Modeling bioremediation in eutrophicated coastal lagoons by the green seaweed Ulva lactuca (Chlorophyceae)

Well-planned (modeled) bioremediation can remove water column dissolved nutrients and constitute a long-term solution for eutrophication processes in aquatic environments. The present study aimed at creating a model to simulate the removal of nutrients from water and sediments with the macroalgae Ulva lactuca in the Piratininga Lagoon, Rio de Janeiro, Brazil. Two bioremediation models were developed: 1) removing nutrients from a static water column and 2) removing nutrients from a renewing water column with diffusion from the sediments and inputs from streams (dynamic). Growth and nutrient absorption rates for U. lactuca were obtained from previous research as 5.14 % d^-1, 4.70 mg N g algae^-1 d^-1, and 0.70 mg P g algae^-1 d^-1, respectively. For the static model, measured concentrations of dissolved N and P in the lagoon were considered, with a cultivation period of 60 days (model step). An initial raft biomass of 70 kg was simulated, yielding 1347 kg of algae by the end of the first 60 days, with a removal of 123.14 kg N 60 d^-1 and 18.42 kg P 60 d^-1. With this operation, the time for nutrient bioremediation would be 8 months. In the dynamic cultivation, the period was equivalent to the water residence time of 40 days (model step). The efficiency and remediation time depend on the initially installed biomass (100-500 kg). Results showed that positioning the rafts by the streams and sewage inputs is recommended. Although there are fundamental differences between models and real ecosystems, the simulation of bioremediation allows better planning, leading to the mitigation of eutrophication.