Temporal variations in diatom abundance and downward vertical flux in the oligotrophic North Pacific gyre

The abundance of diatoms in the water column and the downward vertical flux of diatom cells from the euphotic zone were investigated during a time series of 11 monthly cruises (June 1994–July 1995) to Station ALOHA (22°45′N, 158°00′W) as one component of the Hawaii Ocean Time-series (HOT) Program. The diatom community was studied using light microscopy and by high-performance liquid chromatographic (HPLC) pigment analyses. Distinct diatom assemblages were found in the mixed-layer and in the Deep Chlorophyll Maximum Layer (DCML). Diatom cell abundances in the water column were generally low during the year, except in July 1994, when they increased in the upper euphotic layer. Two lightly silicified species (Hemiaulus hauckii [Grunow] and Mastogloia woodiana [Taylor]) were mainly responsible for this increase. Other less abundant diatom species present in the mixed-layer assemblage showed a similar temporal pattern. H. hauckii contained Richelia-type endosymbionts with heterocysts and was presumably able to fix dinitrogen. Both species of diatoms also were an important component of the vertical diatom flux out of the euphotic zone, which, likewise, was highest in July 1994. During this maximum export period, aggregates of these two species were collected in the drifting sediment traps. In the DCML, diatom abundances and export were low throughout the year, with the exception of one genus (Pseudonitzschia) for which a slight concentration increase was observed in spring. Reflecting the observed diatom cell abundance and vertical flux, fucoxanthin concentrations (a pigment marker for diatoms) did not indicate any significant increase of diatom pigment biomass in the DCML during the year. Ratios of diadinoxanthin to chromophyte pigments suggested that the phytoplankton cells sinking out of the euphotic zone in midsummer originated from the mixed-layer. The attenuation of the pigment vertical fluxes with depth was significantly lower for fucoxanthin, indicating a generally slower decay of diatom flux with depth compared with other phytoplankton groups. Our findings suggest that, in the subtropical North Pacific Ocean, summer conditions seem to favor the development of selected species of diatoms in the mixed-layer and that these assemblages appear to be more important with regard to export production than those in the DCML.     

Interactive effects of iron,irradiance and CO2 on Ross Sea phytoplankton

We conducted a factorial shipboard continuous culture experiment to examine the interactive effects of altered iron, irradiance and CO₂ on the summer phytoplankton community of the Ross Sea, Antarctica. After 18 days of continuous incubation, iron enrichment increased phytoplankton biomass, nutrient drawdown, diatom and Phaeocystis abundance, and some photosynthetic parameters. High irradiance significantly increased the number of Phaeocystis antarctica colonies, as well as P. antarctica abundance relative to diatoms. Iron and light had significant interactive effects on diatom and P. antarctica pigment concentrations, P. antarctica colony abundance, and Si:N, Si:C, and N:P ratios. The major influence of high CO₂ was on diatom community structure, by favoring the large centric diatom Chaetoceros lineola over the small pennate species Cylindrotheca closterium. The ratio of centric to pennate diatoms was significantly responsive to changes in all three variables individually, and to all of their possible two- and three-way combinations. These results suggest that shifts in light, iron, and CO₂ and their mutual interactions all play a role in controlling present day Ross Sea plankton community structure, and need to be considered when predicting the possible future responses of biology and biogeochemistry in this region.     

Diatoms associated with two South African kelp species: Ecklonia maxima and Laminaria pallida

Kelp forests are believed to host a large biomass of epiphytic fauna and flora, including diatoms, which constitute the base of aquatic food webs and play an important role in the transfer of energy to higher trophic levels. Epiphytic diatom assemblages associated with two common species of South African kelps, Ecklonia maxima and Laminaria pallida, were investigated in this study. Primary blades of adult and juvenile thalli of both kelp species were sampled at False Bay in July 2017 and analysed using scanning electron microscopy. Our findings showed that both kelp species hosted relatively low densities of diatoms (ranging from 7 [SD 5] cells mm^?2 on adult specimens of L. pallida to 43 [SD 66] cells mm^?2 on blades of juvenile E. maxima), with Amphora and Gomphoseptatum reaching the highest absolute abundances. Although non-metric multidimensional scaling showed overlapping and largely scattered sample sets, a significant relationship between the diatom communities and the species and age of the host macroalga was detected by two-way PERMANOVA. In general, more abundant and diverse diatom communities were observed on juvenile thalli than on adult thalli, with species belonging to Navicula and Rhoicosphenia contributing significantly to the observed dissimilarity. Due to a significant interaction between species and age effects, however, the overall ability of kelp species, their age, and their interaction to explain the variation in diatom community structure was limited. We suggest that the low densities of epiphytic diatoms were directly related to the sloughing of epithelial cells observed in both kelp species. We further speculate that on such unstable substrata some diatom taxa might adapt to an endophytic life to avoid the antifouling mechanisms developed by their hosts.     

Population dynamics of phytoplankton, heterotrophic bacteria, and viruses during the spring bloom in the western subarctic Pacific

We characterized the community composition of phytoplankton in the western subarctic Pacific from the pre-bloom to the decline phase of the spring bloom with special reference to decreases in the silicic acid concentration in surface waters as an index for diatom bloom development. Furthermore, responses of heterotrophic bacteria and viruses to the spring bloom were also concomitantly investigated. Under pre-bloom conditions when nutrients were abundant but the surface mixed layer depth was relatively deep, chlorophyll (Chl) a concentrations were consistently low and green algae (chlorophytes and prasinophytes), cryptophytes, and diatoms were predominant in the phytoplankton assemblages as estimated by algal pigment signatures. Together with the shallowing of the mixed layer depth and the decrease in silicic acid concentration, diatoms bloomed remarkably in the Oyashio region, though the magnitude of the bloom in the Kuroshio-Oyashio transition (hereafter Transition) region was relatively small. A total of 77 diatom species were identified, with the bloom-forming diatoms mainly consisting of Thalassiosira, Chaetoceros, and Fragilariopsis species. It has become evident that the carotenoid fucoxanthin can serve as a strong indicator of the diatom carbon biomass during the spring diatom bloom. Differences in the species richness of diatoms among stations generally enabled us to separate the Oyashio bloom stations from the Transition and the Oyashio pre-bloom stations. Relatively high values of the Shannon-Wiener index for the diatom species were also maintained during the Oyashio bloom, indicating that a wide variety of species then shared dominance. In the decline phase of the Oyashio bloom when surface nutrient concentrations decreased, senescent diatom cells increased, as inferred from the levels of chlorophyllide a. Although the cell density of heterotrophic bacteria changed little with the development of the diatom bloom, viral abundance increased toward the end of the bloom, suggesting an increased likelihood of mortality among organisms including diatoms resulting from viral infection. This is the first report on the microbial trophodynamics, including viruses, during the spring diatom bloom in the western subarctic Pacific.     

Contribution of benthic and epiphytic diatoms to clam and oyster production in the Akkeshi-ko estuary

The species composition and dynamics of phytoplankton in the water column and its contribution to clam, Ruditapes philippinarum, and oyster, Crassostrea gigas, production were studied in the Akkeshi-ko estuary, eastern part of Hokkaido, Japan. A total 128 taxa of diatoms were identified, with 103 and 102 species occurring on the surface sediments and in the water column, respectively. Amphora sp., Bacillaria paradoxa var paxilifer, Cocconeis scutellum, Navicula sp., Nitzschia sigma, Paralia sulcata, Rhoicosphenia curvata, Synedra ulna and Thalassiosira sp. were most common and dominant in all stations in both water column and surface sediment. Benthic diatoms were most dominant in both water column and surface sediment. The species composition of epiphytes was, in part, similar to diatom assemblage of the water column and also to that of the surface sediment. The benthic and pelagic ratio in the diatom assemblage of the water column was very high. Benthic diatoms were able to resuspend into water column by turbulence stimulated by wind. Chlorophyll a concentration in the water column increased as wind speed increased. Seventy and 67% of the gut contents oysters consisted of benthic diatoms in 2003 and 2004 and also 78 and 87% of clams in 2003 and 2004, respectively. The availability of benthic and epiphytic diatom assemblages contributes significantly to food resources for clam and oyster mariculture due to resuspension by disturbance in the Akkeshi-ko estuary.     

No microzooplankton grazing during a Mediopyxis helysia dominated diatom bloom

The new diatom species Mediopyxis helysia was described to science from clones found in 2003 in the North Sea, northern Wadden Sea, and the Gulf of Maine. Seven years after its first occurrence, we observed Mediopyxis to contribute up to almost 50% of the biovolume of the diatoms during a diatom spring bloom in the western Wadden Sea. Grazing experiments based on the dilution technique could not detect any microzooplankton grazing impact on the bloom community. Mediopyxis is now also well established in the western Wadden Sea and does have the potential to become a dominant species. The reasons for its success remain largely unresolved but avoidance of being grazed might be one factor. Future research on this new species is needed to understand the success and forecast the ecological footprint of this large diatom species arriving in the western European Seas.     

Long-term changes in the seasonality of selected diatoms related to grazers and environmental conditions

The decoupling of trophic interactions could be one of the severe consequences of climate warming in aquatic systems. The timing of phytoplankton blooms, in particular, can affect competition within the plankton community as well as food-web interactions with zooplankton and fish. Using long-term data from Helgoland Roads in the southern North Sea, we examine diatom seasonality, using three representative diatom species combined with environmental and copepod time series over the last four decades. The long-term annual abundances of Guinardia delicatula, Thalassionema nitzschioides and Odontella aurita exhibited interannual variations and dissimilar cyclic patterns during the time period under study (1962-2008). Of the three diatoms, G. delicatula showed a significant trend towards earlier bloom timings for 1962-2008 and a later decline of its abundance over time was found. Grazing and water transparency explained most of the bloom timing fluctuations of the diatoms considered. The annual timing of occurrences of each diatom species was correlated with their preceding concentrations. Earlier bloom timings occurred when autumn/winter concentrations were higher than average and later bloom timings occurred when autumn/winter concentrations were lower than average. Different environmental and predation variables related to the diatom bloom timings were found suggesting that climate warming might not affect the onset of the blooms of the three diatom species in the same manner. The results of the multiple linear regression analyses showed that the timings of decline of the three diatoms were mainly correlated with decreasing nutrient concentrations. Sunshine duration could prolong the duration of the blooms of T. nitzschioides and O. aurita provided that enough nutrients were available. In the case of G. delicatula, however, sunshine duration was negatively correlated with its end of the growth period. G. delicatula and T. nitzschioides showed later decreases in abundances under warmer spring and summer temperatures. Such species specific differences in the sensitivity to the forcing variables could lead to shifts in community structure and could ultimately have wider implications to the overall ecosystem health of the North Sea.     

白令海夏季浮游植物的群落结构与时空变化

通过1999年和2010年夏季同期7月在白令海(169°E~166°W,50°N~67°N)获取的94份浮游植物样品分析,获得了近十年的始末两个时间节点的浮游植物群落结构与时空变化,探讨了浮游植物群落动态及其与环境因素的关联。研究结果显示,共鉴定浮游植物(>10μm)5门58属153种,分为3个生态类群。硅藻是浮游植物的主体,种类多丰度高,占总种类数目的66.7%,占总丰度的95.2%。鉴于样品属性和空间范围的不同,物种组成有细微差别,丰度有较大差异且空间分布明显不同,高丰度区受控于上层营养盐供给和表层环流系统。优势种从北方温带大洋性硅藻演变为广温广盐性与冷水性硅藻,1999年以西氏新细齿状藻为第一优势种,柔弱伪菱形藻次之;2010年以丹麦细柱藻为第一优势种,冷水性的诺登海链藻次之并在陆架和陆坡占优。浮游植物群落结构较为稳定,由深水群落和浅水群落组成。深水群落分布于太平洋西北部和白令海盆,种类组成以温带大洋性的西氏新细齿状藻、长海毛藻、大西洋角毛藻和广布性的菱形海线藻、扁面角毛藻、笔尖根管藻为主,丰度低,种间丰度分配均匀,优势种多元化,物种多样性高;浅水群落分布于白令海陆坡和陆架,主要由冷水性的诺登海链藻、叉尖角毛藻、聚生角毛藻和广布性的丹麦细柱藻、旋链角毛藻组成,丰度高,种间丰度分配不均匀,优势种突出,物种多样性低。白令海夏季浮游植物种类组成及丰度变化直接受控于表层环流、营养盐、春季冰缘线等环境因素。     

Iron deficiency in micro-sized diatoms in the Oyashio region of the Western subarctic Pacific during spring

In order to detect iron (Fe) stress in micro-sized (20–200 μm) diatoms in the Oyashio region, western subarctic Pacific during spring, immunological ferredoxin/flavodoxin assays were applied to samples collected from the surface layer in May 2005. Concomitantly, the community composition of the micro-sized phytoplankton and hydrographic conditions, including dissolved Fe and macronutrient concentrations, were also examined. Chlorophyll (Chl) a concentrations were <2 mg m⁻³ at all sampling stations, except at a station where the Chl a level was 9.0 mg m⁻³ and a micro-sized diatom bloom occurred. A high abundance of ferredoxin in micro-sized diatoms was detected only at a rather near-shore station where dissolved Fe and macronutrient concentrations were higher, indicating that the micro-sized diatoms did not suffer from iron deficiency. On the other hand, flavodoxin in micro-sized diatoms was often observed at the other stations, including the bloom station, where macronutrients were replete but dissolved Fe concentration was low (0.31 nM). A significant amount of chlorophyllide a, a degradation product of Chl a, was also observed at the bloom station, suggesting a decline of the diatom bloom. The micro-sized phytoplankton species at all the stations were mainly composed of the diatoms Thalassiosira, Chaetoceros, and Fragilariopsis spp. Our study indicates that micro-sized diatoms were stressed by Fe bioavailability during the spring season in the Oyashio region     

Cenoses of phototrophic algae of ultrasaline lakes in the Kulunda steppe (Altai krai,Russian Federation)

In 2012, expeditions of the Institute of Microbiology, Russian Academy of Sciences, delivered samples of algo-bacterial mats from Kulunda steppe alkaline lakes (Petukhovskoe alkaline lake, Tanatar VI, and Gorchina III). The filamentous alga Ctenocladus circinnatus (Chlorophyta) acted as an edificator of the mats. The composition of cenoses algocomponents also included chlorophytes Dunaliella viridis and Picocystis salinarum as well as diatoms Anomeoneis sphaerophora, Brachysira brebissonii, B. zellensis, Mastogloia pusilla var. subcapitata, Nitzschia amphibia, N. cf. communis, and Nitzschia sp. 1. The composition and structure of phototrophic algae cenoses (including diatom taxocenes) were described for the investigated lakes for the first time. For the period from 2011 to 2012, the total mineralization significantly increased in lakes. This involved sensible alterations of cenoses. B. zellensis was the most permanent component of diatom taxocenes in both seasons. In the summer of 2011, it was often accompanied by A. sphaerophora and B. brebissonii. In the summer of 2012, A. sphaerophora was found only singularly in Lake Gorchina III, and some biotopes of Lake Tanatar VI were massively inhabited by N. cf. communis, including colonies that had not been previously described for the species. The genetic analysis of three diatoms, which are markedly different from each other in their appearance and were sampled from different lakes but were all determined as Nitzschia cf. communis, showed their complete similarity to each other with the 18S rRNA gene fragment and the highest similarity of all the three diatoms with the species Nitzschia communis.     

Interactions of bacteria with diatoms: influence on natural marine biofilms

Interactions between microfouling components influence the biofilm community and the cascading events, thus playing an important role in the biofouling process. Bacteria and diatoms are among the dominant forms reported in biofilms. Experiments were carried out with natural marine biofilms from a tropical monsoon‐influenced environment to evaluate the interactions between bacteria and diatoms through application of antibiotics (streptomycin and chloramphenicol). Overall, chloramphenicol inhibited diatom communities, whereas streptomycin did not. These antibiotic‐mediated changes in the fouling diatom community were consistent across the seasons. However, the rates at which the fouling communities changed depended on the initial species composition. It was also observed that elevated nutrient concentrations overrode the inhibitory effect of chloramphenicol. Maximum Vibrio enhancement was observed in the enriched conditions during the pre‐monsoon and unenriched conditions during the monsoon (with naturally elevated background nutrient concentrations), highlighting the relevance of nutrient concentrations for Vibrio spp. This has interesting implications for antibiotic‐mediated interactions between fouling diatom and bacterial communities under differing nutrient regimes. Although this study indicates the relevance of ‘cross‐talk’ at the microfouling level, understanding the effects of additional microbial products (e.g. bacteriocins and peptidoglycan) on these community‐level interactions will represent a step forward.     

Benthic diatoms and sulfide fluctuations: Upper basin of Pettaquamscutt River,Rhode Island

The purpose of this study was to determine if seasonal anoxia affects the community composition and abundance of benthic diatoms in an estuarine basin. Subtidal benthic diatoms were collected monthly at 1-m water depth intervals from 2 to 7 m in an estuarine basin of Pettaquamscutt River, Rhode Island, during 1981. Water samples were collected at the same depths to measure temperature, salinity, oxygen and sulfide levels. The basin became stratified above 7 m in June and the interface between oxic and anoxic waters remained at 5 or 5·5 m until December when it rose to above 4 m. Motile, biraphid diatoms dominated on the muddy sediments and live cell counts of these were insignificant below 5·5 m. At shallower depths, abundance was seasonally bimodal. In the spring, a peak began in April at 3 m (later in the season with increasing depth) and a smaller fall peak began in October at 4 and 5 m (later at shallower depths). Highest standing crop in August was at 5·5 m when 1% PAR (Photosynthetically Active Radiation) was at 4 m.The position of the interface between oxic and anoxic waters after stratification of the water column affected both abundance and species composition of benthic diatoms within 1·5 m above it. Navicula gregaria Donkin and N. ammophila Grunow dominated the spring and summer assemblages at all depths, but after September N. gregaria vanished from the basin below 3 m. In fall and winter, distinctly different populations were present at 4–5·5 m and at 2 m. The assemblage at 4 m and below consisted of sulfide-tolerant species of Navicula. Healthy populations of Navicula ammophila Grunow, N. pseudocrassirostris Hustedt, and N. peregrina (Ehrenberg) Kützing together reached 138 × 10³ cells cm^?2 at less than 1% light levels and up to 88 μM sulfide.     

Water column and recent sediment data on diatoms and coccolithophorids,off Portugal,confirm sediment record of upwelling events

Diatom and coccolithophorid abundance and diatom assemblage composition found in the water column along the Portuguese margin, during upwelling and non-upwelling conditions, are compared to the distribution patterns observed in the recent sediments from the same area. The water column results indicate a one order of magnitude increase in phytoplankton biomass during upwelling conditions (summer), with diatoms being the most important contributors. Coccolithophorids, on the contrary, dominate the phytoplankton in winter (non-upwelling). The comparison of the upwelling and non-upwelling spatial distribution of these phytoplankton groups to their sedimentary record reveals the sediment record as a reflection of the upwelling situation, preserving most of its original spatial variability. The comparison between living and fossil diatom assemblages indicates that from the two genera which dominate the summer biologic assemblage, Pseudo-nitzschia and Chaetoceros, Pseudo-nitzschia is not represented in the sediments, while Chaetoceros is the dominant form of the sediment. Thalassiosira which occurs in the same abundance in both seasons, responding to both river and upwelling nutrient input, can not be a reliable indicator of any single process. However, this genus distribution in the sediments can be used as an indicator of continuous nutrient availability.Such results are of great importance for paleoceanographic reconstructions since they constitute a good indication that the sediment record, even though somewhat altered with respect to assemblage composition, does reflect the water column characteristics.     

Water column and recent sediment data on diatoms and coccolithophorids,off Portugal,confirm sediment record of upwelling events

Diatom and coccolithophorid abundance and diatom assemblage composition found in the water column along the Portuguese margin, during upwelling and non-upwelling conditions, are compared to the distribution patterns observed in the recent sediments from the same area. The water column results indicate a one order of magnitude increase in phytoplankton biomass during upwelling conditions (summer), with diatoms being the most important contributors. Coccolithophorids, on the contrary, dominate the phytoplankton in winter (non-upwelling). The comparison of the upwelling and non-upwelling spatial distribution of these phytoplankton groups to their sedimentary record reveals the sediment record as a reflection of the upwelling situation, preserving most of its original spatial variability. The comparison between living and fossil diatom assemblages indicates that from the two genera which dominate the summer biological assemblage, Pseudo-nitzschia and Chaetoceros, Pseudo-nitzschia is not represented in the sediments, while Chaetoceros is the dominant form of the sediment. Thalassiosira, which occurs in the same abundance in both seasons, responding to both river and upwelling nutrient input, can not be a reliable indicator of any single process. However, this genus distribution in the sediments can be used as an indicator of continuous nutrient availability. Such results are of great importance for paleoceanographic reconstructions, since they constitute a good indication that the sediment record, even though somewhat altered with respect to assemblage composition, does reflect the water column characteristics.     

2013年春季崂山湾浮游植物群落及其环境控制

2013 年春季(3、4、5 月)对崂山湾海域浮游植物群落及其理化环境进行了3 个航次的调查研究。共鉴定浮游植物40 属75 种, 硅藻是调查区主要的浮游植物类群; 浮游植物细胞丰度平均为1.27×10⁶个/m³,3 月份的平均丰度显著高于4、5 月份(P<0.01); 优势种主要为冰河拟星杆藻(Asterionopsis glacialis)、加拉星平藻(Asteroplanus karianus)、密联角毛藻(Chaetoceros densus)和夜光藻(Noctiluca scintillans), 浮游植物群落演替明显, 由3 月份硅藻控制逐步过渡到4、5 月份硅/甲藻共同控制; 相对较低的硝酸盐水平是导致4、5 月份浮游植物丰度显著低于3 月份的主要原因; 物种–环境数据的冗余分析(RDA)结果显示, 温度、硝酸盐含量是控制崂山湾春季浮游植物分布的重要环境因子。     

长江口海域春季浮游植物的年际变化

于2014年5月在长江口海域采集海水样品,分析了浮游植物种类组成、细胞丰度、优势种等群落结构特征。共鉴定浮游植物105种,其中硅藻30属56种,甲藻19属41种,以及隶属于金藻、蓝藻等的8个种。浮游硅藻以近岸半咸水种为主,优势种类是中肋骨条藻(Skeletonema costatum);浮游甲藻以沿岸广布种为主,典型代表是东海原甲藻(Prorocentrum donghaiense)。为了更好地了解长江口海域浮游植物的年际变化趋势,本文收集整理了该海域1986—2014年间的春季数据,分析了浮游硅藻、浮游甲藻主要群落特征的年际变化,以及浮游硅藻优势种类细胞粒径的变化趋势。结果表明:(1)浮游硅藻、浮游甲藻的种类数均呈现不同程度的上升趋势,浮游甲藻的上升幅度更为明显;(2)浮游甲藻种类数在浮游植物种类数中的占有比例呈上升趋势,而浮游硅藻所占比例呈下降趋势;(3)浮游硅藻优势种类的细胞粒径呈现小型化趋势。     

夏季强降雨对胶州湾浮游硅藻群落结构的影响

为研究强降水对胶州湾浮游硅藻群落的影响及作用机制,作者利用2012年夏季暴雨前(7月17日)和暴雨后(8月5日)采集的浮游植物样品,对暴雨前后胶州湾海域浮游硅藻的种类组成、丰度和群落结构特征等进行了分析,探讨了台风导致的强降雨天气对浮游硅藻群落结构的影响。结果表明,暴雨前胶州湾海域共记录到硅藻22属54种,暴雨后,共记录到出硅藻18属39种,浮游硅藻种类数显著下降。暴雨前浮游硅藻的平均丰度为2 953.9×104个/m3,优势种有浮动弯角藻(Eucampia zoodiacus)、星脐圆筛藻(Coscinodiscus asteromphalus)、拟旋链角毛藻(Chaetoceros pseudocurvisetus)和劳氏角毛藻(Chaetoceros lorenzianus);暴雨后浮游硅藻平均丰度为111.4×104个/m3,比暴雨前降低了1个数量级,优势种组成也发生变化,有浮动弯角藻、星脐圆筛藻、大洋角管藻(Cerataulina pelagica)和波状石丝藻(Lithodesmium undulatum)。暴雨前后胶州湾海域浮游硅藻平面分布也发生变化,暴雨前在湾西部丰度值较高,暴雨后浮游硅藻平面分布相比暴雨前更加均匀。本研究可为深入探讨海洋环境异常变化对浮游硅藻乃至浮游植物群落的影响提供重要依据。     

福建三沙湾表层沉积硅藻分布特征及其与环境因子的关系

通过对三沙湾24个站位表层沉积物样品进行硅藻分析,共鉴定硅藻31属84种（包括变种和变型）,其中夏季和冬季分别鉴定出31属71种和31属67种,主要属种以中心纲硅藻为主,如爱氏辐环藻（Actinocyclus ehrenbergii）、波状辐裥藻（Actinoptychus undulatus）、横滨盒形藻（Biddulphia grundleri）、中心圆筛藻（Coscinodiscus centralis）等.在生态习性上,底栖海水种占绝对优势.硅藻丰度从湾顶向湾外递减,夏季丰度高于冬季,平均丰度分别为9 003个/g和6477个/g.沉积物粒度分析显示,三沙湾表层沉积物以粉砂质粘土为主,研究表明三沙湾硅藻丰度与平均粒径存在一定相关性,夏季硅藻丰度变化范围为2 104～28 209个/g,粒径范围为5.26～7.36Φ,硅藻丰度与平均粒径变化范围都较大;而冬季硅藻丰度变化在1 929～14 372个/g之间,粒径范围为5.51～6.98Φ,相对夏季其变化较小,论证了水动力是影响硅藻沉积分布的一个重要因素.对影响三沙湾表层沉积硅藻分布的主要环境因子进行探讨,三沙湾表层沉积硅藻主要受潮流、深度和盐度等影响较大.     

Phytoplankton assemblages in the CoOP-WEST coastal upwelling area

Elevated levels of phytoplankton were observed at the Northern California coastal upwelling ecosystem studied as part of the CoOP-WEST project during monthly summer surveys in 2000, 2001 and 2002. The high concentrations of chlorophyll were made up mostly of larger cells; the phytoplankton communities that resulted were dominated by centric diatoms. The highest chlorophyll a concentrations were observed when large diatoms or small colony-forming species dominated the assemblages. In contrast, when smaller nano-flagellates and picoplankton were dominant, total chlorophyll a concentrations were over four or five-fold lower than when diatoms were prevalent, illustrating the importance of large diatoms to total phytoplankton biomass. Each year, when chlorophyll concentrations were greatest, Chaetoceros species created a Chaetoceros-dominated system. A few other genera contributed to the upwelling diatom community, mostly the centric diatoms Asterionella, Thalassiosira, Rhizosolenia (including Guinardia and Proboscia), and the pennate Nitzschia. These diatoms have been described in other coastal upwelling ecosystems, and at this study site form a functional group that are apparently well adapted to the high-nutrient, turbulent conditions that are typical of these coastal regions.     

牡蛎对浮游植物群落下行控制作用的围隔实验研究

牡蛎的滤食作用不但能控制浮游植物丰度,还能通过摄食选择性影响群落结构。通过在较高营养水体的围隔实验表明,牡蛎养殖在持续磷限制条件下,促使浮游植物群落由甲藻向小型硅藻优势转变。实验结束时对照组中浮游植物群落由扁压原甲藻主导,占总丰度的77%;而牡蛎养殖组新月柱鞘藻丰度占比达到93%。牡蛎养殖组中,甲藻去除速率随养殖密度增长,但是硅藻最终丰度在高密度牡蛎组最高。研究结果说明大量的贝类养殖能够提升小型链状硅藻的优势度,增加此类赤潮发生风险。