Investigating the ecology and biogeochemistry of the twilight zone, one of the least studied ecosystems on Earth

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; some of these are presumed to feed on sinking particles of detritus, whereas others are thought to migrate into surface waters to feed on phytoplankton. These trophic interactions influence the storage of carbon in the ocean’s interior, and hence contribute to global climate regulation. However, the current paucity of information on the ecology and physiology of mesopelagic zooplankton hinders our ability to represent their activities in global biogeochemical models, including those used by the IPCC to assess future climate.

We seek a highly motivated student to investigate how the distribution and activities of mesopelagic zooplankton influence the fate of sinking organic matter. Changes in the trophic ecology and physiology of zooplankton with increasing depth will be examined using a combination of laboratory analyses and size-spectrum theory. Increased understanding of who is eating what in the mesopelagic food web will be used to examine the potential routes through which energy and organic matter cycle within this important ecosystem. This will help improve the representation of zooplankton in biogeochemical models.

Methodology
The proposed research will involve examining samples and data collected around South Georgia, in the Benguela upwelling region and along a transect of the Atlantic. There will likely be opportunities for the student to collect additional plankton 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 consequences for the fate of carbon. 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/Matlab programming. Additional training in zooplankton identification, stable isotope techniques and lipid analysis will also be provided.