The impacts of urban expansion on coastal regions can be mitigated through territorial governance instruments, particularly in Brazil, where master plans and the protection of coastal wetlands (e.g., estuaries and mangroves) play a critical role. These ecosystems provide essential regulatory and provisioning services but face increasing pressure from urbanization. To evaluate the effectiveness of Protected Areas (PAs) in conserving mangrove ecosystems, we analyzed a 28-year dataset (1992–2020; n = 552 samples). Additionally, we assessed whether mangroves mitigate urbanization-driven eutrophication in adjacent waters using the Trophic Index (TRIX). Our findings demonstrate that the establishment of the PA in 1992 significantly reduced mangrove loss. Long-term monitoring revealed that the waters within the mangrove ecosystem remained oligotrophic (TRIX < 4.0), whereas adjacent unprotected areas shifted to mesotrophic conditions (TRIX ≥ 4.0), underscoring the wetland’s role in buffering eutrophication. A generalized additive model (GAM) explained over 67 % of the observed variability in trophic state, highlighting the strong influence of land-use change. Simulations based on the current master plan project that a 100 % increase in the urbanized area, could elevate the system to hypereutrophic status (TRIX > 8.0), triggering cascade effects such as: toxic algal blooms, increased hypoxia, shifts in aquatic community structure, biodiversity loss, altered environmental metabolism and, disruption of biogeochemical cycles. These outcomes would compromise the ecosystem services provided by mangroves, emphasizing the need for policy interventions that integrate coastal wetland protection into urban planning.
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