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.

This paper presents a new approach which was developed to find an optimal combination of candidate metrics for creating a fishbased estuarine biotic index (EBI) for defining the quality status of an estuarine area. The key idea was that a powerful index should minimise two prediction errors simultaneously: falsely declaring the status of a site as disturbed while it is not (Type I error) and the reverse, falsely declaring a disturbed site as undisturbed (Type II error). The balance between both errors is an inherent characteristic of an index and can be displayed as a curve. The area under this curve (AUC) is a measure of the misclassification rate (smaller = better). This criterion was the basis for a stepwise approach whereby in each step a metric resulting in the highest reduction of AUC was added. Five metrics were selected and the distribution of their average was the basis to derive the thresholds for the classes of the EBI. This paper presents the fish-based index (EBI) for the brackish Schelde estuary in Flanders (Belgium). The index was calibrated against fyke net data from five sites during the period from 1995 to 2004. The sites ranged in quality from moderately impacted to very disturbed, classes 3 to 5 respectively. Despite there being neither of the highest classes 1 (high) and 2 (good), the EBI presented can serve as an evaluation tool in the highly impacted situation in Flanders as it discriminates well between moderate and highly impacted sites. In addition, its definition complies with the biological status classes of the European Water Framework Directive (WFD).Keywords Brackish estuary AE Fish-based index of biotic integrity AE Schelde AE Flanders IntroductionWorldwide, estuaries suffer from ever increasing human pressure (Dennison et al., 1993;Simenstad

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.

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Keywords:Biotic integrity Fish-based index Meso-and oligohaline estuary Freshwater European Water Framework Directive Zeeschelde a b s t r a c t Fish-based indices monitor changes in surface waters and are a valuable aid in communication by summarising complex information about the environment (Harrison and Whitfield, 2004). A zone-specific fish-based multimetric estuarine index of biotic integrity (Z-EBI) was developed based on a 13 year time series of fish surveys from the Zeeschelde estuary (Belgium). Sites were pre-classified using indicators of anthropogenic impact. Metrics showing a monotone response with pressure classes were selected for further analysis. Thresholds for the good ecological potential (GEP) were defined from references. A modified trisection was applied for the other thresholds. The Z-EBI is defined by the average of the metric scores calculated over a one year period and translated into an ecological quality ratio (EQR). The indices integrate structural and functional qualities of the estuarine fish communities. The Z-EBI performances were successfully validated for habitat degradation in the various habitat zones.