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The most mysterious region in the ocean: How do low-nutrient, low-chlorophyll regions affect the Earth's carbon cycle?
In the vast ocean, the low nutrient and low chlorophyll zone (LNLC) is undoubtedly one of the most mysterious parts. These areas are characterized by a low content of nutrients (such as nitrogen, phosphorus or iron) in the water column, resulting in extremely low photosynthetic productivity, which is reflected in relatively low chlorophyll concentrations. About 75% of the world's oceans are covered by these areas, mainly concentrated in subtropical gyres, but can also be found in the Mediterranean and some inland lakes.
"The existence of low-nutrient, low-chlorophyll zones guides changes in marine ecosystems and plays an important role in the Earth's carbon cycle."
Physical processes lead to limited nutrient availability in these areas, which in turn promotes the growth of small photosynthetic phytoplankton. These LNLC areas are usually not associated with coasts, which receive more nutrients from the land and have upwelling. With the impact of climate change, human beings have gradually deepened their research on low-nutrient and low-chlorophyll areas. The seasonal and inter-decadal changes in these areas have a significant impact on the global carbon cycle.
Chlorophyll and primary production
Chlorophyll is a light-harvesting pigment in photosynthetic organisms that can convert light energy into cellular energy and synthesize organic matter. In water bodies, chlorophyll concentration is often used as an indicator of the abundance of photosynthetic phytoplankton, which in turn reflects the status of primary production. Primary production is the process by which carbon dioxide (either in gaseous or aqueous form) and other elements are converted into organic compounds. In the ocean, primary production is mainly achieved through photosynthesis, and photosynthetic producers are the basis of the aquatic food chain.
Lack of nutrients and environmental conditions limit primary production in the LNLC region. In these areas, the phytoplankton community is dominated by microphytoplankton, which, due to their large surface area, can absorb nutrients more efficiently, thereby supporting limited primary production.
Nutrient cycling in the LNLC region
Phytoplankton grow primarily in the upper ocean (mixed layer), where there is enough light energy to support their growth. However, the supply of nutrients comes from several major sources, including reinjected nutrients from the deep ocean, recycled nutrients from the surface ocean, and externally imported nutrients. The combination of these processes results in limited nutrient availability in the LNLC region, which is jointly influenced by the biological pump, Ekman sinking, and water layer stratification.
“The existence and changes in these areas reveal the delicate balance in how marine ecosystems operate.”
In sinking or stratified LNLC regions, external and recycled nutrient sources remain key to promoting phytoplankton growth, even though primary production rates are generally low. This process makes nitrogen fixation an important mechanism for further adjusting nitrogen levels in the water column to support the regeneration of phytoplankton.
Productivity of the LNLC region
Despite the low primary productivity per unit area in these regions, due to their widespread distribution, it is estimated that approximately 40% of global ocean productivity comes from LNLC regions. Phytoplankton communities in these areas rely heavily on nitrogen fixation and the supply of recycled nutrients to sustain production. Seasonal and transient nutrient supply events can transiently increase primary productivity.
In the subtropical gyre region, vertical mixing during winter and spring brings nutrients from the deep waters into the bright water layer, helping to trigger a transient increase in primary productivity. During these events, larger phytoplankton, such as diatoms, benefit from the increased nutrient diversity.
How does it affect the global carbon cycle
As global warming and ocean stratification intensify, the LNLC region is likely to become more oligotrophic, which will involve other tropical and subtropical waters at risk of reduced productivity. This has further deepened humanity's understanding and attention to the LNLC region. The interrelationship between these regions and the carbon cycle makes them increasingly important areas of research.
"In these mysterious ocean regions, there are still countless unsolved questions waiting for us to explore and understand."
In the future, research on these LNLC regions will not only help us understand changes in the global carbon cycle, but may also provide us with new strategies to cope with climate change. How will changes in these regions shape the future of the Earth? It is worth pondering.
