Quick Dips

The Oceanic Carbon Sink

The meeting of marine water masses around New Zealand is providing information on the impact of human activities on the ocean ecosystem and the global climate system. A series of research voyages is under way, contributing information to the international Joint Global Ocean Flux Studies and Global Ocean Ecosystem Dynamics Research programmes.

The voyages have been focusing on the "subtropical convergence" off the east coast of the South Island, where warm subtropical waters low in nutrient concentrations meet the cool, nutrient-rich waters from the subantarctic. The narrow region is constrained geographically by the Chatham Rise, making it much easier to study than in other areas.

The heart of the research is the question of how carbon is absorbed and transported in the ocean ecosystem, which has major implications for the effects of carbon-related climate change.

Each year human activities emit 7-8 gigatonnes (700-800 billion kilograms) of carbon into the atmosphere, mostly in the form of carbon dioxide. Of this, the world's oceans absorb between 1-3 Gt per year. This carbon is then transported into the deep ocean where it remains for hundreds of years.

Improvements in estimating the oceans' uptake of carbon currently offer the best approach to determining the fate of man-made carbon dioxide emissions. Research is also required to more clearly understand what determines the natural rate of carbon cycling within the oceans.

Very few studies have been conducted in the Southern Hemisphere to establish the rate of carbon absorption by the oceans and identify the regions of highest carbon uptake.

Carbon is transported into the deep ocean via the food web. It is absorbed into the surface waters and is then utilised by phytoplankton (microscopic single-celled plants) as a carbon source for photosynthesis. The phytoplankton are then eaten by zooplankton, some of which are consumed by fish. Carbon is transported into the deep ocean by the sinking of plankton and faecal material.

In the past year, two voyages to investigate the carbon fluxes in the water surrounding New Zealand have been conducted by scientists from the National Institute of Water and Atmospheric Research, and Otago and Auckland Universities. Estimates were made of the rates of transfer of carbon to higher trophic levels by measuring grazing by zooplankton on phytoplankton and rates of carbon sedimentation into the deep ocean.

The first voyage studied carbon fluxes in winter and the second sought information when phytoplankton growth is at its maximum. Initial results indicate that in both winter and spring, the greatest carbon fluxes into the deep ocean occur in the subtropical convergence region. A third voyage is planned for February 1995.

Julie Hall
NIWA -- Ecosystems