1923] Allen: Some Tide-water Collections of Marine Diatomes 415 



of diatoms is very conspicuous throughout the twenty hours. The 

 influence upon temperature and pH in the night is equally evident 

 but it is not so apparent in the daytime. 



The largest number of diatoms (63,856 per liter) was recorded at 

 6 a.m., two hours after low water, and the smallest number (3424 per 

 liter) was recorded at 5 p.m. at the time of low water. pH was lowest 

 (8.32) at low tide, 4 a.m., and highest (8.40) at high tide, 9:30 p.m., 

 (pH not corrected for salinity). Specific gravity was lowest (1.02689) 

 at or near both high tides and highest (1.02724) at 4:30 p.m., a little 

 before low tide. The temperature was lowest (13.6° C.) at 4:30 a.m., 

 just after low tide and highest (15.4° C.) at 1 p.m., three hours after 

 high tide. Tide levels ranged from six feet at 9 :30 a.m. to zero at 

 5 p.m. Variations between the extremes mentioned may be followed 

 by reference to figure 1. 



Study of this graph makes it plain that local topography and 

 climatic conditions have a strong influence. At night, when this 

 influence was least, all of the other four characteristics changed with 

 the tide. In the daytime, when the shallowness of the bay water 

 greatly magnified the solar influence on temperature, both tempera- 

 ture and pH failed to respond to change in the tide. If low tide had 

 come early in the afternoon of that day or later in the evening, it is 

 probable that there would have been traceable change. In a larger 

 or deeper bay solar influence would not have been so immediate or 

 so obvious. 



Although the numbers of diatoms and the specific gravity changed 

 with the tide throughout the twenty hours, it is notable that the 

 change was in a direction opposite to ordinary expectation. Since 

 most bays which have been studied in any way at all comparable to 

 this are continuously supplied with more or less fresh water, the 

 change of tide would serve to dilute sea water and reduce the specific 

 gravity. Therefore samples taken at the entrance should be slightly 

 lower in specific gravity on the ebbing tide and slightly higher on 

 the flowing tide. In the case of this bay, the enormous surface as 

 compared with the mass of water evidently permits such rapid con- 

 centration through evaporation that after several months without 

 fresh-water increment there is a slight accumulation of denser water 

 in the bay, and it is this factor that causes the condition of reversal. 

 A similar (and consequent) explanation applies to the changes in 

 diatoms. This explanation, however, must be taken with some caution 

 on account of the many unevaluated factors influencing biological 



