Maintenance of estuarine water quality by mangroves occurs during flood periods: A case study of a subtropical mangrove wetland

Seasonal changes in water quality were measured in samples taken at various distances from shallow water across mudflat to mangroves during flood period and from mangroves across mudflat to shallow water during ebb period in a subtropical mangrove estuary (Zhangjiang Estuary, Fujian, China). The TN (total dissolved nitrogen), TP (total dissolved phosphorus), COD (chemical oxygen demand), and DOC (dissolved organic carbon) contents during the flood period were significantly higher than those during the ebb period. In contrast, the opposite was true for the POC (particulate organic carbon) content and transparency. The mangroves at Zhangjiang Estuary may trap nutrients at rates of 90.5 g N/m²/yr, 2.2 g TP/m²/yr, and 13.7 g C/m²/yr in the form of DOC, and export POC at a rate of 81.8 g/m²/yr. Our results support the hypothesis that the maintenance of estuarine water quality by mangroves occurs during flood periods.     

An integrated East China Sea–Changjiang Estuary model system with aim at resolving multi-scale regional–shelf–estuarine dynamics

A high-resolution numerical model system is essential to resolve multi-scale coastal ocean dynamics. So a multi-scale unstructured grid-based finite-volume coastal ocean model (FVCOM) system has been established for the East China Sea and Changjiang Estuary (ECS–CE) with the aim at resolving coastal ocean dynamics and understanding different physical processes. The modeling system consists of a three-domain-nested weather research and forecasting model, FVCOM model with the inclusion of FVCOM surface wave model in order to understand the wave–current interactions. The ECS–CE system contains three different scale models: a shelf-scale model for the East China Sea, an estuarine-scale model for the Changjiang Estuary and adjacent region, and a fine-scale model for the deep waterway regions. These three FVCOM-based models guarantee the conservation of mass and momentum transferring from outer domain to inner domain using the one-way common-grid nesting procedure. The model system has been validated using data from various observation data, including surface wind, tides, currents, salinity, and wave to accurately reveal the multi-scale dynamics of the East China Sea and Changjiang Estuary. This modeling system has been demonstrated via application to the seasonal variations of Changjiang diluted water and the bottom saltwater intrusion in the North Passage, and it shows strong potential for estuarine and coastal ocean dynamics and operational forecasting.     

新疆博斯腾湖水体颗粒和溶解有机碳的季节变化及其来源初探

在博斯腾湖选取了13个点位,于2012年5、8、10月测定表层和底层水体中的颗粒有机碳、溶解有机碳、颗粒有机氮和叶绿素a含量.结果显示颗粒和溶解有机碳在表层水体中的浓度与底层相近.博斯腾湖水体中颗粒有机碳的季节变化十分明显,其平均浓度从春季(0.64 mg/L)到夏季(0.71 mg/L)变化不大,但在秋季变化十分显著(浓度达1.58 mg/L).其中西北湖区和湖心区颗粒有机碳的季节变化最明显,东部湖区颗粒有机碳的季节变化相对较小.博斯腾湖水体的颗粒有机碳在春、秋两季主要来自外源输入,在夏季受水体中浮游生物的影响较大.博斯腾湖水体中溶解有机碳也具有一定的季节变化,夏季浓度(平均为9.3 mg/L)略低于春、秋两季(平均为10.3 mg/L).溶解有机碳在河口区的季节变化最强,其夏季浓度明显偏低,主要是由于开都河河水的稀释作用.总体上,博斯腾湖水体中溶解有机碳浓度的变化主要受外部因素的影响.     

Seasonal changes of organic matter origins and anammox activity in the Changjiang Estuary deduced from multi-biomarkers in suspended particulates

Human activity-induced eutrophication and harmful algal blooms are main causes of the expansion of the hypoxic zone in the Changjiang Estuary. Among the many changes in biogeochemical processes, anaerobic ammonium oxidation (anammox) is proposed to play an important role in the nitrogen cycle in hypoxic areas. Ladderane lipids have been used as biomarkers to indicate anammox activity in ecosystems, but the origins of anammox bacteria and ladderanes in suspended particulates are still unclear. In this study, we report the results of a suite of biomarker analyses of suspended particulates across a salinity gradient of the Changjiang Estuary in both the spring and summer to evaluate the origins of the ladderanes and their potential as proxies for anammox activity and hypoxia. The spatio-temporal variations in terrestrial biomarkers (n-alkanes and n-alkanols), marine biomarkers (brassicasterol and dinosterol), and the Terrestrial and Marine Biomarker Ratio and Branched and Isoprenoid Tetraether indices reveal that marine organic matter was dominant in the particulates in both the spring (55%) and summer (86%) seasons. Correlations with both marine and terrestrial biomarkers suggest that ladderanes were mainly produced in the water column, and therefore that ladderane concentrations in suspended particulates in the Changjiang Estuary mainly reflect anammox activity in the water column, although changes in anammox bacterial assemblages may also have played a role in ladderane concentrations. Overall, ladderane results suggest that anammox activity was widespread in the Changjiang Estuary; but higher ladderane concentrations in the summer (especially in the upwelling zone) were correlated with lower dissolved oxygen concentrations, which suggest that they are useful proxies for hypoxia.     

Mechanisms leading to the frequent occurrences of hypoxia and a preliminary analysis of the associated acidification off the Changiiang estuary in summer

Based on literature data and shipboard observations,this study investigated the main environmental characteristics of the seafloor topography,current field,front,and upwelling that are closely related to hypoxia occurrence off the Changjiang estuary.The physical processes of the plume front and upwelling off the Changjiang estuary in summer were coupled.The vertical distribution pattern of the plume front was closely related to the upwelling.By reviewing and analyzing the historical summer hypoxia events off the Changjiang estuary,we statistically demonstrated the spatial structure of the horizontal distribution of the hypoxic zone and investigated the location of occurrence zone of the hypoxia.We found that the dissolved oxygen(DO)concentration on the inner continental shelf off the estuary showed a"V"shape in relation to station depth.Therefore,we noted that the hypoxic water on the inner continental shelf mostly occurred on the slopes with steep seafloor topography.Base on the current understanding of the hypoxic mechanisms off the Changjiang estuary,we analyzed the biogeochemical mechanisms that could cause the steep terrain off the Changjiang estuary to become the main areas prone to summer hypoxia and explained the internal relations between the location of the hypoxic zone on the slopes and the plume front and upwelling.The plume front and upwelling off the Changjiang estuary and their coupling were important driving forces of summer hypoxia.The continuous supply of nutrients affected by the interaction of the plume front extension of the Changjiang Diluted Water(CDW)and upwelling and the favorable light conditions were important mechanisms causing the phytoplankton blooms and benthic hypoxia off the Changjiang estuary in summer.By analyzing oxygen utilization,organic carbon mineralization,and nutrient regeneration in the hypoxic zone,we observed that the significant oxygen utilization process off the Changjiang estuary in summer also mainly occurred near the steep slopes with front and upwelling features and confirmed the apparent nutrient loss in the benthic hypoxic zone.Meanwhile,our analysis revealed that the sediment resuspension in the benthic boundary layer in the mud areas off the Changjiang estuary could also affect the oxygen utilization and mineralization of organic carbon and nutrient recycling and regeneration.This study also demonstrated that the steep terrain off the Changjiang estuary was the main location for summer acidification,and the coupling between the plume front and upwelling on the steep slopes was an important physical driving force inducing summer benthic acidification.Finally,we discussed issues to address in future studies of the hypoxic zone and water acidification off the Changjiang estuary.     

Mechanisms leading to the frequent occurrences of hypoxia and a preliminary analysis of the associated acidification off the Changjiang estuary in summer

Based on literature data and shipboard observations, this study investigated the main environmental characteristics of the seafloor topography, current field, front, and upwelling that are closely related to hypoxia occurrence off the Changjiang estuary. The physical processes of the plume front and upwelling off the Changjiang estuary in summer were coupled. The vertical distribution pattern of the plume front was closely related to the upwelling. By reviewing and analyzing the historical summer hypoxia events off the Changjiang estuary, we statistically demonstrated the spatial structure of the horizontal distribution of the hypoxic zone and investigated the location of occurrence zone of the hypoxia. We found that the dissolved oxygen (DO) concentration on the inner continental shelf off the estuary showed a “V” shape in relation to station depth. Therefore, we noted that the hypoxic water on the inner continental shelf mostly occurred on the slopes with steep seafloor topography. Base on the current understanding of the hypoxic mechanisms off the Changjiang estuary, we analyzed the biogeochemical mechanisms that could cause the steep terrain off the Changjiang estuary to become the main areas prone to summer hypoxia and explained the internal relations between the location of the hypoxic zone on the slopes and the plume front and upwelling. The plume front and upwelling off the Changjiang estuary and their coupling were important driving forces of summer hypoxia. The continuous supply of nutrients affected by the interaction of the plume front extension of the Changjiang Diluted Water (CDW) and upwelling and the favorable light conditions were important mechanisms causing the phytoplankton blooms and benthic hypoxia off the Changjiang estuary in summer. By analyzing oxygen utilization, organic carbon mineralization, and nutrient regeneration in the hypoxic zone, we observed that the significant oxygen utilization process off the Changjiang estuary in summer also mainly occurred near the steep slopes with front and upwelling features and confirmed the apparent nutrient loss in the benthic hypoxic zone. Meanwhile, our analysis revealed that the sediment resuspension in the benthic boundary layer in the mud areas off the Changjiang estuary could also affect the oxygen utilization and mineralization of organic carbon and nutrient recycling and regeneration. This study also demonstrated that the steep terrain off the Changjiang estuary was the main location for summer acidification, and the coupling between the plume front and upwelling on the steep slopes was an important physical driving force inducing summer benthic acidification. Finally, we discussed issues to address in future studies of the hypoxic zone and water acidification off the Changjiang estuary.     

Reevaluation of mixing among multiple water masses in the shelf: An example from the East China Sea

East China Sea (ECS) is bounded by the continent where the fourth largest river of Changjiang discharges large amounts of freshwater to the west and by the Kuroshio in the East and connected to the South China Sea via Taiwan Strait, therefore water characteristics are very complex and undergo great seasonal changes. The dominant source waters in the ECS are found to be Kuroshio Surface Water (KSW), Kuroshio Sub-surface Water (KSSW), Changjiang Diluted Water (CDW), and Taiwan Strait Warm Water (TSWW). Optimum multiparameter analysis (OMP) using temperature, salinity and ²²⁶Ra were applied to quantify the contribution of individual source water to the surface water of the ECS in summer. The successful application of radium isotope in OMP analysis demonstrates the usefulness of ²²⁶Ra in the discrimination of mixing among multiple water sources. In 1987, one interesting phenomenon was that the KSSW entered the surface with the upwelling at the margin of continental shelf, and affected the coastal water obviously. In 1999, the TSWW extended northward continuously up to the Changjiang Estuary.     

Carbon pools and fluxes in the China Seas and adjacent oceans

The China Seas include the South China Sea, East China Sea, Yellow Sea, and Bohai Sea. Located off the Northwestern Pacific margin, covering 4700000 km~2 from tropical to northern temperate zones, and including a variety of continental margins/basins and depths, the China Seas provide typical cases for carbon budget studies. The South China Sea being a deep basin and part of the Western Pacific Warm Pool is characterized by oceanic features; the East China Sea with a wide continental shelf, enormous terrestrial discharges and open margins to the West Pacific, is featured by strong cross-shelf materials transport; the Yellow Sea is featured by the confluence of cold and warm waters; and the Bohai Sea is a shallow semiclosed gulf with strong impacts of human activities. Three large rivers, the Yangtze River, Yellow River, and Pearl River, flow into the East China Sea, the Bohai Sea, and the South China Sea, respectively. The Kuroshio Current at the outer margin of the Chinese continental shelf is one of the two major western boundary currents of the world oceans and its strength and position directly affect the regional climate of China. These characteristics make the China Seas a typical case of marginal seas to study carbon storage and fluxes. This paper systematically analyzes the literature data on the carbon pools and fluxes of the Bohai Sea,Yellow Sea, East China Sea, and South China Sea, including different interfaces(land-sea, sea-air, sediment-water, and marginal sea-open ocean) and different ecosystems(mangroves, wetland, seagrass beds, macroalgae mariculture, coral reefs, euphotic zones, and water column). Among the four seas, the Bohai Sea and South China Sea are acting as CO_2 sources, releasing about0.22 and 13.86–33.60 Tg C yr~(-1) into the atmosphere, respectively, whereas the Yellow Sea and East China Sea are acting as carbon sinks, absorbing about 1.15 and 6.92–23.30 Tg C yr~(-1) of atmospheric CO_2, respectively. Overall, if only the CO_2 exchange at the sea-air interface is considered, the Chinese marginal seas appear to be a source of atmospheric CO_2, with a net release of 6.01–9.33 Tg C yr~(-1), mainly from the inputs of rivers and adjacent oceans. The riverine dissolved inorganic carbon (DIC) input into the Bohai Sea and Yellow Sea, East China Sea, and South China Sea are 5.04, 14.60, and 40.14 Tg C yr~(-1),respectively. The DIC input from adjacent oceans is as high as 144.81 Tg C yr~(-1), significantly exceeding the carbon released from the seas to the atmosphere. In terms of output, the depositional fluxes of organic carbon in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea are 2.00, 3.60, 7.40, and 5.92 Tg C yr~(-1), respectively. The fluxes of organic carbon from the East China Sea and South China Sea to the adjacent oceans are 15.25–36.70 and 43.93 Tg C yr~(-1), respectively. The annual carbon storage of mangroves, wetlands, and seagrass in Chinese coastal waters is 0.36–1.75 Tg C yr~(-1), with a dissolved organic carbon(DOC) output from seagrass beds of up to 0.59 Tg C yr~(-1). Removable organic carbon flux by Chinese macroalgae mariculture account for 0.68 Tg C yr~(-1) and the associated POC depositional and DOC releasing fluxes are 0.14 and 0.82 Tg C yr~(-1), respectively. Thus, in total, the annual output of organic carbon, which is mainly DOC, in the China Seas is 81.72–104.56 Tg C yr~(-1). The DOC efflux from the East China Sea to the adjacent oceans is 15.00–35.00 Tg C yr~(-1). The DOC efflux from the South China Sea is 31.39 Tg C yr~(-1). Although the marginal China Seas seem to be a source of atmospheric CO_2 based on the CO_2 flux at the sea-air interface, the combined effects of the riverine input in the area, oceanic input, depositional export,and microbial carbon pump(DOC conversion and output) indicate that the China Seas represent an important carbon storage area.     

Carbon and nitrogen cycling in the Zhubi coral reef lagoon of the South China Sea as revealed by 210Po and 210Pb

The radionuclides (210)Po and (210)Pb were examined to trace the cycling of particulate organic carbon (POC) and particulate organic nitrogen (PON) in the Zhubi coral reef lagoon. The net export flux of POC to the open sea is 14 mg Cm(-2) d(-1). However, the net exchange of PON has not yet been observed. On average, the vertical export fluxes in the lagoon of POC and PON, as derived from (210)Po/(210)Pb disequilibria, are 43 mg Cm(-2) d(-1) and 13.8 mg Nm(-2) d(-1), respectively. The deficit of (210)Po relative to (210)Pb in particulate matter provides evidence for the degradation of particulate organic matter. According to the mass balance budgets, 310 mg Cm(-2) d(-1) and 121 mg Nm(-2) d(-1) were recycled into dissolved fractions. Based on a first-order kinetics model, the degradation rate constants of POC and PON are 0.28 and 0.30 m(-1), respectively. Thus, (210)Po and (210)Pb can quantify the cycling of carbon and nitrogen in this coral lagoon.     

Microbial food web contributions to bottom water hypoxia in the northern Gulf of Mexico

Nutrients from the Mississippi/Atchafalaya Rivers greatly stimulate biological production in the ‘classical’ food web on the inner shelf of the northern Gulf of Mexico. Portions of this production, especially large diatoms and zooplankton fecal pellets, sink and decompose in the bottom water, consuming oxygen and contributing to the annual development of an extensive zone of bottom water hypoxia, typically >15,000 km² since 1993. The microbial food web is also active in the Mississippi River plume, but consists of small organisms that sink slowly. This ‘recycling’ food web has not been considered as a significant contributor to vertical flux and hypoxia. However, gelatinous zooplankton, especially pelagic appendicularians such as Oikopleura dioica, mediate the conversion of microbial web organisms to organic particles with high sinking rates. When pelagic appendicularians are abundant in coastal regions of the northern Gulf of Mexico, they stimulate the rapid vertical transfer of microbial web productivity in the surface layer, which is only 5–15 m thick in the coastal hypoxic region, to the sub-pycnocline layer that becomes hypoxic each summer. In this paper we present results from two studies examining the significance of this pathway. In both 2002 and 2004, we observed high production rates of appendicularians in coastal waters. Discarded gelatinous houses and fecal pellets from the appendicularian populations often provided more than 1 g m⁻² d⁻¹ of organic carbon for the establishment and maintenance of hypoxia in the northern Gulf of Mexico. This source of organic matter flux is especially important in regions far from the river plumes and during periods of low river discharge. Autotrophic elements of this food web are primarily supported by recycled inorganic nutrients originating in the Mississippi and Atchafalaya Rivers. Sources of dissolved organic matter (DOM) supporting the heterotrophic components of this microbial food web may include in situ production, the Mississippi/Atchafalaya Rivers, and Louisiana's coastal wetlands. If significant, the latter source provides a possible link between Louisiana's high rates of coastal land loss and the large hypoxic zone observed along the coast during summer. Both of the latter DOM sources are independent of phytoplankton production stimulated by inputs of riverine inorganic nutrients.     

Comparison of hypoxia among four river-dominated ocean margins: The Changjiang (Yangtze), Mississippi,Pearl, and Rhône rivers

We examined the occurrence of seasonal hypoxia (O₂<2 mg l⁻¹) in the bottom waters of four river-dominated ocean margins (off the Changjiang, Mississippi, Pearl and Rhône Rivers) and compared the processes leading to the depletion of oxygen. Consumption of oxygen in bottom waters is linked to biological oxygen demand fueled by organic matter from primary production in the nutrient-rich river plume and perhaps terrigenous inputs. Hypoxia occurs when this consumption exceeds replenishment by diffusion, turbulent mixing or lateral advection of oxygenated water. The margins off the Mississippi and Changjiang are affected the most by summer hypoxia, while the margins off the Rhône and the Pearl rivers systems are less affected, although nutrient concentrations in the river water are very similar in the four systems. Spring and summer primary production is high overall for the shelves adjacent to the Mississippi, Changjiang and Pearl (1–10 g C m⁻² d⁻¹), and lower off the Rhône River (<1 g C m⁻² d⁻¹), which could be one of the reasons of the absence of hypoxia on the Rhône shelf. The residence time of the bottom water is also related to the occurrence of hypoxia, with the Mississippi margin showing a long residence time and frequent occurrences of hypoxia during summer over very large spatial scales, whereas the East China Sea (ECS)/Changjiang displays hypoxia less regularly due to a shorter residence time of the bottom water. Physical stratification plays an important role with both the Changjiang and Mississippi shelf showing strong thermohaline stratification during summer over extended periods of time, whereas summer stratification is less prominent for the Pearl and Rhône partly due to the wind effect on mixing. The shape of the shelf is the last important factor since hypoxia occurs at intermediate depths (between 5 and 50 m) on broad shelves (Gulf of Mexico and ECS). Shallow estuaries with low residence time such as the Pearl River estuary during the summer wet season when mixing and flushing are dominant features, or deeper shelves, such as the Gulf of Lion off the Rhône show little or no hypoxia.     

Carbon and nitrogen cycling in the Zhubi coral reef lagoon of the South China Sea as revealed by Po and Pb

The radionuclides ²¹⁰Po and ²¹⁰Pb were examined to trace the cycling of particulate organic carbon (POC) and particulate organic nitrogen (PON) in the Zhubi coral reef lagoon. The net export flux of POC to the open sea is 14 mgC m⁻² d⁻¹. However, the net exchange of PON has not yet been observed. On average, the vertical export fluxes in the lagoon of POC and PON, as derived from ²¹⁰Po/²¹⁰Pb disequilibria, are 43 mgC m⁻² d⁻¹ and 13.8 mgN m⁻² d⁻¹, respectively. The deficit of ²¹⁰Po relative to ²¹⁰Pb in particulate matter provides evidence for the degradation of particulate organic matter. According to the mass balance budgets, 310 mgC m⁻² d⁻¹ and 121 mgN m⁻² d⁻¹ were recycled into dissolved fractions. Based on a first-order kinetics model, the degradation rate constants of POC and PON are 0.28 and 0.30 m⁻¹, respectively. Thus, ²¹⁰Po and ²¹⁰Pb can quantify the cycling of carbon and nitrogen in this coral lagoon.     

The distribution of dissolved lead in the coastal waters of the East China Sea

The distribution of dissolved lead in the coastal waters of the East China Sea was investigated seasonally. The average concentrations in surface waters during the spring and autumn were 0.52 nM and 0.27 nM, respectively. In the spring, the concentration of dissolved Pb in the surface waters and bottom waters ranged from 0.13 to 1.86 nM and from 0.15 to 0.94 nM, respectively. For both the surface water and the bottom water, the highest values were observed at the Yangtze River Estuary. Seasonal variability of D-Pb between spring and autumn in the ECS was observed. These results suggested that riverine inputs and atmospheric inputs may be the main sources of lead in this area, while adsorption and co-precipitation on suspended particles at the river estuary and biological process may be the major sinks.     

Organic carbon transport in the Songhua River,NE China: Influence of land use

Carbon transported by rivers is an important component of the global carbon cycle. Here, we report on organic carbon transport along the third largest river in China, the Songhua River, and its major tributaries. Water samples were collected seasonally or more frequently to determine dissolved organic carbon (DOC) and particulate organic carbon (POC) concentrations and C/N and stable carbon isotopic ratios. Principal component analysis and multiple regression analysis of these data, in combination with hydrological records for the past 50 years, were used to determine the major factors influencing the riverine carbon fluxes. Results indicate that the organic carbon in the Songhua River basin is derived mainly from terrestrial sources. In the 2008–2009 hydrological year, the mean concentrations of DOC and POC were 5.87 and 2.36 mg/L, and the estimated fluxes of the DOC and POC were 0.30 and 0.14 t·km^?2·year^?1, respectively. The riverine POC and DOC concentrations were higher in subcatchments with more cropland, but the area‐specific fluxes were lower, owing to decreased discharge. We found that hydrological characteristics and land‐use type (whether forest or cropland) were the most important factors influencing carbon transport in this system. Agricultural activity, particularly irrigation, is the principal cause of changes in water discharge and carbon export. Over the last 50 years, the conversion of forest to cropland has reduced riverine carbon exports mainly through an associated decrease in discharge following increased extraction of water for irrigation.     

Hypoxia in Manila Bay,Philippines during the northeast monsoon

Herein we present results from one of the first extensive bay-wide oceanographic surveys of Manila Bay, wherein 31 stations were sampled during the northeast monsoon (cold and dry season). A band of hypoxic bottom water (dissolved oxygen <2.8 mg/L) spanned the midsection of the bay from east to west. Bottom nitrate concentrations (5.7–16.8 μM; avg. 11.1 μM) and total organic carbon values in sediments (1.7–3.1%; avg. 2.4%) were high in the midsection, which coincided with the band of hypoxic bottom water. Physical processes and site-specific accumulation of organic material likely lead to hypoxic conditions in Manila Bay, even during the northeast monsoon period when the water column is relatively well mixed. The results of this study complement the previously reported widespread hypoxia that occurs during the rainy season. Thus, hypoxia may be pervasive in the bay throughout the year, although it varies in intensity and spatial extent.     

Water Quality Modeling and Dissolved Oxygen Balance in Streams: A Point Source Streeter‐Phelps Application in the Case of the Harsit Stream

The Harsit Stream flowing into the Black Sea is an important watershed in the northeastern part of Turkey. The monthly averages of dissolved oxygen, carbonaceous biochemical oxygen demand, temperature, pH, total suspended solids, total phosphorus and total nitrogen were obtained for the water samples taken from the Tekke, Akcakale and Haciemin sites of the Harsit Stream. Water quality parameters were analyzed and evaluated according to the WHO standards and the Turkish Water Pollution Control Regulation. Initially, the variations in the dissolved oxygen profile were determined from dissolved oxygen, temperature and carbonaceous biochemical oxygen demand measurements by using the point source Streeter‐Phelps equation. Critical dissolved oxygen deficit values were calculated for the monthly average from January–December 2005, which are below the limit value (4 mg L^–1) excluding April (7.26 mg L^–1) and May (5.98 mg L^–1) in the Harsit Stream, resulting in it having second class water quality in terms of dissolved oxygen concentration, temperature, pH and biochemical oxygen demand. It is also seen that the minimum dissolved oxygen limit value (8 mg L^–1), which is a desired quantity for drinking water, is under the limit value for all months in the case of the water mixture. In particular, during the summer months, the consumption speed of oxygen is greater than the production speed of oxygen with increasing temperature and decreasing discharge, and negative values (anoxia) of dissolved oxygen sag are seen. These situations call for significant precautions and treatment criteria.     

Microbial Mineralization of Dichloroethene and Vinyl Chloride under Hypoxic Conditions

Mineralization of ¹⁴C‐radiolabled vinyl chloride ([1,2‐¹⁴C] VC) and cis‐dichloroethene ([1,2‐¹⁴C] cis‐DCE) under hypoxic (initial dissolved oxygen (DO) concentrations about 0.1 mg/L) and nominally anoxic (DO minimum detection limit = 0.01 mg/L) was examined in chloroethene‐exposed sediments from two groundwater and two surface water sites. The results show significant VC and dichloroethene (DCE) mineralization under hypoxic conditions. All the sample treatments exhibited pseudo‐first‐order kinetics for DCE and VC mineralization over an extended range of substrate concentrations. First‐order rates for VC mineralization were approximately 1 to 2 orders of magnitude higher in hypoxic groundwater sediment treatments and at least three times higher in hypoxic surface water sediment treatments than in the respective anoxic treatments. For VC, oxygen‐linked processes accounted for 65 to 85% of mineralization at DO concentrations below 0.1 mg/L, and ¹⁴CO₂ was the only degradation product observed in VC treatments under hypoxic conditions. Because the lower detection limit for DO concentrations measured in the field is typically 0.1 to 0.5 mg/L, these results indicate that oxygen‐linked VC and DCE biodegradation can be significant under field conditions that appear anoxic. Furthermore, because rates of VC mineralization exceeded rates of DCE mineralization under hypoxic conditions, DCE accumulation without concomitant accumulation of VC may not be evidence of a DCE degradative “stall” in chloroethene plumes. Significantly, mineralization of VC above the level that could reasonably be attributed to residual DO contamination was also observed in several nominally anoxic (DO minimum detection limit = 0.01 mg/L) microcosm treatments.     

Riverine inputs of total organic carbon and suspended particulate matter from the Pearl River Delta to the coastal ocean off South China

A total of 1008 samples were collected from the eight major riverine runoff outlets in the Pearl River Delta (PRD) during 2005-2006 to estimate the fluxes of total organic carbon (TOC) to the coastal ocean off South China. The average dissolved organic carbon (DOC) concentration was 1.67 mg/L with a range of 1.38-2.13 mg/L. Concentrations of particulate organic carbon (POC) ranged from 2.66-4.12% of total suspended particulate matter (SPM). The fluxes of TOC and SPM from the PRD via the eight outlets were 9.2 x 10(5) and 2.5 x 10(7)tons/yr, respectively. Temporal variations in POC and DOC were observed at all outlets due to the large variability in runoff levels because of the seasonality of rainfall, and the riverine discharge amount was an important factor controlling TOC flux. The net contribution of organic carbon from the PRD to the coastal ocean represented approximately 0.1-0.2% of total organic carbon transported by rivers worldwide.     

Total Organic Carbon (TOC) as a Sum Parameter of Water Pollution in Selected Polish Rivers (Vistula,Odra, and Warta)

This paper reports on total organic carbon (TOC) and its fractions dissolved organic carbon (DOC) and particulate organic carbon (POC) studied in different Polish rivers. The samples investigated came from the Vistula, Odra, and Warta rivers, and were compared with similar data on river waters available in the literature. The DOC concentrations ranged from 10.0 to 14.2 mg/L and did not vary during the vegetative season. The POC values considerably increased from May through September and reached a maximum in summer. Results for the years 1991τ1996 evidenced a significant increase in the POC value for the Polish rivers from 10.8 to 24.5 mg/L, in comparison with analogous values for West European rivers and North American ones. The enhanced values of TOC and POC were interpreted as being due to anthropogenic pollution.     

Hypoxia in Manila Bay, Philippines during the northeast monsoon

Herein we present results from one of the first extensive bay-wide oceanographic surveys of Manila Bay, wherein 31 stations were sampled during the northeast monsoon (cold and dry season). A band of hypoxic bottom water (dissolved oxygen<2.8 mg/L) spanned the midsection of the bay from east to west. Bottom nitrate concentrations (5.7-16.8 μM; avg. 11.1 μM) and total organic carbon values in sediments (1.7-3.1%; avg. 2.4%) were high in the midsection, which coincided with the band of hypoxic bottom water. Physical processes and site-specific accumulation of organic material likely lead to hypoxic conditions in Manila Bay, even during the northeast monsoon period when the water column is relatively well mixed. The results of this study complement the previously reported widespread hypoxia that occurs during the rainy season. Thus, hypoxia may be pervasive in the bay throughout the year, although it varies in intensity and spatial extent.