59 



NOTES ON DINOFLAGELLATES AND OTHER 



ORGANISMS CAUSING DISCOLOURATION 



OF THE SAND AT PORT ERIN. 



By E. Catherine Herdman, 



Newnham College, Cambridge. 



(Read 10th December, 1920.) 



In the Annual Reports of 1911 and 1912, accounts were 

 given of greenish-brown patches on the sandy beach at Port 

 Erin. These were shown to be due to the presence, sometimes 

 in large numbers, of an actively motile Dinoflagellate described 

 as Ampliidinium operculatum and sometimes of abundant 

 Diatoms. The discoloured patches of sand were seen to vary 

 in size and position from time to time and, in the individual 

 patches, it seemed that Dinoflagellates were replaced by 

 Diatoms and these, in turn, by more Dinoflagellates. It was 

 observed that the discolouration disappeared shortly before the 

 patch of sand was covered by the rising tide, and only 

 reappeared after that part of the beach was once more exposed. 

 No Amphidinia were present in tow-nettings taken in a few 

 inches of water over the position occupied by the discoloured 

 patches. It was therefore assumed that, at these times, the 

 organisms had gone down to the deeper layers of sand. The 

 variation in extent of the patches seemed to be greater, on 

 the whole, at neaps than at springs, and it was suggested that 

 possibly some correlation existed between the abundance of 

 Dinoflagellates and the state of the tides. A considerable 

 divergence of form was noticed between the individual 

 Amphidinia, but it was thought that they were probably all 

 varieties or possibly different stages in the life history of the 

 one species. 



This summer, in July, August, and September, some 

 further observations were made on the occurrence and 



