Dakin and Latarche — The Plankton of Loiufh Neagh. 75 



characteristic forms of the arctic and also the central European plain plankton. 

 Thus, so far as the phytoplankton is concerned, we have a most interesting 

 mixed flora. 



While Lough Neagh agrees with the Danish lakes in the relative 

 abundance of the phytoplankton, the characteristic species present in both 

 areas are different. Many of our species are also found in large quantities in 

 the Danish lakes, but on the whole there is a considerable difference. Thus 

 Tabellaria does not play such a striking part, nor do apparently the Myxo- 

 phyceae, except at certain seasons. 



The Scottish lochs seem to lae relatively poor in the Myxophyceae. 

 Melosira is rather rare. Stephanodiscus astraea never occurs commonly as 

 a plankton organism, and Fragillaria ci-otonensis is rather rare. On the other 

 hand, as Messrs. West have shown, the Scottish lakes are characterized 

 above everything by an extraordinary abundance of Desmids. So far as 

 Lough Neagh is concerned, the only Desmid occurring in very large quantity 

 is Staurastrum. This occurs in great quantities in September (158,000 in one 

 catch), but is far behind the numbers of other Chlorophyeeae. 



Lough Neagh is by no means a "Desmid lake." Messrs, West have 

 propounded a theory to account for the prevalence of Desmids in certain 

 of the British Lake areas. Wesenberg Lund and James Murray have also 

 discussed the problem. So far as our observations of the district go, the facts 

 entirely support Messrs. West. These workers have noticed that the rich 

 Desmid lakes occur in areas belonging to the older geological formations. 

 Wesenberg Lund has stated that Desmids thrive best in water rich in humic 

 acids from peat bogs, and James Murray has also objected to the theory ; but 

 there seems little doubt at present that Messrs. West's theory agrees best 

 with the facts observed. 



Practically all the water of Lough Neagh is from drainage areas of more 

 recent than Jurassic times ; and we have already seen that Lough Neagh is 

 not a Desmid lake. No doubt a drainage water derived from bogs and peat- 

 beds in general is of considerable importance; and it is very probable that other 

 factors besides the geological formation are determinants, yet the coincidence 

 of Desmid areas with old-formation lakes is most marked. 



The phytoplankton of Lough Neagh is practically at all times in excess of 

 the zooplankton, and to our minds always sufficient as food for the zoo- 

 plankton. Now this brings into our discussion the startling theory propounded 

 by Flitter that the planktonic organisms are not present in sufficient quantity 

 to be food for aquatic animals, and that most of the latter feed on organic 

 carbon compounds in solution. Putter's work itself has been largely confined 

 to marine organisms, and has been seriously questioned, and, to a great extent, 



IM.A. PROC, VOL. xxs., SBCr . B, [L] 



