A comparative study on planktonic ciliates in two shallow mesotrophic lakes (China): species composition, distribution and quantitative importance

The species richness and seasonal development of planktonic ciliates were studied and compared in two shallow mesotrophic lakes, one covered with dense submerged macrophytes, the other macrophyte poor. Considerable differences in ciliate species composition, dominant taxa, abundance and biomass were observed. Ciliates were much more species rich in the macrophyte-rich lake, while they were more abundant numerically in the macrophyte-poor lake. Altogether, 96 species, included in 53 genera, 14 orders were identified. Among them, 80 species (included in 45 genera, 14 orders) observed from the macrophyte-rich lake, against 49 species (36 genera, 12 orders) were from the macrophyte-poor lake. In the macrophyte-rich lake, the mean abundance and biomass were 13.5 cells ml^-1 and 547.10 g l^-1 f.w.; abundance and biomass were higher in spring and winter; naked oligotrichs dominated total ciliate abundance and Peritrichida dominated the biomass. In the macrophyte-poor lake, ciliate mean abundance and biomass were 35.5 cells ml^-1 and 953.39 g l^-1 f.w.; abundances peaked in autumn; Scuticociliates dominated the abundance and Tintinnids dominated the biomass. Possible causes for the observed differences are discussed.     

Seasonal dynamics of phytoplankton and phytobenthos,and associated chemical interactions,in a shallow upland lake (Malham Tarn,northern England)

Seasonal changes of phytoplankton were followed over 3 years (1985–87) in a shallow, unstratified and calcareous upland lake.The phytoplankton was of low to moderate abundance and generally dominated by phytoflagellates. Seasonality involved a winter minimum of abundance, a spring maximum of diatoms, and often brief increases in summer that included blue-greens, especially the colonial Gloeotrichia echinulata. Some components were of benthic origin. Seasonal growth of the main component of the phytobenthos, Chara globularisvar. virgata, caused a regular summer depletion in lake water of Ca²⁺ and HCO₃ ^- (alkalinity) by associated CaCO₃ deposition, and a more extreme (and unusual) depletion of K⁺. Chemical analysis of Chara biomass and of underlying sediments indicated a large benthic nutrient stock, much surpassing that represented by the phytoplankton. Growth in this biomass, and the magnitude of water-borne inputs, influenced the removals of Ca²⁺, K⁺ and inorganic N. The phytoplankton was probably limited by a low-P medium, to which co-precipitation of phosphate with CaCO₃ may have contributed. A vernal depletion of Si was probably limiting to diatom growth, and appeared to be mainly induced by benthic rather than planktonic diatoms. Examples of long-term change in composition of the phytoplankton and phytobenthos are noted and discussed in relation to the interaction of these components, nutrient enrichment, and possible alternative stable states.     

Chlorella-bearing ciliates dominate in an oligotrophic North Patagonian lake (Lake Pirehueico, Chile): abundance, biomass and symbiotic photosynthesis

1. Large mixotrophic ciliates ( Stentor araucanus , S. amethystinus and Ophrydium naumanni ) were a characteristic component of a temperate, oligotrophic lake in North Patagonia. During a 1-year study, the abundance, biomass and primary production of these large Chlorella -bearing ciliates were compared with those of the total plankton community.
2. Mixotrophic ciliates peaked in spring and from late summer to autumn, accounting for 1.6–43% (annual average: 16.3%) and 67–99% (annual average: 92%) of total ciliate abundance and biomass, respectively. Their contribution to total zooplankton biomass, including flagellates, rotifers, ciliates and crustaceans, was 14–76%, or 47% as an annual average. Endosymbiotic algae accounted for up to 25% of total autotrophic biomass (annual mean: 3.9%).
3. Maximum cell-specific photosynthetic rates of S. araucanus and S. amethystinus at light saturation varied between 80 and 4400 pg C ciliate^–1 h^–1 with high values during autumn and winter, and low values during summer. The depth-integrated rates of photosynthesis (0–40 m) of algal endosymbionts contributed 1–25% to total photosynthesis (annual mean: 6.5%).
4. A comparison of calculated ingestion rates with photosynthetic rates of Stentor indicates that photosynthate produced by endosymbionts generally exceeded heterotrophic food supply of Stentor during autumn and winter, but was much lower during summer, when food supply was high.
5. The mixotrophic ciliates represent an important 'link' between nanoplankton and higher trophic levels within the plankton community because of their high heterotrophic biomass and considerable contribution to total photosynthesis.     

Vertical distribution of planktonic ciliates in strongly stratified temperate lakes

The vertical distribution of planktonic ciliates in eight strongly stratified temperate lakes was studied in summer 1998. Ciliate abundance and biomass were highest (mean 39.9 cells ml^–1 and 181.9 g C l^–1) in the epi-, and lowest (mean 8.2 cells ml^–1 and 97.6 g C l^–1) in the hypolimnion. The community of ciliates was dominated by five orders: Oligotrichida, Haptorida, Prostomatida, Scuticociliatida and Peritrichida. The community composition varied greatly with depth. In the epilimnion, the ciliate numbers were dominated by oligotrichs but small algivorous prostomatids, peritrichs and haptorids were also numerous. In the metalimnion, these groups were replaced by scuticociliates and mixotrophic prostomatids. In the hypolimnion species known as benthic migrants appeared. We found a positive significant correlation (p < 0.05) between ciliate numbers and Chl a and bacterial densities. Only in the hypolimnion, the correlation between ciliates numbers and Chl a was not significant.     

Autofluorescence of marine planktonic Oligotrichina and other ciliates

Planktonic ciliates, principally from the suborders Oligotrichina and Tintinnina, were examined by epifluorescence microscopy. This allowed (1) to check if isolated symbiotic plastids demonstrated by TEM in some species could keep and show an important autofluorescence, (2) to count and identify the species presenting this characteristic, and (3) to determine their proportion compared with the other strictly heterotrophic planktonic ciliates. An average of 40.6% of the Oligotrichina species, collected during the fall and winter, diplayed a strong autofluorescence. This indicated chlorophyll contents, in a good state of activity, which most often masked the digestive vacuoles shown by TEM. It seems that the maintenance of plastids, or of symbiotic algae, is common in this sub-order. These autofluorescent ciliates are probably all mixotrophic. We suggest to call them ‘plastidic ciliates’, and to consider their role in marine primary production. Discussion deals with the origin and the role of autofluorescent pigments in the Oligotrichina, compared with the other planktonic ciliates. The interest and the limits of the methods employed up to now, or to be used in this field, are analysed. Finally the evolutionary and ecological significance of such a development of symbiosis among planktonic ciliates is discussed.     

The temporal and spatial distribution of planktonic and benthic protozoan communities in a small productive lake

An investigation into the spatial and temporal distribution of protozoa in Esthwaite Water, a small eutrophic lake in the English Lake District, was carried out from mid-June until late September, 1977, during the period of summer stratification. Quantitative analyses of planktonic protozoan populations were performed on water samples collected at 1 m intervals throughout the water column and individual depth-time distribution profiles were constructed for the major ciliate species. Population density and species succession of the benthic protozoa was also studied.     

Mixotrophic ciliates in an Andean lake: dependence on light and prey of an Ophrydium naumanni population

1. Planktonic ciliates were examined during a spring–summer period (November 1998–April 1999) in the ultraoligotrophic Lake Moreno Oeste (41°5' S and 71°33' W, 758 m a.s.l), which belongs to the Nahuel Huapi System (Patagonia, Argentina). The lake is deep ( Z _max=90 m) and warm monomictic.
2. Sampling was performed at a mid-lake station, where vertical profiles of temperature and light were measured in situ , and samples for bacteria and ciliates enumeration were taken throughout the water column.
3. The peritrich Ophrydium naumanni , a freshwater pelagic ciliate with endosymbiotic Chlorella , was the dominant ciliate in the lake.
4.  Ophrydium naumanni and autotrophic picoplankton exhibit a clear coincidence in their vertical distribution ( P  < 0.05), preferring levels at or near the 1% of surface photosynthetically active radiation (PAR) irradiance. Both may have the same light requirements, or the coincidence may reflect a trophic relationship.
5. Dependence on light and prey by O. naumanni were studied using field experiments, in which we analysed ciliate grazing on bacteria, and in laboratory experiments, in which we compared particle uptake under dark and light conditions.
6.  Ophrydium naumanni was able to ingest particles [latex microspheres and fluorescently labelled bacteria (FLB)] in field and laboratory experiment, indicating that it has the potential to affect bacteria population of Lake Moreno Oeste.
7. Ciliate particle ingestion was observed to be dependent on light availability because under dark conditions, the ingestion was lowered ( P  < 0.05).     

Distribution and production of chironomids (Diptera: Chironomidae) in a shallow, alkaline lake (Neusiedler See, Austria)

From July 1990 to July 1991 the benthic community of the open water zone of Neusiedler See, one of the largest shallow lakes in central Europe, was studied with special reference to the chironomids. Only 16 spp. of chironomids inhabited the sediment of the open water zone. The numerically dominant species were Tanypus punctipennis, Procladius cf. choreus, Microchironomus tener and Cladotanytarsus gr. mancus. Most invertebrates showed a distinct horizontal distribution. Species richness and abundance were highest on muddy and organically rich substrates near the reed belt. Chironomid densities in this area reached 54,000 ind m^–2 and biomass was 2.0 g dw m^–2. The two tanypod species accounted for more than 90% of the standing stock of the macrozoobenthos near the reed belt. The sediment of the open lake and of the eastern part of Neusiedler See was composed of compact clay and sand as a result of the erosion of fine material due to strong waves and currents. Individual densities in these areas were much lower. Production of the numerically dominant species T. punctipennis was estimated using the increment-summation method, whereas production of the remaining species was estimated using an empirically derived multiple regression. Mean annual production of chironomids exceeded 6 g dw m^–2 yr^–1 near the reed belt, but it reached only 0.55 g dw m^–2 yr^–1 in the open lake. These values are rather low compared with other lakes and can be explained by unfavourable sediment conditions due to wave action and by physiological stress due to the water chemistry.     

DNA-based analysis of planktonic methanotrophs in a stratified lake

1. The assemblage of planktonic methanotrophs in a stratified freshwater lake was investigated. Vertical patterns were analysed by denaturing gradient gel electrophoresis, using the primer pair specific for 16S rRNA genes of type I methanotrophs.
2. The resulting banding patterns could be divided into three distinct groups, and sequenced bands were all related to the Methylobacter species. No amplicon was obtained with the primer pair specific for type II methanotrophs.
3. Cloning analysis of the pmoA gene was performed using samples from three water depths (epilimnion, metalimnion and hypolimnion). The compositions of the clone libraries from the three depths were distinct from each other but all three libraries were dominated by clones related to Methylobacter species.     

Long‐term dynamics of submerged macrophytes and algae in a small and shallow,eutrophic lake: implications for the stability of macrophyte‐dominance

1. Submerged macrophyte and phytoplankton components of eutrophic, shallow lakes have frequently undergone dynamic changes in composition and abundance with important consequences for lake functioning and stability. However, because of a paucity of long‐term survey data, we know little regarding the nature, direction and sequencing of such changes over decadal–centennial or longer timescales. 2. To circumvent this problem, we analysed multiple (n = 5) chronologically correlated sediment cores for plant macro‐remains and a single core for pollen and diatoms from one small, shallow, English lake (Felbrigg Hall Lake, Norfolk, U.K.), documenting 250 years of change to macrophyte and algal communities. 3. All five cores showed broadly similar stratigraphic changes in macrophyte remains with three distinct phases of macrophyte development: Myriophyllum–Chara–Potamogeton (c. pre‐1900), to Ceratophyllum–Chara–Potamogeton (c. 1900–1960) and finally to Zannichellia–Potamogeton (c. post‐1960). Macrophyte species richness declined from at least 10 species pre‐1900 to just four species at the present day. Additionally, in the final Zannichellia–Potamogeton phase, a directional shift between epi‐benthic and phytoplankton‐based primary production was indicated by the diatom data. 4. Based on macrophyte–seasonality relationships established for the region, concomitant with the final shift to Zannichellia–Potamogeton, we infer a reduction in the seasonal duration of plant dominance (plant‐covered period). Furthermore, we hypothesise that this change in species composition resulted in a situation whereby macrophyte populations were seasonally ‘sandwiched’ between two phytoplankton peaks in spring and late summer as observed in the contemporary lake. 5. We suggest that eutrophication‐induced reductions in macrophyte species richness, especially if the number of plant‐seasonal strategies is reduced, may constrict the plant growing season. In turn, this may render a shallow lake increasingly vulnerable to seasonal invasions of phytoplankton resulting in further species losses in the plant community. Thus, as part of a slow (over perhaps 10–100s of years) and self‐perpetuating process, macrophytes may be gradually pushed out by phytoplankton without the need for a perturbation as required in the alternative stable states model of plant loss.     

Temporal and vertical distribution of planktonic rotifers in a meromictic crater lake,Lake Nyahirya (Western Uganda)

Quantitative zooplankton sampling was carried out biweekly during 16 months in Lake Nyahirya, a Western Uganda crater lake. Six out of 24 rotifer species recorded from the lake were quantitatively important (Horaella brehmi, Brachionus angularis, B. caudatus, Keratella tropica, Filinia longiseta and F. opoliensis, and three others were common (Brachionus falcatus, Asplanchna sieboldi and Conochiloides natans). Rotifer peaks were correlated with rainy periods and relatively clear water, which coincide with the European spring and autumn. All species were able to deal with hypoxic conditions. A clear niche separation can be observed between different genera and between species of the same genus. The population dynamics of the rotifers seem to be ruled by repeated irregular environmental fluctuations. Rainfall appears to be a primary steering factor.     

Growth and nutrient uptake by two species of Elodea in experimental conditions and their role in nutrient accumulation in a macrophyte-dominated lake

The capacity of Elodea nuttallii (Planch.) St. John and Elodea canadensis Michx. to remove nitrogen from water was evaluated in laboratory experiment. The growth rate of plants and their effect on the nitrogen level of hypertrophic Lake Zwemlust (the Netherlands) as well as on lake water enriched with nitrogen were investigated. The plants grew best in water enriched with up to 2 mg NH₄-Nl^–1 and 2 mg NH₄-Nl^–1 plus 2 mg NO₃ Nl^–1. During a 14 day experiment, plants absorbed from 75% to 90% of nitrogen. Higher nitrogen concentration than 4 mg l^–1 had a negative effect on growth of both species. Elodea nuttallii and E. canadensis prefer NOinf4/^p+ over NOinf4/^p– when both ions were present in water in equal concentrations.     

Vertical distribution of planktonic rotifers in a karstic meromictic lake

Vertical distribution of planktonic rotifers is described in relation to temperature and oxygen in Lake La Cruz, a single-doline, closed karstic lake (121 m diameter and 25 m maximum depth) which shows iron meromixis. Samples were taken by peristaltic pumping at 10 cm depth intervals in the oxycline zone from June 1987 to September 1988. A model of rotifer vertical structure in stratified lakes is proposed. Rotifers concentrate their populations at the depths with intense gradients. As stratification develops some rotifer populations show a downward migration following the thermocline and some others show an upward migration following the oxycline. The production-respiration balance in the lake, and so the position of the oxycline with respect to the thermocline and the layer of maximum production, depends on meteorological conditions. A shift in the dominance of congeneric or related species can occur in consecutive years. In Lake La Cruz, mixing conditions and subterranean inflow in spring were much more intense in 1988 than 1987, and the distance between production and decomposition depths was smaller in 1988. Anuraeopsis miraclei, an oxycline-bound species with high abundance in 1987, was displaced by A. fissa in 1988. A. fissa, which was a metalimnetic species during early summer, reached peak densities (3 × 10⁴ ind l^–1) at the oxycline, equaling the abundance of A. miraclei the preceeding year.     

The development and persistence of alternative ecosystem states in a large,shallow lake

1. Palaeolimnological data were used to investigate drivers of the community of primary producers in Lake Mattamuskeet, North Carolina, U.S.A. This is a large, shallow lake with two basins currently dominated by phytoplankton and macrophytes. The two basins were divided in 1940 by the building of a roadway across the lake, which also corresponded with the divergence in their ecosystem state. 2. Photosynthetic pigments, organic matter and nutrients (P, N, C, S) were analysed in sediment cores from each basin to reconstruct the primary producer community over the past c. 100 years. We sought to answer two questions. First, what changes to the ecosystem resulting from the building of the roadway caused the development of different primary producer communities in the two basins? Second, why have the alternative ecosystem states persisted despite a variety of human perturbations since 1940? 3. K‐means cluster analysis and principal component analysis were applied to identify three sediment types based on photosynthetic pigment data: sediments indicating low productivity (low pigment concentrations), sediments associated with macrophytes (chlorophyll a and b) and with phytoplankton (alloxanthin and aphanizophyll). In addition, other palaeolimnological proxies measured, such as loss on ignition, total phosphorus, total organic carbon/total nitrogen and other nutrients, were different in post‐1940 sediments within the two basins. 4. These differences suggest characteristics, such as nutrient cycling, water depth and other physical changes resulting from roadway construction, combined to establish and maintain the differing communities of primary producers in the two basins. Furthermore, Fe/S dynamics and waterfowl herbivory probably contributed to the development of the two ecosystem states.     

Existence of a macrophyte-dominated clear water state over a very wide range of nutrient concentrations in a small shallow lake

Little Mere, a small shallow lake, has been monitored for four years, since its main source of nutrients (sewage effluent) was diverted. The lake has provided strong evidence for the persistence of a clear water state over a wide range of nutrient concentrations. It had clear water at extremely high nutrient concentrations prior to effluent diversion, associated with high densities of the large body-sized grazer, Daphnia magna, associated with low fish densities and fish predation. Following sewage effluent diversion in 1991, the nutrient concentrations significantly declined, the oxygen concentrations rose, and fish predation increased. The dominance of large body-sized grazers shifted to one of relatively smaller body-sized animals but the clear water state has been maintained. This is probably due to provision of refuges for grazers by large nymphaeid stands (also found prior to diversion). There has been a continued decrease in nutrient concentrations and expansion of the total macrophyte coverage, largely by submerged plants, following effluent diversion. The grazer community of Little Mere has also responded to this latter change with a decline in daphnids and increase in densities of weed-associated grazers. The presence of large densities of such open water grazers was the apparent main buffer mechanisms of the clear water state until 1994. The lake has, so far, maintained its clear water in the absence of such grazers. Thus, new buffer mechanisms appear to operate to stabilize the ecosystem. Little Mere appears to have shifted from previous top-down controlled clear water state to a bottom-up controlled clear water state.     

Increase in photosynthetic efficiency as a strategy of planktonic organisms exploiting deep lake layers

1. The photosynthetic efficiencies of the mixotrophic ciliate Ophrydium naumanni and the autotrophic dinoflagellate Gymnodinium paradoxum were investigated using laboratory and field experiments in Lake Moreno Oeste (41°5′S and 71°33′W, 758 m a.s.l.), in the Nahuel Huapi System (North Patagonia, Argentina). 2. The effect of different underwater light intensities on net primary production (NPP) was assessed during one summer. Additionally, laboratory experiments were carried out to obtain photosynthesis‐irradiance response curves for each species. 3. Ophrydium naumanni and G. paradoxum dominated the metalimnetic (30 m depth) deep chlorophyll maximum (DCM) in the lake. 4. Despite these deep higher abundances, the cell‐specific production of both species was higher at 10 m than at 30 m (DCM) depth. In addition, at 5 m depth, NPP was reduced by PAR + UV‐A radiation. 5. Both species exhibited a positive NPP at very low irradiance but the mixotrophic ciliate was more efficient in exploiting the DCM irradiance level both in situ and at comparable light intensities in laboratory experiments. Light acclimatised O. naumanni showed a higher NPP at lower irradiances and photoinhibition at medium and high irradiances. 6. Under the strong wind‐driven turbulence commonly found in Patagonian lakes, organisms cannot select their position in the epilimnetic water column and will be dragged to potentially harmful UV radiation levels. Thus, metalimnetic DCM colonisation by these two species represents a tradeoff between higher survival and lower cell‐specific NPP.     

Effects of fish and nutrient additions on food-web stability in a charophyte-dominated lake

1. The response of major food‐web constituents to combinations of nutrient addition and zooplanktivorous fish abundance was tested during two subsequent years in the shallow charophyte‐dominated lake Naardermeer in the Netherlands, using in situ enclosures. 2. Treatment effects differed sharply between study years. In 1998, when the summer temperature was low (17–21 °C), high algal biomass only developed at high nutrient levels in the presence of fish, but with no major effect on Chara biomass. In 1999, when the summer temperature was relatively high (20–25 °C), algal blooms occurred at high nutrient levels regardless of fish abundance, and were associated with a drastic decline in Chara biomass. 3. Differences between years in temperature and initial zooplankton composition and biomass were likely to contribute to the varying relative importance of top‐down and bottom‐up effects in these enclosure experiments. 4. The results suggest that when nutrient loads are increased towards levels where the macrophyte‐dominated state is being destabilised, a ‘switch’ is more likely to occur in warm summers.     

Micro‐scale structure in the chemistry and biology of a shallow lake

1. Repeat sampling in daytime within a lily (Nuphar lutea) bed and in open water showed distinct heterogeneities in the three‐dimensional distributions of water chemistry and planktonic organisms on centimetre to decimetre scales. 2. Vertical gradients of physico‐chemical variables that did not exist at dawn developed during the day in both sites, as available nutrients were released from the sediments and were consumed towards the surface. 3. Distributions of algal standing crop suggest limitation by both nutrients and grazing. 4. Marked variability in distributions may question the assumptions often made about the homogeneity of plankton and available nutrient distributions in open water and in macrophyte stands of shallow lakes. Although simple sampling regimes for monitoring of water quality may be adequate for many purposes, they miss a fine structure in the water that is inherently interesting in understanding the underlying processes of plankton function.     

Aquatic macrophytes and phytoplankton of a quarry lake in the upper Vicenza range: preliminary study

Abstract

The work was carried out during 1995 in a quarry-lake and 13 samplings were performed to gather data on the aquatic flora. Sampling locations both along the shores and in the water provided data to define the position and the amount of covering by macrophytes. As regards the microphytes, considered as the sole phytoplankton component, the composition and the seasonal abundance were evaluated.