Through the cooperation of the San Francisco Bay Regional Water Quality Control Board, personnel from the U.S. Environmental Protection Agency's Duluth Environmental Research Lab performed bioassays using water from the South Fork of Strawberry Creek. Bioassays are widely used to evaluate what deleterious effects ambient surface water has on sensitive aquatic indicator organisms. Bioassays are an excellent water quality diagnostic tool because they take into account all site-specific environmental factors inherent in a given water body.
Two bioassays were performed on two different sensitive aquatic indicator organisms. The survival and growth of fathead minnows (Pimephales promelas) was clinically observed, as well as survival and reproduction of Ceriodaphnia sp., a sensitive crustacean. Both bioassays were seven day static renewal tests. The test organisms were placed in separate tanks containing creek water and the water was replaced daily by fresh creek water collected every morning from the South Fork at the Eucalyptus Grove. This location was chosen as a water sampling site because the South Fork supplies two-thirds of the total streamflow under low flow conditions and by sampling far downstream all upstream factors would be taken into account. This site also coincided with a baseline water quality sampling station (#3).
Water samples for chemical analyses were collected at the same time creek water was collected for the bioassays on the first day of the week-long bioassay water sampling. By coordinating this sampling, water quality data was made available for one of the seven water samples used in the bioassays. This data could be used to approximate the concentrations of various water quality constituents at that location during the week and could help correlate any observed detrimental effects in the indicator organisms to a specific water quality parameter. The bioassays were performed July 22-28, 1987. Water quality data for July 22 at station #3 is given in Appendix A.
Both bioassays were conducted according to procedures described in Short-term Methods for Estimating Chronic Toxicity for Effluent and Receiving Waters to Freshwater Organisms (EPA, 1985). No mortality was observed in either the fathead minnows or the Ceriodaphnia sp .. Growth of the juvenile (5-7 day old) fathead minnows was not impaired in comparison to the control groups, based on dry weight analyses. Furthermore, reproduction of the Ceriodaphnia sp. was not impaired in comparison to the control group, based on brood size. This indicates that during the week of July 22, South Fork ambient water had no adverse acute or chronic effects on these sensitive widely-used indicator organisms.