508 Algae. 



usually cultninating in a vernal maximum about the end of April and 

 often declining rapidly to a low level in June. The average monthly 

 production declines gradually during the remainder oi the year to the 

 winter minimum in December." 



„Individual years vary greatly irom these averages as a result of 

 hydrographic, climatic, and other environing conditions in varying com- 

 binations." 



„The course of plankton production in Channel and backwaters 

 throughout the year exhibits a series of recurrent pulses, culminating in 

 maxima and separated by minima, which give the planktograph the 

 appearance of a series of „frequency of error" curves of varying ampli- 

 tudes. These pulses generally have a duration of 3 to 5 weeks, and tend 

 to coincide in their location in all localities coincidently examined by 

 us. This similarity in the direction of movement in production amounts 

 quantitatively to 65 per cent of the possible comparisons in our records. 

 This cyclic movement in production is plainly influenced, accelerated 

 or retarded, or its amplitude extended or depressed, by environmental 

 factors, but is not itself traceable to any one or any combination of them. 

 A brief interval of examination — not more than one week — is essential 

 to a demonstration of the existence of these pulses." 



„Area and depth, within limits of our environment, show little re- 

 lation to plankton production." 



„Age of the water is an important factor in determining production 

 in streams. Young waters from Springs and creeks have but little plank- 

 ton, and even such tributaries asSpoon River (drainage basin 1,870 

 square miles) contain but little plankton, principally of more rapidly 

 developing organisms. This barren water, impounded for 10—30 days in 

 backwater reservoirs such as Phelps Lake, develops an abundant plank- 

 ton. The rate of run-off and replacement of impounded waters deter- 

 mines to some extent the amplitude of production. This is greatest 

 where run-off is least and rate of renewal slowest." 



„Fluctuations in hydrographic conditions constitute the most imme- 

 diately effective factor in the environment of the potamoplankton. Rising 

 levels usually witness a sharp decline in plankton content (per m. 3 ) as 

 barren storm waters mingle with or replace plankton-rich waters of 

 Channel and reservoir backwaters. Falling levels are periods of reco- 

 very and increase in plankton. Stability in hydrographic conditions con- 

 duces to rise in production at all seasons of the year, and instability is 

 always destructive. Winter floods tend to lower plankton production; 

 spring floods increase it." 



„Temperature affects production profoundly. Below 45" the plankton 

 content in the river is only about 9 per cent of that present above this 

 temperature, and in backwaters but 29 to 40 per cent. Minimum pro- 

 duction is at times of minimum temperature. The vernal pulse in pro- 

 duction attends the vernal rise in temperature and culminates at about 

 60°— 70°. With the establishment of the midsummer temperatures (about 

 80') production falls from 44 to 87 per cent in Channel and backwaters. 

 Itrises, however, 63 per cent in Phelps Lake, so that other causes 

 than temperature may be operative in producing the midsummer decline. 

 The autumnal decline in temperatures is accompanied by decline in pro- 

 duction in the Channel and in Quiver Lake, but by an increase in other 

 backwaters, which exhibit a tendency toward an autumnal pulse. The 

 decline to winter minimum occurs in December." 



„An early spring accelerates, and a late spring retards, the vernal 

 pulse, and a late autumn prolongs the autumnal production. Summer 

 heat pulses often attend plankton increases. Minimum temperatures are 

 not prohibitive of large plankton production. The December production 

 in Phelps Lake in 1898(43,14 cm.") exceeds the vernal maximum else- 

 where in all localities but one, but falls much below the summer pro- 

 duction in Phelps Lake. The ice-sheet is not inimical to a considerable 

 plankton production unless Stagnation conditions occur." 



„Light affects plankton production. The half year with more illu- 

 mination and fewer cloudy days produces from 1,6 to 7 times as much 



