Potential use of two subtropical mangrove species (Laguncularia racemosa and rhizophora mangle) for nutrient removal in closed recirculating systems

Six silvofishery systems were constructed to estimate the potential use of mangrove seedlings for nutrient removal. Two systems did not contain seedlings (i.e., control treatment), while the remaining systems were divided into separate treatments using two mangrove species (Laguncularia racemosa and Rhizophora mangle). Each system consisted of two water tanks linked by two hoses. The first tank contained 150 poeciliid fishes, while the second contained a biological filter of gravel and sand on the bottom, as well as a hydroponic arrangement of 34 seedlings. Water exchange between both tanks was performed over a 24-h period, every ten days for seven months, and the concentration of nutrients (NH₄⁺, NO₂^-, NO₃^-, and PO₄^-3) was measured every 8 h. Laguncularia racemosa showed a higher growth rate compared to R. mangle, but there were no differences regarding the growth of fishes among the six systems. Final dissolved inorganic nitrogen removal was 42% in the control treatment and 90% in both treatments using mangroves. Dissolved inorganic phosphorus removal was 45% in the control treatment, 44% in the L. racemosa treatment, and 35% in the R. mangle treatment. Our results indicate that both mangrove species are capable of removing a considerable amount of nitrogen, but phosphorus removal was unsatisfactory.