Spatial and seasonal variation in growth rate and secondary production of Yoldia notabilis in Otsuchi Bay, Japan, with reference to the influence of food supply from the water column

Abstract

A 1 yr field study of a long-llved protobranch bivalve, Yoldia notahiljs Yokoyama, in Otsuchi Bay, northeastern Japan, revealed that there was a large seasonal and spatial variation in growth rate and secondary production, and that primary production in the water column and following organlc supply to the bottom were responsible for the vanation. Both shell and soft tissue growth of each of 9 year-classes were rapid in spring, and slow or even negative in other seasons. The rapid growth coincided well with the occurrence of spring phytoplankton bloom in the water column and the peak of organic flux to the bottom. Mean shell length and mean soft tissue weight of each year-class were larger, and the growth rates of shell and soft tissue as well as the secondary production were higher at the shallower station (10 m deep) than at the deeper station (14 m). The differences in slze, growth rate and production between the 2 stations were consistent with the local difference in food supply from the water column to the bottom, but not related to other environmental factors such as temperature, salinity, organic content in the sediment and density of Y. notabilis. Annual production was 1.38 and 0.89 g ash-free dry wt 0.1 m-'yr-', giving P/B ratios of 0.76 and 0.41 for the shallow and deep station respectively. Food intake by Y. notabilis between February and April 1991 was estimated to be 26.4 and 13.9 g C m-2 (shallow and deep respectively), which amounted to ca 10 % of the primary production and 30 to 40 % of the organic supply to the bottom. These estimates suggested that the Y. notabilis population plays an important role in the energy flow from pelagic to benthic communities.