Phytoplancon is one of the 4 biological quality elements (BQE) used for the assessment of the ecological status of the surface water bodies. The assessment is made in 5 classes : high (blue) - good (green) - moderate (yellow) - poor (orange) - bad (red). The evaluation of this BQE is only relevant for the large rivers where the quantity of phytoplancton is significant and can be evaluated. Therefore phytoplancton is assessed in natural water bodies of type VI an in heavy modified waterbodies (HMWB) presenting a lake character, as the HMWB Sûre (WB III-2.2.1) and the HMWB Our (WB V-1.2). The frequency for phytoplancton monitoring is annually with 6 samples between April and October.