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What Limits Phytoplankton Growth?

Phytoplankton are an essential part of any marine ecosystem. Phytoplankton are more similar to plants than the small animals that make up the rest of the plankton species. Like plants, phytoplankton obtain energy by photosynthesis with the help of cells filled with chlorophyll. It has been estimated that phytoplankton are responsible for one half of the world's photosynthesis.

Rising Ocean Temperature
- Rising ocean temperatures keep essential nutrients in water too deep for plankton growth.
  
  As a general rule, warm water contains fewer nutrients than deeper, cold water. When surface water warms, it further separates from deeper water, inhibiting nutrient distribution. Phytoplankton require many of these deep-water nutrients for multiplying and photosynthesis. An occasional upwelling of deeper water can cause local phytoplankton blooms until the nutrients have been depleted.
Iron Deficiency
- A lack of dissolved iron in nutrient-rich waters delays the growth and reproduction of phytoplankton. Iron is a key nutrient in the production of chlorophyll in phytoplankton. Chlorophyll-containing cells convert the sun's light into sugars and carbohydrates that plankton use for energy.
Seasonal Changes
- Phytoplankton growth peaks during the spring. This rapid growth occurs due to the relative coolness of the water from the winter months. When cool, surface water mixes with nutrient-rich deep water and returns to the surface to feed the next cycle of plankton. This growth occurs until midsummer when the surface waters warm, and plankton have exhausted the nutrient supply from the cold water.
Ozone Depletion
- Ozone depletion exposes phytoplankton to harmful radiation.
  
  Since the 1970s, manmade chemicals have been rapidly depleting the Earth's ozone layer. The ozone layer blocks excessive radiation from the sun from reaching the surface of the Earth. Because of the depletion of the ozone layer over Antarctica, damaging ultraviolet-b rays are affecting phytoplankton growth. Ultraviolet-b ray exposure has inhibited chlorophyll production in phytoplankton by about 56 percent.

Nature