Non-determinism to Probability, or N : P in the community ecology of phytoplankton

A counter view is advanced to the opinion of BULGAKOV & LEVICH (1999) that the ratio of nitrogen to phosphorus (N:P) is an independent factor that reg ulates assemblage composition in the phytoplankton. The initial ratios of r esource availability provide a valuable indication of the capacity of a sys tem to support phytoplankton biomass and they suggest the probable outcome of autogenic processes under relatively steady-state conditions of a chroni c resource limitation Several arguments are put to refute the claim that ra tios drive the assembly processes, for the resources can either saturate th e rate of assembly (no resource limitation of growth rate) or one or other fails to do so (the resource itself limits the growth rate). TILMAN'S exper iments fulfilled these conditions and highly predictable outcomes are obtai ned with multiple growth-rate limiting resources acting on multiple-species assemblages. The models do not predict the outcome while growth rates rema in saturated. This is frequently ignored by proponents of the nutrient-rati o hypothesis. Counter-arguments to the thesis of BULGAKOV & LEVICH hypothesis draw on the results obtained over a number of years from large-scale field experiments , involving the development of nitrogen-fixing cyanobacteria and the dynami c reactions of diatoms to altered nutrient loads and ratios, the concentrat ions of nutrients needed to discriminate the growth performances of differe nt species and the molecular biology of nutrient uptake by plant cells. An alternative view of community assembly suggests that most species of alg ae will grow under a wide range of environmental conditions. Which dominate s depends upon the interaction of many complex factors but outcomes are inf luenced by precedent and inocula. The ability to maintain growth under cond itions of suboptimal nutrient supply may eventually prove to become selecti vely crucial. By then, the community structure may well have been cast. Nei ther before nor after that point does the ratio of one nutrient to another determine the dynamic performance of the contenders.