Control of Lacustrine Phytoplankton by Nutrients: Erosion of the Phosphorus Paradigm

Control of lacustrine phytoplankton biomass by phosphorus is one of the oldest and most stable paradigms in modern limnology. Even so, evidence from bioassays conducted by multiple investigators at numerous sites over the last three decades shows that N is at least as likely as P to be limiting to phytoplankton growth. A number of important flaws in the evidence supporting the phosphorus paradigm have contributed to an unrealistic degree of focus on phosphorus as a controlling element. These include insufficient skeptism in interpretation of: 1) the phosphorus: chlorophyll correlation in lakes 2) the results of whole-lake fertilization experiments, and 3) stoichiometric arguments based on total N:total P ratios for inland waters. A new paradigm based on parity between N and P control of phytoplankton biomass in lakes seems more viable than the P paradigm. The new paradigm renews interest in the degree to which plankton communities are molded in composition by small differences in relative availability of N and P, the mechanisms that lead to a high frequency of N limitation in oligotrophic lakes, and the failure of aquatic N-fixers to compensate significantly for N deficiency under most conditions. A new N/P paradigm still must acknowledge that suppression of P loading often will be the most effective means of reducing phytoplankton biomass in eutrophic lakes, even if N is initially limiting.