Spatial structuring and variability of mesopelagic ecosystems in the southern Indian ocean
The Southern Indian Ocean hosts diverse oceanic environments, pelagic communities, and predator populations that have recently led to the establishment of conservation areas and UNESCO World Heritage recognition. However, predator populations in this region have displayed signs of decline, likely at...
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Other Authors: | , , , , , , , , , |
Format: | Doctoral or Postdoctoral Thesis |
Language: | French |
Published: |
HAL CCSD
2023
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Subjects: | |
Online Access: | https://theses.hal.science/tel-04524647 https://theses.hal.science/tel-04524647/document https://theses.hal.science/tel-04524647/file/142844_IZARD_2023_archivage.pdf |
Summary: | The Southern Indian Ocean hosts diverse oceanic environments, pelagic communities, and predator populations that have recently led to the establishment of conservation areas and UNESCO World Heritage recognition. However, predator populations in this region have displayed signs of decline, likely attributed to the impacts of climate change and prey dynamics. These prey populations consist predominantly of zooplankton and micronekton, playing a crucial role in marine ecosystems by influencing biogeochemical cycles and the transfer of energy and biomass along the trophic web. Nevertheless, the study of their structuring is intricate due to their deep distribution in the mesopelagic zone (200-1000 m) and their daily vertical migrations, making it one of the least explored areas on a global scale. In this thesis, we employ active acoustics, which provides high spatial and vertical resolution for tracking these organisms, to investigate the structuring of mesopelagic ecosystems in two contrasting oceanic systems. We initially examined their responses in a transition zone between these oceans, within the Saint-Paul and Amsterdam Islands region. Our findings indicate that nycthemeral migrations are the primary factor vertically structuring pelagic organisms, even within a pronounced transition zone. Three spatially coherent regions were identified, sharing common environmental and acoustic features. Observations at 38 kHz (commonly used in hydroacoustics) confirmed increased biological activity in the subtropical zone and a minimum in the subantarctic zone. However, our analyses also reveal a distinct acoustic response based on the acoustic frequency considered, in terms of vertical structuring and integrated biomass. Additionally, the low levels measured in the Southern Ocean appear inconsistent with the high biomasses consumed by the large marine predator populations in this region. These observations highlight an acoustic resonance issue in the study of these organisms. To investigate this phenomenon, our second ... |
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