OF DIATOMS AND COPEPODA IN THE IRISH SEA. 125 



fate depends so far as concerns food from the sea. That shoals of migratory 

 fish such as herrings and mackerel are attracted in summer and autumn by 

 dense swarms of certain Copepoda such as Calanus and Temora can no 

 longer be doubted ; and there is abundance of evidence that earlier in the 

 year the young stages of other food-fishes, such as plaice, are nourished first 

 by Diatoms and when larger by the smaller Copepoda. These conclusions, 

 however, do not finish the matter. There remains in the sea the much more 

 minute Protozoa and Protophyta of the Nannoplankton, which to a large 

 extent escape through the meshes of our plankton nets and which are doubt- 

 less of great importance as the food of Copepoda and other large organisms 

 which in their turn nourish fish. 



There are several other interesting lines of further investigation which 

 open up as collaterals from plankton investigation. One of these is the 

 origin of the great vernal phytoplankton maximum. In the spring there 

 is an awakening of the plant-life of the sea comparable to the growing of the 

 grass and the budding of the trees on land. This cannot be due to any rise 

 in temperature, as the sea at the time that the Diatoms start their active 

 growth and reproduction is at its coldest. We have series of observations 

 extending over more than ten years showing that the water of Port Erin bay 

 is generally of lower temperature in March than in December or January. 

 Of the various causes for the rise in the Diatom curve in these cold waters 

 oE early spring I have for some years * regarded with greatest favour the 

 view suggested first by Sir John Murray, viz., the increase in sunlight at 

 that time of year. In that case it would be a photosynthetic phenomenon — 

 the increased solar light energy enabling the Diatoms to obtain from their 

 environment by photosynthesis the materials required for their growth and 

 reproduction. The view that the spring increase in plankton is due to changes 

 in the alkalinity of the water does not in my opinion conflict with the photo- 

 synthesis theory but is supplementary to it. The position in regard to the 

 relation between variations in alkalinity and in the plankton, in our district, 

 is as follows : — 



The sea around the Isle of Man is a good deal more alkaline in spring 

 (say April) than it is in summer (say July) ; and during the years 1912-14 

 Professor Benjamin Moore, by examining samples of sea-water periodically 

 at the Port Erin Biological Station, was able to show f that the alkalinity, 

 which gets low in summer, increases somewhat in autumn, and then decreases 

 rapidly, to disappear during the winter ; and then once more, after several 

 months of a minimum, begins to come into evidence again in March and 

 rapidly rises to its maximum in April or May. This periodic change in 

 alkalinity will be seen to correspond roughly with the changes in the living 



* See " Intensive Study, &c." Part III. p. 260, 1910, Trans. Biol. Soc. Liverpool, xxiv, 

 t Trans. Biol. Soc. Liverpool, xxix. (1915) p. 233, 



