Production of Zostera marina with different shoot size and stand structures in the　Seto Inland Sea, Japan : production in the luxuriant season in 2012

2012年６月に瀬戸内海の３つのアマモ場の，それぞれ異なる株サイズと群落構造を示す計7地点のアマモ群落において生産量を測定，比較した。株あたりの生産量は株サイズに依存し，大きい株が相対的に低い密度で生育する生野島（広島県；安芸灘）のアマモ場で50.0-73.2 mg DW shoot -1 d-1であり，小さい株が密生する平郡島（山口県；伊予灘）の株あたり生産量（7.7-27.4 mg DW shoot -1 d-1）より大きかった。面積あたり生産量において，株あたり生産量の差は株密度により相殺される傾向もみられたが，生野島のアマモ場の生産量（2.89-5.38 g DW m-2 d-1）の方が平郡島のアマモ場の生産量(1.63-2.56 g DW m-2 d-1) よりも大きかった。これら２つのアマモ場では底質に大きな相違がみられ，アマモの群落構造や生産量に影響を与えていると考えられた。すなわち，生野島の底質はほとんど泥により構成され有機物含量も高い一方で，平郡島の底質は中砂・細砂を中心に構成されより厳しい波浪環境を反映していた。

2011年秋季のアイゴの食害による消失から回復途上にある阿波島（広島県；安芸灘）のアマモ場では新たに発芽した実生由来の株がパッチ状の群落を作り，株密度も現存量も低かった。調査した群落の中で生産量は最も低かったが(0.60 g DW m-2 d-1)，その現存量回転率（6.5％ d-1）は，他群落（1.7-3.3％ d-1）のそれよりも大きかった。

In June 2012, production was measured and compared among Zostera marina stands with different shoot sizes and stand structures, at a total of seven stations in three Z. marina beds in the Seto Inland Sea, Japan. Production per shoot depended on the shoot size, and was larger (50.9 - 73.2 mg DW shoot -1 d-1) at the stands in the bed of Ikuno-shima Is. (Hiroshima Pref.; Aki-Nada Sea), where large shoots formed stands with lower densities, than at stands in the bed of Heigun-jima Is. (Yamaguchi Pref.; Suo-Nada Sea) (7.7 - 27.4 mg DW shoot -1 d-1) where small shoots exhibited higher densities. Though the areal production estimated was compensated by shoot density, it was still larger at the bed of Ikunoshima Is. (2.89-5.38 g DW m-2 d-1) than at Heigun-jima Is. (1.63 - 2.56 g DW m-2 d-1). Sediment characteristics were quite different between the two Z. marina beds and considered to affect the stand structures and productivity, i.e., the sediment at Ikuno-shima Is. was muddy and rich in organic matter and the sediment at Heigun-jima Is. was dominated by sand indicating severer physical conditions induced by waves.

In Aba-shima Is. (Hiroshima Pref.; Aki-Nada Sea), the third research site, the Z. marina bed was on the way of recovery after catastrophic damage due to heavy grazing of rabbitfish (Siganus fuscescens) which had occurred in autumn of 2011, and young shoots developed from seeds forming a patchy stand with a low shoot density and biomass. Though the production was lowest (0.60 g DW m-2 d-1), the turnover of the biomass was higher (6.5 % d-1) at the stand in Aba-shima Is. than values (1.7-3.3% d-1) at the other two beds.