Ciliate Abundance Research Articles

Ecological assessment of an algaecidal naphthoquinone derivate for the mitigation of Stephanodiscus within a mesocosm

The novel eco-friendly algaecidal naphthoquinone derivate was used to control harmful algal bloom causing species Stephanodiscus and, its effect was assessed on other undesired and non-targeted microbial communities. We conducted a mesocosm experiment to investigate the effects of this novel algaecide on native microbial communities rearing in water collected from Nakdonggang River. Upon treatment of the mesocosm with the naphthoquinone derivate the concentration of Chl-a decreased from 20.4 μg L−1 to 9.5 μg L−1 after 2 days. The turbidity has also shown decrement (exhibited 15.5 NTU on the 7th day). The concentrations of DOC and phosphate in the treatment were slightly higher than those in the control due to the decomposition of dead Stephanodiscus, whereas the DO and pH in the treated condition were slightly lower than those in the control; which was due to increment of organic acids and higher degradation activity. Results showed that bacterial abundance were not significantly different but community composition were slightly different as revealed by NGS (Next generation sequencing). The variation in HNF (Heterotrophic nanoflagellates) revealed that the bacterial community composition changed following the change in bacterial abundance. During the treatment, the abundance of Stephanodiscus was significantly reduced by more than 80% after 6 days, and the abundance of ciliates and the dominant species, Halteria grandinella, had shown marked decline. The abundance of zooplankton sharply decreased to 5 ind. L−1on the 8th day but increased again by the end of the study period. The Shannon-Wiener diversity index of phytoplankton, ciliates and zooplankton in the treated mesocosm increased significantly after 4, 7 and 8 days, respectively. The marked changes in the ecosystem structure were observed in treatment compare to control. However, the beneficial microalgal populations were not affected which indicated possibility of restoration of treated ecosystem and regain of healthy community structure after certain period.

Spatial and seasonal variations in the planktonic ciliate community and its relationship with environmental factors in Daya Bay, the South China Sea

Abstract Planktonic ciliate composition, abundance and its response to environmental change were investigated during four seasons (winter of 2013, spring, summer and autumn of 2014) in Daya Bay, the South China Sea. A total of 41 species belonging to eight orders were identified, 14 of which were dominant. Planktonic ciliate communities showed a distinct seasonal pattern in ciliate abundance and a clear seasonal shift in the taxonomic composition. The largest number of ciliate species occurred in summer, whereas the highest abundance peaked in spring, mainly due to oligotrichids. In terms of spatial distribution, ciliate species were abundant in the area of artificial reefs, and ciliate abundance was higher in the Dapeng Cove aquaculture area and lower at the Daya Bay Nuclear Power Station. Clustering analysis demonstrated that the seasonal variations of the ciliate community structure were more obvious than spatial variations. Multivariate and univariate analyses illustrated that ciliate abundance was significantly correlated with the water nutrient level and chlorophyll-a concentration, temperature, salinity and dissolved oxygen. Moreover, the dominant abiotic environmental factors affecting the spatial pattern of ciliate communities varied between seasons.

Effects of copepods on natural microplankton communities: do they exert top-down control?

Top-down effects in the pelagic realm are quite well known in freshwater ecosystems. However, our knowledge of these effects in the ocean remains scant. It is known that copepods prefer to prey on ciliates and heterotrophic dinoflagellates, and their high or low abundances can change the structure of microplankton communities. Field studies in subtropical waters have shown parallel increases of mesozooplankton and phytoplankton without a lag, suggesting a top-down effect of mesozooplankton preying upon microzooplankton and releasing primary producers from predation. In the present work, we added copepods at increasing densities to natural plankton in 24 h experiments. A decrease in aloricated ciliates abundance of nearly 50% and increases in the abundances of picoeukaryotes, Synechococcus, Prochlorococcus, diatoms, and chlorophyll a were observed. No effect of nutrient additions was observed in parallel grazing experiments. Thus, a top-down effect of copepods upon microzooplankton explains the observed changes in the abundance of the different phytoplankton groups. Copepods promote important changes down the food web, structuring the community by predation upon microzooplankton. There are biogeochemical consequences of zooplankton variability over short time scales in the ocean.

Response of soil ciliate to diammonium phosphate addition in alpine meadow of southern Gansu, Northwest China

In typical alpine meadow of southern Gansu Province, five sampling sites by fertilizer diammonium phosphate (DAP) addition ranging from 0 g·m-2 to 120 g·m-2 were set in order to examine the response of soil ciliate communities to different concentrations of DAP. An investigation on the abundance and species richness of soil ciliates was carried out by non-flooded Petri dish, observation in vivo and three-level ten-fold dilution methods. Soil physicochemical factors including soil water content, pH value, contents of soil total nitrogen, total phosphorus and organic carbon at different sites were also measured. It was found that there was a total of 129 species belonging to 9 classes, 17 orders, 31 families and 46 genera. Lower species richness and diversity but higher ciliate abundance were observed in the soils with higher concentrations of DAP compared with the control. The dominant group Haptorida was substituted by Colpodida from the control to the soils with the increase of fertilizer application. The results of redundancy analysis indicated that the gradient changes of total phosphorus, soil temperature and water content were the main driving factors for the change of soil ciliate community distribution.

Predator trait evolution alters prey community composition

AbstractPredators have potentially strong effects on prey abundance, growth, life histories, and behavior, but the net direction and strength of these effects depend on traits in the predator population. Trait evolution that occurs on ecological timescales is common and may affect species interactions. Using laboratory microcosms of a bacterivorous ciliate (Colpidium sp.) and a diverse community of bacteria, we tested the hypothesis that predator trait evolution in response to temperature variability and nutrient concentration alters the composition of the prey community using a two‐phase experiment. The first phase consisted of a selection experiment, in which microcosm communities were randomly assigned to one of eight fully factorial treatments of high or low nutrients, variable or stable temperatures, and with or without Colpidium. The second phase was a reciprocal transplant experiment, in which a subsample of each microcosm community from the selection experiment was transferred to one of four common garden environments, consisting of all combinations of high or low nutrients and variable or stable temperature. Nutrient enrichment directly decreased bacterial species richness and altered the community composition, but also indirectly increased bacterial species richness through decreases in ciliate peak density and, subsequently, ciliate abundance. Moreover, the evolutionary effects of nutrient enrichment on predator traits were more pronounced in some contemporary common garden environments, and undetectable in others. Our findings suggest that historical and contemporary environments are equally important to consider in understanding how trait evolution affects community structure.

Correlation between some environmental variables and abundance of <i>Almophrya mediovacuolata</i> (Ciliophora: Anoplophryidae) endocommensal ciliate of an anecic earthworms (Oligochaeta: Annelida) in Bambui (North-West Cameroon)

This study was devoted to accessing the influence of some soil physico-chemical parameters on the abundance of Almophrya mediovacuolata Ngassam, 1983, astome ciliate of the digestive tract of earthworms (EW) of the species Alma nilotica collected along “Fa’a ntsa” stream in Bambui. The survey primarily involved soil samples collection from the same spots of EW collection and preparation for physico-chemical analysis; evaluation in situ of the volumic density (VD) of worms (number /dm3 soil), their dissection, isolation and counting of ciliates with respect to different portion of EW’s gut (fore, mid and hindgut). Furthermore, correlation analysis between soil physico-chemical parameters and biological responses (EW volumic density and ciliate abundance) were performed. The results reveal that EW abundance was positively and significantly correlated with the following physico-chemical parameters: Cation Exchange Capacity CEC (p <0.01) and Mg2+ (p <0.05). A positive and significant correlation was found between Almophrya mediovacuolata and the pH of KCl in the foregut and midgut (p <0.01) while a negative and significant correlation was found between the abundance of Almophrya mediovacuolata and Ca2+ in the foregut (p <0.05). Almophrya mediovacuolata were found mostly in the foregut. This result shows that each portion of the digestive tract of Alma nilotica can be considered as a set of natural microhabitat in which a number of physico-chemical factors generate ecological niches suitable for the survival of different species of microorganisms among which ciliated protozoa. The fore and midgut was noticed to be the preferential zones of Almophrya mediovacuolata.Keywords: Biotic parameters, ciliated protozoa, microhabitat, soil physico-chemical parameters.

Recovery of ciliated protozoan communities in response to environmental change in a shrimp-farming pond in southern China

The temporal dynamics of ciliate community structure in a southern Chinese shrimp aquaculture facility were investigated during the period June–September 2012. A total of 53 species belonging to 37 genera and 17 orders were recorded based on analyses of eight samples. Ciliate abundance peaked between 16 August and 14 September 2012, while the maximum number of species occurred on 26 June 2012. Clear temporal patterns were observed in the ciliate community structure. The patterns of succession of the 10 most abundant species were consistent with the results of a Canonical Analysis of Principal coordinates (CAP) analysis. Correlation analyses showed that these patterns of succession were related to temporal changes in environmental variables. In summary, the results demonstrate that the ciliate community responds predictably to environmental variations and recovers from shrimp cultivation.

Distribution of Ciliates in Intertidal Sediments across Geographic Distances: A Molecular View

The estimation of the diversity and geographic distribution of protists in particular ciliates has long been an ongoing debate. We estimated the distribution of ciliates in intertidal sediments with geographic distance ranging from 10cm to 1,000km, using high-throughput DNA sequencing. Higher community similarity, in particular that of rare ciliate taxa, occurred between replicates than between sites. Statistical analyses showed that the community dissimilarities of both abundant and moderately abundant ciliates had correlations with geographic distance at the scales of 50m, 200km and 1000km, indicating a distance-decay relationship. No significant correlation was observed for rare taxa. The analysis of Bray-Curtis similarity showed a higher pairwise community similarity in abundant taxa than in moderately abundant taxa, and that in rare taxa was the lowest. Abundant taxa usually can disperse in a wider range than rare taxa, though their distribution is restricted by distance to some extent. Rare taxa are potentially more sensitive to changing environments, but no distance-decay relationship could be observed. The data indicate contrasting patterns of geographic distribution of the abundant and rare ciliate taxa and a weak distance-decay relationship for relatively abundant taxa at a scale over 50m.

The Impacts of Crustacean Zooplankton on a Natural Ciliate Community: a Short-term Incubation Experiment

Direct and indirect effects of crustacean zooplankton (cladocerans and copepods) are important regulators of ciliate communities, especially in eutrophic systems. However, it is not clear whether pseudodiaptomids (e.g., Schmackeria), one of the dominant calanoid copepods in Chinese lakes, effectively impacts natural ciliate communities. The impacts of small-bodied cladocerans (e.g., Bosmina) on ciliates are also controversial. We performed an incubation experiment using winter lake water from Lake Chaohu to assess the structuring effects that crustacean zooplankton have on natural ciliate populations. The presence and absence of cladocerans (Bosmina sp.) and copepods (Schmackeria inopinus) were alternated in four treatments. Both Bosmina sp. and Schmackeria inopinus had substantial impacts on ciliate abundance, biomass, and community structure. The response of ciliates was different in the presence of Bosmina sp. compared with Schmackeria inopinus and varied among categories such as the ciliate population, relative body size and functional feeding group. Our results also highlight the importance of interference and exploitative competition among metazooplankton groups.

Algicidal activity of the bacterium, Pseudomonas fluorescens SK09, to mitigate Stephanodiscus hantzschii (Bacillariophyceae) blooms using field mesocosms

ABSTRACTThe main goal of this study was to characterize mortality and trophic interactions of Stephanodiscus hantzschii (small centric diatoms) in the presence of an algicidal bacterium, Pseudomonas fluorescens HYK0210-SK09 (SK09) in outdoor mesocosm experiments with an extremely low temperature and also in direct application in a river. SK09 at an inoculation density of 5 × 106 cells/mL (Experiment 1) and SK09 cells immobilized in activated carbon polyvinyl alcohol sponge carriers (Experiment 2) did not appear to attack S. hantzschii in a species-specific manner at temperatures close to freezing. Furthermore, SK09 did not control the diatom blooms when SK09 cells immobilized in the carriers were directly applied in the river. The dynamics of both the heterotrophic flagellate and ciliate abundances showed similar trends to the activity of algicidal bacteria; moreover the heterotrophic protists could not control the total bacterial density. Inorganic nutrient concentrations and conductivity remained unchanged compared with those in the control group. The results obtained in this study and previous results are important for assessing biological control using algicidal bacteria in the wild.

Short-term Influence of Drilling Fluid on Ciliates from Activated Sludge in Sequencing Batch Reactors.

Spent drilling muds are the liquid residues of rock drilling operations. Due to a high concentration of suspended solids and potentially detrimental chemical properties, they can negatively affect microorganisms participating in wastewater treatment processes. We evaluated the addition of a potassium-polymer drilling fluid (DF) to activated sludge in laboratory sequencing batch reactors (SBRs) for municipal wastewater treatment. Ciliate assemblage, the most dynamic component of eukaryotes in activated sludge, and which is highly sensitive to changes in the system, was evaluated. The average ciliate abundance dropped by about 51% (SBR 2; 1% DF added) and 33% (SBR 3; 3% DF added) in comparison to the control (SBR 1; wastewater only). A decrease in the total number of ciliate species during the experiment was observed, from 25 to 24 in SBR 2 and from 17 to 13 in SBR 3. Moreover, a drop in the number of dominant (>100 individuals mL) ciliate species was observed during the experiment-from eight in the control to five in SBR 2 and four in SBR 3-signaling noticeable changes in the quantitative structure of ciliate species. The species analyzed showed different responses to DF addition. The most sensitive was , which is bacteriovorus. In contrast, two predators, and , showed no reaction to DF addition. Our results indicate that addition of potassium-polymer DF, in doses of 1 to 3% of the treated wastewater volume, had no toxic effects on ciliates, but qualitative and quantitative changes in their community were observed.

Low CO2 Sensitivity of Microzooplankton Communities in the Gullmar Fjord, Skagerrak: Evidence from a Long-Term Mesocosm Study.

Ocean acidification is considered as a crucial stressor for marine communities. In this study, we tested the effects of the IPCC RPC6.0 end-of-century acidification scenario on a natural plankton community in the Gullmar Fjord, Sweden, during a long-term mesocosm experiment from a spring bloom to a mid-summer situation. The focus of this study was on microzooplankton and its interactions with phytoplankton and mesozooplankton. The microzooplankton community was dominated by ciliates, especially small Strombidium sp., with the exception of the last days when heterotrophic dinoflagellates increased in abundance. We did not observe any effects of high CO2 on the community composition and diversity of microzooplankton. While ciliate abundance, biomass and growth rate were not affected by elevated CO2, we observed a positive effect of elevated CO2 on dinoflagellate abundances. Additionally, growth rates of dinoflagellates were significantly higher in the high CO2 treatments. Given the higher Chlorophyll a content measured under high CO2, our results point at mainly indirect effects of CO2 on microzooplankton caused by changes in phytoplankton standing stocks, in this case most likely an increase in small-sized phytoplankton of <8 μm. Overall, the results from the present study covering the most important part of the growing season indicate that coastal microzooplankton communities are rather robust towards realistic acidification scenarios.

Dampened copepod‐mediated trophic cascades in a microzooplankton‐dominated microbial food web: A mesocosm study

AbstractInteractions of bottom‐up factors such as the availability of mineral and organic nutrients, and top‐down factors like predation and viral infection affect microbial communities of the pelagic food web. Hypothesis derived from previous experimental and modeling work suggest some general mechanisms on how these factors may be linked and call for an experiment to test interactions between (1) nitrogen source and diatom cell size, (2) bacterial growth rate limitation by mineral nutrients versus organic carbon, and (3) enhanced versus decreased predation pressure by copepods (Calanus finmarchicus). We performed a mesocosm experiment using a replicated three factor––two level full factorial design ( / , +/− glucose, and +/− copepods). Diatoms, pico‐ and nano‐sized autotrophs, prokaryotes, heterotrophic nanoflagellates, micro‐ and mesozooplankton abundances were monitored for 19 days. The main finding was a system that responded remarkably little to these manipulations with no effects of nitrogen source and only moderate effects of carbon treatments. Zooplankton manipulations had the strongest impact on microbial communities, but, opposite to our hypothesis based on previous studies, microzooplankton and diatoms increased during the first 7–10 days when mesozooplankton abundances were experimentally enhanced. We suggest that high abundances of dinoflagellates and ciliates and occurrence of rotifers, which dominated microzooplankton community in the first half of the experiment complicated trophic linkages between copepods, microzooplankton and diatoms. Apart from this dampened copepod mediated trophic cascade, we found tight couplings between communities of the microbial food web, with maxima of predator communities coinciding with low abundances of prey communities.

The effect of increased nitrogen load on phytoplankton in a phosphorus‐limited lake

Summary Widespread use of artificial fertilisers and the burning of fossil fuels and/or biomass release a large amount of reactive nitrogen into the atmosphere. So far, the effects of increasing nitrogen deposition from the atmosphere have mainly been studied in nitrogen‐limited limnic and marine systems. Interestingly, in phosphorus‐limited lakes, additional nitrogen input might not affect phytoplankton biomass, but rather increase mainly the degree of phosphorus limitation. The resulting effects on plankton communities are difficult to predict and quantify. To estimate the effects of increasing nitrogen load on a spring phytoplankton community in a primarily phosphorus‐limited system, a mesocosm experiment was performed in an oligotrophic lake, in which a gradient of six increasing nitrogen enrichment levels was applied. During the initial phytoplankton growth phase (spring bloom), molar, seston nitrogen:phosphorus ratios increased from 43 to 72 and carbon:phosphorus ratios from 328 to 542 with increasing nitrogen enrichment, indicating increased phosphorus limitation. Three commonly used phytoplankton biomass proxies (phytoplankton biovolume, chlorophyll a and particulate organic carbon) showed only minor responses to nitrogen enrichment. Different groups and species of phytoplankton varied in their responses to the nitrogen enrichment in both the growth phase (spring bloom) and the descending phase (clear water phase). Overall, we detected an effect of nitrogen enrichment on phytoplankton stoichiometry and community composition. The observed changes in the phytoplankton community combined with changes in abundances of heterotrophic nanoflagellates and ciliates indicate bottom‐up driven alterations of the basal food web due to increased nitrogen loads.

Ecological Traits of the Algae-Bearing Tetrahymena utriculariae (Ciliophora) from Traps of the Aquatic Carnivorous Plant Utricularia reflexa.

Trap fluid of aquatic carnivorous plants of the genus Utricularia hosts specific microbiomes consisting of commensal pro- and eukaryotes of largely unknown ecology. We examined the characteristics and dynamics of bacteria and the three dominant eukaryotes, i.e. the algae-bearing ciliate Tetrahymena utriculariae (Ciliophora), a green flagellate Euglena agilis (Euglenophyta), and the alga Scenedesmus alternans (Chlorophyta), associated with the traps of Utricularia reflexa. Our study focused on ecological traits and life strategies of the highly abundant ciliate whose biomass by far exceeds that of other eukaryotes and bacteria independent of the trap age. The ciliate was the only bacterivore in the traps, driving rapid turnover of bacterial standing stock. However, given the large size of the ciliate and the cell-specific uptake rates of bacteria we estimated that bacterivory alone would likely be insufficient to support its apparent rapid growth in traps. We suggest that mixotrophy based on algal symbionts contributes significantly to the diet and survival strategy of the ciliate in the extreme (anaerobic, low pH) trap-fluid environment. We propose a revised concept of major microbial interactions in the trap fluid where ciliate bacterivory plays a central role in regeneration of nutrients bound in rapidly growing bacterial biomass.

Ecology of ciliates in microbial mats in meltwater streams, King George Island, maritime Antarctica

Microbial communities living in microbial mats are known to constitute early indicators of ecosystem disturbance, but little is known about their response to environmental factors in the Antarctic. This paper presents the first major study on ciliates from microbial mats in streams on King George Island (Antarctica). The objective of the study is to investigate the structure and spatial distribution of ciliate communities and to identify the environmental factors determining the structure of the assemblages. Additionally, we compared ciliate communities inhabiting streams on King George Island with other isolated islands of the maritime Antarctic. Samples of microbial mats for ciliate analysis were collected from three streams fed by Ecology Glacier. Sampling was carried out three times from 17 January to 24 February 2012. The total numbers of ciliates were dominated by small taxa recognized as cosmopolitan, also recorded on other islands of the maritime Antarctic. The species richness, abundance, and biomass of ciliates differed significantly between the stations studied, with the lowest numbers in the middle course of the stream and the highest numbers in the microhabitats closest to the glacier and at the site where the stream empties into the pond. Variables that significantly explained the variance in ciliate communities in the transects investigated were total organic carbon, total nitrogen, temperature, dissolved oxygen, and conductivity. Further research is required to explain the impact of biotic factors on the presence of ciliates, including the abundance of bacteria, microalgae, and small metazoa.

Ecology of Ciliates in Microbial Mats in Small Ponds: Relationship to Environmental Parameters (King George Island, Maritime Antarctica)

The objective of the study was to investigate the taxonomic structure and spatial distribution of ciliate communities, and to identify the environmental factors determining the taxonomic structure of the assemblages. Additionally, we compared ciliate communities inhabiting ponds on King George Island (Antarctica) with those on other islands of the maritime Antarctic. The taxonomic richness, abundance and biomass of ciliates in microbial mats varied among ponds increasing with their eutrophication level. As the level of eutrophication increased, there was a decrease in the abundance of predatory and omnivorous ciliates and a decrease in that of algivorous taxa. The canonical correspondence analyses of the spatial distribution of ciliates showed that all environmental variables together explained 76% of the total variance. The Monte Carlo permutation test showed that dissolved orthophosphates, nitrate and total organic carbon had the most significant effect on the distribution of ciliates. The proportions o...

Molecular diversity and distribution pattern of ciliates in sediments from deep-sea hydrothermal vents in the Okinawa Trough and adjacent sea areas

In comparison with the macrobenthos and prokaryotes, patterns of diversity and distribution of microbial eukaryotes in deep-sea hydrothermal vents are poorly known. The widely used high-throughput sequencing of 18S rDNA has revealed a high diversity of microeukaryotes yielded from both living organisms and buried DNA in marine sediments. More recently, cDNA surveys have been utilized to uncover the diversity of active organisms. However, both methods have never been used to evaluate the diversity of ciliates in hydrothermal vents. By using high-throughput DNA and cDNA sequencing of 18S rDNA, we evaluated the molecular diversity of ciliates, a representative group of microbial eukaryotes, from the sediments of deep-sea hydrothermal vents in the Okinawa Trough and compared it with that of an adjacent deep-sea area about 15km away and that of an offshore area of the Yellow Sea about 500km away. The results of DNA sequencing showed that Spirotrichea and Oligohymenophorea were the most diverse and abundant groups in all the three habitats. The proportion of sequences of Oligohymenophorea was the highest in the hydrothermal vents whereas Spirotrichea was the most diverse group at all three habitats. Plagiopyleans were found only in the hydrothermal vents but with low diversity and abundance. By contrast, the cDNA sequencing showed that Plagiopylea was the most diverse and most abundant group in the hydrothermal vents, followed by Spirotrichea in terms of diversity and Oligohymenophorea in terms of relative abundance. A novel group of ciliates, distinctly separate from the 12 known classes, was detected in the hydrothermal vents, indicating undescribed, possibly highly divergent ciliates may inhabit this environment. Statistical analyses showed that: (i) the three habitats differed significantly from one another in terms of diversity of both the rare and the total ciliate taxa, and; (ii) the adjacent deep sea was more similar to the offshore area than to the hydrothermal vents. In terms of the diversity of abundant taxa, however, there was no significant difference between the hydrothermal vents and the adjacent deep sea, both of which differed significantly from the offshore area. As abundant ciliate taxa can be found in several sampling sites, they are likely adapted to large environmental variations, while rare taxa are found in specific habitat and thus are potentially more sensitive to varying environmental conditions.

A first description of ciliate assemblages in a subtropical, eutrophic bay, South China Sea: species assemblage and environmental correlates

The ciliate community together with physical-chemical variables was investigated in a subtropical, eutrophic bay of the South China Sea from January 7–10 (dry season) and April 27–30 (wet season), 2012. Both temperature and salinity exhibited typical seasonality. A total of 27 species of ciliates belonging to 19 genera were identified, and the dominant species were Strombidium globosaneum in January and Codonella rapa in April. The composition and abundance exhibited spatial and temporal fluctuations, with total mean abundance of ciliates in April (1219 ± 919 ind l-1 ), was much higher than that in January (283 ± 137 ind l-1 ). Spatially, high species numbers were found mainly near the mouth of the estuary, and the abundance decreased from inside to the mouth of the bay. Statistical analysis showed that ciliate assemblages were closely related to the environmental variables. The distribution patterns of salinity and chlorophyll a concentration were similar, suggesting that, prey abundance may correlate significantly with ciliate abundance, and salinity may not be the main factor explaining the spatial variation in the ciliate community in the study area.

The effects of zinc on the structure and functioning of a freshwater community: A microcosm experiment.

A major problem with risk assessment of chemicals is the extrapolation of laboratory single-species toxicity tests, which oversimplify the actual field situation by ignoring species interactions, to natural communities. The authors tested if the bioavailability-normalized 5% hazardous concentration (HC5) estimated from chronic planktonic single-species toxicity data (HC5plankton ) for zinc (Zn) is protective for a plankton community and investigated the direct and indirect effects of Zn (at HC5 and HC50) on a freshwater community's structure and function. Microcosms were exposed to 3 different Zn concentrations (background, HC5plankton = 75 μg Zn/L and HC50plankton = 300 μg Zn/L) for 5 wk. The planktonic groups revealed a consistent no-observed-effect concentration for the community of 75 μg Zn/L, similar to or higher than the HC5plankton , thus suggesting its protectiveness in the present study. At 300 μg Zn/L a significant reduction in cladocerans resulted in increases of rotifer, ciliate, and phytoplankton abundance. In addition, the phytoplankton community shifted in dominance from grazing-resistant to edible species. Contrary to the species sensitivity distribution (SSD) prediction, which identified phytoplankton as the most sensitive group, only the total chlorophyll and the abundance of 2 phytoplankton species were adversely affected at 300 μg Zn/L. Thus, although the HC5 estimated from the bioavailability-normalized SSD was overall protective for the plankton community, the SSD was not able to correctly predict the species sensitivity ranking within their community context at the HC50. Environ Toxicol Chem 2016;35:2698-2712. © 2016 SETAC.