Numerical And Graphical Procedures For Estimation Of Community Photosynthesis And Respiration In Experimental Streams

Abstract

A numerical dissolved oxygen (D.O.) routing model DORM is developed to determine• total stream community photosynthesis (P) and community respiration rates (R) through successive routing of two-station diel D.O. measurements in a stream. The model differs from existing procedures for diel curve productivity analysis in that it uses the complete D.O. transport equation, including D.O. surface exchange, longitudinal dispersion, dependence of respiratory rates on water temperature and dissolved oxygen. The model is applied to the experimental field .channels at the USEPA Monticello Ecological Research Station to compute P and R values at different seasons and under different conditions of water temperatures, solar radiation, and pH. A sensitivity analysis shows that computed P and R values are most sensitive to residence times and surface oxygen exchange (reaeration) coefficients. New equations for surface exchange including the effect of wind have been developed and are summarized. A graphical simplified routing procedure produced P and R values which were 82 and 89 per cent, respectively, of those obtained by complete routing, with a standard deviation of les~ than 5 per cent. Nighttime longitudinal D.O. gradients were used to derive respiration rates, R. Maximum values of P and R in the MERS channels were 14.8 g m- 2 day-1 and 10.7 g -2-1 m day , respectively. P/R ratios ranged from 0.3 to 2.1. A seasonal dependence of P and R values was found as expected. Hysteresis in plots of hourly P versus photosynthetically active radiation (PAR) intensity was observed frequently. It was of such magnitude that it could not be caused by errors in surface exchange estimates or other physical processes.