148 SIR W. A. HERDMAN : RESULTS OF CONTINUOUS 
The curve for the temperature of the sea during the same period, on the 
other hand, does not correspond, and shows very little rise while the plankton 
is on the increase. Tt is clear, for example, that change in temperature of the 
sea will not account for the sudden increase in the plankton which began on 
April 9th, after five days of increased sunshine. During the five days, April 
oth to 9th, when the sunshine record increased from 12 5 seconds to ? seconds 
(by the actinometer), the sea-temperature increased only from 7:07 to 1:9? C., 
and this was followed after a week by an eight-fold increase of the plankton 
(from 100,000 to 800,000 diatoms per haul). 
In the spring of 1910 there were more days and more hours of sunshine 
recorded at Port Erin than in any of the previous years, and the diatom 
record was also a high one. January, February, and March have in 1910 
over 20 more hours of sunshine than in 1909, and about 13 hours more than 
the average of the same period in the four preceding years. In short, the 
early months of 1910 had an unusual amount of sunshine, and so had those 
of 1907, and we find that in these two years there was a much greater 
phytoplankton maximum in April than was the case in the two intermediate 
years. As it is possible that it is the sun in March that has most effect upon 
the April phytoplankton, we may quote here from our records the hours of 
sun in March for the four years :— 
1907. 1908, 1909, 1910, 
March sun............... 113 83 77 100 
Plankton maximum ... April May May April 
The individual hauls in April in each of the four years are much higher in 
1907 and 1910—in brief, the diatoms appeared in great abundance earlier 
in April in the two years when there had been most sunshine in March. 
I 
The differences between successive years may be very great in both the 
quantity and the constitution of the plankton. For example, in 1907 one 
haul of the Nansen vertical net on April 5th gave 17 million diatoms, of 
which 14 million were Chetoceras contortum ; and two surface-nets on 
September 12th gave 13 and 16 millions of Rhizosolenia semispina. In the 
following year, however, both these diatoms were comparatively rare at the 
corresponding seasons (April and September). Again, Chetoceras contortum 
and Thalassiosira nordenskioldii, which were mainly responsible for the spring 
maximum in 1907, were much less prominent in 1908; and R. semispina, 
which reached millions in Sept. 1907, was almost absent in Sept. 1908. 
It may be worth while, therefore, to give the following table IT., in which 
are shown, for the series of years, the approximate time of the diatom 
maximum, the character of the plankton when any dominant organism was 
present, the general weather conditions of the year (abstracted from the 
Reports of the Meteorological Office), and the lowest sea temperature in 
degrees Fahr. during March :— 
