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 - the basis of the assessment

These are the organisms living in the water, and the assessment starts with them, 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.

A change to note: Directive (EU) 2026/805 moves specific pollutants to the assessment of chemical status. In Poland this will apply once the directive is transposed into national law (deadline: 21 December 2027); until then the arrangement described above holds.

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).

In classification they tell high status from good - and nothing more. When biology or physico-chemistry falls below good, hydromorphology does not change the result. Heavy alteration of the channel has a different effect: it can lead to part of the waters being designated heavily modified, and then its potential, not its status, is assessed.

How the elements combine into a class

This is not an average of grades. In Poland, the class of ecological status of natural waters is put together according to Annex 12 to the classification regulation, in three steps:

1. Biology: of the assessed biological elements, the worst one counts (the principle of “the worst decides”, one-out-all-out). If it comes out moderate, poor or bad, that is the ecological status, regardless of the rest.
2. Physico-chemistry: when biology is good or high, and the physico-chemical indicators (including specific pollutants) exceed the class II boundary, the status drops to moderate - no lower. For general indicators (oxygen, nutrients, salinity…) a single exceedance within the measurement uncertainty does not lower the class; for specific pollutants there is no such tolerance. Indicators between class I and II lower high status to good.
3. Hydromorphology: counts only when everything else indicates high status - it can lower it to good, nothing more.

That is why a single degraded biological element can “drag down” the whole assessment, while poor physico-chemistry on its own will never give class V. Monitoring is still worth running completely: a missing measurement of one element is a gap in the assessment, not a good result.

In heavily modified and artificial waters the arrangement is the same, only instead of status the ecological potential is assessed.

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 and the classification method (Poland, used as the worked example): Regulation of 25 June 2021 on the classification of surface water bodies (Journal of Laws 2021 item 1475; Annex 12 - classification of status, Annex 13 - of potential) - text in ELI. We do not copy the thresholds here - they change between cycles.

(Addresses and legal status checked on 20 October 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.