Plankton, derived from the Greek ‘planktos’ – to drift or wander – is a term used in order to describe inertly migrant small plants and animals within a marine or freshwater system (phytoplankton (Figure 11) and zooplankton (Figure 12)). Although able to vary their depth of occurrence within the ocean, they are incapable of actively moving independent of ocean currents.
Variations observed in plankton abundance and dynamics over many decades has been seen to correlate with that of Antarctic krill (Euphausia superba), whose population has seen rapid decline over the past 25 years. This coincides with levels of phytoplankton blooms during the summer and the extent of ice algae during winter months (Atkinson et al., 2004). Although krill are perceived as mainly diatom feeders, recent research has shown that through all stages of their life history they are capable of adapting to local food sources. The main taxonomic groups consumed are diatoms, prymnesiophytes (primarily unicellular, pohotosynthetic organisms) and cryptophytes, with their relative abundances being variable seasonally and with fluxes in the level of grazers present.