the deep Cape Basin at Deep Sea Drilling Project (DSDP) Site 361, which we combine with general circulation model results to identify driving mechanisms of organic carbon burial. A multimillion-year decrease (i.e., Early Aptian–Albian) in organic carbon burial, reflected in a lithological succession of black shale, gray shale, and red beds, was caused by increasing bottom water oxygenation due to abating hydrographic restriction via South Atlantic–Southern Ocean gateways. These results emphasize basin evolution and ocean gateway development as a decisive primary control on enhanced organic carbon preservation in the Cape Basin at geological timescales (> 1 Myr). The Early Aptian black shale sequence comprises alternations of shales with high (> 6 %) and relatively low (∼ 3.5 %) organic carbon content of marine sources, the former being deposited during the global Oceanic Anoxic Event (OAE) 1a, as well as during repetitive intervals before and after OAE 1a. In all cases, these short-term intervals of enhanced organic carbon burial coincided with strong influxes of sediments derived from the proximal African continent, indicating closely coupled climate–land–ocean interactions. Supported by our model results, we show that fluctuations in weathering-derived nutrient input from the southern African continent, linked to changes in orbitally driven humidity and aridity, were the underlying drivers of repetitive episodes of enhanced organic carbon burial in the deep Cape Basin. These results suggest that deep marine environments of emerging ocean basins responded sensitively and directly to short-term fluctuations in riverine nutrient fluxes. We explain this relationship using the lack of wide and mature continental shelf seas that could have acted as a barrier or filter for nutrient transfer from the continent into the deep ocean.

Global Dam‐Driven Changes to Riverine N:P:Si Ratios Delivered to the Coastal Ocean

AbstractRiver damming alters nutrient fluxes along the land‐ocean aquatic continuum as a result of biogeochemical processes in reservoirs. Both the changes in riverine nutrient fluxes and nutrient ratios impact ecosystem functioning of receiving water bodies. We utilize spatially distributed mechanistic models of nitrogen (N), phosphorus (P), and silicon (Si) cycling in reservoirs to quantify changes in nutrient stoichiometry of river discharge to coastal waters. The results demonstrate that the growing number of dams decouples the riverine fluxes of N, P, and Si. Worldwide, preferential removal of P over N in reservoirs increases N:P ratios delivered to the ocean, raising the potential for P limitation of coastal productivity. By midcentury, more than half of the rivers discharging to the coastal zone will experience a higher removal of reactive Si relative to reactive P and total N, in response to the rapid pace at which new hydroelectric dams are being built.

Spatiotemporal river flux and composition of nutrients affecting adjacent coastal water quality in Hainan Island, China

Human activities have changed global nutrients cycling and appreciably altered marine ecosystems in riverine-estuarine systems. This is evident by the significant shifts in the supply of nitrogen and phosphorus. However, very little is known about the concentrations and fluxes and of these nutrients and their seasonal changes in Hainan Island, China. We examined Hainan Island, which is the second largest island in China, to assess the spatiotemporal concentrations and fluxes of nutrients using seasonal data on a whole-of-system scale covering river estuaries and adjacent coastal water (2017). The annual average total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) concentrations were 77.23 ± 31.36 μmol/L and 3.25 ± 1.67 μmol/L, respectively. The annual fluxes of pollutants from the discharge of coastal rivers on Hainan Island into coastal waters were 1.8 × 104 t for TDN and 1.1 × 103 t for TDP, respectively. The month with the largest contribution to the annual pollutants load was October, which contributed 13.60% of TDN and 17.79% of TDP. In addition, the largest river fluxes were Nandu River, which contributed 7.5 × 103 t of TDN and 364 t of TDP. The annual average concentrations of pollutants in the surface coastal water were 7.60 ± 10.71 μmol/L for dissolved inorganic nitrogen (DIN) and 0.24 ± 0.31 μmol/L for dissolved inorganic phosphorus (DIP). The annual average N/P ratios in coastal rivers and seawater were 30 and 65, which were significantly higher than the Redfield ratio (N/P = 16). The DIN and DIP concentrations of most monitoring stations met the national first-class seawater standards, but the problem of nutrient pollution in some local coastal water areas was still outstanding. The coastal water quality monitoring should consider the influence of tropical storms influence on Hainan Island. The effective improvement of coastal water quality should be based on land–ocean coordination monitoring of coastal water quality and the riverine nutrient flux of Hainan Island.

High-frequency measured turbidity as a surrogate for phosphorus in boreal zone rivers: appropriate options and critical situations

In situ high-frequency measured turbidity can potentially be used as a surrogate for riverine phosphorus (P) concentrations to better justify the effectiveness of nutrient loss mitigation measures at agricultural sites. We explore the possibilities of using turbidity as a surrogate for total phosphorus (TP) and particulate phosphorus (PP) in four snowmelt-driven rivers draining agricultural clayey catchments. Our results suggest slightly stronger relationship between in situ measured turbidity and PP than between turbidity and TP. Overall, linear TP and PP regressions showed better error statistics in the larger catchments compared with their sub-catchments. Local calibration of the in situ sensors was sensitive to the number of high P concentration discrete water samples. Two optional calibration curves, one with and one without influential data, resulted in a 17% difference in the estimated mean TP concentrations of a snowmelt storm contributing 18% of the annual discharge volume. Accordingly, the error related to monthly mean TP estimates was the largest in spring months at all sites. The addition of total dissolved phosphorus (TDP) improved the model performance, especially for sites where the TDP/TP ratio is large and highly variable over time. We demonstrate how long-term discrete samples beyond sensor deployment can be utilized in the evaluation of the applicability range of the local calibration. We recommend analysing the validity of P concentration estimates, especially during high discharge episodes that contribute substantially to annual riverine nutrient fluxes, since the use of surrogates may introduce large differences into the P concentration estimates based on selected local calibration curves.

Submarine groundwater discharge and associated nutrient and carbon inputs into Sydney Harbour (Australia)

The effects of urbanization and scales on submarine groundwater discharge (SGD) remain poorly understood. Here, we used radium isotopes to quantify SGD-derived fluxes of nitrogen, phosphorus and dissolved organic carbon (DOC) into Sydney Harbour estuary, Australia. Sydney is the most populated city of Oceania, with several localised cases of historical groundwater pollution. We sampled top and bottom waters at the harbour scale (~20 km) and also at four small scale embayments (~2 km). A decreasing gradient in radium isotope concentrations from upstream to downstream was observed. Mass balances constructed with 224Ra, 223Ra, and 226Ra revealed that total SGD ranged from 42 to 121 × 104 m3 d−1 depending on assumptions and isotope. These fluxes were related mostly to saline SGD (recirculated seawater), and are equivalent to >60 times the mean annual freshwater river discharge into Sydney Harbour (0.68 × 104 m3 d−1). The estimated SGD rates (2.2 ± 1.5 cm d−1) were comparable to the global average radium-derived-SGD in other urban estuaries (~3.1 cm d−1). No obvious relationships were observed between SGD and scale in Sydney Harbour. However, higher SGD rates estimated for embayments closer to the ocean indicate that a combination of waves, tides and urbanization control SGD. SGD derived fluxes exceeded maximum riverine nutrient fluxes by a factor of 2 for DOC, 6 for PO43−, 40 for NH4+ and 1.3 for NOX. Previous work has suggested that nutrients enter Sydney Harbour primarily through rivers or stormwater following episodic rain events. Our results imply that diffuse saline SGD can also be an important but overlooked source of nutrients, potentially sustaining primary productivity in times of no river flow.

Riverine nutrient fluxes and environmental effects on China's estuaries

An increase in riverine nutrient fluxes significantly influences the estuarine ecosystem. This study collected nutrient data in most of China's rivers from 1963 to 2015 to estimate the nutrient fluxes from major rivers and analyze interannual variability of nutrient fluxes and estuarine environmental effects. The results showed that the nutrient fluxes from the Yangtze River increased annually from 1963 to 2012. The trend of nutrient fluxes from the Yellow River was consistent with that from the Jiulong River, i.e., nutrient fluxes increased from 1998 to 2007 and then decreased. The areal nutrient fluxes from China's major rivers were higher than those from major world rivers, while the areal nutrient yield rates per capita were lower than those from major world rivers. We also found that China's estuaries were predominantly phosphorus-limited and slowly moving toward lower dissolved silica over dissolved inorganic nitrogen (DSi:DIN) ratios with time. Meanwhile, the nutrient limitation of phytoplankton growth in most of China's estuary systems was moving toward a higher incidence of phosphorus and silicon limitations as a result of increased DIN fluxes, and this would likely alter phytoplankton communities. Furthermore, the decreases in the DSi:DIN ratio and dissolved silica over dissolved inorganic phosphate (DSi:DIP) ratio, and the increases in both DIN and DIP fluxes, caused increased red tide blooms.

Advances in NANI and NAPI accounting for the Baltic drainage basin: spatial and temporal trends and relationships to watershed TN and TP fluxes

In order to assess the progress toward eutrophication management goals, it is important to understand trends in land-based nutrient use. Here we present net anthropogenic nitrogen and phosphorus inputs (NANI and NAPI, respectively) for 2000 and 2010 for the Baltic Sea watershed. Overall, across the entire Baltic, between the 5-year periods centered on 2000 and 2010, NANI and NAPI decreased modestly by −6 and −4%, respectively, but with substantial regional variation, including major increases in the Gulf of Riga drainage basin (+19 and +58%, respectively) and decreases in the Danish Straits drainage basin (−25 and −40% respectively). The changes were due primarily to changes in mineral fertilizer use. Mineral fertilizers dominated inputs, at 57% of both NANI and NAPI in 2000, increasing to 68 and 70%, respectively, by 2010. Net food and feed imports declined over that period, corresponding to increased crop production; either fewer imports of food and feedstocks were required to feed humans and livestock, or more of these commodities were exported. A strong linear relationship exists between regional net nutrient inputs and riverine nutrient fluxes for both periods. About 17% of NANI and 4.7% of NAPI were exported to the sea in 2000; these relationships did not significantly differ from those for 2010. Changes in NANI from 2000 to 2010 across basins were directly proportional rather than linearly related to changes in total N (TN) fluxes to the sea (i.e., no change in NANI suggests no change in TN flux). Similarly, for all basins except those draining to the Baltic Proper, changes in NAPI were proportional to changes in total P (TP) fluxes. The Danish Straits decreased most between 2000 and 2010, where NANI and NAPI declined by 25 and 40%, respectively, and corresponding fluxes of TN and TP declined 31 and 18%, respectively. For the Baltic Proper, NAPI was relatively unchanged between 2000 and 2010, while riverine TP fluxes decreased 25%, due possibly to lagged effects of fertilizer reduction resulting from socio-political changes in the early 1990s or improvements in sewage treatment capabilities. For most regions, further reductions in NANI and NAPI could be achieved by more efficient production and greater substitution of manure for imported mineral fertilizers.

Estuarine circulation-driven entrainment of oceanic nutrients fuels coastal phytoplankton in an open coastal system in Japan

We investigated interactions among seasonal fluctuations in phytoplankton biomass, riverine nutrient flux, and the fluxes of nutrients entrained by estuarine circulation in Tango Bay, Japan, to determine the influence of freshwater inflows to an open bay on coastal phytoplankton productivity. The riverine nutrient flux was strongly regulated by river discharge. Estuarine circulation was driven by river discharge, with high fluxes of nutrients (mean nitrate + nitrite flux: 5.3 ± 3.5 Mg [mega grams]-N day−1) between winter and early spring, enhanced by nutrient supply to the surface water via vertical mixing. In contrast, low-nutrient seawater was delivered to the bay between late spring and summer (1.0 ± 0.8 Mg-N day−1). Seasonal fluctuations in phytoplankton biomass were affected by the entrained fluxes of oceanic nutrients and variation in the euphotic zone depth, and to a lesser degree by the riverine nutrient flux. Bioassays and stoichiometric analyses indicated that phytoplankton growth was limited by nitrogen and/or phosphorus. Both the entrainment of oceanic nutrients and the euphotic zone depth affected the duration and magnitude of blooms. Our findings show that, unlike semi-enclosed bays, seasonal variations in coastal phytoplankton in an open coastal system are primarily fueled by the entrainment of oceanic nutrients and are influenced by both freshwater inflow and coastal conditions (e.g. vertical mixing and wind events).

Expansion of photic-zone euxinia during the Permian–Triassic biotic crisis and its causes: Microbial biomarker records

The incursion of euxinic waters into the ocean-surface layer is hypothesized to have been an important killing agent during the end-Permian mass extinction. However, both the causes and extent of oceanic euxinia during this crisis remain poorly known, making assessment of its role in the mass extinction difficult. Here, we document the distribution of aryl isoprenoids (AIs), which are biomarkers of obligate anaerobic green sulfur bacteria (Chlorobiaceae) that are indicative of photic-zone euxinia, in 12 Permian-Triassic Boundary (PTB) sections with a wide distribution globally. Profiles of AI abundance for the 12 study sections show significant spatio-temporal variation. No AIs were identified in the shallowest sections, but AIs are present both prior to and following the mass extinction in intermediate-depth sections and following the mass extinction in deep-water sections. This pattern suggests a combination of upward and oceanward expansion of photic-zone euxinia during the PTB crisis, possibly fueled by elevated riverine nutrient fluxes as a consequence of climatic warming, terrestrial ecosystem destruction, and enhanced erosion. This hypothesis is supported by the close association of AIs with elevated abundances of moretanes and dibenzofuran (DBF), which are biomarkers for terrestrial erosion. Expansion of oceanic euxinia at intermediate water depths may have established a long-term reservoir that fed episodic incursions of H2S-bearing waters into shallow-marine environments, delaying the recovery of marine ecosystems during the Early Triassic. The biomarker records of the present study thus provide significant evidence of terrestrial-marine linkage during the PTB crisis.

Rivers in the Anthropocene: Global scale modifications of riverine nutrient fluxes by damming

The damming of rivers represents one of the major anthropogenic disturbances of the natural cycles of water and nutrient elements on the continents. Currently, more than 50% of the world's stream and river flow crosses one or more dams before reaching the oceans. This fraction could climb up to 90% by 2030. The associated modifications of both the absolute and relative riverine fluxes of nutrients have far-reaching ecohydrological implications, from individual ecosystems to the global biogeosphere. While dam reservoirs usually act as sinks of macronutrients along the river continuum, their effects on riverine fluxes and chemical speciation differ markedly from one nutrient element to another. Dams can thus fundamentally alter nutrient limitation patterns and water quality in river-floodplain systems and receiving water bodies, including lakes and coastal marine areas. Here, we briefly review recent research addressing the impact of dams on riverine nutrient fluxes and stoichiometry, and identify some of the research challenges ahead.

Modeled long-term changes of DIN:DIP ratio in the Changjiang River in relation to Chl-α and DO concentrations in adjacent estuary

In the past four decades (1970–2013), nitrogen and phosphorous inputs to the Changjiang River basin, mainly from human activities, have increased 3-fold and 306-fold, respectively. The riverine nutrient fluxes to the estuary have also grown exponentially. Dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorous (DIP) fluxes of the Changjiang River increased by 338% and 574% during 1970–2013 period, and red tides and benthic hypoxia have been observed in the outflow region of the Changjiang River in the East China Sea (ECS). We assumed that time series changes in the DIN:DIP ratio from the Changjiang River could have a significant impact on Chlorophyll-α (Chl-α) concentration in the surface sea water and dissolved oxygen (DO) concentration in the bottom sea water of the Changjiang estuary. Our study showed that the DIN:DIP ratio from the Changjiang River increased from 76 to 384 between 1970 and 1985, and decreased from 255 to 149 between 1986 and 2013. The observed Chl-α concentration increased by 146% from 1992 to 2010 in the Changjiang estuary, and was negatively related to the DIN:DIP ratio in 1992–2010. Bottom sea water DO concentration decreased by 24.6% during 1992–2010 and a “low oxygen zone” (122°∼123°E, 32°∼33°N) was observed during summer since 1999. The anthropogenically enhanced nutrient inputs dominated river DIN and DIP fluxes and influenced Chl-α concentrations as well as bottom DO concentrations in the estuary. Scenarios emphasizing global collaboration and proactive environmental problem-solving may result in reductions in the river nutrient exports and in Chl-α and DO concentration in the Changjiang estuary by 2050.

Seasonal nutrient chemistry in mountainous river systems of tropical Western Peninsular India

Nutrient chemistry was studied in three mountainous rivers (Mandovi, Zuari and Netravati), across western peninsular India (WPI) during south-west monsoon (SWM), post-monsoon and pre-monsoon seasons of year 2011–2012. Nutrients in rainwater were dominated by dissolved inorganic nitrogen (DIN) and urea, dependent upon industrial emissions and the rainfall location. Nutrients in river water were enriched with DIN and Si(OH)4 as compared with during the SWM season. During pre-monsoon, the river water was enriched with dissolved organic nitrogen (DON), whereas enrichment of a particular nutrient species was not consistent during the post-monsoon season in all three studied river systems. Nutrients exports to the estuarine water were dependent upon the geomorphological feature of river catchment and on prevalent anthropogenic activities. The significant observed contribution of DON to total dissolved nitrogen in Mandovi (62%), Zuari (83%) and Netravati (69%) is important, in view of alterations in biogeochemical processes of adjacent Arabian Sea. Furthermore, the yield of nutrient such as was found to be a function of runoff per catchment area in studied river systems, is comparable with other tropical rivers of world, which highlights the importance of mountainous rivers along WPI in their contribution to the Global riverine nutrient flux.

Different contributions of riverine and oceanic nutrient fluxes supporting primary production in Ishikari Bay

We computed a ratio of riverine nutrient flux (RNF) to bottom nutrient flux (BNF) to determine the relative importance of oceanic and riverine nutrient fluxes on primary production dynamics in Ishikari Bay, which is composed of oligotrophic subarctic coastal water. Across spring, summer and autumn, the RNF:BNF ratio (R:B ratio) was significantly greater than 1.0, especially in spring and autumn for DIN and Si(OH)4, suggesting that riverine nutrients mostly supported primary production. A strong inverse relationship (r=−0.927) between Chl a and salinity in autumn and a corresponding increase in the apparent utilization of DIN and primary production indicated that the contribution of DIN from the Ishikari River on primary production was maximal in autumn. However the R:B ratio for PO4 was significantly less than 1.0, especially in summer (0.1) and autumn (0.3), suggesting a larger contribution of bottom upwelling nutrient sources. In spring, when the ratio was close to 1 (0.8), PO4 supply from both bottom (upwelling) and surface (river) was equivalent, since PO4 concentration of river end-member was the lowest. Although riverine nutrient fluxes were a major source of DIN and Si(OH)4 nutrient supply in the bay, oceanic nutrient contribution from bottom upwelling and horizontal advection was a major source of PO4. While riverine nutrients significantly fuel primary production, the estuarine circulation process may contribute significantly to compensating for the inadequate supply of riverine PO4 in an oligotrophic system like Ishikari Bay. Also, unlike the usual estuarine system in which nutrient concentration at a deeper layer is high due to the regeneration of nutrients at depth, concentration in Ishikari Bay was very low due to an influence of oligotrophic waters. We conclude that riverine nutrient flux contributes a large portion of the total flux in Ishikari Bay.

Numerical simulation of nutrient and phytoplankton dynamics in Guangxi coastal bays, China

The increasing riverine pollutants have resulted in nutrient enrichment and deterioration of water quality in the coastal water of Guangxi Province, China. However, the quantitative relationship between nutrient loads and water quality responses, which is crucial for developing eutrophication control strategies, is not well studied. In this study, the riverine fluxes of nutrients were quantified and integrated with nutrient cycling and phytoplankton dynamics by using box models for Guangxi coastal bays. The model concepts and biogeochemical equations were the same; while most model parameters were specific for each bay. The parameters were calibrated with seasonal observations during 2006–2007, and validated with yearly averaged measurements in 2009. The general features of nutrient and phytoplankton dynamics were reproduced, and the models were proved feasible under a wide range of bay conditions. Dissolved inorganic nitrogen was depleted during the spring algal bloom in Zhenzhu Bay and Fangcheng Bay with relatively less nutrient inputs. Phosphorus concentration was high in spring, which decreased then due to continuous phytoplankton consumption. Chlorophyll-a concentration reached its annual maximum in summer, but was the minimum in winter. Eutrophication was characterized by both an increase in nutrient concentrations and phytoplankton biomass in Lianzhou Bay. Either about 80% reduction of nitrogen or 70% reduction of phosphorus was required to control the algal bloom in Lianzhou Bay. Defects of the models were discussed and suggestions to the environmental protection of Guangxi coastal bays were proposed.