Nico Koedam

Primary producers sustaining macro-invertebrate communities in intertidal mangrove forests

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. We determined carbon and nitrogen stable isotope ratios (δ13C, δ15N) in sediments, primary producers, and 22 invertebrate species from an intertidal mangrove forest located along the southeast coast of India in order to determine the contribution of mangrove leaf litter and other carbon sources to the invertebrate community. Organic matter in sediments under the mangrove vegetation was characterized by relatively high δ13C values and low C:N ratios, indicating that mangrove-derived organic matter was not the principal source and that imported phytodetritus from the mangrove creeks and adjacent bay significantly contributed to the sediment organic matter pool. Invertebrates were found to display a wide range of δ13C values, most being 3–11‰ enriched relative to the average mangrove leaf signal. The pulmonate gastropod Onchidium sp. showed unusually low δ15N values (–5.6±0.9‰), but further work is needed to adequately explain these data. 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 suggests that local and imported algal sources are a major source of carbon for benthic invertebrate communities in intertidal mangrove forests. These results provide new insights into carbon utilization patterns in vegetated tropical intertidal habitats and 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.

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.

Identification of new, conserved, non‐ribosomal peptide synthetases from fluorescent pseudomonads involved in the biosynthesis of the siderophore pyoverdine

Pyoverdines, the main siderophores of fluorescent pseudomonads, contain a peptide moiety, different for each pyoverdine, and an identical chromophore. While it has been shown that non‐ribosomal peptide synthetases (NRPSs) are involved in the biosynthesis of the peptide chain of pyoverdines, this was not demonstrated for the biosynthesis of the chromo‐phore part. We found that PvsA, from Pseudomonas fluorescens ATCC 17400, and PvdL (PA2424), from Pseudomonas aeruginosa are similar NRPSs and functional homologues, necessary for the production of pyoverdine. Transcriptional lacZ fusions showed that pvdL is co‐transcribed with the upstream PA2425 gene, encoding a putative thioesterase, and is iron‐regulated via PvdS. Similarly, RT‐PCR analysis revealed that expression of pvsA is repressed by iron. Analysis of the adenylation domains of PvsA, PvdL and their homologues, revealed that their N‐terminus starts with an acyl‐CoA ligase module, followed by three amino acid activation domains. Computer modelling of these domains suggests that PvsA in P. fluorescens and PvdL in P. aeruginosa are orthologues involved in the biosynthesis of the pyoverdine chromophore.

Involvement of Phenazines and Anthranilate in the Antagonism of Pseudomonas aeruginosa PNA1 and Tn5 Derivatives Toward Fusarium spp. and Pythium spp.

Pseudomonas aeruginosa PNA1, isolated from the rhizosphere of chickpea in India, suppressed Fusarium wilt of chickpea, caused by Fusarium oxysporum f. sp. ciceris, and Pythium damping-off of bean, caused by Pythium splendens. When grown in culture, PNA1 produced the phenazine antibiotics phenazine-1-carboxylic acid and oxychloraphine, and inhibited mycelial growth of F. oxysporum f. sp. ciceris, P. splendens, and certain other phytopathogenic fungi. Two mutants (FM29 and FM13) deficient in phenazine production were obtained following transposon mutagenesis of PNA1. The transposon in the genome of FM29 was localized to phnA, which is thought to encode a subunit of anthranilate synthase II involved in the phenazine biosynthesis. The FM13 mutation was complemented by trpC, which encodes indole glycerol phosphate synthase in the tryptophan biosynthesis pathway; consequently, FM13 could not grow on a minimal medium in the absence of tryptophan. Neither FM29 nor FM13 suppressed Fusarium wilt of chickpea to the ...

Distribution pattern of the flora in a peri-urban forest: an effect of the city–forest ecotone

We studied the spatial variation of plant assemblages in a 4383 ha deciduous forest in the periphery of Brussels. All vascular plant species were recorded following a grid-map of 1 km 2 -cells totally or partly covered by the forest. A total of 414 plant species was observed. Species composition and plant functional groups in forest edges and the neighbouring forest interior are studied. The study quantifies plant functional groups (e.g. ancient forest plants, true forest species, rare species, geophytes, C, S or R-strategists) in the plots and tests for relationships with the geographical position in the forest according to a city-to-forest gradient. Our results led us to identify distinct characteristics of forest flora based on the distance to the actual city border. The study revealed significant outer edge effects on the flora. Plant composition of the forest interior and forest edge can be separated by ordination on species composition, suggesting that these two biotopes have a distinct species assemblage. χ 2 -test and Two-Way-Indicator-Species-Analysis detected significant edge-associated species that were recorded exclusively in the edge or were the most frequent in the forest edge. We found that forest stands close to urban areas support populations of indicators of disturbance (pioneer species, C and R-strategists and aliens) to plots which are distant from the city. However, our observations also suggest that species groups with high conservation value (e.g. ancient forest species, or rare species) may also be more represented at the edge than in the core of the forest. Moreover, no forest specialists which would occur only in the interior zone were found. These observations are in disagreement with the hypothesis that true forest plants and species groups of high conservation value would be more frequent in the forest than on the borders. These findings are applicable to the management of forests in an urban context.

Mangrove species zonation and soil redox state, sulphide concentration and salinity in Gazi Bay (Kenya), a preliminary study

The relationship between soil redox state, sulphide concentration, salinity and spatial patterns of mangrove species distribution was investigated in the mangrove forest of Gazi Bay (Kenya). Field measurements were conducted to examine the relationship between species distribution along a band transect of 280 m and soil redox potential (Eh) and sulphide patterns, as well as the indirectly related (through flooding regimes) soil salinity. Of the three major species Avicenniamarina, Ceriops tagal and Rhizophoramucronata present along the transect, only the distribution of the latter correlated with the measured soil variables, R. mucronata being absent from the less‐reduced zone with high salinity. Bruguieragymnorhiza and Heritieralittoralis occur in minor populations, they are restricted to the saline, sulphide‐poor and less‐reduced substrates. From the results it is concluded that soil redox potential (Eh), sulphide concentration and salinity may contribute to structure mangroves through the distribution of dominant species, however in combination with other environmental conditions and processes of vegetation dynamics.

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.