Abstract:
A technique for estimating gross primary production and respiration of periphyton communities, in situ, in small turbulent streams was developed using a photosynthesis-respiration chamber in a modified "light and dark bottle" method. Trays containing portions of the substrate bearing the benthic community were embedded in the stream, level with the bottom. A clear plexiglas cover could be clamped to a tray to make it part of a closed system in which the water was mixed, circulated, and exchanged to facilitate measurement of changes in the concentration of dissolved oxygen. The rates of gross primary production and respiration of the periphyton communities in two coastal streams in Oregon were estimated periodically during the six months from July through
December 1964 by hourly determinations of the changes in concentration of dissolved oxygen in the chamber. The rates so determined were used to demonstrate seasonal trends in both processes and to obtain quantitative relationships between these processes and the environmental parameters known to control them. These relationships were determined by formulating multiple regression
equations for both components of community metabolism using light intensity, water temperature, and plant biomass as the independent variables for gross production and the latter two plus the
wet weight of the animal fraction of the community for respiration computations. Respective R² values of 0.971 and 0.800 for each stream indicated that most of the variability in the gross production process was accounted for and that the relationships involved were not the same in both streams. A single R² value of 0.821 was obtained from the measurements of community respiration since differences in the variability between streams were not statistically significant. The resulting equations were also used to predict production and respiration on a monthly basis. A portable illumination totalizer was used to measure incident solar radiation striking the streams in gram-calories per cm². The influence of the canopy and surrounding topography in determining available light was found to be significant and varied for each stream.
Approximately 75 to 95 percent of the total light energy was absorbed and reflected and did not reach the communities in the streams. The predicted range of gross production (0.34-l.28 g O₂/m²/day) was substantially lower than results reported from other in situ investigations using the oxygen method. The most apparent reasons for these low levels were low light availability and instability of the bottom substrate. An asymptotic gross photosynthesis-light intensity curve was
obtained by plotting actual dissolved oxygen changes against observed light intensities. The shape of the curve was similar to that obtained in a laboratory community. Predicted rates of respiration (0.22-1.60 g O₂/m²/day) were consistently higher than equivalent estimates of gross production until November, when the animal fraction of the community was reduced. For most of the season both streams were heterotrophic, obtaining a significant amount of their energy from the surrounding watershed.
