长江口上升流区营养盐动力学

近年来，东中国海赤潮灾情严重，且74.7%的赤潮集中在 30°30′～32°00′N、122°15′～123°10′E的“赤潮高发区”。在研究该区赤潮成因时，长江口沿岸上升流的影响越来越受到人们的关注，并被一些专家观测和研究。但目前为止，针对该区营养盐动力学特征及其对叶绿素a影响的研究较少，且不系统。 本文根据2004年四个季度月的调查资料，系统地探讨了长江口上升流区营养盐动力学特征；估算了上升流的营养盐通量，并和陆源输入通量进行了比较。初步探讨了上升流对该区营养盐结构和浮游植物生长的影响。为深入研究长江口富营养化和赤潮形成机制提供了参考。 结果表明春季在122°20′～123°00′E，31°00′～32°00′N以北海域存在低温、高盐、低溶解氧的沿岸上升流。它不但可把底层高含量磷酸盐输送到10m层以上海区，而且还为上层海区输入了相对低含量硝酸盐和硅酸盐，从而改善了上层营养盐结构，使得营养盐比值接近Redfield Ratios，同时还改善了上层的透明度；从而有利于浮游植物的繁殖。夏季上升流受到强大的长江冲淡水压制，表现不如春季明显，主体水团出现在122°20′～123°00′E，31°15′～31°50′N海区10m层以下。 在秋、冬季，上升流现象被更强的对流现象所掩盖，表现为台湾暖流表层水的入侵。表、底层水域不仅温、盐度分布十分接近，而且营养盐结构差异也较小。冬季台湾暖流水中的磷酸盐含量远比秋季高，与春、夏季上升流水团中磷酸盐含量接近。硝酸盐和硅酸盐含量比秋季稍高，比春、夏季上升流水团中的含量稍低。 叶绿素a季节性分布表明，在春、夏季的10m层以下水域，叶绿素a受到透明度限制，含量相差不大；而在表层和10m层之间，春季叶绿素a的含量远高于夏季，说明春季的营养盐结构和自然条件更有利于浮游植物的繁荣生长。在秋季台湾暖流水影响的区域，表层叶绿素a含量较夏季稍低。而冬季该区叶绿素a含量则是最低的。 对长江口上升流水团春季营养盐通量的计算结果表明，上升流水团中磷酸盐输送通量远远高于长江径流输入，是其径流通量的两倍以上，可能会成为影响该海区磷酸盐分布以及浮游植物生长的一个值得关注的因素。 关键词：上升流，营养盐动力学，营养盐结构，叶绿素a，长江冲淡水

In recent years, frequent outbreaks of red tides in East China Sea became a growing concern, of which 74.7% took places in the area of 122°15′–123°10′E/30°30′–32°00′N in the Changjiang (Yangtze River) estuary, where several researches have been made on coastal upwelling in eutrophication in the region. However, available knowledge on the nutrient dynamic and its influence on chlorophyll-a distribution in the estuary are not systematic and comprehensive so far. Dynamics of nutrient movement and its influence on chlorophyll-a distribution in upwelling area of Changjiang estuary were investigated in February, May, August and November 2004. In spring, the upwelling was in low temperature, high salinity, and low dissolved oxygen in the region of 122°20′–123°00′E/31°00′–32°00′N. It is found for the first time that, besides bringing up high concentration of phosphate, the upwelling also brought up relatively low concentrations of nitrate and silicate to the upper layers above 10m. Nutrients transported by upwelling modified the nutrient ratios that close to the Redfield Ratios, and enhanced the nutrient structure in the upper layers. Also, the spring upwelling improved the sunlight transparency in the euphotic zone in favor of phytoplankton growth, and the chlorophyll-a distribution changed correspondingly. Affected by strong influence of Changjiang diluted water (CDW), the summer upwelling was weaker than the spring one, and could hardly affect the upper layers. In summer, the upwelling covered mainly a narrower latitudinal belt of 122°20′–123°00′E/31°15′–31°50′N and affected the nutrient distribution in the layers below 10m. The upper layers were darkened by CDW bearing thick suspended particulates, which became one of the limiting factors to the growth of phytoplankton. Chlorophyll-a content in the summer upwelling area was much lower than that in spring, indicating that the dynamics of nutrients and natural environment in spring are more beneficial to the regional phytoplankton growth. During autumn and winter, the nutrient distribution was affected by the shelf-intrusion of the Kuroshio surface water. Nutrient concentration and nutrient ratio between the bottom and the surface layers were close for the waters transportation which eliminated the seawater stratification. In the region of 122°30′–123°05′E, there existed an area with low chlorophyll-a contents and the correlation between nutrient ratios and chlorophyll-a distribution was discussed. The calculated estimation of nutrient flux indicates that the phosphate flux in spring upwelling was two times higher than that of the Changjiang river water. In contrast, the fluxes of nitrate and silicate were much lower than the inputs from Changjiang river water. Key words: upwelling; nutrient dynamics; nutrient structure; Chlorophyll-a; Changjiang diluted water.