Nitrogen Conversion in a Bivalve Culture System

Bivalve spat were grown in an on-shore upwelling nursery using a landlocked heliothermic marine basin (Norwegian oyster-poll) as a food production system and thermal source. Several manipulations, involving artificial fertilization (N, P and Si), were performed in order to enhance the production capacity. Based on data from a monitoring programme (May-August) on physical, chemical and biological variables in the system, main paths of nitrogen flow and dynamics of bivalve production and nitrogen conversion efficiency were described. The conversion efficiency of the system, bivalve N production over estimated new N, of which 86% was fertilizer nitrogen, was 16·2% for the experimental period of 93 days. During this period the decrease in efficiency from levels of 22–25% to 8% was probably due to the transition from nitrate-limited to light- and grazing-limited phytoplankton production. The food utilization efficiency, bivalve N production over available particulate N in the nursery, was 19·8% for the experimental period. The efficiency increased in July from 19·4% for the first two weeks to 27·0% during late July. This was probably due to a higher food value of the phytoplankton community in late July, dominated by Skeletonema costatum (Bacillariophyceae) and Nitzschia sp., than the phytoplankton community in early July, dominated by Fragilaria sp.