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Sediment bioassays, chemical contaminants and benthic ecology: New insights or just muddy water?
Marine Environmental Research 27 (1989) 73-75
Editorial
SEDIMENT BIOASSAYS, CHEMICAL CONTAMINANTS AND BENTHIC ECOLOGY: NEW INSIGHTS OR JUST M U D D Y WATER? Sediment bioassays are used widely in the United States to make decisions about the treatment of contaminated sediments and may be used in decisions aimed at controlling the sources of contaminants to coastal waters. These assays involve placing organisms on test sediments or in suspensions of these sediments for a prescribed period and determining mortality or some measure of sublethal effect. In addition sediment bioassay results are being compared to concentrations of chemicals in sediments and the indigenous fauna to draw conclusions about population and communitylevel effects of anthropogenic chemicals. Two such 'synoptic' methods are the sediment triad(Chapman et al., 1987) and the apparent effects threshold (PTI, 1988). Both are similar in that they infer that observed increases in metals and organic contaminants in a set of sediment samples are causing any correlated increased mortalities in assays or decreased abundance of organisms from the same sediments. In the sediment triad approach three factors (assay results, contaminants and infaunal abundance) are correlated on a three-axis graph. In the apparent effects threshold biological and chemical data from a large number of stations in~ one area (e.g. Puget Sound, Washington) are compared. A 'threshold' of effects for each chemical is defined as that concentration of a substance in sediments above which there is always an associated negative biological effect (from assays or ecological analysis). The synoptic 73 Marine Environ. Res. (27) (1989)--~ 1989 Elsevier Science Publishers Ltd, England. Printed
in Great Britain
74
Editortal
approaches have been applied in several coastal areas and are being considered for adoption by several governmental entities in the United States. If these assays and synoptic approaches measure the effects of anthropogenic chemicals in ecosystems then they represent a major breakthrough in understanding marine contamination. Their assumptions about chemical toxicity, however, deserve further consideration. At the heart of these measures are two factors: (1) the ability of organisms to survive in sediments and sediment suspensions that have elevated concentrations of some substances and are also fine-grained and enriched in organic carbon and (2) the causes of ecological changes in soft-bottom benthic communities. For the assays, some of the test organisms come from sandy habitats and are ill-adapted to survive in muds. Mud suspensions might be expected to clog and abrade sensitive tissues, such as gills, leading to injury and death. Some data show that mortality in these assays are more closely related to the proportion of fine-grained sediments or total organic carbon content than any of the chemical contaminants measured (Long & Buchman, 1989). For the ecological data the presence and abundance of organisms is interpreted solely in terms of chemical toxicity. Because there is almost no mechanistic understanding of the toxicity that occurs in sediment bioassays, considerable caution is appropriate in their application (see Swartz, 1988). Much can be done to increase our understanding of chemical toxicity associated with sediment-bound chemicals, particularly the relationships between concentrations of chemicals in sediments and porewaters and bioavailability. With increased knowledge more enlightened use of sediment bioassays may be possible. The synoptic approaches are drawing conclusions about toxicity based on correlations that are subject to other ecological interpretations. For example, benthic ecologists have long known that transitions from sandy and silty sediments to muddy sediments are accompanied by changes in species composition, numbers and diversity of benthic organisms. The organic loading to muddy sediments is usually higher than to coarser sediments, and certain species can better exploit this food supply. Other factors being equal, sand-to-mud transitions are also accompanied by increases in the concentrations of metals and hydrophobic organic contaminants due to increases in binding capacity of the finer sediments. To ascribe ecological changes in these transitions to the accompanying increases in these chemicals would seem to be at least arguable in all cases and plainly wrong in most cases. Clearly, much work needs to be done to separate the ecological effects of contaminants from those of organic enrichment (e.g. Spies et al., 1988). Both the triad and the apparent effects threshold concepts were initially developed with data from the 48-h bivalve larvae sediment bioassay
Editorial
75
(Chapman & Morgan, 1983) and Rhepoxinius abronius sediment bioassay (Swartz et al., 1985). Recent data from San Francisco Bay suggest that both assays may be highly sensitive to proportions of fine-grain sediment and total organic carbon content of sediment (Long & Buchman, 1989). Previous data on the Rhepoxinius abronius assay had suggested a similar effect (Dewitt el al., 1988). It is clear that in coastal waters that are not experiencing chemical toxicity that sampling a variety of sediment types will provide data that will show increases in contaminants that correlate with decreases in some species, and possibly increased mortalities in some sediment bioassays. Therefore, the principles underlying these methods are not convincingly established. This leads to a high probability of false positives and drawing erroneous conclusions about chemical toxicity of sediments. U n d e r such circumstances the triad and the apparent effects threshold do not, at this time, seem suited for use in regulatory decision-making regarding disposal, treatment or disposition of sediments carrying measurable amounts of contaminant chemicals.
References Chapman, P. M. & Morgan, J. D. (1983). Sediment bioassays with oyster larvae. Bull. Environ. Contamin. ToxicoL, 31, 438--44. Chapman, P. M., Dexter, R. N. & Long, E. R. (1987). Synoptic measures of sediment contamination, toxicity and infaunal community composition (the sediment quality triad) in San Francisco Bay. Mar. Ecol. Prog. Set., 37, 75-96. Dewitt, T. H., Ditsworth, G. R. & Swartz, R. C. (1988). Effects of natural sediment features on survival of the phoxocephalid amphipod Rhepoxinius abronius. Mar. Environ. Res., 25, 99-124. Long, E. R. & Buchman, M. F. (1989). An evaluation of candidate measures of biological effects for the national status and trends program. NOAA Techn. Mem. NOS ONA 45. Seattle, Washington. 105 pp. + appendices. PTI (1988). The apparent effects threshold. Briefing report to the EPA Science Advisory Board. PTI Environmental Services, Bellvue, Washington. September, 1988, USA. 57 pp. Spies, R. B., Hardin, D. D. & Toal, J. P. (1988). Organic enrichment or toxicity? A comparison of the effects of kelp and crude oil in sediments on the colonization and growth of benthic infauna. J. Exp. Mar. Biol. Ecol., 124, 261-82. Swartz, R. C. (1988). Toxicological methods for determining the effects of contaminated sediment on marine organisms. In Fate and Effects of Sedimentbound Chemicals in Aquatic_Systems. ed. K. L. Dickson, A. W. Maki & W. A. Brungs. Pergamon Press, New York, pp. 183-97. Swartz, R. C., DeBen, W. A., Phillips, J. K., Lamberson, J. O. & Cole, F. A. (1985). Phoxocephalid amphipod bioassay for marine sediment toxicity. In Aquatic Toxicology and Hazard Assessment: Seventh Symposium. ed. R. D. Cardwell, R. Purdy & R. C. Bahner. ASTM STP 854, American Society for Testing and Materials, Philadelphia. R. B. Spies
Editorial
SEDIMENT BIOASSAYS, CHEMICAL CONTAMINANTS AND BENTHIC ECOLOGY: NEW INSIGHTS OR JUST M U D D Y WATER? Sediment bioassays are used widely in the United States to make decisions about the treatment of contaminated sediments and may be used in decisions aimed at controlling the sources of contaminants to coastal waters. These assays involve placing organisms on test sediments or in suspensions of these sediments for a prescribed period and determining mortality or some measure of sublethal effect. In addition sediment bioassay results are being compared to concentrations of chemicals in sediments and the indigenous fauna to draw conclusions about population and communitylevel effects of anthropogenic chemicals. Two such 'synoptic' methods are the sediment triad(Chapman et al., 1987) and the apparent effects threshold (PTI, 1988). Both are similar in that they infer that observed increases in metals and organic contaminants in a set of sediment samples are causing any correlated increased mortalities in assays or decreased abundance of organisms from the same sediments. In the sediment triad approach three factors (assay results, contaminants and infaunal abundance) are correlated on a three-axis graph. In the apparent effects threshold biological and chemical data from a large number of stations in~ one area (e.g. Puget Sound, Washington) are compared. A 'threshold' of effects for each chemical is defined as that concentration of a substance in sediments above which there is always an associated negative biological effect (from assays or ecological analysis). The synoptic 73 Marine Environ. Res. (27) (1989)--~ 1989 Elsevier Science Publishers Ltd, England. Printed
in Great Britain
74
Editortal
approaches have been applied in several coastal areas and are being considered for adoption by several governmental entities in the United States. If these assays and synoptic approaches measure the effects of anthropogenic chemicals in ecosystems then they represent a major breakthrough in understanding marine contamination. Their assumptions about chemical toxicity, however, deserve further consideration. At the heart of these measures are two factors: (1) the ability of organisms to survive in sediments and sediment suspensions that have elevated concentrations of some substances and are also fine-grained and enriched in organic carbon and (2) the causes of ecological changes in soft-bottom benthic communities. For the assays, some of the test organisms come from sandy habitats and are ill-adapted to survive in muds. Mud suspensions might be expected to clog and abrade sensitive tissues, such as gills, leading to injury and death. Some data show that mortality in these assays are more closely related to the proportion of fine-grained sediments or total organic carbon content than any of the chemical contaminants measured (Long & Buchman, 1989). For the ecological data the presence and abundance of organisms is interpreted solely in terms of chemical toxicity. Because there is almost no mechanistic understanding of the toxicity that occurs in sediment bioassays, considerable caution is appropriate in their application (see Swartz, 1988). Much can be done to increase our understanding of chemical toxicity associated with sediment-bound chemicals, particularly the relationships between concentrations of chemicals in sediments and porewaters and bioavailability. With increased knowledge more enlightened use of sediment bioassays may be possible. The synoptic approaches are drawing conclusions about toxicity based on correlations that are subject to other ecological interpretations. For example, benthic ecologists have long known that transitions from sandy and silty sediments to muddy sediments are accompanied by changes in species composition, numbers and diversity of benthic organisms. The organic loading to muddy sediments is usually higher than to coarser sediments, and certain species can better exploit this food supply. Other factors being equal, sand-to-mud transitions are also accompanied by increases in the concentrations of metals and hydrophobic organic contaminants due to increases in binding capacity of the finer sediments. To ascribe ecological changes in these transitions to the accompanying increases in these chemicals would seem to be at least arguable in all cases and plainly wrong in most cases. Clearly, much work needs to be done to separate the ecological effects of contaminants from those of organic enrichment (e.g. Spies et al., 1988). Both the triad and the apparent effects threshold concepts were initially developed with data from the 48-h bivalve larvae sediment bioassay
Editorial
75
(Chapman & Morgan, 1983) and Rhepoxinius abronius sediment bioassay (Swartz et al., 1985). Recent data from San Francisco Bay suggest that both assays may be highly sensitive to proportions of fine-grain sediment and total organic carbon content of sediment (Long & Buchman, 1989). Previous data on the Rhepoxinius abronius assay had suggested a similar effect (Dewitt el al., 1988). It is clear that in coastal waters that are not experiencing chemical toxicity that sampling a variety of sediment types will provide data that will show increases in contaminants that correlate with decreases in some species, and possibly increased mortalities in some sediment bioassays. Therefore, the principles underlying these methods are not convincingly established. This leads to a high probability of false positives and drawing erroneous conclusions about chemical toxicity of sediments. U n d e r such circumstances the triad and the apparent effects threshold do not, at this time, seem suited for use in regulatory decision-making regarding disposal, treatment or disposition of sediments carrying measurable amounts of contaminant chemicals.
References Chapman, P. M. & Morgan, J. D. (1983). Sediment bioassays with oyster larvae. Bull. Environ. Contamin. ToxicoL, 31, 438--44. Chapman, P. M., Dexter, R. N. & Long, E. R. (1987). Synoptic measures of sediment contamination, toxicity and infaunal community composition (the sediment quality triad) in San Francisco Bay. Mar. Ecol. Prog. Set., 37, 75-96. Dewitt, T. H., Ditsworth, G. R. & Swartz, R. C. (1988). Effects of natural sediment features on survival of the phoxocephalid amphipod Rhepoxinius abronius. Mar. Environ. Res., 25, 99-124. Long, E. R. & Buchman, M. F. (1989). An evaluation of candidate measures of biological effects for the national status and trends program. NOAA Techn. Mem. NOS ONA 45. Seattle, Washington. 105 pp. + appendices. PTI (1988). The apparent effects threshold. Briefing report to the EPA Science Advisory Board. PTI Environmental Services, Bellvue, Washington. September, 1988, USA. 57 pp. Spies, R. B., Hardin, D. D. & Toal, J. P. (1988). Organic enrichment or toxicity? A comparison of the effects of kelp and crude oil in sediments on the colonization and growth of benthic infauna. J. Exp. Mar. Biol. Ecol., 124, 261-82. Swartz, R. C. (1988). Toxicological methods for determining the effects of contaminated sediment on marine organisms. In Fate and Effects of Sedimentbound Chemicals in Aquatic_Systems. ed. K. L. Dickson, A. W. Maki & W. A. Brungs. Pergamon Press, New York, pp. 183-97. Swartz, R. C., DeBen, W. A., Phillips, J. K., Lamberson, J. O. & Cole, F. A. (1985). Phoxocephalid amphipod bioassay for marine sediment toxicity. In Aquatic Toxicology and Hazard Assessment: Seventh Symposium. ed. R. D. Cardwell, R. Purdy & R. C. Bahner. ASTM STP 854, American Society for Testing and Materials, Philadelphia. R. B. Spies