Nutrient limitation and phytoplankton growth: a historical perspective

Eystein Paasche
Section for Marine Botany, Department of Biology
University of Oslo, Blindern, 0316 Oslo, Norway
Email: e.paasche@bio.uio.no

The two decades from ca. 1960 to 1980 saw a number of methodological and conceptual breakthroughs regarding nutrient-growth relationships, leading to conclusions that form the basis for our present understanding. Improved nutrient analyses were essential for this entire development. On the methodological side, the formulation of semi-defined growth media opened an avenue for experimentation with cultures of true plankton algae. The introduction of quantitative expressions describing nutrient-limited growth laid the foundations for later modelling. This research was aided by the application of the chemostat to microalgal research. Use of mesocosms opened the way for realistic studies of plankton communities under controlled conditions. The introduction of the 15N isotope to measure nitrogen uptake in field studies allowed new insight into the nutritional basis of primary production. On the conceptual side, the proposal that coastal waters are generally nitrogen-limited shifted attention away, for a time, from other potentially limiting nutrients (silicon, phosphorus, iron). The distinction between new and regenerated production offshore proved important for an understanding of the nutrient supply to coastal waters as well. Pulse uptake of nutrients in response to nutrient patchiness was recognised as a mechanism allowing plankton algae to grow in a nutrient-poor environment. Associated with this was the identification of variable C:N:P ratos in cultured algal species, raising the possibility that nutrient status might be judged by elemental analysis of plankton samples. At the species level, the ideas of nutrient-driven succession and resource competition did much to stimulate laboratory and field experimentation. Examples of all this will be given in my presentation.