The importance of the oceans in the carbon cycle (problematics)
Due to human activities, the atmospheric concentration of carbon dioxide (CO2) continues to rise and will probably lead to atmospheric global warming, commonly called the greenhouse effect. Only half of the anthropogenic emissions of CO2 remains in the atmosphere; the rest is absorbed by the oceans and terrestrial ecosystems. In the oceans, part of the carbon uptake is accomplished by the phytoplankton, a veritable submarine prairie, that fixes carbon dioxide by the process of photosynthesis and produces organic plant matter that is available for other marine organisms. Photosynthesis and the transfer of this organic matter to the deep waters comprise the oceanic biological pump.
The oceans and climate change
The biological pump is very sensitive to the physical conditions of the ocean surface that are themselves strongly influenced by atmospheric conditions. It is therefore not difficult to imagine an interaction between climate change, oceanic conditions, and this biological pump that could affect the current trend of the greenhouse effect. A climate change, by modifying the relative abundance of different planktonic species, could affect the efficiency of the biological pump for transferring organic carbon in the surface waters toward deep waters. The rules that govern the oceanic carbon cycle and its response to atmospheric warming are very complex. The Gulf of St. Lawrence itself is too small to have a significant influence on the increase of atmospheric CO2, but the results obtained here may be generalized to other comparable coastal ecosystems.
The Greenhouse Effect
The greenhouse effect results from the selective way in which different atmospheric components act on solar radiation (from the sun) and on terrestrial radiation (solar radiation reflected by the earth). The solar radiation with very short wavelengths is absorbed by the oxygen and ozone in the upper atmosphere. Most of the solar radiation that reaches the earth's surface is within the range of the visible light; these rays pass through the carbon dioxide and water vapor of the atmosphere and heat the earth's surface. The earth reflects back most of this energy in the infrared range (radiation with longer wavelengths). Before it can escape into space, part of this radiative energy is dissipated as heat, being absorbed by water vapor, carbon dioxide, methane, and other more complex components in the upper atmosphere. The result is a warming of the lower layers of the atmosphere; this is the process known as the "greenhouse effect," an analogy to the greenhouses used in gardening. More simply, the visible solar rays can pass through the atmosphere while the infrared rays reflected by the earth are blocked and dissipated in the form of heat. The equilibrium between the radiative losses and gains depends largely on the concentrations of the different atmospheric components. Therefore, an increase in their concentrations has direct consequences on the overall warming of the atmosphere.
Current research on this domain
Our understanding of the phenomena involved is still too rudimentary to give accurate predictions. It is therefore essential that we set up monitoring programs in the marine environment. With time series of data spanning several years, we can separate natural variations from climate changes that can affect the biology of the oceans. To try to improve our knowledge, oceanographers from several countries, including Canada, have participated in an international program known by the acronym JGOFS, which included large-scale experiments in different oceanic regions. At the Maurice Lamontagne Institute, scientists developed different carbon flow models for the Gulf of St. Lawrence. Here we will present a simplified version of the model (annual budget) and more detailed seasonal models (winter-spring / summer-fall) that summarize the results from two years of intensive sampling.
The results presented here come from research conducted between 1992 and 1995 within the Canadian contingent of the International Joint Global Ocean Flux Study Program (JGOFS's Canadian coordinator: Bruce D. Johnson) with financial support from both the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canadian Department of Fisheries and Oceans (DFO; Green Plan Program). The work results from a collaboration among scientists from DFO (A. Vézina, T. Packard, C. Savenkoff, N. Silverberg, J.-C. Therriault), GIROQ (Groupe de recherche interuniversitaire en océanographie du Québec; L. Legendre, B. Klein), McGill University in Montréal (G. Ingram, A. Mucci), the Institut des sciences de la mer de Rimouski (S. Demers, G. Desrosiers, S. Roy, B. Sundby), and Memorial University in Newfoundland (R. Rivkin, D. Deibel).
The text and figures were adapted from the following articles:
Savenkoff, C., A. F.Vézina and, J.-C. Therriault. 1997.Le cycle du carbone dans le golfe du Saint-Laurent. Nouvelles des Sciences, the information bulletin published by the Maurice Lamontagne Institute, Fisheries and Oceans Canada-Laurentian Region. Vol. 8, no. 7, pp. 4-7.
Savenkoff, C., A. F. Vézina, S. Roy, B. Klein, C. Lovejoy, J.-C. Therriault, L. Legendre, R. Rivkin, C. Bérubé, J.-E. Tremblay, and N. Silverberg. 2000. Export of biogenic carbon and structure and dynamics of the pelagic food web in the Gulf of St. Lawrence. I. Seasonal variations. Deep-Sea Research II, 47, pp. 585-607.
Vézina, A. F., C. Savenkoff, S. Roy, B. Klein, R. Rivkin, J.-C. Therriault, and L. Legendre. 2000. Export of biogenic carbon and structure and dynamics of the pelagic food web in the Gulf of St. Lawrence. II. Inverse analysis. Deep-Sea Research II, 47, pp. 609-635.
Adaptation for Internet: Robert Siron and Claude Savenkoff
Infographics: Johanne Noël
English translation: Laure Devine