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The Measurement of Primary Production’ JOHN H. RYTHER 1l~ooti.r Hole Oceanographic Institution, ll~oods Hole, .l/trs.s. ABSTRACT A discussion is presented of the measurement, of primary product,ioll t)y short -term c\ - pcrimental methods and by measurement of in situ environmental changes occurring over longer periods of time. The methods described include the measurement of photosynthesis by oxygen production, carbon dioxide assimilation and the associated pH change, and up- take of radioactive carbon (CY). In situ changes in the standing crop and the concentra- tion of inorganic plant nutrients are also discussed, and the possibility explored of relating the rate of uptake of radioactive inorganic tracers to primary production. The relationship between chlorophyll and photosynthesis is reviewed and some nen- experimental evidence is presented which indicates the possibility of using chlorophyll concent ration as an indes of primary production. In the discussions of each method emphasis is placed on sensi- tivity, reliability, and the interpretation of the data. The production of organic matter is carried out in water, as elsewhere, by plants through the familiar process of photosyn- thesis. The problem of measuring primary production in natural waters then resolves itself, with certain reservations, to the measurement of the photosynthetic rates of aquatic plant communities. The present paper will be concerned with the methods by which production may be measured. Emphasis will be placed on theoretical considerations rather than the applied aspects of such measurements. The actual techniques which may be em- ployed are governed to a large extent by the particular environment to be studied and the producer populations living therein, and various details of t,he problem require unique solutions. The productivitNy of natural waters is often judged on t’he basis of the quantity of organic matter which they support at any one moment. This instantaneous population or “standing crop” in itself is a rather poor index of production without some indica- tion of the time factor involved in its forma- tion. Large lads of eel grass, kelp, drifting Sargaswm, ct c. may consist of organic matter \vhi,ah has been synthesized over a 1 Contribution So. 825 from the Woods Hole Oceanographic Institution. This work was car- ried out with the aid of a research grant from the National Science Foundation. period of months if not’ years. The a(~fw-11 production rate of suc*h impressive popula- tions may be quite Ion- and may be OLYY- shadowed by tfhatJ of relatively small stand- ing crops of phyt,oplankton which are subject to decimation from many sourws. Production must therefore be considered as a rate rather than a st’atic quantity. The most direct and in many cases thrl most satisfactory method of measuring production is simply by the increase or de- crease in the standing crop over a measured interval of time. In this case production is measured in.terms of biomass, and may 1~~ expressed variously as the increase in, 1) the area of bottom wvered, 2) volume of producer organisms, 3) displacement vol- ume, 4) wet or damp weight, 5) dry weight, 6) ash-free dry weight) (i.e., loss on ignition). This method, in one form or another, has been widely used in studying t,he production of macroscopic benthic populations whicbh follow a regular seasonal pattern of growth and are relatively undisturbed by predation or other decimating factors. It is quitcl unsuitable for measuring production of the phytoplankton since cahanges in its biomass reflect merely the net change resulting both from production and removal by prcda- tion, water movement, sinking, etc. In the latter case it is desirable to meas- ure as nearly as possible the instantaneous production rat,e. In practiw this means 72