Conectividade hidrológica do contínuo fluviomarinho : extremos climáticos, oscilações globais e implicações no sistema costeiro semifechado

Abstract

This thesis investigates the hydrological connectivity of the river-to-sea continuum from watershed hydrology to the Semi-Enclosed Coastal System (SCSF), using the São Marcos Estuarine Complex (CESM) in Maranhão, Brazil, as a case study. The research was divided into two main stages: (1) analysis of river-to-sea continuum (RSC) indicators, and (2) identification of potential hydroclimatic hazards to the water quality of the SCSF. The study covered the period from 1984 to 2022, considering parameters such as physiography, rainfall, hydrological flows, socio-environmental systems, and coastal oceanographic parameters. Three main tributary basins of the CESM were analyzed: the Pindaré River (RP), Grajaú River (RG), and Mearim River (RM). The analysis of climate extremes indicated a reduction in rainfall intensity in the eastern CESM region (mainly composed of the Cerrado biome) and an increase in the intensity of extremes and consecutive dry days in the western region (mainly composed of the Amazon biome). The Pindaré River is the main tributary, with an annual discharge of 6.81 km³·yr⁻¹, which is twice the discharge of the other sub-basins. The RM showed greater hydrological stability during dry years, whereas RP and RG presented higher variability. High 8 flow events were 50% more frequent than low-flow events throughout the study period. Standardized Precipitation Index (SPI) analyses revealed that increased seasonality and consecutive dry years are heightening the vulnerability of these basins. ENSO events, such as El Niño and La Niña, caused up to 30% variation in annual runoff, particularly in RP and RG, which are also impacted by deforestation and land-use changes. River fragmentation, measured by the CARFI index, varied among the sub-basins, with the highest value observed in RM (24.24). Land-use and land-cover analysis indicated a reduction of ~30% in natural cover in the watershed and ~27% in the surroundings of rivers and water bodies. Changes in river hydrology, particularly during low-flow years, affected salinity and dissolved oxygen (DO) in the CESM, with RM influencing the largest variations. Elevated phosphate levels were observed during low-flow years compared to regular-flow years. The macrotidal regime and the morphometry of the SMEC suggest a limited discharge capacity, making it sensitive to pollutant retention and eutrophication. Additionally, the system faces cumulative pressures from port activities, bauxite mining, and nutrient-rich discharges from the urban expansion of São Luís Island. Low-flow conditions can concentrate pollutants, while high-flow events transport them downstream, exacerbating environmental risks. Climatic phenomena with the strongest correlations to local precipitation parameters were the Atlantic Multidecadal Oscillation (AMO), Atlantic Meridional Mode (AMM), Multivariate ENSO Index (MEI), and North Atlantic Oscillation (NAO), especially with the trend component of climate extreme indices. These correlations were spatially heterogeneous, distinguishing east-west regions and areas of differing altimetries. Between 1993 and 2022, 269 hydroclimatic disasters were recorded in the CESM watershed, with higher incidence in the RG and RM basins, associated with extreme rainfall events. The evolution of RSC governance increasingly focused on natural resource management, civil defense, and resilience to natural disasters, reflecting a response to hydroclimatic disasters. The increase in dry conditions in the CESM watershed results in heightened saline intrusion, mangrove encroachment into upper estuarine zones, and elevated phosphate levels. These findings underscore the need for adaptive governance strategies that integrate hydrological variability and human pressures to enhance ecological resilience and promote sustainable management. By preserving hydrological connectivity and implementing integrated management practices, this research provides a framework 9 applicable to similar systems worldwide, helping mitigate climate change impacts and protecting coastal socio-economic activities