CONCURRENT TRENDS IN SMALL WATERSHED SNOW COVER, SOIL MINERALIZATION RATES, AND EXPORT OF DISSOLVED ORGANIC CARBON AND NITROGEN INTO LAKE SUPERIOR

We began continuous study of the Calumet (176 – ha ) and Wallace Lake (155- ha, on Isle Royale) watersheds in 1979 and 1982 respectively. Both drain directly into Lake Superior. Studies focus on winter hydrology and biogeochemistry. The two watersheds differ in vegetation (northern hardwoods, boreal), but since ~1995 both show similar time trends in snowpack moisture loss, moving in the time of peak stream snowmelt discharge, and increased stream dissolved carbon and nitrogen outputs. Winter air temperatures have increased ~1^o C yr^-1 , Calumet soil temperatures prior to snow cover increased ~0.2^o C yr^-1 , and soil temperatures beneath the snowpack have warmed. Annual precipitation has declined >1 cm yr^-1 with >90% of the decline in winter. Snow cover, winter precipitation, peak snowpack water equivalent, snowmelt (-0.8 cm yr^-1), and annual runoff (-1.04 cm yr^-1) have all declined. However, winter runoff has increased relative to precipitation indicating the more rapid and earlier runoff. At Calumet shallow subsurface lateral snowmelt flow occurs during ~10% of the winter but accounts for ~30% of total winter discharge. The more rapid runoff increases shallow subsurface snowmelt lateral flow and in turn stream water dissolved inorganic and organic nitrogen and organic carbon concentrations and watershed outputs. Long-term data from the Wallace Lake watershed show soil inorganic nitrogen pools average three times annual precipitation inputs, and net and gross soil nitrification rates peak in spring concurrent with peak stream runoff and nitrogen export. At both sites most of the annual warming and drying now occurs in November which >20 years ago was the peak precipitation month. As a result of November snowpack decline, soils now freeze, stimulating nitrification rates, and providing larger mobile nitrogen pools for export during snowmelt.