The rate of ammonia absorption in the dark and the amount of orthophosphate extracted by boiling water treatments have been used to follow the transient nature of the nitrogen and phosphorus nutrition of algae and aquatic weeds. It is suggested that only terminal portions of aquatic weeds be used for nutritional bioassays because of nutritional differences between young and old portions of the same plant. Available nitrogen compounds related to rainfall have been shown to influence the nitrogen nutritional status of Spirogyra and Cladophora. In applications of bioassays the importance of testing each species of plants separately is shown by contrasting results obtained with nitrogen-fixing (phosphorus-limited) and non-fixing (nitrogen-limited) blue-green algae from the same environment. (Modified author abstract)