AGU: Fall meeting, 1996
Research on the spatial and temporal variability of solute
contributions from ephemeral hillslope streams to alpine surface waters
was conducted in 1995 and 1996 in the Green Lakes Valley of the Colorado
Front Range as part of the Long Term Ecological Research Program (LTER).
Recent measurements of nitrate concentrations in alpine
surface waters of the Colorado Front Range raises concern of nitrogen
saturation and stream acidification in these sensitive ecosystems. The
source of
NO3-
remains largely unknown, but may result from
a combination of snowmelt storage/release, nitrification in soils,
and infrequent summer precipitation.
We investigate whether ephemeral discharge from talus-mantled
hillslopes contributes a substantial amount of the
NO3-
reaching high elevation streams during the growing season.
Samples of subtalus
flow withdrawn along linear transects downslope between melting
snowfields and valley margins were analyzed for
NO3-
ammonium, other major cations and anions, and ANC.
Pits excavated
in associated talus formations yeilded samples of soils and fines
which were analyzed for inorganic, organic, and total resident
nitrogen and carbon, and revealed substantial interstitial ice
which was melted and analyzed similar to subtalus water.
A time series of samples drawn from sites of subtalus flow reveals
a decrease from high
NO3-
concentrations
(>50 ueq/L)
associated with peak snowmelt to lower concentrations
(5.2 to 16.8 ueq/L)
during the growing season.
Late-season
increases to > 20 ueq/L
associated with significant precipitation
may be augmented by potential nitrification by microbial populations
harboured in fine materials within talus.
This hypothesis is further
supported by large amounts of organic N (1.5 - 31.5 ug/g)
in subtalus fine gravels and sands and sufficient available carbon to
support microbial activity.
The data indicate that talus formations
mantling large portions of alpine
watersheds may contribute substantial amounts of nitrate
to surface waters
and play a significant role in controlling
the hydrochemistry of these catchments.