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Free-living (FLC) and particle-associated (PAC) bacterial communities contribute differentially to the cycling of matter in the environment. In this work we have for the first time studied the taxonomic composition and diversity of FLC and PAC in the oligotrophic freshwater Lake Baikal during the under-ice plankton peak. The communities on the bottom ice surface and in the water column under the ice were studied. Analysis by light microscopy showed that the bottom surface of the ice was dominated by green algae Chlorella spp. A complex of microalgae developed in the water column: green algae Monoraphidium griffithii, Monoraphidium contortum, Chlorella spp., chrysophyte Dinobryon cylindricum, haptophyte Chrysochromulina parva, and diatom Synedra acus. Algal abundance on the ice sub-surface was several orders of magnitude higher than that in the water column. According to pyrosequencing data of the bacterial 16S rRNA V3-V4 region, diversity of FLC (as measured by Shannon, Simpson, Chao1 and ACE indices) was higher than that of PAC. Proteobacteria (Burkholderiaceae) dominated the FLC associated with ice; FLC of the water column under ice were dominated by Actinobacteria (Sporichthyaceae and Ilumatobacteraceae) and Verrucomicrobia (Methylacidiphilaceae). PAC associated with ice were dominated by Bacteroidetes (Flavobacteriaceae), while water column-associated PAC were dominated by Verrucomicrobia (FukuN18_freshwater_group, Methylacidiphilaceae) and cyanobacteria (Cyanobiaceae). The difference between free-living and particle-associated, as well as between water column and ice sub-surface bacterial communities, were explored on operational taxonomic unit (OTU) and phylotype levels.

To assess the current state of phytoplankton in the littoral area of Lake Baikal and provide a baseline for future comparisons, we sampled spring plankton communities from the 44 littoral and 3 pelagic stations covering all three basins of the lake. The study examined chemical parameters of water (NH4+, NO2−, NO3−, PO4−3, Si, COD), species composition, abundance, and biomass of phytoplankton in Lake Baikal during late spring 2017. Sharp spatial heterogeneity was observed in the distribution of phytoplankton biomass along the western (399 ± 72 mg/m3) and eastern (1319 ± 220 mg/m3) shores of the lake. The phytoplankton were diverse, with 79 species; dominant algae were different from site to site and from south to north throughout the lake. In Southern and Central Baikal, we recorded an intense bloom of the diatom Synedra acus subsp. radians (28–1400 cells/mL), similar to that observed for the past 10 years, while the chrysophyte Dinobryon cylindricum dominated in Northern Baikal. The diatoms Aulacoseira baicalensis, A. islandica, and Stephanodiscus meyeri that were dominant in the 1960s–1990s were not numerous in 2017 (0.5–10 cells/mL). This change in dominant species indicates structural changes in the phytoplankton of Lake Baikal, which have led to the disappearance of the main distinctive feature of the Baikalian phytoplankton – the alternation of extremely high (with the algal biomass over 1000 mg/m3) and extremely low (less than 100 mg/m3) productivity years. The ecological equilibrium appears to have shifted towards a new steady state.

The roles of temperature and light on grazing and photosynthesis were examined for Dinobryon sociale, a common freshwater mixotrophic alga. Photosynthetic rate was determined for D. sociale adapted to temperatures of 8, 12, 16, and 20°C under photosynthetically active radiation light irradiances of 25, 66, and 130 μmol photons · m(-2) · s(-1) , with concurrent measurement of bacterial ingestion at all temperatures under medium and high light (66 and 130 μmol photons · m(-2) · s(-1) ). Rates of ingestion and photosynthesis increased with temperature to a maximum at 16°C under the two higher light regimes, and declined at 20°C. Although both light and temperature had a marked effect on photosynthesis, there was no significant difference in bacterivory at medium and high irradiances at any given temperature. At the lowest light condition (25 μmol photons · m(-2) · s(-1) ), photosynthesis remained low and relatively stable at all temperatures. D. sociale acquired the majority of carbon from photosynthesis, although the low photosynthetic rate without a concurrent decline in feeding rate at 8°C suggested 20%-30% of the carbon budget could be attributed to bacterivory at low temperatures. Grazing experiments in nutrient-modified media revealed that this mixotroph had increased ingestion rates when either dissolved nitrogen or phosphorus was decreased. This work increases our understanding of environmental effects on mixotrophic nutrition. Although the influence of abiotic factors on phagotrophy and phototrophy in pure heterotrophs and phototrophs has been well studied, much less is known for mixotrophic organisms.

Using microcosm experiments, we investigated the interactive effects of temperature and light on specific growth rates of three species each of the phytoplanktonic genera Cryptomonas and Dinobryon. Several species of these genera play important roles in the food web of lakes and seem to be sensitive to high water temperature. We measured growth rates at three to four photon flux densities ranging from 10 to 240 μmol photon · m-2 · s-1 and at 4-5 temperatures ranging from 10°C to 28°C. The temperature × light interaction was generally strong, species specific, and also genus specific. Five of the six species studied tolerated 25°C when light availability was high; however, low light reduced tolerance of high temperatures. Growth rates of all six species were unaffected by temperature in the 10°C-15°C range at light levels ≤50 μmol photon · m-2 · s-1 . At high light, growth rates of Cryptomonas spp. increased with temperature until the temperature optimum was reached and then declined. The Dinobryon species were less sensitive than Cryptomonas spp. to photon flux densities of 40 μmol photon · m-2 · s-1 and 200 μmol photon · m-2 · s-1 over the entire temperature range but did not grow under a combination of very low light (10 μmol photon · m-2 · s-1 ) and high temperature (≥20°C). Among the three Cryptomonas species, cell volume declined with temperature and the maximum temperature tolerated was negatively related to cell size. Since Cryptomonas is important food for microzooplankton, these trends may affect the pelagic carbon flow if lake warming continues.

Polar marine ecosystems are characterized by low water temperatures, sea ice cover, and extreme annual variation in solar irradiance and primary productivity. A review of the available information from the Arctic suggests that mixotrophy (i.e. the combination of photosynthetic and phagotrophic modes of nutrition in one cell) is wide spread among plankton. In the central Arctic Ocean (AO) in summer, mixotrophic flagellates such as Micromonas and Dinobryon can account for most of bacterivory. Planktonic ciliates with acquired phototrophy form the bulk of microzooplankton biomass in both the ultra-oligotrophic deep basins of AO and its productive shelf seas. With the exception of the diatom bloom in the marginal ice zone, mixotrophic ciliates often dominate total chlorophyll in the mixed layer in summer taking advantage of the 24-h insolation. Their relatively high growth rates at low temperatures indicate that they are important component of primary and secondary production. The key Arctic copepod species preferentially feed on chloroplast-bearing ciliates, which form an important link in the planktonic food web. The limited year round data indicate that are available show that mixotrophic plankton persist in the water column during the long polar winter when irradiance is low or absent and ice cover reduces further light penetration. These observations suggest that at high latitudes an alternative food web based on mixotrophy may dominate the pelagic lower food web during much of the year.

International audience; Like many large freshwater ecosystems in Europe, Lake Bourget suffered from eutrophication during the second part of the 20th century and since the 1980s has been partially restored by reductions in nutrient loadings. Here, we analyse a data set comprised of field measurements of physicochemical and biological variables in Lake Bourget covering the period from 2004 to 2011 and complement this data set with laboratory experiments, to gain an understanding of the changes in phytoplankton community structure during recent years and drivers of these changes. Between 1995 and 2008, Lake Bourget was characterised by the proliferation of the red-coloured filamentous and toxic cyanobacterium Planktothrix rubescens, comprising 34.1-52.6% of the total phytoplanktonic biomass between 2004 and 2008. In 2009, although the contribution of P.rubescens to the total biomass was still considerable (25.3%), it was significantly lower (P

Morpho-functional classifications of phytoplankton are increasingly used in ecological studies of different aquatic ecosystems. The aim was to compare three different classifications: (a) functional groups (FG); (b) morpho-functional groups (MFG); and (c) morphologically based functional groups (MBFG), and to detect how well can the biomass of phytoplankton groups based on different classifications be explained by environmental conditions of two different karstic lakes in the National Park Plitvice Lakes. Phytoplankton biomass in Lake Kozjak was lower than in Lake Prosce (total avg. 1.13 and 2.59 mg l−1, respectively). Several diatoms (Cyclotella spp., Discostella stelligera, Synedra sp., Ulnaria ulna var. acus) with Dinobryon divergens were dominant in Kozjak, while Prosce was characterized by Dinobryon sociale, D. divergens, Cryptomonas sp., Stephanodiscus sp. and Fragilaria crotonensis. The best results in describing the phytoplankton succession were obtained using MBFG approach although it is less sensitive in describing variability of individual descriptive algal groups in this environment than FG and MFG. Nonetheless, because of distinct differences among dominant phytoplankton groups in these systems, MBFG classification was shown to be applicable, as supported by statistical analyses. FG and MFG classifications showed a high level of similarity, thus proving to be applicable to deep karstic lakes.

1. Humic lakes with a high external supply of DOC and low input of nutrients can often support a high biomass of metazoan zooplankton. In such lakes, autotrophic algae compete with bacteria for ino ...

The aim of the study was to perform an ecological status assessment based on the phytoplankton in the vendace-type Lake Dejguny in accordance with the requirements of the Water Framework Directive. The phytoplankton analyses were conducted during three growth seasons in 2006, 2007, and 2008. The phytoplankton multimetric Phytoplankton Metric for Polish Lakes (PMPL) was used to assess the lake’s ecological status. The average total biomass of phytoplankton ranged from 1.6 to 3.9 mg dm-3, while the average biomass of Cyanoprokaryota ranged from 0.4 to 1.4 mg dm-3, with the minimum noted in 2008. The phytoplankton assemblages were dominated mainly by filamentous Cyanoprokaryota and pennate Bacillariophyceae in 2006 and 2008, or exclusively by pennate Bacillariophyceae in 2007. The relatively low total biomass and Cyanoprokaryota biomass, as well as seasonal phytoplankton dynamics with the dominant taxa of Tabellaria flocculosa, Dinobryon sociale, and D. divergens confirmed the lake’s mesotrophic state and some of the features of reference conditions in this lake, whereas the predominance of filamentous species Planktothrix agardhii and Planktolyngbya limnetica was characteristic of eutrophic conditions. However, a clear tendency towards progressive eutrophication stemming from the significant domination of filamentous Cyanoprokaryota was observed, and according to the PMPL the assessment indicated that Lake Dejguny had a good ecological status in 2006 and 2007 and even a high one in 2008.

This paper presents the preliminary results of a study of the phytoplankton structure and dynamics and physicochemical properties of Lake Swarzędzkie in 2011. The subject of the study is a shallow, elongated, post-glacial lake located in western Poland. The surface area is 93.7 ha with a maximum depth of 7.2 m. Its poor water quality led to the implementation of chemical and biological restoration procedures in an attempt to improve it. The highest concentration of total nitrogen was 13.07 mg l-1 N. Secchi depth (SD) was low in the summer with a minimum value of 0.5 m in September. The most abundant group of phytoplankton were cyanobacteria, and the maximum value of chlorophyll a concentration was 278.0 μg l-1. The dominant species were Pseudanabaena limnetica and Aphanizomenon gracile. Cyanobacteria were the most abundant phytoplankton until November. Maximum diatom, cryptophyte, and chrysophyte density was noted in the spring, and the most abundant were Nitzschia acicularis, Cryptomonas marssonii, Rhodomonas lacustris, Erkenia subaequiciliata, and Dinobryon sociale. One-hundred and thirty-six phytoplankton taxa belonging to nine taxonomic groups were identified in Lake Swarzędzkie. The highest number of taxa was noted among chlorophytes (57 taxa), cyanobacteria (19), diatoms (16), and chrysophytes (12), while other taxonomic groups were represented by smaller number of taxa.

Dinobryon spp. are common components of the phytoplankton in temperate lakes. While these chrysophytes link bacterial carbon as well as primary production to the larger food web, few studies have examined their vertical distribution over a season. Data were collected over a 2.5 yr period in mesotrophic Lake Lacawac in eastern Pennsylvania (USA) to examine water col- umn attributes relating to seasonality of the mixotrophic alga D. cylindricum, and specifically to address a hypo thesis that abundance and vertical distribution in the genus are associated with temperature. This information was used to guide laboratory experiments examining temperature and light effects on growth, feeding, and vertical migration of cultured Dinobryon. Data from Lake Lacawac and the literature indicated that high abundances of Dinobryon were associated with a narrow range of temperature (9�18°C) relative to temperatures where they were present (3�26°C). High abundances of several species of Dynobryon were associated with this tempera- ture range, and occurred from late winter to late spring, depending on latitude and depth in the water column. Although high Dinobryon abundances were not tied directly to levels of photosyn- thetically active radiation in Lake Lacawac, a UV-exclusion experiment in a nearby oligotrophic lake indicated a temperature-dependent negative effect of UV radiation. UV may limit the occur- rence of Dinobryon populations in surface waters of clear oligotrophic lakes. Laboratory experi- ments showed that both light and temperature affected growth and feeding rates of Dinobryon, and that maximum feeding and growth rates occurred within the temperature range where high Dinobryon abundances were observed in field studies.

* Globally, freshwater ecosystems are warming at unprecedented rates and northern temperate lakes are simultaneously experiencing increased runoff of humic substances (brownification), with little known consequences for future conservation of biodiversity and ecosystem functioning. * We employed an outdoor mesocosm experiment during spring and summer to investigate the combined effects of gradually increasing warming and brownification perturbations on the phytoplankton community structure (biodiversity and composition) and functioning (biomass). * While we did not observe overall significant treatment effects on total phytoplankton biomasses, we show that predicted increases in warming and brownification can reduce biodiversity considerably, occasionally up to 90% of Shannon diversity estimates. Our results demonstrate that the loss of biodiversity is driven by the dominance of mixotrophic algae (Dinobryon and Cryptomonas), whereas several other phytoplankton taxa may be temporarily displaced from the community, including Cyclotella, Desmodesmus, Monoraphidium, Tetraedron, Nitzschia and Golenkinia. * The observed loss of biodiversity coincided with an increase in bacterial production providing resources for potential mixotrophs along the gradient of warming and brownification. This coupling between bacterial production and mixotrophs was likely a major cause behind the competitive displacement of obligate phototrophs and supports evidence for the importance of consumer–prey dynamics in shaping environmental impacts on phytoplankton communities. * We conclude that warming and brownification are likely to cause a profound loss of biodiversity by indirectly affecting competitive interactions among phytoplankton taxa. Importantly, our results did not show an abrupt loss of biodiversity; instead the reduction in taxa richness levelled off after exceeding a threshold of warming and brownification. These results exemplify the complex nonlinear responses of biodiversity to environmental perturbations and provide further insights for predicting biodiversity patterns to the future warming and brownification of freshwaters.

Freshwater organisms synthesize a wide variety of fatty acids (FAs); however, the ability to synthesize and/or subsequently modify a particular FA is not universal, making it possible to use certain FAs as biomarkers. Herein we document the occurrence of unusual FAs (polymethylene-interrupted fatty acids; PMI-FAs) in select freshwater organisms in the Laurentian Great Lakes. We did not detect PMI-FAs in: (a) natural seston from Lake Erie and Hamilton Harbor (Lake Ontario), (b) various species of laboratory-cultured algae including a green alga (Scenedesmus obliquus), two cyanobacteria (Aphanizomenon flos-aquae and Synechococystis sp.), two diatoms (Asterionella formosa, Diatoma elongatum) and a chrysophyte (Dinobryon cylindricum) or, (c) zooplankton (Daphnia spp., calanoid or cyclopoid copepods) from Lake Ontario, suggesting that PMI-FAs are not substantively incorporated into consumers at the phytoplankton-zooplankton interface. However, these unusual FAs comprised 4-6% of total fatty acids (on a dry tissue weight basis) of native fat mucket (Lampsilis siliquoidea) and plain pocketbook (L. cardium) mussels and in invasive zebra (Dreissena polymorpha) and quagga (D. bugensis) mussels. We were able to clearly partition Great Lakes' mussels into three separate groups (zebra, quagga, and native mussels) based solely on their PMI-FA profiles. We also provide evidence for the trophic transfer of PMI-FAs from mussels to various fishes in Lakes Ontario and Michigan, further underlining the potential usefulness of PMI-FAs for tracking the dietary contribution of mollusks in food web and contaminant-fate studies. © 2011.

With the discovery of a high molecular diversity of protists, a discrepancy between morphological and molecular species richness estimates became apparent. Solving the current concerns requires a comparative analysis of different sequences combined with morphological analyses of single cells originating from preserved field samples. We refined a single-cell PCR (SC-PCR) protocol for analyzing cells from field samples preserved with Lugol’s iodine solution. We linked microscopic screening with multiplex PCR targeting the SSU rDNA, internal transcribed spacer 1 (ITS1), 5.8S rDNA, internal transcribed spacer 2 (ITS2), and the mitochondrial cytochrome oxidase 1 (CO1) in a single PCR reaction. Using this method, we investigated the intraspecific molecular variation in Dinobryon populations originating from two lakes in the Salzkammergut area of Austria. All investigated genetic markers showed two separated clusters within the investigated populations of Dinobryon divergens O. E. Imhof, indicating a reproductive isolation of the two coexisting populations. Based on these findings, we describe a lineage, which is morphologically similar to D. divergens but, based on the molecular data, is reproductively isolated.

Plankton in mountain lakes are confronted with generally higher levels of incident ultraviolet radiation (UVR), lower temperatures, and shorter growing seasons than their lower elevation counterparts. The direct inhibitory effects of high UVR and low temperatures on montane phytoplankton are widely recognized. Yet little is known about the indirect effects of these two abiotic factors on phytoplankton, and more specifically whether they alter zooplankton grazing rates which may in turn influence phytoplankton. Here, we report the results of field microcosm experiments that examine the impact of temperature and UVR on phytoplankton growth rates and zooplankton grazing rates (by adult female calanoid copepods). We also examine consequent changes in the absolute and relative abundance of the four dominant phytoplankton species present in the source lake (Asterionella formosa, Dinobryon sp., Discostella stelligera, and Fragilaria crotonensis). All four species exhibited higher growth rates at higher temperatures and three of the four species (all except Dinobryon) exhibited lower growth rates in the presence of UVR versus when shielded from UVR. The in situ grazing rates of zooplankton had significant effects on all species except Asterionella. Lower temperatures significantly reduced grazing rates on Fragilaria and Discostella, but not Dinobryon. While UVR had no effect on zooplankton grazing on any of the four species, there was a significant interaction effect of temperature and UVR on zooplankton grazing on Dinobryon. Discostella and Dinobryon increased in abundance relative to the other species in the presence of UVR. Colder temperatures, the presence of zooplankton, and UVR all had consistently negative effects on rates of increase in overall phytoplankton biomass. These results demonstrate the importance of indirect as well as direct effects of climate forcing by UVR and temperature on phytoplankton community composition in mountain lakes, and suggest that warmer climates and higher UVR levels may favor certain species over others.

Vertical distribution of phytoplankton is highly influenced by physical and chemical factors, but the knowledge about these aspects remain unknown in Colombian Amazon lakes. In this work, the relations between the physico-chemical variables and the vertical distribution of phytoplankton community of a clear water lake (Lake Boa) are analyzed. Samples were taken at every 30 cm from surface to bottom in the center of the lake at different times of the hydrological cycle. Phytoplankton transformed from a diluted community dominated by cyanobacteria in high waters to one with elevated abundance of coccal chlorophytes in low waters. In the low water phase, the vertical distribution of Cryptomonas spp., Peridinium cf. umbonatum and Chlamydomonas spp. had significant statistical associations with the redox potential (RP), reactive phosphorus (PO4), and hydrogen sulfide (H2S) (p ≤ 0.05). During the flood stage the conditions were more varied and significant associations were detected between the vertical arrangement of Botryococcus braunii, Oscillatoria splendida, Dinobryon serturlaria, Euglena acus, and Trachelomonas volvocina with conductivity, dissolved oxygen (DO), pH, temperature, dissolved organic carbon (DOC), PO4, RP, and ammonia (NH4). The vertical structure of the Lake Boa’s phytoplankton responds to annual changes produced by the flood pulse, and to the vertical environmental variations, which are stronger during the high water phase.

Summary 1. We applied Reynolds's approach to the study of phytoplankton ecology through functional associations of species to identify possible algal species, which associate with the freshwater cyanobacterium Synechococcus. Previously an association among Synechococcus spp. and small-celled chlorophytes (association Z) has been recognised by evaluating phytoplankton associations according to functional criteria. 2. Biomass data for phytoplankton and picocyanobacteria from Lago Maggiore spanning more years were organised in a matrix and a cluster analysis was performed. The results showed four groups separated at a linkage distance of 0.20. Mixotrophic species which clustered with Synechococcus spp. were Ceratium hirundinella, Chrysochromulina parva, Cryptomonas erosa, Cryptomonas ovata, Dinobryon bavaricum, Dinobryon sociale, Rhodomonas minuta and Uroglena americana. The redundancy analysis (RDA) consolidated the association of Synechococcus with C. hirundinella and R. minuta showing greater probability of occurrence than random aggregates of species. 3. The association Synechococcus spp. –C. hirundinella also appeared from temporal variation of their biomass. In early summer both these species increased at the same time; later, the peak of C. hirundinella (potential predator) coincided with a minimum of Synechococcus spp. suggesting a possible predator-prey interaction. This implied that phytoplankton assemblages which form a functional group cannot only have similar adaptations and requirements but can also exhibit trophic interactions. 4. We propose to enlarge the association Z and create an association ZMX (where MX stands for mixotrophs) which would include Synechococcus spp. and C. hirundinella as the most representative of the mixotrophic species found in the oligotrophic Lago Maggiore.

【In order to grasp the structure and dynamics of phytoplankton communities, chlorophyll-a (Chl-a) and cell abundance were measured at 20 stations during the period from August 9 to August 21, 2003 in the southeastern Barents Sea on surface and subsurface chlorophyll maximum depth (SCM). Surface temperatures were varied from minimum $-0.7^{\circ}C(st. 18)$ to maximum $10.4^{\circ}C(st.1)$ . Salinities were varied from minimum 29.9 psu(st. 18) to maximum 35.8 psu(st.2). The maximum nutrient(phosphate, nitrate, silicate) concentrations were $0.12{\mu}M,\;0.11{\mu}M,\;7.53{\mu}M$ and minimum concentrations were $0.01{\mu}M,\;0.03{\mu}M,\;1.43{\mu}M$ , respectively. On SCM physical environmental factor were almost similar. Chl-a concentrations ranged from 0.23 to $2.13{\mu}g\;chi-a\;l^{-1}$ at SCM. Nano- and pico phytoplankton were the important contributors for increase of the Chl-a. It was about seven times difference between highest concentration to lowest. Phytoplankton communities were composed of diatoms, dinoflagellates, cryptophyceae, silicoflagellate, and prymnesiophyceae showing 37 taxa at surface and 38 taxa at SCM. Picophytoplankton was the most dominant in all stations and all layers, but the second groups were 2 and/or 3 taxa. Phytoplankton abundance ranged from minimum $4.3{\times}10^5\;cells\;l^{-1}$ (st. 20) to maximum $2.4{\times}10^6\;cells\;l^{\-1}$ . (st. 17) at surface water. As a result, phytoplankton might be controlled by physical factors such as North Atlantic ocean currents and northern melt water among environmental factors in Barents Set h addition the dominant species were nano- and pico phytoplankton such as Phaeocystis, Cryptomonas and Dinobryon in the study area.】

Dynamics of phytoplankton communities were investigated at five stations in Youngsan River from December 1995 to November 1996. Phytoplankton were identified to the total of 466 taxa, which were composed of 136 genera, 438 species, 27 varieties and 1 forma. The standing crops ranged 1,163-50,765 cells during the investigation periods. The variation of classes was in order to Chrysophyceae - Bascillariophyceae - Chlorophyceae at St. 1 and was only Bacillariophyceae at the other stations. The dominant species were 12 taxa including Aulacoseira ambigua, A. garnulata, Cyclotella meneghiniana, C. pseudostelligera, Chlorella vulgaris, Dinobryon sertularia, Flagilaria elliptica, Monoraphidium contortum, Micractinium pusillum, Nitzschia palea, Scenedesmus grahneisii and Stephanodiscus hantzschii f. tenuis. The standing crops of dominant species ranged from 10.6% to 94.7%. The genus Dinobryon in Chrysophyceae was dominant species at St. 1 in December 1995, but not recorded in the other months. The dominant species were composed with the planktonic diatoms from winter to spring and were the tychoplanktonic and the benthic species at St. 2-5 from summer to fall. The relationships between total standing crops and water temperature, pH, , , and showed low positive or negative coefficients. Stephanodiscus hantzschii f. tenuis had low positive or negative coefficients with water temperature, , , and . The fluctuations of standing crops in the Youngsan river phytoplankton community were not caused by a single factor but controlled by the complex factors with interaction between phytoplankton community and envirowmental factors.

Photoresponsive behavior in phytoplanktonic flagellates potentially controls depth regulation, vertical migration, and the accumulation of cells in preferred conditions and hence has major implications for photosynthesis,growth, and species competition. This study used microscale laboratory chambers with cell track analysis and mesoscale lake studies to investigate the responses of five phylogenetically contrasting freshwater flagellates to gradients of light. Laboratory results demonstrated that these species differed in their light preferences despite being grown at the same photon irradiance. Preferred photon irradiances were 20-120, 20, 65-120, 4-20, and >200 μmol photons.m - 2 .s - 1 , respectively, for Ceratium furcoides (Levander) Langhans, Chlamydomonas moewusii Gerloff, Dinobryon sertularia Ehrenberg, Euglena gracilis Klebs, and Plagioselmis nonnoplanctica (Skuja) Novarino. Analysis of the response of individual cells showed that in all species, photoresponsive preference was the result of positive and negative phototaxis combined with step-up and step-down photophobic reactions. There was no evidence for photokinesis or cell memory. Only in C. furcoides did the preferred photon irradiance alter with growth conditions. In C. furcoides, D. sertularia, and P. nannoplanctica, irradiance preference matched the optimal irradiance for growth, whereas in the two remaining species a lower than optimal irradiance was preferred, suggesting that light may be used as an indicator of other ecological conditions. Mesoscale experiments in a lake demonstrated that the laboratory microscale measurements provide information relevant to understanding ecological distributions. Behavioral responses to light contribute to the delineation of vertical niche separation and provide a method for predicting the spatial and temporal distribution of flagellates.

The biomass and bacterial ingestion rates of mixotrophic phytoflagellates were examined on diurnal and seasonal scales in oligotrophic Lake Skarlen (S. Sweden). Ingestion rates were measured using fluorescent microspheres in situ on three depths every 6(th) hour during 24 hours in summer in order to quantify diurnal variations and once at midday during autumn, winter and spring. Mixotrophic phytoflagellates composed a large part of the phytoplankton biomass (18-31 %) throughout the year. Nonetheless, calculations suggest that the mixotrophs only consumed between 0.5 and 4.4 % of the bacterial carbon production, with the highest consumption percentage in winter under ice. Two groups of algae, Cryptomonas spp. and Dinobryon spp., were commonly found with ingested prey and both exhibited significant diurnal differences in bacterial ingestion rates with the highest mean rates measured in late afternoon (0.50 and 0.27 bacteria cell(-1) hour(-1), respectively) and very low rates during night (0.09 and 0.06 bacteria cell(-1) hour). In Cryptomonas spp., ingestion rates also varied.somewhat over the year and bacterial ingestion was highest during winter (0.57 bacteria cell(-1) hour), possibly as a result of the lengthy impaired light climate. (Less)

In four deep karstic lakes (Lake Vransko, Lake Visovacko, Kozjak and Prosce) several functionally well-adapted phytoplankton groups interchange in dominance during the seasonal cycle from April to September and likely tolerate the constraining conditions of nutrients deficiency more successfully as an aggregated group than as taxonomic units. This paper considers the functional approach by Reynolds et al. (2002), a method capturing much of the species ecology and which is successfully used for describing phytoplankton successions. Deep karstic lakes are attributed with the early spring (April) formation of stratification, low phytoplankton biomass, and with maximum biomass accumulating in July. The co-occurrence of functional groups in those lakes is evident but typically distinct according to different biomass proportions. The functional groups A, B, C, D, P, E and LO, represented by different Cyclotella, Ulnaria/Fragilaria, Dinobryon, Ceratium/Peridinium species, co-occurred in 50% and up to 90% of phytoplankton biomass in all investigated lakes. By observing the co- occurrence across the Croatian karstic region we: (i) demonstrate the regional distribution of phytoplankton assemblages in relation to low nutrient environment, and (ii) build a simple uniform pattern which successfully describes a variety of phytoplankton in deep karstic lakes and provides information of reliable indicators for those systems.

1. Algal taste and odour is usually associated with open water blooms and eutrophic systems. However, some algal species can produce high biomass under ice-cover, even at low nutrient concentrations, that can impact water quality. This paper describes a winter odour outbreak in oligotrophic Glenmore Reservoir (Calgary, Alberta, Canada), the major algal species, volatile organic compounds (VOCs) and some treatment implications. 2. Using sensory, chemical and microscope analyses, we monitored odour, algal biomass and taxa, bacteria and major nutrients. In a preliminary assessment of the effectiveness of standard water treatment with this type of algal biomass and odour, we used bench-scale tests and sampled raw water from the Glenmore treatment plant at successive treatment stages. 3. In the winter of 1999–2000 Glenmore ice-cover was delayed, nutrients were characteristically low (TP

We studied the occurrence and dynamics of transparent exopolymer particles (TEP) and 4'-6'-diamidino-2-phenylindole (DAPI)-stained microaggregates (MA) and the bacterial colonization of the latter by fluorescence in situ hybridization with rRNA-targeted oligonucleotides in Lake Constance, Germany, from June to December 1996. MA were categorized into the size classes 8 to 20, 20 to 60 and >60 µm. Total abundance of TEP varied between 71 and 2554 ml -1 and that of MA between 310 and 1526 ml -1 with the largest proportion in the size class 8 to 20 µm (140 to 959 ml -1 ). Numbers of TEP and of MA >60 µm, never exceeding 100 ml -1 , decreased with depth and were cor- related to the biomass of total phytoplankton, diatoms, chrysophytes and dinoflagellates. Numbers of the smaller-sized MA did not systematically decrease with depth. Four types of DAPI-stained MA were identified: aggregates composed of bacterial and of cyanobacterial colonies, aggregates fluo- rescing yellow (DAPI yellow particles), and phytoplankton and zooplankton fragments fluorescing faintly blue after DAPI staining. The latter and DAPI yellow particles were most abundant. The majority of MA was densely colonized by bacteria. At least 20% of the DAPI-stainable cells were detected by the Bacteria-specific probe EUB338 and a mean between 44 and 56% of the various types of MA. β-Proteobacteria dominated aggregates composed of remains of diatoms and often also on fragments. On DAPI yellow particles bacteria of this phylogenetic lineage and the Cytophaga/Flavobacteria cluster were equally abundant. γ-Proteobacteria usually constituted only minor proportions except on fragments of Dinobryon spp. and of zooplankton origin on which they constituted up to 62% of the DAPI-stainable cells. α -Proteobacteria were not detected at all. Together, all group-specific probes constituted >95% of the bacteria detected by the EUB338 probe on MA except for bacterial aggregates on which only 59% were detected. The different spatio-tem- poral dynamics of TEP and MA and the bacterial colonization of the latter suggested that TEP and MA > 60 µm were early decomposition products mainly of diatoms and other algae whereas smaller MA were more degraded stages of detrital particulate organic matter.

Deep chlorophyll maxima (DCM) were found in all five Laurentian Great Lakes during August, 1998. Chlorophyll profiles were consistent over large areas in Lakes Superior and Michigan, while distinct inter-site differences were apparent in the other three lakes. Shade adaptation appeared to be primarily responsible for increases in chlorophyll at depth in Lakes Huron and Ontario, while in Lake Superior increases in phytoplankton biovolume were also noted. Deep living phytoplankton populations in the latter lake exhibited improved nutrient status at depth, where concentrations of both soluble phosphorus and silica were higher. Phytoplankton community composition in the DCM differed from that previously reported for the lakes, most notably in the reduced populations of Cyclotella, relative to the epilimnion, seen at most sites. Filamentous chlorophytes were often more abundant at depth, as were certain species of Dinobryon.

Temporal changes in the plankton community and trophic relations between the metazooplankton and its potential prey (bacteria, flagellates, ciliates and algae) were studied following the flooding of a new reservoir (Sep, France). The high dissolved organic matter (DOM) concentrations favoured the development of heterotrophic and mixotrophic organisms, which dominated the unicellular plankton after flooding. The proportion of strictly autotrophic organisms increased with time, reaching 67 % of the biomass of protistan plankton at the end of the study. A small-sized chrysomonade (2-5 μm) accounted for 53 % of the abundance of the heterotrophic nanoflagellates (HNF) throughout the study. The Chrysophyceae, Chrysidalis sp. accounted for 76 % of the mean relative abundance of all pigmented flagellates combined. Some mixotrophic species such as Dinobryon cylindricum and Cryptomonas ovata mainly occurred at the start of the study. The prostomatid ciliate Urotricha furcata, which dominated in terms of numbers immediately after flooding, was later replaced by mixotrophic species, an oligotrichid Pelagohalteria viridis then a peritrichid, Vorticella sp. The rotifers Polyarthra sp., Synchaeta sp. and Hexarthra mira were the first metazooplankton organisms to colonise the pelagic environment. They were replaced in the 3 rd year by Cladocera (Daphnia longispina and Ceriodaphnia quadrangula) and Copepoda, firstly calanoids (Eudiaptomus gracilis) and then cyclopoids (Cyclops vicinus and Acanthocyclops robustus). This study emphasizes the importance of the organisms of the microbial food web in the nutrition of the metazooplankton in the period following the flooding of a newly impounded reservoir.

Three previously undescribed species of the loricate chrysophyte genus Dinobryon (D. brucei K.H. Nicholls sp. nov., D. dillonii K.H. Nicholls sp. nov. and D. praecambrianum K.H. Nicholls sp. nov.) have been found in several lakes and ponds in Ontario, Canada. All possess a lorica morphology distinct from other members of the genus, thus allowing their designation as separate species. Stomatocysts for two of the new species, D. dillonii and D. praecambrianum are described, but the stomatocyst for D. brucei is still unknown, despite the presence of abundant vegetative material in two lakes.

The importance of protists as bacterivores in a coastal community and an offshore community of Lake Erie was compared during the summer of 1994. Bacterial density, cell size, and empirical conversion factors for bacterial productivity were highly variable at both sites and greater at the coastal site (P < 0.01). Bacterial productivity at the coastal site was 25-50 times higher than at the offshore site. Bacterivory was estimated in situ by fluorescently labeled native bacteria. Per-cell grazing rate and filtering rate for each taxon were routinely determined. Heterotrophic nanoflagellates (HNAN) were the most important protistan bacterivores at the offshore site, while HNAN and ciliates were similarly dominant bacterivores at the coastal site. Mixotrophic bacterivory was important only at the offshore site where Dinobryon was the dominant bacterivore. Bacterial carbon flux through protists was higher at the coastal site by an order of magnitude. Offshore protists grazed virtually the entire bacterial production, while coastal protists usually grazed less than half of the bacterial production. These results suggest that coastal and offshore sites differed fundamentally in the significance of protists to carbon flux through the microbial loop to higher trophic levels.

The temporal and spatial developments of 15 phytoplankton species widely distributed in European and North American lakes, have been studied by informations in the literature and especially by their behaviour during a 5 years development in the Danish lake Grane Langso. The species were characterized by temporal differences in maximum abundance, the density at the maximum, the number of maxima per year, the length of the vegetative period and the constancy of the annual maximum based on long-term studies. In several cases the phytoplankton species can be characterized by one individual behaviour in one group of lakes, while the same species behaves differently in another group of lakes. The results suggest that several species include two or more types with different tolerances and environmental requirements. Most of the species included are Chrysophytes, mainly species of Uroglena and Dinobryon. A remarkable feature of Grane Langso's Chrysophytes is their predilection to accumulate in the bottom water during the summer. A similar behaviour was not demonstrated by Chrysophytes in other lakes.

SUMMARY 1 Studies carried out in Lake Baikal in late spring (late May-early June) 1991 showed marked differences in the species composition and abundance of phytoplankton in different regions of the lake. The south and north basins were characterized by small forms of algae. The middle basin, Maloe More and the shallow waters of the Selenga had species with large cells including Aulacoseira islandica subsp. helvetica, Dinobryon cylindricum and D. divergens. 2 Areas of high biomass were correlated with shallow waters and river inputs. This was especially apparent in the region of the Selenga delta. The vertical distribution of phytoplankton indicated the non-synchronous start of the spring homothermy throughout the lake. Phytoplankton were concentrated in the upper 100 m layer with subsurface maxima resulting from the sinking of large algae. 3 The concentration of phytoplankton biomass in general at this time characterized the lake as moderately productive.

During the spring clear-water phase of 1993, an enclosure experiment was performed in the mesotrophic Schohsee (Plon, FRG) in order to assess the impact of crustacean zooplankton on the rotifer and phytoplankton community. Among the crustacean plankton, calanoid and cyclopoid cope- pods were abundant, but Daphnia 'longispina' reached the highest densities in this experiment. The colonial rotifer Conochilus unicomis was not affected by crustacean plankton. The two most abundant species, Synchaeta peclinata and Keratella cochlearis, increased exponentially when macrozooplankton had been excluded from the enclosures, but did not increase when crustaceans were present. Birth and death rates of K.cochlearis could be reliably determined in this field experiment, suggesting that this rotifer species was mainly controlled by exploitative competition rather than by mechanical interfer- ence or predation. Daphnia 'longispina' generally grazed selectively on the smaller ciliates and algae, thus depriving the rotifers of their phytoplankton resources. The dominant alga, the chrysophycean Dinobryon, increased, whether crustaceans were present or not, but appeared to be grazed upon to a certain extent despite its considerable cell size and colonial organization.

The vertical distribution of chlorophyll was examined in relation to physical and chemical parameters in oligomesotrophic Lake Samish, Washington state, during the latter half of a growing season. A majority of chlorophyll resided in the metalimnion in late July, with metalimnetic populations dominated by diatoms (Cydotella bodanica) and chrysophytes (Mallomonas caudata, Dinobryon scrtularia). A silica gradient appeared to be most important in determining the vertical position of these populations. Later in the season, segregation of nutrients and light, in the face of a deepening epilimnion, led to transient accumulations of phytoplankton at the top of the metalimnion. It appeared that losses due to heterotrophic activities accentuated the lower boundary of this chlorophyll peak.

The marine chrysophyte Dinobryon balticum (Schzutt) Lemm. was one of the dominant members of the phytoplankton community (1.8×103 cells-L−1) in June and July in Conception Bay, Newfoundland. Dinobryon balticum colonies were common only in samples from June and July. The cells were concentrated at 5 m (X±SD=1.11±4 × 105 cells.L−1) and at 40 m (3.32±2×104.L−1) depths. Colonies were composed of up to 560 cells with a mean (±SD) colony size of 10 ± 1 cells at 5 m and 40 ± 8 cells at 40 m. Fluorescent latex bead-uptake experiments conducted with field samples indicated that this marine species was capable of phagotrophy and that twice as many Dinobryon cells were ingesting beads at 40 m than at 5 m, although the ingestion rates for those cells actively ingesting beads were similar at both depths. This chrysophyte was found in association with bacteria-and nutrient-rich microhabitats of microaggregates and fecal pellets. The cells and colonies observed in this study appeared to be healthy, as demonstrated by their appearance and their ability to ingest beads.

The dynamics of chrysophyte populations and the onset of encystment in relation to several physical, chemical, and biological variables were studied in two Canadian lakes, Lake Opinicon (LO) and Upper Rock Lake (URL). LO is shallow and polymictic during the ice-free season, whereas URL is deep and dimictic. Seasonal patterns of many limnological variables, as well as species composition and seasonal dynamics of chrysophyte assemblages, were broadly similar in both lakes. Three groups of chrysophytes were recognized: 1) taxa that were more abundant during ice cover and/or early spring and occasionally in late fall (Dinobryon spp. Ehrenberg and Synura spinosa Korsh), 2) taxa that seemed to prefer moderately warm water proglena americana Calkins and Ochromonas sp. Wyssotzki) and had maximum densities in late summer and occasionally in the fall, and 3) taxa that occurred in low numbers and did not exhibit any discernible seasonality in the two lakes (Mallomonas spp. Perty). Encystment rarely occurred during ice cover in both lakes. Stomatocyst concentrations were highest shortly after ice breakup (late April or early May), declined to trace levels by June, and remained at those levels until the end of the sampling season (late November). Encystment rates (cyst concentrations) were higher in dimictic URL. Encystment in both lakes coincided with concurrent changes in physical and chemical variables and chrysophyte vegetative cell densities, but it was not possible to pinpoint the exact conditions responsible for triggering encystment. Only a small proportion of chrysophytes encysted. Cyst production within a lake may vary from year to year.

Particle uptake by two strains of the freshwater phytoflagellate Dinobryon was studied using cells grown in clonal batch cultures and presented with fluorescent microspheres. Clearance rates were found to be strongly dependent on temperature, with maximum clearance rate observed at around 20°C. Clearance rate over the diurnal light dark cycle was observed to be slightly higher during the dark phase. Cultures quickly died when placed in total darkness. cells of one strain grown for two weeks under dim irradiance (18 μEm -2 s -1 ) showed reduced clearance rates. For the other strain no relation was observed between clearance rate and irradiance

This study compares the composition and dynamics of phytoplankton communities and of periphyton communities growing on Plexiglas slides in a temperate oligotrophic lake. Ordination methods were used in the comparisons, which covered a three-month period during the summer. Plankton and periphyton communities differed considerably, and shared only six common taxa. An ephemeral Dinobryon pulse occurred simultaneously in both communities, and represented the largest source of variability in community composition. Recovery from the pulse was faster in the phytoplankton than in the periphyton. Attachment to substrata, through its effect on cell turnover times and nutrient cycling, is hypothesized to provide a temporal refuge for plankton species during periods of unfavorable environmental conditions.

6 pages, 3 figures, 2 tables, We report the presence and abundance of Dinobryon faculiferum (Chrysophyta) in NW Mediterranean coastal waters, as well as the first estimations of the grazing impact of this mixotrophic species on bacteria. In June 2006, D. faculiferum was detected at an abundance of 37 cell mL–1 (1.2% of total protistan abundance). By grazing with cell-specific rates of 7.8 bacteria cell h–1, this single species explained 4.5% of the total bacterial grazing losses, This study was supported by the ESTRAMAR (CTM2004-12631/MAR) and PROBA (CSIC-CONICET Programa "Luis Santaló") projects of R.M. Work of J.M.G. was supported by projects SUMMER (CTM2008-03309/MAR) and STORM (CTM2009-09 352/MAR) from Spanish MCy

Planktivorous fish can affect phytoplankton indirectly by control of herbivore communities, and directly by excretion of soluble nutrients. A field mesocosm study showed that nutrient regeneration by sockeye salmon (Oncorhynchus nerka) fry substantially increased biomass of zooplankton and phytoplankton relative to fishless controls. Plankton communities in enclosures treated with only fish-regenerated nutrients but not fish predation responded differently to two densities of fish. Enclosures with low fish densities exhibited significant increases in zooplankton biomass and small increases in phytoplankton biomass. Nutrients resulting from high fish densities stimulated blooms of ungrazable Dinobryon, and extremely low zooplankton biomass when compared with fishless controls. Phosphorus excretion rates by zooplankton and fish explained 55% of the variance in total plankton biomass in predation-free enclosures and 95% of the variance in chlorophyll a concentrations for enclosures with predators and fish-regenerated nutrients. The results of this experiment suggest that algal production is greatly enhanced by excretion of nutrients by fish, while grazer control is less important.

An enclosure experiment was conducted to assess the effects of a zooplankton elimination on the structure of a phytoplankton community. Phytoplankton biomass and production were higher in grazer-free enclosures, while the productivity per unit biovolume was lower. Exclusion of zooplankton favoured the majority of algal species, especially chrysophyceans (Dinobryon spp.) and the diatom Rhizosolenia, while mucilagineous green-algae were disfavoured. Middle sized algae (ESD 15–50 µm) and those with the largest Surface Area/Volume ratio were proportionally most favoured by the elimination of grazers.

The life cycle of a previously undescribed chrysophyte, assigned to the new genus Rhizoochromonas, is described. It includes a small motile stage with heterokont flagellation which invades a Dinobryon lorica. Reproduction by cell division of the nearly spherical rhizopodial vegetative stage frequently leads to expulsion of the host protoplast through overcrowding of the lorica. Endogenous cysts (stomatocysts) are also formed within the Dinobryon lorica. The new family, Brehmiellaceae, is established to accommodate pseudopodial/rhizopodial chrysophytes with heterokont flagellation in the motile stage. Rhizoochromonas endoloricata gen. et sp. nov. has been found at two widely separated softwater locations in Ontario, and at one it constituted a major component of the planktonic flora during the autumns of six successive years.

The three-dimensional structure of the flagellar apparatus of Dinobryon cylindrioum Imhof. (UTEX no. LB 2266) was determined using serial section reconstruction. Four microtubular rootlet systems (R1, R2, R3, and R4)and a rhizoplast are present, following the general pattern found in other chrysophytes. The R1 rootlet, containing seven microtubules, originates at the basal body of the long flagellum that bears mastigonemes (F1). The R1 rootlet forms an arc which curves in clockwise direction (when viewed from the anterior end of the cell) approximately halfway around the pit from which the short smooth flagellum (F2) emerges. Numerous microtubules cascade from the exterior-facing side of this rootlet to the tail of the cell. The R2 rootlet originates between the F1 and F2 basal bodies, is attached to the F1 basal body by a fibrous connection, and forms a clockwise arc above the R1 rootlet. This rootlet extends approximately one quarter of the way around the pit. The R3 rootlet system originates as a trough-shaped band of six microtubules spanning the distance between the proximal ends of the F1 and F2 basal bodies. The six-membered rootlet splits into two parts, designated R3 and R3. Both parts circle the pit in counter-clockwise direction, pass beneath the F2 basal body, and descend into the cell alongside the chloroplast. The R4 rootlet originates in fibrous material, passes diagonally over the top of the F2 basal body, forms a clockwise loop at least three quarters of the way around the pit to the interior of the R3 and R3 rootlets, and ends in the cytoplasm. Similarities of rootlet origins and other details of the flagellar apparatus of D. cylindricum with those of other heterokont organisms reinforce the idea that these organisms are phylogenetically related.

Seasonal fluctuation of phytoplankton with special reference to chrysophytes was studied in four ponds and lakes, Doro - ike, Hoshino - ike, Sengari Reservoir and Yasuba- ike, which are situated in the southern part of Hyogo Prefecture, Japan, and are in different stages of eutrophication. The chrysophytes increased in cell number from autumn to spring. The maximum density of the chrysophytes was recorded in March in all the ponds and lakes ; 5, 900 cells·ml-1 in Doro-ike Pond, 13, 000 cells·ml-1in Hoshino - ike Pond, 190 cells·ml-1 in Sengari Reservoir and 6, 900 cells·ml-1 in Yasuba- ike Pond. The chrysophytes were an abundant algal group in the low temperature period and Mallomonas akrokomos, Synura mammillosa, Chrysococcus rufescens and Dinobryon sociale were dominant in the freezing time. Many chrysophyte species occurred not only in oligotrophic waters but also in eutrophic ones, and the number of species increased with eutrophication. The chrysophytes appeared all the year round in oligotrophic waters, whereas, in eutrophic waters, they appeared in the low temperature period with higher density than in oligotrophic waters. But in heavily polluted waters where water blooms of the cyanophytes occurred, the number of species and the density were very low. Based on these, the chrysophytes are regarded as representatives of eutrophic waters as well as of oligotrophic ones.

Ochromonas sensu lato is the largest genus in the Chrysophyceae, containing over 100 names. Ochromonas species are biflagellate, naked, plastid-bearing single cells, distinguished from loricate, scaled, colonial and colorless genera. Most, if not all, species of Ochromonas are mixotrophic, i.e., they photosynthesize but they also engulf bacteria and other small prey. Preliminary evidence from SSU rRNA sequences show that Ochromonas is a polyphyletic genus. Ochromonas tuberculata is the most distinct from all other Ochromonas species. The other Ochromonas species (examined thus far) are scattered in three clades. For example, O. danica and O. sphaerocystis are sister to Poterioochromonas stipitata and P. malhamensis. Four additional species (identified by light microscopy as O. elegans Doflein, O. globosa Skuja, O. ovalis Dolfein, O. sociabilis Pringheim) have SSU rRNA sequences identical to P. malhamensis. Of these, only O. sociabilis has been transferred to Poterioochromonas. Thus, at least some species may be synonymous with others. Two clades of marine species are also known, one containing coastal species and the other containing open ocean species. A number of genera (some also polyphyletic) are interspersed amongst the Ochromonas species (e.g., Chrysolepidomonas, Chrysonephele, Chrysoxys, Cyclonexis, Dinobryon, Epipyxis, Uroglena, Uroglenopsis). The goal of this research (just beginning) is to establish a monophyletic Ochromonas, probably by assigning some species to other genera (existing or new). One major problem is that the type species, O. triangulata Vysotskii, hasn't been observed in over 100 years, and it is unclear which of several clades of Ochromonas contains the type. Results will be discussed.

The development of phytoplankton assemblages in the two main basins of the deep (Zmax=350 m) and large (49 km3) subalpine Lake Garda (Northern Italy) was investigated through monthly samplings from 1998 to 2000. The algal development was strongly typified by the alternation, from spring to autumn, of Fragilaria crotonensis and Mougeotia sp. and by the increasing importance of filaments of the complex Planktothrix rubescens/agardhii in summer and autumn. These three “master species” are characterised by higher biovolumes and/or more regular and wide annual development in comparison to the other dominant taxa. The simultaneous application of ordination (NMDS-Non metric Multi Dimensional Scaling) and cluster analysis techniques revealed an ordered and coherent temporal succession of phytoplankton assemblages in the two main basins of the lake. These temporal and spatial regularities arise from the high inertia and resilience against perturbations that characterise the deep and large lakes, and contrast sharply with the less ordered or chaotic and unpredictable seasonal assemblages that may be found in small and shallow lakes. It is stressed that, owing to the use of different methodologies, the modifications of the phytoplankton assemblages in the last fifty years must be interpreted with particular caution. The only demonstrable differences seem to indicate that, with the speeding up of eutrophication processes in the 1960s and 1970s, detectable populations of Planktothrix and irregular increases of Conjugatophyceae (Mougeotia sp., Closterium aciculare) were established. During and after the 1980s, the increase of phosphorus concentrations in the lake and the progression towards more mesotrophic conditions was accompanied by the appearance of further substantial populations of cyanobacteria (Planktolyngbya limnetica, Anabaena lemmermannii). In contrast with these signs of alteration, Fragilaria crotonensis, Asterionella formosa, Ceratium hirundinella and Dinobryon spp. were among the most abundant taxa from the 1950s to the present time. These modifications are largely consistent with the changes observed during the 1990s in consequence of the different extent of the deep vertical mixing. During complete overturn, with the maximum spring replenishment of nutrients in the euphotic layers, a greater development of Mougeotia sp. and Oscillatoriales was observed. The historical trend and the ecological results allowed elucidation of the trophic characteristics of many important species developing in Lake Garda and, in general, in the deep subalpine lakes.

Unusually large virus-hke particles (VLPs) with tails were found in water samples from Norwegian and Danish coastal waters. Size of the VLP heads were 340 to 400 nm and the tails were 2.2 to 2.8 vm long. The VLPs occurred at a maximum concentration of about 104 ml-'. The possible hosts are unknown. Most viruses measure between 20 and 350 nm (Bitton 1980). The largest are found among the animal viruses and the smallest among the plant viruses and the bacteriophages. Filamentous viruses may be as long as 1000 nm or longer, but the diameter of these viruses is 10 nm or less (Bradley 1967, Laskin & Lechevalier 1973). The viral population of natural marine waters is dominated by forms with heads < 60 nm in diameter (Bergh et al. 1989). Most of these small viruses are thought to be bacteriophages (Bratbak et a1 1990). Viruses in the size range of 100 to 150 nm usually make up < 1 to 10 % of marine viral communities. Water samples in w h c h we observed unusually large virus-like particles (VLPs) were collected at 0.2 m depth in Raunefjorden, western Norway in April 1991 (Bratbak et al. 1990) and during a mesocosm experiment in Knebelvig, Denmark in July 1991 (Nybroe et al. 1992). The mesocosm experiment was set up in the beginning of July and sampled every 2 or 3 d for 3 wk. Samples preserved with 1 O/O glutaraldehyde were harvested by centrifugation onto electron microscope grids, stained with 2 % uranyl-acetate, and inspected in a JEOL100CX transmission electron microscope as previously described (B~rsheim et al. 1990, Bratbak et al. 1990). The VLPs (Fig. l a ) we observed in the mesocosm experiment were found in 2 of the 4 enclosures investigated. A maximum concentration of ca 104 ml-' was observed on Day 5 of the experiment. The particles were also found in the sample taken 3 d later, but not in earlier or later samples. The head structure of the particles appears to have an octahedral form and the size of the head, as it appears on the electron micrographs, was 370 nm (SD = 17) X 340 nm (SD = 12). The tail structure was ca 2.2 pm (SD = 0.3) long and 80 to 85 nm wide. The VLP found in the free water masses of Raunefjorden was similar in morphology to the VLP found in the enclosure study (Fig. lA , B) but the head structure was slightly larger (400 X 370 nm) and the tail structure was about 2.8 pm long and 55 to 60 nm wide. The only comparable VLP we know of has a head diameter of 390 nm and a tail of about 1 pm and has been found in cultures of the filamentous green algae Uronema gigas (Dodds & Cole 1980). Fig. 1. Transmission electron micrograph of the unusually large virus-like particles. (A) Sample from mesocosm experiment in Denmark; (B) sample from Raunefjorden, Norway. Arrow: virus-like particles of a more typical size (< 100 nm); b: bacterium O Inter-Research 1992 0171-8630/92/0085/0201/$ 03.00 202 Mar. Ecol. Prog. Ser. 85: 201-202, 1992 Compared to microorganisms such as bacteria and flagellates, viruses will generally appear as electrondense structures after positive staining with uranylacetate due to their high nucleic acid content. As shown in Fig. 1A. B the large VLPs have a high electron density in the head structure. We do not know of any biological or non-biological structure that gives a similar appearance after this preparation and we therefore assume that these particles are indeed viruses. The host of the viruses found in the enclosure study must be a planktonic organism because they were produced within the enclosures where only planktonic organisms were present. The size of these large viruses limits the number of possible host organisms. The viruses we observed resemble tailed bacteriophages but the size of the bacteria (Fig. 1) found together with the viruses makes them unlikely hosts. The eucaryotic community found together with the virus in the enclosure study was numerically dominated by unidentified autotrophic and heterotrophic forms that were 1 to 5 pm in diameter. The most abundant larger phytoplankton forms included the diatoms Rhizosolenia fragilissima and Probiscia alata; some unidentified cryptophyceans; some dinophyceans (including Gymnodim.um sp. and Scripsiella sp.); and the chrysophyceans Dinobryon balticum and D. petiiolatum. The most abundant protozoans included colorless flagellates, and the ciliates Urotricha sp., Strombidium spp., and Mesodinium rubrum. The roles of viruses in natural waters have just begun to be revealed and we are still groping to find our way with new methods and approaches. Our observation of the large VLP is mainly qualitative and its host is unknown. The finding does emphasize that the choice of method for detecting and counting viruses may be crucial. Large viruses such as those described here would probably have been lost if any prefiltration had been applied (Cottrell & Suttle 1991, Paul et al. 1991) and they would probably have been considered small bacteria if they had been observed in the epifluorescence microscope after staining with DAPI (Hara et al. 1991). This note was submitted to the editor Acknowledgements. This study was supported by the EEC MAST program, contract no. 0020, the Danish Natural Research Council, Danish Technical Research Council (J.-nos. 11-8630 and 16-4806), National Agency of Environmental Protection in Denmark (HAV9O Marine Research Programme in Denmark contract no. 1.23), the Royal Norwegian Council for Scientific and Industrial Research (contract 27744), the Norwegian Council for Fisheries Research, and the Norwegian Council for Science and the Humanities. The electron microscope work was done at the Laboratory for Electron ~Microscopy, Science Faculty, Univ. of Bergen.

Jöri Lake III (2512 m a.s.l., zm = 22 m, A = 57.81 ´ 103 m2, V = 601.1 ´ 103 m3) is situated in the Vereina region in the eastern part of the Swiss Alps. We studied microbial grazing on bacteria and bacterial productivity during the ice-free period. The lake normally gets thermally stratified for two months between July and September. In 1996, chlorophyll-a concentrations varied from 0.5 to 2.0 μg l-1 with maximum values just below the thermocline (6 m depth), in 1997, they were between 0.6 and 5.0 μg l-1 with maximum values at 10 m depth – several meters below the thermocline. Bacterial densities varied between 0.7 and 1.7 ´ 106 ml-1 with maxima in the thermocline, one to two meters above the chlorophyll maximum. The areal bacterial biomass (volume beneath 1 m2 to a depth of 8 m) was 10 μg C l-1 which remained more or less constant for the periods investigated. In 1997, bacterial growth rate and production rates were determined using [3H]-thymidine incorporation. The rates were as low as 0.002 to 0.006 h-1 and 0.01 to 0.03 μg C l-1 h-1, respectively. We found a carbon ratio of bacteria, phytoplankton, and autotrophic picoplancton (APP) of 1.5:1.1:1 which shows a rather high abundance of bacteria and autotrophic picoplankton (APP) compared to larger phytoplankton. Bacterial growth followed a temperature dependence similar to the one observed for bacteria from Lake Zürich, a prealpine and mesotrophic lake which was studied for comparison. Microbial food web in Jöri Lake III was not top down controlled during the periods of our study and mixotrophic algae like Dinobryon cylindricum var. alpinum and autotrophic nanoflagellates (ANF) were the dominant bacterial grazers observed.

Contrasting zooplankton assemblages consistently produced different compositional shifts in a phytoplankton community. Two experiments in 120-liter enclosures were used to assess the responses of the algae to two different-sized zooplankton communities. Grazing by a mixture of small copepods, Bosmina longirostris, and rotifers led to increased growth of phytoplankters with greatest axial linear dimensions < 25 pm and ratios of surface area to volume ~2.6, such as Chlamydomonas and Chlorococcales. Larger phytoplankton taxa such as Asterionellafirmosa, Closteriopsis Iongissimus, and Synedra sp. declined in the presence of small zooplankters. In contrast, a mixture of large zooplankters dominated by Daphnia pulex and Diaptomus oregonensis caused declines in phytoplankters with greatest axial linear dimensions ~60 pm and ratios of surface area to volume ~2.75, while larger algae such as Aphanocapsa and Dinobryon increased. Discriminant analyses showed that volume and surface area were the most effective characters for determining the response of algae to grazers. Zooplankton community structure was important in determining the responses of algal assemblages to-grazing.

The reproductive significance of siliceous cysts (statospores) produced by the common vernal chrysophyte Dinobryon cylindricum Imhof has been investigated under defined culture conditions. Three types of statospores have thus far been induced in culture: 1) uninucleate, asexual; 2) binucleate, asexual (potentially autogamic); 3) binucleate, sexual (zygotic). The production of each type of cyst responds differently to an array of nutrient deficiencies (P, N, vitamins, micrometals). An individual clone may be capable of participating in the production of all or only a subset of these types of resting cysts. All D. cylindricum statospores are morphologically identical regardless of their reproductive significance. Sexual reproduction leading to zygotic statospore formation is anisogamous, heterothallic, and involves a gametogenic hormone (erogen) that is apparently continuously released from female clones. Only a single bipolar mating group is documented here and clonal compatibility varies considerably within the mating group. The dynamics of encystment for each type of statospore has been determined relative to the growth of the vegetative cell population. Statospores may be produced either during the exponential phase (intrinsic encystment) or stationary phase (extrinsic encystment) of culture growth depending on the clones involved. The effect of both asexual and sexual resting cyst production on the net growth rate and dynamics of natural chrysophyte populations is discussed. Statospores appear to represent a more flexible reproductive strategy than the resistant zygospores produced by the other common groups of planktonic microalgae.

Simultaneous measurements of inorganic carbon fixation and phagotrophic particle uptake by Dinobryon in a metalimnetic algal abundance peak showed that this alga depended more strongly on ingested bacteria for nutrition than on photosynthesis. Measurements of the grazing rate at different depths in Lac Gilbert, Quebec, showed that the particle ingestion rate depends on water temperature rather than light availability. Phagocytosis of bacteria proceeded at a similar rate both day and night in most lakes where Dinobryon was found. Since other chrysomonad genera (Chlsophaerella, Uroglena, Catenochrysis, Ochromonas, Chromulina, and Chrysococcus) were also found to ingest particles, it will not be possible to estimate grazing on bacteria by counting nonpigmented cells and ignoring those containing chlorophyll. In oligotrophic Lac Bowker, 30% of the phytoplankton cells were actively ingesting small particles, and it was these phytoplankton and not the less numerous zooplankton that were responsible for most bacterial grazing in the lake. Phagotrophy by algae appears to be an important but unexpected pathway for energy flow in lakes. The flagellated Chrysophyceae, or chrysomonads, bear affinities to both plants and animals and have been claimed for study by zoologists as well as by botanists (Allen 1969). Coexistence of autoand heterotrophy in the chrysomonads and the widespread tendency toward complete loss of chlorophyll have fostered the suggestion that these algae may model progenitors of the Metazoa (Provasoli and Pintner 1953). The diversity and versatility of the group are strongly expressed in the genus Uchromonas, which is capable of phagotrophy, heterotrophy, photoheterotrophy, and photoautotrophy (Pringsheim 19 5 5; Aaronson and Baker 1959). However, Ochromonas is ’ This work was supported by a Natural Sciences and Engineering Research Council of Canada operating grant to J. Kalff and an NSERC postgraduate scholarship to D. Bird. A contribution to the McGill Limnology Research Centre, Lake Memphremagog Project. one of only a few chrysomonads to have been studied extensively in the laboratory, and the nutritional capabilities of other chrysomonads are virtually unknown, especially in their natural setting. Pascher (1943) observed under the microscope that the colonial chrysophycean alga Dinobryon could capture and digest small particles, and thus concluded that at least some part of this alga’s nutrition was heterotrophic. This unusual phagotrophic ability was not studied further until the recent discovery that several Dinobryon spp. are not only phagotrophic in nature, but can be major grazers of bacteria in lakes (Bird and Kalff 1986). Here we present further work on four different aspects of the capabilities of this alga: first, the relative importance of bacterivory and photosynthetic carbon fixation to Dinobryon’s nutrition; second, a comparison of day and night grazing rates; third, the relationship of grazing

A new species of Dinobryon is described from Australian freshwaters. It is characterized by distinctive, anteriorly-tapered unguentariform loricae grouped in compact colonies, and by the external attachment of daughter loricae.