Resistance of Hong Kong waters to nutrient enrichment: assessment of the role of physical processes in reducing eutrophication

Hong Kong waters receive high nutrient loading from year-round sewage effluent and Pearl River discharge during the summer wet season. We assessed the role of physical processes in reducing eutrophication by calculating a eutrophication reduction index for four different hydrographical areas and four seasons. We used outdoor incubation experiments to assess the response of phytoplankton when physical (mixing and dilution) processes and mesozooplankton grazing were reduced. The primary regulator of phytoplankton growth in low nutrient eastern waters (reference site) shifted from nutrients in the wet season to increased vertical mixing in the dry season. In the highly flushed western waters and Victoria Harbour, the majority (>86 %) of the eutrophication impacts were reduced by strong hydrodynamic mixing (turbulence, vertical mixing, and flushing effects) all year. In southern waters, eutrophication effects were severe (chlorophyll a of up to ~73 μg L−1) and was regulated by the ambient phosphate (PO4) concentration (~0.1 μM) during summer. In contrast, 62–96 % of the potential eutrophication impacts were reduced by physical processes during other seasons. Bioassays also revealed that the yield of chlorophyll from dissolved inorganic nitrogen (DIN) that was taken up by phytoplankton [1.1–3.3 g Chl (mol N)−1] was not significantly different in both N- and P-limited cases. In contrast, the uptake ratios of DIN:PO4 (26:1–105:1) and Chl:P ratios [42–150 g Chl (mol P)−1] in the P-limited cases were significantly (p < 0.05, t test) higher than the N-limited cases [~16DIN:1P and 22–48 g Chl (mol P)−1]. The C:Chl ratios ranged from 32 to 87 g g−1. These potential ranges in ratios need to be considered in future nutrient models.