Last time, we talked about phytoplankton and its importance, not only as the food base for all aquatic life, but also as the planet's second lung.
Zooplankton is the next link in the food chain, made up of micro-organisms from the animal kingdom. Like phytoplankton, it is invisible (or almost invisible) to the naked eye. Unlike phytoplankton, it is able to move actively, but is unable to oppose the current. They can be unicellular or multicellular, like the Daphnia, the largest zooplankton in Lake Geneva.
Zooplankton feed on phytoplankton and are eaten by fish (especially young fish, known as fry), as well as by shellfish in Lake Geneva. It is therefore an essential link between primary producers (autotrophs) and secondary consumers (heterotrophs).
Zooplankton is made up of two groups: permanent zooplankton and temporary zooplankton. Permanent zooplankton spend their entire lives in planktonic form (such as Daphnia), while temporary plankton live in planktonic form during the larval phase of their development, before becoming adult crustaceans or molluscs. For example, the larva of the Zebra Mussel is part of the zooplankton of Lake Geneva.
In Lake Geneva, the total number of zooplankton species is around 200; the diversity of zooplankton is much lower than that of phytoplankton. However, compared with the number of fish species in the lake - just over twenty - this figure is still impressive!
Unlike phytoplankton, zooplankton can survive at depth. When the sun sets over Lake Geneva, they rise to the surface to feed on phytoplankton, then descend to the depths during the day to avoid predators and the heat. This is not the only technique used by zooplankton to avoid becoming prey; many are transparent so that they cannot be seen!
Zooplankton have an extremely high reproduction rate. Take the Daphnia for example. Females can reproduce every 4 to 5 days from the age of 11 days. This is an effective strategy for maintaining its population, despite its short lifespan (30 days on average) and numerous predators.
It's a whole world in motion, hidden from our view... but that doesn't make it any less important!
Information from CIPEL, UniGe and ActuEnvironnement
Photo credit: Aqualis
