Impact of estuarine fronts on the dispersal of piscivorous birds in the German Bight

Survey data on seabird distribution at sea, hydrographic data and optical s atellite data collected for the German Eight were used to analyse the varia bility of the distribution of wintering red-throated diver and black-throat ed diver Gavia stellata/arctica in relation to oscillations of the Jutland Coastal Current (JCC) and associated surface fronts. Data collected from hy drographic stations were summarised by means of principal component analysi s; the first component, reflecting characteristics of the JCC, provided a s atisfactory quantitative measurement of the average meso-scale habitat used by both species. The pelagic range of divers clearly followed the outer es tuarine front between surface North Sea water and the JCC, which was locate d between the 20 and 30 m depth contours. Despite a highly transient traili ng edge of the JCC, no divers were ever observed in North Sea water. Hydrog raphic as well as composited satellite CZCS (Coastal Zone Color Scanner) da ta indicated the presence of an inner front, spanning a stronger surface sa linity gradient than the outer estuarine front. The inner estuarine front w as located between mixed estuarine water, with salinities between 32 and 34 psu, and the core of the estuarine water mass from the river Elbe, with sa linities below 32 psu. Seven-year composite images, produced from all avail able CZCS-Chl (chlorophyll) data taken over the German Eight during the Nim bus-7 mission, revealed the mean distribution of the inner estuarine front as a sharp gradient between the 15 and 20 m depth contours, extending less than 10 km in longitude. The variability of the inner estuarine front was m easured over a series of cruises, and it indicated a quasi-stable structure ; the prevailing position of the surface front was in a zone stretching fro m 07 degrees 30' E at Horns Reef (55 degrees 45' N) to 07 degrees 50' E sou th of Amrum Bank (54 degrees 30' N), except during easterly winds when the front was advected offshore as far as 6 degrees 50' E. The recorded patches of high densities of divers were almost confined to areas within 5 km dist ance from the mean frontal zone. The link between divers and the inner estu arine front seemed rather persistent, as peak densities coincided with the location of this front during all cruises, while lower densities were norma lly observed in the core Elbe water and in the mixed water outside the fron t. The authors suggest that the quasi-stability and strength of the inner e stuarine front give rise to a predictable location of food resources (i.e, small fish) for divers in the German Eight. Our study highlights the potent ial for frontal structures of the JCC to influence the marine ecosystem of the southeastern North Sea.