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

Abstract

ABSTRACT: Survey data on seabird distribution at sea, hydrographic data and optical satellite datacollected for the German Bight were used to analyse the variability of the distribution of wintering red-throated diver and black-throated diver Gavia stellata / arctica in relation to oscillations of the JutlandCoastal Current (JCC) and associated surface fronts. Data collected from hydrographic stations weresummarised by means of principal component analysis; the first component, reflecting characteristicsof the JCC, provided a satisfactory quantitative measurement of the average meso-scale habitat usedby both species. The pelagic range of divers clearly followed the outer estuarine front between surfaceNorth Sea water and the JCC, which was located between the 20 and 30 m depth contours. Despite ahighly transient trailing edge of the JCC, no divers were ever observed in North Sea water. Hydro-graphic as well as composited satellite CZCS (Coastal Zone Color Scanner) data indicated the pres-ence of an inner front, spanning a stronger surface salinity gradient than the outer estuarine front. Theinner estuarine front was located between mixed estuarine water, with salinities between 32 and34 psu, and the core of the estuarine water mass from the river Elbe, with salinities below 32 psu.Seven-year composite images, produced from all available CZCS-Chl (chlorophyll) data taken overthe German Bight during the Nimbus-7 mission, revealed the mean distribution of the inner estuarinefront as a sharp gradient between the 15 and 20 m depth contours, extending less than 10 km inlongitude. The variability of the inner estuarine front was measured over a series of cruises, and itindicated a quasi-stable structure; the prevailing position of the surface front was in a zone stretchingfrom 07°30’E at Horns Reef (55°45’N) to 07°50’E south of Amrum Bank (54°30’N), except duringeasterly winds when the front was advected offshore as far as 6°50’E. The recorded patches of highdensities of divers were almost confined to areas within 5 km distance from the mean frontal zone. Thelink between divers and the inner estuarine front seemed rather persistent, as peak densities coincidedwith the location of this front during all cruises, while lower densities were normally observed in thecore Elbe water and in the mixed water outside the front. The authors suggest that the quasi-stabilityand strength of the inner estuarine front give rise to a predictable location of food resources (i.e. smallfish) for divers in the German Bight. Our study highlights the potential for frontal structures of the JCCto influence the marine ecosystem of the southeastern North Sea.KEY WORDS: Estuarine fronts · German Bight · Seabird foraging · Red-throated/black-throated diver ·