Common characteristics HNLC regions cover 20% of the world’s oceans and are characterized by varying physical, chemical, and biological patterns. These surface waters have annually varying, yet relatively abundant macronutrient concentrations compared to other oceanic provinces. This trace metal limitation leads to communities of smaller sized phytoplankton. Compared to more productive regions of the ocean, HNLC zones have higher ratios of silicic acid to nitrate because larger
diatoms, that require silicic acid to make their opal silica shells, are less prevalent. The distribution of trace metals and relative abundance of macronutrients are reflected in the plankton community structure. For example, the selection of phytoplankton with a high surface area to volume ratio results in HNLC regions being dominated by nano- and picoplankton. This ratio allows for optimal utilization of available dissolved nutrients. Larger phytoplankton, such as diatoms, cannot energetically sustain themselves in these regions. Common picoplankton within these regions include genera such as
prochlorococcus (not generally found in the North Pacific),
synechococcus, and various
eukaryotes. Grazing protists likely control the abundance and distribution of these small phytoplankton. The generally lower net primary production in HNLC zones results in lower biological draw-down of atmospheric carbon dioxide and thus these regions are generally considered a net source of carbon dioxide to the atmosphere.
North Pacific The discovery and naming of the first HNLC region, the
North Pacific, was formalized in a seminal paper published in 1988. Iron is supplied to the North Pacific by dust storms that occur in Asia and Alaska as well as iron-rich waters
advected from the continental margin, sometimes by eddies such as
Haida Eddies. Concentrations of iron however vary throughout the year. Ocean currents are driven by seasonal atmospheric patterns which transport iron from the
Kuril-Kamchatka margin into the western Subarctic Pacific. This introduction of iron provides a subsurface supply of micronutrients, which can be used by primary producers during upwelling of deeper waters to the surface. Seafloor depth may also stimulate phytoplankton blooms in HNLC regions as iron diffuses from the seafloor and alleviates iron limitation in shallow waters. Research conducted in the Gulf of Alaska showed that areas with shallow waters, such as the south shelf of Alaska, have more intense phytoplankton blooms than offshore waters. The region was fertilized by raining volcanic dust containing
soluble iron. In the days following, phytoplankton blooms were visible from space. Even though the North Pacific is an HNLC region, it produces and exports to the ocean interior a relatively high amount of particulate biogenic silica compared to the North Atlantic, which supports significant diatom growth. New production is a term used in
biological oceanography to describe the way in which nitrogen is recycled within the ocean. Thus the Equatorial Pacific is considered one of the three major HNLC regions. Like other major HNLC provinces, the Equatorial Pacific is considered nutrient-limited due to lack of trace metals such as iron. The Equatorial Pacific receives approximately 7-10 times more iron from
Equatorial Undercurrent (EUC) upwelling than from inputs due to settling atmospheric dust. Climate reconstructions of
glacial periods using
sediment proxy records have revealed that the Equatorial Pacific may have been 2.5 times more productive than the modern equatorial ocean. In other words, enhanced regional upwelling, rather than iron-rich atmospheric dust deposition, may explain why this region experiences higher primary productivity during glacial periods. Compared to the North Pacific and Southern Ocean, Equatorial Pacific waters have relatively low levels of
biogenic silica and thus do not support significant standing stocks of diatoms. Iron deposited in the North Atlantic is incorporated into
North Atlantic Deep Water and is transported to the Southern Ocean via
thermohaline circulation. Eventually mixing with the
Antarctic Circumpolar Water, upwelling provides iron and macronutrients to the Southern Ocean surface waters. Therefore, iron inputs and primary production in the Southern Ocean are sensitive to iron-rich Saharan dust deposited over the Atlantic. Because of low atmospheric dust inputs directly onto Southern Ocean surface waters, chlorophyll α concentrations are low. Light availability in the Southern Ocean changes dramatically seasonally, but it does not seem to be a significant constraint on phytoplankton growth. and explorations of the Southern Drake Passage region have observed this phenomenon around the
Crozet Islands,
Kerguelen Islands, and
South Georgia and the South Sandwich Islands. These areas are adjacent to shelf regions of Antarctica and islands of the Southern Ocean. The micronutrients required for algal growth are believed to be supplied from the shelves themselves. In the Southern Ocean, prevailing low temperatures are believed to have a negative impact on phytoplankton growth rates. Phytoplankton growth rate is very intense and short lived in
open areas surrounded by sea ice and permanent
sea-ice zones. Grazing by herbivores such as krill,
copepods and
salps is believed to suppress phytoplankton standing stock. Unlike the open waters of the Southern Ocean, grazing along continental shelf margins is low, so most phytoplankton that are not consumed sink to the sea floor which provides nutrients to
benthic organisms. == Hypotheses ==