Ecological vs chemical status of waters - what the difference is

Knowledge base · July 3, 2026

This is one of the most common misunderstandings when reading water assessments: ecological status and chemical status are sometimes confused or taken as one. Yet they are two separate assessments, made by different criteria and on different scales - and only together do they give the full picture of a water body. Understanding this difference greatly helps in interpreting monitoring results.

Ecological status - about the functioning of the ecosystem

Ecological status tells you how well the water lives: whether its ecosystem is close to its natural state. It is assessed in five classes (high → bad), and the result is decided above all by organisms (biological elements), supported by physico-chemical and hydromorphological indicators. It is an assessment “about life in the water” - sensitive to, among other things, eutrophication, channel alteration or disturbances of flow.

Chemical status - about specific substances

Chemical status looks at something different: at the presence of priority substances and other particularly dangerous pollutants (e.g. heavy metals, PAHs, some pesticides), assessed against the environmental quality standards (EQS). The scale here is simpler - two classes: good or failing to achieve good. It is an assessment “about what is dissolved and dangerous”, independent of how lush the life in the water is.

Why separately

Because these two things need not go together. A river can have good chemical status (no exceedances of priority substances) and at the same time moderate ecological status - e.g. because of an excess of nutrients or a regulated channel. The reverse also happens. Separating the assessments makes it possible to accurately pinpoint the problem and choose the right measures.

How it combines into a whole

The overall status of a water body is the worse of the two results - “good” only when both are at least good (the principle of “the worst decides”).

A note on concepts: for heavily modified and artificial waters, instead of ecological status the ecological potential is assessed. Chemical status is assessed in the same way regardless of whether a water body is natural or modified.

Sources

• Water Framework Directive - Directive 2000/60/EC, Annex V (definitions of classes and quality elements).
• Priority substances and EQS - Directive 2013/39/EU.
• National classification thresholds are set by implementing regulations and are revised more often than the directive itself - which is why we do not copy them here.

In practice

Both assessments rest on systematic measurements - biological and physico-chemical - run at fixed points and compared over time. If you collect such data yourself, LimnoLog lets you keep it in one place, with guidelines for the individual indicators. More on individual measurements you will find in the Method Guide; on what ecological status is made up of - in the article on elements of assessment.