–  Migrating fish species with different swimming capacities and energy use show different capacities for passing obstacles between habitats, such as culverts and fish ladders. Here, we present an integrated study on swimming capacity and energetic use in seven European freshwater fish species with different ranges of migration (brown trout Salmo trutta L., European perch Perca fluviatilis L., roach Rutilus rutilus L., common carp Cyprinus carpio L., gudgeon Gobio gobio L., bullhead Cottus gobio L. and stone loach Barbatula barbatula L.). Critical (Ucrit), optimal (Uopt) and maximum (Umax) swimming speed and oxygen consumption (MO2) were analysed and showed values correlated to migration capacity with highest swimming capacities in trout and roach and lowest in stone loach and bullhead. The resulting data can be used to make estimates of maximum passable water speeds in culverts. In conclusion, long‐distance migrators show higher swimming capacities and can potentially clear obstacles easier than short distance migrators with lower swimming capacities. Even small obstacles (<25 cm) could be a barrier for genetic exchange between populations in short‐distance migrators.

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Recently, a significant increase in macrophyte growth has been observed in many lowland rivers in Flanders, mainly due to eutrophication and an improvement of the water quality. This growth strongly influences the channel roughness (Manning-n). The first purpose of the project was a better understanding of the complex relationship between biomass development and discharge capacity in lowland rivers. In order to avoid the backing up of water upstream, the whole vegetation body is usually mowed annually. This project also investigated a lighter ecological management of aquatic plants in which only a part of the vegetation is removed in separate and alternating blocks, seeking a compromise between sufficient discharge and conserving large parts of the macrophyte vegetation with all its functions. Beside laboratory experiments, field survey was undertaken in the Grote Caliebeek, a tributary of the Kleine Nete in Flanders, Belgium. The results indicated that the presence of macrophytes in lowland rivers slowed down the waterflow and resulted in a raised water level. The upstream water level followed biomass development at a certain discharge level. The mowing experiments and the field survey indicated that alternating weed cutting patterns can reduce fall in an effective way. This research emphasizes the possibilities of alternating weed cutting patterns in order to deal with water flow problems. In the long term there will be the need for a better understanding of the ecological relationships in the search for a sustainable integrated method of controlling aquatic vegetation.

The Belgian coastal plain and the Schelde estuary are threatened by sea-level rise. While of great economic importance with a threatened population of some 0.8 million (of a total population of 10 million), assessments of these risks are limited. This article describes the physical characteristics of the coast and undertakes a qualitative interpretation of its vulnerability. Low-lying polders are the most vulnerable to sea-level rise where a major problem is water drainage during rainy periods; their varying vulnerability to sea-level rise and increase in rain intensity is assessed, including the relationship between drainage levels and saltwater seepage. Freshwater lenses developed within the dunes are also vulnerable to sea-level rise, leading to threats to drinking water supplies from saltwater intrusion. Belgian coastal defence structures and their effectiveness are discussed. Historical sea-level rise during the past century, wave and wind data, and the evolution of erosion and accretion along the coast are interpreted. For Antwerpen, a harbour city on the river Schelde, the effects of sea-level rise are far from clear. Included here are historical data on changes in tidal amplitude during the 20th century. Future research needs should focus on the quantitative interpretation of data to understand the effect of sea-level rise on beach erosion, flood risk, and fresh and salt groundwater distribution. Furthermore, a thorough socio-economic study should be undertaken to assess the vulnerability of the Belgian coast and the Schelde estuary. ADDITIONAL INDEX WORDS: Belgian coastal plain, saltwater intrusion, gravitational and artificial water drainage, coastal defences. PHYSICAL CHARACTERISTICS OF THE BELGIAN COASTAL PLAIN Geographical Position and Economic RelevanceThe Belgian coastline is on the southern North Sea and comprises an open wave-exposed coast and the more sheltered upper reaches of the Schelde estuary. The wave-exposed coast (65-km long) has a nearly continuous dune belt that protects the hinterland. This dune barrier consists of two arcs, one stretching from Dunkerque (France) to Wenduine and a second arc from Wenduine to Breskens (The Netherlands). Its continuity is interrupted by the estuaries of the river IJzer in the west and het Zwin in the east and the harbours of Oostende, Blankenberge, and Zeebrugge. The total coastal zone population is ca. 0.4 million (i.e., ca. 4% of the Belgian population), which is concentrated in the towns of Brugge, Oostende, Blankenberge, Veurne, Nieuwpoort, and Knokke (Figure 1).During summer holidays the population increases by ca. 0.3 million resident tourists and on peak days by 0.25 million