No abstract

27Eutrophication impacts may vary spatially and temporally due to different 28 physical processes. Using a 22-year time series data set , a comparison of 29 eutrophication impacts between two eutrophic harbors, Victoria and Tolo Harbours, in 30 Hong Kong with very different hydrodynamic conditions was conducted. In the 31 highly-flushed Victoria Harbour (Victoria), the highest Chl a (13 μg L -1 ) occurred due 32 to stratification in summer as a result of the input of the eutrophic Pearl River 33 discharge, but the high flushing rate restricted nutrient utilization and the further 34 accumulation of algal biomass. In other seasons, vertical mixing induced light 35 limitation and horizontal dilution led to low Chl a (< 2 μg L -1 ) and no spring bloom. 36 Few hypoxic events (DO < 2 mg L -1 ) occurred due to strong tidal mixing. Therefore, 37 Victoria is resilient to nutrient enrichment. In contrast, in the weakly-flushed Tolo 38 Harbour (Tolo), year long stratification, the long residence times and weak tidal 39 currents favored algal growth, resulting in a spring diatom bloom and high Chl a (up 40 to 30 μg L -1 ) all year and frequent hypoxic events in summer. Hence, Tolo is 41 susceptible to nutrient enrichment and it responded to nutrient reduction since sewage 42 treatment resulted in a 32-38% decrease in algal biomass in Tolo, but not in Victoria. 43 A significant (11-22%) reduction in bottom DO in the both harbors after sewage 44 treatment was due to a decrease in the organic loading from sewage treatment or the 45 diversion.46

The size-fractionated phytoplankton biomass and primary production were investigated in four contrasting areas of Hong Kong waters in 2006. Phytoplankton biomass and production varied seasonally in response to the influence of the Pearl River discharge. In the dry season, the phytoplankton biomass and production were low (<42 mg chl m −2 and <1.8 g C m −2 day −1 ) in all four areas, due to low temperatures and dilution and reduced light availability due to strong vertical mixing. In contrast, in the wet season, in the river-impacted western areas, the phytoplankton biomass and production increased greater than five-fold compared to the dry season, especially in summer. In summer, algal biomass was 15-fold higher than in winter, and the mean integrated primary productivity (IPP) was 9 g C m −2 day −1 in southern waters due to strong stratification, high temperatures, light availability, and nutrient input from the Pearl River estuary. However, in the highly flushed western waters, chl a and IPP were lower (<30 mg m −2 and 4 g C m −2 day −1, respectively) due to dilution. The maximal algal biomass and primary production occurred in southern waters with strong stratification and less flushing. Spring blooms (>10 μg chlaL −1 ) rarely occurred despite the high chl-specific photosynthetic rate (mostly >10 μg C μg chla) as the accumulation of algal biomass was restricted by active physical processes (e.g., strong vertical mixing and freshwater dilution). Phytoplankton biomass and production were mostly dominated by the >5-μm size fraction all year except in eastern waters during spring and mostly composed of fast-growing chain-forming diatoms. In the stratified southern waters in summer, the largest algal blooms occurred in part due to high nutrient inputs from the Pearl River estuary.