What is Causing the Recent Increases in NO3 in Loch Vale Surface Waters?

J.S. Baron
USGS, Natural Resource Ecology Laboratory, CSU, Fort Collins, CO

T.R.Schmidt
USGS, Mineral Resources Team, Denver CO

A. Krcmarik
USGS, Fort Collins Science Center, Fort Collins CO

Nitrate concentrations at the outlet to Loch Vale watershed maintained average concentrations of ~1.0 mg/L from 1982 through 1998. In 1999 concentrations began to increase. Five-year running mean concentrations were nearly twice as high for the years centered around 2003 and 2004, and remain elevated above the 1982-1998 value. Annual precipitation has been at or below the longterm Loch Vale mean value of 102 cm/year since 1998, Mean annual temperatures appear to have increased since 1998, and N deposition is increasing at a rate of about 2% each year. None of these trends match the stream chemistry pattern, yet precipitation, temperature, and N deposition probably all contribute in some way. Since nitrogen is an essential and often limiting nutrient, biogeochemical processes may also be important to elucidating stream chemistry. We used structural equation modeling in an attempt to sort out the causal effects of climate, deposition, and ecosystem processes on stream chemistry. We found interactions among nearly all watershed components except lake algae, and no clear driver for the change in stream nitrate concentrations. It appears both precipitation and temperature influence forest soil processes that in turn influence stream chemical composition. Alpine soil processes were less connected to stream chemistry than forests. In addition to the direct influence of atmospheric nitrogen deposition, precipitation and temperature had strong, and different, direct influences on stream nitrate. Climate variability is therefore influencing watershed biogeochemistry in a way that complicates our efforts to track the
effects of atmospheric nitrogen deposition trends in Loch Vale.