NITROGEN CONVERSION IN A BIVALVE CULTURE

Bivalve spat were grown in an on-shore upwelling nursery using a landl ocked heliothermic marine basin (Norwegian oyster-poll) as a food prod uction system and thermal source. Several manipulations, involving art ificial fertilization (N, P and Si), were performed in order to enhanc e the production capacity. Based on data from a monitoring programme ( May-August) on physical, chemical and biological variables in the syst em, main paths of nitrogen flow and dynamics of bivalve production and nitrogen conversion efficiency were described. The conversion efficie ncy 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 utili zation efficiency, bivalve N production over available particulate N i n the nursery, was 19.8% for the experimental period. The efficiency i ncreased in July from 19.4% for the first two weeks to 27.0% during la te July. This was probably due to a higher food value of the phytoplan kton community in late July, dominated by Skeletonema costatum (Bacill ariophyceae) and Nitzschia sp., than the phytoplankton community in ea rly July, dominated by Fragilaria sp.