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.

<p> The net ecosystem CO2 exchange and Sphagnum net primary production of an abandoned block-cut bog were measured in the field and in the laboratory using gas exchange techniques. Environmental variables were measured concurrently. Seasonal Sphagnum growth and decomposition were determined using crank wires and litter bags.</p> <p> The bog was a net source of CO2 during the summer months. While, Sphagnum did grow over the study period, the Sphagnum net primary production was offset by peat respiration. Sphagnum net primary production and net ecosystem CO2 exchange at the abandoned block-cut bog are within the ranges found for natural peatlands.</p> <p> Sphagnum photosynthesis was greatest at wet sites and at the start and end of the study period when the water table was higher and the temperature was cooler. Ericaceous shrub cover strongly affected Sphagnum growth. Sites with vascular plant cover photosynthesized at approximately twice the rate of sites where cover was removed.</p> <p> Laboratory results indicate that drying and wetting cycles negatively affect Sphagnum net primary production and net ecosystem CO2 exchange. Sphagnum and peat respiration increased 4 to 14 fold upon rewetting whereas Sphagnum photosynthesis did not recover until 20 days of saturation.</p> <p> The results emphasize the importance of stable moisture availability for the development of a new acrotelm. The peatland will likely remain a source of CO2 until the acrotelm is able to counterbalance the peat respiration.</p> / Thesis / Master of Science (MSc)