High-Nutrient, low-chlorophyll

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. Iron is a critical phytoplankton micronutrient necessary for enzyme catalysis and electron transport. 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. 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 to reliably 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 is the process by which autotrophs use light to convert carbon from aqueous carbon dioxide to sugar for cellular growth. Light catalyzes 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 discovered by Redfield, Ketcham, and Richards (RKR) and is known as the Redfield Ratio. Photosynthesis (forward) and respiration (reverse) is represented by the equation: 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. 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.