Scientists are investigating what role climate change plays in the decline of fish stocks throughout the North Atlantic. Previously, overfishing has been cited as the primary reason for dwindling fish stocks but now scientists are investigating another, potentially greater threat that could be contributing to stock decline.

Zooplankton levels have fallen by 90 percent during the last 40 years (National Geographic). And because it provides a vital food source for many fish species including cod and haddock, declining zooplankton could be responsible for the recently observed population crashes in fish stocks.

Plankton are microscopic organisms that drift on the oceans' currents. They include organisms such as diatoms, dinoflagillates, krill, and copepods as well as the microscopic larva of crustaceans, sea urchins, and fish. Plankton also include tiny photosynthetic organisms that are so numerous and productive that they are responsible for generating more oxygen than all other plants on Earth combined.

Plankton are categorized into the following groups based on their trophic role (the role they play within their food web):

• phytoplankton - Phytoplankton are the primary producers of the planktonic world. They are photosynthetic plankton and include organisms such as diatoms, dinoflagellates, and cyanobacteria.
• zooplankton - Zooplankton are the consumers of the planktonic world. As such, they feed on other plankton to obtain the energy and nutrients they need to survive. Zooplankton include the larvae of fish, crustaceans.
• bacterioplankton - Bacterioplankton are the recyclers of the planktonic world. They are free-floating bacteria and archaea that serve to break-down and recycle waste material in the seas.

Plankton can also be categorized by whether or not it spends its entire life as a microscopic organism:

• holoplankton - Holoplankton are organisms that are planktonic for the entirety of their life cycle.
• meroplankton - Meroplankton are organisms that are planktonic for only part of their life cycle, for example, only during the larval stage of their development.

Zooplankton and phytoplankton together form the basis of many marine food webs, so if they experience a decline, it will be reflected further up the food chain. If global warming is indeed affecting levels of plankton in the world's oceans, it could mean there will be far reaching effects felt at every trophic level in the oceans' food webs.

Find out more:

• Atlantic Fish Crisis May Be Due to Global Warming (National Geographic)
• Plankton (Wikipedia)
• Phytoplankton and Zooplankton (Enchanted Learning)
• What is Plankton? (Australian Museum)

Photo © Miguel Angelo Silva / iStockPhoto.