.1 how energy is obtained, how efficiently it is used. 

 what ])ortion of it is available for reproduction and 

 Lirowtb, and how much of it is passed on to higher 

 trophic levels through predation. These many factors 

 may be brought together in the following equation to 

 show what happens to the gross energy intake, l\ at 

 any trophic level : 



F.+R + D + ir + l^ + , + b 



One can make a number of derivations from this 

 ecjuation ; but what is of particular interest and im- 

 portance is the amount of energy of each trophic level 

 that is transferable to the next. This is represented 

 by the equation : 



!,_,, = J,^ — E 



n - IV 



Each factor in the equation must be measured at each 

 trophic level, and for each species. Since primary 

 production is basic and concerns the capture of en- 

 ergy' by plants, it will be considered first. 



Primary production 



\'arious methods are employed for measuring 

 primary productivity, each procedure having certain 

 advantages and disadvantages (Ryther 1956). Fur- 

 ther work in evaluating and improving these meth- 

 ods or developing new ones is desirable. 



A common procedure for analyzing aquatic habi- 

 tats is to take equal samples of green phytoplankton, 

 ordinarily inseparably mi.xed with bacteria and zoo- 

 plankton, and suspend during daylight hours in both 

 transparent and blackened bottles at the same depth 

 at which obtained. Photosynthesis of course does not 

 occur in the blackened bottle, and there is a loss of 

 oxygen, resulting from respiration, R. and decompo- 

 sition, E -\- D -\- PV. In the transparent bottle, 

 photosynthesis occurs in addition to respiration and 

 decomposition, bringing a production of carbohy- 

 drates. There will either be an increase in oxygen 

 concentration, or the loss of oxygen will not be so 

 great as in the blackened bottle. The difference in 

 the final oxygen content of the two bottles will be a 

 measure of gross ('rod action: I \. 



If the o.xygen content of the water is measured at 

 the beginning of the experiment, then the loss of 

 oxygen in the blackened bottled subtracted from the 

 difference in oxygen content of the two bottles at the 

 end of the experiment will represent the net pro- 

 ductivity. This net productivity may also be deter- 

 mined from the difference in the oxygen content of 

 the transparent bottle between the beginning and the 

 end. To obtain net production for an entire daily 

 cycle, the consumption of oxygen for respiration and 

 decomposition over 24 hours must be subtracted from 

 the gross photosynthetic output during daylight 



iiours. Tiie use of black and wiiite bottles for meas- 

 uring productivity has been criticized because of a 

 possible difference in the amount of oxygen utilized 

 by Imctcria in the two bottles (Nielsen 1952, Pratt 

 and Herkson 1959), but the occurrence of a signifi- 

 cant difference has been denied (Ryther 1956). 



One may use the amount of carbon dioxide, rather 

 than oxygen, absorbed during a period of time as a 

 measure of photosynthesis, if correction is made for 

 the carbon dioxide given off in respiration and de- 

 composition. Changes in the amount of COj in the 

 water may be calculated from the differences in pH, 

 the hydrogen-ion concentration (Verduin 1956). Net 

 l)roduction during daylight hours may be measured 

 by introducing a known amount of C'^On into a vol- 

 ume of water where the amount of carbon dioxide 

 already present is known. The amount of C'^ ab- 

 sorbed by the phytoplankton can be accurately deter- 

 mined by use of counters applied to phytoplankton 

 collected and dried at the end of the period. Then the 

 proportion of the radio-active carbon absorbed to the 

 amount introduced can be applied to the total CO2 

 initially present to get the total amount absorbed 

 (Nielsen 1952). 



In fertile eutrophic lakes there is a continual sink- 

 ing of dead organic material, derived chiefly from the 

 plankton, from the epilimnion into the hypolimnion. 

 Tiie decomposition of this material absorbs oxygen 

 from the hypolimnion and liberates CO2 to produce 

 a stagnation period during the summer months. The 

 amount of o.xygen deficit, or carbon dioxide incre- 

 ment, and the rate at which it forms can be measured 

 to furnish a rough index of the lake's net productivity 

 during the period between spring and autumn over- 

 turns (Hutchinson 1938, Ruttner 1953). Such esti- 

 mates are in error by the amount of organic material 

 brought in by streams, and they will vary in com- 

 parative usefulness depending on the volume ratio of 

 hypolimnion to epilimnion (Hutchinson 1957). 



Since nitrogen and phosphorus are metabolized 

 more rapidly by plants in the manufacture of food 

 during the growing season than they are regenerated 

 from decomposing material, the rate and extent of 

 the depletion of nitrates and phosphates in freely cir- 

 culating bodies of water or in the epilimnion of strati- 

 fied lakes serve as an index of the amount of organic 

 matter produced (Hutchinson 1957). The rate of ac- 

 cumulation and regeneration of these substances in 

 the hypolimnion from the dead organisms that sink 

 into it may also be used to get an approximation of 

 primary production (Waldichuk 1956). These meas- 

 urements are not exact since they do not account for 

 the repeated regeneration and reutilization of the sub- 

 stances in the photic zone during the season nor their 

 transference to and storage in the bodies of animals. 



The rate of photosynthesis varies in relation to the 

 amount of chlorophyll present and to light intensity 



Exchanges, productivity, and yield 203 



