The influence of wind-induced mixing on the vertical distribution of buoyant and sinking phytoplankton species

In this study we exploit recent advances in high-resolution autonomous monitoring to investigate the impact of short-term variations in wind-induced mixing on the surface biomass and vertical distribution of buoyant and sinking phytoplankton species. An autonomous platform (the Automatic Water Quality Monitoring Station) moored in a Mediterranean reservoir provided minute-by-minute records of wind speed and the phytoplankton fluorescence during winter and summer. This information was then used here to quantify the impact of short-term changes in the weather on the vertical distribution of diatoms and cyanobacteria. Additionally, we apply an empirical model to determine the extent of entrainment of diatoms and cyanobacteria within the turbulent upper layers of the water column. During winter, the surface time series of fluorescence was positively correlated with the short-term variations in wind speed. In contrast, during the summer, fluorescence was negatively correlated with wind speed. In the latter case, turbulence overcame the flotation velocity of buoyant cyanobacteria, thus homogenizing their vertical distribution and decreasing surface biomass. In both cases, the dynamic response of surface phytoplankton biomass to short-term changes in wind stress was rapid, within the minute scale. As far as we know from the literature, this is the first study in which the interaction between wind stress and surface phytoplankton fluorescence has been quantified on such a fine temporal scale. Finally, relevance for forecasting and reservoir management is pointed out.     

Diel variation in the vertical distribution of fish and plankton in Lake Kinneret: a 24-h study of ecological overlap

Diel vertical migration (DVM) behaviour is a predator avoidance mechanism observed within many zooplankton species in the presence of zooplanktivorous fish. A 24-h survey was carried out in June 1998 to investigate diel variation in the vertical distribution of fish, zooplankton and phytoplankton (chlorophyll) in Lake Kinneret, Israel. Fish revealed diel variation in vertical distribution but had no spatial overlap with zooplankton, and consequently no apparent influence on zooplankton dispersal. Zooplankton revealed some diel variation in distribution being affected by thermocline and oxycline position and movement of the internal the internal seiche wave. Cyclopoid species closely follow the movement of the seiche wave implying that, due to their greater motility, they are following conditions that are suitable to them. The Cladocera species and small rotifers only partly, which may be part of their phototaxic behaviour. Physical forces like convection, horizontal and vertical forcing probably have a role in contributing to a homogeneous distribution of the plankton by preventing stratification or interfering with the more motile zooplankton which may be attempting to migrate.     

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。     

Phytoplankton development and turbulent mixing in Lake Kinneret (1992-1996)

We have utilized data from a recently developed three-dimensionalvelocity fluctuation meter to compute the dissipation of turbulentkinetic energies (TKE) and the intensity of turbulent mixingin horizontal and vertical planes in the pelagic, epilimnicwater of Lake Kinneret, Israel. These characteristics of wind-inducedturbulent movement have been monitored from January 1992 throughDecember 1996. The turbulence parameters were strongly correlatedto wind energy inputs, calculated daily as 5 day cumulativeinputs. There have been dramatic changes in the annual and seasonaldevelopment of phytoplankton, together with unusually high levelsof primary production in this lake since 1994. We observed differentpatterns of vertical and horizontal turbulent movement and ofTKE dissipation rates during the years when ‘unusual’phytoplankton development occurred (1994–1996) comparedto ‘normal’ years (1992, 1993). The first appearanceof the filamentous cyanobacterium Aphanizomenon in this lakein August–September 1994 coincided with a period of markedlylower rates of TKE dispersion and a shift from vertical to horizontaldominance of the turbulent eddy spins. The absence of a regularwinter-spring bloom of the dinoflagellate, Peridinium, in 1996occurred when dissipation rates of TKE were extremely high,while record high amounts of dinoflagellates (1994, 1995) appearedwhen dissipation rates were very low. Correlations were shownbetween phytoplankton parameters (chlorophyll, primary productionand the ratio of primary production to chlorophyll) and boththe dissipation rate of TKE and the intensity of water turbulentmixing in the vertical plane. We suggest that the changes inthe ‘turbulence climate’ of Lake Kinneret were animportant factor in determining shifts in phytoplankton successionand the population composition of the algal assemblage.     

The phytoplankton community of a eutrophic reservoir

The dynamics of the phytoplankton community of a eutrophic reservoir are described for a two year period. Fifty-eight species were recorded, 25 of them common. Bacillariophyta dominated during the winter and early spring and Chlorophyta during late spring, to be replaced by a bloom of Cyanophyta. The mean and peak biomass of phytoplankton was 8.6 mg 1^–1 and 40.8 mg 1^–1 in 1981, and 8.3 mg 1^–1 and 37.6 mg 1^–1 in 1982. Temperature accounted for 67.3% and pH for 8% of the variation in total phytoplankton biomass over the two year period, using a multiple regression technique.Both horizontal and vertical patchiness, measured as an index of mean crowding, were recorded in the reservoir. Horizontal aggregations were associated with spring blooms of Chlorophyta and summer blooms of Cyanophyta, while vertical aggregations were most marked during the summer bloom of Cyanophyta. Concentrations of phytoplankton were influenced by wind, the prevailing southwesterly wind accumulating algae in the northeasterly arm of the reservoir during much of the year.     

Factors influencing the spatial distribution of zooplankton and fish in Loch Ness, UK

• 1 The vertical and horizontal distribution of phytoplankton, zooplankton and fish in Loch Ness, Scotland, were monitored during one day‐time and one night‐time survey in July 1992. The vertical samples were collected at a site located at the northern end of the loch and the horizontal samples along a longitudinal transect.
• 2 The vertical distribution surveys demonstrated that the phytoplankton, the zooplankton and the fish were concentrated in the top 30 m of water above the seasonal thermocline. Within this layer, Cyclops stayed much closer to the surface than Eudiaptomus but both species moved towards the surface at night.
• 3 The most important factor influencing the horizontal distribution of the phytoplankton was the north‐ south gradient in productivity. The sub‐catchments surrounding the north basin contain a greater proportion of arable land than those to the south and the concentrations of nitrate‐nitrogen and phytoplankton chlorophyll increased systematically from south to north.
• 4 Zooplankton distribution patterns were influenced by wind‐induced water movements and the dispersion of allochthonous material from the main inflows. The highest concentrations of Cyclops were recorded in the north, where there was more phytoplankton, and the highest concentrations of Eudiaptomus in the south, where there were higher concentrations of non‐algal particulates.
• 5 There was no spatial correlation between total zooplankton and total fish abundance but the highest concentrations of small (1–5 cm) fish were recorded in the south where there was a large patch of Eudiaptomus. The number of Eudiaptomus at specific locations within this patch were, however, negatively correlated with the numbers of small fish. These results suggest that the fish were actively foraging within the patch and were depleting their zooplankton prey in the areas where they were most abundant.

     

Modeling microzooplankton and macrozooplankton dynamics within a coastal upwelling system

A simple nutrient–phytoplankton–zooplankton (NPZ)pelagic ecosystem model coupled to a two-dimensional primitiveequation circulation model with explicit mixed-layer physicsis configured in a coastal setting to study the biological responseto idealized wind-driven upwelling conditions. Conventionalecosystem model parameterization, which assumes macrozooplanktonas the target grazers, leads to upwelling-induced phytoplanktonblooms that exhaust available nutrient supply and whose zonalscale increases with wind duration. Offshore zooplankton maximaresult from upwelled water with greater total nitrogen concentrationsthan initial ambient surface water. Substantial vertical mixingin the surface boundary layer sets the vertical scale of theproductivity. Phytoplankton sinking contributes to a nearshoreaccumulation of total nitrogen, and enhances the magnitude andduration of the phytoplankton bloom. The system responds differentlywhen the zooplankton are parameterized to represent microzooplankton.The phytoplankton and zooplankton maxima have more limited zonalextent, are more independent of the duration of wind forcing,and near-surface nutrient levels remain high over most of thedomain. When winds are relaxed, the diminished offshore transportreveals the underlying biological oscillations in the microzooplankton-parameterizedecosystem, and reduced vertical mixing decouples surface fromsubsurface dynamics. In contrast, the macrozooplankton systemrelaxes to a steady state supporting limited phytoplankton andlarge zooplankton levels in the upwelling region.     

Environmental forcing on the interactions of plankton communities across a continental shelf in the Eastern Atlantic upwelling system

To investigate whether phytoplankton is the main factor determining mesozooplankton distribution in a continental shelf affected by upwelling, oceanographic surveys were conducted off NW Portugal in 2002 and 2003. During four different seasons, we investigated how the mesoscale relationship between these two communities was forced by environmental conditions across the shelf. The horizontal and vertical distribution of phytoplankton was shaped by wind stress over the water. Diatoms dominated mixed and upwelled waters, whereas dinoflagellates prevailed with thermal stratification. Mesozooplankton was less influenced by wind forcing and concentrated mostly at mid-shelf, on the outer margin of main phytoplankton patches. We found that this pattern, under strong thermal stratification conditions, can be associated with localized grazing “hot spots”. Copepods were dominant, contributing to the mesozooplankton community variability between cruises whereas phytoplankton presented a clearer seasonal pattern. Nevertheless, the distribution and abundance of mesozooplankton were directly correlated with all phytoplankton groups at the inner-mid-shelf, while no correlation was observed offshore the outer-shelf. The relative composition of mesozooplankton did not vary between depth strata, whereas a cross-shelf separation occurred between nearshore and widespread clusters of species. This reflected a dependence on phytoplankton and reflected the high variability of oceanographic conditions of the study area.     

Spatial distribution of motile phytoplankton in a stratified reservoir: the physical controls on patch formation

Changes in the spatial distribution of the dinoflagellate Ceratiumhirundinella were observed in a stratified, medium-sized (16km²) Argentinean reservoir over several days. A fluorescenceprofiling technique was used to identify persistent patchinessin the distribution of the dinoflagellate. A three-dimensionalnumerical model was used to reconcile a range of different unsteadyprocesses and prove that the initial source of heterogeneityin the system was the vertical migration of Ceratium. Once migrationestablished vertical heterogeneity, the dominant influence onthe patch dynamics alternated between control by migration andcontrol by mixing and transport. This led to the developmentof persistent horizontal patchiness. The analysis revealed thatthe region of the lake inhabited by Ceratium was highly predictableand from this result it was determined that physical processes(with some influence from migration) control the habitat ofthis dinoflagellate rather than biological/chemical gradients.When the spatial habitat of a particular phytoplankton speciescan be isolated in this manner, the resources available to thespecies can be more accurately determined by further study.The results are particularly applicable to the study of motile/buoyantplankton in aquatic systems that are periodically subject tomoderate or strong wind forcing events.     

Factors influencing the nonrandom abscission of Solidago canadensis seeds

 为揭示加拿大一枝黄花(Solidago canadensis)种群扩散机制, 明确种子的脱落及风传扩散在其种群蔓延中的作用, 在人工环境下测定了不同湍流强度、风速和湿度处理下种子脱落的差异, 并对脱落种子与未脱落种子进行形态学特征对比。结果表明: 加拿大一枝黄花的种子脱落受湍流、风速和湿度等因素的共同影响。水平气流下种子的脱落阈值为5.1 m·s^–1, 并随着风速增加, 种子的脱落率增加。与模拟水平气流相比, 模拟垂直气流下种子的脱落阈值显著偏小。相对于层流状态, 湍流的存在显著提高了种子的脱落率, 平均增幅超过300%; 但单纯提高湍流强度对种子脱落率的影响不显著。增加湿度则显著降低种子的脱落率。种子形态学特征对比结果表明, 脱落种子的冠毛数量和冠毛夹角显著高于未脱落种子。该研究结果为研究加拿大一枝黄花种子脱落规律和风传扩散机制提供了科学依据, 也为其他入侵性杂草种子的扩散机制及入侵过程提供了借鉴。     

The spatial distribution of different phytoplankton functional groups in a Mediterranean reservoir

A new high-resolution spectrofluorimetric probe and an automatic water-quality monitoring station (AWQMS) have been used to record seasonal variations in the spatial distribution of three functional groups of phytoplankton in a Mediterranean water-supply reservoir. In comparison with classical methods, the combined use of these innovative techniques enables development of faster and less laborious spatial distribution surveys, thus favouring higher-frequency and spatially more detailed measurements, and, consequently, a better understanding of phytoplankton dynamics. The results show that the observed variations can be explained by the interaction between the buoyancy properties of the phytoplankton and the mixing characteristics of the reservoir. During the winter, when the lake was isothermal and the phytoplankton was dominated by diatoms, there was no significant spatial variation. In the spring, when the phytoplankton was dominated by chlorophytes there was also very little variation but some motile species formed patches when the wind speed was low. The most pronounced non-uniform distributions of phytoplankton were observed during the summer when the phytoplankton community was dominated by positively buoyant cyanobacteria. Then there was a very strong link between the vertical and horizontal gradients which were also related to the prevailing meteorological conditions.     

The effects of diel changes in circulation and mixing on the longitudinal distribution of phytoplankton in a canyon‐shaped Mediterranean reservoir

1. The near‐surface distribution of phytoplankton cells along the thalweg of a canyon‐shaped reservoir (El Gergal, southern Spain) during two surveys is described and interpreted as the result of time‐varying large‐scale circulation patterns, vertical mixing processes and the physiological capacity of algal cells to regulate its position in the water column. 2. Vertical gradients of chlorophyll‐a concentration developed in the water column during the day but disappeared at night, as a result of the shoaling and deepening of the diurnal mixed layer (dml). The changes in the depth of the dml are largely controlled in El Gergal by convectively driven mixing processes. The longitudinal circulation changes, in turn, as a result of weak and diurnal land‐sea breezes. The distribution of algal cells was patchy at all times but did not change during any of the surveys. 3. An expression is proposed to estimate time scales for the development of horizontal patchiness T_P based on simple concepts of transport. It is shown that T_P is in the order of a week, indicating that horizontal patchiness does not respond immediately to hourly changes in the controlling factors. The magnitude of T_P, though, depends on how the vertical distribution of chlorophyll‐a and longitudinal currents change on subdiurnal time scales. In particular, T_P is sensitive to the lag existing between the momentum and heat fluxes through the free surface, driving circulation and vertical mixing.     

How do sinking phytoplankton species manage to persist?

Phytoplankton require light for photosynthesis. Yet, most phytoplankton species are heavier than water and therefore sink. How can these sinking species persist? Somehow, the answer should lie in the turbulent motion that redisperses sinking phytoplankton over the vertical water column. Here, we show, using a reaction-advection-diffusion equation of light-limited phytoplankton, that there is a turbulence window sustaining sinking phytoplankton species in deep waters. If turbulent diffusion is too high, phytoplankton are mixed to great depths, and the depth-averaged light conditions are too low to allow net positive population growth. Conversely, if turbulent diffusion is too low, sinking phytoplankton populations end up at the ocean floor and succumb in the dark. At intermediate levels of turbulent diffusion, however, phytoplankton populations can outgrow both mixing rates and sinking rates. In this way, the reproducing population as a whole can maintain a position in the well-lit zone near the top of the water column, even if all individuals within the population have a tendency to sink. This theory unites earlier classic results by Sverdrup and Riley as well as our own recent findings and provides a new conceptual framework for the understanding of phytoplankton dynamics under the influence of mixing processes.     

Turbulence disturbs vertical refuge use by Chaoborus flavicans larvae and increases their horizontal dispersion

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Spatial Heterogeneity of Cyanobacteria and Diatoms in a Thermally Stratified Canyon‐Shaped Reservoir

Phytoplankton communities in lakes and reservoirs are seldom homogeneously distributed but usually aggregate in patches and gradients. In this study we have combined the use of in vivo spectrofluorometry and acoustic Doppler current profiling to investigate the effect of water movements on the spatial distribution of cyanobacteria and diatoms in a thermally stratified reservoir in SW Spain. The distinctive canyon‐shaped morphometry of the reservoir (El Gergal) favoured the development of a “conveyor belt” pattern of circulation aligned with the long axis of the reservoir. Under non‐regulated conditions, the spatial distribution of phytoplankton was almost entirely dependent on the interactions between advective transport and the buoyancy properties of the different functional groups of phytoplankton. The positively‐buoyant cyanobacteria accumulated near the surface and were then transported downwind by the surface drift currents. In contrast, the negatively‐buoyant diatoms sank in the water column and were transported upwind by the sub‐surface return currents. When deep water was abstracted from the reservoir, these distribution patterns were modified. The results are discussed in relation to the problem of acquiring representative water samples from the reservoir and the application of a simple empirical model to optimize the location of the station used for routine cyanobacteria sampling on the reservoir. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Circulation patterns; their investigation and prediction

The effect of wind on the water currents in Rutland Water is being studied with a view to predicting algal distribution patterns so that, by exercising different abstraction options, water treatment problems can be avoided. Investigations using a scale, physical model of the reservoir in a wind tunnel suggest that under most wind directions the circulations tend to separate the water body into three distinct, topographic regions. Water currents in the reservoir itself have been plotted by tracking the paths of drogues using theodolites. This has indicated that in the central basin the circulation may be of the conveyor belt type, rather than the solely horizontal type of the model. Surface chlorophyll readings are shown to indicate wind-driven water currents. Algal distribution has been predicted from wind data; however, the analysis requires modification in order to apply to all wind conditions. The effect of wind on the vertical distribution of algae and its implication for abstraction are discussed.     

Winds under the Rain Forest Canopy: The Aerodynamic Environment of Gliding Tree Frogs

Movement of air under the canopy of a forest affects the gliding of animals such as frogs, snakes, geckos, and squirrels; the dispersal of pollen, seeds, and spores; as well as convective transport of heat and carbon dioxide. Wind speed profiles were measured under the canopy of a lowland rain forest during the morning, afternoon, and night at three sites in Costa Rica to determine the aerodynamic environment in which tree frogs maneuver while gliding. During the course of a day, average and maximum wind speeds were highest in the morning and midday, and lowest at night. Wind speeds under the canopy were highest near the top of the canopy and were lowest near the canopy floor in the morning and afternoon, and exhibited little variation with respect to height at night. Turbulence intensity (a common measure of gustiness) was constant (ca 1) for all times of day, heights in the canopy, and sites, but the absolute magnitudes of wind gust speeds were higher during the day than at night. Power spectral densities revealed that most of the variation in wind speeds occurred at frequencies that could potentially affect the gliding of tree frogs. Tree frogs (and many other gliding animals), however, glide at night and thereby avoid the higher wind speeds that occur by day. Computer simulations of the dynamic motions of frogs while gliding revealed that the night levels of wind gusts have little effect on the direction of gliding of tree frogs.     

Macrozooplankton and the persistence of the deep chlorophyll maximum in a stratified lake

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极端干旱区大气边界层厚度时间演变及其与地表能量平衡的关系

主要采用ECMWF的地表和大气产品分析了中国西北极端干旱区大气边界层厚度与地表能量通量的时间变化特征,同时,结合探空加强观测分析了大气边界层演变的可能因素.得出:西北极端干旱区大气边界层厚度呈现出季节性的年际和年代际变化,夏季大气边界层厚度呈下降趋势,春、秋季节呈现出先增加后降低的趋势,冬季以阶段性降低趋势为主,20世纪80年代是大气边界层厚度的转折时期;感热通量是极端干旱区大气边界层发展的主要热力因素;由于夏季净辐射量、地气温差、粗糙度以及风速等因子随时间演变而呈降低趋势,潜热通量呈增加趋势,导致了边界层高度形成的热力作用减弱,边界层厚度降低;同时,粗糙度和风速也是大气边界层发展的主要动力因素,由于边界层粗糙度和风速降低,促使垂直风切变减小,湍流动力作用减弱,也会导致边界层厚度降低.     

Spatial distribution of phytoplankton in Lake Baikal, Spring 1991

• 1 Studies carried out in Lake Baikal in late spring (late May-early June) 1991 showed marked differences in the species composition and abundance of phytoplankton in different regions of the lake. The south and north basins were characterized by small forms of algae. The middle basin, Maloe More and the shallow waters of the Selenga had species with large cells including Aulacoseira islandica subsp. helvetica, Dinobryon cylindricum and D. divergens.
• 2 Areas of high biomass were correlated with shallow waters and river inputs. This was especially apparent in the region of the Selenga delta. The vertical distribution of phytoplankton indicated the non-synchronous start of the spring homothermy throughout the lake. Phytoplankton were concentrated in the upper 100 m layer with subsurface maxima resulting from the sinking of large algae.
• 3 The concentration of phytoplankton biomass in general at this time characterized the lake as moderately productive.