ASTM E2122-02(2013)

5.1 The ecological importance of bivalves, their wide geographic distribution, ease of handling in the laboratory and the field, and their ability to filter and ingest large volumes of water and sediment particles make them appropriate species for conducting field bioassays to assess bioaccumulation potential and associated biological effects. The test procedures in this guide are intended to provide guidance for conducting controlled experiments with caged bivalves under “natural,” site-specific conditions. It is important to acknowledge that a number of “natural” factors can affect bivalve growth and the accumulation of chemicals in their tissues (Section 6, Interferences). This field bioassay can also be conducted in conjunction with laboratory bioassays to help answer questions raised in the field exposures. The field exposures can also be used to validate the results of laboratory bioassays.

5.2 The ultimate resources of concern are communities. However, it is often difficult or impossible to adequately assess the ecological fitness or condition of the community or identify and test the most sensitive species. Bivalves are recommended as a surrogate test species for other species and communities for the following reasons: (1) They readily accumulate many chemicals and show sublethal effects associated with exposure to those chemicals (2); (2) they accumulate many chemicals through multiple pathways of exposure, including water, sediment, and food (22, 23, 24, 25, 26, 27), and (3) caged bivalves have been shown to represent effects on the benthos more accurately than traditional laboratory tests (28, 29). Although bivalve species might be considered insensitive because of their wide use as indicators of chemical bioavailability, it has been suggested that sensitivity is related to the type of test, end points being measured, and duration of exposure (2). In short-term toxicity assessments in which survival is the end point, bivalves may appear to be more tolerant to and less affected by chemicals because of their ability to close their valves for short periods and avoid exposure (30, 31, 32, 33) . However, studies comparing the mortality end point in bivalves and other test species have found bivalves to be equally (34, 35) or more sensitive (36, 37) than the other species (Table 1). When the bivalve growth end point was compared to the mortality end point in other test species, the bivalve growth end point was more sensitive (18, 28, 29, 38, 39).

5.3 Bivalves are an abundant component of many soft bottom marine, estuarine, and freshwater environments. Intertidal marine bivalves make up a significant portion of many habitats and provide habitats for many additional species. It is important to monitor freshwater bivalves for the following reasons: they are among the first taxa to disappear from benthic communities impacted by chemicals; they have been shown to be more sensitive than several other major taxa in laboartory tests.(48) The threatened and endangered status of many freshwater bivalve species also make them an important group to monitor.

5.4 If practical, the species to be used in a field bioassay should be one that is endemic to the area under investigation. In many cases, the specific area under investigation may not support bivalves due to a variety of factors including high concentrations of chemicals, competition or predation, or lack of suitable habitat or substrate. Under these conditions, it may be desirable to use a species that would normally be found in the environment if all conditions were favorable; however, it may be necessary to use a surrogate species, that is, a species that can tolerate the environmental conditions but is not normally found in the area, if native species are unavailable in the test area.

5.5 Bivalves generally utilize one of two primary modes of feeding: filter-feeding or deposit feeding. However, all known deposit-feeding bivalves are facultative in that they can either deposit- or filter-feed. Filter-feeders assimilate dissolved organics as well as suspended particulate matter, including plankton and suspended sediments, from the water column and have the potential for exposure to chemicals associated with this ingested material. Facultative deposit-feeding bivalves can be exposed to chemicals associated with sediments as they ingest sediments. They also ingest particulate material from the water column as they filter feed. As such, bivalves are capable of integrating exposure to chemicals dissolved in water and sorbed on sediment particles on the bottom or in suspension. It should be acknowledged that bivalves transplanted in the overlying water above sediment or transplanted directly on or in sediment may not exclusively accumulate or be affected by chemicals in a particular medium. That is, bivalves in or on sediment may still filter and accumulate chemicals from overlying water. Conversely, bivalves transplanted in the water column may filter suspended sediment and accumulate chemicals from that sediment. Bivalves can also assimilate chemicals as they ventilate overlying water.

5.6 Field bioassays are conducted to obtain information concerning the bioavailability of chemicals in the water column or bedded sediments and subsequent biological effects on bivalves after short- and long-term exposure to water and sediment under site-specific conditions. These bioassays do not necessarily provide information about whether delayed effects will occur, although a post-exposure observation period could provide such information. Sublethal post-exposure observations may include gonad development, spawning success, gamete survival, and development. The decision to conduct post-exposure studies in the field or in the laboratory depends on the observations being made, test conditions required, and experimental logistics.

5.7 The in-situ exposures described in this guide could be followed by laboratory measurements, such as scope for growth (2), filtration rate (55), byssal thread production (56, 57, 58), and biomarkers (59, 60).

5.8 The bivalve field bioassay can be used to determine the spatial or temporal trends of chemical bioavailability in water and sediment and effects due to exposure to those chemicals. Spatial comparisons of parameters of concern can be made by distributing the caged bivalves along physical and chemical gradients at scales commensurate with the desired level of discrimination. For example, station locations might be distributed along a known physical or chemical gradient in relation to the boundary of a disposal site (61, 62, 63, 64, 65), sewage outfall (66), or effluent pipe or at stations identified as containing elevated concentrations of chemicals in water or sediment as identified in a reconnaissance survey (3, 67, 68). This can be accomplished by placing caged bivalves along horizontal transects or at different depths in the water column. Temporal comparisons can be made by conducting before-and- after studies. For example, the effectiveness of dredge activities, effluent diffuser construction, effluent reduction, or remedial action can be determined by conducting field bioassays before the action, during the action, and after the action.

5.9 The relative bioavailability of chemicals from the various pathways of exposure (that is, aqueous phase, suspended particulate matter, sediment) and subsequent effects can be determined by simultaneously deploying bivalves with different feeding strategies and making supplementary measurements. A combination of filtration and the use of sediment traps followed by chemical analysis of the various environmental compartments can be used to identify the relative contribution of the aqueous phase, suspended particulate matter, and sediment. Lipid bags or semi-permeable membrane devices (SPMDs), which predominantly collect the dissolved fraction of chemicals, could also be used ( 69, 70, 71, 72, 73). The bioaccumulation of chemicals and effects among different bivalve species deployed side-by-side can be compared and used to help explain the spatial variability of chemical contamination, particularly if the different species are placed in different locations (that is, in the water column, on top of the sediments, within the sediments). This field assessment approach could be supplemented with laboratory studies designed to answer specific questions regarding dissolved versus particulate pathways of exposure.

5.10 Results of bivalve field bioassays might be an important consideration when assessing the hazards of materials to aquatic organisms (see Guide E1023) or when deriving water or sediment quality guidelines for aquatic organisms (15, 74). They might also be useful for establishing tissue residue criteria. Bivalve field bioassays can be useful in making decisions regarding the extent of remedial action needed for contaminated sites. They also provide a convenient method for manipulative field experiments, hypothesis testing, and monitoring specific sites before, during, and after cleanup operations (67, 68).