High Frequency Radars Observe Krill Feeding: Using Lagrangian Coherent Structures to Detect Food Web Focusing

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 subme-soscale oceanographic features transporting and concentrating plankton. Concentrations of plankton at the base of the food web into large patches increase prey availability for intermediate and upper trophic levels creating productive regions known as biological hotspots. Coastal biological hotspots are characterized by highly nonlinear oceanographic features, with spatiotemporal scales that can be resolved by coastal High Frequency Radars (HFR). These features have the potential to be transport hubs, delivering and/or concentrating constant supplies of plankton. The following study uses HFR observed surface currents around a known coastal biological hotspot, Palmer Deep Canyon Antarctica. Here we quantify prey concentrating ocean features, and describe their ecological significance by comparing them to biological observations. To quantify physical prey concentrating features, a Lagrangian Coherent Structure (LCS) technique known as Finite Time Lyapunov Exponents was applied to HFR observed surface currents. It was found that Antarctic Krill (Euphausia superba) collocate with physical prey concentrating features more often than randomly generated krill swarms (null model). It was also found that subsurface krill swarms collocate with LCS even more often than surface krill swarms. This quantified relationship between ocean movement and food web dynamics can be applied to predictions of spatial ecology and changing prey conditions. This novel application of HFR and LCS technology will expand the use of HFR to fisheries ecology. Results will inform future work in the identification of biophysical interactions in more complex food webs, emphasize the role of transport in the maintenance of the biological hotspot around Palmer Deep.