In the context of global climate change, warming and eutrophication are two major global change stressors that affect the marine ecosystem, especially in coastal waters. Phytoplankton, the primary producer in the ocean, responds rapidly to and can readily reflect the environmental changes in marine ecosystems. In this thesis, I used both statistical models and cultural experiments to study the combined effect of temperature and nutrient availability on the abundance and physiology of phytoplankton, with specific focus on diatoms and dinoflagellates. I used statistical models to reveal the effect of temperature and anthropogenic input on diatoms and dinoflagellates in the Pearl River Estuary (PRE), a highly urbanised subtropical estuarine ecosystem. By analyzing a 18-years monthly monito...[ Read more ]

In the context of global climate change, warming and eutrophication are two major global change stressors that affect the marine ecosystem, especially in coastal waters. Phytoplankton, the primary producer in the ocean, responds rapidly to and can readily reflect the environmental changes in marine ecosystems. In this thesis, I used both statistical models and cultural experiments to study the combined effect of temperature and nutrient availability on the abundance and physiology of phytoplankton, with specific focus on diatoms and dinoflagellates. I used statistical models to reveal the effect of temperature and anthropogenic input on diatoms and dinoflagellates in the Pearl River Estuary (PRE), a highly urbanised subtropical estuarine ecosystem. By analyzing a 18-years monthly monitoring dataset, I observed an increasing trend of diatom to dinoflagellate ratio and nitrate concentration in the PRE over the past 18 years. The models predicted that warming and nitrate enrichment will promote the increase in diatom cell density in PRE, while the dinoflagellate cell density will only increase with warming when nutrients are depleted. I also conducted a series of multi-stressors cultural experiments to investigate how phosphorus limitation and warming can affect the nitrogen uptake in the phytoplankton. The experimental result demonstrated that P-limitation inhibits the nitrate uptake (but not ammonium uptake) of two phytoplankton cultures, Thalassiosira weissflogii (diatom) and Amphidinium carterae (dinoflagellate). An increase in temperature was found to exacerbate the inhibitory effect of phosphorus limitation on Thalassiosira weissflogii, in terms of nitrate uptake and carbon fixation, while Amphidinium carterae is less vulnerable to phosphorus limitation when temperature increased.