Short‐term Responses of Nutrient‐deficient Algae to Nutrient Addition1

Changes in net photosynthesis, dark respiration, ATP content, and some other aspects of composition were measured following phosphate or ammonium addition to cultures of the green alga Scenedesmus quadricauda (Turp.) Bréb. deficient in phosphorus or nitrogen. The deficient nutrient was rapidly taken up. Light‐saturated net photosynthesis was depressed below the pre‐addition rate during nutrient uptake and did not increase markedly above that rate until several hours after uptake was complete. Dark respiration, on the other hand, was markedly stimulated during uptake of the deficient nutrient and to a lesser extent after uptake was complete. Phosphate addition to P‐deficient cells caused a large increase in the ATP content within 2–4 h of addition, whereas ammonium addition to N‐deficient cells caused much less or no increase in ATP content. Short‐term enrichment experiments to detect nutrient limitations are evaluated in the light of these and similar results taken from the literature. The available evidence shows that photosynthetic responses cannot reliably be used in short‐term enrichments but changes in respiration or ATP content may be useful in certain circumstances.