Microb Ecol (1989) 17:111-136

MICROBIAL ECOLOGY 9 Springer-VerlagNew York Inc. 1989

Review The Role of Ciliated Protozoa in Pelagic Freshwater Ecosystems John R. Beaver and Thomas L. Crisman Universityof Florida, Department of EnvironmentalEngineeringSciences, Gainesville, Florida 32611, USA

Abstract. The abundance and biomass ofciliates are both strongly related to lake trophic status as measured by chlorophyll a concentrations. Taxonomic replacements occur with increasing eutrophication such that largebodied forms (predominantly oligotrichs) are progressively replaced by smaller-bodied ciliates (mainly scuticociliates). Highly acidic lakes display a more pronounced dominance of large-bodied forms when contrasted with less acidic lakes of comparable trophy. Community structure of ciliate populations is determined largely by lake trophy with acidic oligotrophic systems being characterized by reduced diversity and species richness compared with hypereutrophic systems. The temporal and spatial distribution of small (< 100 um) ciliate populations is ascribed to lake thermal regimes which provide localized concentrations of food resources. Likewise, in extremely productive lakes, very large (> 100 ~m) meroplanktonic ciliates enter the water column during midsummer after the development of thermal stratification and associated profundal deoxygenation. Laboratory studies indicate that large zooplankton (crustaceans) are capable of utilizing ciliates as a food source, but there is little direct evidence from field studies documenting this trophic link. Ciliates can be voracious grazers of both bacterioplankton and phytoplankton, and each species has a distinct range of preferred particle size which is a function of both mouth size and morphology. Myxotrophic ciliates may be important components in some plankton communities, particularly during periods of nutrient limitation or after their displacement from the benthos of eutrophic lakes. Evidence regarding the importance of planktonic ciliated protozoa in nutrient regeneration and as intermediaries in energy flow is discussed. Introduction Planktonic ciliated protozoa have been ignored historically in studies of freshWater plankton ecology, although it is now apparent that they both form an integral part of the planktonic food web [95] and contribute significantly to the total zooplankton standing crop [50, 87]. This plankton group has been overlooked by both phytoplankton and zooplankton specialists. Logically, protozoa

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Table 1. Summary of data on annual abundances of planktonic ciliates in lakesa

Trophic state

Range of observed abundances (cells/ml)

Chlorophyll a (mg/m3) values associated with trophic state

Ultraoligotrophic Oligotrophic Mesotrophic Eutrophic Hypereutrophic

2.4 2.3-10.8 18.0-70.9 55.5-145.1 90.0-215.0

56

Modified from Porter [91] should be studied by zooplankton ecologists, but they typically collect samples with nets through which m o s t ciliates pass. The methodology used to enumerate planktonic protozoa is most suitable to p h y t o p l a n k t o n specialists; however, they usually only record autotrophic taxa. F u r t h e r m o r e , the preservation techniques e m p l o y e d for both p h y t o p l a n k t o n and zooplankton (ethanol, Lugol's solution, formalin) are extremely disruptive to ciliates and render their cells unrecognizable. T h e general data base on planktonic ciliates in lake e n v i r o n m e n t s is scant. Recently, d e v e l o p m e n t o f i m p r o v e d preservation techniques and an increased enthusiasm on the part o f limnologists and p r o t o z o a n ecologists have contributed to a m o d e s t but growing list o f studies dealing with ciliate distributions. Although some authors have suggested that m i c r o z o o p l a n k t o n (predominantly ciliates) contribute significantly in nutrient regeneration [22, 70, 71], others have questioned this role [ 112]. The purpose o f this paper is to review the data base on the structure and role o f ciliated p r o t o z o a accumulated primarily during the last 10 years, with special emphasis on the role o f ciliates as trophic linkages in freshwater planktonic food webs.

Ciliate Distribution in Lake Types Abundance and Biomass of Small (< 100 izm) Planktonic Ciliates in Lakes The relationship between the abundance and biomass o f the total ciliate comm u n i t y and trophic state has been investigated by several researchers [9, 50, 87]. Included a m o n g the limnological variables c o m m o n l y used to assess trophic state, both total phosphorus and chlorophyll a concentrations are significantly related to ciliate abundance, with the latter being the most accurate predictor o f total ciliate abundance in lakes [9, 87]. Typically, oligotrophic lakes are characterized by m o d e s t ciliate populations ( < 10 cells/ml), whereas m o r e productive lakes exhibit greater abundance (Table 1). Total biomass o f ciliate populations parallels that o f abundance, increasing progressively along a gradient o f increasing eutrophy (Fig. 1). H o w e v e r , a m a r k e d latitudinal difference

Ciliated Protozoa in Freshwater Systems

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Fig, 1. Comparison of total ciliate abundance (A) and biomass (B) for Quebec lakes (0, [87]) vs Florida lakes (O, [9]) as a function of trophic state. Values fit the regression models y = mx + b Wherey = log~0ciliate biomass (mg d.w./m 3)or ciliate abundance (no./liter) and x = log~ochlorophyll a (rng/m3). Intercepts are not significantly different (P < 0.0668), but 95% confidence limits do not overlap in mesotrophic lakes and higher trophic states. does exist i n a n n u a l m e a n ciliate a b u n d a n c e p e r u n i t c h l o r o p h y l l , w h e r e b y SUbtropical lakes ( F l o r i d a ) are c o n s i d e r a b l y h i g h e r t h a n c o m p a r a b l e t e m p e r a t e lakes ( Q u e b e c ) i n t h e m e s o t r o p h i c a n d e u t r o p h i c range.

Taxonomic Replacements Associated with Increasing Eutrophication Three o r d e r s o f c i l i a t e s - - H a p t o r i d a (e.g., Mesodinium, Askenasia), Scuticociliatida (e.g., Cyclidiurn), a n d O l i g o t r i c h i d a (e.g., Strombidium, Strobilidium,

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Fig. 2. Shannon-Weaver diversity indices and species richness for 30 Florida lakes (from [9]).

OA = oligotrophicacidic, ON = oligotrophicnonacidic, M = mesotrophic,E = eutrophic, and H = hypereutrophic.

Halteria)--typically dominate planktonic communities. Although the mean annual abundance of all three orders generally increases along the trophic gradient, their rank dominance changes markedly. Oligotrichs are usually the dominant order in oligotrophic situations [12, 64, 69], but are progressively replaced by the scuticociliates in more productive lakes [ 12, 86]. The occurrence of large numbers of scuticociliates in oligotrophic lakes is usually limited to the often observed fall maximum in total ciliate abundance [12, 64]. The contribution of haptorids does not appear to change markedly with trophic state [ 12]. Ciliates in the size class 20-30/~m (mostly scuticociliates) are numerically dominant in most Florida lakes (Fig. 3), but their importance generally increases with higher trophic state [12]. Conversely, the importance of the 40-50 ~tm size class (mostly oligotrichs) decreases with higher productivity. In contrast to the subtropics, small ciliates (18-24 ~zm) are more important in oligotrophic Canadian lakes [50, 51] than in comparable oligotrophic Florida lakes [12]. It is unlikely that the observed changes in ciliate composition and size with increasing eutrophication are due directly to physiochemical exclusion, as most ciliates are extremely tolerant of the range of conditions found in freshwater lakes [ 16].

Community Structure of Ciliate Populations Relative to Trophic State In addition to the overall trends in order composition described above, ciliate diversity and species richness are positively related to lake productivity [9].

Ciliated Protozoa in Freshwater Systems

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Fig. 3. Size partitioning of ciliate communities by troph[c classification for 30 Florida lakes. Size classes include 50/~m (n). Lake

groupings as in Figure 2. The number of species present is significantly lower in acidic oligotrophic lakes as COmpared with other trophic categories in Florida (Fig. 2). This subgroup had a mean number of species of 10.8 (range 8-13) compared with 18.2 (range 16-2 I) in nonacidic oligotrophic lakes. At the opposite end of the trophic Spectrum, hypereutrophic lakes exhibited a mean of 24.5 species (range 2127), and were statistically different from lower trophic ranges. Ciliate diversity follows the same pattern with highest diversity in hypereutrophic lakes and significantly reduced diversity in acidic oligotrophic lakes. Many ciliate taxa are distributed ubiquitously in all lake types worldwide including Strombidium viride (Oligotrichida) and Cyclidium glaucoma (ScutiCociliatida). Other ciliate species display a more restricted range. For example, Stentor niger is a good indicator of acidic oligotrophic lakes in Florida [9] and large ciliates (e.g., Loxodes and Spirostomum) are found primarily in the plankton of very productive ponds and lakes [4, 43].

Community Structure of Planktonic Cifiates Relative to Lake Acidity ~fhe historical development of acid precipitation and the biotic response in softwater lakes of Scandinavia [20] and the United States [24] have been well

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J . R . Beaver and T. L. Crisman

Table 2. Annual mean percentage composition o f ciliate c o m m u nities examined for response to lake acidification a pH interval

n

6.0-7.0 5.5-6.0 5.0-5.5 5.0

8 2 7 4

Haptorida 28.3 21.7 14.9 11.9

+ 7.3 + 1.7 + 14.2 _+ 6.7

Oligotrichida 37.7 48.3 49.1 73.2

_+ _+ _+ _+

18.0 12.9 18.7 16.6

Scuticociliatida 17.9 26.0 10.6 5.2

+ + + +

13.9 7.8 14.8 9.1

F r o m Beaver and C r i s m a n [13]

documented, but major differences have been noted in zooplankton community structure between comparably acidic temperate and subtropical lakes [21]. Increased acidity is accompanied by a reduction in both the number of species and the abundance of total zooplankton in both temperate [67] and subtropical [27] lakes. However, important macrozooplankton taxa such as Daphnia are often eliminated from highly acidic temperate lakes [ 106] but not in comparable subtropical lakes [21 ]. Beaver and Crisman [13] investigated planktonic ciliate communities in softwater subtropical lakes (pH 4.7-6.8) and noted that both the abundance and biomass of total ciliates generally decreased with increased acidity. The sharpest reduction was seen in lakes < 5.0 pH. The orders Oligotrichida, Haptorida, and Scuticociliatida co-dominated ciliate assemblages of lakes > pH 5.0 but were replaced by the Oligotrichida in more acidic lakes (Table 2). The percentage contribution of ciliated protozoa to total zooplankton biomass decreased with decreasing pH [21 ], although only minor taxonomic replacements were observed for rotifers and crustaceans. In addition to the observed compositional shift, smaller ciliates (especially 20-30/zm) were progressively replaced by larger ciliates (40-50/~m) with increasing acidity (Fig. 3). The smaller size class contained most of the scuticociliates (Cyclidium), whereas a majority of the oligotrichs (Strombidium, Strobilidium) and some of the haptorids (Askenasia) fell into the larger size class. Recent investigations on acidic Florida systems [9; R. W. Bienert, Jr. 1987, Ph.D. Thesis, University of Florida] suggest that a relatively large-sized ciliate (150-200 #m), Stentor niger (Heterotrichida), appears to be a good indicator of acidic conditions. This species was limited primarily to lakes < 5.5 pH, and it contributed an average of 64% of total ciliate biomass in those acidic systems in which it was found [9]. S. niger is quite sensitive to distortion during preservation, and precise identification requires collection of live specimens. Its reaction to a variety of preservatives gives it the appearance of detrital material, and unless inspected closely it may be easily overlooked. The functional role of S. niger in the microbial food web of lakes in which it dominates is probably very substantial as all specimens were densely packed with zoochlorellae. Its myxotrophic mode of nutrition (utilization of particulate, photosynthetically fixed, and dissolved organic carbon) [92, 94] is likely an adaptation to nutrient-limited conditions. The fact that peak densities of this species occur only during thermal stratification in summer further supports this interpretation (R. W. Bienert, Jr. 1987. Ph.D. Thesis, University of Florida).

Ciliatecl Protozoa irt Freshwater Systems

117

t~ick and Drews [ 17] suggested that physiochemiea[ exclusion may be important for limiting the distribution of ciliates, with many species reaching their tolerance limit between pH 4.0 and 5.0. Direct physiological exclusion must exert some influence on ciliate distributions, but we believe that food availability is the principal governing factor. Bacterial biomass in temperate lakes often declines with decreasing pH [17], and those ciliate taxa dependent on bacteria as their principal food should also decline with increased acidity. The reduced importance of scuticociliate and haptorid taxa in such acidic lakes l~robably reflects the inefficiency of these ciliates to concentrate bacteria from a dilute suspension [34]. In contrast, the larger-bodied oligotrichs can subsist due to their ability to utilize nannoplanktonic algae in addition to bacteria. It is important to note that our understanding of ciliate community structure in acidic lakes is limited to subtropical Florida lakes. Comparable data from the temperate zone do not exist.

Ciliate Distribution in the Plankton

Temporal Variation of Small Ciliate Populations The seasonal dynamics of small planktonic ciliate populations are poorly underStood; however, recent evidence indicates that lake thermal regimes (especially timing and duration of thermal stratification) may indirectly influence protozoa by leading to localized enhancement of food resources within the water column [10, 86]. Oligotrophic lakes, which are often deep, are characterized by maxima of total abundance and biomass in the fall [10, 65, 69, 82] (Fig. 4). The population Pulses in these nutrient-poor systems are invariably dominated by the oligotrichs. Members of the Haptorida and Scuticociliatida, although poorly represented, generally peak simultaneously with the oligotrichs [10, 65]. Mesotrophic lakes, which are usually moderately deep and nutrient enriched, often display a bimodal pattern of ciliate abur~dance and biomass dominated by the Scuticociliatida [10, 105]. Population peaks in both temperate and subtropical lakes occur in late spring and early fall, corresponding to the periods of initial therraal stratification and destratification, respectively. I~etailed seasonality data for small ciliates in productive lakes are sparse, but it appears that protozoan communities often peak during summer [ 10] and are associated with metalimnetic and hypolimnetic plates of senescing algal and bacterial cells [86, 88]. Food resources probably are the major regulator of ciliate populations (diVersity, abundance, biomass) in general and the temporal succession of ciliate cOmmunities in particular [86]. Systems poor in nutrients tend to have popUlatic3n surges associated with thermal overturn--the time of highest productivity. Populations tend to be densest in the epilimnion instead of being asSociated with metalimnetic plates of algae and bacteria [65, 69]. Like oligotrophic Systems, mesotrophic lakes also experience a peak of abundance and biomass tl~aring fall associated with mixis; however, their additional late spring surge is 0robably due to the accumulation of organic matter in the metalimnion after

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Fig. 4. Representative seasonal cycles for total abundances and dominant orders of small (< 100/~m) ciliates, illustrated by data from three Florida lakes-Lake Annie (oligotrophic), Lake Francis (mesotrophic), and Lake Wauberg (hypereutrophic). Top panels display total abundance and bottom panels are Scuticociliatida (Q) and Oligotrichida (O). All values in cells/ml.

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Ciliated Protozoa in FreshwaterSystems

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Stratification [ 105] and the subsequent development of bacterial densities exceeding the threshold concentrations necessary for growth of scuticociliate and other bactivorous ciliate populations [33]. Finally, unlike lower trophic states, populations of small-bodied ciliates in eutrophic-hypereutrophic lakes display midsummer maxima that are likely associated with the accumulation in the lower water column of detritus and bacteria from the photic zone column. In addition, the relative shallowness of most productive systems dictates a more rapid warming of both the water Column and sediments, which hastens growth rates of both ciliates and bacteria

[41].

Various components of limnetic ciliate communities are significantly correlated with their food resources both temporally [10, 50] and spatially [86, 105]. Consequently, the interrelated factors of lake depth, nutrient concentrations, and the thermal regimes of lakes would substantially affect potential food SOurces for ciliates and determine patterns in seasonal succession.

Spatial Distribution within the Water Column of Small Ciliate Populations Relatively little information is available on the factors controlling the distribution of small ciliate taxa within the water column. The notable exceptions are investigations conducted in a eutrophic Georgia reservoir [86, 88]. In this system, the vertical distribution of ciliate abundance was nearly uniform during Winter mixis, although there was a tendency towards slightly higher values near the surface. After development of stable thermal stratification, protozoan numbers increased in the warming epilimnion and decreased in the hypolimnion. A bloom of scuticociliates began in the metalimnion and hypolimnion during .midsummer and was sustained until early fall. Peak abundances were recorded JUst below the thermocline. Concurrent with this bloom of scuticociliates in the lower water column, the density and diversity of epilimnetic ciliates inCreased until destratification. The vertical distribution of individual species in this reservoir was also influenced by stratification. During winter mixis few vertical distribution gradients were evident, but after stratification and subsequent oxygen reduction in the hypolimnion, individual taxa were confined to specific sections of the Water column. Small oligotrichs were restricted to the epilimnion, whereas scuticociliates and an omnivorous Coleps sp. were largely found in the metalinanion and hypolimnion and were significantly correlated with high algal and bacterial activity [86]. Strombidium viride, howeverl was always located near the surface because its symbiotic zoochlorellae imposed a partial dependence on a light source. This suggested that nutrition is the major determinant for the vertical distribution of small ciliates, because myxotrophic ciliates were found in the photic zone and primarily bactivorous forms (e.g., scuticociliates) Were located in the metalimnion where bacterial densities were high due to the accumulation of sedimented algal cells and detritus. A similar relationship between abundance of ciliates and thermal stratification has been demonstrated for two large oligotrophic lakes. Hunt and Chein

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J.R. Beaverand T. L. Crisman

[69] reported that small ciliates, principally oligotrichs, were distributed throughout the water column of a temperate lake during holomixis but tended to concentrate in the epilimnion as the thermocline deepened during summer. Hecky and Kling [65] reported that most ciliates in tropical Lake Tanganyika were confined to the epilimnion during stratification and frequently their biomass equalled or exceeded phytoplankton biomass because of the contribution of myxotrophic ciliates to total autotrophic standing crop. Sorokin and Paveljeva [105] reported that ciliates formed a substantial portion of the zooplankton biomass in a mesotrophic Soviet lake. Maximum ciliate concentrations were found in the metalimnion with biomass reaching approximately 3 g C/m 3, close to the total biomass of other zooplankton forms. Numerical abundance in this region ranged from 87 to 192 ciliates/ml. The development of metalimnetic protozoan plates coincided with elevated bacterial densities associated with decaying algal cells. Large increases in the predatory rotifer Asplanchna were noted within specific regions of the water column subsequent to ciliate population blooms, strongly suggesting a predator-prey relationship.

Temporal and Spatial Distribution of Populations of Large Meroplanktonic Ciliates The best understood area of freshwater protozoan ecology is the vertical and seasonal distribution of large (> 100 ~tm) ciliates in very productive water bodies [cf. 3, 45, 6 I]. The occurrence of large ciliates in the plankton is a predictable event directly related to thermal stratification and the resulting deoxygenation and accumulation of reducing compounds in the hypolimnion and at the sediment-water interface. Goulder [55, 56, 59, 60] extensively investigated the distribution of large ciliates within the water column of shallow hypereutrophic Priest Pot and determined that ciliate densities were correlated with oxygen concentrations. During summer stratification, the occurrence of otherwise hibernal benthic species in the hypolimnion was related to anoxia at the sediment-water interface which displaced these ciliates into the water column [56]. Typically, the large benthic ciliates Frontonia leucas, Spirostomum teres, S. minor, and Loxodes magnus regularly migrated from anoxic sediments during summer stratification and moved progressively higher in the water column (56 m from the sediments) as the zone of deoxygenation expanded upwards [4, 43, 44] (Fig. 5). As stratification deteriorated, this population migrated downwards and returned to the sediments. A second association of large thiobiotic ciliates that are obligate anaerobes (Brachonella spiralis, Caenomorpha medusula, and Metopus es) appeared and occupied a region of the hypolimnion just previously vacated by the first group. Finally, peritrich ciliates (Vorticella, Epistylis), which are epiphytic and epizootic on large phytoplankton and zooplankton, usually remained in the epilimnion [4]. Consequently, there was an absence of significant spatial overlap within the water column by the large ciliates [3] (Fig. 5). Finlay [42] demonstrated that vertical migration both within the sediments and into the water column of hypereutrophic lakes is governed by the development of the redox discontinuity profile.

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The ability o f these large ciliates to survive anaerobic conditions indicates that they are facultative or obligate anaerobes. Goulder [57] suggested that these ciliates may undergo pronounced diel vertical migration to the overlying oxygenated water; however, this has not been documented. An alternative explanation for the occurrence of large ciliates in anoxic waters is that oxygen may be present at very small concentrations in the hypolimnion due to eddy diffusion and production by photosynthetic bacteria and phytoplankton [5]. The micropatchiness and extremely low levels o f oxygen escape detection. Finlay et al. [49] reported the capacity of Loxodes for anaerobic respiration using nitrate reduction on the mitochondrial membrane as an energy source, thereby having no need for oxygen. Thus, ciliates located near the anoxic-oxic boundary may have an energetic advantage by simultaneously using aerobic and anaerobic respiration. Finlay and Fenchel [47] noted for species of Loxodes that increased light intensities aggravated oxygen toxicity. Exposure of cells to light at the oxicanoxic boundary induced downward migration. Photosensitivity and oxygen toxicity are thus biochemically linked, and removal of Loxodes from a supply o f oxygen inhibits photosensitivity. This tendency towards microaerophily by Loxodes and similar ciliates allows them to occupy an area of the water column devoid o f potential predators and competitors (rotifers and crustaceans) as the latter are eliminated from the hypolimnion because of their dependence on oxygen [48, 96]. Large ciliates (> 100 ~m) in hypereutrophic lakes therefore display pronounced vertical distribution within the water column. Some taxa live exclusively in the oxygenated epilimnion [4], others may be found only in the anoxic hypolimnion [45], and yet another group straddles the boundary beween oxygenation-deoxygenation [4] (Fig. 6).

Functional Role of Ciliates in the Freshwater Plankton

Prey for Large Zooplankton The possibility that ciliates are a prey item for macrozooplankton has been recognized [94], but the extent o f this interaction is uncertain because protozoa are rarely recognizable in the gut contents of metazoa. Many laboratory studies have determined that rotifers [52], cladocerans [94, 115], and copepods [1, 75] effectively ingest ciliates as prey. A major criticism of most of these studies is that they often are limited to the genus Paramecium, which is not a com m on component of the plankton. Moreover, prey and predator concentrations are experimentally elevated over those encountered in situ to achieve some measurable effect. The studies that have used com m on representatives of the ciliate plankton, Cyclidium glaucoma [94] and Halteria grandinella [1 ], allow some qualitative assessment of a possible trophic relationship. Porter et al. [94] demonstrated that Daphnia magna had higher rates of filtering and ingestion when fed Cyclidium than when fed Paramecium, although assimilation rates were comparable.

Ciliated Protozoa in Freshwater Systems

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Archbold and Berger [ 1] have shown in laboratory studies that crustaceans and naupliar stages of cyclopoid copepods are effective predators on Halteria grandinella. There is also some in situ evidence for macrozooplankton cropping of ciliate populations [ 14, 105]. Finally, the larval stages of some herbivorous fish are believed to use ciliates as a food source [75]. The importance of this predation link is that ciliates may transform the Picoplankton (bacteria) into a larger form available to higher trophic levels. t]ecause most metazoa are inefficient at grazing bacteria [26, 91, 93], this relationship likely represents a significant but poorly quantified carbon and energy pathway in the freshwater plankton.

Planktonic Ciliates as Grazers of Phytoplankton Ample evidence suggests that algivorous activity by ciliates is common [ 16, 58, 66]. Peak abundance of potentially algal grazing ciliates (large-bodied oligotrichs) and their algal prey often coincide within the water column. Nauwerck [85] recorded peaks in oligotrich numbers simultaneously with those of three Small chlorophytes--Chlamydomonas, Stichococcus, and Chlorella. Other investigators have observed the same relationship in tropical and subtropical lakes [65, 74]. It has been hypothesized that oligotrichs may influence the population dyt~amics of phytoplankton by grazing nannoplanktonic chlorophytes [ 10]. Hein-

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bokel and Beers [66] have demonstrated that marine oligotrichs may graze as much as 20% of the phytoplankton crop daily. In addition, Fenchel [33] has shown that ciliates that feed on larger (> 1 ~m) particles compare favorably with metazoan suspension feeders in the ability to concentrate particles from dilute suspension. Finally, Finlay and Berninger [46] noted that different species of Loxodes partition their algal food sources on the basis of particle size, and that this greatly influences spatial and seasonal distributions.

Planktonic Ciliates as Grazers o f Bacteria Most free-living ciliates are bactivorous, with each species having a distinct particle size range which is retained and ingested [34]. The size spectrum preferred by each species is a function of mouth size and morphology [35]. Ciliates that feed on the smallest (

The role of ciliated protozoa in pelagic freshwater ecosystems.

The abundance and biomass of ciliates are both strongly related to lake trophic status as measured by chlorophylla concentrations. Taxonomic replaceme...
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