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One of the most internationally used bioassays for toxicity screening of chemicals and for toxicity monitoring of effluents and contaminated waters is the acute toxicity test with daphnid crustaceans, and in particular that performed with Daphnia magna.Standard methods have been developed for this assay that were gradually endorsed by national and international organisations dealing with toxicity testing procedures, in view of its application within a regulatory framework. As for all toxicity tests, the organisms used for the acute D. magna assay have to be obtained from live stocks which are cultured in the laboratory on live food (micro-algae).Unsurprisingly the various standard protocols of this particular assay differ – at least to a certain extent – with regard to the test organism culturing conditions. In addition, some technical aspects of the toxicity test such as the effect criterion (mortality of immobility), the exposure time, the type of dilution water, etc., also vary from one standard to another.Although this particular assay is currently used in many countries, the technical and biological problems inherent in year-round culturing and availability of the biological material and the culturing/maintenance costs of live stocks restrict its application to a limited number of highly specialised laboratories.This fundamental bottleneck in toxicity testing triggered investigations which brought forward the concept of “microbiotests” or “small-scale” toxicity tests. “Culture/maintenance free” aquatic microbiotests with species of different phylogenetic groups were developed in the early 1990s at the Laboratory for Environmental Toxicology and Aquatic Ecology at the Ghent University in Belgium.These assays which were given the generic name “Toxkits”, are unique in that they employ dormant stages (“cryptobiotic eggs”) of the test species, which can be stored for long periods of time and “hatched” at the time of performance of the assays.One of these microbiotests is the Daphtoxkit F magna, which is currently used in many laboratories worldwide for research as well as for toxicity monitoring purposes.The microbiotest technology has several advantages in comparison to the “traditional” tests based on laboratory cultures, especially its independence of the stock culturing burden. However, the acceptance (or possible non-acceptance) of performing assays with test organisms obtained from “dormant eggs” should be clearly dictated by the “sensitivity” and “precision” criteria of the former assays in comparison to the latter.The first part of this review therefore thoroughly reviews the scientific literature and of data obtained from various laboratories for assays performed with either D. magna test organisms obtained from lab cultures or hatched from dormant eggs.Attention has focused on data of quality control tests performed on reference chemicals, and in particular on potassium dichromate (K2Cr2O7) for which an acceptability range of 0.6–2.1 mg·L–1 has been set in ISO standard 6341 for the 24 h EC50 of the acute D. magna assay.Mean EC50s, standard deviations and variation coefficients were calculated from the collected data, all of which are presented in tables and figures and discussed in detail.The major conclusions drawn from the analysis of the large number of quality control (QC) data on the acute D. magna toxicity test are that :(1) Virtually all results from assays performed with Daphnias taken from lab cultures or with Daphnia microbiotests are within the acceptability range set by ISO standard 6341 for the reference chemical potassium dichromate.(2) The mean 24 h EC50s of the Daphnia microbiotests performed in different laboratories are within the range of the mean EC50s of the assays based on lab cultures, and the variation coefficients (20 to 30%) are similar.(3) The precision – in terms of the long term in house variability – of the quality control Daphnia microbiotests is as good as that of the QC tests based on lab cultures.The review further reports on intra-laboratory sensitivity comparison studies performed during the last 15 years on pure chemicals and on natural samples, with both laboratory cultured organisms and Daphnias hatched from dormant eggs. These studies carried out in different laboratories showed EC50 correlation coefficients of 0.86 to 0.98, corroborating a similar sensitivity of the two types of test organisms.The third part of the review reports and analyses data on proficiency ringtests on the acute D. magna assay which have been organised in different countries since 2002 with either reference chemicals or with natural samples, and in which part of the laboratories performed their assays with Daphnia microbiotests and others with lab cultured Daphnias.The conclusions drawn from all the ringtests indicate that the sensitivity of Daphnia neonates hatched from dormant eggs is similar to that of test organisms taken from lab cultures and that in most cases the precision of the Daphnia microbiotest is superior to that of the assays based on lab cultures.The review finally addresses the issue of possible sensitivity differences of Daphnias hatched from dormant eggs which are produced by different D. magna strains.From these investigations it appeared that the EC50s from assays performed with Daphnias hatched from dormant eggs of different strains did not differ significantly from those from assays undertaken with daphnids from lab cultures.The obvious advantages of Daphnia microbiotests over tests with Daphnias stemming from lab cultures have led to the worldwide use of these culture/maintenance free and low cost small-scale assays in both research and toxicity monitoring.The Daphnia microbiotest is in current use in several countries for toxicity testing in a regulatory framework, and recent calculations indicate that about 10 000 acute D. magna assays are now performed annually with neonates hatched from dormant eggs.The use of dormant eggs to obtain test organisms independently of stock culturing has recently also been accepted in international standards for toxicity testing. ISO standard 20665 (2008) related to the determination of chronic toxicity with Ceriodaphnia dubia, and ISO standard 20666 (2008) for the determination of the chronic toxicity with Brachionus calyciflorus in 48 h, both indicate that the assays can be conducted with organisms hatched from dormant eggs.On the basis of the extensive scientific evidence provided in this review that is justifiably supported by the two ISO methods mentioned above, the authors therefore recommend that the use of Daphnias hatched from dormant eggs should also be incorporated in national and international standards, as an alternative to the use of Daphnias taken from laboratory cultures.

The scientific literature reports dozens of bioassay-based methods for the assessment of the sediment quality, many of them even officially standardized by national or international organizations (ASTM, OECD, ISO, EPA, etc.). Each one offers specific advantages, and suffers of possible disadvantages, but practically all have two major drawbacks: they are usually time-consuming (especially in the case of chronic bioassays), and fail to univocally identify the actual cause of the detected toxicity or the detected impairment of the investigated biological characteristic(s). Therefore, this paper will review the possible use of "sediment quality guidelines" (SQG), i.e. "numerical chemical concentrations intended to be either protective of biological resources, or predictive of adverse effects to those resources, or both".

Sediment acid volatile sulfide (AVS) concentrations were measured in wadeable streams of a wide variety of ecoregions of western Europe (84 sites in 10 countries and nine ecoregions) to better understand spatial distribution and ecoregion relationships. Acid volatile sulfide has been shown to be a major factor controlling the bioavailability and toxicity of many common trace metals, such as Cd, Cu, Ni, Pb, and Zn. Sediment characteristics varied widely. The ratio of the sum of the simultaneously extracted metals (SEM) to AVS ranged from 0.03 to 486.59. The sigmaSEM-AVS ranged from -40.02 to 17.71 micromol/g. On a regional scale, sediment characteristics such as dominant parent soil material showed significant trends in AVS distribution and variation by ecoregion. Total Fe and Mn were correlated weakly with SEM concentrations. Three AVS model approaches (i.e., the SEM:AVS ratio, SEM-AVS difference, and carbon normalization) were compared at threshold exceedance levels of SEM/AVS > 9, SEM-AVS > 2, and SEM-AVS/foc > 150 micromol/g organic carbon (OC). Only 4.76% of the sediments exceeded all three AVS thresholds; 22.6% of the sediments exceeded two models; and 13% of the sediments exceeded one model only. Using the SEM:AVS, SEM-AVS, and fraction of organic carbon models, and including site-specific data and regional soil characteristics, ecoregions 1 (Portugal), 3 (Italy), 4 (Switzerland), and 9 (Belgium/Germany) had the highest potential metals toxicity; ecoregions 13 and 8 (Belgium/France) showed the lowest potential toxicity. However, because AVS can vary widely spatially and temporally, these data should not be considered as representative of the sampled ecoregions. The general relationship between AVS levels and sediment characteristics provides some predictive capability for wadeable streams in the European ecoregions.

A field study was conducted to validate concentrations of zinc in freshwater sediments that are tolerated by benthic macroinvertebrate communities and to determine whether a relationship exists with the acid volatile sulfide (AVS)-simultaneously extracted metal (SEM) model. In both the lake and riverine systems, one sediment type was high in AVS and one low in AVS, which resulted in zinc-spiked sediments that ranged from low to high SEM to AVS ratios. The colonization trays were sampled seasonally, ranging from 6 to 37 weeks of exposure, and were evaluated using several appropriate benthic indices. Results of the field evaluations at the four test sites confirmed the validity of the AVS-SEM model, predicting benthic macroinvertebrate effects correctly 92% of the time. In sediments where the SEM to AVS ratio or the AVS and organic (OC)-normalized fractions exceeded 8 and 583 micromol/g of OC, toxicity was observed from the zinc-spiked sediments. Conversely, when the SEM to AVS ratio or OC-normalized AVS fractions were less than 2 or 100 micromol/g of OC, no toxicity was observed. In the range of 148 to 154 micromol/g of OC, toxicity varied in two treatments. Total zinc concentrations in sediments showed no relationship to benthic effects. The most impaired benthic community occurred in the high-gradient stream sediments, which had low OC and AVS concentrations and SEM to AVS ratios of 33 and 44 in the spiked sediments. Five to six benthic metrics were depressed at SEM to AVS ratios of 8.32 and 9.73. The no-observed-effect level appeared to be near a SEM to AVS ratio of 2, with slight to no effects between ratios of 2.34 and 2.94. No sites with ratios of less than 2 showed any adverse effects.

Cadmium enters the aquatic environment from numerous sources, e.g., via the atmosphere, from which the cadmium released by combustion, mainly of fossil fuels, is deposited. Since some fertilizers contain up to 40 mg/kg cadmium, wash-out from agricultural land is another source. Zinc ores contain up to several percent of cadmium, and there may be high concentrations in raw zinc, zinc alloys and zinc compounds, so that cadmium may enter the aquatic environment from mining residues, solid wastes and wastewater discharges. Once in the aquatic environment, cadmium is highly mobile. Its dissolved species are extremely labile, and are the first to be released, e.g., when salinity increases in estuaries. Sediments reflect the metal loading in the past, and their analysis can be the key to an understanding of the fate of anthropogenic discharges into the hydrosphere. The internal cadmium loading of aquatic systems can be estimated from the remobilization rates of cadmium from sediments.