长江入海泥沙减少对长江口海域有害藻华的影响

Harmful algal blooms (HABs) are increasingly frequent in coastal waters in China Seas as well as around the world over the last several decades, among which the Changjiang Estuary and its adjacent sea areas is the most frequent HAB occurrence area. The accelerated coastal eutrophication, which resulted from the increasing anthropogenic loadings of nutrients, is commonly assumed to be the primary cause for this increase. However, another neglected major impact of human activities in the catchments (impounding) is the drastic reduction of riverine sediment load into the sea, which is the controlling factor of phytoplankton growth of --light availability. Based on analysis of historical data, field investigation and experiments, this proposal aims to research on the impacts of the reduction in riverine sediment load on the content and temporal-spatial distributions of suspended sediment in the Changjiang Estuary; and to demonstrate the potential relationship between HABs and the reduced suspended sediment in the Changjiang Estuary; and finally to quantitatively discriminate the contributions of the reduction in riverine sediment load and increase in riverine nutrient flux on increase in HABs in the Changjiang Estuary, respectively. The results of this work may provide scientific bases for the prediction and prevention of HABs in the Changjiang Estuary.

近几十年来，全球有害藻华在不断增加，我国有害藻华的发生频率、规模和发生区域也在急剧增大，而长江口及其邻近海域是有害藻华发生频率最高的海区之一。目前普遍认为人类活动造成的河口-近海富营养化程度的加剧，是导致长江口海域乃至全球有害藻华增加的主要原因。然而，在大中型河口区域，流域人类活动除了导致河口富营养化程度加剧之外，另一个被忽视的重大影响则是流域人类活动（筑坝）导致入海泥沙的急剧减少，而后者恰恰是影响藻华生物生长的最重要的条件之一—光照的关键控制因素。本项目基于长江口海域历史资料与遥感反演、现场调查与实验，研究长江入海泥沙减少对长江口海域悬沙含量及其时空分布格局的影响，进而揭示长江口海域悬沙含量及其时空分布格局变化与有害藻华时空变化之间的内在联系，定量辨析泥沙减少和营养盐增加对长江口海域有害藻华的各自贡献，为长江口海域有害藻华的预测、防治等提供科学依据。

流域人类活动引起的长江入海营养盐（N和P）通量的增加被普遍认为是导致长江口海域赤潮频发的主要原因。然而，另一个被忽视的流域人类活动重大影响是筑坝等活动导致长江入海泥沙的急剧减少，这恰恰是影响河口藻华生物生长的光照条件的关键因素。本项目研究表明，20世纪80年代以来长江流域筑坝拦截等水利工程导致长江入海泥沙通量大幅度减少了70%，并进而导致长江口海域悬沙含量显著降低、等值线明显向口门方向位移。悬沙分布格局的这些变化对长江口海域初级生产过程及赤潮产生了显著影响。20世纪80年代以来以叶绿素a浓度表征的长江口浮游植物生物量增大了数倍，这是长江口悬沙含量减少与营养盐浓度增大二者协同作用的结果；但是，长江口叶绿素a浓度峰区明显向低盐度的口门方向位移、且峰区所对应的盐度范围也明显扩大，则是悬沙浓度减少所致。与此相对应地，20世纪80年代以来长江口及其邻近海域的赤潮爆发频率显著增加，且与入海泥沙通量的年际变化之间具有镜像关系和显著负相关关系，同时近10几年来长江口海域赤潮高发区也向低盐口门区方向位移。因此认为，长江入海泥沙通量的降低不仅增大了长江口及其邻近海域赤潮发生频率，而且使得赤潮高发区向低盐度的口门区位移。