Vertical motions associated with mesoscale ocean eddies modulate the light and nutrient environment, stimulating phytoplankton biomass and chlorophyll anomalies. Populations within eddies may be trapped by the horizontal circulation or laterally diluted by mixing with surrounding waters. Conventionally, eddy boundaries are determined using Eulerian methods from daily satellite sea level anomaly fields. However, Eulerian methods do not delineate the bounds of trapped water masses, which is an important consideration when interpreting eddy-induced changes in chlorophyll concentration. Integrated Lagrangian particle tracking methods, on the other hand, more accurately identify coherent structure boundaries that trap fluid masses for time scales relevant to phytoplankton bloom evolution. From two decades of remote sensing observations in the North Pacific Subtropical Gyre, we compared coincident Eulerian and Lagrangian eddy atlases to assess the impact of eddy trapping on chlorophyll concentration. We found more positive chlorophyll anomalies within Lagrangian coherent vortices than in Eulerian eddy boundaries and outside-eddy waters. Yet, there are striking differences seasonally and regionally within the gyre. Chlorophyll is most enriched within coherent boundaries of the Hawaiian Lee eddies and in the region southeast of the Islands in fall and winter. Our results suggest that typical Eulerian analyses underestimate the role of mesoscale eddies in enhancing chlorophyll by not accounting for lateral mixing. Consequently, quantifying the contribution of mesoscale eddies toward open ocean primary production is more complex than previously assumed.

Vertical motions associated with mesoscale ocean eddies modulate the light and nutrient environment, stimulating anomalies in phytoplankton biomass and chlorophyll. Phytoplankton populations can be subsequently trapped by the horizontal circulation or laterally diluted. In a time-varying flow, Lagrangian methods can be used to quantify eddy trapping, also known as Lagrangian coherency. From two decades of remote sensing observations in the North Pacific Subtropical Gyre, we compared coincident Eulerian and Lagrangian eddy atlases to assess the impact of eddy trapping on chlorophyll concentration. We found higher chlorophyll within Lagrangian coherent boundaries than in Eulerianeddies and outside-eddy waters. Yet, there are differences regionally and seasonally. For example, chlorophyll is most enriched within coherent boundaries of the Hawaiian Lee eddies and to the south of 23N in fall and winter. Our results suggest that by not accounting for lateral dilution, Eulerian analyses may underestimate the role of mesoscale eddies in enhancing chlorophyll.