[Department of Ocean Science] MPhil Thesis Defense Seminar: Exploring the Dissolved Organic Matter (DOM) Chemistry in Hong Kong Coastal Waters: Implications for Coastal Aquaculture Effects and Water Quality

Abstract: 

Coastal aquaculture releases dissolved organic matter (DOM), but its influence in open coastal systems is difficult to separate from hydrological variability and other DOM sources. This thesis examines how aquaculture activity, hydrological forcing, and source mixing shape DOM dynamics and water quality in Hong Kong coastal waters. Monthly monitoring of 36 fish farms was combined with fluorescence characterization, fish-feed incubation experiments, and Fast expectation-maximization microbial source tracking (FEAST) model. The study area covers the Pearl River-influenced southwestern region and more oceanic northeastern region. Results show that fish culture zones can function as semi-independent biogeochemical systems, with DOM characteristics and water quality differing from adjacent waters. In the southwest, riverine inputs promoted synchronous variation between DOM and nutrients, whereas in the nutrient-poor northeast, DOM degradation appeared to play a more direct role in nutrient dynamics and water quality responses. To place these fish-farm signals within the broader coastal DOM pool, source tracking was further applied. FEAST identified beach groundwater as a persistent dominant DOM source while the aquaculture-derived DOM was consistently detectable but relatively minor. Overall, this thesis provides critical insights into how DOM dynamics can be used to evaluate aquaculture influence and source mixing in hydrologically complex coastal waters.