Peridinium (P. limbatum, £. willei ) and ap- 

 parently some blue-greens are also. In some 

 blue-greens, however, such as Chamaesiphon low 

 light and current may be more important than low 

 temperature . We have recently found Chamaesi - 

 phon abundant in mid-Peruvian streams at summer 

 temperatures . The Chlorophyceae seem more 

 eurythermal . Many green algae have good vege- 

 tative growth at fairly low temperatures , if light 

 is adequate, and they continue growth at summer 

 temperatures . The larger species of Spirogyra and 

 Oedoqonium and many of the Chlorococcales seem 

 mesothermal (eurythermal) . Among the diatoms 

 Melosira granulata and Eunotia pectinalis are much 

 more eurythermal than most species. In brown- 

 water streams in North Carolina Eunotia pectinalis 

 replaces the genus Fraqilaria of northern waters in 

 late winter and spring, and it is abundant after the 

 water temperature well exceeds 20°. West (1909) 

 cites Rhizosolenia as a diatom growing at warmer 

 temperatures than most. We agree with this and 

 also suggest that Terpsinoe may be another . Many 

 species of fresh-water algae which are most abun- 

 dant in spring and autumn may be mesothermal but 

 data is lacking which will clearly separate light 

 from temperature effects . 



There are probably few megathermal algae in 

 spite of the fact that certain blue-greens espe- 

 cially grow in hot springs . Some workers report 

 that most thermal species grow as well at ordi- 

 nary room temperatures as at high temperatures. 

 Other reports differ, however. Synechococcus 

 eximus is said to make maximum growth at 79°C 

 and will not survive at a temperature below 70°. 

 Oscillatora filiformis is said to tolerate a tem- 

 perature above 85°C and it will survive at a tem- 

 perature no lower than 59°C . 



Recently we (Schumacher and Whitford) have 

 been collecting algae in winter in North Carolina. 

 We find that 15° seems to be the critical tempera- 

 ture for many microthermal species . A species of 

 Chrysophyceae, Phaeosphaera perforata is perhaps 

 the most remarkable. It has been under observa- 

 tion for more than ten years . It grows only at 

 temperatures between 4 and 10°C, and disappears 

 rapidly as the temperature approaches 15°. It is 

 very abundant in Coastal Plain streams in Febru- 

 ary and March following cold winters , and fairly 

 common in bogs and brooks somewhat later at 

 Raleigh, 150 miles inland. We do not have con- 

 clusive evidence, but apparently it is rare near 

 the coast southward because water temperatures 

 do not drop below 10 for a long enough period for 

 it to become abundant. 



Another species in the Xanthophyceae , 

 Chlorosaccus fluidus Luther grows in somewhat 



Whitford, unpublished data . 



similar habitats at about the same temperatures. 

 The diatom, Meridion circulare seems another 

 microthermal species. It is found growing with 

 us only in winter and early spring except in the 

 mountains . Single living but depauperate cells 

 can occasionally be collected in the ooze of 

 stream bottoms at any season but the typical half- 

 wheel colonies occur only at temperatures below 

 15°. Actinella punctata Lewis, a supposedly rare 

 diatom, has been collected in ten counties in the 

 Coastal Plain. It is an abundant epiphyte on 

 algae and stems during the cold months but, like 

 Meridion, is present only as an occasional single 

 cell in the ooze during warm seasons . Chae- 

 tophora incrassata is common in brooks in late 

 winter but disappears rapidly when the water tem- 

 perature exceeds 15°. Fragillaria is very rare at 

 lower elevations except in late winter but is com- 

 mon in streams at high elevations , where tempera- 

 tures remain low, in summer. 



The factor of water quality is the most com- 

 plex and diverse especially if one includes such 

 things as water color, turbidity, and pH, in addi- 

 tion to mineral content, and dissolved gases. 

 Nevertheless they are all closely related and in- 

 teracting and have to be considered together. 

 There is a voluminous literature on water quality 

 and it is useless to try to summarize it in a paper 

 of this length. There have been numerous attempts 

 to generalize by using types of flora, or groups of 

 organisms as indicators of over all conditions . 

 Frequently single supposedly important factors 

 have been considered so important that such terms 

 as Galciphilic and halophytic have been coined . 

 Much investigation has been done on phosphorus 

 and iron as limiting factors. Undoubtedly these 

 as well as nitrate are often limiting. There is no 

 doubt that lack of available silicon limits diatom 

 growth, for instance. In communities of micro- 

 organisms, however, conditions can change so 

 rapidly that stenoeclous species (that is those 

 with a narrow range of tolerance) may become 

 abundant or disappear with amazing rapidity. As 

 Hutchinson said (1944) it is the interaction of a 

 complex of both physical and chemical factors 

 which produces both seasonal fluctuations and 

 sporadic blooms . 



The terms eutrophic and oligotrophic have 

 been most used to indicate general productivity, 

 but we cannot yet reduce them to positive specifi- 

 cations . We can only say that a eutrophic habitat 

 is one with a high pH where available organic mat- 

 ter is rapidly reduced to liberate an abundance of 

 the vital mineral elements . The oligotrophic habi- 

 tat has a low pH and the mineral nutrients are low. 

 Thus pH seems the best single indicator of the type 

 of algal flora. At least several American phycolo- 

 gists agree with this statement. The pH results, 

 of course , from the interaction of a complex of 

 factors . 



