Influência da salinidade na heterogeneidade de paisagens estuarinas do domínio morfoclimático do semiárido brasileiro

Abstract

Along the northern coast of Northeast Brazil, some estuaries are vulnerable to hypersaline conditions due to hydroclimatic, morphostructural, and pedological aspects. Varying salt concentrations tend to condition the development of particular landscapes at the regional level. This research aims to delineate the influence of salinity on the formation of landscapes of tropical estuarine plains, with emphasis on the morphoclimatic domains of the Brazilian semiarid region. For this purpose, soil samples were collected in the Ceará and Rio Grande do Norte estuaries; subsequently, granulometry, electrical conductivity, pH (water), organic matter, macronutrients, and micronutrients were analyzed. The following indexes were obtained: sum of bases (SB), effective cation exchange capacity (t), cation exchange capacity at pH 7.0 (CTC), saturation for bases (V %), saturation for exchangeable aluminium (m %), and the percentage of exchangeable sodium (PST). The analysis of the variation of physical and chemical parameters was carried out using statistics and geostatistics. Orbital images were processed to identify the multitemporal diversity of the floristic composition and exposed soils. Altimetric models were generated using orbital sensors, and drainage information was extracted, aiming at the integrated association of these conditions. Although the pH of these soils are predominantly alkaline, there have been records of acidic pH values as well. Furthermore, Na+ concentration was the highest in the serum complex, which was followed by Mg2+ concentration; these ion concentrations exhibited the highest surface records in the Apodi/Mossoró (921.03 cmol/dm3) and Piranhas/Assú (805.5 cmol/dm3) estuaries, as compared to those in other hypersaline areas of the world. In these estuaries, high salt concentrations were observed due to precipitated evaporation. The annual water deficiency in the study area reached up to 1,829.8 mm, as observed in the Piranhas/Assú estuary. In the Acaraú estuary, salinity directly influences the expansion of glycophytic and halophytic plants, with mangroves located in regions containing up to 40 cmol/dm3 of sodium in intertidal and subtidal compartments. In this estuary, from latitude 2°52‟ S to the south, the decrease in sodium was associated with the greater earthly influence on plains above the fluvial- marine flood quota, and allowed the mangrove halophytic vegetation to be replaced by glycophytic plants; the mangrove fringes were subsequently observed to border the channels. In some open profiles in the Piranhas/Assú estuaries, Porto do Capim, and Camurupim estuaries, organic matter levels were higher in the subsurface, thereby suggesting the presence of past mangrove environments; restricted drainage, groundwater, and high evapotranspiration promote salt movement in the soil profiles. It was found that the Holocene hypersaline plains result from three conditions: a momentary stage associated with mangroves; covering of an old mangrove environment by sandy sediments, with high salinity on the surface and/or subsurface; disconnected environments from the mangroves, with hypersaline conditions that do not allow mangrove fixation, and may present desert landscapes. Therefore, in terms of the scale of analysis (from local to regional), this research contributes to the advancing knowledge about natural and human-made elements that determine estuarine landscapes, and serves as a tool to achieve compatible conservation with different forms of coastal hypersaline environments.