Sedimentation flux from mariculture of oyster (Crassostrea gigas) in Ofunato estuary, Japan

Abstract

Seasonal patterns of Chl a in water samples, sedimentation rates (total dry weight per area per day) and content of total carbon, total nitrogen, and total phosphorus in sediment trap samples, as well as in sediment samples, were measured at several stations in Ofunato estuary, Japan. High rates of sedimentation to the bottom were observed in March and September, corresponding to elevated concentrations of Chl a . In the middle part of the estuary, the peaks of sedimentation rate and fluxes of chemical elements through the 20-m deep layer in September amounted to 23 g m −2 d −1 , 2200 mg C m −2 d −1 , 290 mg N m −2 d −1 , and 28 mg P m −2 d −1 , coinciding with oxygen depletion in deeper layers. Seasonal changes in sedimentation can be explained by marked increases in biodeposits from oysters cultured in the surface layers. However, no marked seasonal changes in chemical elements were found within the sediment, suggesting high degradation rates of biodeposits at the sediment-water interface. Biodeposits from culture rafts were estimated using a population dynamic model for the Japanese oyster. This model gave predictions in agreement with observed seasonal changes in biodeposition fluxes through the 20-m layer in September, with a minimum estimation of 5.1 g m −2 d −1 with uniform seawater dispersion, and a maximum estimation of 390 g m −2 d −1 without dispersion.