Bart T.A. Bossuyt

Field validation of sediment zinc toxicity

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.

Tolerance and acclimation to zinc of field-collected Daphnia magna populations

The zinc tolerance of two Daphnia magna populations collected at a zinc contaminated site was studied. One clone was isolated from each population in order to determine interclonal variation in zinc tolerance. 48hEC50-values, life table parameters, carapace lengths and cellular energy allocation (CEA) were used as test endpoints and compared with the results obtained with a standard laboratory clone. The natural clones were more tolerant to acute zinc toxicity (up to a factor of 4) and exhibited a higher reproduction rate (factor 2) and carapace length (factor 1.2). The optimal zinc concentrations for the natural clones ranged from 80 to 200 microg Zn/l. When cultured without zinc, the natural clones gradually lost their zinc tolerance. Therefore, the environmental relevance of using toxicity data obtained with organisms (natural, as well as laboratory clones) acclimated to culture media containing no or very small amounts of zinc can be questioned.

Influence of multigeneration acclimation to copper on tolerance, energy reserves, and homeostasis of Daphnia magna straus

A multigeneration acclimation experiment was performed with Daphnia magna exposed to copper to assess possible changes in tolerance and to establish the optimal concentration range (OCEE) of this species. The hypothesis was tested that as the bioavailable background concentration of an essential metal increases (within realistic limits), the natural tolerance (to the metal) of the acclimated/adapted organisms and communities will increase. During 18 months the daphnids were exposed to six different, environmentally relevant, copper background concentrations ranging between 0.5 and 100 microg Cu L(-1) (7 x 10(-15) and 3.7 x 10(-9) M Cu2+). An increase in acute (effect concentration resulting in 50% immobility: 48-h EC50) and chronic copper (effect concentration resulting in 50% or 10% reproduction reduction: 21-d EC50, 21-d EC10) tolerance was observed with increasing exposure concentration. The 48-h EC50 increased significantly from 204 +/- 24 microg Cu L(-1) to 320 +/- 43 microg Cu L(-1). A nonsignificant change from 48.0 (47.9-48.0) microg Cu L(-1) to 78.8 (66.3-93.6) microg Cu L(-1) was noted in the chronic toxicity assays. The optimal concentration range was assessed using different biological parameters (i.e., net reproduction [R0]), energy reserves (Ea), body length measurements, filtration rates, and body burdens. After three generations of acclimation the OCEE ranged between 1 and 35 microg Cu L(-1) (2 X 10(-14) to 80 x 10(-12) M Cu2+). Body burden measurements revealed an active copper regulation up to 35 microg Cu L(-1) (80 pM Cu2+). It can be concluded that acclimation of D. magna to copper does occur in laboratory experiments, even at realistic copper background concentrations (10(-11) - 10(-9) M Cu2+). However, it is suggested that this phenomenon is of less importance in the context of regulatory risk assessments. An optimal copper concentration range for D. magna was observed between 1 and 35 microg Cu L(-1) (10(-14) - 10(-11) M Cu2+), indicating that copper deficiency can occur in routine laboratory cultures.

Variability of the protective effect of sodium on the acute toxicity of copper to freshwater cladocerans

The acute biotic ligand model (BLM) is proposed by the U.S. Environmental Protection Agency (U.S. EPA) to incorporate bioavailability in calculating aquatic life criteria for Cu in freshwater. This approach currently assumes that the effects of water chemistry on acute Cu toxicity can be described with one single set of identical BLM parameters for all organisms. An important water characteristic is the concentration of Na, which protects aquatic organisms against Cu toxicity. Based on physiological considerations related to the mechanism of Cu toxicity and the possible role of Na therein, we hypothesized that an interspecies variability of the protective effect of sodium on Cu toxicity might exist among freshwater organisms. To test this hypothesis, acute 48-h toxicity assays with Cu were conducted with 16 field-collected cladoceran populations and a laboratory-reared clone of Ceriodaphnia dubia at Na concentrations of between 0.077 and 10 mM. Increased Na protected all but one population. Contrary to what the BLM predicts, however, an upper limit to this protective effect was observed for some populations at Na concentrations of greater than 4 mM. This may suggest that processes other than just Cu-Na competition at a single unidentate biotic ligand site may be involved in Cu toxicity. Between populations, conditional stability constants for binding of Na to the biotic ligand (log K*(NaBL)), which quantify the protective effect of Na, varied between 2.2 and 4.4. Higher log K values generally were associated with more sensitive populations. Although a full mechanistic explanation for our observations is lacking, our data may potentially be used to refine the U.S. EPA procedure to establish site-specific water-quality criteria for Cu.

Relevance of generic and site‐specific species sensitivity distributions in the current risk assessment procedures for copper and zinc

Species sensitivity distributions (SSD) were constructed using acute toxicity data of various cladoceran species collected in five different aquatic systems. The aim of this research was to study the relative acute cladoceran community sensitivity in different aquatic systems. Current risk assessment procedures are based upon hypothetical communities and do not take into account variation in species composition and tolerance between aquatic communities. Two metals, copper and zinc, were used as model toxicants. To establish comparative sensitivity, a standard medium (International Organization for Standardization [ISO]) was used. The generic SSD (log-normal distribution) based on toxicity data obtained in this standard medium for all species (collected at all sites) resulted in a hazardous concentrations that protects 95% of the species occurring in a (hypothetical) ecosystem (i.e., hazardous concentration protecting 95% of the species of the hypothetical ecosystem [HC5]) of 6.7 microg Cu L(-1) (90% confidence limits: 4.2-10.8) and 559 microg Zn L(-1) (375-843). This generic SSD was not significantly different from the site-specific SSDs (i.e., constructed with species only occurring at a specific site). Mean community sensitivity (the geometric mean of 48-h 50% effective concentration [EC50] values of species within a community) among sites varied within a factor of 2 (between 17.3 and 23.6 microg Cu L(-1) for Cu and between 973 and 1,808 microg Zn L(-1) for Zn), and HC5s varied within a factor of 4 for copper (between 4.5 and 17.3 microg Cu L(-1)) and 7 for zinc (between 194 and 1,341 microg Zn L(-1)). For copper, the HC50 of our generic SSD was significantly lower than the one based on literature toxicity data of cladoceran species (which were recalculated to the hardness of our standard medium). In contrast, no significant differences were observed between the generic SSD and the literature-based SSD for zinc. It is suggested that the community sensitivity of different cladoceran populations is similar among aquatic systems and is not dependent on the species composition.