UiothriXv 



water surface 



Gomphonema 



DiatOTTlQ 



other on the stream profile determine the laws of 

 distribution of the animal population as well. 

 Succeeding riffles or shallows frequently carry the 

 greater volume of water on alternating sides of the 

 stream, so that erosion is greater first against the 

 right bank and then against the left. This asym- 

 metrical pattern results in asymmetric distribution 

 of the benthos biocoenoses of the pools, with an 

 accompanying break in their continuity at every 

 riffle . Ulothrix spp . , Stigeoclonium tenue and 

 Diatoma vulgare are all characteristic of riffles 

 and regularly drop out as massive components of 

 the vegetation wherever pool conditions obtain. 

 Spirogyra spp., Euglena spp . and other mostly un- 

 attached forms naturally collect where current is 

 minimal. Within a riffle or shallows itself, cer- 

 tain areas are apparently much more favorable than 

 others for the larger algae. Growth of Diatoma 

 vulgare has been observed to be inhibited in the 

 portions of the riffle downstream from large rocks, 

 where water movement is relatively slow. (Fig. 2) 



Fig . 1 . Diagram of spring zonation of prin- 

 cipal algae on rocks of shallows (average depth 

 ca . 15 cm.) in the Saline River, southeastern 

 Michigan. Dark area represents Ulothrix tenuis - 

 sima filaments, present only on rocks in swift 

 water immediately below the water surface where 

 the current is broken by protruding emergent rocks 

 as at A. Irregular oval patches represent Gom- 

 phonema olivaceum occupying the upstream face of 

 rocks such as A and B in rapid current but slightly 

 below the Ulothrix level. Irregular short lines 

 represent the Diatoma vulgare community which is 

 partly intermingled with the Gomphonema but is 

 dominant at a slightly lower level. Rocks such as 

 C, below a given depth are not colonized by mas- 

 sive Gomphonema colonies or by Ulothrix. Arrow 

 shows direction of water movement. Note that this 

 zonation is not uniquely related to depth, but is 

 also influenced by current rate . 



-2 H. 



DISTRIBUTION WITHIN THE BASIN AS A WHOLE 



Most small streams consist of a series of 

 alternating shallow areas ("shallows", "riffles") 

 and deep areas ("pools") . The shallow areas 

 naturally receive greater abrasion by the water and 

 the molar agents it carries. The water flows faster 

 here and in a thinner sheet, and significant chemi- 

 cal differences are to be expected between the 

 shallows and pools, although little effort has been 

 made to demonstrate them. Many algae are con- 

 fined to shallow parts of headwaters streams, just 

 as others are characteristic of the slower and 

 deeper waters of pools or deeps. Peculiarities of 

 the deeps and shallows as they alternate with each 



Fig . 2 . Diagram of colonization by Diatom a 

 vulgare in a shallows of the Saline River, Michigan. 

 Heavy continuous line represents the water's edge. 

 Heavy broken lines are isobaths at 1 and 2 ft. 

 depths . R = large, emergent rocks; pattern of 

 short lines represents the Diatoma vegetation. 

 Note that this Diatoma is limited to shallow (swift) 

 water, and that it is absent from shallow areas 

 near the stream edge and from areas to the lee of 

 emergent rocks where water movement is much 

 slower. Arrow represents direction of water move- 

 ment . 



Eddy presented the view that the amount of 

 plankton in river water is dependent upon the 



