HR: 11:20h
AN: OS22A-05    [Abstracts]
TI: Modern Environmental Changes on Amapa Coastal Plain under Amazon River Influence
AU: * Santos, V F
EM: valdenira.ferreira@iepa.ap.gov.br
AF: Instituto de Pesquisas Cientificas e Tecnologicas do Estado do Amapa-IEPA, Centro de Pesquisas Aquaticas-CPAq Rod. JK, km 10, sn, Macapa, a 68903-000, Brazil
AU: Figueiredo, A G
EM: alberto@igeo.uff.br
AF: Universidade Federal Fluminense, Departamento de Geologia/LAGEMAR, Niteroi, RJ 24210-340, Brazil
AU: Silveira, O M
EM: silveira@ufpa.br
AF: Universidade Federal do Para-UFPA, Centro de Geociencias Av. Augusto Correa, 01 Guama, Belem, PA 66075-110, Brazil
AU: Polidori, L
EM: laurent.polidori@esgt.cnam.fr
AF: Ecole Superieure des Geometres Topographes – ESGT/CNAM, 1, boulevard Pythagore Campus Universitaire, Le Mans, 72000, France
AB: The Amazonian coastal environment is very dynamic compared to other coasts. It is situated at the edge of the Earth's largest forest, and is segmented by fluvial systems, with the biggest being the Amazon River. The rivers are particularly influenced by the Intertropical Convergence Zone (ITCZ), which controls the water and particle discharge, and the flooding regime. Moderate and strong El Nino conditions correlate with low-precipitation periods, and La Nina events cause precipitation to increase. These variables and others related to the Amazon dispersal system create an interesting area for the study of global and regional environmental changes. The Araguari River floodplain on the Amapa coast is influenced by natural processes of global scale such as ENSO events and ITCZ, and by local processes such as Amazon River discharge, tides and tidal bore (pororoca). Anthropogenic processes such as extensive water-buffalo farming also promote environmental changes. Time- series analyses of remote sensing images and suspended sediment have shown that the maximum turbidity zone inside Araguari River is related to the pororoca phenomenon. The pororoca remobilizes sediment from the river bottom and margins, developing sediment suspension >15 g/l as it passes - creating fluid muds. The pororoca also introduces Amazon- and shelf-derived sediment into the Araguari estuary. Measurements during eight spring-tide cycles indicate erosion of 3 cm of consolidated mud and deposition of 1 cm. The pororoca also influences the remobilization and cycling of nutrients and consequently affects the distribution of benthic organisms, including benthonic foraminifera and thecamoebians. For more than a century, the coastal plain has had water-buffalo farming (>42,000 animals today), which modifies the drainage system and affects sedimentary processes. Areas with more buffalo trails have higher suspended-sediment concentration (SSC) during the dry season and lower SSC during the rainy season. This relationship is reversed in drainages without the influence of the herd. The development of small channels, one meter deep by one meter wide, induced by the buffalos can evolve into a large drainage network in a short period of time. The Santana Creek started as a buffalo trail and in three decades it evolved into a network with a main channel 200 m wide and 6 km long. Despite the Amazon River having the largest influence in the region, the Araguari River also has a considerable water discharge of 2.4x103 m3/s and particulate discharge of 7x105 tons/y, and is able to impose changes to the Amapa coastal environments and inner shelf. The natural closure of the Araguari north arm during middle of the 19th century has induced the development of a wide coastal plain in the Cape Norte region, and decreased fresh water to the Carapaporis channel. On the other hand, deforestation for farm development and buffalo farming has influenced the hydrologic regime, sediment and nutrient balance.
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 1640 Remote sensing (1855)
DE: 1834 Human impacts
DE: 1861 Sedimentation (4863)
DE: 3020 Littoral processes
SC: Ocean Sciences [OS]
MN: 2007 Joint Assembly