Prey and predator overlap at the edge of a mesoscale eddy: fine-scale, in-situ distributions to inform our understanding of oceanographic processes

Abstract

Eddies can enhance primary as well as secondary production, creating a diverse meso-and submesoscale seascape at the eddy front which can affect the aggregation of plankton and particles. Due to the coarse resolution provided by sampling with plankton nets, our knowledge of plankton distributions at these edges is limited. We used a towed, undulating underwater imaging system to investigate the physical and biological drivers of zoo-and ichthyoplankton aggregations at the edge of a decaying mesoscale eddy (ME) in the Straits of Florida. Using a sparse Convolutional Neural Network we identified 132 million images of plankton. Larval fish and Oithona spp. copepod concentrations were significantly higher in the eddy water mass, compared to the Florida Current water mass, only four days before the ME's dissipation. Larval fish and Oithona distributions were tightly coupled, indicating potential predator-prey interactions. Larval fishes are known predators of Oithona, however, Random forests models showed that Oithona spp. and larval fish concentrations were primarily driven by variables signifying the physical footprint of the ME, such as current speed and direction. These results suggest that eddy-related advection leads to largely passive overlap between predator and prey, a positive, energy-efficient outcome for predators at the expense of prey. Eddies are ubiquitous features of the ocean, turning mechanical energy into trophic energy 1. The footprint of a mesoscale eddy can extend 100-300 km in diameter and can last for several weeks to months 2. Through their upwelling effect, cyclonic mesoscale eddies (MEs) have been shown to enhance primary 3,4 and secondary production 5-7. This enhanced productivity may increase growth 8 and survival 9 of larval fishes, which normally experience up to 99% mortality due to starvation and predation 10. Eddies may also physically retain larval fishes 11 , leading to higher larval fish concentrations inside eddies, relative to outside ambient waters, and are considered effective vectors for the transport of zoo-, and ichthyoplankton 12-14. As such, mesoscale eddies play an important role in the connectivity of holo-and meroplankton populations 15. Eddy divergence and convergence patterns in the ocean lead to a cascading flow of energy from large to small scales 16 , with turbulent frictional coupling inducing smaller anti-cyclonic eddies at the periphery of larger cyclonic eddies thereby creating a feature-and energy-rich seascape 17. Upwelling occurs in the centre of cyclonic MEs during their spin-up phase (termed a 'forced' eddy), but during the decay/spin-down phase (termed a 'free' eddy), this switches to downwelling at the core with upwelling occurring at the eddy edge 1,18. In both instances, due to its frontal character, the eddy edge is an important feature for predator-prey interactions 1. Less motile prey are often passively aggregated at the eddy edge and can be exploited by higher trophic levels and top predators

A bird's-eye view on turbulence: seabird foraging associations with evolving surface flow features

“…10-100 km) physical features [10,11]. Mesoscale eddies can provide physical mechanisms to transport [12] or aggregate prey [13], thereby providing foraging opportunities for wide-ranging marine predators, from seabirds to sharks [14][15][16][17].…”

Detection of the Deep-Sea Plankton Community in Marine Ecosystem with Underwater Robotic Platform

“…Underwater plankton imaging has the capacity to detect patterns of the plankton distributions that we would be unable to be tackled by sampling with nets. [21]. Therefore, we consider applying machine vision technology to underwater images or videos is currently a feasible method for studying plankton.…”

Ocean-Surface Heterogeneity Mapping (OHMA) to Identify Regions of Change

“…There have been numerous studies which have identified areas of front persistence, or high front frequency (for example see [5,15]), as regions of enhanced regional biodiversity, and species abundance. Their surface expressions, which are the only features detectable using satellite sensors, represent far more extensive structures beneath the surface, shaping the distribution of life beneath the surface [16][17][18], and driving the region's ecosystem functioning [6]. STH features vary regionally.…”