Farid Dahdouh-Guebas

Sources of organic carbon in mangrove sediments: variability and possible ecological implications

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 δ13C (between −29.4 and −20.6‰) showed a wide range of values. Lower stocks of organic carbon coincided with low C/N (atom) ratios and less negative δ13C values, indicating import of marine or estuarine particulate suspended matter. High organic carbon stocks coincided with high C/N ratios and δ13C 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.

Utilization of mangrove wood products around Mida Creek (Kenya) amongst subsistence and commercial users

Mida Creek (Kenya) comprises mangrove forests and other marine resources that are of economic, ecological, and environmental importance to the local village communities. In total 116 households (100 of which could be used for numerical analysis), which are estimated to correspond to a coverage of ca. 30% of the total Mida Creek population, were interviewed to assess the human reliance on mangrove resources in Mida Creek. The survey indicates that mangroves are a major resource of wood for house construction, fuel wood, charcoal, and boat building. Minor uses of mangrove products include pharmaceutical and medicinal applications, tanning material, and furniture making. Rhizophora mucronata, Ceriops tagal, and Bruguiera gymnorrhiza are the major resources for house construction and fuel wood, while Sonneratia alba and Xylocarpus granatum were reported to be useful for boat building and medicinal uses respectively. The survey further describes harvesting activities and house construction, and reveals species preferences within this one particular use. As a result of depletion of the supply and the banning of mangrove harvesting, the local people are turning to other wood materials and to poaching. In our view, local utilization patterns rather than global usefulness data are required to establish a conservation policy of both mangroves and users’ subsistence requirements.ResumenLa ensenada de Mida (Kenya) posee basques de manglar y otros recursos marinos de importancia económica, ecológica y ambiental para las comunidades de los pueblos. En total 116 hogares (100 de los cuales podrían ser usados para análisis numéricos) corresponden a una cobertura total del 24% de la totalidad de la población de ensenada de Mida. El sondeo indica que los manglares son la mayor fuente de madera para la construcción de casas y botes, combustible y carbón de madera. Usos menores de los productos de manglar incluyen aplicaciones farmacéuticas y medicinales, material de teñido y muebles. Rhizophora mucronata, Ceriops tagal y Brugeuiera gymnorrhiza son las principales fuentes para combustible y construcción de casas, mientras que Sonneratia alba y Xylocarpus granatum fueron reportados como aptos para la construcción de botes y usos medicinales respectivamente. El sondeo describe adicionalmente actividades de consecha y construcción de casas revelando preferencias de especies para este caso en particular. Como resultado de la destructión del recurso y la prohibitión de la cosecha de mangle, los pobladores se han inclinado por el uso de otras maderas y a la pesca furtiva. Consideramos que se requieren patrones de utilizatión local más que datos globales para establecer una políica de conservatión de manglares y requerimientos para la subsistencia de los usuarios.

Restoration and management of mangrove systems - a lesson for and from the East African region

The restoration of mangroves has received a lot of attention world wide for several reasons. Firstly, the long ignored ecological and environmental values of mangrove forests have been documented for many mangrove areas in the world. Secondly, there is a high subsistence dependence on natural resources from mangrove forests. In addition, large losses of mangroves have occurred throughout the world leading to coastal erosion, decline of fishery resources and other environmental consequences, some of which in need of urgent attention. Finally, governments throughout the world are showing commitments towards sustainable use of mangrove areas. This paper outlines the activities of mangrove restoration and management around the world with particular emphasis on Eastern Africa. As noted here, extensive research has been carried out on the ecology, structure and functioning of the mangrove ecosystem. However, the findings have not been interpreted in a management framework, thus mangrove forests around the world continue to be over-exploited, converted to aquaculture ponds, and polluted. We strongly argue that links between research and sustainable management of mangrove ecosystems should be established.

The Use of Remote Sensing and GIS in the Sustainable Management of Tropical Coastal Ecosystems

The sustainable use and management of important tropical coastal ecosystems (mangrove forests, seagrass beds and coral reefs) cannot be done without understanding the direct and indirect impacts of man. The ecosystems resilience and recovery capacity following such impacts must be determined. The efficacy of mitigation measures must also be considered. Remote sensing and geographic information systems (GIS) are excellent tools to use in such studies. This paper reviews the state of the art and application of these tools in tropical coastal zones, and illustrates their relevance in sustainable development. It highlights a selected number of remote sensing case-studies on land cover patterns, population structure and dynamics, and stand characteristics from South-East Asia, Africa and South-America, with a particular emphasis on mangroves. It further shows how remote sensing technology and other scientific tools can be integrated in long-term studies, both retrospective and predictive, in order to anticipate degradation and to take mitigating measures at an early stage. The paper also highlights the guidelines for sustainable management that can result from remote sensing and GIS studies, and identifies existent gaps and research priorities.There is a need for more comprehensive approaches that deal with new remote sensing technologies and analysis in a GIS-environment, and that integrate findings collected over longer periods with the aim of prediction. It is also imperative to collect and integrate data from different disciplines. These are essential in the spirit of sustainable development and management, particularly in developing countries, which are often more vulnerable to environmental degradation.

Colonization of non-planted mangrove species into restored mangrove stands in Gazi Bay, Kenya

Recruitment of non-planted mangrove species into Rhizophora mucronata, Sonneratia albaand Avicennia marina reforested stands (all of them 5 years old) was investigated to assess possibilities for natural colonization. Corresponding bare (denuded or open without mangroves) and natural (relatively undisturbed) sites were used as controls. Interstitial water salinity and temperature (measured at low tide) were lower, whereas sediment organic matter content was higher in the areas with mangrove cover. Also, the bare sites were more sandy, whereas those with mangrove cover had more clay and silt. There was no apparent recruitment of non-planted mangrove species into the bare areas, but the reforested stands of S. alba, A. marina, and R. mucronata had 5400, 4000 and 700 recruits ha −1 , respectively of different mangrove species. The results therefore suggest that mangrove reforestation has facilitated natural colonization of sites, most likely by altering local hydrodynamics.

Regeneration status of mangrove forests in Mida Creek, Kenya : a compromised or secured future ?

Abstract The structure and regeneration patterns of Mida Creek mangrove vegetation were studied along belt transects at 2 forest sites of Mida Creek (3°20′S, 40°00′E): Uyombo and Kirepwe. Based on the species importance values, the dominant mangrove tree species in Mida were Ceriops tagal (Perr.) C. B. Robinson and Rhizophora mucronata Lamk. Tree density varied from 1197 trees ha−1 at Kirepwe to 1585 trees ha−1 at Uyombo and mean tree height was higher at the former site compared to the latter. The size-class structure at both localities of Mida showed the presence of more small trees than large ones. Spatial distribution pattern of adults and juveniles varied greatly between sites and they showed a close to uniform pattern (Morisitas Index I0 ≪ 1) for trees, but a tendency to random distribution (I0 = 1) for juveniles. The present paper shows that unmanaged but exploited mangroves do not necessarily disappear, but change qualitatively from locally preferred R. mucronata to the less preferred C. tagal. Whereas the effects of this change on the ecological function of the mangrove cannot be estimated yet, the economical function of the mangrove has evidently weakened.

Food preferences of Neosarmatium meinerti de Man (Decapoda: Sesarminae) and its possible effect on the regeneration of mangroves

Neosarmatium meinerti is a crab which commonly inhabitsthelandward Avicennia marina fringe in East Africanmangroves.Stomach content analysis of individuals collected at Gazi BayandMida Creek (Kenya) showed that their diet mainly consisted ofmangrove leaves, completed with little animal matter.Experimentsdesigned to offer the crabs a choice of leaves or propagulesof thevarious mangrove species present indicated that N. meinertihas no distinct preference for any of them. Nor is itattracted byspongy material impregnated with the extract of propagulesfrom anyparticular mangrove species. However, at least for Rhizophoramucronata, this crab species has a slight preference forfreshlygathered propagules when presented both matured and freshlygathered ones. There is little evidence that the plant-deriveddietof N. meinerti consists of one mangrove species only,thoughit is probable that this crab will restrict its feeding areawithinthe landward A. marina fringe. Because of the lack ofspecificity in its feeding behaviour, N. meinerti canalso bea threat to the successful regeneration or restoration ofmangrovesthrough seedling or propagule planting.

Mangrove species and stand mapping in Gazi bay (Kenya) using quickbird satellite imagery

This paper presents an automated method for mangrove stand recognition (delineation and labeling) and species mapping based on fuzzy per‐pixel classification techniques of a QuickBird satellite image. The four dominant mangrove species in Gazi Bay (Kenya) are mapped with an overall accuracy of 72 percent, where the two socio‐economically most important species are mapped with user accuracies above 85 percent. Mangrove stand maps were compared to visual delineations done by an expert interpreter and the quality was based on the quantity of overlap one has with the other. An overall correspondence up to 86 percent was achieved.

Spatial variations in macrobenthic fauna recolonisation in a tropical mangrove bay

Recolonisation by crab species and sediment-infauna taxa (at class level) in artificially regenerated mangrove stands of Avicennia marina, Rhizophora mucronata and Sonneratia alba (5 yr old) were studied using respective bare sites (open without mangroves or denuded) and natural sites (relatively undisturbed) as controls. The controls were chosen based on site history, physical proximity and tidal inundation class in reference to the particular reforested mangrove stand and samples randomly taken. A number of environmental variables were measured; interstitial water salinity and temperature (measured at low tide) were lower, whereas sediment organic matter content was higher in the areas with mangrove cover, with the natural sites having the highest content. The bare sites were generally sandier, whereas the areas with mangrove cover had higher proportions of clay and silt. Generally, there was a higher crab density in the reforested sites than in the bare sites, whereas crab species diversity (Shannon diversity index) did not vary from one site to another for any of the mangrove species. In terms of crab species composition, the reforested sites were more similar (Sørensen similarity coefficient) to the natural sites and less to the bare controls. For sediment-infauna, the reforested sites had a significantly higher density than the respective bare controls, while the natural sites had the highest density. The number of sediment-infauna taxa in both the reforested and natural sites of all the mangrove species was similar and higher than in the comparable bare sites. The results suggest that the reforested sites are supporting more faunal recolonisation, and therefore becoming more akin to the natural mangrove sites in terms of the investigated functional indicators. The findings seem to support the use of artificial mangrove regeneration (in areas where natural regeneration has been impeded by physical conditions or otherwise) as an effective management tool in the restoration and conservation of the functional integrity of degraded mangrove habitats.Key words: Crabs, Environmental variables, Kenya, Recolonisation, Restored mangroves, Sediment-infauna

Recent Changes in Land-Use in the Pambala–Chilaw Lagoon Complex (Sri Lanka) Investigated Using Remote Sensing and GIS: Conservation of Mangroves vs. Development of Shrimp Farming

Shrimp aquaculture is currently one of the major threats to mangroves, their destruction causing both environmental and social problems. This study investigated the mangrove area in the Pambala–Chilaw Lagoon complex (07°30′ N, 079°49′ E) in Sri Lanka. Using air-borne remote sensing, a land-use map was constructed in a geographical information system of the study area and pond managers were interviewed about the functioning of their shrimp farms. Based on fieldwork during the four years after taking the aerial photographs, an updated GIS-based land-use map was constructed and compared the original situation. This revealed that shrimp farms had expanded with by 25 ha in four years, mainly at the expense of mangrove forest (approx. 13 ha) and coconut plantations (approx. 11 ha). Official documents from the World Conservation Union advise conservation of the undisturbed habitats in Pambala, but this studys observations do not corroborate an eventuation of this. Other authors reported political patronage as the main cause of this adverse situation. Since the shrimp industry depends on various ecological services provided by the mangrove ecosystem in order to maintain production (cf. ecological footprint concept) the mangrove destruction is counter-productive and these observations are therefore alarming for the aquaculturists as well. The low ratio (2.6 : 1) of remaining mangrove to shrimp pond area suggests that the industry may experience severe problems, particularly as mangrove areas continue to be reclaimed for aquaculture or other land uses, unless drastic measures are taken.