Field and laboratory evaluation of bioassays for nitrogen and phosphorus with algae and aquatic weeds

The rate of absorption of ammonia in the dark and the amount of orthophosphate extracted by boiling water can be used to follow changes in the nutritional status of nitrogen and phosphorus in algae and aquatic weeds with relation to changes in supply of these elements. Measurements of alkaline phosphatase activity carried out in phosphorus-free media can also be used to follow changes in the phosphorus nutrition of aquatic plants; growth under conditions of surplus available phosphorus reduces (by dilution) their alka-line phosphatase activity. Only terminal portions of aquatic weeds should be used for nutritional bioassays because of nutritional differences between young and old portions of the same plant. The importance of testing each species of plants separately is shown by contrasting results obtained with nitrogen-fixing (phosphorus-limited) and nonfixing ( nitrogen-limited) bl ue-green algae from the same environment. These methods provide simple but useful bioassays for studies of eutrophication