南麂列岛海洋自然保护区浮游植物的种类多样性及其生态分布

于2006年5月至2007年2月之间,对南麂列岛海域的浮游植物类群进行了4个季节的调查,分析了该海域浮游植物的种类组成、优势种类、群落结构以及水平分布等特征参数的季节变化。共鉴定浮游植物80种,隶属于4个门,硅藻种类最多,甲藻其次。浮游植物可划分为3个生态类群,以广温类群为主。春季和夏季分别以三角棘原甲藻和中肋骨条藻为绝对优势种,秋冬季的优势种类组成多样化。共鉴定57种赤潮生物,占浮游植物种类数的71.25%。调查期间,三角棘原甲藻和中肋骨条藻分别于春季和夏季形成赤潮。浮游植物的物种丰富度呈现春、夏、秋、冬递减的趋势。浮游植物细胞丰度的年平均值为1.03×106cells/L,春夏季显著高于秋冬季。春季和夏季时,浮游植物高值区集中在南麂岛西北近岸海域;秋季和冬季时,浮游植物高值区相对集中在南麂岛东南近岸海域。浮游植物群落的多样性指数(H')以秋季最高,冬季最低。春季的三角棘原甲藻赤潮期间,水体中N/P值显著升高;夏季的中肋骨条藻赤潮期间,水体中N/P值显著降低。     

海洋围隔生态系中赤潮发生前后浮游植物群落结构的初步分析

1990年5至7月,在国家海洋局第三海洋研究所陆基水池中进行了了围隔生态系内富营养化引起的生态效应实验。实验表明,富营养的围隔袋中,浮游植物群落结构逐渐发生了变化,组成和数量都有很大的变动。形成水华和赤潮。并发生了种群的演替。水华以硅藻开始,形成高潮之后便衰退。随后微鞭毛藻和甲藻开始较大量繁殖。金藻和甲藻在实验中形成赤潮,最高值达到6.7×10~(10)cells/m~3。甲藻和硅藻的生长似乎互相抑制。实验初期,在硅藻繁盛时,甲藻的种、属和数量都很少。而在演替的后期,硅藻衰落时,甲藻占优势。数量最高达2.6×10~8cells/m~3。实验演示了发生在硅藻水华之后的甲藻赤潮发展过程,与1986年厦门西海域发生的裸甲藻赤潮的发展过程相似。实验结果为进一步研究甲藻赤潮的形成规律及其预测预报积累了资料和经验。     

1998年秋季渤海中部及其邻近海域的网采浮游植物群落

研究了1998年秋季覆盖渤海中部和渤海海峡及其邻近海域30个测站两遍大面调查的网采浮游植物物种及其群落特征。经初步分析,共发现浮游植物3门类38属84种(不包括未定名物种),物种主要以硅藻为主,但甲藻在群落中的比重也很明显,在个别站位上会成为优势类群。优势物种主要为偏心圆筛藻(Coscinodiscus excentricus)、梭状角藻(Ceratium fusus)、洛氏角毛藻(Chaetoceros lorenzianus)、浮动弯角藻(Eucampia zodiacus)、叉状角藻(Ceratium furca)、佛氏海线藻(Thalassionema frauenfeldii)、中华半管藻(Hemiaulus sinensis)、尖刺伪菱形藻(Pseudo-nitzschia pungens)、布氏双尾藻(Ditylum brightwellii)、粗刺角藻(Ceratium horridum)、掌状冠盖藻(Stephanopyxis palmeriana)和萎软几内亚藻(Guinardia flaccida)。细胞丰度的平面分布由主要浮游硅藻的分布所决定,其高值区分布在渤海湾、渤海中部、莱州湾北部和渤海海峡南部。浮游甲藻在渤海湾北部和莱州湾北部等局部区域形成高值区,并影响浮游植物的平面分布。两遍大面站调查期间渤海浮游植物群落的结构是相对保守的,渤海浮游植物群落季节和周年的变化主要是物种演替过程。调查期处于浮游植物群落的负增长时期,其主要原因是物理水文作用的结果。与1982年的历史同期浮游植物群落相比,角毛藻属衰退,浮游甲藻、圆筛藻属和浮动弯角藻兴起。但与1992年的历史同期浮游植物群落相比,群落优势种组成相近。同1982年和1992年同期资料相比,浮游植物细胞丰度和平面分布大致相等。渤海秋季浮游植物群落结构在近20a是相对保守的,影响秋季浮游植物群落发展的主要因素是物理水文作用。     

北部湾海洋原甲藻的形态特征及其系统进化分析

为探讨广西北部湾海洋原甲藻(Prorocentrum micans)的形态特征及其系统进化意义,利用光学显微镜、分子生物学方法,对海洋原甲藻BBW-01 藻株的形态特征进行了描述,并分析了其分子系统进化关系.结果表明,各地理株系的海洋原甲藻的形态特征相似,仅在细胞大小上存在差异.海洋原甲藻BBW-01 与采自于广东大亚湾的海洋原甲藻形态特征最为接近,其壳板后端的7 个呈“V”字形对称排列的大孔可作为海洋原甲藻鉴定的重要指标.18S rDNA 序列同源检索和系统进化分析表明,海洋原甲藻BBW-01 与源自中国南海的海洋原甲藻的亲缘关系最近,并与其他2 株海洋原甲藻聚成一支,属于浮游、兼性浮游类原甲藻.因此,对赤潮原因种的准确识别有助于预防和减轻赤潮对海洋环境和人类带来的危害.     

我国东海赤潮原甲藻应属哪种？

阐述并讨论了中国东海长江口外经常发生的原甲藻赤潮原因种的种类认知问题．论述了自从Stein提出具齿原甲藻(Prorocendrum dentatum Stein)新种后迄今中外专家报告该种的描述及分布,并对比了具齿原甲藻与长江口的原甲藻的异同．研究证实我国东海的原甲藻与具齿原甲藻有较大的差别,主要表现在体积大小、藻体末端形态以及藻体前端突起的大小和形态等方面．本文汇集了国内外对具齿原甲藻与我国原甲藻的研究成果,对比认为。我国长江口外经常发生赤潮的原甲藻为东海原甲藻(Prorocendrum donghaiense Lu)．文中还讨论了由陆斗定发表的东海原甲藻需要补充修正的观点．在过去的数年中,我国东海长江口海域频繁爆发大面积的原甲藻赤潮,但对于赤潮原因种原甲藻(Prorocentrum)的定种问题存在两种不同的观点,一种观点认为本种是具齿原甲藻,另一种观点认为是东海原甲藻．为了阐明此种原甲藻的命名问题,于2002年11月在深圳召开了题为“我国东海赤潮原因种原甲藻分类学国际研讨会”．认为是P．dentatum的依据是长期以来国际上通常把类似我国东海的原甲藻定为P．dentatum,其根据是它的前端有突起等．而持不同观点的专家认为我国东海的这种原甲藻在细胞长度上与Stein描述的原P．dentatum差异很大,并且除少数样品末端尖伸外,大多数细胞末端是钝圆的．专家们还认为在日本、韩国等亚洲海域分布的被报告为P．dentatum的原甲藻与我国东海的此种原甲藻为同一种．综观各种观点,作者认为东海本种原甲藻应为东海原甲藻.     

还原态氮源在微小原甲藻大规模赤潮中的重要性--动力学模型

1998年春末夏初,在美国的切萨皮克海湾的Choptank河出现了由微小原甲藻引发的大规模的赤潮。我们做了一系列与该藻赤潮发生机制有关的生理学特征实验。其中与氮吸收有关的生理学参数被应用于微小原甲藻赤潮发生动力学模型。为说明几个关键的生态及生理学过程在微小原甲藻赤潮发展和持续过程重的重要性,我们用该模型测试了几个关键过程点。模型测试的结果表明,河流输入充足的氮源是引起微小原甲藻赤潮的关键因素,而输入营养盐的组成结构对赤潮的发生并不起主要作用。然而在赤潮形成后,赤潮的维系依赖于还原态的氮源。在赤潮的维持过程中,微小原甲藻的倾向于吸收还原态的氮源的生理学特征起了很大作用。模型进一步表明微小原甲藻在低光照或黑暗条件下对氮的吸收仍然保持相当的吸收速率有利于该藻赤潮的发展。     

1999年春季渤海中部及其邻近海域的网采浮游植物群落

研究了 1999年春季覆盖渤海中部、渤海海峡及其邻近海域 30个测站两遍大面调查的浮游植物物种及其群落特征 ,共发现浮游植物 4门 33属 6 5种 (不包括未定名物种 ) ,以硅藻为主。优势种主要为 :冰河拟星杆藻 (Asterionellopsisglacialis)、虹彩圆筛藻 (Coscinodiscus oculus- iridis)、太阳双尾藻 (Ditylum sol)、刚毛根管藻 (Rhizosolenia setigera)、布氏双尾藻 (Ditylumbrightwellii)、中肋骨条藻 (Skeletonema costatum)、膜状缪氏藻 (Meuniera membranacea)、派格棍形藻 (Bacillaria paxillifera)、加拉星平藻 (Asteroplanus karianus)、圆海链藻 (Thalassiosira rotula)、卡氏角毛藻 (Chaetoceros castracanei)和尖刺伪菱形藻(Pseudo- nitzschia pungens)。细胞丰度的平面分布主要由浮游硅藻的分布所决定 ,其高值区分布在渤海中部靠近渤海海峡处。浮游甲藻在渤海海峡局部区域形成高值区。两遍大面站调查期间渤海浮游植物群落的结构变化较大 ,原因主要是春季以小细胞为主体的浮游植物群落物种演替较快 ,另外春季的大风过程对浮游植物的群落结构影响较大。两遍大面站调查期间浮游植物细胞丰度呈增长趋势 ,温度和硅酸盐浓度的增加是导致调查期浮游植物生长的重要因素。E5站浮游植物群落的分析表明 :春     

海南岛以南海域浮游植物群落特征研究

为掌握海南岛以南海域浮游植物群落特征, 1998–1999年对其进行了4个航次的现场调查。经显微镜检, 共鉴定出浮游植物290种(包括23个变种和5个变型)。在此基础上, 作者通过统计和聚类分析方法, 研究了这一区域浮游植物的物种组成、丰度变动和群落结构等特征。结果显示, 海南岛以南海域浮游植物物种丰富, 以硅藻门和甲藻门为主; 物种组成的时空差异显著。其优势种的暖水性、高盐性或广盐性特征明显。冷季以广温种小舟形藻(Navicula subminuscula)占优势, 随气温回升, 暖水性种类优势地位突出。不同季节浮游植物丰度差异小, 以硅藻门丰度占优势, 但与冷季相比, 暖季中甲藻门和蓝藻门丰度明显上升。4月和9月的高丰度中心位于西部海域北部湾湾口附近, 1月和12月则出现在中东部水域。聚类结果显示各浮游植物群落组分无明显的斑块分布特征。群落物种多样性表现为4月和9月高于1月和12月; 群落稳定性以12月份为最差; 中南部水域群落稳定性较差。总之, 海南岛以南海域浮游植物群落具独特的热带开阔海域生物区系特征。冷暖季群落特征有明显差异。海域水文条件对群落特征的影响复杂。     

2004年东海原甲藻赤潮爆发的现场调查和分析

2004年4～5月对东海赤潮高发区开展了赤潮原因种的大面观测并对期间爆发的特大赤潮进行跟踪调查,在56个站位采集了171份样品。表层东海原甲藻(Prorocentrum donghaiense)变化范围为2.5×10³～6.0×10⁷cells·L^-1,最大值出现在122.94°E,30°N的rb12A站;中层东海原甲藻变化范围为1.0×10³～5.32×10⁶cells·L^-1,最大值出现在rf40站。从水平分布看,东海原甲藻呈不均匀分布,从垂直分布看,赤潮爆发前原甲藻细胞在水体中层密集,大量增殖后上升到表层,爆发赤潮。     

东海原甲藻修订及与相关原甲藻的分类学比较

分析了东海原甲藻(Prorocentrum donghaiense Lu)显微结构,并与具齿原甲藻(Prorocentrum dentatum Stein)模式种和Schiller的钝头原甲藻的描述等进行了比较,结果表明,它们之间的形态结构和个体大小具有很大的差别,这些差异远超出了同种个体因环境不同所造成的形态变化范围．从细胞形态及其表面结构可以判断,日本、韩国海区所记录并报道的“P．dentatum”与我国东海的东海原甲藻应属同一种．因此可以认为,我国东海赤潮高发区以及在韩国、日本海区的出现的高生物量(high biomass bloom—forming species)赤潮原甲藻不是Stein所发表的具齿原甲藻。而是东海原甲藻,并对其进行了进一步修订,其种名应为东海原甲藻Prorocentrum donghaiense Lu．     

Seasonal dynamics of bacterioplankton community structure in a eutrophic lake as determined by 5S rRNA analysis.

Community structure of bacterioplankton was studied during the major growth season for phytoplankton (April to October) in the epilimnion of a temperate eutrophic lake (Lake Plusssee, northern Germany) by using comparative 5S rRNA analysis. Estimates of the relative abundances of single taxonomic groups were made on the basis of the amounts of single 5S rRNA bands obtained after high-resolution electrophoresis of RNA directly from the bacterioplankton. Full-sequence analysis of single environmental 5S rRNAs enabled the identification of single taxonomic groups of bacteria. Comparison of partial 5S rRNA sequences allowed the detection of changes of single taxa over time. Overall, the whole bacterioplankton community showed two to eight abundant (>4% of the total 5S rRNA) taxa. A distinctive seasonal succession was observed in the taxonomic structure of this pelagic community. A rather-stable community structure, with seven to eight different taxonomic units, was observed beginning in April during the spring phytoplankton bloom. A strong reduction in this diversity occurred at the beginning of the clear-water phase (early May), when only two to four abundant taxa were observed, with one taxon dominating (up to 72% of the total 5S rRNA). The community structure during summer stagnation (June and July) was characterized by frequent changes of different dominating taxa. During late summer, a dinoflagellate bloom (Ceratium hirudinella) occurred, with Comamonas acidovorans (beta-subclass of the class Proteobacteria) becoming the dominant bacterial species (average abundance of 43% of the total 5S rRNA). Finally, the seasonal dynamics of the community structure of bacterioplankton were compared with the abundances of other major groups of the aquatic food web, such as phyto- and zooplankton, revealing that strong grazing pressure by zooplankton can reduce microbial diversity substantially in pelagic environments.     

Impact of warming on phyto‐bacterioplankton coupling and bacterial community composition in experimental mesocosms

Global warming is assumed to alter the trophic interactions and carbon flow patterns of aquatic food webs. The impact of temperature on phyto‐bacterioplankton coupling and bacterial community composition (BCC) was the focus of the present study, in which an indoor mesocosm experiment with natural plankton communities from the western Baltic Sea was conducted. A 6°C increase in water temperature resulted, as predicted, in tighter coupling between the diatom‐dominated phytoplankton and heterotrophic bacteria, accompanied by a strong increase in carbon flow into bacterioplankton during the phytoplankton bloom phase. Suppressed bacterial development at cold in situ temperatures probably reflected lowered bacterial production and grazing by protists, as the latter were less affected by low temperatures. BCC was strongly influenced by the phytoplankton bloom stage and to a lesser extent by temperature. Under both temperature regimes, Gammaproteobacteria clearly dominated during the phytoplankton peak, with Glaciecola sp. as the single most abundant taxon. However, warming induced the appearance of additional bacterial taxa belonging to Betaproteobacteria and Bacteroidetes. Our results show that warming during an early phytoplankton bloom causes a shift towards a more heterotrophic system, with the appearance of new bacterial taxa suggesting a potential for utilization of a broader substrate spectrum.     

Seasonal dynamics of phytoplankton in a lagoon-type lake Izmenchivoye (Southeast Sakhalin)

The seasonal changes in taxonomic structure, dynamics of number, and biomass of phytoplankton in the Izmenchivoye lagoon lake (southeast Sakhalin) were studied. In all, 266 species and intraspecies taxa of microalgae were revealed. The greatest species diversity (according to the Shannon index) was observed in May, August and October (H = 2.76–2.89), the least (H = 0.5–0.86), in April and January of 2006. The monthly average number varied from 997 up to 84 282 cells/l, and biomass from 18.98 up to 878.62 mg/m³. The average annual number of phytoplankton and its biomass were 32 650 cells/l and biomass 172.13 mg/m³ respectively. The maximum number was registered in August, 2005, and maximum biomass was recorded in January, 2006. Winter, spring and summer peaks of number coinciding with those of biomass were registered. For the first time, winter bloom of phytoplankton was registered in inland waters of the Sakhalin Island. In the winter and spring the basic input to formation of the parameters was composed of diatoms; in summer and autumn it was composed of by flagellates (dinophyta and cryptophyta).     

Some ecological effects of artificial circulation on a small eutrophic lake with particular emphasis on phytoplankton II. Kezar lake experiment, 1969

A small eutrophic New Hampshire lake was artificially circulated (mixed) from 28 May to 15 September, 1969, inclusive, to impede the annual bloom of a noxious blue-green alga; yet Aphanizomenon flos-aquae bloomed immediately after mixing was commenced. The bloom collapsed in early July; it was succeeded by heavy growth of predominantly chlorophycean taxa. In an in vitro experiment Aphanizomenon flos-aquae did reattain bloom proportions when the influence of artificial circulation was removed. Other phytoplankton exhibited population pulses only when the dominance of cyanophycean and chlorophycean taxa were in transition. Mixing maintained uniform vertical populations of all phytoplankton. Changes in water transparency attended fluctuations in phytoplankton abundance. An isothermic condition was maintained over the test period, which increased the lake's heat budget, and most chemical nutrients were distributed isometrically in the water column. Increased concentrations were exhibited by Ca, Cl, Cu, K, Mg, SiO₂ and Zn. Sodium was not affected by mixing. Levels of Fe, Mn, Zn, phosphate and ammonia, nitrate, and organic nitrogen were influenced by phytoplankton. Mixing could not maintain orthograde profiles of dissolved O₂ and CO₂ when dense populations of phytoplankton prevailed. Variations in chlorophyll-a followed closely changes in phytoplankton abundance. Its degradation to pheo-pigments appeared to be less for a bloom of Aphanizomenon flos-aquae than during dense growth of chlorophycean taxa. Rates of photosynthesis were considerably greater when the latter algae were predominant. Extracellular release of organic carbon usually increased with depth; it amounted to 19.4 percent of the total carbon fixed in the euphotic zone.     

神农架地区典型草本群落中的昆虫访花行为比较

对神农架地区不同海拔高度(1,000–2,600 m)的典型草本群落中的昆虫访花行为进行了初步的监测和统计。在9个样地中共观察到鳞翅目、膜翅目、双翅目11科昆虫访花, 访花者的类群在不同海拔高度的群落中有所不同, 但明显与植物群落的物种组成有关。对昆虫访花频率的监测发现, 不同物种组成的草本群落中, 所有昆虫访花的总频率和不同类群的昆虫访花频率都有差异, 正在开花的物种较多的群落中通常能吸引相对较多的访花者。同一种植物在不同的群落背景中受访频率差异显著, 如花被不甚显著的酸模(Rumex acetosa)在某些群落中未观察到访花者, 在另一些群落中却有昆虫频繁访问。而花被显著的血见愁老鹳草(Geranium henryi)在各群落中都有频繁的昆虫访问。一些植物(如广布野豌豆Vicia cracca)虽然在不同群落中都有昆虫访问, 但访花者类群却有较大差异。作者认为, 在植物群落中, 不同植物的花在吸引昆虫传粉方面存在着一定的相互作用, 在研究特定植物的虫媒传粉时应重视该植物种群所处的群落背景所起的作用。     

横岗水库浮游植物种类组成与时空分布

于2005年5月和11月调查了横岗水库的水质和浮游植物,对浮游植物群落结构进行了分析.两次采样期间,浮游植物群落数量组成与种类结构发生明显变化.5月份浮游植物平均细胞数量高达275.41×10⁶cells·L^-1,平均生物量为31.07mg·L^-1;其中优美平裂藻(Oscillatoria.elegans)和银灰平列藻(Oscillatoria.glauca)为绝对优势种,占总细胞数的90%;平裂藻的大量出现导致浮游植物生物量由球体等效半径在10～20μm的种类占主导,但0～5μm区间的小型藻类仍维持较高的生物量.11月份,浮游植物平均细胞数量降低到12.69×10⁶cells·L^-1,平均生物量为20.78mg·L^-1.蓝藻门有16种,优美平裂藻、银灰平列藻的优势度明显下降;绿藻种类数量增加到48种,在新出现的25个种类中,鼓藻科(Desmidiaceae)、盘星藻属(Pediastrum)和栅藻属(Scenedesmus)有较高的细胞数;优势种类没有变化;大细胞种类的生物量明显增加,特别是等效半径大于20μm的种类.整体上看,浮游植物数量和生物量从河流入水口到大坝处有递减趋势,这种趋势在11月份更为明显,这与采样时期水库水动力过程的梯度相一致.     

Changes in community structure and photosynthetic activities of total phytoplankton species during the growth,maintenance, and dissipation phases of a Prorocentrum donghaiense bloom

The potential interactions between the bloom-forming dinoflagellates and other phytoplankton during the algal bloom cycle are interesting, while the causes for the phytoplankton community changes were not fully understood. We hypothesized that phytoplankton community structure and photosynthetic activities of total phytoplankton have their special characteristics in different phases of the algal blooms. To test this hypothesis, a survey covering the process of a Prorocentrum donghaiense bloom in coastal waters between Dongtou and Nanji Islands was carried out from 9 to 20 May 2016, and the changes in the phytoplankton community and photosynthetic activities of total phytoplankton were determined. Surface seawater was sampled for microscopic analysis of phytoplankton composition and pulse amplitude modulated (PAM) chlorophyll fluorescence analysis of photosynthetic activities of the total phytoplankton species. A total of 25, 31, and 19 phytoplankton species were identified in its growth (9–12 May), maintenance (13–18 May) and dissipation phases (19–20 May), respectively. Diatoms were dominant in terms of species number while dinoflagellates were predominant at cell abundance. Dinoflagellates were the major dominant species during three phases of the bloom based on the dominance (Y) value, whereas the dominant species extended to dinoflagellates and diatoms including P. donghaiense, Coscinodiscus argus, Gonyaulax spinifera, Cyclotella sp. and Scrippsiella trochoidea in the dissipation phase. In the maintenance phase, the average cell abundances of the total phytoplankton and P. donghaiense were consistent with the chlorophyll a (Chla) concentration in the seawater; for the diversity indices of total phytoplankton species, Simpson index (C) was the highest while Shannon index (H′) and Pielou evenness index (J′) were the lowest. Furthermore, photosynthetic activities of the total phytoplankton species represented by the effective quantum yield (F_q'/F_m') and the maximum relative electron transport rate (rETR_max) in the maintenance phase were higher than those in the growth and dissipation phases. The results indicated that the characteristics of phytoplankton community structure and photosynthetic activities could be regarded as criteria in predicting the phases of algal blooms.     

大庆新华湖藻类植物群落结构与环境因子的相关性

2014年5-10月对大庆新华湖藻类植物群落结构与环境因子的关系进行了初步研究, 为湖泊状况的动态监测提供基本数据。调查期间, 共发现藻类植物135种, 隶属5门63属, 其中硅藻门56种, 绿藻门49种, 蓝藻门15种, 裸藻门12种, 甲藻门3种。新华湖优势种共24种, 包括绿藻门14种, 蓝藻门6种, 硅藻门3种, 裸藻门1种, 且季节更替明显, 可以初步推断新华湖藻类植物群落组成为绿藻-硅藻型。新华湖藻类植物细胞丰度变化范围为37.57×10⁶-72.37×10⁶ cells·L^-1, 平均值为52.13×10⁶ cells·L^-1。多样性指数变化为: Simpson生态优势度指数(D)在0.642-0.928之间, Shannon-Weaver多样性指数(H')在1.698-3.1之间, Pielou均匀度指数(J)在0.324-0.561之间, 3种指数变化趋势一致, 均在秋季最高, 春季最低。研究表明, 水温、pH值、总磷、生化需氧量、溶解氧和水动力是影响新华湖藻类植物群落结构的主要因子。     

Seasonal succession of phytoplankton in Lake Yogo over 2 years: effect of artificial manipulation

We studied the seasonal variation in concentrations of nutrients and phytoplankton in Lake Yogo for 2 years, from May 2000 to May 2002, in order to clarify the seasonal succession of phytoplankton and the effect of various manipulations on it. It was revealed that in spite of the installation of aeration systems and the pumping of mesotrophic water from Lake Biwa during the summer season, the trophic state of Lake Yogo overall has not improved during the past few decades. However, the pumping of water from Lake Biwa did affect the concentrations of nutrients and the periods of cyanobacterial bloom during the summer. The pumping period was different in each year, and the cyanobacterial bloom occurred during the period without pumping in both years. The aeration destratification was not strong enough to prevent cyanobacterial blooms. Cyanobacteria and Bacillariophyceae contributed most to the phytoplankton biomass in both years. Aphanizomenon, Anabaena, and Microcystis were the main genera among cyanobacteria. The bloom of Aphanizomenon or Anabaena occurred early in the summer, and was then replaced by Microcystis. Aphanizomenon was almost always present, and often formed bloom even in winter. The seasonal succession of Bacillariophyceae was almost the same in both years and was well categorized: winter-growing species such as Aulacoseira pusilla (F. Meister) Tuji et Houki and species of Thalassiosiraceae, spring-growing species such as Asterionella formosa Hassall, Fragilaria crotonensis Kitton, and Synedra cf. acus, and fall-growing species such as Aulacoseira ambigua (Grunow) Simonsen, and Aulacoseira granulata (Ehrenb.) Simonsen.     

The phytoplankton community of Punta Morales, Nicoya Gulf, Costa Rica

Three daily samplings of the phytoplankton community were made at two consecutive days in March, April, May, September, October, November and December 1997, at Punta Morales, Golfo de Nicoya, Costa Rica. Samples were collected during each tide at depths of 50% and 10% of light penetration using a Niskin bottle. A total of 43 taxa were identified. Centric diatoms, pennates and flagellates represented 90% of total phytoplankton abundance. In the phytoplankton fraction (cells > 30 microm), diatoms were the most abundant group, and Skeletonema costatum (32%) dominated. In nannophytoplankton (cells < 30 microm), Chaetoceros (23.7%) was the most abundant taxon, followed by flagellates (23%) and Cylindrotheca closterium (13.1%). These results agree with previous surveys and suggest that a typical net phytoplankton community persist through time in the Punta Morales zone. The number of nannophytoplankton fraction cells varied seasonally and suggests quantitative changes in species abundance, with possible modifications of cellular size or chain length in filamentous species. The codominance between S. costatum and Chaetoceros spp. during the rainy season suggested the ocurrence of an early ecological sucession, and nutrients could be the factor generating such population changes.