Long-Term and Seasonal Changes in Nutrients,Phytoplankton Biomass,and Dissolved Oxygen in Deep Bay,Hong Kong |
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| Authors: | Jie Xu Kedong Yin Joseph H W Lee Hongbin Liu Alvin Y T Ho Xiangcheng Yuan Paul J Harrison |
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| Institution: | (1) Atmospheric, Marine, and Coastal Environment (AMCE) Program, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, People’s Republic of China;(2) Australian Rivers Institute, Griffith University, Nathan Campus, Brisbane, Queensland, 4111, Australia;(3) Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, People’s Republic of China;(4) Key Laboratory of Tropical Marine Environmental Dynamics, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China |
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| Abstract: | Deep Bay is a semienclosed bay that receives sewage from Shenzhen, a fast-growing city in China. NH4 is the main N component of the sewage (>50% of total N) in the inner bay, and a twofold increase in NH4 and PO4 concentrations is attributed to increased sewage loading over the 21-year period (1986–2006). During this time series, the
maximum annual average NH4 and PO4 concentrations exceeded 500 and 39 μM, respectively. The inner bay (Stns DM1 and DM2) has a long residence time and very
high nutrient loads and yet much lower phytoplankton biomass (chlorophyll (Chl) <10 μg L−1 except for Jan, July, and Aug) and few severe long-term hypoxic events (dissolved oxygen (DO) generally >2 mg L−1) than expected. Because it is shallow (~2 m), phytoplankton growth is likely limited by light due to mixing and suspended
sediments, as well as by ammonium toxicity, and biomass accumulation is reduced by grazing, which may reduce the occurrence
of hypoxia. Since nutrients were not limiting in the inner bay, the significant long-term increase in Chl a (0.52–0.57 μg L−1 year−1) was attributed to climatic effects in which the significant increase in rainfall (11 mm year−1) decreased salinity, increased stratification, and improved water stability. The outer bay (DM3 to DM5) has a high flushing
rate (0.2 day−1), is deeper (3 to 5 m), and has summer stratification, yet there are few large algal blooms and hypoxic events since dilution
by the Pearl River discharge in summer, and the invasion of coastal water in winter is likely greater than the phytoplankton
growth rate. A significant long-term increase in NO3 (0.45–0.94 μM year−1) occurred in the outer bay, but no increasing trend was observed for SiO4 or PO4, and these long-term trends in NO3, PO4, and SiO4 in the outer bay agreed with those long-term trends in the Pearl River discharge. Dissolved inorganic nitrogen (DIN) has
approximately doubled from 35–62 to 68–107 μM in the outer bay during the last two decades, and consequently DIN to PO4 molar ratios have also increased over twofold since there was no change in PO4. The rapid increase in salinity and DO and the decrease in nutrients and suspended solids from the inner to the outer bay
suggest that the sewage effluent from the inner bay is rapidly diluted and appears to have a limited effect on the phytoplankton
of the adjacent waters beyond Deep Bay. Therefore, physical processes play a key role in reducing the risk of algal blooms
and hypoxic events in Deep Bay. |
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