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Marine Ecology Progress Series

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MEPS 214:279-287 (2001)  -  doi:10.3354/meps214279

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

Henrik Skov*, Erik Prins

Ornis Consult A/S, Vesterbrogade 140, 1620 Copenhagen V, Denmark

ABSTRACT: Survey data on seabird distribution at sea, hydrographic data and optical satellite data collected 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 Jutland Coastal Current (JCC) and associated surface fronts. Data collected from hydrographic stations were summarised by means of principal component analysis; the first component, reflecting characteristics of the JCC, provided a satisfactory quantitative measurement of the average meso-scale habitat used by both species. The pelagic range of divers clearly followed the outer estuarine front between surface North Sea water and the JCC, which was located between the 20 and 30 m depth contours. Despite a highly transient trailing edge of the JCC, no divers were ever observed in North Sea water. Hydrographic as well as composited satellite CZCS (Coastal Zone Color Scanner) data indicated the presence of an inner front, spanning a stronger surface salinity gradient than the outer estuarine front. The inner estuarine front was 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 salinities below 32 psu. Seven-year composite images, produced from all available CZCS-Chl (chlorophyll) data taken over the German Bight during the Nimbus-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 measured over a series of cruises, and it indicated a quasi-stable structure; the prevailing position of the surface front was in a zone stretching from 07°30¹E at Horns Reef (55°45¹N) to 07°50¹E south of Amrum Bank (54°30¹N), except during easterly winds when the front was advected offshore as far as 6°50¹E. The recorded patches of high densities of divers were almost confined to areas within 5 km distance from the mean frontal zone. The link between divers and the inner estuarine front seemed rather persistent, as peak densities coincided with the location of this front during all cruises, while lower densities were normally observed in the core Elbe water and in the mixed water outside the front. The authors suggest that the quasi-stability and strength of the inner estuarine front give rise to a predictable location of food resources (i.e. small fish) for divers in the German Bight. Our study highlights the potential for frontal structures of the JCC to influence the marine ecosystem of the southeastern North Sea.

KEY WORDS: Estuarine fronts · German Bight · Seabird foraging · Red-throated/black-throated diver · Gavia stellata/arctica

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