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accurately described. Similar investigations have also been carried out with 

 regard to other plankton organisms of frequent occurrence. A good basis has 

 thus been furnished for the very necessary further work in this field. 



The investigations hitherto made also indicate, that the different species 

 may be placed together in certain groups, in regard to their seasonal appearance, 

 besides their return year after year in almost the same order, certain species 

 being most frequently found at the same time. The more numerous of these 

 are thus the species which characterise the plankton at a giveti time, and we 

 may therefore speak of plankton communities. As regards the area invetigated, 

 the phytoplankton is as a rule predominant to such a degree, that it becomes 

 natural to characterise the plankton in particular thereby. This is not the place 

 to enter upon any closer description of the plankton communities, but it should 

 be mentioned that throughout the greater part of the area we find the winter 

 condition of the sea distinguished by a plankton sparse in number and of varied 

 nature. In the spring the Diatoms increase largely in numbers, so that the spring 

 plankton may be characterised as a rich Diatom plankton. Within this are 

 again varying kinds; at first certain species are predominant, yielding place 

 later on to others. Towards summer the quantity of Diatoms decreases, and the 

 plankton is then generally characterised by the Peridinians, especially the Ce- 

 ratia, and is also less in quantity than in the spring. Under very distinct coast- 

 al conditions, however, this Peridinian period is less definitely pronounced. 

 Its duration varies somewhat; at most places within our area it is followed by 

 a new Diatom period, an autumn flowering; this happens, however, only to a 

 slight degree, or not at all, in the waters nearest to the ocean. After the autumn 

 flowering, the sea sinks back into its winter rest. In the true Baltic, the annual 

 changes in the composition of the plankton differ from the foregoing, inasmuch 

 as a plankton is found there in summer and autumn which is characterised by 

 blue-green algœ. Within the remaining divisions of the area, also, variations 

 naturally occur, corresponding to the varying geographical distribution of the 

 species. 



This annual alternation of flowering periods, during which the plankton 

 is very rich in quantity, with periods of decline, is peculiar to the coastal waters, 

 in contrast to the open ocean waters, where the quantity of the plankton never 

 attains such heights. It is beyond doubt that there exists a correlation between 

 the rich plankton of the coastal waters of north-western Europe and Iceland 

 and the great stock of fish in these regions, although we do not as yet know all 

 the intermediate links and factors which also contribute hereto. 



