Investigation of metal toxicity to tropical biota: Recommendations for revision of the Australian water quality guidelines
Supervising Scientist Report 127
Markich SJ and Camilleri C
Supervising Scientist, 1997
ISBN 0 642 24327 1
- SSR127 - Investigation of metal toxicity to tropical biota: Recommendations for revision of the Australian water quality guidelines (PDF - 3,970 KB)
The current Australian water quality guidelines for the protection of aquatic life are based predominantly on toxicological data derived from North American temperate studies. This is due to a lack of relevant toxicological data for Australian species. Thus, there is a clear requirement to assess the suitability of the current guidelines with respect to Australian biota and environmental conditions. A major objective of this study was to collate all known metal (including metalloid) toxicity data for aquatic biota in tropical Australia, as part of a broader study to test whether temperate North American metal toxicity data are applicable to the tropics of Australia.
The specific objectives of this study were to:
- review available data on the toxicity of metals to aquatic biota in tropical Australia;
- identify metals considered to be priority toxicants to aquatic biota in tropical Australia; and
- employ previously developed toxicity testing protocols for two tropical freshwater species to obtain preliminary toxicity data for two priority metals.
From the literature review, it was concluded that insufficient metal toxicity data exist for Australian tropical species. Data were absent for a range of metals (eg Ag, As, Al, Cr, Hg, Ni, Sb and Se) listed in the current Australian water quality guidelines. Aluminium, Cd, Co, Cu, Ni, Mn, Pb, U, V and Zn were identified as priority metals of potential ecotoxicological concern in aquatic ecosystems of tropical Australia, largely as a consequence of mining activities, but also from urban impacts. Instead of testing the toxicity of the priority metals for which data do not currently exist (ie Al, Co, Ni and V), it was deemed more important to conduct further experimental work on Cu and U, in the context of elucidating the relatively high variability in the toxic response of these two metals. As a result, Cu and U were selected and toxicity tests conducted using two tropical freshwater species (green hydra (Hydra viridissima) and gudgeon fish (Mogurnda mogurnda)) from the Australian wet/dry tropics using test protocols designed to maximise the greatest sensitivity of metal response in the shortest period of time.
Population growth and survival were selected as the toxicological endpoints for H. viridissima and M. mogurnda, respectively. A four-parameter logistic regression model provided the best fit for the sigmoidal relationship between the selected responses of each organism and the measured total metal concentration. Using this regression model, the EC50 (median effect concentration) and an LC50 (concentration at 50% survival) were calculated for H. viridissima and M. mogurnda, respectively, for both Cu and U. The BEC10 (10% bounded effect concentration), an alternative statistical measure to the no-observed-effect-concentration (NOEC) and the MDEC (minimum detectable effect concentration), an alternative measure to the lowest-observed-effect-concentration (LOEC), were also calculated. A standardised synthetic test water (ie soft, slightly acidic, with a low buffering and complexation capacity), typical of sandy-braided streams throughout the Australian wet/dry tropics, was used for the experimental studies to maximise the toxic response to the organisms.
Results from the experimental work showed that H. viridissima were more sensitive to both Cu and U than were M. mogurnda. However, the difference in sensitivity was not equivalent for each metal. For example, the MDECs of Cu and U (as UO2) for H. viridissima were 1.8 and 61 µg L-1, respectively, whereas for M. mogurnda they were 13.4 and 1298 µg L-1, respectively. H. viridissima was about eight times more sensitive to Cu than U, whereas M. mogurnda was about twenty times more sensitive. Once differences between the sublethal and lethal endpoints of the two organisms were corrected by statistical extrapolation, H. viridissima was approximately seven times more sensitive than M. mogurnda to U, but only about three times more sensitive to Cu. Both species were more sensitive to Cu than U. These results are generally consistent with those from previous studies when differences in key water quality variables, including water hardness, alkalinity, pH and dissolved organic carbon (DOC), are considered.
The current Australian water quality guidelines do not specify a protection level for U, or several other metals (eg Co, Mn and V), considered to be of potential toxicological concern in tropical Australian waters. Based on the results of this study, a guideline value of 5 µg L-1 is recommended for total U in (tropical) Australia. Similarly, a guideline value of 1 µg L-1 is recommended for total Cu in very soft waters (ie hardness <10 mg L-1 as CaCO3). However, this guideline value approximates background (ie negligible anthropogenic disturbance) Cu concentrations measured in freshwater systems throughout tropical Australia.
The incorporation of key water quality variables, such as water hardness, alkalinity, pH and DOC (in the form of fulvic and humic acids), into further revisions of the Australian and New Zealand water quality guidelines could effectively address the issue of including bioavailable metal concentrations, instead of total concentrations. However, it is expected that only water hardness (which also incorporates alkalinity) will be used to quantitatively modify guideline values for selected metals (ie Cd, Cr(III), Cu, Ni, Pb and Zn) in the near future. It is recommended that further work should address the effects of pH and DOC (in the form of fulvic and humic acids) on the uptake and toxicity of Cu and U, as well as other priority metals of toxicological concern in tropical Australia. This information will potentially provide greater accuracy in determining the impact of metals in freshwater ecosystems, and thus, greater refine ecological risk assessments.
To provide a preliminary assessment of the suitability of the current Australian water quality guidelines for protecting aquatic life, a comparison of the toxicity of Cu and U to freshwater crustaceans and fish from North America and tropical Australia was performed. Copper and U were selected because they provided the most comprehensive toxicological dataset for tropical Australian species. The comparative study showed an overlap in the range of Cu and U toxicity values for both freshwater species of Crustacea and fish from tropical Australia and temperate North America, under conditions of comparable water chemistry. The level of confidence was greatest for Cu toxicity data on fish and least for U toxicity data on Crustacea. In general, few U toxicity data are available, particularly for North American species. Additional Cu toxicity data on tropical Australian species of crustaceans are required before reliable comparisons can be made with North American.
Based on several Australian toxicity studies, including this study, it was concluded that the Australian water quality guidelines, derived largely from North American toxicity data, are appropriate for Australian species and conditions when key water quality variables (eg temperature, water hardness and alkalinity) are considered. It is therefore proposed that North American toxicity data can be used to derive Australian water quality guidelines for protecting aquatic life in circumstances where toxicity data are either absent or scant for Australian aquatic life.