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

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 N2 fixation within the eddy core, which was likely driven by enhanced subsurface iron supply, ultimately resulting in increased phosphate consumption. Eddy‐enhanced N2 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. Plain Language Summary: Cyclonic eddies are oceanographic features leading to upward transfer of deep‐water nutrients that support phytoplankton growth in the sunlit surface ocean. In the low‐nutrient Northwest Pacific, cyclonic eddies are considered to be an extremely important nutrient source in alleviating nutrient limitation; however, their biogeochemical effects are not well‐examined, in particular in the upper layers where nutrients are below the detection limit of conventional techniques. By examining a cyclonic eddy with four sequential transect observations, our results showed that the eddy core had higher nutrient concentrations in the lower euphotic zone compared to the eddy edge, consistent with previous studies; in contrast, concentrations of phosphate in the upper surface layer had significantly lower values within the core relative to those at the edge. Surface rate measurements suggested that this lower phosphate was potentially driven by elevated N2 fixation, likely due to enhanced iron supply from depth, thereby resulting in additional consumption of excess surface phosphate. However, this increased nutrient supply did not lead to the enhancement of total phytoplankton biomass across the entire euphotic zone. Key Points: Lower phosphate concentrations were observed above the nutricline within the eddy core in comparison to the edgeEnhanced N2 fixation within the eddy core is proposed to have driven increased phosphate consumptionNo substantial total phytoplankton biomass increase was found within the eddy core [ABSTRACT FROM AUTHOR]