Phytoplankton

Phytoplankton are a group of free floating, microscopic organisms predominantly classified as algae. Over 4,000 species of phytoplankton have been identified and this list is rapidly growing. Phytoplankton are mostly single cellular organisms and all are autotrophic (i.e., they contain photosynthetic pigments). These pigments allow phytoplankton to use the sun’s energy to convert CO2 and inorganic nutrients, through photosynthesis, into biological molecules such as proteins and carbohydrates.

This process of creating new biological molecules is called primary production. Phytoplankton are the only primary producers in the open oceans, and thus form the basis of the food chain in over 70 percent of the world’s surface area. Specific aspects of primary production are often considered more carefully. The net primary production (NPP) refers to the amount of organic carbon available after respiration has been subtracted from the total amount of photosynthesis.

It is an important term because this is the amount of carbon that is available to the rest of the foodweb, and is the upper limit on respiration. The net community production (NCP) is the difference between net primary production and heterotrophic respiration. It is measured by gross changes in oxygen or biomass in a specific time. The new production is the fraction of primary production driven by newly available nitrogen. This is principally the nitrate and nitrite that becomes available when deep ocean waters are brought up into the euphotic zone, but could include sources from the atmosphere or river inputs.

The global population of phytoplankton is not evenly distributed throughout the world’s oceans. This distribution is the result of growth being limited by the availability of nutrients. High concentrations of phytoplankton are seen in upwelling areas, such as the Benguela Current off the southwest coast of Africa.

Phytoplankton and Climate Change 

CO2 does not limit phytoplankton growth. CO2 is not thought to limit phytoplankton growth in any region of the ocean. Primary production, in general, is limited by the availability of inorganic nutrients. Therefore, it is not expected that increased atmospheric concentrations of CO2 will have a significant impact on the phytoplankton population in the oceans.

The “biological pump” locks away CO2 The biological pump is the mechanism by which anthropogenic CO2 is taken from the atmosphere and stored in the deep ocean, which transfers carbon from the surface to the deep ocean, across the barrier of the permanent thermocline. The pump is powered by phytoplankton fixing carbon and sinks to the deep ocean. This removes CO2 from the surface ocean, causing more CO2 to be drawn from the air to maintain equilibrium. Carbon is locked away from the atmosphere as a result of being taken below the permanent thermocline.

This contrasts with primary productivity on land, which builds plants (such as trees) and in turn animals, locking CO2 away from the atmosphere. Although increasing levels of CO2 do not have a fertilizing effect on the oceans, the longer growing season in temperate and polar regions is expected to lead to an increase in primary productivity.