FILAMENT FORMATION IN THE DIATOM MELOSIRA GRANULATA1

The frequency and position of separation valves in filaments of Melosira granulata were observed both in laboratory cultures and in natural habitats. The numerical relationship between mean filament length in a population and the proportion of separation valves produced (the separation valve index-SVI) has also been determined. The data indicate that filament length is controlled through the production of separation valves and that the SVI gives an indication of the mean filament length. Two series of cell divisions (the first producing the separation valves) are necessary before separation of the filament can occur. Paris of separation valves occur predominantly towards the middle of the filament. We suggest that the SVI may prove to be a useful indicator of mean filament length, and hence of ecological change, even when the filaments are no longer intact, as, for example, in sediment core samples and on permanent slides of acid-cleaned collections.     

Phytoplankton responses to experimental enhancement of grazing pressure and nutrient recycling in a small Andean lake

1. Three series of field experiments with different zooplankton species composition and biomass were performed in a small lake in the south Andes. We attempted to measure the responses of phytoplankton species resulting from grazing mortality and stimulation of growth by nutrient recycling.
2. Nanoflagellates contributed substantially to total phytoplankton cell abundance. Chrysochromulina parva represented 93.4%, 92.2% and 95.9% of total phytoplankton density in December, January and February, respectively. This fraction was reduced in all treatments with increasing zooplankton biomass.
3. A negative relationship was obtained between C. parva cell numbers and increase in dissolved P. On the other hand, a significant positive relationship between the abundance of the diatom Aulacoseira granulata and P concentration was observed. These results indicate that the ungrazed diatom was able to capitalise on the increase in nutrient availability.
4. As a net result of the increase or decrease of algal species we observed a change in the nano:net phytoplankton relationship. The outcome of three‐day incubations with increased zooplankton biomass was an increasing importance of net phytoplankton.
5. The results indicate the importance of the indirect effects of zooplankton (through nutrient recycling) in the increase in diatoms, and the role of grazing as a growth‐limiting factor for the flagellate C. parva .     

The phytoplankton of an enriched, soft-water lake subject to intermittent hydraulic flushing (Grasmere, English Lake District)

SUMMARY. 1. The abundance and species composition of the phytoplankton of Grasmere were monitored over a 12-year period following the commissioning of a small sewage treatment works, upstream of the lake.
2. Although Grasmere has been subject to increased phosphorus-loading and has quickly developed many features associated with eutrophication, the composition of its plankton has retained the characteristics of a mesotrophic, soft-water lake: a vernal diatom maximum, generally dominated by Asterionella , is followed by summer growths of nanoplanktonic species, of various colonial Chlorophyceae, before a substantial return to Asterionella-dominance in the autumn. In cooler summers there have been episodes of renewed diatom growth. followed by truncated recapitulations of the essential summer sequence. Anabaena solitaria was relatively common in drier summers when dissolved inorganic nitrogen fell to low concentrations, although many of the 'larger' algal species associated with stratified eutrophic lakes (Aphanizomenon, Microcystis, Ceratium spp.) have failed to become abundant in Grasmere.
3. Mean biomass levels (as indicated by chlorophyll concentration) have increased in response to the enrichment, though they do not conform well with most chlorophyll/phosphorus regression-models.
4. The key factor resisting a more complete transition to a typical eutrophic plankton appears to be the efficiency of episodic flushing of the lake during periods of high fluvial discharge sustained by heavy rainfall over the extensive, mountainous catchment. The probabilities that these limnological conditions might fail to deter the development of 'nuisance' blooms are briefly assessed.     

The control of sub-surface maxima of diatoms in a stratified lake by physical, chemical and biological factors

The sedimentation patterns of Asterionella formosa Hass. wereobserved in two lake basins, in one of which a mid-water populationmaximum of the diatom occurred. Comparison of the alga's distributionwith vertical profiles of temperature and dissolved reactivesilicon suggested that the formation of a mid-water maximumwas associated with a sharp silicon gradient. Sinking velocitiesof the diatom were reduced at the maximum compared to thoseat lesser or greater depths. The suspension of cultures at differentdepths, in either nutrient-replete medium or exposed to theprevailing nutrient conditions, following thermal stratification,confirmed that an increase of sinking velocity occurred in silicon-or light-deficient populations. It is suggested that the mid-watermaximum of A.formosa was formed by differential colony sinkingvelocities with depth in response to variations in silicon availability.The formation of a strong silicon gradient was dependent uponthe growth of the population being restricted to the epilimnionby the onset of thermal stratification.     

湖南省大通湖百余年环境演化历史及营养物基准的建立

科学有效地治理退化湖泊需要知晓湖泊的演化历史,并设立合理的参照目标(即环境基准)来及时评判治理效果.湖南省大通湖位于经济发达的长江中下游地区,发挥着重要的湖泊水生态系统服务功能.在强烈的人类活动干扰下,该湖近年来生态系统退化严重,但其水环境演变的历史缺少详细的记录.研究对大通湖沉积钻孔的年代、烧失量、化学元素、沉积物总...     

Interaction effects of fish, nutrients, mixing and sediments on autotrophic picoplankton and algal composition

1. We examined the effects of nutrients, turbulent mixing, mosquitofish, Gambusia affinis Baird and Girard and sediments on algal composition, algal biomass and autotrophic picoplankton (APP) abundance in a 6-week experiment of factorial design in twenty-four 5-m³ outdoor mesocosms during late autumn 1995.
2. Turbulent mixing decreased surface temperature and increased turbidity, which also was increased by the addition of sediments. Total algal biomass was significantly enhanced by nutrients and mixing, and decreased by the sediment treatment. In the mixing × nutrient treatment, algal biomass increased more than expected from the individual effects, while the fish × mixing and mixing × sediment treatments increased algal biomass less than expected.
3.  Cryptomonas (cryptomonad) blooms were observed in the unmixed, high nutrient treatment; Synedra (diatom) blooms were observed in the high nutrient, high sediment treatment; Ulothrix (green algae) blooms were observed in the mixed, high nutrient, low sediment treatment.
4. Eukaryotic APP abundances were increased by sediment addition and by turbulent mixing, and increased synergistically by mixing × sediment and mixing × nutrient interactions. Prokaryotic APP abundances were decreased by nutrient enhancement and by a mixing × nutrient interaction. There were no main effects of fish on APP abundance, but fish were involved in some of the two–way interactions.
5. The large number of significant interaction effects indicates that APP and other phytoplankton are regulated by a complex set of interdependent factors which should be considered simultaneously in studies of phytoplankton population dynamics and community composition.     

Host-Specificity and Dynamics in Bacterial Communities Associated with Bloom-Forming Freshwater Phytoplankton

Many freshwater phytoplankton species have the potential to form transient nuisance blooms that affect water quality and other aquatic biota. Heterotrophic bacteria can influence such blooms via nutrient regeneration but also via antagonism and other biotic interactions. We studied the composition of bacterial communities associated with three bloom-forming freshwater phytoplankton species, the diatom Aulacoseira granulata and the cyanobacteria Microcystis aeruginosa and Cylindrospermopsis raciborskii. Experimental cultures incubated with and without lake bacteria were sampled in three different growth phases and bacterial community composition was assessed by 454-Pyrosequencing of 16S rRNA gene amplicons. Betaproteobacteria were dominant in all cultures inoculated with lake bacteria, but decreased during the experiment. In contrast, Alphaproteobacteria, which made up the second most abundant class of bacteria, increased overall during the course of the experiment. Other bacterial classes responded in contrasting ways to the experimental incubations causing significantly different bacterial communities to develop in response to host phytoplankton species, growth phase and between attached and free-living fractions. Differences in bacterial community composition between cyanobacteria and diatom cultures were greater than between the two cyanobacteria. Despite the significance, major differences between phytoplankton cultures were in the proportion of the OTUs rather than in the absence or presence of specific taxa. Different phytoplankton species favoring different bacterial communities may have important consequences for the fate of organic matter in systems where these bloom forming species occur. The dynamics and development of transient blooms may also be affected as bacterial communities seem to influence phytoplankton species growth in contrasting ways.     

Increased nutrient loading and rapid changes in phytoplankton expected with climate change in stratified South European lakes: sensitivity of lakes with different trophic state and catchment properties

We hypothesised that increasing winter affluence and summer temperatures, anticipated in southern Europe with climate change, will deteriorate the ecological status of lakes, especially in those with shorter retention time. We tested these hypotheses analysing weekly phytoplankton and chemistry data collected over 2 years of contrasting weather from two adjacent stratified lakes in North Italy, differing from each other by trophic state and water retention time. Dissolved oxygen concentrations were higher in colder hypolimnia of both lakes in the second year following the cold winter, despite the second summer was warmer and the lakes more strongly stratified. Higher loading during the rainy winter and spring increased nutrient (N, P, Si) concentrations, and a phytoplankton based trophic state index, whilst the N/P ratio decreased in both lakes. The weakened Si limitation in the second year enabled an increase of diatom biovolumes in spring in both lakes. Chlorophyll a concentration increased in the oligo-mesotrophic lake, but dropped markedly in the eutrophic lake where the series of commonly occurring cyanobacteria blooms was interrupted. The projected increase of winter precipitation in southern Europe is likely to increase the nutrient loadings to lakes and contribute to their eutrophication. The impact is proportional to the runoff/in-lake concentration ratio of nutrients rather than to the retention time, and is more pronounced in lakes with lower trophy.     

AN ECOLOGICAL BASIS FOR EXTRACELLULAR CARBONIC ANHYDRASE IN MARINE UNICELLULAR ALGAE

Extracellular carbonic anhydrase (CA_e) is expressed by many, but not all, autotrophic species of aquatic unicellular protists. We measured CA_e activities in unicellular marine algae characteristic of either high nutrient spring, fall, and winter blooms or low nutrient summer populations to provide ecological/evolutionary information about the enzyme. Highest activities occurred in spring bloom and opportunistic diatoms exposed to long photoperiods (16 h) when pH was highest and CO₂ was lowest. Lower activities were recorded for a fall-bloom diatom exposed to the long photoperiod, and lowest values were found under all culture conditions for one diatom and a number of flagellated species typical of summer low nutrient environments. Other potential sources of variance in measurements of CA_e activity were examined. Maximum activities of CA_e were recorded for the diatom, Skeletonema costatum (Greville) Cleve, during late exponential phase of growth and within 8 h of the beginning of the photoperiod. We concluded that ecological factors are important in determining CA_e activities in marine unicellular protists. Potential functions of CA_e in the metabolism of marine unicellular algae are discussed.     

Relationship Between Surface Sediment Diatoms and Summer Water Quality in Shallow Lakes of the Middle and Lower Reaches of the Yangtze River

The relationship between surface sediment diatoms and summer water quality was investigated at 49 lakes in the middle and lower reaches of the Yangtze River. Lakes ranging from oligomesotrophic to hypereutrophic were examined, providing an obvious nutrient gradient. With the shift from mesotrophic to eutrophic levels, diatom multi-ecotypes dominated by epiphytic and facultative planktonic taxa were replaced by nutrient-tolerant planktonic taxa, such as Cyclotella meneghiniana Skvortzow, C. atomus Hustedt,Cyclostephanos Round, and Stephanodiscus Ehrenberg etc., reflecting the nutrient changes in the lake.The relationship between diatoms and summer water quality indices was explored further using numeric analysis. Canonical correspondence analysis (CCA) with forward selection and a Monte Carlo permutation test revealed that of all 25 summer water environmental variables, total phosphorus (TP), chlorophyll a (Chzl a), Secchi depth (SD), dissolved inorganic phosphorus, C1-, SO42-, Mg2 , CO32-, and water depth were significant variables (P＜0.05) in explaining diatom distributions. Of these, TP, Chl a, SD, and C1-, were the most important variables. The result of the correlation analysis also showed that a significant correlation exists among these variables, implying that these indices are either interconnected or independent in explaining the diatom data. For phosphorus-limited sites, TP was the most significant variable affecting the diatoms, also affecting changes in Chl a, SD, and iron concentrations. The independence of Chl a may be related to algal competition induced by lake eutrophication, resulting in the feedback to diatom community.In addition to TP, SD can be related to sediment disturbance by wave action and the growth of macrophytes in large shallow lakes. These relationships between diatom ecotypes and water quality provide the basis for a future quantitative reconstruction of historic lake nutrient evolution in the study area and will also provide a wealth of modern ecological knowledge that can be used to interpret fossil diatom records.     

Relationship Between Surface Sediment Diatoms and Summer Water Quality in Shallow Lakes of the Middle and Lower Reaches of the Yangtze River

The relationship between surface sediment diatoms and summer water quality was investigated at 49 lakes in the middle and lower reaches of the Yangtze River. Lakes ranging from oligomesotrophic to hypereutrophic were examined, providing an obvious nutrient gradient. With the shift from mesotrophic to eutrophic levels, diatom multi-ecotypes dominated by epiphytic and facultative planktonic taxa were replaced by nutrient-tolerant planktonic taxa, such as Cyclotella meneghiniana Skvortzow, C. atomus Hustedt, Cyclostephanos Round, and Stephanodiscus Ehrenberg etc., reflecting the nutrient changes in the lake. The relationship between diatoms and summer water quality indices was explored further using numeric analysis. Canonical correspondence analysis (CCA) with forward selection and a Monte Carlo permutation test revealed that of all 25 summer water environmental variables, total phosphorus (TP), chlorophyll a (Chl a), Secchi depth (SD), dissolved inorganic phosphorus, Cl–, SO42–, Mg2+, CO32–, and water depth were significant variables (P<0.05) in explaining diatom distributions. Of these, TP, Chl a, SD, and Cl–, were the most important variables. The result of the correlation analysis also showed that a significant correlation exists among these variables, implying that these indices are either interconnected or independent in explaining the diatom data. For phosphorus-limited sites, TP was the most significant variable affecting the diatoms, also affecting changes in Chl a, SD, and iron concentrations. The independence of Chl a may be related to algal competition induced by lake eutrophication, resulting in the feedback to diatom community. In addition to TP, SD can be related to sediment disturbance by wave action and the growth of macrophytes in large shallow lakes. These relationships between diatom ecotypes and water quality provide the basis for a future quantitative reconstruction of historic lake nutrient evolution in the study area and will also provide a wealth of modern ecological knowledge that can be used to interpret fossil diatom records.     

68 A new paradigm for the aphanizomenon bloom problem in upper klamath lake

Upper Klamath Lake (UKL) is the largest lake in Oregon (area 287 km², avg. depth 4.2 m). It is naturally eutrophic and regularly suffers nuisance summer blooms of cyanobacteria, principally Aphanizomenon flos‐aquae (AFA). Sediment coring studies show that AFA was absent or minimal until about 1880 when a steady increase began, culminating in the blooms of recent decades. These studies show concomitant increases in sediment N (～20%) and P (～50%) along with shifts in the algal flora indicating increased eutrophication. These changes correlate with increased human impacts, such as deforestation, construction, roadbuilding etc., and especially the ditching, diking and draining of adjacent wetlands for conversion to agriculture. Agricultural nutrient runoff, especially P, has been often cited as the cause of the AFA blooms, and most attention has been focused on the dynamics of UKL during the summer bloom. We propose that a more significant factor may be the loss of early‐season suppression of AFA because of the loss of the lake‐associated wetlands, which originally constituted 42% of the lake area, and which have declined in area by 66.3% since the late 1800's. The melting of snow and ice in the spring would flush into the lake a surge of wetland plant decomposition products, most significantly organic acids and humic substances. We propose that formerly these wetland effluents caused a complex of effects on lake pH, solar UV transparency, photochemical interactions, nutrient availability, and Daphnia grazing dynamics, which would have combined to prevent the development of any AFA bloom.     

70 Use of molecular probe to detect taste/odor-causing cyanobacteria in the phoenix drinking water distribution system

Upper Klamath Lake (UKL) is the largest lake in Oregon (area 287 km², avg. depth 4.2 m). It is naturally eutrophic and regularly suffers nuisance summer blooms of cyanobacteria, principally Aphanizomenon flos-aquae (AFA). Sediment coring studies show that AFA was absent or minimal until about 1880 when a steady increase began, culminating in the blooms of recent decades. These studies show concomitant increases in sediment N (∼20%) and P (∼50%) along with shifts in the algal flora indicating increased eutrophication. These changes correlate with increased human impacts, such as deforestation, construction, roadbuilding etc., and especially the ditching, diking and draining of adjacent wetlands for conversion to agriculture. Agricultural nutrient runoff, especially P, has been often cited as the cause of the AFA blooms, and most attention has been focused on the dynamics of UKL during the summer bloom. We propose that a more significant factor may be the loss of early-season suppression of AFA because of the loss of the lake-associated wetlands, which originally constituted 42% of the lake area, and which have declined in area by 66.3% since the late 1800's. The melting of snow and ice in the spring would flush into the lake a surge of wetland plant decomposition products, most significantly organic acids and humic substances. We propose that formerly these wetland effluents caused a complex of effects on lake pH, solar UV transparency, photochemical interactions, nutrient availability, and Daphnia grazing dynamics, which would have combined to prevent the development of any AFA bloom.     

西江颗粒直链藻种群生态特征

西江肇庆段是珠江干流汇入珠三角河网水域的咽喉通道,结合长期定点连续采样和空间季节性调查,重点对2009年西江干流肇庆段的颗粒直链藻(Aulacoseira granulata)的种群生态学特征进行系统分析。结果显示:全年中,颗粒直链藻在总种群中所占百分比均值为25.37%,与总种群丰度存在极显著正相关关系,这表明颗粒直链藻是西江浮游植物群落的重要优势种。调查期间发现了颗粒直链藻的4种形态,包括原变种(var.granulata)、极狭变种(var.angustissima)、弯曲变种(var.curvata)和极狭变种螺旋变型(var.angustissima f.spiralis)。其中,原变种占有绝对优势地位,其相对百分组成的年均值为91.47%,推测为原变种的形态结构比其它3种形态更容易适应外界环境变化,进而形成优势群体。颗粒直链藻总密度的周年变化呈现明显的双峰型,两个峰值分别出现在8月和11月,主要得益于适宜的水温、相对稳定的水体和较高的营养盐水平,这些均有助于颗粒直链藻的生长繁殖。第一个高峰之前,颗粒直链藻总密度未出现明显的波动,这很可能是由于水体径流量增大导致的稀释作用和频繁降雨导致的水体剧烈波动对颗粒直链藻增长的负作用掩盖了水温上升所带来的益处。从空间分布来看,颗粒直链藻总丰度的最大值一般出现在S1、S5和S6站点,这与营养盐最大值的空间分布格局是一致的。     

Eutrophication,recovery and temperature in Lake Mjøsa: detecting trends with monitoring data and sediment records

1. How climate warming may interact with other pressures on aquatic ecosystems is an important issue for research and management. We combined lake monitoring data with a palaeolimnological study to explore the combined effects of eutrophication and subsequent oligotrophication with a long‐term temperature increase in epilimnetic waters. Our goals were (i) to evaluate how well sediment‐based reconstructions reflect the instrumental observations, (ii) to use the palaeo‐record to characterise a reference state for the lake and (iii) to explore whether data from the sediment record can aid in separating the effects of nutrient load and temperature in a large and deep lake. 2. Lake Mjøsa is a large and deep lake in south‐eastern Norway. Eutrophication symptoms peaked in the 1970s, which led to extensive measures to reduce the phosphorus load. A monitoring programme has run continuously from 1972. Monitoring has documented a marked decrease in phosphorus load and algal biomass and also revealed an increase in epilimnetic temperature and extended summer stratification. 3. Records of algal pigments and diatoms were extracted from sediment cores taken from 236 m depth. The pigment record documented dramatic changes in lake production consistent with the monitoring record. The diatom record reflected well the eutrophication history of the lake and also demonstrated that the assemblage of the recent recovery stage differs from that of the pre‐eutrophication period. 4. Ordination of diatom assemblages over time constrained by proxies for nutrient load and temperature indicated that the diatom assemblage correlated with both factors, which together accounted for 60% of the variation in diatom composition. No interaction was detected between these factors. The results suggest that the diatom assemblage has responded to varying nutrient loads as well as to changes in temperature and/or factors that correlate with temperature. 5. Reconstructions of algal biomass and total phosphorus content mirrored known changes through the monitoring period, although the absolute phosphorus estimates were too high relative to the instrumental record. The sediment record from Lake Mjøsa provides a baseline for lake production in terms of algal pigments and organic contents, and for the diatom assemblage composition in a pristine stage.     

Artificial mixing prevents nuisance blooms of the cyanobacterium Microcystis in Lake Nieuwe Meer, the Netherlands

1. Artificial mixing in the hypertrophic Lake Nieuwe Meer was successful in preventing blooms of the cyanobacterium Microcystis. During the 2 years of artificial, deep mixing the number of colonies of Microcystis per litre and also per m² was lower than in the two preceding control years. Hardly any nuisance scums of Microcystis occurred in the lake.
2. The phytoplankton shifted from a cyanobacteria-dominated community in summer to a mixed community of flagellates, green algae and diatoms. Reduced sedimentation losses in the mixed lake, probably in combination with a lower pH, favoured non-buoyant algae, while the entrainment of cyanobacteria in the turbulent flow nullified their advantage of buoyancy.
3. The chlorophyll concentrations were much lower in the mixed lake, but the euphotic depth did not show clear differences between the years. The chlorophyll content integrated through depth (m^–2) increased in the artificially mixed lake.
4. The deep lake normally stratified in summer, but artificial mixing of the lake in 1993 resulted in a homogeneous temperature and oxygen distribution with depth. In spring 1994, the mixing was applied intermittently with a reduction of 75% of the energy costs, while the mixing was still sufficient to prevent stratification.
5. Determination of the buoyancy state of the colonies on a sunny and calm day showed that the buoyancy loss was low close to the bubble plumes, and high at some distance from these plumes. This suggests that Microcystis could escape the mixing at some distance from the plumes, and could synthesize more carbohydrates during its stay in the upper illuminated layer of the lake than the deep mixed colonies close to the bubble plumes. Determination of the buoyancy state appeared to be a good and simple method to investigate the extent of entrainment of colonies in the turbulent flow.     

Artificial mixing prevents nuisance blooms of the cyanobacterium Microcystis in Lake Nieuwe Meer, the Netherlands

1. Artificial mixing in the hypertrophic Lake Nieuwe Meer was successful in preventing blooms of the cyanobacterium Microcystis. During the 2 years of artificial, deep mixing the number of colonies of Microcystis per litre and also per m² was lower than in the two preceding control years. Hardly any nuisance scums of Microcystis occurred in the lake.
2. The phytoplankton shifted from a cyanobacteria-dominated community in summer to a mixed community of flagellates, green algae and diatoms. Reduced sedimentation losses in the mixed lake, probably in combination with a lower pH, favoured non-buoyant algae, while the entrainment of cyanobacteria in the turbulent flow nullified their advantage of buoyancy.
3. The chlorophyll concentrations were much lower in the mixed lake, but the euphotic depth did not show clear differences between the years. The chlorophyll content integrated through depth (m^–2) increased in the artificially mixed lake.
4. The deep lake normally stratified in summer, but artificial mixing of the lake in 1993 resulted in a homogeneous temperature and oxygen distribution with depth. In spring 1994, the mixing was applied intermittently with a reduction of 75% of the energy costs, while the mixing was still sufficient to prevent stratification.
5. Determination of the buoyancy state of the colonies on a sunny and calm day showed that the buoyancy loss was low close to the bubble plumes, and high at some distance from these plumes. This suggests that Microcystis could escape the mixing at some distance from the plumes, and could synthesize more carbohydrates during its stay in the upper illuminated layer of the lake than the deep mixed colonies close to the bubble plumes. Determination of the buoyancy state appeared to be a good and simple method to investigate the extent of entrainment of colonies in the turbulent flow.     

Climate drivers of diatom distribution in shallow subarctic lakes

相似文献   

Relationships between heterotrophic nanoflagellates and the demographic response of Daphnia longispina in a eutrophic lake with poor food quality conditions

1. Based on both field data and laboratory studies, the summer population of Daphnia longispina living in a stratified eutrophic lake was examined in relation to the abundance of algae, nanoflagellates and picocyanobacteria.
2. In early July, the Daphnia population replaced Bosmina and remained the dominant zooplankter during summer 1994. Its development in July was concomitant with an increase of edible algae but, despite the apparent abundance in available food, the Daphnia population decreased throughout August suggesting poor food conditions.
3. From mid-August to the beginning of September, the biomass of inedible phytoplankton was greater than that of the smaller, edible fraction. Consequently the average rate of increase, birth rates and fecundity of Daphnia remained low. Although the biomass of heterotrophic nanoflagellates was consistently low, the demographic parameters of Daphnia were correlated throughout this period with these protozoans.
4. Life table experiments run in the laboratory showed that epilimnetic food supported both the growth in length of individual Daphnia and an increase in fecundity, but metalimnetic food supported only individual growth. D. longispina probably failed to reproduce because of the abundance of detritus mixed with the heterotrophic nanoflagellates in the metalimnetic water at that period of the year. The vertical migration of Daphnia into these deeper layers could be caused by a predator avoidance mechanism.