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Ocean regions with little phytoplankton
High nutrient, low chlorophyll (HNLC) regions are regions of the ocean where the abundance of phytoplankton is low and fairly constant despite the availability of macronutrients. Phytoplankton rely on a suite of nutrients for cellular function. Macronutrients (e. g., nitrate, phosphate, silicic acid) are generally available in higher quantities in surface ocean waters, and are the typical components of common garden fertilizers. Micronutrients (e. g., iron, zinc, cobalt) are generally available in lower quantities and include trace metals. Macronutrients are typically available in millimolar concentrations, while micronutrients are generally available in micro to nanomolar concentrations. In general, nitrogen tends to be a limiting ocean nutrient, but in HNLC regions it is never significantly depleted. Instead, these regions tend to be limited by low concentrations of metabolizable iron. Between the 1930s and '80s, it was hypothesized that iron is a limiting ocean micronutrient, but there were not sufficient methods reliably to detect iron in seawater to confirm this hypothesis. In 1989, high concentrations of iron rich sediments in nearshore coastal waters off the Gulf of Alaska were detected. However, offshore waters had lower iron concentrations and lower productivity despite macronutrient availability for phytoplankton growth. Light provides the energy for the photosynthetic process and nutrients are incorporated into organic material. For photosynthesis to occur, macronutrients such as nitrate and phosphate must be available in sufficient ratios and bioavailable forms for biological utilization. The molecular ratio of 106(Carbon):16(Nitrogen):1(Phosphorus) was deduced by Redfield, Ketcham, and Richards (RKR) and is known as the Redfield Ratio. Photosynthesis (forward) and respiration (reverse) is represented by the equation:

{106CO2} + {16HNO3} + {H3PO4} + {122H2O} <=> {(CH2O)106(NH3)16(H3PO4)} + {136O2}

Photosynthesis can be limited by deficiencies of certain macronutrients. However, in the North Pacific, the Equatorial Pacific, and the Southern Ocean macronutrients are found in sufficient ratios, quantities and bioavailable forms to support greater levels of primary production than found. Macronutrient availability in HNLC regions in tandem with low standing stocks of phytoplankton suggests that some other biogeochemical process limits phytoplankton growth.

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.

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. Iron fertilization studies conducted at repeated intervals over the span of a week have produced a larger biological response than a single fertilization event. The biological response size tends to depend on a site’s biological, chemical, and physical characteristics. In the Equatorial and North Pacific, silica is thought to constrain additional production after iron fertilization, while light limits additional production in the Southern Ocean. The large bloom response and community shift has led to environmental concerns about fertilizing large sections of HNLC regions. One study suggests that diatoms grow preferentially during fertilization experiments. Some diatoms, such as pseudo nitzschia, release the neurotoxin domoic acid, poisoning grazing fish. Dust deposition might not result in phytoplankton blooms unless settling dust is in the correct bioavailable form of iron. Additionally, iron must be deposited during productive seasons and coincide with the appropriate RKR ratios of surface nutrients. Aeolian dust has a larger influence on Northern Hemisphere HNLC regions because more land mass contributes to more dust deposition. Due to the Southern Ocean's isolation from land, upwelling related to eddy diffusivity provides iron to HNLC regions.

Formulated by John Walsh in 1976, the grazing hypothesis states that grazing by heterotrophs suppresses primary productivity in areas of high nutrient concentrations. Predation by microzooplankton primarily accounts for phytoplankton loss in HNLC regions. Grazing by larger zooplankton and advective mixing are also responsible for a small proportion of losses to phytoplankton communities. Constant grazing limits phytoplankton to a low, constant standing stock. Without this grazing pressure, some scientists believe small phytoplankton would produce blooms despite micronutrient depletion because smaller phytoplankton typically have lower iron requirements and can absorb nutrients at a slower rate. The extent to which each factor contributes to low production may differ in each HNLC region. Iron limitation allows for smaller, more iron frugal phytoplankton to grow at rapid rates, while grazing by microzooplankton maintains stable stocks of these smaller phytoplankton.

To effectively remove anthropogenic carbon from the atmosphere, iron fertilization would need to result in significant removal of particulate carbon from the surface ocean and transport it to the deep ocean. and only a 15 25 ppm decrease in atmospheric carbon dioxide would result with sustained global iron fertilization. Pronounced community shifts to diatoms have been observed during fertilization, and it's still unclear if the change in species composition has any long term environmental effects.

The following is completely theoretical. Testing would be required to determine feasibility, optimum iron concentration per unit area, carbon sequestration by area over time, need for other micro nutrients, amount of energy required to maintain such a system, and the potential amount of energy produced by the system. This system considers economic feasibility (profitability of bio fuel products and carbon credits) and risk management.

Grazing results in algae being consumed by micro zooplankton. This predation results in less than 7 10% of carbon being taken to the bottom of the ocean. Growing algae in floating farms could allow these HNLC areas to grow algae for harvest without the problem of predation. Algae grown in floating farms would be recycled through grazing if there was a catastrophic failure of a floating farm, which would limit any environmental damage.

Algae grown in floating farms could be harvested and used for food or fuel. All biological life is made up of lipids, carbohydrates, amino acids, and nucleic acids. Whole algae could be turned into animal feed, fertilizer, or bio char. Separating the lipids from the algae could also create bio diesel from the lipid content and bio char from the rest. Of course, the algae could be pumped to the bottom of the ocean, below any grazing pressure for sequestration.

In a controlled floating farm, the harvest could be sampled to record the amount of algae per unit volume which will indicate the amount of carbon being sequestered. If this carbon is sequestered at the bottom of the ocean, this figure could be used to accurately create carbon credits. Sequestering carbon dioxide on the ocean floor could destroy the unstudied ecosystem and cause undiscovered lifeforms to go extinct. Carbon sequestration on land does so with desiccated algae. Without sufficient sources of water, bacteria and other life will have a difficult time digesting the sequestered algae. Biofuels, not sold and used as renewable fuel, could be sequestered in abandoned oil wells and coal mines. The volume of bio diesel and mass of bio char would provide an accurate figure for producing (when sequestering) and selling (when removing from wells or mines) carbon credits.