The phytoplankton development in the Barents Sea is typical for a high latitude region with a pronounced maximum in biomass and productivity during spring. During winter and early spring (January-March) both phytoplankton biomass and productivity are quite low. The spring bloom is initiated during mid-April to mid-May and may vary strongly from one year to another. The bloom duration is typically about 3–4 weeks and it is followed by a reduction of phytoplankton biomass mainly due to the exhaustion of nutrients and grazing by zooplankton. Later in autumn when the increasing winds start to mix the upper layer and bring nutrients to the surface a short autumn bloom can be observed. However, the time
development of this general description can vary geographically. For instance, the spring bloom at the ice edge in the Barents Sea can sometimes take place earlier than in the southern regions due to early stratification, a product of the ice melting.
Daily Net Primary Production (NPP) and open water area (OW) were calculated from satellite data as described in detail in Arrigo and Van Dijken (2015). Satellite-derived surface Chl a (Sat Chl a, Level 3, 8 day binned) was based on SeaWiFS retrievals for the years 1998 through 2002. MODIS/Aqua data are used 2003 onwards, using the latest reprocessing (R2014.0 for SeaWiFS and R2014.0/R2014.0.1 for MODIS/Aqua)). The work done here is in collaboration with Professor Kevin Arrigo and Gert van Dijken from the Stanford University, USA.
Interannual and Seasonal variability of Chl a
One of main aims in our ongoing work is to validate satellite data using observations. Previously published results from the Fugløya- Bjørnøya (FB) section show that the seasonal dynamics and magnitude of the Satellite Chl a concentration is highly correlated with the observed in situ Chl a concentrations, both for the upper 20 m and 50 m (Dalpadado et al., 2014). In their study, the seasonal dynamics of in situ Chl a and meso- zooplankton biomass at the FB section show that the development of zooplankton starts with a lag time of one month after the initiation of the phytoplankton bloom, and that maximum biomass occurs from June through September. We have spatial in situ Chl a observations mostly from the autumn months (Aug. and Sep.) in the Barents Sea. Comparing near real-time satellite and in situ observations show that there are significant positive relationships between the two variables, providing that both types of data have a good coverage (Figure 3.2.1).
Figure 3.2.1. Near real-time satellite data from autumn on chlorophyll concentrations together with locations of in situ observations (maps) and relationship between the two variables (graphs).
As remote sensing data provide good spatial and temporal coverage, we use s these data to explore interannual variability of chlorophyll concentrations. Satellite data from the Barents Sea showed that there is large year-to-year variability of Chl a concentrations, with a general increasing trend over the years (Figure 3.2.2).
Figure 3.2.2. Interannual variability of satellite based Chl a - average annual mean.
Net Primary Production (NPP)
Figure 3.2.3 Polygon regions in the Barents Sea. Note that two of the polygons (Central Bank and Bear Island Trench) are additional divided in the Zooplankton work: Central Bank is further divided into two regions (Great and Central Banks) and Bear Island Trench into three (Bear Island Trench, Hopen Deep and Thor Iversen Bank).
Remote sensing data were explored using the polygon areas shown in Figure 3.2.3.
Satellite based Net Primary Production (NPP) of the total polygon area show that there is significant interannual variability of the net primary production during the period 1998–2016. However, the general trend shows that NPP has increased over the years in the Barents Sea (Figure 3.2.4). The increase is mainly due to the fact that ice coverage has been reduced leading to larger ice-free areas and longer growth period (Figure 3.2.7-9; Dalpadado et al., 2014; Arrigo and Van Dijken, 2011; 2015). Furthermore, the mean production per unit area in general has also increased over the years. Our results show that the mean daily production rate (mg C M-2 day-1) averaged over the growing season has increased considerably since 2010, from 446 (1998–2009) to 596 mg C M-2 day-1 (2010–2016) during the first half of the year (not shown). The NPP in the eastern regions (South East and North east polygons) has increased significantly during the study period (Figure 3.2.3). The NPP in the Svalbard region was highest among the northern polygons, showing also an increasing trend over the years (Figure 3.2.5). The NPP in the south west polygon showed large interannual variability, with no marked increasing trends (not shown).
The new production (NP) estimated using nitrogen consumption (seasonal draw- down of nitrate in the water column) for the Fugløya-Bjørnøya (FB) and Vardø Nord (VN) sections from March to June show that the results were comparable to satellite NPP values (Rey et al., in prep). In their study, primary production is of the same order in both sections indicating that most production from March to June is based on winter nitrate. Their results show that the yearly NP was about 60% of the total NPP production indicating that nearly half of the annual production occurs during the spring bloom and is fuelled by winter nutrients. A study by Sakshaug et al. (2009) also show that new production can contribute ca. 50% of the total production in the Barents Sea.
Figure 3.2.4. Annual net primary production (NPP- satellite based) in the Barents Sea. Note that 2016 data are processed only until mid-September.
Figure 3.2.5 Annual net primary production (NPP- satellite based) in the South East and North East polygons.
Figure 3.2.6. Annual net primary production (NPP- satellite based) in 3 northern polygon areas.
Open Water Area (OWA)
The concentration of sea ice in the Arctic has dropped by ca. 9% per decade since 1978 and has been accompanied with reduced sea-ice thickness and duration (Arrigo and Van Dijken, 2015 and references therein). Due to reduction of sea ice, the OWA (maximum ice free waters in late summer or autumn) in the Barents Sea has increased over the years, possibly leading to higher NPP in the region. Satellite based Open Water Area (OWA) estimates confirm this (Figure 3.2.7A). The increase is most significant in the North East and South East polygons (Figure 3.2.7B).
Figure 3.2.7. Open water area A), whole Barents Sea and B) Eastern Barents Sea
There was a significant relationship of increasing satellite-based NPP with increasing OWA (Figure 3.2.7). In addition, NPP was also related to increasing chlorophyll a concentration (Figure 8). The increasing trend in NPP shown in Figure 3.2.8 is therefore a reflection of an increase in both OWA and average biomass of phytoplankton (Chlorophyll a).
Figure 3.2.8. Relationship between satellite derived NPP and Chl a and Open Water Area (OWA).
We have defined the length of the growing season as the number of days between start and end of seasonal phytoplankton growth. Start of growing season = first day that there is a valid chlorophyll pixel in a polygon. End of growing season = last day that there is a valid chlorophyll pixel in a polygon. Remote sensing results showed that the growing season has increased from an average of 166 to 174 days between the periods 1998-2005 and 2006-2016 (Figure 3.2.9).
Figure 3.2.9. Average length of the growing season (number of days) in the Barents Sea.
- Validations performed on in situ observations and satellite data show that for the Barents Sea, the model by Arrigo et al. (2008) gives reasonable results that compare well with observed measurements (Dalpadado et al., 2014, Rey et al. (in prep) and ongoing TIBIA work)
- Spatially integrated production (NPP) has increased over the years in most polygon regions. A noteworthy increase is observed in the Eastern regions (North East and South East), where sea ice coverage has diminished over the years.
- There is a highly significant relationship between Chlorophyll, Open water area (ice-free area) and integrated production (NPP). The increase in ice-free area provides improved habitat for phytoplankton growth as the growing season (length of number of days with open water) has increased.
- Our investigations reveal that the major part of annual production has taken place by day 200. Published work also show that nearly half of the annual production occurs during the spring bloom and is fuelled by winter nutrients