Prospective scenarios for water quality and ecological status in Lake Sete Cidades (Portugal): The integration of mathematical modelling in decision processes

The design of alternative strategies for water and ecological quality protection at the Lake Verde of Sete Cidades should be coupled with the assessment of future trophic states. Therefore, a mathematical model was developed to make prospective scenarios to reduce the risk of environmental degradation of the lake, and a modified Psenner scheme was used to characterize P distribution in the sediments. The model was able to describe thermal stratification, nutrient cycling (P, NH₄ and NO₃), dissolved O₂, and phytoplankton dynamics in the water column and adjacent sediment layers. Internal P recycling, resulting from thermal stratification and sediment anoxia, was identified as the main cause for the increase of P concentration in the hypolimnion followed by slow transfer to the epilimnion (about 20 μg/L annual average). Cyanobacteria blooms during spring were explained by the availability of P and increased water temperature verified during this season. The most sensitive model parameter was sediment porosity. This parameter has a direct effect in dissolved O₂ and P profiles and also in phytoplankton biomass. Finally, different water quality restoration scenarios were identified and their effectiveness assessed. Without the adoption of remediation measures (scenario control), Lake Verde water quality would deteriorate with annual average concentrations of total P and phytoplankton biomass (dry matter) reaching 34 μg/L and 2 mg/L, respectively, after 10 years of simulation. The reduction of P loads (scenario PORAL) into the lake would improve water quality comparatively to the scenario control, reducing the annual average concentrations of total P from 34 μg/L to 26 μg/L and of phytoplankton from 2 mg/L down to 1.4 mg/L after 10 years of simulation. In scenario sediments, corresponding to a decrease in the organic content of the sediments, a reduction in the concentrations of total P and phytoplankton is expected in the first two years of simulation, but this effect, would be attenuated throughout the years due to organic matter sedimentation. The best strategy is obtained by combining external and internal measures for P remediation. Finally, it is recommended that the model be used to integrate the results of water quality monitoring and watershed management plans.     

云南大理地区1989—2019年期间气候变化及对洱海水质的影响

依据1989年至2019年云南大理地区所辖12个气象站点的气候数据和洱海水质监测资料等文献,分析大理地区气候变化特征和洱海富营养化变化趋势,并总结洱海水质综合营养状态指数与降水量、气温的相互关系。结果表明,1989年至2019年期间,大理地区的年平均气温呈波动上升趋势,气候变暖明显,冬季气温升温幅度最大;年降水量总体呈波动下降趋势,秋季降水量减少最为显著。洱海水质的综合营养状态指数及单因子总氮、总磷、高锰酸盐指数等总体呈升高趋势,而水体透明度呈降低趋势;进一步可分成2个阶段,即2003年之前呈快速上升或下降变化趋势;2003年之后呈波动稳定趋势。洱海综合营养状态指数与年平均气温呈正相关,与年降水量呈负相关关系;总氮、水体透明度分别与年平均气温正相关和负相关,与年降水量则呈负相关和正相关;而总磷与冬季平均气温、高锰酸盐指数与夏季或冬季平均气温均呈正相关关系。年内变化上,洱海污染指数、综合营养状态指数在最近的2015—2019年期间呈现6—10月份明显增高,显示非汛期水质明显好于汛期状况。总之,1989—2019年期间,受大理地区气温升高、降水量减少导致入湖水量减少的影响,洱海综合营养状态指数呈升高趋势,湖泊富营养化进程加剧状况没有得到改善,洱海水环境仍然比较脆弱。     

Phosphorus fractions in sediment profiles and their potential contributions to eutrophication in Dianchi Lake

Dianchi Lake is a eutrophic lake in southwestern China. Sediment and the bottom water samples were taken from six sites in the east, west, south, north and center of the lake, respectively, in December 2002. Total phosphorus (TP) concentrations in sediments were high and reached a maximum value of 6.66 g/kg. There was a soluble reactive phosphorus (SRP) concentration gradient at the sediment–water interface. In the present study, sediment P was divided into loosely adsorbed P (NH₄Cl-P), redox-sensitive P (BD-P), metal oxides bound P (NaOH-P), calcium bound P (HCl-P), and organic P (Org-P). At three of the six sites selected, the concentrations of different P forms in sediments followed the order: NaOH-P, Org-P>HCl-P>BD-P>NH₄Cl-P in the profile, and in the southern lake the order was HCl-P>NaOH-P, Org-P>BD-P>NH₄Cl-P in the top 15 cm layers of the sediments. The sediment profiles showed that different forms of P had an increasing trend upward toward the sediment surface. There is a considerable potential for release from the sediment into the overlying water and sediment P could be the dominant factor determining the trophic status of the lake if the external load is reduced. An erratum to this article can be found at     

Mercury speciation and distribution in Aha Reservoir which was contaminated by coal mining activities in Guiyang, Guizhou, China

Two sampling campaigns were carried out in March and August 2005 representing dry and wet seasons, respectively, to investigate the distribution patterns of Hg species in the water column and sediment profiles at two sampling stations in Aha Reservoir located in Guiyang, Southwestern China. Aha Reservoir has been contaminated by Hg due to small scale coal mining activities. Mercury concentrations in both water and sediment were elevated. A clearly seasonal variation of dissolved Hg (DHg), particulate Hg (PHg) and total Hg (THg) concentrations in the water column was observed. The concentrations of these Hg species in the wet season were significantly higher than in the dry season. Runoff input and diffusion of Hg from sediments could be the reasons for elevated concentrations of these Hg species in the wet season. The contaminated sediment is acting as a secondary contamination source for both inorganic Hg (IHg) and methylmercury (MeHg) to the overlying water. The cycling of Mn in the sediment governs the diffusion process of IHg to the water column. In the dry season (winter and spring), Mn occurs as MnO₂ because the uppermost part of sediment is in an oxic condition and Hg ions are absorbed by MnO₂. In the wet season (summer and fall), the uppermost part of the sediment profile is in a reduced condition because of stratification of the water column and MnO₂ is reduced to Mn²⁺, which results in transformation of Hg²⁺ into porewater as Mn²⁺ became soluble. This causes a higher diffusive flux of IHg from sediment to overlying water in the wet season. Both sampling stations showed a consistent trend that THg concentrations decreased in the uppermost part of sediment cores. This demonstrated that the measures taken to reduce ADM contamination to Aha Reservoir also reduced Hg input to the reservoir. Methyl Hg diffusive fluxes from sediment to overlying water were higher in the wet season than the dry season demonstrating that high temperatures favor Hg methylation processes in sediment.     

Water quality associated with survival of submersed aquatic vegetation along an estuarine gradient

The decline of submersed aquatic vegetation (SAV) in tributaries of the Chesapeake Bay has been associated with increasing anthropogenic inputs, and restoration of the bay remains a major goal of the present multi-state “Bay Cleanup” effort. In order to determine SAV response to water quality, we quantified the water column parameters associated with success of transplants and natural regrowth over a three-year period along an estuarine gradient in the Choptank River, a major tributary on the eastern shore of Chesapeake Bay. The improvement in water quality due to low precipitation and low nonpoint source loadings during 1985–1988 provided a natural experiment in which SAV was able to persist upstream where it had not been for almost a decade. Mean water quality parameters were examined during the growing season (May–October) at 14 sites spanning the estuarine gradient and arrayed to show correspondence with the occurrence of SAV. Regrowth of SAV in the Choptank is associated with mean dissolved inorganic nitrogen <10 μM; mean dissolved phosphate <0.35 μM; mean suspended sediment <20 mg l^?1; mean chlorophylla in the water column <15 μg l^?1; and mean light attenuation coefficient (Kd) <2 m^?1. These values correspond well with those derived in other parts of the Chesapeake, particularly in the lower bay, and may provide managers with values that can be used as target concentrations for nutrient reduction strategies where SAV is an issue.     

Sediment-water oxygen and nutrient exchanges along the longitudinal axis of Chesapeake Bay: Seasonal patterns,controlling factors and ecological significance

Sediment-water oxygen and nutrient (NH₄ ⁺, NO₃ ^?+NO₂ ^?, DON, PO₄ ^3?, and DSi) fluxes were measured in three distinct regions of Chesapeake Bay at monthly intervals during 1 yr and for portions of several additional years. Examination of these data revealed strong spatial and temporal patterns. Most fluxes were greatest in the central bay (station MB), moderate in the high salinity lower bay (station SB) and reduced in the oligohaline upper bay (station NB). Sediment oxygen consumption (SOC) rates generally increased with increasing temperature until bottom water concentrations of dissolved oxygen (DO) fell below 2.5 mg l^?1, apparently limiting SOC rates. Fluxes of NH₄ ⁺ were elevated at temperatures >15°C and, when coupled with low bottom water DO concentrations (<5 mg l^?1), very large releases (>500 μmol N m^?2 h^?1) were observed. Nitrate + nitrite (NO₃ ^?+NO₂ ^?) exchanges were directed into sediments in areas where bottom water NO₃ ^?+NO₂ ^? concentrations were high (>18 μM N); sediment efflux of NO₃ ^?+NO₂ ^? occurred only in areas where bottom water NO₃ ^?+NO₂ ^? concentrations were relatively low (<11 μM N) and bottom waters well oxygenated. Phosphate fluxes were small except in areas of hypoxic and anoxic bottom waters; in those cases releases were high (50–150 μmol P m^?2 h^?1) but of short duration (2 mo). Dissolved silicate (DSi) fluxes were directed out of the sediments at all stations and appeared to be proportional to primary production in overlying waters. Dissolved organic nitrogen (DON) was released from the sediments at stations NB and SB and taken up by the sediments at station MB in summer months; DON fluxes were either small or noninterpretable during cooler months of the year. It appears that the amount and quality of organic matter reaching the sediments is of primary importance in determining the spatial variability and interannual differences in sediment nutrient fluxes along the axis of the bay. Surficial sediment chlorophyll-a, used as an indicator of labile sediment organic matter, was highly correlated with NH₄ ^?, PO₄ ^3?, and DSi fluxes but only after a temporal lag of about 1 mo was added between deposition events and sediment nutrient releases. Sediment O:N flux ratios indicated that substantial sediment nitrification-denitrification probably occurred at all sites during winter-spring but not summer-fall; N:P flux ratios were high in spring but much less than expected during summer, particularly at hypoxic and anoxic sites. Finally, a comparison of seasonal N and P demand by phytoplankton with sediment nutrient releases indicated that the sediments provide a substantial fraction of nutrients required by phytoplankton in summer, but not winter, especially in the mid bay region.     

Scales of nutrient-limited phytoplankton productivity in Chesapeake Bay

The scales on which phytoplankton biomass vary in response to variable nutrient inputs depend on the nutrient status of the plankton community and on the capacity of consumers to respond to increases in phytoplankton productivity. Overenrichment and associated declines in water quality occur when phytoplankton growth rate becomes nutrient-saturated, the production and consumption of phytoplankton biomass become uncoupled in time and space, and phytoplankton biomass becomes high and varies on scales longer than phytoplankton generation times. In Chesapeake Bay, phytoplankton growth rates appear to be limited by dissolved inorganic phosphorus (DIP) during spring when biomass reaches its annual maximum and by dissolved inorganic nitrogen (DIN) during summer when phytoplankton growth rates are highest. However, despite high inputs of DIN and dissolved silicate (DSi) relative to DIP (molar ratios of N∶P and Si∶P>100), seasonal accumulations of phytoplankton biomass within the salt-intruded-reach of the bay appear to be limited by riverine DIN supply while the magnitude of the spring diatom bloom is governed by DSi supply. Seasonal imbalances between biomass production and consumption lead to massive accumulations of phytoplankton biomass (often>1,000 mg Chl-a m^?2) during spring, to spring-summer oxygen depletion (summer bottom water <20% saturation), and to exceptionally high levels of annual phytoplankton production (>400 g m^?2 yr^?1). Nitrogen-dependent seasonal accumulations of phytoplankton biomass and annual production occur as a consequence of differences in the rates and pathways of nitrogen and phosphorus cycling within the bay and underscore the importance of controlling nitrogen inputs to the mesohaline and lower reaches of the bay.     

Phosphorus fractionation in surficial sediments of Pandoh Lake,Lesser Himalaya,Himachal Pradesh,India

The fractionation of P in Pandoh Lake surface sediments has been investigated for the first time in order to understand its environmental availability and sources, and the eutrophication status of this lake. Inorganic-P is present mainly as authigenic-P (step-III). The authigenic P concentration is higher in winter relative to the summer and monsoon seasons and ranged from 35.9 to 46.9 μg/g. The loosely sorbed or exchangeable-P (step-I), Fe(III)-bound-P (step-II) and detrital inorganic-P (step-IV) were higher in the monsoon season and varied from 3.70 to 11.1 μg/g, 16.9 to 32.0 μg/g and 9.89 to 17.0 μg/g, respectively. Organic-P reached a maximum in the summer season and ranged from 8.00 to 14.9 μg/g. Authigenic-P and detrital inorganic-P show seasonal changes, as pH influences the interaction between P and CaCO₃ in the water column. In the winter season, phosphate is precipitated out of the water column and fixed in the sediments as a result of an increase in pH. Calcite-bound-P in the sediments may be redissolved by decreasing pH in the summer season. Relatively high rates of mineralization during the monsoon results in the seasonal pattern of organic-P fractionation to sediment as follows: monsoon = winter < summer. Iron, Ca, organic matter and silt and clay contents seem to play a significant role in regulating the seasonal P budget. Principal component analysis (PCA) was used to identify the factors which influence sedimentary P in the different seasons.     

Environmental status of several Shubenacadie River headwater lakes,Nova Scotia,Canada

The Shubenacadie River basin is the largest watershed in Nova Scotia, Canada, encompassing an area of approximately 2,800 km² and supporting one of the most rapidly expanding populations in Atlantic Canada. A comprehensive study was carried out to assess the effect of recent development in the basin on the headwater lakes. Information on the environmental status of the lakes can be further used in the development of a management framework for the basin with respect to water quality and quantity objectives.Water and sediment quality were investigated in four of the Shubenacadie River headwater lakes. In addition, trophic status of the lakes was assessed by using dissolved phosphorus and oxygen concentrations. The surface area and mean depth of the lakes ranged from 0.83 to 1.13 km² and 4.3 to 6.6 m, respectively. Three of the studied lakes were thermally stratified during the summer. The concentration of dissolved oxygen decreased significantly in the hypolimnion during the stratification period, although the lakes were generally classified as oligotrophic. The water quality is typical for lakes of the area. The pH of the water ranged between 6.1 and 7.3 during the study period. Major ions were chloride, sodium, and sulphate. A significant increase of As, Hg, Zn, Pb, Cu, Ni and Co was found in surface sediments in all four lakes. However, the concentration of these elements in lake water was lower than recommended guidelines for aquatic life and human consumption.     

A Modeling and Field Approach to Identify Essential Fish Habitat for Juvenile Bay Whiff (Citharichthys spilopterus) and Southern Flounder (Paralichthys lethostigma) Within the Aransas Bay Complex, TX

The goal of this study was to use an ecosystem-based approach to consider the effect of environmental conditions on the distribution and abundance of juvenile bay whiff and southern flounder within the Aransas Bay Complex, TX, USA. Species habitat models for both species were developed using boosted regression trees. Juvenile bay whiff were associated with low temperatures (<15 °C, 20–23 °C), moderate percent dry weight of sediments (25–60 %), salinity >10, and moderate to high dissolved oxygen (6–9 mg O₂/l, 10–14 mg/l). Juvenile southern flounder were associated with low temperatures (<15 °C), low percent dry weight of sediment (<25 %), seagrass habitat, shallow depths (<1.2 m), and high dissolved oxygen (>8 mg O₂/l). Our results indicate that conservation measures should focus along the eastern side of Aransas Bay and the north corner of Copano Bay to protect essential fish habitat. These findings provide a valuable new tool for fisheries managers to aid in the sustainable management of bay whiff and southern flounder and provide crucial information needed to prioritize areas for habitat conservation.     

Seasonal and long-term trends in the water quality of Florida Bay (1989–1997)

Analysis of 6 yr of monthly water quality data was performed on three distinct zones of Florida Bay: the eastern bay, central bay, and western bay. Each zone was analyzed for trends at intra-annual (seasonal), interannual (oscillation), and long-term (monotonic) scales. the variables TON, TOC, temperature, and TN∶TP ratio had seasonal maxima in the summer rainy season; APA and Chla, indicators of the size and activity of the microplankton tended to have maxima in the fall. In contrast, NO₃ ⁻, NO₂ ⁻, NH₄ ⁺, turbidity, and DO_sat, were highest in the winter dry season. There were large changes in some of the water quality variables of Florida Bay over the study period. Salinity and TP concentrations declined baywide while turbidity increased dramatically. Salinity declined in the eastern, central, and western Florida Bay by 13.6‰, 11.6‰, and 5.6‰, respectively. Some of the decrease in the eastern bay could be accounted for by increased freshwater flows from the Everglades. In contrast to most other estuarine systems, increased runoff may have been partially responsible for the decrease in TP concentrations as input concentrations were 0.3–0.5 μM. Turbidity in the eastern bay increased twofold from 1991 to 1996, while in the central and western bays it increased by factors of 20 and 4, respectively. Chla concentrations were particularly dynamic and spatially heterogeneous. In the eastern bay, which makes up roughly half of the surface area of Florida Bay, Chla declined by 0.9 μg l⁻¹ (63%). The hydrographically isolated central bay zone underwent a fivefold increase in phytoplankton biomass from 1989 to 1994, then rapidly declined to previous levels by 1996. In western Florida Bay there was a significant increase in Chla, yet median concentrations of Chla in the water column remained modest (∼2 μg l⁻¹) by most estuarine standards. Only in the central bay did the DIN pool increase substantially (threefold to sixfold). Notably, these changes in turbidity and phytoplankton biomass occurred after the poorly-understood seagrass die-off in 1987. It is likely the death and decomposition of large amounts of seagrass biomass can at least partially explain some of the changes in water quality of Florida Bay, but the connections are temporally disjoint and the process indirect and not well understood.     

Application of excitation emission matrix fluorescence monitoring in the assessment of spatial and seasonal drivers of dissolved organic matter composition: Sources and physical disturbance controls

The environmental dynamics of dissolved organic matter (DOM) were characterized for a shallow, subtropical, seagrass-dominated estuarine bay, namely Florida Bay, USA. Large spatial and seasonal variations in DOM quantity and quality were assessed using dissolved organic C (DOC) measurements and spectrophotometric properties including excitation emission matrix (EEM) fluorescence with parallel factor analysis (PARAFAC). Surface water samples were collected monthly for 2 years across the bay. DOM characteristics were statistically different across the bay, and the bay was spatially characterized into four basins based on chemical characteristics of DOM as determined by EEM-PARAFAC. Differences between zones were explained based on hydrology, geomorphology, and primary productivity of the local seagrass community. In addition, potential disturbance effects from a very active hurricane season were identified. Although the overall seasonal patterns of DOM variations were not significantly affected on a bay-wide scale by this disturbance, enhanced freshwater delivery and associated P and DOM inputs (both quantity and quality) were suggested as potential drivers for the appearance of algal blooms in high impact areas. The application of EEM-PARAFAC proved to be ideally suited for studies requiring high sample throughput methods to assess spatial and temporal ecological drivers and to determine disturbance-induced impacts in aquatic ecosystems.     

Long-Term Changes in Water Quality and Productivity in the Patuxent River Estuary: 1985 to 2003

We conducted a quantitative assessment of estuarine ecosystem responses to reduced phosphorus and nitrogen loading from sewage treatment facilities and to variability in freshwater flow and nonpoint nutrient inputs to the Patuxent River estuary. We analyzed a 19-year dataset of water quality conditions, nutrient loading, and climatic forcing for three estuarine regions and also computed monthly rates of net production of dissolved O₂ and physical transport of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) using a salt- and water-balance model. Point-source loading of DIN and DIP to the estuary declined by 40–60% following upgrades to sewage treatment plants and correlated with parallel decreases in DIN and DIP concentrations throughout the Patuxent. Reduced point-source nutrient loading and concentration resulted in declines in phytoplankton chlorophyll-a (chl-a) and light-saturated carbon fixation, as well as in bottom-layer O₂ consumption for upper regions of the estuary. Despite significant reductions in seaward N transport from the middle to lower estuary, chl-a, turbidity, and surface-layer net O₂ production increased in the lower estuary, especially during summer. This degradation of water quality in the lower estuary appears to be linked to a trend of increasing net inputs of DIN into the estuary from Chesapeake Bay and to above-average river flow during the mid-1990s. In addition, increased abundance of Mnemiopsis leidyi significantly reduced copepod abundance during summer from 1990 to 2002, which favored increases in chl-a and allowed a shift in total N partitioning from DIN to particulate organic nitrogen. These analyses illustrate (1) the value of long-term monitoring data, (2) the need for regional scale nutrient management that includes integrated estuarine systems, and (3) the potential water quality impacts of altered coastal food webs.     

Phosphorus in suspended matter and sediments of a hypertrophic lake. A case study: Lake Dianchi,China

From June 2004 to December 2004, Lake Dianchi, which had large scale of cyanobacterial blooms was investigated in order to study P-fractionation in the suspended matter and the sediment. The investigation improves our understanding of phosphorus in Lake Dianchi and the relationship between phosphorus and cyanobacterial blooms. It contributes to the available literature on the behavior of P in hypertrophic lakes. The distribution of P-fractions in Lake Dianchi was not uniform from northwest to south, but was closely related to the trophic status of the whole lake. The concentrations of total phosphorus, labile P (NH₄Cl-P), Organic P (NaOH-NRP) and loss on ignition in suspended matter were positively correlated with the strength of cyanobacterial blooms. Total phosphorus in suspended matter was relatively stable for almost half an year and closely related to Chl. a concentration. The main content of organic phosphorus is in the cyanobacterial blooms. The concentrations of phosphorus bound to metal oxides and carbonates (NaOH-SRP and HCl-P) in sediment were similar to NaOH-SRP and HCl-P in the corresponding suspended matter. The latter two forms of P in suspended matter were not affected by cyanobacterial blooms, indicating that the inorganic phosphorus is derived from the sediment after resuspension from the sediment due to wind and wave action. The contribution of the different P-fractions to TP in sediment and in suspended matter indicates that NH₄Cl-P in the suspended matter is an important buffer for maintaining dissolved phosphorus in water.     

A Simple Index of Trophic Status in Estuaries and Coastal Bays Based on Measurements of pH and Dissolved Oxygen

Measurements of pH and dissolved oxygen saturation in summer from 90 Irish estuaries and coastal bays were used to develop a simple index of trophic state. The index is based on the assumption that large fluctuations in both these variables are likely to be a characteristic feature of eutrophication. In this paper, we use a simple index to capture the relative variation in both pH and dissolved oxygen saturation in each water body during a period of 3 years. We show that the index is in good agreement with other trophic status schemes and strongly correlated with parameters indicative of trophic state such as chlorophyll a and biochemical oxygen demand. We conclude that the index could be used as a simple screening tool to group individual water bodies into broad categories that reflect their trophic state.     

Stable Carbon Isotope Biogeochemistry and Anthropogenic Impacts on Karst Ground Water, Zunyi, Southwest China

Natural and anthropogenic impacts on karst ground water, Zunyi, Southwest China, are discussed using the stable isotope composition of dissolved inorganic carbon and particulate organic carbon, together with carbon species contents and water chemistry. The waters can be mainly characterized as HCO₃–Ca type, HCO₃ · SO₄–Ca type, or HCO₃ · SO₄–Ca · Mg type, according to mass balance considerations. It is found that the average δ¹³C_DIC values of ground waters are higher in winter (low-flow season) than in summer (high-flow season). Lower contents of dissolved inorganic carbon (DIC) and lower values of δ¹³C_DIC in summer than in winter, indicate that local rain events in summer and a longer residence time of water in winter play an important role in the evolution of ground water carbon in karst flow systems; therefore, soil CO₂ makes a larger contribution to the DIC in summer than in winter. The range of δ¹³C_DIC values indicate that dissolved inorganic carbon is mainly controlled by the rate of carbonate dissolution. The concentrations of dissolved organic carbon and particulate organic carbon in most ground water samples are lower than 2.0 mg C L⁻¹ and 0.5 mg C L⁻¹, respectively, but some waters have slightly higher contents of organic carbon. The waters with high organic carbon contents are generally located in the urban area where lower δ¹³C_DIC values suggest that urbanization has had an effect on the ground water biogeochemistry and might threaten the water quality.     

Seasonal variations of arsenic at the sediment–water interface of Poyang Lake,China

Arsenic species including arsenite, arsenate, and organic arsenic were measured in the porewaters collected from Poyang Lake, the largest freshwater lake of China. The vertical distributions of dissolved arsenic species and some diagenetic constituents [Fe(II), Mn(II), S(−II)] were also obtained in the same porewater samples in summer and winter. In sediments the concentration profiles of total As and As species bound to Fe–Mn oxyhydroxides and to organic matter were also determined along with the concentrations of Fe, Mn and S in different extractable fractions. Results indicate that, in the summer season, the concentrations of total dissolved As varying from 3.9 to 55.8 μg/L in sediments were higher than those (5.3–15.7 μg/L) measured in the winter season, while the concentrations of total As species in the solid phase varied between 10.97 and 25.32 mg/kg and between 7.84 and 30.52 mg/kg on a dry weight basis in summer and winter, respectively. Seasonal profiles of dissolved As suggest downward and upward diffusion, and the flux of dissolved As across the sediment–water interface (SWI) in summer and winter were estimated at 3.88 mg/m² a and 0.79 mg/m² a, respectively. Based on porewater profiles and sediment phase data, the main geochemical behavior of As was controlled by adsorption/desorption, precipitation and molecular diffusion. The solubility and migration of inorganic As are controlled by Fe–Mn oxyhydroxides in summer whereas they appear to be more likely controlled by both amorphous Fe–Mn oxyhydroxides and sulfides in winter. A better knowledge of the cycle of As in Poyang Lake is essential to a better management of its hydrology and for the environmental protection of biota in the lake.     

Excess air during aquifer storage and recovery in an arid basin (Las Vegas Valley, USA)

The Las Vegas Valley Water District in Nevada, USA, has operated an artificial recharge (AR) program since 1989. In summer 2001, observations of gas exsolving from tap water prompted a study that revealed total dissolved gas (TDG) pressures approaching 2?atm with a gas composition that it is predominantly air. Measurements of TDG pressure at well heads and in the distribution system indicated two potential mechanisms for elevated TDG pressures: (1) air entrainment during AR operations, and (2) temperature changes between the winter recharge season and the summer withdrawal season. Air entrainment during pumping was investigated by intentionally allowing the forebay (upstream reservoir) of a large pumping station to drawdown to the point of vortex formation. This resulted in up to a 0.7?atm increase in TDG pressure. In general, the solubility of gases in water decreases as the temperature increases. In the Las Vegas Valley, water that acquired a modest amount of dissolved gas during winter artificial recharge operations experienced an increase in dissolved gas pressure (0.04?atm/°C) as the water warmed in the subsurface. A combination of air entrainment during AR operations and its amplification by temperature increase after recharge can account for most of the observed amounts of excess gas at this site.     

Microelectrode Study of Oxygen Uptake and Organic Matter Decomposition in The Sediments of Xiamen Western Bay

Sediment cores were sampled from Xiamen Western Bay at five sites during the summer and winter of 2006 and Hg–Au microelectrodes were used to make on board measurements of the concentration gradients of dissolved oxygen, Mn²⁺, and Fe²⁺ within the sediments. The O₂ concentrations decreased sharply from about 200 μmol L⁻¹ in the bottom seawater to zero within a depth of a few millimeters into the sediment. Dissolved Mn²⁺ was detected below the oxic zones with peak concentrations up to 600 μmol L⁻¹, whereas dissolved Fe²⁺ had peak concentrations up to 1,000 μmol L⁻¹ in deeper layers. The elemental contents of organic carbon and nitrogen within the sediments were analyzed and their C/N ratios were in the range of 9.0 to 10.1, indicative of heavy terrestrial origin. Sediments from two sites near municipal wastewater discharge outlets had higher organic contents than those from the other sites. These high organic contents corresponded to shallow O₂ penetration depths, high dissolved Mn²⁺ and Fe²⁺ concentrations, and negative redox potentials within the sediments. This indicated that the high organic matter content had promoted microbial respiration within the sediments. Overall, the organic content did not show any appreciable decrease with increasing sediment depths, so a quadratic polynomial function was used to fit the curve of O₂ profiles within the sediments. Based on the O₂ profiles, O₂ fluxes across the seawater and sediment interface were estimated to be in the range 6.07 to 14.9 mmol m⁻² day⁻¹, and organic carbon consumption rates within the surface sediments were estimated to be in the range 3.3 to 20.8 mgC cm⁻³ a⁻¹. The case demonstrated that biogeochemistry within the sediments of the bay was very sensitive to human activities such as sewage discharge.     

Enhanced Benthic Nutrient Flux During Monsoon Periods in a Coastal Lake Formed by Tideland Reclamation

Sediment and water quality were investigated in an artificial coastal lake (Saemangeum Lake, Korea) that was formed by constructing a 33-km long sea-dyke offshore from the mouths of two adjacent rivers, which discharge into the Yellow Sea. Sediment showed drastic increases in fine fraction (silt and clay) after the dyke construction. TN, TP, and OC contents in the sediment showed the similar spatial variation to that of fine fraction. A mixing model indicated benthic fluxes of nutrients such as PO₄, NH₄, and SiO₂, which were considerably elevated during the summer monsoon season. It is revealed that this phenomenon was associated with the development of strong salinity stratification, elevated water temperature, and increased groundwater discharge. However, a change in the sedimentation environment due to dyke construction is suggested as the primary reason for the enhanced benthic fluxes.