Linking centennial scale anthropogenic changes and sedimentary records as lessons for urban coastal management

Coastal eutrophication and urban flooding are increasingly important components of global change. Although increased seawater renewal by barrier openings and channelizing are common mitigation measures in coastal lagoons worldwide, their effects on these ecosystems are not fully understood. Here, we evaluated the relationships between human interventions in the watershed, artificial connections to the sea, and the sediment burial rates in an urban coastal lagoon (Maricá lagoon, Southeastern Brazil). Sediment accretion along with nutrient and carbon burial rates were determined in two sediment cores representing the past ∼120 years (²¹⁰Pb dating) and associated with anthropogenic changes as indicated by historical records and geoinformation analyses. Lagoon infilling and eutrophication, expressed by the average sediment accretion, TP, TN, and OC burial rates, respectively, increased ∼9-18, 13-15, 11-14 and 11-12-fold from the earliest (<1950) to the most recent (2000-2017) period. These multi-proxy records confirm mechanistic links between deforestation, urbanization, and untreated sewage discharges. In addition, our findings reveal artificial connections to the sea may contribute to lagoonal eutrophication and infilling, particularly when not integrated with sewage treatment and forest conservation or reforestation in the watershed. Therefore, increased seawater renewal by physical interventions commonly considered as mitigation measures may in contrast cause severe degradation in coastal lagoons, causing harmful consequences that should be not neglected when implementing management practices.