Tuning the tide: creating ecological conditions for tidal marsh development in a flood control area

The Schelde estuary, characterised as a turbid, polluted and eutrophic system, has nowadays reached a turning point in the restoration of its water quality. During the past century, human activities have reduced the intertidal areas, essential in the estuarine ecosystem for nutrient cycling and the self-cleaning capacity. Today, in combination with a master plan to protect the population from storm surges, an opportunity rises to restore areas with a tidal influence. One specific option of combining safety and ecology is the creation of flood control areas (FCA) under the influence of a controlled reduced tide (CRT). These specific areas will differ in many ways from fully tidal areas. However, these areas can fulfill important ecological functions with effects on aeration, nitrification, denitrification, sedimentation and primary production in the estuary. Opportunities for ecological development within a CRT have been investigated for a specific case. The ecology within a CRT showed to be very case specific, depending e.g. on the morphology of the area, the sluice design and the local water quality. Depending on the sluice design, water quality can be improved and sedimentation can be influenced. Possible measures to design a CRT with a rich habitat variation are discussed.     

Modelling of water and sediment quality in the Scheldt estuary

In 1987 the Tidal Waters Division (Ministry of Transport, Public Works and Water Management, The Netherlands) initiated the SAWES project (Systems Analysis WEstern Scheldt). In the framework of this project, a water quality model for the Scheldt estuary was made. The model aims to establish the relation between pollution inputs and the concentration of pollutants in the abiotic compartments and to establish the fate of wasted substances. The model includes oxygen, nitrogen and trace metals. Due to the low oxygen content of the upper Scheldt estuary, a new model approach for trace metals had to be developed, taking into account precipitated metal sulphides. The calibration, verification and sensitivity analysis of the model provided a good understanding of the chemistry of the estuary. Afterwards the model was used to support policy making by computing how the water and sediment quality in the estuary respond to reduced inputs of waste.     

Water quality modelling of the western Scheldt estuary

In 1987 the National Institute for Coastal and Marine Management (theNetherlands) initiated the SAWES project (Systems Analysis WEstern Scheldt).The main goals of the project were to acquire knowledge with respect to thepollution problems in the Scheldt estuary and to apply this knowledge tomanagement issues of the Scheldt on an operational level. The main interestwas to understand quantitatively the relation between inputs of pollutingsubstances and effects on the ecosystem. In the framework of the project, awater quality model for the Scheldt estuary was made. In 1995, the modelarea was expanded to include the Belgian coastal region at the mouth of theestuary. The model calculates the fate of discharged substances andestablishes the relation between pollution inputs and the resulting waterquality, including general water quality (oxygen, pH, alkalinity, major ionchemistry and nutrients) as well as pollutant concentrations. The modelincorporates all chemical processes which affect these concentrations,including the precipitation/dissolution of metal sulphides which to a largeextent controls the fate of trace metals. Based on calibration andverification exercises, it has been shown that the model provides a goodrepresentation of the physical and chemical processes taking place withinthe estuary and can therefore be used to support policy development for theestuary. For example, the model can compute how the water and sedimentquality in the estuary will respond to reduced inputs of waste or to suchhuman activities as dredging. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ecological Rehabilitation of the Schelde Estuary (The Netherlands–Belgium; Northwest Europe): Linking Ecology, Safety Against Floods, and Accessibility for Port Development

The Long‐Term Vision for the Schelde estuary was determined as “the development of a healthy and multifunctional estuarine water system that can be utilized in a sustainable way for human needs.” This Dutch–Flemish managerial plan sets quality targets for the condition of the estuary by the year 2030 and the management measures to achieve them. Targets were developed and integrated from three central perspectives: accessibility of the ports, safety against floods in the densely populated catchment, and ecosystem health. This study focuses on the ecological rehabilitation of the estuary and the creation of sustainable nature, seeking possible alliances with security measures against floods, navigation requisites for port activities, and enhancement of the estuary's educational and recreational values. The estuary and its valley were subdivided into ecologically relevant zones. Key parameters were identified, and a conceptual rehabilitation model was developed, based on a problem analysis. Goals were set in a semiquantified way for most attributes of the estuarine functioning and prioritized for each zone. Rehabilitation measures with maximal contribution to the priority goals were identified for each zone. Spatial analysis of the study area indicated optimal areas for the implementation of these measures. To exemplify the array of possibilities on an ecosystem level, two different rehabilitation plans were proposed, each from a different approach. The potential contribution to the rehabilitation of the estuary was compared for both alternatives.     

Nutrient biogeochemistry in the water column (N,P, Si) and porewater (N) of sandy sediment of the Scheldt estuary (SW-Netherlands)

The Scheldt river drains a densely populated and industrialized area in northern France, western Belgium and the south-west Netherlands. Mineralization of the high organic load carried by the river leads to oxygen depletion in the water column and high concentrations of dissolved nitrogen and phosphorus compounds. Upon estuarine mixing, dissolved oxygen concentrations are gradually restored due to reaeration and dilution with sea water. The longitudinal redox gradient present in the Scheldt estuary strongly affects the geochemistry of nutrients. Dissolved nutrients in the water column and dissolved nitrogen species in sediment porewaters were determined for a typical summer and winter situation. Water column concentration-salinity plots showed conservative behaviour of dissolved Si during winter. During summer (and spring) dissolved Si may be completely removed from solution due to uptake by diatoms. The geochemistry of phosphorus was governed by inorganic and biological processes. The behaviour of nitrogen was controlled by denitrification in the anoxic fluvial estuary, followed by nitrification in the upper estuary (prior to oxygen regeneration). In addition, nitrogen was taken up during phytoplankton blooms in the lower estuary. Dissolved inorganic nitrogen species in porewaters from the upper 20 cm of sediments were obtained from a subtidal site in the middle of the lower estuary. Dissolved nutrient concentrations were low in the upper 10–15 cm of the sandy and organic poor (<1% POC) sediments mainly as a result of strong sediment mixing. The porewater profiles of ammonium and nitrate were evaluated quantitatively, using a one-dimensional steady-state diagenetic model. This coupled ammonium-nitrate model showed ammonification of organic matter to be restricted to the upper 4 to 7 cm of the sediments. Total nitrification ranged from 3.7–18.1 mmol m^?2 d^?1, converting all ammonium produced by ammonification. The net balance between nitrification and denitrification depended on the season. Nitrate was released from the sediments during winter but is taken up from the water column during summer. These results are in good agreement with data obtained from the independently calibrated water column model for the Scheldt Estuary (VAN GILSet al., 1993).     

黄河干流与河口湿地生态需水研究进展

通过回顾黄河干流和河口湿地生态需水相关研究的发展历程及主要研究成果,从生态需水研究的研究对象及目标、研究内容、研究方法及应用等方面归纳总结了黄河干流和河口生态需水的研究现状和面临挑战。目前,有关黄河干流和河口湿地的生态需水方面的研究已取得一定的成果,对黄河流域生态需水基础理论基本规律的认识相对清晰,但对黄河干流和河口湿地生态系统认识的不足导致目前生态需水计算方法不统一,计算结果存在一定的误差,从而导致黄河生态需水在流域水资源配置与管理实践中难以达到预期结果。今后仍需在逐步积累的实测资料的基础上进行细化的生态需水研究,重点开展基于河流和湿地生态系统完整性的生态需水研究;基于生态-水文响应关系并综合其他保护目标的生态水文过程研究;还要考虑经济、社会、环境"三赢"的权衡以及未来水沙条件等因素的不确定性,寻求合理的能应用到实际水量配置方案中的生态需水量等。并将生态需水成果与黄河水量生态调度有效结合起来,在水量调度实践中予以论证。     

The Environmental Reserve and its role in retaining the diversity of birds at the Thukela Estuary

In 1998, a new Water Act was promulgated in South Africa. The basic premise of the Act is that a river and its associated estuary are recognised as having a legitimate right to water as a resource. These aquatic ecosystems are entitled to the quantity and quality of water that will allow them to maintain their basic ecological functions. This is the Environmental Reserve. It is considered that aquatic ecosystems must sustain these functions as they, in turn, will guarantee and prolong the sustainability of the resource. This paper briefly outlines the basic Environmental Reserve methodology and the processes that are followed to achieve sustainability of the resource, specifically pertaining to estuaries where a scenario based approach is used. The Reserve determination for the Thukela Estuary on the north-east coast of South Africa and data relating to its aquatic birds are used to illustrate how the process is undertaken. Birds are one of five key biotic components used in the Reserve determination methodology. These data are evaluated in conjunction with the impacts of various flow scenarios proposed for implementation in the Thukela System and the resulting scenario-based impacts are assessed. From this a flow scenario, which would allow the estuary to remain within the current class of ecological functioning and at the same time allow maximum abstraction from the system for other uses, can be determined.     

Dissolved and particulate trace metal geochemistry in the scheldt estuary,S. W. Netherlands (water column and sediments)

The geochemistry of dissolved and particulate trace metals has been studied in the water column and the sediments of the Scheldt estuary between 1987 and 1990. A strong seasonal influence on the behaviour of dissolved Cd, Cu and Zn is observed, related to the redox conditions in the upper estuary and phytoplankton activity in the lower estuary (which are both seasonally dependent variables). The dissolved trace metal concentrations in the fresh water end-member are remarkably low during spring and summer, due to metal sulphide precipitation in the anoxic Scheldt river. However, the dissolved concentrations increase rapidly with increasing salinity, due to oxidation of metal sulphides that are present in the suspended matter, accompanied by (e.g. chloro-)complexation of the released metals. Readsorption of Cd and Zn occurs in the lower estuary during the spring phytoplankton bloom. During winter, when the Scheldt river is not completely anoxic, much higher dissolved trace metal concentrations are observed in the fresh water end-member since metal sulphide precipitation in the water column is precluded. Rapid trace metal removal is observed in the low salinity, high turbidity zone, due to adsorption onto suspended matter and freshly precipitated iron and manganese oxyhydroxides. Upon further mixing, desorption is apparent, due to a similar oxidation-complexation mechanism as observed during spring and summer. Pore water infusion may also contribute to the enrichment of dissolved Cd, Cu and Zn in the mid-estuarine region. The trace metal contents of the suspended matter and the sediments show a continuous decrease with increasing salinity. This behaviour is to a very large extent due to physical mixing of contaminated fluvial particulates and relatively unpolluted marine particulates. Desorption of Cd, Cu and Zn can be identified but is of minor importance compared to the conservative mixing process. The distribution of dissolved Cd, Cu and Zn in the pore waters of the mid-estuarine region reflects the impact of early diagenetic processes. Trace metal peaks are observed near the sediment-water interface, and at greater depth in the manganese and iron reduction zones. These peaks are attributed to oxidation of reduced trace metal compounds (e.g. sulphides) and reduction of the (iron and manganese) oxide carrier phases, respectively. At greater depth, the dissolved trace metal concentrations are much lower due to metal sulphide precipitation in the sulphate reduction zone. Analysis of a large sediment dataset indicates severe trace metal pollution of the Scheldt estuary at the end of the fifties. A major reduction of the pollution by As, Cr, Hg, Pb, and Zn has occurred in the seventies, and of Cd and Cu in the eighties. The Ni pollution has increased over the time period considered. In spite of this improvement, the present-day pollution status of the Scheldt estuary is still reason for concern.     

Nutrient fluxes and water quality in the drainage network of the Scheldt basin over the last 50 years

and use and urban activity in the catchment of the Scheldt river system have deeply changed during the last 50 years, modifying in turn the water quality of the drainage network and the fluxes of nutrient transferred to the estuary and to the sea. Based on the RIVERSTRAHLER model, developed for establishing the link between the biogeochemical functioning of large river systems and the constraints set by the meteorology, the morphology of the drainage network and the human activity in the watershed, we reassembled the data available to document these constraints in the Scheldt basin since the last half of the XXth century and we used them to reconstruct the variations of nutrient and oxygen concentrations at the outlet of the Scheldt drainage network. We compared the results with the water quality data at the entrance of the estuarine zone available since the 1960s. Both model results and observational data show a very severe deterioration of water quality (with deep oxygen depletion) in the beginning of the 1960s, while a clear trend to improvement is apparent since the late 1980s. The budget of nutrient loadings from the watershed, retention within the drainage network and delivery to the estuarine zone is established on an annual basis for the 50 last years. The yearly fluxes of nutrient delivered by the river to the estuary and the sea show a severe depletion of silica with respect to nitrogen compared with the requirements of diatoms, and a clear shift from the early 1990s from nitrogen to phosphorus potential limitation. Seasonal variations of nutrient delivery are however much more pronounced for nitrogen, with much less inputs during the dry seasons, while phosphorus inputs, mainly from point sources are more constant, so that nitrogen limitation can still occur during summer. Compared with similar budget estimations carried out for the Seine river system, the Scheldt basin, in spite of its much higher population density, does not deliver higher specific fluxes of nutrient (presently about 2000 kgN/km²/yr, 80 kgP/km²/yr and 1000 kgSi/km²/yr), owing to very efficient processes of nutrient retention.     

Average annual growth and condition of mussels as a function of food source

Prior to 1986, before completion of a storm surge barrier, the average annual condition of mussels at culture plots in the Eastern Scheldt was better in the western part near the North Sea, than in the central part of the estuary. No such difference was observed in 1986 and 1987.To predict the effects of the barrier on mussel culture, the condition of the mussels, which is an index of growth rate, is analysed and correlated with food sources for the period 1981–84. In the western and central part of the Eastern Scheldt, mussel condition correlates strongly with average annual primary production, but not with chlorophyll-a concentrations. This suggests a direct link between primary production and the growth of mussels.In the western part of the estuary, the relation between condition and primary production has a less steep slope than in the central part. Import of food from the North Sea is suggested to act as an additional food source in the west of the Eastern Scheldt; hence the better condition values of the mussels, also in years with relatively low primary production.The storm surge barrier reduces the water exchange with the North Sea. Reduced import of food and consequently lower mussel condition are expected in the western part of the estuary. Preliminary data from 1987 confirm this prediction.     

Ecohydrology as a new tool for sustainable management of estuaries and coastal waters

Throughout the world, estuaries and coastal waters have experienced degradation. Present proposed remedial measures based on engineering and technological fix are not likely to restore the ecological processes of a healthy, robust estuary and, as such, will not reinstate the full beneficial functions of the estuary ecosystem. The successful management of estuaries and coastal waters requires an ecohydrologybased, basin-wide approach. This necessitates changing present practices by official institutions based on municipalities or counties as an administrative unit, or the narrowly focused approaches of managers of specific activities (e.g., farming and fisheries, water resources, urban and economic developments, wetlands management and nature conservationists). Without this change in thinking and management concept, estuaries and coastal waters will continue to degrade, whatever integrated coastal management plans are implemented. To help in this process of change there is a need to (1) develop a profound understanding of the effects of biota and biotic processes on mediating estuary response to changing hydrology, sediment and nutrient flux and of the biota on hydrology at the river basin scale, and (2) to develop science-based remediation measures at the river basin scale, with elements of ecohydrology and phytotechnology at their core, to strengthen the ability of the biota to sustain and adapt to human-induced stresses.This revised version wa published online in March 2005 with corrections to the issue cover date.     

基于陆海统筹的九龙江-厦门湾海岸生态过渡带综合监测体系构建

海岸带是陆地和海洋之间的生态过渡带,生态保护和开发利用矛盾突出。而生态与环境监测是海岸带环境污染治理与生态保护的重要基础,是海岸带可持续发展的关键。在分析目前海岸带监测存在的问题基础上,以九龙江-厦门湾为研究对象,通过遥感和生物监测、标准衔接、采样和分析仪器以及在线监测系统研发等技术集成,构建了从污染源、环境质量到生态系统以及景观层次的一体化综合生态监测体系。该体系基于常规监测,建立了基于生态系统的河流-河口（近海）生物、水体和沉积物的生态环境一体化监测;探索制定了适用于九龙江河口不同盐度区的营养盐基准/标准系列推荐值;基于营养盐污染入海总量控制目标,构建了河流入海污染物通量在线监测系统;通过遥感监测和实地调查相结合,实现了从关键生态系统到景观的海岸带综合生态监测。基于综合监测体系,构建了兼顾陆海的河口湾区域生态安全评价指标体系,实现区域生态系统可持续发展或生态安全的动态评价。因此,通过上述系统的集成,成功实现了从陆域（流域）到河口（近海）一体化综合生态监测,可为海岸带地区的生态质量改善、污染防治、主要污染物排放总量控制、生态安全评价、生态保护与修复等提供科学支撑。     

Assessment of the subtidal macrobenthic community functioning of a temperate estuary following environmental restoration

Biological Traits Analysis (BTA) is a recently proposed method for addressing ecological functioning based on traits exhibited by members of biological assemblages. This multi-trait approach was applied to the soft-bottom subtidal macrobenthic communities of the Mondego estuary (Portugal), aiming to assess its functioning following a management measure implemented in this system. In particular, the response of benthic assemblages to restoration efforts was investigated over a 5-year period, testing for temporal differences before and after management, in order to assess the effectiveness of this recovery action.BTA revealed to be a useful approach providing valuable information on the functioning of the subtidal benthic communities. Overall, results suggested that there have been some changes in the ecosystem over the study period, although the success of the management measure at the benthic functional level revealed unclear. The climatic variability experienced in the estuary over the monitoring period seemed to have played a significant role in masking the potential effects of restoration. Furthermore, evidence suggested a possible persistence in the benthic functioning despite the occurrence of shifts in taxonomic composition, assured by the potential ability of different species with an alike set of traits to perform similar roles in the ecosystem.To best of our knowledge, this study constituted one of the first attempts to investigate the effects of a management measure in an estuary by means of Biological Traits Analysis. As such, it can thus be useful as a guideline for further management actions in the Mondego estuary extendable to other poikilohaline estuaries as well, and to provide insights on the BTA application to this type of ecosystems.     

Molluscan diversity in tidal marshes along the Scheldt estuary (The Netherlands, Belgium)

The mollusc fauna of 64 sites in 31 tidal marshes was surveyed along a salinity gradient from freshwater to marine conditions in the river Scheldt (Belgium–The Netherlands). A total of 10649 specimens involving 31 taxa were identified. Salinity turned out to be a major factor in mollusc assemblages in the Scheldt estuary, but other factors can not be excluded. In the marine part five species were common, compared to the brackish part where only Assiminea grayana was abundant. In the freshwater zone species richness was highest (24). There was a significant correlation between flooding frequency and species richness in the tidal freshwater marsh `Durmemonding'. Finally, the survey confirmed the distribution of the amphibious hygromiid snail Pseudotrichia rubiginosa, a species which in Belgium only occurs in the marshes of the tidal freshwater part of the Scheldt and its tributaries.     

Diatoms, silicic acid and biogenic silica dynamics along the salinity gradient of the Scheldt estuary (Belgium/The Netherlands)

The Scheldt estuary (Belgium/The Netherlands) was sampled along the entire salinity gradient from 2003 to 2005 for silicic acid (DSi), biogenic silica (BSi), suspended particulate matter (SPM) and pigments. Net DSi consumption and/or release within the estuary were investigated by comparing measured DSi concentrations with (fully-transient) model simulations of the concentrations that would have been obtained in case of conservative transport. The DSi consumption was at maximum in May due to diatoms of presumably marine origin blooming in the lower estuary. DSi consumption decreased rapidly in July, probably because of the grazing pressure of copepods also of marine origin, and DSi was released from late summer onwards. Multiple regression analyses showed that most of the BSi did not follow the dynamics of the living diatoms but rather that of the SPM. They also suggested that diatoms were more silicified in the upper estuary than in the lower estuary. Phytoliths were not expected to contribute significantly to the BSi pool. As BSi dynamics strongly differed from those of diatoms and DSi, this study highlighted the importance of taking BSi into account when investigating estuarine silica dynamics. This study also revealed the fundamental role of the coupling between the biogeochemical and ecological functioning of the lower estuary and that of the adjacent coastal zone. This contrasts with the classical consideration that estuaries act as one-way filters for dissolved and particulate material of riverine origin.     

基于自然地理特征的长江口水域分区

河口分区是营养盐基准制定的前提,也是进行河口资源管理的得力工具。基于长江口2005-2006年的调查数据,参考国内外相关研究成果,结合长江口本身的特征,提出以自然地理特征为基础,利用层级分区方法,对长江口水域进行分区。经一、二级分区后,长江口水域可划分为长江口过渡区、长江口外近海区、杭州湾和舟山海区等4个海区。4个海区的自然地理特征各具特色,且各海区间的分界线具有明确的地理学意义。采用单因素方差分析法对各分区的水体特征和沉积物特征进行一致性检验,不同分区间的差异性较为显著,说明分区结果较为合理。     

Restoration of the Sieperda Tidal Marsh in the Scheldt Estuary, The Netherlands

Because of land reclamation, reinforcement of dikes, and the deepening of shipping channels, large areas of tidal marshes have been removed or eroded from the Scheldt estuary during the last two centuries. Tidal wetland restoration contributes toward compensating this loss of habitat. Not all restoration projects are meticulously planned, however; some are forced by nature. During a severe storm in 1990, a dike was breached in the brackish part of the Scheldt estuary and returned tidal influence to the Sieperda polder. In the 10 years since the dike breach, the former polder has changed into a brackish tidal marsh. Here we report on the geomorphologic and ecological developments that have taken place in the marsh. Tidal intrusion into the former polder turned crop fields into mudflats and changed pastures into salty marsh vegetation. The digging of a new creek improved marsh hydrology and enhanced tidal intrusion further into the marsh. Macrofauna typical of estuarine mudflats established rapidly in the developing marsh. Vegetation succession took place rapidly. Within 5 years, large areas of mudflats became covered with marsh vegetation. Birds characteristic of salt marshes were observed breeding or seen foraging in the marsh. The number of wading birds declined as areas of mudflat became overgrown. It is demonstrated that tidal flow is the engine to tidal marsh restoration. Tidal influence caused geomorphologic changes, which directed ecological developments in the former polder.     

Behaviour of selected PCBs,PAHs and γ-HCH in the Scheldt estuary,S.W. Netherlands

The behaviour of some organic micropollutants in the abiotic compartments water, particulate matter and bottom sediments of the Scheldt estuary, S.W. Netherlands, was studied between 1986 and 1989. Special attention was given to two individual PolyChlorinated Biphenyl congeners (PCB 52 and PCB 153), two Polycyclic Aromatic Hydrocarbons (PAHs) fluoranthene and benzo(a)pyrene and γ-HCH. Dissolved PCBs and γ-HCH behave conservatively during estuarine mixing. The concentration of γ-HCH in the fresh-water is extremely high in spring compared to the other seasons. Dissolved fluoranthene does not behave conservatively. The concentration of dissolved fluoranthene in the freshwater endmember shows a seasonal dependence with highest concentrations in winter and lowest in summer. Benzo(a)pyrene could hardly be detected in the dissolved phase with the methods used. PCBs and PAHs in particulate matter and sediments behave essentially conservatively as a result of the mixing of riverine particulates with high, and marine particulates with low organic micropollutant contents. Particulate fluoranthene is removed at low salinities under (nearly) anoxic conditions probably by microbial degradation. The rivers Scheldt and Rupel appear to be the major sources for the compounds studied. However, in particular the organic micropollutant contents of the sediments indicate that important emissions along the estuary were or are still present. Measurements of individual PCBs and PAHs in the <63 μm fraction of a dated sediment core from a salt marsh in the eastern part of the Scheldt estuary show that the recent input of PCBs and PAHs into the estuaryis probably 2–3 times lower than the maximum input in the mid-1960s. The PCB profile shows postdepositional congener-selective mobilization caused by advective transport. The PAH assemblage is remarkably uniform in the sediment core. The PAH ratios indicate that combustion of coal is the main source of PAHs in the Scheldt estuary.     

Fish communities’ response to implementation of restoring measures in a highly artificialized estuary

Over the years, the Mondego estuary has undergone various anthropogenic impacts. One of the most serious was the closing of the communication between the two arms of the system (north and south), in the 1990s, which promoted eutrophication and a consequent water quality decline in the south arm. Several mitigation measures were subsequently implemented, in particular the re-establishing of the communication between the two arms in 2006, increasing water flow and reducing water residence time in the south arm. The present study aimed to evaluate the impact of management measures on the ecological and conservation condition of the Mondego estuary, through a longitudinal assessment of the structure and composition of the fish communities over a decade. The Mondego fish community showed important modifications over the years, in terms of structure, ecological quality and conservation value. The fish community status improved following the reconnection of both arms. In the south arm those changes appear to be more evident than in the other estuarine areas, where an inverse pattern was observed in the last few years. A redistribution of the fish species within the system may have been responsible for those unexpected alterations in the north arm and upstream area.