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. [1][2] Instead, these regions are limited by low concentrations of metabolizable iron. [1] 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. [1][2]
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The iron hypothesis proposes that a lack of iron in the ecosystem prevents phytoplankton from achieving an algal bloom.[2] Iron is necessary for nitrogen fixation which is required for the production of amino acids [1][3]. 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.[3]
Small phytoplankton can be preyed upon by micro-zooplankton 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.[3] Since phytoplankton biomass is kept in check, nutrients are never depleted, causing the high nitrate concentrations that define HNLC regions.
Antarctic Paradox is a term in ecology 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[4] and deep mixing reduces the amount of light available for photosynthesis.[5]