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

Hao-Ran Zhang, Yuntao Wang*, Peng Xiu, Fei Chai

*Corresponding author:

ABSTRACT: The subarctic northeast Pacific Ocean (SNEP) has been identified as a high-nitrate, low-chlorophyll region where, due to iron (Fe) limitation, the 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 region near Ocean Station Papa (OSP) of SNEP in 2012 show prominent variations in chl a in the upper ocean (0-150 m). The chl a peaks in late summer and autumn and at a value >3 times that in winter; these observations at fine resolution can help us better investigate 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. The seasonal variability of chl a in the upper layer is reproduced by the model, and the result is reasonably comparable with the BGC-Argo observations in 2012 in the ocean near the OSP. Phytoplankton growth is limited in winter by the deep mixed layer, which reduces light availability, but gradually increases later in the year due to the higher light availability associated with the shoaling of the mixed layer and increasing solar radiation. In spring and early summer, the scarcity of Fe limits phytoplankton growth, especially diatom growth, and the high grazing pressure on small phytoplankton from overwintering microzooplankton inhibit the accumulation of phytoplankton biomass. Chl a accumulates throughout the summer and reaches its peak in late summer and autumn because mesozooplankton begin to consume microzooplankton, which reduces microzooplankton grazing on phytoplankton. Although the model study may be restricted to the seasonal pattern of BGC-Argo observations only in 2012, this study nevertheless fully describes the seasonal variability in chl a in the SNEP and is crucial to understanding mechanisms underlying phytoplankton dynamics in high-nitrate, low-chlorophyll regions.