On the Summer Plankton in the Waters of the Western Aleutian Islands in 1928

Abstract

About 30 samples were collected around the Near and the Rat Islands of the Western Aleutian Islands during the IV voyage of the Hakuhô-maru in July and August, 1928. They were disposed to me for quantitative and qualitative studies, by which the following results were obtained: (1) Quantitative distribution of plankton. Aline drawn between the Near and the Rat Islands divides the investigated area into 2 parts: one extending along the coast of the Bering Sea and the other along the Pacific coast. The former is far richly populated by plankton than the latter. The poorly populated regions are located along the eastern coast of the Near Islands, close to the southern part of the Rat Islands, and most extensively between these two islands, Densely populated regions are found between these poorly populated regions in the Pacific on one hand, and along the north-eastern part of the Near Islands and around the Rat Islands in the Bering Sea on the other. The above mentioned fact coupled with the state of isotherms and isohalines of the surface and 50-meterlayers seems to show that the surface water masses are descending into the lower layer just in the region poorly populated by plankton, thus preventing the water masses coming from the south and the north from mingling with each other in the deeper layer. On the contrary, the densely populated regions seem to lie at the places where the deeper water mass is rather upwelling or the water masses form the northe and the south are stagnant. Possibly, in the poorly populated regions the water may lack some nutrient salts, while either the mechanical accumulation by the stagnation of the water mass or the rapid growth of plankton by the abundant supply of the nutrient salts may account for regions of dense population (see text-fig. ang table B). (2) Qualitative result. The plankton in this area is rather monotonous and few in number of species (see Table A on p. 72). Chaetoceros is mainly observed in the poorly populated regin (I a-c) as well as in the fairly populated region (II a-c), and most of them are found along the Pacific coast. The densely populated region on the Pacific side has Nitzschia seriata as the representative species in addition to Thalassiothrix longissima and Coscinodiscus. The densely populated regions along the northern part of the Rat Islands (III) shows Thal. long. as the leading species. The densely populated region on the north-eastern part of the Near Islands (IV) contains prominently and abundantly Rhizosolenia and Denticula sp. Among the animal plankton, Eucalunus elongatus, Cyttarocyis Ehrenbergii and Codonellopsis morchella prevail mainly on the Pacific side, but on the contracy Calanus pulmchrus and the genus P ?? rafavella in the Bering Sea. The difference of percentage of each species, especially of plankton animals, which may be far more sensible than the phytoplankton, shows to some extent the grade of mixture of the Bering Sea water and Pacific water in the investigated area. The line between the Near and the Rat Islands divides this area into 2 regions not only quantitatively but also qualitatively. Most plankton plants of this area are common to the southern seas of the Pacific side of Japan, but is may be mentioned that these species which prevail in the Summer plankton here generally abundant among the Spring plankton in the southern seas.