Habitat-specific solute retention in two small streams: an intersite comparison

We measured uptake rates of phosphorus, nitrate, calcium, and dissolvedorganic carbon within two headwater streams, one in the Appalachian mountains of NorthCarolina and the other in the Cascade range of Oregon. The major physical differencesbetween these two streams are parent geology and local geomorphic structure. Uptake rateswere measured following low-level nutrient releases during summer 1987 in 20-m reacheswith different geomorphology. The relative importance of biotic vs. abiotic sorption ofnitrogen and phosphorus by sediments from the two streams was assessed by laboratoryexperiments.
Nitrate-N uptake rates were high for the western stream (11.9 pg-m-2.min-') and lowfor the eastern stream (3 9 pg • m-2- min-') during the summer. The debris dam reaches inOregon 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 (18.6Ag • m-2- min-9, primarily through biotic processes. SRP uptake lengths were short for thecobble (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 acombination 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 Appalachianstream (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 enhancedretention of dissolved organic carbon in both streams (60% of all uptake in the easternstream 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 Oregonstream were expected based on geographic location and parent geology; streams in thisarea drain catchments of volcanic origin and tend to have low N:P (atomic; 1.8 for thewestern stream) ratios, indicating potential N limitation. Streams flowing over graniticbedrock, such as the eastern stream, tend to have lower P availability (N:P = 15.5). Thecombined 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 Puptake lengths and a strong impact of P spiraling on ecosystem dynamics. In the westernstream, strong biotic control of N uptake combined with strong N demand result in shortN uptake lengths. This is especially true at sites of downed timber that retain both FPOMand CPOM, creating a high N demand (shortening N spirals).
Key words: calcium; dissolved organic carbon; geology; geomorphology,- nitrate; phosphorus; re-tention• spiraling; uptake lengths; uptake rates.