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This study presents insights on the productivity of an under-valued, over-exploited and sewage polluted peri-urban mangrove through litter fall studies on three common mangrove species over a period of two years. These species R. mucronata, A. marina and S. alba. The study site has been exposed to raw sewage for decades, dozed with sewage every tidal cycle, with the loading exponentially reducing with distance from the source. We observed a strong correlation between the leaf C:N ratio and leaf δ15N signature. Higher C:N ratio for R. mucronata corresponding with lower leaf δ15N (3.88 ± 0.64‰) signature, and lower C:N ration for A. marina and S. alb (6.48 ± 0.03‰ and 6.76 ± 0.24‰ respectively) corresponding with higher δ15N signature. This reflects species specific response to raw sewage exposure. This shows that the forest has a more open N cycle, favouring δ15N accumulation within the system. However, the level of sewage exposure did not appear to impact on litterfall rates. The mean annual litter fall was estimated at 12.16 ± 2.89 t ha-1yr-1 for the whole stand. Litter fall, in both content and quantity was highly seasonal, with high rates occurring in the dry North Easterly Monsoon (NEM) season, January-April (ca. 5.10 ± 1.36 g DW m-2 day-1) and lower rates in the cool and wet South Easterly Monsoon (SEM) season, June-October (ca. 2.53 ± 0.47 g DW m-2 day-1). Productivity varied significantly between species, R. mucronata recording the highest annual rate of 15.34 ± 3.34 t ha-yr-1. No significant differences in litter fall was observed between A. marina and S. alba, (11.44 ± 2.90 and 9.69 ± 5.26 t ha-1yr-1 respectively). This study shows that sewage exposure does not necessarily translate into elevated productivity in mangroves, but may alter leaf nitrate content depending on species, possibly altering the decay of litter, affecting nutrient cycling within the system.

Kleptoplasty is a remarkable form of symbiosis whereby photosynthetically functional algal chloroplasts are retained by the host organism. Certain sacoglossan sea slugs form such associations and some species such as Elysia viridis (Montagu, 1804) can survive for months without access to their food algae. We report evidence for light-dependent assimilation of ammonium, nitrite, and urea (but not nitrate) in E. viridis with functional kleptoplasts. N uptake was decreased in slugs with reduced chloroplast densities, and after exposure to inhibitors of glutamine synthetase and glutamate synthetase. Incorporation of 15N-labeled substrates was traced into individual hydrolyzable amino acids, with highest incorporation in glutamine-glutamate. N assimilation by kleptoplasts in multicellular hosts constitutes an additional mechanism which may contribute to the unique and long-lived functionality of these symbioses. © Inter-Research 2008. ispartof: Aquatic Biology vol:4 issue:1 pages:15-21 status: published

This study presents an assessment of a unique exploitation of a peri-urban mangrove. The diversity of respondents, spread over wide areas, some with no apparent links with the ecosystem, coupled with legal and illegal resource exploitation, in the midst of inadequate but punitive enforcement meted on impoverished communities, pose significant challenges in conducting questionnaire surveys. Potentially limiting if not inhibiting information flow. Mangrove wood is exploited at both subsistence and commercial scales, firewood being the most ubiquitous form of use at both urban and rural settings. Forest assessments indicate the lack of preferred or specific harvesting sites. Though harvesting location is mostly dependent on profession. R. mucronata is the most harvested mangrove species. The study shows that resource exploitation is intense and “efficient” in an urban setting due to the economic drive and high demand emanating from a high human population. This has given rise to practices that mitigate against traditional norms, promoting destructive unsustainable harvesting. The outcome is the gross under-valuing of ecosystem goods and services in addition to degrading the ecosystem. The management of mangroves for wood extraction in urban areas may not be a viable and/or sustainable option, as it conflicts with functions of mangrove ecosystems, otherwise important in „resource limited‟ urban environment. It is recommended that a participatory and adaptive management, considering multiple uses and users is the viable way to manage peri-urban mangroves. this will ensure social and ecological resilience in the long-run. However, this may require specific legislative, education and institutional interventions. Integrating local ecological knowledge, may further promote a participatory approach and expedite the formulation of sustainable management plans for peri-urban mangroves. Mohamed, M. O. S., Kairo, J. G., Dahdouh-Guebas, F. and Koedam N. 2008. How Sustainable is the Utilization of Mangrove Products in Peri-Urban Mombasa, Kenya? Ecology and Society, Manuscript #: ES-2008-2617, Version 1 . Published

This research deals mainly with the changes in land cover and their impact on the environment (the soil, the infiltration and drainage of water, evapotranspiration, flora and fauna) in and around Chilaw Lagoon, a Sri Lankan mangrove site. This requires research on the space-time dynamics in soil use and land cover to estimate and predict the human influences on the ecological changes. It examines how the fishing and the shrimp farms can (co-)exist on a sustainable manner in particular. A tool getting more and more crucial in the detection, description, quantification and observance of changes in the landscape and land use, is remote sensing (e.g. Dahdouh-Guebas et al., 2005a). In combination with geographical information systems (GIS) and field work, this is an effective management tool, particularly useful in the detection of ecological degradation (Dahdouh-Guebas et al., 2005b). It is very important to detect cryptic ecological degradation to mitigate loss of ecological functions, such as the protection function of mangroves (Dahdouh-Guebas et al., 2005c; Dahdouh-Guebas and Koedam, 2006). In addition, mangroves function as a source for timber and non-timber forest products for subsistence users, and as a nursery for numerous marine species (Ewel et al., 1998). Our research implements the following activities: (1) Characterisation of the actual land cover by means of remote sensing (Ikonos images) before the field work; (2) Verification on the field of the results of the image classification: land cover, land use, habitats, vegetation assemblages, individual species; (3) Field work on the vegetation structure; (4) Evaluation of the perception of the local population concerning the actual situation of the wood and fishery resources and in particular the cultivation of shrimps; and (5) By analysis of the image processing, the ecological footprint of the shrimp farms and the fishery, a calculation will be attempted of the area of the mangrove forests that are needed for a sustainable management of the shrimp farms and of the mangrove forests. There appears to be a conflict between the shrimp farms and the mangrove forests. One cuts down mangroves to build shrimp farms. But, on the other side shrimp farms cannot survive without mangroves because the gravid females used for stocking the shrimp ponds are dependent on the mangroves. A first expected result is an extensive classification of a very high Ikonos satellite image of Chilaw-Lagoon, with an interpretation key. This will give a good view on land cover and land use in the lagoon which will be used for land management. A second expected result is to give guidelines, based on the ecological footprint of shrimp farms, for reforestation of mangroves. And by this way to give a solution for the conflicts between adjacent land use. The research is ongoing, but this presentation discusses the state-of-the-art in the light of the above objectives and the expected results.

In response to widespread global degradation of mangrove forests, many initiatives have been undertaken to reforest degraded sites. However, many of these replantings are monospecific, raising questions regarding habitat change and reduced ecological functions in mangrove plantations compared to natural mixed mangrove forests. The main objective of this study was to determine whether colonising mangrove juveniles can be recruited into the adult tree layer and thus enhance the structural complexity of previously reforested monospecific stands. Vegetation structure of originally monospecific replanted mangrove stands (Sonneratia alba and Rhizophora mucronata) that were 8 yr old was assessed. Instantaneous juvenile (seedling/sapling) diversity and abundance were determined. Adjacent natural stands were used as reference sites. The R. mucronata stand had the same species richness as its natural reference and thus had a higher Complexity Index due to recruitment of non-planted species into the adult tree population than in a previous assessment when it was monospecific. Juvenile species richness and diversity were similar between the R. mucronata stands (reforested and natural); however, the total juvenile density was significantly higher (p < 0.05) in the natural stand (7390 ± 660 juveniles ha–1) than in the reforested stand (2048 ± 667 juveniles ha–1). In contrast, the adult vegetation layer of the S. alba reforested stand remained monospecific, suggesting 100% mortality of previously observed non-planted mangrove juveniles. Many replanted mangroves around the globe are monocultures and are thus likely to have low thresholds to as they do not benefit from the ‘portfolio effect’ provided in mixed species stands. Successful recruitment of non-planted species into replanted monospecific mangroves stands may therefore enhance ecosystem productivity and stability. However, more research needs to be conducted on traits of different mangrove species and their contribution to ecosystem productivity and sensitivities to environmental perturbations.

The main objectives of this experimental study relates to the study of vegetation structure dynamics; (1) it contributes to the scientific understanding concerning the hydrochorous dispersal o fpropagules, and (2) it attempts to predict possible shifts in mangrove assemblages under different 'Intergovernmental Panel for Climate Change (IPCC)' scenarios of sea level rise. Collected data was implemented in a Geographical Information System (GIS) to perform all spatial analyses required for this study. The main results show the suitability maps of a small area (0.1 ha) which indicate the areas that can be colonised by different species ofpropagules. Secondly, modelling of the different IPCC scenarios (whole bay) gave the overall trend of frontal regression in the seaward zones while the landward zones expand into the terrestrial habitats. When considering the maximum scenario, +88cm by the year 2100, the landward expansion is clearly hindered by a talud which the colonizing species can not cross. In this situation the more landward assemblages dominated by Avicennia will not survive unless they adapt to the replaced inundation class. The GIS analyses take into account the available information derived from the fieldwork, but alterations related to degradation and/or sea level changes (e.g. changes in topography, erosion patterns, golf action,...) are beyond the scope of this explorative study.

Recruitment of non-planted mangrove species into Rhizophora mucronata, Sonneratia alba and 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.

Stable carbon isotope analysis of tree rings has become a widely used proxy in environmental and palaeoclimatological studies. In those studies, a-cellulose has often been the preferred material because of its singular composition and its immobility in wood. However, cellulose extraction is a time-consuming procedure and since the development of on-line isotope ratio mass spectrometers has become the time-limiting step in the isotopic analysis of wood samples for dendrochemical purposes. In this study we evaluate the necessity of cellulose extraction for isotopic analysis of tree rings in a tropical mangrove tree, Rhizophora mucronata Lam. Comparison between the d13C of extracted a-cellulose and bulk wood material revealed a highly significant linear relationship (d13Cbulk wood = 0.92 (± 0.08) * d13Ca-cellulose - 2.91 (± 2.04); p 13C as compared to bulk wood. The slope of the regression was not significantly different from one (p d13Cbulk wood - d13Ca-cellulose slopes for earlywood and latewood or between the slopes for samples from trees growing in contrasting environmental conditions. These results indicate that bulk wood can be used instead of a-cellulose when measuring stable carbon isotopes in the sapwood of R. mucronata in the context of a dendrochronological investigation.

Tropical trees are often excluded from dendrochronological investigations because of a lack of distinct growth ring boundaries, causing a gap in paleoclimate reconstructions from tropical regions. The potential use of time series of vessel features (density, diameter, surface area and hydraulic conductivity) combined with spectral analysis as a proxy for environmental conditions in the mangrove Rhizophora mucronata was investigated. Intra-annual differences in the vessel features revealed a trade-off between hydraulic efficiency (large vessels) during the rainy season and hydraulic safety (small, more numerous vessels) during the dry season. In addition to the earlywood-latewood variations, a semiannual signal was discovered in the vessel density and diameters after Fourier transformation. The similarity in the Fourier spectra of the vessel features and the climate data, in particular mean relative humidity and precipitation, provides strong evidence for a climatic driving force for the intra-annual variability of the vessel features. The high-resolution approach used in this study, in combination with spectral analysis, may have great potential for the study of climate variability in tropical regions.

In February - March 2002 fieldwork was carried out in a classic zoned mangrove forest at Gazi Bay (Kenya) and included topographical measurements and a collection of the vegetation data in the work field, but also successive propagule counting along five line transects. The main objective of the present experimental study relates to the study of vegetation structure dynamics and contributes to an overall view on where propagules can establish from the moment they detach themselves from their parental tree. The latter particularly concerns the juvenile vegetation layer of the species Rhizophora mucronata and Ceriops tagal. This part of the research framework gives the possibility to start working in a GIS environment and resolving several questions regarding the impact of degradation and sea level changes on the dispersal of propagules at the study area in question. The GIS-analyses take into account the available information derived from the fieldwork, but alternations that go hand in hand with degradation and/or sea level changes (e.g. changes in topography, erosion patterns, rates of sediment supply, golf action,…) are beyond the scope of this explorative study. The main findings that result from these analyses are that the study area at present has the potential to successfully rejuvenate and renew, although increasing anthropogenic pressures could have severe consequences on the dispersal of the propagules within this mangrove stand through the loss of aerial roots masses which were shown to provide stranding areas to propagules. Also, the hypothetical scenario that represents a minimum sea level rise of 4.8 mm.y-1 could alter the distribution pattern of the juvenile vegetation layer within a time span of 20 years, which could lead to notable floristic modifications from a regional point of view.

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In this methodological study, the applicability of aerial photographs for monitoring mangrove vegetation dynamics at high resolution was investigated. Vegetation maps of three mangrove forests in Sri Lanka (Galle, Rekawa and Pambala) were produced based on visual analysis of aerial photographs. The visual analysis was aided by applying an interpretation key constructed during a first fieldwork mission. Image attributes used for the identification of individual trees included: gray values, texture, form and size of the crowns and the presence or absence of a shaded side. For the identification of species assemblages, the vegetation structure (i.e. the distribution of individual trees) appeared to be an important attribute. The accuracy and reliability of the vegetation maps were investigated during a second fieldwork mission. The aerial photographs proved to be very useful for the production of genus-based vegetation maps. The error analysis showed that density estimations (quantitative identification) based on aerial photography was not sufficiently accurate for the objectives of the study, but that the overall identification of vegetation assemblages (qualitative identification) coincided most satisfactory with the ground-truth data. In addition to the applicability of aerial photography in monitoring mangroves, the importance of aerial photography in the management of the mangrove ecosystem is clearly highlighted.

An integrated research framework on mangrove vegetation structure dynamics, regeneration and restoration was presented in the Forests and Water Issue of ETFRN News 33 (Dahdouh-Guebas, 2001b). It discussed the broad framework in which changes in the vegetation structure of mangrove forests, amongst other research topics, were studied over several decades using remote sensing techniques. This contribution emphasizes ‘why’ such monitoring is necessary and ‘how’ this remote sensing aspect is carried out.

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.

In contrast to the large number of studies on the trophic significance of mangrove primary production to the aquatic foodweb, there have been few attempts to provide an overview of the relative importance of different primary carbon sources to invertebrates in the intertidal mangrove habitats. Mangrove sediments from three different mangrove ecosystems (Coringa Wildlife Sanctuary in the Godavari Delta, Andhra Pradesh, India, and Galle and Pambala, Sri Lanka) were analysed for their organic carbon and total nitrogen content, elemental ratios (C:N) and carbon isotopic composition. Organic carbon content (0.64 - 31.67% dry weight), C/N ratios (7.0 - 27.3) and d13C (between -29.37 and -20.61‰) showed a wide range of values. Lower stocks of organic carbon coincided with low C/N (atom) ratios and less negative d13C values, indicating import of marine or estuarine particulate suspended matter. High organic carbon stocks coincided with high C/N ratios and d13C values close, but not equal, to those of the mangrove vegetation. 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. At one such site (Coringa Wildlife Sanctuary), 23 invertebrate species and different primary producers were collected and analysed for their carbon and nitrogen stable isotopic composition (d13C, d15N) in order to determine the contribution of mangrove leaf litter and other carbon sources to the invertebrate community. ~heskn vertebrates were found to display a wide range of d13C values, most being 3-1 1‰ enriched relative to the average mangrove leaf signal. Two invertebrate species displayed remarkable stable isotope signatures: the pulmonate gastropod Onchidium sp. had extremely low d15N values (-5.58 +/- 0.86‰) and Polycladia hosting endosymbiotic algae showed low d13C values (-36.2 +/- 0.8‰), but further work is needed to adequately explain these. A compilation of stable isotope data from various sources indicates that significant assimilation of mangrove-derived carbon is only detectable in a limited number of species, and stresses that local and imported algal sources can be a major source of carbon for benthic invertebrate communities in intertidal mangrove forests. These findings show a striking similarity with results from temperate salt marsh ecosystems where local plant production has often been found to contribute little to intertidal foodwebs, and thus provide new insights into carbon utilization patterns in vegetated intertidal ecosystems.

The identification of populations of Bruguiera sexangula, Bruguiera gymnorhiza and their putative hybrids in the field is difficult using only morphological and phenological characters. Using a PCR based technique, RAPD (Random Amplified Polymorphic DNA), the genetic variation of Bruguiera populations was studied from contrasting climatic and geographic regions along the southwest coastal region of Sri Lanka. Out of 45 primers screened, 20 primers allowed us to observe polymorphism, not only between species (interspecific) but also within the species (intraspecific). Analysis of RAPD data appears to be helpful in determining the genetic relationship among populations of B. gymnorhiza and B. sexangula. RAPD markers revealed that the two species are well separated without any hybrid position between the two taxa though they occur in mixed stands. Although sampling sizes of populations of this study were small, genetic variation among B. gymnorhiza and B. sexangula populations could be observed. For B. sexangula, it was possible to differentiate each of the three populations, even when using a small number of primers.

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