珠江口近百年来富营养化加剧的沉积记录

由于土地利用和人类活动加剧所导致的营养元素由河流输入的增加是引起河口港湾地区富营养化趋势增强的重要原因,由此引发的赤潮在中国沿海地区呈现越来越频繁的趋势。本文通过珠江口两个沉积柱状样(ZJ6和ZJ10)揭示了该水域近百年来的富营养化加剧趋势。由柱状样中的TOC/TN比值判断,TOC是陆源和水生两种来源的混合物。在假设陆源和水生有机质C/N比值分别为5和20后,计算了在沉积物中两种来源有机碳的含量得到:两钻孔柱状样中水生有机碳、总氮、生物硅、陆源有机碳沉积通量自20世纪20年代到90年代呈明显升高趋势,分别增加了2.0、3.6、2.9、12.0倍(ZJ6)和5.4、6.8、5.5、10.6倍(ZJ10)。这一趋势与中国珠江三角洲地区在此期间,特别是50年来生产力的迅速提高趋势相对应。两钻孔柱状样中生物硅沉积通量的增加幅度逐渐超出水生有机碳沉积通量的增加幅度,表明硅藻是富营养化的敏感藻类。目前,Si相对于N、P还不是珠江口水域的限制性营养元素,但若不对水域的营养物进行有效管理以平衡营养元素间的比例关系和减弱富营养化趋势,珠江口的浮游生物种群结构和底层水的溶解氧含量将受到严重影响。

SEDIMENTARY RECORDS OF ACCELERATED EUTROPHICATION FOR THE LAST 100 YEARS AT THE PEARL RIVER ESTUARY

Changes in delivery of river born nutrients owing to land use changes and anthropogenic emissions, are known to result in eutrophication in many enclosed bays and seas. Red tide, caused by eutrophication, occurred more and more frequently in last twenty years along coastal zones in China. Here, we present evidence of accelerated eutrophication at the Pearl River mouth for the last 100 years, obtained by quantifying organic carbon and biogenic silica within two dated sediment cores. Organic matter within the cores is mixture of aquatic and terrestrial sources deduced from TOC/TN ratios. Different sources are calculated by assuming that the C/N ratio of aquatic and terrestrial organic matter is 5 and 20 respectively. Both accumulation rates of aquatic organic carbon, total nitrogen, biogenic silica and terrestrial organic carbon show increasing trends from about 1920's to 1990's, and have increased by as much as 3.0, 4.6, 3.9 and 13.0 times in core ZJ6 and 6.4, 7.8, 6.5 and 11.6 times in core ZJ10. The trends correspond to the accelerated development of Chinese productivity during the period, especially during the last 50 years. The increase of biogenic silica accumulation rates outdid that of aquatic organic carbon within both cores, indicating diatom blooms are sensitive and respond first to eutrophication. Silicate is not limited nutrient relative to nitrate and phosphate at present, but if nutrient management is not sufficient to balance the nutrient contents in the water and reduce eutrophication, the species composition of phytoplankton and the amounts of oxygen in the bottom water would be affected.$$$$