19 
107000,000 per m.*%, 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- 
moaey, aud 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, running 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 hes 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. 
SCHIZOPHYCEA. 
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 alge of the plankton. In contrast with the 
plankton of Lake Michigan, there is a noticeable decrease in the 
proportion of Anabena and Clathrocystis. Rivularia, Glototrichia, 
and Aphanizomenon flos-aque, often reported in fresh-water plank- 
(3) 
