De ontwikkelingen in de polder Breebaart resultaten van de monitoring in 2003 en 2004 en een vergelijking met 2001 en 2002

Abstract

In January 2001 a reduced tidal regime was introduced in Polder Breebaart, a polder located in the northern-east part of the Province of Groningen (The Netherlands). This reduced tidal regime was achieved by building a culvert in the seawall, connecting the polder with the seawater from the Dollard, a part of the Ems-estuary. The aim was (1)to develop and restore a brackish water area with natural abiotic and biotic processes. (2)to create an opportunity for migratory fishes to migrate from the sea into freshwater and vice versa. (3) to increase environmental awareness and public support for conservation. A monitoring program was carried out in order to follow the changes in morphological, physical and biological processes. The results of the bottomdepth measurements show that sediment accumulates in the channel. This channel had been dug out as part of the restoration operation. In the first two years (2001-2002) sedimentation was about 30 cm. In order to stop this deposition, the maximum water level in the polder was lowered approximately 15cm from April 2003 onward, to force higher water velocities at outgoing tide causing more resuspension. Despite this, sedimentation was still about 15cm in the period 2003-2004. The distribution of salinity-concentrations within the polder shows a weak fresh-to-salt gradient over a short distance of approximately 600m between the outflow of the freshwater supply of the fish ladder and the culvert in the seawall. Within time salinity varies from 0,7 to 24. This salinity-range is similar to that of a nearby location in the Dollard, outside the polder. Nutrient concentrations (phosphate, nitrate) and concentrations of suspended matter correspond likewise between the two sites. The original vegetation at the borders of the channel disappeared completely. Species adapted to salt water appeared, such as Glasswort (Salicornia sp.), Annual seablite (Suaeda maritima), Lesser sea spurrey (Spergularia salina) and Sea aster (Aster tripolium). The rather rare species of Cotula coronopifolia (Common brassbuttons) settled on several locations on the transition to the grassland. Benthic fauna consisted mainly of about four species (max 8) of which the ragworm Nereis diversicolor was the most important one. In 2004 total density of benthic individuals decreased drastically compared with 2003. Moreover nearly 95% of these individuals consisted of ragworms. The ragworm is