Apport du traitement d'images dans le suivi de l'influence des teneurs en nutriments sur la croissance des lentilles d'eau (Lemna minor)

The growth of duckweed (Lemna minor) is an important component in the process in wasterwater floating macrophyte ponds. An excess or shortage of this biomass may be responsible for the dysfunction of such ponds. Modeling these duckweed ponds through mass balances based on Petersen's matrix may be helpful in facilitating the optimal management of such facilities. This study sought at assessing the efficiency of digital image processing in the growth monitoring of Lemna minor under different concentration of nitrogen and phosphorus. The experiments were carried out in a growth chamber (phytotron) using an experimental pilot involving initial fresh Lemna minor biomass (1 g), nitrogen and phosphorus concentration varying between 5 and 64 mg N-NH4+.L-1, and 1 and 24 mg P-PO43-.L-1, respectively. The digital image processing was achieved in addition to gravimetric methods (fresh weight and/or dry weight). Our results showed that image processing method allowed a continuous and non destructive monitoring of duckweeds biomass. The growth of Lemna minor progressively decreases when nitrogen and phosphorus were greater than 16 mg N-NH4+.L-1 and 6 mg P-PO43-.L-1. The maximum growth rate μmax are of the order of 0.07 d-1, the kinetic constants Ks and Ki are respectively 3.83 mg.L-1 and 204 mg L-1 for nitrogen, 1.26 mg.L-1 and 13.3 mg.L-1 for phosphorus. Such results are interesting as they could serve in the optimal management of Lemna minor and the modeling of biological reactors.