Nutrient and microbiological characteristics of fine benthic organic matter in mountain streams

Fine benthic organic matter (FBOM) was collected over a 10-mo period from 14 lst-order
streams in the Cascade Mountains of western Oregon to investigate 1) relationships between FBOM
substrate quality and microbial activity, 2) links between organic matter sources and FBOM substrate
quality, and 3) how FBOM is influenced by riparian vegetation, elevation, and season. Streams drained
forests in 3 successional age classes: old-growth forest dominated by Douglas-fir (Pseudotsuga menziesii)
and western hemlock (Tsuga heterophylla), and young regenerating stands, either 10 y old with
a large riparian herbaceous component, or 30 y old and surrounded by deciduous trees such as red
alder (Alnus rubra).

Seasonal trends showed a major autumn depression in carbon:nitrogen ratios (C:N) and an increase
in microbial activities, a likely result of increased leaf inputs after an early fall storm. Decreases in
C:N, total C, total N, and organic P were correlated with reciprocal increases in respiration, Pglucosidase
and phosphatase activities, and acetylene reduction, all of which are relative indicators
of microbial activity. Lower C:N and higher denitrification potentials, respiration rates, [3-glucosidase
and phosphatase activities, and mineralizable N were observed in young stands compared to old
growth, suggesting higher quality FBOM and faster decomposition rates in young stands. An exception
to this trend was acetylene reduction, which was greater in FBOM from old-growth streams.
Significantly lower C:N at high elevations (1220-1280 m) versus low elevations (580-800 m) suggested
the presence of more herbaceous vegetation and alder in high-elevation riparian zones. Lower total
N and total C, and elevated denitrification potentials, acetylene reduction, respiration rates, and phosphatase
activity at low elevations (580-800 m) suggested greater decomposition rates at low elevations.
Organic P was 3.6 and 2.2 mg P/ g organic matter at high and low elevations, respectively, a significant
difference probably resulting from the young geologic age of parent material at high elevations. Data
from this study suggest a potential link, mediated by shifts in FBOM, between headwater forest
management and dynamics of stream food webs.

Key words: fine benthic organic matter, mountain streams, microbial indicators, tree harvest effects,
elevation effects, Douglas-fir, red alder.