Marine ecosystems are vital to human society: as a source of food, for economic growth and for their potential to mitigate climate change. With marine ecosystems threatened by climate change and overfishing, there is a need for sustainable fisheries management, which has been the basis for an ecosystem-based approach to management. This has led to considerable interest in end-to-end ecosystem models, where the physical effects of the environment and the population dynamics of all marine organisms are coupled together into one framework. In this thesis, I studied an end-to-end model which coupled together a box-component model representing phytoplankton and zooplankton, with a size-structured fish community model.
I investigated the potential artefacts in model results, caused by numerical methods or by model architecture. I found that care needs to be taken with the choice of numerical method used to simulate size-structured models, as the choice of numerical resolution can yield numerically stable results but can also affect large-scale behaviours of the system, such as the slope and mathematical stability of the size-spectra solutions. With regards to model architecture, coupling together two submodels which differ in structure and resolution can lead to large-scale behaviours of the system which appear plausible and consistent with empirical data, but which impose serious discrepancies in the underlying life-histories of the fish.
By distinguishing model artefacts from ecosystem-effects, the interactions and feedbacks between the higher and lower trophic level organisms can be investigated. I studied the potential impact of climate change upon the marine ecosystem, and in particular, upon the seasonal dynamics of phytoplankton. I found that under a warming climate, the spring phytoplankton bloom occurs earlier and for a longer duration, and the model predicts the loss of the autumn phytoplankton bloom. These changes were not solely due to the direct effect of temperature, but also due to the indirect effect of the interactions of the fish population with zooplankton. The effect of fishing upon the marine ecosystem was also explored with this end-to-end model, with two potential fishing strategies applied to the system. Regardless of the choice of fishing strategy, intensive exploitation of fish stocks can lead to a significant shift in the dynamics of phytoplankton. The phytoplankton's dynamics change from stable annual patterns to unpredictable periodic behaviours.
This thesis has developed an end-to-end model which uses a minimal framework to study the interactions of organisms at different trophic levels, and highlights the importance of these interactions and the associated feedbacks under different scenarios. It combines important theoretical insights into the consequences of model architecture and model-derived artefacts upon ecosystem-scale behaviours, at the same time as highlighting the potential for end-to-end models as a practical and flexible management tool
