This figure shows that the atmosphere-ocean exchanges of carbon, in the form of carbon dioxide (CO2), are dominated by photosynthesis in phytoplanktonic organisms, which incorporates CO2 into the plant tissues, and respiration and decomposition, which subsequently releases and returns the CO2 to the environment. Contrary to primary production, which only takes place in the surface waters, respiration can occur at all depths and even within the sediment.
Flux in g C m-2 yr-1
It appears that 90% of the carbon fixed by phytoplankton is remineralized or horizontally transported within the first 100 to 150 m of the water column. Since the carbon released in this surface layer can rapidly return to the atmosphere, it has no impact on the atmospheric composition or the climate. In fact, only 10% of the carbon reaches the deep waters, where it can be trapped for decades. It is this sequestration that affects the exchange of CO2 between the ocean and the atmosphere.
The figure also shows that a large fraction of the organic carbon produced in the surface remains unaccounted for. In fact, the amount of carbon that leaves the euphotic zone is much larger than the values measured by scientists during their oceanographic missions. We don't know what proportion of the missing carbon is remineralized in the cold intermediate layer, is exported horizontally with the surface currents, or is consumed in the upper trophic levels.
As this simplified model shows, the phytoplankton production can follow one of two paths. The first passes through the microbial network, which is composed of small phytoplankton (cell size < 5 µm in diameter), bacteria, and protozoans, and then to the zooplankton. This pathway is very efficient for recycling and keeping carbon in the surface waters. We have observed that this pathway is especially active in the summer and fall. The second pathway involves the rapid sedimentation of large phytoplankton (cell size > 5 µm in diameter; mostly diatoms) during spring blooms and/or by large-scale grazing of zooplankton on phytoplankton. This pathway is more efficient for exporting a large amount of organic matter to deep waters, by the sedimentation of fecal pellets, for example. This is seen at the beginning of the winter and in the spring. Even though there are large seasonal differences in these two trophic pathways, it seems that the transfer of the phytoplankton production through the microbial network dominates over the annual scale in the Gulf of St. Lawrence.