Oligohaline Wetlands Research Articles

Potential methane production in oligohaline wetlands undergoing erosion and accretion in the Mississippi River Delta Plain, Louisiana, USA

Methane (CH4) is steadily increasing in the atmosphere from different sources including wetlands. However, there is limited landscape level CH4 flux data in deltaic coastal systems where the availability of freshwater is impacted by the combined effect of climate change and anthropogenic impacts. Here we determine potential CH4 fluxes in oligohaline wetlands and benthic sediments in the Mississippi River Delta Plain (MRDP), which is undergoing the highest rate of wetland loss and most extensive hydrological wetland restoration in North America. We evaluate potential CH4 fluxes in two contrasting deltaic systems, one undergoing sediment accretion as result of a freshwater and sediment diversions (Wax Lake Delta, WLD), and one experiencing net land loss (Barataria-Lake Cataouatche, BLC). Short- (<4 days) and long-term (36 days) incubations using soil and sediment intact cores and slurries were performed at different temperatures representing seasonal differences (10, 20, 30 °C). Our study revealed that all habitats were net sources of atmospheric CH4 in all seasons, and CH4 fluxes were generally the highest for the 20 °C incubation. The CH4 flux was higher in the marsh habitat of the recently formed delta system (WLD) with total carbon content of 5–24 mg C cm−3 compared to the marsh habitat in BLC, which had high soil carbon content of 67–213 mg C cm−3. This suggests that the quantity of soil organic matter might not be a determining factor in CH4 flux. Overall, benthic habitats were found to have the lowest CH4 fluxes indicating that projected future conversions of marshes to open water in this region will impact the total wetland CH4 emission, although the overall contribution of such conversions to the regional and global carbon budgets is still unknown. Further research is needed to expand the CH4 flux studies by simultaneously using several methods across different wetland habitats.

Salinity exerted little effect on decomposition of emergent macrophytes in coastal peatlands

Climate-warming-driven sea level rise may change the vegetation composition of coastal wetlands, influencing ecosystem functions including photosynthetic activity, biomass production, litter decomposability and nutrient cycling. Litter decomposition rates of the most dominant macrophytes, Phragmites australis, Carex sp. and Schoenoplectus tabernaemontani, in two nearby freshwater and oligohaline peatlands were compared to determine the impact of salinity on litter decomposition. Electrical conductivity difference of 1.2–8.0 mS cm-1 (mean difference = 4.8 mS cm-1) between freshwater and oligohaline peatlands did not significantly affect the decomposition of macrophyte litter. Between species, S. tabernaemontani lost a significantly higher mass (29%) in the first month of decomposition compared to P. australis and Carex sp. Overall, litter decay rates for S. tabernaemontani was k = 0.0064–0.0078 d-1 while P. australis and Carex sp. both had k = 0.0017–0.0028 d-1. After one year, S. tabernaemontani litter was totally decomposed while the other species had 40–60% mass remaining relative to their initial dry mass confirming that P. australis and Carex sp. contribute to peat formation while S. tabernaemontani does not. Apparent differences in decomposition kinetics between S. tabernaemontani and the other species tested indicated that litter quality, especially C:P, C:N, N:P and C:Ca, is of great importance for both microbial colonization (initial phase) as well as for sustaining decomposition after colonization. This study shows that litter quality controls litter decomposition more than narrow salinity ranges. Hence, we suggest that sea level rise, combined with increased precipitation, may not have significant impact on the decomposition of these macrophytes in the future. However, sea level rise may lead to changes in vegetation composition replacing the peat-forming species by S. tabernaemontani which favors relatively deep, oligohaline wetlands. This would eventually affect carbon sequestration rates.

Disproportionate variations in denitrification and anaerobic ammonium oxidation across freshwater-oligohaline wetlands in Min River Estuary, Southeast China.

Spatial and temporal variations in soil denitrification and anaerobic ammonium oxidation (anammox) across the freshwater-oligohaline wetlands in subtropical estuary have not been well understood. In this study, continuous-flow soil core incubation combined with nitrogen isotope tracer was used to determine denitrification and anammox rates across freshwater-oligohaline tidal wetlands in Min River Estuary, Southeast China. Areal rates of denitrification and anammox varied from 3.89 to 19.0 μmol m-2 h-1 and from 0.15 to 1.11 μmol m-2 h-1, respectively, across these wetlands and throughout sampling months. Denitrification rates were higher in warm months (July, September) than in cool months (November, January), whereas anammox did not vary significantly across the sampling months. Average denitrification rates throughout the sampling months were higher in freshwater than in oligohaline wetlands, while anammox rates did not vary among the wetlands. Relative contribution of anammox (Ra) to N2 production (including denitrification and anammox) varied from 1.03 to 18.3% across the sampling months and wetlands. Denitrification rates differed significantly across the wetlands and sampling months. Anammox rates and Ra did not vary significantly among the sampling months. Denitrification rates were positively correlated with water content, total organic carbon (TOC), total nitrogen, dissolved organic nitrogen, NH4+, NOx-, Fe2+, and Fe2+/Fe3+, but negatively related to pH. Anammox rates showed negative relationships with water content and TOC. Water content, temperature, and pH were crucial for organic carbon and Fe2+ availability with important implications on denitrification and anammox. Therefore, denitrification rates vary significantly, whereas anammox rates do not vary significantly across freshwater-oligohaline wetlands in the Min River Estuary.

Low-level saltwater intrusion alters soil diazotrophic community structure in a subtropical estuarine wetland

Although diazotrophs play a critical role in the nitrogen (N) cycle by supplying new N, the response and mechanism of nifH gene diversity and community structure to minor changes in salinity remain unclear. Here, we investigated the diazotrophic communities in both rhizospheric and non-rhizospheric soils along a low-level salinity gradient across one freshwater (0.08 ± 0.02‰) and two oligohaline (1.27 ± 0.09 and 3.31 ± 0.14‰) wetlands by sequencing the nifH gene. The nifH gene alpha diversity was similar along the salinity gradient, but community structure differed markedly. The most abundant genera across the samples were within the Deltaproteobacteria class, including Geobacter, Desulfuromonas, Pelobacter, and Desulfovibrio. The nifH gene diversity (both alpha and beta diversity) and community structure did not differ significantly between rhizospheric and non-rhizospheric soils. Furthermore, soil salinity, pH, and nutrient availability (i.e., NH4+-N, NO3−-N, and C:N ratio) had synergistic effects on the diazotrophic community structure. In conclusion, the transition from freshwater to oligohaline water significantly altered the diazotrophic community structures but not diazotrophic diversity. Our study shows that saltwater intrusion may affect estuarine N fixation capacity and N cycling processes by reshaping these diazotrophic communities.

Coupled relationships among anammox, denitrification, and dissimilatory nitrate reduction to ammonium along salinity gradients in a Chinese estuarine wetland

Salinization in estuarine wetlands significantly alters the balance between their nitrogen (N) removal and retention abilities but these processes have not yet been characterized effectively. In the present study, the potential rates of sediment denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) were mapped using N isotope tracing methods along salinity gradients across the Yellow River Delta wetland (YRDW) in China. The contribution of anammox to total dissimilatory N transformations in YRDW was merely 6.8%, whereas denitrification and DNRA contributed 52.3% and 40.9%, respectively. The potential rate of denitrification (5.82 μmol/kg/h) decreased significantly along salinity gradients and markedly exceeded DNRA potential rate (2.7 μmol/kg/h) in fresh wetlands, but was lower than that of DNRA in oligohaline wetlands (3.06 and 3.18 μmol/kg/h, respectively). Moreover, a significantly positive relationship between salinity and DNRA/denitrification was obeserved, indicating that increased salinity may favor DNRA over denitrification. Furthermore, total sulfur (TS) content and ratio of total organic carbon to total nitrogen (C/N) increased with the salinity gradient and showed evident positive relationships with the DNRA/denitrification ratio. In this study, we proved that increased salinization resulted in the dominance of DNRA over denitrification, possible through the addition of S and alteration of the C/N in estuarine wetlands, leading to increased N retention in estuarine wetlands during salinization, which would enhance the eutrophication potential within wetlands and in downstream ecosystems.

Soil organic carbon controls dissimilatory nitrate reduction to ammonium along a freshwater-oligohaline gradient of Min River Estuary, Southeast China

Quality and source of soil organic carbon controlling DNRA in estuarine wetlands have not been well understood. We used a 15N isotopic tracer approach to investigate DNRA rates and determined the effects of organic carbon quality and source on magnitudes and distribution of DNRA along a freshwater-oligohaline gradient of Min River estuary, Southeast China. DNRA rates ranged from 0.45 to 2.92 nmol g−1 h−1 and were significantly higher in summer than in winter. DNRA was well predicted by dissolved organic carbon (DOC), organic carbon isotope (δ13Corg), total OC as the main predictor variables, which explained 63%, 6.0% and 7.0% of DNRA variances, respectively. Water content and temperature were crucial for DOC availability with important implications on DNRA. Soil δ13Corg increased from freshwater to oligohaline wetlands and showed a positive correlation with DNRA. We therefore conclude that high DOC availability enhances DNRA and can best predict spatial distribution in subtropical estuary.

Consideraciones sobre la comunidad de diatomeas en relación a gradientes de altitud y salinidad en humedales de la Puna y los Altos Andes (Catamarca y Jujuy, Argentina)

Introducción y objetivos: En la Puna y Prepuna Argentina existe un importante número de humedales de altura netamente dulces hasta francamente hiperhalinos. Este aporte tuvo como objetivos integrar la información recabada en ocho años de estudio (1998 y 2005) y analizar la distribución de los taxones a lo largo de gradientes de altura (2600-4700 m snm) y salinidad (0,2-104 g L-1).&#x0D; M&amp;M: Analizamos las diatomeas halladas en 57 humedales ubicados en las provincias de Catamarca y Jujuy entre los 2340 y 4683 m snm de altura.&#x0D; Resultados: Identificamos 73 géneros de diatomeas con 375 taxones infragenéricos y a muchos de ellos los registramos por primera vez en el país (29) o en la región de estudio (331). Los géneros mejor representados en cuanto al número de especies fueron: Nitzschia (43), Navicula (37), Pinnularia (21), Surirella (17), Planothidium (15), Halamphora (13), Craticula (12), Amphora (11) y Stauroneis (10). En los humedales oligohalinos, la riqueza específica fluctuó entre 7 y 69, en los mesohalinos entre 18 y 54 y en los polihalinos entre 29 a 49. Respecto a la altura, entre los 4301-4500 m snm, ocurrió una importante disminución del número de especies.&#x0D; Conclusiones: La riqueza específica no parece tener una relación directa con la altitud o la salinidad por lo que postulamos que serían otros factores los que condicionarían la diversidad en los ambientes de altura analizados. En este aporte damos a conocer nuevos rangos de tolerancia a la salinidad y la altura para numerosos taxones.

Salt effects on carbon mineralization in southeastern coastal wetland soils of the United States

Widespread saltwater intrusion into freshwater coastal ecosystems could dramatically alter the fate of stored carbon as well as the rate of future soil carbon (C) sequestration. We carried out a large survey of soil C at 51 freshwater wetland sites across the coastal plain of North Carolina. We then conducted a microcosm experiment with soils from a subset of eight sites that spanned the full range of observed soil solution salinity, ranging from oligohaline (ca. 0.09–2.84 ppt) to brackish (ca. 5.8–15 ppt) conditions. Each soil in the microcosm experiment was exposed to four salinity treatments, i.e., deionized water (DIW), artificial sea water with sulfate (ASW), artificial sea water without SO42− (ASW − SO4), and deionized water with SO42− (DIW + SO4) under both oxic and anoxic conditions. Here we focused on whether background salinity affected C use efficiency (or the rate of C mineralization divided by the mass of soil C). Across all 51 surveyed sites, neither soil organic C content nor microbial biomass was correlated with soil salinity. Soil organic C content was however the primary predictor of microbial biomass and rates of C mineralization, and soil chloride concentrations were positively correlated with soil organic C for the eight selected sites. We found that C use efficiency was higher in oligohaline soils, suggesting that organic C compounds in the oligohaline wetland soils may contain more labile fractions. In the microcosm experiment, the addition of marine salts, with or without SO42− ions, inhibited soil C use efficiency. Sulfate added alone had little effect on C use efficiency, but substantially inhibited CH4 production efficiency in anoxic assays for the sites with low soil Cl− concentration. Sulfate additions had no effect on CH4 production for soils from sites with prior salinity exposure. Ionic stress exerted larger inhibitory effect on both aerobic and anaerobic C mineralization than the SO42− effect in the short-term, regardless of the prior exposure to marine salts or not, while SO42− exerted far greater control on the CH4 potential only in sites that have not experienced saltwater intrusion before. Overall our findings suggest that long-term effects of salinity on shifts in C mineralization may differ from short-term experimental responses, likely due to other factors (e.g., microbial tolerance, hydrology, temperature) under field conditions.

Anaerobic organic carbon mineralization in tidal wetlands along a low-level salinity gradient of a subtropical estuary: Rates, pathways, and controls

To examine how the rates and pathways of anaerobic organic carbon mineralization (AOCM) of tidal freshwater wetlands change with low-level increases in salinity, we investigated the rates and controls of microbial iron and sulfate reduction, methane production, and total AOCM in tidal wetlands along a freshwater to oligohaline (0.1–3.3 ppt) gradient in the Min River Estuary in southeastern China. Porewater chloride was found to be strongly correlated with total organic carbon (TOC), carbon to nitrogen (C:N) ratios, and porewater geochemistry (sulfate, pH, ammonium, and dissolved methane levels). Furthermore, a higher plant biomass, larger iron oxides pool, and lower sulfide levels were observed in the oligohaline wetlands. The contribution of microbial sulfate reduction to AOCM increased from 16% to 67%. In contrast, the contribution of microbial iron reduction and methane production declined from 52% to 22% and 12% to 2%, respectively, along the increasing salinity gradient. No consistent changes were found in the total AOCM rates. The rates of methane production were primarily controlled by the C:N ratios and concentrations of porewater ammonium and amorphous iron oxides, while the microbial sulfate and iron reduction rates were mainly controlled by belowground biomass, water content, and concentrations of porewater chloride and sulfate. Our findings provide insight into the potential consequences of modest saltwater intrusion; these may not alter the rates of AOCM in the Min River Estuary, but could change the dominant AOCM pathway from microbial iron reduction to sulfate reduction and accelerate sulfidic effects in this historically freshwater wetland ecosystem.

Wetland Plant Community Responses to the Interactive Effects of Simulated Nutrient and Sediment Loading: Implications for Coastal Restoration Using Mississippi River Diversions

The Mississippi River Delta Complex (MRDC) has experienced extensive wetland loss in the last century due, in part, to flood control levees that have isolated the lower Mississippi River and its sediment resource from adjacent wetlands. Reconnecting the River to these wetlands through diversions is being used and proposed on a larger scale for the future, to reduce wetland loss rates. However, some currently operating diversions (e.g., Caernarvon and Davis Pond) have been implicated in causing negative impacts on wetland ecosystem structure and function due to increased nutrient loads in diverted Mississippi River water combined with insufficient sediment delivery. Initial assessments of these concerns were carried out in a greenhouse setting where six nutrient enrichment treatment levels (control, NO3, NH4, PO4, SO4, and Combo [NO3 + NH4 + PO4 + SO4]) were applied with and without sediment addition to 60 marsh sods from a Sagittaria lancifolia-dominated oligohaline wetland at rates simulating the Davis Pond Diversion of the Mississippi River. After 25 months, independent enrichment with N (regardless of form) and sediment was generally beneficial to wetland structure and function, while SO4 enrichment had the opposite effect, regardless of sediment addition. Simultaneous application of N and P (i.e., the Combo treatment level) ameliorated the negative impacts of SO4-loading, but the concurrent application of sediment did not, likely because the loading rate was based on a diversion that was designed to deliver water and not to maximize sediment input. Nonetheless, sediment input is critical to the sustainability of MRDC wetlands experiencing high rates of deterioration. Thus, optimizing future diversions to maximize sediment delivery, along with continued surveillance of negative nutrient effects, are recommended management decisions.

Contemporary Deposition and Long-Term Accumulation of Sediment and Nutrients by Tidal Freshwater Forested Wetlands Impacted by Sea Level Rise

Contemporary deposition (artificial marker horizon, 3.5 years) and long-term accumulation rates (210Pb profiles, ~150 years) of sediment and associated carbon (C), nitrogen (N), and phosphorus (P) were measured in wetlands along the tidal Savannah and Waccamaw rivers in the southeastern USA. Four sites along each river spanned an upstream-to-downstream salinification gradient, from upriver tidal freshwater forested wetland (TFFW), through moderately and highly salt-impacted forested wetlands, to oligohaline marsh downriver. Contemporary deposition rates (sediment, C, N, and P) were greatest in oligohaline marsh and lowest in TFFW along both rivers. Greater rates of deposition in oligohaline and salt-stressed forested wetlands were associated with a shift to greater clay and metal content that is likely associated with a change from low availability of watershed-derived sediment to TFFW and to greater availability of a coastal sediment source to oligohaline wetlands. Long-term accumulation rates along the Waccamaw River had the opposite spatial pattern compared to contemporary deposition, with greater rates in TFFW that declined to oligohaline marsh. Long-term sediment and elemental mass accumulation rates also were 3–9× lower than contemporary deposition rates. In comparison to other studies, sediment and associated nutrient accumulation in TFFW are lower than downriver/estuarine freshwater, oligohaline, and salt marshes, suggesting a reduced capacity for surface sedimentation (short-term) as well as shallow soil processes (long-term sedimentation) to offset sea level rise in TFFW. Nonetheless, their potentially large spatial extent suggests that TFFW have a large impact on the transport and fate of sediment and nutrients in tidal rivers and estuaries.

Haloform formation in coastal wetlands along a salinity gradient at South Carolina, United States

Environmental contextNatural haloform emissions contribute to stratospheric ozone depletion but there are major unknown or underestimated sources of these gases. This study demonstrates that soil and water at tidal wetlands are important haloform sources, and emissions peak at the forest–marsh transition zone. The low-lying forested wetlands of the south-eastern United States that are facing sea-level rise and seawater intrusion may become hotspots for haloform emission. AbstractSoil haloform emissions are sources of reactive halogens that catalytically deplete ozone in the stratosphere but there are still unknown or underestimated haloform sources. The &gt;200000ha of low-lying tidal freshwater swamps (forests and marshes) in the south-eastern United States could be haloform (CHX3, X=Cl or Br) sources because sea-level rise and saltwater intrusion bring halides inland where they mix with terrestrial humic substances. To evaluate the spatial variation along the common forest–marsh salinity gradient (freshwater wetland, oligohaline wetland and mesohaline saltmarsh), we measured chloroform emissions from in situ chambers and from laboratory incubations of soil and water samples collected from Winyah Bay, South Carolina. The in situ and soil-core haloform emissions were both highest in the oligohaline wetland, whereas the aqueous production was highest in mesohaline saltmarsh. The predominant source shifted from sediment emission to water emission from freshwater wetland to mesohaline saltmarsh. Spreading out soil samples increased soil haloform emission, suggesting that soil pores can trap high amounts of CHCl3. Soil sterilisation did not suppress CHCl3 emission, indicating the important contribution of abiotic soil CHCl3 formation. Surface wetland water samples from eight locations along a salinity gradient with different management practices (natural v. managed) were subjected to radical-based halogenation by Fenton-like reagents. Halide availability, organic matter source, temperature and light irradiation were all found to affect the radical-based abiotic haloform formation from surface water. This study clearly indicates that soil and water from the studied coastal wetlands are both haloform sources, which however appear to have different formation mechanisms.

Plant–plant interactions in a subtropical mangrove‐to‐marsh transition zone: effects of environmental drivers

AbstractQuestionsDoes the presence of herbaceous vegetation affect the establishment success of mangrove tree species in the transition zone between subtropical coastal mangrove forests and marshes? How do plant–plant interactions in this transition zone respond to variation in two primary coastal environmental drivers?LocationSubtropical coastal region of the southern United States.MethodsWe conducted a greenhouse study to better understand how abiotic factors affect plant species interactions in the mangrove‐to‐marsh transition zone, or ecotone. We manipulated salinity (fresh, brackish or salt water) and hydrologic conditions (continuously saturated or 20‐cm tidal range) to simulate ecotonal environments. Propagules of the mangroves Avicennia germinans and Laguncularia racemosa were introduced to mesocosms containing an established marsh community. Both mangrove species were also introduced to containers lacking other vegetation. We monitored mangrove establishment success and survival over 22 mo. Mangrove growth was measured as stem height and above‐ground biomass. Stem height, stem density and above‐ground biomass of the dominant marsh species were documented.ResultsEstablishment success of A. germinans was reduced under saturated saltwater conditions, but establishment of L. racemosa was not affected by experimental treatments. There was complete mortality of A. germinans in mesocosms under freshwater conditions, and very low survival of L. racemosa. In contrast, survival of both species in monoculture under freshwater conditions exceeded 62%. The marsh species Distichlis spicata and Eleocharis cellulosa suppressed growth of both mangroves throughout the experiment, whereas the mangroves did not affect herbaceous species growth. The magnitude of growth suppression by marsh species varied with environmental conditions; suppression was often higher in saturated compared to tidal conditions, and higher in fresh and salt water compared to brackish water.ConclusionsOur results indicate that herbaceous marsh species can suppress mangrove early seedling growth. Depending on species composition and density, marsh plants can slow mangrove landward migration under predicted climate change scenarios as salinity in freshwater and oligohaline wetlands increases with rising sea levels. Change in the relative coverage of mangrove forests and marshes will depend on both the ability of marsh species to migrate further inland as mangroves advance, and the ability of shoreline mangroves to adjust to rising sea level through accretionary processes.

Biogeochemical Recovery of Oligohaline Wetland Soils Experiencing a Salinity Pulse

Oligohaline wetlands exist at a dynamic interface along a river-to-estuary gradient where salinity changes frequently due to fluctuating tidal cycles and weather events. Large storms can cause short-term saltwater intrusion into these low-lying coastal areas, exposing oligohaline wetland plants and soils to above-normal salinities and severe stress on wetland organisms. The objective of this study was to determine how oligohaline wetland soils respond to a single pulse saltwater intrusion, similar to what may occur during hurricane-associated storm surge. Soil response variables were measured in intrusion-Impacted and Reference soils before, during, and after a 6-wk saltwater intrusion event to determine potential impacts on microbial biomass and activity and associated nutrient dynamics. We observed no significant change in basal CO₂ or CH₄ production in oligohaline soils (interstitial salinity = 1.3 ± 0.1 practical salinity units [psu]; mean ± 1 SD) exposed to 20 psu saltwater, while substrate-induced methanogenesis was negatively correlated with salinity. Microbial biomass C (MBC) responded positively to saltwater intrusion by doubling in concentration at the 0- to 5-cm depth interval. Saltwater intrusion had no impact on porewater nutrient concentrations; however, extractable NH₄⁺ decreased as salinity increased. Although some significant changes in microbial activity, abundance, and nutrient availability occurred due to saltwater intrusion, these impacts were generally transient, with post-intrusion conditions resembling pre-intrusion conditions. These results suggest that short-term and transient saltwater intrusion may have little longer-term effect on wetland soil biogeochemistry. However, compounding effects of frequent saltwater intrusion pulses due to strong, regularly occurring storm events could cause longer-lasting shifts in biogeochemical functioning of these wetlands.

Invertebrate Communities of Low-Salinity Wetlands: Overview and Comparison between Phragmites and Typha Marshes Within the Hudson River Estuary

Benthic macroinvertebrate abundance, taxonomic composition, and surface flooding dynamics were compared among high and low elevation stands of narrow-leaved cattail (Typha angustifolia) and invasive common reed (Phragmites australis) at Iona Island Marsh, an oligohaline wetland, and Piermont Marsh, a mesohaline wetland, within the Hudson River National Estuarine Research Reserve during 1999 and 2000. Overall, the benthic macroinvertebrate community at both sites was similar in composition and abundance to those documented from other low-salinity systems. Macroinvertebrate taxa richness was lowest in mesohaline common reed, but similar among common reed and cattail habitats in oligohaline wetlands. Total macroinvertebrate densities were greater at high-elevation compared to low-elevation reed stands at the mesohaline site during summer 1999 and spring 2000. Total macroinvertebrate densities were similar among both oligohaline vegetation types during all seasons, except for spring 2000, when lower densities were observed in low-elevation common reed. A weak positive relationship between macroinvertebrate density and depth of flooding suggests that surface hydrology may be influencing the observed patterns of macroinvertebrate density among the vegetation stands. These results suggest that benthic macroinvertebrate abundance and diversity may not necessarily be impaired in low-salinity wetlands experiencing invasion by common reed unless the change in vegetation is accompanied by a measurable alteration to physical conditions on the marsh surface (i.e., elevation and flooding dynamics).

Tidal marsh plant community response to sea-level rise: A mesocosm study

Recent evidence from field surveys suggests that periodic saline intrusions into tidal freshwater wetlands may not reduce plant species richness and that reductions in richness may not occur until a more consistent brackish salinity regime develops. In a greenhouse experiment using tidal wetland mesocosms, we tested the hypothesis that plant species richness of coastal marshes (wetlands dominated by herbaceous plants) would be similar under oligohaline (0.5–5) and freshwater conditions (<0.5) (psu; practical salinity units), provided that propagules of salt-tolerant species were present. We also hypothesized that plant community responses would depend on inundation frequency. Species richness was highest in fresh and oligohaline treatments (<5, 11–13 species per mesocosm) and decreased with increasing salinity up to 12. Inundation frequency did not significantly influence species richness. Aboveground biomass decreased weakly as salinity increased, but the difference between salinity treatments was significant only at the 0.1 level. Flooding did not significantly alter aboveground biomass, and belowground biomass was not significantly influenced by either salinity or flooding treatments (P<0.05). Biomass of individual species and non-metric multidimensional scaling (NMS) analysis indicated a shift toward salt-tolerant plant communities with increasing salinity, but detected little influence of inundation. These findings suggest that species composition shifts may prevent strong reductions in biomass due to loss of individual species as salinity increases, and that community composition will change more rapidly in response to increases in salinity than to inundation increases. Also, saltwater intrusions into low-salinity marshes of up to 6 will not create an immediate and significant reduction in species richness assuming propagules of salt-tolerant plants are available and given the tolerance of many Fresh/Oligohaline species observed in this study to salinity levels ≥6. However, the species composition of fresh and oligohaline wetlands can shift toward a less species-rich brackish plant community within a single growing season under prolonged salinity exposures of 12.

Nitrogen and Phosphorus Differentially Affect Annual and Perennial Plants in Tidal Freshwater and Oligohaline Wetlands

To test species composition and biomass responses to excess nutrients, herbaceous plants of tidal freshwater and oligohaline wetlands in a Chesapeake Bay subestuary were fertilized with nitrogen (N), phosphorus (P), both N and P (N + P), or not fertilized (Control) for 4 years. In marshes, the N treatment increased abundance measures of perennials but decreased those of annuals while the P treatment increased annuals and decreased perennials. In swamps, however, perennials increased in response to P. Total herbaceous aboveground biomass production was not limited by N, P, or N + P in marshes or swamps. These findings suggest that annual species are more susceptible than perennials to P limitation, possibly due to lack of a large perenniating root organ and lower susceptibility to mycorrhizal inoculation. Furthermore, eutrophication effects are likely to vary between swamp and marsh habitats and depend on whether the dominant nutrient supplied is nitrogen or phosphorus.

Mercury concentrations in oligohaline wetland vegetation and associated soil biogeochemistry

Concentrations of mercury were determined in above- and below-ground tissues of dominant plant species, as well as soils, in the wetlands of Lake Maurepas, Louisiana. Indicators of wetland soil biogeochemical status, such as soil redox potential, pore-water nutrient concentrations, and pore-water total sulfides, were also determined. Total mercury concentrations in plant tissues were within the typical range for vegetation not exposed to mercury contamination. Similarly, total mercury concentrations in soils were typical of uncontaminated wetlands within this geographic region. Soil methyl mercury levels in this study are slightly lower than those reported in other studies of nearby wetlands. This may reflect the less extensive geographic sampling in this study, or the low water levels in the Lake Maurepas system immediately prior to and during this study, which would have altered soil biogeochemical status. This is corroborated by measurements of soil redox potential and soil pore-water nitrogen and sulfur constituents conducted during this study that suggest minimal sulfate reduction was occurring in surficial soils. This study indicates that the wetlands surrounding Lake Maurepas are typical of many uncontaminated oligohaline wetlands in the southeastern U.S. in regard to mercury concentrations.

Physical and chemical characteristics of two wetlands in Lake Danao Natural Park, Ormoc City

The study was conducted to characterize the wetlands located at Sitio Quinto Limbo and Tasakan in Lake Danao Natural Park, Ormoc City based on some physicochemical parameters. The two wetlands are generally shallow. Salinity ranged from 0.3 to 0.5 ppt which is characteristic of an oligohaline wetland. Sediment and water were slightly acidic. Concentrations of ammonium ions were lower than the normal value. Levels of nitrate and phosphate ions exceeded the normal value of 1mg/L and 0.025 mg/L, respectively. Lead ion concentrations were generally within the normal range (10-100 mg/kg). Mean cadmium ion concentration was normal (0.01-1.0 mg/kg) although in some sites concentrations beyond the normal range were observed. Based on the findings, it is recommended that conservation program be urgently implemented in the study area to ensure the conservation of the wetlands and their resources.

Ecosystem Characteristics and Management Considerations for Coastal Wetlands in Mexico

Ecological parameters characterizing physico-chemical and biological conditions were collected from a total of 29 representative wetlands that occur between the near-shore terrestrial ecosystems and the outer edge of the tidal zone along the coasts of Mexico. This total includes about 40% of the total extent of coastal wetlands in Mexico. It is possible to recognize four distinct groups of wetlands based largely on the salinity gradients: oligohaline (0–10 ppt), estuarine (10–30 ppt), euhaline (30–40 ppt), and hyperhaline (>40 ppt). Estuarine wetlands are the most abundant and widespread group. Oligohaline wetlands are more common in the south and the hyperhaline wetlands are more common along the north coasts of Mexico. Euhaline systems are in specialized localities scattered along both coasts. Human activities and development pressures continue to threaten the coastal wetlands in Mexico. This paper presents the first survey of ecological characteristics of the coastal wetlands in Mexico. These data highlight the range of variability that exists, demonstrates problems such as eutrophication, and provides a basis for future more detailed biomonitoring, management and conservation programs for these highly vulnerable and economically important ecosystems in Mexico.