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

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 redthroated 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°30' E at Horns Reef (55°45' N) to 07°50' E south of Amrum Bank (54°30' N), except during easterly winds when the front was advected offshore as far as 6°50' E. 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 Resale or republication not permitted without written consent of the publisher

Spatio-temporal distribution of floating objects in the German Bight (North Sea)

“…The slightly enhanced abundances of floating wood near Helgoland (similar as for seaweed-see above) are probably related to accumulations close to the North Frisian front (Skov and Prins, 2001; Fig. 1).…”

“…about 50 km offshore) salinity fluctuates interannually between 31 and 33 psu (Wiltshire et al, 2008) while at about 75 km offshore the long-term mean salinity is N33 psu (Heyen and Dippner, 1998). The White Bank area is under the influence of central North Sea waters with relatively stable salinities of 34-35 psu (Skov and Prins, 2001). In the north-eastern sector, water moves from the German Bight into the northern North Sea and into the Skagerrak (Fig.…”

“…Two major frontal systems separate the more haline central North Sea water of the outer German Bight from the estuarine coastal waters along the 30 m depth contour (Krause et al, 1986;Budéus, 1989;Becker et al, 1992). A permanent front is generated by the plume of the river Elbe off the North Frisian coast (Krause et al, 1986;Dippner, 1993;Skov and Prins, 2001). A thermal front occurs seasonally parallel to the East Frisian coastline (Krause et al, 1986;Budéus, 1989;Dippner, 1993).…”

“…Accumulations of floating objects around Helgoland Island had previously been mentioned by Franke et al (1999). The high densities in waters near Helgoland could be related to accumulations in the quasistable estuarine front in this area (Skov and Prins, 2001, Fig. 1), while the accumulation in the western area of the East Frisian sector might also receive input of floating seaweeds from the British Channel (see also Vandendriessche et al, 2006).…”

Selective foraging within estuarine plume fronts by an inshore resident seabird

“…Within these areas, mixing, and nutrient retention enhance primary productivity, which in turn attract and aggregate zooplankton (Grimes and Finucane, 1991). Entrainment of phytoplankton and zooplankton attract foraging fish, making estuarine plume fronts important nearshore foraging features for marine predators, particularly seabirds (Grimes and Kingsford, 1996;Skov and Prins, 2001;Zamon et al, 2014). However, the dynamic nature of these water masses result in large physical and physico-chemical fluctuations (e.g., temperature, salinity and dissolved oxygen), which influence local prey distribution, composition and biomass (Grimes and Finucane, 1991;Wagner and Austin, 1999).…”