{3.7 gC assimilated/hr/g chlorophyll in marine phy- 

 toplankton at light saturation) (Ryther and Yentsch 

 1957). The amount of chlorophyll in the plankton 

 may be determined photometrically for all depths and 

 calculated in terms of unit area of surface. 



Measurement of productivity in flowing waters, 

 such as streams, presents special problems. Most of 

 the vegetation is benthic rather than planktonic. De- 

 termination of the rates of photosynthesis and oxygen 

 use are best made by direct measurement of changes 

 in the concentration of oxygen in the water as be- 

 tween day and night. Because of photosynthesis, 

 there is a net increase in oxygen concentration dur- 

 ing the daytime. 



At night, photosynthesis ceases, but oxygen use 

 continues, so the oxygen loss gives a measure of the 

 rate of respiration and decomposition, and this pre- 

 sumably remains the same throughout the 24-hour 

 daily cycle. Adding the average hourly night loss to 

 the average hourly gain during the day and multiply- 

 ing by the hours of daylight gives the total gross pro- 

 duction for the 24-hour day. To obtain the net pro- 

 duction for the entire day, the hourly loss at night 

 must be multiplied by twenty-four, and subtracted 

 from the total gross production. Corrections need to 

 be made, however, for the greater diffusion of oxygen 

 from the air into the water at night, when oxygen 

 concentration in the water is lowered, than during the 

 day, when it is higher. There may actually be dif- 

 fusion of oxygen out of the water during the daytime 

 when the photosynthetic rate is high. Additional cor- 

 rections will also be necessary for import of oxygen 

 from ground water and surface drainage and trans- 

 portation of oxygen and carbon dioxide downstream 

 by swift currents (Odum 1956, 1957). 



On land, the annual net production of herbaceous 

 plants that grow from seed or underground parts is 

 approximately equivalent to the biomass of the vege- 

 tation at its maximum stage of growth, provided no 

 appreciable amount has been lost or consumed by 

 animals. Exclosures may be erected to prevent graz- 

 ing by larger animals. The measurement of total pro- 

 duction is inexact when the maximum standing crop 

 occurs before the terminus of the growing season. 

 The annual crop divided by the length of the grow- 

 ing season gives net productivity in terms of average 

 daily increment. Daily productivity varies, however, 

 with the stage of growth. At one site, the cumula- 

 tive productivity of common cat-tail, from March 29 

 to July 2, when the largest crop was reached, aver- 

 aged 8.17 grams carbon per square meter per day, 

 but for a short period of maximum growth. May 

 4 to 28, was 23.48 (Penfound 1956). Seasonal bio- 

 mass production of grasses is increased by a moderate 

 amount of grazing, so in measuring primary pro- 

 ductivity under natural conditions, the stimulating or 

 mhibiting effect of animal consumption should be 

 given proper evaluation. 



The annual woody increment of trees and shrubs 

 is proportional to the increase in diameter or width 

 of growth rings. Mature trees may be felled, unit 

 samples of branches, trunk, and roots dried and 

 weighed, and the annual woody production since 

 germination calculated (Ovington 1957). Attempts 

 have been made to measure respiratory losses (Moller 

 et al. 1954), and the annual production of foliage, 

 seeds, acorns (Downs and McQuilken 1944), and 

 nuts, but measurement methods need to be improved 

 (Baldwin 1942). 



Secondary production 



When an animal species is represented by a low 

 overwintering population, or an immature stage, the 

 maximum biomass obtained in each generation is the 

 approximate net production for that generation. 

 However, this does not account for continued repro- 

 duction and growth of individuals after the maxi- 

 mum biomass of the population is attained, nor does 

 it account for excreta, natural deaths, or the kill of 

 predators. If the population of the species is main- 

 tained at a more or less uniform level throughout the 

 year, then the mean biomass times the number of 

 generations gives the net production, with again the 

 e.xception of the factors mentioned above. Lindeman 

 ( 1941 ) considered the phytoplankton turnover, or the 

 production of a new generation, to occur every week 

 from May through September and every two weeks 

 through the rest of the year, the zooplankton to re- 

 place itself bi-weekly through the year, Chaoborus 

 to have three generations per year ; midge flies, two ; 

 and various aquatic beetles and bugs, one generation 

 per year. Juday (1940) estimated that the mean 

 standing crop of both phytoplankton and zooplankton 

 replaced itself every two weeks throughout the year. 

 To obtain gross productivity, the respiratory rates of 

 these animals must also be measured. 



Although shortcut methods may often be prac- 

 ticable, we need more accurate data, based on careful 

 field observations and experiments, of food require- 

 ments, reproductive rates, growth rates, mortality, 

 and so on, for individual species. The energy intake 

 and requirements of individual species can often be 

 measured under experimental conditions by present- 

 ing a known amount of food to one or several indi- 

 viduals and determining the amount consumed dur- 

 ing a period of time. This is preferable to measuring 

 the oxygen intake of resting animals. The influence 

 of various environmental factors, size and age of the 

 animals, density of populations, etc., on the food con- 

 sumption can be ascertained and often also the pro- 

 portion utilized for existence, growth, and other ac- 

 tivities (ground animals, Bornebusch 1930; Tubifex, 

 Ivlev 1939; grasshoppers, Smalley 1960; rotifers. 



204 Ecological processes and dynamics 



