Phytoplancon is one of the 4 biological quality elements (BQE) for the assessment of the ecological status of the natural water bodies. The evaluation of this BQE is only relevant for the large rivers where the quantity of phytoplancton is significant and can be evaluated. The natural water bodies of typology VI present significant phytoplancton concentrations to allow an assessment. Some of the heavy modified waterbodies (HMWB) present significant phytoplancton concentrations too, these are the HMWB Sûre (WB III-2.2.1) and the HMWB Our (WB V-1.2). The annual frequency for phytoplancton monitoring is 6 samples between April and October.