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 O 2 consumed per μ g 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 O 2 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 O 2 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.