High Frequency Radars as Ecological Sensors: Using Lagrangian Coherent Structures to Quantify Prey Concentrating Features

Food resources in marine food webs are often concentrated into patches within biological hotspots, separated by large expanses of barren ocean. Inhomogeneous, or “patchy” distribution of plankton is in part due to mesoscale and submesoscale oceanographic features transporting and locally concentrating plankton. Concentrations of plankton at the base of the food web increase prey availability for intermediate and upper trophic levels creating productive regions known as biological hotspots. Submesoscale oceanographic features within coastal biological hotspots occur at spatiotemporal scales that can be resolved by coastal High Frequency Radars (HFR). To identify areas with enhanced passive particle (plankton) transport and concentration, four popular mathematical techniques known as Coherent Structure Analyses (CSA) are applied to HFR observed ocean surface currents. Each CSA technique quantifies the stretching (pulling drifters apart) and shrinking (pushing drifters together) of the flow, yet each has its own unique calculation of “stretching” and “shrinking”. All four CSA have been applied to ocean datasets in previous studies, however results from the four techniques have not been compared and evaluated as valid ecological metrics in energetic coastal flows. The following study will bring clarity to what makes each type of coherent structure differ from the others when applied to plankton transport and concentration in a coastal marine ecosystem. Results show some coherent structures define boarders of transport while others define centers of attraction. The following study uses HFR observed surface currents around a known biological hotspot, Palmer Deep Antarctica to quantify prey concentrating ocean features, and describe their ecological significance by comparing them to biological observations. Transport features quantified by CSA have the potential to be transport hubs, delivering and concentrating constant supplies of plankton. Results have the potential to inform the identification of prey concentrating features in larger, more complex food webs, and expand the use of coastal HFR data.