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.Lalli, C.M.; Parsons, T.R. (2004) Biological Oceanography: An Introduction (2nd Ed.) Elsevier Butterworth Heinemann, Burlington, MA, p. 55. Instead, these regions tend to be limited by low concentrations of metabolizable iron. Iron is a critical phytoplankton micronutrient necessary for enzyme catalysis and electron transport.
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. This pattern was observed in other oceanic regions and led to the naming of three major HNLC zones: the North Pacific Ocean, the Equatorial Pacific Ocean, and the Southern Ocean.
The discovery of HNLC regions has fostered scientific debate about the ethics and efficacy of iron fertilization experiments which attempt to draw down atmospheric carbon dioxide by stimulating surface-level photosynthesis. It has also led to the development of hypotheses such as grazing control which poses that HNLC regions are formed, in part, from the grazing of phytoplankton (e.g. dinoflagellates, ciliates) by smaller organisms (e.g. protists).
Primary production
Primary production is the process by which autotrophs use light to convert carbon from aqueous carbon dioxide to sugar for cellular 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.
Since primary production and phytoplankton biomass cannot currently be measured over entire ocean basins, scientists use chlorophyll α as a proxy for primary production. Modern satellite observations monitor and track global chlorophyll α abundances in the ocean via remote sensing. Higher chlorophyll concentrations generally indicate areas of enhanced primary production, and conversely lower chlorophyll levels indicate low primary production. This co-occurrence of low chlorophyll and high macronutrient availability is why these regions are deemed "high-nutrient, low-chlorophyll."
In addition to the macronutrients needed for organic matter synthesis, phytoplankton need micronutrients such as trace metals for cellular functions. Micronutrient availability can constrain primary production because trace metals are sometimes limiting nutrients. Iron has been determined to be a primary limiting micronutrient in HNLC provinces. Recent studies have indicated that zinc and cobalt may be secondary and/or co-limiting micronutrients.
Global distribution
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. While HNLC broadly describes the biogeochemical trends of these large ocean regions, all three zones experience seasonal phytoplankton blooms in response to global atmospheric patterns. On average, HNLC regions tend to be growth-limited by iron and variably, zinc. 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. Unlike the Southern Ocean and the North Pacific, the Equatorial Pacific experiences temporal silicate availability which leads to large seasonal diatom blooms.
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. HNLC areas are of interest to geoengineers and some in the scientific community who believe fertilizing large patches of these waters with iron could potentially lower dissolved carbon dioxide and offset increased anthropogenic carbon emissions. Analysis of Antarctic ice core data over the last million years shows correlation between high levels of dust and low temperature, indicating that addition of diffuse iron-rich dust to the sea has been a natural amplifier of climate cooling.
North Pacific
The discovery and naming of the first HNLC region, the North Pacific, was formalized in a seminal paper published in 1988. The study concluded that surface waters of the eastern North Pacific are generally dominated by picoplankton despite the relative abundance of macronutrients. In other words, larger phytoplankton, such as diatoms which thrive in nutrient-rich waters, were not found. Instead, the surface waters were replete with smaller pico- and nanoplankton. Based on laboratory nutrient experiments, iron was hypothesized to be a key limiting micronutrient.
The Pacific Ocean is the largest and oldest body of water on Earth. The North Pacific is characterized by the general clockwise rotation of the North Pacific gyre, which is driven by trade winds. Spatial variations in tradewinds result in cooler air temperatures in the western North Pacific and milder air temperatures in the eastern North Pacific (i.e., Subarctic Pacific). 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. Volcanic ash from the eruption of the Kasatochi volcano in August 2008 provided an example of natural iron fertilization in the Northeast Pacific Ocean. The region was fertilized by raining volcanic dust containing soluble iron. In the days following, phytoplankton blooms were visible from space.
Limitations in trace metal concentrations in the North Pacific limit diatom blooms throughout the entire year. 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.
Equatorial Pacific The Equatorial Pacific is an oceanic province characterized by nearly year-round upwelling due to the convergence of trade winds from the northeast and southeast at the Intertropical Convergence Zone. The Equatorial Pacific spans nearly half of Earth’s circumference and plays a major role in global marine new primary production.Chavez, F P., and J R Toggweiler, 1995: Physical estimates of global new production: The upwelling contribution In Dahlem Workshop on Upwelling in the Ocean: Modern Processes and Ancient Records, Chichester, UK, John Wiley & Sons, 313-320. New production is a term used in biological oceanography to describe the way in which nitrogen is recycled within the ocean. In regions of enhanced new production, nitrate from the aphotic zone makes its way into surface waters, replenishing surface nitrate supply. Despite nitrogen availability in Equatorial Pacific waters, primary production and observed surface ocean biomass are considerably lower compared to other major upwelling regions of 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. During these glacial periods, the Equatorial Pacific increased its export of marine new production, thus providing a sink of atmospheric carbon dioxide. The science of paleoceanography attempts to understand the interplay of glacial cycles with ocean dynamics. Paleo-oceanographers currently challenge the Aeolian Dust hypothesis which suggests that the atmospheric transport of iron-rich dust off Central and South America controls the intensity of primary production in the Equatorial Pacific. One study suggests that because EUC upwelling provides most of the bioavailable iron to the equatorial surface waters, the only method to reverse HNLC conditions is by enhancing upwelling. 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.