Deploy tracemetal sampler Matt Druce

Deploying the trace metal sampler. Photo: Matt Druce

Tangaroa voyage: Phytoplankton in the Ross Sea

29 March 2023

Prof Cliff Law (NIWA, Otago University) led a major piece of ASP Project 3 research on the 2023 Tangaroa voyage, looking at the biogeochemical controls of phytoplankton in the Ross Sea region.

What are phytoplankton, and why do we study them?

Phytoplankton are floating microscopic algae that use sunlight, nutrients and carbon dioxide to create their energy through photosynthesis. Through this process they are the base of the Antarctic food web, analogous to grass on a farm.

This video explains plankton in the Ross Sea.

Understanding phytoplankton productivity improves our knowledge on how Antarctic food webs and biodiversity might change with warming, and ultimately how the oceanic influence on climate may be altered. For example, phytoplankton play a critical role in marine carbon sequestration to the seafloor which acts as a control of atmospheric carbon dioxide.

On the 2023 research voyage

During the voyage 56 profiles of water column structure using the ship's conductivity, temperature and depth (CTD) instrument were collected with discrete measurements of multiple parameters characterising the planktonic and chemical system of the surface ocean. This was supplemented by 26 profiles using a dedicated sampler for the measurement of dissolved trace metals, with processing carried out in a specialised ultraclean container laboratory. Dissolved iron in surface waters is at vanishing low concentrations in the Southern Ocean, but it is a major determinant of phytoplankton abundance. Consequently these measurements are central for model development and projection of how phytoplankton productivity may change in the future.

In addition to water column measurements, four experiments were undertaken to complement previous experiments on the 2021 RV Tangaroa voyage. These experiments manipulated phytoplankton in laboratory “mesocosms” to determine how they may respond to changing temperature and nutrient supply. The team examined the effects of nutrients from dust, seeps, and also ice on phytoplankton productivity and composition, and determined how changes in temperature and iron may influence fatty acids.

Overall, a total of 333 trace metal concentration and 130 trace metal isotope samples were collected, including characterisation of sea ice and glacial ice. Analyses will continue over the next year back in New Zealand.

Sea ice sampling Jenny Visser

Sea ice sampling from the workboat. Photo: Jenny Visser

The Tangaroa voyage supports ASP research into the biological and physical aspects of Ross Sea ecology that provides fundamental information essential to understanding threats from the changing environment. Field work of this kind is critical to development and validation of the projection models that the Antarctic Science Platform is developing to better understand climate-related threats to the Ross Sea MPA and further afield.

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Prof Cliff Law: “To understand the Southern Ocean you have to be there.”