What are the roles of phytoplankton and zooplankton in carbon cycling?

Phytoplankton and zooplankton play crucial roles in carbon cycling, primarily through photosynthesis, respiration, and the biological pump.

Phytoplankton, microscopic plants that float in the upper part of the ocean, are the primary producers in the marine food web. They use sunlight to photosynthesise, a process that converts carbon dioxide and water into glucose and oxygen. This process is a significant part of the carbon cycle as it removes carbon dioxide, a greenhouse gas, from the atmosphere and converts it into organic matter. This organic carbon is then transferred through the food web as phytoplankton are consumed by other marine organisms, such as zooplankton.

Zooplankton, tiny animals that also float in the ocean, consume phytoplankton and other smaller zooplankton. Through this consumption, they incorporate the carbon stored in their food into their own bodies. When zooplankton respire, they release some of this carbon back into the water as carbon dioxide. However, a significant portion of the carbon remains in their bodies and is transferred up the food chain when they are eaten by larger organisms.

Both phytoplankton and zooplankton contribute to the biological pump, a process that transports carbon from the surface ocean to the deep ocean. When these organisms die, their bodies sink towards the ocean floor, taking the carbon stored within them with it. This process effectively sequesters carbon in the deep ocean for hundreds to thousands of years, preventing it from re-entering the atmosphere and contributing to climate change.

In addition, some species of zooplankton migrate vertically in the water column on a daily basis, consuming carbon-rich food at the surface during the night and excreting it at depth during the day. This behaviour, known as diel vertical migration, also contributes to the transport of carbon to the deep ocean.

In summary, phytoplankton and zooplankton play vital roles in the carbon cycle. Through photosynthesis, respiration, and the biological pump, they help regulate the amount of carbon dioxide in the atmosphere, thereby influencing Earth's climate.