have In common that they do not possess any sharp 

 geographical distribution. They occur where their 

 claims upon the habitat are fulfilled". 



Some species are more stenoecious in habi- 

 tat requirements than others; that is, they will 

 multiply and become abundant only within a narrow 

 range of habitat conditions. These are the so- 

 called rare species which are sometimes very 

 abundant when found. Sometimes they are thought 

 to be indigenous to a particular locality because 

 they have been found a few times in one locality 

 only. Is it not possible that most of them are 

 really widespread? Suppose a species is just 

 barely able to survive during unfavorable seasons 

 or conditions and becomes abundant only when the 

 habitat is exactly favorable. It would long be re- 

 garded as a rare species . If this theory is true how 

 do species survive during unfavorable periods. 

 Fritsch (1931) has discussed this problem. He 

 postulates that they may survive as spores or other 

 "resting cells, " but also suggests that they sur- 

 vive in the bottom ooze of the littoral zone, in case 

 of plankton species . We have considerable evi- 

 dence that this is true of both lotic and lenitic 

 species . Synura uvella can always be found dur- 

 ing the warmest weather in massive net collections 

 from North Carolina lakes. The colonies are very 

 rare and contain only a few cells, but they are 

 there . We have found occasional living but depau- 

 perate cells of Actinella in the bottom ooze of 

 streams where the water temperature reaches 27 C . 

 Batrachospermum seems to survive the summer as 

 very tiny rhizoidal, microscopic colonies on stones 

 in streams . I could give many other such in- 

 stances . 



Many species of algae are more euryecious 

 in habitat requirements . They are abundant enough 

 under fairly favorable conditions to be collected 

 widely or at all seasons of the year. These are 

 the so-called cosmopolitan species. I suppose 

 Mlcrasteria radiata is likely to turn up in any col- 

 lection in the world where desmids are found at 

 all. Perhaps this species is just somewhat more 

 tolerant of unfavorable conditions than some, but 

 like them only becomes abundant when conditions 

 are exactly right . 



I propose a theory of micro-habitats in both 

 time and space for species of fresh-water algae. 

 It Is suggested that stenoecious species survive 

 unfavorable periods or in unfavorable places as 

 resting cells or as vegetative cells in very small, 

 uncollectable numbers, that more euryecious 

 species survive in the same way but in numbers 

 great enough to be more often collected; but that 

 all species become abundant only under precisely 

 suitable conditions . This theory would explain 

 the sudden blooming of plankton species, the 

 marked seasonal pulses of others, and also the 

 apparent rarity of certain species and the general 

 and cosmopolitan distribution of others. 



If an ecological approach were made on this 

 basis, that is on the basis that each species or 

 perhaps small group of species occupies a micro- 

 habitat, I believe it would help solve many of our 

 ecological problems. It would help in the matter 

 of an ecological classification, because the 

 smallest group of species occupying a particular 

 micro-habitat would be our smallest community. A 

 sufficient body of data regarding these habitats 

 would enable us to group them logically into larger 

 units. It would help solve the problem of seasonal 

 pulses and of sporadic blooms. When a narrow 

 band of blooming Microcystis occurs across one of 

 our larger lakes, the habitat factors in this area 

 must be just slightly different from those on each 

 side. If we knew the exact habitat of one of the 

 rare species we could predict where in the world it 

 could likely be found . We already have a fairly 

 large body of ecological data some of which might 

 be interpreted on this basis. 



In conclusion, I should like to suggest to 

 phycologists; first, that students of flora try to re- 

 cord and publish more than bare date and locality 

 records, at least on apparently interesting species; 

 and second that there be more studies of species 

 ecology. Only from a backlog of accurate and de- 

 tailed data on species can a really good ecology 

 of fresh-water algae ever be derived. 



The ecology of fresh-water algae will, for 

 many years, be very productive both of important 

 data and of new ideas . I strongly recommend it to 

 students with an interest in the algae . 



Summary 



The contribution of floristic studies and of 

 limnology to algal ecology has been pointed out 

 and also that certain concepts and terms have been 

 borrowed from these disciplines. 



The importance of such habitat factors as 

 light, temperature, water quality, and speed of 

 current have been discussed together with some of 

 the problems encountered in obtaining accurate 

 data . 



The fact that there is no entirely satisfactory 

 system of community classification was empha- 

 sized . 



The problems of seasonal distribution and of 

 world distribution of fresh-water algae were briefly 

 discussed. It was indicated that data are lacking 

 for good correlation of seasonal distribution with 

 habitat factors, and likewise that we cannot yet 

 hypothesize accurately as to the character of world 

 floras . 



A theory was presented that fresh-water algae 

 occupy micro-habitats and species frequently are 

 not abundant enough to be collected except when 

 growing under precisely suitable conditions . 



