14 



P. T. CLEVE, THE PLANKTON OK THE NOETH SEA AND THE SKAGERAK. 



The (i«(i_y//u<s-p]ankton consists chiefly of southern neritic forms, but contains a 

 certain number of northern forms as Chcetoceros debilis, Skeletonema costatum a. o. There 

 cannot, on the other hand, be any doubt that this kind of plankton is brought into the 

 Skagerak with the Jutland Current and from the southern North Sea. Therefore it seems 

 necessary to admit, that the northern forms have migrated from the northern part of the 

 North Sea and, through the submarine channels S. and E. of the Dogger Bank, penetrated 

 towards the Continental coasts. These submarine channels really seem to exercise a 

 very great influence on the distribution of the plankton above the 50-metre plateau of 

 the bottom and also on the migration of the fishes. 



November 1900. 



A. The North Sea. In that month a large coUection of samples were taken by 

 steamers crossing the North Sea in different directions. The microscopical examination 

 of the plankton proves that the prevailing types were tripos- and c??V/y?>n<s-plankton. The 

 former kind occurred chiefly between 58°— 59° N. 0= E. and 55°— 56° N. 1° E., most 

 abundantly between 55° and 57° N., and especially W. of the Danish Peninsula. The 

 didyinus-\>\?inkion prevailed in the southern North Sea, from Holland to Skagen, where it 

 became intermingled with triposplankton. 



The plankton was collected in all kinds of water, containing 35 to 28 p. m. sali- 

 nity. I tried, as in the former cases, to make out what species characterize the one or 

 other kind and with the following result: 



' Water of 35 p. in. salinity contained Acanthometron catervatum and Chwtoceros 



atlanticus, which may be considered as characteristic, as other forms also occurred in 

 the 34 p. ra. salinity. 



Water of 34 p. m. salinity contained chiefly tripos-plankton. That also was the 

 case with some samples from the 33 p. m. water, both having in common a number of 

 species of almost equal frequency in both kinds. 



Common to both kinds of water were the following forms: 



Forms of southern origin. 

 Centropagus typicus + , 

 Oithona plumifera rr, 

 O. similis + , 

 Paracalanus 2'><^i-rvus + , 

 Sagitta hipunctata + , 

 Anvpliorella Steenstrupi r, 

 Codonella ventricosa + , 

 Dictyooysta elegans rr, 

 Dictyocha Jibula r, 

 Distephanus speculum r, 

 Ceratium furoa c, 

 C. fusus c, 

 C. macroceros cg, 

 C. tripos cc. 



Forms of nortliern origin. 

 Calanus finmarchicus + , 

 Metridia lucens rr, 

 Pseudocalaniis elongatus c, 

 Temora longicornis c, 

 Cyttarocylis denticulata r. 

 Tintinnus acuminatus, r, 

 Dinophysis acuta r, , 



Gonyaulaa: spinifera rr, 

 Peridinium pallidum r, 

 P. piellucidum rr, 

 Xanthidium hystrix rr, 

 Asterionella japonica rr, 

 Chcetoceros decipiens r, 

 Coscinodiscus radiatus r. 



