Despite being a relatively remote and well conserved area, the sub-Antarctic ecoregion faces pressing global threats from climate change, the ozone hole and introduced species. Its freshwater ecosystems are one of the least studied components of this biome, but they are known to confront a host of invasive taxa including trout and beavers. We set out to understand the basic characterization and dynamics of organic matter processing and decomposition in sub-Antarctic streams under natural forest (NF) conditions and in ponds constructed by North American beavers (Castor canadensis) (BP). We found these streams have a naturally stable benthic organic matter regime throughout the year with a peak in leaf input from Nothofagus pumilio in autumn. Beaver ponds significantly increased the retention of organic matter and caused significantly higher decomposition rates, probably associated with increased density and biomass of Hyalella spp. As expected, leaf decay rates for N. pumilio, a deciduous species, were higher (NF: -0.0028 day- ± 0.0001 SE; BP: -0.0118 day-1 ± 0.0009 SE) than N. betuloides (a broad-leaf evergreen) (NF: -0.0018 day-1 ± 0.0005 SE; BP: -0.0040 day-1 ± 0.0003 SE). Overall these results indicate that the naturally low decomposition rates (slower than 89% of a global survey of decay rates) for these cold, oligotrophic streams are being modified by introduced beavers to resemble more temperate latitudes.