. The ecology of algae : a symposium held at the Pymatuning Laboratory of Field Biology on June 18 and 19, 1959 . X ALGAE From Bartsch, 1958 L_ INFLUEtJCE OF QUANTITY OF ALGAE ON AUGUST 26, 1955 FiQure 12 million cells per ml. The algae added to one por- tion of the dilution water-sewage mixture were killed by short exposure to heat at 70°C; the others were living. values with living and dead algae were compared with values obtained when the medium contained sewage alone (Figure 13) . Although oxygen demand with dead cells was a lit- tle greater, it is believed that the ultimate
. The ecology of algae : a symposium held at the Pymatuning Laboratory of Field Biology on June 18 and 19, 1959 . X ALGAE From Bartsch, 1958 L_ INFLUEtJCE OF QUANTITY OF ALGAE ON AUGUST 26, 1955 FiQure 12 million cells per ml. The algae added to one por- tion of the dilution water-sewage mixture were killed by short exposure to heat at 70°C; the others were living. values with living and dead algae were compared with values obtained when the medium contained sewage alone (Figure 13) . Although oxygen demand with dead cells was a lit- tle greater, it is believed that the ultimate first- stage (Lg) in either case would have been about the same. Under identical laboratory condi- tions, when dead Chlorella cells were added to samples in relative quantities of 1 , 10, and 66 (18,330, 183,300, and 1,222,000 cells/ml. re- spectively), the resulting values were , 3 .73 , and 7 .74 ppm. respectively . On the basis of the Chlorella tests , it appeared that the contribution to the five-day, 20°C ap- proximated 0 .2 ppm. per billion cells . Some workers believe that dense populations of algae in polluted waters are always beneficial and desirable. This view undoubtedly is supported by the common observation of high dissolved oxy- gen concentrations in their presence and failure to note the nocturnal depressions. Concurrent rapid production of a more stable mass of organics in the form of algae from readily putrescible wastes in ef- fect postpones satisfaction of the oxygen demand to some later time at a different place. When condi- tions are favorable, such postponement may be de- sirable; when unfavorable, as in examples cited, algae make existing pollution conditions even worse. The oxygen released to the water in algal photosynthesis is momentarily beneficial in spite of the frequent inefficiency in its use. To evaluate equitably the algal source of oxygen, the processes by which algae along with the other biota also con- sume oxygen
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