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 developed 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' final effluent. As with other mariculture systems, only similar to 25% of the food-N was used for fish growth. The dominant processes affecting N within the system were excretion by fish of ammonia-N (30%), dissolved organic nitrogen (DON) (30%) and faeces-N (10%), rapid transformation 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 food input). Nitrification (10%) competed with the seaweed for the available ammonia-N. There was only minimal uptake of nitrate by the seaweed. Anoxic conditions developed in the sedimentation tank. Denitrification (5%) and bacterial ammonification, including sulphate reduction, were observed. The system showed only minor seasonality, probably because the major processes (food input and U, lactuca uptake) had only a slight dependence on temperature. Our quantitative understanding of the processes affecting N within the system was confirmed by a parallel study in which a computer simulation model was developed which was able to predict well the observed changes in ammonia-N, total oxidised N and U. lactuca growth. There was an excess of phosphorus in the food supplied. It was removed by the seaweed (9 to 21%) but to a lesser degree than ammonia-N. The residual organic matter in the sedimentation tank was enriched in P, probably because of the presence in the food of bone meal which is resistant to bacterial decay. The final effluent discharged 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 mariculture technology.