Phytoplankton and Primary Production

WALTER HELBLING, E; VIRGINIA ESTELA VILLAFAÑE

Encyclopedia of Life Support Systems (EOLSS)

Eolss Publishers

Lugar: Oxford, UK; Año: 2007; p. 1 - 1

Primary productivity in aquatic systems is mostly carried out by phytoplankton—unicellularorganisms that move passively in the water column. Primary production is dependent on thephotosynthetic process in which cells use solar energy to combine water and carbon dioxide intoorganic compounds that will be available to higher trophic levels. There are a number of factors thatregulate and limit primary production in aquatic ecosystems, being the most important solarradiation and nutrients. Particularly, solar radiation—quantity and quality—strongly affects thephotosynthetic process. Solar radiation attenuates with depth (as a function of particulate anddissolved materials) so that there is delimited zone where phytoplankton photosynthesis canoccur—the euphotic zone (conventionally, the depth of 1% surface irradiance). The general depthdistributionof phytoplankton photosynthesis may reflect a photo-inhibitory process at the surfacebecause of high radiation levels; then a subsurface maximum is found and, at depth, photosynthesisis again reduced because of low radiation levels. The quality of solar radiation causes assortedresponses in phytoplankton: While PAR (photosynthetically active radiation, 400-700 nm) isresponsible for the bulk of photosynthesis, UVR (280-400 nm) is generally considered an inhibitorof this process. Still, it is very difficult to draw generalizations, as responses are clearly dependenton the species, the amount and quality of radiation, as well as on the time scale considered to assessphytoplankton primary production. The principal limiting nutrients in aquatic environments arenitrogen, phosphorous and silicon. It is generally seen that if their concentration is low,phytoplankton cannot increase their biomass and thus photosynthesis remains low. On the otherhand, when nutrient concentrations increase (e.g. due to agricultural runoff in coastal areas orupwelling effect) total photosynthesis, and therefore phytoplankton biomass, will increase. Otherlimiting nutrients are trace metals such as iron, that seem to be responsible for limitingphotosynthesis in large areas of the Southern Ocean and other areas such as the equatorial Pacific.