HNLC
Encyclopedia
HNLC stands for "high-nitrate
, low-chlorophyll
" - a term used in marine ecology to describe areas of the ocean where the number of phytoplankton
(standing stock) are low and fairly constant in spite of high macro-nutrient concentrations (nitrate, phosphate, silicic acid). In general, some essential inorganic substances, like nitrate, may be present in oceanic waters in concentrations low enough to be limiting to plant production, but in HNLC regions the level of nitrate is never significantly depleted. Instead, these regions are limited by low concentrations of metabolizable iron. There are two popular explanations for the existence of HNLC regions: the ‘iron hypothesis’ and the ‘grazing control hypothesis'. HNLC regions cover 20% of the world’s oceans in three major areas: equatorial Pacific Ocean
, subarctic Pacific Ocean and the Southern Ocean
.
. Iron is necessary for nitrogen fixation
which is required for the production of amino acids . This iron limitation inhibits the growth of larger phytoplankton, such as diatoms. Instead, HNLC regions are characterized by small phytoplankton flagellates which can take up iron at lower concentrations due to greater surface area to volume ratios.
known as protozoan grazers. Protozoa are capable of reproductive rates equal to or exceeding that of their phytoplankton prey, therefore preventing any significant increase in phytoplankton populations, including blooms. Since phytoplankton biomass is kept in check, nutrients are never depleted, causing the high nitrate concentrations that define HNLC regions.
that describes the phenomenon that in vast areas of the Southern Ocean
there are plenty of nutrients for phytoplankton
to thrive but still, the phytoplankton do not grow much. These areas have been termed as HNLC (high nitrate, low chlorophyll) areas. Suggested reasons for this are that iron
concentrations are low and deep mixing reduces the amount of light available for photosynthesis.
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
, low-chlorophyll
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
" - a term used in marine ecology to describe areas of the ocean where the number of phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
(standing stock) are low and fairly constant in spite of high macro-nutrient concentrations (nitrate, phosphate, silicic acid). In general, some essential inorganic substances, like nitrate, may be present in oceanic waters in concentrations low enough to be limiting to plant production, but in HNLC regions the level of nitrate is never significantly depleted. Instead, these regions are limited by low concentrations of metabolizable iron. There are two popular explanations for the existence of HNLC regions: the ‘iron hypothesis’ and the ‘grazing control hypothesis'. HNLC regions cover 20% of the world’s oceans in three major areas: equatorial Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
, subarctic Pacific Ocean and the Southern Ocean
Southern Ocean
The Southern Ocean comprises the southernmost waters of the World Ocean, generally taken to be south of 60°S latitude and encircling Antarctica. It is usually regarded as the fourth-largest of the five principal oceanic divisions...
.
Iron hypothesis
The iron hypothesis proposes that a lack of iron in the ecosystem prevents phytoplankton from achieving an algal bloomAlgal bloom
An algal bloom is a rapid increase or accumulation in the population of algae in an aquatic system. Algal blooms may occur in freshwater as well as marine environments. Typically, only one or a small number of phytoplankton species are involved, and some blooms may be recognized by discoloration...
. Iron is necessary for nitrogen fixation
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
which is required for the production of amino acids . This iron limitation inhibits the growth of larger phytoplankton, such as diatoms. Instead, HNLC regions are characterized by small phytoplankton flagellates which can take up iron at lower concentrations due to greater surface area to volume ratios.
Grazing control hypothesis
Small phytoplankton can be preyed upon by micro-zooplanktonZooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
known as protozoan grazers. Protozoa are capable of reproductive rates equal to or exceeding that of their phytoplankton prey, therefore preventing any significant increase in phytoplankton populations, including blooms. Since phytoplankton biomass is kept in check, nutrients are never depleted, causing the high nitrate concentrations that define HNLC regions.
Antarctic Paradox
Antarctic Paradox is a term in ecologyEcology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
that describes the phenomenon that in vast areas of the Southern Ocean
Southern Ocean
The Southern Ocean comprises the southernmost waters of the World Ocean, generally taken to be south of 60°S latitude and encircling Antarctica. It is usually regarded as the fourth-largest of the five principal oceanic divisions...
there are plenty of nutrients for phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
to thrive but still, the phytoplankton do not grow much. These areas have been termed as HNLC (high nitrate, low chlorophyll) areas. Suggested reasons for this are that iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
concentrations are low and deep mixing reduces the amount of light available for photosynthesis.
See also
- Antarctic krillAntarctic krillAntarctic krill, Euphausia superba, is a species of krill found in the Antarctic waters of the Southern Ocean. It is a shrimp-like crustacean that lives in large schools, called swarms, sometimes reaching densities of 10,000–30,000 individual animals per cubic metre...
- Iron fertilizationIron fertilizationIron fertilization is the intentional introduction of iron to the upper ocean to stimulate a phytoplankton bloom. This is intended to enhance biological productivity, which can benefit the marine food chain and remove carbon dioxide from the atmosphere. Iron is a trace element necessary for...
- Iron HypothesisIron HypothesisThe Iron Hypothesis was formulated by oceanographer John Martin, based on theories by Joseph Hart and first tested in 1993. In 1988, Nature published the results of Martin's experiments as well as his speculations on climate change...
- John Martin (oceanographer)John Martin (oceanographer)John Martin , was an oceanographer.Born in Old Lyme, Connecticut, he is best known for his research on the role of iron as a phytoplankton micronutrient, and its significance for so-called "High-Nutrient, Low Chlorophyll" regions of the oceans...
- Marine snowMarine snowIn the deep ocean, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to the aphotic zone below. The term was first coined by the explorer William Beebe as he...
- Primary productionPrimary production400px|thumb|Global oceanic and terrestrial photoautotroph abundance, from September [[1997]] to August 2000. As an estimate of autotroph biomass, it is only a rough indicator of primary production potential, and not an actual estimate of it...