Eutrophication and trophic status - how the fertility of waters is assessed
If there is one process that keeps those who care for lakes awake at night, it is eutrophication - the gradual enrichment of water with nutrients, leading to lush algal growth, a fall in transparency and oxygen deficits. Understanding what trophic status is and how it is described is the key to interpreting many lake-monitoring results.
What eutrophication is
Eutrophication is a rise in the fertility of water, driven mainly by phosphorus and nitrogen. An excess of these elements - most often from field run-off, sewage or catchment development - stimulates the production of algae and plants. The effects include blooms (including cyanobacterial ones), a fall in transparency and, as the excess organic matter decomposes, oxygen deficits, especially near the bottom. Eutrophication can be natural (slow), but it is its anthropogenic acceleration that is the real problem.
Trophic status - a scale of fertility
Trophic status describes the level of fertility of a water body. Classically, the following are distinguished:
- oligotrophic - poor in nutrients, water clear, well oxygenated;
- mesotrophic - intermediate;
- eutrophic - fertile, prone to blooms, with oxygen deficits near the bottom;
- hypertrophic - extremely fertile.
What it is measured with
Trophic status is described by several linked indicators:
- total phosphorus and total nitrogen - the elements that drive fertility,
- chlorophyll-a - a measure of algal biomass,
- transparency - e.g. measurement with a Secchi disc.
The indices that bind them (e.g. Carlson’s TSI) turn these quantities into a single, comparable number. In practice, signals of eutrophication can also include intensifying cyanobacterial blooms and a fall in oxygenation.
The link with water-status assessment
Nutrients and their effects belong to the supporting elements of ecological-status assessment (more in the article on elements of assessment). Strong eutrophication usually “drags down” the status class - which is why limiting the inflow of phosphorus and nitrogen is one of the main aims of water protection.
In practice
The picture of eutrophication is built from a series of measurements of several indicators at once - nutrients, chlorophyll, transparency and oxygen - compared across the season and over the years. If you run such monitoring, LimnoLog lets you keep these indicators in one project and observe their trend on a shared chart. On how fertility fits into the status classes, we write in the article Classes and status of water quality.
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