The literature indicates a strong correlation between inundation of previously oxidized soils, as can occur on a floodplain, and increased microbial methylation of mercury. There is special concern ...over the potential for increased methylmercury levels in the Yolo Bypass, a 24,000‐ha floodplain for California's Sacramento River and its tributaries. The objective of our first study component was to compare methylmercury accumulation between juvenile Chinook salmon Oncorhynchus tshawytscha in the Yolo Bypass and those in the Sacramento River during a winter 2005 flood event. For each location, we tested accumulation in two groups of hatchery Chinook salmon juveniles: (1) free‐ranging, coded‐wire‐tagged fish that were released into the floodplain and river and recaptured by downstream sampling and (2) fish that were reared in enclosures at fixed locations in both the river and floodplain. We found that free‐ranging juvenile Chinook salmon in the floodplain accumulated 3.2% more methylmercury per day than did free‐ranging fish in the river. However, fish in the floodplain grew 0.7% more per day than fish in the river. Variance in growth and in methylmercury content was significantly higher in the free‐ranging fish than in the enclosure‐reared fish, suggesting suboptimal rearing conditions in the enclosures. In a second study component, we analyzed methylmercury levels of free‐ranging Chinook salmon released in the Yolo Bypass during hydrologically variable years (2001–2003 and 2005); the objective was to determine whether interannual differences in the primary source of floodwater to the Yolo Bypass were associated with different patterns of mercury accumulation in Yolo Bypass Chinook salmon. Fish in the Yolo Bypass showed different patterns of methylmercury accumulation in 2001, 2002, 2003, and 2005. Methylmercury accumulated linearly with time in years when Cache Creek provided the primary source of flood flow but followed a quadratic pattern in years when flood flow was dominated by the Sacramento River.
The generality of the trophic cascade has been an intensely debated topic among ecologists. We conducted a meta-analysis of 54 separate enclosure and pond experiments that measured the response of ...the zooplankton and phytoplankton to zooplanktivorous fish treatments. These results provide unequivocal support for the trophic cascade hypothesis in freshwater food webs. Zooplanktivorous fish treatments resulted in reduced zooplankton biomass and increased phytoplankton biomass. The trophic cascade was weakly dampened at the level of the phytoplankton. However, the response of the phytoplankton to the trophic cascade was highly skewed, with very strong responses in slightly more than one-third of the cases and weak responses in the others.
Eleven of 41 brands of skin whiteners that were collected in Phnom Penh, Cambodia, and processed with a screening kit contained more than 2000 μg/g mercury. Risk analysis indicates that these 11 ...brands were toxic. Nine of 19 of these skin whiteners analyzed with cold vapor atomic absorption (CVAA) spectrophotometry exceeded Association of Southeast Asian Nations (ASEAN) guidelines for cosmetic good manufacturing practice limit on mercury of 1 μg/g. The most contaminated whitener analyzed by CVAA had 12,590 μg/g mercury. The mercury-containing products were labeled as produced in Thailand, China, Taiwan, Vietnam, the United States, and an unidentified country. Eight other products (antifungal, steroids, and antibiotics) were sold as additives to be mixed into whitener concoctions. In the 19 samples analyzed with CVAA, there was a significant association between the mercury content and a label "for export only." Labeling of sampled products varied from detailed to slight, with none containing Khmer instructions. Variability in mercury content of some products appeared to reflect copying of brand names with very similar packaging.
1. A series of experiments examined growth of Daphnia magna on three algal diets (Rhodomonas minuta, Scenedesmus acutus and Synechococcus sp.) at varying physiological states nitrogen and phosphorus ...(P) limitations to test whether variation in algal fatty acid and/or elemental composition can predict Daphnia growth.
2. These algae differed widely in their essential fatty acid (EFA) composition while phosphorus (P) or nitrogen limitation had only a small influence on their ω3‐polyunsaturated fatty acid (PUFA) content.
3. Individually, algal ω3‐PUFA content explained 69% of the variation in the normalised growth of D. magna, while algal phosphorus content explained 11% of the variation. Quantitative models for D. magna growth used both algal ω3‐PUFA content and algal C : P ratio as food quality indices. Together, algal ω3‐PUFA content and C : P ratio explained 70% of the variation in the normalised growth rate of D. magna.
4. Our results indicate that EFA influenced algal food quality much more strongly than P. The EFA and mineral P impacts appear to be independent.
Introduction. We evaluated periphytic algal and microbial communities to assess the influence of human and cattle impact on Sierra water quality. Methods. 64 sites (lakes and streams from Lake Tahoe ...to Sequoia National Park, California) were sampled for suspended indicator bacteria and algae following standardized procedures. The potential for nonpoint pollution was divided into three categories: cattle-grazing areas (C), recreation use areas (R), or remote wildlife areas (W). Results. Periphyton was found at 100% of C sites, 89% of R sites, but only 25% of W sites. Eleven species of periphytic algae were identified, including Zygnema, Ulothrix, Chlorella, Spirogyra, mixed Diatoms, and Cladophoria. Mean benthic algae coverage was 66% at C sites compared to 2% at W sites (P<0.05). The prevalence of E. coli associated with periphyton was 100% at C sites, 25% of R sites, and 0% of W sites. Mean E. coli CFU/gm of algae detected was: C = 173,000, R = 700, W = 0. (P<0.05). Analysis of neighboring water for E. coli bacteria >100 CFU/100 mL: C = 91%, R = 8%, W = 0 (P<0.05). Conclusion. Higher periphytic algal biomass and uniform presence of periphyton-attached E. coli corresponded to watersheds exposed to summer cattle grazing. These differences suggest cattle grazing compromises water quality.
Pinpointing erosional ‘hot spots’ or sediment source areas within a watershed can be difficult because of the highly non-linear and episodic nature of effective runoff and sediment transport. ...Continuous monitoring of stream networks can provide insight into sediment source areas not possible from routine sampling by capturing episodic events and integrating entire storm events. This paper presents the results of a three-year study of spring snowmelt runoff in watersheds of Lake Tahoe, California, USA, using a network of nine nephelometric turbidometers. We estimate that montane badlands, comprising 1.2% of the Ward Creek watershed (25 km
2), contributed 10–39% of snowmelt-derived suspended sediment load. The badlands are comprised of poorly-vegetated and deeply-incised gullies formed in parent material of Miocene volcanic mudflow breccia. The larger (29.5 km
2) Blackwood Creek watershed also showed high relative erosion rates from volcanic terrain. Tributaries draining forested granitic and metasedimentary headwater regions had minimal sediment loads. Monitoring networks indicated that temporary main channel storage regulated release of fine sediment eroded from steep headwater slopes. This lag between erosion and delivery may lead to underestimation of gully erosion contribution to sediment budgets.
The Sierra Nevada Mountain range serves as an important source of drinking water for the State of California. However, summer cattle grazing on federal lands affects the overall water quality yield ...from this essential watershed as cattle manure is washed into the lakes and streams or directly deposited into these bodies of water. This organic pollution introduces harmful microorganisms and also provides nutrients such as nitrogen and phosphorus which increase algae growth causing eutrophication of otherwise naturally oligotrophic mountain lakes and streams. Disinfection and filtration of this water by municipal water districts after it flows downstream will become increasingly costly. This will be compounded by increasing surface water temperatures and the potential for toxins release by cyanobacteria blooms. With increasing demands for clean water for a state population approaching 40 million, steps need to be implemented to mitigate the impact of cattle on the Sierra Nevada watershed. Compared to lower elevations, high elevation grazing has the greatest impact on the watershed because of fragile unforgiving ecosystems. The societal costs from non-point pollution exceed the benefit achieved through grazing of relatively few cattle at the higher elevations. We propose limiting summer cattle grazing on public lands to lower elevations, with a final goal of allowing summer grazing on public lands only below 1,500 m elevation in the Central and Northern Sierra and 2,000 m elevation in the Southern Sierra.
We developed a time series model of Secchi depth for Lake Tahoe incorporating a mechanistic understanding of interannual variability with sufficient simplicity to allow data-based parameter ...estimation. Secchi depth is still occasionally over 40 m at Lake Tahoe, but mean annual Secchi depth has declined nearly 10 m since 1967, prompting a large-scale restoration program. Year-to-year variability is extremely high, obscuring restoration actions and compliance with water quality standards. The model focused on Secchi depth during summer, when the lake is least transparent and most heavily used. Interannual variability for the summer season is driven largely by precipitation differences. The model offers a means for determining compliance with water quality standards when precipitation anomalies may persist for years. As demonstrated by means of an ex-post forecast, increasing Secchi depths during 1999-2001 were simply climate-driven and do not represent a recovery of the lake. The long-term trend for summer is most likely due to the accumulation of allochthonous mineral suspensoids.
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