Abstract
The North Pacific Subtropical Gyre (NPSG) is the largest ecosystem on Earth, and it plays a critical role in global ocean productivity and carbon cycling. Here, we report a rare and striking ~2000-km-long phytoplankton bloom that lasted over one month in the western part of the NPSG in summer 2003. The bloom resulted from the co-occurrence of a northward-shifted North Equatorial Current (NEC) supplying additional phosphate, and strong eddy activity that fueled productivity and spread chlorophyll mainly through horizontal stirring. The extensive one-month bloom had a maximum Chl concentration of six times the summer mean value and collectively fixed an additional five teragrams (5 × 1012 g) of carbon above the summer average. An increase in the pCO2 during the bloom suggests that most of the additionally fixed carbon was rapidly consumed.
Highlights
The western part of the North Pacific Subtropical Gyre is a hotspot of eddies[1]
The one-month bloom began around July 4 and reached its peak on July 12, 2003, with a Chl concentration of 0.4 mg/m3, which is at least six times higher than the summer mean value from 1998 to 2014 (Fig. 1b,c)
From the Argo and CTD’s vertical temperature profiles, we demonstrate that there was a possible deepening of the vertical mixing when the bloom occurred in the wNPSG in 2003
Summary
The western part of the North Pacific Subtropical Gyre (wNPSG, 130°E to 180°E centred along 22°N) is a hotspot of eddies[1]. Strong vertical stratification in summer inhibits the mixing of the surface water with the deep, nutrient-rich water. These conditions result in a chronically nutrient-limited surface layer, making the NPSG the least biologically productive region per unit area in the global ocean[10]. Despite being nutrient-limited in summer, phytoplankton blooms have been reported in the central and eastern part of the NPSG11–13. The mechanisms driving these blooms include nitrogen fixation by diazotrophs[14, 15], eddy interactions[16], internal waves[17], and mixing at the subtropical front[18]. We analysed the drifter data and performed an eddy composite analysis to show that the stirring induced by the strong eddy currents was the main mechanism sustaining the summertime bloom
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