Excerpt from Kesterson Reservoir 2000 Biological Monitoring: Prepared for U. S. Bureau of Reclamation Mid-Pacific Region
Biological monitoring of Kesterson Reservoir has been conducted since 1987 in order to determine selenium concentrations in abiotic and biotic elements. In 1988, with the drying out of Kesterson and filling of low-lying areas, studies shifted to focus on the upland environment and the ephemeral pools that form during the rainy winter months. Selenium-caused effects to wildlife (such as embryo deformities, reduced egg viability, or mortality) have not been found during those studies although elevated selenium continues to be found in bird eggs and other wildlife. Based on aquatic bird toxicity data, the selenium chemistry data generally indicate a low-level risk of avian reproductive effects. However, due to the generally small numbers of breeding birds present at Kesterson, low-level effects would be difficult to detect. Based on the results of selenium monitoring at Kesterson since 1988, the U.S. Bureau of Reclamation (Reclamation), the Central Valley Regional Water Quality Control Board (RWQCB), U.S. Fish and Wildlife Service (Service), and CH2M Hill in 1995 agreed to change the monitoring program at Kesterson from annual comprehensive monitoring to more focused annual monitoring of selected biota that were judged to be at the highest risk, with more comprehensive sampling of biota every three years. The 1998 sample year was the first year that complete monitoring was conducted after the 1995 decision, and only limited sampling was conducted in 1999 and 2000 (as planned). The next planned comprehensive monitoring is to be conducted in 2001.
Figure 1 - 1 shows the Cumulative rainfall for water years (July 1 to June 30) at Kesterson from 1989 through 2000. 1998 was an El Nino year and, therefore, an unusually wet year (Figure 1 - 1); rainfall and ephemeral pools persisted far into the growing season for plants and the breeding season for birds. The excess rainfall caused flooding and delayed plant growth and bird nesting as well as apparently reducing nesting success at Kesterson and other areas of California (A. Erichsen, University of California, Davis; R. Anderson, Swainson's Hawk Technical Action Committee; S. Buranek, National Park Service; personal communications). Shorebird nesting attempts at Kesterson were higher than any other year since 1988, in contrast to the low number of nesting attempts made by many terrestrial bird species.
Rainfall in 1999 and 2000 was lower than in 1998 and the area covered by rainwater pools was limited, with only a few pools persisting into March and April. There was little biological activity at these pools during the avian reproductive period and no bird nesting associated with the pools was observed. Killdeer (Charadrius vociferus) were the only shorebird species found nesting at Kesterson in both 1999 and 2000. During the 2000 nesting season (March - June), killdeer began nesting during the first part of March as in 1998, but in lower numbers (Figure 1 - 2A). However, terrestrial plants, invertebrates, and vertebrate prey items appeared to be abundant in both 1999 and 2000 (based on observations during monitoring). Cavity-nesting species studied (i.e., American kestrels [Falco sparverius] and barn owls [Tuto alba]) appeared to have similar success (as a percent of successful nests) from 1998 to 2000, whereas the cup-nesting terrestrial species studied (loggerhead shrikes [Lanius ludovicianus]) had greater success in 1999 and 2000 than in 1998.
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