6 ASTRID CLEVE-EULER, QUANTITATIVE PLANKTON RESEARCHES IN THE SKAOER RAK. 



plankton per litre sea-water in, let us say, a year. But the present stånd of our know- 

 ledge does not allow anything of the kind, since the tables drawn ref er only to the actual 

 frequency of the plankton at a given moment and since we knöw neither the ehanges in 

 frequency from day to day nor the real production at any moment at all; the frequency 

 observed being equal to the difference between that production and all various kinds of 

 simultaneous losses. 



Considering what has already been said about what organisms are reached by the 

 method used, these investigations will be found to bear 011 all the phytoplankton, with an 

 exception for a tribe of small algae, the Coccolithophorideae, whose calcar shells get dissol- 

 ved by the acide solution of Fleming, and among animals över the Protozoa. 



A source of error, that makes the counting more or less difficult and may, at times, 

 render the numbers obtained somewhat uncertain, i. e. too small, is the presence of sand 

 and organic detritus in most samples, but to a very variable extent. In a great many 

 cases, it is the increasing amount of such stuffs, that makes is impossible to examinethe 

 residue after so large a water-portion, that would seem desirable on account of the scan- 

 tiness of the vegetation. »Sharp-edged sand-grains are of course most numerous in 

 the vicinity of sandy banks and shores. The detritus is to some extent formed of various 

 foreign particles, as coal-flaws, wood-elements etc., 1 but a large and regular part consists 

 of excrements and dead rests of organisms, dissolving into the water with the aid of mi- 

 croorganisms. In periods of great »flowerings» of the diatoms, this same detritus is in- 

 considerable, compared to the rich quantities of living organisms; at other times, when 

 the water is almost empty, as in late summer, it plays a more apparent part. We cannot, 

 for the present, af förd a right appreciation of its import ance as a nutrition for other orga- 

 nisms than microbes. In any case, we may presume, as things stånd, that the primary 

 nutrition of the Seas, those real meadows of algae, growing up in the upper layers of the 

 sea-water at regularly returning periods of winter and spring, is not for the major part eaten 

 alive, but, on the contrary, dies away, when its time is out, and gets destroyed by sapro- 

 phytes. 



Series I. August 15th 16th 1912. 



This series, collected two months after the latest onc investigated by Mr Gran 

 from the same boundary, clearly illustrates the scantiness of the plankton in summer- 

 time. In late summer, our waters are previously known to be very poor, especially in 

 diatoms, and to display a vegetation chiefly consisting of Ceratia, to j udge from the net- 

 fishings. Some numbers are given in the following tables. 



To begin with S. Extra II, at the mouth of the Kattegat, therc is among the dia- 

 toms in the surface-layer only one species worth mentioning, viz. Rhizosohnia alaia 

 f. gracillima, the well-known summer-form, pointed out alread}^ by Cleve as charac- 



1 It is evident, that such a detritus is copious especially in the outfreshed surface-layers, formed in 

 spring with the aid of the Baltic Current and of affluents from land. 



