Question

In lecture, I mentioned most lakes are phosphorus limited. However, the most severely eutrophic lakes are often nitrogen limited, Explain how intense phytoplankton production can cause lakes to flip from being phosphorus limited to being nitrogen limited. Mention hypoxia, cyanobacteria, nitrogen-fixation, iron-redox reactions, sediment phosphorus release and denitrification.

Answer 1

Phosphorus is essential for growth of plants and other organisms. When concentration of phosphorus mostly from high breeding rate e.g., pigs, cattle etc. rise in water it causes severe growth of plants and algae as well as phytoplanktons. As a results layers of phosphorus start to release phosphorus as well due to positive gradient. These reactions in water are iron-redox reactions that are crucial for the release of phosphorus.

When too much phosphorus accumulates, it produces a degrading growth feedback in plants, algae and phytoplanktons that then shift toward excessive nitrogen available from fertilizers and cause a flip from P to N. Cynobacteria now dominates and denitrification as well as nitrification takes place simultaneously.

Severe eutrophication induces hypoxia in water that cause a major damage to plants biodiversity in the environment.