432 FAGER [chap. 19 



is then corrected for respiration by assuming a ratio of photosynthesis to 

 respiration of 10/1. It has, however, been shown that the ratio can vary from 

 20/1 to 1.4/1, depending on the physiological state of the phytoplankton and 

 environmental conditions. The light-dark bottle method of productivity 

 measurement has two disadvantages : it may require very long periods during 

 which it is uncertain what may happen to the phytoplankton in the unnaturally 

 restricted space and in the presence of solid surfaces ; it measures only gross 

 production, and net production must be estimated by some assumption con- 

 cerning respiration of the phytoplankton. The use of 14 C has many advantages 

 — speed, direct measurement of photosynthesis, etc. — but it gives an estimate 

 somewhere between gross and net production. The value of the correction 

 which should be used to get gross production may vary depending on the 

 physiological state of the phytoplankton. It will be noted that a major problem 

 in the estimation of net production is the correction for respiration of the 

 phytoplankton ; this cannot be easily separated from that of the zooplankton 

 and bacteria in the sample. As mentioned earlier, Ryther has estimated that 

 net production is usually in the range 50-75% of gross production, but this is a 

 rather large spread if one wishes to strike any sort of energy balance and the 

 estimate is based on certain assumptions which may not be valid under all 

 conditions. 



Another problem which arises when measurements are infrequent is that the 

 productivity measurement may be only a somewhat sophisticated measure of 

 the standing crop at the moment of measurement. For it to have a meaning 

 as production over a period of time, it is necessary to assume that, during the 

 period, environmental conditions do not change in such a way as to affect 

 photosynthesis and that the standing crop does not change in numbers, species 

 composition, or any other manner that will affect the rate of photosynthesis. 

 Such assumptions may be difficult to justify unless measurements are made 

 with considerable frequency (see Steemann Nielsen, Chapter 7, page 129). 

 Attempts have been made to take into account certain of the changes by 

 setting up more or less complicated equations for the rate of change of plant 

 material in a vertical water column under unit surface area (Riley, Stommel 

 and Bumpus, 1949; Gushing, 1959; Steele, 1959). These involve estimates of 

 photosynthetic rates, grazing rates by zooplankton, sinking rates of the phyto- 

 plankton, mixing, changes in nutrients (usually phosphorus), etc. As Steele 

 points out, however, there is an element of circularity in some of the calculations 

 and a rather serious doubt as to whether certain of the estimates are relevant to 

 conditions in the ocean. For example, laboratory determinations of the filtering 

 rate oiCalanus, upon which the estimate of grazing rate has been based, range 

 from 1 to 240 ml per animal per day while Calanus respiration rates combined 

 with phytoplankton concentrations in the sea require rates of 1 to 3 liters per 

 animal per day. There is some evidence that part of the discrepancy may be 

 due to the smallness of the volumes of water used in the laboratory experiments. 



Cushing (1955) has used a method which overcomes some of these problems, 

 although it can, unfortunately, be used only on diatoms and thus is in- 



