Habitat-Specific Solute Retention in Two Small Streams: An Intersite Comparison

We measured uptake rates of phosphorus, nitrate, calcium, and dissolved organic carbon within two headwater streams, one in the Appalachian mountains of North Carolina and the other in the Cascade range of Oregon. The major physical differences between these two streams are parent geology and local geomorphic structure. Uptake rates were measured following low-level nutrient releases during summer 1987 in 20-m reaches with different geomorphology. The relative importance of biotic vs. abiotic sorption of nitrogen and phosphorus by sediments from the two streams was assessed by laboratory experiments. Nitrate-N uptake rates were high for the western stream (11.9 g im-22 min-1) and low for the eastern stream (3.9 Ag m-2 minm) during the summer. The debris dam reaches in Oregon were the most retentive of N (shortest uptake length of 17 m). Conversely, sol- uble reactive phosphorus (SRP) uptake rates were higher for the eastern stream (18.6 Ag m- 2 min), primarily through biotic processes. SRP uptake lengths were short for the cobble (32 m), debris dam (35 m), and rock outcrop (40 m) reaches in the eastern stream. Uptake of SRP in either stream was not related to sediment size fraction but rather to a combination of sediment infiltration rates and quality of organic material. Calcium (Ca) uptake lengths were long in the Oregon stream (1278 m) but short in the Appalachian stream (106 m). Surprisingly, the eastern stream was more retentive of Ca than nitrate (shorter uptake lengths for Ca) during this time period. Debris dams greatly enhanced retention of dissolved organic carbon in both streams (60% of all uptake in the eastern stream and 81 % in the western stream), although retention was greater in the eastern stream. The lower uptake rate of phosphorus and higher uptake rate of nitrate in the Oregon stream were expected based on geographic location and parent geology; streams in this area drain catchments of volcanic origin and tend to have low N:P (atomic; 1.8 for the western stream) ratios, indicating potential N limitation. Streams flowing over granitic bedrock, such as the eastern stream, tend to have lower P availability (N:P = 15.5). The combined results of laboratory and field measurements indicate that in the eastern stream, strong biotic control of P uptake coupled with high P demand result in relatively short P uptake lengths and a strong impact of P spiraling on ecosystem dynamics. In the western stream, strong biotic control of N uptake combined with strong N demand result in short N uptake lengths. This is especially true at sites of downed timber that retain both FPOM and CPOM, creating a high N demand (shortening N spirals).