东海赤潮高发区春季溶解氧和pH分布特征及影响因素探讨

DISTRIBUTION OF DISSOLVED OXYGEN AND pH IN FREQUENT HAB AREA OF THE EAST CHINA SEA IN SPRING 2002

根据2002年4月27日—5月2日长江口邻近海域的大面调查,分析了东海溶解氧及pH值的分布特征,并对长江口外溶解氧低值区的成因及其与赤潮发生的关系进行了初步探讨。结果表明,调查海域pH值呈近岸低、外海高的分布趋势,溶解氧整体处于过饱和状态,呈近岸高、外海低的分布趋势。4月下旬在调查海区东南部底层已开始出现溶解氧低值区,面积约为15400km2,该水域表观耗氧量AOU一般在1.50mg/L以上,并伴随有氧的亏损发生,形成原因主要是水交换较弱和有机物分解耗氧。溶解氧低值区可能是有机碎屑的沉降汇集区,随着夏季温度的升高及长江丰水期的到来,有机碎屑有可能在台湾暖流的影响下产生西、北向的爬升而造成溶解氧低值区扩大和溶解氧含量的进一步降低。

This paper introduces the results of the first cruise on China harmful algal blooms （HAB or red tide） research, one of the national basic research programs of China. The cruise was carried out in Changjiang （Yangtze River） River estuary and adjacent areas of the East China Sea, located from 29.0°-32.0°N and 122.0°-123.5°E, on April 27 to May 2, 2002. Twenty-eight survey stations among 7 sections were implemented during the cruise. Niskin bottles were used for sampling dissolved oxygen （DO）, pH, nutrients and Chl-α. In the field survey, temperature, salinity, the distributions of DO and pH in frequent-HAB area of in the East China Sea were studied, and the cause of low-oxygen area near the estuary and its link to HAB were also discussed. The pH values measured in the survey area in April to May, 2002, were in the range of 7.99 to 8.48, averaged at 8.23 ± 0.12, lower in coastal area and higher in outside area; while the DO distribution showed an opposite horizontal trend ： higher in coastal area but in outer sea. In overall, DO was oversaturated ranging between 5.14 and 11.17mg/L, averaged at （8.14 ± 1.06）mg/L. The apparent oxygen utilization （AOU） was between -2.83 and 2.55 mg/L, （ - 0.19 ± 0.88）mg/L in average. The highest DO area was located in the coastal area form the estuary to Hangzhou Bay in the south, and the DO had negative correlation with salinity and temperature, indicating some physical processes （such as water runoff） had largely influenced the DO distribution in the area. Generally, AOU values were positive, demonstrating that organic matter were decomposed strongly and generated considerably huge oxygen depletion in bottom water. Judging from the relationship of DO and pH vs. salinity, Chl-α and nutrients, biological and chemical processes were more important in distributing the DO in the outer sea than in the costal area. Furthermore, the distributions of DO and pH showed similar trends of decrease from surface to bottom, from north to south in bottom water. A lower-oxygen area of about 10 to 20m in thickness in about 15400 km^2 occurred in the spring featured with oxygen depletion in bottom water, southeast part of the survey area, and the outside sea in 50m isobath （50 to 60m in depth）, with AOU greater than 1.50mg/L in general. Similarly, there was a lower pH area in southwest part of survey area, in about 20m in thickness and pH lower than 8.05, extending from Zhoushan Islands to the 50m isobath southeastward at bottom. The lowest area of pH was superimposed with low-oxygen area to some degree. On the other hand, pH positively correlated with Chl-α and DO, and AOU was positively correlated with phosphate and silicate. These facts indicate that biological process instead of runoff or outersea water exchange, was the main controlling factor to pH and DO in middle to bottom water in this area. Additionally, in lower-oxygen area, biological and chemical processes but runoff or outersea water exchange were responsible for the DO and pH distributions. Most probably, the oxygen was consumed in the decomposition of organic matters due to weak water mass exchange. It is believed by the authors that the low-oxygen area was the conservation pool of sinking organic particles and cyst or inactive cells of the red tide algae. In summer high-water-level period of Changjiang River, particulate organism and cyst or inactive cells in the surface sediment are likely to float up northward and westward, causing the expansion of low-oxygen area and the DO reduction, under the influence of Taiwan Worm Current. Furthermore, it is possible that the cyst or inactive cells would move upward to the surface layer and bloom if the condition favors.