Approaches for determining phytoplankton nutrient limitation

Aquatic primary productivity is frequently limited by the availability of n utrients. The ability to identify factors limiting algal growth is of consi derable importance to our understanding of the ecology of aquatic plants an d to water management practices. Methods used to identify limiting resource s in the past have included a) analysis of nutrient availability, b) elemen tal composition and cell quotas for various nutrients, c) bio-assays monito ring growth of test species or of natural populations following nutrient en richment and d) measurements of various physiological parameters, such as e nhancement of respiration and dark carbon fixation rates and perturbation o f photosynthetic rate following re-supply of nutrients. In this paper we briefly review the merits and methodological limitations o f these approaches for the assessment of the nutrient status of algal popul ations. We discuss how an understanding of biochemical and metabolic change s induced by nutrient limitation has led to the development of rapid and si mple tools to monitor the nutrient status of aquatic plants and algae. In p articular, we describe the use of transient changes in chlorophyll a fluore scence as a potential tool for rapid assessment of algal nutrient status an d the development of molecular probes specific to nutrient limited cells, s uch as flavodoxin as a diagnostic tool for Fe-limitation.