Annex V: Ecological quality is classified as high, good or moderate. High status is defined as "no or very minor anthropogenic alterations to the values of the physico-chemical and hydromorphological quality elements" normally associated with that type of water body under undisturbed conditions. There should be "no, or only very minor, evidence of distortion" of biological quality elements.
For good ecological status, "low levels of distortion" of biological elements resulting from human activity are acceptable, but these should deviate "only slightly" from those associated with those found in that type of water body under undisturbed conditions.
Moderate status water bodies are considerably more disturbed than good status waters. Moderate status allows values of biological quality elements to "deviate moderately" from those under undisturbed conditions, and "moderate signs of distortion" will be apparent due to human activity.
What does good ecological status look like?
Annex V includes the following guidelines:
Algae: "Slight changes" in composition and abundance of algae occur compared to undisturbed systems, but this does not include "any accelerated growth". There are no "undesirable disturbances" to the balance of organisms or the physico-chemical quality of the water or sediment.
Fish: "Slight changes" in species composition and abundance compared to undisturbed systems. The age structure of fish communities may show signs of disturbance due to human impacts via physico-chemical or hydromorphological quality elements, with some age classes possibly being absent in some species due to reproductive failure.
Physical and chemical conditions, including pH, temperature and nutrient levels, should be "within the ranges established so as to ensure the functioning of the ecosystem and the achievement of the values specified above for the biological quality elements".
Pollutants: Levels will not be in excess of chemical quality standards to be set by Member States according to the method prescribed in Annex V. The method requires setting standards based on acute or chronic toxicity data for organisms representing algae, zooplankton/daphnia and fish. The size of the safety factors required depends on the quality and quantity of the data.
Major derogations:Article 4(4): The timetable for achieving good ecological status may be extended by up to 12 years for reasons of technical feasibility or disproportionate expense. Extensions may be longer where natural conditions do not permit more timely improvement.
Article 4(5): Member States may aim for less stringent standards than good ecological status where this is infeasible or disproportionately expensive, provided that the highest standards of ecological or, for groundwaters, chemical quality are achieved, and that the environmental and socio-economic needs served by "such human activity" cannot be achieved by other means which are a better environmental option not entailing disproportionate cost.