Higher Phytoplankton Biomass Drives Recent Increases in Arctic Production

The reduction in sea ice extent in the Arctic Ocean over the last few decades has resulted in both a longer phytoplankton growing season and increased open water habitat for phytoplankton growth. Consequently, annual net primary production (NPP) over the entire Arctic Ocean increased 30% between 1998 and 2012, the year that summer sea ice extent reached its historical minimum. During that time period, the largest increases in annual NPP were restricted to the interior Arctic shelves (Laptev, Kara, Siberian), where NPP increased 70-112% over the 15-year study and loss of sea ice was most severe (4.2-5.4%/yr). In contrast, NPP on the outflow shelves showed a much smaller increase or even a decrease in response to much smaller declines in sea ice cover (0-0.9%/yr). However, a more recent study has shown that since reaching its minimum sea ice extent in 2012, the rate of increase in open water area in the Arctic Ocean has slowed considerably, decreasing from 88,000 km² yr^-1 between 1998 and 2012 to only 620 km² yr^-1 between 2012 and 2018. In addition, the average number of open water days in the Arctic only increased by five days between 1998 and 2018. Nevertheless, annual NPP continued to increase at rate of 6.8 Tg C/yr between 1998 and 2018, peaking at 391 Tg C/yr in 2018. Notably, despite the flattening in the trend in open water area between 2012 and 2018, the rate of increase in annual NPP over the Arctic during that time period (13.5 Tg C/yr) was double that between 1998 and 2012 (6.4 Tg C/yr). Whereas the increase in annual NPP between 1998 and 2012 was due primarily to the decline in sea ice and an increase on open water phytoplankton habitat, the increase in annual NPP between 2012 and 2018 was due to increases in phytoplankton biomass. Prior to 2012, concentrations of chlorophyll a (Chl a) across the Arctic Ocean were relatively constant, increasing by only 0.002 mg m^-3 (or 0.27%) per year. However, between 2012 and 2018, mean Chl a concentration in Arctic Ocean waters increased at a rate of 4.3% per year, 16 times faster than the rate prior to 2012. The largest increases in Chl a between 1998 and 2018 were measured on the inflow shelves of the Chukchi Sea and Barents Sea, and at the recently exposed shelf break of the Laptev Sea, suggesting that increased nitrogen supply may be responsible for the increase in NPP since 2012.