A comparison of eutrophication impacts in two harbours in Hong Kong with different hydrodynamics
Eutrophication impacts may vary spatially and temporally due to different physical processes. Using a 22-year time series data set (1986–2007), a comparison was made of eutrophication impacts between the two harbours with very different hydrodynamic conditions. Victoria Harbour (Victoria) receives sewage effluent and therefore nutrients are abundant. In the highly-flushed Victoria, the highest monthly average Chl a (13 μg L−1) occurred during the period of strongest stratification in summer as a result of rainfall, runoff and the input of the nutrient-rich Pearl River estuarine waters, but the high flushing rate restricted nutrient utilization and further accumulation of algal biomass. In other seasons, vertical mixing induced light limitation and horizontal dilution led to low Chl a (< 2 μg L−1) and no spring bloom. Few hypoxic events (DO < 2 mg L−1) occurred due to re-aeration and limited accumulation at depth due to flushing and vertical mixing. Therefore, Victoria is resilient to nutrient enrichment. In contrast, in the weakly-flushed Tolo Harbour (Tolo), year long stratification, long residence times and weak tidal currents favored algal growth, resulting in a spring diatom bloom and high Chl a (10–30 μg L−1) all year and frequent hypoxic events in summer. Hence, Tolo is susceptible to nutrient enrichment and responded to nutrient reduction after sewage diversion in 1997. Sewage diversion from Tolo resulted in a 32–38% decrease in algal biomass in Tolo, but not in Victoria. There has been a significant increase (11–22%) in bottom DO in both harbours. Our findings demonstrate that an understanding of the role of physical processes is critical in order to predict the effectiveness of sewage management strategies in reducing eutrophication impacts.