A MODEL APPROACH FOR SIZE-SELECTIVE COMPETITION OF MARINE-PHYTOPLANKTON FOR FLUCTUATING NITRATE AND AMMONIUM

Phytoplankton size-selective competition for fluctuating nutrients was studied with the use of a numerical model, which describes nitrate an d ammonium uptake, nitrate reduction to ammonium uptake, nitrate reduc tion to ammonium uptake, nitrate reduction to ammonium, and growth as a function of cell size under fluctuating nitrogen limitation. The onl y size-dependent parameter in the model was the cell nutrient quota. R elated to this, the cell surface area per biomass was negatively corre lated to cell volume, and the vacuole volume per biomass ratio was pos itively correlated to cell volume. Simulations showed an inverse corre lation between the maximum specific growth rate adn cell size under st eady-state conditions. With nitrate as the limiting nitrogen source, n itrogen quotas were always higher than with ammonium at the same speci fic growth rate. Net passive transport of ammonium due to unspecific d iffusion of ammonia across the plasma membrane decreased the affinity for ammonium, whereas the affinity for nitrate was not influenced. Tra nsient state-specific ammonium uptake was not dependent on cell size. However, small algae always have the highest specific growth rate in a mmonium-controlled systems according to our model. Transient state nit rate uptake rate was positively correlated to cell size because larger algae have a higher vacuole volume per biomass, in which nitrate can be stored. Despite their lower maximum growth rate, larger algae becam e dominant during simulations under fluctuating nitrate supply when am plitude of and the period between nitrate pulses were high enough. Res ults from model simulations were qualitatively validated by earlier ob servations that large diatoms become dominant under fluctuating condit ions when nitrate is the main nitrogen source.