Processos geomorfológicos e desenvolvimento dos manguezais: o exemplo da Ilha Nova-São Caetano de Odivelas-PA

Abstract

The jagged configuration of the NE coast of Pará, with numerous bays and indentations, is typical of a "rias" coast (Barbosa & Pinto 1973), and offers favorable conditions for the development of exuberant coastal and estuarine mangrove forests. At the mouth of the Mojuim River, in the municipality of São Caetano de Odivelas, Ilha Nova is a good example of mangrove forest establishment, which has been occurring for at least two decades (a fact proven by analysis of photographs of 1977 areas). This work is a contribution to the FUNTEC/SECTAM Project "Mangroves of the Paraense Coast". Its main objective is the study of the interactions between geomorphological and sedimentological processes and the identification of mangrove succession gradients. The characterization of the vegetation was done by phytosociological methods according to the type of vegetation; the FITOPAC 1 program was used for data processing; as for sediment collection, a mud auger and vibration sampler were used. The results show that hydrodynamic and geomorphologic processes condition the asymmetric shape of the island as well as the mangrove succession gradients both in terms of accretion ("ladder" and "mushroom" patterns) and erosion ("toothpick" pattern) geomorphological processes. The "ladder" pattern develops south of the island in a sector of recent accretion, where Rhizophora mangle (average height 3m) colonizes the muddy depressions (10 to 30cm of surface sediment). The "mushroom" pattern, occurs in the western part of the island and is defined by the concentric distribution of the vegetation, the predominant species is Avicennia germinans (average height 3m), surrounded by strata of Spartina sp. that colonizes sandy sediments. In the eastern part where the action of erosive processes is quite accentuated, the "paliteiro" pattern occurs, the main species is Avicennia germinans (average height 10m). In this area of higher topography the vegetation is supported by more consolidated sediments with an oxidation profile varying from 70cm to 1m in gradual contact with a package of fine sand. "Tlaser", "wavy" and "Ienticular bedding" structures, typical of tidal-dominated environments, are observed on the sandbank (Reineck & Singh 1980). Interpretation of the sedimentary sequence of the Island shows that it evolved from a sandy estuarine bar that was rapidly colonized by mangrove vegetation. This shows that Ilha Nova is an excellent example of the positive ecological role played by mangroves.