Cory's shearwater - a warm loving sea-bird species
Common dolphin (Delphinus delphis)
Close-up photo of photophores on the head and arms of the squid Histioteuthis reversa (eye in upper right corner).
Author: Erik Olsen (Institute of Marine Research) and Henrik Skov (DHI Water and Environment)
Most of Thursday has been spent at station 24, on the top of the Mid-Atlantic Ridge, although "on top" is a bit misleading as the middle of the ridge is a very deep (up to 4500 m deep) underwater valley with steep walls and high sea-mounts on either side. Station 24 was located on the eastern slope of this long valley, just south of a sea-mount.
The significant increase of the surface water-temperatures from 9.5 to 15.5 degrees C is an indication that the G.O. Sars cruise has passed the Sub-Polar front and is now moving into the warmer water masses of the subtropical gyre. The rapid shift in water temperatures has had a profound effect on the species composition of most plankton, cephalopods, fish and marine top predators. In general, our biological samples today showed a further shift towards more warm-water species.
In the zooplankton samples more southerly species of krill were found, and the samples of both the plankton trawl and the Underwater Video Profiler showed low abundance of copepods. In the cephalopods the dominance of the cold-water Gonatus sp. ceased in favour of warm-water species. Entering the warmer water masses also introduced four new cephalopod species. Of these the squid (Histioteuthis reversa) was particularly stunning, being covered with rows of brilliant photophores. The photophores on this species are not simple bulb-like light-emitters, but are equipped with reflectors and slits to regulate the color and angle of light. These photophores are shown in the picture to the right. Among the fish the shift over the front was indicated by an increase in species number in the catch, and an appearance of warm-water species like Diaphus species (lanternfishes).
The change in prey species seem to have had a profound negative effect on the number of large whales being recorded from the top deck. The community of top predators is now characterised by species associated with the Gulf Current system like Cory's Shearwater (Calonectris diomedea) and Common Dolphin (Delphinus delphis). As recorded by the Underwater Video Profiler station 24 was further characterised by a high concentration of phytoplankton, especially diatom chains.