NITROGEN AND PHOSPHORUS CYCLING AND TRANSFORMATIONS IN A PROTOTYPE NONPOLLUTING INTEGRATED MARICULTURE SYSTEM, EILAT, ISRAEL

An intensive mariculture system for the growth of sea bream has been d eveloped which uses seaweed Ulva lactuca to remove dissolved nutrients (N and P) from the circulating water. This allows water to be safely recycled to the fish and results in a comparatively 'non-polluting' fi nal effluent. As with other mariculture systems, only similar to 25% o f the food-N was used for fish growth. The dominant processes affectin g N within the system were excretion by fish of ammonia-N (30%), disso lved organic nitrogen (DON) (30%) and faeces-N (10%), rapid transforma tion of DON to ammonia-N (16%) in the fish tank and uptake of ammonia- N (17 to 39%) by the seaweed (all percentages expressed relative to fo od input). Nitrification (10%) competed with the seaweed for the avail able ammonia-N. There was only minimal uptake of nitrate by the seawee d. Anoxic conditions developed in the sedimentation tank. Denitrificat ion (5%) and bacterial ammonification, including sulphate reduction, w ere observed. The system showed only minor seasonality, probably becau se the major processes (food input and U, lactuca uptake) had only a s light dependence on temperature. Our quantitative understanding of the processes affecting N within the system was confirmed by a parallel s tudy in which a computer simulation model was developed which was able to predict well the observed changes in ammonia-N, total oxidised N a nd U. lactuca growth. There was an excess of phosphorus in the food su pplied. It was removed by the seaweed (9 to 21%) but to a lesser degre e than ammonia-N. The residual organic matter in the sedimentation tan k was enriched in P, probably because of the presence in the food of b one meal which is resistant to bacterial decay. The final effluent dis charged contained 20 to 27% of the N supplied (and 39 to 47% of the P) which is less than half the N discharged from conventional maricultur e technology.