Understanding the mixing generated by zooplankton

The project

Lake Erie (City of Toledo) turns green. The Guardian (2014)

Everybody knows that Water is an essential element on Earth and we cannot live without it. In particular the deterioration of its quality can pose a problem for our population and future generations, as well as has a direct impact on aquatic life. According to UN "there is enough freshwater on the planet for seven billion people but it is distributed unevenly and too much of it is wasted, polluted and unsustainably managed" (UN, January 2015). Lakes nowadays are used as a source of drinking water but also for other recreational activities (such as fishing, swimming, ...) and awareness raised about their importance and their protection.

As an example the image above shows an algae blooms in Lake Erie, the tenth largest globally lake belonging to Great Lakes in North America. As an effect of the agricultural pollution (fertiliser) in the drainage basin, the water turned green and now it contains toxins that make the water unsuitable for drinking. The condition, if permanent, may affect the habitat as well. In this example the phenomenon was enhanced by human activities in the area which released nutrients into the lake.

Generally speaking, when plants die, they decompose in the deeper layers and the process subtracts oxygen from the water and releases back nutrients and carbon dioxide as well. On the other hand oxygen on the surface is produced due to photosynthetic activity. But the presence of the thermocline (the ideal line of maximum temperature/density gradient - click the link and read the post for a quick and clear explanation) prevents the mixing between the lower O2-depleted and the upper oxygen-rich layers. If the thermocline persists for many years (this is more likely for deep lakes), it will further deteriorate the water quality. Such condition may kill fishes and may release more poisonous pollutants from the lake sediment. This phenomenon is usually enhanced by human activities in the area.

It is clear that the mixing plays an important role in determining the water quality since it affects the transport of pollutants, nutrients and is responsible for accumulation of dissolved substances, like in Lake Erie. As a result, the understanding of its dynamic in a lake is an essential and important factor.

17314236034_8807a85be3_o In particular the major interest of my research is focused on zooplankton (see a Daphnia on the left). Yes but, why such tiny but abundant organisms may be important for the mixing? Because the majority of zooplankton lives in the darker and bottom water of a lake to avoid their visual predators (fishes) and swims for several meters to the surface to feed on pythoplankton during the night always hiding from them. During this vertical migration the zooplankton could generate mixing by swimming and crossing the thermocline while the lake is stratified during the summer months.

Recently several studies in oceans have suggested that this behaviour is likely, however it is still a highly debated concept. The topic has also raised very recently the attention of few newspapers:

The research wants to study the same process but in fresh water basins.