Small-scale diel vertical migration of zooplankton in the High Arctic

Diel vertical migration (DVM) of zooplankton is considered less prominent at high latitudes where diel changes in irradiance are minimal during periods of midnight sun and polar night, leaving zooplankton without a temporal refuge and thus eliminating a key advantage of DVM. One of the shortcomings of previous DVM studies of zooplankton based on net sampling is that the depth resolution often has been too coarse to detect vertical migrations over short distances. We investigated DVM of zooplankton during August 2010 in drifting sea ice northeast of Svalbard (~81.5°N, ~30.5°E). Classical DVM behaviour (midnight rising, midday sinking) was observed between 20 and 80 m in young copepodite stages (CI–III) of Calanus finmarchicus and Calanus glacialis. The copepods Microcalanus spp., Pseudocalanus spp., Oithona atlantica, Oithona similis and Triconia borealis, alongside Eukrohnia hamata, Limacina helicina, and Fritillaria spp., all displayed signs of DVM. We conclude that zooplankton exhibit DVM in ice-covered waters over rather short distances to optimise food intake in the presence of predators.     

Seasonal vertical distribution and diel migration of zooplankton in a temperate stratified lake

The investigation of the vertical distribution of the zooplankton community in the temperate Lake Trichonis during four seasons in 2005, showed the existence of vertical segregation among species, ontogenetic stages and sexes within and between the major groups. In each season, the two or three more abundant rotifer species distributed at separate depth layers, while this feature was maintained during the entire 24 h period, since no diel vertical migrations (DVM) were performed. In contrast, the crustacean community, comprised mainly by the calanoid copepod Eudiaptomus drieschi and the cladoceran Diaphanosoma orghidani, showed various patterns of DVM, being more pronounced in spring and summer. Females of E. drieschi distributed deeper than males, while the copepod nauplii were found mainly in the surface layer in all four seasons. Temperature was the most important abiotic factor affecting directly and indirectly the vertical distribution and migration of various species. During stratification, the metalimnion was the most productive layer in Lake Trichonis, having maximum values of dissolved oxygen and low transparency due to high concentration of organic matter and phytoplankton. The DVM patterns of the crustaceans indicate that the metalimnion acts probably as a daylight refuge against predation by Atherina boyeri, which is the dominant planktivorous fish in the lake.     

Small-scale distribution and diel vertical migration of zooplankton in a shallow lake (Lake Naardermeer,the Netherlands)

Groundwater is a major influence on the hydrological, chemical and thermal regime of chalk streams in the southern U.K. However, little is currently known about the nature of the sediment delivery system within these chalk stream systems, even though sediment-related problems have been increasingly cited as a cause of habitat degradation and of declining salmonid stocks. To address this knowledge gap, suspended sediment fluxes were monitored at 4 sites within the Hampshire Avon catchment between February 1999 and August 2000. Maximum suspended sediment concentrations ranged from nearly 45 mg l^–1 to 260 mg l^–1. Over the study period, annual suspended sediment loads ranged from 644 to 6215 t yr^–1 and annual specific sediment yields ranged from 1.4 to 12.5 t km^–2 yr^–1. The results show that, relative to other U.K. rivers, the study chalk streams are characterised by low suspended sediment concentrations and loads and less episodic behaviour.     

浮游动物昼夜垂直迁移机理的主要假说及其研究进展

对有关浮游动物昼夜垂直迁移(DVM)机理的实验、假说以及理论模型方面的研究进.展进行了综述。昼夜垂直迁移通常指常规迁移(傍晚上升,拂晓下降),其行为过程不仅影响浮游动物的垂直分布,而且也间接地影响其水平分布及生活史特征,对浮游动物在一定水域的种群维持和补充具有重要的意义。与垂直迁移机理有关的假说主要有光驱动假说、逃避捕食者假说、能量和资源利用假说等,其中逃避捕食者假说已得到大量的实验证实。其理论模型研究一般在两个时间尺度上进行,一个是短期的行为机制研究,另一个从长期的生活史策略的角度考虑。随着实验技术的进步和理论及建模工具的发展,这两个时间尺度的研究正逐渐统一到一个框架下进行。     

Living planktonic foraminifera in the Fram Strait (Arctic): absence of diel vertical migration during the midnight sun

The timing of vertical migration in planktonic foraminifera (ex. ontogenetic, diel) is still an open debate. This work aims to investigate the diel vertical migration (DVM) of Neogloboquadrina pachyderma (N. pachyderma) and Turborotalita quinqueloba (T. quinqueloba) in the Arctic during the midnight sun. N. pachyderma and T. quinqueloba dominate the total assemblage in the cold Polar Water and warmer North Atlantic Water masses, respectively. Foraminifera were collected at several depths along the Fram Strait. Afterwards sampling was performed at the same station for 24 h at continuous and discrete time intervals. Results show no evidence of planktonic foraminifera DVM since there was no significant variability in the abundance and size distribution during the 24-h collection period. This finding provides information to improve the interpretation of foraminifera in paleoclimatic works. This is especially relevant in the Fram Strait as paleoclimatic studies in this region are fundamental to investigating the history of the Atlantic water inflow into the Arctic Ocean.     

Distribution and diel vertical migration of the euphausiid Thysanoessa macrura in Gerlache Strait,Antarctica

Thysanoessa macrura was found throughout Gerlache Strait, Antarctica, during four surveys carried out from 30 October to 23 November 1989, with the highest abundance being 332 individuals m^–2 (0–290 m). Reproduction had begun just before the surveys took place, as indicated by the presence of females with attached spermatophores and of larvae. Thirteen-month old females were reproductive. Larvae in 9 depth strata between 0–290 m were dominated by calyptopis stages, and developed from calyptopis 1 to furcilia 1 during November. Larval abundance was not correlated to chlorophyll a concentration, which showed a consistent east-west gradient in Gerlache Strait with highest concentrations (>30 mg chlorophyll a m^–3) in bays of the Antarctic Peninsula. Survival of larvae appeared to not be affected by phytoplankton abundance. Older T. macrura showed strong diel vertical migration between the surface at night and depths to 120 m during mid-day. Larvae were consistently found in the chlorophyll a-rich upper 50 m during night (90%) and day (81%), while adults and juveniles were found in the upper 50m at night (83%), but only 16% remained there during the day.     

Community structure and diel migration of zooplankton in shallow brackish lakes: role of salinity and predators

Diel horizontal migration (DHM), where zooplankton moves towards macrophytes during daytime to avoid planktivorous fish, has been reported as a common migration pattern of zooplankton in shallow temperate freshwater lakes. However, in shallow eutrophic brackish lakes, macrophytes seem not to have the same refuge effect, as these lakes may remain turbid even at relatively high macrophyte abundances. To investigate the extent to which macrophytes serve as a refuge for zooplankton at different salinities, we introduced artificial plants mimicking submerged macrophytes in the littoral zone of four shallow lakes, with salinities ranging from almost freshwater (0.3) to oligohaline waters (3.8). Furthermore, we examined the effects of different salinities on the community structure. Diel samples of zooplankton were taken from artificial plants, from areas where macrophytes had been removed (intermediate areas) and, in two of the lakes, also in open water. Fish and macroinvertebrates were sampled amongst the artificial plants and in intermediate areas to investigate their influence on zooplankton migration. Our results indicated that diel vertical migration (DVM) was the most frequent migration pattern of zooplankton groups, suggesting that submerged macrophytes were a poor refuge against predation at all salinities under study. Presumably, this pattern was the result of the relatively high densities of small planktivorous fish and macroinvertebrate predators within the submerged plants. In addition, we found major differences in the composition of zooplankton, fish and macroinvertebrate communities at the different salinities and species richness and diversity of zooplankton decreased with increasing salinity. At low salinities both planktonic/free-swimming and benthic/plant-associated cladocerans occurred, whilst only benthic ones occurred at the highest salinity. The low zooplankton biomass and overall smaller-bodied zooplankton specimens may result in a lower grazing capacity on phytoplankton, and enhance the turbid state in nutrient rich shallow brackish lakes.     

The photobehaviour of Daphnia spp. as a model to explain diel vertical migration in zooplankton

Many pelagic animal species in the marine environment and in lakes migrate to deeper water layers before sunrise and return around sunset. The amplitude of these diel vertical migrations (DVM) varies from several hundreds of metres in the oceans to approx. 5–20 m in lakes. DVM can be studied from a proximate and an ultimate point of view. A proximate analysis is intended to reveal the underlying behavioural mechanism and the factors that cause the daily displacements. The ultimate analysis deals with the adaptive significance of DVM and the driving forces that were responsible for the selection of the traits essential to the behavioural mechanism. The freshwater cladoceran Daphnia is the best studied species and results can be used to model migration behaviour in general. Phototaxis in Daphnia spp., which is defined as a light-oriented swimming towards (positive phototaxis) or away (negative phototaxis) from a light source, is considered the most important mechanism basic to DVM. A distinction has been made between primary phototaxis which occurs when light intensity is constant, and secondary phototaxis which is caused by changes in light intensity. Both types of reaction are superimposed on normal swimming. This swimming of Daphnia spp. consists of alternating upwards and downwards displacements over small distances. An internal oscillator seems to be at the base of these alternations. Primary phototaxis is the result of a dominance of either the upwards or the downwards oscillator phase, and the direction depends on internal and external factors: for example, fish-mediated chemicals or kairomones induce a downwards drift. Adverse environmental factors may produce a persistent primary phototaxis. Rare clones of D. magna have been found that show also persistent positive or negative primary phototaxis and interbreeding of the two types produces intermediate progeny: thus a genetic component seems to be involved. Also secondary phototaxis is superimposed on normal swimming: a continuous increase in light intensity amplifies the downwards oscillator phase and decreases the upwards phase. A threshold must be succeeded which depends on the rate and the duration of the relative change in light intensity. The relation between both is given by the stimulus strength versus stimulus duration curve. An absolute threshold or rheobase exists, defined as the minimum rate of change causing a response if continued for an infinitely long time. DVM in a lake takes place during a period of 1-5-2 h when light changes are higher than the rheobase threshold. Accelerations in the rate of relative increase in light intensity strongly enhance downwards swimming in Daphnia spp. and this enhancement increases with increasing fish kairomone and food concentration. This phenomenon may represent a ‘decision-making mechanism’ to realize the adaptive goal of DVM: at high fish predator densities, thus high kairomone concentrations, and sufficiently high food concentrations, DVM is profitable but not so at low concentrations. Body axis orientation in Daphnia spp. is controlled with regard to light-dark boundaries or contrasts. Under water, contrasts are present at the boundaries of the illuminated circular window which results from the maximum angle of refraction at 48–9° with the normal (Snell's window). Contrasts are fixed by the compound eye and appropriate turning of the body axis orients the daphnid in an upwards or an obliquely downwards direction. A predisposition for a positively or negatively phototactic orientation seems to be the result of a disturbed balance of the two oscillators governing normal swimming. Some investigators have tried to study DVM at a laboratory scale during a 24 h cycle. To imitate nature, properties of a natural water column, such as a large temperature gradient, were compressed into a few cm. With appropriate light intensity changes, vertical distributions looking like DVM were obtained. The results can be explained by phototactic reactions and the artificial nature of the compressed environmental factors but do not compare with DVM in the field. A mechanistic model of DVM based on phototaxis is presented. Both, primary and secondary phototaxis is considered an extension of normal swimming. Using the light intensity changes of dawn and the differential enhancement of kairomones and food concentrations, amplitudes of DVM could be simulated comparable to those in a lake. The most important adaptive significance of DVM is avoidance of visual predators such as juvenile fish. However, in the absence of fish kairomones, small-scale DVMs are often present, which were probably evolved for UV-protection, and are realized by not enhanced phototaxis. In addition, the ‘decision-making mechanism’ was probably evolved as based on the enhanced phototactic reaction to accelerations in the rate of relative changes in light intensity and the presence of fish kairomones.     

Sampling and analysis of gut contents in relation to environmental variability and diel vertical migration by herbivorous zooplankton

The accuracy of standard sampling and analysis procedures forestimating ingestion by herbivorous zooplankton was assessedusing models. Artificial environments were created in a computermodel, allowing for depth-dependent variability in temperature,chlorophyll and primary production. Model zooplankton were simulatedwithin these artificial environments using individual-basedmodels. The model zooplankton feed and defaecate at rates determinedby temperature and food concentrations, and also exhibit dielvertical migration (DVM) according to a variety of migrationmodels. The computer model was run for different combinationsof these nine environmental and five DVM models. Data were ‘sampled’from the model output, similar to field sampling of mesozooplanktongrazing. Daily ingestion was calculated from the gut ‘samples’using standard procedures for analysing gut fluorescence. Thesample results were compared with the actual ingestion valuesin the model, and some causes of discrepancies were noted. (i)If incorrect temperatures were assumed when calculating thegut evacuation rate (K), then estimates of ingestion were wrongby up to 40%. (ii) Non-uniform food environments gave errorsof up to 30% because of the large variability of measured gutcontents among individuals. (iii) Sampling from only part ofthe total depth range (e.g. at the chlorophyll maximum) resultedin estimates of ingestion being only 5% of the real value. Thissampling practice should be discouraged, because the sampleis not random. (iv) If sampling is not frequent enough, errorscan be as large as 45%, but more usually were 10% for realisticsampling frequencies. We describe an analysis procedure thatuses Monte Carlo-type simulations in a computer spreadsheetto estimate population consumption. These calculations takeinto account natural variability due to populations, samplesand assumptions. We urge that results should be presented asranges of possible values, rather than as single ‘mean’values, to allow for easier recognition of meaningful differencesamong samples and systems.     

Tintinnid distributions in the Strait of Magellan (Chile)

In the Strait of Magellan and in the adjacent Pacific and Atlantic Ocean shelf tintinnids were studied during three oceanographic cruises (November 1989, March–April 1991, and April 1995). Total tintinnid abundances were higher in the first cruise and dramatically decreased in the other two late summer cruises. As a general rule, abundances were higher at the surface and in the Atlantic sector. A total of 47 tintinnid species were found. There was not a single species, which could be identified as indicator of a specific water mass. For each cruise, cluster analysis applied on a species/samples matrix identified station sets characterized by a specific tintinnid association. Angosturas entrance was characterized by agglutinated lorica tintinnids, typical of the Atlantic waters, while the Pacific sector by hyaline species. In Punta Arenas basin, a peculiar tintinnid community was clearly identified, although water masses derived from the mixing process between the Pacific and Atlantic waters. Overall, tintinnid communities maintained the structure typical of the water mass despite advection and/or mixing. The main result of this study was to demonstrate that the structure of the tintinnid species association could be used as indicator of the origin of water masses.     

The summer zooplankton community at South Georgia: biomass,vertical migration and grazing

Zooplankton abundance and biomass were determined during January 1990 at two stations to the north-west of South Georgia using a Longhurst Hardy Plankton Recorder (LHPR). At both shelf and oceanic station sites, zooplankton biomass, (excluding Euphausia superba), was found to be ca. 13 g dry mass m^–2. Copepods and small euphausiids dominated the catches. These estimates are over 4 times higher than values generally reported for the Southern Ocean and may reflect firstly, the high productivity of the study area, secondly, the time of year, summer, when biomass for many species is maximal, and thirdly, the high sampling efficiency of the LHPR. Principal components analysis disclosed similarities and differences between adjacent depth strata in terms of abundance, biomass and species composition. At both stations most variability occurred in the mixed layer (0–60 m) and thermocline (60–120 m) with depth horizons below this being more homogeneous. Diel migrations were observed for most taxa with abundance increasing in the mixed layer at night. At the oceanic station, species and higher taxa belonging to the mesopelagic community were generally well spread throughout this domain and, with the exception of Pleuromamma robusta and Metridia curticauda, showed little evidence of migration. The grazing impact of the epipelagic community (copepods and small euphausiids) was estimated to remove 3–4% of the microbial standing stock day^–1 and a conservative 25% and 56% of daily primary production at the oceanic and shelf stations respectively.     

Comparison of fish and phantom midge influence on cladocerans diel vertical migration in a dual basin lake

Diurnal vertical migrations (DVM) behaviour of cladocerans was investigated in two mesotrophic Irish lakes connected by a canal, characterised by interesting differences in the presence of zooplanktivorous predators. In Doon Upper, fish (mostly juvenile perch and roach) and a little-studied phantom midge Mochlonyx fuliginosus (Chaoboridae) were found, but Doon Lower was solely inhabited by fish. As the presence of diverse predators may alter spatial avoidance behaviour of zooplankton prey in different ways, the aim of this study was to determine whether and how two predator types, fish and phantom midge larvae, have changed DVM pattern of cladocerans during day and night in Doon lakes. Two sampling series of phytoplankton, zooplankton, fish, and water physical analyses were conducted on 09–10 June and 19–20 September 2007 in both lakes. In the study conducted in June, under a similar distribution of M. fuliginosus and juvenile fish in Doon Upper, a reverse migration of Daphnia galeata was observed as a strategy allowing them to avoid both types of predators. However, in September, when M. fuliginosus lived in a 24 h refugium below the oxycline as a response to increasing predation risk posed by YOY fish penetrated the upper strata of water during day and night, reverse migrations of D. galeata were not clear. In Doon Lower, normal migration was observed as an advantageous behavioural response against visual predators (fish), in both large and small Cladocera species: D. galeata, Diaphanosoma branchyurum and Bosmina sp. Thus, our results indicate dissimilar migration patterns of D. galeata depending on the presence of one (Doon Lower) or two predators with different predation behaviour (Doon Upper).     

The effect of different zooplankton grazing patterns resulting from diel vertical migration on phytoplankton growth and composition: a laboratory experiment

Diel vertical migration (DVM) of herbivorous zooplankton is a widespread behavioural phenomenon in freshwater ecosystems. So far only little attention has been paid to the impact of DVM on the phytoplankton community in the epilimnion. Some theoretical models predict that algal population growth in the epilimnion should depend on the herbivores migration and grazing patterns: even if migrating zooplankton consume the same total amount of algae per day in the epilimnion as non-migrating zooplankton, nocturnal grazing should result in enhanced algal growth and favour algal species with high intrinsic growth rates over species with lower intrinsic growth rates. To test these hypotheses we performed experiments in which several algal species were confronted with different feeding regimes of Daphnia. In the experiments algal growth did not only depend on the absolute time of grazing but was comparatively higher when grazing took place only during the night, even when the grazing pressure was the same. Furthermore, algal species with higher intrinsic growth rates had higher advantages when being grazed upon only discontinuously during the night than algal species with a smaller intrinsic growth rate. The grazing pattern itself was an important factor for relative algal performance.     

Ovigerity,selective predation,and variable diel vertical migration in Euchaeta elongata (Copepoda: Calanoida)

Summary We present a statistical analysis of a previously published (Yen, 1983) but heretofore unanalyzed data set on the vertical distributions and diel vertical migration (DVM) of adult females of the marine planktonic copepod Euchaeta elongata in Dabob Bay, Washington, USA. Non-ovigerous females were strongly migratory on all four dates sampled, residing between 75–175 m during the day and at shallower depths during the night, commonly entering the upper 50 m of the water column. In contrast, ovigerous females were non-migratory or weakly migratory, largely remaining between 100–175 m both day and night, and entering the upper 50 m of the water column only rarely. Thus non-ovigerous females always migrated much more strongly, as measured by both amplitude of migration and the proportion of animals migrating, than did ovigerous females. These results led us to hypothesize that differential susceptibility to visually orienting predators was the cause of these differences in DVM behavior in female E. elongata, and we subsequently undertook an experimental study of the feeding selectivity of the copepod's natural predator, Pacific herring (Clupea harengus pallasi). Pacific herring exhibited a highly significant preference for ovigerous over nonovigerous adult female E. elongata. The demographic consequences of variable DVM in adult female E. elongata were investigated by way of life table analyses. Results indicated that under conditions of thermal stratification of the water column there is a distinct demographic disadvantage (reduced rate of realized population growth) incurred by non-migratory or weakly migratory ovigerous females due to delayed egg development at cooler subsurface temperatures. We conclude that ovigerous female E. elongata remain at depth both day and night to avoid visually orienting predators, and that such behavior must afford the copepod a demographic advantage of no less than a 26% reduction in adult mortality to offset the demographic cost of delayed egg development.     

Population structure, vertical distribution and diel migration of Sagitta setosa (Chaetognatha) in the south-western part of the Black Sea

The population structure of Sagitta setosa from the south-westernpart of the Black Sea was examined between May 1994 and December1996. In addition, the vertical distribution and diel verticalmigration of S.setosa were studied for juveniles (     

On the combined analysis of proximate and ultimate aspects in diel vertical migration (DVM) research

Although evolutionary ecologists agree that proximate and ultimate aspects are two sides of one coin, they are seldom interested in studies on physiological and behavioural mechanisms at the base of ecological phenomena. Nevertheless, these mechanisms are objects of selection and evolved to realise adaptive significances. This paper is a plea to bring both fields closer together, and, by means of an example of Diel Vertical Migration of Daphnia, some proximate and ultimate aspects are discussed. It is argued that light changes, not fish kairomone, is the primary cause for an individual to swim downwards at dawn and upwards at dusk. However, what is called a causal factor might differ when ecosystems or individuals are studied. In addition, causality in ecology is not simple, and has the character of a `set of necessary conditions'. To illustrate the importance of proximate analyses in DVM, two basic response mechanisms are discussed: Photobehaviour system 1 and 2. The physiological character of these systems leads to a fixed type of migration or to a phenotypically induced DVM, respectively. The adaptive significance of the first might be a reduction of the hazardous effects of UV radiation and of the second a lowering of mortality due to visually hunting predators.     

Responses of zooplankton body size and community trophic structure to temperature change in a subtropical reservoir

Understanding the effects of global warming on trait variation and trophic structure is a crucial challenge in the 21st century. However, there is a lack of general patterns that can be used to predict trait variation and community trophic structure under the ongoing environmental change. We investigated the responses of body size and community trophic structure of zooplankton to climate related factors (e.g., temperature). Isotopic niche breadth was applied to investigate the community trophic structure across a 1‐year study from a subtropical reservoir (Tingxi Reservoir) in southeastern China. Body size and community isotopic niche breadth of zooplankton were larger during water mixing than stratification periods and correlated significantly with water temperature change along the time series. The contributions of intra‐ and intertaxonomic components to body size and community trophic structure variation showed significant relationships with the temperature change going from the mixing to stratification periods. Water temperature imposed direct effect on body size, while direct and indirect effect on the community trophic structure of zooplankton occurred through trophic redundancy along time series. Water temperature and community properties (e.g., body size, trophic redundancy, or trophic interaction) showed complex interactions and integrated to influence community trophic structure of zooplankton. Our results can expand the knowledge of how elevated temperature will alter individual trait and community trophic structure under future climate change.     

Species diversity, spatial distribution, and assemblages of zooplankton within the Strait of Magellan in austral summer

The main aim of this work was to identify zooplankton assemblages by means of statistical testing and associate them with hydrographic properties of the Strait of Magellan and its microbasins. Zooplankton samples were collected by the R/V Cariboo in late austral summer 1991. Nineteen stations were sampled by BIONESS from the surface layer to 900 m depth, along the main longitudinal axis of the Strait of Magellan. There was a marked regional-scale pattern in zooplankton species diversity and richness, related to the hydrographical features of the Strait and its sub-basins. Six groups of samples were identified by cluster analysis in terms of zooplankton structure, and related to sea-water properties of temperature, salinity, and Chl a (Sperman’s correlation coefficient). Plankton assemblages of the Strait of Magellan seem closely linked to the remote sub-Antarctic and adjacent ocean biota, but populations may have independent evolutionary lines and population dynamics.     

Pyrosoma atlanticum (Tunicata, Thaliacea): diel migration and vertical distribution as a function of colony size

A large population of the colonial pelagic tunicate Pyrosomaatlanticum occurred in April 1991 in offshore waters of theLigurian Sea (Northwestern Mediterranean). The high numbersof colonies caught allowed their vertical distribution and dielmigration in the 0–965 m water column to be describedas a function of their size. Daytime depths and amplitudes ofthe migration were correlated with colony size. The amplitudeof the migration ranged from 90 m for 3-mm-length colonies to760 m for 51-mm-length colonies, with a mean amplitude of 410m for the whole population, all sizes pooled. The results ofhorizontal hauls at a given depth around sunrise and sunsetshowed a marked diurnal symmetry of the migratory cycle relativeto noon, and that migration of the population was not cohesive.For example, the larger the colonies, the later after sunsetthey reached the upper layers during their upward migration.     

Seasonal variations in the diel vertical distribution of phytoplankton and zooplankton in a shallow pond

The seasonal succession of phytoplankton diversity, and the variations in the diel vertical distribution of phyto‐ and zooplankton were investigated in a small shallow pond (1.7 m water depth) in 2003. It was inferred that the water tended to stratify weakly in the daytime from February to June. In February and April, the green alga Golenkinia radiata Chodat dominated the phytoplankton assemblage. The cell density of G. radiata greatly decreased in April, when rotifers increased near the bottom. The vertical mixing was attenuated in June, large populations of the euglenoids (Lepocinclis salina Fritsch, Phacus acuminatus Stokes, Trachelomonas hispida (Perty) Stein et Deflandre) developed, and the cyanobacterium Aphanizomenon flos‐aquae var. klebahnii Elenk. appeared at low density. Euglenoids and A. flos‐aquae were mostly distributed in the bottom layer. In late September, when the water was mixed throughout the day, euglenoids and A. flos‐aquae were distributed evenly throughout the water column. The zooplankton (cyclopoid copepods and rotifers) densities in September were the lowest throughout the year. The vertical mixing increased in November, and the phytoplankton community was composed of A. flos‐aquae, P. acuminatus, T. hispida and the green alga Ankistrodesmus falcatus (Corda) Ralfs. In November, at the final stage of water bloom of A. flos‐aquae, its population density decreased with depth. The two euglenoids exhibited similar cell distributions at 0.8 m and 1.6 m during 1–3 November. A. falcatus was distributed evenly throughout the water column; however, when the vertical mixing lessened, the cells at the surface started to sink. Copepod nauplii and rotifers appeared at high densities in November. Seasonal variation in the phytoplankton community structure in the pond seemed to be related to the vertical mixing of the water. In addition, zooplankton, especially rotifers, might play an important role in initiating a spring clear‐water phase and in the bloom collapse of A. flos‐aquae.