Graduate School of Biosphere Science, Hiroshima University, 1-4-4 Kagamiyama, Higashi-Hiroshima 739-8528, Japan
An artificial midlayer seafloor (AMS) was settled in July 2005 to promote aerobic decomposition of sinking organic matter from suspended oyster culture in Etajima Bay, Japan. The AMS consisted of a steel frame and polyethylene mesh cages in which oyster shells were stuffed. The top of the AMS was 3 m from the seabed so that organic particles from the suspended oyster culture decomposed under aerobic conditions. Particulate matter, benthic fauna and flora on the AMS as well as sediments under the AMS were monitored for 6 months from July 2005 to January 2006. During the first 4 months after deployment, carbon content in the sediments under the AMS was decreased. Based on the budget analyses of carbon, it was estimated that 6-8% of the sinking particulate carbon was trapped on the AMS and 5-10% was reduced by bacterial decomposition. As a result, it was estimated that 6-9% of sinking particulate carbon was reduced. From 5 months and onward, however, the removal rate of particulate matter decreased. This could be due to a decrease in bacterial decomposition activity in the winter season, and additional particulate load by increases in attached algae and suspension feeder on the AMS.