Eventos extremos de precipitação e o enriquecimento de nutrientes em uma baía costeira conservada: lições para o planejamento ambiental frente às mudanças globais

Abstract

Coastal regions are prominent as preferential areas for the accumulation of organic and inorganic compounds derived from both autochthonous and allochthonous sources. The dynamics of these aquatic systems are characterized by the interaction and processing of materials deposited in sediments, providing valuable records of the evolution of environmental conditions over time. Human activities, combined with climate change, have accelerated the degradation of these environments on a global scale. Land-use and land-cover changes exacerbate structural and functional alterations in coastal ecosystems. In this context, preserved areas serve as valuable natural laboratories for investigating the regional and global impacts of climate change in synergy with anthropogenic activities. This study aims to investigate environmental and anthropogenic changes in a relatively preserved environment (Saco do Mamanguá, Paraty, RJ) through the integrated analysis of geochemical, granulometric, precipitation, and land-use and land-cover variables over recent decades. Based on sediment accumulation patterns, phosphorus content, and elemental ratios (XRF), four phases of environmental development were identified: 1888 to 1956, 1956 to 1986, 1986 to 2007, and 2007 to 2016. These phases were used to propose an integrated interpretation of environmental indicators, comparing sedimentary behavior in response to the evolution of the drainage basin and anthropogenic interventions. The results highlight trends demonstrating the interaction of local and global scales. Between 1968 and 2016, a high frequency of extreme precipitation events was recorded, totaling 483 occurrences, with 192 occurring from the late 1990s onward, particularly after ~2007. This indicates the worsening effects of climate change at the regional scale. From 2007 onwards, there was an abrupt increase in the total accumulation rate (TAS) and in the elemental ratios Ti/Ca, Fe/Ca, and K/Ca, indicative of increased input of terrigenous sediments. Concurrently, the accumulation rates of phosphorus fractions showed a significant increase between ~2011 and ~2013, alongside granulometric changes, with higher proportions of coarser material during the same period. This phenomenon is linked to the rise in extreme precipitation events, coupled with increased deforestation and tourism pressure in the Mamanguá region, suggesting that these factors intensified erosion processes and directly affected sedimentary and geochemical dynamics in Saco do Mamanguá. This study underscores the importance of preserving coastal ecosystems like Saco do Mamanguá to better understand how environmental impacts, across multiple scales, influence natural processes within the context of increasing anthropogenic pressures and climate change.