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Marine Ecology Progress Series

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MEPS 680:33-50 (2021)  -  DOI:

Modeling the seasonal variability of phytoplankton in the subarctic northeast Pacific Ocean

Hao-Ran Zhang1, Yuntao Wang1,*, Peng Xiu2, Fei Chai1,3

1State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 300012, PR China
2State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, PR China
3School of Marine Sciences, University of Maine, Orono, ME 04469, USA
*Corresponding author:

ABSTRACT: The subarctic northeast Pacific Ocean (SNEP) is a high-nitrate, low-chlorophyll region where, due to iron (Fe) limitation, chlorophyll a (chl a) concentration is persistently low throughout the year despite abundant nitrate. Observations from a Biogeochemical-Argo float (BGC-Argo) deployed in the SNEP in 2012 show prominent variations in chl a in the upper ocean (0-150 m). Chl a peaks in late summer and autumn at a value >3 times that in winter; these observations at a fine resolution can help us disentangle the mechanisms driving the full seasonal cycle of phytoplankton. A one-dimensional physical-biogeochemical coupled model was applied at OSP to investigate the factors driving phytoplankton dynamics in the SNEP. Seasonal variability of chl a in the upper layer is reproduced by the model; results are reasonably comparable to the BGC-Argo observations in 2012 near OSP. Phytoplankton growth is limited in winter by the deep mixed layer, which reduces light availability; light gradually increases later in the year due to shoaling of the mixed layer and increasing solar radiation. In spring and early summer, Fe limitation restricts phytoplankton growth, especially diatoms, and grazing pressure on small phytoplankton from overwintering microzooplankton inhibits accumulation of phytoplankton biomass. Chl a accumulates throughout the summer, peaking in late summer and autumn as mesozooplankton consume microzooplankton, which reduces microzooplankton grazing on phytoplankton. Although the model study was restricted to the seasonal pattern of BGC-Argo observations in 2012, it nevertheless fully describes seasonal variability in chl a in the SNEP and is crucial to understanding mechanisms underlying phytoplankton dynamics in high-nitrate, low-chlorophyll regions.

KEY WORDS: Iron limitation · High-nitrate low-chlorophyll · Microzooplankton grazing · Physical-biogeochemical coupled model · Phytoplankton dynamics · Seasonal variability

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Cite this article as: Zhang HR, Wang Y, Xiu P, Chai F (2021) Modeling the seasonal variability of phytoplankton in the subarctic northeast Pacific Ocean. Mar Ecol Prog Ser 680:33-50.

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