The influence of Elbe river water on the Wadden Sea of Sylt (German Bight, North Sea)

Abstract

(1) The transportation period of Elbe river water, admixed with the North Frisian coastal water, from the estuary to the outer Wadden Sea of Sylt was investigated using Elbe river floods (January, 1975). The time difference between maximum salinity decrease at light-vessel “Elbe 1” off the Elbe estuary and that off List (Wadden Sea of Sylt) is one month. This is equivalent to a mean residual current velocity of 4.8 cm/s, under the condition of winds from SSW with an average speed of 10.8 m/s, and assuming that the Elbe water travels the shortest way north. (2) Taking this transportation period into consideration, the quantitative relationship between the salinity off List (Y) (from 3 years' measurements) and both Elbe water discharge volumes (X1) (of the previous month) and local freshwater surplus (X2) was calculated. It could be shown that the multiple linear regression funktionY=a+b1X1+b2X2 describes the relationship optimally (multiple and partial correlation coefficients significant at the 0.1% level). The partial regression coefficientb2 of this equation is by one order of magnitude higher thanb1; 10 m3/s of local freshwater surplus would decrease the salinity off List to 0.2443‰, whereas 10 m3/s of Elbe water would decrease it to 0.0275‰. Without the influence of these freshwater sources a mean salinity of 31.92‰ would be found off List. (3) Since the Elbe river discharged 52 times more freshwater during the investigation period than the local freshwater input (26 times more in winter — November to April), the Elbe water would decrease the salinity in the Wadden Sea of Sylt sixfold more in absolute terms (threefold more in winter) than the local freshwater surplus. This dominant influence of the Elbe water on the Wadden Sea of Sylt, situated 130 km north of the Elbe estuary, must have considerable ecological significance, since dissolved and finely suspended particulate matter from the Elbe river (at least the “conservative” components) will be carried to the Wadden Sea of Sylt at the same rate as follows from the calculations using salinity.