Microcosm Bioassays Research Articles

Do functional traits and biochemical biomarkers of the nematode Oncholaimus campylocercoides De Coninck and Schuurmans Stekhoven, 1933 affected by fluoranthene and polystyrene microplastics? Results from a microcosm bioassay and molecular modeling

The current experiment measured the multifaceted effects of polystyrene and fluoranthene, acting alone or in a mixture, on the meiobenthic nematode species Oncholaimus campylocercoides. This Oncholaimid was first experimentally selected from an entire nematode assemblage taken from the Jeddah coasts (Saudi Arabia). Several discernible changes were found in morphometry and functional traits after exposure to single and combined treatments. An increase in the activity of the biochemical biomarkers catalase and glutathione S-transferase was also observed following the exposure of males and gravid females of O. campylocercoides to 37.5 ng fluoranthene·g−1 dry weight (DW) and 62.5 mg polystyrene·kg−1 DW paralleled by a higher vulnerability of females. Moreover, the reproduction and feeding of this species were impaired, starting from 37.5 ng fluoranthene·g−1 and 62.5 mg polystyrene·kg−1, respectively. These results have been confirmed by good binding affinities and molecular interactions of fluoranthene and polystyrene with both GLD-3 and SDP receptors.

Ocean warming and CO2-driven acidification can alter the toxicity of metal-contaminated sediments to the meiofauna community

Interactive effects of trace metal contamination, ocean warming, and CO2-driven acidification on the structure of a meiofaunal benthic community was assessed. Meiofauna microcosm bioassays were carried out in controlled conditions in a full factorial experimental design which included three fixed factors: metal contamination in the sediment (3 levels of a mixture of Cu, Pb, Zn, and Hg), temperature (26 and 28 °C) and pH (7.6 and 8.1). Metal contamination caused a sharp decrease in the densities of the most abundant meiobenthic groups and interacted with temperature rise, exacerbating deleterious effects for Nematoda and Copepoda, but mitigating effects for Acoelomorpha. CO2-driven acidification resulted in increased acoelomorphs density, but only in sediments with lower levels of metals. Copepod densities, in turn, were lower in the CO2-driven acidification scenario regardless of contamination or temperature. The results obtained in the present study showed that temperature rise and CO2-driven acidification of coastal ocean waters, at environmentally relevant levels, interacts with trace metals in marine sediments, differently affecting the major groups of benthic biota.

Mesocosm and Microcosm Bioassays to Examine Effects of Mine-Influenced Sediments on the Growth of Wild Rice (Zizania palustris L.)

Mesocosm and Microcosm Bioassays to Examine Effects of Mine-Influenced Sediments on the Growth of Wild Rice (Zizania palustris L.)

Microbial methylation potential of mercury sulfide particles dictated by surface structure

Environmental contamination by mercury in its organometallic form, methylmercury, remains a major global concern due to its neurotoxicity, environmental persistence and biomagnification through the food chain. Accurate prediction of mercury methylation cannot be achieved based on aqueous speciation alone, and there remains limited mechanistic understanding of microbial methylation of particulate-phase mercury. Here we assess the time-dependent changes in structural properties and methylation potential of nanoparticulate mercury using microscopic and spectroscopic analyses, microcosm bioassays and theoretical calculations. We show that the methylation potential of a mercury sulfide mineral ubiquitous in contaminated soils and sediments (nanoparticulate metacinnabar) is determined by its crystal structure. Methylmercury production increases when more of nano-metacinnabar’s exposed surfaces occur as the (111) facet, due to its large binding affinity to methylating bacteria, likely via the protein transporter responsible for mercury cellular uptake prior to methylation. During nanocrystal growth, the (111) facet diminishes, lessening methylation of nano-metacinnabar. However, natural ligands alleviate this process by preferentially adsorbing to the (111) facet, and consequently hinder natural attenuation of mercury methylation. We show that the methylation potential of nanoparticulate mercury is independent of surface area. Instead, the nano-scale surface structure of nanoparticulate mercury is crucial for understanding the environmental behaviour of mercury and other nutrient or toxic soft elements. The environmental behaviour of mercury and other toxic soft elements is in part dictated by the surface structure of nanoparticulates, according to a combination of microcosm bioassays and theoretical calculations.

Roles of Nutrient Limitation on Western Lake Erie CyanoHAB Toxin Production.

Cyanobacterial harmful algal bloom (CyanoHAB) proliferation is a global problem impacting ecosystem and human health. Western Lake Erie (WLE) typically endures two highly toxic CyanoHABs during summer: a Microcystis spp. bloom in Maumee Bay that extends throughout the western basin, and a Planktothrix spp. bloom in Sandusky Bay. Recently, the USA and Canada agreed to a 40% phosphorus (P) load reduction to lessen the severity of the WLE blooms. To investigate phosphorus and nitrogen (N) limitation of biomass and toxin production in WLE CyanoHABs, we conducted in situ nutrient addition and 40% dilution microcosm bioassays in June and August 2019. During the June Sandusky Bay bloom, biomass production as well as hepatotoxic microcystin and neurotoxic anatoxin production were N and P co-limited with microcystin production becoming nutrient deplete under 40% dilution. During August, the Maumee Bay bloom produced microcystin under nutrient repletion with slight induced P limitation under 40% dilution, and the Sandusky Bay bloom produced anatoxin under N limitation in both dilution treatments. The results demonstrate the importance of nutrient limitation effects on microcystin and anatoxin production. To properly combat cyanotoxin and cyanobacterial biomass production in WLE, both N and P reduction efforts should be implemented in its watershed.

Health risk assessment of heavy metals (Cu, Zn, and Mn) in wild oat grown in soils amended with sediment dredged from the Joumine Dam in Bizerte, Tunisia

The Joumine Dam located in northern Tunisia has lost more than 20% of its initial storage capacity due to sedimentation, meaning that sediment management is necessary. The sediments at the reservoir bottom act as a sink for nutrients and chemicals originating from the upper agricultural lands and take the form of suspended particles. We proposed that the dredged sediments could be used to amend arid to semiarid soils, as this would partially cover the financial burden of dredging works and reduce the volume of these deposits. However, to check the feasibility of using the sediments as a fertilizer, it was necessary to assess the potential health risks from contaminants in the sediments. Therefore, the present study aimed to evaluate the human health risk (i.e., the hazard quotient, HQ) from heavy metals consumed due to the ingestion of Bromus ramosus (wild oat) grown in soil amended with the Joumine Dam sediments. Plant growth was monitored in macrocosm (amendment rate of 1.17%) and microcosm (amendment rate ≤ 10%) bioassays to elucidate the metal concentrations in roots, stems, leaves, and seeds. Zn, Cu, and Mn concentrations were analyzed in the plants grown in the macrocosm experiments, while the follow-up was only performed for Zn in the plants grown in the microcosm experiments. The human exposure to soil pollutants (HESP) evaluation model was adopted to evaluate the health risk (HQ) to humans through direct and indirect oral exposure to heavy metals in wild oat. At the macrocosm scale, Cu was found to be the main source of risk (HQ = 1.86) to children. At the microcosm scale, utilization of the sediment reduced the mobility and bioavailability of copper in the soil, thus decreasing the potential health risk from this metal.Graphic abstract

Lethal effects of leaf leachate from the non-native invasive shrub Amur honeysuckle (Lonicera maackii) on a model aquatic organism (Hyalella azteca)

ABSTRACTThe invasive shrub Lonicera maackii (Amur honeysuckle) is a problematic species in the eastern United States and there is growing evidence that materials from this species have toxic effects on some organisms. We used a sequence of microcosm bioassays to assess the influence of L. maackii leaf leachate on the macroinvertebrate Hyalella azteca, which is a standard aquatic organism for toxicity assessment. In a laboratory setting, H. azteca were exposed to a leaf leachate dilution series (6.25%, 12.5%, 25%, 50%, 100%) in 48-h toxicity tests. This was repeated throughout the growing season to assess the potential for changes in leaf toxicity due to phenology. Strong toxic effects were found when H. azteca was exposed to a L. maackii leachate from autumn (P < 0.05) and, in fact, all organisms died when exposed to any level of concentration in most trials. Mean percent survival also decreased significantly in all dilutions in the spring (P < 0.05 for all treatments); however, little toxicity was detected in growing season trials. These results suggest (a) strong toxic effects of L. maackii foliage on a model aquatic organism that (b) varies throughout the year, potentially in relationship to biochemical changes associated with phenology.

Impact of nutrient enrichment on productivity of coastal water along the SE Mediterranean shore of Israel - A bioassay approach

The coastal waters of the southeastern Mediterranean-Sea (SEMS) are routinely enriched with naturally-occurring and anthropogenic land-based nutrient loads. These external inputs may affect autotrophic and heterotrophic microbial biomass and activity. Here, we conducted 13 microcosm bioassays with different additions of inorganic NO3-(N), PO4-(P) and Si(OH)4-(Si) in different seasons along the Mediterranean coast of Israel. Our results indicate that cyanobacteria are mainly N-limited, whereas N or Si (or both) limit pico-eukaryotes. Furthermore, the degree to which N affects phytoplankton depends on the ambient seawater's inorganic N and N:P characteristics. Heterotrophic bacteria displayed no response in all treatments, except when all nutrients were added simultaneously, suggesting a possible co-limitation by nutrients. These results contrast the N+P co-limitation of phytoplankton and the P-limitation of bacteria in the open waters of the SEMS. These observations enable the application for a better science-based environmental monitoring and policy implementation along the SEMS coast of Israel.

Contribution of airborne microbes to bacterial production and N2 fixation in seawater upon aerosol deposition

AbstractAerosol deposition may supply a high diversity of airborne microbes, which can affect surface microbial composition and biological production. This study reports a diverse microbial community associated with dust and other aerosol particles, which differed significantly according to their geographical air mass origin. Microcosm bioassay experiments, in which aerosols were added to sterile (0.2 µm filtered and autoclaved) SE Mediterranean Sea (SEMS) water, were performed to assess the potential impact of airborne bacteria on bacterial abundance, production, and N2 fixation. Significant increase was observed in all parameters within a few hours, and calculations suggest that airborne microbes can account for one third in bacterial abundance and 50–100% in bacterial production and N2‐fixation rates following dust/aerosol amendments in the surface SEMS. We show that dust/aerosol deposition can be a potential source of a wide array of microorganisms, which may impact microbial composition and food web dynamics in oligotrophic marine systems such as the SEMS.

Biomass, Nutrient, and Trace Element Accumulation and Partitioning in Cattail ( L.) during Wetland Phytoremediation of Municipal Biosolids.

Biomass and contaminant accumulation and partitioning in plants determine the harvest stage for optimum contaminant uptake during phytoremediation of municipal biosolids. This wetland microcosm bioassay characterized accumulation and partitioning of biomass, nutrients (N and P), and trace elements (Zn, Cu, Cr, and Cd) in cattail ( L.) in a growth room. Four cattail seedlings were transplanted into each 20-L plastic pail containing 3.9 kg (dry wt.) biosolids from an end-of-life municipal lagoon. A 10-cm-deep water column was maintained above the 12-cm-thick biosolids layer. Plants were harvested every 14 d over a period of 126 d for determination of aboveground biomass (AGB) and belowground biomass (BGB) yields, along with contaminant concentrations in these plant tissues. Logistic model fits to biomass yield data indicated no significant difference in asymptotic yield between AGB and BGB. Aboveground biomass accumulated significantly greater amounts of N and P and lower amounts of trace elements than BGB. Maximum N accumulation in AGB occurred 83 d after transplanting (DAT), and peak P uptake occurred at 86 DAT. Harvesting at maximum aboveground accumulation removed (percent of the initial element concentration in the biosolids) 4% N, 3% P, 0.05% Zn, 0.6% Cu, 0.1% Cd, and 0.2% Cr. Therefore, under the conditions of this study, phytoremediation would be most effective if cattail is harvested at 86 DAT. These results contribute toward the identification of the harvest stage that will optimize contaminant uptake and enhance in situ phytoremediation of biosolids using cattail.

Functional response of the fungus Hirsutella rhossiliensis to the nematode, Heterodera glycines.

Functional response is a key index in determining the population fluctuation in predation. However, the lack of operable research system limits the studies on functional response of fungal predators. Hirsutella rhossiliensis is a dominant parasite of the soybean cyst nematode, Heterodera glycines. In a soil microcosm bioassay, we determined fungal biomass at different days within 21 days after inoculation, and parasitism rate of H. glycines by the fungus was determined. The functional response of H. rhossiliensis to H. glycines was established and found to be Holling's type III, which was influenced by mycelial densities. Meanwhile, we conducted anti-fungal analysis of metabolic fractions extracted from H. rhossiliensis to explain the potential mechanism of the intraspecific competition illustrated by functional response. The result of anti-fungal experiments indicated that the fungal predators had more complicated interaction at population level than expected, which might be regulated by self-inhibition metabolite(s). This study was the first functional response study of fungal predators in microcosm. With the increasing recognition of emerging fungal threats to animal, plant, and ecosystem health, the methodologies and hypotheses proposed in this study might inspire further research in fungal ecology.

Comparison of laboratory batch and flow-through microcosm bioassays

Since 1997, we have been developing a protocol for ecotoxicological bioassays in 2-L laboratory microcosms and have applied it to the study of various pollutants and ecotoxicological risk assessment scenarios in the area of urban facilities and transport infrastructures. The effects on five different organisms (micro-algae, duckweeds, daphnids, amphipods, chironomids) are assessed using biological responses such as growth, emergence (chironomids), reproduction (daphnids) and survival, with a duration of exposure of 3 weeks. This bioassay has mainly been used as a batch bioassay, i.e., the water was not renewed during the test. A flow-through microcosm bioassay has been developed recently, with the assumption that conditions for the biota should be improved, variability reduced, and the range of exposure patterns enlarged (e.g., the possibility of maintaining constant exposure in the water column). This paper compares the results obtained in batch and flow-through microcosm bioassays, using cadmium as a model toxicant. As expected, the stabilization of physico-chemical parameters, increased organism fitness and reduced variability were observed in the flow-through microcosm bioassay.

Dynamics of phytoplankton community structure in the South China Sea in response to the East Asian aerosol input

Abstract. Recent studies have demonstrated atmospheric deposition as an important source of bioreactive compounds to the ocean. The South China Sea (SCS), where aerosol loading is among the highest in the world, however, is poorly studied, particularly on the in situ response of phytoplankton community structures to atmospheric deposition. By conducting a series of microcosm bioassays at different hydrographical locations and simulating different aerosol event scales, we observed both positive and negative responses to the input of East Asian (EA) aerosol with high nitrogen (N) and trace metal contents, in terms of biomass, composition and physiological characteristics of phytoplankton communities. High levels of aerosol loading relieved phytoplankton nitrogen and trace metal limitations in SCS, and thus increased total phytoplankton biomass, enhanced their physiological indicators (e.g. photosynthetic efficiency) and shifted phytoplankton assemblages from being dominated by picoplankton to microphytoplanton, especially diatoms. However, under low levels of aerosol loading, the composition shift and biomass accumulation were not apparent, suggesting that the stimulation effects might be counterbalanced by enhanced grazing mortality indicated by increased abundance of protist grazers. Trace metal toxicity of the aerosols might also be the reason for the reduction of picocyanobacteria when amended with high EA aerosols. The magnitude and duration of the deposition event, as well as the hydrographical and trophic conditions of receiving waters are also important factors when predicting the influence of an aerosol deposition event. Our results demonstrated different responses of phytoplankton and microbial food web dynamics to different scales of atmospheric input events in SCS and highlighted the need for achieving an accurate comprehension of atmospheric nutrient on the biogeochemical cycles of the oceans.

An Assessment of the Impact of Chromium-Amended Sediment on a Marine Nematode Assemblage Using Microcosm Bioassays

Microcosms were used to assess the impact of chromium on free-living marine nematodes. Nematodes were exposed to three chromium concentrations (500 ppm (dm, dry mass), 800 ppm (dm) and 1,300 ppm (dm)), and effects were examined after 4 weeks. Results showed significant differences between univariate measures of control nematodes and those from medium- and high-chromium microcosms. Most, decreased significantly with increasing level of chromium contamination. The medium-chromium treatment seems to be the minimal concentration that could has a negative effect on nematodes. Results from multivariate analyses demonstrated that responses of nematode species to chromium treatments were varied: Leptonemella aphanothecae was eliminated at all doses tested and seemed to be intolerant species to chromium; Daptonema normandicum and Sabatieria longisetosa which significantly increased at 500 ppm chromium (dm) appeared to be "opportunistic" species at this dose whereas the two Bathylaimus species (Bathylaimus capacosus and Bathylaimus tenuicaudatus) which increased at all the doses tested seemed to be "chromium resistant". As we think such "opportunistic" and "resistant" species could be used as sensitive indicators of unsafe marine food. The use of microcosms has allowed the effects of the chromium on nematodes to be assessed individually, which was not possible in the field.

Seasonal phytoplankton response to increased temperature and phosphorus inputs in a freshwater coastal lagoon, Southern Brazil: a microcosm bioassay

Aim: the present study aimed at assessing the response of phytoplankton biomass (as chlorophyll-a) to simulated conditions of increased water temperature and phosphorus (P) inputs in Peri lagoon, a subtropical coastal lagoon dominated by Cylindrospermopsis raciborskii most of the year; Methods: microcosm experiments were conducted in thermostatic light chambers during 3 and 7 days in austral summer, autumn and winter 2009. Control temperatures were tested against 3 and 5 °C rises in water temperature in each season. In each temperature treatment, three P concentrations were tested: control (non-enriched, ~1.5 µg.L -1 PO 4 3+ ) and enrichments of four (+6 µg.L -1 PO 4 3+ ) and eight (+12 µg.L -1 PO 4 3+ ) times natural concentrations; Results: the results showed that P enrichments alone did not increase chlorophyll-a concentrations, but temperature increases significantly elevated phytoplankton biomass in autumn and winter microcosms. Water temperature increases were followed by significant elevations in the oxygen saturation levels in all microcosms and seasons. The combined effect of increased temperature and P enrichments resulted in the highest chlorophyll-a levels also in autumn and winter. Summer microcosms seem to have been negatively affected by the experimental conditions (too high water temperatures); Conclusions: the bioassays showed that global temperature rises can significantly elevate the phytoplankton biomass in Peri coastal lagoon, especially in colder months and if followed by increased P inputs, what can lead to major ecological consequences to the water body and to water supply issues in the region.

Effects of two chelating agents (EDTA and DTPA) on the autochthonous vegetation of a soil polluted with Cu, Zn and Cd

Chelating agents are thought to be useful for the remediation of metal-polluted soils with adequate organic matter, but the effects of these substances on the plants or seed bank of plant communities that occur in the soils are relatively unknown. In this work, the effects of two chelating substances (EDTA and DTPA) on a wet grassland plant community affected by the presence of the abandoned copper mine "Fernandito" (Garganta de los Montes, Madrid) were compared. A microcosm bioassay (6 months) was designed using the soil's top layer containing the grassland's seed bank. This soil showed a high Cu pollution level, significant contents of Zn and Cd (1120, 190, and 15 ppm, respectively), a pH of 5 and an OM content of 6.2%. The soil was subjected to three different treatments: a) untreated soil (control), b) the addition of 1 g/kg EDTA, or of c) 1 g/kg DTPA. The results presented here are those related to the plant cover, species richness, aboveground and subterranean biomass and chemical composition of the most abundant plants. Neither EDTA nor DTPA caused intense negative effects on the plants rather they significantly increased the amount of copper accumulated in aboveground parts and roots. In particular, Agrostis castellana and Corrigiola telephiifolia extracted high amounts of copper when grown in the soil with added EDTA, although they showed some nutritional imbalances (lower P contents). In contrast, lower metal concentrations were detected in plants grown in the DTPA amended soil.

An assessment of the impact of copper mine tailings disposal on meiofaunal assemblages using microcosm bioassays

Microcosms were used to assess the impact of copper mine tailings disposal on the littoral meiofaunal assemblages of the Atacama region of northern Chile. The specific purpose was to establish a cause and effect relationship between the elevated copper concentrations and altered meiofaunal assemblages observed at the study sites. Meiofaunal assemblages were exposed to a series of copper concentrations to assess general toxicity, both densities and taxa diversities decreased with increasing copper. Natural coarse sediments were mixed with a tailings substitute to assess the physical impact of the tailings dumping on meiofaunal assemblages. Meiofaunal assemblage densities increased with increasing amounts of tailings substitute, entirely due to an increase in surface utilising foraminiferans. However, taxa diversities decreased as the interstitial spaces became blocked. Finally, the microcosms were used to conduct bioassays of sediments and seawaters from the impacted sites. The sediments from the impacted sites proved to be toxic resulting in reduced meiofaunal densities and taxa diversities. Seawater samples did not prove to be significantly toxic. The use of microcosms has allowed the effects of the physical and chemical components of tailings to be assessed individually, which was not possible in the field. Additionally, it allowed a cause and effect relationship to be established between elevated concentrations of porewater copper observed in the field and the reduced densities and taxa diversities of the meiofaunal assemblages observed at the same sites.

A sediment quality triad assessment of the impact of copper mine tailings disposal on the littoral sedimentary environment in the Atacama region of northern Chile

A sediment quality triad (SQT) assessment was made of the impact of copper mine tailings disposal on littoral meiofaunal assemblages in the Atacama region of northern Chile. This situation is unusual in that the disposal is direct into the high-energy coastal system and not via a river estuary or other low-energy environment. This situation also allows for the examination of the impact of copper mine tailings in the absence of confounding effects from other pollutants. The three components of the SQT were: 1. an analysis of the bioavailable metals in both the sedimentary porewater and the adjacent seawater, 2. a microcosm bioassay of both sediments and seawaters using meiofaunal assemblages, and 3. quantitative field samples of the meiofaunal assemblages. Twelve study sites with varying degrees of impact were used, including three reference sites. The study identified that both the meiofaunal assemblage densities and taxa diversities decrease with increasing levels of bioavailable copper, that the Foraminifera and Harpacticoida are sensitive to copper, and that otoplanid Turbellaria are often characteristic of impacted sites; tailings also have both chemical and physical impacts on the environment. In some cases the physical impact of tailings is more important in excluding some organisms e.g. the interstitial polychaete, Saccocirrus sonomacus, from a site than is their chemical impact.

Studies of the microbial P-cycle during a Lagrangian phosphate-addition experiment in the Eastern Mediterranean

Microbial uptake of orthophosphate was studied before and during a Lagrangian experiment where orthophosphate was added to the surface mixed layer in the Cyprus Gyre, Eastern Mediterranean, a region previously hypothesized to be characterized by P-limited growth of both phytoplankton and heterotrophic bacteria. The addition of ca. 110 nM orthophosphate to a ca. 16 km2 patch in situ led, within 1 day, to an increase in particulate-P from 8 to ca. 15 nM, a result in good agreement with a previous microcosm bioassay indicating this system to have a maximum capacity for orthophosphate consumption of between 10 and 25 nM phosphate. In samples of unperturbed water taken before the addition, outside, or below the experimental patch, orthophosphate turnover time (Tt) was <4 h, argued to be consistent with the assumption of diffusion-limited phytoplankton growth. Upon addition, Tt increased to 94 h. Estimates of maximum potential uptake rate (Vmax) for orthophosphate in unperturbed water exceeded by more than one order of magnitude the biological P-requirement (ν) as obtained from stoichiometric conversion of C-based primary and bacterial production values to estimated P-requirement. Upon addition of orthophosphate, Vmax decreased to a level comparable to ν. The observations are consistent with the assumption of P-starved cells before and P-replete cells with excess external orthophosphate after the addition. Orthophosphate uptake in unperturbed water was dominated by<1 μm organisms (mean ±SD between samples 0.56±0.03 μm). In samples with higher turnover time, orthophosphate uptake was shifted towards larger organisms, culminating after 5 days with a near doubling in mean size (1.08 μm). The size distribution of particulate-P standing stock had a mean size of 10 μm, indicating the presence of a substantial biomass of micro-organisms larger than those involved in P-uptake. Comparison of the measured particulate-P with microscope-based biomass estimates indicated a microbial food web dominated by heterotrophic organisms (70% of particulate-P), distributed with ca. 25% of total particulate-P in heterotrophic bacteria, ca. 40% in heterotrophic flagellates, and ca. 5% in ciliates. Concentration of bioavailable phosphate (Sn) estimated from the relationship Sn=νTi indicated Sn values<1 nM PO4 before the addition, increasing afterwards. Estimates of the sum Kt+Sn for the 0.6–0.2 μm size fraction were in the range 1–7 nM PO4 before and outside patch, suggesting this sum to be dominated by the half-saturation constant Kt. Kt+Sn increased to 69 nM after addition, then dropped over the following week back to background levels. As reported elsewhere in this volume, there was a decline in the observed chlorophyll concentrations, but a positive response in copepods. Less clear than the effects at the level of osmotroph physiology were the subsequent responses expected in the food web. Two possible mechanisms are discussed: (1) a positive response in bacterial production and the subsequent food chain of bacterial predators, and (2) a positive response in phytoplankton predators due to a shift in food quality rather than in food quantity.

Variation, replication, and power analysis ofMyriophyllumspp. microcosm toxicity data

Myriophyllum spp. have been proposed as a new standard laboratory aquatic macrophyte test species for the registration of pesticides. The main objectives of this investigation were to determine the power of Myriophyllum sibiricum and Myriophyllum spicatum toxicity data derived from an outdoor microcosm bioassay, to evaluate the variation of 10 different aquatic plant endpoints and to calculate the minimum detectable difference for these endpoints, to determine the replication required to detect ecologically significant changes from control for these endpoints, and to make recommendations for future field studies with Myriophyllum spp. Control data from four different studies that characterized haloacetic acid toxicity with Myriophyllum spp. for durations of three to six weeks during the summer of 1999 with five treatment levels (n = 3), including control, were examined. Endpoint coefficient of variation ranged, on average, from 6 to 28%. Node number and plant length endpoints were consistently the most statistically powerful for both plant species. It was possible to detect approximately 30% change from control in both endpoints with high statistical power (beta = 0.2, alpha = 0.05, n = 3). The range of minimum detectable differences was 40 to 60% for the other endpoints. Replication to detect a > or = 25% change from control would require an n of 2 to 21, depending on the endpoint. Myriophyllum sibiricum had slightly lower coefficients of variation and thus required fewer replicates than M. spicatum to be statistically significantly different from control values. Variation within microcosm studies was not significantly different from that of controlled laboratory studies, implying that most of the variation observed in the field is inherent in the plants. Based on statistical sensitivity, ecological relevance, and toxicological sensitivity, we recommend using plant length and root endpoints as indicators of toxicity under field conditions.