Disentangling Advection and Lagrangian Evolution of Surface Chlorophyll in a Nearshore Submarine Canyon Using Satellite Remote Sensing and High-Frequency Radar

Abstract

Palmer Deep submarine canyon on the western Antarctic Peninsula hosts permanent penguin breeding rookeries and is characterized by elevated chlorophyll-a compared to the surrounding continental shelf. Particle residence times within the canyon are shorter than phytoplankton doubling times, which points to the ecosystem's productivity being tied primarily to advection of externally generated biomass into the canyon. This view is supported by recent observational studies showing alignment of attractive flow structures with phytoplankton patches. While residence times are short, they vary in space and are longer than the timescale for submesoscale instabilities with strong vertical motions (an inertial period), allowing for biological sources to be regionally or episodically important. Here we use measurements of ocean surface velocities (from high-frequency radars) and chlorophyll (from satellites) to calculate the Eulerian, Lagrangian, and horizontal advection terms of the surface chlorophyll budget. The Lagrangian term (including biological sources) is generally comparable in magnitude to advection, but the latter is more important on the canyon's western flank. We then compare joint distributions of relative vorticity and strain conditioned on a particle's net chlorophyll change. In general, parcels experiencing a net increase (decrease) in chlorophyll experience greater cyclonic (anticyclonic) vorticity. Although high-vorticity features significantly influence parcel motion, trajectories generally align with an estimate of the balanced flow, which is often characterized by a cyclone over the central canyon and eastern flank. Without subsurface data we cannot confirm whether the Lagrangian change truly indicates biological accumulation but we offer some interpretations.