Mudanças na sedimentação no estuário do rio Jaguaribe (CE) devido a mudanças nos usos do solo

Abstract

The Jaguaribe river is the river with the biggest whatershed of Ceará and has a catchment area of 75,000 km ² which occupies over 50% of the state, however, land use changes that occurred in the watershed of this river and global climate change could be causing the decrease of the depth of the estuary which can make impossible for ship to navigate in the estuary and affect the aquatic fauna. The focus of this study is the deposited sediment in islands within the estuary and the surrounding areas which were considered as possible sources of sediment to these islands. The sampling plan consisted in 6 point within the estuary of the Jaguaribe river where were taken sediment cores and further 8 points where were taken superficial sediment from the bottom of the river, from areas in the margin of the estuary and from the dune fields near the estuary. The sampling locals were chosen based on the mapping of the estuary, this mapping used images from the satellites Landsat 5, Quickbird II and Kompsat 2 and cover the period of time between the years of 1988 and 2008. This mapping showed that the existing islands in the estuary suffered great changes during this period, overall there was an increase of 31.5 hectares in the area colonized by mangrove vegetation in the islands. The period of greatest growth was the period between 1992 and 2003 where there was an increase of 6 hectares with a speed of 2.1 hectare per year. In addition to mapping was performed an estimative of sediment load from various land uses in the watershed of the lower estuary, the estimative showed that the main activities that contribute sediment to the estuary is agriculture (282,322 t / year), with the main contribution being from plantations of beans, cassava and corn. Following are the urban areas (115,076 t / year), the shrimp farms (13,475 t / year) and livestock (1,374 t / year). Unlike all the other activities found in the study area, shrimp farming is the only activity in which the sediment load is released directly into the estuary. The diameter of the sediment from the samples showed a predominance of sand on the other classes and also showed that in a same profile may be different types of sediment, with layers with large amounts of silt and clay and layers formed primarily of coarser grains, the appearance of this sediment is similar from the sediment removed from the margin of the estuary and indicates that the several areas of erosion found scattered along the margin of the river may be possible source of great importance within this context.