Upper Ocean Biogeochemistry of the Oligotrophic North Pacific Subtropical Gyre: From Nutrient Sources to Carbon Export

Abstract

Subtropical gyres cover 26%–29% of the world's surface ocean and are conventionally regarded as ocean deserts due to their permanent stratification, depleted surface nutrients, and low biological productivity. Despite tremendous advances over the past three decades, particularly through the Hawaii Ocean Time‐series and the Bermuda Atlantic Time‐series Study, which have revolutionized our understanding of the biogeochemistry in oligotrophic marine ecosystems, the gyres remain understudied. We review current understanding of upper ocean biogeochemistry in the North Pacific Subtropical Gyre, considering other subtropical gyres for comparison. We focus our synthesis on spatial variability, which shows larger than expected dynamic ranges of properties such as nutrient concentrations, rates of N2 fixation, and biological production. This review provides new insights into how nutrient sources drive community structure and export in upper subtropical gyres. We examine the euphotic zone (EZ) in subtropical gyres as a two‐layered vertically structured system: a nutrient‐depleted layer above the top of the nutricline in the well‐lit upper ocean and a nutrient‐replete layer below in the dimly lit waters. These layers vary in nutrient supply and stoichiometries and physical forcing, promoting differences in community structure and food webs, with direct impacts on the magnitude and composition of export production. We evaluate long‐term variations in key biogeochemical parameters in both of these EZ layers. Finally, we identify major knowledge gaps and research challenges in these vast and unique systems that offer opportunities for future studies.

Climate variability shifts the vertical structure of phytoplankton in the Sargasso Sea

“…Our results indicate the understudied subsurface community, below the mixed-layer and hidden from satellite view, is important for controlling the variability of column integrated phytoplankton carbon in regions such as the Sargasso Sea. This subsurface community is also supported by new nutrients and may contribute significantly to new production and export production (Bouman et al, 2020;Dai et al, 2023). Considering its position in the water column, closer in space to the daily vertical migrations of zooplankton, this subsurface community may sustain different trophic pathways within the marine ecosystem than that of the community in the surface mixed-layer, with implications for secondary production, trophic energy transfer, and fisheries.…”

Enhanced Phosphate Consumption Stimulated by Nitrogen Fixation Within a Cyclonic Eddy in the Northwest Pacific

“…The subtropical Northwest Pacific, characterized by low nutrients and low phytoplankton biomass (Dai et al., 2023; Martiny et al., 2019), is increasingly impacted by eddies (Chelton, Schlax, & Samelson, 2011; Y. Liu et al., 2012; Qiu & Chen, 2010). Accordingly, upwelling of deep waters in this region via cyclonic eddies could facilitate an enhanced nutrient supply into the euphotic zone, potentially temporarily alleviating limitations on bulk phytoplankton growth imposed by N, phosphorus (P), and/or iron (Fe) availability (Browning et al., 2022; Q. Li et al., 2015; Moore et al., 2013; Yuan et al., 2023).…”

“…The subtropical Northwest Pacific, characterized by low nutrients and low phytoplankton biomass (Dai et al, 2023;Martiny et al, 2019), is increasingly impacted by eddies Y. Liu et al, 2012;Qiu & Chen, 2010).…”

Ocean’s largest chlorophyll-rich tongue is extending westward

“…Yet, following decades of climate change, the long-term variation in the CRT remains unclear, despite various changes in atmospheric circulation and ocean currents have been observed in the equatorial Paci c 9-14 . This is noteworthy, especially considering that the oligotrophic subtropical gyres in the global ocean were suggested to be expanding in recent two decades due to strengthened vertical strati cation induced by global warming [15][16][17][18] . It thus prompts a crucial question: is the CRT in the equatorial Paci c, located right in-between the expanding north and south Paci c subtropical gyres, inevitably shrinking?…”