9.1 Fish
As pointed out in Section 7.0, there are very limited data from the literature which describe the effects of DGS on marine fish. The data which do exist indicate that marine fish display the same signs of GBT as do fresh water fish. Furthermore, the data imply that water delta P thresholds for these signs are essentially the same as for fresh water fish. For example, adult sea bass and striped mullet have a survival threshold near a delta P value of 114 mm Hg (Grey et al. 1985). This is nearly the same as that derived from the graphical analysis of Section 6.1 for cutthroat and rainbow trout.
For larval stages of striped bass, Cornacchia and Colt (1984) reported swim bladder over-inflation thresholds of about 22 mm Hg, which is essentially that which would be predicted by Equation 4 and that found for rainbow trout by Shrimpton et al. (1990a and b).
9.1.1 Factors Affecting Guideline Derivation and Application
Those factors which would affect the derivation and application of a DGS guideline for marine fish are essentially the same as those described in Section 8.1.1 for fresh water fish.
9.1.2 Rationale
The limited data describing the effects of DGS on marine fish suggest that they are affected by DGS in the same way as fresh water fish and that the delta P thresholds for the signs of GBT are very similar to those for fresh water fish. Thus, the rational which was used in Section 8.1.2 will be applied in the derivation of DGS guidelines for marine fish.
9.1.3 Guideline Derivation
In situations involving water depths of one metre or more, the maximum delta P should not exceed 76 mm Hg. For sea level conditions, this corresponds to a TGP% of about 110%. For water depths less than one metre, the guideline should be based on Equation 4 which describes the threshold for swim bladder over-inflation as a function of water depth and pO2 levels. Figure 23 shows the results of combining the two guideline criteria. As indicated in the figure, delta P levels greater that 76 mm Hg are unsafe for aquatic invertebrates regardless of water pO2 or water depth. For water depths less than one metre, the maximum allowable delta P is a function of water pO2.
In situations where there is a natural background level of DGS, the same guidelines should apply to any man-made alterations to the dissolved gas regime of marine environments. That is, any changes to the dissolved gas environment, in combination with natural background levels, should not exceed the above guidelines. If natural levels are higher than the recommended guidelines it must be recognized that these levels may also be harmful to fish. Therefore, there is no justification for introducing comparable DGS levels of man-made origin to marine environments.
In hatchery environments Equation 4 may not apply. With the much higher fish densities in hatcheries, accompanied by declines in pO2 along the rearing facility and surface feeding, fish may spend more time near the water surface and become subjected to higher stress levels than in wild environments. Therefore, it is recommended that the guideline for hatcheries be set at a maximum delta P of 24 mm Hg (i.e., the threshold for swim bladder over-inflation under sea level normoxic conditions and zero water depth). This corresponds to a sea level TGP% of 103%. If pO2 levels in the hatchery drop to 100 mm Hg, the guideline should be a maximum delta P of 0 mm Hg.
9.1.4 Guideline Application
The application of the above DGS water quality guidelines to man-made alterations of marine environments must focus on fisheries habitat. The first step in applying the guideline is to assess the habitat which is available for use by the various marine fish species. This includes assessment of habitats for spawning, rearing, and adult holding along with information on the temporal usage of these habitats. These data, along with information on water depth (which may vary over the year) and pO2 levels (which may also vary over the year as well as diurnally), provide the necessary information for application of the guideline criteria. The conditions described in Section 9.1.1 will also have to be considered in establishing guideline compliance.
9.2 Invertebrates
The limited data which describe the effects of DGS on marine invertebrates indicate that they have approximately the same sensitivity to DGS as do adult fish. For example, Bisker and Castagna (1985) found that of three species of clams studied, the threshold for 100% survival of the most sensitive species was at a water delta P of about 114 mm Hg. Clams floated to the surface at somewhat lower levels of delta P.
9.2.1 Factors Affecting Guideline Derivation and Application
With the exception of those factors involving the swim bladder, most of the other factors described above for marine fish would apply to guidelines for marine invertebrates.
9.2.2 Rationale
Because guidelines derived for marine fish are based on responses of fish species which are more sensitive than marine invertebrates (e.g., striped bass - Cornacchia and Colt 1984), the water quality guidelines derived for fish should also be protective of marine invertebrates.
9.2.3 Guideline Derivation
The guideline for marine invertebrates should be the same as for freshwater and marine fish species. In situations involving water depths of one metre or more, the maximum delta P should not exceed 76 mm Hg. For sea level conditions, this corresponds to a TGP% of about 110%. For water depths less than one metre, the guideline should be based on Equation 4 which describes the threshold for swim bladder over-inflation as a function of water depth and pO2 levels. Figure 23 shows the results of combining the two guideline criteria. As indicated in the figure, delta P levels greater that 76 mm Hg are unsafe for marine invertebrates regardless of water pO2 or water depth. For water depths less than one metre, the maximum allowable delta P is a function of water pO2.
In situations where there is a natural background level of DGS, the same guidelines should apply to any man-made alterations to the dissolved gas regime of marine environments. That is, any changes to the dissolved gas environment, in combination with natural background levels, should not exceed the above guidelines. If natural levels are higher than the recommended guidelines it must be recognized that these levels may also be harmful to marine invertebrates. Therefore, there is no justification for introducing comparable DGS levels of man-made origin to a marine environment.
9.3 Plants and Algae
As pointed out earlier, there are no data available which would indicate the effects of DGS on marine plants and algae. As a result, guidelines cannot be derived. However, based on the discussion of Section 7.3, it is not anticipated that marine plants and algae would be any more sensitive to DGS than fish.