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

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.

Automated summary

The subarctic northeast Pacific Ocean (SNEP) is a high-nitrate, low-chlorophyll region where chlorophyll a concentration is persistently low due to iron (Fe) limitation. Observations from a Biogeochemical-Argo float (BGC-Argo) and a physical-biogeochemical coupled model at OSP show that phytoplankton growth is influenced by seasonal variability and iron limitation, with grazing pressure also playing a significant role in phytoplankton dynamics.