A FINE-GRAINED SEDIMENT BUDGET FOR THE SYLT-ROMO TIDAL BASIN

A budget for net accumulation of fine-grained sediment (< 63 mu m) has been set up for the Sylt-Romo tidal basin. Net accumulation within th e basin was computed from Pb-210 core dating and mapping of the intert idal and supratidal surface sediments. It was found that a yearly mean value of 58.10(3) tons of sediment was deposited in the tidal basin. The largest sediment source for the net input of fine-grained sediment is the North Sea contributing about 64 % to the net budget; the fluvi al input and primary production contribute 14 % and 15 %, respectively . Local salt marsh erosion accounts for about 5 % of the budget and at mospheric deposition for only 2 %. The total amount of sediment deposi ted in the investigated area was low compared with earlier investigati ons in the Wadden Sea. This is explained partly by the intensive dikin g of the natural salt marshes fringing the area in the past, and partl y by the exposed conditions of most of the intertidal flats. An index describing the trapping efficiency of the water exchanged between the North Sea and the Sylt-Romo tidal area is defined as the ratio between yearly net sediment input from the North Sea and yearly exchanged wat er volume between the tidal basin and the sea. This index shows that i n the Sylt-Romo tidal basin, fine-grained suspended sediment ''filters '' out of the exchanged sea water at a rate that is 12 times lower tha n in the Gradyb tidal basin. It is concluded that the net deposition o f fine-grained sediment in a tidal basin is mainly a function of physi ographical and hydrodynamical parameters and to a lesser degree of sed iment availability.