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.     

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.     

Size fractionated phytoplankton production in two humic shallow lakes with contrasting coverage of free floating plants

We conducted a 1-year survey in two humic shallow lakes from the floodplain of the Lower Paraná River, Laguna Grande Lake (LGL) and a relictual oxbow lake (ROL). We aimed to test two hypotheses: (1) the efficiency in light use of picoplankton (0.2–3 μm) is greater as light restriction increases and (2) the contribution of picoplankton to the total productivity is higher when the total photosynthetic biomass is lower. We performed P–E curves for picoplankton and nano- and microplankton (>3 μm) using the ¹⁴C assimilation technique. The light environments of the water bodies differed mainly owing to the development of free floating plants on the surface of the ROL and the dominance of phytoplankton in LGL. Primary productivity patterns in LGL were seasonality driven whilst in the ROL they were related to the coverage of floating macrophytes, which promoted light limitation and a lower productivity. In LGL, nano- and microplankton were in general more productive and the relative contribution of picoplankton to the total phytoplankton production decreased with the increase in total photosynthetic biomass. Hence, our study extends previously observed patterns to subtropical shallow lakes, where seasonality and free floating plants may influence the dynamics of phytoplankton production.     

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.     

Ecological Characteristics of Autotrophic Picoplankton in a Prealpine Lake

The seasonal distribution of autotrophic picoplankton in Lake Constance was investigated over four consecutive years. Cell numbers varied seasonally and vertically over four orders of magnitude (10² to 10⁶ cells ml⁻¹). A horizontal variation by a factor of 3 in abundance and biomass across the different parts of the lake was found during summer stratification. Picoplankton peaks occurred during the phytoplankton spring bloom and in late summer. Low values were characteristic for the clear-water phase in early summer and for autumn-winter. This seasonal pattern differed from that of larger phytoplankton in Lake Constance and from the seasonal distribution of picoplankton known from other lakes and marine environments. Picoplankton was predominated by chroococcoid cyanobacteria of about 0.6 μ³ biovolume. The average cell size increased from winter until early summer. Using HPLC pigment analysis, we identified zeaxanthin and β-carotene as typical picoplankton pigments. Results of the pigment analyses suggest that algae others than picocyano-bacteria may be more prominent in the picoplankton size class than derived from routine epifluorescence counting.     

Production of new organic carbon and its distribution between autotrophic picoplankton, bacteria, extracellular organic carbon and phytoplankton in an upland lake

• 1 Picoplankton community production (0.2–2μm) was investigated over 3 months, June-September 1991, in Llyn Padarn, a mesotrophic upland lake in north Wales.
• 2 The picoplankton was differentiated into autotrophic algae (<1–3μm) and heterotrophic bacteria (<0.2–1 μm) using differential filtration through a 1 μm pore size Nuclepore filter.
• 3 Efficient separation of these distinct metabolic constituents of picoplankton was obtained. A good correlation (r= 0.81, P < 0.001) was found between physical separation of bacterial and picoalgal cells from fluorescence microscopy and the distribution of heterotrophic metabolic activity between different cell size fractions measured by uptake of ¹⁴C-glucose.
• 4 Picoplankton community production was differentiated into the ‘absolute’ autotrophic production by picoalgae, corrected for overestimation due to retention of bacteria with the picoalgae, and the heterotrophic component, bacterial uptake of ‘extracellular organic carbon’ (EOC), derived from the entire phytoplankton community.
• 5 The heterotrophic contribution to picoplankton community production ranged from 88 to 1%, mean value 55% of total. Autotrophic picoplankton production was dominant in June and July, but in August and September heterotrophic uptake of EOC was the major input to picoplankton community production.
• 6 During the 3 months, the mean contributions to plankton production were autotrophic picoplankton 10.3%, heterotrophic bacterial uptake of EOC 9.7%, EOC in lake water 11.6% and phytoplankton (>3μm) 68.3%.
• 7 Bacteria accounted for about half the picopfankton community production via uptake of EOC. Thus although autotrophic picoplankton were ubiquitous, it is likely that their contribution via primary production to the carbon balance of planktonic environments has been overestimated in previous studies.

     

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.     

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.     

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.     

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.     

Phytoplankton and primary production in clear-vegetated,inorganic-turbid,and algal-turbid shallow lakes from the pampa plain (Argentina)

Shallow lakes often alternate between two possible states: one clear with submerged macrophytes, and another one turbid, dominated by phytoplankton. A third type of shallow lakes, the inorganic turbid, result from high contents of suspended inorganic material, and is characterized by low phytoplankton biomass and macrophytes absence. In our survey, the structure and photosynthetic properties (based on ¹⁴C method) of phytoplankton were related to environmental conditions in these three types of lakes in the Pampa Plain. The underwater light climate was characterized. Clear-vegetated lakes were more transparent (K _d 4.5–7.7 m⁻¹), had high DOC concentrations (>45 mg l⁻¹), low phytoplankton Chl a (1.6–2.7 μg l⁻¹) dominated by nanoflagellates. Phytoplankton productivity and photosynthetic efficiency (α ~ 0.03 mgC mgChla ⁻¹ h⁻¹ W⁻¹ m²) were relatively low. Inorganic-turbid lakes showed highest K _d values (59.8–61.4 m⁻¹), lowest phytoplankton densities (dominated by Bacillariophyta), and Chl a ranged from 14.6 to 18.3 μg l⁻¹. Phytoplankton-turbid lakes showed, in general, high K _d (4.9–58.5 m⁻¹) due to their high phytoplankton abundances. These lakes exhibited the highest Chl a values (14.2–125.7 μg l⁻¹), and the highest productivities and efficiencies (maximum 0.56 mgC mgChla ⁻¹ h⁻¹ W⁻¹ m²). Autotrophic picoplankton abundance, dominated by ficocianine-rich picocyanobacteria, differed among the shallow lakes independently of their type (0.086 × 10⁵–41.7 × 10⁵ cells ml⁻¹). This article provides a complete characterization of phytoplankton structure (all size fractions), and primary production of the three types of lakes from the Pampa Plain, one of the richest areas in shallow lakes from South America. Handling editor: J. Padisak     

The distributions of autumn picoplankton in relation to environmental factors in the reservoirs along the Wujiang River in Guizhou Province,SW China

Autumn picoplankton (Synechococcus, picoeukaryotes and heterotrophic bacteria) and environmental factors have been investigated in a series of reservoirs along the Wujiang River in Guizhou Province, SW China. The average abundances of Synechococcus, picoeukaryotes and heterotrophic bacteria was 10⁴, 10² and 10⁶ cells ml⁻¹, respectively. In autumn meso-eutrophic reservoirs, thermal stratification was clear and abundances of different picoplankton groups in release water was low; whereas these phenomena were not obvious in autumn hypereutrophic reservoir. Picoplankton numbers decreased with increasing water depth and showed a positive correlation with water temperature, which reflected the importance of light and temperature on the picoplankton growth. Contribution of Synechococcus to total phytoplankton production and contribution of picoeukaryotes to total phytoplankton production asynchronous changed with varying trophic states. Synechococcus preferred meso-eutrophic reservoirs over hypereutrophic reservoir and picoeukaryotes showed no preference for the investigated reservoirs in autumn. Handling editor: L. Naselli-Flores     

Phytoplankton structure and dynamics in a turbid Antarctic lake (Potter Peninsula, King George Island)

Qualitative and quantitative changes in the freshwater phytoplankton from one of the largest lakes in Potter Peninsula were studied during the summer period of 1995/1996. The main limnological features were analysed. This research constitutes the first study dealing with the phytoplankton structure and dynamics from lakes in Potter Peninsula. Due to the mixing of the water column by the constant wind action during the ice-free period, neither physical, chemical nor biological parameters showed differences between depths. Soluble reactive phosphorus remained low (26 μg l⁻¹) while total phosphorus concentrations were up to 232 μg l⁻¹. Suspended solids and total phosphorus differed significantly between the two basins in which the lake is divided. Sixty-five taxa were identified. The Bacillariophyceae was the dominant class followed by Chrysophyceae. Achnanthes lanceolata var. haynaldii and Hydrurus foetidus (cysts) were the dominant species. Tychoplanktonic species were the best represented in terms of abundance and species richness. The principal component analysis carried out with abiotic variables showed a temporal gradient characterised by the augmentation of suspended solids, total phosphorus, conductivity, pH and NH₄-N to the end of the summer period. The Principal Component Analysis performed with biotic variables also showed a temporal trend related to a decline in phytoplankton density and an enhancement of chlorophyll-a despite a slight increase of light limitation during the study period. This inverse relation may be due to the presence of picoplankton in chlorophyll-a samples. The potential effect of grazing on the phytoplankton structure is a subject to be tested in future studies. Accepted: 7 February 1999     

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.     

Crustacean zooplankton size structure in aquaculture lakes: is larger size structure always associated with higher grazing pressure?

Crustacean zooplankton size structure in 27 aquaculture lakes was studied to test the hypothesis that larger size structure is associated with higher grazing pressure. Mean body length of crustaceans was positively correlated with increasing Chl a (r ² = 0.40, P = 0.000) and TP (r ² = 0.38, P = 0.000), contrary to the empirical studies. However, the ratio of zooplankton to phytoplankton biomass decreased significantly with increasing TP (r ² = 0.27, P = 0.005) and mean body length (r ² = 0.46, P = 0.000). Meanwhile, size structure showed no significant effect in explaining residual variations of phosphorus–chlorophyll relationship (P = 0.231). These results indicate that larger size structure was not always associated with higher zooplankton grazing pressure. It is likely that in aquaculture lakes crustacean zooplankton size structure was of minor importance in control of phytoplankton biomass, and it was mainly regulated by fish predation. The results showed in our study and the empirical studies might be a reflection of two different stages of lake eutrophication and fish predation intensity. Handling editor: S. Dodson     

Responses of Two Coastal Algae (Skeletonema costatum and Chlorella vulgaris) to Changes in Light and Iron Levels1

Iron (Fe) is essential for phytoplankton growth and photosynthesis, and is proposed to be an important factor regulating algal blooms under replete major nutrients in coastal environments. Here, Skeletonema costatum, a typical red-tide diatom species, and Chlorella vulgaris, a widely distributed Chlorella, were chosen to examine carbon fixation and Fe uptake by coastal algae under dark and light conditions with different Fe levels. The cellular carbon fixation and intracellular Fe uptake were measured via ¹⁴C and ⁵⁵Fe tracer assay, respectively. Cell growth, cell size, and chlorophyll-α concentration were measured to investigate the algal physiological variation in different treatments. Our results showed that cellular Fe uptake proceeds under dark and the uptake rates were comparable to or even higher than those in the light for both algal species. Fe requirements per unit carbon fixation were also higher in the dark resulting in higher Fe: C ratios. During the experimental period, high Fe addition significantly enhanced cellular carbon fixation and Fe uptake. Compared to C. vulgaris, S. costatum was the common dominant bloom species because of its lower Fe demand but higher Fe uptake rate. This study provides some of the first measurements of Fe quotas in coastal phytoplankton cells, and implies that light and Fe concentrations may influence the phytoplankton community succession when blooms occur in coastal ecosystems.     

Phytoplankton and Epiphyte Development and Their Shading Effect on Submerged Macrophytes in Lakes of Different Nutrient Status

The annual mean light intensity at the depth limit of the Littorella vegetation was 24–33% of the subsurface light intensity, despite large variations in each attenuation component (lake water, phytoplankton, and epiphytes). In oligotrophic, silicate-poor lakes, the light attenuation above the submerged vegetation was dominated by the water itself, which accounted for 65–72% of the total attenuation. Phytoplankton and epiphytes were equal in importance to each other. In oligotrophic, silicate-rich lakes and lakes receiving a nitrogen supply above background level, the epiphytes were more abundant, accounting for about 50% of the light attenuation. In one lake with a high nutrient supply, the epiphytes were responsible for 86% of the light attenuation. A new method of measuring the effect of shading by the epiphytic community on submerged macrophytes is presented. The light attenuation caused by the phytoplankton and the epiphytes was investigated and related to the depth distribution of the submerged angiosperm, Littorella uniflora. It is shown that the biomass of the epiphytes increased more than the biomass of the phytoplankton in response to an external or internal nutrient loading. Shading by epiphytes is of decisive importance for the depth distribution of Littorella at increasing nutrient supply.     

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.     

The Saline Lakes of Saskatchewan IV. Primary Production by Phytoplankton in Selected Saline Ecosystems

Primary production by phytoplankton, efficiency of photosynthesis, and chlorophyll-a concentrations were determined for seven saline lakes that varied widely in ionic concentration and composition. The investigations were done during the summer months of 1972 and 1973. Productivity ranged from 0.001 to 11.135 g C m⁻³ day⁻¹ and 0.053 to 7.968 g C m⁻² day⁻¹. Highest productivities were measured in two lakes that supported blooms of Aphanizomenon flos-aquae and Nodularia spumigena, respectively. Species of Cyanophyceae, Bacillariophyceae and Chlorophyceae dominated the phytoplankton of the study lakes. Active chlorophyll-a ranged from 0.01 to 116 mg m⁻³. Integral photosynthetic efficiency estimates were <1% except during phytoplankton blooms when they were considerably higher. The overall range of 0.03 to 3.8% is concordant with estimates for other lacustrine ecosystems. The extinction of light caused by photo-synthetic processes, or in situ efficiency, was <1% in the trophogenic zone for most lakes but, it was considerably higher during blooms. In situ efficiencies invariably increased with depth in ail lakes.     

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.