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496 " study suggests that impact on large animals should be assessed on a project-by-project basis with attention given to the probable distribution of snowpack increase between the winter range and the higher elevation parts of the target. Recent studies suggest that most and per- haps all of the snowpack increase available in the San Juan Mountains will be at the higher elevations, above the winter range. Studies of the effect of snowpack on small mammals were made in the alpine zone (Stoecker, 1976) and at several forest zone sites (Sleeper et al., 1976). In the alpine zone, the pocket gopher is the animal of principal importance, since the amount of earth it moves is sig- nificant for local vegetation and for geomorphic proc- esses. There is some suggestion that snow cover may restrict the gopher winter home-range size, but no clear effect on distribution or activity was found. At the most intensively studied forest site, there was a marked decline in population density of chipmunks and deer mice after heavy-snowpack winters with full recovery of the chipmunks and partial recovery of the deer mice during the following summer. At other sites, however, either no effect or even a weak positive rela- tionship between deer mice and snowpack was found, so the effects are believed to vary from site to site and may depend upon differences in site dryness. No effects on voles were found. 7) SUBALPINE LAKE In the Medicine Bow Mountains, effect of increased spring runoff on a subalpine lake was studied (Parker and Condit, 1975). It was concluded that increased runoff would tend to decrease the standing crop of phytoplankton, the amount of the decrease depending on a variety of factors such as wind, spring air tem- perature, etc., not influenced by snowpack augmenta- tion. The opinion was expressed that the lake would not suffer any perturbation as a result of snowpack augmentation that would not occur in its absence. 8) TROUT PRODUCTION This was also studied III the Medicine Bow Mountains (Baxter and Loar, 1975). Though a drastic snowpack increase would probably decrease trout production, it appears that moderate increase would have no ap- preciable effect. Production of trout and the benthic invertebrates upon which they feed is apparently in- fluenced as much by variations in the pattern of runoff and by late summer air temperature as it is by the volume of snowpack. 9) SUMMARY In their syntheses and discussion of the San Juan Ecology Project, the investigators concluded as follows (Caine et af., 1976, pp. 12-13): The results of the San Juan Ecology Project suggest that there should be no immediate, large-scale impacts on the terrestrial ecosystems of these mountains following an addition of up to 30 percent of the normal snowpack, but with no addition to maximum snowpacks. Further, much of the work reported here suggests that compen- sating mechanisms within the studied ecosystems are Vol. 58, No.6, June 1977 such that any impacts would be buffered, at least for short periods of time, and of lesser magnitude than the changes in snow conditions required to produce them. However, some parts of the mountain ecosystem are much more susceptible to changes in snow conditions than others, so important local effects are possible. In general, these susceptible components are very small parts of the entire system but their economic or esthetic value may be much greater than their mass or area suggest. Remedial action is possible in most of these cases but has not been studied in this project. Our work has shown three ecosystem components to be most sus- ceptible to increased snowfall: (I) snowbank situations at elevations above treeline; (2) elk herds (in other mountain ranges other big game species may be similarly affected); and (3) some small mammal populations, es- pecially the deer mouse. Not all of these impacts are necessarily deleterious; an increase in the area of snow- bank edge habitats in alpine area may, for example, increase the niches available for rare plant species. Finally, even in the small areas where we predict great- est impacts from increased snowfall, the changes involved are unlikely to approach the magnitude of other man- made impacts on mountain ecosystems. However, it should be remembered that they may act in phase with other man-made impacts and with natural climatic changes, in which case the total effect could be much greater than our studies suggest. e. Impact on the natural environment- Seeding agents Of the various seeding agents, silver iodide is the only one perceived as having potential environmental im- pact. This perception comes mostly from the strong bacteriocidal action of some forms of silver. Following the recommendations of Cooper and Jolly (1969), Sky- water investigations of potential silver impacts were directed at microbial and aquatic compartments (since these were the most important and most accessible to study) as well as at monitoring the silver in cloud- seeding study areas. Figure 3 summarizes in a tentative, generalized way the silver content of the principal environmental com- partments in the contiguous United States, the concen- tration of silver in each compartment, and the rates of exchange between them, based on interpretations of current literature. The soil compartment (including also mud and vegetable litter), calculated for the top 20 em comprising the root zone, contains by far the largest quantity of silver, at concentrations that may become quite high in mineralized areas. Living matter of all sorts from microbes and fungi to animals (shown as "Living" in Fig. 3), which has on the average a slight tendency to concentrate silver from the soil, contains the next largest quantity. The exchange between living matter and soil through uptake and decomposition (designated by the diamonds related to the bottom scale) dominates all other exchanges by at least an order of magnitude. The rates of exchange are slow enough so that the contents of these two compartments must change only slowly. The silver concentration and content in lakes and rivers (shown in Fig. 3 as "Water") are determined mainly by depositional and erosional exchanges with the soil and by runoff to the sea, whereas uptake and decomposition exchanges with plants playa lesser role.