Physical drivers of food web focusing in marine ecosystems
Creators: name: Veatch, Jacquelyn
Publisher: name: No Publisher Supplied
Year: 2024

description: Food resources (plankton) in the ocean are notoriously patchy, being concentrated in some areas and sparse in others. Such heterogeneity ripples up to the food web to the distribution of higher trophic-level creatures who utilize these patches to increase their foraging efficiency. This bottom-up control on spatial ecology is known as food web focusing. My dissertation has analyzed the role of horizontal advection in the creation and maintenance of these large patches from daily to seasonal scales.To quantify advective transport and concentration, Lagrangian Coherent Structures (LCS) were used. LCS analysis applied to velocity fields will detect distribution patterns of attracting features accumulating simulated plankton (passive drifters) - features that cannot be identified and tracked from the velocity fields alone. Two LCS techniques were tested, (1) a single particle tracking technique based on integrated movement of all particles, Relative Particle Density (RPD) and (2) a paired particle tracking technique based on the relative motion of two particles, Finite Time Lyapunov Exponent (FTLE). Both techniques quantify attracting features, but with different approaches. To observe the connection between advective transport and food web focusing in coastal biological hotspots, Palmer Deep Canyon, Antarctica was used as an ecological laboratory. The SWARM field team and I deployed three High Frequency Radars (HFR) around this known penguin foraging area. We then used gliders, small boat towed surveys, and GPS tagged penguins to map the distribution of phytoplankton, krill swarms, and penguin foraging behavior onto HFR observed surface currents. LCS techniques were applied to HFR observed surface currents, assigning scalar quantities to areas of attraction within the study region. These scalar quantities vary in space and time with the ocean currents.It was found that (i) phytoplankton patches and krill swarms are often association with strong LCS features; (ii) this association is stronger when the water column is stratified; (iii) krill both above and below the mixed layer depth are often associated with strong LCS and (iv) penguins forage along strong LCS features, suggesting food web focusing is occurring in the system initiated by coastal ocean transport of phytoplankton. These conclusions show the importance of horizontal advection in the spatial ecology of coastal ecosystems. It was also found that (v) paired particle FTLE more accurately quantify the attractive features associated with food web focusing than single particle RPD. These results clarify the ecological relevance of the different classes of LCS techniques, and suggest that the use of the more computationally complex paired particle technique may be necessary to identify physical mechanisms of food web focusing.To explore the utility of FTLE in understanding more complex food webs over larger scales, I used a 10-year HFR time series covering the continental shelf from North Carolina to Massachusetts provided by the Mid-Atlantic Region Association Coastal Ocean Observing System (MARACOOS). FTLE applied to these data produce a climatology of attractive ocean features that bear the seasonality and interannual variability of the Mid-Atlantic Bight.Results suggest that (i) strong attracting features are more common in spring and summer than fall and winter. This is likely because fall and winter experience more storm/high wind events where most of the surface ocean is flowing in the same direction. Such uniformity of flow in winter and fall does not lend to strong attracting features, creating fewer attracting features in these seasons comparatively. These seasonal patterns in hydrography could be synchronized with local phenology that rely on attracting features to concentrate prey or transport planktonic larvae. Additionally, (ii) Nantucket Shoals region consistently had a high occurrence of strong attracting features throughout each season, which may be a reason why this region is known to be a foraging ground for whales.These conclusions provide a deeper understanding of the role of ocean currents in the maintenance coastal biological hotspots and set the groundwork for the use of LCS to understand food web focusing in larger food webs such as the Mid-Atlantic Bight.