The Davis Strait - DCE - Nationalt Center for Miljø og Energi

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(phytoplankton) to consumers at higher trophic levels e. g. fish and their
larvae; whales, primarily the bowhead whale (Balaena mysticetus) (Laidre et
al. 2007, Laidre et al. 2010): and seabirds, e. g. little auk (Alle alle), a specialised
zooplankton feeder on the large copepods of the genus Calanus
(Karnovsky et al. 2003). Most of the higher trophic levels in the Arctic marine
ecosystem rely on the lipids that are accumulated in Calanus (Lee et al.
2006, Falk-Petersen et al. 2009). Consequently, a great deal of the biological
activity, e.g. spawning and growth of fish, is synchronised with the life cycle
of Calanus. Zooplankton not only supports the large, highly visible components
of the marine food web but also the microbial community. Regeneration
of nitrogen and carbon through excretion by zooplankton is crucial for
bacterial and phytoplankton production (Daly et al. 1999, Møller et al. 2003).
Zooplankton, mainly the Calanus copepods, play a key ecological role in
supplying the benthic communities with high quality food with their large
and fast-sinking faecal pellets (Juul-Pedersen et al. 2006). Thus, vertical flux
of faecal pellets sinking down to the seabed sustains diverse benthic communities
such as bivalves, sponges, echinoderms, anemones, crabs and fish
(Turner 2002, and references therein).
4.2.2 The importance of Calanus copepods
Earlier studies on the distribution and functional role of zooplankton in the
pelagic food-web off Greenland, mainly in relation to fisheries research,
have revealed the prominent role of Calanus. The species of this genus feed
on algae and protozoa in the surface layers and accumulate surplus energy
in form of lipids, which are used for overwintering at depth and to fuel reproduction
the following spring (Lee et al. 2006, Falk-Petersen et al. 2009,
Swalethorp et al. 2011). Most of the higher trophic levels rely on the lipids
accumulated in Calanus mainly as wax esters. These can be transferred
through the food web and incorporated directly into the lipids of the consumer
through several trophic levels. For instance, lipids originating from
Calanus can be found in the blubber of beluga and sperm whales, which feed
on fish, shrimps and squid (Smith & Schnack-Schiel 1990, Dahl et al. 2000)
and in the bowhead whale (B. mysticetus) and northern right whales (Eubalaena
glacialis), which feed mainly on Calanus (Hoekstra et al. 2002, Zachary
et al. 2009). Consequently, many biological activities – e.g. spawning and
growth of fish – are synchronised with the life cycle of Calanus. In larvae of
the Greenland halibut (Reinhardtius hippoglossoides) and sandeel (Ammodytes
sp.) from the West Greenland shelf, various copepod species, including
Calanus were the main prey item during the main productive season (May,
June and July). They constituted between 88% and 99% of the biomass of ingested
prey (Simonsen et al. 2006).
Vertical distributions of the Calanus species are influenced strongly by ontogenetic
vertical migrations that occur between the dark winter season and
the light summer season. For the most of the light summer season Calanus is
present in the surface waters. During summer and autumn, Calanus begins
to descend to deep-water layers for winter hibernation, changing the plankton
community structure in the upper water column from Calanus to smaller
copepod and protozooplankton dominance. The grazing impact on phytoplankton
by the smaller non-Calanus copepod community after Calanus has
left the upper layer can be considerably higher than in spring. This is a result
of shorter generation time and more sustained reproduction as well as relaxed
food competition and predation by Calanus (Hansen et al. 1999, and
references therein). The importance of small non-Calanus population in eco-