Microzooplankton (<200 μm, dominated by protists) are dominant consumers of phytoplankton and bacterioplankton and their grazing is affected by various prey cell properties. In this thesis, response of protist feeding on prey cell properties was investigated, with a special focus on the diel variability of the grazing activity. Effect of prey size on microzooplankton grazing was investigated with dilution experiments with nutrient enrichment at two stations with contrasting trophic conditions in Hong Kong. The enhanced grazing on enlarged bacteria in nutrient treatment and a higher stimulation of grazing on large (>5 μm) phytoplankton at the WE station with larger protist grazers indicated the effect of prey size on protist grazing. The generality of diel variability of carbon (C) to nitrogen (N) ratio in phytoplankton was investigated in Chlorella autotrophica growing over a series of P- and N-limited growth rates in cyclostat culture system. Although with different extent of diel variations across growth rates, C. autotrophica exhibited a general diel pattern of increasing C:N during day and decreasing C:N during night at all studied growth rates. The effect of prey C:N on diel grazing of protist was investigated with grazing experiments with isolated ciliate and dinoflagellate feeding on mono-diet of green algae with contrasting C:N at both day and night time. Both graz...[ Read more ]

Microzooplankton (<200 μm, dominated by protists) are dominant consumers of phytoplankton and bacterioplankton and their grazing is affected by various prey cell properties. In this thesis, response of protist feeding on prey cell properties was investigated, with a special focus on the diel variability of the grazing activity. Effect of prey size on microzooplankton grazing was investigated with dilution experiments with nutrient enrichment at two stations with contrasting trophic conditions in Hong Kong. The enhanced grazing on enlarged bacteria in nutrient treatment and a higher stimulation of grazing on large (>5 μm) phytoplankton at the WE station with larger protist grazers indicated the effect of prey size on protist grazing. The generality of diel variability of carbon (C) to nitrogen (N) ratio in phytoplankton was investigated in Chlorella autotrophica growing over a series of P- and N-limited growth rates in cyclostat culture system. Although with different extent of diel variations across growth rates, C. autotrophica exhibited a general diel pattern of increasing C:N during day and decreasing C:N during night at all studied growth rates. The effect of prey C:N on diel grazing of protist was investigated with grazing experiments with isolated ciliate and dinoflagellate feeding on mono-diet of green algae with contrasting C:N at both day and night time. Both grazers fed at higher rates on prey with higher C:N than on prey with lower C:N, with the dinoflagellate grazer exhibiting compensatory feeding for acquisition of limiting N. The effect of prey cell properties on diel grazing of heterotrophic nanoflagellate (HNF) on picoplankton was investigated in the South China Sea and Hong Kong coastal water with 2-point dilution experiments. Results showed that prey C:N played a role in influencing the HNF grazing on picophytoplankton, while both prey size and C:N potentially influenced the diel grazing on heterotrophic bacteria.