1. Primary productivity by aquatic macrophytes is frequently limited by availability of inorganic C, which the leaves take up from the water Aquatic macrophytes don’t respond to N and P additions in the water column and only to sediment fertilizers when sediment is very nutrient poor. Eg sand or gravel Mud always has plenty of N and P.

4. Primary productivity by aquatic macrophytes is frequently limited by availability of inorganic C Potamogeton pectinatus is switching to bicarbonate as carbon dioxide becomes depleted in the weed bed Callitriche stagnalis does not have this capability The ability to use bicarbonate depends on the presence of the enzyme carbonic anhydrase which catalyses the hydration of carbon dioxide

5. Hard water has more dissolved inorganic C (carbonates and bicarbonates) than soft water. Production, biomass and species diversity of aquatic plants is always higher in hard water lakes and ponds than softwater systems.