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We measured the grazing and growth response of the mixotrophic chrysomonad flagellate Poterioochromonas malhamensis on four closely related picocyanobacterial strains isolated from subalpine lakes in central Europe. The picocyanobacteria represented different pigment types (phycoerythrin-rich, PE, and phycocyanin-rich, PC) and phylogenetic clusters. The grazing experiments were conducted with laboratory cultures acclimated to 10 µmol photon/m 2 /sec (low light, LL) and 100 µmol photon/m 2 /sec (moderate light, ML), either in the dark or at four different irradiances ranging from low (6 µmol photon/m 2 /sec) to high (1,500 µmol photon/m 2 /sec) light intensity. Poterioochromonas malhamensis preferred the larger, green PC-rich picocyanobacteria to the smaller, red PE-rich picocyanobacterial, and heterotrophic bacteria. The feeding and growth rates of P. malhamensis were sensitive to the actual light conditions during the experiments; the flagellate performed relatively better in the dark and at LL conditions than at high light intensity. In summary, our results found strain-specific ingestion and growth rates of the flagellate; an effect of the preculturing conditions, and, unexpectedly, a direct adverse effect of high light levels. We conclude that this flagellate may avoid exposure to high surface light intensities commonly encountered in temperate lakes during the summer.

Planktonic ciliates are major components of pelagic food webs in both marine and freshwaters. Their population dynamics are controlled 'bottom-up' by prey availability and 'top-down' by microcrustacean predators. In oceans, copepods are the main ciliate predators while in lakes cladocerans are the typical predators. The efficacy by which these functionally different predators control ciliate population dynamics is debated. We, therefore, investigated experimentally the grazing of three microcrustacean predators with different feeding modes on five freshwater ciliates. We then performed a meta-analysis to assess if our findings can be generalised for aquatic ecosystems. We hypothesized that top-down control is stronger in lakes than in the ocean. We find that: (i) average ingestion rates of marine and freshwater microcrustaceans do not differ; (ii) clearance rates of freshwater cladocerans decrease with ciliate size but increase with ciliate size in freshwater copepods; (iii) clearance rates of the marine microcrustaceans is unrelated to ciliate cell size. These findings have implications for the functioning of freshwater and marine food webs: (i) the ciliate—microcrustacean link is stronger in lakes than in the ocean, and (ii) globally top-down control of ciliates is unlikely in the ocean. [ABSTRACT FROM AUTHOR]

We combined profiling of the bloom-forming and potentially toxigenic cyanobacterium Planktothrix rubescens using a multiparameter probe equipped with a phycoerythrin sensor ( in vivo fluorometry, IVL) in Lake Mondsee, Austria, with flow cytometric live analyses of discrete samples taken from several depths in the upper 20 m of the water column. Results obtained by IVL and acoustic flow cytometry (AFC) were compared to microscopic analyses of integrated (0–21 m) water samples using fixed material. This comparison was made because the integrated samples are used for the Austrian monitoring programme according to the EU Water Framework Directive. We demonstrate that AFC provides quantitative analyses of the filaments of P. rubescens that are significantly correlated to IVF and microscopic analyses, allowing rapid (within hours) and more precise calculation of P. rubescens biomass than estimates derived from IVL. Our analysis shows that vertically integrated water samples provide unreliable information on the concentration of P. rubescens in the upper surface waters and on the peak concentration of P. rubescens within the water column. We conclude that the protocol that we developed is superior to the current monitoring practice. [ABSTRACT FROM AUTHOR]

We assessed long-term aquatic biodiversity patterns, with a focus on functional diversity (FD), by using sedimentary Chironomidae and Cladocera assemblages in four mountain lakes in the Austrian Alps. The objectives were to detect whether the invertebrate communities exhibited similar recent diversity trends in climate-sensitive alpine lakes, to observe how lake-specific limnological shifts affect biodiversity, and to test the relationship between taxonomic diversity (TD) and FD for paleolimnological biodiversity evaluations. We applied traditional TD indices to the sedimentary assemblages and used an FD index that was based on functional traits of the encountered taxa of both invertebrate groups. There were some similar long-term trends in invertebrate FD and TD among the lakes; e.g. chironomid diversity increased in two lakes and cladoceran diversity in three lakes toward the twentieth and twenty-first century, but in general the diversity patterns were variable between lakes. Overall, FD had a positive relationship with TD suggesting that functionality can be used in paleolimnological studies as a measure of biodiversity. However, cladoceran taxon richness did not correlate with FD in two lakes suggesting that TD is not always a surrogate for FD. Furthermore, TD of chironomids tended to increase more pronouncedly than FD in the top core sections indicating that increasing TD does not necessarily enhance FD and that different taxonomic entities may play similar functional roles in ecosystems. Accordingly, when evaluating biodiversity and ecological redundancy, functionality of the species pool should be carefully considered and highlighted also in paleolimnological research. [ABSTRACT FROM AUTHOR]

Abstract: Acidic mining lakes (pH <3) are specific habitats exhibiting particular chemical and biological characteristics. The species richness is low and mixotrophy and omnivory are common features of the plankton food web in such lakes. The plankton community structure of mining lakes of different morphometry and mixing type but similar chemical characteristics (Lake 130, Germany and Lake Langau, Austria) was investigated. The focus was laid on the species composition, the trophic relationship between the phago-mixotrophic flagellate Ochromonas sp. and bacteria and the formation of a deep chlorophyll maximum along a vertical pH-gradient. The shallow wind-exposed Lake 130 exhibited a higher species richness than Lake Langau. This increase in species richness was made up mainly by mero-planktic species, suggesting a strong benthic/littoral – pelagic coupling. Based on the field data from both lakes, a nonlinear, negative relation between bacteria and Ochromonas biomass was found, suggesting that at an Ochromonas biomass below 50μgCL−1, the grazing pressure on bacteria is low and with increasing Ochromonas biomass bacteria decline. Furthermore, in Lake Langau, a prominent deep chlorophyll maximum was found with chlorophyll concentrations ca. 50 times higher than in the epilimnion which was build up by the euglenophyte Lepocinclis sp. We conclude that lake morphometry, and specific abiotic characteristics such as mixing behaviour influence the community structure in these mining lakes. [Copyright &y& Elsevier]

Cell volume, growth and production rates of two common planktonic freshwater ciliates, the prostomatid Coleps spetai and the oligotrich Rimostrombidium lacustris, were investigated in clonal laboratory cultures. The clones were isolated from oligo-mesotrophic alpine Lake Mondsee, Austria, during summer and kept in culture with the small cryptophyte Cryptomonas sp. as food. All parameters investigated revealed significant clonal differences among both species. The extent of the clonal differences was comparable to differences observed earlier between similar planktonic ciliate species. The ecological relevance of varying clonal growth rates was evaluated using a simple numerical model. The model outcome suggests that differences in growth rates by 10% may significantly alter the clonal composition in the course of a ciliate peak in temperate lakes. The experimental results and the model outcome are discussed in the context of the functional diversity of freshwater ciliates. It is concluded that the morphospecies concept, which is the most widely used concept by both ciliate taxonomists and ecologists, may severely underestimate the ecological plasticity and the functional diversity of aquatic ciliates. [ABSTRACT FROM AUTHOR]

The abundance and photosynthetic activity of picocyanobacteria in the oligotrophic alpine lake Traunsee were measured at a station located close to the outlet of industrial soda waste and at a mid-lake reference station during spring, 1999 through to autumn, 2000. Picocyanobacterial numbers measured by flow cytometry in Traunsee (0.7–13.2 × 104 ml-1) were comparable to those of other oligotrophic lakes, and there was no significant difference between the contaminated and the reference sampling location. Picoplankton (<2 μm) photosynthetic rates measured in vitro by the 14C-technique were significantly reduced at the contaminated site relative to the reference station at low photosynthetically available radiation (10 μE m-2 s-1), while no difference between these two stations was found at moderately high light intensity (100 μE m-2 s-1). The investigation was complemented by laboratory experiments with cultured picocyanobacteria. Three Synechococcus spp. strains were exposed to water taken from either of the two Traunsee stations and from a control station located in neighbouring Attersee. Cell-specific photosynthetic activity measured by 4-h in vitro incubations revealed no significant difference among the three stations investigated. Growth rates of the same three Synechococcus spp. strains were measured by flow cytometry over several days in the laboratory. One strain, in particular, was sensitive to water taken from the contaminated site; growth rate of this strain was significantly reduced, relative to when exposed to water taken from the reference station. Taken together, our results demonstrate that picocyanobacteria are highly sensitive bioindicators of contaminant stress. The overall impact of the emissions from the industrial outlet on the picocyanobacteria was, however, relatively minor. [ABSTRACT FROM AUTHOR]

We describe single-cell and single-colony sorting protocols which allowed for rapid establishment of a diverse culture collection of clonal autotrophic picoplankton (APP) isolates originating from oligotrophic and oligo-mesotrophic subalpine lakes. Overall sort recoveries, expressed as the percentage of sorted microwells exhibiting APP growth, ranged from 5% to 17% depending on the type of APP, but the growth success varied greatly (from 0% to 68%) depending on the origin of the sorted sample. We applied two direct sequencing and two denaturing gradient gel electrophoresis (DGGE) protocols to identify and characterize the genetic purity of 21 of our picocyanobacteria cultures, namely, direct sequencing of the 16S rRNA gene and cpcBA-IGS region, and DGGE analyses involving a 194-bp fragment of the internal transcribed spacer (ITS) and a ca. 500-bp fragment of the phycocyanin (PC) operon (cpcBA-IGS, novel protocol described herein). Of those 21 picocyanobacteria cultures obtained by single-cell/single-colony sorting and subsequently characterized genetically/screened for genetic purity, only one culture was composed of multiple picocyanobacterial strains. [Copyright &y& Elsevier]