Phytoplankton composition indicators for the assessment of eutrophication in marine waters: Present state and challenges within the European directives

► Few phytoplankton composition indicators have been proposed for European directives. ► Flaws revealed by taxonomy-based indicators requires completer composition indicators. ► Low cost-effectiveness of data acquisition tools hampers their standardization. ► Pressure-impact relationships should be set without impact of natural variability. ► Reference conditions and boundaries should be based on ecological principles.