Non-determinism to Probability, or N : P in the community ecology of phytoplankton
Reynolds, C. S.
published: Sep 16, 1999
ArtNo. ESP141014601009, Price: 29.00 €
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 regulates assemblage composition in the phytoplankton. The initial ratios of resource availability provide a valuable indication of the capacity of a system to support phytoplankton biomass and they suggest the probable outcome of autogenic processes under relatively steady-state conditions of a chronic resource limitation. Several arguments are put to refute the claim that ratios drive the assembly processes, for the resources can either saturate the 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 experiments fulfilled these conditions and highly predictable outcomes are obtained with multiple growth-rate limiting resources acting on multiple-species assemblages. The models do not predict the outcome while growth rates remain saturated. This is frequently ignored by proponents of the nutrient-ratio 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 dynamic reactions of diatoms to altered nutrient loads and ratios, the concentrations of nutrients needed to discriminate the growth performances of different species and the molecular biology of nutrient uptake by plant cells. An alternative view of community assembly suggests that most species of algae will grow under a wide range of environmental conditions. Which dominates depends upon the interaction of many complex factors but outcomes are influenced by precedent and inocula. The ability to maintain growth under conditions of suboptimal nutrient supply may eventually prove to become selectively crucial. By then, the community structure may well have been cast. Neither before nor after that point does the ratio of one nutrient to another determine the dynamic performance of the contenders.