Exchanges of carbon, in the form of carbon dioxide (CO2), between the atmosphere and the oceans 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. A minor change in these opposite flows (photosynthesis and respiration) can modify the equilibrium and thus the proportion of anthropogenic carbon (carbon that is produced by human activities) that stays in the atmosphere.
The microscopic algae that make up the phytoplankton manufacture their organic tissue by combining inorganic materials like carbon dioxide (CO2) and nutrients that are present in the water. This manufacture of living organic matter from inert matter is the result of a biochemical process known as photosynthesis, which takes place using solar energy captured by chlorophyll, a pigment present in all green plants. Because of this, photosynthesis can only take place in the ocean's euphotic zone. The primary production (or phytoplankton production) resulting from the photosynthesis is the starting point of all the food webs in the ocean.
Since photosynthesis requires solar energy, the phytoplankton can develop only in the surface water, where the solar rays penetrate to a certain depth. This illuminated surface layer is called the euphotic zone, and all of the ocean's primary production occurs there. The depth of the euphotic zone is variable, depending on ambient conditions and the amount of particles suspended in the seawater; its lower limit corresponds to the depth where 1% of the surface light remains. In the Gulf of St. Lawrence, the euphotic zone usually includes the top 20 to 30 meters of water.
Phytoplankton is a general term that includes all microscopic plants (unicellular algae usually between 1 and 100 µm) that grow in both fresh and salt water. Diatoms and dinoflagellates are among the most common phytoplankton groups in the St. Lawrence. During their development, these microscopic algae are transported with the surface water by ocean currents. A bit like a submarine prairie, the phytoplankton is the basic food source for herbivorous marine organisms like zooplankton and certain invertebrates, themselves a source of food for higher organisms (for example, fish and marine mammals). The phytoplankton is thus the base of the food webs. This primary production determines the productivity of the marine ecosystems.
In this study, phytoplankton is separated into two size classes: large phytoplankton (cell size > 5 µm in diameter; mostly diatoms) and small phytoplankton (cell size < 5 µm in diameter).
|Biomass (mg C m-2)|
|Large phytoplankton (68%)||2751|
|Small phytoplankton (32%)||489|
|Primary production (mg C m-2 j-1)|
|Large phytoplankton (56%)||794|
|Small phytoplankton (44%)||346|
|Biomass (mg C m-2)|
|Large phytoplankton (26%)||280|
|Small phytoplankton (74%)||636|
|Primary production (mg C m-2 j-1)|
|Large phytoplankton (23%)||97|
|Small phytoplankton (77%)||344|
In addition to small-celled phytoplankton (< 5 µm), bacteria and protozoa compose the microbial network. While bacteria are unicellular microorganisms that are neither plants nor animals, protozoa are unicellular heterotrophs (animals) that include flagellates, dinoflagellates, and ciliates covered in this study.
Plankton by definition includes all aquatic organisms that live suspended in the water. These organisms are plants (phytoplankton) or animals (zooplankton). As shown by this simplified carbon cycle model, the phytoplankton production can take one of two pathways (or food webs). The first passes through the microbial network and then to the zooplankton. The second pathway is dominated by the larger phytoplankton (> 5 µm in diameter, mostly diatoms) and by massive consumption of the phytoplankton by heterotrophic organisms: flagellates, dinoflagellates, ciliates, and zooplankton. Even though there are seasonal differences in the importance of these two trophic networks in the Gulf of St. Lawrence, it is the microbial network that dominates over the annual scale.
The microbial network is composed of very small phytoplankton cells (< 5 µm in diameter), protozoans (unicellular organisms including flagellates, dinoflagellates, and ciliates), and bacteria. The pathway through the microbial network is very efficient for recycling and holding carbon in the surface waters. In the Gulf of St. Lawrence, this pathway is mostly seen in the summer and fall; warmer and more stable water favors the development of algae like flagellates, which are able to remain in the surface waters for long periods.
Zooplankton is the term used to designate all the animal organisms whose size is larger than 63 µm. The zooplankton can be separated into three size classes: small zooplankton (from 63 to 500 µm), medium zooplankton (500 µm to 5 mm), and large zooplankton (> 5 mm).
The small zooplankton group includes early larval stages of zooplankton (nauplii and copepodites).
The medium zooplankton group is made up mostly of copepods-organisms that belong to the crustacean family. Two of the most common copepod species in St. Lawrence marine waters are Calanus finmarchicus and Oithona similis.
Large zooplankton includes euphausids (krill), chaetognaths, amphipods, cnidarians and ctenophores (jellyfish), mysids, tunicates, and small fish (ichthyoplankton). In these simplified budgets, we consider only the first two size classes.
The consumption of phytoplankton by zooplankton is generally more rapid than the microbial network and results in a large quantity of organic matter. In the Gulf of St. Lawrence, this pathway for the primary production is observed from the beginning of the winter and in spring, when colder and well-mixed water favors the proliferation of diatoms.
The model shows that the fate of a large proportion of the organic carbon produced at the surface remains unknown. In fact, the amount of carbon that leaves the euphotic zone is much higher than the values measured by scientists during oceanographic missions using sophisticated sampling equipment like sediment traps. We do not know what proportion of this missing carbon is remineralized in the cold intermediate layer, exported horizontally with surface currents, or consumed in the upper trophic levels.
Other research efforts in the St. Lawrence estuary will examine this issue. [PDF Format , 375KB (in French)]
In summer, the waters of the Gulf of St. Lawrence can be divided vertically into three layers: the warm and less saline (temperature > 5.0°C, salinity = 29.0 to 31.5) surface layer includes the euphotic zone and is located in the upper 30 meters; the intermediate layer is characterized by very cold water (thermal minimum: T = -2.0 to 2.0°C, S = 31.5 to 33.0) and descends to about 150-200 m; the deep water layer is warmer and more saline (T > 3°C, S = 33.0 to 34.5). There are only two layers in the winter: the surface and intermediate layers mix to make a single cold water mass while the deep water remains distinct.
Part of the primary production is exported from the euphotic zone to the underlying layers (the intermediate and deep water layers), mostly in the form of very small particles. The particle content varies as a function of the composition, the magnitude, and the size structure of the planktonic food webs. In general, however, the particle amounts decrease with depth. This decrease results from the combined effects of bacterial activity and the fragmentation, solubilization, ingestion, or decomposition (respiration) of particles during their long descent toward the deep waters.
Only 10% of the carbon makes it to the deep water where it can be trapped for decades and thus affect the CO2 exchanges between the ocean and the atmosphere. This may appear very inefficient as a method of climate regulation, but one must consider that this small flux at a local scale would become enormous when considered at a global scale.
Respiration is a metabolic process that decomposes organic matter and releases the CO2 contained in the living organisms. Contrary to primary production, which occurs only in the surface, respiration can occur at any depth and even within the sediments. Since the recycling rate of CO2 to the atmosphere depends on the depth at which it is liberated, the deeper the CO2 is freed the longer its residence time in the ocean will be. Our simplified annual model indicates that 90% of the carbon fixed by the phytoplankton is remineralized or horizontally transported in the top 100-150 m of water. The carbon released in this surface layer can rapidly return to the atmosphere. Thus globally, this carbon remineralization has no effect on the composition of the atmosphere and therefore on the climate.
A small amount of the particles that fall toward the sea floor is not decomposed and will be gradually buried in the sediments and thus removed from the carbon cycle. The marine sediment, except for the surface layer that can still be decomposed, mixed by bioturbation, or disturbed by deep waters, is considered as a trap for the carbon. The carbon, once sequestered in the deeper sediments, is incorporated into the earth's crust and constitutes stable geological layers. This "non-exchangeable" carbon is thus trapped for several hundreds of millions of years (the formation of coal or oil reserves is the best example of the fossilization process at the geological scale).
To balance the different mass-balance equations, the model calculates a residual flow of 705 mg C m-2 d-1 out of the euphotic layer that adds to the sedimentation flux of detrital, phytoplanktonic, and zooplanktonic matter. This residual flux takes place via passive processes (i.e., entrainment and advection), where each group is exported in proportion to its biomass.
From the Lower Estuary (nursery area for Calanus finmarchicus, the dominant large zooplankton species), a large number of Calanus are transported by the currents resulting from the runoff of the St. Lawrence and Great Lakes drainage systems towards the Gulf. The summer zooplankton biomass is supported directly by this advection. This transport of zooplankton production has been termed a Calanus pump (Plourde and Runge, 1993). We simulated this effect in the model by forcing an additional flux (residual flux) towards the large zooplankton compartment only. This change results in an inflowing residual of 155 mg C m-2 d-1 as estimated by the model.
Organic matter export or particulate sedimentation flux out of the euphotic layer is made up of products resulting from the metabolism of living organisms (e.g., debris, cellular lysis, egestion, fecal pellets, dead organic matter).