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Why is 75% of the world's oceans low in nutrients and chlorophyll? Explore the secrets of these mysterious waters!
About 75% of the world's oceans are labeled low nutrient and low chlorophyll areas (LNLCs). The specificity of these areas is fascinating, and their existence not only affects the functioning of marine ecosystems, but also plays an important role in the global carbon cycle. Exploring the reasons for these secrets will lead us to a deeper understanding of the mystery and fragility of the ocean, and at the same time, provide important clues for future protection and utilization.
Characteristics of LNLC Regions
The so-called low-nutrient and low-chlorophyll areas refer to waters that are rich in fewer nutrients (such as nitrogen, phosphorus and iron), which directly leads to lower primary productivity in these areas, which is manifested in low chlorophyll concentrations. These areas are often described as oligotrophic and are concentrated mainly in subtropical eddies, but also in the Mediterranean and some inland lakes.
LNLC regions exist because physical processes limit nutrient availability, causing nutrients to recycle primarily in the photic zone and select for smaller phytoplankton species.
Physical and biological factors of nutrient cycling
Phytoplankton growth occurs primarily in the mixed layer, the upper ocean, where there is sufficient light energy and turbulent mixing at the surface increases the vertical uniformity of temperature, salinity, and density along the ocean. In these LNLC regions, nutrient availability comes primarily from three sources: reintroduction of deep ocean water, recycling from the surface ocean, and “external” nutrients from the atmosphere or land. The formation of LNLC regions is the result of multiple processes, including biological pumps, Ekman sinking, and water column stratification.
The biological pump creates a nutrient gradient by exporting organic matter from the surface ocean to the deep ocean, which leads to increased nutrient scarcity in the LNLC region.
Productivity of the LNLC region
Although primary production per unit area is generally lower in LNLC regions, an estimated 40% of global ocean productivity originates from these regions. This is because the LNLC region is large and most of it is located in the subtropical vortex. The waters in these areas are generally warmer and more stratified, which limits the delivery of new nutrients and makes the main source of nutrients the "regeneration cycle."In the LNLC area, although the new production is low, the organic matter output through the biological pump will still be reused to a certain extent through the biological cycle.
Variability of LNLC regions
Although LNLC regions are generally nutrient-poor, these areas are still dynamic and experience changes with the seasons. For example, the North Atlantic subtropical gyre shrinks in the winter and expands in the summer. Long-term trends show that the subtropical vortex in the Northern Hemisphere is expanding, while the vortex in the Southern Hemisphere is weakening. Climate change exacerbates the process of thermostratification, which limits the availability of nutrients and thus alters patterns of primary production.
As oceans warm, curiosity about the future of the LNLC region grows, and many researchers worry that this will affect the global ecological balance.
Specific LNLC region examples
North Atlantic Subtropical Vortex
As the centre of the North Atlantic subtropical gyre, the Sargasso Sea, although normally oligotrophic, has significantly higher primary production rates than expected due to nitrogen fixation and seasonal dynamics. The North Atlantic receives its phosphate supply from the Arctic Ocean, and nitrogen-fixing cyanobacteria such as Trichodesmium provide nitrates.
North Pacific Subtropical Vortex
This is one of the largest eddies in the world, where primary production is limited by nitrogen, phosphorus and iron, and is influenced by ENSO and PDO. The study shows that during the 1997-1998 ENSO event, major components of production patterns shifted, affecting physical and biological processes in the region.
Mediterranean Sea
The oligotrophic character of the Mediterranean is driven by its unique anti-tidal cycle. The waters here show a strong decreasing nutrient gradient from west to east, with a higher demand for nitrogen and phosphorus becoming relatively important.
Monitoring LNLC Areas
Due to the remoteness and size of the LNLC region, monitoring the properties of these waters is extremely challenging. Data collection requires facing harsh environmental conditions and must be supported by international cooperation and resources, which affects the reliability of the data.
The combination of ground-based instruments and satellite imagery provides valuable information about these mysterious waters, but future monitoring will require greater international support and awareness.
As we delve deeper into the mysteries of these low-nutrient, low-chlorophyll areas, we can't help but wonder how the composition of these mysterious waters and the ecosystems they host will affect our future marine environment?
