The major systemic transformation that eutrophication imposes is in the performance of the Nutrient Cycle, and it is these changes that cause the observable and measurable changes to the water body which are troublesome degradations from our perspective.
The Nutrient Cycle is still robust and productive in a eutrophic water body, producing lots of Biomass. It’s just the wrong Biomass that is being produced. Instead of producing the Biomass characteristic of a biodiverse productive Food Chain at the apex of which are fish, Biomass is dominated by algae and toxic CyanoHAB producing cyanobacteria.
In Systems Theory terms we seek to elucidate the changes in the Attractors operating on the system, since reversing these changes to restore the desired Attractor profile is the only effective way to reverse eutrophication.
Two critical shifts in system Attractors manifest as stratification of the water column and the establishment of anaerobic benthic conditions. This stratified anaerobic environment drives fundamental changes in the Nutrient Cycle:
- A productive Food Chain is made up of organisms that respire aerobically so they require aerobic conditions or high levels of dissolved oxygen in the water. In a eutrophic water body stratification leads to an anaerobic benthic layer which gradually increases over time. The anaerobic zone starts at the bottom (benthic) and rises to take over more and more of the water column. This “crowds out” aerobic life-forms by reducing the volume of water that is oxygenated and can support their respiration.
- The anaerobic benthic zone causes some fundamental changes in the chemistry of the nutrients Nitrogen (N) and Phosphorus (P).
- It causes P that has precipitated in the sediment to remobilize in the water column and become available again as a nutrient.
- It causes N to be present as ammonia rather than Nitrate. Nitrate is readily converted to Nitrogen gas and discharged from the water to the atmosphere (which is 78% Nitrogen) and thus eliminated from the water. Ammonia is not.
- Cyanobacteria have gas vesicles that they can inflate and deflate in order to navigate their way up and down a stratified water column. This means that they can descend to the nutrient rich benthic sedimentary zone at night to load up on N and P and then ascend to the surface during the day to take full advantage of the sunlight that provides the energy for photosynthesis. This allows the cyanobacteria to rapidly utilize nutrients in a eutrophic water body to bloom explosively.
- Cyanobacteria are able to assimilate N from ammonia which most other photosynthesizers (algae, aquatic plants etc) cannot do. This therefore gives them a competitive advantage to dominate the eutrophic environment.
- As invasive aquatic weeds, algae and cyanobacteria begin to dominate the Nutrient Cycle a positive feedback loop is established whereby through annual recycling this Biomass dies off and falls to the bottom where it decomposes into the sediment. This replenishes the sedimentary nutrient store, making the nutrients available again for weed, algae and cyanobacteria proliferation the following year. Additional nutrient inflows during the year further increase the total nutrient levels in the water body.