Yields of Dissolved Organic C,N, and P. from Three High-Elevation Catchments,Colorado Front Range,U.S.A.

Ecosystem dynamics in high-elevation watersheds are extremely sensitive to changes in chemical, energy, and water fluxes. Here we report information on yields of dissolved organic C, N, and P for the 1999 snowmelt runoff season from three high-elevation catchments in the Colorado Front Range, U.S.A.: Green Lake 4 (GL4) and Albion townsite (ALB) on North Boulder Creek and the Saddle Stream (SS), a tributary catchment dominated by alpine tundra. Dissolved organic carbon (DOC) concentrations in stream waters ranged from <1 to 10 mg C L^-1, with the highest values occurring at the SS site. Dissolved organic nitrogen (DON) concentrations ranged from below detection limits to 0.28 mg N L^-1 and were again highest at the tundra-dominatedsite. Dissolved organic phosphorus (DOP) concentrations were at or near detection limits throughout the season in all three catchments indicating a strong terrestrial retention of P. OnlyDOC showed a significant relationship to discharge. Yields of DOC in the three catchments ranged from 10.6 to 11.8 kg C ha^-1 while yields of DON and DOP ranged from 0.32 to 0.41 and 0.02 to 0.08 kg ha^-1, respectively. The relatively highyield of organic N and P relative to C from the highest elevationsite (GL4) was somewhat surprising and points to either: (1) a source of dissolved organic material (DOM) in the upper reaches of the catchment that is enriched in these nutrients or (2) theselective uptake and processing of organic N and P downstream ofthe sampling site. Additionally, seasonal changes in the relativeimportance of DOM precursor materials appear to result in changesin the N content of DOM at both the GL4 and ALB sites.     

Use of Natural 35S to Trace Sulphate Cycling in Small Lakes,Flattops Wilderness Area,Colorado, U.S.A.

Measurements of the cosmogenically-produced ³⁵S, a radioisotope of sulphur (t_1/2 = 87 days), are reported for the Ned Wilson Lake watershed in Colorado. The watershed contains two small lakes and a flowing spring presumed to be representative of local ground water. The watershed is located in the Flattops Wilderness Area and the waters in the system have low alkalinity, making them sensitive to increases in acid and sulphate deposition. Time series of ³⁵S measurements were made during the summers of 1995 and 1996 (July–September) at all three sites. The system is dominated by melting snow and an initial concentration of 16–20 mBq L^-1 was estimated for snowmelt based on a series of snow samples collected in the Rocky Mountains. The two lakes had large initial ³⁵S concentrations in July, indicating that a large fraction of the lake water and sulphate was introduced by meltwater from that year's snowpack. In 1995 and 1996, ³⁵S concentrations decreased more rapidly than could be accounted for by decay, indicating that other processes were affecting ³⁵S concentrations. The most likely explanation is that exchange with sediments or the biota was removing ³⁵S from the lake and replacing it with older sulphate devoid of ³⁵S. In September of 1995 and 1996, ³⁵S concentrations increased, suggesting that atmospheric deposition is important in the sulphate flux of these lakes in late summer. Sulphur-35 concentrations in the spring water were highly variable but never higher than 3.6 mBq L^-1 and averaged 2 mBq L^-1. Using a simple mixing model, it was estimated that 75% of the spring water was derived from precipitation of previous years.     

Dynamics of P, Al, and Fe during High Discharge Episodic Acidification at the Bear Brook Watershed in Maine, U.S.A.

Phosphorus (P), aluminum (Al), and iron (Fe) stream chemistry were assessed for high discharge snowmelt events at the Bear Brook Watershed, Maine (BBWM) during December 2001 and February 2002 and compared with results from a January 1995 study of the same streams. The West Bear catchment has been subjected to artificial acidification since 1989. The East Bear catchment is the untreated reference. Total (acid soluble) Al, Fe, and P were positively correlated with discharge during the 2001–2002 events. However, dissolved P concentrations remained low (≤0.1 μmol L^-1) during high discharge events as pH decreased in both streams.For example, in 2001, total P concentration increased to 1.7 μmol L^-1 during the rising limb of the hydrograph in West Bear, approximately five times the value in East Bear. During the same event, in West Bear and East Bear dissolved Al concentrations increased to 21 and 6.3 μmol L^-1, respectively, while total Al concentrations increased to 166 and 30 μmol L^-1, respectively. Dissolved Fe concentrations remained ≤0.9 μmol L^-1 in both streams during all study events. However, total Fe concentrations in 2001 increased to 239 and 4.1 μmol L^-1 for West Bear and East Bear, respectively. Total Al and Fe declined parallel to total P after peaking during all study periods. Nearly all of the base cations were in dissolved form during the three events, indicating that total Al in West and East Bear Brooks is not associated with primary minerals such as feldspars. We conclude that particulate Al, Fe, and P are chemically linked during transport at high discharge in these episodically and chronically acidified streams.