The Schelde is a lowland river originating in the northern part of France (St. Quentin), and entering the North Sea near Vlissingen, The Netherlands. The estuary covers about half of its length (355 km) as the tidal influence is stopped by sluices near Gent 160 km upstream. We focused on the Zeeschelde, the estuarine part in Flanders comprising a mesohaline, an oligohaline and a freshwater tidal zone. The Zeeschelde is subject to severe eutrophication as it receives high inputs from domestic, industrial and agricultural activities. The ecological values and nature conservation interests of the Zeeschelde are taken into consideration by a series of (inter)national policy instruments, aiming at a sustainable management and conservation of this aquatic environment. As a result several management plans apply also to the Zeeschelde or to parts of it. The most far-reaching plans are the Long Term Vision for the Schelde estuary (LTVS) and the updated Sigmaplan which combine ecological rehabilitation and sustainable habitat creation with flood control measures and navigation requisites. Compliance with almost all national and international agreements requires monitoring of biota. In the WFD fish is one of the biotic quality elements to be used in order to assess the ecological status of transitional waters. Species composition, abundance and the proportion of disturbance-sensitive species should be quantified. Any distortion attributable to anthropogenic impact is calculated by means of the Ecological Quality Ratio (EQR), representing the difference between monitored data and reference conditions. The fish-based assessment tool that we developed was designed to comply with these criteria. In addition it can be used on a metric level to assess fish species of special interest under the Habitats Directive. The fish assemblages in the Zeeschelde were described based on sampling results recorded over a period of 13 years. An overview was provided of the temporal and spatial variation in those assemblages along the salinity gradient in the Zeeschelde estuary (Chapter 2). The species richness and abundance increased over these years in the different salinity zones of the Zeeschelde. Between 1991 and 2008 a total of 71 fish species were recorded within this part of the estuary. Each salinity zone is characterised by a typical fish assemblage, although some species are shared between all three zones. The observed increase since 2007 in species richness in the freshwater and oligohaline zones coincides with a remarkable increase in dissolved oxygen. Guild specific qualitative Maximal and Good Ecological Potential (MEP/GEP) lists were composed for the different zones within the Zeeschelde estuary and its tidal tributaries (Chapter 3). The geographical range and ecological demands of the detected fish species were assessed. The outcome was decisive for acceptance within these lists, which served to develop a fish-based index for the Zeeschelde. In chapter 4 the ecological goals and associated habitat needs were described for fish populations in estuaries. The Zeeschelde was presented as a case study for the description of ecological goals for the fish species listed in the MEP/GEP lists. In order to make the method more widely applicable we first classified fishes into guilds, relevant for the formulation of ecological goals. Next we described guild-specific ecological goals and defined habitat needs linked with a proper functioning of the estuarine ecosystem. The habitat needs ensure the completion of all lifecycle stages: spawning, breeding, feeding and growth to maturity. A hierarchical approach was adopted to define the goals and habitat needs: from a regional scale to habitat level. On a regional and basin wide scale the essential habitat need is connectivity, on an estuarine scale this is space and on a habitat scale diversity is most important. The proposed ecological goals need further quantification. However in general the rehabilitation of marshes and mudflats and the enhancement of flood control areas as fish habitats, with special attention for connectivity with the estuary, will significantly increase the carrying capacity of the Zeeschelde for most of the relevant populations. In Chapters 5 and 6 two essential habitat needs are discussed in detail.In chapter 5, we modelled the environmental constraints controlling the movements of anadromous and catadromous fish populations that migrate through the tidal watershed of the river Schelde. For remaining diadromous populations (flounder, three-spined stickleback, twaite shad, thinlip mullet, European eel and European smelt) a data driven logistic model was parameterized. We modelled the presence/absence of fish species in samples taken between 1995 and 2004 as a function of temperature, dissolved oxygen, river flow and season. We demonstrated that it is possible to make acceptable predictions about the future spatiotemporal distribution of migrant fishes, even if only relatively limited information is available. An important management issue that derived from our study is that it is essential to avoid at all times DO concentrations below 5 mg l-1 in the freshwater and brackish tidal estuary of the watershed. Restoration of habitats such as marshes and mudflat areas will enhance aeration of the water and help to avoid severe DO drops. The use of tidal marshes for fish and the influence of creek characteristics on the visiting fish assemblages were assessed (Chapter 6). As expected the influence of the salinity gradient is reflected in the different fish assemblages. We caught a high proportion of juveniles suggesting that the creeks are a juvenile habitat. The highest fish abundance was recorded in summer (after hatching) because then juveniles seek shelter in the creeks. It was also observed that the visit frequency was related to creek dimensions and inundation time. Larger creeks, lower in the tidal frame and with a more complex structure, as they include side creeks and permanent pools, are of higher interest for fish. We also observed a positive effect of rivulets on the mudflat adjoining the tidal marsh as they guide the fish towards the creeks. These observations are important for the design of tidal wetland restoration projects. In chapters 7 and 8 different approaches to define a fish-based evaluation tool to assess the ecological quality status of an estuary (the Zeeschelde) were described. The fish index comprises metrics which are ecologically relevant variables that are sensitive to human pressures. A first step in the selection of these metrics consisted in assessing how they evolve along a pressure gradient (graphical selection).In chapter 7 a new concept in the index development was introduced i.e. the balance between type I (false positive) and type II (false negative) errors. The magnitude of these errors was expressed as the area under the curve (AUC). Graphical screening assured the selection of metrics responsive to anthropogenic degradation. We scored metrics by judging the metric value variation in the best available site (quintiles). A forward stepwise regression selected the metric with the best balance between the type I and type II error. Metric selection was continued until the lowest AUC was obtained. To define the EBI thresholds we fixed the maximum type I error of each integrity class threshold at 10%. It was a major concern that not all quality classes can be discriminated because of unbalanced pre-classification data. Secondly the final index had a high type II error, although we believe both types of error should be small. Therefore in the next chapter a different approach was used in order to obtain a better index.In chapter 8 we described the development of a Zone specific fish-based multimetric Estuarine index of Biotic Integrity (Z-EBI) based on fish surveys data from the Zeeschelde estuary (Chapter 2). Again we pre-classified sites using indicators of anthropogenic impact and selected metrics showing a monotone response with pressure classes for further analysis. Metric values were calculated using pooled annual data within one salinity zone and expressed as catch per unit effort. Metrics were selected using a Principal Component Analysis (PCA) combined with a redundancy test. We defined thresholds for the Good Ecological Potential (GEP) from salinity zone specific references developed in chapter 3. andapplied a modified trisection for the other thresholds (moderate, poor and bad). The Z-EBI is defined by the average of the metric scores calculated over a one year period within each zone and translated into an Ecological Quality Ratio (EQR) to comply with the European Water Framework Directive (WFD). The indices integrate structural and functional qualities of the estuarine fish communities and can be used to assess the ecological quality of the Zeeschelde. We successfully validated the Z-EBI performances for habitat degradation in the various habitat zones. With this new index we encompass small temporal and spatial variations within the estuary. It accounts for the seasonal variation and covers the complete salinity zone, which is an improvement compared to the previous index. The developed indices are able to make the distinction between impacted and unimpacted (GEP) status.Our results showed that the ecological status of the Zeeschelde at present varies from bad to moderate. A comparison of the average scores obtained with EBI and Z-EBI indicated that in those cases where a different appreciation appeared, the EBI scores lower. This confirms our view that local and temporal appreciations are too sensitive to small variations, which do not necessarily represent an overall negative impact on the ecosystem functioning. Implementing rehabilitation and conservation measures will improve the ecological quality status of the Zeeschelde.At present the Z-EBI corresponds best with the demands from the different legislations and provides the most holistic information from an ecological point of view. |
