Definition (by USGS)
Eutrophication is a process whereby water bodies, such as lakes, estuaries, or slow-moving streams receive excess nutrients that stimulate excessive plant growth (algae, periphyton attached algae, and nuisance plants & weeds). This enhanced plant growth, often an algal bloom, reduces dissolved oxygen in the water when dead plant material decomposes and can cause other organisms to die.
The level of eutrophication of a water body is expressed as a trophic state. The trophic state of a water body is derived from multiple parameters. Besides nutrient concentrations such as phosphorus and nitrogen, other parameters such as chlorophyll-a and secchi-depth are also used. Based on these criteria, the condition of a water body is categorised into oligotrophic (nutrient poor), mesotrophic and eutrophic (nutrient rich) states.
Eutrophication can result in visible cyanobacterial or algal blooms, surface scums, floating plant mats and benthic macrophyte aggregations. Concentrations of phosphorus of < 0.1 mg / l are sufficient to cause a cyanobacterial (algal) bloom. The decay of this organic matter may lead to oxygen depletion in the water, which in turn can cause secondary problems such as fish kills and liberation of toxic substances or phosphates that were previously bound to oxidized sediment. Phosphate released from sediments accelerates eutrophication.
Causes of eutrophication
Some lakes are naturally eutrophic, but in many other cases the excess nutrient input results from anthropogenic origins such as municipal wastewater discharges, industrial effluents and runoff from fertilizers and manure spread on agricultural areas. Nutrient enrichment seriously degrades aquatic ecosystems and impairs the use of water for drinking, industry, agriculture and recreation.
Phosphorus as a limiting nutrient
A limiting nutrient is a nutrient necessary for plant growth, but available in a concentration insufficient to support continued growth. Once supply of this nutrient is exhausted, plant growth ceases regardless of other remaining nutrient quantities.
Any nutrient can become a limiting nutrient in an ecosystem. For algae, the most obvious limiting nutrients are nitrogen, certain metals and phosphorus. Nitrogen removal is an expensive process, involving high energy and chemical costs and specialised equipment. Certain microorganisms, including algae, are able to fix atmospheric nitrogen opportunistically.
Removal of metals may disrupt the local ecology, especially that of aquatic plants. Phosphorus limitation is the most practical means of preventing the growth of toxic blue green algae.
Implications of phosphorus as a limiting nutrient
The figure below illustrates the effect of an increase in the trophic status of a water body on the algal population. In a healthy oligotrophic lake, it is normal for diatoms and small green algae to be present. However, as the trophic level of the water increases, a shift is seen in the algal population from small benign algal species, to larger filamentous green algae and toxic blue green algae. The presence of a large bloom of blue green algae in a lake indicates that it has reached a hypertrophic state. Limiting FRP in the system will cause a positive shift in the algal population back to the benign species, thus reducing the systems productivity and trophic status. Phoslock targets FRP, and in doing so, provides a permanent and sustainable management solution to eutrophication.
Common eutrophication control tools and consequences of not treating
Common Remediation options
e.g. WTP Biological Nutrient Removal
- Only really suitable for point sources
- N removal is expensive
- High energy (chemical costs) & equipment
- Nitrogen is refixed opportunistically
- Not suitable for very low levels & natural waterways
- Remobilised under some conditions
- Al & Fe - pH & sludge problems
- Algicides - chemical or biological are unsustainable and have risky ecological impacts
- Aeration - expensive
- Removal of nutrient rich water from the hypolimnion - unsustainable
Consequences of not treating
Eutrophication and blue green algal proliferation will only worsen without treatment. Some serious consequences of non-treatment are:
- toxin release
- ecology disruption with BGA
- Epiphyte impact on macrophytes
- DO stress with pH changes
- Shift from oligotrophic to eutrophic & exotic species
Corrosion & biodeposition risk
- Taste / odour