Ulva lactuca Biofilters for Marine Fishpond Effluents I. Ammonia Uptake Kinetics and Nitrogen Content

The potential of the seaweed Ulva lactuca as a biofilter for effluents of intensive marine fishponds in arid regions has been studied in Eilat, Israel. One kg (wet wt) m-2 of Ulva in 1 m2 tanks 60 cm deep, removed, during the day, over 90 percent of the ammonia from the fishpond effluents, at inflow fluxes up to 10 μmoles L_1h-1 (0.14 mole m-2d-1). At fluxes of 40 μmoles L-1 h-1(0.58 mole m-2d-1), nearly 40 percent were removed. Night time ammonia removal remained efficient at low ammonia fluxes. Ammonia uptake rate and percent nitrogen (N) in dry weight (DW) of the seaweed showed hyperbolic correlations with ammonia flux through the tanks. The data fitted the Michaelis-Menten kinetics, with a maximal uptake rate of 28 μmoles L-1h-1 (0.4 mole m-2d-1). Pulse ammonia uptake rate was approximately four times that of continuous uptake. Maximal percent N in DW of U. lactuca, when stocked at 1 kg m-2, was 4.07. Increased stocking density (2, 4 and 6 kg m-2) did not significantly affect ammonia uptake rate per tank. However, N-content was significantly higher, averaging 5.5 percent of DW. The data indicate that at low ammonia fluxes (below 10 μmoles L-1h_1) U. lactuca was N-limited and that at higher fluxes the algae became gradually light limited. Our results indicate that 10 m2 of Ulva biofilter can remove from the effluents of our intensive grow-out ponds over 90 percent of the ammonia produced by 1 kg of daily feed ration or by approximately 75 kg of fish.