en aquaculture aquaculture پژوهشي Orginal research papers Nutrients especially nitrate and phosphate are the main limiting factors of algal growth in aquatic ecosystems. The present study was conducted to examine the effects of different concentrations of nitrate and phosphate simultaneously on growth of macro algae, <em>Ulva rigida</em>. For this purpose, algae were fed various concentrations of nitrate and phosphate during 28 days in 300 L tanks in the form of 4 experimental treatments with three replicates. The experimental treatments were: T₁: 10 mg L^-1 NO₃+5 mg L^-1 PO₄, T₂: 20 mg L^-1 NO₃+10 mg L^-1 Po₄PO₄, T₃: 30 mg L^-1 NO₃+15 mg L^-1 PO₄, T₄: 40 mg L^-1 NO₃+20 mg L^-1 PO₄ and one control treatment was considered. According to the results, all experimental treatments showed increased growth compared to the control treatment (<em>p</em><0.05). There were no significant differences in growth of algae between T₃ and T₄ (<em>p</em>>0.05). The highest values of gain weight (88.02&plusmn;0.74 g, nearly 17 times more than the initial weight) and daily growth percent (60.57&plusmn;1.29 %) were observed in algae of T₃ and then T₄ (daily growth percent: 42.95&plusmn;1.56 %). In conclusion, our results suggest that a concentration of 30 mg L^-1 NO₃+15 mg L^-1 PO₄ is more suitable for the growth of macro algae, <em>U. rigida</em>. Macro algae, Ulva rigida, Concentration, Nitrate, Phosphate 59 66 http://jifro.ir/browse.php?a_code=A-10-2223-1&slc_lang=en&sid=1 A. Shakouri aarash220@yahoo.com 100319475328460032493 100319475328460032493 Yes Department of Marine Biology, Faculty of Marine Sciences, University of Chabahar Maritime and Marine Sciences, Chabahar, Iran. G.M. Balouch gm_soupak@yahoo.com 100319475328460032494 100319475328460032494 No Department of Marine Biology, Faculty of Marine Sciences, University of Chabahar Maritime and Marine Sciences, Chabahar, Iran.