500 



monas fermo were found at Marseilles above the dam, and the num- 

 ber at Morris and below the dam at Marseilles was much larger than 

 at stations either above the former or below the latter point. It was 

 further characteristic of this section that one-celled green algae and 

 green flagellate Protoj:oa were nearly absent in its waters except for 

 survivors of the Lake Michigan contribution. Even the hardier 

 Lake Michigan diatoms were distinctly waning in numbers, their 

 moribund condition being plainly shown by the fading and breaking 

 down of their chloroplasts. On the other hand, many diatoms come 

 through this section alive, particularly the two species of Tabellaria 

 (fenestrata and flocculosa) together with some of the Synedras and 

 Naviculas. 



The microplankton collections of the section from Marseilles to 

 Starved Rock agree with the longshore collections to the effect that 

 the septic organisms were diminishing rapidly here, SpJicFrotilits and 

 CarcJiesiuni disappea.ring from both the waters and margins, but 

 appearing in dredgings from the bottom of the channel, to which 

 these organisms had seemingly settled to die. Oikomonas tenno 

 also, the most abundant bacterium-eating protozoan, gave us an aver- 

 age of 2IO to the cubic centimeter above the dam at Marseilles, but 

 fell to an average of 54 to the centimeter at Starved Rock. The 

 Lake Michigan diatoms, on the other hand, continued in nearly con- 

 stant numbers, as did also the clean-water unicellular alg^e and green 

 Protozoa. 



In the Starved Rock-Chillicothe section of the river the outstand- 

 ing feature of the plankton was the marked increase in diatoms and 

 other chlorophyll-bearing imicellular organisms, plant and animal, 

 which became sufficiently abundant by the time Hennepin was reached 

 to give the water a characteristic greenish tinge. This was largely 

 due, however, to the picking up numbers of a single species of dia- 

 tom, Melosira gramilata var. spinosa, a form common in the Illinois 

 River and especially in its backwaters. This "greening up" of the 

 water below La Salle was noticed, in fact, on a down-stream trip in 

 June, 1910, and was conspicuous to the naked eye in both 191 1 and 

 1 91 2, most noticeably so at the lowest water-levels. Rise of water 

 and an increased turbiditv due to flooding rains in early August, 191 1, 

 obscured it for a time. It will presently be seen that these indications 

 of a radical change about Hennepin in the biological contents of the 

 water, are supported by the products of longshore collections, for it 

 is here that the foul-water blue-green algre began distinctly to dimin- 

 ish, that the Cladophoras and Sfigeocloniimi hihricuni began to take 

 the place of Sfigcoclouium foiitc as the most abundant green algse, 



