19 



10,000,000 per m. 3 , and reach their maximum development (over 

 600,000,000) in winter months (December to February), especially 

 during low water and more stable conditions, as in January, Feb- 

 ruary, and December, 1898 (Pt. I., PL XII.). At such times the 

 temperature is at or near 32. With flood conditions and rise in 

 temperature the numbers fall below 100,000,000, Tunning from 

 10,000,000 to 50,000,000 during most of the summer. The decline 

 is due in part to the dilution by flood waters, and largely to the 

 retreat up the stream of the crest of the wave of bacterial activity 

 caused by the Peoria pulse of sewage. As noted in the discussion 

 of the chemical conditions, in Part I., this wave lies considerably 

 above Havana during the warmer months. Summer floods, as in 

 June and September, 1897, are wont to wash into the river large 

 quantities of these organisms, bringing the numbers up to 300,- 

 000,000 at times. The figures above cited give but a feeble repre- 

 sentation of the real conditions in the river during this period of 

 maximum. Many of these organisms become attached to objects 

 along shore, and accumulate in great quantity in quieter waters 

 along the channel. They form a serious menace to the fishing 

 industry, since they accumulate in a day or two upon the fyke-nets 

 in quantity so great that their- weight and resistance to the current 

 are sufficient to break down the nets. Their effect upon the consti- 

 tution of the plankton is seen in the marked increase in certain 

 ciliates which accompanies the maximum of these organisms. 



SCHIZOPHYCEyE. 



Nine forms were recorded, though a number of others which 

 occurred but rarely in the plankton remained unidentified. The 

 average number (combined silk and filter-paper records, but omit- 

 ting the former when the latter are available) is 85,909,985 per m. 3 

 This group contributes to the plankton throughout the year, and 

 though numerically abundant is quantitatively less important, 

 owing to the small size of its most abundant member, Microcystis. 

 This species and Oscillatoria constitute quantitatively the greater 

 part of the blue-green algas of the plankton. In contrast with the 

 plankton of Lake Michigan, there is a noticeable decrease in the 

 proportion of AnabcBua and Clathrocystis. Rivularia, Gloiotrichia, 

 and Aphanizomenon flos-aquce, often reported in fresh- water plank- 



(3) 



