chapter  15
32 Pages

1Chapter 5 A Comparison of Eutrophication Processes in Three Chinese Subtropical Semi-Enclosed Embayments with Different Buffering Capacities

Abstract .......................................................................................................................................... 368 15.1 Introduction .......................................................................................................................... 368 15.2 Geographic Locations and Geomorphologic Setting ........................................................... 369 15.3 Oceanographic Setting and Drivers of Change .................................................................... 369 15.4 Bays: Physical Processes and Anthropogenic Influences ..................................................... 371

15.4.1 Port Shelter ............................................................................................................... 371 15.4.2 Tolo Harbour ............................................................................................................. 372 15.4.3 Deep Bay................................................................................................................... 377

15.5 Eutrophication Processes ...................................................................................................... 377 15.5.1 Chlorophyll a ............................................................................................................380 15.5.2 Dissolved Oxygen ..................................................................................................... 383 15.5.3 Nutrient Stoichiometry and Nutrient Limitation ...................................................... 383

15.6 Management Strategy and Perspectives ............................................................................... 386 15.6.1 Tolo Harbour ............................................................................................................. 386 15.6.2 Port Shelter ............................................................................................................... 387 15.6.3 Deep Bay................................................................................................................... 387

15.7 Climate Change Effects ........................................................................................................ 388 15.8 Summary and Conclusions ................................................................................................... 389 Acknowledgments .......................................................................................................................... 392 References ...................................................................................................................................... 394

Phytoplankton biomass and bottom dissolved oxygen are controlled by processes such as residence time/exchange between a bay and the open ocean, vertical mixing/stratification, the most limiting nutrient/nutrient ratios, light penetration and grazing as well as climate variability. We examined the eutrophication processes and the drivers that regulate these processes by comparing three semi-enclosed bays (Port Shelter, Tolo Harbour, and Deep Bay) with different buffering capacities using long-term water quality data, including chlorophyll and dissolved oxygen along with salinity, temperature, and nutrients. The outer part of Port Shelter is wide open to southern coastal waters and has a residence time of about 20 days, but it is relatively pristine, with little input of sewage effluent. Nutrient concentrations are as low as chlorophyll a values. Tolo Harbour in northeast waters is a land-locked bay/inlet with high nutrients, a long 28-day residence time, and year-round stratification; hence, it is very vulnerable to eutrophication. Phytoplankton biomass has not reached the potential maximum that could be produced from the high ambient nutrient concentrations, suggesting that top-down control by grazing as well as turbidity is affecting the system. Deep Bay, in western waters between mainland China and Hong Kong, receives sewage input from the city of Shenzhen with up to 500 µM DIN at the head of the bay. However, there is no hypoxia and phytoplankton biomass is not as high as one would expect from such high nutrient levels. Turbidity, light limitation, and top-down control by benthic grazing likely play an important role in reducing the magnitude and frequency of phytoplankton blooms. Deep Bay receives the highest nutrient loads of the three bays, but eutrophication impacts are greatly reduced due to the aforementioned controls.