人工護岸構造物の優占生物が大阪湾沿岸域の富栄養化に及ぼす影響 １． 垂直護岸でのムラサキイガイの成長と脱落

Abstract

Some areal investigations were conducted along the coast of Osaka Prefecture from 1989 to 1991 and effects of Mytilus populations on the coastal bottom environment were examined on the basis of their distribution, production and elimination. Mytilus edulis galloprovinciallis predominated on vertical walls or block mounds of the northern and eastern coasts of Osaka Bay, and its maximum standing stock reached 230.4 kg m&lt-1&gt in wet body weight at a breakwater outside Osaka South Outer-Port in July 1990. The total biomass of the mussels along the eastern coasts of Osaka Bay in July 1990 was estimated as ca. 1.2 × 104 t in wet body weight, ca. 5.5 × 103 t in wet tissue weight and ca. 90 t in dry tissue nitrogen. Population density and biomass of M. edulis galloprovinciallis markedly declined in Ozaki Harbor in late summer. As a result, elimination of the mussels (108.4 gN m&lt-2&gt) exceeded by 1.7 times than their production (65.1 gN m&lt-2&gt) in May to September, 1990. These findings suggested that the bulk of the mussels was deposited and decomposed on the sea bed in summer when oxygen saturations of the bottom seawater were low. For dense population of M. edulis galloprovinciallis on the vertical walls of the eutrophic Osaka Bay, more attention should be paid to their negative effects on the sea bottom environment by their mortality and decomposition in summer, rather than the positive effects through their filtering and purifying functions of the sea water.