Living in a fluid-dynamical landscape : how do marine predators respond to turbulence?
Creators: name: Della Penna, A
Publisher: name: University Of Tasmania
Year: 2023

description: Marine top predators play a fundamental role in maintaining the structure and functioning of healthy marine ecosystems. In the last decades the development of bio-logging (i.e. deployment of autonomous recording tags on free-living animals) has radically changed the study of top predators and their interactions with their environment. Combinations of sensors measuring position (Argos and GPS), environmental properties (water temperature, light) and proxies for foraging behavior (accelerometers) have enabled relating migrations of large fish, marine mammals, sea turtles and seabirds to basin scale patterns of ocean currents, temperature, and productivity. However, what influences marine predators' movement at smaller spatial and temporal scales, such as the ones they experience during their foraging trips, is still largely unknown. This project analyses the interaction between marine top predators (elephant seals and macaroni penguins) and sub-mesoscale (few days-months, 10-100 km) ocean dynamics. This is achieved by combining in-situ observations, bio-logging data, remote-sensing, ecological modelling and a Lagrangian approach (i.e. based on the tracking of water parcels). The study is conducted in the sub-Antarctic region around the Kerguelen Plateau (Indian Sector of the Southern Ocean). This sector of the Southern Ocean provides a useful natural laboratory for this work in that (i) several marine predators species have large colonies on the island, (ii) the area is located in a highly dynamical ocean regime dominated by the Antarctic Circumpolar Current, (iii) the trophic web in the area is relatively simple and (iv) production is dominated by iron limitation, making it possible to disentangle physical and ecological effects. First, I examine whether the mechanical effect of mesoscale currents on the trajectories of large fast-swimming marine predators is significant. I find that, in spite of what is often assumed, foraging marine predators are transported by the horizontal ocean dynamics for a non-negligible part of their foraging trip, exhibiting a 'quasi-planktonic' behaviour. This phenomenon is more likely to occur on sub-meso and mesoscale fronts that are favourable for animal foraging. Secondly, I focus on how mesoscale turbulence affects the distribution of ocean productivity by combining a Lagrangian approach with previous biogeochemical and ecological knowledge of the region. A novel result is that waters that have been recently (‚Äöv†¬¿ 20 days) in contact with the Kerguelen plateau (which is assumed to be the major source of iron of the region) are more likely to manifest a system dominated by large diatoms. I parametrise and spatialise a simple model describing phytoplankton community structure in the area to predict which dynamical niches are more favourable for high concentrations of diatoms. This result is important because qualitative assessment of the regional food web shows that the growth of fish larvae and crustaceans (that occupy a large fraction of top predators' diet) benefit from phytoplankton communities dominated by large diatoms. The findings of this project support and complement previous studies about the role of mesoscale dynamics on marine life and open challenging questions about an end-to-end‚ÄövÑvp approach to bio-logging observations and ecosystem-based conservation management.