Sierra Nevada Network

Stressor: Air Pollution

Air Contaminants and Atmospheric Deposition

Circulation Pattern Creating the "Fresno Eddy" (Click to enlarge)

The southern and central Sierra Nevada are subject to some of the worst air quality in the United States, particularly during the summer months. The San Joaquin Valley, west of the Sierra Nevada parks, is a trap for air pollutants originating in the valley as well as pollutants from cities along the central California coast that are carried in on prevailing winds. Southward-flowing air currents enter California at the San Francisco Bay and move through the valley until they reach the mountains at the southern end of the basin, causing an eddy to form in the vicinity of Visalia and Fresno, just west of the southern Sierra Nevada (See Map to right).. Thermal inversions frequently trap air over the valley at night during the summertime. Airborne pollutants are then transported into the mountains when this air rises during the day. As a result, Sequoia and Kings Canyon have some of the worst air quality found in any NPS unit in the country. Yosemite and Devils Postpile are also impacted, but to a lesser degree.

One of the most damaging air pollutants is ozone. Research suggests chronic ozone pollution can lead to shifts in forest structure and composition. If current ozone concentrations remain relatively constant, or increase, they may affect the genetic composition of pine and sequoia seedling populations and contribute to increased susceptibility to fatal insect attacks, death rates, and decreased recruitment. The effects of chronic ozone pollution on other species are not yet known.

There are resultant biological effects of nutrient deposition on aquatic and terrestrial ecosystems, and this enrichment can have considerable effects on sensitive organisms or
communities (e.g., lichens and phytoplankton)—even at very low levels of atmospheric deposition.

High-elevation aquatic ecosystems in the Sierra Nevada are particularly sensitive to change from atmospheric deposition because the waters are oligotrophic and have a low
buffering capacity. In Yosemite, correlations between higher nitrate concentrations in sensitive surface waters and areas of higher nitrogen deposition have been observed. In contrast, decreased exports in dissolved nitrogen were observed in Emerald Lake in Sequoia.National Park. The decrease was attributed to increased phosphorus inputs that caused a switch from a lake dominated by phosphorus limitation to one dominated by nitrogen limitation. Sickman et al. described two trends in nitrate concentrations in Emerald Lake. During snowmelt, nitrate pulses (i.e., peak values during April) were related to snowpack depth–the deeper the snowpack the greater the nitrate pulse. There is little variation in precipitation concentrations, therefore, the quantity of precipitation (i.e. snowpack depth) is the determining factor.

The second pattern, and the one most relevant to phytoplankton, is a decline in summer/autumn lake nitrate concentrations to zero between the 1980s and 1990s. This late season decline occurred despite the fact that N deposition did not decrease. Instead, increased phosphorus loading allowed the phytoplankton to fully utilize nitrate during the summer/autumn seasons, driving them into a N-limited trophic state. The cause of increased phosphorus loading is unknown, but inputs from atmospheric deposition, soils, and, sediments are likely reasons and the subject of ongoing research. Mid-elevation, mixed-conifer watersheds in Sequoia’s Giant Forest have shown net retention of nitrogen, with stream concentrations often below detection limits. Giant sequoia forests are particularly effective at immobilizing nitrogen and reducing leaching losses; they may be adapted to even more nutrient poor environments than other coniferous ecosystems.

The consequences of increased nitrogen deposition and retention on terrestrial plant communities in the Sierra Nevada are unknown, but greater foliar biomass production, resulting in enhanced litter accumulation on the forest floor (fuel) and in aboveground biomass (stand densification), may increase the risk of severe fire damage. nitrogen pollutants are likely to be important in causing changes in lichen communities—e.g., shifts to nitrophilous species or changes in abundance. Increased levels of soil nitrogen caused by atmospheric nitrogen deposition can increase the dominance of non-native invasive plants and decrease diversity of native plant communities. Enhanced growth of invasive species from increased nitrogen has been observed in coastal sage scrub of Southern California, and is implicated in exacerbating invasion of Mediterranean nonnative grasses. Changes in the alpine plant community of the Rocky Mountains from nitrogen deposition have been observed. With continued urbanization of California’s Central Valley, with increasing livestock operations, and with the possibility of transpacific N transport from Asia, it is probable that N deposition and its ecosystem effects in the High Sierra will increase in the next several decades.

High elevation lakes and streams in the parks are very dilute and sensitive to change from atmospheric deposition of nutrients, toxic substances, and acids. While chronic acidification is currently not a problem, episodic depression of acid-neutralizing capacity occurs during snowmelt and episodic acidification occurs during what are known as “dirty rainstorms”, i.e., rainstorms of summer and early fall. If acid deposition increases–which is likely due to rapid population growth in the San Joaquin Valley–episodic acidification will become more frequent and may alter aquatic communities. Recent research suggests Sierra Nevada waters may be fairly resilient and able to buffer current and potentially increased inputs. The actual threat to water quality posed by episodic acidification, however, is unknown.

Sequoia, Kings Canyon, and Yosemite are downwind of one of the most productive agricultural areas in the world, the San Joaquin Valley. Every year, millions of pounds of pesticides (net weight of active ingredient) are applied to crops — 9,872,707 pounds in 2003 alone (Pesticide Use Database, managed by California Department of Pesticide Regulation). Pesticides volatilize, i.e., become suspended in the atmosphere as particulate matter (atmospheric contaminants), then drift into the parks on prevailing winds. Organophosphates have been found in precipitation up to an elevation of 1,920 meters in Sequoia. Some synthetic chemicals are endocrine disruptors (hormonal mimics) in concentrations of parts per trillion, potentially leading to altered wildlife reproductive capacity, longevity, behavior, and cancer and mutations. Synthetic chemical drift also may play a role in decline of mountain yellow-legged frogs and other amphibians in the Sierra Nevada. While there is correlation between ecosystem effects and synthetic chemical presence, the mechanism for specific pesticide effects has not been established.

This article is an excerpt from the Sierra Nevada Network: Vital Signs Monitoring Plan (2007).