On The Hypoxia In The Pearl River And Changjiang River Estuaries

Hypoxia,defined here as dissolved oxygen(DO)below a given criterion(operationally 2-3 mg O2 L-1),is one of the most important environmental issues of global significance.Hypoxic events have been reported both in the upper Pearl River Estuary(PRE)and East China Sea(ECS)off Changjiang(Yangtze)estuary(CJE).However,much remains unknown about these two hypoxic systems.For example,the current status and long term trend of the hypoxic zone in the lower PRE have not been fully explored.The dissolved organic matter and the initial DO condition in the source water involved in the hypoxia development in ECS off the CJE are neither clear.This dissertation sought to examine these important questions associated with the coastal hypoxia based on a series of field surveys and long term monitoring data.We evaluated the DO status and long term trend of the lower PRE and adjacent water off Hong Kong based on the monthly surveys from April 2010 to March 2011 along with 25-year monitoring data from Hong Kong Environmental Department.It clearly showed the evolution of the hypoxia that begun to emerge in the bottom layer in the lower reach of PRE off Hong Kong in June,and fully developed in July and August when the hypoxic water occupied?1000 km2.The hypoxia was dissipated in September/October.The development of oxygen-depleted zone was closely coupled with phytoplankton blooms in the surface water featured by supersaturation of DO,suggesting the importance of the autochthonous organic matter in fueling bottom DO consumption when they settle down through the pycnocline.Long term monitoring data collected in the study area adjacent to Hong Kong by Hong Kong Environment Protection Department showed a decreasing trend of-2 ?mol kg-1 a-1 of the annual minimum DO in the bottom water during the past 25 years from February 1990 to December 2014.Associated was the increment at a rate of?1.3 ?mol kg-1 a-1 in the annual maximum surface dissolved inorganic nitrogen(DIN),suggesting again that eutrophication has been the most plausible driver stimulating the formation of hypoxia in this region.Our monthly cruises along with the decadal monitoring data therefore demonstrated an emerging large hypoxic zone driven primarily by anthropogenic eutrophication.Being dynamic in hydrological circulation and thus good water exchange of the region,this new development may suggest threshold of the environmental stressors such as eutrophication which holds important and expensive consequence to the regional environment.We also examined the contribution of dissolved organic matter to the oxygen consumption in the hypoxia in the ECS off the CJE based on a data set collected in the summer of 2011,The DOC and dissolved inorganic carbon(DIC)concentrations ranged from-60 to?120 ?mol L-1 and?1660 to?2120 ?mol kg-1 respectively.The spatial distribution of DOC and DIC was largely shaped by the mixture of three water masses,i.e.Changjiang River plume(CRP),East China Sea surface water(ESW)and East China Sea bottom water(EBW).Biological alteration of DOC and DIC were observed in euphotic layer.-20%net community production was converted to DOC with prominent spatiotemporal variations.Nevertheless,net DIC production and quasi-conservative DOC were observed beneath the pyconalcline,suggesting the particulate organic matter largely accounts for the DO consumption/DIC increment and DOC contributes little to the DO consumption.On the basis of field observations conducted in summer 2009 and 2011,we also investigated the non-local drivers associated with the initial dissolved oxygen(DO)levels that had significant impact on the development of summer hypoxias in the ECS off the CJE.The bottom water of the Changjiang River plume(CJP)can be traced back to the Luzon Strait where the South China Sea subsurface water mixing with Kuroshio subsurface water.The mixing ratio and afterward oxygen consumption during its travel to CJP area thus determine the initial DO level for CJP bottom water.Along the pathway of bottom-hugging water on the East China Sea Shelf,we found systematic increases in nitrate,phosphate and apparent oxygen utilization and their increments followed the Redfield stoichiometry underscoring the importance of organic supply and consumption during travel.Obviously,these serial non-local factors exert synergistic control preparing a fundamentally low initial DO to facilitate the bottom water hypoxia although the residence time of CJP bottom water is only?11 days.In the context of global warming and continuously increasing atmospheric and riverine nutrient inputs,the CJP hypoxia is anticipated to magnify and be more frequently occurred.