中国科学院海洋研究所机构知识库

本文通过分析长江口及邻近海域表层沉积硅藻群落的时空分布特征及其和环境变量之间的关系，结合同期上层水体参数，构建了适用于该海域的硅藻-DIN（溶解无机氮）定量转换关系，通过现场数据验证了该模型的可靠性。结合硅藻-DIN定量转换模型和柱状沉积物沉积硅藻数据，在测年技术的支持下，定量重建了区域内水体DIN的历史变化情况，辅以沉积物粒径、TOC（总有机碳）、TN（总氮）和BSi（生物硅）的历史变化情况，讨论了人类活动对长江口及邻近海域富营养化的影响。获得的主要结论如下：

1. 长江口及邻近海域表层沉积物以黏土质粉砂、粉砂质砂和砂为主。区域内粒径组成总体上呈现明显的“南细、北粗，近岸细、离岸粗”的分布特征。沉积物颗粒越细，对有机质的富集能力越强；沉积物中BSi与TOC、TN均有显著的相关性（P < 0.01），证明硅藻是长江口及邻近海域表层沉积物有机质的主要贡献者之一。

2. 34个表层沉积物样品中，共鉴定出藻类（taxa）114种，隶属于16科33属，其中硅藻113种，硅鞭藻1种。硅藻的绝对丰度没有明显的分布特征，范围为0.24×10³-49.32×10³ 个/克 干重，平均值为16.69×10³ 个/克 干重。香浓-威纳指数（H’）在3.02-5.03之间，平均值为4.03，丰富度指数（D）在2.80-5.28之间，平均值为4.07，二者从近岸到远岸的呈现降低趋势。共筛选出17个硅藻优势种属，其中具槽帕拉藻（Paralia sulcata）的相对丰度介于4.87 %-56.12 %之间，平均值为20.07 %，为17个优势种最高。沉积物粒粒径越细，沉积硅藻丰度越高。沉积硅藻的多样性呈现近岸-远岸降低的趋势，沉积硅藻组成也呈现明显的近岸-远岸的差异。盐度、DIN、温度和水深是影响硅藻属种组成和分布的最显著的因子，4个环境因子均能够独立影响硅藻的属种组成和分布，可以在后续的硅藻-环境因子定量转换模型中得到应用。

3. 通过最小二乘加权平均回归方法（WA-PLS），结合32个表层沉积硅藻数据和同期上覆水年均DIN数据，建立了硅藻-DIN定量转换模型，具有良好的转换能力。经过留一法（leave-one-out）检验后，其相关系数（R²）达到0.910，预测均方差（RMSEP）仅为0.096。利用2012年长江口及邻近海域和黄海的22个表层沉积硅藻数据和DIN数据验证后发现，该模型在长江口及邻近海域的18个样品中反演效果较好，R²达到0.85。而黄海海域的4个样品中的验证效果较差，推导值和实测值差别最多可达13倍，说明该转换模型具有一定的适用范围。

4. DH8-2柱状沉积物全长50 cm，以黏土质粉砂为主，整体粒度较细，没有出现大的波动。根据放射性同位素²¹⁰Pb和¹³⁷Cs测年结果显示，该柱状沉积物平均沉积速率为0.996 cm/a，共包含了从1962-2012年共50年的沉积信息。整个柱状沉积物中，共鉴定出硅藻70种，隶属17科30属。其中中心纲38类，隶属9科15属；羽纹纲32类，隶属8科15属。共筛选出12个优势种，其中，星形柄链藻（Podosira stelliger）相对丰度在0.49-68.18 %之间，平均值为24.15 %，为12个优势种最高。自1972年以来，TOC、TN、BSi以及硅藻的丰度和多样性均出现了显著的增高，细胞直径大的底栖硅藻种（如星形柄链藻等）含量减少，直径较小的浮游硅藻种（如斯氏海链藻等）含量增加，喜好富营养化环境的硅藻种（如具槽帕拉藻等）含量增加，显示出水体营养盐浓度的增加和结构的改变。

5. 借助硅藻-DIN定量转换模型，重建得出DH8-2站点水体DIN的含量从1962年（5.82 μmol/L）-2012（9.85 μmol/L）年期间增长了近70 %，在这50年中，该区域的营养盐浓度有了显著的增加，富营养化加重。进一步分析发现，DIN含量在1962-1972年处于一个较低的水平（平均值5.95 μmol/L）；1972-1998年间，DIN含量显著上升（平均值7.93 μmol/L），显示出该时期人类活动影响的加剧，大量的营养盐输入导致水体DIN含量增高，并导致水体中营养盐结构发生了显著改变；1998-2012年，DIN含量持续上升，但增幅减小（平均值9.71 μmol/L）。这段时期，沉积硅藻群落结构的变化显示水体营养盐结构进一步改变，东海甲藻赤潮的显著增加证实了这一点。

This study analyzed the temporal and spatial distribution characteristics of surface sediment diatom fossils and their relationship with environmental variables in the Changjiang River Estuary (CRE) and its adjacent waters. A Diatom-DIN (dissolved inorganic nitrogen) quantitative transfer model was developed by combining surface sediment diatom fossils data and corresponding upper water parameters. The diatom fossils in a sediment core were also investigated to reconstruct the historical changes of DIN by the Diatom-DIN quantitative transfer model with the support of core chronologies. We discussed the effect of human activities on eutrophication in the CRE and adjacent waters by DIN reconstruction and historical changes of sediment grain size and total organic carbon (TOC), total nitrogen (TN) and biological silicon (BSi). Major results and conclusions are as the following:

1. Surface sediments in the CRE and its adjacent waters were mainly composed by clayey silt, silty sand and sand. In the investigated area, the grain size of surface sediment appeared to have an obvious characteristic: fine in south and near shore area, coarse in north and offshore area. Fine grained sediments are more favorable for the preservation of organic matter (TOC, TN and BSi). The distribution characters of BSi had significant positive correlations with TOC and TN (P < 0.01), which means diatoms should be one of the main sources of organic matter.

2. A total of 114 taxa were identified in 34 surface sediments, including 72 centric diatom taxa, 41 pennate diatom taxa, and 1 silicoflagellate species (Dictyocha fibula). The diatom fossils absolute abundance (D_abs) ranged from 0.24×10³ valves/g DW to 49.32×10³ valves/g DW, with an average of 16.69×10³ valves/g DW, the D_abs did not significantly differ between the inshore and offshore areas. The Shannon-Weaver index (H’) ranged from 3.0-5.0, with an average of 4.0. The richness index (D) ranged from 2.8-5.3, with an average of 4.1. Both parameters showed a downward trend from inshore to offshore areas. 17 dominant species were selected, the relative abundance of Paralia sulcata ranged from 4.9 % to 56.1 %, with the highest average abundance of 20.1 % among the 17 dominant taxa. The D_abs was significantly affected by grain size, diatom fossils were better preserved in fine sediments. The diversity of diatom fossils increased with increasing nutrient concentration, grain composition had weaker influenced on species diversity, the composition of diatom fossils showed apparent coastal-offshore difference. Salinity, DIN, temperature and water depth affected the composition and distribution of diatom taxa independently (P < 0.05), which can be used to establish transfer models for paleoenvironmental reconstruction in the CRE and its adjacent waters.

3. The diatom fossils data form 32 surface sediments and the corresponding upper water DIN data were used to establish a Diatom-DIN quantitative transfer model by Weighted-Averaging Partial Least Squares (WA-PLS) calibration model. The reliability of the function was tested by leave-one-out statistical test, with RMSEP = 0.096 and R² = 0.910 between observed and inferred values, showing its good predictive ability. Further test by 2012 field survey data (22 samples) also proved that the function had a certain range of application, the prediction results were more accurate in the CRE and its adjacent (n = 18, R² = 0.85) than in the Yellow Sea area (4 samples).

4. Core DH8-2 was mainly composed by clayey silt, no large fluctuations occurred in grain size. The ²¹⁰Pb and ¹³⁷Cs dating of core DH8-2 showed that the mean sedimentation rate was 0.996 cm/a, in corresponding to the sedimentation duration of 50 years (1962-2012). A total of 70 diatom taxa were identified in core DH8-2, including 72 centric taxa, 41 pennate taxa. 12 dominant species were selected, the relative abundance of Podosira stelliger ranged from 0.49 % to 68.18 %, this species had the highest average abundance of 24.15 % among the 12 dominant taxa. Since 1972, the contents of TOC, TN and BSi, the D_abs and diversity all showed significant increase, benthic species decreased, phytoplankton species increased, diatom with large cell diameters have been gradually replaced by diatom of smaller diameter, diatom species prefer the eutrophic environment increased, these phenomena indicated increase in nutrient concentration and changes of nutrient structure.

5. The reconstruction results by Diatom-DIN transfer model showed a significant increase of DIN content since 1962 (5.82 μmol/L) to 2012 (9.85 μmol/L) in DH8-2 sampling area. During the 50 years, the concentration of nutrient in the area has increased significantly, eutrophication has increased. Further analysis found that the DIN content was at a low level (5.95 μmol/L in average) during 1962 to 1972. The DIN content increased significantly (7.93 μmol/L in average) during 1972-1998 period, indicating intensified human activity in this period, large amounts of nutrient input led to significant increase of DIN content, the structure of nutrients also changed. The DIN content continued to rise during 1998-2012 (9.71 μmol/L in average), but the increasing rate was lower than last period. The composition and structure of diatom fossils obviously changed in this period, indicating a further variation in nutrient structure, which was confirmed by the outbreaks of red tides dominated by dinoflagellates start form 1998.