Photosynthesis vs respiration
Photo credits: C. Sardet, S. Mirshak and N. Sardet / Les chroniques du plancton
Plants capture and store solar energy through photosynthesis. During photosynthesis, living plants convert minerals (e.g. nitrogen, phosphorus) and carbon dioxide into the oxygen that we breathe and sugar molecules that they use for food. Since plants produce their own food from simple substances present in their surrounding, they have been considered as primary producers. By providing the oxygen as well as the food that requires the non-photosynthetic species, primary producers sustain all of the Earth’s life forms.
The non-photosynthetic species are called consumers and for example, when a cow grazes the grass, the cow converts the sugar and the oxygen produced by the grass into carbon dioxide and energy through respiration. Respiration is the opposite process of photosynthesis and interestingly, even plants respire. Respiration allows the transformation of sugar (obtained by photosynthesis) into chemical energy required for cells functioning (e.g. division, growth). Plants photosynthesis produce more sugar and oxygen than they consume it through respiration. The surpluses of sugar must be stored or used for plants growth. Thus, they become part of leaves, roots, tubers, fruits or tree trunks. Since sugar is the photosynthesis carbon product of the carbon dioxide naturally present in the environment or released by people when they burn coal, oil, and other fossil fuels, plant productivity plays a major role in the global carbon cycle.
Oceanic primary production
Plants are not the only primary producers and in the oceans, microscopic algae, seaweeds and seagrasses fulfill the same role. Since seaweeds and seagrasses grow on seafloor in presence of light, their distribution is limited to coastal environment. On contrary, as drifting cells, algae are susceptible to grow in both coastal and offshore environments. This ubiquity of algae explains their main role in the oceanic primary production (they provide 95% of it). Besides, along evolution, algae appeared well before plants. Thus, we principally owe our breathable atmosphere to algae’s work rather than seaweeds and plants one. Presently, the primary production from land and oceans are equivalent with 50 x 1015 gram of Carbon per year produced by each environment. We will further discuss more in detail the conditions of the algae primary production.
The plankton bloom
Algae production is most of the time pulsative and not continuous. Their growth occurs under certain favorable environmental conditions apart from which, the remaining population is mainly in dormancy state (metabolic activity minimized for survival) and extremely low. When growing conditions are reunited, a population explosion could occur, which is called algal bloom or phytoplankton bloom (from “phyto” meaning “plant” and “plankton” meaning drifting). Sometimes more than one species bloom at the same time. Blooms are often visible, which allow their monitoring through remote sensing. Indeed, high concentrations of phytoplankton in the water column can cause the water to appear blue-green, green, brown or even red, depending upon the pigments found in the species experiencing the bloom. Pigments are substances in phytoplankton that absorb the sun’s energy, which is needed to drive the process of photosynthesis.
The right condition are needed for a phytoplankton bloom
A phytoplankton bloom is susceptible to occur when the environmental factors such as water temperature and salinity must be just right and essential nutrients and light must be available in the correct amounts.
In the “ocean current” section we saw that the thermohaline circulation is based on buoyancy/density variations. The density of a water mass is controlled by its salinity, temperature and in a lesser extent, pressure. Generally, from the surface to seafloor, several water masses are superimposed with increasing density with depth. The density differential between to water masses is an impassable barrier for drifting organisms like algae. Thus, a surface water mass with high nutrient content is a good blooming place since algae are literally stuck in a water mass rich in nutrients and light.
A bloom ends when the surface water mass in which it is located no longer possesses enough nutrients to sustain it. To allow a new bloom to occur, the surface water mass must mix with the underlying nutrients-rich water masses (which is called delaminate), then to restructure in a surface water mass newly nutrient and light-rich. In winter, the strong cold wind reduces the temperature of the surface water mass, which consequently increases its density. The low differential density between the surface with the underlying water masses associated with the mixing effect of wind allows their mixing. The surface and underlying water masses can then form one mass of equitably distributed nutrients. In spring, the increasing temperature affects mostly the surface waters, which induce a lamination by the temperature (the temperature dilate the water, reducing its density). This water mass will then sustain a phytoplanktonic bloom.
The two most important nutrients for phytoplankton growth are the elements nitrogen (N) and phosphorus (P), which are found naturally in aquatic environments in various concentrations. Iron, zinc and manganese are also essential, but they are needed only in very small quantities. Since algal species require a different amount of each nutrient, their distribution often controls which species that will be susceptible to blooming.
That primary production will further be eaten (by microscopic crustaceans or filter-feeders as oysters), degraded (by bacteria) or preserved and contribute to the marine sediment.