Quantitative Studies on the Smaller Macrobenthos Inhabiting Various Topographical Environments around the Sagami Bank in the Deep-Sea System of Sagami Bay

Abstract

A survey was made in an area of complicated topography within the deep-sea system of Sagami Bay Samples were obtained quantitatively with the Smith-McIntyre bottomsampler of 1/10 m² and were washed through a screen of 10 mm mesh The biomass, the number of individuals and the number of species were studied at each station in relationship to submarine topography All the items showed more or less parallel features in three topographical environments High figures were found on the top of the bank and low ones in the canyon, while those values obtained in the sloping environment were intermediate and variable The percent composition of the quantitatively important animal groups was rather constant throughout the top of the bank, and in this hahitat the faunal diversity was higher than in the others From the distributional patterns of the species, and also from the species composition and dominancy at each station, the benthic communities of the bank-top and of the canyon were known to be distinct In the sloping environment, however, the benthic assemblages at some stations were related to the bank-top community, either being its impoverished phase or with several elements peculiar to the slope, while at the remaining stations they were rather peculiar but related to the canyon community All the aspects of the benthic ecology mentioned above were found to be closely related, both in their general and special features, to the nature of the sediments, in which three major types were recognized corresponding to the above three topographical environments From the nature of the sediment, water movement, if any, can be expected on the top of the bank, and this is reflected in the preponderance of plankton- and/or ceston feeders making up this benthic community Within the canyon, there were found the impoverishment of benthic community which is supposed to be due to the stagnant environment The biomasses of the whole area were compared to those in other regions in the form of cumulative curves In the deep-sea system of Sagami Bay which is well surrounded by land, the biomasses were only slightly lower than those of the shallow-sea system on the shelves in neighbouring regions, and those of a shallow enclosed bay were an order of magnitude higher than these figures Those in the offshore parts of the continental shelf, remote from the coast, were nearly an order of magnitude lower