Blum: Water Currents & Algae 355 



tearing the thallus away from the stone, or transporting both stone and thallus 

 to a slower part of the stream (Picken, 1936). 



Influence of the current on algal reproduction. In their reproduction current 

 algae take full advantage of the medium of dispersal which is at their doorstep. 

 It is commonly observed that many algae which colonize stream bottoms 

 achieve in certain seasons almost saturation coverage of available and favor- 

 able sites. Thanks to the mixing done by the current these algae are able to 

 introduce their reproductive units into what must be a very high percentage 

 of rock fissures, cracks, scratches, and roughened areas, into enough, at least, 

 of such depressions to permit subsequent growth from the colonizing cells to 

 cover close to 100 per cent of the available surface. In southern Michigan 

 streams which I investigated colonization of rock surfaces is very rapid, and 

 successful in very high percentages of the space available. The winter dom- 

 inant diatoms Gomphonema olivaceum and Diatoma vulgare, for example, achieve 

 good growth in winter on newly submerged rock surfaces in as little as 10 days. 

 Both of these forms were at the same time colonists and seasonal dominants, 

 no evidence being found of succession before the establishment of the com- 

 munities they represent. The period within which G. olivaceum colonized 

 bare rock surfaces extended from late November to early April, and coloniza- 

 tion seemed to be possible at any time within this period (Blum, 1954). 



Evidence that planktonic forms reproduce as they are carried downstream 

 has been presented by various workers but there remains the suspicion that 

 much of the actual cell division occurs on the bottom and that the apparent 

 increase in phytoplankton downstream is largely the result of more extensive 

 nutrient beds there and of more dense populations of benthic individuals, 

 many of which rise every day into the plankton. I observed the vegetative 

 dissemination of Spirogyra and Oscillatoria communities on warm summer 

 days in the Saline River in southern Michigan. These communities were 

 especially characteristic of cjuiet shoals or bays. Here the algae remained 

 on the bottom in contact with nutrient-rich silt deposits, as masses of filaments 

 easily visible from a distance. The surface waters of such shoals or bays is 

 usually in slow circular movement set up by the main current of the stream, 

 which by-passes the shoal or the bay in a tangent to the circular current which 

 it produces there. At times of rapid photosynthesis, individual masses of the 

 algal filaments are detached and buoyed upward by trapped oxygen bubbles. 

 Once the algal mass has quit the floor of such a shoal, it is carried slowly along 

 in the eddying surface water. After moving for some time in this circular 

 manner it may eventually be picked up by the tangential current of the main 

 stream which removes it definitively from the shoal. As the algal mass travels 

 downstream, it disseminates live filaments along the way. The progress of 

 these filaments is arrested on obstructions or on new shoal areas or other sedi- 

 ments downstream, which in this way are themselves colonized. The elevation 

 of algal masses by entrapped bubbles can be observed from about noon until 

 about 2 to 3 p.m. on sunny days in summer and the movement downstream 

 of these floating masses can be observed throughout an entire afternoon. 



The evolution of current-inhabiting algae. I believe the first attempt to clas- 

 sify the body types of current algae was made by Cedergren (1938). His 



