PROCESSES IN NUTRIENT BASED PHYTOPLANKTON ECOLOGY

Fluctuations in the free intracellular amino acid pools following an ammonia perturbation to ammonium limited Skeletonema costatum and Gymnodinium simplex provides evidence which suggests that the enzyme glutamine synthetase (EC.6.3.1.2) acts as the primary ammonium assimilating enzyme in marine phytoplankton under nitrogen limitation.

Limiting nutrient patchiness (ammonium) is examined as a factor affecting both phytoplankton physiology and competition. It is shown that temporal patchiness in the supply of the limiting nutrient sets up periodicities in cellular carbon fixation and in vivo chlorophyll a fluorescence. Populations grown in a patchy limiting nutrient environment appear better adapted to take up nutrient pulses than do populations grown under conditions of homogeneous distributions of the limiting nutrients. It is also shown that the patchiness of the limiting nutrient effects the outcome of species competition with the winners being those species best able to optimize uptake under that particular patchy regime.

A theoretical framework is developed to explore the effects of limiting nutrient patchiness on phytoplankton growth. This work shows that the degree of patchiness in the environment can affect individual growth rates and thus alter community structure even though there is no change in the average ambient nutrient concentration. In addition the apparent K(,s)('') for growth, for patch adapted populations, may be lowered significantly by making the distribution of the nutrient patchy with respect to time.

A qualitative model is proposed relating nutrient supply, light and temperature and their effects on phytoplankton community structure.