Eddies and fronts influence pelagic communities across the eastern Pacific ocean

Abstract

The behaviors of micronekton and zooplankton across the eastern Pacific Ocean are poorly known, as pelagic fauna in this region are rarely sampled. One particularly understudied aspect of these community dynamics is their distribution and responses to mesoscale oceanographic features. To explore this, we conducted active acoustics and oceanographic surveys at two locations within the eastern Pacific Ocean with Saildrone uncrewed surface vehicles. Using 38 and 200 kHz data collected by these platforms, we evaluated the vertical structure and migration patterns of micronekton and large zooplankton in the upper 1000 m of the water column within survey areas at the western and eastern ends of this region. We found stronger and shallower acoustic backscatter and stronger DVM intensity within the eastern area, likely driven by its significantly lower midwater oxygen and higher surface productivity compared to the western area. The passage of cyclonic and anticyclonic eddies through the sampling areas was found to correlate with changes in the vertical structure of micronekton in both survey areas by shoaling and deepening acoustic scattering layers, respectively. Further, the strength of mesopelagic scattering layers was greatly increased along the northern frontal boundary of a strong, westward near-surface current in our eastern study area. Our results highlight the considerable natural variability of pelagic community dynamics on daily to weekly scales in the eastern Pacific Ocean in addition to broader regional patterns. Such variation is inherent to open ocean conditions and plays a large role in aggregation and predator–prey dynamics. Describing the interactions between mesoscale features and pelagic fauna may help us understand how these communities will respond to a changing ocean environment— induced by anthropogenic activities and broader climate-related shifts. This is the first study to use Saildrones to examine micronekton in the open ocean and provides some of the only remote sampling of pelagic prey communities in the eastern Pacific Ocean.