Threats to Intertidal Soft-Sediment Ecosystems

Abstract

ONE OF THE MANY FACTORS threatening the ecological integrity of the Wadden Sea, a coastal shallow water ecosystem in the northern part of the Netherlands, is bottom disturbance due to fishing, particularly mechanical dredging for shellfish. Because of its far-reaching national and international ecological impacts, fishing has been a major policy issue for several decades. Tidal flats occur along the edges of shallow seas with soft-sediment bottoms, in areas where the tidal range is at least a meter or so (Eisma 1998; van de Kam et al. 2004).The lowest intertidal areas are largely barren, except for seagrass meadows (Zostera) and reefs formed by shellfish such as oysters (Ostrea) or tubeworms (e.g., Sabellaria). Intertidal areas are inundated at least once per day; higher up are salt marshes, which are more rarely and irregularly inundated. In tropical regions, and even some benign temperate areas such as northern New Zealand, the upper parts of intertidal areas may be covered by mangrove forests rather than salt marsh. Mangroves also have a tendency to cover the regularly inundated parts of intertidal soft sediments, thus reducing the extent of mudflats in many tropical areas. No intertidal deposits occur at high latitudes, farther north than 70° to 73°, as coastlines are either ice covered for most of the year or disturbed by moving ice too frequently for soft-sediment deposits to build up. Soft-sediment systems worldwide provide living space for a group of highly specialized migrant shorebirds, relatively long-legged small to medium-size birds that breed spread out over boreal to arctic areas in the Northern Hemisphere during June and July and migrate considerable distances to much smaller coastal intertidal ecosystems the world over. From August to May, the birds are found in large concentrations in the Wadden Sea. Long-distance migrant shorebirds are particularly susceptible to the effects of human encroachment on coastal habitats, overexploitation of marine resources, and global climate change (Piersma and Baker 2000;