OBSERVATIONS ON PLANT PLANKTON. 



391 



In December, on the west coast outside the Clyde sea-area, 

 the characteristic diatoms were Coscinodiscm cuncinnns, Biddiilphia 

 mobilieiifiis, Ditylum Brii/htwellii, while species of Chatoccros and 

 Naricula were casually present. Both Cerathnn Tripos and C. Fusiis 

 appeared sparingly in all the captures. Within the Clyde area, 

 S/idetoneina became next abundant to Coscinodiscus, after it Biddul- 

 phiii, and next IJIueluceros decipiens, while Ceratium Tripos and C. 

 Fiisus were more plentiful than in the waters outside. Compared 

 with these, other diatoms and plant organisms were of purely 

 casual occurrence. These appeared more or less in every haul of 

 the nets, and in fairly steady proportions, as cited. Not a single 

 specimen of Bhizosolenia was obtained in December, while sometimes 

 in August it was the predominant generic form, e.(j. in Kilbrennan 

 Sound. 



At all seasons of the year, especially near the shore, a small 

 proportion of the diatoms captured are dead, their cell-contents 

 either disorganised or gone. This is particularly noticeable in such 

 large forms as (.'oscinodiscus concinmis and Biddnlphia mobiliensis ; 

 but it occurs frequently in other forms, such as Chatucerus di'cipiens, 

 and, above ail, in Skeletonema costatum. I believe this phenomenon, 

 which at first puzzled me greatly, to be due to the decline or loss 

 of salinity of the water. My reasons for this belief are these : 

 Such diatoms are mostly found at or near the surface and in the 

 vicinity of land where a considerable mixture of fresh with salt 

 water occurs. This may be seen in any of our western sea lochs, 

 especially after rains, where a sheet of practically fresh water lies 

 on the top very slowly mixing with the salt water. It is well 

 known that seaweeds, in general, are extremely sensitive to varia- 

 tions in the degree of salinity. My belief is strengthened by the 

 following observation : In April, on ascending Loch Fyne, the 

 nearer we came to the top the larger the quantity of dead diatoms 

 were found. This was particularly noticeable in Slcelctonenui, which 

 almost monopolised the surface ; and on tow-netting in the waters 

 between Inveraray and Cairndhu at the head of the loch, where 

 there was a thick sheet of fresh water on the surface, owing to 

 recent heavy rains, it was impossible to find a living coll in the vast 

 mass of Skeletonema. This observation was confirmed in Loch 

 Etive in summer. This loch was full of Skeletonema, some at the 

 surface, but most in from 3 to 5 fathoms. A large quantity of fresh 

 water was entering the loch, and the diatoms at the surface were in 

 large proportion dead, while those in the lower layers were living. 

 The observation is the more noteworthy, as I am led by other 

 reasons to think this diatom mostly abounds in waters of a fairly 

 low degree of salinity, and I have elsewhere taken the low degree 

 of salinity of this loch in August as possibly accounting for the 

 presence of Skeletonema in it at that season. However, this last 

 idea is a purely speculative one, and does not in any way affect my 

 general contention that the great numbers of dead diatoms in the 

 sea are to be accounted for by the addition of fresh waters. 



Diatoms as Food of Marine Animals. — When Ave reflect that 

 the whole bulk of animal life ui the ocean must be dependent on 



