东海近海海域营养盐分布特征及其与赤潮发生关系的初步研究

根据2002年4月25日～5月2日的调查数据,分析了东海近海海域营养盐(NO3--N,PO4^3-P,SiO3^2-Si,NH4-N等)的分布特征,并初步探讨了营养盐分布与赤潮发生的关系．结果表明,调查海区营养盐浓度较高,与国家一类海水水质相比,无机氮和无机磷的超标率分别为46％和60％,长江口及杭州湾附近海域富营养化程度比较严重．调查海域由于受沿岸长江等河流输入的影响。营养盐浓度自近岸向外海快速递减,等值线几乎与海岸线平行．根据调查结束后赤潮发生的特点和区域,表明营养盐浓度的增加,尤其是DIN和PO4^3-P的增加,与赤潮的发生有一定关系。但本次赤潮并不是发生在营养盐浓度最高的海区,因此,富营养化并不是诱发本次赤潮发生的唯一环境因素．     

东海两种赤潮生物种间竞争的围隔实验

1998年5月和2002年5月在东海赤潮高发区进行了两次添加营养盐诱发赤潮的围隔实验,结果表明,1998年5月实验开始时,具齿原甲藻和中肋骨条藻分别占群落总量的85％和11％,对照围隔中,具齿原甲藻一直处于优势,高峰时所占比例接近99％．加磷围隔中,中肋骨条藻显示出很强的竞争能力,第五天的数量与具齿原甲藻相等．2002年5月用4个围隔装置进行实验,在不同营养状况下两个种的增殖速率不同,营养盐浓度高的围隔中,中肋骨条藻的增殖速率比具齿原甲藻快得多,在营养盐限制条件下,具齿原甲藻比中肋骨条藻存活的要好。表明在营养盐丰富情况下,中肋骨条藻以其快速的繁殖在竞争中取得优势地位,导致中肋骨条藻形成的赤潮持续时间较短,消亡也快,而具齿原甲藻赤潮形成期较长,但在营养盐很低的情况下,还能维持较长时问．     

九龙江河口区养虾塘沉积物-水界面营养盐交换通量特征

通过对九龙江河口区陆基养虾塘水样和沉积物样品采集分析及结合室内模拟实验,探讨了虾塘在不同养殖阶段沉积物-水界面营养盐通量时间变化特征及其主要影响因素。虾塘沉积物向上覆水体释放NO_x~--N(NO_2~--N和NO_3~--N)、NH_4~+-N和PO_4~(3-)-P能力均呈现随养殖时间推移而降低的特征。沉积物在养殖中期和后期分别呈现对上覆水体NO_x~--N和PO_4~(3-)-P的吸收现象,但总体表现为释放(平均通量分别为(1.87±1.15)、(1.58±0.52)mg m~(-2)h~(-1)和(1.22±0.62)mg m~(-2)h~(-1))。沉积物-水界面溶解无机氮交换以NH_4~+-N为主(沉积物平均释放通量为(46.18±13.82)mg m~(-2)h~(-1))。沉积物间隙水与上覆水间的营养盐浓度差(梯度)及温度对上述交换通量的时间动态特征具有重要调控作用。研究结果表明养殖初期或中期沉积物较高的无机氮(尤其是NO_2~--N和NH_4~+-N)释放是养殖塘水质恶化的一个极具潜力的污染内源,可能会对虾的健康生长产生负面效应,控制沉积物无机氮释放是养虾塘养殖初期和中期重要的日常管理活动之一。     

长江口地区大气湿沉降中营养盐的初步研究

为了解大气湿沉降对于赤潮发生的影响,2000年5月至2001年4月在嵊泗群岛采集了64个雨水样品,并分析了其中的N、P、Si等营养盐的浓度．结果表明,营养盐月平均浓度之间相差较大,与风向即不同的污染物来源和降水量等因素有关;营养盐季节通量分布不均,季节性明显．除冬季外,其他3个季节均能成为赤潮的诱发因子;NO3--N、NH4+-N、NO2--N、PO4^3-P和SiO3^2-Si的年均浓度依次为20．23、30．14、0．1l、0．045和3．43μmol·L-1,年通量分别为2．67×10^8、3．98×10^9、0．014×10^8、0．0059×10^8和0．45×10^8mol,与河流输入量相比,湿沉降对营养盐的年输入量较小．     

赤潮过程浮游植物与营养物质时间变化率研究

利用2000年大亚湾澳头海域赤潮定点连续调查资料及其多年现场调查资料,采用灰色回归模型,综合分析赤潮发生过程水体中浮游植物细胞密度与营养物质(NO3-、NH4+、PO4^3-、SiO3^2-、Fe)的时间变化率关系,分析了叶绿素a含量与浮游植物细胞密度相互关系．结果表明,预测值与实测基值本一致,复相关系数范围在0．51～0．83．当水体叶绿素浓度为5．8μg·dm-3,预示可能发生赤潮,通过采样分析水体叶绿素a含量或利用水色卫星遥感资料反演水体叶绿素浓度,计算浮游植物细胞密度,为赤潮的预测预报提供简便有效的方法．此外．本水域初级生产力由磷控制．     

亚热带河口陆基养虾塘水体溶解性碳浓度及沉积物-水界面碳通量时空动态特征

以福建闽江和九龙江河口陆基养虾塘为研究对象,通过野外原位观测和室内模拟培养实验,开展了河口陆基养虾塘养殖期间水体溶解性有机碳(DOC)和溶解性无机碳(DIC)及养虾塘沉积物-水界面碳交换通量变化特征的研究。结果表明:时间变化上,养虾塘水体溶解性碳浓度及沉积物-水界面碳通量在闽江河口呈现8月中旬10月中旬6月中旬的特征,在九龙江河口表现为随养殖阶段推移而增加的趋势;空间变化上,闽江河口养虾塘水体溶解性碳浓度及沉积物-水界面碳通量显著高于九龙江河口;沉积物释放溶解性碳速率与水体溶解性碳浓度呈现显著正相关关系,沉积物碳释放过程是引起养虾塘水体溶解性碳浓度时空变化的重要因素。表明河口区水产养虾塘碳循环研究时需考虑不同形态碳生物地球化学循环的时空差异性。     

阿哈水库沉积物-水界面重金属扩散通量

为了解阿哈水库水体中重金属的污染现状,选取了4个典型点位进行夏、冬两季采样分析,研究了上覆水体及沉积物孔隙水中Co、Ni、Cu、Zn、As、Cr及Pb 7种重金属的时空分布规律及其界面扩散通量。结果表明:阿哈水库上覆水体中溶解性重金属的均值含量符合我国《地表水环境质量标准》(GB 3838—2002)Ⅰ类标准与《生活饮用水卫生标准》(GB 5749—2006),未受到重金属污染。夏季分层期,底层水缺氧导致水体As、Co、Cr含量增高,而冬季混合期重金属的垂向变化较小。沉积物孔隙水中重金属浓度均值高于上覆水,且Co与Ni、As呈显著正相关,其在水库中的迁移转化过程受铁锰循环的共同影响; Cu、Zn与Pb存在正相关关系,具有相似的污染来源。分析沉积物-水界面重金属扩散通量显示,Zn保持负扩散,表明沉积物对Zn具有很好的净化吸附效果;而Co、Ni、As、Pb保持正扩散,特别是夏季As的扩散通量达2.07μg·cm~(-2)·a~(-1),对上覆水体可能产生较大的释放风险。研究证实,近十年来阿哈水库水体重金属的污染控制取得了显著的效果。     

东海赤潮高发区春季叶绿素a和初级生产力的分布特征

2002年4-5月对东海海域进行了综合调查,分析了海区叶绿素a和初级生产力的分布特性．结果表明,大面站表层平均叶绿素a浓度为1．086mg·m-3．分级叶绿素a结果显示．春季东海浮游植物以微型和微微型(<20μm)占优势,其对海区叶绿素a的贡献为64％,超微型浮游植物(<5μm)占浮游植物生物量的27％．营养盐分布和浮游动物的摄食压力影响了叶绿素a及其粒级结构的分布．平均初级生产力为10．091mg·m-3·h-1。赤潮跟踪的R-03、RL-01、RG-01站的平均初级生产力为399．984mg·m-3·h-1．光和营养盐成为叶绿素和初级生产力平面分布的主要限制因子．表层叶绿素a和初级生产力均在调查海区的123·E纵断面冲淡区产生高值区．DC-11站浮游植物生物量异常高,表层叶绿素a达到9．082mg·m-3,初级生产力为128．79mg·m-3·h-1．但并未出现水色异常．     

陆源物质输送对赤潮高发区的影响-以铝为例

近年来长江口附近海域富营养化问题日趋严重,赤潮爆发次数增多,而富含营养物质的长江水的输入可能是海内富营养化的主要物质来源．Al在海洋中停留时间较短且不易受到人为活动的影响,常用Al作为陆源输入及不同水团运动的有效示踪元素．结果表明。研究海域中溶解态Al的浓度秋季高于夏季,且与水体的盐度和悬浮颗粒物浓度表现出一定的关系．赤潮高发区内溶解态Al的浓度一般大于0．04pmol.L-1,明显表现出受到陆源输入的影响．赤潮发生时由于其自身的絮凝及赤潮生物颗粒物表面的吸附作用使水体中的溶解态Al含量平均下降约40％,而在赤潮发生过程中溶解态Al形态变化及机理尚需进一步深入研究．     

太湖春季沉积物间隙水中磷的分布特征及界面释放的影响

通过对太湖春季不同湖区水体和沉积物间隙水中磷的分布特征研究,探讨了间隙水中磷的释放对上覆水环境的影响。结果表明,颗粒态磷(PP)和溶解态有机磷(DOP)是太湖水体中主要的磷形态,占总磷的58%～95%。不同湖区沉积物间隙水磷的剖面变化可能与生态特征及水动力引起的沉积物-水界面的扰动强度密切相关。湖心和西部沿岸沉积物扰动强烈,致使间隙水中磷含量向上逐渐降底,而梅梁湾和贡湖间隙水磷的垂向变化不大。东太湖和竺山湖沉积物界面间隙水中磷含量偏高,可能是由于表层沉积物的有氧环境使Fe2+被氧化固定下来,并促进了总磷(TP)和溶解态活性磷(DRP)的扩散释放。总体而言,间隙水中各形态磷具有向上覆水体释放的趋势,其中DRP的扩散潜力最大,竺山湖沉积物-水界面DRP扩散通量高达11.42mg.m-2.d-1,表明春季浮游植物的复苏生长对DRP的迫切需求。     

长江口锋面附近咸淡水混合对浮游植物生长影响的现场培养

通过2009年6月调查航次,获得了营养盐等参数断面分布,表明咸淡水混合是控制营养盐分布的主要因素。为了解不同盐度梯度下浮游植物生长与营养盐吸收的关系,采集两个站位水样分别代表长江冲淡水(C1站)和外海水(I10站),按C1站水样比例,分100%、75%、50%、25%、0%不同比例混合进行现场模拟咸淡水混合培养,有以下认识:(1)平行结果表明培养过程中活体荧光极大值在100%混合组,且淡水比例越低,指数生长期0—48 h内生长速率越低,100%、75%、50%、25%组分别为1.18/d、1.12/d、1.14/d、0.77/d;(2)低于26盐度的水体中PO34-在48 h内可被迅速消耗而产生限制作用,是控制浮游植物生长的潜在限制因子;(3)除0%组外,各混合组DIN/P(DIN:溶解无机氮,Dissolved Inorganic Nitrogen,DIN=NO3-+NO-2+NH4+)比值在浮游植物指数生长期有升高趋势,100%组DIN/P比值增加了一倍。各组培养48 h后DIN/Si比值逐渐降低至原来的0.7左右,初始DIN/Si小于一定时间内硅藻吸收的(ΔDIN/ΔSi)比,是造成各组DIN/Si比值减小的原因。以上结果表明咸淡水混合过程中形成的营养盐梯度可造成浮游植物生长程度和速率差异,且可因局部浮游植物旺发而改变海水营养结构。     

胶州湾夏秋季大气湿沉降中的营养盐及其入海的生态效应

为揭示大气湿沉降对胶州湾营养盐的输送通量及其生态效应,分别于2015年6—8月(夏季)、9—11月(秋季)采集胶州湾降水样品,测定了降水中不同形态N、P、Si的浓度。结果表明,降水中不同形态营养盐的浓度变化较大,且均与降水量呈负相关关系,其中NH4-N和NO3-N的浓度较高,溶解有机氮(DON)占溶解态总氮(DTN)含量的25.9%,而NO_2-N,PO_4-P和SiO_3-Si的浓度均很低。溶解无机氮(DIN)、DON、PO_4-P以及SiO_3-Si的湿沉降通量分别为141.7、61.87、0.35 mmol m~(-2)a~(-1)和0.12 mmol m~(-2)a~(-1)。受降水量和营养物质来源制约,各项营养盐湿沉降通量时间变化显著。农业活动导致的无机氮排放构成了胶州湾湿沉降DIN的主要来源。大气湿沉降DIN、DON、PO_4-P和SiO_3-Si分别占胶州湾总输入负荷的9.04%、10.24%、0.57%和0.17%,湿沉降输入的PO_4-P在夏、秋季分别可以支持0.575 mgC m~2d~(-1)和1.42 mg C m~2d~(-1)的新生产力;雨水中DIN/P比值高达1 617,突发性强降雨带来的营养盐输入会加剧表层水体的P限制和Si限制,对胶州湾浮游植物群落结构和粒级结构产生重要影响。大气湿沉降是胶州湾生源要素生物地球化学过程的重要一环,对营养物质收支的贡献及可能引发的生态效应不容忽视。     

秦皇岛海域褐潮生消过程中营养盐特征

2009—2015年在秦皇岛海域发生的褐潮给当地海水养殖业和滨海旅游业造成巨大损失,也对海洋生态系统造成破坏性影响。营养盐是藻类生长的重要生源要素,研究其在褐潮生消过程中的变化特征,对于揭示褐潮发生的营养学机制具有重要意义。基于2014年4—6月在秦皇岛褐潮多发海域30个站位的调查数据,对褐潮发生前后营养盐特征及其与抑食金球藻的关系进行了研究。结果表明: 4、5、6月溶解态氮(DN)浓度分别为265.65、355.36和323.71 μg·L^-1,其中,溶解态有机氮(DON)浓度分别为196.98、242.88和177.69 μg·L^-1,在DN中的占比分别为74.2%、68.3%和54.9%;4、5、6月溶解态磷(DP)浓度分别为15.95、11.39和11.14 μg·L^-1,4、5月PO₄^3--P在DP中占比较大,分别为74.8%和80.9%,6月溶解态有机磷(DOP)占比升至66.2%,PO₄^3--P占比降至33.8%;4、5、6月SiO₃^2--Si浓度分别为70.95、181.13和120.68 μg·L^-1。除DON和5月无机氮(DIN)外,其他营养盐浓度的平面分布均整体呈近岸高、离岸低的趋势,高值区多出现在河口。通过R型-因子分析和营养盐结构分析发现,4月,褐潮处于发展阶段,DOP可能是抑食金球藻生长的主要控制因子;5月,褐潮处于维持阶段,水温成为主要控制因子,水温大于12 ℃即可发生褐潮;6月,褐潮开始消亡,PO₄^3--P对浮游植物群落结构具有更大的影响力。DON为诱发褐潮爆发的关键水质因子,其阈值浓度为 150 μg·L^-1,且DON/DIN值应大于1。     

Temporal variations in microbenthic metabolism and inorganic nitrogen fluxes in sandy and muddy sediments of a tidally dominated bay in the northern Wadden Sea

Factors controlling seasonal variations in benthic metabolism (O₂ flux) and dissolved inorganic nitrogen (DIN) fluxes were examined during a 12–14 month period at three intertidal Wadden Sea stations. Since the flux measurements were made as small-scale laboratory core incubations, the results are primarily related to the microbenthic community (microalgae, bacteria, micro-, meio- and small macrofauna) and cannot be considered representative of the total benthic community in the Wadden Sea. Furthermore, it has to be emphasized that light intensity during day-time simulations were constant and saturating at all times. Benthic primary production and oxygen uptake appeared to be temperature dependent with a ‘seasonal Q₁₀’ of 1.7–1.8 and 2.7–4.3, respectively. Inundation had no effect on oxygen fluxes as evidenced by similar sediment respiration with and without water cover. A stronger temperature dependence of primary production in muddy than in sandy sediment indicated that the overall control in the latter may be complex due to factors like macrofaunal grazing and nutrient availability. Benthic respiration may not be controlled by temperature alone, as sedimentary organic matter content correlated significantly with both temperature and benthic respiration. Annual gross primary production in high intertidal sandy sediment was 10 and 50% higher than in low intertidal sandy and muddy sediments, respectively. Since annual benthic community respiration was 2 times higher in muddy than sandy sediments, the annual net primary production was about 0 in the former and 17–19 mol C m^?2 yr^?1 in the latter. However, heterotrophic contribution by larger faunal components as well as removal of organic carbon by waves and tidal currents, which are not included here, may balance the budget at the sandy stations. There was no or only weak relationships between (light and dark) DIN exchange and factors like temperature, sedimentary organic content, and oxygen fluxes. Factors related to nutrient fluxes, such as denitrification and nutrient concentration in the overlying water, may have hampered any such relationships. In fact, DIN fluxes at all three stations appeared to be strongly controlled by DIN concentrations in the overlying water. On an annual basis, the sediment appeared to be a net sink for DIN.     

Carbon and Nitrogen Cycling in a Shallow Productive Sub-Tropical Coastal Embayment (Western Moreton Bay, Australia): The Importance of Pelagic–Benthic Coupling

Climatic variables, water quality, benthic fluxes, sediment properties, and infauna were measured six times over an annual cycle in a shallow sub-tropical embayment to characterize carbon and nutrient cycling, and elucidate the role of pelagic–benthic coupling. Organic carbon (OC) inputs to the bay are dominated by phytoplankton (mean 74%), followed by catchment inputs (15%), and benthic microalgae (BMA; 9%). The importance of catchment inputs was highly variable and dependent on antecedent rainfall, with significant storage of allochthonous OC in sediments following high flow events and remineralization of this material supporting productivity during the subsequent period. Outputs were dominated by benthic mineralization (mean 59% of total inputs), followed by pelagic mineralization (16%), burial (1%), and assimilation in macrofaunal biomass (2%). The net ecosystem metabolism (NEM = production minus respiration) varied between ?4 and 33% (mean 9%) of total primary production, whereas the productivity/respiration (p/r) ranged between 0.96 and 1.5 (mean 1.13). Up to 100% of the NEM is potentially removed via the demersal detritivore pathway. Dissolved inorganic nitrogen (DIN) inputs from the catchment contributed less than 1% of the total phytoplankton demand, implicating internal DIN recycling (pelagic 23% and benthic 19%) and potentially benthic dissolved organic nitrogen (DON) fluxes (27%) or N fixation (up to 47%) as important processes sustaining productivity. Although phytoplankton dominated OC inputs in this system, BMA exerted strong seasonal controls over benthic DIN fluxes, limiting pelagic productivity when mixing/photic depth approached 1.3. The results of this study suggest low DIN:TOC and net autotrophic NEM may be a significant feature of shallow sub-tropical systems where the mixing/photic depth is consistently less than 4.     

Spatio-temporal variability in benthic mineralization processes in the eastern English Channel

The effect of phytodetritus derived from Phaeocystis sp. bloom on benthic mineralization processes has been determined at four intertidal stations along the French coast of the eastern English Channel. Sites were chosen to offer a diversity of sediment types, from permeable sandy beach to estuarine mudflats. Sediment Oxygen Demand (SOD) as well as total fluxes of Dissolved Inorganic Nitrogen (DIN) at the sediment–water interface were determined by using whole core incubation technique and diffusive fluxes were predicted from interstitial water concentrations. In the absence of phytodetritus deposits, a marked gradient of granulometric characteristics and organic matter contents were observed, and resulted in more intensive mineralization processes in muddy sediments. Highly significant correlations (P < 0.05) were evidenced between SOD and porosity, bacterial biomass, Organic Carbon and Organic Nitrogen, evidencing the direct link between sediment texture, organic matter accumulation and microbial activity. The spring bloom led to a massive input of organic matter in surficial sediments and mineralization rates significantly increased while higher DIN release towards the water column was observed. A modification of the mineralization pathways was evidenced but clearly depended on the sediment type. With a global view, benthic mineralization processes in the intertidal zone provided significant a part of DIN inputs in the coastal zone while water column was depleted in nutrients.     

Degrading seagrass (<Emphasis Type="Italic">Posidonia oceanica</Emphasis>) ecosystems: a source of dissolved matter in the Mediterranean

Diurnal variation of dissolved oxygen (DO), organic and inorganic carbon (DOC, DIC), nitrogen (DON, DIN), and phosphorus (DOP, DIP) flux across the sediment–water interface was assessed in fish farm impacted and pristine seagrass (Posidonia oceanica) meadows in the Aegean Sea (Greece). DIC consumption decreased by 52% and DO production decreased by 60% in the light, suggesting reduced photosynthetic performance of the plant community under the fish cages probably due to organic matter loading. In light there was 4 and 15 times higher release of dissolved inorganic and organic matter, respectively, compared to dark incubations under the cages, indicating that fish farming impact is more intense during daytime. DO was taken up, while DIC was released in the dark in both stations, representing a direct measure of mineralization. Dissolved inorganic matter flux (as the sum of DIN and DIP fluxes) was positively related to DIC flux, rendering mineralization as the main driver of nutrient flux under the cages. On average, the impacted meadow released DIN and DIP both in light and dark, while efflux of dissolved organic matter (as the sum of DOC, DON, and DOP fluxes) increased by 132% in the light and by 21% in the dark, implying that the degrading seagrass meadow is a source of dissolved matter to the surrounding water. Shoot density and leaf production were negatively correlated with both diel DIN and DIP fluxes, showing that meadow regression is accompanied by DIN and DIP release from the sediment. Hence, nutrient efflux can adequately illustrate meadow deterioration and, therefore, can be used as indicator of P. oceanica community health.     

胶州湾生源要素的大气沉降及其生态效应研究进展

作为受人类活动影响显著的典型半封闭海湾,胶州湾生源要素的大气沉降研究在揭示人为污染物质排放对海湾生态系统影响方面具有典型性.从大气干/湿沉降生源要素的研究方法、通量及其影响因素、入海生态效应3个方面入手,系统总结了胶州湾生源要素大气干、湿沉降及其生态效应的研究进展:1)与国内外有关海湾、河口和边缘海相比,胶州湾大气干、湿沉降氮的浓度和通量都较高,溶解无机氮(DIN)是其主要组分,溶解有机氮(DON)占总溶解态氮(TDN)含量的22%～31%,而P和Si的浓度和沉降通量都很低.2) (NO₃^--N+NO₂^--N)的大气沉降量比陆源输入量略高,而NH₄⁺-N、PO₄^3--P以及SiO₃^2--Si的大气沉降量所占比例很低.3)大气总悬浮颗粒物(TSP)浓度、排放源强度、降水量以及气象条件是影响胶州湾大气沉降生源要素的主要因素.4)大气沉降的生源要素可促进胶州湾初级生产力的提高和改变表层海水的营养盐结构,进而导致浮游植物群落结构的改变和优势种由硅藻向甲藻的演替,从而对胶州湾生态系统产生重要影响.对今后的研究提出建议:1)建设胶州湾大气干/湿沉降监测网;2)精确量化不同形态和粒径生源要素颗粒的干沉降速率;3)量化大气沉降生源要素入海的生态效应,并深入剖析其生物地球化学机制;4)甄别大气沉降生源要素通过间接方式入海的机制、通量及影响因素.深入研究胶州湾生源要素的大气沉降,对阐明人类活动对海湾生态系统的影响有积极意义,也有助于加深对陆架边缘海生源要素生物地球化学循环过程的理解.     

Increased nutrient loads from the Changjiang (Yangtze) River have led to increased Harmful Algal Blooms

Based on observations collected during 15 cruises from 2002 to 2007 and on historical data, annual and seasonal variations in dissolved inorganic nutrients in the Changjiang (Yangtze) River estuary and the adjacent area were investigated. The importance of nutrients and nutrient fluxes to Harmful Algal Blooms (HABs) in the East China Sea (ECS) was also studied. The results showed an increasing trend in dissolved inorganic nitrogen (DIN) over the past fifty years. The changes in the PO₄-P concentration fluctuated from 1959 to 2000, but the level has been almost stable since then. By contrast, the SiO₃-Si concentrations decreased significantly over the past fifty years. Regarding seasonal variations, nutrients usually exhibited high values during autumn and winter, and the lowest values occurred in mid-summer. A strong positive relationship was observed between the DIN and PO₄-P concentrations and the frequency or scale of HABs, indicating that eutrophication played a crucial role in the occurrence of HABs. As for nutrient fluxes, DIN and PO₄-P fluxes have climbed sharply since 1980. In the ECS, the DIN increased sharply, but PO₄-P concentrations only changed slightly. The increased nutrient discharges, especially DIN, could also have caused the strong increase in the frequency and area of HABs. The areas of hypoxic bottom water off the Changjiang River estuary presented an increasing trend over the past fifty years, which may also be attributable to eutrophication. Changes in the nutrient composition were also investigated in the present study. The N/P ratio increased dramatically over the past fifty years, while the Si/N ratio decreased. The percentage of diatoms decreased from 99% to 73%, while dinoflagellates increased from less than 1% to over 25% during the past fifty years. Therefore, long-term changes in nutrients, especially excess DIN and increasing N/P, could be responsible for the shift in the phytoplankton community structure.