Growth inhibitory effect of dissolved organic matter from fresh free-floating Ulva spp. fronds on asari clam Ruditapes philippinarum

Green tides caused by Ulvales group macroalgae have been reported from many eutrophic waters around the world. The asari clam Ruditapes philippinarum is an important fisheries resource in Japan and co-exists in coastal waters with Ulvales macroalgae. This study examined a possible impact of fresh Ulva fronds on the growth of juvenile, adult, and spat asari clams by conducting rearing experiments. Significant inhibition of juvenile and spat growth, but not adult growth, was observed when fresh Ulva fronds were floated on rearing water at 42.7 g/l (or 0.68 g/cm 2 ), a density per area corresponding to the vegetation at Ajina flat on June 2006. This growth inhibition was experimentally observed at a level of 1.8 mg C/l (or a 0.47 mg C/l increase against blank rearing water) of dissolved organic matter (DOM) released from Ulva fronds. On the other hand, coastal waters with Ulva blooms contained higher levels of DOM (mean ± SD) at 2.3 ± 0.8 mg C/l (or 1.6 mg C/l higher than neighboring waters with sparse Ulva vegetation), suggesting that similar inhibition of asari clam growth may be occurring in the natural water environment with prevalent Ulva blooms.