Marcelo Cancela Lisboa Cohen

Studies on Holocene mangrove ecosystem dynamics of the Bragança Peninsula in north-eastern Pará, Brazil

Abstract Three sediment cores from the Braganca Peninsula located in the coastal region in the north-eastern portion of Para State have been studied by pollen analysis to reconstruct Holocene environmental changes and dynamics of the mangrove ecosystem. The cores were taken from an Avicennia forest (Bosque de Avicennia (BDA)), a salt marsh area (Campo Salgado (CS)) and a Rhizophora dominated area (Furo do Chato). Pollen traps were installed in five different areas of the peninsula to study modern pollen deposition. Nine accelerator mass spectrometry radiocarbon dates provide time control and show that sediment deposits accumulated relatively undisturbed. Mangrove vegetation started to develop at different times at the three sites: at 5120 14C yr BP at the CS site, at 2170 14C yr BP at the BDA site and at 1440 14C yr BP at the FDC site. Since mid Holocene times, the mangroves covered even the most elevated area on the peninsula, which is today a salt marsh, suggesting somewhat higher relative sea-levels. The pollen concentration in relatively undisturbed deposits seems to be an indicator for the frequency of inundation. The tidal inundation frequency decreased, probably related to lower sea-levels, during the late Holocene around 1770 14C yr BP at BDA, around 910 14C yr BP at FDC and around 750 14C yr BP at CS. The change from a mangrove ecosystem to a salt marsh on the higher elevation, around 420 14C yr BP is probably natural and not due to an anthropogenic impact. Modern pollen rain from different mangrove types show different ratios between Rhizophora and Avicennia pollen, which can be used to reconstruct past composition of the mangrove. In spite of bioturbation and especially tidal inundation, which change the local pollen deposition within the mangrove zone, past mangrove dynamics can be reconstructed. The pollen record for BDA indicates a mixed Rhizophora/Avicennia mangrove vegetation between 2170 and 1770 14C yr BP. Later Rhizophora trees became more frequent and since ca. 200 14C yr BP Avicennia dominated in the forest.

Temporal changes of mangrove vegetation boundaries in Amazonia: Application of GIS and remote sensing techniques

The present work analyses a 25-year timeseries of radar and satellite images, andidentifies areas with losses and gains ofvegetation coverage along ∼166 km of Parascoastline (North Brazil) and in the innerparts of the Bragança peninsula. Thegeomorphology of this region hassignificantly changed in recent years. Aresult of these changes has been a retreatof the mangrove vegetation along thecoastline, mainly due to landward sandmigration, which covers the mudflat andasphyxiates the vegetation. Image analysissuggests that the loss of vegetationcoverage has been the dominating process inthe last 25 years, occurring on ∼ 42% ofthe Bragança coastline and adjacentareas. Vegetation has remained stable along∼ 39% of the coastline, while mudsedimentation has allowed mangrovedevelopment along ∼ 19% of it. On the otherhand, during this period of time, mangroveshave invaded 3.4 km2 (∼ 38%) of theelevated herbaceous flats in the highestsectors of the Bragança peninsula.Despite other possible causes for mangrovedeath along the coastline, such as littoraldrift currents or cyclical changes incoastal sediment dynamics, the invasion ofmangrove into herbaceous elevated flats inthe central peninsula cannot be attributedto these factors. The current dynamics ofvegetation coverage change seem to becompatible with a long-term trend relatedto the predicted rates of sea-level rise.

Sediment porewater salinity, inundation frequency and mangrove vegetation height in Bragança, North Brazil: an ecohydrology-based empirical model

An empirical model based on an ecohydrological approach was developed, which allows the integration of hydrographical, topographical and physicochemical information with vegetation characteristics of mangroves and marshes of the Bragança Peninsula, North Brazil. A synoptic distribution of sediment porewater salinity was produced with isolines derived from predicted porewater salinity values from a GIS-based, multiple regression model. It includes a non-linear function of calculated inundation frequencies, the measured salinity distribution in the adjacent estuaries and sediment porewater salinities determined in the dry season. An excellent fit between measured and predicted porewater salinities was obtained. Highly significant correlations were found between vegetation height, inundation frequency and predicted porewater salinities. The relationship between vegetation height and predicted porewater salinities indicated the existence of at least three data ‘clusters’, which probably respond to different height to diameter-in-breast-height proportions according to age and environmental stress conditions. The porewater salinities produced by the model proved to be a solid proxy for the average stress situation of mangrove vegetation. The model offers good potential as a tool for analysing wetland dynamics, particularly at a synoptic, basin level.

Implications of mangrove dynamics for private land use in Bragança, North Brazil: a case study

This work analyses effects of recent variations in the tidal inundation frequency in a mangrove ecosystem in the Bragança peninsula, North Brazil, and its implications for land occupation and use. Field data, time series of remote sensing images and local legislation were analysed focusing on the potential socio-economic impact of a changing environmental setting due to a rise in relative sea level. In the investigated period (1972–1997), vegetation changes along the coastline indicate net losses of mangrove coverage. In the central part of the peninsula, a topographically higher herbaceous plain constituting part of a farm presents an active progression of mangrove forest into an area previously dominated by grasses and herbs. This area measured 8.8 km2 in 1972 but was gradually reduced to 5.6 km2 in 1997, while progressively replaced by a monospecific stand of the black mangrove,Avicennia germinans. A linear extrapolation indicates that the elevated plain may be completely covered by mangrove by 2035. Current Brazilian legislation prohibits the extraction of mangrove trees without an officially approved management plan. Thus, the usable area of the farm has suffered a reduction by ca. 36% over 25 yr and we predict that is could be entirely replaced by mangroves in the next 35 yr. In this case study, legislation and ecosystem characteristics are analysed and a management plan discussed which could represent income alternatives for affected resource users at the local and regional level.

Mid- and late-Holocene sedimentary process and palaeovegetation changes near the mouth of the Amazon River

The integration of sedimentary facies, pollen, spores, carbon and nitrogen isotopes records, C/N ratio and radiocarbon dates allowed the identification of changes in vegetation and the sources of organic matter accumulated on tidal flats near the mouth of the Amazon River during the mid and late Holocene. Data from the margin of Amazon River indicate marine influence related to mangrove presence over a tidal mud flat between 5560–5470 cal. yr BP and 5290–5150 cal. yr BP. Afterward, the mangrove area shrank following the return of more humid conditions and increase of Amazon River discharge. A common reworking process of the tidal flat through the lateral migration of a meandering creek occurred in the study site, with later development of transitional vegetation under freshwater influence. Following the natural vegetation succession under stable climate and hydrological conditions, the expansion of ‘várzea’ (flooded freshwater vegetation) forests occurred since 600–560 cal. yr BP until the present. Furthermore, regarding the tidal flats located west of the mouth of Amazon River, these stable conditions also allowed the mangrove maintenance over mudflats with deposition of marine organic matter during at least the last 2350–2300 cal. yr BP.

Mangrove inundation and nutrient dynamics from a GIS perspective

A digital elevation model describing topography, tide elevation and inundation degree and frequency of a mangrove forest in North Brazil is discussed in relation to existing phosphate and physicochemical data in waters of an adjacent tidal creek. Due to smooth topography, an increase of 20 cm in tidal height above average neap tides increases flooded area from about 50 to 80%. Analysis of the relationship between microtopography, tidal height and flooding rate showed that in the upper 60 cm of the mangrove forest, increases of 20 cm in topographical height resulted in a doubling of the inundation frequency. This can be particularly relevant for the analysis of nutrient mobilization and vegetation structure of infrequently inundated wetlands. Throughout the year, low-tide phosphate in creek water was inversely proportional to the maximum area flooded during high tide, this correlation being higher during the dry season. Similarly, the inverse relationship between flooded areas and low-tide/high-tide pH ratios was highly significant during the dry season and the beginning of the rainy season. Although the high correlations obtained are based on data pairs obtained at high and low tide, it has to clarified whether the association between inundation degree and creek water pH is relevant for the stability of P compounds in sediment on the short scale of a tidal cycle.

Model of wetland development of the Amapá coast during the late Holocene

Os tipos de vegetacao atual, sequencias sedimentares, dados de polen e datacoes por radiocarbono obtidas em tres testemunhos de sedimento da planicie costeira de Calcoene foram utilizados para estabelecer uma historia paleoecologica durante o Holoceno superior das zonas umidas costeiras do Amapa conforme as mudancas no regime de inundacao, nivel do mar e clima. Baseado nestes tres registros, quatro fases de desenvolvimento da vegetacao sao apresentadas e provavelmente refletem a interacao entre o fluxo de energia na acumulacao do sedimento e a influencia das aguas salobras e doces na vegetacao. Este trabalho sugere alternâncias entre periodos caracterizados por influencias marinha e fluvial. O perfil longitudinal nao revelou a ocorrencia de manguezais nos sedimentos depositados por volta de 2100 anos A.P. Durante a segunda fase, a lama preencheu progressivamente as depressoes e canais de mare. Provavelmente, os manguezais iniciaram seu desenvolvimento nas margens dos canais, e os campos herbaceos nos setores elevados. A terceira fase e caracterizada por uma interrupcao no desenvolvimento dos manguezais e a expansao da vegetacao de varzea devido a uma diminuicao na influencia das aguas marinhas. A ultima fase e representada pela expansao de manguezais e varzeas. A correlacao entre os padroes atuais de distribuicao das unidades geobotânicas e a paleovegetacao indica que os manguezais e as florestas de varzea estao migrando sobre os campos herbaceos nos setores topograficamente mais elevados do litoral em estudo, o que pode estar relacionado a um aumento do nivel relativo do mar.

Morphological and vegetation changes on tidal flats of the Amazon Coast during the last 5000 cal. yr BP

The analysis of morphology, sedimentary facies, palynology, carbon and nitrogen isotopes, C/N ratio and radiocarbon data from tidal flats sediments influenced by the Amazon River indicates that vegetation development during the last 5000 cal. yr BP was controlled by morphological changes associated with marine and fluvial process. The proximal portion of the tidal flat corresponds to abandoned channels surrounded by terrestrial vegetation since 5280–5160 cal. yr BP. During the last 2840–2750 cal. yr BP, autochthonous organic material became prevalent as well as the increase in contribution of terrestrial organic matter mainly from ‘várzea’ vegetation. Herbaceous plants already colonized the tidal flat before 3170–2970 cal. yr BP, and after 2630–2310 cal. yr BP and 1350–1290 cal. yr BP they were replaced by mangrove vegetation following the development of secondary channels connected to the sea with brackish water influence. Furthermore, the erosion of the herbaceous field to form chenier ridges/tidal mixed flat after 3170–2970 cal. yr BP and before 220–140 cal. yr BP in the distal portion of the tidal flat suggests an overall interpretation of a retrogradation cycle. This process was driven by a relative sea-level rise influenced by the increase of Amazon discharge during the late Holocene.

The growth of the Doce River Delta in northeastern Brazil indicated by sedimentary facies and diatoms

The combination of diatom analyses with sedimentary facies has been limited in studies aimed at reconstructing paleoenvironments and the history of sea-level fluctuations in coastal settings. The present study integrated facies and diatom analyses, together with 14C dating and geomorphological information derived from remote sensing, to analyze the Quaternary deposits of a paleoestuary nearby the Doce River Delta in northeastern Brazil. The goal was to demonstrate the potential of diatoms as a paleoenvironmental proxy and improve the history of Holocene sea-level fluctuations in that region. The deposits, ranging from 7425–7509 to 1313–405 cal yr bp, were arranged in an overall regressive succession recorded by the upward superposition of estuarine channel, estuarine central basin, lake/ria and fluvial channel/marsh facies associations. Marine, marine/brackish and freshwater diatoms were found in these deposits. The highest concentration of diatoms was associated with the low-energy estuarine central basin deposits formed when the relative sea level was high. In the mid-Holocene, ca. 5000 cal yr bp, the coastline began its progradation due to a relative drop in sea level. This change in coastal dynamics was marked by a decrease in the concentration of diatoms and the prevalence of freshwater species over marine and marine/brackish water species. As a result of the Doce River Delta progradation, the studied estuarine channel was closed due to the accumulation of sand as beach ridges and barriers at the river mouth, a process that ultimately led to the estuary being replaced by a lake/ria.

Drivers of Temporal Changes in Mangrove Vegetation Boundaries and Consequences for Land Use

In this chapter, we deal with the main abiotic factors influencing large-scale features of the mangrove ecosystem, including changes in boundaries, coverage and land use. A topographic profile of the Braganca peninsula is presented, including the distribution of the main vegetation units and the relationship between porewater salinities in surface sediments and mangrove vegetation height. Departing from these basic patterns, the influence of inundation frequency and sediment salinity on wetland structure is treated using an empirical, GIS-based model. This integrates high-resolution topographical and tidal information, field data of porewater and estuarine salinity and vegetation structure, allowing a realistic appraisal of the distribution of porewater salinity and vegetation height at synoptic scale. The evolution of coastal vegetation in the last decades shows net coverage losses along the coastline. Vegetation death was mainly caused by erosion and/or landward sand migration, as well as by deposition on top of older mud sediments. Simultaneously, mangroves have rapidly invaded the elevated herbaceous flats in the highest sectors of the Braganca peninsula. The current dynamics of vegetation change are compatible with the predicted rates of sea-level rise. We discuss management strategies derived from mangrove-marsh ecotone shifts involving a potential conflict between private property use right and environmental legislation, and present an example of a decision support scheme combining mangrove protection and use.