CYANOBACTERIAL BUOYANCY REGULATION: THE PARADOXICAL ROLES OF CARBON1

In stratified lakes, dominance of the phytoplankton by cyanobacteria is largely the result of their buoyancy and depth regulation. Bloom-forming cyanobacteria regulate the gas vesicle and storage polymer contents of their cells in response to interactive environmental factors, especially light and nutrients. While research on the roles of nitrogen and phosphorus in cyanobacterial buoyancy regulation has reached a consensus, evaluations of the roles of carbon have remained open to dispute. We investigated the various effects of changes in carbon availability on cyanobacterial buoyancy with continuous cultures of Microcystis aeruginosa Kuetz. emend. Elenkin (1924), a notorious bloom-former. Although CO₂ limitation of photosynthesis can promote buoyancy in the short term by preventing the collapse of turgor-sensitive gas vesicles and/or by limiting polysaccharide accumulation, we found that sustained carbon limitation restricts buoyancy regulation by limiting gas vesicle as well as polysaccharide synthesis. These results provide an explanation for the positive effects of bicarbonate enrichment on cyanobacterial nitrogen uptake and bloom formation in lake experiments and may help to explain the pattern of cyanobacterial dominance in phosphorus-enriched, low-carbon lakes.     

Buoyancy Regulation and the Potential for Vertical Migration in the Oceanic Cyanobacterium Trichodesmium

Diel protein and carbohydrate content in Trichodesmium thiebautii was measured to evaluate the relationship to buoyancy status. Carbohydrate:protein ratio was the best predictor of buoyancy and fit a cosine curve with increasing values during the day and decreasing values at night in cycles that paralleled observed diel buoyancy patterns. This ratio also increased in short-term experiments as a function of light and increased in parallel with decreasing positive buoyancy. We used changes in this ratio to estimate the potential for vertical migration. Whereas limited vertical excursions in the upper 70 m are possible, deeper migrations appear unlikely unless respiration rates decrease significantly. N:P ratios in sinking and floating colonies were used to test for the P acquisition at depth (vertical migration). We noted that pooled N:P ratios were not significantly different between sinking and ascending colonies (N:P = 65.6 and 66.3, respectively) collected along the northern Australian coast, much like published results from north of Hawaii. Highly significant differences (p <0.0001) were observed in the western Gulf of Mexico between sinking and ascending colonies (N:P = 87.0 and 43.5, respectively) and provide the best direct evidence to date of vertical migration for P acquisition. Our physiological data on compositional changes during buoyancy reversals suggest a complex relationship between light and nutrients. It appears likely that light and P metabolism interact to regulate the vertical extent of migrations, with deep vertical migration regulated by P metabolism superimposed on a mixed-layer light-driven migration. The variability in N:P ratios suggests that care should be taken in assuming buoyancy reversals always result in P acquisition in this oceanic cyanobacterium.     

Palaeolimnological evidence for the independent evolution of neighbouring terminal lakes,the Murray Darling Basin,Australia

Phytoplankton composition and production are highly unpredictable within an estuary, due to the high variability of forcing factors, such as freshwater flow, salinity, nutrients and light. The Guadiana estuary has shown sharp inter-annual differences in freshwater flow, related to variable precipitation, which is expected to affect nutrient loadings, light availability and phytoplankton succession. Water retention due to dam construction will further enhance changes in river flow and ecosystem dynamics. The main goal of the present study was to describe and relate phytoplankton succession and environmental conditions, namely nutrients and light, in the Guadiana upper estuary (south-western Iberian Peninsula), a dam regulated temperate estuary. From March 2004 to October 2005, water samples were collected in three stations along a longitudinal transect covering the upper estuary. Several water variables were determined and phytoplankton composition was studied through inverted and epifluorescence microscopy. A typical freshwater phytoplankton succession was observed, from a diatom spring bloom to cyanobacteria dominance in the summer, and a second diatom bloom in the autumn. Neither nutrients nor light availability seemed to be related to the observed succession, especially the seasonal variation of diatom abundance. During summer, nutrient concentrations (especially Si) were high and non-limiting, whilst light was available in the mixing layer. However, diatoms were present in low numbers. Grazing pressure was probably responsible for the regulation of diatom seasonal succession in the Guadiana upper estuary, which should be addressed in future studies. Handling editor: K. Martens     

具齿原甲藻的生态特征及赤潮成因浅析

在东海的长江口和舟山海域,2000-2002年连续3年的5月发生罕见的大规模赤潮,赤潮生物为具齿原甲藻,为了解该种的生态特征及赤潮频发原因,本文分析近赤潮应急监测资料及野外生态调查资料,结果表明,该种适温,适盐范围分别为15-25℃和14-32,最适温度,盐度分别为18-22℃和22-31,该种运动能力较强,在海流交汇的紊乱水体中具有强适应能力,具有昼夜垂直移动特性,水温,硝酸盐含量,海流及上升流,磷限制环境下的种间竞争优势对诱发和维持赤潮具有重要作用.     

大型浅水富营养化湖泊中蓝藻水华形成机理的思考

湖泊富营养化依然是我国目前以及今后相当长一段时期内的重大水环境问题。研究蓝藻水华的形成机制 ,对于科学预测湖泊中蓝藻水华的产生 ,并采取相应措施减少其带来的影响具有重要的生态和环境意义。为探索富营养化湖泊中蓝藻水华形成机理 ,综述了目前对我国大型浅水湖泊蓝藻水华成因研究现状和对水华形成机理的一般认识。分析了导致蓝藻水华形成的化学、物理和生物等主要环境因素 ,论述了蓝藻 ,尤其是微囊藻成为水华优势种的可能原因。认为对水华的形成需要全面认识 ,营养盐浓度的升高可能仅是蓝藻水华形成、且人们可以加以控制的因素之一 ;在探索水华成因时 ,不能仅仅局限于夏季蓝藻水华发生时环境特征的研究与观察 ,而应该提前关注蓝藻的越冬生理生态特征、春季复苏的生态诱导因子及其阈值以及在复苏后 ,蓝藻如何在生长过程中形成群体 ,并逐步成为湖泊水生生态系统中的优势种乃至形成水华的过程。并需要对蓝藻越冬的生存对策、蓝藻群体的形成的条件、蓝藻在春季复苏的触发条件及其生态阈值、以及蓝藻在与其它藻类种群竞争中取胜的生理生化特征有足够的认识。蓝藻水华的“暴发”是表观现象 ,其前提还是藻类一定的生物量 ,且是一个逐渐形成的过程。根据生态学的基本理论和野外对水华形成过程的原位观测     

太湖水华程度及其生态环境因子的时空分布特征

湖泊水华是全世界面临的严重生态环境问题之一,对人类和生态系统健康都有重大影响。由于湖泊水华受流域面源、点源污染、气候、水文因子以及湖泊生态系统自身特征等众多因素影响,水华是否爆发、其严重程度及时空分布特征呈现明显的复杂性。以我国太湖为研究区域,基于近年的水华及水环境的监测数据,用自组织特征映射神经网络对太湖不同监测点的水华程度进行了自动聚类分析。结果表明,太湖水华程度呈现为明显的无水华、轻度、中度和重度水华4类。不同程度水华的叶绿素a、水温、COD_(Mn)、营养盐、浮游植物生物量以及藻种(蓝藻、绿藻、硅藻)结构的时空差异显著,不同变量间的关系复杂,有助于深入认识太湖近年水华发生的时空变异特性。     

Phytoplankton population dynamics in perennially ice-covered Lake Fryxell, Antarctica

Phytoplankton were collected over five austral summers (1987–88through 1991–92) to examine seasonal and annual fluctuationsin species composition and biovolume in Lake Fryxell, a perenniallyice-covered lake located in the Dry Valleys of South VictoriaLand, Antarctica. Lake Fryxell has perennial gradients of salinity,dissolved oxygen and nutrients. We found that algal speciesdiversity was low (56 taxa were collected), confirming the resultsof previous short-term studies. The phytoplankton consistedprimarily of cryptophyte and chlorophyte flagellates, and filamentouscyanobacteria. The presence of filamentous cyanobacteria, whichhave not been reported as abundant in this lake by previousworkers, may represent a significant ecological change. Eachaustral summer, one dominant species contributed >70% ofthe total biovolume; Chroomonas lacustris was dominant in 1987–88,while Cryptomonas sp. dominated the phytoplankton in the remaining4 years. No species succession occurred during the austral summer.Some common taxa were vertically stratified (Oscillatoria limnetica,Phormidium angustissimum, Pyramimonas sp., Oscillatoria sp.),while others showed no distinct vertical stratification (Chlamydomonassubcaudata, Cryptomonas sp.). The stratification of the phytoplanktonreflects the gradients of nutrients and light, and the stabilityof the water column.     

Maintenance of Coastal Surface Blooms by Surface Temperature Stratification and Wind Drift

Algae blooms are an increasingly recurrent phenomenon of potentially socio-economic impact in coastal waters globally and in the coastal upwelling region off northern Baja California, Mexico. In coastal upwelling areas the diurnal wind pattern is directed towards the coast during the day. We regularly found positive Near Surface Temperature Stratification (NSTS), the resulting density stratification is expected to reduce the frictional coupling of the surface layer from deeper waters and allow for its more efficient wind transport. We propose that the net transport of the top layer of approximately 2.7 kilometers per day towards the coast helps maintain surface blooms of slow growing dinoflagellate such as Lingulodinium polyedrum. We measured: near surface stratification with a free-rising CTD profiler, trajectories of drifter buoys with attached thermographs, wind speed and direction, velocity profiles via an Acoustic Doppler Current Profiler, Chlorophyll and cell concentration from water samples and vertical migration using sediment traps. The ADCP and drifter data agree and show noticeable current shear within the first meters of the surface where temperature stratification and high cell densities of L. polyedrum were found during the day. Drifters with 1m depth drogue moved towards the shore, whereas drifters at 3 and 5 m depth showed trajectories parallel or away from shore. A small part of the surface population migrated down to the sea floor during night thus reducing horizontal dispersion. The persistent transport of the surface bloom population towards shore should help maintain the bloom in favorable environmental conditions with high nutrients, but also increasing the potential socioeconomic impact of the blooms. The coast wise transport is not limited to blooms but includes all dissolved and particulate constituents in surface waters.     

Effects of Light and Wind Speed on the Vertical Distribution of Microcystis aeruginosa Colonies of Different Sizes during a Summer Bloom

Different dynamics, of the vertical distribution of Microcystis aeruginosa colonies of different sizes, were examined in a large shallow lake (Taihu Lake) during a summer bloom. During the observation the Microcystis colonies, larger than 120 μm, were mainly concentrated in the upper layer both under calm and windy conditions. Colonies from 36 μm to 120 μm showed a clear relationship to wind conditions whereas colonies less than 36 μm showed a nearly uniform vertical distribution at the different depths. There was no clear relationship between the vertical distribution of colonies larger than 36 μm and light intensity. However, at solar noon, colonies less than 36 μm were mainly distributed in the middle layers of the water column. The results indicate that small colonies are easily affected by the wind‐induced mixing, and they show an ability for active movement in response to light intensity under calm conditions. In contrast, large colonies tend to show little diurnal repositioning to light intensity and thereby are mainly concentrated in the surface layer. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

OBSERVATIONS ON THE SWIMMING AND BUOYANCY OF SOME THECOSOMATOUS PTEROPOD GASTROPODS

Cymbulia peroni produces forward propulsion only on the upstrokeof its ‘wingbeat’ the downstroke opposing slightnegative buoyancy. The species' torpedo-shaped pseudoconchaprovides stability as well as buoyancy. Corolla ovata producespropulsive power on the downstroke of its ’wings‘and shows periods of active upward swimming alternating withpassive downward gliding/parachuting. Both cymbuliid speciesare gelatinous slow movers, close to neutral buoyancy, and areincapable of effective diurnal vertical migration. Two cavoliniidspecies were also studied; Cilo pyramida and Cuvierina columnella.Both were dense and sank quickly if they ceased active swimming.Both had postural responses to slow sinking, but vigorous activitywas required to hover in the water column. Clio could not climbrapidly enough to carry out effective diurnal vertical migration,but was capable of fast horizontal swimming during which thetriangular shell appeared to act as a hydrofoil. The smallerCuvierina could not swim quickly in a horizontal direction,but was capable of swimming vertically at rates sufficient tomaintain station with moving isolumes and may therefore carryout significant diurnal vertical migrations. (Received 28 September 1989; accepted 12 December 1989)     

Stratification by cyanobacteria in lakes: a dynamic buoyancy model indicates size limitations met by Planktothrix rubescens filaments

The ability of the Planktothrix rubescens to stratify in Lake Zürich is related to the size and shape of the cyanobacterial filaments. Detailed measurements made in the lake are used in a dynamic computer model of buoyancy regulation to investigate the vertical movements of filaments tracking the depth at which the irradiance would support neutral buoyancy. The movement of the filament lags behind the constantly changing target depth owing to (a) the time taken for the filament to respond to the irradiance by changing its density and (b) the time it takes to move by sinking down or floating up through the water column. The model simulates the stratification depth over a 5-month period of the summer from the continuous measurements of irradiance and weekly measurements of light attenuation and temperature, without any further adjustment over the period. Models using filaments of the size observed in Lake Zürich explain several details of the observed depth changes: smaller planktonic cyanobacteria (e.g. Limnothrix sp.) are unable to migrate fast enough and larger ones (e.g. Anabaena spp.) will overshoot and become entrained in the epilimnion. The model can be used to simulate recruitment of Planktothrix filaments from different depths after vernal stratification. Recruitment of filaments from depths down to 45 m will contribute to the metalimnetic population increase in early July.     

垂向湍流、温、光对浅水湖泊下沉藻类垂向分布的影响

垂向湍流、温、光等是影响藻类生长和垂向分布的关键因素。基于大型浅水湖泊（太湖）的背景物理场（辐射、水深、水温和消光等）,利用藻类生长-扩散模型的敏感性试验,探讨物理过程对浅水湖泊中下沉藻的垂向分布的影响机制。水温是次表层叶绿素峰值（Subsurface Chlorophyll Maxinmum：SCMax）或谷值（Subsurface Chlorophyll Minimum：SCMin）形成的限制性条件;当水温大于阈值时,随深度的增大,过强光能抑制藻类的生长转变为弱光限制藻类的生长是SCMax形成的关键;垂向湍流的裹挟作用是弱化SCMax强度的关键过程。该研究有利于厘清物理过程对藻类群落演替的作用机制,强化对水生态系统的修复效果。     

Modelling the distribution of Anabaena and Melosira in a stratified river weir pool

We have developed a model to predict the stratification dynamics and its influence on the distribution of algal species in a river with strong diurnal changes in temperaturestratification. While the phytoplankton concentrations vary inresponse to buoyancy and light limited growth, the populationsare also redistributed vertically due to diurnal mixing causedby surface evaporation and night time cooling. Theparameterization of the vertical eddy diffusivity coefficientwithin the model takes into account the generation of mixing byheat losses at the surface and the suppression of mixing bythermal stratification. The model is successfully tested against extensive temperatureand algal concentration data in Maude weir pool on theMurrumbidgee River, Australia. Given literature values offloating/sinking rates and maximum growth rates for thecyanobacterium Anabaena and the diatom Melosira, wewere able to reproduce extremely well the time evolution ofconcentrations of each species over long periods of low flow. This revised version was published online in July 2006 with corrections to the Cover Date.     

Implications of seasonal mixing for phytoplankton production and bloom development

Based on a 1D model considering phytoplankton and nutrients in a vertical water column, we investigate the consequences of temporal and spatial variations in turbulent mixing for phytoplankton production and biomass. We show that in seasonally mixed systems, the processes controlling phytoplankton production and the sensitivity of phytoplankton abundance to ambient light, trophic state and mixed-layer depth differ substantially from those at steady state in systems with time-constant diffusivities. In seasonally mixed systems, the annually replenished nutrient pool in the euphotic zone is an important factor for phytoplankton production supporting bloom development, whereas without winter mixing, production mainly depends on the diffusive nutrient flux during stratified conditions. Seasonal changes in water column production are predominantly determined by seasonal changes in phytoplankton abundance, but also by seasonal changes in specific production resulting from the transport of nutrients, the exploitation of the nutrient pool and the increase in light shading associated with phytoplankton growth. The interplay between seasonal mixing and the vertical distribution of mixing intensities is a key factor determining the relative importance of the processes controlling phytoplankton production and the sensitivity of the size and timing of the annual maximum phytoplankton abundance to the abiotic conditions.     

Dinoflagellate growth rate in water columns of varying turbidity as a function of migration phase with daylight

Photosynthetic dinoflagellates occasionally exhibit nonphototacticphase relationships between their diurnal vertical migrationand the daylight cycle. The cause of this variability in specificfield cases is generally unclear. A computer model, MIGTIM,is reported that incorporates light dependence of the initialslope () and maximum photosynthesis (P_max) of the photosynthesisversus light intensity (P-I) curve. MIGTIM is used to determinethe migration phase that optimizes growth rate of sinusoidallymigrating populations under different conditions of photosyntheticlight dependence, turbidity and migration depth. Migration phaseprovides little selective advantage for shallow populationsor for populations in low turbidity water columns. Migrationphase provides a strong selective advantage for deep populationsor for populations in high turbidity water columns. A comparisonbetween MIGTIM output and published field observations suggeststhat field populations experienced optimum growth rate underthe observed environmental and biological conditions. The analysissupports the contention that, under appropriate conditions,water column turbidity is an important component of the complexequation that determines migration phase and that this componentacts through the organism's cumulative diurnal photosyntheticresponse.     

四川丘陵区水库浮游植物功能群季节演替特征及驱动因子

为了解四川丘陵水库浮游植物群落结构、演替规律和影响因素,于2016—2017年分4个季节对该区域10个中小型水库的浮游植物功能类群演替与水环境变化关系进行了分析。研究期间10个水库共检出浮游植物9门104属188种,分属于24个功能群;在全年或各季节占优势的功能群,多为喜高营养或喜静水,耐低光、不耐冲刷和高光的类群,包括低光静水类群S1、S2、SN和Y,高营养静水类群TC、X1和X2,高营养低光类群J和C,及具有广适性的LO。其中,全年优势度指数Y>0.02的功能群有6个,高低依次为S1>TC>SN>S2>LO>J。单因素方差分析表明:不同季节优势功能群存在明显差异(P<0.05),其中夏季为S1+TC+SN+LO+S2,秋季为S1+S2+J+SN+X1+LO+X2,冬季为S1+X2+C+X1+J+Y,春季为S1+SN+H1+J+X1+X2。丝状蓝藻的种类组成(包括S1、S2、SN、TC和H1)在四季间有所更替,但其优势度并未改变;季节间水库分层的形成与打破,及水体光照强度、牧食强度和营养水平等因素的变化,导致了该地区水库浮游植物功能群之间的此...     

湖泊蓝藻水华发生机理研究进展

蓝藻水华是富营养化湖泊常见的生态灾害,通过产生毒素、死亡分解时使水体缺氧和破坏正常的食物网威胁到饮用水安全、公众健康和景观,会造成严重的经济损失和社会问题,揭示其发生机理是进行防治的基础。综述了蓝藻水华发生机理的主要假说和证据,主要分为环境因子(营养盐、氮磷比、温度、微量元素、浮游动物牧食、水文和气象条件等)和生理生态特性(伪空泡、胶质鞘、CO2浓缩机制、适应低光强、贮藏营养物质、防晒、产毒素和固氮等)两个方面;评述了主要新理论,展望了今后的研究。到目前为止的研究表明寻找一两个关键因子并不能阐明蓝藻水华的发生机理。现存的理论或假说尽管已经在蓝藻水华的防治实践中产生重要作用,但仍然未能清楚地阐释其发生的客观规律。认为蓝藻水华是在各种环境因子(外因)的耦合驱动下,水华蓝藻由于其独特的生理生态特性(内因),产生巨大的生物量而在浮游植物群落中占绝对优势,在合适的水文气象条件下集聚于水表而形成。因此水华机理的研究应同时关注水华蓝藻的生理生态学规律和蓝藻水华发生的各种环境条件。不同环境因子协同影响水华蓝藻的不同生理生态特性的表达,从而影响水华的发生过程,将可能是以后研究的重点。蓝藻水华机理的研究在微观方面正趋向于应用分子生物学手段分析蓝藻生理过程,宏观方面则将广泛应用遥感遥测技术观测全湖蓝藻的变化规律。今后加强对水华蓝藻生理生态特性的基因表达与调控和环境多因子耦合作用于蓝藻水华过程的研究将有重要意义。蓝藻水华的机理研究包括现象、过程和原因3个层次的问题,通过大量的现象和过程的研究,不断揭示其发生过程中水华蓝藻的群落演替、种群发展、细胞活性和分子机理等变化规律,才能找到其发生的真正原因,为其防治提供理论依据和更好的治理措施。在蓝藻水华防治方面,控制营养盐和生态修复可能将是今后很长时间内最根本最有效和最具操作性的方法。     

A numerical model of nitrogen flxation and its application to Lake Valencia,Venezuela*

SUMMARY. 1. A numerical model for calculation of daily and annual nitrogen fixation in lakes is presented. The model is based on empirically-derived equations for the rates of nitrogen fixation by heterocysts (nitrogen-fixing cells) in relation to light and on functions for the vertical and tetnporal distributions of heterocysts and light in a lake. 2. Applications of the model to Lake Valencia, Venezuela, between December 1980 and December 1981 indicated that nitrogen fixation is largely a surface phenomenon in this lake: 80% of diurnal fixation occurred within 1m of the water surface. 3. Nitrogen fixation is largely restricted to periods of lake stratification, when the phytoplankton have sufficient light for growth, but dissolved inorganic nitrogen is scarce. Nitrogen fixation was maximal late in the stratification period of 1981: 85 % of fixation occurred within the last 3 months of the 9-month period. 4. The annual nitrogen fixation in Lake Valencia is 26 kg ha^?1, which is comparable to the nitrogen fixation in temperate eutrophic lakes with seasonal blue-green algal blooms. However, nitrogen fixation accounted for only 23% of the total nitrogen supply to Lake Valencia in 1981.