Monitoring bioremediation of an impacted coastal lagoon through nature-based solutions: a case study of the Maricá Lagoon (RJ, Brazil)

Abstract This study evaluates bioremediation using effective microorganisms (EM) to improve quality in the eutrophic Maricá Lagoon (RJ, Brazil). The intervention, implemented in late 2021, consisted of the regular application of EM in one of the lagoon’s main fluvial contributors: the Mumbuca-Ubatiba drainage basin. To evaluate bioremediation, physicochemical and biogeochemical variables were monitored in the water column and sediments from 2020 to 2023. Whereas the drainage basin was sampled monthly, the lagoon was sampled every 3 months. Dissolved inorganic nitrogen (DIN) concentrations showed strong spatial variability in the system, reaching up to 2412 µM in the eastern margin (Mumbuca-Ubatiba)—dominated by ammonium (> 80% of DIN)—and up to 234 µM in the western margin (São Bento channel), where nitrate predominated (> 70%). Chlorophyll-a concentrations declined from 240 µg L⁻ 1 in 2020 to 11.44 µg L⁻ 1 in 2023 in Mumbuca-Ubatiba basin. Improvements associated with bioremediation were more evident in sedimentary variables in the lagoon. Total organic carbon decreased from 7.79% (2021) to 2.72% (2023), while total phosphorus showed an overall decreasing trend despite a peak of 2826 mg kg⁻ 1 in August 2023. The Trophic State Index (TSI) revealed a general improvement in water quality, with conditions shifting from hypereutrophic (TSI ≥ 59.1) and supereutrophic (58.2–59.0) to mesotrophic levels (53.2–55.7) in several stations. Despite improvements, persistent hypoxia (DO < 2 mg.L −1 ) and continuous high nutrient inputs indicate that bioremediation alone is insufficient to fully restore system conditions. These findings demonstrate that EM-based bioremediation should be integrated with broader environmental governance strategies for long-term effectiveness.