The Quality of Rocky Bottoms index (CFR): A validated method for the assessment of macroalgae according to the European Water Framework Directive

•The CFR index was adjusted using a continuous scoring system.•It was applied at intertidal and subtidal areas of Spain, France and Portugal.•It was tested against several anthropogenic pressures.•The precision and accuracy of the index were improved.•An application for the automatic calculation of the CFR index is provided online.

The Quality of Rocky Bottoms index (CFR by its Spanish acronym) is a multimetric method used for the assessment of macroalgae communities in accordance to the European Water Framework Directive. In order to improve the precision and accuracy of the assessments, the index was adjusted using a continuous scoring system. The index was tested at 184 intertidal and 57 subtidal stations located in the Atlantic coasts of Spain, Portugal and France. The anthropogenic pressure level of the stations was estimated according to a semiquantitative scale based on the type of discharge, distance to the contaminant source and its magnitude. 70.5% of the stations were correctly assigned to their expected quality and only 5.8% were critically misclassified. The linear regression and weighted kappa analyses between the pressure levels and the CFR results showed highly significant correlations and very good agreement levels both at intertidal and subtidal areas.