Autotrophic Picoplankton Abundance Research Articles

Unusual behaviour of phototrophic picoplankton in turbid waters.

Autotrophic picoplankton (APP) abundance and contribution to phytoplankton biomass was studied in Hungarian shallow lakes to test the effect of inorganic turbidity determining the size distribution of the phytoplankton. The studied lakes displayed wide turbidity (TSS: 4–2250 mg l-1) and phytoplankton biomass (chlorophyll a: 1–460 μg l-1) range, as well as APP abundance (0 and 100 million cells ml-1) and contribution (0–100%) to total phytoplankton biomass. Inorganic turbidity had a significant effect on the abundance and contribution of APP, resulting in higher values compared to other freshwater lakes with the same phytoplankton biomass. Our analysis has provided empirical evidence for a switching point (50 mg l-1 inorganic turbidity), above which turbidity is the key factor causing APP predominance regardless of phytoplankton biomass in shallow turbid lakes. Our results have shown that turbid shallow lakes are unique waters, where the formerly and widely accepted model (decreasing APP contribution with increasing phytoplankton biomass) is not applicable. We hypothesize that this unusual behaviour of APP in turbid waters is a result of either diminished underwater light intensity or a reduced grazing pressure due to high inorganic turbidity.

The effect of elevated CO2 on autotrophic picoplankton abundance and production in a eutrophic lake (Lake Taihu, China)

The effects of elevated CO2 concentrations on the community structure and primary production of the autotrophic picoplankton of a eutrophic lake were studied in Lake Taihu, China. We conducted in situ experiments with three CO2 concentrations (270, 380 and 750ppm) over four seasons during 2012 and 2013. Our results showed that phycocyanin-rich picocyanobacteria were dominant in winter and that photosynthetic picoeukaryotes were prevalent during the other three seasons. CO2 elevation could significantly increase the abundance of photosynthetic picoeukaryotes in all seasons except winter, but did not have any influence on picocyanobacterial abundance. CO2 enrichment caused an increase in the primary production of the picoplankton community in most seasons, and significant differences were observed among the treatments in summer and winter. In addition, the contribution of picoplankton to total primary production significantly increased under higher CO2 concentrations in winter. The increase in the abundance of photosynthetic picoeukaryotes and the primary production of picoplankton under high CO2 concentrations may reduce the transfer of matter and energy to higher trophic levels and increase the importance of the microbial food web.

Horizontal and vertical distribution of marine virioplankton: a basin scale investigation based on a global cruise.

Despite the fact that marine viruses have been increasingly studied in the last decade, there is little information on viral abundance and distribution on a global scale. In this study, we report on a global-scale survey covering the Pacific, Atlantic, and Indian Oceans on viral distribution using flow cytometry. Viruses were stained with the SYBR Green I, which targets only dsDNA viruses. The average viral abundance was 1.10±0.73×107 ml−1 in global surface oceans and decreased from the areas with high chlorophyll concentration (on average, 1.47±0.78×107 ml−1) to the oligotrophic subtropical gyres (on average, 6.34±2.18×106 ml−1). On a large-spatial-scale, viruses displayed significant relationships with both heterotrophic and autotrophic picoplankton abundance, suggesting that viral distribution is dependent on their host cell abundance. Our study provided a basin scale pattern of marine viral distributions and their relationship with major host cells, indicating that viruses play a significant role in the global marine ecosystem.

Planktonic Assemblages in a Coastal Mediterranean Area Subjected to Anthropogenic Pressure

In marine environment phytoplankton and picoplankton are responsible for a bulk of production and nutrient cycling and may give important information about the different seawater habitats. In the present paper, the temporal changes of phytoplankton as well as autotrophic and heterotrophic picoplankton abundance and biomass in two coastal areas in the Gulf of Taranto (Mediterranean Sea) subjected to different levels of anthropogenic pressure were studied and related to the main environmental variables. The two analysed areas were significantly different as regards the abiotic conditions which also varied temporally. Univariate analyses revealed that larger phytoplankton and heterotrophic picoplankton abundance and biomass varied as well. The multivariate analyses showed a complex distribution of the whole planktonic assemblages, which varied in time and space without a decipherable pattern, presumably due to the peculiar spatial-temporal dynamics of the sole autotrophic picoplankton abundance. Significant correlations between planktonic assemblages and environmental variables were discussed by taking into account also the potential role of the considered planktonic components as useful environmental monitoring parameters.

The plankton community in Norwegian coastal waters—abundance, composition, spatial distribution and diel variation

The purpose of the present study was to explore the composition and variation of the pico-, nano- and micro-plankton communities in Norwegian coastal waters and Skagerrak, and the co-occurrence of bacteria and viruses. Samples were collected along three cruise transects from Jæren, Lista and Oksøy on the south coast of Norway and into the North Sea and Skagerrak. We also followed a drifting buoy for 55 h in Skagerrak in order to observe diel variations. Satellite ocean color images (SeaWiFS) of the chlorophyll a (chl a) distribution compared favorably to in situ measurements in open waters, while closer to the shore remote sensing chl a data was overestimated compared to the in situ data. Using light microscopy, we identified 49 micro- and 15 nanoplankton sized phototrophic forms as well as 40 micro- and 12 nanoplankton sized heterotrophic forms. The only picoeukaryote (0.2–2.0 μm) we identified was Resultor micron (Pedinophyceae ). Along the transects a significant variation in the distribution and abundance of different plankton forms were observed, with Synechococcus spp and autotrophic picoeukaryotes as the most notable examples. There was no correlation between viruses and chl a, but between viruses and bacteria, and between viruses and some of the phytoplankton groups, especially the picoeukaryotes. Moreover, there was a negative correlation between nutrients and small viruses (Low Fluorescent Viruses) but a positive correlation between nutrients and large viruses (High Fluorescent Viruses). The abundance of autotrophic picoplankton, bacteria and viruses showed a diel variation in surface waters with higher values around noon and late at night and lower values in the evening. Synechococcus spp were found at 20 m depth 25–45 nautical miles from shore apparently forming a bloom that stretched out for more than 100 nautical miles from Skagerrak and up the south west coast of Norway. The different methods used for assessing abundance, distribution and diversity of microorganisms yielded complementary information about the plankton community. Flow cytometry enabled us to map the distribution of the smaller phytoplankton forms, bacteria and viruses in more detail than has been possible before but detection and quantification of specific forms (genus or species) still requires taxonomic skills, molecular analysis or both.

Diel Changes in Phytoplankton Composition and Abundance in the Surface and Sub‐Surface Strata from a Shallow Eutrophic Pond

AbstractRepresentative phytoplankton assemblages were identified in a eutrophic pond over a 24 hour period. One assemblage characterized species in the surface (neuston) layer and another consisted of algae from 2, 5, and 20 cm sub‐surface depths. The surface layer (0 cm) included a similar, but less diverse assemblage of species, and a lower abundance of cells per unit volume, than those at the lower depths. At each of the sub‐surface depths (2–20 cm), the major phytoplankton components initially followed similar patterns of abundance in reference to the time and depth of sample collections then later differed in their abundance levels. The dominant algae were chlorophytes, cryptophytes, diatoms, and cyanobacteria. Mean concentrations of total phytoplankton, over the 24 hours for the surface, were 3.3 × 103 cells ml–1, compared to 36.9 × 103 cells ml–1 for depths 2–20 cm. The autotrophic picoplankton abundance was recorded separately from the phytoplankton with mean concentrations of 472.9 × 103 cells ml–1 in the surface layer and 623.0 × 103 cells ml–1 for the three sub‐surface depths. Photos of representative species from these surface layers are presented. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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 (Kd 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 Kd 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 Kd (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.

Bottom-up or top-down – How is the autotrophic picoplankton mainly controlled? Results of long-term investigations from two drinking water reservoirs of different trophic state

In two reservoirs of different trophic state, the abundance of autotrophic picoplankton was continuously assessed for 7 and 11 years, respectively. Simultaneously, other important environmental and biotic factors (nutrients, temperature, phyto- and zooplankton) were analysed. The mesotrophic Saidenbach Reservoir receives five- to sevenfold higher phosphorus imports, and exhibits twice higher phosphorus contents, and tenfold higher phytoplankton concentrations than the oligotrophic Neunzehnhain Reservoir. Despite these differences, the quantities and dynamics of autotrophic picoplankton (APP) showed remarkable similarities in both reservoirs. The APP consisted almost exclusively of Phycoerythrin-rich cyanobacteria. A notable growth of cyanobacteria never started before the onset of stratification and finished no later than the beginning of the autumnal turnover. Although the soluble reactive phosphorus (SRP) concentrations in the epilimnion were always near the limit of detection, there was no evidence for growth restriction of APP by nutrients in either of the reservoirs. However, the data reveal a strong grazing impact of all potential grazers, especially of the daphnids. This zooplankton group always showed a strict behaviour opposite of the APP during its growth periods. The grazing influence of rotifers and protozoans is less pronounced but also obvious, notable by the frequently observed inverse relationships between their quantities and those of APP. It is assumed that during the stratification period in the summer the APP dynamics is mainly top-down controlled.

Autotrophic picoplankton of Lake Baikal: composition, abundance and structure

Investigations of Baikalian phytoplankton in 1960s showed that some very small blue-green algae with a cell diameter of 1.5 μm develop in great abundance in Lake Baikal in summer. They were described as a new endemic species – Synechocystis limnetica Popovsk. S. limnetica was found as mass dominant species of the autotrophic picoplankton. Knowledge on species composition of Baikalian picoplankton is limited to this species. Autotrophic picoplankton (APP) communities were studied monthly from 1997 to 2001 at the monitoring station in southern Lake Baikal. Microscopic examinations of picoplanktonic samples based on the fluorescence of pigments, cell morphology and ultrastructure revealed that Lake Baikal АРР is dominated by chroococcoid phycoerythrin-rich cyanobacteria of genus Synechococcus. Cyanobacteria of this genus were represented by various morphotypes throughout the study: coccoid and ellipsoid Synechococcus spp. 0.85 × 1.1 μm by size, constituting the main mass of autotrophic picoplankton, their proportion varying during the year from 74 to 97%; Synechococcus-species with short and long rod-shaped form of cells, reach maximum (8–18%) in the autumnal period. Endemic species of Lake Baikal – S. limnetica contribute 1–4% in APP communities. Baikalian APP was found to include free aggregated cells of Synechocystis- and Synechococcus-type, to 5% in autumn, as well as the colonial species Aphanothece clathrata f. brevis, the contribution of which to total autotrophic picoplankton fraction changes significantly in seasonal and annual aspects. Picoplanktonic green algae are not especially abundant in Lake Baikal, and, on average did not exceed 8% of the total APP. The sizes of picoalgae are 1.25 × 1.75 μm, both single cells of green algae; aggregated forms are noted in the picoplankton. Maximal abundance of autotrophic picoplankton was registered in a near-surface layer in late July (7.36 × 105 cells ml−1), and the minimum number (2.03 × 104 cells ml−1) in May, during the period of spring isothermal mixing.

Picophytoplankton contribution to phytoplankton community structure in the Gulf of Venice (NW Adriatic Sea)

AbstractThe size distribution of the phytoplankton community has been analyzed on a monthly basis in the Gulf of Venice (NW Adriatic Sea), with the aim to assess the seasonal variation of the contribution of autotrophic picoplankton (APP) to total phytoplankton. The investigation was carried out in two stations characterized by different influences of fluvial inputs. APP was mainly made up of Synechococcus , the larger fraction (Utermöhl Fraction Phytoplankton, UFP: µm as maximum linear dimension) of diatoms and nanoflagellates. The average APP abundance (46 × 103 cells ml–1) and biomass (8.1 µg C dm–3) indicate that APP was a persistent and significant component of the NW Adriatic phytoplankton. The highest APP abundance (up to 270 × 103 cells ml–1) and biomass (up to 49 µg C dm–3) were recorded from summer to autumn, UFP peaked from late winter to late spring. The mean APP contribution to total phytoplankton abundance and biomass was 98% and 31% respectively.The seasonal variation of total phytoplankton biomass was mainly driven by the UFP fraction; however, a seasonal shift in the relative importance of UFP and APP occurred from spring to summer. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Seasonal Dynamics of Picocyanobacteria and Picoeukaryotes in a Large Shallow Lake (Lake Balaton, Hungary)

AbstractThe abundance and composition of autotrophic picoplankton (APP) were studied between February 2003 and March 2004 in Lake Balaton. Water samples were taken fortnightly in the eutrophic western basin and mesotrophic eastern basin. Our study, which took more than one year, revealed pronounced seasonal pattern of the picoplankton abundance and composition. According to our results there were three types of picoplankton in Lake Balaton: 1. Phycoerythrin‐rich coccoid cyanobacteria (PE), dominant summer picoplankters in the mesotrophic lake area; 2. Phycocyanin‐rich cyanobacteria (PC), the most abundant summer picoplankters in the eutrophic lake area; 3. Picoeukaryotes, dominant winter picoplankters in the whole lake. The observed abundance of picoeukaryotes (3 × 105 cells ml–1) was one of the highest ever found. Our study confirms that in Lake Balaton the colonial autotrophic picoplankton (colonial APP) become dominant in summer in the nutrient limited period. We have found strong negative relationship between the concentrations of available nitrogen forms (NH4–N, NO3–N, urea‐N) and the colonial APP abundance. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The significance of autotrophic and heterotrophic picoplankton in hypertrophic ecosystems

Hypertrophic aquatic ecosystems are biologically important because they represent an environment where homeostatic mechanisms are strongly reduced and extreme oscillations occur in physical and chemical parameters as well as in the growth of many planktonic organisms. Hypertrophic lakes may represent not only the ultimate state of eutrophication, but also systems where abundance and production of picoplanktonic organisms (0.2–2 μm in diameter) may theoretically be greatest. In this review we examined the microbial ecology literature for studies on hypertrophic lakes and assessed the significance of heterotrophic bacteria (BACT) and autotrophic picoplankton (APP). Our main conclusions are as follows: (a) The relative contribution of APP to total phytoplankton biomass is strongly reduced in hypertrophic systems. (b) The absolute abundance and biomass of APP are highly variable among these systems; depending mainly on differences in light quality and quantity, the existence of nitrogen limitation and presence of pollutants. (c) The abundance of BACT does not increase as rapidly as algal biomass with increasing trophy. (d) A clear dichotomy exists between lakes dominated by Microcystis where bacterial biomass and production are relatively low, and Planktothrix-dominated lakes where bacterial biomass and production are high. (e) Bacterial production per unit biomass tends to increase from oligotrophic to hypertrophic systems supporting the previous hypothesis that bacteria do not play a lesser role in lake metabolism at the upper end of the trophic scale. In general, however, our knowledge of the microbial ecology of hypertrophic systems has advanced very little since the 1980s, largely because most studies of these systems have focused on their remediation.

The relationship between autotrophic picoplankton (APP) - the smallest autotrophic component of food web and the trophic status and the depth of lakes

The occurrence and dynamics of autotrophic picoplankton (APP) in relation to the trophic status of a lake is discussed basing on results from 9 lakes situated in northeastern Poland and southern Finland. The lakes varied in the concentration of basic nutrients, chlorophyll a, phytoplankton biomass, pH depth and the composition of APP.A positive relationship between the productivity of the lakes and the APP biomass was confirmed. Shallow lakes were characterised by a significantly higher abundance and biomass of APP than deeper lakes, regardless of their trophic status. The abundance of APP as well as its contribution to the total phytoplankton biomass in the studied shallow lakes was also higher, than it could be expected from the level of basic nutrients in those lakes.Results imply that, when compared to deep lakes of a comparable trophic level, shallow and occasionally turbid water environment may create favourable conditions for APP, enabling it to occur in high numbers and thus influence the carbon cycling causing an increase of the carbon flow through the microbial food webs.

The dynamics of microbial and herbivorous food webs in a coastal sea with special reference to intermittent nutrient supply from bottom intrusion

Seasonal changes in abundance of planktonic microorganisms, together with some physico-chemical variables, were monitored monthly from May 1999 to March 2002 in the surface water of a coastal bay where nutrients are mainly supplied by intermittent intrusions of deeper water (bottom intrusion). No significant bottom intrusion was detected in 1999 but large or frequent bottom intrusions were found from June to October in 2000, and again from mid-June only to late July in 2001. These results indicate that there is a different nutrient supply every year, and peaks in the abundance of dominant eukaryotic phytoplankton (diatoms and dinoflagellates) roughly corresponded to the occurrences of bottom intrusions. By contrast, there was a cyclic seasonal pattern of autotrophic picoplankton (APP) cell density, which reached maxima in August of every year at very similar levels (4.0-5.0 × 105 cells ml–1). Thus, the seasonal abundance of APP was apparently independent of the occurrence of bottom intrusions. Seasonal changes in cell densities of heterotrophic bacteria showed similar trends to the APP, and temperature-dependent growth of both was indicated. The present study suggests that the matter cycling in the bay varies as a result of shifts in the dominant food linkages, from a microbial food web to a herbivorous food web, due to intermittent nutrient supplies from bottom intrusions.

Autotrophic picoplankton in Lake Baikal: Abundance, dynamics, and distribution

The autotrophic picoplankton of Lake Baikal was discovered more than 30 years ago; the productivity of the endemic member of the group, Synechocystis limnetica, is of particular interest. In the course of long-term investigations, the abundance of this species was established using the sedimentation technique. Maximum abundance was reached at 104 cells per ml in summer. In addition to estimating the distribution of this species throughout a defined lake area during the spring-summer period, the character of interrelations with large phytoplankton was revealed. Baikalian autotrophic picoplankton was studied from 1997 to 2000 using epifluorecsence and scanning electron microscopy. The study of seasonal and annual dynamics of picoplankton abundance was conducted monthly at the monitoring station in southern Baikal. The spatial distribution of autotrophic picoplankton was observed in the period when Lake Baikal was free of ice; the lake area was defined by 50 stations. Microscopic examination of planktonic samples revealed that the phycoerythrin-rich cyanobacteria of genus Synechococcus generated the largest biomass of autotrophic picoplankton in Lake Baikal. Morphotype diversity of the representatives of genus Synechecoccus was revealed in the study: Synechococcus spp. of coccoid and ellipsoid forms were dominant by abundance. The coccoid and ellipsoid forms had diameters of 0.9 and 0.7 to 1.3 μm, respectively. The portion of picoplanktonic algae in the Baikalian picoplankton community is not high and does not exceed 10% in the summer time when growth is highest. The autotrophic picoplankton abundance throughout the year in the 0–50 m layer varies by more than one hundred times. Maximal concentration of picocyanobacteria occurs in late July–August at 10 to 25 m depths where it reached 106 cells ml−1. The minimal concentration (103 cells ml−1) was observed in December at the same depths. A small peak of the picoplankton growth occurred in April of each year when the total numbers of picocyanobacteria increased 2 to 4 times after winter. In 1999 the peak rose to 105 cells ml−1 . During the investigation period the highest indices of autotrophic picoplankton numbers were observed in 1999 and the lowest in 1997. The spatial distribution of this group in the whole Baikal aquatorium was characteristically heterogeneous; its abundance varied by 2 to 10 times in different parts of the lake. The largest fluctuation occurred in August and the least in September. The maximum picoplankton abundance was observed in the epilimnion during the periods of thermal stratification. In the course of homothermy a comparably homogeneous distribution of picocyanobacteria was revealed up to 300 m depth, but the highest numbers were found in the upper 30 m layer.

Autotrophic and Heterotrophic Picoplankton in Wetlands: Differences with Lake Patterns

AbstractThis 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.

Potential effects of abiotic factors on the abundance of autotrophic picoplankton in four boreal lakes

The seasonality of autotrophic picoplankton (APP) in four boreal lakes of varying trophic state and light availability was studied. Red-fluorescing cyanobacteria (CyAPP) dominated the APP in three lakes, but in the fourth, most humic lake, Valkea Kotinen, eukaryotic picoalgal cells (EuAPP) predominated. The most productive, least humic, and stratified lake (Ormajarvi) was the only one in which low numbers of orange-fluorescing cyanobacteria were present in winter (7 X 10 2 cells ml -1 ) and early spring (1 X 10 2 cells ml -1 ) under the ice cover. The numbers of CyAPP did not correspond to the trophic status of the lakes. The highest average and maximal number of CyAPP were recorded in shallow and moderately productive Jylisjarvi (6.2 × 10 5 cells ml -1 ) while the lowest number (8 × 10 4 cells ml -1 ) was recorded in the deepest lake, Paajarvi, with fairly similar water quality. Light availability and water temperature seemed to be more important abiotic factor in regulating CyAPP and EuAPP density than nutrient concentrations. In the deepest lakes, light and temperature correlated very strongly with density of CyAPP in spring, while in the shallowest lake this correlation only occurred in autumn. Although light and temperature were occasinally significantly correlated with EuAPP cell density, no common trend throughout all the seasons (as seen for prokaryotic APP) was found.

Which factors determine the abundance and distribution of picocyanobacteria in inland waters? A comparison among different types of lakes and ponds

In a limnological survey including 45 lakes and ponds from central-eastern Spain, we studied the main factors associated with the abundance of autotrophic pico-plankton (APP), mostly formed by unicellular picocyanobacteria (APP-Pcy). The study covers a wide range of trophic conditions, salinity, water flow regimes and lake typologies. Phycoerythrin (PE)-rich picocyanobacteria (PE-APP) were abundant in the metalimnion and upper hypolimnion of deep stratified lakes, exhibiting low nutrient availability in the epilimnion. Among these lakes, higher PE-APP abundances were found in lakes with higher retention time during periods of stability, whereas lakes with low retention time lacked conspicuous Pcy populations. They usually formed deep chlorophyll maxima in the nutrient-richer metalimnetic layers, especially when epilimnetic nutrient exhaustion occurred during stratification. The ratios of PE with respect to other photosynthetic pigments in deep lakes usually increased with depth. In these lakes, the available light is selectively enriched in the yellow-green range as depth increases, and these wavelengths can be selectively harvested by PE. Phycocyanin (PC)-rich APP without PE, mostly unicellular rod-shaped cyanobacteria, were only abundant in some hypertrophic shallow lakes, although in many lakes with highly eutrophic conditions their presence was very low or undetectable.

Seasonal changes in the abundance of autotrophic picoplankton and some environmental factors in hypereutrophic Furuike Pond

Seasonal change in the abundance of autotrophic picoplankton (APP) was investigated once or twice a week in relation to some environmental variables in a hypereutrophic pond, from July 1999 to June 2000. Cell density of APP ranged between 0.3×105 and 10.1×105 cells ml−1, overlapping the lower range of APP abundances given in the literature for hypereutrophic systems. The pattern of seasonal change in concentration of dissolved inorganic phosphorus (0.3–20.3 μ mol P l−1) was similar to that of cell density of APP, suggesting that phosphorus limitation on APP abundance. By contrast, nitrogen limitation seemed unlikely since the pattern of seasonal change in concentration of dissolved inorganic nitrogen was different from that of APP cell density. We could not find any coupled oscillations between APP abundance and heterotrophic nanoflagellates, or between that of APP and ciliates. The dominant ciliate taxa, based on their cell densities, were Cinetochilum margaritaceum, Cyclidium glaucoma, Halteria grandinella, Strobilidium sp. and Urotricha spp. The relative contribution of the < 2 μm fraction to total chlorophyll concentration was seasonally high (up to 16.2%), indicating seasonal importance of APP abundance as food for heterotrophs.

Phytoplankton community structure in non-stratified lakes of Pomerania (NW Poland)

The species composition, abundance and biomass of autotrophic picoplankton and larger phytoplanktonic organisms were analysed in 17 non-stratified lakes of Pomerania. Water samples were collected in August in 1995–1999. The composition and structure of phytoplankton varied among the lakes. Two groups of lakes could be distinguished: with a low and with a high species richness. Phytoplankton abundance ranged from about 1 × 102 to 1.5 × 106 cells ml−1 and was usually dominated by green algae, cyanobacteria, chrysophytes and dinoflagellates. Phytoplankton biomass ranged from 0.04 to 16.9 mg l−1 and in half of the lakes was dominated by cyanobacteria or dinoflagellates. Biomass of the smallest size fraction i.e. autotrophic picoplankton (APP) was always small in comparison with that of larger phytoplanktonic organisms. Cyanobacterial cells were usually the main components of APP, while eukaryotic APP were rare. Colonial forms accounted for 6–93% of the total APP abundance. In terms of total phytoplankton biomass, organisms from one of the other fractions, i.e. nanoplankton or microplankton, were usually more important. As the community structure of phytoplankton showed clear differences among lakes, special attention was paid to the variation in the relative abundance and biomass of some species or groups of species.