A SIMULATION-MODEL OF LAGOON ALGAE BASED ON NITROGEN COMPETITION AND INTERNAL STORAGE

Simulation modeling was used to explore the importance of resource com petition to algal community structure of shallow coastal lagoons. We u sed modified Michaelis-Menten equations to describe the kinetics of ni trogen uptake for three co-occurring algal functional forms: phytoplan kton, foliose algae, and cyanobacterial mats. Uptake rates and subsequ ent growth were modified by differential internal storage and recyclin g of nitrogen from decaying tissues. Analysis of model behavior indica ted that a two-step model uncoupling nutrient uptake and growth was ca pable of producing a wide variety of dynamic behavior not possible wit h simple uptake kinetics. A nutrient storage term allowed for time lag s between nutrient input and growth, and resulted in periodic changes in dominance between foliose algae and mats. Analyses of the most impo rtant parameters of the model, nutrient uptake and growth rates, indic ate that our model can be adapted for algal communities composed of di fferent dominant algal forms. The model demonstrates a dynamic respons e in both magnitude and timing of algal growth when the parameter valu es for nutrient uptake are manipulated. From this we hypothesize that the dynamic relationships between the algal groups are similar in diff erent communities, but that the parameter values in the Michaelis-Ment en equations for each algal form must be tailored to each system. We v alidated the model with three field experiments from the published lit erature. First, we compared model predictions to results of a replicat e microcosm experiment modeling lagoonal communities treated with 20 n itrogen loading rates spanning three orders of magnitude. The model ad equately predicted the dynamics of the simplified microcosm community within the range of loading rates where nitrogen was limiting (0-0.270 mg.L-1.d-1). As predicted by the model and confirmed with the experim ents, microalgal mats dominated when nitrogen addition was low, but we re replaced by foliose algae at higher N loading levels (0.068-0.090 m g.L-1.d-1). Phytoplankton never dominated the community. Second, we co mpared the observed and predicted communities for a competition experi ment excluding two of the algal forms. The magnitude of the phytoplank ton biomass in each treatment is accurately predicted but predictions of timing are less accurate. The final biomass of foliose algae and ma ts is predicted accurately. Finally, we applied the model to an experi ment using algal communities of San Francisco Bay. Results matched pre dictions when the kinetic parameters for foliose algae were changed to those for Ulva, the green macroalga from San Francisco Bay. These res ults support the hypothesis that nitrogen storage is important in unde rstanding growth and nutrient utilization patterns of foliose algae an d benthic cyanobacterial mats and the subsequent impacts on community structure.