7.1: Primary Production
Primary production is the creation of new organic matter from inorganic substrates, and it is this organic matter that serves as the base of the food web for most marine consumers. Primary production generally refers to the process of photosynthesis, or the utilization of light energy to produce chemical fuels that is undertaken by plants and algae according to the reaction:
\[\ce{6CO2 + 6H2O → C6H12O6 + 6O2}\]
Here, powered by light energy, carbon dioxide and water combine to produce glucose and oxygen. However, primary production is also carried out by bacteria in the absence of light through chemosynthesis . Instead of light providing the energy for the reaction, the energy comes from the oxidation of inorganic materials, such as hydrogen sulfide (see section 4.11 on hydrothermal vents). Here we will concentrate on photosynthesis because it plays a much larger role in total oceanic productivity than chemosynthesis.
The organisms responsible for oceanic primary production include a wide diversity of marine plants and algae. While many people may be more familiar with the larger seagrasses and macroalgae (seaweeds), by far the greatest amount of photosynthesis in the ocean comes from microscopic algae, the phytoplankton . The term “plankton” refers to organisms that drift with the currents, and the phytoplankton are the free-floating algae that undergo photosynthesis (contrast this with the zooplankton , who are the drifting animals). In section 7.2 we will take a closer look at the organisms responsible for oceanic primary production.
The total amount of organic material created by the producers is called the gross primary productivity , or total production . However, the primary producers consume a portion of this organic matter themselves through respiration, so the total amount that is left to support the consumers in the ocean is called net production (gross productivity – respiration). Gross production can be divided into two components, new production and regenerated production. New production is supported by nutrients brought in from outside of the local ecosystem through processes such as upwelling or ocean currents. Regenerated production results from the recycling of nutrients within an ecosystem.
Overall, marine productivity is similar to terrestrial production. Marine net production is about 35-50 billion metric tons per year, while terrestrial production reaches 50-70 billion tons per year. However, the biomass responsible for that production in the ocean is about 1-2 billion metric tons, compared to 600-1000 billion metric tons of biomass in terrestrial systems. So the oceans are producing almost as much organic material as terrestrial producers, but are doing it from only a fraction of the amount of producer biomass. One reason for this discrepancy is that the phytoplankton are constantly being consumed, while much of the terrestrial biomass is much longer-lived than the plankton.