ORIGINS AND OXYGEN CONSUMPTION OF PARTICULATE ORGANIC MATTER DEPOSITING IN OYSTER FARMS

Abstract

 Suspended oyster aquaculture is one of sources of particulate organic matter (POM) potentially facilitating formations of hypoxia in bottom water of inner bays. While non-feeding oyster aquaculture is generally considered to less affect surrounding environments, substantial amount of POM, such as feces, pseudofeces and exfoliated sessile-organisms on oyster shells and aquaculture facilities deposit to sea bottom. We examined the chemical compositions (carbon and nitrogen stable isotopes, fatty acid compositions) and oxygen consumption rate of POM depositing inside and outside oyster farms in Shizugawa Bay. As a result, the local depositional flux of POM was 2 to 18 times larger inside oyster farms than outside them. It was also found that both the oxygen consumption rate per particulate organic carbon (POC) and the amount of algae fatty acid per POC of depositing POM was lower inside the oyster farms than outside them. Furthermore, those of depositing POM were also lower than those of oyster biodeposits and sessile organisms on oyster shells and farming facilities. These results demonstrate that in the oyster farms, labile fractions of POM was decomposed and significantly reduced in the sedimentation process, and POM depositing to sea floor had relatively higher contents of refractory fractions of POM as well as lower rates of oxygen consumptions per unit mass of POM. It was further estimated that the oxygen consumption of POM from oyster farms accounted for ca. 10% of the total oxygen consumption in the entire area of the inner part of the bay on average.