Cross-basin differences in the nutrient assimilation characteristics of induced phytoplankton blooms in the subtropical Pacific waters

To better understand the nutrient assimilation characteristics of subtropical phytoplankton, deep-water addition incubation experiments were carried out on surface waters collected at seven stations across the subtropical North and South Pacific Ocean. These deep-water additions induced phytoplankton blooms with nutrient drawdown at all stations. The drawdown ratios of dissolved inorganic nitrogen (DIN) to phosphate (inline-formulaPO₄) varied from 14.1 to 30.7 at the inline-formulaPO₄-replete stations in the central North Pacific (CNP) and eastern South Pacific (ESP). These ratios were similar to the range represented by the canonical Redfield ratio (16) through to typical particulate inline-formulaN:P ratios in the surface subtropical ocean (28). In contrast, lower inline-formulaDIN:PO₄ drawdown ratios (7.7–13.3) were observed in induced blooms at the inline-formulaPO₄-depleted stations in the western North Pacific (WNP). The inline-formulaDIN:PO₄ drawdown ratios in the inline-formulaPO₄-replete ESP were associated with eukaryote-dominated blooms, while those in the inline-formulaPO₄-depleted WNP were associated with eukaryotic and cyanobacterial blooms. The surplus inline-formulaPO₄ assimilation, relative to DIN, by phytoplankton in the WNP was not expected based on their typical cellular inline-formulaN:P ratio and was likely due to the high inline-formulaPO₄ uptake capability as induced by low-inline-formulaPO₄-adapted phytoplankton. The low- and high-inline-formulaP^* (inline-formula $M14inlinescrollmathml=chem{\mathrm{normal\; PO}}_{\mathrm{normal\; 4}}-$ 42pt13ptsvg-formulamathimgbc5ca4f3082fc0837f02eab52e876b9b bg-18-897-2021-ie00001.svg42pt13ptbg-18-897-2021-ie00001.png  DIN/16) regimes geographically corresponded to the low and high inline-formulaDIN:PO₄ drawdown ratios in the WNP and the CNP or ESP, respectively. The basin-wide inline-formulaP^* distribution in the oligotrophic Pacific surface waters showed a clear regional trend from low in the WNP (inline-formula<50 nM) to high in the ESP (inline-formula>100 nM). These results suggest that the subtropical phytoplankton blooms as observed in our experiments could be an important factor controlling inline-formulaP^* as well as the commonly recognized dinitrogen fixation and denitrification characteristics.