Vertical Accretion and Profile Changes in Abandoned Man-Made Tidal Marshes in the Dollard Estuary, the Netherlands
Keywords:Salt marsh, vertical accretion, ditching, drainage, grazing, levee formation, restoration, Wadden Sea
In the Wadden Sea, an increasing area of the man-made tidal marshes, which cover over 17,000 ha, are becoming nature reserves or parts of national parks. Consequently, management aims altered from reclaiming land towards restoring natural-like marshes. Within this scope, maintenance 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 to study vertical accretion rates and to evaluate marsh-profile changes. Elevations were surveyed in 1984 and in 1991/1992 along transects with 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 that ran from the seaward marsh edge to the inland seawall, distance from the intertidal mudflats affected vertical accretion rates more than did the 1984 marsh elevation. As a consequence of a gradient in grazing intensity, vegetation structure (density and height) decreased inland and was probably an important auxiliary factor in determining vertical accretion patterns. After abandonment of the drainage system in 1984, the number of levees increased along minor creeks (former ditches), as did elevation differences at many existing levees. Levee development was more pronounced inland, which may be explained by the greater differences in vegetation structure between inland levees and marsh interiors (between minor creeks) as a result of the gradient in grazing intensity. Levee development, together with formation of badly drained depressions, increased elevation differences and abiotic and biotic diversity in the marshes. Vertical accretion rates in the Dollard marshes ranged from 6.6 mm/yr to 11.4 mm/yr among the four marsh sections. These values are relatively low compared to those of other man-made marshes, which might be a consequence of abandoning the drainage system.