Pools and lakes 431 



In alt small ponds and ditches with reasonable exposure to sunlight the 

 general periodicity is much, the same : diatoms being the dominant winter types 

 and Zygnemacetv the dominant spring types (with abundance of diatoms). It 

 is the summer phase which is so very variable and there is no doubt that 

 meteorological conditions exert a profound influence. Therefore in temperate 

 countries the controlling factors are, and must be, irregular. The determina- 

 tion of the LIMITING FACTORS will probably afford a solution of the whole 

 problem. It is usually one factor which exercises a decisive influence and 

 this factor is the limiting factor. 



In the United States, Transeau ('13) has made a large number of careful 

 observations on the periodicity of the AlgaB of small bodies of water in 

 Illinois and has concluded that ' on the basis of their periods of greatest 

 abundance, the duration of the vegetative cycles, and the times of reproduc- 

 tion, freshwater Algse in general may be divided into seven classes.' These 

 are : winter annuals, spring annuals, summer annuals, autumn annuals, 

 perennials, ephemerals and irregulars. The accompanying chart illustrates 

 some of Transeau 's observations. 



D. Algal Associations of Pools and Lakes. 



So far as can be ascertained there is no zoning in the algal associations of 

 pools and lakes. There are the Algas around the shores of the lake con- 

 stituting the benthos and those which occur free-floating in the waters of the 

 lake constituting the plankton. In the smaller pools many of the benthic 

 species are found in the ' plankton,' which is thus composed of a mixture of 

 forms many of which are not by any means true plankton organisms. This 

 so-called ' plankton ' of small pools is often designated as heleoplankton. The 

 general alga-flora varies very considerably, depending largely upon the geo- 

 logical formations and the altitude. 



1. Benthos. The alga-flora of the benthic region of pools and lakes 

 varies much according to the nature of the banks, whether rocky, sandy 

 or marshy. The algal associations occur mostly among various aquatic 

 macrophytes to which many of the species are attached. 



In the more reedy lakes (Edogonium and Bulboch&te are not uncommon, 

 species of the first-named genus sometimes dominating all the filamentous 

 types. Several species of Coleochtete are common epiphytes, C. scutnta being 

 the most abundant. Several species of Ulothrix may occur, but not very 

 commonly, and Ch&tophora is distinctly a vernal type. C. pisiformis is 

 epiphytic and is generally distributed, whereas the curious colonies of 

 G. incrassata (fig. 188 A) are unattached and more locally distributed. Other 

 more or less vernal types are represented by various species of Microspora 

 (M. amcena, M. floccosa and others), and Tribonema bombycinum is frequent. 



