Models of Plankton Biomass Spectra

The goal of this thesis is to study the causes and consequences of the size structure of marine planktonic ecosystems. To this end, a theoretical (continuous) size spectrum model was developed. Phytoplankton and zooplankton biomass is represented in the model as functions of their respective biomass size spectrums. Thus, the model allows the natural recovery of the continuous size spectrum for planktonic ecosystems. The model was developed under the assumption that the structure of planktonic ecosystems is largely dependent on organism size. The generality of the model allows the use of either allometric or random encounter models for the specification of the size-dependent functions in the system. A "distributed grazing" size-dependent dynamic simulation model is presented as a special (discrete) case of the "general" size spectrum model proposed. The model consists of multiple phytoplankton size classes and one zooplankton size class representative of the entire herbivore community. Zooplankton grazing is described by a dynamic function that distributes grazing pressure among phytoplankton size classes according to their relative biomass. Allometric theory was used to specify the size-dependence of the maximum phytoplankton growth rates. Simulations were carried out to study the dynamics of phytoplankton size structure under different allometric structures, representative of ecosystem diversity. Complementing the analysis, simulations were carried out to study the effects of nutrient enrichment processes on phytoplankton size structure. Size structure varied systematically as a function of the allometric structure and nutrient enrichment process chosen. Two parameters were explored: 1) spectral slope, which indicates the relative contribution of small and large cells to total phytoplankton biomass; and 2) transient spectral shape, which indicates the effects of enrichment on phytoplankton succession as the system goes from its original to a new steady state condition. The model provides a natural and simple way of simulating the transient phytoplankton size structure under nutrient enrichment scenarios. It is argued that this will prove very useful in the analysis of plankton size structure dynamics in natural (e.g. spring blooms) and artificial (e.g. ocean fertilization) enrichment processes. Thesis Supervisor: Sallie W. Chisholm Title: Professor of Civil and Environmental Engineering, and Biology