Stephen Esosa Osakpolor

• Ecosystems are interconnected through the exchange of resources known as subsidies. Subsidies have the potential to affect the receiving ecosystem, altering its productivity and trophic cascade. The boundary between aquatic and terrestrial ecosystems provides a clear distinction between aquatic and terrestrial organisms and is a particularly interesting location for studying resource subsidies. Process-based models can aid in predicting the effects of anthropogenic stressors on food webs and understanding the functioning of meta-ecosystems. The goal of this thesis is to contribute to the development of theories on how changes in subsidies affect recipient ecosystems using aquatic-terrestrial interface as a case study. In this thesis, a review of process-based food web models applied to the aquatic-terrestrial interface (aquatic-terrestrial models) and theoretical meta-ecosystems (theoretical models) was carried out (chapter 2). Results show that the models have enhanced our understanding of how terrestrial subsidies affect aquatic ecosystem. General understanding of how subsidies affect the stability and functions of meta-ecosystems was also enhanced. However, existing aquatic-terrestrial models focused primarily on how subsidies from terrestrial ecosystems affect aquatic ecosystems, with none considering reciprocal flows. Furthermore, the quality characteristics of subsidies were not taken into account, despite potential differences from alternative local resources. Therefore, chapters 3 and 4 developed theories using terrestrial ecosystems with aquatic subsidies as a case study. Chapter 3 focused on how changes in subsidy quality affect the recipient ecosystem and hypothesized that changes in subsidy quality have a cascading effect on the recipient ecosystem (subsidy quality hypothesis). However, the model predictions were most sensitive to the input rate of inorganic nutrients in the recipient ecosystem, indicating that ecosystems are controlled by both top-down (TD) and bottom-up (BU) processes. Chapter 4 shows that the TD and BU processes of ecosystems interact antagonistically. The generated theories can be integrated into empirical research by testing predictions, assumptions, using model equations, and adopting the framework. This thesis improves our understanding of the impacts of subsidies on recipient ecosystems. Future meta-ecosystem models may consider the cross-ecosystem flow of information to further enhance our understanding of meta-ecosystems. Additionally, aquatic-terrestrial models developed to predict algae blooms may consider developing trait-based models to improve predictions.