Intertidal Mangrove Sediments Research Articles

Deciphering environmental forcings in the distribution of meiofauna and nematodes in mangroves of the Atlantic-Caribbean-East Pacific and Indo-West Pacific regions

Mangroves develop under environmental conditions and anthropogenic pressures whose impact on benthic meiofauna remains poorly understood. It is unclear how meiofauna communities are structured according to local sedimentary conditions. This study was designed to characterize the community structure of meiofauna and nematodes (dominant taxa) and the associated environmental forcings in intertidal mangrove sediments from Mayotte (Indo-West-Pacific), Martinique and Guadeloupe (Caribbean). Sediment cores were sampled at the end of the dry season at low tide on adult mangrove stands with similar immersion time. In each sediment layer, we analyzed redox potential, pH, porewater salinity, grain size, organic matter, metals, organic contaminants, prokaryotes and meiofauna. Our results show that sediments far from cities and agricultural fields trapped site-specific contaminants due to local water transport processes. Some metals, PAHs or pesticides exceeded toxicity thresholds in most of the studied stations, thus being harmful to benthic fauna. The sedimentary environment acts as a filter selecting specific meiofauna communities at station scale only in the Caribbean. In Mayotte, horizontal homogeneity contrasts with vertical heterogeneity of the sedimentary environment and the meiofauna. Nematode genera showed particular distribution patterns horizontally and vertically, suggesting the presence of sediment patches suitable for a restricted pool of genera on each island. Results in the Caribbean are consistent with nested diversity patterns due to environmental filtering. Conversely, horizontal homogeneity at Mayotte would reflect greater dispersal between stations or more spatially homogeneous anthropogenic pressures. The nematode genera present at depth may not be the most specialized, but the most versatile, capable of thriving in different conditions. Terschellingia and Daptonema showed contrasted responses to environmental forcing, likely due to their versatility, while Desmodora showed uniform responses between study areas, except when toxicity thresholds were exceeded. Our results emphasize that a given genus of nematode may respond differently to sedimentary conditions depending on sites.

Effects of polypropylene microplastics on carbon dioxide dynamics in intertidal mangrove sediments

Microplastics (MPs) in soil can influence CO2 dynamics by altering organic carbon (OC) and microbial composition. Nevertheless, the fluctuation of CO2 response attributed to MPs in mangrove sediments is unclear. This study explores the impact of micro-sized polypropylene (mPP) particles on the carbon dynamics of intertidal mangrove sediments. In the high-tide level sediment, after 28 days, the cumulative CO2 levels for varying mPP dosages were as follows: 496.86 ± 2.07, 430.38 ± 3.84 and 447.09 ± 1.72 mg kg−1 for 0.1%, 1% and 10% (w/w) mPP, respectively. The CO2 emissions were found to be increased with a 0.1% (w/w) mPP level and decreased with 1% and 10% (w/w) mPP at high-tide level sediment, suggesting a tide level-specific dose dependence of the CO2 emission pattern in mangrove sediments. Overall, results indicated that the presence of mPP in mangrove sediments would potentially affect intertidal total CO2 storage under given experimental conditions.

Degradation of BDE-47 in mangrove sediments under alternating anaerobic-aerobic conditions

Polybrominated diphenyl ethers (PBDEs) resistant to degradation have significant environmental impacts. Anaerobic reductive debromination and aerobic oxidation of PBDEs by microorganisms are main removal mechanisms during natural attenuation, but previous studies often focused on the process under either aerobic or anaerobic condition leading to unsatisfactory removal. The present study aims to remove PBDEs by employing alternating anaerobic-aerobic condition, which is common in inter-tidal mangrove sediments, and elucidate the degradation pathways. During 40-week experiment, BDE-47 reduced with an accumulation of tri-BDEs and di-BDEs as debromination products in all sediments. However, the removal percentages of BDE-47 and the concentrations of debromination congeners varied among flushing regimes. Sediments under less frequent flushing regime (longer duration of aerobic period) had significantly lower concentration and proportion of debromination products, especially BDE-17, than that under more frequent regime (longer anaerobic period). BDE-17 then went through aerobic degradation pathway, as evidenced by the accumulation of its hydroxylation form. Microbial analyses further revealed that less frequent regime favored accumulation of biphenyl dioxygenase gene for aerobic degradation, while more frequent tidal regime promoted growth of dehalogenating bacteria for reductive debromination. This study first time demonstrated that PBDEs in contaminated sediments could be removed under alternating anaerobic-aerobic conditions.

Tidally driven sulfidic conditions in Peruvian mangrove sediments

The seasonal influence of tidal regimes on sulfidic conditions was studied in intertidal environments from a mangrove estuary in Northern Peru. Along two sampling stations, creek water and sediment cores were collected during the dry and wet seasons at all tidal phases (ebb, low, flow, and high tides). Physical-chemical parameters were measured in the creek water (temperature, salinity, pH, Eh, and DO), whereas pH, redox potential (Eh), and total organic matter contents were obtained from the sediment cores. In addition, total dissolved sulfide content ∑ (H2S, HS−, H2−) was measured from sediment pore water. During the dry and wet seasons, the creek water pH, Eh, and dissolved oxygen were lowest in low tide, whereas oxygenated conditions and higher pH and Eh values prevailed in high tide. The total organic matter content in sediments was higher during the dry season, with the highest contents observed in the seaward station. Higher average ∑H2S (landward station, 243.1 ± 234.9 μM L−1; seaward station, 544.9 ± 174.4 μM L−1) were noted during wet season compared to dry season (landward station, 5.3 ± 4.5 μM L−1; seaward station, 430.2 ± 435.1 μM L−1). These ∑H2S contents increased towards the bottom of the sediment column, reflecting the anaerobic decomposition of the organic matter and sulfate reduction. This study provides insight to the geochemical dynamics of intertidal mangrove sediments that are sensitive to fluctuating reducing and sulfidic conditions, oscillating at time scales of minutes to hours.

Mangrove Methane Biogeochemistry in the Indian Sundarbans: A Proposed Budget

Biogeochemical cycling of CH4 was investigated at Lothian Island, one of the relatively pristine islands of Indian Sundarbans and its adjacent Saptamukhi estuary, during June 2010 to December 2012. Intertidal mangrove sediments were highly anoxic and rich in organic carbon. Mean rates of methanogenesis were 3547 and 48.88 µmol m-3 wet sediment d-1, for intertidal (up to 25 cm depth) and sub-tidal sediments (first 5 cm depth), respectively. CH4 in pore-water was 53.4 times more supersaturated than in adjacent estuarine waters. This resulted in significant CH4 efflux from sediments to estuarine waters - via advective and diffusive transport. About 8.2% of the total CH4 produced in intertidal mangrove sediments was transported to the adjacent estuary through advective flux, which was 20 times higher than diffusive CH4 flux. Mean CH4 concentrations in estuarine surface and sub-surface waters were 69.9 and 56.1 nM, respectively, with a dissolved CH4 oxidation rate in estuarine surface waters of 20.5 nmol L-1 d-1. An estimated 0.09 Gg yr-1 of CH4 is released from estuaries of Sundarbans to the regional atmosphere. The mean CH4 mixing ratio over the forest atmosphere was 2 ppmv. On annual basis, only 2.75% of total supplied CH4 to the forest atmosphere was transported to the upper atmosphere via biosphere-atmosphere exchange. Mean CH4 photo-oxidation rate over the forest atmosphere was 3.25 x 10-9 mg cm-3 d-1. Using new and and previously published data we present for the first time, a CH4 budget for Sundarbans mangrove ecosystem which in part, revealed the existence of anaerobic CH4 oxidation in the mangrove sediment column.

Acidic drainage drives anomalous rare earth element signatures in intertidal mangrove sediments

Sedimentary rare earth element (REE) signatures can provide powerful insights into nearshore biogeochemical processes and anthropogenic influences. Despite this, there is limited research investigating REE behaviour in sediments influenced by coastal acid sulfate soils (CASS). Here, we explore REE abundance and fractionation in intertidal mangrove sediments that received CASS drainage for ~15–20y within the Hastings Catchment in NSW, Australia. Sediments close to the CASS discharge point (<200m) were compared with those further downstream (1300–1600m), and at a nearby control site. Average ΣREE concentrations were highest near the CASS discharge point (148–186mg/kg), and decreased with distance downstream (111–146mg/kg) and in control sediments (70mg/kg). Reactive Fe concentrations (defined by 1M HCl extractability) were also significantly higher in surface sediments (0–6cm) near the CASS discharge point. Middle-REE (MREE) enrichments dominated fractionation patterns at all sites (>1.5), with a high proportion (63–100%) of REEs residing in the reactive (1M HCl extractable) sediment fraction. Interestingly, the degree of MREE enrichment was significantly correlated with Ce anomalies (r2=0.72, P<0.001) and the heavy-REE (HREE) to light-REE (LREE) ratios (HREE/LREE, r2=0.74, P<0.001) in the reactive sediment fraction, only in those sites situated closest to the CASS drainage. The observed high MREE enrichments, positive Ce anomalies (>1) and HREE/LREE ratios (>1) are consistent with reactive Fe(III) oxides/oxyhydroxides driving REE retention in these sediments. This study indicates that CASS drainage alters REE signatures in receiving sediments by (1) providing a source of REEs, thereby enhancing sedimentary REE concentrations, and (2) causing accumulation of reactive Fe(III) phases with a high affinity for REEs. Together, these two factors drive the development of distinctive REE signatures in CASS-impacted sediments. The recognition of such signatures may provide a promising tool for identifying coastal sediments receiving anthropogenic CASS drainage inputs.

Authigenic pyrite formation and re-oxidation as an indicator of an unsteady-state redox sedimentary environment: Evidence from the intertidal mangrove sediments of Hainan Island, China

Two cores of intertidal mangrove sediments from the Tanmen and Qinglan Harbors on Hainan Island, China, were investigated for their geochemical characteristics of carbon, nitrogen, iron and sulfur and the pyrite morphology and framboidal pyrite size distribution.A modified sequential iron extraction procedure revealed extremely high FeHR/FeT ratios (0.81±0.07, n=28). The pyrite results determined by the nitric acid digestion and chromium reduction method show a strong correlation (r=0.91, n=28), indicating that most of the chromium-reducible sulfur is pyrite, whereas the proportion of elemental sulfur is minor. The organic carbon concentrations and the atomic C/N ratios demonstrate that the organic carbon in the mangrove sediments is derived predominantly from higher plants. The chromium-reducible sulfur (CRS) values show a good linear logarithmic correlation with the total organic carbon (TOC), indicating that the process of sulfate reduction increases rapidly with the concentration of TOC at Qinglan Harbor (QL), which has low TOC contents (<5wt%). In contrast, sulfate reduction increases slowly with high TOC (>5wt%) at Tanmen Harbor (TM). These data suggest that pyrite formation at the QL site is controlled by the TOC contents, whereas at the TM site, the primary factor controlling the pyritization process is the supply rate of sulfate. Both sites have significantly high sulfate contents (average 1.67±0.45wt% and 0.80±0.32wt% at Tanmen and Qinglan, respectively), which are isotopically depleted in 34S (average −6.15±7.17‰ and −6.72±7.33‰ at Tanmen and Qinglan, respectively) suggesting that the sulfate is mainly from the reoxidation of reduced sulfides (mainly pyrite) instead of seawater sulfate during burial.The distributions of pyrite textures suggest that the pyrite in the mangrove swamps is formed mainly as framboids and only a few pyrite crystals are formed directly as euhedral crystals. The high mean diameters and standard deviations (7.0±4.6μm and 6.1±4.1μm at Tanmen and Qinglan, respectively) are compatible with previous observations in oxic and dysoxic environments. At the TM site, we observed a notable double-hump phenomenon in the framboid size distribution and a greater number of secondary framboids between the 50 and 100cm layers, suggesting different pyrite growth events than are present at QL site. There are remarkable increases in the sulfur isotopic fractionations between the CRS and the acid-dissolvable sulfate in several layers of the QL site (e.g.,>10‰ at 10, 20, 80 and 110–120cm depths), demonstrating that the reoxidized sulfate could then be re-reduced to create greater 34S depletions in sulfides relative to the acid-dissolvable sulfate. In addition, the small mean diameters and standard deviations of framboid size in these layers are much closer to those of dysoxic sediments (e.g., the Peru margin) where the redox conditions may be suitable for the re-reduction of partial sulfate derived from the reoxidation of reduced sulfur. Generally, these data provide evidence that the sudden redox turnover caused by the oxygen input during ebb tides or bioturbation occurs in both mangrove swamps, leading to a significant reoxidation of reduced sulfurs.

Geochemical heterogeneity of a coastal mangrove wetland

Preliminary geochemical studies of intertidal mangrove sediments from Polaris Point, Guam (13°30'N 144°45'E) showed a composition transition from a terrestrial to a marine environment. Seven mangrove core samples were taken in a transect across the mangrove swamp during the October wet season. Mineralogy and major (Si, Al, Fe, Mn, Mg, Ca, P) and minor elements (Ga, Zn, Ni, Co, Mn, Cr, Pb, Rb, Sr, V, Y, Zr) distribution in the mangrove swamp were determined using techniques of X-ray diffraction (XRD) and X-ray fluorescence (XFR). Element concentrations largely reflect the sediment mineralogy which includes terrestrially-derived volcanic material, marine-derived carbonate and clay minerals. High concentrations of the metals Ni, Co, Mn and Cr are indicative of the basaltic source , although some residual contamination may exist from a previous oil spill. Movement of a subsurface salt-wedge over a 12-hour tidal period is thought to be reflected by a negative correlation between the tidal height (m) and groundwater salinity (ppt), and a positive correlation between groundwater pH, Eh (mV) and Mn concentration (ppm). these results suggest further investigation of the temporal and spatial variations of transition elements in the mangrove swamp, together with the causes of these variations, is warranted.

Characterization of phosphorus fractions in the sediments of a tropical intertidal mangrove ecosystem

Solid phases of phosphorus fractions in the surface and core sediments were studied to understand the biogeochemical cycling and bioavailability of phosphorus in the Pichavaram intertidal mangrove sediments of India. Total P in surface and core sediments ranged between 451–552 and 459–736 μg g−1 respectively and Fe bound P was the dominant fraction. Low levels of Fe bound P in the mangrove zone than the two estuarine zones may be because of high salinity inhibition of phosphate adsorption onto the Fe-oxides/hydroxides. Post-depositional reorganization of P was observed in surface sediments, converting organic P and Fe bound P into the authigenic P. High levels of organic P in the mangrove zone is primarily due to intensive cycling and degradation of organic matter and adsorption of phosphate on the organic molecules. The burial rates and regeneration efficiency of P in the intertidal mangrove ecosystem ranged from 5.41 to 7.27 μmol P cm−2 year−1 and 0.122 to 0.233 μmol P cm−2 year−1, respectively. High burial efficiency (≈99%) of P proves the earlier observation of limiting nature of P for the biological productivity. Further, bioavailable P (exchangeable P + Fe bound P + organic P) constituted a considerable proportion of sedimentary P pool of which an average accounted for 55 and 50% in surface and core sediments respectively. The results indicate that significant amount of P is locked in sediments in the form of authigenic P and detrital P which makes P as a limiting nutrient for the biological productivity.