Understanding the ecology and biogeochemistry of midwater ecosystems

Project Rationale :
The mesopelagic zone of the ocean (~50-1000 m beneath the surface), often referred to as the ‘twilight zone’, remains one of the most poorly understood ecosystems on Earth. It hosts a diverse community of animals, many of which undertake daily vertical migrations to feed at shallower depths under the cover of darkness. The depth at which mesopelagic organisms remineralize organic matter affects the residence time of carbon in the ocean, and hence influences how the oceans interact with global climate. However, the current paucity of information on mesopelagic food web structure and the physiology of the resident animals hampers our ability to quantify and understand how they influence carbon cycling in the ocean.
This PhD aims to better understand the fate of organic matter within the mesopelagic zone. We seek a highly motivated student to investigate who is eating whom in the mesopelagic food web, providing information on the potential routes through which energy and organic matter cycle. Additional analysis of zooplankton size-spectra will be used to examine the efficiency with which organic matter is transferred within the twilight zone.

Methodology:
The proposed research will involve examining samples collected on research cruises around South Georgia and in the Benguela upwelling region. There is strong potential for the student to collect additional samples/conduct experiments on forthcoming research cruises (e.g. North Atlantic, Southern Ocean) during their PhD. The composition of fatty acids within the tissues of all species sampled will be examined for biomarkers indicative of herbivory, carnivory, detritivory and omnivory. Stable isotope analysis of 12C/13C signatures of individual fatty acids will be used to explore the potential sources of organic carbon (e.g. photoautotrophic vs chemoautotrophic) and how this changes throughout the upper 1000 m of the ocean. FlowCam analysis of preserved zooplankton samples, and the resulting biovolume-spectra, will be used to investigate trophic transfer efficiency and the potential for carbon export. We encourage the student to build on the proposed work and extend it into an area of their own innovation. Possible ideas include: i) looking at the use of essential polyunsaturated fatty acids to cope with changes in environmental temperature and pressuring during vertical migration; ii) developing a numerical model to explore the biogeochemical implications of trophic interactions within the mesopelagic zone.

Training:
The INSPIRE DTP programme provides comprehensive personal and professional development training alongside extensive opportunities for students to expand their multi-disciplinary outlook through interactions with a wide network of academic, research and industrial/policy partners. The student will be registered at the University of Southampton and hosted at the National Oceanography Centre, in the Ocean Biogeochemistry and Ecosystems group. The Ocean Biogeochemistry and Ecosystems group is renowned globally as one of the leading centres of excellence in biological carbon pump research with plankton ecologists, numerical modellers, remote sensing specialists, theoreticians and particle flux geochemists working together to address the most significant problems in biological oceanography. Specific training will include: experimental design, zooplankton sampling techniques, size-spectrum analysis, data synthesis, statistical skills and R programming. Additional training in zooplankton identification, stable isotope techniques and lipid analysis will also be provided.