Elements of ecological status assessment

Knowledge base · July 4, 2026

Ecological status is not a single number “out of thin air” - it is the result of assessing three groups of elements that together describe the condition of the water. Knowing what goes into the assessment makes it easier to plan your own monitoring and to understand why a given water body received one class rather than another.

The three groups are not equal in rank - they differ in their role in the assessment and in what they are able to detect:

⚠️ Good physico-chemistry does not rescue poor biology. A set of field measurements can look exemplary and the ecological status still come out moderate if the communities are impoverished - because biology remembers months and a water sample remembers an hour.

1. Biological elements - decisive

These are the organisms living in the water, and it is they that are decisive for ecological status

• because they reflect long-term conditions most fully. Depending on the water type, the following are assessed, among others:

• phytoplankton (algae suspended in the water column),

• phytobenthos (mainly diatoms growing on the bottom),

• macrophytes (aquatic plants),

• benthic macroinvertebrates (bottom-dwelling invertebrates),

• ichthyofauna (fish).

The set of elements differs for rivers and lakes. Who is who among these groups of organisms is explained by benthos, plankton, macrophytes. For the practical side of the measurements we refer you to mapping macrophytes, bioindication with macroinvertebrates and fish monitoring.

2. Physico-chemical elements - supporting

They support the biological assessment and help explain its result. They include, among others:

• oxygen conditions - dissolved oxygen, BOD, COD, total organic carbon (TOC),
• nutrients - nitrogen and phosphorus compounds (key to eutrophication),
• salinity - including electrolytic conductivity,
• acidification - reaction (pH),
• thermal conditions - temperature,
• (in lakes) transparency - e.g. measurement with a Secchi disc,
• specific synthetic and non-synthetic pollutants.

Most of them we describe in the Method Guide - e.g. dissolved oxygen, pH and conductivity or transparency.

3. Hydromorphological elements - supporting

They describe the “physical skeleton” of a watercourse or water body:

• the hydrological regime (the amount and dynamics of flow),
• continuity (passability for organisms and sediment),
• morphological conditions (the shape of the channel, banks, bottom).

They matter most in distinguishing very good status from good and in assessing modified waters.

How the elements combine into a class

The principle of “the worst decides” (one-out-all-out) applies: the class of ecological status is determined by the weakest-performing element. That is why a single neglected indicator can “drag down” the whole assessment - and why it is worth running monitoring completely and consistently.

Sources

• The set of quality elements and how they combine into an assessment: Directive 2000/60/EC (Water Framework Directive), Annex V - definitions of biological, hydromorphological and physico-chemical elements and the classification rules.
• National boundary values (Poland, used as the worked example): Regulation of 25 June 2021 on the classification of surface water bodies (Journal of Laws 2021 item 1475) - text in ELI. We do not copy the thresholds here - they change between cycles.

(Addresses checked on 18 August 2026.)

In practice

The assessment combines many indicators from different groups, measured at fixed points and compared over time - a lot of data to organise. LimnoLog lets you gather it in a single project (including biological indicators as custom ones), with guidelines and charts. On how the classes combine into the overall status, we write in the article Classes and status of water quality.