Freshwater Microcosms Research Articles

Ecological effects of the herbicide linuron in tropical freshwater microcosms

Effects of a single application of the photosynthesis-inhibiting herbicide linuron (0, 15, 50, 150, and 500 μg/L) on the ecology of outdoor plankton-dominated microcosms were studied in Thailand. As a result of the decreased photosynthesis, DO and pH decreased while EC, alkalinity and nutrient concentrations increased. Chlorophytes belonging to the genera Scenedesmus, Coelastrum and Pediastrum were the most sensitive taxa, whereas several other chlorophytes, diatoms, and cryptophytes increased in abundance. Tolerant taxa appeared to be less digestable for several zooplankton taxa, which subsequently decreased in abundances. Chamaesiphon sp. (Cyanobacteria) was the most susceptible periphyton species. As a consequence of functional redundancy, effects of the herbicide on the chlorophyll- a content of periphyton and especially phytoplankton did not always reflect the effects noted on community level.

A comparison of specific growth rates of periphytic diatoms of varying cell size under laboratory and field conditions

Diatom species grown under non-limiting nutrient availability in multispecific biofilms were sampled from glass substrates immersed in the field and in experimental freshwater microcosms, and their growth responses were determined. The major species that developed on the substrates were common to both experiments and the specific growth rates (k) ranged from 0.06 to 0.41 division day−1. An inverse relationship between k and cell sizes was observed, which is in accordance with allometry results reported by several authors. Although found in lower amounts, the large, slow-growing species accounted significantly in total community biovolume, underlying their significance for ecological purposes. From growth characteristics data, life history strategies were drawn for the dominant species recorded, from small pioneer species to large taxa that are more favoured by high resource supply. Kinetic data measured in both laboratory and in situ experiments stressed that the difficulties to mimic the field in laboratory experiments may have a strong impact on growth kinetics.

Functional responses of prokaryotes and viruses to grazer effects and nutrient additions in freshwater microcosms

For aquatic systems, there is little data on the interactions between viruses, prokaryotes, grazers and the availability of resources. We conducted a microcosm experiment using a size fractionation approach to manipulate grazers, with a purpose to examine the effects of inorganic and organic nutrients on viral and prokaryotic standing stocks and activities, and on prokaryotic community composition as assessed by fluorescent in situ hybridization (FISH) method. Experiments were performed during periods of severe phosphate (P)-limiting conditions in the oligotrophic Sep Reservoir (Massif Central, France). In the absence of nutrient addition, the presence of grazers in microcosms stimulated prokaryotic growth and viral proliferation, likely through nutrient and substrate enrichment. Addition of nutrients had a stronger effect on viral infection of prokaryotes than grazing. Addition of P led to the most pronounced increase in prokaryotic abundance, production and growth efficiency, thus providing direct evidence of P limitation of prokaryotes. Enhanced prokaryotic activity in P treatments also stimulated viral abundance and viral-induced lyses of prokaryotes. Changes in prokaryotic community composition due to nutrient additions were evident in the grazer-free samples. Prokaryotic populations hybridizing for the probes bacteria, beta-Proteobacteria and alpha-Proteobacteria responded to nutrient enrichment with significant increases in their relative abundances, whereas cells hybridizing for Archaea and Cytophaga-Flavobacterium (now known as Bacteroidetes) probes failed to show any functional response. Cells hybridizing for the latter cluster increased towards the end of incubation period in the control samples (that is, without nutrient additions) with grazers present, suggesting the development of grazing resistant forms. From our nutrient enrichment microcosm experiments, we conclude that the presence of grazers is a stimulating factor for prokaryotic growth and viral proliferation in the plankton, probably through nutrient regeneration process.

Influence of industrial contamination on mobile genetic elements: class 1 integron abundance and gene cassette structure in aquatic bacterial communities

The acquisition of new genetic material via horizontal gene transfer allows bacteria to rapidly evolve. One key to estimating the contribution of horizontal gene transfer to bacterial evolution is to quantify the abundance of mobile genetic elements (MGEs) in bacterial communities under varying degrees of selective pressure. We quantified class 1 integrase (intI1) gene abundance in total community DNA extracted from contaminated and reference riverine and estuarine microhabitats, and in metal- or antibiotic-amended freshwater microcosms. The intI1 gene was more abundant in all contaminant-exposed communities indicating that relative gene transfer potential is higher in these communities. A second key to assessing the contributions of MGEs to bacterial evolution is to examine the structure and function of the MGE-associated gene pool. We determined that the gene cassette pool is a novel and diverse resource available for bacterial acquisition, but that contamination has no discernible effect on cassette richness. Gene cassette profiles were more similar within sites than among sites, yet bacterial community profiles were not, suggesting that selective pressures can shape the structure of the gene cassette pool. Of the 46 sequenced gene cassette products, 37 were novel sequences, while the 9 gene cassettes with similarity to database sequences were primarily to hypothetical proteins. That class 1 integrons are ubiquitous and abundant in environmental bacterial communities indicates that this group of MGEs can play a substantial role in the acquisition of a diverse array of gene cassettes beyond their demonstrated impact in mediating multidrug resistance in clinical bacteria.

Effects of Chlorpyrifos, Carbendazim, and Linuron on the Ecology of a Small Indoor Aquatic Microcosm

To validate the use of small indoor microcosms for the risk assessment of pesticides, the fate and effects of chlorpyrifos, carbendazim, and linuron were studied in 8.5-liter indoor freshwater microcosms. Functional and structural responses to selected concentrations were evaluated and compared with responses observed in larger-scale model ecosystem studies. Overall, the microcosms adequately displayed the chain of effects resulting from the application, although they did not always predict the exact fate and responses that were observed in larger semifield studies. Because closed systems were used that did not contain sediment and macrophytes, pesticides were relatively persistent in the present study. Consequently, calculated toxicity values were generally more comparable with those reported in studies with long- than with short-term exposure. Carbendazim had a higher overall no-observed-effect concentration (NOEC) compared with experiments performed in larger systems because macroinvertebrate taxa, the most sensitive species group to this fungicide, were not abundant or diverse. Future refinements to the test system could include the addition of a sediment compartment and sensitive macroinvertebrate taxa. However, the simple design offers the potential to perform experiments under more controlled conditions than larger and, consequently, more complex model ecosystems, while maintaining relatively high ecologic realism compared with standard laboratory tests. Further implications for risk-assessment studies are discussed in an ecotoxicologic and methodologic context.

Time Course Transformations and Fate of Mercury in Aquatic Model Ecosystems

Model aquatic ecosystems have been used to study the natural mechanisms involved in the distribution and transformation of inorganic mercury (IHg) in the different compartments and its interactions with the biota. Laboratory incubations in indoor freshwater microcosms, presenting a simple biological organization, were carried out at various spiked concentrations (3, 25 and 257 nmol l−1 of IHg, as mercuric chloride) and from a single initial contamination of the water column. The different compartments of the model ecosystems (water, sediment, macrophytes Elodea canadensis and snails Lymnaea stagnalis) were investigated for mercury distribution and speciation during a 2-month experimental period. The principal results obtained have evidenced different Hg biogeochemical pathways including biotic IHg methylation and reduction and transfer to the biota. A fast transfer of IHg from the water to the aquatic organisms and to the sediment was first observed with IHg half-lives of 24 h and 8 days, respectively. IHg methylation, clearly related to biogenic processes, was also demonstrated in all contaminated microcosms after 1 week of exposure. Finally, gaseous mercury species were determined in the different microcosms and significant biological induced production of elemental Hg (Hg°) and dimethyl Hg (DMHg) was observed. This overall investigation, based on the time courses evolution of IHg and in situ produced monomethylmercury (MMHg) concentrations allows to determine uptake and elimination rate constants for IHg as well as the bioaccumulation kinetics of MMHg in macrophytes and snails. The applicability of these aquatic model ecosystems to provide real insights for pollution impact and ecotoxicological risk assessments has been demonstrated.

Impact of 17 α-ethinylestradiol on the plankton in freshwater microcosms—II: Responses of phytoplankton and the interrelation within the ecosystem

Effects of 17 α-ethinylestradiol (EE) on phytoplankton were investigated in a lentic freshwater microcosm study. Treatment with EE caused a shift in the species composition as shown by a principle response curve. Whereas densities of Cyanophyceae, Dinophyceae, and Chrysophyceae increased, those of Conjugatophyceae and Cryptophyceae decreased. Furthermore, relative density of Chlorophyceae declined after EE treatment. The changes showed taxa-specific time dependencies. Some species, especially the cyanobacterium Cyanobium parvum, bloomed after the treatment. EE treatment promoted total abundance and biomass of phytoplankton. Although the number of species per microcosm increased, the diversity indices (Shannon–Wiener, Simpson) tended to lower values. The ecosystem only partly recovered during the investigated post-treatment period of 6 weeks. Probably at least the main effects were caused indirectly, i.e. via decrease of grazing zooplankton (crustaceans). The relation of EE to variation of phytoplankton composition was closer than those of other abiotic factors, indicating the relevance of its impact. EE also probably affected nutrients of the phytoplankton.

Impact of 17 α-ethinylestradiol on the plankton in freshwater microcosms—I: Response of zooplankton and abiotic variables

We investigated effects of 17 α-ethinylestradiol (EE) in vertebrate free 230 L still water microcosms. Zooplankton composition and physico-chemical variables were observed during 4 weeks of pre-application, 6 weeks of dosing via controlled release, and a 12 weeks post-treatment period. In the treated microcosms, time-weighted averages of EE concentration ranged between 7 and 220 ng/L during the dosing period, with concentration maxima up to 724 ng/L. EE exposure resulted in a decrease of species numbers and diversity (Shannon–Wiener, Simpson). Abundances of cladocerans, copepods, and, less unambiguously, rotifers declined. Strongest affected groups were the offspring of cladocerans and copepods and, on species level, the cladoceran species Daphnia longispina and Chydorus sphaericus as well as the rotifer species Keratella quadrata and Polyarthra sp. EE apparently affected the phosphate cycle as indicated by increased phosphate concentrations in the water. During post-treatment period, the treated microcosms recovered, but especially the highest treated microcosms did not fully re-approximate to the controls. Whereas EE affected cladocerans and copepods directly, shifts of rotifers may (partly) be caused indirectly, e.g. by competition with crustaceae. Although not providing an absolute proof, the traits of direct and indirect effects on different taxonomic groups and larval stages as well as the time course of the effects indicate that effects primarily resulted from endocrine activity of EE.

The fate of isoproturon in a freshwater microcosm with Lemna minor as a model organism

Degradation, bioaccumulation and volatile loss of the 14C-labeled phenylurea herbicide isoproturon (IPU) was examined in a freshwater microcosm with the free floating macrophyte species Lemna minor during a 21-day exposure time. Isoproturon volatilisation was very low with 0.13 ± 0.01% of the initially applied herbicide. Only a minor amount of the herbicide was completely metabolised, presumably by rhizosphere microorganisms and released as 14CO 2. In total, about 9% isoproturon was removed from the aquatic medium during 21 days. The major portion of the pesticide was removed by bioaccumulation of Lemna minor (5.0 ± 0.8%) and the bioconcentration factor (BCF) based on freshweight was 15.8 ± 0.2. However, this study indicated a high persistence of IPU in freshwater ecosystems and a potential hazard due to bioaccumulation in non-target species. The novel experimental system of this study, developed for easy use and multiple sampling abilities, enabled quantitatively studying the fate of isoproturon and showed high reproducibility with a mean average 14C-recovery rate of 97.1 ± 0.7%.

Coselection for microbial resistance to metals and antibiotics in freshwater microcosms

Bacterial resistances to diverse metals and antibiotics are often genetically linked, suggesting that exposure to toxic metals may select for strains resistant to antibiotics and vice versa. To test the hypothesis that resistances to metals and antibiotics are coselected for in environmental microbial assemblages, we investigated the frequency of diverse resistances in freshwater microcosms amended with Cd, Ni, ampicillin or tetracycline. We found that all four toxicants significantly increased the frequency of bacterioplankton resistance to multiple, chemically unrelated metals and antibiotics. An ampicillin-resistant strain of the opportunistic human pathogen Ralstonia mannitolilytica was enriched in microcosms amended with Cd. Frequencies of antibiotic resistance were elevated in microcosms with metal concentrations representative of industry and mining-impacted environments (0.01-1 mM). Metal but not antibiotic amendments decreased microbial diversity, and a weeklong exposure to high concentrations of ampicillin (0.01-10 mg l-1) and tetracycline (0.03-30 mg l-1) decreased microbial abundance only slightly, implying a large reservoir of antibiotic resistance in the studied environment. Our results provide first experimental evidence that the exposure of freshwater environments to individual metals and antibiotics selects for multiresistant microorganisms, including opportunistic human pathogens.

Impact of triphenyltin acetate in microcosms simulating floodplain lakes. II. Comparison of species sensitivity distributions between laboratory and semi-field

The study objectives were to shed light on the types of freshwater organism that are sensitive to triphenyltin acetate (TPT) and to compare the laboratory and microcosm sensitivities of the invertebrate community. The responses of a wide array of freshwater taxa (including invertebrates, phytoplankton and macrophytes) from acute laboratory Single Species Tests (SST) were compared with the concentration-response relationships of aquatic populations in two types of freshwater microcosms. Representatives of several taxonomic groups of invertebrates, and several phytoplankton and vascular plant species proved to be sensitive to TPT, illustrating its diverse modes of toxic action. Statistically calculated ecological risk thresholds (HC5 values) based on 96 h laboratory EC50 values for invertebrates were 1.3 microg/l, while these values on the basis of microcosm-Species Sensitivity Distributions (SSD) for invertebrates in sampling weeks 2-8 after TPT treatment ranged from 0.2 to 0.6 microg/l based on nominal peak concentrations. Responses observed in the microcosms did not differ between system types and sampling dates, indicating that ecological threshold levels are not affected by different community structures including taxa sensitive to TPT. The laboratory-derived invertebrate SSD curve was less sensitive than the curves from the microcosms. Possible explanations for the more sensitive field response are delayed effects and/or additional chronic exposure via the food chain in the microcosms.

Ecological Effects of Spring and Late Summer Applications of Lambda-Cyhalothrin on Freshwater Microcosms

The aim of the study was to compare the effects of the pyrethroid insecticide lambda-cyhalothrin (treated at 10, 25, 50, 100, 250 ng active ingredient a.i./L) on a drainage ditch ecosystem in spring and late summer. Microcosms (water volume approximately 430 L) were established using enclosures in a 50-cm-deep experimental ditch system containing communities typical of macrophyte-dominated freshwater ecosystems. Effects on macroinvertebrates, zooplankton, phytoplankton, macrophytes, and community metabolism were assessed and evaluated using univariate and multivariate statistical techniques. The macroinvertebrate community responded most clearly to treatment and, as anticipated, insects and crustaceans were among the most sensitive organisms. Statistical analysis showed that the underlying community structure was significantly different between the spring and summer experiments. However, the most sensitive species (Chaoborus obscuripes and Gammarus pulex) were abundant in spring as well as in late summer. In spring and late summer, only slight and transient effects were observed at the community level in the 10-ng/L treatment. Overall, the study did not show substantial differences in the responses of sensitive taxa between spring and late summer treatments, and effects thresholds were similar irrespective of season of treatment.

Effect of salinity on temporal and spatial dynamics of ammonia-oxidising bacteria from intertidal freshwater sediment

Temporal and spatial dynamics within an ammonia-oxidising community from intertidal, freshwater sediments were studied in microcosms simulating flooding twice a day with fresh, brackish and marine waters. The microcosms had been filled with the upper 5 cm of intertidal freshwater sediment from the river Scheldt. Changes in community composition were examined by denaturing gradient gel electrophoresis of amplified DNA from the community. In the first week of incubation the initially present members of the Nitrosomonas oligotropha lineage were replaced by other members of the same lineage in the top layer of the sediment subjected to flooding with freshwater. Prolonged incubation extended niche differentiation to a depth of 5 cm. In the microcosms flooded with saline media, the initially present members of the N. oligotropha lineage were replaced by strains belonging to the Nitrosomonas marina lineage, but only in the top 1 cm. Shift in community composition occurred earlier in the marine microcosms than in the brackish microcosms and was slower than the change in the freshwater microcosms. Irrespective of the nature of the flooding medium, shifts in community composition were always consistent among replicate microcosms. We conclude that salinity is an important steering factor in niche differentiation among ammonia-oxidising bacteria and also that changes within the community of this functional group of bacteria may occur at different rates.

Population dynamics of transgenic strain Escherichia coli Z905/pPHL7 in freshwater and saline lake water microcosms with differing microbial community structures

Populations of Escherichia coli Z905/pPHL7, a transgenic microorganism, were heterogenic in the expression of plasmid genes when adapting to the conditions of water microcosms of various mineralization levels and structure of microbial community. This TM has formed two subpopulations (ampicillin-resistant and ampicillin-sensitive) in every microcosm. Irrespective of mineralization level of a microcosm, when E. coli Z905/pPHL7 alone was introduced, the ampicillin-resistant subpopulation prevailed, while introduction of the TM together with indigenous bacteria led to the dominance of the ampicillin-sensitive subpopulation. A high level of lux gene expression maintained longer in the freshwater microcosms than in sterile saline lake water microcosms. A horizontal gene transfer has been revealed between the jointly introduced TM and Micrococcus sp. 9/pSH1 in microcosms with the Lake Shira sterile water.

Source and supply of terrestrial organic matter affects aquatic microbial metabolism

Aquatic ecosystems are connected to their surrounding watersheds through inputs of terrestrial-derived dissolved organic matter (DOM). The assimilation of this allochthonous resource by recipient bacterioplankton has consequences for food webs and the biogeochemistry of aquatic ecosystems. We used laboratory batch experiments to examine how variation in the source and supply (i.e. concentration) of DOM affects the productivity, respiration and growth efficiency of heterotrophic lake bacterioplankton. We created 6 different DOM sources from soils beneath near- monotypic tree stands in a temperate deciduous-coniferous forest. We then exposed freshwater microcosms containing a natural microbial community to a 1100 µM supply gradient of each DOM source. Bacterial productivity (BP) and bacterial respiration (BR) increased linearly over the broad gradient, on average consuming 7% of the standing pool of dissolved organic carbon (DOC). Bacter- ial metabolism was also influenced by the chemical composition of the DOM source. Carbon-specific productivity declined exponentially with an increase in the carbon:phosphorus (C:P) ratio of the dif- ferent DOM sources, consistent with the predictions of ecological stoichiometry. Together, our short- term laboratory experiments quantitatively describe the metabolic responses of freshwater bacterio- plankton to variation in the supply of terrestrial-derived DOM. Furthermore, our results suggest that dissolved organic phosphorus (DOP) content, which may be linked to the identity of terrestrial vege- tation, is indicative of DOM quality and influences the productivity of freshwater bacterioplankton.

Grazer effects on prokaryotes and viruses in a freshwater microcosm experiment

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 41:115-124 (2005) - doi:10.3354/ame041115 Grazer effects on prokaryotes and viruses in a freshwater microcosm experiment T. Sime-Ngando1,*, A. S. Pradeep Ram1,2 1Laboratoire de Biologie des Protistes, Université Blaise Pascal (Clermont-Ferrand II), UMR CNRS 6023, 63177 Aubière Cedex, France2Present address: Centre for Ecological Research, Kyoto University, 509-3 Otsuka, Kamitanakami-hirano, Otsu 520-2113 Shiga, Japan *Email: telesphore.sime-ngando@univ-bpclermont.fr ABSTRACT: We conducted a short-term course experiment using a size-fractionation approach to manipulate grazers and test for their effects on viral and prokaryotic standing stocks and activities and on prokaryotic community composition, as assessed by the FISH (fluorescence in situ hybridisation) method. Experimental samples were collected in the Sep Reservoir during severe P-limiting conditions. The presence of grazers in microcosms appeared to be a stimulating factor for prokaryotic growth and viral proliferation, likely through the related nutrient and substrate enrichments. About 60% of the total prokaryote abundance was detected by FISH, with a dominance of Eubacteria and beta-Proteobacteria throughout the experiment, i.e. in the presence and in the absence of grazers. The relative abundances of the minor phylotypes remained unchanged in the absence of grazers, but significantly increased with time when grazers were present, indicating a grazer-mediated reduction in resource competition in prokaryotic assemblages. However, grazer effects resulted in an apparently greater alteration of the bacterial size structure (i.e. the occurrence of large, grazing-resistant cells) than of the relative abundances of the phylotypes analysed. The main findings suggest that, at least on a short-term scale (i.e. ≤1 prokaryotic net generation time): (1) There is a synergy between grazer, prokaryotic and viral activities in oligotrophic conditions, through a cascading effect from grazer-mediated resource enrichment, with (2) a higher impact on the prokaryotic size structure than on the relative abundances of the major phylotypes, and (3) a significant acquisition of competitive advantages by the less adundant phylotypes. These findings are discussed in light of the recent ‘phage kills winner’ theory. KEY WORDS: Plankton · Lakes · Microcosms · Viruses · Prokaryotes · Community composition · Grazer effects · Microbial ecology Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 41, No. 2. Online publication date: November 25, 2005 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2005 Inter-Research.

Diversity and vertical distribution of magnetotactic bacteria along chemical gradients in freshwater microcosms

The vertical distribution of magnetotactic bacteria along various physico-chemical gradients in freshwater microcosms was analyzed by a combined approach of viable cell counts, 16S rRNA gene analysis, microsensor profiling and biogeochemical methods. The occurrence of magnetotactic bacteria was restricted to a narrow sediment layer overlapping or closely below the maximum oxygen and nitrate penetration depth. Different species showed different preferences within vertical gradients, but the largest proportion (63–98%) of magnetotactic bacteria was detected within the suboxic zone. In one microcosm the community of magnetotactic bacteria was dominated by one species of a coccoid “ Alphaproteobacterium”, as detected by denaturing gradient gel electrophoresis in sediment horizons from 1 to 10 mm depth. Maximum numbers of magnetotactic bacteria were up to 1.5 × 10 7 cells/cm 3, which corresponded to 1% of the total cell number in the upper sediment layer. The occurrence of magnetotactic bacteria coincided with the availability of significant amounts (6–60 μM) of soluble Fe(II), and in one sample with hydrogen sulfide (up to 40 μM). Although various trends were clearly observed, a strict correlation between the distribution of magnetotactic bacteria and individual geochemical parameters was absent. This is discussed in terms of metabolic adaptation of various strains of magnetotactic bacteria to stratified sediments and diversity of the magnetotactic bacterial communities.

Response of zooplankton and phytoplankton communities to creosote-impregnated Douglas fir pilings in freshwater microcosms.

Creosote has been used extensively as an industrial wood preservative for the protection of marine pilings, railway ties, and utility poles and is a common source of polycyclic aromatic hydrocarbons (PAHs) into aquatic environments. At present, there is little information by which to judge the potential for creosote leached from impregnated pilings to cause toxicity to biota in aquatic environments. The objective of the current study was to assess the effects of creosote on zooplankton and phytoplankton populations in freshwater microcosms in relation to changes in the concentration and composition of PAHs leached from creosote-impregnated Douglas fir pilings during an 83-day exposure period. The study consisted of single microcosms containing one half, one, two, three, four, and six treated pilings. Two microcosms that received untreated pilings were used as controls. The total surface area of pilings in each microcosm was normalized by adding the appropriate number of untreated pilings. Samples were collected periodically between -14 and 83 days pre- and postexposure to determine aqueous concentrations of 15 priority PAHs and to assess the response of zooplankton and phytoplankton communities. Plankton community response to creosote was analyzed using principle responses curves. Peak aqueous concentrations of sigmaPAH occurred at day 7, ranging from 7.3 to 97.3 microg/L. Zooplankton abundance decreased in all microcosms after introduction of the impregnated pilings, with the magnitude of response varying as a function of aqueous creosote concentration. Using inverse regression, a no-observed-effect concentration for the zooplankton community of 11.1 microg/L was estimated. In contrast, algal abundance and diversity increased in all treatments between 7 and 21 days and attained levels up to twice that in control microcosms. This trend most likely reflected decreased grazing pressure because of the decrease in zooplankton populations, but it may also have reflected growth stimulation resulting from exposure to various constituents within the creosote mixture. Our results indicate that creosote leached from impregnated pilings deployed under typical conditions (e.g., wharves) may cause transient toxicity to benthic or limnetic communities shortly after deployment, but this likely poses few long-term risks to aquatic freshwater plankton communities.

Aquatic risk assessment of a realistic exposure to pesticides used in bulb crops: a microcosm study.

The fungicide fluazinam, the insecticide lambda-cyhalothrin, and the herbicides asulam and metamitron were applied to indoor freshwater microcosms (water volume approximately 0.6 m3). The treatment regime was based on a realistic application scenario in tulip cultivation. Concentrations of each pesticide were equal to 0%, 0.2%, 0.5%, 2%, and 5% spray drift emission of label-recommended rates. Contribution of compounds to the toxicity of the pesticide package was established by expressing their concentrations as fractions of toxic units. The fate of the compounds in the water, and responses of phytoplankton, zooplankton, periphyton, macroinvertebrates, macrophytes, decomposition, and water quality were followed for 13 weeks. The half-lives of lambda-cyhalothrin, metamitron, and fluazinam were 1 to 2 d; that of asulam was >30 d. No consistent effects could be demonstrated for the 0.2% treatment regime that was therefore considered the no-observed-effect concentration community (NOEC). The macroinvertebrate populations of Gammarus pulex, Asellus aquaticus, and Proasellus meridianus were the most sensitive end points, followed by species of copepods and cladocerans. Responses mainly were due to lambda-cyhalothrin. The 0.5% treatment regime resulted in short-term effects. Pronounced effects were observed at the 2% and 5% treatment levels. At the end of the experiment, the macrophyte biomass that consisted of Elodea nuttallii, showed a decline at the two highest treatment levels, asulam being the causal factor (NOEC: 0.5% treatment level). Primary production was reduced at the 5% treatment level only. In our experiment, the first-tier risk assessment procedure for individual compounds was adequate for protecting sensitive populations exposed to realistic combinations of pesticides. Spray drift reduction measures seem to be efficient in protecting aquatic ecosystems in agricultural areas.

Response of freshwater microcosm communities to nutrients, fish, and elevated temperature during winter and summer

Under conditions of stress, shallow freshwater ecosystems can undergo a state change characterized by the rapid loss of macrophytes and subsequent dominance of phytoplankton. Elevated water temperature may promote such change. Here we report the impact of two warming regimes (continuous 3°C above ambient and 3°C above ambient during summer only), with two nutrient loadings and the presence or absence of fish, on 48 microcosm ecosystems created to mimic shallow pond environments. We found that warming did not significantly encourage phytoplankton blooms, even in combination with increased nutrients and fish. Instead, macrophyte communities remained dominant. Macrophyte‐associated invertebrates (gastropods and ostracods) increased in numbers in the warmed microcosms, potentially helping to stabilize the macrophyte communities. Nevertheless, warming produced trends in water chemistry that could be problematic. It increased phosphorus concentrations, total alkalinity, and conductivity. It decreased pH and oxygen saturation and increased the frequency of severe deoxygenation. These trends were largely independent of the other experimental treatments and support the suggestion that moderate warming has the potential to exacerbate existing eutrophication problems.