Continuous automated measurement of rates of photosynthesis and respiration in an undisturbed river community1

A precise, continuous function (a Fourier series) describing undisturbed net community production throughout a day has been obtained from automatically recorded and proccssecl oxygen concentration data from two stations in a river. The linear relationship found between light and productivity confirmed the precision of the method and was used to give a nearly continuous estimate of community photosynthesis, respiration, and gross productive efficiency. Respiration and efficiency may remain constant for one to several days, or may change over a few hours. Day-to-day variation in community photosynthesis and respiration may preclude extrapolation of a number of isolated single-day measurements to determine seasonal or other long term trends, Most methods for estimating aquatic productivity have depended on incubation measurements of 14C uptake or 02 production, but errors are caused by containment of the sample, and time resolution is limitecl by the incubation period (Vollenweider 1969). Attempts to measure productivity without perturbation by incubation have been based on the fact that the rate of change of free oxygen and inorganic carbon concentration depends almost entirely on community respiration, photosynthesis, physical mixing and transport, and cxchangc with the atmosphere. Thus it is theoretically possible to measure net community productivity (photosynthesis minus rcspiration) from the rate of change of inorganic carbon or oxygen concentration if rates of mixing, transport, and exchange with the atmosphere are known. Stated differently, net community productivity can in theory be estimated by using a solution of the partial differential equation for oxygen or inorganic carbon mass-balance. This avoids errors due to containment and incubation, but attempts to use free concentration measurements have been limited by problems in accounting for mixing and transport and in solving the mass-balance equation (Horn’ This work was supported by National Science Foundation Grant GB 32914, Recorders were provided by National Oceanic and Atmospheric Administration, Department of Commerce Grant N22-9-72 ( N ) an made available by M. Garstang. d berger and Kelly 1972). These attempts have also only been used to give measurements at discrete times or daily mean values. Although measurements of unperturbed productivity have been attempted in lakes (Winberg 1960; Welch 1968; Eley 1970) the best known method for using free concentration measurements is the upstreamdownstream diurnal measurement method pioneered by Odum ( 1956) (hereafter referred to as the Odum method). That method estimates production from the change of oxygen concentration during the passage of parcels of water along the length of a river segment (see Owens 1965). The method is equivalent to use of an approximate finite difference solution of the massbalance equation and thus it is noise enhancing; that is, it is sensitive to errors in measurements of concentration (Hornberger and Kelly 1972 and below). The oxygen mass-balance equation has long been used by sanitary engineers to estimate oxygen depletion, but they have been Iittle concerned with productivity measurement, although Strccter ( 1922) recognized the importance of the net productivity term in the equation. O’Connor and DiToro (1970) presented a solution of the equation that included diurnal variation in oxygen due to photosynthesis, but they did not consider how the equation might bc used to estimate production. Koivo and Phillips ( 1971) used an exact solution of LIMNOLOGY AND OCEANOGRAPHY 305 MARCH 1974, V. 19(2)