Determining the optimum levels of macroalgal density and salinity for removing aquaculture effluent has gained increasing research interest in recent years because of the growing concerns over environmental sustainability. Here, we determined the effects of macroalgal density and salinity on the uptake of NO2−, NO3−, NH3, and PO43− by Caulerpa lentillifera from the effluent of Poecilia latipinna using spectrophotometry. Laboratory experiments were conducted to measure nutrient uptake at five different macroalgal density levels (10, 20, 30, 40, and 50 g/L) and three salinity levels (20, 30, and 40 ppt) with and without aeration. Quadratic regression analysis revealed significant nonlinear and linear effects of macroalgal density on the uptake of NO2−, NO3−, NH3, and PO43−, where the maximum uptake was predicted to occur at the macroalgal densities of 31.6, 33.3, 50.0, and 20.0 g/L, respectively. Likewise, the effects of salinity on the uptake of NO2−, NO3−, NH3, and PO43− were significant and nonlinear where the maximum uptake was predicted to occur at the salinity levels of 29.1, 30.7, 29.5, and 29.5 ppt, respectively. The result of the effects of aeration was mixed but somewhat indicated a positive effect on the nutrient uptake within the 24 h period. Our results could help aquaculturists to minimize the excessive nutrients by C. lentillifera from aquaculture effluent while achieving long-term sustainable aquaculture production.

Aquaculture is one of the fastest growing food producing industries in the world. Aquaculture effluent contains high concentrations of inorganic nutrients. Reduction of these inorganic nutrients in aquaculture effluent is crucial for fulfilling the effluent standards or reuse of aquaculture effluent. This study investigated the effective use of green macroalga Caulerpa lentillifera as a bioremediatory species for nutrient removal from aquaculture effluent by conducting an on-station experiment and measurements. The effluent of a fish culture unit was circulated through a macroalgal culture unit every four days for a total of 60 days, allowing 15 circulations. Concentrations of inorganic nutrients (NO2−-N, NO3−-N, NH3-N, and PO43−) were measured in the integrated system consisting of a fish unit, settling unit, macroalgal unit and extra tank for water circulation in triplicates. Multiple linear regression analysis revealed that the application of the bioremediation system led to a significant reduction in nutrient concentrations within one day, and slightly further in the following two days. On average over the 15 circulations, the first one day of application decreased the concentrations of NO2−-N, NO3−-N, NH3-N, and PO43− by 0.247 mg/L, 81.6 mg/L, 0.682 mg/L, and 0.352 mg/L, respectively. Furthermore, the C:N ratio of macroalgae decreased during the 60-day application period, providing evidence of the nutrient uptake by macroalgae. Based on the European Union (EU) standard and quality criteria of France and the Joint FAO/WHO Expert Committee (JFWEC), the macroalgae grown in the integrated system were at the safe level for human consumption in terms of contents of Cd, Pb, and As. The results of our study imply that recirculating aquaculture systems utilizing C. lentillifera for biofiltration have the potential for effective treatment of aquaculture effluent integrating fish and macroalgae production.

Cryptocoryne wendtii is commonly used for aquaria and water gardens and hence is an economically important aquatic plant with a high demand in local and export markets. There are fifty species of genus Cryptocoryne out of which fourteen species including C. wendtii are endemic to Sri Lanka. Although Sri Lanka exports around 300 aquatic plants, exportation of endemic and threatened species collected from the wild is prohibited, but the cultivated plants. C. wendtii ('Brown') shows poor growth when cultivated in common soil which is used for normal propagation of other ornamental aquatic plants. Hence, this study was undertaken to find an effective substratum for C. wendtii, which is important for aquatic plant industry.C. wendtii was grown in five different media prepared with top soil and river sand in different proportions, both under emerged and submerged conditions. Results were analyzed using one-way analysis of variance. Plants grown in river sand had the highest dry weight gain and percentage survival compared to plants grown in top soil and river sand media prepared according to 1:1, 1:2, 1:3 ratios irrespective of either they were grown emerged or submerged, demonstrating that river sand enriched with commercially available fertilizer is the best medium to grow C. wendtii.