Draft of my sections for HNLC Page
Introduction
High-nutrient, low chlorophyll (HNLC) is 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. It has been hypothesized that iron has been a limiting nutrient in the ocean since the 1930s (Martin, 1992). The importance of iron utilization to regulate plant life in the ocean was known at the time, but because scientists did not know much about the amount of iron in the ocean because of how difficult it was to study (Martin, 1992). In 1989, Dr. John H. Martin published a study conducted in the Gulf of Alaska to have a better understanding of iron concentration in the ocean (Martin, 1992). This study found high loads of iron-rich sediments in nearshore coastal waters, which was expected, and observed surprisingly low iron concentrations in offshore surface waters (Martin, 1992). This then lead to observations and experiments done in other oceans around the world that lead to the discovery of three major HNLC regions: equatorial Pacific Ocean, North Pacific Ocean and the Southern Ocean. These regions cover 20% of the world’s oceans with each individual region having different physical characteristics, biological communities, and limiting nutrients. Two other popular explanations for the existence of HNLC regions besides the Iron Hypothesis is the Grazing Control Hypothesis and Antarctic Paradox Hypothesis.
North Pacific
The Pacific Ocean is the largest and oldest body of water on the Earth, with the North Pacific Ocean being the deepest point on the planet. The North Pacific Ocean is characterized by the clockwise direction of the North Pacific gyre that is composed of the North Equatorial current along 15 degrees N that is driven by the trade winds and then becomes the warm Kuroshio Current located near Japan. This body of water then moves northward as the Aleutian Current and then returns to the North Equatorial Current. Because of the decrease in evaporation due to low temperatures, the lowest counts of salinity can be measured in the North Pacific at about 32 parts per thousand.[1] The tradewinds in the North Pacific vary spatially with colder temperatures off of Northern Asia in the and mild winters off of Canada in the winter.
Dr. John H. Martin, who is most well known for his work on the role of iron as a micronutrient for phytoplankton, conducted his most early research on the Iron Hypothesis in the waters near Alaska. This study found high loads of iron-rich sediments in nearshore coastal waters, which was expected, and observed surprisingly low iron concentrations in offshore surface waters. This work in Alaska lead to the conclusion that the open ocean water was infertile in regards to phytoplankton production and began the observation and discovery of other HNLC regions. (Martin, 1992) The key sources of iron in this HNLC region is the continental margin. Scientists have conducted research to have a better understanding of what other factors may be impacting primary production in this HNLC regions. One of the main ways iron is thought to be supplied to the North Pacific Ocean is from the continental margin. It is suggested that labile iron is advected from the Kuril/Kamchatka margin to the Western Subartic Pacific to provide a subsurface supply of iron that is shallow enough to be utilized when upwelling and vertical mixing occurs in this region so that it the iron may be provided to the HNLC region.[2] The seafloor depth may also impact phytoplankton blooms in these HNLC regions. Research conducted in the Bering Sea showed that areas with shallow waters such as the Continental shelf of the eastern Bering Sea, have more intense phytoplankton blooms than deep waters such as the deep western Basin of the Bering Sea.[3] This may be due to iron diffusing out of the seafloor that alleviates the limitation of iron in these shallower waters.[3]
Biological characteristics: ecology (community makeup), micronutrients
Limitations in trace-metal concentrations in the North Pacific keeps diatoms from blooming throughout the entire year.[4] Even though the North Pacific Ocean is considered HNLC, it shows a high production of biogenic silica and export flux of silica.[4] This is observed because of the higher silicon (Si) concentrations and Si:N uptake ratios.[4] The overall excess of silicic acid utilization over nitrate is reinforced because the dissolution rate of biogenic silica is lower than the rate nitrogen degrades and the inhibition of the uptake of nitrate due to ammonium.[4]
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- ^ "Pacific Ocean". Encyclopedia Britannica. Retrieved 2017-11-03.
- ^ Lam, Phoebe J.; Bishop, James K. B. (2008-04-01). "The continental margin is a key source of iron to the HNLC North Pacific Ocean". Geophysical Research Letters. 35 (7): L07608. doi:10.1029/2008gl033294. ISSN 1944-8007.
- ^ a b Tyrrell, T.; Merico, A.; Waniek, J. J.; Wong, C. S.; Metzl, N.; Whitney, F. (2005-12-01). "Effect of seafloor depth on phytoplankton blooms in high-nitrate, low-chlorophyll (HNLC) regions". Journal of Geophysical Research: Biogeosciences. 110 (G2): G02007. doi:10.1029/2005jg000041. ISSN 2156-2202.
- ^ a b c d Pondaven, P.; Ruiz-Pino, D.; Druon, J.N.; Fravalo, C.; Tréguer, P. "Factors controlling silicon and nitrogen biogeochemical cycles in high nutrient, low chlorophyll systems (the Southern Ocean and the North Pacific): Comparison with a mesotrophic system (the North Atlantic)". Deep Sea Research Part I: Oceanographic Research Papers. 46 (11): 1923–1968. doi:10.1016/s0967-0637(99)00033-3.