PLANKTON. 



THE PHYTO-PLANKTON OF THE CLYDE SEA-AREA. 



THE observations on this subject have been only few in number, and 

 practically the whole of our knowledge is contained in the Report of Observa- 

 tion on Plant Plankton by George Murray, F.R.S., in the Fifteenth Annual 

 Report of the Fishery Board for Scotland, 1897. There is still much work to 

 be done before we shall have a thorough knowledge of the minute free-float- 

 ing vegetation of this area. 



The most striking character of this plankton, as of that of many other 

 marine areas, is the great seasonal changes which occur, and the regular yearly 

 alternation of Diatoms and Peridinieae. During the early months of the year 

 the Diatoms are extraordinarily abundant in this district and in all the 

 surrounding coastal waters. They continue in enormous numbers for some 

 time and then, towards the end of March, or beginning of April, they begin 

 to diminish in number ; they appear to decrease steadily for some time and 

 then their number remains about constant till the next yearly increase. 



The Peridinieae, on the other hand, are very few while the Diatoms are in 

 the ascendant, but as the Diatoms fall off in number they begin to increase, 

 reaching their maximum about August and lingering on till December, as 

 described in Mr. Murray's paper quoted above. 



During the most active period of diatom life the predominant form in the 

 Clyde Sea-Area is certainly Skeletonema costatum. This form exists in such 

 enormous quantities at the surface that Mr. Murray describes how a bow- 

 net may, in a few minutes, be one- third filled with a sediment consisting 

 principally of this species. In the layer beneath, at this time of year, species 

 of Coscinodiscus, Biddulphia, Ditylum, Rhizosolenia, etc., abound, reaching 

 their maximum at about 5 fathoms and extending down to 25 fathoms. 



The Peridinieae of the later months show a much smaller variety of forms ; 

 those most frequently met with are species of Ceratium, namely : C. tripos, 

 fusus, and /Mm*, of which the first named seems to be most common in the 

 Clyde Sea-Area. The genus Peridinium is also represented by certain forms, 

 but the species have not yet been exactly determined. 



In the summer, when the Peridinieae take the first place as far as actual 

 quantity is concerned, the Diatoms, though much less in total amount than 

 in the earlier months, show a considerable increase in the number of forms ; 

 of which various species of Chaetoceros are the most noticeable. In late 

 months, such as December, Coscinodiscus concinnus seems to be the most 

 abundant form, and after it come Skeletonema costatum, Biddulphia 

 mobiliensis, Chaetoceros decipiens, roughly in order of abundance, while 

 Ceratium tripos and C. fusus are fairly plentiful. 



At all seasons of the year, especially near the shore and at the top of lochs 

 and generally at the surface after heavy rain, a number of the Diatoms met 

 with either possess only disorganised contents, or are mere skeletons. 

 This is found particularly in the common species Skeletonema costatum, and is 

 also especially noticeable in large forms like Coscinodiscus concinnus and 

 Biddulphia mobiliensis, but it also occurs frequently in the case of many 

 other Diatoms. This phenomenon is doubtless due to a decrease in the 

 salinity brought about by the influx of fresh water through streams or by 

 heavy rain, which produces a lighter fresh water layer at the surface. 



Of forms other than the Diatoms and Peridinieae there is very little 

 knowledge ; small floating green algal cells, probably simple members of the 

 Chlorophyceae, are fairly often met with, and present a quite un worked field. 



A list, drawn up by Professor Cleve, is appended showing the forms met 

 with in hauls from nine stations on the West Coast. The species obtained 

 are very generally the same as those found in the Clyde Sea-Area. 



