Search

Mangrove forests are ecologically and economically important and frequently dominating protected coastal areas in the tropics and subtropics at suitable intertidal zones and are often subjected to disturbances that disrupt the structure of an ecosystem, that change resource availability and that create patterns in vegetation by producing a mosaic of seral stages that ecologists have long recognised as important to landscape-level patch mosaics. Several good reasons justify the need for pursuing a predictive understanding of the ecology of mangrove species competition including the role of disturbance events and the aftermath. A predictive understanding can challenge our assumptions concerning the factors that control plant distribution and abundance and provide techniques for predicting rates of species change ranges in response to disturbances. The aim of this study was to evaluate and predict the impact of canopy disturbances on Gazi Bay mangrove forests and the subsequent vegetation patterns both spatially and temporally. The use of a simple 1D cellular automaton provided a detailed and nearly comprehensive parameterisation of the model by forest structure parameters belonging to the standard measurements of mangrove field surveys. In the study presented, the field data were obtained for disturbance impacts at various spatial scales considering not only the spatial extent of the disturbance but also its particular location. For this, multiple sampling transects were selected a priori, based on the vegetation patterns observed on Quickbird satellite image (2002) of Gazi, to reflect major ecological zones and vegetation transitions in space. Earlier field studies already revealed different population trajectories in some cases for the same pairwise species interactions, which are consistent with the hypothesis that different scales of disturbances may affect succession trends. Simulation experiments supported these findings by demonstrating that varying disturbance impacts determine coexistence or mutual exclusion of the interacting species and occasionally leading to equilibrium shifts to alternative states. We suggest the consideration of simulation experiments as a good proxy for predicting mangrove species dynamics not neglecting the need of further evaluation based on the transient ecodynamics.

Mangrove sediments from three different mangrove ecosystems (Coringa Wildlife Sanctuary in the Godavari Delta, Andhra Pradesh, India, and Galle and Pambala, south-west Sri Lanka) were analysed for their organic carbon content, elemental ratios (C:N) and carbon stable isotope composition. Organic carbon content (0.6 - 31.7% dry weight), C/N ratios (7.0 - 27.3) and delta(13)C (between -29.4 and -20.6parts per thousand) showed a wide range of values. Lower stocks of organic carbon coincided with low C/N (atom) ratios and less negative delta(13)C values, indicating import of marine or estuarine particulate suspended matter. High organic carbon stocks coincided with high C/N ratios and delta(13)C values close, but not equal, to those of the mangrove vegetation. The variations observed in this study and published literature data could be adequately described by a simple two-end mixing model, whereby marine/estuarine suspended matter and mangrove litter were taken as end members. Thus, while in some mangrove ecosystems or vegetation zones, organic carbon stocks can be very high and are almost entirely of mangrove origin, there also appear to be cases in which deposited estuarine or marine suspended matter is the dominant source of organic carbon and nitrogen in mangrove sediments. This situation is remarkably similar to that observed in temperate salt marsh ecosystems where the importance of local vascular plant production to the sediment organic carbon pool is equally variable. The observed high variability in organic matter origin is thought to have a major impact on the overall carbon dynamics in intertidal mangrove ecosystems. ispartof: Hydrobiologia vol:495 issue:1 pages:33-39 status: published

In the Parc National du Banc d'Arguin (P.N.B.A.) in Mauritania, the mangroves of the West-African coast reach their northernmost distribution and comprise exclusively Avicennia germinans. As a biogeographically marginal, monospecific mangal in an area where freshwater availability has decreased over the last decades, concern about the survival of the mangroves has been expressed. As yet, a description was lacking and no data regarding the fitness of A. germinans in the P.N.B.A. were available. The mangrove and associated vegetation nearby Cape Timiris (southern border of the P.N.B.A.) was mapped in January 1998 and described for the adult, young and juvenile vegetation layer, along the lagoon perimeter of about 4 km. Physiognomic aspects of the mangroves were compared with those of a mangrove formation on the tidal island of Eizin further North and with those of the northernmost mangrove system, in Iouik. Four different formations were found (tall trees, wide trees, shrub and sebkha formations) with `trees' as small as 30 cm flowering profusely. There were no site-related differences in leaf morphological characteristics. Propagules were available in large numbers but germinated successfully only where protected from the main Sahara wind currents and from the sun. Experiments to investigate the effects of predation or aridity (air exposure) on germination showed an absence of predator influences and that drought did affect viability of propagules. Release of propagules at the time of spring tides may favour colonisation of new areas. Future management plans can consider the collection and broadcasting of fresh propagules, as well as favouring free play of hydrodynamics (including flooding, breaching of barriers) in the system. Except for inappropriate topographical conditions (mangroves growing in terrestrial locations, with little chance for propagule survival),A. germinans did not show signs of reduced vitality at its biogeographical limit.

Our current knowledge on production, composition, transport, pathways and transform nations of organic carbon in tropical mangrove environments is reviewed and discussed. Organic carbon entering mangrove foodwebs is either produced autochthonously or imported by tides and/or rivers. Mangrove litter and benthic microalgae tire Usually the most important autochthonous carbon sources. Depending on local conditions, phytoplankton and seagrass detritus imported with tides may represent a significant supplementary carbon input. Litter handling by the fauna not only affects microbial carbon transformations, but also the amount of organic carbon available for export. Most mangrove detritus that enters the sediment is degraded by microorganisms. Aerobic respiration and anaerobic sulfate reduction are usually considered the most important microbial respiration processes, but recent evidence suggests that iron respiration may be important in mangrove sediments as well. Organic carbon that escapes microbial degradation is stored in sediments and in some mangrove ecosystems, organic-rich sediments may extend to several meters depth. Many mangrove forests also lose a significant fraction of their net primary production to coastal waters. Large differences occur between mangrove forests with respect to litter production and export. Mangrove-derived DOC is also released into the water column and can add to the total organic carbon export. Numerous compounds have been characterized from mangrove tissues, including carbohydrates, amino acids, lignin-derived phenols, tannins, fatty acids, triterpenoids and n-alkanes. Many of these may, together with stable isotopes, exhibit a strong source signature and are potentially useful tracers of mangrove-derived organic matter. Our knowledge on mangrove carbon dynamics has improved considerably in recent years, but there are still significant gaps and shortcomings. These are emphasized and relevant research directions are Suggested. (C) 2007 Elsevier B.V. All rights reserved. ispartof: Aquatic Botany vol:89 issue:2 pages:201-219 status: published