The Seasonal Dynamics and Composition of Photosynthetic Picoplankton Communities in Temperate Lakes in Ontario,Canada

The seasonal abundance and composition of photosynthetic picoplankton (0.2-2 μm) was compared among five oligotrophic to mesotrophic lakes in Ontario. Epilimnetic picocyanobacteria abundance followed a similar pattern in all lakes; maximum abundance (2-4 × 10⁵ cells · ml⁻¹) occurred in late summer following a period of rapid, often exponential increase after epilimnetic temperatures reached 20 °C. In half of the lakes picocyanobacteria abundance was significantly correlated with temperature, while in other lakes the presence of a small spring peak resulted in a poor correlation with temperature. In all lakes there was a significant correlation between epilimnetic abundance and day of the year. Correlations with water chemistry parameters (soluble reactive phosphorus, total phosphorus, particulate C: P and C: N) were generally weaker or insignificant. However, in the three lakes with the highest spring nitrate concentrations, a significant negative correlation with nitrate was observed. During summer stratification, picocyanobacteria abundance reached a maximum within the metalimnion and at or above the euphotic zone (1% of incident light) in all lakes. These peaks were not related to nutrient gradients. The average total phytoplankton biomass ranged from 0.5 g m⁻³ (wet weight) in the most oligotrophic lake to 1.4 g m⁻³ for the most mesotrophic with picoplankton biomass ranging from 0.01 g m⁻³ to 0.3 g m⁻³. Picocyanobacteria biomass comprised 1 to 9 % of total phytoplankton biomass in late summer, but in one year for one lake represented a maximum of 56%. Other photosynthetic picoplankton (unidentified eukaryotes, Chlorella spp. Nannochloris spp.), although less abundant (10³ cells · ml⁻¹) than picocyanobacteria, represented biomass equal or greater than that of the picocyanobacteria in spring and early summer. On average, half of the photosynthetic picoplankton biomass was eukaryotic in the more coloured lakes, while in the clear lakes less than 20% was eukaryotic. Among the lakes there was a significant positive correlation between the average light extinction coefficient and the proportion of eukaryotic biomass of the picoplankton. In mesotrophic Jack's Lake, the contribution of picoplankton to the maximum photosynthetic rate ranged from 10 to 47% with the highest values in the spring (47%) and late summer (33%), as a result of eukaryotic picoplankton and picocyanobacteria respectively. Picocyanobacteria cell specific growth rates were high during July (0.6-0.8 day⁻¹) and losses were close to 80% of the growth rate. Thus, despite low biomass, photosynthetic picoplankton populations appeared to turn over rapidly and potentially contributed significantly to planktonic food webs in early spring and late summer.     

Algal picoplankton production and contribution to food-webs in oligotrophic British Columbia lakes

The pelagic communities of two contrasting oligotrophic lakes in British Columbia were studied to determine why an interior, dimictic lake (Quesnel) supports a greater biomass of zooplankton and produces larger planktivorous sockeye salmon (Oncorhynchus nerka) than a coastal warm-monomictic lake (Sproat). The ultra-oligotrophic status and differing planktivore densities in Sproat Lake increased the relative importance of algal picoplankton, diminished the abundance of large zooplankton, and increased the significance of rotifers and other small-bodied zooplankton. These picoplankton based food webs result in longer, indirect and less efficient pathways of carbon flow from phytoplankton to fish. In contrast, Quesnel Lake is a more productive oligotrophic lake and its pelagic food webs are based more on nanoplankton and small microphytoplankton that support larger-bodied zooplankton (Daphnia, Diaptomus), and a more direct and efficient two-step transfer to fish. The greater variability of the annual recruitment of sockeye fry in interior lakes may keep zooplankton communities in a non-steady state, this in turn may perpetuate the occurrence of quadrennial cyclic dominance in adult salmon returning to these systems.     

Autotrophic Picoplankton: Community Composition,Abundance and Distribution across a Gradient of Oligotrophic British Columbia and Yukon Territory Lakes

Autotrophic picoplankton communities were examined in eleven oligotrophic lakes from a broad geographic region of western Canada, representing a variety of physico-chemical and biological conditions. During our study, several of the lakes were treated with additions of inorganic nitrogen and phosphorus fertilizers. Picoplankton communities in most lakes were dominated (>70%) by unicellular or colonial coccoid cyanobacteria, provisionally identified by morphological and autofluorescence properties as Synechococcus. Also common in some lakes were red-fluorescing cyanobacteria and Chlorella-like eucaryotes. Autotrophic picoplankters contributed from 36-63% to total chlorophyll, from >2-26% to total phytoplankton carbon, and from 29–53% to total photosynthesis. Average populations ranged from >5-10,000 cells·ml⁻¹ in winter and early spring to 65-75,000 cells · ml⁻¹ in summer and fall. Peak densities in most lakes occurred in August-September and most populations were within the epilimnion or metalimnion/hypolimnion boundary. Subsurface peaks were prevalent only in untreated, strongly stratified lakes. Eucaryotic picoplankters became dominant in acidic (pH < 6.2), humic lakes. Colonial picoplankters were more common in more productive interior lakes in August, and though present, were uncommon in coastal systems. Picoplankton populations exhibited large increases under ice in a Yukon lake, and their abundance and seasonal distribution showed little relation to temperature or to light. Fertilization of lakes resulted in picoplankton population increases (>2x) and the elimination of subsurface peaks. Nutrients were considered to be one of the major factors controlling population abundance in these oligotrophic lakes with average pH < 6.5.     

Seasonal Changes in Chlorophyll-containing Picoplankton Populations of Ten Lakes in Northern England

Key features of photosynthetic picoplankton populations were compared during 1988 in ten lakes in northern England ranging from oligotrophic to slightly eutrophic; two of the three eutrophic lakes were shallow and lacked a thermocline. Measurements were made at 0.5 m depth of temperature, total chlorophyll a, chlorophyll-containing picoplankton cell density, mean picoplankton cell volume and percentage of phycoerythrin-rich cells in the total picoplankton population. All lakes showed maxima for total chlorophyll concentration and picoplankton cell density in mid- to late summer. The maximum value for picoplankton density ranged from 3.4 × 10³ (Esthwaite Water) to 1.3 × 10⁶ cells ml⁻¹ (Ennerdale Water). There was a significant negative relationship (p < 0.05) between log₁₀ of maximum picoplankton cell density and maximum total chlorophyll, the latter being taken as an indicator of lake trophic status. The ratio of maximum to minimum picoplankton density during the year in a particular lake ranged from 39 to 2360 and showed no obvious relationship to lake type. Overall, the seasonal range in picoplankton density was about one order of magnitude greater than the range in total chlorophyll a, but there were considerable differences between lakes. Phycoerythrin-rich picoplankton as a percentage of total picoplankton reached a maximum in summer in all lakes. Values were always very low (<5%) in the two shallow eutrophic lakes, but reached 97% and over in the four most oligotrophic lakes. In two of the oligotrophic lakes, Wast-water and Ennerdale Water, phycoerythrin-rich picoplankton was a major component of the summer phytoplankton biomass.     

Phototrophic Picoplankton in Temperate Lakes: Seasonal Abundance and Importance along a Trophic Gradient

The seasonal development of autotrophic picoplankton was investigated in seven Danish lakes representing a eutrophication gradient. Highest cell abundance between 1.5 to 6 × 10⁵ cells ml⁻¹ were found in mid-summer. Minor peaks were observed in spring. In winter, densities were below 10³ ml⁻¹. The highest relative picoplankton contribution to total autotrophic biomass also occurred in mid-summer. In the eutrophic lakes and one humic lake the average seasonal contribution of picoplankton to total chlorophyll was below 1% increasing to 5-8% in the meso- and oligotrophic clear water lakes. During short periods the proportion of picoplankton did reach 25%. The higher relative importance of picoplankton in less productive lakes was not due to higher actual chlorophyll concentrations, but due to a much more pronounced response by larger algae at higher nutrient loading. Both cyanobacteria and eukaryote organisms were present as picoplankton. Only eukaryotes were found in one eutrophic lake and an acidic, humic lake. In the eutrophic lakes eukaryote picoplankton was dominant; both with respect to cell densities and biovolume, whereas cyanobacteria dominated the two meso-oligotrophic lakes. Autotrophic picoplankton were present in all lake types, however their importance seemed to be less in most eutrophic lakes than in less productive, meso-oligotrophic lakes.     

Abundance and distribution of picoplankton in tropical, oligotrophic Lake Kivu, eastern Africa

1. We used flow cytometry to characterize freshwater photosynthetic picoplankton (PPP) and heterotrophic bacteria (HB) in Lake Kivu, one of the East‐African great lakes. Throughout three cruises run in different seasons, covering the four major basins, phycoerythrin‐rich cells dominated the PPP. Heterotrophic bacteria and PPP cell numbers were always high and spatial variations were modest. This represents an important difference from temperate and high latitude lakes that show high fluctuations in cell abundance over an annual cycle. 2. Three populations of picocyanobacteria were identified: one corresponded to single‐cells (identified as Synechococcus by epifluorescence microscopy, molecular methods and pigment content), and the two other that most probably correspond to two and four celled colonies of the same taxon. The proportion of these two subpopulations was greater under stratified conditions, with stronger nutrient limitation. 3. High PPP concentrations (c. 10⁵ cell mL^?1) relative to HB (c. 10⁶ cell mL^?1) were always found. Lake Kivu supports relatively less bacteria than phytoplankton biomass than temperate systems, probably as a consequence of factors such as temperature, oligotrophy, nutrient limitation and trophic structure. 4. A review of PPP concentration across aquatic systems suggests that the abundance of Synechococcus‐like cyanobacteria in large, oligotrophic, tropical lakes is very high. 5. Photosynthetic picoplankton cell abundances in the oligotrophic tropical lakes Kivu and Tanganyika are comparable to those of eutrophic temperate lakes. This apparently contradicts the view that PPP abundance increases with increasing eutrophy. More data on PPP in tropical lakes are needed to explore further this particular pattern.     

Nutrient and Grazer-Mediated Effects on Picoplankton and Size Structure in Phytoplankton Communities

This study examines the factors which contribute to the abundance of algal picoplankton in lakes. A three-year field study of a meso-eutrophic lake was compared with observations from oligotrophic and highly eutrophic lakes in the region. Trophic state alone (oligotrophic vs. eutrophic) was not a good predictor of the importance of picoplankton; smaller cells were relatively abundant when phosphorus was limiting other phytoplankters, but also when nitrogen was in surplus. Subsequent field experiments found that picoplankton growth was stimulated by N, but not by P additions. This relationship was strongly affected by light and grazer levels. Grazers apparently mediate the effects of nutrient deficiency, and favor the growth of larger algal size classes, especially nanoplankton. The flux of P within experimental enclosures was controlled by picoplankton abundance under low nutrient conditions, but was a function of total phytoplankton biomass under P surplus.     

Carbon-14 Uptake by Picoplankton and Total Phytoplankton in Eight New Zealand Lakes

Eight New Zealand lakes were surveyed for ¹⁴C uptake by phytoplankton as a function of light intensity. The results support the view that the photosynthetic picoplankton is an important contributor to primary productivity in oligotrophic lakes but is relatively unimportant in more eutrophic lakes. A comparison of carbon uptake vs. light intensity characteristics (P vs. I) of the picoplankton size class vs. that of the total phytoplankton community supports the view that the picoplankton size class may be adapted to utilization of dimmer light.     

Comparing microscopic counts and pigment analyses in 46 phytoplankton communities from lakes of different trophic state

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Picoplankton photosynthesis and diurnal variations in photosynthesis-irradiance relationship in a eutrophic and meso-oligotrophic lake

The abundance and relative importance of autotrophic picoplankton were investigated in two lakes of different trophic status. In the eutrophic lake, measurements of primary production were performed on water samples in situ and in a light incubator three times during the day whereas for the oligotrophic lake, only one measurement of primary production was performed on water samples in the incubator. Dark-carbon losses of phytoplankton from Lake Loosdrecht were investigated in time series. Cell numbers of autotrophic picoplankton in eutrophic Lake Loosdrecht (3.2 × 10⁴ cells ml^–1) were lower than in meso-oligotrophic Lake Maarsseveen (9.8 and 11.4 × 10⁴ cells ml^–1 at the surface and bottom respectively). In the phytoplankton of both lakes the ratio of picoplankton production increased with decreasing light intensity. In Lake Loosdrecht depth-integrated contribution of picoplankton to total photosynthesis was less than 4%. The P-I-relationship showed diurnal variations in light saturated photosynthesis, while light limited carbon uptake remained constant during the day. Dark carbon losses from short-term labelled phytoplankton during the first 12 hours of the night period accounted for 10–25% of material fixed during the preceeding light period.     

Systematics and Ecology of Chlorophyte Picoplankton in German Inland Waters along a Nutrient Gradient

The community structure and succession of autotrophic picoplankton in several oligotrophic to hypertrophic German freshwater ecosystems were studied with emphasis on the occurrence and characterization of chlorophyte picoplankton. Depending on the trophic status and the time of the year, the relation of green eukaryotic picoplankton to picocyanobacteria, the contribution of the picoplankton to the total phytoplankton biomass, and the succession and dominance of picoplankton groups changed considerably. A significant correlation between the picoplankton abundances, their biomass and their biomass contribution could not be found. Although the chlorophyte picoplankton were similar with respect to their ultrastructure, phylogenetic analyses of the rbcL genes revealed that these organisms evolved independently within several green algal lineages. The most common picoplanktonic green algae in the lakes that were studied belong to the genera Choricystis and Pseudodictyosphaerium. Considering the new molecular biological findings, the systematics of picoplanktonic green algae from freshwater and marine habitats are discussed.     

The picoplankton in Antarctic lakes of northern Victoria Land during summer 1989–1990

Summary Photoautotrophic picoplankton is reported from some lakes located near the Italian Antarctic station of Terra Nova. Observations, carried out by both flow cytometry on water samples and electron microscopy on micro-organisms in cultures from each lake, have confirmed the occurrence in all the environments studied of this fraction accounting, in several cases, for more than the 50% of the phytoplankton, measured as chlorophyll. Cultures of the picoplankton fraction from these waters contained known prokaryotic (Synechococcus) and eukaryotic (Chlorella) genera as well as two unidentified entities, possibly prochlorophytes.     

Trophic coupling of the microbial and the classical food web in Lake Baikal,Siberia

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Determination of Eubacterial and Cyanobacterial Size and Number in Lake Baikal Using Epifluorescence

Chroococcoid cyanobacteria, (mean size = 0.79 μm, likely Synchetocystis limnetica Popovsk) and total eubacteria (mean size = 0.33 μm), from Lake Baikal, USSR, were enumerated using epifluorescence microscopy and sized with image analysis. Bacterial densities ranged from 0.44 · 10⁶ cells ml⁻¹ at 250 m to 2.3 · 10⁶ cells ml⁻¹ at the surface. Mean eubacterial abundance was 1.3 · 10⁶ cells ml⁻¹. Cyanobacterial densities were more variable, ranging from 0.42 · 10⁴ cells ml⁻¹ at 250 m to 9.8 · 10⁴ cells ml⁻¹ at the surface, with a mean abundance of 2.7 · 10⁴ cells ml⁻¹. The cyanobacteria, in particular, occurred in clusters resembling “marine snow”. Our results indicate that Lake Baikal picoplankton size and density are similar to other large lakes but may have a more diverse community structure than in other large oligotrophic lakes.     

The Role of Autotrophic Picocyanobacteria in Calcite Precipitation in an Oligotrophic Lake

A 1-year field study monitoring depth profiles of picoplankton and physicochemical data in the oligotrophic Lake Lucerne (Switzerland) showed that picocyanobacteria play an important role in the CaCO₃ precipitation process. Laboratory experiments with Mychonastes and Chlorella, isolated from Lake Lucerne and Synechococcus using ion selective electrodes, scanning electron microscopy and X-ray powder diffraction clearly demonstrated the potential of picoplankton for fast and effective CaCO₃ precipitation. The combination of a field study with laboratory experiments confirmed the previous reports of picocyanobacteria triggering the CaCO₃ precipitation in hardwater oligotrophic lakes. Electron micrographs of particles from the water column often reveal the size and shape of picoplankton cells covered by calcite. In addition the results from the laboratory approach indicated that algae and bacteria induced different precipitation mechanisms. Experiments with Mychonastes and Chlorella produced crystalline calcite completely covering the cells. Experiments with the cyanobacteria Synechococcus, however, yielded amorphous, micritic CaCO₃, indicating a different precipitation process.     

Seasonality of planktonic ciliated protozoa in 20 subtropical Florida lakes of varying trophic state

The planktonic ciliate populations of 20 Florida lakes ranging from oligotrophic to hypereutrophic were examined monthly for one year. Oligotrophic lakes displayed abundance peaks during fall mixis and biomass peaks in late winter and fall. Mesotrophic systems exhibited a spring-fall bimodality in ciliate abundance with a biomass maxima occurring during fall. Eutrophic/hypereutrophic lakes had pronounced abundance and biomass maxima during summer, with the large ciliates Plagiopyla nasuta and Paramecium trichium often contributing heavily to the midsummer biomass peak. Members of the Oligotrichida numerically dominated abundance and biomass peaks in oligotrophic lakes while the Scuticociliatida dominated the communities of higher trophic states. Total ciliate abundance and biomass were strongly correlated with chlorophyll a concentrations as were various ciliate taxonomic groups. The relationship between ciliate seasonal distribution in these subtropical lakes with lake thermal regimes and trophic state is discussed.     

Vertical and seasonal distribution of picoplankton and functional nitrogen genes in a high‐altitude warm‐monomictic tropical lake

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High Abundances of Potentially Active Ammonia-Oxidizing Bacteria and Archaea in Oligotrophic,High-Altitude Lakes of the Sierra Nevada,California, USA

Nitrification plays a central role in the nitrogen cycle by determining the oxidation state of nitrogen and its subsequent bioavailability and cycling. However, relatively little is known about the underlying ecology of the microbial communities that carry out nitrification in freshwater ecosystems—and particularly within high-altitude oligotrophic lakes, where nitrogen is frequently a limiting nutrient. We quantified ammonia-oxidizing archaea (AOA) and bacteria (AOB) in 9 high-altitude lakes (2289–3160 m) in the Sierra Nevada, California, USA, in relation to spatial and biogeochemical data. Based on their ammonia monooxygenase (amoA) genes, AOB and AOA were frequently detected. AOB were present in 88% of samples and were more abundant than AOA in all samples. Both groups showed >100 fold variation in abundance between different lakes, and were also variable through time within individual lakes. Nutrient concentrations (ammonium, nitrite, nitrate, and phosphate) were generally low but also varied across and within lakes, suggestive of active internal nutrient cycling; AOB abundance was significantly correlated with phosphate (r² = 0.32, p<0.1), whereas AOA abundance was inversely correlated with lake elevation (r² = 0.43, p<0.05). We also measured low rates of ammonia oxidation—indicating that AOB, AOA, or both, may be biogeochemically active in these oligotrophic ecosystems. Our data indicate that dynamic populations of AOB and AOA are found in oligotrophic, high-altitude, freshwater lakes.     

Autotrophic and Heterotrophic Picoplankton in Wetlands: Differences with Lake Patterns

This study describes the occurrence, importance and seasonal patterns of picoplankton in two wetlands (TDNP and La Safor), and compares them to a system of fifteen interconnected lakes (Ruidera). In TDNP we performed a six‐year monthly study in three sites of the wetland. Bacterial abundance increased throughout time and the autotrophic picoplankton (APP) range was wide (up to 33 × 10⁶ cells/ml). The annual averaged APP contribution to total picoplankton and phytoplankton biovolumes was 0.5–22% and 0.03–6% respectively. There were large differences among sites in terms of APP absolute and relative abundance and seasonal patterns. In La Safor, the APP relative contribution to picoplankton and phytoplankton biovolumes was 0–25% and 0–40%, respectively, while in the Ruidera lakes was 0–47% and 0–5%, respectively. In the three systems there was a significant correlation between bacterial abundance and chlorophyll a but the slopes of the linear regressions were different. No significant relationships were found of APP abundance and trophic status in the wetlands, but were noted in the lake system. There was no clear relationship of APP contribution to total phytoplankton biomass to the trophic gradient in wetlands. In the lakes, the higher contribution of APP was found in those with higher trophic levels.     

The relative importance of viral lysis and nanoflagellate grazing for prokaryote mortality in temperate lakes

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