Climate-Driven High Primary Production and Contrasting Export Production in the Eastern North Pacific Subtropical Gyre

Joo-Eun Yoon,Ju-Hyoung Kim,Il-Nam Kim

doi:10.3389/fmars.2021.710540

Abstract

The eastern North Pacific subtropical gyre (NPSG) contributes significantly to global primary production (PP) and export production (EP). In this study, we have investigated the impact of the North Pacific gyre oscillation (NPGO) mode on the temporal changes in the relationship between PP and EP in the eastern NPSG, using long-term time series of oceanographic observations at Station ALOHA. The positive NPGO phases (N2+: 1998–2004, N4+: 2007–2013), exhibiting a deeper mixed layer depth (MLD), coincided with high PP. Moreover, the N2+ phase showed high EP, associated with an increase in the nano-sized phytoplankton group, and inorganic and organic nitrogen-to-phosphorus ratios. However, multiple physical and biogeochemical factors, such as thermocline depression, increase in pico-sized phytoplankton groups, smallest-sized mesozooplankton, and heterotrophic bacteria, have induced low EP during the N4+ phase, despite deep MLD and increased PP conditions. Enhanced stratification under prolonged warming indicates that the surface eastern NPSG may experience a permanent shift toward small cells.

Highlights

The eastern region of the North Pacific subtropical gyre (NPSG), a well-known crucial open-ocean ecosystem, is characterized by basin-scale ocean circulation
To consider the major sources of sinking POC caught in sediment traps deployed at 150 m, we investigated biogeochemical and ecological parameters for the upper 100 m of the water column, which reflect the organic carbon produced in the upper winter mixed layer depth (MLD) (December, January, and February; 81.5 ± 25.8 m) (Lampitt et al, 2008; Yoon et al, 2018)
Previous studies have reported that the positive phase of North Pacific gyre oscillation (NPGO), which is associated with an intensified high-pressure system over the eastern NPSG, is characterized by cool T, whereas the negative phase exhibits the opposite

Summary

INTRODUCTION

The eastern region of the North Pacific subtropical gyre (NPSG), a well-known crucial open-ocean ecosystem, is characterized by basin-scale ocean circulation. We analyzed the physical and biogeochemical datasets of the eastern NPSG based on water samples obtained by the Hawaii Ocean time-series (HOT) group at Station ALOHA at approximately monthly intervals starting from January 1992 (Karl, 1999). The biogeochemical and ecological responses of the eastern NPSG to the NPGO mode were analyzed by comparing the variability in biological (PP), geological [contents of sedimenttrap-based particulate organic carbon flux (POC150), particulate organic nitrogen flux (PON150), and particulate organic phosphorus flux (POP150)], chemical [dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), low-level nitrate + nitrite (LLN), and low-level phosphate (LLP)], and ecological (surface phytoplankton size fractions, heterotrophic bacteria abundance, and smallest-sized mesozooplankton biomass) factors. A two-sample Student’s t-test was applied with the null hypothesis that there was no difference in averages between the two groups, using MATLAB’s ttest function (MathWorks, 2021)

RESULTS AND DISCUSSION

DATA AVAILABILITY STATEMENT

CONCLUSION AND SUMMARY