ULVA-LACTUCA BIOFILTERS FOR MARINE FISHPOND EFFLUENTS .2. GROWTH-RATE, YIELD AND C-N RATIO

The seaweed Ulva lactuca L. was cultured in effluents from intensive marine fishponds in Eilat, Israel. The specific growth rate and the yield of U. lactuca in nutrient-rich fishpond effluents were higher than in pulse-enriched fresh seawater. Both yield and specific growth rate of the seaweed showed positive hyperbolic correlations with N-flux which fitted the Michaelis-Menten kinetics. Maximal dry weight (DW) yield was 55 g m-2 d-1 and the maximal specific growth rate was 18% d-1. The C:N ratio in plant tissue showed a negative hyperbolic correlation with N-flux, dropping from over 15 to approximately 7 (w:w) when grown at low and high N-fluxes, respectively. As the stocking density increased up to 6 kg m-2 under N-sufficient conditions, both specific growth rate and yield decreased significantly, as did mostly also the C:N ratio. The data suggest the seaweed to be N-limited at low N-fluxes and light limited at high N fluxes, especially at high stocking densities. The high yields obtained show that fishpond effluents supply U. lactuca with all its required nutrients including inorganic carbon. For high yield and high nitrogen content, U. lactuca should be kept at densities of 1-2 kg m-2 and at ammonia fluxes of about 0.5 moles m-2 d-1. Nitrogen content can be increased further, but at reduced yield, by increasing the stocking density to 4 or 6 kg m-2.