3.1 Spatial and Temporal Variabilities of Biomass and Daily Rates of PP of PSBs

Chl a concentrations and daily rates of PP during the PSB increased at latitudes greater than 75°N from 2003 to 2013 in the Arctic Ocean (Figures 1a and 1b). Reduction in chl a concentration observed around 72° to 75°N is likely due to a reduction in nutrients either explained by the occurrence of under‐ice blooms or an increase in stratification at these latitudes. We also observed low chl a concentrations and daily rates of PP during the PSB above 81°N in the Arctic basin where daily PAR is low (Figure 3a), and low concentrations of nutrients prevail all year long (Codispoti et al., 2013). Regarding the spatial distribution of PSBs, there was a strong variability in the total number of annual PSBs observed in the Arctic Ocean over the 2003–2013 time period (see the supporting information, Figure S4). A small number of annual PSBs are recorded in northern areas of the Arctic Ocean (>75°N), which were more covered by sea ice early in the time series. Moreover, at pan‐Arctic scale, there was a strong variability in the number of observations (i.e., chl a concentrations) for a given pixel during the PSB (see the supporting information, Figure S5). The frequent occurrence of clouds or low‐lying fog just after ice melt, especially during late summer and early fall, limited the number of available chl a data and made the ocean color observations patchy (Perrette et al., 2011).

We observed a strong regional variability of the PP trends in the PSB within the eight different sectors of the Arctic Ocean. Positive and significant annual trends in PP during the PSB were found in 11 (p value ≤ 0.05) and 3 (0.05 < p value ≤0.1) out of the 28 subsectors with sufficient observations from 2003 to 2013. The highest increase in PP was observed in the Barents (53.54 mg C m−2 day−1 year−1 between 73° and 76°N) and Kara (75.70 mg C m−2 day−1 year−1 between 82° and 85°N) subsectors (Figure 2). The Chukchi, Beaufort, Baffin, and Greenland sectors showed the lowest trends of annual PP of about 15 mg C m−2 day−1 year−1 over the 2003–2013 time period and for different ranges of latitudes between 70° and 82°N (Figure 2). Overall in the Arctic Ocean, we observed a significant increase in PP during the PSB of about 31% between 2003 and 2013 (an annual increasing trend of 21.43 mg C m−2 day−1 year−1, p value <0.01, R2 = 0.65; see the supporting information, Figure S1). Moreover, the spatial distribution of chl a concentration shows that high values occur mostly on the shelves compared to the central basin where the waters are considered as oligotrophic (Ardyna et al., 2011; Coupel et al., 2011, 2015; see the supporting information, Figure S6).

The sea ice breakup was observed later in the season at latitudes above 80°N in the Arctic Ocean (day of the year 195–250 on average, i.e., mid‐July to early September) compared to lower latitudes (day of the year 170–210 on average, i.e., mid‐June to late July). We found positive and significant annual trends in daily PAR beneath the sea surface during the PSB in four out of the 28 defined subsectors of the Arctic Ocean with sufficient observations from 2003 to 2013, such as the Barents, Kara, and Laptev sectors (see the supporting information, Figure S7). This can be explained by the fact that the breakup happened earlier in the season and closer to the summer solstice (Figure 3b; i.e., at the maximum of daily solar radiation, 21 June), resulting in a longer photoperiod and in an increase of PAR at these latitudes.

Our results suggest that for some subsectors located in the Barents, Kara, and Laptev sectors, an increase in daily PAR due to an earlier ice breakup contributes (i) to an increase in the accumulation of phytoplankton biomass due to favorable bloom dynamics (Behrenfeld et al., 2016) and (ii) to an increase in the daily rates of PP during the open‐water PSB observed between 2003 and 2013. This observation is also in accordance with the study of Zhang et al. (2010) who showed concomitant increases in PAR and marine PP in the Arctic Ocean between 1988 and 2007 with a coupled 3‐D pan‐Arctic model. The ongoing shift in the timing of ice breakup has therefore important consequences for PSB PP in the Arctic. However, other factors than PAR that could affect the phytoplankton bloom, such as nutrient availability and its potential replenishment in the water column, could explain the increase in daily rates of PP during the open‐water PSB for other subsectors and should be examined in the future (Popova et al., 2010; Rainville et al., 2011; Tremblay et al., 2015; Tremblay & Gagon, 2009). The observed spatially heterogeneous responses of the annual trends of PP suggest a dependence of PP on multiple confounding factors driving the Arctic ecosystem dynamics that needs to be better understood and characterized.

A mismatch between the spring bloom burst of phytoplankton and the activity of zooplankton grazers resuming winter diapause could also explain this increase in the daily rates of PP during the open‐water PSB, since the life cycles of phytoplankton and zooplankton grazers are tightly linked to the timing of the blooms (Behrenfeld et al., 2016; Ji et al., 2013; Leu et al., 2011; Søreide et al., 2010). The PSB is usually followed by a long‐delayed peak in zooplankton biomass, dominated by large copepods of the genus Calanus that overwinters at depth (Ashjian et al., 2003; Campbell et al., 2009). Because of a low grazer biomass and activity at the onset of the PSB, much of the PP may not be consumed in the water column, as it has been documented, for example, in the Chukchi and Beaufort Seas where more than half of the PP may not be consumed by the zooplankton (Campbell et al., 2009; Sherr et al., 2009).

The increasing PP in PSBs might have contributed to a significant fraction of increased open ocean PP that has notably been observed in recent Arctic studies (Arrigo & van Dijken, 2015; Bélanger et al., 2013; Pabi et al., 2008). It is also important to note that these observations do not account for PP that could occur within or beneath sea ice cover or for PP located at the subsurface chlorophyll maximum. Major phytoplankton under‐ice blooms have recently been observed in different regions of the Arctic Ocean (Arrigo et al., 2012; Fortier et al., 2002; Mundy et al., 2009). The occurrence of such under‐ice blooms, invisible to satellite remote sensing, may increase in the future in an Arctic Ocean dominated by thinner first year ice (Lowry et al., 2014; Mundy et al., 2009).