渤海湾是环渤海地区社会经济发展的核心地带之一，环湾区域人口高度密集，工、农业生产十分活跃。高强度人类活动给渤海湾生态环境带来了巨大压力，导致海水富营养化不断加剧，赤潮灾害频发。在“十三五”规划中，渤海湾的水质污染治理是蓝色海湾整治工程的重要内容。研究海水富营养化历程、浮游植物群落的历史变化特征、以及两者间的相关关系，是在渤海湾建立陆源减排方案，开展水环境治理和赤潮灾害防治工作的重要依据。本研究通过在渤海湾开展的调查航次，以及20世纪50年代至21世纪10年代期间的历史调查资料，研究了渤海湾营养盐和浮游植物群落的时空分布特征与响应关系。主要研究结果如下：
    （1）营养盐的时空分布特征
    调查结果表明，溶解无机氮（Dissolved inorganic nitrogen: DIN）、溶解磷酸盐（Dissolved inorganic phosphorus: DIP）和溶解硅酸盐（Dissolved silicate: DSi）均呈现出显著的时空差异。DIN在春季、夏季和秋季的平均浓度分别为15.32 μmol/L、7.37 μmol/L和10.42 μmol/L，空间分布较为相似，高值区主要分布于渤海湾西侧近岸海域，与陆源输入密切相关。DIP在三个季节的平均浓度分别为0.16 μmol/L、0.19 μmol/L和0.13 μmol/L，春季高值区主要出现在近岸海域，夏季和秋季的高值区主要出现在曹妃甸附近海域，与陆源输入、沉积物释放等因素相关。DSi在三个季节的平均浓度分别为3.06 μmol/L、3.34 μmol/L和4.69 μmol/L，春季高值区主要出现在海河口附近海域和南岸近岸海域，夏季主要出现在北岸和湾口海域，秋季则主要出现在曹妃甸西侧海域，并向湾口延伸，与沉积物释放和海流等因素密切相关。基于聚类分析结果，可将渤海湾营养盐的分布以15 m等深线为界，分为西侧海域和东侧海域两个区间，其中西侧海域以陆源影响为主，在三个季节均具有较高DIN和DIN/DIP，东侧海域则以海源影响为主，存在季节性的高DIP、DSi、DSi/DIN和DSi/DIP。
    （2）浮游植物群落的时空分布特征
    本次调查共鉴定得到浮游植物109种，其中硅藻77种，甲藻29种，金藻2种，绿藻1种。春季、夏季和秋季的浮游植物平均丰度分别为0.74×10⁴ cells/L、16.31×10⁴ cells/L和0.89×10⁴ cells/L。春季和秋季主要由硅藻组成，优势种为柔弱伪菱形藻（Pseudo-nitzschia delicatissima）、诺氏海链藻（Thalassiosira nordenskioeldii）和具槽帕拉藻（Paralia sulcata）等；夏季甲藻较多，部分甲藻成为优势种，如米氏凯伦藻（Karenia mikimotoi）和叉角藻（Ceratium furca）。从空间分布上来看，春季的高值区主要出现在渤海湾北岸和沧州附近海域，夏季高值区主要位于从西北角向南岸延伸的海域，秋季高值区位于天津附近、南岸和湾口北侧海域。统计分析表明，海水温度、盐度和营养盐是影响浮游植物季节差异和空间分布的重要因素。基于聚类分析结果，渤海湾浮游植物群落可大致以20 m等深线分为西侧和东侧海域两个区间，其中西侧海域甲藻类群的比例较高，与相对较高的DIN浓度以及高的DIN/DIP密切相关，而东侧海域则以硅藻类群为主，与相对较高浓度的DIP和DSi密切相关。
    （3）营养盐与浮游植物的相关性分析
    通过比较1950s~2010s的历史资料，发现渤海湾营养盐的变化特征基本表现为：DIN平均浓度（5月、8月和10月，下同）持续升高而DIP和DSi平均浓度降低；DIN/DIP持续升高，DSi/DIN显著降低，主要营养盐限制因子从DIN转变为DIP或DSi。生活废水排放、海水养殖等人类活动是导致营养盐浓度及比例变化的重要因素。在季节水平上，DIN在夏季的变化幅度相对于春季和秋季较小，而DSi在夏季的变化幅度较大，这可能与海河流域在夏季降水量减少较快减缓了N和Si向近海的输运有关；DIP在夏季的变化相对平缓，可能与夏季底层海水低氧促进了P从沉积物释放进入水体有关。
    浮游植物平均细胞丰度与叶绿素a平均浓度均表现为先减少后增加的趋势。浮游植物群落结构组成的变化主要体现为甲藻以及小细胞硅藻的增多。DIN和DIN/DIP的不断升高是造成浮游植物群落变化的重要原因。在季节水平上，春季浮游植物细胞丰度出现了先增加后减少的变化过程，夏季则为先减少后增加，其变化拐点均位于20世纪90年代，这部分研究尚需更多观测资料进行验证。
    通过研究渤海湾营养盐和浮游植物的时空变化特征，提出渤海湾可大致以15~20 m等深线分为西侧和东侧海域，分别是陆源因素主要影响区和海源因素主要影响区，建议渤海湾水环境治理工作重点放在西侧近岸海域，以陆源N输入削减为主。通过对历史调查资料的综合分析，发现营养盐和浮游植物的变化存在季节差异与不同影响因素，研究结果为深入认识渤海湾生态环境在人类活动和气候变化共同作用下的演变特征提供了科学依据。

    The coastal zone surrounding the Bohai Bay is one of the important economically developing areas around the Bohai Sea, with high density of populations and rapidly developing agriculture and industry. High-intensity anthropogenic activities brought intense pressures on eco-environments of the Bohai Bay, resulting in heavy eutrophication and frequent red tides. According to the 13th Five-Year Plan, seawater pollution control of the Bohai Bay was an important task of the “Blue Bay Regulations”. It is important to study the progress of eutrophication, the historical changes of phytoplankton assemblages, and their correlations, which would provide scientific supports for the scenarios of terrestrial pollution control to improve seawater quality and reduce red tide risks. Based on the three cruises conducted in the Bohai Bay and previous investigations during 1950s~2010s, this study analyzed the temporal and spatial distributions of nutrients and phytoplankton assemblages and their correlations. The main results in this study are summarized as follows:
    (1) Temporal and spatial variations of nutrients
    The investigations showed significant temporal and spatial variations of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and dissolved silicate (DSi). The DIN concentration in spring, summer and autumn was 15.32 μmol/L, 7.37 μmol/L, and 10.42 μmol/L, respectively. Spatial distributions of DIN in the three seasons showed similarities with high values ordinarily observed in the western inshore waters, which was largely related to terrestrial inputs. The DIP concentration was 0.16 μmol/L in spring, 0.19 μmol/L in summer, and 0.13 μmol/L in autumn, respectively. DIP exhibited high values in the inshore waters in spring, near Caofeidian in summer and autumn, mainly associated with terrestrial inputs, sediment release, etc. The DSi concentration was 3.06 μmol/L in spring, 3.34 μmol/L in summer, and 4.69 μmol/L in autumn, respectively. The distribution of DSi was largely related to sediment release and circulation with high values in spring observed near the Haihe Estuary and the southern sea areas, those in summer distributed in the north and the mouth of the Bohai Bay, and those in autumn distributed from the west of Caofeidian to the mouth of the Bohai Bay. Based on the cluster analysis of nutrients, the Bohai Bay could be partitioned into western and eastern parts with 15 m depth as the separation. The western sea areas were primarily influenced by terrestrial factors, and showed consistent high values of DIN and DIN/DIP in the three seasons, while the eastern sea areas were mainly affected by marine factors, and showed seasonal high values of DIP, DSi, DSi/DIN and DSi/DIP. 
    (2) Temporal and spatial variations of phytoplankton assemblages
    A total of 109 species were identified, in which 77 species were Bacillariophyta, 29 species were Dinophyta, 2 species were Chrysophyta and 1 species was Chlorophyta. The phytoplankton abundance was 0.74×10⁴ cells/L in spring, 16.31×10⁴ cells/L in summer, and 0.89×10⁴ cells/L in autumn, respectively. Spring and autumn were dominated by diatoms, and dominant species included Pseudo-nitzschia delicatissima, Thalassiosira nordenskioeldii, Paralia sulcata, etc., while in summer, dinoflagellates dominated, and dominant species included Karenia mikimotoi, Ceratium furca, etc. Spatially, phytoplankton abundance showed significant seasonal variations with high values in spring distributed near the northern and Cangzhou coastal waters, those in summer observed from the northwest sea areas to the southern inshore waters, and those in autumn distributed near Tianjin, the southern inshore waters and the northern mouth of the Bohai Bay. Statistical analysis indicated that temperature, salinity, and nutrients were important factors responsible for the seasonal and spatial variations of phytoplankton assemblages. According to the cluster analysis of phytoplankton, the Bohai Bay could be partitioned into western and eastern sea areas with 20 m depth as the separation. The western inshore waters showed higher proportions of dinoflagellates, closely related to relatively high values of DIN and DIN/DIP, while the eastern inshore waters were dominated by diatoms, associated with relatively high values of DIP and DSi.
    (3) Correlations between nutrients and phytoplankton assemblages
    Through the analysis with historical investigations during 1950s~2010s, the changes of nutrients showed an increase in DIN and decreases in DIP and DSi in average concentrations of the three seasons (May, August and October, the same below); DIN/DIP increased continually, while DSi/DIN declined significantly, leading to the shift from DIN limitation to DIP or DSi limitation. Anthropogenic activities, e.g., domestic wastewater discharge and marine culture, were important factors relating to those changes in nutrients concentrations and ratios. Seasonally, compared to spring and autumn, summer showed a gentle increase in DIN and a sharp decrease in DSi, which was possibly related to the situation that more significant decline of precipitation in summer weakened the transportation of N and Si from land to inshore waters, while the gentle decrease of DIP in summer was possibly associated with more P released from sediment due to low dissolved oxygen conditions. 
    Phytoplankton abundance and chlorophyll a first decreased and then increased in average values during 1950s~2010s. Dinoflagellates and small size diatoms increased in phytoplankton composition. The increases of DIN and DIN/DIP were important factors associating with the above historical changes of phytoplankton assemblages. Seasonally, phytoplankton abundance in spring first increased and then decreased, while that in summer first decreased and then increased, inflection points of which were both observed around 1990s. More observations were necessary to verify the seasonal variations of historical changes of phytoplankton abundance. 
    This study elaborated temporal and spatial variations of nutrients and phytoplankton assemblages in the Bohai Bay. The results innovatively showed that the Bohai Bay could be partitioned into western and eastern sea areas with 15~20 m depth as the separation, which were mainly affected by terrestrial and marine factors, respectively. It suggested that the further seawater quality improvement should be conducted focusing on the western Bohai Bay and terrestrial N inputs reducing. Furthermore, this study analyzed the historical changes of nutrients and phytoplankton assemblages based on previous investigations, and found their changes exhibited seasonal variations and different influencing factors, which would provide more understandings to the evolution characteristics of the Bohai Bay under anthropogenic activities and climate change.