南亚热带大型贫营养水库浮游植物群落结构与季节变化——以新丰江水库为例

新丰江水库是我国第四大的水库，也是广东省最大的水库和重要的水源地。于2004～2005年2月一次调查了新丰江水库水文、水质和浮游植物分布，分析了浮游植物群落季节动态特征。新丰江水库浮游植物生物量比较低，在0．037—1．497mg·L^-1之间变化。浮游植物种类较多，11次采样共检到158种。在丰度上，水库浮游植物主要以小环藻、蓝纤维藻、小球藻和纤维藻等优势种为主，而在生物量上则以微小多甲藻为优势。浮游植物组成随季节变化而不同，春季以硅藻、甲藻和绿藻为优势类群；夏季以蓝藻、绿藻和硅藻为优势类群；秋季蓝、绿藻减少而硅藻和甲藻增加。2004年的浮游植物季节性变化更为明显，有从硅藻-绿藻优势（2月和4月份），到蓝藻-绿藻优势（6月和8月份），到混合优势（10月份）和金藻优势（12月份）这样一个变化过程。2005年硅藻的相对丰度比2004年高出很多。两年浮游植物组成的差异与两年的降水量有关。水动力学对丰水期（6～8月份）浮游植物组成结构有较大影响，导致硅藻和绿藻相对丰度的增加。与温带贫营养型水库相比，新丰江水库的浮游植物群落具有春季和秋季种类多、夏季的蓝藻种类丰富的特点。从细胞大小分布上看，小于20μm浮游植物是生物量的主要贡献者，其次是大于45μm的浮游植物。在粒径小于20μm的浮游植物中，微小多甲藻是最主要的贡献者。浮游植物群落的大小分布受水动力学条件和营养盐浓度动态的影响。     

三峡水库坝前水域浮游植物群落时空动态研究

于2012—2013年对三峡水库坝前水域浮游植物组成、优势种、密度、生物量及多样性进行了周年季度调查,共鉴定浮游植物151种属,其中绿藻门71种,硅藻门47种,蓝藻门20种,隐藻门和甲藻门各4种,裸藻门和金藻门各2种,黄藻门1种。浮游植物优势种的季节更替明显,夏季和秋季优势种类为硅藻和绿藻,冬季为硅藻、蓝藻和绿藻,春季为绿藻、硅藻和隐藻。浮游植物年均密度和生物量分别为3.95×106 ind./L和4.078 mg/L,空间差异表现为支流和库湾远高于干流,季节动态表现为夏季最高,春季和冬季次之,秋季最低。三峡水库坝前水域浮游植物多样性指数偏低,水体污染类型属于α-中污染。研究为客观了解三峡水库坝前水域浮游植物群落的空间分布与季节变化特征提供一定的参考依据。     

稻鱼共作生态系统浮游植物群落结构和生物多样性

稻鱼共作技术是中国传统农业的精华,通过对稻鱼共作系统水体浮游植物的种类组成、密度、生物量及多样性进行分析,明确该系统中浮游植物数量变化特性,为进一步开发利用这一经典农艺提供理论基础和实践依据。结果表明,稻田生态系统中浮游植物群落包含蓝藻门、甲藻门、隐藻门、裸藻门、绿藻门和硅藻门等6门,共38属93种。稻鱼共作显著增加稻田水体浮游植物的密度和生物量,降低硅藻和蓝藻的优势度,增加绿藻和裸藻的优势度,提高了稻田水体浮游植物多样性指数。     

鹤地水库浮游植物群落的结构与动态

鹤地水库位于雷州半岛北部(21°42'～22°22'N,109°54'～110°25'E),是一座中营养化的大型水库.为了研究其浮游植物群落的结构与变化特点,在水库设置5个采样点,并于2003年2、7、9、12月对其采样.鹤地水库浮游植物生物量变化为O.156～2.548 mg L~(-1),主要由蓝藻和硅藻组成.5个采样点的浮游植物生物量具有明显的季节变化,且变化趋势相同,即丰水期的生物量高于枯水期,主要是由于丰水期水温较高以及入库河水带入的营养盐.5个采样点的浮游植物生物量从主要入库河流至大坝区呈下降趋势,与磷浓度的降低直接相关.浮游植物优势种主要以热带代表性种类为主,且有明显的季节变化,枯水期主要为硅藻的根管藻(Rhizosolenia sp.)、小环藻(Cyclotella sp.)、颗粒直链藻(Melosira granulata)以及模糊直链藻(Melosira ambigua)等.丰水期为蓝藻的拟柱孢藻(Cylindrospermopsis raciborskii)、湖泊假鱼腥藻(Pseudanabaena limnetica)等,浮游植物优势种类的变化主要受磷浓度的影响.浮游植物前8个优势种的生物量占浮游植物群落生物量的850%～92%,显著低于温带地区浮游植物群落结构稳定的湖泊.     

安徽沱湖夏季浮游植物群落结构特征与环境因子关系

为了揭示沱湖浮游植物群落结构特征及其与水环境因子的关系,于2016年7月(夏季),对沱湖流域上游至下游11个采样点浮游植物种类组成、细胞丰度、生物量等进行调查研究。结果显示,沱湖共有浮游植物96种(含变种),隶属8门48属,其中绿藻门(Chlorophyta)和硅藻门(Bacillariophyta)种类最多,绿藻门有23属39种,占总种数的40.63%,硅藻门有7属20种,占总种数的20.83%;其次为裸藻门(Euglenophyta),有5属17种,占总种数的17.71%,蓝藻门(Cyanophyta) 8属14种,占14.58%;甲藻门(Pyrrophyta) 2属2种,隐藻门(Cryptophyta) 1属2种,各占总种数的2.08%,黄藻门(Xanthophyta)与金藻门(Chrysophyta)均有1属1种,均占总种数的1.04%。绿藻和硅藻类物种在沱湖浮游植物群落结构中处于优势地位,沱湖夏季浮游植物种类组成表现为绿藻-硅藻型。沱湖夏季浮游植物细胞丰度与生物量从上游到下游呈逐渐增加趋势,细胞丰度与生物量平均值分别为4.022×106cells/L与3.046 mg/L,蓝藻门和绿藻门类群为沱湖浮游植物细胞丰度主体,硅藻门和裸藻门类群为沱湖浮游植物生物量的主体;上游浮游植物多样性指数与均匀度指数均略高于下游采样点,沱湖水质呈β中污型-无污染型,上游水质优于下游水质。浮游植物群落结构与水环境因子的典型对应分析(CCA)结果表明,电导率、透明度、水深及pH值等环境因子与沱湖夏季浮游植物群落结构有较强的相关性。     

上海公园水体夏季浮游植物群落与环境因子的关系

为了解公园水体浮游植物群落状况,分析浮游植物物种分布和环境因子之间的关系,揭示浮游植物物种对生态环境的需求,于2008年7月和9月对上海市11个公园水体浮游植物群落进行了调查,对获得的浮游植物数据和环境因子数据进行典范相关分析(CCA),并绘制了物种与环境因子关系的二维排序图。结果表明：调查期间共鉴定出浮游植物384种,隶属于8门,浮游植物密度范围为2.01×10⁵～57.60×10⁵ cells·L^-1;群落组成以蓝藻、裸藻、硅藻和绿藻为主,主要优势种有细微颤藻、无常蓝纤维藻、尾裸藻、颗粒直链藻、梅尼小环藻、普通小球藻、四尾栅藻等;7月影响浮游植物分布的主要环境因子依次为铵态氮、溶解氧、水温和总磷,而9月的pH值、水温、溶解氧、透明度和总氮含量对浮游植物的分布产生影响较大;其中,透明度和浮游动物量是影响隐藻、甲藻和硅藻藻类生物量的主要环境因子,而蓝藻、裸藻、绿藻主要受水体氮磷营养盐浓度和溶解氧的影响。     

淀山湖浮游植物群落特征及其演替规律

为探明淀山湖浮游植物群落结构演变与富营养化之间的关系,于2004-2006年对上海市最大天然淡水湖泊淀山湖的浮游植物进行逐月采样调查,分析其群落结构特征.共采集到淀山湖浮游植物84属205种,主要由绿藻(种类数占50%)、硅藻(20%)、蓝藻(13%)、裸藻(13%)等组成.相邻两月之间种类相似性系数呈现冬春季高、夏秋季低的趋势:优势种为银灰平裂藻(Merismopedia glauca)、小席藻(Phormidium tenus)、铜绿微囊藻(Microcystis aeruginosa)、具缘微囊藻(M.marginata)、湖泊鞘丝藻(Lyngbya limnetica)、微小色球藻(Chroococcus minutus),颗粒直链藻最窄变种(Melosira granulata var.angustissima )、啮蚀隐藻(Cryptomonas erosa)、小球藻(Chlorella vulgate)和四尾栅藻(Scenedesmus quadricauda)等.浮游植物群落细胞数量主要由蓝藻(42.73%)、绿藻(37.75%)、硅藻(12.67%)和隐藻(6.06%)组成;生物量主要由硅藻(36.75%)、蓝藻(16.78%)、绿藻(16.36%)和隐藻03.53%)等组成.淀山湖浮游植物群落结构季节演替模式不同于PEG(Plankton Ecology Group)模型,其中蓝藻从春末开始大量出现,夏季大量繁殖,一直延续到秋初.综合文献资料看出,淀山湖浮游植物群落已从1959年的硅藻一金藻型、1987-1988年的隐藻-硅藻型演变为2004-2006年的蓝藻-绿藻型;数量由1959年的103 ind./L上升至2004-2006年的1.11×107 cells/L.演替的总体趋势表现为:贫中营养型的金藻、甲藻比例下降,富营养型的蓝藻、隐藻和微型绿藻增加.浮游植物数量和群落结构的演变指示了淀山湖水体的富营养化进程.     

东小磨虾池及其沿岸浮游植物群落物种多样性

１９９１年６～９月东小磨虾池及其沿岸浮游植物群落中出现过的浮游植物门类有硅藻、甲藻、蓝藻、绿藻、金藻和裸藻等,种类在９４种以上,以硅藻为主,达５９种。其中国内以前未记录过的海洋种类４种。虾池与沿岸的浮游植物群落物种多样性之间存在着一些差异。ＳｈａｎｎｏｎＷｅａｎｅｒ多样性指数的范围在０．１２～３．１７     

珠三角河网水域栅藻的时空分布特征

珠三角河网水域是珠江之水流入南海的必经之地,对2012年该水域浮游植物群落中的栅藻种类丰富度和生物量的时空特征及其环境影响因子进行系统阐析。调查期间共发现栅藻24种(包括变种、变型),其中具刺和无刺种类分别为11种和13种。出现率高且生物量贡献率大的物种仅3种,包括被甲栅藻原变种(Scenedesmus armatus var.armatus)、被甲栅藻博格变种双尾变型(S.armatus var.boglariensis f.bicaudatus)和二形栅藻(S.dimorphus),分析认为这3个物种在栅藻种群中的生态优势很可能与其形态特征存在一定的关联。栅藻种类丰富度的季节分布呈现丰水期高于枯水期的特征,空间分布呈现河网中部站位低于外侧站位的特征。此外,种类丰富度的空间分布格局存在季节差异,丰水季节呈现河网中部站位明显低于外侧站位的特征,而枯水季节西江沿线站位种类丰富度明显偏低。分析认为,水温和径流是影响栅藻种类丰富度季节变动的关键因素;同时,径流也是影响总种类丰富度空间分布格局的关键因素,而水温和营养盐是影响不同季节种类丰富度的空间格局的重要因素。栅藻种群生物量的季节分布呈现丰水期高于枯水期的特征,空间分布表现为珠江桥站位的生物量明显高于其它站位。分析认为,径流和水温是决定种群生物量季节变动的关键因素,而营养盐、水体稳定性和盐度是决定生物量空间分布格局的关键因素。栅藻生物量与种类丰富度的关系基本符合单峰模型,且偏重于单峰模型的上升区,丰富了全球多样性模式的分级特征和区域特征。     

博斯腾湖浮游植物群落结构特征及其影响因子分析

2011年对博斯腾湖大湖区17个采样站位的浮游植物及水体主要理化因子进行了4次系统调查。结果表明,在17个站位共鉴定出浮游植物127种(属),其中优势种(属)9种。浮游植物群落全年均以硅藻为主导,冬、春季节,浮游植物组成呈硅藻-甲藻型,优势类群主要为贫-中营养型浮游藻类,到夏、秋季节逐渐形成硅藻-绿藻型,以富营养型的浮游藻类为优势类群。浮游植物总平均生物量为(2.51±2.95)mg/L,生物量季节变动显著,峰值出现在夏季,冬季最低。基于Canoco的多变量分析表明:环境变量共解释了浮游植物群落总变异的54.5%,水温是影响浮游植物分布最重要的环境因子,其次为枝角类丰度。水中氮含量是影响浮游植物丰度的主要因子,同时浮游植物对水体有机物含量也有较大的影响。     

Spatial and seasonal changes of net plankton and zoobenthos in Lake Tonle Sap, Cambodia

To clarify spatial and seasonal differences in net plankton and zoobenthos in Lake Tonle Sap, Cambodia, quantitative surveys were carried out at 14 stations in the north and south basins in high- and low-water seasons during 2003–2005. In the phytoplankton communities, a diatom Aulacoseira granulata dominated throughout the lake in the high-water seasons, while blue-green algae, mostly composed of Microcystis, surpassed other algae in the low-water season when the lake water was very turbid and the Secchi disk readings were only a few centimeters. In the low-water seasons, a bloom of floating blue-green algae occurred everywhere, especially prominent in the coastal areas. Protozoans and rotifers dominated the zooplankton communities. In the open-water stations, diversity was higher in high-water seasons in phytoplankton, while it was not significantly different between seasons in zooplankton. Composition of plankton communities in Lake Tonle Sap appears to have changed little since the 1950s, at least in phytoplankton, while the phytoplankton density appears to be higher in the present study. Among the macrozoobenthos, mollusks, oligochaetes and chironomids dominated in density, and mollusks exceeded others in biomass in both basins and seasons. The total densities of macrozobenthos were not high, being fewer than 1,300 m⁻² throughout the stations and seasons. Possible reasons for the low zoobenthos abundance in the lake may include high predation pressures by benthivorous fish or unfavorable unstable and flocculant substrates.     

Downstream changes in phytoplankton composition and biomass in a lowland river–lake system (Warnow River, Germany)

To determine longitudinal changes in phytoplankton composition and biomass in the Warnow River (Germany), single water parcels were followed during their downstream transport in August and October 1996 and April 1997. In summer, the phytoplankton assemblage was dominated by centric diatom and cyanobacteria species. Stephanodiscus hantzschii, Pseudanabaena limnetica, Planktothrix agardhii and Aulacoseira granulata var. angustissima were the most frequent species. In autumn, small centric diatoms dominated the whole river course. Irrespective of the season, in the fluvial lakes of the upper river, a substantial increase of phytoplankton biomass was observed. Shallow upstream river stretches were associated with large biomass losses. In the deep, slow flowing lower regions, total biomass remained constant. Longitudinal changes in biomass reflected downstream variations in flow velocity and river morphology. Cyanobacteria, cryptophytes and diatom species were subjected to large biomass losses along fast flowing, shallow river sections, whereas chlorophytes were favoured. Diatoms and cryptophytes benefited from low flow velocity and increased water depth in the downstream river. Changes in water depth and flow velocity have been found as key factors that cause the longitudinal differences in phytoplankton composition and biomass in small rivers.     

Monitoring of algae in Dutch rivers: does it meet its goals?

Observations on phytoplankton in the lower reaches of the rivers Rhine and Meuse were carried out in the framework of the national monitoring programme of the main water systems in The Netherlands. Seasonal changes in density and species composition were analysed to detect the major variables and to discuss whether this monitoring meets its goals. Phytoplankton reached peak densities of 140 and 65 μg L^-1 Chl a in the Rhine and Meuse, respectively. Overall density was correlated with seasonal variation in water discharge rather than with nutrient concentrations, that are high in both rivers. The position of sampling sites in relation to the downstream development of the plankton was very important, especially in the regulated Meuse. Despite hydrographic and chemical differences between the two rivers, many species, predominantly diatoms and green algae, were shared. The occurrence of the diatom Skeletonema subsalsum in the Rhine and the green algae Neodesmus danubialis, Micractinium pusillum and Pseudotetrastrum punctatum in the Meuse (1992, but not 1996) was interpreted as a feature related, respectively, to the high salinity of the Rhine and specific riverine conditions of the Meuse. In general the potamoplankton was characterised as an opportunistic assemblage exploiting the high nutrient contents and disturbed hydrography of both rivers. Nevertheless, the phytoplankton contributed to the characterisation of the present river communities, ones that differ rather strongly in historic records. Although year to year variation in phytoplankton density is mainly related to variation in water discharge, phytoplankton biomass did increase as a result of eutrophication over a period of decades. The widespread nature of many of today's potamoplankton species in the two rivers render these organisms less useful for indicating short-term changes in water quality. However the importance of phytoplankton in trophic relationships merits its inclusion in the monitoring of riverine ecosystems. This revised version was published online in June 2006 with corrections to the Cover Date.     

滇池北部湖区浮游植物时空格局及相关环境因子

为研究滇池北部湖区浮游植物的时空格局,探讨影响格局形成的关键影响因子,2008年12月至2009年6月,在滇池北部湖区选择6个点位,共开展了24次采样,调查并分析了浮游植物群落及其相关理化参数。通过比对滇池全湖周年监测数据,研究发现,北部湖区叶绿素a浓度从12月中旬开始下降,2月中旬降至最低值26.5μg/L,5月中旬达到最大值530.5μg/L。调查期间共鉴定出浮游植物74属97种,其中绿藻53种,蓝藻20种,硅藻17种,隐藻2种,金藻2种,甲藻1种,裸藻2种。蓝藻生物量在12月、3月至6月期间占优势,12月份优势种为绿色微囊藻(Microcystis viridis),3月至5月为水华束丝藻(Aphanizomenonflos-aquae),6月为惠氏微囊藻(Microcystis wesenbergii)。硅藻在1月占优势,优势种为颗粒直链藻最窄变种(Melosira granulata var.angustissima)。研究结果表明,水温高于14℃以上,微囊藻生物量迅速增加;束丝藻生物量随温度增加而增加,高于22℃时迅速减少。北部湖区6个点位浮游植物的空间差异显著,D1、D4号点微囊藻、束丝藻生物量显著高于其他四个点(P<0.01),D2、D5号点广缘小环藻生物量显著高于其他四个点(P<0.01)。研究结果显示,在富营养化严重的滇池北部湖区,水温可能为影响浮游植物种类组成及生物量的关键因子,浮游植物水平分布的差异主要由气象因素和水体营养盐共同决定。     

Meroplankton dynamics in a saline,turbulent, turbid shallow lake (Neusiedlersee,Austria and Hungary)

Phytoplankton species composition, horizontal distribution, seasonal- and long-term dynamics are investigated in relation to some environmental factors, based on about 700 samples taken between 1968 and 1992 in the shallow, turbid, turbulent, saline Neusiedlersee (Austria/Hungary). We deduced:

1. The phytoplankton is relatively poor in species which is attributable to high salinity and turbidity. Blue-green algae of picoalgal size, meroplanktonic diatoms, green algae either with gelée or elongated form are the most important groups in the plankton;
2. No consequent horizontal differences in the distribution of phytoplankton biomass were found; the high contribution of diatoms (69%) and green algae (22%) is characteristic;
3. Despite the low percentage contribution (0.54%) of phytoplankton dry weight in total seston, the two variables correlate closely because of simultaneous resuspension after wind actions. The prevalence of both low phytoplankton/seston ratio and significant correlation between these two variables is indicative to the presence of a meroplankton;
4. Phytoplankton biomass, especially that of diatoms, varies within wide limits in the short term as a combined effect of growth/loss, transport by horizontal water currents and periodic resuspension from the sediment surface. Diatoms like Fragilaria, Surirella and Campylodiscus are involved especially. Parameters of population dynamics (growth rate, annual cycles, length of stationary phases, standing crop) were estimated based on moving averages. Growth rates were smaller than those of ‘normal’ planktonic species; however, the similarities of seasonal pattern suggest that the carrying capacity of the lake is very constant;
5. Picoalgal biomass is very high in the lake; its contribution to total biomass can exceed 75%, especially in spring. The large cellular chlorophyll a content of Neusiedlersee's phytoplankton can most probably be explained by this high picoalgal standing crop, which is not included in the routine biomass estimations;
6. Concerning long-term changes of phytoplankton, periodic appearances and disappearances are very characteristic. Annual average biomass of most species is growing for several (3–5) years, then declining with a similar rate and these periods are recurrent. Climatic drying-out periods, during which water level lowers, conductivity increases, nutrients and their ratios change following trend-like periodicities, and they are supposed to be responsible for the observed longterm periodicity of dominant species. An increased nutrient load in the seventies was superimposed on this cyclicity;
7. Most of the species can be found in smaller or larger amounts in the littoral microhabitats (canals, inner ponds) of the lake in periods when they are absent or very rare in the open water. Thus, these littoral microhabitats play a very important role in the survival and recruitment of planktonic populations to the open water.

     

背角无齿蚌(Anodonta woodiana)对浅水系统底栖和浮游藻类竞争关系的影响

底栖藻类和浮游藻类之间的竞争关系对浅水生态系统的结构、功能具有重要的影响,双壳类可通过滤食控制浮游藻类,从而改变底栖藻类与浮游藻类之间的竞争结果。论文通过比较放养背角无齿蚌(Anodonta woodiana)(蚌处理组)与不放养背角无齿蚌(对照组)系统中底栖藻类、浮游藻类的生物量和优势种等的变化,研究了滤食性双壳类对底栖藻类和浮游藻类间竞争的影响。结果表明,背角无齿蚌可显著降低浮游藻类生物量,提高水体透明度和沉积物表面光照条件,从而显著提高底栖藻类的生物量;背角无齿蚌也改变了浮游藻类的优势种,使优势种由蓝藻转变成硅藻。因此,滤食性双壳类有利于促进浅水生态系统从混水态向清水态转变,本研究结果对富营养化浅水湖泊修复与管理具有一定的参考意义。     

The influence of macrophytes on a phytoplankton community in experimental conditions

The impact of submerged macrophytes or their extracts on planktonic algae was studied under experimental conditions. Live Ceratophyllum demersum L., its extract, and extracts of four other plant species induced modifications in the phytoplankton dominance structure. These modifications were: a decline in the number of Oscillatoria limnetica Lemm., which was the most numerous cyanobacterian species, and a decline in biomass and percentage contribution of all cyanobacteria to total algal biomass. This was accompanied by an increase in biomass and percentage contribution of green algae, especially Chlorella sp. and Chlamydomonas sp. Also, there was an increase in biomass and percentage contribution of nanoplankton (under 50 µm) to total phytoplankton biomass.The isolation of planktonic algae from direct influence of C. demersum by means of dialysis membranes caused an increase in number, biomass and percentage contribution of cyanobacteria. Release of organic compounds of over 3000 daltons by macrophytes apparently contributed to a decline of cyanobacteria by changing the phytoplankton dominance structure.     

An elementary,structural analysis of river phytoplankton

Summary A structural analysis of river phytoplankton has been carried out based upon published studies on 67 rivers. When available on a yearly basis to account for seasonal variability, five structural features have been chosen: species composition, species richness, species dominance, diversity and biomass (total and per taxonomic groups). Despite the high number of reported studies, most of them cover only some of the aforementioned features. As a result of the low amount of studies, tropical rivers are underepresented. No size distribution studies have been carried out on river phytoplankton. The average species richness amounts to 126, being higher in temperate rivers. Roughly one half of each flora is comprised of sporadic species. No statistically significant relationship between species richness and latitude has been found despite the fact that tropical rivers appear to house fewer species than temperate rivers. Also, one half of the support in the floras are either benthic or tychoplanktonic. Diatoms comprise the majority of species numbers in the whole data set but are substituted by desmids in tropical rivers and by green algae when benthic species are not taken into account. There appears to be lower biomass in river phytoplankton than in lakes. Diatoms are also the major taxonomic group comprising total biomass in rivers but they share clearly a lower fraction in tropical rivers. On an average basis, diatoms appear to be more dominant in rivers than in lakes. The time course of diatom dominance occurs close to the summer solstice in tropical rivers whereas is much more lagged in temperate sites. The diversity of river phytoplankton is highly scattered (0.40–4.40 bits ind^–1).     

Seasonal changes on planktonic diatom communities along an inshore-offshore gradient in the Gulf of Gabes (Tunisia)

We investigated the composition, biomass and cell size dynamics of the marine planktonic diatom species along a coastal-open sea gradient in relation to the hydrological characteristics, during four oceanographic cruises in July 2005, May–June 2006, September 2006 and March 2007 in the Gulf of Gabes. The study of the marine planktonic diatoms throughout the sampling period showed the presence of 40 different species belonging to 22 pennate and 18 centric diatom species. Centric diatoms were more abundant than the pennate ones; 56% and 44% of total diatom abundance, respectively. Diatoms were very abundant, representing about 60% of the total phytoplankton abundance, with an exception in July 2005 (19%) during which Dictyochophyceae were the most dominant group (41% of the total phytoplankton abundance). Diatoms, which were dominant in the coastal samples, mainly proliferate in the semi-mixing conditions (May–June 2006), whereas they declined in the offshore area, most likely due to silicate shortage. In this period, in spite of the high abundance of diatom planktonic cells, only diatom biomass was correlated with silica amount, proving that biomass was a better ecological bio-indicator than abundance. The results suggest that the marine planktonic diatoms taxa were generally adapted to specific hydrological structure. In fact, the dominance index of diatoms showed that the biodiversity of diatoms increased gradually along the coastal-open sea gradient except in May–June 2006 during which a slight decrease along the inshore towards offshore areas was observed. This index increase depended on a coastal-open sea distance during thermal stratification (July 2005 and September 2006) and mixing periods (March 2007).