Lagrangian flow properties along filament-like structures and its impact on the distribution and composition of phytoplankton

Ismael Hernandez-Carrasco, CSIC-IMEDEA, Mediterranean Institute for Advanced Studies, Oceanography and Global Change, Esporles, Spain, Alejandro Orfila Forster, Mediterranean Institute for Advanced Studies (IMEDEA, UIB-CSIC), Esporles, Spain, Vincent Rossi, CNRS-MIO, Mediterranean Institute of Oceanography, Marseille, France, Eva Alou-Font, Balearic Islands Coastal Observing System, SOCIB, Palma de Mallorca, Spain, Verónica Morales-Márquez, IMEDEA (Mediterranean Institute for Advanced Studies), TMOOS, Palma De Mallorca, Spain and Veronique Garcon, CNRS-LEGOS, Toulouse, France
Abstract:
Physical factors induced by the turbulent flow play a crucial role in regulating marine ecosystems. Here we show that the combination of complementary Lagrangian diagnostics provides an improved description of the geometrical and kinetic properties of the flow, which facilitates the interpretation of physical mechanisms affecting phytoplankton composition, dynamics and patchiness. The influence of small-scale dynamics (O(3.5-25) km, i.e. spanning upper submesoscale and mesoscale processes) on phytoplankton in surface waters derived from satellite chlorophyll-a (chl a) is studied using Lagrangian metrics computed from High-Frequency Radar currents over the Ibiza Channel. Attracting small-scale flow structures are associated to filaments of accumulated negative divergence where particles and chl a standing stocks cluster. Regions of accumulated positive divergence, representing large accumulated upward vertical velocities and suggesting accrued injection of subsurface nutrients, match areas with large chl a concentrations. Furthermore the composition of phytoplankton community was studied regarding the sustained kinematic properties of the flow during the formation of an intense filament. In this regard new metrics accounting for the Lagrangian evolution of the turbulent and topological properties along fluid parcel trajectories have been developed. Here we provide observational evidences, during the SHEBEX cruise (May 2015) in the western Mediterranean and altimetry-derived Lagrangian computations, that fluid parcels associated with high Lagrangian turbulent kinetic energy and Lagrangian positive vorticity host higher relative abundance of larger pennate diatoms (i.e. Pseudo-nitzschia spp). Our findings indicate that the combination of these diagnostics provides an improved description of the turbulent dynamics showing that the Lagrangian properties of the flow have important biological consequences.
Back to: (Sub)mesoscale Physical/Biogeochemical Interactions II