Low-nutrient, low-chlorophyll (LNLC) regions are aquatic zones that are low in nutrients (such as nitrogen, phosphorus, or iron) and consequently have low rate of primary production, as indicated by low chlorophyll concentrations. These regions can be described as oligotrophic, and about 75% of the world's oceans encompass LNLC regions. A majority of LNLC regions are associated with subtropical gyres but are also present in areas of the Mediterranean Sea, and some inland lakes. Physical processes limit nutrient availability in LNLC regions, which favors nutrient recycling in the photic zone and selects for smaller phytoplankton species. LNLC regions are generally not found near coasts, since coastal areas receive more nutrients from terrestrial sources and upwelling. In marine systems, seasonal and decadal variability of primary productivity in LNLC regions is driven in part by large-scale climatic regimes (e.g. El Niño-Southern Oscillation) leading to important effects on the global carbon cycle and the oceanic carbon cycle.
The naming of these regions follows the same format as the more widely known high-nutrient, low-chlorophyll (HNLC) regions, which display high macro-nutrient concentrations but unusually low primary production and chlorophyll concentrations, usually due to lack of micro-nutrients, usually iron-limitation. While the LNLC terminology appears synonymous with oligotrophic, literature in recent decades has predominantly used the term to define oligotrophic ocean areas (as opposed to lakes, rivers, or other bodies of water) in discussions about dust and iron fertilization. Occasionally, LNLC refers to "low nitrate, low chlorophyll," which has a similar meaning.
Chlorophyll and primary production
Chlorophylls are the light harvesting pigments employed in photosynthesizing organisms that facilitate the conversion of light into cellular energy for synthesis of organic matter. In aquatic systems, chlorophyll concentrations are frequently used as a proxy measurement for photosynthetic plankton (phytoplankton) abundance and thus the amount of primary production. Chlorophyll concentrations can serve as an estimate for primary productivity.
Primary production is the process of converting CO2, either gaseous or aqueous, and other elements into organic compounds. The extent of global primary production therefore controls how much energy is available to in the biosphere. Most primary production in the ocean occurs via photosynthesis, by organisms called phototrophic primary producers. Phototrophs are the foundation of most aquatic food webs, and primary production in the ocean accounts for about half of Earth's carbon fixation.
In addition to light and CO2, photosynthesis requires nutrients, such as nitrate, phosphate, silicate, and iron in order for phytoplankton to grow. Primary productivity is therefore controlled by the availability of the limiting nutrient (Liebig's law of the minimum). The Redfield Ratio (106C:16N:1P) describes the general relative proportions of carbon, nitrogen, and phosphorus found in marine biomass. If a given region is limited by a nutrient, the relative proportion of nutrients can deviate from the Redfield Ratio. Due to the low nutrient levels in LNLCs, primary production in these regions is usually limited by one or more nutrients.
Physical and biological determinants of nutrient cycling
Phytoplankton growth occurs in the upper ocean, or mixed layer, where sufficient light energy is available and turbulent mixing near the surface increases vertical homogeneity in ocean tracers like temperature, salinity, and density. Nutrient availability in the mixed layer, often a limiting factor for phytoplankton growth, is derived from three primary sources: 1) reintroduced nutrients from deep ocean water that is mixed into surface waters (i.e., new production), 2) recycled nutrients in the surface ocean (i.e., regenerated production), also known as the regeneration loop, and 3) 'external' nutrients introduced from terrestrial or atmospheric input and/or biological processes (such as, nitrogen fixation). LNLC regions result from a combination of processes that limit nutrient availability, primarily: the biological pump, Ekman downwelling, and stratification. The biological pump creates a nutrient gradient, or nutricline, in LNLC regions by exporting organic matter from the surface ocean to the deep ocean before that organic matter (OM) can be respired in the photic zone, thus also exporting the OM-associated nutrients from the photic zone. Nutrients transported to depth below the pycnocline can be reintroduced to the surface through ocean mixing processes (including Ekman upwelling and turbulent diffusion) that bring deep, nutrient-rich waters back to the ocean surface and stimulate primary production. LNLCs don't generally occur in regions with significant upwelling or turbulent diffusion because these areas generally see greater nutrient availability and thus primary production.
Lack of reintroduced nutrients can be exacerbated by Ekman downwelling, caused by convergent wind forcing and Ekman transport, which moves water masses downward and removes nutrients from the surface ocean by exporting them out of the photic zone. Downwelling locations are prime LNLC regions, like coastal areas with landward Ekman transport or subtropical gyres.
Stratification of the water column due to density gradients associated with temperature (see thermocline) or salinity (see halocline) also limits reintroduced nutrients from deep water by creating less dense water above more dense water that prevents ocean mixing, only allowing minimal nutrient delivery through the slower turbulent diffusion rates. Physical mixing processes caused by wind or storm events may however introduce nutrients from deeper in the water column into the photic zone and stimulate transient phytoplankton blooms.
While primary production is limited in downwelling or stratified LNLC regions, other nutrient sources become important for maintaining phytoplankton growth, like recycled surface water nutrients and external nutrients (aforementioned sources 2 and 3). Lower nutrient levels favor the growth of picophytoplankton, whose small size increases heterotrophic nutrient recycling (known as the microbial loop) and reduces the loss of nutrients from the biological pump due to the slower sinking rates of smaller particles.
In order to compensate for the low nitrogen/nutrient availability, nitrogen fixation is favored as a mechanism for bringing 'external' nutrients into the surface ocean. Nitrogen fixation is the microbial mediated process that converts biologically unavailable dissolved nitrogen gas, which is readily available in the atmosphere and mixed into oceans through diffusion and wave action mixing, into biologically available ammonium. Catalyzed by the enzyme nitrogenase, nitrogen fixation requires iron and often solar energy, making iron a common limiting nutrient in LNLCs. In areas far from terrestrial sources, iron is predominantly delivered through aeolian dust, naturally derived from arid landmasses (e.g. deserts) or volcanic eruptions, that can travel long distances to supply this critical micronutrient to LNLC regions like open ocean gyres.
Productivity in LNLC regions LNLC regions are estimated to cover around 75% of the global ocean, and despite the generally low per area rates of primary production, an estimated 40% of global ocean productivity occurs in these locations due to their large geographic extent.W.H. Berger, 1989. Global maps of ocean productivity. W.H. Berger, V.S. Smetacek, G. Wefer (Eds.), Productivity of the Ocean: Present and Past, Wiley, Chichester (1989), pp. 429-455
A majority of the ocean's LNLC regions are within subtropical gyres, although the Mediterranean Sea is another large LNLC region (for more see Example section). In general, LNLC regions have relatively warm, stratified water columns that limit the delivery of new nutrients to the photic zone via upwelling, thus favoring the nutrient "regeneration loop" where the majority of nutrients are recycled between primary producers, grazers and heterotrophic bacteria. Because the regeneration loop is favored in LNLC regions, there are low rates of new production and limited organic matter exported via the biological pump. However, these regions can experience seasonal and episodic nutrient delivery events that increase primary productivity, favoring the "export pathway" (biological pump), where both dissolved and particulate organic matter exit the system, some of which is sequestered in the deep ocean.
In subtropical gyres, and more generally in areas with low nutrient availability, phytoplankton communities are generally dominated by picoplankton (cells between 0.2 and 2μm). Picoplankton are smaller than other plankton that dominate higher nutrient regions, and their small volume means they have a larger surface area to volume ratio. Picoplankton can therefore take up nutrients more efficiently than larger plankton with smaller surface area to volume ratios. Picoplankton in subtropical gyres are highly grazed upon by nano- (2-20μm) and micro- (20-200μm) plankton, resulting in fast nutrient cycling, especially of nitrogen and phosphorus, and less export of organic matter and associated nutrients to the deep ocean via the biological pump.
Vertical mixing of the water column in winter and spring from storms in LNLC regions can deliver nutrients from deeper in the water column and favor transient increases in primary productivity within ocean gyres. During these events, larger phytoplankton species, such as diatoms, are favored when more nutrients are present, resulting in increased rates of organic matter export to the deep ocean via sinking particulates.
While generally low, observed primary productivity in gyres is higher than would be expected based on measured nutrient supply. The excess primary production can be explained by nitrogen fixing organisms that supply the remaining bioavailable nitrogen needed to support observed seasonal primary productivity. Low organic nitrogen levels in surface waters selects for nitrogen-fixing bacteria called diazotrophs. Nitrogen fixing organisms are often iron limited because nitrogenase has two iron-sulfur clusters. Thus, iron delivery via aeolian dust is often associated with increased primary productivity in the subtropical gyres and in the Mediterranean Sea.
Variability of LNLC regions Even with their overall low nutrient availability, LNLC regions are dynamic and experience seasonal variability in size and productivity. For example, the size of the North Atlantic Subtropical Gyre shrinks in winter and expands in summer. Long-term trends indicate that subtropical gyres in the northern hemisphere (North Pacific and North Atlantic) are expanding over time, and the subtropical gyres in the southern hemisphere (South Pacific) are growing weaker and more inconsistent. All LNLC subtropical gyres appear to have long-term trends of increasing sea-level height anomalies (SLA) and sea surface temperature (SST). Increasing SLA is indicative of deepening of both the thermocline and the nutricline, which reduces light and nutrient availability for the phytoplankton community in the mixed layer. Increased SST within LNLC regions contributes to increased stratification of the water column, which generally reduces nutrient inputs to the photic zone from deeper waters. Natural climatic variability events such as El Nino/Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) influence SST in the global ocean with implications for primary productivity and phytoplankton communities. For example, the PDO warm phase increases SST in the equatorial pacific, causing increased stratification of the surface ocean that reduces nutrient availability and favors nitrogen fixing organisms.