Composition, distribution, and life history strategies of mesopelagic fauna in a changing Arctic Ocean

Priou, Pierre (2023) Composition, distribution, and life history strategies of mesopelagic fauna in a changing Arctic Ocean. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Micronekton and zooplankton inhabit the mesopelagic zone (200-1,000 m depth) which represents a unique environment in the global ocean where low-light levels structure predator-prey interactions. Because of the central position of mesopelagic organisms in food webs and the strong vertical gradient in light intensity, these animals conduct light-mediated diel vertical migrations (DVM) to maximize feeding while limiting mortality through visual predation. DVM contribute to the biological carbon pump by actively transporting carbon from the upper ocean to greater depths. The extreme light climate prevailing at high latitudes is hypothesized to prevent DVM during the midnight sun and polar night periods and hence prevent the establishment of viable mesopelagic populations in the Arctic Ocean. Yet, recent observations of mesopelagic layers in the European Arctic and the Central Arctic Ocean challenge this paradigm. There remain uncertainties regarding the spatial extent, species composition, life history strategies, and environmental drivers of the occurrence of mesopelagic organisms in the Arctic Ocean. Here, I used acoustic data collected at different spatial and temporal scales from multiple regions of the Arctic Ocean to investigate the distribution, structure, and dynamics of Arctic mesopelagic organisms. I demonstrate that mesopelagic organisms are commonly found in the Arctic Ocean and identified two distinct species assemblages that may be representative of larger biogeographic provinces. The seasonal vertical distribution of mesopelagic organisms generally followed a light comfort zone – a narrow range of light intensities. The seasonal cycle was characterized by two phases of active feeding on Calanus copepods in spring and autumn interspersed by oversummering and overwintering periods of vertical segregation with Calanus during the polar night and midnight sun. While the light comfort zone hypothesis provides a comprehensive framework for understanding the structure and dynamics of the mesopelagic zone, not all size classes and taxa adhere to this hypothesis. In the Arctic, mesopelagic organisms exhibit complex and flexible behaviours to accommodate a broad range of changing environmental conditions. I conclude that the mesopelagic ecological niche is widespread in the Arctic Ocean. Future studies of the biological carbon pump and of trophic interactions in the Arctic should consider and include the mesopelagic niche.

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