Production and Respiration of a Bivalve Theora lubrica in Northern Bingo-Nada, the Seto Inland Sea

瀬戸内海備後灘北部域の泥質底において、大型底生動物群集内で卓越するシズクガイ(個体数比率:61-67%)の生産量と呼吸量を推定した。シズクガイは、6月に最高密痩を示すが(2,038個体/㎡)、その後は急減し8月にはほぼ消滅した、着底・加入個体は夏季の一時期を除いて周年存在し、殻長組成より7個の同時出生群が分離された。シズクガイの生産量と呼吸量を同時出生群ごとに求めると、それぞれ21.1g dry weight/㎡/year、 80.6g dry welght/㎡/year を示し、後者の季節的変動幅は前者より大きく、また生物量や水温の動きとほぼ対応していた。しかし、水温上昇期の6-7月の生産量は、生物量が最大となるにもかかわらず増加しておらず、これは呼吸量によるエネルギー損失が上昇したためであると推察される。

Density of a small semelid bivalve Theora lubrica, which occupied 61-67 % of the macrobenthic community in northern Bingo-Nada in 1986-1987, reached a maximum in June (2,038 ind./m^2) and thereafter decreased markedly to near zero in mid summer. The bivalves immediately after settlement were collected during a year except August, and divided into seven cohorts on the basis of seasonal change in the size compositions. Estimates of the production and respiratory energy loss were made on each cohort, and totaled to 21.1 g dry weight/m^2/year and 80.6 g dry weight/m^2/year, respectively. Seasonal change of the respiratory energy loss, which was influenced by the biomass and sediment temperature, was larger than that of the production. It is assumed to be caused mainly by a large amount of the respiratory energy loss that the production hardly increased in June - July, when the biomass was maximum and ambient temperature was on the rise.