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In June 1993, we conducted field studies to evaluate in-situ effects on aquatic invertebrates in a prairie ecosystem in central North Dakota. Based on preliminary laboratory tests (2), the suppressant foams Ansul Silv-Ex and Phoschek WD881 are known to be more toxic to aquatic organisms than liquid retardants. Laboratory-determined effective concentrations of two commonly used foams were tested in order to fulfill the following objectives: 1) to determine in-situ effects on aquatic macroinvertebrate communities, and 2) to determine the toxicity of these chemicals to a selected indigenous invertebrate found in the aquatic system.

Description of Study Area

Methods

To provide a relatively uniform community of aquatic invertebrates for the exposure, artificial substrate trays constructed of 1/2" mesh aquaculture netting and a base of 1/8" polyethylene and 3/4" PVC pipe were deployed on May 10, 1993 for a colonization period of 4 weeks. The trays contained 5 g of pre-weighed, air-dried Cottonwood (Populus deltoides) leaves. A total of 120 substrate trays were deployed with a nylon pull cord attached to the conduit pipe at each of the 24 limnocorral locations, 4 to be enclosed in each limnocorral (1 in each quadrant) and one reference tray outside each limnocorral. Substrates were sampled by pulling the nylon cord vertically and placing the trays in zip-lock bags with 90% ethanol preservative. On June 4, the 24 trays outside the limnocorrals were sampled before limnocorral placement to provide a reference for determination of community similarity and allow detection of disturbance effects associated with limnocorral placement. At 24 hours before the exposure, limnocorrals (8 Silv-Ex, 8 Phos-Chek WD-881, 8 control) were placed in Fish Lake to enclose the remaining 96 colonized artificial substrates, which were sampled after the 96 hr. exposure.

To determine the toxicity of these chemicals to an indigenous invertebrate, the water boatman Cenocorixa sp. (Hemiptera: Corixidae) was collected from Fish Lake on June 5 prior to chemical addition. Ten Cenocorixa sp. were placed in 15 L polyethylene chambers fitted with 1 mm mesh netting, and suspended from the frame in each limnocorral 1 hr after chemical addition. Number of organisms remaining alive was recorded daily throughout the test.

Chemical concentrations representing the lowest observable effect level (LOEL) based on laboratory test data for Daphnia magna (3) were used for the Fish Lake exposure (Silv-Ex = 6 mg/L, Phoschek WD881 = 4.7 mg/L). Chemicals were pre-weighed in the laboratory and added as a liquid to numbered limnocorrals in a randomized fashion.

Self-contained dataloggers (DataSonde II™ units, Hydrolab Inc., Austin, Texas; use of brandnames does not constitute endorsement by the U.S. Government) were used to measure pH, dissolved oxygen, conductivity, and water temperature at hourly intervals throughout the exposure. Units were suspended at mid-depth in Fish Lake and within randomly selected limnocorrals (2 Silv-Ex, 2 Phos-Chek WD881, 2 control). One liter water samples were also collected from each limnocorral and the open water areas between 0700-0900 h for determination of dissolved oxygen, Ammonia, orthophosphorus (EPA method 365.1, Colorimatic Automated Ascorbic Acid Method), and pH daily throughout the test period. Nitrate, nitrite, chlorides, sulfates, alkalinity, and hardness were measured once before and once after the 96 hr exposure (4).

Results

Aquatic macroinvertebrate taxa richness in Fish Lake was extremely low; only 18 taxa were collected in the artificial substrate trays, and the 5 most dominant taxa were chironomids (Table 1). Seven reference trays and those in 5 of the limnocorrals (3, 21-24) were covered with sediment and could not be used for analysis. Taxa richness, relative abundance, and mean number of organisms did not differ among treatments (Figures 1, 2). Mean number of organisms in reference trays was lower as compared to trays inside the enclosures, however disturbance effects due to placement of the limnocorrals were not detected. The Pinkham and Pearson Similarity Index indicated that for both total number of organisms and relative abundance, treatments did not differ from controls (Figure 3).

After 24 hr, exposure to 6 mg/L Silv-Ex resulted in significantly higher mortality of water boatmen (Cenocorixa sp.) than in controls (P = 0.003, Figure 4). The most dramatic decrease in survival (69%) occurred during the first 24 hr, but survival continued to decrease throughout the 96-h Silv-Ex exposure until only 11% of the organisms remained. Contrastingly, the 4.7 mg/L Phoschek WD881 treatment did not cause mortality significantly different from that of controls (Figure 4). However, organisms showed impaired movement that suggested a sublethal response related to chemical exposure.

Discussion

Chemical concentrations used in this study were not high enough to cause reductions in invertebrate species abundance or taxa richness. It is unlikely that our inability to detect subtle community changes was due to the sampling technique used; artificial substrates often provide samples with low variability (7), as was the case in this study. With the exception of limnocorrals 2 and 13, taxa richness and numbers of organisms in most samples taken during this study were within a relatively small range (Figures 1, 2), and no effects due to chemical exposure were detected. The community which colonized the leaf substrates may not have included species that were sensitive to reductions in surface tension. Future determinations of community effects should include exposure to higher chemical concentrations.

Using a field application example, where chemical spillage or overspray might occur, the concentration of 6 mg/L Silv-Ex used in our study is equivalent to a spill of 1.5 L of a 1% Silv-Ex mixture directly into one 2500 L limnocorral, or a spray coverage rate of 0.75 gal/100 ft2. Assuming even coverage, if a 1% Silv-Ex mixture was applied at the recommended application rate for grassland prairie ecosystems (5 gal/100 ft2), the resulting concentration in one limnocorral would be 40 mg/L. This concentration is 167% higher than that producing significant mortality of larval fathead minnows (Pimephales promelas) in both field and laboratory exposures (8). Since dilution of an affected area would be rapid, it is unlikely that a concentration this high would actually occur. Nonetheless, water boatmen and other invertebrates that utilize the water surface could suffer adverse effects from direct chemical exposure. Even though these organisms and other invertebrates such as daphnids and amphipods are neither economically or recreationally important aquatic resources, they are an integral part of the food chain essential to the support of higher trophic levels such as fish and birds.

References