SEDIMENTARY RECORDS OF ACCELERATED EUTROPHICATION FOR THE LAST 100 YEARS AT THE PEARL RIVER ESTUARY
珠江口近百年来富营养化加剧的沉积记录

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.$$$$