Yuan, Zhongwei, Browning, Thomas J. , Du, Chuanjun, Shen, Hui, Wang, Lei, Ma, Yifan, Jiang, Zong‐Pei, Liu, Zhiyu, Zhou, Kuanbo, Kao, Shuh‐Ji and Dai, Minhan (2023) Enhanced Phosphate Consumption Stimulated by Nitrogen Fixation Within a Cyclonic Eddy in the Northwest Pacific. Journal of Geophysical Research: Oceans, 128 (11). 10.1029/2023JC019947. DOI 10.1029/2023JC019947.

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Abstract

Mesoscale eddies are common in the subtropical Northwest Pacific, however, relatively little is known about their spatial variability and temporal evolution, and how these impact upper ocean biogeochemistry. Here we investigate these using observations of a cyclonic eddy carried out along four sequential transects. Consistent with previous observations of cyclonic eddies, the eddy core had doming isopycnals, bringing elevated nutrient waters nearer to the surface. However, we also found that the upper layer of the eddy above the nutricline had significantly lower phosphate concentrations within its core relative to its edge. We attributed this to elevated N 2 fixation within the eddy core, which was likely driven by enhanced subsurface iron supply, ultimately resulting in increased phosphate consumption. Eddy‐enhanced N 2 fixation was additionally supported by the elevation of nitrate + nitrite to phosphate ratios below the euphotic zone. Moreover, we observed that while the upward displacement of isopycnals within the eddy core led to an increase in phytoplankton biomass in the lower euphotic zone, there was no significant increase in total phytoplankton biomass across the entire euphotic zone. Cyclonic eddies in the subtropical North Pacific are projected to be becoming more frequent, implying that such dynamics could become increasingly important for regulating nutrient biogeochemistry and ultimately productivity of the region.

Key Points

Lower phosphate concentrations were observed above the nutricline within the eddy core in comparison to the edge

Enhanced N2 fixation within the eddy core is proposed to have driven increased phosphate consumption

No substantial total phytoplankton biomass increase was found within the eddy core

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