- The purpose of this study was to investigate the effects of
logging on the productivity, structure and biomass of a periphyton
community developed in mountain streams. The three streams used
in this investigation were located in the coast range of Lincoln
County, Oregon. They were selected for similarity of habitat, terrestrial
canopy, and benthic community, one dominated by diatoms
In 1966, Deer Creek watershed was subjected to a selective
pattern of logging with a buffer strip of canopy left standing along the
stream. Needle Branch watershed was subjected to clearcut logging
resulting in the removal of all the canopy, and the watershed of Flynn
Creek was left in its natural condition and used as a control.
Clearcut logging had a profound impact on the aquatic environment
of Needle Branch. As indicated by light reading taken during
primary productivity runs, the stream received a mean value of seven
times more light in 1967, July-September, as compared to similar
months in 1964, prior to logging. This resulted in a high of 50
langleys/hr as compared to 13 langleys/hr recorded prior to logging.
Concurrently, the mean temperature of the stream, as measured during
productivity runs, increased 6 C, with a high of 26 C recorded as
compared to 14. 8 C prior to logging.
The impact of these environmental changes resulted in a periphyton
community quite different from that existing in the stream
prior to logging. As indicated by the communities developed on
glass artificial substrates, this stream, which supported a periphyton
community of diatoms prior to logging, was changed to a habitat
supporting a mixed community of filamentous algae and diatoms with
the filamentous algae becoming dominant. The diatom flora of the
stream changed in species composition and became more uniform
throughout, after logging.
Samples of the communities taken from the natural substrate
and glass artificial substrates were similar, but dissimilarities arose
in the relative abundance of the species. Artificial substrates cut
from native rock supported a community of greater similarity to that
on the natural substrate, than existed between the communities developed
on glass artificial substrates and the natural substrate.
The index of the autotrophic organisms in Needle Branch (mg
chlorophyll a/m²) was lower after logging, as compared to that found in the stream prior to logging. Mean concentrations,for cornparable
months, of 51.3 mg chlorophyll a/m² were recorded before
logging, compared to 9.10 mg chlorophyll a/m² recorded after
logging. The index of the autotrophic organisms obtained from the
organic matter collected on glass artificial substrates showed little
relationship to that obtained from the substrate of the stream.
Respiration of the community after logging was higher than
during the equivalent period prior to logging. Mean respiration for
July-September, 1964,was 1.08 g 0₂/m²/day while after logging the
reduced biomass had a respiration rate of 1.28 g 0₂ /m²/day.
After logging, gross primary production was higher in Needle
Branch than in Flynn Creek (control watershed),, but slightly less
than recorded for Needle Branch prior to logging. Needle Branch
maintained gross primary production comparable to the prelogging
period, with a smaller biomass, by increased efficiency of photosynthesis
per mg chlorophyll a.
The photosynthesis-respiration ratio (P/R ratio) indicated that
prior to logging, Needle Branch was autotrophic for all seasons except
the fall. After logging, the stream became progressively more
heterotrophic from spring to summer.