KONGL. SV. VKT. AKADEMIENS IIANDLINGAK. 15AN1). 34. N:0 2. 



V. The Skagerak in 1899. 



The seasonal changes in tho plankton will be exauiined for certain periods witli 

 reference to the prevailing plankton types. 



Period 1. January. The prevailing types are the tripos- and concinnus-plankton. 

 Halosphcera oecurred in the north at Vaderö, where also Clione limacina and Thalassiothrix 

 longissima were seen onee, the last named abundantly. Some southern forms, whieh are 

 to be considered as reranants from the last period of 1898, for instance Protn pedata, 

 Paracalanus parvus, Sagitta, Ceratium b ucepl talarn, Dltylum Brightwellii a. o. disappeared, 

 and aretie or boreal forms increased in frequency towards the next period. 



Period II, February — March. This period is characterized by the abundance of 

 aretie and northern species. The Coscinodiscus concinnus continued to be common duririg 

 February. Animals were on the whole rare, but diatoins exceedingly abundant, during 

 March in the whole Skagerak and Cattegat to Öresund. The most important forms were 

 the following: 



Biddulphia aurita. Coscinodiscus oculus iridis. 



Chcetoceros constrictus. C polychordus. 



C. contortus. Nitzschia seriata. 



C. debilis. Rhizosolenia semispina. 



C. diadema. Skeletonema costatam. 



C. socialis. Thalassiosira gravida. 



C. scolopendra. T. Nordenskiöldii. 



C. teres. 



Most of these forms disappeared before the end of March, Chcet. debilis and C. 

 diadema before the middle of April; C. constrictus remained to the end of May and C. 

 contortus was more or less common to the end of the year. C. decipiens oecurred among 

 these boreal forms more or less sparingly, but it increased in abundance during the 

 next period. 



The area of distribution of these species is from Iceland to the Färöes and the 

 Shetlands, and, as they were not seen in the North Sea between Scotland and Scandinavia, 

 it may be assumed, that they arrived into the Skagerak from the deep »Norske Rende». 



Period III. April. This period is remarkable for the abundance of chmto-plankton. 

 Connected with this kind seemed to be Dinobryum pellucidum and Rhizosolenia semispina. 

 Chcetoceros constrictus continued to be common, but other northern forms became scarce. 

 At the end of the month Rhizosolenia styliformis appeared suddenly at Måseskär in great 

 abundance and in its company a number of southern forms. 



Period IV. May — June. This period commenced, as stated above, with the appear- 



ance of Rhizosolenia styliformis. In its company a number of southern forms arrived as: 



Oithona similis. Chcetoceros danicus. 



Chcetoceros curvisetus. C. Schiittii, 



