Mangrove Organic Matter Research Articles

Factors influencing mangrove carbon storage and its response to environmental stress

Mangrove forests serve as significant carbon sinks and play a crucial role in mitigating climate change. Currently, the response of mangroves to intensified climate change and human activities, and the factors that influence the magnitude of carbon storage in their sediments remain uncertain. To address these questions, two sediment cores were collected from the mangrove reserve in Pearl Bay, Guangxi, China. The activity of 210Pb in the sediment, grain size, bulk elemental composition, stable carbon isotopes, lignin, and different sediment organic matter (OM) fractions were investigated to determine the local mangrove’s response to climate change and human activities, as well as the factors influencing its carbon storage. The results showed mangrove forests with lower tidal ranges, slower sedimentation rates, and where OM predominantly originated locally tend to have larger carbon stocks. The mangrove OM (MOM) decreased progressively from the bottom to the top of the cores, indicating that the mangroves in Pearl Bay have possibly undergone degradation, which was further substantiated by the decrease in lignin content. Based on these results, the entire cores were divided into two stages: stable stage 1 (1963–2001) and degradation stage 2 (2001–2020). The cause of the mangrove degradation is likely due to the impact of human activities; however, these impacts are anticipated to gradually lessen in the future due to mangrove protection policies. Our results indicate that lignin can track and predict mangrove growth trends and provide guidance for the sustainable management of mangrove ecosystems.

Taraxerol abundance as a proxy for in situ mangrove sediment

Mangrove sediments are valuable archives of relative sea-level change if they can be distinguished in the stratigraphic record from other organic-rich depositional environments (e.g., freshwater swamps). Proxies for establishing environment of deposition can be poorly preserved (e.g., foraminifera) in mangrove sediment. Consequently, differentiating mangrove and freshwater sediment in the stratigraphic record is often subjective. We explore if biomarkers can objectively identify mangrove sediment with emphasis on their utility for reconstructing relative sea level. Our approach is specific to identifying in situ sediment, which has received less attention than identifying allochthonous mangrove organic matter. To characterize mangrove and non-mangrove (freshwater) environments, we measured n-alkane, sterol, and triterpenoid abundances in surface sediments at three sites in the Federated States of Micronesia. Elevated taraxerol abundance is diagnostic of sediment accumulating in mangroves and taraxerol is particularly abundant beneath monospecific stands of Rhizophora spp. Taraxerol was undetectable in freshwater sediment. Other triterpenoids are more abundant in mangrove sediment than in freshwater sediment. Using cores from Micronesian mangroves, we examine if biomarkers in sediments are indicative of in situ deposition in a mangrove, and have utility as a relative sea-level proxy. Taraxerol concentrations in cores are comparable to surface mangrove sediments, which indicates deposition in a mangrove. This interpretation is supported by pollen assemblages. Downcore taraxerol variability may reflect changing inputs from Rhizophora spp. rather than diagenesis. We propose that taraxerol is a proxy that differentiates between organic sediment that accumulated in mangrove vs. freshwater environments, lending it utility for reconstructing relative sea level.

Selective associations of organic matter components with ferrihydrite: Implications for Fe-organic matter preservation in tidal flat wetlands

The association of organic matter (OM) with iron (Fe) is a critical mechanism to preserve OM in intertidal coastal ecosystems such as salt marshes and mangroves. The present study aimed to investigate the influence of the organic carbon to Fe molar ratio (OC/Fe), ionic strength, and OM composition on Fe-OM association. Fe-OM was prepared using humic acids extracted from salt marsh and mangrove sediments by the adsorption process in the OC/Fe molar ratios of 0.01–1.0 and by the coprecipitation process in the OC/Fe molar ratios of 0.1–9.9. Sample properties were studied by X-ray diffraction, scanning electron microscopy, ultraviolet–visible spectroscopy, attenuated total reflection Fourier transform infrared spectroscopy, three-dimensional fluorescence excitation emission matrix spectroscopy, and pyrolysis–gas chromatography/mass spectrometry. The results showed that selective associations of OM components with ferrihydrite were affected by the association mode (e.g., adsorption and coprecipitation), OC/Fe molar ratio, OM composition and ionic strength. Abundant aliphatic compounds could be adsorbed on ferrihydrite. Moreover, ferrihydrite was prone to preserve humic-like fluorescent components through the adsorption process. The protein-like fluorescent components and aliphatic compounds had high affinity for ferrihydrite during coprecipitation with low OC/Fe molar ratios. An increase in the OC/Fe molar ratio resulted in coprecipitation of aromatic compounds with ferrihydrite. Salinity-induced flocculation caused more amount of aromatic and polycyclic aromatic compounds to associate with ferrihydrite during the coprecipitation process. Compared to the salt marsh OM, more N-compounds and less aliphatic compounds in the mangrove OM were coprecipitated with ferrihydrite. This study provided important insights into the mechanisms of OM preservation with ferrihydrite in tidal wetland ecosystems.

Dissolved and particulate carbon export from a tropical mangrove‐dominated riverine system

AbstractDespite being a major component in the mangrove carbon (blue carbon) budget, “outwelling” flux (or export to the sea) has gained little attention relative to other biogeochemical fluxes and reservoir carbon stock estimations. This study aims to estimate lateral exchange fluxes of dissolved and particulate organic carbon (DOC, POC) and dissolved inorganic carbon (DIC) from the watershed through a microtidal mangrove‐dominated estuary to the coastal sea in Panay Island, Philippines. Along the estuarine transect, consistent addition of DOC, DIC, and POC at higher salinities were attributed to mangrove organic matter input. Upstream groundwater input (carbonate weathering) and downstream mangrove organic matter decomposition (possibly sulfate reduction) were the main controls on DIC. DOC corresponded to relatively pure mangrove sources in creek water, while POC was a mixture of detrital and algal organic matter. The mangrove system acted as net exporter of carbon to the sea in both dry and wet seasons. From short‐term observations, outwelling fluxes of mangrove‐derived DOC, DIC, and POC contributed 27–53%, 8–31%, and 42%, respectively, to their estuarine outflow. Unlike other studies, such low percentage for DIC might result from other external nonmangrove input (e.g., watershed carbonate weathering). Overall estuarine carbon flux was dominated by DIC (90–95%) with only minor contribution from DOC. The approach utilized in this study to estimate lateral carbon flux specific to a small mangrove setting can be useful in delineating blue carbon budgets that avoid geographical and methodological biases.

Metal Binding and Sources of Humic Substances in Recent Sediments from the Cananéia-Iguape Estuarine-Lagoon Complex (South-Eastern Brazil)

The Cananéia-Iguape estuarine–lagoon complex (São Paulo state, Brazil) is a natural laboratory to study metal binding by humic substances (HS) in subtropical settings. This transitional environment is evolving into a freshwater environment due to water input from the Ribeira River, funneled through the Valo Grande Canal (Iguape). Past mining activities in the Ribeira River basin and maritime traffic are suspected to be potential sources of trace metals in the system. In this study, the trace metal contents of Free Humic Acids (FHA), Bound Humic Acids (BHA), and Fulvic Acids (FA) extracted from sedimentary organic matter were investigated. Moreover, the sources of HS were traced using their stable carbon isotope compositions and C/N ratios. The results suggested a mixed marine–terrestrial source of FHA, BHA, and FA. Copper and Cr were the most abundant trace metals bound to HS. On average, Cu showed concentrations of 176, 115, and 37.9 μg g−1 in FHA, BHA, and FA, respectively, whereas Cr showed average concentrations of 47.4, 86.3, and 43.9 μg g−1 in FHA, BHA, and FA, respectively. Marine FHA showed the highest binding capacity for trace metals, whereas terrestrial FA derived from the decay of mangrove organic matter showed the lowest binding capacity.

Molecular Identification of Cellulolytic Bacteria From Mangrove Sediment at Tin Minning Region In West Bangka

Cellulose as an abundant source of glucose in Indonesia requires acceleration of decomposition utilizing cellulolytic bacteria. Cellulolytic bacteria can be obtained from the isolation of mangrove organic matter, such as sediments. Muntok Sub-district is one of the regions with the most tin mining in West Bangka Regency also has mangroves in the coastal area. Exploration of cellulolytic bacteria in mangroves with different environmental characteristics encourages researchers to find new bacterial strains that produce cellulase enzymes with new properties. Thirteen isolates were successfully isolated from three locations. Tembelok mangrove sediments produced Seven bacterial isolates, Peltim Mangrove samples produced three isolates and from Sukal Mangrove three isolates were obtained. Seven isolates showed clear zones in the Lugol test and three isolates including were gram-positive bacteria. Molecular test with 16S rRNA analysis showed TBL1 isolate has 85% similar identity of Vibrio parahaemolyticus strain HY3 and TBL2 isolate has 98% similar identity of Bacillus amyloliquefaciens strain HS8. Bacillus amyloliquefaciens potential to further study as cellulose degrading bacteria for feed ingredients.

Variability in the organic carbon stocks, sources, and accumulation rates of Indonesian mangrove ecosystems

Mangrove ecosystems are an important natural carbon sink that accumulate and store large amounts of organic carbon (Corg), in particular in the sediment. However, the magnitude of carbon stocks and the rate of carbon accumulation (CAR) vary geographically due to a large variation of local factors. In order to better understand the blue carbon sink of mangrove ecosystems, we measured organic carbon stocks, sources and accumulation rates in three Indonesian mangrove ecosystems with different environmental settings and conditions; (i) a degraded estuarine mangrove forest in the Segara Anakan Lagoon (SAL), Central Java, (ii) an undegraded estuarine mangrove forest in Berau region, East Kalimantan, and (iii) a pristine marine mangrove forest on Kongsi Island, Thousand Islands, Jakarta. In general, Corg stocks were higher in estuarine than in marine mangroves, although a large variation was observed among the estuarine mangroves. The mean total Corg stock in Berau (615 ± 181 Mg C ha−1) is twice as high as that in SAL (298 ± 181 Mg C ha−1). However, the Segara Anakan Lagoon displayed large within-system variation with a much higher Corg stock in the eastern (483 ± 124 Mg C ha−1) than in the central lagoon (167 ± 36 Mg C ha−1). The predominant accumulation of autochthonous mangrove organic matter likely contributed to the higher Corg stocks in Berau and the eastern SAL. Interestingly, the CAR distribution pattern in SAL is opposite to that of its Corg stocks. The central SAL that receives high sediment inputs from the hinterland has a much higher CAR than the eastern SAL (658 ± 311 g C m−2 yr−1 and 194 ± 46 g C m−2 yr−1, respectively), while Berau has one of the highest CAR (1722 ± 183 g C m−2 yr−1) ever measured. It appears that these large differences are driven by the environmental setting and conditions, mainly sediment dynamics and hydrodynamics, landform, and vegetation conditions. It is inferred that quantifying carbon accumulation in sediments is a useful tool in estimating the present-day carbon storage of mangrove ecosystems. This is a precondition for taking measures under REDD+ (Reducing Emissions from Deforestation and Forest Degradation and the role of conservation, sustainable management of forests and enhancement of forest carbon stocks in developing countries) schemes.

Exchange between macrophyte and mangrove organic matter input in estuarine systems: drought implications

Paq proxy was applied on mid and long-chain n-alkanes in order to develop an index to assist in the long-term assessment of changes and exchanges between aquatic communities in estuarine environments as a result of increased saline incursion. New boundaries are proposed according to n-alkanes abundance in the leaves of local mangrove tree species for better definition of emergent aquatic plant end-member. Molecular distribution and δ13C isotopic signatures of n-alkanes were also used to support the parameter. A predominance of odd carbon-numbered long chain (> C27 n-alkanes) indicated a main terrestrial input, although mid-chain n-alkanes (23 and 25 carbons) were also identified. Short-chain n-alkanes (< 19) were absent or found in a very small concentration. Carbon Preference Index and C/N ratio reinforced a terrestrial origin for the sedimentary organic matter. δ13C signatures of n-alkanes for C23, C25, C29 and C31 were characteristic of mangrove input, ranging from -27.57 ‰ (C23) to -34.40 ‰ (C29) for Piraquê-Mirim River (PM) and from -27.36 ‰ (C23) to -32.84 ‰ (C31) for Piraquê-Açu River (PA) without any particular trend. Paq proxy was successfully used to identify rises in marine macrophytes (mid-chain n-alkanes) input mainly in the confluence and PM probably as consequence of drought periods. The confluence presented a mixing sources of OM. PA was identified as a stable mangrove environment while PM was characterized by pulses of marine macrophytes input and should be managed with care in respect to its catchment for water use and dam constructions as a shortage of fresh water can alter the estuarine ecology.

Top-meter soil organic carbon stocks and sources in restored mangrove forests of different ages

Knowledge of the changes in the soil organic carbon (OC) stock and its sources with forest age is crucial to understanding the carbon processes and carbon sequestration benefits associated with mangrove restoration. The present study compared the soil OC contents and stocks among three restored Kandelia obovata mangrove forests with different ages (12, 24 and 48 years) in South China. The contributions of mangrove and allochthonous suspended particulate organic matter (SPOM) in tidal water to soil OC were also estimated. The results showed increases in soil OC content and density with forest age, and the OC stocks in the top meter of soil were 13.0 ± 0.5 kg C m−2, 15.1 ± 0.7 kg C m−2 and 17.0 ± 0.9 kg C m−2 at the 12-year site, 24-year site and 48-year site, respectively. We also found a higher soil labile OC content at the 48-year site. The higher soil OC content was accompanied by more 13C-depleted OC in the older forests, and the significantly negative relationship between soil OC contents and δ13C values indicated that the increase in soil OC content relied on the input of 13C-depleted mangrove organic matter. Isotope mixing calculations showed that the soil OC was dominated by SPOM at the 12-year site but was dominated by both mangrove-derived OC and SPOM at the 48-year site. Moreover, the findings also suggested that the effect of mangrove restoration on the soil OC content and stock was more substantial in the upper soil layers.

Sedimentary composition and organic carbon sources in mangrove forests along the coast of northeast Vietnam

The objective of this research was to examine the patterns of sedimentation and sedimentary organic carbon content in mangrove forests along the coast of northeast Vietnam. Stable isotope ratios (δ13C), C/N ratios, and sediment texture (sediment grain size, sorting, skewness and sediment facies) were analyzed in three sediment cores from different mangrove forests. Results showed that sediments were mainly composed of fine sand, silt, and clay. Mud content decreased from estuarine islets through to the fringe, and to riverine mangrove forests. Total organic carbon (TOC) content, δ13C values, and C/N ratios ranged from 0.3 to 6.8%, –30.1 to −24.9‰, and 12 to 42, respectively. Results from bivariate analysis of sedimentary parameters clearly indicated that hydrological energy in the riverine mangrove forest was relatively lower than that observed from the fringe and estuarine islet mangrove forests. Co-variation between δ13C and C/N ratios showed that sedimentary organic carbon in riverine mangrove forests mainly originated from mangrove organic matter, while fringe and estuarine islet mangrove forests tended to accumulate higher proportions of marine phytoplankton. Results underscored the significance of mangrove stands and geomorphological settings in influencing sedimentation and organic carbon burial processes in mangrove sediments.

Carbon accumulation and storage capacity in mangrove sediments three decades after deforestation within a eutrophic bay

A dated sediment core from an eutrophic mangrove area presented non-significant differences in carbon accumulation rates before (55.7±10.2gm−2yr−1) and after three decades of deforestation (59.7±7.2gm−2yr−1). Although eutrophication effects appear to compensate the loss of mangrove organic matter input, the results in this work show a threefold lower carbon accumulation than the global averages estimated for mangrove sediments. The effects of increasing eutrophication and enhanced sediment dry bulk density observed after deforestation (~30% higher) did not result in higher carbon stocks. Moreover, the lower TOC:OP (<400) and C:N (~20) molar ratios, as well as increased nutrient accumulation, reflect the dominance of phytoplankton-derived organic matter after deforestation, resulting in less-efficient sedimentary carbon sinks. These results indicate that the organic material deposited from eutrophication may not compensate mangrove deforestation losses on carbon accumulation in mangrove ecosystems.

Geochemical behavior of phosphorus and iron in porewater in a mangrove tidal flat and associated phosphorus input into the ocean

A thorough understanding of the distribution and geochemical behavior of phosphorus (P) and iron (Fe) across the sediment-water interface (SWI) is essential for estimating the exchange flux between sediment porewater and overlying seawater. In this study, in situ high-resolution diffusive gradients in thin films (DGT) technique, ZrO-Chelex DGT, were applied to investigate the depth profiles of dissolved reactive phosphorus (DRP) and ferrous iron (Fe2+) in sediment porewater in a mangrove tidal flat of the Jiulong River estuary, China. The concentrations of DRP and Fe2+, which ranged from 0.014 to 0.288mgL−1 and 0.003–0.619mgL−1, respectively, were highly heterogeneous across these profiles. The positive correlation between DRP and Fe2+ concentrations in most parts of the profiles verified the key role of Fe-redox cycling in controlling DRP variations. However, in some parts of the profiles, this coupling relationship was disturbed by the enrichment of mangrove organic matter or biological activity, such as that of macrobenthos and microorganisms. The recirculated seawater fluxes driven by tides and wave setups were calculated to be 23.15m3 m−1 d−1 and 1.49 × 10−3 m3 m−1 d−1, respectively. Based on these calculated fluxes and end-member concentrations, the net DRP flux into the ocean was calculated to be approximately 1.57mmolm−2 d−1, which is much larger than the molecular diffusion flux of DRP (1.79 × 10−3mmolm−2 d−1). These results indicate that the advection of recirculated seawater is a significant pathway to deliver DRP into the ocean and that this kind of internal phosphorus from mangrove tidal flats or muddy coastal zones should be considered in future nutrient budgets of estuaries or coastal oceans.

Small-scale variability in geomorphological settings influences mangrove-derived organic matter export in a tropical bay

Abstract. Organic matter (OM) exchanges between adjacent habitats affect the dynamics and functioning of coastal systems, as well as the role of the different primary producers as energy and nutrient sources in food webs. Elemental (C, N, C : N) and isotope (δ13C) signatures and fatty acid (FA) profiles were used to assess the influence of geomorphological setting in two climatic seasons on the export and fate of mangrove OM across a tidally influenced tropical area, Gazi Bay (Kenya). The main results indicate that tidal transport, along with riverine runoff, plays a significant role in the distribution of mangrove organic matter. In particular, a marked spatial variability in the export of organic matter from mangroves to adjacent habitats was due to the different settings of the creeks flowing into the bay. Kinondo Creek acted as a mangrove retention site, where export of mangrove material was limited to the contiguous intertidal area, while Kidogoweni Creek acted as a flow-through system, from which mangrove material spreads into the bay, especially in the rainy season. This pattern was evident from the isotopic signature of primary producers, which were more 13C-depleted in the Kinondo Creek and nearby, due to the lower dilution of the dissolved inorganic carbon (DIC) pool, typically depleted as an effect of intense mangrove mineralisation. Despite the trapping efficiency of the seagrass canopy, suspended particulate OM showed the important contribution of mangroves across the whole bay, up to the coral reef, as an effect of the strong ebb tide. Overall, mixing model outcomes indicated a widespread mixed contribution of both allochthonous and autochthonous OM sources across Gazi Bay. Moreover, FAs indicated a notable contribution of brown macroalgae and bacteria in both sediment and suspended pools. These results suggest that ecological connectivity in Gazi Bay is strongly influenced by geomorphological setting, which may have far-reaching consequences for the functioning of the whole ecosystem and the local food webs.

Genetic diversity and population structure among sorghum (Sorghum bicolor, L.) germplasm collections from Western Ethiopia

The mangrove sediments are rich in organic matter and humic substances, responsible for important functions such as reducing the toxicity of heavy metals, nutrient stabilization, serving as drain to atmospheric carbon and increasing the plant growth. In this study, we observed the effect of humic substances, humic acids and fulvic acids isolated from sediments of a mangrove forest, sampled from the Municipal Ecological Station Ilha do Lameirao (EEMIL, located in Vitoria, Espirito Santo State, Brazil), on the growth and root acidification of Rizophora mangle and Laguncularia racemosa seedlings. For this, R. mangle and L. racemosa propagules were transferred to pots and then treated with different humic materials. The evaluation in root architecture change was performed by analyzing the main root axis length, lateral root length, density of lateral roots, fresh and dry mass weight of roots and estimated H + -ATPase activity by specific root acidification measurement. The results indicate that all humic materials extracted from mangrove organic matter were able to modify the root architecture systems of the studied plants. In addition to inducing an increase in the number of lateral roots and root branching, it also stimulated specific root acidification when compared to control groups. There is a good potential in developing technologies for the production of seedlings of mangrove plant species treated with biostimulants based in humic materials isolated from the mangrove ecosystem itself. Keywords: Humic substances, humic acids, fulvic acids, mangrove

Mercury dilution by autochthonous organic matter in a fertilized mangrove wetland

A dated sediment core from a highly-fertilized mangrove wetland located in Cubatão (SE Brazil) presented a negative correlation between mercury (Hg) and organic carbon contents. This is an unusual result for a metal with well-known affinity to organic matter. A dilution of Hg concentrations by autochthonous organic matter explained this observation, as revealed by carbon stable isotopes signatures (δ13C). Mercury dilution by the predominant mangrove-derived organic matter counterbalanced the positive influences of algal-derived organic matter and clay contents on Hg levels, suggesting that deleterious effects of Hg may be attenuated. Considering the current paradigm on the positive effect of organic matter on Hg concentrations in coastal sediments and the expected increase in mangrove organic matter burial due to natural and anthropogenic stimulations of primary production, predictions on the influences of organic matter on Hg accumulation in mangrove wetlands deserve caution.

Sources and distributions of terrigenous organic matter in a mangrove fringed small tropical estuary in South China

The sources and distributions of terrigenous organic matter (OM) were investigated in a small tropical estuary in the Hainan Island, South China. Plants, suspended particulate matter (SPM), and surface sediments samples in the estuary and coast were collected. Bulk properties [organic carbon (OC%), total nitrogen (TN%), stable carbon isotope (δ 13C) and grain size] and lignin phenol concentrations were measured. OC% of mangrove plants was (43.4±2.1)%, which is similar to the values reported form an grove plants in other regions. OC% of sediment samples ranged from 0.07% to 1.42%, and they were related to the sediment texture. Lignin phenols in the sediment ranged from 5.16 mg/100 mg OC in the uppermost station to 0.51 mg/100 mgOC in the coast. The molar ratio of organic carbon to total nitrogen(C/N) (∼7) and δ 13C (∼−31.1×10−3) of riverine SPM revealed that the major OM sources of riverine SPM were aquatic OM (phytoplankton and/or bacteria). Moreover, the lower lignin concentration (Λ8) and higher (Ad/Al)v of lignin phenols suggest that terrestrial OM in riverine SPM were mainly from soil. Further more, C/N ratio, δ 13C and lignin phenols reveal that mangrove plants were the predominant OM sources of mangrove surface sediment. Based on the δ 13C and lignin phenols, it can be concluded that the major OM sources in estuarine and coastal surface sediments were marine phytoplankton, riverine SPM and mangrove surface sediment. In addition, the higher (Ad/Al)v of lignin phenols in those coastal sediments indicate that sea grass might be a potential OM source in coastal sediments, however, the lower (Ad/Al)v in the estuarine sediments in turn suggests that sea grass could not be transported to the mangrove fringed region. A three-end-member model which is based on lignin concentrations and δ 13C was applied to evaluate the contribution of mangroves to the organic matter preserved in the surface sediments. Around the mangrove fringed region, mangrove could contribute more than 50% to the sedimentary OM, and this value is much higher than riverine OM. Nevertheless, mangrove OM could not be efficiently transported to the coastal region. Our study suggests that mangrove forest is an important OM source in this small estuary.

Importance of seagrass as a carbon source for heterotrophic bacteria in a subtropical estuary (Florida Bay)

A stable carbon isotope approach was taken to identify potential organic matter sources incorporated into biomass by the heterotrophic bacterial community of Florida Bay, a subtropical estuary with a recent history of seagrass loss and phytoplankton blooms. To gain a more complete understanding of bacterial carbon cycling in seagrass estuaries, this study focused on the importance of seagrass-derived organic matter to pelagic, seagrass epiphytic, and sediment surface bacteria. Particulate organic matter (POM), seagrass epiphytic, seagrass ( Thalassia testudinum) leaf, and sediment surface samples were collected from four Florida Bay locations with historically different organic matter inputs, macrophyte densities, and primary productivities. Bulk (observed and those reported previously) and compound-specific bacterial fatty acid δ 13C values were used to determine important carbon sources to the estuary and benthic and pelagic heterotrophic bacteria. The δ 13C values of T. testudinum green leaves with epiphytes removed ranged from −9.9 to −6.9‰. Thalassia testudinum δ 13C values were significant more enriched in 13C than POM, epiphytic, and sediment samples, which ranged from −16.4 to −13.5, −16.2 to −9.6, and −16.7 to −11.0‰, respectively. Bacterial fatty acid δ 13C values (measured for br14:0, 15:0, i15:0, a15:0, br17:0, and 17:0) ranged from −25.5 to −8.2‰. Assuming a −3‰ carbon source fractionation from fatty acid to whole bacteria, pelagic, epiphytic, and sediment bacterial δ 13C values were generally more depleted in 13C than T. testudinum δ 13C values, more enriched in 13C than reported δ 13C values for mangroves, and similar to reported δ 13C values for algae. IsoSource mixing model results indicated that organic matter derived from T. testudinum was incorporated by both benthic and pelagic bacterial communities, where 13–67% of bacterial δ 13C values could arise from consumption of seagrass-derived organic matter. The IsoSource model, however, failed to discriminate clearly the fraction of algal (0–86%) and mangrove (0–42%) organic matter incorporated by bacterial communities. These results indicate that pelagic, epiphytic, and sediment surface bacteria consumed organic matter from a variety of sources. Bacterial communities incorporated consistently seagrass-derived organic matter, the dominant macrophyte in Florida Bay, but seagrass δ 13C values alone could not account fully for bacterial δ 13C values.

Role of grapsid crabs, Parasesarma erythrodactyla, in entry of mangrove leaves into an estuarine food web: a mesocosm study

The influence of the crab Parasesarma erythrodactyla on the entry of the organic matter derived from Avicennia marina mangrove leaves in a sub-tropical mangrove ecosystem of southeast Queensland, Australia, was simulated using tidal mesocosms. Degradation of mangrove leaf organic matter was followed by analysing the fatty acid composition, carbon, and nitrogen isotopic signatures of the surface sediment and suspended particulate organic matter (SPOM) with and without the presence of crabs. Assimilation of mangrove organic matter by P. erythrodactyla was also assessed by stable isotope and fatty acid analyses in tissues and faeces. Results of the chemical tracer analyses question the adaptability of P. erythrodactyla to a diet comprised exclusively of mangrove leaves, and suggest that these organisms were dependent on additional food sources in their natural environment. Crab processing of senescent leaves significantly accelerated the transfer of mangrove organic matter to the surface sediments, as shown by a higher C/N ratio, a higher contribution of long-chain fatty acids and a more depleted C isotopic signature of sediment samples in the mesocosms with crabs compared to those without crabs.

Mangrove macrobenthos: Assemblages, services, and linkages

Macrobenthic assemblages are relatively poorly known compared to other components of the mangrove ecosystem. Tropical mangroves support macrobenthic biodiversity resources yet to be properly documented and interpreted. Some methodological challenges, such as the generally high spatial heterogeneity and complexity of the habitat, evidently reduce sampling efficiency and accuracy, while also leaving some microhabitats under-sampled. Macrobenthic assemblage structure seems to be influenced by local environmental conditions, such as hydroperiod, organic matter availability and sediment characteristics. Brachyurans, gastropods and oligochaetes dominate in the sediment, with the former two groups also common on hard surfaces provided by tree trunks, while insects and arachnids inhabit the canopy. Traditionally, studies of mangrove macrobenthos have focused on assemblage structure or the biology of individual species, but more complex inter-specific interactions and the inter-relationship between habitat and the biota are recently being addressed. Brachyuran crabs are the best-studied macrobenthos group, but many issues about their role in mangrove ecosystem dynamics are still controversial. Despite many species of mangrove macrobenthos being referred to as ‘trophic dead ends’, most serve as important links between recalcitrant mangrove organic matter and estuarine secondary production, through feeding excursion by mobile nekton during the high tide, and macrobenthos-mediated processing and exportation of organic matter. A significant difference in the standing crop biomass of forests between the Indo-west-Pacific (IWP)' and Atlantic-east-Pacific (AEP) mangroves may be related to the difference in species richness of mangrove as well as macrobenthos diversity in the two bioregions. Such differences in assemblage structure may also result in different ecosystem functioning, but the nature of the links is, however, yet to be explored. There is also a strong need for evaluating mangrove macrobenthos assemblages as a component of the connected biotic resources in the land-sea continuum, rather than as an isolated faunal group.

Inter-specific and geographical variations in the fatty acid composition of mangrove leaves: implications for using fatty acids as a taxonomic tool and tracers of organic matter

Fatty acid compositions of the leaves of six species of mangroves were studied to ascertain their use as biomarkers for determining the fate of mangrove organic matter in the habitat and as taxonomic tool. Mangrove leaves were collected from three locations in the western Pacific Ocean: Moreton Bay (MB) (Australia), Hong Kong (China) and Okinawa (Japan). In MB, samples were collected from two sites separated by 15 km: Logan River Estuary (LRE) and Jabiru Island. In addition, along the LRE, leaves were collected from five stations at ∼2–3 km apart. Results show that the analysis of the entire fatty acid profiles of the mangrove leaves is a promising taxonomic tool as the profiles of most species were sufficiently different to be separated in an non-metric multidimensional scaling plot. In addition, geographically separated populations of the same species could also be identified by their fatty acid profiles. In most cases, two non-ubiquitous groups of fatty acids dominated in the mangrove leaves: the polyunsaturated 18:2ω6 and 18:3ω3 and the long chain fatty acids (≥24:0). With respect to the relative contributions of these fatty acids, three groups of species were identified, in which one or both groups of fatty acids may potentially be used as markers of the mangrove organic matter in the estuarine environment.