Does Above-Normal Precipitation Reduce the Impact of Mountain Cattle Grazing on Watershed Algae and Bacteria?
Tóm tắt
Cattle grazing is believed to have a harmful impact on high elevation Sierra Nevada mountain (Sierra) watersheds. During normal precipitation years, the prevalence of suspended stream and small lake aquatic coliforms and E. coli is significantly increased in cattle-grazed areas, and periphytic algae biomass is amplified. In addition, large numbers of E. coli are found attached to algae only in cattle-grazed areas. During 2011 the Sierra experienced 200 % above-normal precipitation, mostly as snow. Delayed snowmelt and higher volume stream flows provided a unique opportunity to determine if cattle grazing remained harmful to the aquatic environment. A total of 45 remote sites, including both lakes and streams, were sampled during the summer of 2011. Risk determinants for non-point pollution were divided into four categories: (1) cattle grazing areas (C), (2) recreation areas with predominant impact from pack animals (PA), (3) recreation areas used only by humans (H), and (4) remote wildlife areas (W). Algal biomass was accessed, and samples collected for taxonomy and analysis of microbes suspended or attached to algae. Ten species of algae were identified including Cladophora, Chlorella, Spirogyra, Zygnema, Ulothrix, and mixed Diatoms. Mean benthic coverage area of algae was 30 % at C sites compared to only 2 % at W sites (P<0.0001). The prevalence of E. coli attached to algae was 90 % at C sites, 23 % at PA sites, 0 % at H sites and 0 % at W sites. Mean E. coli CFU/gm of algae found by pollution risk category was (×1000): C=267±215.5, PA=29.4±50.8, H=0, W=0 (P<0.0001). Number of sites at which suspended aquatic E. coli was detected (>100 CFU/100 mL): C=7 (70 %), and none at PA, H, and W sites (P<0.0001). Conclusions: The fecal indicator E. coli was detected suspended in water only in C areas. Significantly higher periphytic algal biomass and uniform presence of algae-attached E. coli was detected in C watersheds. The 200 % above-normal precipitation year did not decrease the impact of non-point pollution by cattle in the Sierra watershed.
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