The Barents Sea has a number of water masses with the relatively warmer and more saline (>35) Atlantic Water which flows through the southern part of the Barents Sea, and the colder, less saline (34.4-35) Arctic Water to the north. The boundary between these two water masses is marked by the Polar Front, and the different physical and chemical properties of these water masses influence the growth and development of the resident phytoplankton species (Loeng and Drinkwater, 2007).
Seasonal changes in sea-ice formation and melt, freshwater inputs into coastal waters and seasonal changes in solar radiation also result in the formation of stratified layers with different populations of phytoplankton compared to those lower in the water column, Recent declines in the extent, thickness and duration of ice cover in the Northern Barents Sea are expected to result in a poleward movement of phytoplankton species, and an earlier date for the spring bloom of both open water and sea-ice algal communities (Wassmann, 2011).
Current phytoplankton gross primary production averages about 90 g C m-2 y-1 ( 19%) in the Barents Sea (Wassmann et al. 2006, Wassmann 2011), with lower values (up to about 60 g C m-2 y-1) found under the northern and north-eastern sea-ice covered regions. However there is much spatial and inter-annual variability, both due to the variability of the physical conditions and also the occurrence of phytoplankton bloom forming species (notably diatoms, and the prymnesiophytes Phaeocystis pouchetti and Emiliania huxleyi). A review of estimates of gross primary production for different regions of the Barents Sea is provided by Wassmann et al. (2006). Gross primary production is more variable towards northern and eastern regions of the Barents Sea and least variable in the region north of Norway (Wassmann, 2011). The Norwegian Institute for Marine Research maintains two regular ship sampling transects in this latter region, the Fulgøya-Bjørnøya transect (FB) and the Vardø-Nord transect (VN).
Species succession follows a general pattern over the growing season, though again there is much interannual variability along transects (FB and VN) surveyed. The mean pattern for the FB transect for the period 2008-2012 is shown in Figure 2.4.6. Cell numbers of all species are low in the winter period. With increasing solar radiation and stratification of the surface ocean, phytoplankton numbers begin to increase in spring typically beginning in the coastal waters (Loeng and Drinkwater, 2007).
On average, diatoms form the first peak during April, followed by flagellate and ciliate species in May. A second peak of diatoms occurs during June-July, together with peak dinophyte and cryptophyte cell numbers. Late summer is characterized by high numbers of flagellate species (Rey, 2004).
In general, phytoplankton species in the coastal pelagic zone have a more complicated annual cycle compared to open shelf areas. For instance, monitoring by the Murmansk Marine Biological Institute (MMBI) of subarctic coastal systems in the eastern Barents Sea shows two periods of peak abundance taking place; one in early spring and one in late spring. Moreover, once seasonal stratification has established the summer stage sets in; this starts with a peak in early-summer and ends with another peak in autumn.
In these coastal waters, the start of spring phytoplankton activity (mid-March) is linked to the emergence of early-spring diatoms, namely Thalassiosira hyalina (Grun.) Gran, T. cf.gravida Cl., Navicula pelagica Cl., N. septentrionalis (Grun.) Gran, Nitzschia grunowii Hasle, and Amphora hyperborea (Grun.). Сell abundance in this period is low and can range from several dozens to several hundred cells l-1. The first spring maximum takes place in mid-April and occurs due to early-spring neritic arcto-boreal diatom species such as Thalassiosira, Chaetoceros, Navicula, and Nitzschia. Parameters of quantitative phytoplankton development reach their maximums that then remain for a few days. Phytoplankton abundance during early-spring bloom ranges from several hundred thousand to 2 million cells l-1, and biomass ranges from 1 to 3 mg l-1. At this period, the core of the community is concentrated in the upper 10-cm layer. Species forming the first maximum phytoplankton bloom are: Thalassiosira cf. gravid; Т. Nordenskioeldii; Chaetoceros socialis; C. furcellatus; and Navicula vanhoeffenii.
The second spring maximum (late May to early June) is linked to continental freshwater runoff, and the beginning date, quantitative characteristics, and qualitative structure varies from year to year, depending on when the maximum runoff takes place. In most cases, the phytoplankton activity repeats the first spring event, potentially with reduced number of dominants. However, in years with the low freshwater runoff, Phaeocystis pouchetii dominates in the bloom in the pelagic zone. The summer period (end of June – end of August) is marked with more of dinophyte microalgae in the phytoplankton community. The autumn succession cycle (from mid-September to early October) is usually associated to emergence of spring diatom forms in the pelagic zone. In this period, diatoms of the genus Chaetoceros and dinophytes of such genera as Ceratium, Dinophysis, and Protoperidinium dominate in the pelagic zone. Abundance does not exceed 2000 cells l-1 with biomass of less than 5 μg l-1.
During the winter period (mid-November through mid-March), the entire phytoplankton community is in a dormant stage. Phytoplankton in the pelagic zone mainly consists of large oceanic dinophyte algae of cosmopolitan and arcto-boreal origin. Abundance ranges from several to dozens of cells l-1. Ceratium longipes, C. tripos, Dinophysis norvegica, and Protoperidinium depressum form the core of the dominant complex.