Biogeoquímica dos sedimentos lamosos e sua influência no padrão de distribuição da vegetação, no manguezal de Bragança, NE do Pará.

Abstract

The present study investigates the relationship between sediment-plant and biogeochemical processes involved in the development and spatial distribution of vegetation in the Bragança mangrove peninsula, exposed to macrotidal regime (spring range 4 m). Therefore, three transects were established at three different topographic levels of that peninsula being colonized by different patterns of vegetation: Transect 1 (T1) Transect 2 (T2) and herbaceous plateau (PHb). The vegetation in T1 is composed mainly by Rhizophora mangle and Avicennia germinans and it is situated in a semi-diurnal flooding regime forest. The transect T2 is less frequently flooded and is characterized by a stand of young Avicennia trees and herbaceous vegetation. In the highest part of this transect with low inundation frequency and high salt stress, the vegetation is dominated by shrubs of A. schaueriana coexisting with herbaceous vegetation of Sesuvium portulacastrum and Sporobolus virginicus. The PHB, localized in the zone with lowest inundation frequency is colonized mainly by tree species of A. schaueriana in the form of shrubs and S. portulacastrum and S. virginicus with sporadic presence of B. maritima. In these sites were performed measurements of the flooding frequency; floristic and fitossociologic inventory in mangrove forest and in the herbaceous halophyte vegetation. Also sediment cores (0-30 cm) and vegetation (leaves, stems and roots) were collected. The sediment samples were subjected to granulometrical, mineralogical and chemical (total sample) analyses, and determination of the humidity, organic matter and salinity as well as cation exchange capacity (CEC) besides determination of the total, inorganic, organic and bioavailable phosphorus. In the vegetation samples, chemical analysis of total phosphorus was performed. The variance analysis (ANOVA) One Way (Post Hoc Fisher's test) was applied to test the mean of the variables (humidity, salinity, organic matter, total-P, inorg.-P, org.-P and bioavailable-P. Discriminant analysis was used to measure the degree of influence of environmental variables (humidity, salinity, organic matter and total-P) in the discrimination of sites sampled. The correlation analysis of Pearson was used to evaluate the degree of interaction between discriminated variables with the hydrological conditions and forest structure data. Fine sediments (clay) in T1 and T2 suggest areas of slow deposition, while in the plateau high levels of sand suggest its formation on a paleodune with intense eolic reworking of sandy and tidal input of finer sediments. The main mineralogy of the sediments is composed of quartz (dominant in the sand and silt) and clay minerals: kaolinite and illite, other minerals of authigenic origin are pyrite, jarosite and vivianite and possibly smectite and k-feldspar. The main mineralogical and chemical composition of major and trace elements indicate the influence of sediments and soils of the Barreiras Formation on the genesis of sediments mangroves and marine influence in their chemical composition. In T1 (the more flooded sector), the dominant species is R. Mangle, while in T2 (less flooded zone) the Avicennia genus is predominant. In the herbaceous plateau the dominance of the S. portulacastrum species, is five times higher than that of S. virginicus, indicating that Sesuvium has greater ability to survive in environments with salt stress. The flooding gradient results in highly significant positive correlation with humidity. This trend is reflected in the tree height, volume and basal area correlated significantly with sediment phosphorus availability in T1 and CEC in T2. The results of Discriminant Analysis show that in T1 the humidity is the variable that most contributes in the discrimination of (Rhizophora and Mixed) forest of the Avicennia forest, while organic matter (OM) discriminated the Mixed forests and Rhizophora forests. In T2, the humidity, total-P and salinity are the variables that most contribute to the discrimination of sites in this transect. In the profiles, along the plain, the total-P and organic mater were the most important variables for discrimination of T1, T2 and PHb. The distribution of vegetation reflects different ecophysiological responses to environmental gradients. Therefore, the combination of organic matter, salinity, and phosphorus and, the high cation exchange capacity of sediments represent a significant role in the colonization of vegetation in the peninsula of Bragança.