Plankton, the name is derived from a Greek root "wanderer." Plankton range in size from microscopic bacteria and plants to larger animals as big as a jellyfish. Plankton have limited or no swimming ability and thus wander their way by waves, currents and tides.

Plankton can be divided into three major size classes:

1. phytoplankton -  microscopic plants and bacteria
2. zooplankton -  microscopic animals
3. macrozooplankton -  larger fish eggs and larvae and pelagic invertebrates

Plankton are often used as indicators of environmental and aquatic health because of their high sensitivity to environmental change and short life span.

• Phytoplankton are useful indicators of high nutrient conditions due to their propensity to multiply rapidly in the right conditions.
• Zooplankton are useful indicators of future fish health because they are a food source for organisms at higher trophic levels, such as finfish.

These are photographs, taken in 2002, of zooplankton found in St Peter's very own Lake Hallett.