Approaches for determining phytoplankton nutrient limitation

Aquatic primary productivity is frequently limited by the availability of nutrients. The ability to identify factors limiting algal growth is of considerable importance to our understanding of the ecology of aquatic plants and to water management practices. Methods used to identify limiting resources in the past have included a) analysis of nutrient availability, b) elemental composition and cell quotas for various nutrients, c) bio-assays monitoring growth of test species or of natural populations following nutrient enrichment and d) measurements of various physiological parameters, such as enhancement of respiration and dark carbon fixation rates and perturbation of photosynthetic rate following re-supply of nutrients. In this paper we briefly review the merits and methodological limitations of these approaches for the assessment of the nutrient status of algal populations. We discuss how an understanding of biochemical and metabolic changes induced by nutrient limitation has led to the development of rapid and simple tools to monitor the nutrient status of aquatic plants and algae. In particular, we describe the use of transient changes in chlorophyll a fluorescence as a potential tool for rapid assessment of algal nutrient status and the development of molecular probes specific to nutrient limited cells, such as flavodoxin as a diagnostic tool for Fe-limitation.