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

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4.1.3 The spring phytoplankton bloom in Davis Strait
The spring phytoplankton bloom (i.e. spring bloom) usually begins in
Southwest Greenland in late March/early April. In the ice-covered areas
timing of its onset is determined by withdrawal of the Davis Strait pack ice
(the West Ice). However, most of the southeastern part of the assessment area
generally has open water all year around due to the warm West Greenland
Current. In the southwestern part of the assessment area the sea ice retreats
in March and the northern part usually has open water April-May. Sea
ice is therefore generally not considered a limiting factor for initiation of the
spring bloom in the assessment area.
A multidisciplinary ecological survey programme (2005-2009) is presented
in the annual ‘Nuuk Basic’ reports, presenting sampling from the inner
Godthåbsfjord to Fyllas Banke, southwest Greenland (Juul-Pedersen et al.
2008, Rysgaard et al. 2008, Juul-Pedersen et al. 2009) and described in detail
for the year 2006 (Arendt et al. 2010). The following biological descriptions
focus mainly on the Fyllas Banke area, which is an area of key importance in
the assessment area. Based on measurements of phytoplankton concentration
for 2010, elevated values occur in March and early April with peak sea
surface concentrations in late April and early May (see also Fig. 4.1.1). Phytoplankton
biomass then decreases throughout the summer, usually associated
with the pycnocline. In late summer in August there is usually a minor
secondary bloom peak.
High concentrations of chlorophyll a (chl a) were frequently measured at the
outer Fyllas Banke. A high integrated phytoplankton biomass (chl a converted
to carbon) in the central parts of Fyllas Banke was measured to 4857mg C
m -2 in the upper 50 m (Arendt et al. 2010). High chl a biomass was also
found in another shallow water area at Store Hellefiskebanke, northeast of
the assessment area. The shallow banks keep the phytoplankton in the photic
zone where net growth is possible. Strong tidal mixing may also feed the
upper layers with nutrients (i.e. upwelling), which boosts the bloom even
more. Upwelling areas are, for example, found at the fishery banks in South
and West Greenland e.g. Fyllas Banke and Store Hellefiskebanke. Upwelling
areas may, besides enhanced production, also retain copepods, which again
are utilised by fish larvae (Simonsen et al. 2006). Therefore, the bank areas
are important for increased primary productivity and carbon cycling caused
by nutrient-rich upwelling events from wind and tidal motions in the Davis
Strait.
4.1.4 Productivity at polynyas and shear zones
Polynyas are predictable open-water areas in otherwise ice-covered waters
in winter and spring. Part of the assessment area has open water all year
around, and thereby acts much like a polynya; although always open to the
south. In polynyas primary production starts much earlier than in icecovered
areas, which means they often are preferred feeding areas for marine
mammals and seabirds. However, the mere presence of open water
makes polynyas attractive for resting seabirds and for mammals that are dependent
on open waters for breathing. Many migrating seabirds also use polynyas
as staging grounds on their way to breeding grounds further north.