The distribution of macrophytic plant communities, defined by phytosociological numerical methods, in the irrigation system of the Lower river Po Plane was correlated with water characteristich. Water samples were collected periodically within phytocoena well-characterized from the phytosociological viewpoint. Nymphaeid-dominated communities (all.Nymphaeion) usually occur in deeper waters than elodeid- myriophyllid-, and ceratophyllid-dominated vegetation types (all.Potamogetonion). Hydrochemically, all of the studied waterbodies can be classified as rich in electrolytes and in bicarbonates. The distribution of phytocoena reflects a gradient in nutrient content: theNymphoidetum peltataecharacterizes waters moderately rich; theMyriophyllum spicatumcommunity and theTrapetum natantisfairly rich; thePotamogetonetum pectinatiand theCeratophyllum demersumcommunity very rich in nutrients.