VERTICAL ACCRETION AND PROFILE CHANGES IN ABANDONED MAN-MADE TIDAL MARSHES IN THE DOLLARD ESTUARY, THE NETHERLANDS

In the Wadden Sea, an increasing area of the man-made tidal marshes, w hich cover over 17,000 ha, are becoming nature reserves or parts of na tional parks. Consequently, management aims altered from reclaiming la nd towards restoring natural-like marshes. Within this scope, maintena nce of the drainage system was discontinued in a 460-ha nature reserve in man-made tidal marshes in the Ems Dollard Estuary, the Netherlands . We collected elevation data in four sections of the nature reserve t o study vertical accretion rates and to evaluate marsh-profile changes . Elevations were surveyed in 1984 and in 1991/1992 along transects wi th a total length of 9,700 m. Generally, vertical accretion rates were negatively correlated with (a) marsh elevations of 1984, (b) distance from the intertidal mudflats, (c) distance from main creeks, and (d) in many cases, distance from minor creeks. At most of the transects th at ran from the seaward marsh edge to the inland seawall, distance fro m the intertidal mudflats affected vertical accretion rates more than did the 1984 marsh elevation. As a consequence of a gradient in grazin g intensity, vegetation structure (density and height) decreased inlan d and was probably an important auxiliary factor in determining vertic al accretion patterns. After abandonment of the drainage system in 198 4, the number of levees increased along minor creeks (former ditches), as did elevation differences at many existing levees. Levee developme nt was more pronounced inland, which may be explained by the greater d ifferences in vegetation structure between inland levees and marsh int eriors (between minor creeks) as a result of the gradient in grazing i ntensity. Levee development, together with formation of badly drained depressions, increased elevation differences and abiotic and biotic di versity in the marshes. Vertical accretion rates in the Dollard marshe s ranged from 6.6 mm/yr to 11.4 mm/yr among the four marsh sections. T hese values are relatively low compared to those of other man-made mar shes, which might be a consequence of abandoning the drainage system.