«Research Article The mangrove forest at the Bucatu Lagoon, Northeast Brazil: structural characterization and anthropic impacts Rômulo Romeu Nóbrega ...»
Mongabay.com Open Access Journal - Tropical Conservation Science Vol.6 (2):254-267, 2013
The mangrove forest at the Bucatu Lagoon,
Northeast Brazil: structural characterization
and anthropic impacts
Rômulo Romeu Nóbrega Alves1,*, Roberto Sassi2 and Gindomar
Departamento de Biologia, Universidade Estadual da Paraíba, Avenida das Baraúnas, 351, Campus
Universitário, Bodocongó, 58109-753, Campina Grande, PB, Brasil.
Universidade Federal da Paraíba, DSE/Núcleo de Estudos e Pesquisas dos Recursos do Mar (NEPREMAR)/CCEN, CEP 58051-900 - João Pessoa, PB, Brasil.
*Corresponding author Email: email@example.com Abstract We investigated the Bucatú microbasin in order to characterize the structure and composition of its surrounding mangrove area by recording the main anthropic interferences on the vegetation. This study was performed along 10 plots (1000 m2 in total), perpendicular to the estuarine canal. Deposition and accumulation of sediments on the plants root system were evaluated using auger holes and photographs. Four plant species, Laguncularia racemosa (L.) Gaertn. (White Mangrove), Rhizophora mangle L. (Red Mangrove), Conocarpus erectus L.(Silver-leaved Buttonwood), and Annona glabra L. (Pond-apple) were recorded. The structural parameters obtained were low, and a great number of dead plants were observed, most of them of the species L. racemosa. Sediment deposition rates of 10-15 cm were recorded. Silting in the mangrove area and alterations of water flow in the drainage basin of the Bucatu River were the main anthropic effects observed, exerting a high negative impact on the mangrove ecosystem we investigated.
Keywords: Coastal lagoon, environmental impact, Laguncularia racemosa, Rhizophora mangle Resumo A microbacia de Bucatú foi estudada visando caracterizar a estrutura e composição do manguezal que a margeia e relacionar essas características com as interferências humanas que vem ocorrendo na área. Os estudos no manguezal foram conduzidos ao longo de 10 parcelas (1000 m2), delimitadas perpendiculares ao canal estuarino. A deposição e o acúmulo de sedimentos sobre os sistemas radiculares das espécies de mangue foram avaliados através de tradagens e registros fotográficos. Foram registradas quatro espécies vegetais: Laguncularia racemosa (L.) Gaertn. (Mangue Branco), Rhizophora mangle L. (Mangue vermelho), Conocarpus erectus L. (Mangue de Botão) e Annona glabra L.(Panã). Os parâmetros estruturais obtidos foram baixos, sendo registrada uma elevada densidade de plantas mortas, sobretudo pertencentes à espécie L. racemosa. Taxas de deposição de sedimento entre 10-15 cm foram registradas no manguezal.
Assoreamento na área de mangue e alterações do fluxo hídrico na bacia de drenagem do rio Bucatú são os principais impactos antrópicos que vem agindo negativamente e de forma expressiva no manguezal estudado.
Palavras-chave: Sedimentation mangrove; Laguncularia racemosa; Coastal lagoon; Environmental impact Tropical Conservation Science | ISSN 1940-0829 | Tropicalconservationscience.org Mongabay.com Open Access Journal - Tropical Conservation Science Vol.6 (2):254-267, 2013 Received: 17 November 2012; Accepted: 1 April 2013; Published: 24 June 2013.
Copyright: © Rômulo Romeu Nóbrega Alves, Roberto Sassi and Gindomar Gomes Santana. This is an open access paper. We use the Creative Commons Attribution 3.0 license http://creativecommons.org/licenses/by/3.0/ - The license permits any user to download, print out, extract, archive, and distribute the article, so long as appropriate credit is given to the authors and source of the work. The license ensures that the published article will be as widely available as possible and that the article can be included in any scientific archive. Open Access authors retain the copyrights of their papers. Open access is a property of individual works, not necessarily journals or publishers.
Cite this paper as: Alves, R. R. N., Sassi, R. and Santana, G. G. 2013. The mangrove forest at the Bucatu Lagoon, Northeast Brazil: structural characterization and anthropic impacts. Tropical Conservation Science Vol. 6(2):254-267. Available online: www.tropicalconservationscience.org Introduction Coastal lagoons are shallow, calm bodies of water with restricted access to the sea . Limited mangrove habitats occur in association with these ecosystems, making them important environments for maintaining the diversity of organisms thriving in those places or using them during their life cycles to obtain food, reproduce or as shelters against predators.
Mangrove habitats are formed by a special association of plants and animals that live in the intertidal zone of low tropical coasts, along estuaries, river deltas, inland brackish waters, lakes and lagoons and their associated estuaries, which are among the most productive areas of our planet [2, 3]. These environments work as an integrated system, in which the vegetation is mainly responsible for the productive dynamics of tropical estuaries and adjacent areas.
Substantial reductions have been reported recently in mangrove areas worldwide , due mainly to anthropic activities . Information on the structure of mangrove areas is greatly needed in order to comprehend the processes of succession, primary production and the reaction of mangrove habitats to stresses caused by natural processes and humans .
It is noteworthy that most of the studied mangrove areas are associated with large estuaries, mainly the ones with apparent economic and social importance. On the other hand, the mangrove habitats of micro-estuaries have been a neglected research area. Sand bars generally block the lagoonal coastal systems, obstructing the efficient drainage of surrounding mangrove areas that are only flooded during high tides. Consequently, such ecosystems have a more compacted soil, often covered by a dense layer of litter. Some tree and/or bush plants, not normally found in mangrove habitats, thrive in those areas .
In Brazil, microestuaries occur in several northeastern states, many of them with welldeveloped mangrove forests that have experienced varying degrees of anthropic impacts .
The structural characterization of mangrove areas is a valuable tool to estimate the ecosystem’s response to environmental conditions caused by alterations of local habitats. In the State of Paraíba there are several small lagoons, forming natural scenery that attracts tourists and people during their leisure time. Those environments have been impacted by varied sources and degrees of human interferences. Here we characterize the mangrove habitat associated with the lagoon system of the Bucatú river, State of Paraíba, with respect to its structure and flora. We also examine the main human interferences in the entire area and their likely effects on the mangrove habitat structure.
Tropical Conservation Science | ISSN 1940-0829 | Tropicalconservationscience.org Mongabay.com Open Access Journal - Tropical Conservation Science Vol.6 (2):254-267, 2013 Methods Characterization of the area The microbasin of the Bucatu River is ca. 856.2 ha, located on the southern Paraíba coast, in the municipality of Conde, ca. 17 km from João Pessoa, the capital of Paraíba., The basin's geographic coordinates are between 7º18'19.85" and 7º19'27.37"S, and between 34º47'47.14" and 34º48'46.67"W (Figures 1 and 2). It is bordered on the north and west by the Maceió de (lagoon of) Tabatinga, on the south by the microbasin of Mucatu River, on the east by the Atlantic Ocean. The microbasin of the Bucatu River, in geological terms lies predominantly on the Plio-Pleistocene sediments of the ‘Formação Barreiras’. The area is geomorphologically subdivided into three well-defined dominions: the Low Coastal Plains, Fluvio-marine plain, and Coastal Plain. The source of the Bucatu River is located at ca. 90 m above the sea level. The edges of the deforested slopes, on the left riverside of its upper course, have been slightly eroded by intense runoff, consisting of a sandy-clayey material .
Fig. 1. Map with localization of the study area.
The methods we used are as follows. The structural characterization of the vegetation at the microbasin of the Bucatu River was performed using the method described by Schaffer-Novelli and Cintrón . Sampling in the mangrove habitat was carried out using ten plots 10 x 10 m, perpendicular to the estuarine canal, delimited with nylon ropes. The first three plots were located close to the river mouth and the others situated further upstream.
All alive and dead plants in each plot were identified and their diameter at breast height (DBH) was measured to within 1 cm by using a calliper made up of a plastic 50-cm rule and wood.
Individuals smaller than 1.30 m had their diameter measured at soil surface instead of DBH, though we have maintained the DBH denomination for practical purposes . The tree heights were measured with telescoping aluminum tubes of known length. A tape measure was used for measuring the stilt-roots.
From the measurements of the plants carried out in each plot, we calculated the density of alive and dead individuals of each species, mean DBH, basal area of alive and dead individuals, and their mean height. With respect to characterization of structural abundance  we estimated: the absolute density (AD, ind/ha), specific relative density (RD, %), specific absolute Tropical Conservation Science | ISSN 1940-0829 | Tropicalconservationscience.org Mongabay.com Open Access Journal - Tropical Conservation Science Vol.6 (2):254-267, 2013 frequency (AF, %), absolute total frequency (ATF, %), specific relative frequency (RF, %), specific basal area (BA, m2/ha), specific relative basal area (Rba, %), and specific importance value index (IVI, %). The data obtained were statistically analyzed using the computer program ‘Excel 9.0’ and ‘Statistica 4.0’. The distributions concerning the heights and diameters of sampled plants in the plots were analyzed through frequency histograms.
All plants sampled from the mangrove habitats and their margins were pressed at the locale they were collected and then were oven-dried in the laboratory at 50oC for 72 hours. They were subsequently identified and the dried specimens were housed in the Herbarium Professor Lauro Pires Xavier (JPB/DSE/CCEN/UFPB).
The deposition and accumulation of sediments on the root system of mangrove plant species were evaluated with auger holes and photographs. A chi-square test (χ2, 5% significance) was applied to determine the proportion of plant species and their state (alive and dead individuals).
The anthropic interferences recorded in the present study were based mainly on field observations, photographic documentation, and soil sampling with augers.
Fig. 2. Study area, with details of the Bucatu river mouth (Photo A), and parts of the dead mangrove habitat: flooded (Photo B) and covered by silt (Photo by Rômulo Alves).
Tropical Conservation Science | ISSN 1940-0829 | Tropicalconservationscience.org Mongabay.com Open Access Journal - Tropical Conservation Science Vol.6 (2):254-267, 2013 Results In the 10 plots analyzed, 517 specimens of plants were recorded, belonging to four distinct species: Laguncularia racemosa (L.) Gaertn. (Mangue Branco), Rhizophora mangle L. (Mangue vermelho), Conocarpus erectus L. (Mangue de Botão) e Annona glabra L. (Panã) The most abundant species was L. racemosa, followed by R. mangle. The species C. erectus and A. glabra occurred only in two plots. Despite the notable abundance of L. racemosa, followed by R.
mangle, in the mangrove ecosystem at Bucatu Lagoon, they had similar densities when considering just the alive individuals. The total absolute density showed high numbers of alive and dead plants, 1290 and 3880 individuals/ha, respectively (Table 1).
Table 1. Density of alive and dead individuals and basal area (m2/ha) per class of DBH (cm) in the plots of the study area in the mangrove habitat at Bucatu Lagoon, State of Paraíba.
The distribution of individuals according to DBH showed a general predominance of tree trunks with ≤2.5 cm diameter (Figure 3). Among the alive plants, the classes of specimens with DBH
2.5 cm and 10 cm comprised most of the total basal area of the study area, 1.681 m2/ha (Table 1). Such distribution of individuals per DBH class indicates that most alive individuals (n = 129) are concentrated in the class 0.5 – 1 cm, though they also had a distribution up to the DBH class 10 cm, but in small proportions. Among the 338 dead plants, the individuals had a maximum 3 cm DBH, with a larger concentration of classes smaller than 2.5 cm. In the three upstreams plots there were only dead plants, most of them belonging to the diameter class ≤2.5 cm, the greatest portion of the dead basal area, totalling 1.042 m2/ha. The basal area of dead plants was 3.5 times larger than the basal area of alive plants in the diameter class ≤2.5 Tropical Conservation Science | ISSN 1940-0829 | Tropicalconservationscience.org Mongabay.com Open Access Journal - Tropical Conservation Science Vol.6 (2):254-267, 2013 cm (Table 1). Dead specimens of L. racemosa had the most expressive basal area (Table 2) and relative density in percentage, in the plots numbers 7 to 10 (Table 4).
The mean tree height in the plots was up to 3.5 m (Table 3), presenting a maximum 5 m and minimum 0.7 m in height. In figure 4 the distribution of individuals is shown by classes of tree height. The height class 1.0 – 1.5 m included the largest number of alive individuals, whereas the classes 2.0 – 2.5 m and 2.5 – 3.0 m contained the largest number of dead individuals, mainly of L. racemosa specimens. The chi-square test between the alive or dead plant condition and the species, showed that there is a significant association among the variables Tropical Conservation Science | ISSN 1940-0829 | Tropicalconservationscience.org Mongabay.com Open Access Journal - Tropical Conservation Science Vol.6 (2):254-267, 2013 (χ2 = 222.2; df = 1; p 0.01), emphasizing that 86% of L. racemosa individuals were dead. The other plant species had a 3% value of dead individuals (Table 5).
9.5 - 10 0.5 - 1.0 1.0 - 1.5 1.5 - 2.0 2.0 - 2.5 2.5 - 3.0 3.0 - 3.5 3.5 - 4.0 4.0 - 4.5 4.5 - 5.0 5.0 - 5.5 5.5 - 6.0 6.0 - 6.5 6.5 - 7.0 7.0 - 7.5 7.5 - 8.0 8.0 - 8.5 8.5 - 9.0