72 Proceedings of the Royal Irish A cademy. 



GEXEKAL OBSERVATIONS ON THE PLANKTON OF 

 LOUGH NEAGH. 



A general observation of any charts showing the distribution of aquatic 

 organisms will at once show that there are far more species present in the sea 

 than in fresh water. Some groups of animals like the Echinodermata, 

 Coelenterata, Porifera, Brachiopoda, and Tunicata are practically eiitirely 

 marine. Other groups are represented by a vast majority of marine forms. A 

 few groups of animals are characteristic of fresh water. Until the Lough 

 Neagh research was taken up, the authors had studied almost entirely the 

 marine plankton. The distribution and variation of marine forms had been 

 observed at Kiel, Heligoland, Naples, and the Irish Sea. This familiarity 

 with the marine plankton has enabled us, perhaps, to note with particular 

 interest many small characters of the fresh-water plankton of Lough Neagh 

 that would have been passed by perhaps without much notice by the fresh- 

 water planktologist. We do not say that he would have missed them, or even 

 under-estimated them. Many features, however, will undoubtedly have been 

 seen by us from a different point of view ; and, as a matter of fact, it is rare 

 in literature to find a discussion of fresh-water plankton by the marine 

 planktologist. 



The first point that aroused our attention in Lough Neagh was the quantity 

 of plankton present in the water. The quantity of plankton present in the 

 Lough Neagh water is by no means characteristic of fresh water ; but, unfor- 

 tunately, no quantitative measurements have been made by using a similar 

 net under similar conditions in different lake-areas and in the sea We have, 

 however, used our net in the Irish Sea at all seasons of the year, and throughout 

 a year in Lough Neagh. As before mentioned, so great is the quantity that 

 the water of Lough Neagh is coloured by the plankton. 



The hauls of a Nansen net off the Isle of Man are given by Herdman, Scott, 

 and Dakinin their Irish Sea work of 1909. 



The following table shows the volume of plankton in the catches ; and it 

 must be remembered that the net has been hauled through three times the 

 depth of our Lough Neagh catches. 



Irish Sea Plankton, 1909.' 



Nansen Net Hants. . 



March, 0-2-0-7 cc. ; April, 0-7-18-7 cc. (average about 4-0 cc.) ; 



May, 14 cc.-2'5 cc. ; June, 5'5-l'5 cc; 



August, O'l-l'O (average about O'icc). 



