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Growing evidence suggests that global climate change promotes the dominance of mixotrophic algae especially in oligotrophic aquatic ecosystems. While theory predicts that mixotrophy increases trophic transfer efficiency in aquatic food webs, deleterious effects of some mixotrophs on consumers have also been reported. Here, using a widespread mixotrophic algal genus Dinobryon, we aimed to quantify how colonial taxa contribute to secondary production in lakes. We, therefore, studied the dietary effects of Dinobryon divergens on Cladocera (Daphnia longispina) and Copepoda (Eudiaptomus gracilis), representing two main taxonomic and functional groups of zooplankton. In feeding experiments, we showed that Dinobryon was largely grazing resistant and even inhibited the uptake of the high-quality reference food in Daphnia. Eudiaptomus could to some extent compensate with selective feeding, but a negative long-term food quality effect was also evident. Besides, Eudiaptomus was more sensitive to the pure diet of Dinobryon than Daphnia. Low lipid content and high C:P elemental ratio further supported the low nutritional value of the mixotroph. In a stable isotope approach analysing a natural plankton community, we found further evidence that carbon of Dinobryon was not conveyed efficiently to zooplankton. Our results show that the increasing dominance of colonial mixotrophs can result in reduced dietary energy transfer to consumers at higher trophic levels. In a wider perspective, global climate change favours the dominance of some detrimental mixotrophic algae which may constrain pelagic trophic transfer efficiency in oligotrophic systems, similarly to cyanobacteria in eutrophic lakes.

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.

Recent studies have revealed how the freshwater biota of Lake Baikal responds to climate change and anthropogenic impacts. We studied phyto- and zooplankton, as well as phyto- and zoobenthos, in the open coastal waters of the southern basin of the lake and of Listvennichny Bay. A total of 180 aquatic organism taxa were recorded. The response of the Baikal ecosystem to climate change can be traced by changes in the species composition of planktonic communities of the lake’s open coasts in summer. The key species were thermophilic the Anabaena lemmermannii P. Richt. (Fij = +0.7) blue-green algae, the Asplanchna priodonta Gosse (Fij = +0.6) rotifers in 2016, the Rhodomonas pusilla (Bachm.) Javorn. (Fij = +0.5) cold-loving algae, and the Cyclops kolensis Lilljeborg (Fij = +0.9) copepods in the past century. The proportion of Chlorophyta decreased from 63% to 17%, the Cyanophyta increased from 3% to 11% in the total biomass of phytoplankton, and the proportion of Cladocera and Rotifera increased to 26% and 11% in the biomass of zooplankton, respectively. Human activity makes an additional contribution to the eutrophication of coastal waters. The Dinobryon species, the cosmopolitan A. formosa and F. radians, dominated phytoplankton, and filamentous algae, Spirogyra, dominated at the bottom in the area with anthropogenic impact. The trophic level was higher than at the unaffected background site: the saprobity index varied from 1.45 to 2.17, the ratio of eutrophic species to oligotrophic species ranged from 1:2 to 3:1, and the ratio of mesosaprobiont biomass to endemics biomass ranged from 2:1 to 7:1. Currently, the boundaries of eutrophication zones of shallow waters in Lake Baikal are expanding, and its coastal zone has acquired features typical of freshwater bodies of the eutrophic type.

Seasonal variations in species composition and growth dynamics of phytoplankton in Uzuncayir Dam Lake (Tunceli, Turkey) were studied in relation to ecological factors by collecting horizontal water and phytoplankton samples for over a year. A total of 46 phytoplankton taxa were recorded, where Bacillariophyta was the most abundant in species number representing 24 taxa. Diatoms were the most conspicuous phytoplankton in terms of abundance as well as occurrence frequency. Charophyta, Chlorophyta, Cyanobacteria, Ochrophyta, Miozoa, and Euglenozoa were the least abundant. However, some Diatoms (Gyrosigma attenuatum, Hippodonta hungarica, Meridion circulare, Nitzschia clausii, Nitzschia frustulum, Pantocsekiella ocellata, Rhoicosphenia abbreviata), Charophytes (Cosmarium granatum, Mougeotiopsis calospora), Cyanobacteria (Leiblenia epiphytica, Planktothrix agardhii); Dinoflagellates (Ceratium hirundinella, Parvodinium africanum, Parvodinium inconspicuum, Peridinium cinctum), and Ochrophyta (Dinobryon pediforme) were also noticeable, with their occurrence in all seasons. Canonical correspondence analysis (CCA) and principal component analysis (PCA) were employed to analyze the relationship between occurrence and abundance of phytoplankton species as well as the ecological factors. The results showed that pH and the concentrations of NH4, NO3, and PO4 influenced seasonal occurrence and abundance of phytoplankton species in the lake. Uzuncayir Dam Lake may be classified as an oligotrophic lake with the mean value of nitrate 2.62 mg L-1 and phosphate 0.23 mg L-1 as supported by the occurrence of various phytoplankton groups. Findings related to pollution indices yielded that Uzuncayir Dam Lake is on the edge of the moderate state of organic pollution.

Harriman and Bear Mountain State Parks, New York, contain a rich and previously under-documented algal flora. We report 382 algal taxa (excluding diatoms), including 294 taxa not previously reported from the parks. Our inventory suggests that the algal flora has changed significantly since it was last studied in 1918 with a general increase in the number of desmid taxa (Zygnematophyceae) and changes in the species composition of other green algae. We hypothesize that these changes are primarily due to normal succession in constructed lakes. Some of the species encountered are infrequently reported from North America. Consequently Harriman and Bear Mountain State Parks represents an important locality for their study. Factors possibly contributing to the apparent rarity of species are discussed. A new species of Dinobryon (Chrysophyceae: Dinobryaceae) is described: Dinobryon sertularioides. Two new taxonomic combinations are also made: Stauridium tetras var. tetraodon (Chlorophyceae: Hydrodictyaceae) and Cosmarium verrucosum var. alatum (Zygnematophyceae: Desmidiaceae).

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.

This study assessed the water quality of Mengkuang Reservoir, Penang, Malaysia, by its trophic status according to a Carlson Modified Trophic Index (CMTSI) and by its biological parameters using Shannon–Wiener diversity index (H′) and saprobic index. The study conducted from August 2005 to July 2006 showed that mean values of CMTSI (nutrients), CMTSI (chlorophyll a) and CMTSI (Secchi depth) were 27.18 ± 8.73, 40.63 ± 7.12 and 41.74 ± 6.38, respectively. The mean values of CMTSI indicated that the reservoir was oligotrophic. Mean value of H′ was 2.15 bits/individual, showing that the water quality based on H′ value was in class III (slightly polluted). Saprobic index value (2.24) also revealed that the reservoir was moderately polluted (class II). The occurrence of Anabaena, Microcystis, Oscillatoria, Nostoc, Dinobryon, Chroococcus, Staurastrum paradoxum and Mallomonas which are indicators of toxic and polluted waters was also recorded. This study therefore showed the importance of phytoplankton composition and community structure as a reliable and important tool to assess the degree of pollution in Mengkuang Reservoir. Long-term assessments of biological and chemical parameters in the reservoir are necessary, and phytoplankton community structure as bioindicator provides unique information about the ecosystem. This information is potentially useful as an early warning sign of deteriorating condition and thus gives insight into the overall ecology of lakes and will assist in the future conservation and management of this lentic ecosystem.

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.

Detailed studies of the Chrysophyceae class of algae have not been undertaken in Serbia thus far. The golden algae usually occur during the winter and spring months. Chrysophyceae of the Zasavica River in Serbia were studied at two localities from December 2012 to Jun 2013. In our research, 26 taxa were recorded and the genus with the highest diversity was Mallomonas (15 species). The most abundant species during the whole study period were Synura uvella, Dinobryon divergens and Dinobryon sociale. At the beginning of summer, Chrysophyceae disappeared from the phytoplankton community. [Projekat Ministarstva nauke Republike Srbije, br. 176020 i br. III 43004]

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.

A significant outbreak of fishy odor occurred in a reservoir located in Inner Mongolia, China, in the winter of 2011, and the odor rating, algal density and concentrations of some potential odorous compounds were monitored over a period of two months. The peak odor rating of the fishy odor was 7 according to flavor profile analysis. Among the dominant algal species (two diatom and one chrysophyte species) observed during the survey, the chrysophyte Dinobryon sp. was the most abundant species, with the peak density recorded at 88,520 cells/mL. Seven potential algal metabolites including heptanal, 2,4-heptadienal, 2,4-decadienal, nonanal, 2-octenal, 2,6-nonadienal and hexanal were detected. The principal component analysis result showed that n-hexanal, n-heptanal and 2,4-decadienal, possibly the metabolites of diatoms, and 2,4-heptadienal, possibly the metabolite of Dinobryon sp., might have contributed to the fishy odor episode. This study demonstrated that the fishy odor episode in this reservoir might be caused by the abnormal growth of chrysophytes and diatoms under the ice-cover.

* 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.

Este artículo contiene 11 páginas, 2 tablas, 5 figuras, Many phototrophic flagellates ingest prokaryotes. This mixotrophic trait becomes a critical aspect of the microbial loop in planktonic food webs because of the typical high abundance of these flagellates. Our knowledge of their selective feeding upon different groups of prokaryotes, particularly under field conditions, is still quite limited. In this study, we investigated the feeding behavior of three species (Rhodomonas sp., Cryptomonas ovata, and Dinobryon cylindricum) via their food vacuole content in field populations of a high mountain lake. We used the catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) protocol with probes specific for the domain Archaea and three groups of Eubacteria: Betaproteobacteria, Actinobacteria, and Cytophaga-Flavobacteria of Bacteroidetes. Our results provide field evidence that contrasting selective feeding exists between coexisting mixotrophic flagellates under the same environmental conditions and that some prokaryotic groups may be preferentially impacted by phagotrophic pressure in aquatic microbial food webs. In our study, Archaea were the preferred prey, chiefly in the case of Rhodomonas sp., which rarely fed on any other prokaryotic group. In general, prey selection did not relate to prey size among the grazed groups. However, Actinobacteria, which were clearly avoided, mostly showed a size of 0.5 m, markedly smaller than cells from the other groups. I, This research was supported by the Spanish Ministry of Science project ECOFOS (CGL2007-64177/BOS) with a predoctoral scholarship to M.A.B., Spanish Ministry of the Environment project EGALA (OAPN-124/2010), and a Generalitat de Catalunya research grant (SGR 2014-1249).

Liuxihe reservoir is a deep, monomictic, oligo-mesotrophic canyon-reservoir in the subtropical monsoon climate region of southern China. Phytoplankton functional groups in the reservoir were investigated and a comparison made between the succession observed in 2008, an exceptionally wet year, and 2009, an average year. The reservoir shows strong annual fluctuations in water level caused by monsoon rains and artificial drawdown. Altogether 28 functional groups of phytoplankton were identified, including 79 genera. Twelve of the groups were analyzed in detail using redundancy analysis. Because of the oligo-mesotrophic and P-limited condition of the reservoir, the dominant functional groups were those tolerant of nutrient (phosphorus) deficiency. The predominant functional groups in the succession process were Groups A (Cyclotella with greatest axial linear dimension10 μm), B (Cyclotella with greatest axial linear dimension10 μm), LO (Peridinium), LM (Ceratium and Microcystis), E (Dinobryon and Mallomonas), F (Botryococcus), X1 (Ankistrodesmus, Ankyra, Chlorella and Monoraphidium) and X2 (Chlamydomonas and Chroomonas). The development of groups A, B and LO was remarkably seasonal. Group A was dominant during stratification, when characteristic small size and high surface/volume ratio morphology conferred an advantage. Group LO was dominant during dry stratification, when motility was advantageous. Group B plankton exhibited a high relative biomass during periods of reduced euphotic depth and isothermy. Groups LM, E, F, X1 and X2 occasionally exhibited high relative biomasses attributable to specific environmental events (e.g. drawdown, changes in zooplankton community). A greater diversity of phytoplankton functional groups was apparent during isothermy. This study underscores the usefulness of functional algal groups in studying succession in subtropical impoundments, in which phytoplankton succession can be significantly affected by external factors such as monsoonal hydrology and artificial drawdown, which alter variables such as retention time, mixing regime and thermal structure and influence light and nutrient availability.

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.

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.

Here we introduce a method for quantitative analysis of planktonic protists and microalgae from preserved field samples combining morphological and small-subunit (SSU) rRNA gene sequence analysis. We linked a microscopic screening with PCR of single cells using field samples preserved with Lugol's iodine solution. Cells possessing a rigid cell wall were incubated with Viscozyme and subsequently with proteinase K for cell disruption; this was unnecessary for fragile cells. The addition of sodium thiosulfate to the PCR tube considerably decreased the inhibiting effect of the fixative (iodine) on the PCR and thus allowed for successful single-cell PCR even of long DNA fragments (up to as many as 3,000 base pairs). We further applied the protocol to investigate the dominant SSU rRNA genotypes in distinct flagellate morphospecies originating from different samples. We hypothesized that despite the morphological similarity, protist morphospecies in different habitats or sampled during different seasons are represented by different genotypes. Our results indicate species-specific differences: the two species Ochromonas sp. and Dinobryon divergens were represented by several different genotypes each, and for the latter species, the dominating genotype differed with habitat. In contrast, Dinobryon pediforme , Dinobryon bavaricum , and Synura sphagnicola were exclusively represented by a single genotype each, and the respective genotype was the same in different samples. In summary, our results highlight the significance of molecular variation within protist morphospecies.

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.

We studied the phytoplankton seasonality in the subarctic Lake Saanajarvi, Finnish Lapland, in two successive years with slightly different weather conditions. The total number of taxa studied during the period was 148. Characteristic phytoplankton species were chrysophytes Uroglena sp., Chrysococcus spp., Dinobryon spp and diatoms Cyclotella spp. The results were analysed in relation to weather patterns and physico-chemical variables measured from the lake during the 2 years. The seasonal dynamics of phytoplankton were characterized by (1) maxima in total densities during autumn and minima in winter; (2) different species reaching maximum and minimum densities during different seasons; (3) close to equilibrium state during strong thermal stratification in 1997 with dominance of only a few taxa; and (4) two annual maxima in species diversity at the beginning of the thermal stratification and during the autumn overturn. According to canonical ordinations, calcium buffer capacity, nutrients and temperature all play a role in regulating algal biomass and species compositions. With regard to physical factors, the length of the mixing cycle, thermal stability of the water column and water temperature seem to have a major control over the plankton dynamics. The length of the ice-free season seems to be more decisive for biomass production than the thermal stability during this period, which, in turn, appears to affect the algal biodiversity.

Species diversity and richness, and seasonal population dynamics of phytoplankton, planktonic protozoa, and bacterioplankton sampled from the epilimnion of Crystal Bog in 2000, were examined in order to test the hypothesis that these groups' diversity and abundance patterns might be linked. Crystal Bog, a humic lake in Vilas County, Wisconsin, is part of the North Temperate Lakes Long-Term Ecological Research Site. Phytoplankton and planktonic protozoa were identified and enumerated in a settling chamber with an inverted microscope. Bacterial cells were enumerated with the use of fluorescence 4', 6'-diamidino-2-phenylindole (DAPI)-staining procedures, and automated ribosomal intergenic spacer analysis (ARISA) was used to assess bacterioplankton diversity. Bacterial cell counts showed little seasonal variation and averaged 2.6 x 10(6) cells/mL over the ice-free season. Phytoplankton and planktonic protozoan numbers varied by up to two orders of magnitude and were most numerous in late spring and summer. Dinoflagellates largely dominated Crystal Bog throughout the ice-free period, specifically Peridiniopsis quadridens in the spring, Peridinium limbatum in summer, and Gymnodinium fuscum and P. quadridens in fall. Brief blooms of Cryptomonas, Dinobryon, and Synura occurred between periods of dinoflagellate domination. The dominant dinoflagellate, Peridinium limbatum, was calculated to have a growth rate of 0.065 day(-1) and a doubling time of 10.7 days. Heterotrophic nanoflagellates (HNFs) were a consistent component of the planktonic protozoa; seasonal patterns were determined for three genera of HNFs (Monosiga, Bicosoeca, and Desmarella moniliformis). Three genera of ciliates (Coleps, Strobilidium, and Strombidium) comprised the greater part of the planktonic protozoa in Crystal Bog. The number of species of planktonic protozoa was too low to calculate a diversity index. Shannon-Weaver diversity indices for phytoplankton and bacterioplankton in the epilimnion followed very similar seasonal patterns in this lake, supporting the hypothesis that in freshwaters, diversity patterns of these groups are linked.

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.

Photosynthesis and microbial plankton dynamics of two lakes in the Kongsfjorden catchment of Spitzbergen (Svalbard Archipelago, 78°N) were investigated during the summer of 2000. One of the lakes (Tvillingvatnet) served as the water supply for the village of Ny Alesund. The other was a deeper, larger unnamed lake in the Ossian Sarsfjella reserve—named Lake OS in this study. Both lakes can be classified as oligotrophic on the basis of chlorophyll a and inorganic nutrient concentrations. Chlorophyll a concentrations ranged between 0.33 and 1.65 µg l-1 in Tvillingvatnet and 0.2 and 0.3 µg l-1 in Lake OS. The phytoplankton was dominated by chrysophytes and cryptophytes, with the diatom Rhizosolenia and a number of dinoflagellate species. Rates of photosynthesis were typically low, between 24.5 µg and 1.0 mg l-1 day-1 in Tvillingvatnet (photosynthetic efficiency 0.004–0.26), and between 3.1 and 29.5 µg l-1 day –1 in Lake OS (photosynthetic efficiency 0.0019–0.0085). Among the phytoflagellates (PNAN) there were a number of mixotrophs which reached their peaks of abundance before other PNAN. Mixotrophy appeared to provide a competitive advantage. Grazing rates for the mixotroph Dinobryon ranged between 0.063 and 1.12 pg C cell-1 day-1 in Lake OS and between 0.22 and 1.11 pg cell-1 day-1 in Tvillingvatnet, with rates increasing between July and August. However, Dinobryon removed less than 1% of bacterial biomass day-1, while the heterotrophic nanoflagellates (HNAN) removed up to 28% of bacterial biomass day-1. Bacterial concentrations were low, with a maximum of 28.8×108 l-1 in Tvillingvatnet and 23.6×108 l-1 in Lake OS. Ciliated protozoan and rotifer diversity in the plankton was low.

A year-long survey of Lough Bunny, a karst lake in County Clare, Ireland, was undertaken in 1992-3. Water transparency (as determined by Secchi disc depth), plant nutrients, chlorophyll a concentrations and phytoplankton were examined. High water transparency indicated that the euphotic zone extended over the majority of the lake and may, at times, have reached the deepest parts of the lake. Silicate-silicon and nitrate + nitrite-nitrogen showed marked seasonal changes with lowest values recorded in May 1992 and August 1992 respectively. Molybdate reactive phosphate-phosphorus showed no seasonality in concentration and was often below the limit of detection of the technique used. The maximum value obtained was 3.9tg P04-P dm3. Chlorophyll a concentrations were also low throughout the year, ranging from 0.5pg dm -3 to 2.6gg dm -3, and these also showed no seasonal pattern of change. Cryptophyta were present in all samples and were often the most numerous group in the phytoplankton. Small dinoflagellates occurred throughout the year, with Ceratium species appearing in the summer months. Cyanobacteria were common and most abundant in late autumn. Dinobryon species were found throughout the year but mostly as single thecate cells and only occasionally as large arbuscular colonies. Diatoms were not well represented in the plankton. The Chlorophyta showed greatest diversity of species but were never abundant. Quantitative counts showed cell numbers in all groups to be low, which is reflected in the low chlorophyll a concentrations. On this evidence it is suggested that the lake is oligotrophic. A comparison with two published lists of the lake's plankton was made. There are extensive beds of Charophyta in the lake. A list of species is given. We suggest that utihisation of plant nutrients by the charophytes and benthic diatoms may be responsible for the water clarity and small phytoplankton populations. C. Pybus (corresponding author; email: colin.pybus@gmit.ie)

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.

Protists in the central Arctic Ocean are adapted to the harsh environmental conditions of its various habitats. During the Polarstern cruise ARK-XXVI/3 in 2011, at one sea-ice station, large aggregates accumulated at the bottom of the melt ponds. In this study, the protist assemblages of the bottom layer of the sea-ice and melt-pond aggregate were investigated using flow cytometry and 454-pyrosequencing. The objective is to provide a first molecular overview of protist biodiversity in these habitats and to consider the overlaps and/or differences in the community compositions. Results of flow cytometry pointed to a cell size distribution that was dominated by 3–10 µm nanoflagellates. The phylogenetic classification of all sequences was conducted at a high taxonomic level, while a selection of abundant (≥1% of total reads) sequences was further classified at a lower level. At a high taxonomic level, both habitats showed very similar community structures, dominated by chrysophytes and chlorophytes. At a lower taxonomic level, dissimilarities in the diversity of both groups were encountered in the abundant biosphere. While sea-ice chlorophytes and chrysophytes were dominated by Chlamydomonas/Chloromonas spp. and Ochromonas spp., the melt-pond aggregate was dominated by Carteria sp., Ochromonas spp. and Dinobryon faciluiferum. We suppose that the similarities in richness and community structure are a consequence of melt-pond freshwater seeping through porous sea ice in late summer. Differences in the abundant biosphere nevertheless indicate that environmental conditions in both habitats vary enough to select for different dominant species. Keywords: 18S rDNA; Arctic Ocean; biodiversity; next-generation sequencing; flow cytometry To access the supplementary material for this article, please see supplementary files in the column to the right (under Article Tools) (Published: 5 November 2014) Citation: Polar Research 2014, 33 , 23466, http://dx.doi.org/10.3402/polar.v33.23466

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

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.

Morphological observations of the siliceous cell coverings of five taxa of Synurophyceae are made based on specimens collected from acidic, seepage lakes situated in either the Ocala National Forest, Florida, U.S.A. or the Cape Cod National Seashore, Massachusetts, U.S.A. All but one of the lakes, Lake Dorr, were clearwater in nature. A new taxon, Mallomonas corymbosa var. poseidonii var. nov., is described based on differences in bristle structure. New morphological features of four additional taxa, Synura echinulata, Mallomonas mangofera f. foveata, M. acaroides var. muskokana, and M. duerrschmidtiae are presented and discussed in relation to their existing taxonomic descriptions. Other taxa of Synurophyceae, Dinobryon spp., Aulacoseira spp., Asterionella ralfsii, and dinofiagellates were common constituents of the phytoplankton from the seven study lakes at the times of collection.

Examination of 17 samples collected by a 20 ?m meshed meshed net in Kongsfjorden, Svalbard, 8–19 July 1988, showed a dominance of dinoflagellates and the chrysophyte Dinobryon Balticum in the surface layers, whereas the diatom and the haptophyte Phaecystis pouchetii abundance increased with depth. The diatom Pseudo-nitzschia granii appeared together with P. pouchetii through the whole water column, and Actinocyclus curvatulus was one of the few diatoms present also in the surface samples. Two samples, from 15 and 50 m, respectively, were cleaned of organic material and mounted in Naphrax for a more critical identification of the diatoms. We were able to group the species according to habitats, especially types of ice. The planktonic Thalassiosira antarctica var. borealis, T. hyalina, T. nordenskioeldii, Bacterosira bathyomphaia, Chaetoceros furcellatus, C. socialis and Fragilariopsis oceanica were present mainly as resting stages representing a post-bloom situation. These species and T. gravida appear early in the season and may have started to grow already under the ice. Fragilariopsis cylindrus and F. oceanica seem to have a closer affinity to ice than Thalassiosira and Chaetoceros spp. although they are common in the plankton. Some Nitzschia species which are usually regarded as typical sea-ice diatoms and have thicker and older ice as the main habitat were present only in small cell numbers in the plankton samples. The last component, evidently introduced from Atlantic water in the Norwegian Sea, consisted of diatoms with a more oceanic distribution, e. g. Fragilariopsis pseudonana and a small form of Thalassiosira bioculata.

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 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.

Developmental and seasonal changes in the preferred prey and the diet composition of the invertebrate predator Chaoborus punctipennis were determined in Plastic Lake, an acidified (pH 5.6) lake in south-central Ontario, Canada. All instars consumed rotifers (mainly Keratella cochlearis, Ploesoma sp., and Asplanchna priodonta), and instars III and IV fed preferentially on crustaceans (mainly bosminids and copepods). Phytoflagellates (Peridinium sp. and Dinobryon sp.), however, numerically dominated the diet of all instars examined (II–IV), and were consumed by instar II larvae in excess of their relative availability. On 40 and 20% of the sampling dates, instars III and IV, respectively, consumed phytoflagellates in accordance with their relative abundance in the lake. Although the contribution of phytoflagellates to the biomass-based diet of C. punctipennis was low, on one occasion phytoflagellates formed almost half of the diet biomass of instar II larvae. A review of the literature shows that in lakes where phytoflagellate densities are high (≥ 100–200/mL), phytoflagellates contribute ≥ 50% of the diet biomass of all instars of Chaoborus spp. These findings indicate that Chaoborus spp. are omnivores that frequently feed on phytoflagellates even when alternative animal prey are abundant. Consumption of phagotrophic phytoflagellates by Chaoborus spp. and other large invertebrate omnivores, such as Mysis spp., Epischura spp., and cyclopoid copepods, may increase the transfer efficiency of organic carbon from the microbial food web to the upper trophic levels in fresh waters. In acidified lakes, consumption of large dinoflagellates by Chaoborus spp. and other invertebrate omnivores may ameliorate the hypothesized bottleneck impeding the flow of carbon between phytoplankton and zooplankton.

Following the U.S. Environmental Protection Agency's specifications for pesticide registration tests, 12 experimental ponds were constructed. Colonization and succession of the pond phytoplankton communities were investigated during the first year after filling. Taxon richness and densities and biomass as chlorophyll-a were measured. In addition, water-quality data were collected and analyzed. The dominant taxonomic groups were the Cyanophyceae, Chlorophyceae (with Desmidiaceae dominating), Dinobryon (in the Chrysophyceae), Dinophyceae, and Bacillariophyceae. Similar successional patterns in all 12 ponds occurred; however, the community structure between ponds was not similar at any time. Although the ponds had statistically similar environmental characteristics, they varied in their community structure, indicating that after one year they were not mature enough for use as replicate test systems. The heterogeneity of the pond phytoplankton communities indicates that meso-cosms need to be managed to produce replicate experimental units.

In enclosure and whole-lake experiments, I tested whether life-form shift (recruitment) is a passive process induced by turbulence, a seasonal phenomenon, or a behavior that can be induced by alterations in environmental variables. The number of algal cells recruited from the sediment varied considerably during the experimental period. The most important migrating genera in this study were: Cryptomonas, Dinobryon, Gonyostomum, Gymnodinium, Peridinium, and Synura. An obvious conclusion is that it is not the same factor in each case that causes life-form shift, but that different triggering factors operate in different algal species. Turbulence and temperature were similar in all treatments and therefore did not cause the considerable fluctuations and trends in algal recruitment in the enclosures. This suggests that life0form shift is not a passive process driven by wind and temperature-induced currents. In the enclosure experiment, alterations in the light régime explained a major part (up to 53%) of the variation in recruitment for most genera. For Gymnodinium this was corroborated in the whole-lake experiment, where the depth of the euphotic zone explained 41% of the variation in recruitment. For Cryptomonas, however, 64% of the variation in recruitment was explained by the depth of the "oxycline", whereas 52% of the variation in recruitment of Synura were explained by the depths of the euphotic zone and the oxyline. Peridinium pusillum and P. wisconsinense showed low recruitment at high zooplankton abundance and high recruitment at low zoo-plankton abundance in the lake experiment, as well as in the enclosure experiment. Thus, the hypothesis that the presence of grazers can induce shifts in behaviour of some algal groups cannot be rejected.

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.

Samples of surface plankton in Lake Nojiri were collected monthly from July 1988 to October 1991.In zooplankton 26 species were found and 25 species in phytoplankton. The seasonal succession of main species was as follows.Bosmina longirostrisobserved through the year, but dicreased in winter.Diaphanosoma brachyrum, Bosminopsis deitersi, Daphnia galeata, Eodiaptomus japonicusandPolyphemus pediculus appearedfrom summer to autamn, andThermocyclops hyalinusfrom spring to summer. On the other hand, Kellicottia longispina, Polyarthra vulgarisand Keratella quadratawere found from winter to spring.Ceratium hirundinella, Melosira italics, Cyclotella comtaandFragilaria crotonensiswere obsreved through the year.Asterionella formosaappeared three season except in autumnThe prevalent species wereKeratella quadratain autumn, Ceratium hirundinellafrom spring to autumn, Dinobryon cylindricumin spring, Fragilaria crotonensisfrom autumn to early winter, Melosira italicaandAsterionella formosain winter.In this Lake, Uroglenainduced a fresh water red tide from June to July in 1989. but never appeared again.Uroglena americanaobserved twice in a year from the middle of spring to the middle of summer and from the middle of autumn to winter.