340 



FRESH-WATER ECOLOGY 



period of drought may, however, counteract the influence of sun- 

 shine. 



A study of algal periodicity in some ponds near Sheffield, 

 together with the results of fortnightly analyses, has suggested a 

 correlation with the nitrate factor for some species. The maximum 

 for this occurs in December whilst there is a minimum in June, 

 and it was observed that Volvox received a severe check when the 

 nitrate was high and only reproduced at times of low nitrate value. 

 Ulothrix reappeared yearly in these ponds, whilst Euglena annually 



1906 



a buna. 



AbuYul. 



FUtheT 

 tATe 



5e^t Oct Nou Pec. Jan Feb Mar Af:>y J^v Ju.tj£ Juiv: Au^. 

 20 T1TT7T 17 5 17 I 12 1 15 3 15 1 14 I 13 1 15 i 



T r 



T — 1 — I — I — 1 — I — I — I — I — I — I — I — I — I — I — r 



T r 





■ VLYUkS 



iPedcOomuta 

 rvpEoboTLtml 



Temja AU 



xrATLins 

 •C'29'18'20'20'4' '^1 ' 10' 7 ' 6 '16' 12'I2'20' 8 '24'3r23' 33'30'28'35" 



'C 28 20 15 13 7 5 9 7 1 9 12 19 22 II 24 28 16 29 24 24 33 



Fig. 199. Abundance and frequencies of the most important algae in a pond 

 near Indiana University from 1906 to 1907. (After Brown.) 



attained to a maximum between July and August soon after the 

 nitrate minimum. 



A similar study by Brown (1908) of some pools near the Uni- 

 versity of Indiana, revealed the fact that the species tended to 

 attain their maximum abundance in autumn and spring. In one 

 pond (fig. 199) the phases were as follows: 



Phase 



Autumn 

 Winter 

 Early spring 

 Late spring 

 Summer 



Dominants 

 Closterium, Euglena, Oedogonium 

 Spirogyra spp. 

 Spirogyra sp. 



Spirogyra, Euglena, Oedogonium 

 No one species 



