Hydrology of saline grasslands in the polders of “De Schelde”

For economic reasons, the Flemish government and the Antwerp port authorities are enlarging the Harbour of Antwerp. Therefore, they create new docks and industrial sites in the polders of “Het Waasland”, situated on the left bank of the Schelde - in front of Antwerp. Two decades earlier however, the Flemish government protected important parts of these polders by assigning them the status of Special Protection Area (SPA) under the EU Bird directive. In such cases, the Bird directive demands compensation measures for the loss of protected bird habitats. Among these protected habitats are the very rare saline grasslands. Because these grasslands need very specific hydrological and ecological conditions to develop, it isn’t possible to create them wherever we want to do. To find possible sites for the re-creation of these saline grasslands, we carried out a detailed study of the hydrology, geomorphology and ecology of the “Waaslandpolders”. The results indicate that the saline conditions originate from discharging groundwater passing through a saline peat layer. These conditions are related to the deposits of coarse grain soils, situated at the landside of dikes near former creeks. In the polders of “Het Waasland”, three other parts have the same origin and lie outside the future extents of the harbour. Only in one of them, we found the same special hydrological conditions present in the existing grasslands. The presence of these conditions says nothing about the occurrence of saline vegetation in the future. Therefore, we now study the development of saline grasslands

The Lippensbroek polder as a case study for the inner dike restoration of ecosystem structures and functions

In the framework of the SIGMA plan, Flemish authorities (Waterways and Maritime Affairs Administration, division Sea Scheldt) plan a large controlled inundation area (CIA) in polders (Kruibeke-Bazel-Rupelmonde, 578 ha) along the freshwater - oligohaline border of the Scheldt estuary. Beside fulfilling an important role in enhancing safety against flooding, a CIA could contribute to the restoration of degraded ecosystem functions and habitats by implementing specific management options. However, as the outcome of some management types on ecosystem structures and functions are still uncertain, preliminary projects on a smaller scale must yield adequate information to allow, if necessary, appropriate adaptations. In the pilot project ‘Lippensbroek’, a management scenario for the northern part of the future Kruibeke-Bazel-Rupelmonde CIA will therefore be tested beforehand on an innerdike area of 10 ha. Carefully designed sluices will allow the exchange of limited amounts of Scheldt water, causing a semidiurnal submersion of ca. 0.5 m. Additionnaly, submersions with larger water quantities will occur on a less regular base during storm floods. We discuss the present and future hydrologic regime of the Lippensbroek polder, together with the necessity of a multidisciplinary program to monitor the restoration of ecosystem structures (fresh water tidal marshes, mudflats, creeks, etc.) and functions (nutrient cycling, storage capacity enhancement, sediment retention, biodiversity and habitat support, etc.)

Silica cycling in a freshwater tidal marsh

Silica plays a major role in eutrophication of coastal waters around the world. Mechanisms controlling the production and fate of silica in estuarine systems are far from understood. Major indications have been reported that intertidal areas may be an important reservoir of silica in estuarine systems. This project intends to clarify the role of a freshwater marsh in the silica cycle within the Schelde estuary. Different silica pools in the marsh (vegetation, sediment, pore-water, groundwater and surface water) are quantified on a two-monthly basis in different vegetation types. Dissolved Si, taken up by plants, is stored as amorphous biogenic silica, and is unavailable to the estuarine ecosystem until these plants decompose. Although the monitoring has not yet been carried out over the intended full year period, Phragmites australis and Urtica dioica could already be identified as the major vegetation sinks for dissolved silica in the marsh. Biogenic silica in surface sediments in the marsh increased from winter to summer. In spring and summer, the marsh becomes a sink for BSi, as diatoms and decomposing material are imported into the marsh. Mass-balances carried out may-june 2002 confirmed this theory. BSi was netto imported into the marsh. In contrary, it was observed that dissolved Si was netto exported from the marsh. The marsh seems to act as a reactor, transforming imported BSi to DSi, and thus makes this silica again available to the estuarine ecosystem. In the future, mass-balances will be carried out in all four seasons, to further clarify this theory. Interactions between the different silica pools will be studied by decomposition and dissolution experiments, both in situ and ex situ. In the end, these major goals will allow to construct an integrated view of the role of freshwater tidal marshes in the silica cycle within an estuarine system, by focusing on retention and processing of silica within the marsh

Kleine mantelmeeuwen <i>Larus fuscus</i> in de zuidelijke Noordzee in augustus en september 1995 = Lesser Black-backed Gulls in the southern North Sea in August and September 1995

The Institute of Nature Conservation conducted seabird counts onboard of the 'R.V. Belgica' between 28 August and 8 September, intended to estimate the stock of Lesser Black-backed Gulls in the southern North Sea, In total 1182 km were surveyed in the area situated between 51° and 54°30' latitude, using the standard strip-transect method. In order to compensate for underestimation caused by congregation of gulls around fishing vessels, the density of the fishing vessels was estimated by using a line transect method. In total 9193 Lesser Black-backed Gulls were recorded, of which 612 were located 'in transect' during the strip-transect counts (353.9 km²). It was appraised that the average density of birds was 1.73/km². The overall density of vessels was approximately 0,0053/km² in a selected area of 57 000 km², with an mean flocksize of 96.6 Lesser Black-backed Gulls per ship. Summing up these two figures, it can be estimated that 100 000 - 150 000 Lesser Black-backed Gulls occurred in the southern North Sea in this period. The highest densities were found off the Belgian and Dutch coast, between Zeebrugge and Katwijk. Another patch with high density was located off NE Norfolk. The majority was associated with fishing vessels (74%, n=9193), although a group of self-feeding birds was seen off the English coast