INVESTIGATION OF THE PLANKTON OF THE IRISH SEA. 147 
the surface and from 6:3 to 6'7 c.c. at 20 fathoms, the deeper water showing 
in all cases less than the surface at the same station *. 
More recently (1912-14), Prof. Denjamin Moore and others have shown 
that there are considerable variations in the alkalinity of the sea-water during 
the year, and that this periodic change corresponds roughly with the plank- 
tonic cycle, the connection between the two being due to the reduction in the 
amount of carbon dioxide present, eaused by the metabolic processes of the 
diatoms and rendering the water more alkaline. 
The more or less sudden disappearance of the spring diatoms after the 
maximum is difficult to explain, unless it be due to the increase in alkalinity in 
the water, which, according to Moore, is a result of the photosynthetic activity 
of the phytoplankton. Other contributory causes may be, as has been pointed 
out by Brandt and others, the exhaustion of necessary inorganic food-matters, 
such as nitrogen or phosphorus compounds or of silica. For example, in 
1909 the diatom maximum came to an end suddenly between May 24th and 
28th, and the rapid diminution in numbers was not accompanied by any 
recorded change in either temperature or salinity or in generak weather 
conditions. Alkalinity records were not started till a couple of years later, 
but it may be that the disappearance of the diatoms is a purely vital 
phenomenon due to their own metabolic activity in changing the constitution 
of the sea-water in which they are living. Another possible factor is that 
the increase in solar energy which favours the phytoplankton rise in early 
spring has now passed the optimum for these organisms and may be harmful, 
but that also is part of their metabolic activity. 
Although one may arrive at the general conclusion that variations in the 
amount of the plankton from year to year must be due ultimately to 
meteorological conditions either at the spot or elsewhere, either at the time 
or earlicr, it is not easy to demonstrate the connection between cause and 
effect in detail. Records of temperature of sea and air, twice daily, and of 
sunshine and other weather conditions have been kept for many years at the 
Port Erin Biological Station, and during 1909 we took determinations of the 
sun-light with a “ Wynne” actinometer on days when plankton was being 
colleeted from the vacht; and, although there may be no obvious relation 
between the weather conditions of the day and the plankton catch, it seems 
possible to correlate the plankton curves with the sunshine records of 
previous weeks or months. For example, in the Report for 1909 will be 
found the records in detail for three years, and curves for the monthly 
averages of 1908 and 1909, showing a close correspondence between the 
sunshine and the plankton and giving in both cases a maximum in May. 
* For further details in regard to all the hydrographic observations, see Report III. for 
1909. 
