36 PROF. W. A. HERDMAN : COMPAKISON OF SUMMER PLANKTON 



It will be noticed, in comparing the Locli Ranza plankton with the Irish 

 Sea hauls in April and September, that in species present and their abun- 

 dance, the Scottish July gathering is much more like the September than 

 the April phyto-plankton — for example, in both July and September the most 

 abundant form is Rhizosolenia semispina, a species absent in April. But, as 

 has been pointed out above, the phyto-plankton gatherings north of Mull 

 show resemblances to the vernal rather than to the autumnal phyto-plankton 

 o£ the Irish Sea. 



It is probably premature to hazard any suggestion as to the explanation of 

 this curious difference between the summer planktons in the Hebridean and 

 the Irish Sea respectively. It may be that the great vernal maximum which 

 dies away in May and June in the Irish Sea passes off more slowly further 

 north, and is still found lingering on in some parts of the Hebrides until the 

 end of July, or possibly even longer. Or it may be that, in some of these 

 deep northern channels, the Diatoms that elsewhere constitute our vernal 

 maximum remain on in comparative abundance throughout the greater part 

 of the year. Still a third possible explanation is that the Diatoms con- 

 stituting these July phyto-plankton gatherings may have invaded the 

 Hebridean seas from the North Atlantic at some period subsequent to the 

 vernal maximum. It is obvious that such a question can only be determined 

 by frequent periodic observations carried on throughout the year by means 

 of vertical hauls at fixed localities. Such series of observations have still to 

 be m.ade in the Scottish seas. Mr. George Murray, in 1896, Look some series 

 of horizontal tow-net gatherings at various localities round the coast of 

 Scotland for the Scottish Fishery Board, and he published a brief report * 

 the following year on his results. His observations were made in March, 

 July, August, and December, and were widespread, including several of the 

 localities I have sampled ; but they seem to deal mainly, if not wholly, with 

 the surface of the sea, and in any case do not include vertical hauls in deep 

 water. But it is interesting to note that his results, so far as they go, are 

 not inconsistent with the observations I have given above. Murray states 

 that after the vernal maximum the Diatoms diminish, but do not disappear, 

 and are to be found throughout the summer in local banks. Skeletonema 

 costatum he notes as the most abundant and characteristic form on the 

 surface generally in April, and in Loch Etive in August : this species 

 occurred only occasionally in our July hauls. 



The table he gives shows that there were fewer species of Diatoms in Loch 

 Nevis and Loch Hourn than in the sea between Rum and Ardnamurchan — 

 there being 28 in the latter column and only 7 in that for Loch Nevis. 

 So far this comparison agrees with the abundant phyto-plankton gatherings 

 we obtained off Rum and Canna, &c., and the comparatively small catches 



* S. F, B. Report for 1896, vol. xv. part iii. p. 212. 



