Environmental characteristics in Japanese surf clam Pseudocardium sachalinense bed off Yoichi coast, west Hokkaido, Japan

Sedimentary condition, bottom disturbance and macrobenthic structure of the shallow sandy bottom off Yoichi west Hokkaido, Japan, were studied in relation to a surf clam Pseudocardium sachalinense bed. Surveys were conducted in April, July and November 2001, and February 2002. Median diameter of the sediment tended to be low with water depth ranged from 0.09 to 0.46mm without seasonal change. Total organic carbon (TC and total nitrogen of the sediment tended to increase with water depth in addition to be high from April to July (O.42-5.04 and 0.04 - 0.43mg/ g, respectively) and low from November to February (0.29 - 1.35 and 0.04 - 0. 15mg/g DW, respectively). Bottom disturbance of calculafing Shields number (PS ) as an index tended to be low with water depth and fluctuated markedly with seasonal change of wave action and effect of topographical features. According to principal component analysis using sedimentary condition and bottom disturbance, five sedimentary types (1. 2, 3, 4 and 5) were divided. Type 1, very high TC and low PS , and type 2, low TC and high c occurred in 6 - 12m depth in parts from April to July and from November to February, respectively, although type 3, high TC and low PS distributed in 6-15m depth through the year. Type 4, low TC and very high PS , and type 5, low TC and PS were found in 3m depth from April to July and from November to February, respectively. Five macrobenthic communities (A, B, C, D and E) were identified according to cluster analysis using the similarity index (C sub(p)) , and characterized as dominant presence of Umbonium costatum and Goniada maculata in A, Scaphechinus griseus and juvenile of P. sachalinense in B, juvenile of Felaniella usta in C, Eohaustrius eous and S. griseus in D, and Nephtys caeca and Urothoe grimaldii in E. A - community was constantly found in 6 - 9m depth with sediment of type 1, 2 and 3, and B - and D - communities occurred in 3m depth with sediment of type 4 and 5 through the year. In 12- 15m depth, distribution patterns of the dominant species in each community changed seasonally by their recruitment. It was suggested that ' the sedimentary condition were suitable for burying and feeding of the clam but mortality due to the bottom disturbance and competition with S. griseus would occur in the habitat.