A simulation model for the growth of freshwater phytoplankton populations was used to examine the ecological processes that shape communities. The computer program for the model (PROTECH: Phytoplankton RespOnses To Environmental CHange) was described (Chapter 2) and formally assessed by a verification study (Chapter 3), a sensitivity analysis (Chapter 4) and a validation analysis (Chapter 5). This work corrected many program bugs and confirmed the suitability of using PROTECH for examining phytoplankton community assembly by testing the following hypotheses:

i) under optimum conditions, the fittest species dominates by virtue of its initial performance;
ii) autogenic processes lead to low diversity;
iii) variability promotes diversity.

The first hypothesis (Chapter 6) was supported because the simulated species with the strongest relevant traits consistently dominated the community, even with a thousand-fold reduction in its starting biomass. In the disturbance-free conditions simulated in the investigation of the second hypothesis (Chapter 7), diversity decreased from its initial value, supporting the hypothesis. The final hypothesis (Chapter 8) was partially supported, with variability (disturbances) increasing diversity until the level of disturbance reached a critical point, beyond which, the diversity decreased again. This pattern in diversity changes was very similar to that suggested by the Intermediate Disturbance Hypothesis but there were significant differences in the actual shape of the response.

Finally, PROTECH was adapted from a lake-simulating model to one that simulated the key characteristics of a Scottish sea-loch (Chapter 9). Changes were made to the physical and biological functions. The simulated output was similar to field data and thus passed a basic Turing test.

It was concluded that, within the constraints involved in the use of a model, the information and insight gained from using an adaptable model such as PROTECH contributed to the understanding of the processes that control the assembly of phytoplankton communities.