NOTES ON DINOFLAGELLATES 15 



NOTES ON DINOFLAGELLATES AND OTHER 

 ORGANISMS CAUSING DISCOLOURATION OF 

 THE SAND AT PORT ERIN. II. (1921). 



By E. Catherine Herdman, 

 Zoology Department, University of Liverpool. 



(Read 26 May, 1922) 



During the spring and summer of 1921, further work* was 

 carried out on the dinoflagellates and other micro-organisms 

 found on Port Erin beach. Observations were taken on a total 

 of 94 days— March 19th to April 19th (inclusive), July 6th to 

 July 26th (inclusive), August 4th to August 30th (inclusive) and 

 September 17th to September 30th (inclusive). The whole of 

 the summer was an exceptionally hot and dry one, and this 

 probably had a profound influence on the relative abundance 

 of the various organisms under consideration. The general 

 impression received was that, as the summer advanced, the 

 dinoflagellates— most markedly Amphidinium lierdmani, 

 Kofoid, and to a lesser degree A. asymmetricum, Kofoid— 

 became much less abundant than in former years, while the 

 diatoms flourished under the abnormal conditions. 



A comparison of the daily records with the meteorological 

 and tidal conditions leads to general conclusions, which are in 

 harmony with those expressed by R. D. Laurie in a paper 

 read to Section D of the British Association in 1913. There 

 is a daily rhythm depending on a combination of the stimuli 

 of light and tide, the optimum in the case of the former stimulus 

 being a dim light, such as occurs in the morning and evening or 

 when the sky is overcast, while, in the case of the latter 

 stimulus, some species are at the surface only at the time of 

 low water. These movements are evidently dependent on the 



•See Trans. L.B.8. 1912, 1913 and 1921 



