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

Fuminori Hashihama,Hiroshi Ogawa,Ken Furuya,E Malcolm S Woodward,Iwao Tanita,Taketoshi Kodama,Hiroaki Saito,Philip W Boyd,Jota Kanda,Saori Yasui-Tamura

doi:10.5194/bg-18-897-2021

Abstract

Abstract. 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 (PO4) varied from 14.1 to 30.7 at the PO4-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 N:P ratios in the surface subtropical ocean (28). In contrast, lower DIN:PO4 drawdown ratios (7.7–13.3) were observed in induced blooms at the PO4-depleted stations in the western North Pacific (WNP). The DIN:PO4 drawdown ratios in the PO4-replete ESP were associated with eukaryote-dominated blooms, while those in the PO4-depleted WNP were associated with eukaryotic and cyanobacterial blooms. The surplus PO4 assimilation, relative to DIN, by phytoplankton in the WNP was not expected based on their typical cellular N:P ratio and was likely due to the high PO4 uptake capability as induced by low-PO4-adapted phytoplankton. The low- and high-P* (=PO4- DIN/16) regimes geographically corresponded to the low and high DIN:PO4 drawdown ratios in the WNP and the CNP or ESP, respectively. The basin-wide P* distribution in the oligotrophic Pacific surface waters showed a clear regional trend from low in the WNP (<50 nM) to high in the ESP (>100 nM). These results suggest that the subtropical phytoplankton blooms as observed in our experiments could be an important factor controlling P* as well as the commonly recognized dinitrogen fixation and denitrification characteristics.

Highlights

The surface waters of the subtropical oceans are characterized by strong stratification, low nutrients, and low phytoplankton biomass (Karl, 2002)
dissolved inorganic nitrogen (DIN) concentrations at the surface were consistently lower than 50 nM, while PO4 concentrations varied geographically and were extremely low in the western North Pacific (WNP) (< 10 nM; stations A, 2, and 5), intermediate in the central North Pacific (CNP) (53 nM; station 8), and high in the eastern South Pacific (ESP) (> 100 nM; stations 15, 18, and 21)
Total fucoxanthin (Tfuco) content in eukaryotes should be higher in the low-photosynthetically active radiation (PAR) regime than in the high-PAR regime, while Zea content in cyanobacteria should be higher in the high-PAR regime than in the low-PAR regime

Summary

Introduction

The surface waters of the subtropical oceans are characterized by strong stratification, low nutrients, and low phytoplankton biomass (Karl, 2002). In this regime, primary production is largely sustained by regenerated production (f ratio: ∼ 0.1, Dugdale and Goering, 1967; Eppley and Peterson, 1979) and driven by small phytoplankton such as the picocyanobacteria Prochlorococcus and Synechococcus (Waterbury et al, 1979; Chisholm et al, 1988). F. Hashihama et al.: Nutrient dynamics of subtropical phytoplankton blooms port production (Benitez-Nelson et al, 2007; McGillicuddy et al, 2007; Dore et al, 2008; Wilson and Qiu, 2008; Karl et al, 2012; Villareal et al, 2012; Hashihama et al, 2014). The mechanisms that bring about the development of these blooms are not simple, but fundamentally they involve nutrient supply with physical forcing (Wilson et al, 2013; Toyoda and Okamoto, 2017)

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