Where Have All the Copepods Gone

The omnivorous marine copepod, Calanus finmarchicus, plays a pivotal role in the North Atlantic's food chain. It serves as rich source of fatty acids and lipids for the many species of fish that feed on it.

A few years ago, I blogged about the possible impact climate change may be having on marine zooplankton and now I'd like to dig a bit deeper to find out how one species in particular, Calanus finmarchicus, is faring.

In the 1960s, Calanus accounted for as much as 70% of the zooplankton biomass in the North Sea. But by the 1990s, that number had declined to 50%. The question was, what had caused the Calanus population to crash?

Research by the UK's Fisheries Research Services Marine Laboratory shows that the key to understanding the Calanus decline lies 500 to 2,500 m below the surface of the sea in the bottom waters where dormant pre-adult Calanus spend the winter.

The life cycle of Calanus is rather interesting. They start out as microscopic eggs (0.05mm in diameter) and develop through eleven progressive stages before finally reaching their adult form. The process from egg to adult takes somewhere between 30 and 40 days and adults are approximately 2mm in length (Zooplankton, 2004).

In the late summer, some of the pre-adult Calanus enter a dormancy period and sink downward some 500 to 2,500m below the surface. In springtime, the come out of their diapause and swim to the surface and develop into adult form (Zooplankton, 2004).

Bottom water is a deep layer of cold water that is formed when cold water at the surface of the Greenland Sea sinks. The volume of bottom water has been declining since 1960 as a result of global warming. Rising temperatures and higher inputs of freshwater (from melting ice) have resulted in a weakning of the process by which bottom waters are formed.

Less bottom water means that fewer Calanus are overwintering. And fewer overwintering Calanus mean less food resources for organisms higher on the food chain that rely on zooplankton for their survival.

Find out more: Zooplankton and Climate Change (FRS Marine Laboratory)