Methane flux from mangrove sediments along the Southwestern coast of Puerto Rico

Although the sediments of coastal marine mangrove forests have been considered a minor source of atmospheric methane, these estimates have been based on sparse data from similar areas. We have gathered evidence that shows that external nutrient and freshwater loading in mangrove sediments may have a significant effect on methane flux. Experiments were performed to examine methane fluxes from anaerobic sediments in a mangrove forest subjected to secondary sewage effluents on the southwestern coast of Puerto Rico. Emission rates were measured in situ using a static chamber technique, and subsequent laboratory analysis of samples was by gas chromatography using a flame ionization detector. Results indicate that methane flux rates were lowest at the landward fringe nearest to the effluent discharge, higher in the seaward fringe occupied by red mangroves, and highest in the transition zone between black and red mangrove communities, with average values of 4 mg CH₄ m^?2 d^?1, 42 mg CH₄ m^?2 d^?1, and 82 mg CH₄ m^?2 d^?1, respectively. Overall mean values show these sediments may emit as much as 40 times more methane than unimpacted pristine areas. Pneumatophores ofAviciennia germinans have been found to serve as conduits to the atmosphere for this gas. Fluctuating water level overlying the mangrove sediment is an important environmental factor controlling seasonal and interannual CH₄ flux variations. Environmental controls such as freshwater inputs and increased nutrient loading influence in situ methane emissions from these environments.     

Recent accretion in mangrove ecosystems based on137Cs and210Pb

Accretion rates were measured in fringe and basin mangrove forests in river and tidally dominated sites in Terminos Lagoon, Mexico, and a basin mangrove forest in Rookery Bay, Florida, USA. Accretion rates were determined using the radionuclides²¹⁰Pb and¹³⁷Cs. Consolidation-corrected accretion rates for the Rookery Bay cores, ranged from 1.4 to 1.7 mm yr^?1, with an average rate of 1.6 mm yr^?1. Rates at the Mexico sites ranged from 1.0 to 4.4 mm yr^?1, with an average of 2.4 mm yr^?1. Determination of rates in these mangrove forests was greatly affected by the consolidation corrections which decreased the apparent accretion rate by over 50% in one case. Accretion rates at basin sites compare favorably with a reported 1.4 to 1.6 mm yr^?1 rate of sea-level rise, indicating little or no subsidence at inland locations. Accretion rates in fringe sites are generally greater than basin sites, indicating greater subsidence rates in these sediments over longer time intervals.     

Mangrove zonation in the dry life zone of the Gulf of Fonseca, Honduras

The structural attributes and zonation patterns of mangrove forests in a dry environmental setting were evaluated in relation to gradients of soil resources, soil regulators, and hydroperiod between October 2000 and August 2001 in the Gulf of Fonseca, Honduras. Transects perpendicular to the edge of tidal channels were established at four mangrove sites, each sectioned into three mangrove zones (fringe, transition, and scrub) based upon distinct tree-height gradients. Porewater sulfide concentrations among zones were normally below levels of detection (<0.03 mM) and mean redox values ranged from 163.4±9.9 to −42.4±15.8 mv, indicating slightly reducing conditions in all zones. Mean porewater NO_x ⁻ and PO₄ ³⁻ concentrations were not significantly different among zones and ranged from 3.3±0.5 to 4.5±0.4 μM and from 0.05 ±0.02 to 0.18±0.04 μM, respectively. Soil nitrogen concentrations were 1–3 mg g⁻¹ in all zones, and mean nitrogen:phosphorus atomic ratios were <20 (range: 12.9–14.9), indicative of fertile wetland soils. Mean basal areas were significantly different among mangrove zones (fringe > transition) and were coupled with differences in height within species and zones.Avicennia germinans occupied areas with higher elevations associated with higher salinities ranging from 80 to 140 g kg⁻¹ in the dry season.Rhizophora mangle dominated lower elevations where salinities ranged from 38 to 57 g kg⁻¹ in both the dry and wet seasons. Spatial analysis on mangrove seedlings along transects confirmed that seedling distribution along the intertidal profile was statistically correlated with conspecific adults, indicating that survival and growth of seedlings in different intertidal locations are closely matched to canopy membership. The sharp transition (<100 m) in forest structure and distribution of mangrove species along the intertidal zone of subhumid coastal environments demonstrate common zonation patterns where species distribution and species-specific habitat suitability are mediated closely with tolerance to soil regulators (high soil salinity) rather than resource availability.     

Strontium isotope geochemistry of groundwater affected by human activities in Nandong underground river system, China

The Nandong Underground River System (NURS) is located in a typical karst area dominated by agriculture in SE Yunnan Province, China. Groundwater plays an important role in the social and economical development in the area. The effects of human activities (agriculture and sewage effluents) on the Sr isotope geochemistry were investigated in the NURS. Seventy-two representative groundwater samples, which were collected from different aquifers (calcite and dolomite), under varying land-use types, both in summer and winter, showed significant spatial differences and slight seasonal variations in Sr concentrations and ⁸⁷Sr/⁸⁶Sr ratios. Agricultural fertilizers and sewage effluents significantly modified the natural ⁸⁷Sr/⁸⁶Sr ratios signature of groundwater that was otherwise dominated by water-rock interaction. Three major sources of Sr could be distinguished by ⁸⁷Sr/⁸⁶Sr ratios and Sr concentrations in karst groundwater. Two sources of Sr are the Triassic calcite and dolomite aquifers, where waters have low Sr concentrations (0.1-0.2 mg/L) and low ⁸⁷Sr/⁸⁶Sr ratios (0.7075-0.7080 and 0.7080-0.7100, respectively); the third source is anthropogenic Sr from agricultural fertilizers and sewage effluents with waters affected having radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.7080-0.8352 for agricultural fertilizers and 0.7080-0.7200 for sewage effluents, respectively), with higher Sr concentrations (0.24-0.51 mg/L). Due to the overlapping ⁸⁷Sr/⁸⁶Sr ratios, it is difficult to distinguish the sources of Sr in groundwater samples contaminated by agricultural fertilizers or sewage effluents based only on their ⁸⁷Sr/⁸⁶Sr ratios. However, ⁸⁷Sr/⁸⁶Sr ratios do provide key information for natural and anthropogenic sources in karst groundwater.     

Origins of high nitrate in groundwater in Tanzania

Dodoma is located in a semi-arid climate (mean annual rainfall 550 mm) in an area of crystalline basement rock. The groundwater contains high nitrate with NO₃⁻ concentrations averaging 150 mg l⁻¹ and total mineralization between 1000–3000 mg l⁻¹. Factor analysis has been used for the identification of factors that bring about the chemical character of groundwater. Three rotated factors, explaining 77.2% of the total data variance, were extracted. The first factor accounts for 51.0% of the variance and shows high positive correlation with Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl⁻, SO₄²⁻, HCO₃⁻ and SEC and is attributed to the leaching of surface and soil salts together with mineral dissolution. The second factor accounts for 14.1% of the variance and is positively correlated with NO₃⁻ and negatively correlated with pH; this is explained by the nitrification process taking place on the sewage effluents. The third factor is highly positively correlated with ¹⁸O and ²H, accounting for 12.1% of the variance and is a consequence of evaporation processes. Nitrate concentrations greater than 100 mg l⁻¹ are commonly found in both deep and shallow groundwaters. It is concluded that the high nitrate concentrations have originated from the sewage effluents that are advecting and dispersing through macropores under bypass flow mechanisms.     

Mangrove development and its response to environmental change in Yingluo Bay (SW China) during the last 150 years: Stable carbon isotopes and mangrove pollen

Located at the interface of terrestrial and marine ecosystems, mangroves are particularly sensitive to environmental change. They provide a sedimentary sink for organic carbon, whereby cores can provide detailed records of mangrove species. We aimed to trace the history of mangrove development over the past 150 years in Yingluo Bay, SW China. Sedimentation rates (avg. 0.32 and 0.37 cm/year) were calculated on the basis of ln(²¹⁰Pb_ex) vs. mass depth, and offset the rate of relative sea level rise (0.22–0.24 cm/year), leading to a seaward expansion of new mangrove habitats. Chemical tracers (δ¹³C_org and C:N) and an isotope mixing model were utilized to trace the contribution of mangrove-derived organic matter (MOM). Changes in the relative abundance of pollen from mangrove plants was used to compensate for diagenetic alteration of the stable isotope values and potential overlaps in isotope values for different sources of organic matter. The result of Pearson correlation analysis showed that the MOM was moderately positive correlated with total mangrove pollen, indicating that stable carbon isotopes and mangrove pollen provide similar information for tracing mangrove ecosystems. Based on results from this study, compositional changes in mangrove communities could be divided into two main stages: a degradation period (1870–1930 AD) and a flourishing period (1930–2011 AD), corresponding to colder temperature and warmer temperature, respectively. Owing to the location being far away from any industrial area and human activity, temperature may be a key factor for mangrove development.     

Recharge and contamination sources of shallow and deep groundwater of pleistocene aquifer in El-Sadat industrial city: isotope and hydrochemical approaches

Hydrochemical and isotopic researches were conducted in El-Sadat City groundwater system to identify groundwater alteration, recharge, residence time and extent of pollution. The groundwater salinity gradually increases as the groundwater moves from northeastern to southwestern parts of the city. Groundwater generally shows mineralization decreasing with depth, indicating that the possibility of recent water penetration far below the surface is limited. Shallow groundwater has an elevated level of nitrate, which is attributed to anthropogenic sources due to intensive agricultural activity. The limit of high nitrate water may mark the maximum penetration of groundwater from the surface, which is found in depths <100 m. The northeastern and southwestern industrial areas are highly contaminated by some heavy metals, which may originate from some local industrial effluents. The sewage oxidation ponds seem to show no effect on groundwater; hence, these ponds are not a point source for these heavy metals. Dissolved ions depict five different hydrochemical facies, and stable isotopes define the recharge mechanisms, the origin of groundwater and the hydraulic confinement of deep groundwater. The deep groundwater is untritiated and has long residence times (in the order of thousands of years). Three different hydrochemical groups have been recognized and mapped in El-Sadat City, based on the chemical and isotopic information of the groundwater. These groups have different levels of contamination. The deep groundwater samples are significantly less impacted by surface activities and it appears that these important water resources have very low recharge rates and would, therefore, be severely impacted by overabstraction. The extensive exploitation of groundwater for drinking water supply would shortly be reflected by a gradual decline of the groundwater table in El-Sadat City. Amelioration of groundwater quality requires further management strategies and efforts in the forthcoming years.     

Carbon dynamics of estuarine ponds receiving treated sewage wastes

The carbon dynamics of six estuarine ponds at Morehead City, North Carolina, were measured from August 1969 through August 1970. Three waste ponds received secondary treated sewage effluent and three served as controls. Total organic levels were determined by combustion with a Beckman total carbon analyzer. Inorganic levels were calculated from temperature, pH, and carbonate alkalinity. Carbon dioxide diffusion across the air-water interface was measured with a floating plastic dome and an infrared gas analyzer. Carbon metabolism was determined from diurnal changes in total inorganic carbon. From October through April the wastereceiving ponds were characterized by an intense bloom of a small alga,Monodus guttula. During the bloom organic levels reached 75 mg C per 1. The warmer months were characterized by a more diverse flora and organic concentrations averaging 25 mg per 1. Organic levels in the control ponds seldom were greater than 10 mg per 1. CO₂ diffused constantly out of the control ponds due to input of high alkalinity water supersaturated with inorganic carbon. During the bloom in the waste ponds diffusion of CO₂ from the atmosphere was as high as 0.5 g C per m² per day. CO₂ diffused into the water constantly because the high uptake rate of inorganic carbon by the small algae reduced the levels in the water to less than 5 mg per 1. Gross production of the waste receiving ponds avearaged 2,86 g C per m² per day during the bloom and 2.44 during the non-bloom period. The average productivity of the control ponds was 1.7. Corrections for CO₂ diffusion were significant in the control ponds and in the waste ponds during theMonodus bloom. Availability of carbon from diffusion and respiratory regeneration seemed to limit phytoplankton productivity during the bloom. The annual budgets for the two sets of ponds were distinctly different. For the control ponds, the input of organic matter was nearly equal to losses indicating almost no net production of organic material. A high net loss of organic material and a net gain of inorganic carbon for the waste ponds reflected the net productive nature.     

Groundwater pollution by Cystine manufacturing industrial effluent around the factory

 The pollution potential of Cystine industrial effluents on groundwater pollution is assessed. The results indicate that the Cystine industrial effluent which is partially treated and stored in lagoons contains a high concentration of inorganic solids and soluble organics. The percolation of these effluents caused groundwater pollution impairing the water quality. Received: 20 April 1999 · Accepted: 7 June 1999     

Seasonal distribution of cadmium among components of sewage treatment ponds: an eco-tech for heavy metal remediation

The study was designed to quantify cadmium accumulation in different components of sewage treatment ponds during different seasons and to assess risk for human consumption perspective. The study estimated cadmium concentration in water, sludge, Eichhornia, plankton and tilapia fish from anaerobic, facultative, maturation-1 and -4 ponds during pre-monsoon, monsoon and post-monsoon periods. It resulted that cadmium accumulated among different components of anaerobic, facultative, maturation-1 and -4 ponds ranging 0–18, 0–10, 0–7 and 0–15.4 ppb, respectively. During monsoon, highest accumulation was observed in tilapia in both facultative and maturation ponds, but during post-monsoon, plankton community showed highest value in all. The highest bio-magnification of cadmium was recorded during monsoon with varying degrees (facultative pond: 4.39, maturation pond-1: 3.03 and maturation pond-4: 7.08). Cadmium concentration in tilapia lied within WHO’s safe level and may be recommended for human consumption. The concentration of cadmium was estimated by Flame Atomic Absorption Spectrophotometer. The above findings occurred due to chelation, adsorption and sedimentation, absorption and bio-accumulation, ionization, leaching through sediment and rainfall. Water pH (6.7–8.8), dissolved oxygen (0–17 mg L^?1), total solids (251–650 mg L^?1), iron (Fe²⁺) concentration (0.61–4.87 mg L^?1) and sedimentation rate (278.9–2,409.6 g day^?1 m^?3) were conducive for the distribution of cadmium into different ecosystem components of treatment ponds. These ponds reclaimed 28.57–61.11 % of sewage-cadmium and may be promoted as a low-cost eco-tech for sewage treatment.     

Impact of discharge wastewater effluents on the physico-chemical qualities of a receiving watershed in a typical rural community

The qualities of the treated final effluents of a wastewater treatment plant located in a rural community of the Eastern Cape Province of South Africa were assessed over the duration of 12 months. Parameters measured include pH, temperature, electrical conductivity, salinity, turbidity, total dissolved solid, dissolved oxygen, chemical oxygen demand, nitrate, nitrite and orthophosphate levels and these were simultaneously monitored in the treated final effluents and the receiving watersheds using standard methods. Unacceptably, high levels of the assayed parameters were observed in many cases for chemical oxygen demand (7.5–248.5 mg/L), nitrate (1.82–13.14 mg/L), nitrite (0.09–1.3 mg/L), orthophosphate (0.07–4.81 mg/L), dissolved oxygen (4.15–11.22 mg/L) and turbidity (3.68–159.06 NTU) during the study period and are severally outside the compliance levels of the South African Guidelines and World Health Organization tolerance limits for effluents intended for discharge through public sewers into receiving watersheds. The study has revealed that there was an adverse impact on the physico-chemical characteristics of the receiving watershed as a result of the discharge of inadequately treated effluents from the wastewater treatment facility. This poses a health risk to several rural communities which rely on the receiving water bodies primarily as their sources of domestic water. There is need for the intervention of appropriate regulatory agencies to ensure production of high quality treated final effluents by wastewater treatment facilities in rural communities of South Africa.     

Distribution and Trophic Importance of Anthropogenic Nitrogen in Narragansett Bay: An Assessment Using Stable Isotopes

Narragansett Bay has been heavily influenced by human activities for more than 200 years. In recent decades, it has been one of the more intensively fertilized estuaries in the USA, with most of the anthropogenic nutrient load originating from sewage treatment plants (STP). This will soon change as tertiary treatment upgrades reduce nitrogen (N) loads by about one third or more during the summer. Before these reductions take place, we sought to characterize the sewage N signature in primary (macroalgae) and secondary (the hard clam, Mercenaria mercenaria) producers in the bay using stable isotopes of N (δ¹⁵N) and carbon (δ¹³C). The δ¹⁵N signatures of the macroalgae show a clear gradient of approximately 4‰ from north to south, i.e., high to low point source loading. There is also evidence of a west to east gradient of heavy to light values of δ¹⁵N in the bay consistent with circulation patterns and residual flows. The Providence River Estuary, just north of Narragansett Bay proper, receives 85% of STP inputs to Narragansett Bay, and lower δ¹⁵N values in macroalgae there reflected preferential uptake of ¹⁴N in this heavily fertilized area. Differences in pH from N stimulated photosynthesis and related shifts in predominance of dissolved C species may control the observed δ¹³C signatures. Unlike the macroalgae, the clams were remarkably uniform in both δ¹⁵N (13.2 ± 0.54‰ SD) and δ¹³C (−16.76 ± 0.61‰ SD) throughout the bay, and the δ¹⁵N values were 2–5‰ heavier than in clams collected outside the bay. We suggest that this remarkable uniformity reflects a food source of anthropogenically heavy phytoplankton formed in the upper bay and supported by sewage derived N. We estimate that approximately half of the N in the clams throughout Narragansett Bay may be from anthropogenic sources.     

Discharge diversion in the Patía River delta,the Colombian Pacific: Geomorphic and ecological consequences for mangrove ecosystems

In the Patía River delta, the best-developed delta on the western margin of South America, a major water diversion started in 1972. The diversion of the Patía flow to the Sanquianga River, the latter a small stream draining internal lakes from the Pacific lowlands, shifted the active delta plain from the south to the north and changed the northern estuarine system into an active delta plain. The Sanquianga Mangrove National Park, a mangrove reserve measuring 800 km², lies in this former estuary, where major hydrologic and sedimentation changes are occurring. Overall, major environmental consequences of this discharge diversion in terms of geomorphic changes along distributary channels and ecological impacts on mangrove ecosystems are evidenced by: (1) distributary channel accretion by operating processes such as sedimentation, overbank flow, increasing width of levees, sedimentation in crevasses, interdistributary channel fill, and colonization of pioneer mangrove; (2) freshening conditions in the Sanquianga distributary channel, a hydrologic change that has shifted the upper estuarine region (salinity <1%) downstream; (3) downstream advance of freshwater vegetation, which is invading channel banks in the lower and mixing estuarine zones; (4) die-off of approximately 5200 ha of mangrove near the delta apex at Bocas de Satinga, where the highest sediment accumulation rates occur; and (5) recurrent periods of mangrove defoliation due to a worm plague. Further analyses indicate strong mangrove erosion along transgressive barrier islands on the former delta plain. Here tectonic-induced subsidence, relative sea-level rise, and sediment starving conditions due to the channel diversion, are the main causes of the observed retreating conditions of mangrove communities. Our data also indicate that the Patía River has the highest sediment load (27 × 10⁶ t yr⁻¹) and basin-wide sediment yield (1500 t km⁻² yr⁻¹) on the west coast of South America. Erosion rates from the Patía catchment have been more pronounced during the decades of 1970–1980 and 1990–2000, as a result of land degradation and deforestation. The high sediment and freshwater inputs into the mangrove ecosystem create additional stress (both at ongoing background levels and, occasionally, at dramatic levels), which may periodically push local environmental parameters beyond the thresholds for mangrove survival. The future environmental state of the Sanquianga Mangrove National Reserve deserves more scientific and governmental attention.     

Behavior of Gd-DTPA in simulated bank filtration

The behavior of Gd-DTPA during bank filtration was simulated in a 30 m column filled with Pleistocene sand and flushed by surface water from a lakeside in Berlin, Germany. The surface water is about a 1:1 mixture of river water and effluents from a sewage treatment plant. Throughout 34 days this water was continuously spiked with Gd-DTPA at a level of 60 μg/L. The broad plateau of the Gd-DTPA pulse declined by 15.4% within 34 days by transmetallation. Nine percentage of the total decline is caused by Y and rare earth elements; the remaining part is attributed to Cu²⁺ which is the most influential metal in surface water. All other metals also contributing to transmetallation are combined with Cu to Cu equivalents because only the rate constant of transmetallation of Cu²⁺ is known. The analytical results of the column effluents prove the pseudo-first-order kinetics of transmetallation based on reversible sorption of metals by pools in the column sediment and disprove biodegradation at noticeable levels. The mass ratio of water to tracer is <10¹⁰.     

The effectiveness of artificial recharge in combating seawater intrusion in Salalah coastal aquifer, Oman

The Salalah central sewage treatment plant has been designed to treat 20,000 m³/day at the first stage and two further stages to double the initial capacity. The plant currently (2005) treats more than 15,000 m³/day effluents to a tertiary level, and after chlorination phase, the effluents are recharged into tube wells in a line parallel to the coast. The process aims to help stabilize the seawater interface and a part to be recovered from hand-dug wells/boreholes further inland and downstream. A three-dimensional flow and solute advection transport model was developed to assess the effectiveness of the proposed recharge scheme and to track the solute transport with respect to the design system. The advection transport model predicted that in 2020 the maximum pathlines of the injection fluids would reach the abstraction wells that are located 600 m, southward of the injection bores in about 1-year travel time in the case of the no-management interference and more than that southward under management interference. The developed flow predicted the wedge of the saline intrusion in 2019 is tracked up to 2.7 and 3.4 km from the shoreline with the injection and without the injection, respectively under constant underflow. The injection scheme is effective in pushing back the saline zone front by 700 m. This study argues that the treated wastewater would help to increase the water levels at the vicinity of the injection line and to reduce the influence of saline inflows from the coast. The reclaimed sewage recharge scheme is examined in the case of the Salalah coastal aquifer using groundwater simulation, which can also be applied to other regions with similar conditions.     

Delineation of contamination plume around oxidation sewage-ponds in Southwestern Nigeria

Integrated surface electrical resistivity and electromagnetic (EM) surveys were conducted in a hard-rock terrain of Southwestern Nigeria in the vicinity of active oxidation sewage treatment ponds. The aim was to detect soil contamination due to the spread of sewage effluent, locate possible leachate plumes and conductive lithologic layers, and access the risk of groundwater pollution in the vicinity of the sewage-ponds. Dipole–dipole resistivity profiling and very low frequency (VLF) data were acquired at 10-m intervals along five 200-m long east-west geophysical traverses. Resistivity sections obtained revealed four subsurface geologic layers comprised of lateritic clay, clayey sand/sand, weathered/fractured bedrock, and competent bedrock. A distinct low resistivity zone corresponding to the contamination plume (labeled B) was delineated from all the resistivity sections. This low zone extends into the weathered bedrock and possibly suggests contamination of this layer. The filtered real component of the processed VLF data detected three distinct anomaly zones that are representative of fractured zones filled with conductive fluids and/or lithologic boundaries that possibly serve as conduits for the movement of contaminated effluents. The results obtained from the two methods suggest possible contamination of the subsurface soil layers and groundwater in the vicinity of the sewage-ponds. The existence of this contaminated plume poses a serious threat to the ecosystem and health of the people living in the vicinity of the sewage-ponds.     

Late Quaternary cycles of mangrove development and decline on the north Australian continental shelf

Mangrove communities in the Australian tropics presently occur as narrow belts of vegetation in estuaries and on sheltered, muddy coasts. Palynological data from continental-shelf and deep-sea cores indicate a long-term cyclical component of mangrove development and decline at a regional scale, which can be linked to specific phases of late Quaternary sea-level change. Extensive mangrove development, relative to today, occurs during periods of marine transgression, whereas very diminished mangrove occurs during marine regressions and during rarer periods of relative sea-level stability. Episodes of flourishing mangrove cannot be linked to phases of humid climate, as has been suggested in studies elsewhere. Rather, the cycle of expansion and decline of mangrove communities on a grand scale is explained in terms of contrasting physiographic settings characteristic of continental-shelf coasts during transgressive and regressive phases, in particular by the existence, or lack, of well-developed tidal estuaries. Copyright © 1999 John Wiley & Sons, Ltd.     

Wetland-water column exchanges of carbon, nitrogen, and phosphorus in a southern Everglades dwarf mangrove

We used enclosures to quantify wetland-water column nutrient exchanges in a dwarf red mangrove, (Rhizophora mangle L.) system near Taylor River, an important hydraulic linkage between the southern Everglades and eastern Florida Bay, Florida, USA. Circular enclosures were constructed around small (2.5–4 m diam) mangrove islands (n=3) and sampled quarterly from August 1996 to May 1998 to quantify net exchanges of carbon, nitrogen, and phosphorus. The dwarf mangrove wetland was a net nitrifying environment with consistent uptake of ammonium (6.6–31.4 μmol m⁻² h⁻¹) and release of nitrite +nitrate (7.1–139.5 μmol m⁻² h⁻¹) to the water column. Significant flux of soluble reactive phosphorus was rarely detected in this nutrient-poor, P-limited environment. We did observe recurrent uptake of total phosphorus and nitrogen (2.1–8.3 and 98–502 μmol m⁻² h⁻¹, respectively), as well as dissolved organic carbon (1.8–6.9 μmol m⁻² h⁻¹) from the water column. Total organic carbon flux shifted unexplainably from uptake, during Year 1, to export, during Year 2. The use of unvegetated (control) enclosures during the second year allowed us to distinguish the influence of mangrove vegetation from soil-water column processes on these fluxes. Nutrient fluxes in control chambers typically paralleled the direction (uptake or release) of mangrove enclosure fluxes, but not the magnitude. In several instances, nutrient fluxes were more than twofold greater in the absence of mangroves, suggesting an influence of the vegetation on wetland-water column processes. Our findings characterize wetland nutrient exchanges, in a mangrove forest type that has received such little attention in the past, and serve as baseline data for a system undergoing hydrologic restoration.     

Evaluating freshwater lens morphology affected by seawater intrusion using chemistry-resistivity integrated technique: a case study of two different land covers in Carey Island, Malaysia

Freshwater lenses are vital to small island communities but are susceptible to seawater intrusion due to the physical changes in the shoreline land cover. The effect of seawater intrusion and irrigation water on a coastal unconfined aquifer beneath naturally preserved mangrove and deforested mangrove-barren belt was investigated in Carey Island. Analysis of the total dissolved solids (TDS) and earth resistivity (ER) using a geochemistry-electrical integrated technique gave a TDS–ER relationship capable of predicting freshwater lens morphology affected by sea-irrigation water. The study result shows freshwater was fourfold thicker in close proximity of the mangrove forest than the mangrove barren area; the further the shoreline from the mangrove thickest section, the less vulnerable was the seawater intrusion and the more fresh the irrigation water, and hence the greater the freshwater availability potential.     

Use of Multiple Chemical Tracers to Define Habitat Use of Indo-Pacific Mangrove Crab, <Emphasis Type="Italic">Scylla Serrata</Emphasis> (Decapoda: Portunidae)

The mangrove or mud crab, Scylla serrata, is an important component of mangrove fisheries throughout the Indo-Pacific. Understanding crab diets and habitat use should assist in managing these fisheries and could provide additional justification for conservation of the mangrove ecosystem itself. We used multiple chemical tracers to test whether crab movements were restricted to local mangrove forests, or extended to include adjacent seagrass beds and reef flats. We sampled three mangrove forests on the island of Kosrae in the Federated States of Micronesia at Lelu Harbor, Okat River, and Utwe tidal channel. Samples of S. serrata and likely food sources were analyzed for stable carbon (δ¹³C), nitrogen (δ¹⁵N), and sulfur (δ³⁴S) isotopes. Scylla serrata tissues also were analyzed for phosphorus (P), cations (K, Ca, Mg, Na), and trace elements (Mn, Fe, Cu, Zn, and B). Discriminant analysis indicated that at least 87% of the crabs remain in each site as distinct populations. Crab stable isotope values indicated potential differences in habitat use within estuaries. Values for δ¹³C and δ³⁴S in crabs from Okat and Utwe were low and similar to values expected from animals feeding within mangrove forests, e.g., feeding on infauna that had average δ¹³C values near −26.5‰. In contrast, crabs from Lelu had higher δ¹³C and δ³⁴S values, with average values of −21.8 and 7.8‰, respectively. These higher isotope values are consistent with increased crab foraging on reef flats and seagrasses. Given that S. serrata have been observed feeding on adjacent reef and seagrass environments on Kosrae, it is likely that they move in and out of the mangroves for feeding. Isotope mixing model results support these conclusions, with the greatest mangrove ecosystem contribution to S. serrata diet occurring in the largest mangrove forests. Conserving larger island mangrove forests (> 1 km deep) appears to support crab foraging activities.