Role of lipids in the maintenance of neutral buoyancy by zooplankton

Many types of zooplankton contain large proportions of lipids. Usually, these lipids represent energy storage compounds, but it has also been suggested that lipids play a role in buoyancy regulation. Lipids are thought to determine the overwintering depth of large calanoid copepods, and it has been widely assumed that these organisms overwinter at some 'depth of neutral buoyancy'. However, lipids are generally more compressible, and have a larger thermal expansion than seawater. This means that any depth of neutral buoyancy will be inherently unstable. Model results show that the ascent rates attributable to this instability are small at depth (where temperature changes are small), suggesting a simple way for animals to remain at depth while overwintering. However, model results also show that the buoyant properties of a copepod (or any small plankter) are extremely sensitive to its relative biochemical composition. This presents problems to maintaining vertical position, but may also be useful for vertical migrations.