considered . A fourth factor , the effect of current , 

 seems of considerable importance in streams and 

 on wave- swept shores . 



Since algae are photosynthetic plants , no 

 one can deny that light is a very important habitat 

 factor. Algae are thus restricted to the photic 

 zone. The depth of this zone varies not only 

 locally with the turbidity or color of the water, 

 with the cloud cover, and in small streams, as 

 pointed out by Blum (1956), with the leaf canopy; 

 but also with season and latitude. Rodhe (1955) 

 has recently postulated, however, that some 

 species may grow heterotrophically in arctic winter 

 darkness. Welch (1952) cites two marine obser- 

 vations indicating that the effective day length at 

 some depth (10 to 40 m) is markedly less than near 

 the surface. This effect is probably of less im- 

 portance in fresh waters except at high latitudes 

 or in winter. 



As is true of other habitat factors, we do not 

 have nearly enough data on the effect of light on 

 algal distribution. It has not been easy to make 

 under-water light measurements. Until recently 

 there was no standard or commercially made meter 

 for under-water measurements . Light and dark 

 bottle experiments have been most common. Total 

 diurnal incident light measurements are still diffi- 

 cult to make. Most fresh-water algae are probably 

 "shade plants . " Exceptions are algae growing on 

 soils, in seeps, or epiphytically on land. Certain 

 species of Zygnemataceae, Mesotaeniaceae, and 

 Desmidiaceae growing in seeps seem to need full 

 sunlight. Tilden (1935) proposed a theory on the 

 relation between algal pigments, especially the 

 accessory pigments, and algal evolution and dis- 

 tribution. This theory has never gained general 

 acceptance except in one part not original with 

 Tilden. It seems to have been adequately proved 

 that the accessory pigments in some red algae 

 render the light penetrating to some depth, more 

 effective in photosynthesis . This seems to be 

 true of fresh-water Rhodophyceae as well as 

 marine species . Batrachospermum . Hildenbrandtia , 

 Chantransia , and Thorea have been found growing 

 at from ten to thirty meters in depth. The fresh- 

 water red algae are said to be red only when grow- 

 ing in deep water. I have collected Thorea ra - 

 mosissima , deep red in color, growing in Vaucheria 

 mats at ten meters depth in Silver Springs, Florida, 

 (Whitford 195 6) . We had thought that Batracho - 

 spermum is a cool water genus, but we have good 

 evidence that low light is an equally important 

 factor. Batrachospermum is found at cool seasons 

 and persists longest in cool shady streams, but it 

 is probable that low light is a more important 

 factor than low temperature . Plants are found later 

 in the spring under shady banks and in the shade 

 of stones than elsewhere, although the water tem- 

 perature here is identical with that in sunny 

 reaches. We were surprised to find Batracho - 



spermum much less widespread in the higher North 

 Carolina mountains in summer than water tempera- 

 tures would lead one to believe. On the other 

 hand, at least one species, J., macrosporum , is 

 abundant in the deep brown waters of Coastal 

 Plain streams throughout the summer. It is most 

 abundant in shady places . Water temperature may 

 exceed 27°C where it is growing.^ 



We have made another observation which 

 seems of some interest. In North Carolina spring 

 and autumn are fairly long . In autumn we notice 

 a reversal of the spring flora as regards species 

 and time of abundance . Late spring species come 

 in earliest in autumn and early spring species 

 later. We note, however, that abundance of indi- 

 viduals in autumn is usually less than in spring 

 although the period of the same water temperatures 

 are approximately equal. In autumn some species 

 seem to be caught between a fairly low tempera- 

 ture requirement and a high light requirement. In 

 spring light intensity and duration are increasing 

 as water temperatures become favorable but in 

 autumn total incident light is decreasing as tem- 

 peratures become favorable for certain species. 

 We definitely note that if bright clear weather pre- 

 vails in autumn we get a more nearly exact rever- 

 sal of the spring flora than when it is cloudy. 

 Oedoqonium kurzii , an important stream species 

 with us, most species of Draparnaldia , as well as 

 some plankton algae such as Asterionella and 

 Dinobryon have a low temperature, high light re- 

 quirement . 



It seems apparent that many green algae are 

 high-light species and red algae are low-light 

 species. Diatoms and Chrysophyceae seem more 

 indifferent in light requirement . Perhaps they will 

 respond to fairly high light intensities if other 

 conditions are ideal . The high diatom populations 

 in the cool Florida springs in summer seems to be 

 an indication that this is true (Whitford 1956) . 

 Blue-greens probably respond more to high summer 

 temperatures than to high light intensities . This 

 would be an interesting laboratory problem as will 

 be pointed out later . 



Oberdorfer (1928) published detailed data on 

 the distribution of algae in relation to the light 

 factor in Lake Constance. His work and that of 

 others has been summarized by Fritsch (1931). 



Schiller (1930) showed by means of culture 

 experiments that cold water (11-12°C) is fre- 

 quently more productive than warm (23-25°C) . The 

 productivity of the northern fishing banks is well 

 known and apparently diatoms are more abundant in 

 northern lakes than in those at lower latitudes . 

 The diatoms and Chrysophyceae are in general 

 microthermal (oligothermal) . Some species of 



^Schumacher and Whitford, unpublished data. 



