A Lagrangian perspective of mesoscale biophysical interactions in the subtropical ocean

Abstract

The most kinetic energy in the ocean is at the mesoscale, which includes highly dynamic physical perturbations that persist for months, a biologically relevant timescale for phytoplankton growth and bloom development. Importantly, mesoscale currents and the associated biological responses (i.e., biophysical interactions) are not spatiotemporally static, so they are difficult to characterize. In this thesis, we interpret phytoplankton observations in an objective Lagrangian manner, or with a frame of reference that follows the motion of water parcels experienced by drifting organisms. We build a Lagrangian coherent eddy tracking algorithm that identifies the boundaries of water masses trapped for a month or longer. Using this tool, we assess the variability of the lateral advective properties of eddies across the North Pacific Subtropical Gyre, finding that only half of the remotely sensed eddies identified from the traditional, Eulerian sea level anomaly method trap waters for these timescales. We then statistically compare satellite-observed chlorophyll-a anomalies associated with eddies that trap versus mix across their boundaries. Lagrangian coherent vortices have more anomalous biological signatures in the gyre, so we argue that the role of leaky eddies in altering biogeochemistry may be underestimated due to lateral dilution. We also highlight substantial regional and seasonal variability in the dominant biophysical interactions within the oligotrophic regime, helping to explain inconsistencies of in situ eddy observations across this region. Lastly, we show how the Lagrangian water mass histories of in situ samples shape the phytoplankton community in the open ocean, quantified with amplicon sequencing and internal genomic standards. In non-eddy waters, we found that cyanobacteria are advantaged over eukaryotic phytoplankton when lateral mixing is minimized for several months. In or near mesoscale eddies, where vertical perturbations are a source of new nutrients, eukaryotic phytoplankton gene abundance has no dependence on the lateral mixing histories. The results suggest dispersal and niche generation drive phytoplankton variability but in different ways in and outside eddies. This thesis emphasizes how Lagrangian tools reveal mesoscale structures (otherwise invisible with Eulerian reference frames) that trap, transport, and transform ecosystems, generating phytoplankton patchiness and variability in the surface ocean.