Tierra del Fuego, at the southernmost tip of South America, is influenced by ozone depletion. The landscape of southern and western Tierra de! Fuego is dominated by peatlands; they are important locally and in the context of global climate change, because they store large quantities of organic carbon. To determine the influence of solar ultraviolet-B radiation (UV-B) on a Tierra de! Fuego peatland, we selectively filtered solar UV-Bin ten pairs of plots. Polyfluorine filters were used to create the Near-Ambient-UV-B Treatment ( 90% solar UV-B), and polyester filters to create the Reduced-UV-B Treatment ( 17% solar UV-B). These filters were first installed in October 1996, and were maintained, September-March, for six years. Following previous plant growth measurements and samples of selected microorganisms under the two UV-B treatments (1996-1999), this dissertation is an account of the more detailed measurements made during the second three-year period of treatments (1999-2001 ). Seasonal sampling of the plant community, microfungi, microfauna, and biogeochemistry of the water and nutrients held by the Sphagnum capitulum was introduced, in an attempt to better understand ecosystem function. Solar UV-B reduced Sphagnum height growth, but this was compensated by more compressed and densely packed Sphagnum capitula. Emergent vascular plants, Nothofagus, Empetrum, and Tetroncium, were more affected than Sphagnum by nearambient UV-B. Solar UV-B altered the Sphagnum-capitulum microenvironment, resulting in: more dissolved organic carbon and phosphorous, higher electrical conductivity, and greater acidity under near-ambient UV-B. Additionally, the populations of testate amoebae and some species of fungi were consistently increased; however, microfungal diversity and rotifer, nematode, and mite populations decreased under near-ambient UV-B. Generally, Sphagnum minimizes the leaching of nutrients by effectively holding water at the capitulum. Solar UV-B altered Sphagnum-capitulum morphology, increased the volume of water held, and made this water more acidic and richer in nutrients. Based on these results, if current trends in ozone depletion were to persist over several decades, a reduction in vascular plant growth, and changes in the trophic relationships of the microorganismal community of the Sphagnum capitulum, would be predicted. These responses have the potential to affect peatland carbon storage and nutrient cycling in Tierra del Fuego.