Eutrophication and trophic status

Knowledge base · July 5, 2026

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.

Four levels are distinguished classically - and each can be recognised by three measurable quantities at once:

⚠️ The three columns sometimes disagree - and that is information, not an error. Chlorophyll a tells you the most here. Example: transparency points to eutrophy and phosphorus to mesotrophy. First check whether the three results come from the same day and station. A mismatch can also be a trace of a sampling or analysis error. Then look at chlorophyll. If it comes out low, like phosphorus, something other than algae is most often dimming the light: mineral suspension after a flood, humic substances in brown water, or freshly stirred sediment. If it comes out high, like transparency, there are many algae, and that may indicate that phosphorus limits their growth. Without chlorophyll you cannot choose between these explanations. Each is a hypothesis to check, not a diagnosis: the relationship between the indicators changes over the season, so a series says more than a single day. More on reading discrepancies: Carlson’s TSI.

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.

Sources

• The boundaries in the table follow the Carlson TSI equations (Carlson R. E., A trophic state index for lakes, Limnology and Oceanography 22(2), 1977) - the same ones behind our TSI calculator. The chlorophyll and phosphorus values are the inverses of the TSI equations at 40, 50 and 70, not separately adopted thresholds - so the article and the tool speak with one number.
• Other systems draw the lines elsewhere (the OECD classification of 1982 uses different phosphorus and chlorophyll ranges). When you state a trophic status, always say under which system - the same water can be “mesotrophic” in one and “eutrophic” in another.

🧮 Tool: Carlson TSI calculator - enter transparency, chlorophyll-a or total phosphorus and see the result for each component separately, with its class according to the table above. No sign-up.

📊 See it live: Global Rivers & Lakes Watch - a public dashboard from three stations: the Maumee River (Ohio, USA), the river Main in Frankfurt and the LéXPLORE research platform on Lake Geneva. Temperature, oxygen, pH, conductivity, turbidity and chlorophyll-a are fetched automatically from USGS, WSV Pegelonline and Eawag Datalakes; plus correlation tiles and a station map. No login.

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.