T. Maris

THE SCHELDT ESTUARY: A DESCRIPTION OF A CHANGING ECOSYSTEM

Estuaries are naturally highly dynamic and rapidly changing systems, forming a complex mixture of many different habitat types. They are very productive biomes and support many important ecosystem functions: biogeochemical cycling and movement of nutrients, mitigation of floods, maintenance of biodiversity and biological production. Human pressure on estuaries is very high. On the other hand, it is recognized that estuaries have a unique functional and structural biodiversity. Therefore, these ecosystems are particularly important for integrating sound ecological management with sustainable economics. These opportunities are explored for the Scheldt estuary, a well-documented system with an exceptional tidal freshwater area. In this article a description of the Scheldt estuary is presented, illustrating that human influence is intertwined with natural dynamics. Hydrology, geomorphology, trophic status and diversity are discussed, and possible future trends in both natural evolution and management are argued.

Spatial and temporal patterns of water quality along the estuarine salinity gradient of the Scheldt estuary (Belgium and The Netherlands): results of an integrated monitoring approach

This paper presents the results of 7 years of integrated monitoring along the Scheldt estuary. The combination of two datasets resulted in a full description of the estuaries water quality parameters from the mouth to the upper boundary, including an extended fresh water tidal part. A synthesis of the monitoring results and all relevant ecological knowledge on the Scheldt allowed to identify opportunities to optimize its management. The results show that the effect of discharge on salinity has a distinct maximum in the polyhaline to mesohaline transition area. Oxygen conditions, nitrogen removal and phytoplankton regulation can be enhanced and improved through management measures within the estuary. To lower carbon and phosphorous loads however measures should be taken within the catchment. To restore most of its ecological functions the estuary needs more space. Optimal locations to address specific functions can be derived from the monitoring results.

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.

Estuaries as filters: the role of tidal marshes in trace metal removal.

Flux calculations demonstrate that many estuaries are natural filters for trace metals. Yet, the underlying processes are poorly investigated. In the present study, it was hypothesized that intertidal marshes contribute significantly to the contaminant filter function of estuaries. Trace metal concentrations and sediment characteristics were measured along a transect from the subtidal, over an intertidal flat and marsh to a restored marsh with controlled reduced tide. Metal concentrations in the intertidal and restored marsh were found to be a factor two to five higher than values in the subtidal and intertidal flat sediments. High metal concentrations and high accretion rates indicate a high metal accumulation capacity of the intertidal marshes. Overbank sedimentation in the tidal marshes of the entire estuary was calculated to remove 25% to 50% of the riverine metal influx, even though marshes comprise less than 8% of the total surface of the estuary. In addition, the large-scale implementation of planned tidal marsh restoration projects was estimated to almost double the trace metal storage capacity of the present natural tidal marshes in the estuary.

Modeling the impacts of multiple environmental stress factors on estuarine copepod populations.

Many studies have focused on natural stress factors that shape the spatial and temporal distribution of calanoid copepods, but bioassays have shown that copepods are also sensitive to a broad range of contaminants. Although both anthropogenic and natural stress factors are obviously at play in natural copepod communities, most studies consider only one or the other. In the present investigation, we modeled the combined impact of both anthropogenic and natural stress factors on copepod populations. The model was applied to estimate Eurytemora affinis densities in the contaminated Scheldt estuary and the relatively uncontaminated Darß-Zingst estuary in relation to temperature, salinity, chlorophyll a, and sediment concentrations of cadmium, copper, and zinc. The results indicated that temperature was largely responsible for seasonal fluctuations of E. affinis densities. Our model results further suggested that exposure to zinc and copper was largely responsible for the reduced population densities in the contaminated estuary. The model provides a consistent framework for integrating and quantifying the impacts of multiple anthropogenic and natural stress factors on copepod populations. It facilitates the extrapolation of laboratory experiments to ecologically relevant end points pertaining to population viability.

Test of some ecological concepts on the longitudinal distribution of zooplankton along a lowland water course

The distribution of zooplankton communities from the source to the mouth of a lowland water course, the Scheldt, was used to test some concepts on the longitudinal distribution of organisms along both estuarine (Remane’s diversity concept) and riverine reaches (River Continuum Concept (RCC), Riverine Ecosystem Synthesis (RES)). Substantial zooplankton densities were found in both April and June samples all along the water course, and not only in the lower reaches as suggested by the RCC. Based on the zooplankton communities, and using hierarchical classifications and Generalized Linear Model, several successive zones influenced by different physical and chemical variables and trophic conditions could be distinguished along the Scheldt, in agreement with the RES model: the non-tidal riverine, the tidal freshwater, the tidal salinity gradient, and the marine zones. Over the entire freshwater reach, rotifers were numerically dominant during both samplings; crustaceans became more abundant in June than in April, specifically in the downstream reaches. Highest total zooplankton abundance and diversity were found in the tidal freshwater reach, lowest in the brackish water reach, in accordance with Remane’s concept. An influence of the tributaries on zooplankton communities is suggested, directly by import or dilution of zooplankton communities or through changing environmental conditions.

The Scheldt Estuary: An Overview of the Morphodynamics of Intertidal Areas

Along the Scheldt estuary, intertidal areas such as intertidal flats and intertidal marshes have important functions including flood water storage, water quality regulation and provision of important habitats. The functioning of intertidal marshes is determined by short-term geomorphic processes: the marshes are regularly flooded and sediments are deposited on the marsh surface, a process through which marshes gain elevation. The spatial patterns and rates of sedimentation on marshes are mainly governed by temporal and spatial variations in tidal characteristics and flow patterns, which are strongly influenced by marsh vegetation, leading to a micro-topography typical for tidal marshes. Strong human modifications of the Scheldt estuary by embankments, channelization and dredging have been increasing the tidal range, especially in the more upstream regions. As part of an integrated management plan for the Scheldt estuary—the so-called Sigmaplan in Flanders, new intertidal areas have been created and more than 2000 ha are planned to be ready by 2030. This intertidal flat and marsh creation is realized in some cases as flood control areas with a reduced tidal exchange between the estuary and the flood control area, where the geomorphologic processes are comparable to natural marshes, although we note some important differences.