Biochemical oxygen demand and algae: fractionation of phytoplankton and nonphytoplankton respiration in a large river

Mass balance equations for dissolved oxygen in streams are formulated to account for, among other variables, algal respiration (R), and biochemical oxygen demand (BOD). The oxygen consumption measured in primary productivity‐respiration analyses is not R but is total community oxygen consumption (TCOC), and BOD measurements are complicated by undefined algal components. Ultimate BOD was found to be 0.24 mg of O2 consumed per μg chlorophyll a and carbonaceous BOD was 0.20 per μ‐g chlorophyll a in excess of background BOD. The results were similar for live and dead algae. Phytoplankton respiration was fractionated from nonphytoplankton oxygen consumption (NPOC) by the regression of respiration against chlorophyll a to obtain a y intercept of zero chlorophyll. The intercepts, NPOC, closely matched O2 consumption measured when phytoplankton biomass was very low. Phytoplankton respiration, calculated as the residual of the difference between TCOC and NPOC, ranged from 0.2 to 1.5 (mean = 0.88) mg O2 per mg chlorophyll a per hour, close to the literature value of 1 (in cultures). Depth‐integrated (DI) phytoplankton respiration was 1/4 to 1/3 of DI gross primary productivity and 1–3% of maximum primary productivity. The separation of phytoplankton R and NPOC permitted the demonstration that R probably is not a simple function of productivity.