of the Fishery Board for Scotland. 



213 



banks of them. Peridiniem, especially the species of Ceratium, are very 

 few in number during the maximum period of diatom life, but they come 

 upon the scene with the waning of the diatom season, reach a maximum 

 about August, and linger on in fair quantities until December at least. 

 Early in April, during four or five days' work in Loch Fyne, when diatoms 

 were exceedingly plentiful, I succeeded in obtaining only one specimen 

 of Ceratium Tripos, which, at other seasons, especially summer, is the 

 predominant plant organism in the western lochs. During the period of 

 most active diatom life, the surface of the sea is almost monopolised by a 

 diatom Skeletonema costatum ; while species of Coscinodiscus, Biddulpliia, 

 Ditylum, Rhizosolenia, etc., abound in the layers beneath, the greatest 

 quantity of these being generally obtained at 5 fathoms, very few 

 below 25 fathoms. The surface of the Clyde sea-area so teemed with 

 Skeletonema in April that a tow-net could be one-third filled in a few 

 minutes with a scum consisting of it. and other diatoms in smaller quan- 

 tity. When I revisited the same stations in the end of July, I could not 

 obtain a single Skeletonema, and its absence (at all depths) continued to 

 puzzle me during my examination of the west coast in August, except for 

 a few specimens obtained near Oban, and its remarkable occurrence in 

 this, Loch Etive. While (with this one exception) it could not be found out- 

 side, the surface of Loch Etive simply swarmed with Skeletonema in 

 abundance, equal to if not greater than that of the Clyde sea-area in 

 spring. While it occurred as a surface organism in spring, it was far more 

 plentiful in Loch Etive at 5 fathoms than at the surface in summer. 

 There was no local, low temperature condition that would serve to explain 

 this, Loch Etive bemg, in point of fact, nearly two degrees warmer than 

 the sea outside. In its greater abundance at 5 fathoms than at the surface 

 it agrees with all other diatoms during the summer and winter, — at all 

 seasons, in fact, except during the period of maximum occurrence. If one 

 sets nets at the surface, 5 fathoms, 10 fathoms, and 20 fathoms, the 

 result will be the greatest capture of diatoms in general at 5 fathoms, a 

 nearly equal (smaller) quantity (often of the very same organisms) at the 

 surface, and at 10 fathoms, and a much slighter quantity at 20 fathoms. 

 Excluding March and April, I found no notable exception to this rule, except 

 in several hauls in December, in the Dunoon basin, when I obtained more 

 from 10 than from 5 fathoms. From the character of the special pigment 

 diatomine, which masks the chlorophyll in diatoms, and its resemblance 

 to the corresponding pigment in P/twophyceae, one would expect this 

 vertical distribution of diatom life, since its maximum occurrence is nearly 

 at the same depth beneath the surface as the Laminarian zone. It is 

 plain that free-floating organisms like diatoms, from being subject to 

 currents and immersions of layers of water, must be liable and adaptable 

 to certain changes of depth, and to the varying quantity and quality of 

 sunlight that reaches them at such depths, just as shore algae are subject 

 to such periodical changes in the rise and fall of the tides. 



In March to April, the plant plankton of the east coast, judging by a 

 hurried examination of it in bad weather, closely resembled that of the 

 west coast outside the Clyde sea-area. The characteristic diatoms were, 

 in the order of their abundance, Coscinodiscus concinnus, Chcetoceros borealis, 

 Ditylum Brightwellii, Chaitoceros decipiens, Rliizosolenia Shrubsolii, 

 Skeletonema costatum, and Coscinodiscus radiatus ; while, of other organ- 

 isms, Ceratium Tripos, in places here and there, almost rivalled Ditylum, 

 but generally was about equal to Rliizosolenia. On entering the Clyde 

 sea-area, however, Ceratium became much more scarce ; and in Loch Fyne, 

 as said above, only one specimen was obtained in several days' tow-netting. 

 On the other hand, Skeletonema, from being scarce, at once leapt easily 



