52 ASTRID CLEVE-EULER, QUANTITATIVE PLANKTON RESEARCHES IN THE SKAGER RAK. 



years have proved to vary within very wide limits. Both the existence of a general 

 schema of vegetation and the strong excentrieities displayed in single years stånd in 

 close connection with the hydrographic eharacter of this basin, bcing at once the place 

 of some dominant currents, never missing in tlieir season, and of other, most irregular 

 and varying, water-movements. 



Regularly returning cnrrents, that manifest themselves by bringing feorward special 

 plankton-vegetations, are the Baltic Current along the Swedish and Norwegian coasts 

 in winter, and the Jutland Current, ronnding the point of Skagen inanopposite direction. 

 The former ponrs a dilute water of 18 — 30 %o salinity över the considerably salter medium, 

 filling the basin in summer and autumn. Between the different kinds of water, a mixing 

 layer is formed, found to constitute a most favourable medium for luxuriant diatom- 

 generation. 



1. The winter-association of diatoms, thus regularly produced in a rather dilute 

 and cold water, may keep the name of Sira-plankton, given to it by Cleve in consequence 

 of the predominance of disc-shaped cells, united by slime-filaments to chains, of the 

 genus Thalassiosira, in the first place, and other similar forms. This Sira-plankton is no 

 doubt endogenetic in our Baltic Avaters; its råte of frequency, however, varies strongly, 

 as it depends in a very marked way on the light available, as will be f urther shown below. 

 Now the light is naturally depending on the situation of the mixture-layer, and the 

 mightiness of the Baltic Current will accordingly be of great importance for as well the 

 distribution as the density and total produetion of the Sira-plankton. Other essential 

 constituents of this plankton-type are Biddulphia aurita, Sceleionema and Thalassio- 

 thrix nitzschioides. 



2. The Sira-plankton requiring a very low temperature, 1 ° or less, for optimal prospe- 

 rity, it disappears for the greatest part at the end of February and is replacéd by a new 

 planktontype, equally developing in the upper, dilute waters of the Kattegat and North- 

 eastern Skager Rak. This is the Chaeto-plankton of Cleve, so named because of some 

 species of Chaetoceras being abundant in it. But as the quantitative analyse has shown 

 Leptocylindrus danicus to be by far the ruling species of this spring- vegetation, this 

 might perhaps better be termed the Leptocylindrus-a,ssociabtioii. It pref ers a temperature 

 of about5°and fillsthe Baltic coastal waters to an exceedingly variable extent, coming up 

 in good years to a frequency of some 3 — 400,000 vegetal cells per litre. Towards the last 

 week of April, it will tade away, thus marking the end of the great diatom-produetion. 



In both of the associations described, diatoms are quite ruling components. Never- 

 theless, two other groups of organisms are regularly present in some thousand cells per 1 

 at highest, namely naked peridinians of simple strueture, the Gymnodinia, and small 

 Protozoa of the genera Mesodinium and Laboece. In spite of their constant and rich occur- 

 ring in our plankton, these groups were unknown before the pioneering investigations of 

 Lohmann, because they could not be caught by the tow-net. They are, however, strictly 

 confined to the surface-layer, where the light is good. 



3. Låter in the year, towards and during the summer, an other plankton-type is 

 generated by the same surface-water, now heated up to 15° — 18°. It is generally known 

 as Tripos-plankton, Ceratium tripos being a characteristic member of it. Other Ceratia 



