Assessing the potential for biotic communities to recolonise freshwater wetlands affected by sulfidic sediments

1. The formation of sulfidic sediments in response to factors such as secondary salinisation and fertiliser usage is an emerging concern for the management of many freshwater wetlands. However, fundamental knowledge regarding the influence of sulfidic sediments on the aquatic biota is still lacking. 2. This study investigated the potential for biota to recolonise wetlands affected by sulfidic sediments, by assessing zooplankton hatching and aquatic plant germination following inundation with freshwater. Sediment samples were collected from 16 wetlands in the southern Murray‐Darling Basin, Australia, that ranged in condition from non‐impacted to possessing a known history of sulfidic sediments and/or acidification. 3. Principal Components Analysis indicated that the wetlands separated out into five different groups based on their sediment chemistry: non‐impacted, sulfidic, sulfidic and highly saline (sediment EC 46 800–209 000 μS cm^?1), sulfidic and potentially acidic (sediment pH 5.81–6.45 and ANC 0.07–0.31% CaCO₃), and sulfidic and acidic (sediment pH 4.37 and ANC 0.00% CaCO₃). 4. A viable dormant propagule bank was present in all wetlands, but the taxon richness of zooplankton and aquatic plants was significantly lower in wetlands affected by sulfidic sediments compared with those that were non‐affected. 5. This suggests that zooplankton and aquatic plants will be capable of recolonising wetlands that have accumulated sulfidic sediments via their propagule banks if the appropriate remediation measures are undertaken, although the communities developing are likely to be less diverse compared with those in non‐affected wetlands.     

Influence of water-level fluctuation duration and magnitude on sediment–water nutrient exchange in coastal wetlands

This study examined the influence of water-level fluctuation (WLF) on sediment–water nutrient exchange in the Laurentian Great Lakes. Water levels in the Laurentian Great Lakes have been below the long-term mean for the past 15 years, causing the exposure of sediments that previously have been either continuously inundated or periodically exposed. The magnitude, duration, and frequency of WLF, as well as land-use history, each can influence the amount and type of sediment–water nutrient exchange. We collected sediment cores from relatively pristine coastal wetlands located on Beaver and Garden Islands in northern Lake Michigan. Sediment cores were taken from different water depths to simulate WLF magnitude; desiccation time was experimentally manipulated to simulate WLF duration. At these relatively pristine wetlands, desiccation time and water depth significantly influenced flux. However, nutrient exchange did not behave in a consistent fashion; phosphorus, nitrate, ammonium, and sulfate flux varied based on sediment exposure history and desiccation time. Sediment–water nutrient exchange rates also were compared to prior measurements taken from more impacted coastal wetlands in southern Lake Michigan and Saginaw Bay in Lake Huron. This comparison revealed a stronger influence of anthropogenic stress than desiccation time, with impacted wetland sediments releasing more soluble reactive phosphorus, sulfate, and ammonium, and retaining more nitrate, than pristine wetlands. Our results indicate that WLFs have the potential to influence sediment–water nutrient exchange, which may influence system productivity, but environmental context can override this influence.     

Linkages between organic matter mineralization and denitrification in eight riparian wetlands

Denitrification (N₂ production) and oxygen consumption rates were measured at ambient field nitrate concentrations during summer in sediments from eight wetlands (mixed hardwood swamps, cedar swamps, heath dominated shrub wetland, herbaceous peatland, and a wetland lacking live vegetation) and two streams. The study sites included wetlands in undisturbed watersheds and in watersheds with considerable agricultural and/or sewage treatment effluent input. Denitrification rates measured in intact cores of water-saturated sediment ranged from 20 to 260 mol N m^-2 h^-1 among the three undisturbed wetlands and were less variable (180 to 260 mol N M^-2 h^-1) among the four disturbed wetlands. Denitrification rates increased when nitrate concentrations in the overlying water were increased experimentally (1 up to 770 M), indicating that nitrate was an important factor controlling denitrification rates. However, rates of nitrate uptake from the overlying water were not a good predictor of denitrification rates because nitrification in the sediments also supplied nitrate for denitrification. Regardless of the dominant vegetation, pH, or degree of disturbance, denitrification rates were best correlated with sediment oxygen consumption rates (r ² = 0.912) indicating a relationship between denitrification and organic matter mineralization and/or sediment nitrification rates. Rates of denitrification in the wetland sediments were similar to those in adjacent stream sediments. Rates of denitrification in these wetlands were within the range of rates previously reported for water-saturated wetland sediments and flooded soils using whole core¹⁵N techniques that quantify coupled nitrification/denitrification, and were higher than rates reported from aerobic (non-saturated) wetland sediments using acetylene block methods.     

Changes in biotic communities developing from freshwater wetland sediments under experimental salinity and water regimes

1. Reduction in diversity of both freshwater aquatic and terrestrial ecosystems has been attributed to salinity increase and such increases are a symptom of changes to land use. Hydrological alteration to ground and surface water are likely to be associated with salinity increase and its influence on biodiversity. However the combined effects of salinity and hydrology on aquatic biodiversity have not been elucidated fully in either field or experimental situations. 2. The effect of salinity and water regime on the biota in sediments from seven wetlands from inland south‐eastern Australia was tested experimentally using germination of aquatic plant seeds (five salinity and two water levels) and emergence of zooplankton eggs (five salinity levels). Salinity levels were <300, 1000, 2000, 3000, 5000 mg L^?1 and water regimes were damp (waterlogged) and submerged. 3. Aquatic plant germination and zooplankton hatching was not consistent for all seven wetland sediments. Four of the wetland sediments, Narran Lakes, Gwydir Wetlands, Macquarie Marshes and Billybung Lagoon showed similar responses to salinity and water regime but the other three wetland sediments from Lake Cowal, Great Cumbung Swamp and Darling Anabranch did not. 4. As salinity increased above 1000 mg L^?1 there was a decrease in the species richness and the abundance of biota germinating or hatching from sediment from four of the wetlands. 5. Salinity had a particularly strong effect in reducing germination from sediments in damp conditions when compared to the flooded conditions. In parallel, salts accumulated in the sediment in damp conditions but did not in flooded conditions. 6. There is potential for increasing salinity in freshwater rivers and wetlands to decrease the species richness of aquatic communities and thus of the wetland community as a whole, resulting in loss of wetland biodiversity. This reduction in diversity varies between wetlands and is at least partly related to hydrology. For aquatic plants the reduction in diversity will be more marked for plants germinating from seed banks at the edges of wetlands where plants are not completely submerged than for the same seed bank germinating in submerged conditions.     

Vertical distribution and diversity of sulfate-reducing prokaryotes in the Pearl River estuarine sediments, Southern China

The vertical distribution and diversity of sulfate-reducing prokaryotes (SRPs) in a sediment core from the Pearl River Estuary was reported for the first time. The profiles of methane and sulfate concentrations along the sediment core indicated processes of methane production/oxidation and sulfate reduction. Phospholipid fatty acids analysis suggested that sulfur-oxidizing bacteria (SOB) might be abundant in the upper layers, while SRPs might be distributed throughout the sediment core. Quantitative competitive-PCR analysis indicated that the ratios of SRPs to total bacteria in the sediment core varied from around 2–20%. Four dissimilatory sulfite reductase ( dsrAB) gene libraries were constructed and analyzed for the top layer (0–6 cm), middle layer (18–24 cm), bottom layer (44–50 cm) and the sulfate-methane transition zone (32–42 cm) sediments. Most of the retrieved dsrAB sequences (80.9%) had low sequence similarity with known SRP sequences and formed deeply branching dsrAB lineages. Meanwhile, bacterial 16S rRNA gene analysis revealed that members of the Proteobacteria were predominant in these sediments. Putative SRPs within Desulfobacteriaceae, Syntrophaceae and Desulfobulbaceae of Deltaproteobacteria , and putative SOB within Epsilonproteobacteria were detected by the 16S rRNA gene analysis. Results of this study suggested a variety of novel SRPs in the Pearl River Estuary sediments.     

The spatial distribution of soil organic carbon in tidal wetland soils of the continental United States

Tidal wetlands contain large reservoirs of carbon in their soils and can sequester carbon dioxide (CO₂) at a greater rate per unit area than nearly any other ecosystem. The spatial distribution of this carbon influences climate and wetland policy. To assist with international accords such as the Paris Climate Agreement, national‐level assessments such as the United States (U.S.) National Greenhouse Gas Inventory, and regional, state, local, and project‐level evaluation of CO₂ sequestration credits, we developed a geodatabase (CoBluCarb) and high‐resolution maps of soil organic carbon (SOC) distribution by linking National Wetlands Inventory data with the U.S. Soil Survey Geographic Database. For over 600,000 wetlands, the total carbon stock and organic carbon density was calculated at 5‐cm vertical resolution from 0 to 300 cm of depth. Across the continental United States, there are 1,153–1,359 Tg of SOC in the upper 0–100 cm of soils across a total of 24 945.9 km² of tidal wetland area, twice as much carbon as the most recent national estimate. Approximately 75% of this carbon was found in estuarine emergent wetlands with freshwater tidal wetlands holding about 19%. The greatest pool of SOC was found within the Atchafalaya/Vermilion Bay complex in Louisiana, containing about 10% of the U.S. total. The average density across all tidal wetlands was 0.071 g cm^?3 across 0–15 cm, 0.055 g cm^?3 across 0–100 cm, and 0.040 g cm^?3 at the 100 cm depth. There is inherent variability between and within individual wetlands; however, we conclude that it is possible to use standardized values at a range of 0–100 cm of the soil profile, to provide first‐order quantification and to evaluate future changes in carbon stocks in response to environmental perturbations. This Tier 2‐oriented carbon stock assessment provides a scientific method that can be copied by other nations in support of international requirements.     

Seasonal Cycling of Fe and S in a Constructed Wetland: The Role of Sulfate-Reducing Bacteria

The role of sulfate-reducing bacteria (SRB) in the cycling of Fe and S was studied in a young constructed wetland located in Kanata, Ontario, Canada. The wetland is a surface-flow system composed of three consecutive cells. Sediments and water samples were collected over the course of 1 year within each cell. Sediments were analyzed for the presence of SRB (using a lactate-rich medium), whereas surface and porewaters were analyzed for their concentrations of dissolved Fe and sulfate and for pH, E_h, and dissolved organic carbon. Lactate-using SRB were present at all three sites within the wetland, and the populations were largest (10¹⁰ colony-forming units per gram of sediment) during the cold winter months, where the temperature of the water was 1°C. The presence of high-SRB populations also corresponded to highly anoxic conditions within the sediments and to a decrease of sulfate concentrations, suggesting that cold temperature did not affect the activity of SRB. Our results indicate that Fe and S cycling in the young constructed wetland was active throughout the year, especially in the cold winter months, where large SRB populations were encountered. This suggests that Fe removal in wetlands can be effective in temperate climates, even though the temperature of the water decreases drastically during the winter.     

Quantitative analyses of ammonia-oxidizing Archaea and bacteria in the sediments of four nitrogen-rich wetlands in China

With the rapid development of ammonia-synthesizing industry, the ammonia-nitrogen pollution in wetlands acting as the sink of point and diffuse pollution has been increased dramatically. Most of ammonia-nitrogen is oxidized at least once by ammonia-oxidizing prokaryotes to complete the nitrogen cycle. Current research findings have expanded the known ammonia-oxidizing prokaryotes from the domain Bacteria to Archaea. However, in the complex wetlands environment, it remains unclear whether ammonia oxidation is exclusively or predominantly linked to Archaea or Bacteria as implied by specific high abundance. In this research, the abundance and composition of Archaea and Bacteria in sediments of four kinds of wetlands with different nitrogen concentration were investigated by using quantitative real-time polymerase chain reaction, cloning, and sequencing approaches based on amoA genes. The results indicated that AOA distributed widely in wetland sediments, and the phylogenetic tree revealed that archaeal amoA functional gene sequences from wetlands sediments cluster as two major evolutionary branches: soil/sediment and sediment/water. The bacteria functionally dominated microbial ammonia oxidation in different wetlands sediments on the basis of molecule analysis, potential nitrification rate, and soil chemistry. Moreover, the factors influencing AOA and AOB abundances with environmental indicator were also analyzed, and the results addressed the copy numbers of archaeal and bacterial amoA functional gene having the higher correlation with pH and ammonia concentration. The pH had relatively great negative impact on the abundance of AOA and AOB, while ammonia concentration showed positive impact on AOB abundance only. These findings could be fundamental to improve understanding of the importance of AOB and AOA in nitrogen and other nutrients cycle in wetland ecosystems.     

Can the use of zooplankton dormant stages from natural wetlands contribute to restoration of mined wetlands?

Wetlands are among the most diverse environments on the planet and are strongly threatened by human activities. Their restoration and/or mitigation of human impacts, therefore, relies on information that can aid to the management of impacted wetlands so that they return to a (semi-) natural state. We investigate in this study the relationship between dormant stages of zooplankton and clay removal in areas subjected to mining. We evaluate whether a gradual increase in topsoil addition from donor natural wetlands to the sediment of mined wetlands influenced the zooplankton community. Eight wetlands were sampled in the Sinos River floodplain, four natural and four mined. In the laboratory, four field sediment samples were incubated for zooplankton hatching in five treatments comprising sediments from: mined wetlands, natural wetlands, and three treatments containing mined sediments added with low (5%), medium (20%) and high (40%) quantities of sediment from natural wetlands. Hatching consisted of 61 individuals distributed across eight zooplankton taxa. Copepod nauplii were the most abundant (31.1%) followed by Epiphanes sp. (29.5%) and Ovalona glabra (16.4%). While natural wetlands provided 42.6% of the hatched zooplankton, mined wetlands had just 6.5%. Zooplankton richness and abundance were higher in natural wetland sediments compared with mined and added sediment wetlands. To some degree, the sediment soil donation from natural to mined wetlands was considered viable. As long as prior studies are performed to test the size and quality of the dormant banks present in the sediment of candidate donor wetlands, sediment from donor wetlands may aid in the establishment of a more diverse community in disturbed systems.

     

Book reviews

Wetlands have been widely applied for water quality amelioration. Enzymatic analysis was applied in a study of decomposition in constructed wetlands. We hypothesise that soil enzyme activities would be lower in wetland sediment than adjacent upland and that the lower soil enzyme activities are partly responsible for the water quality amelioration. Four soil enzyme activities (β-glucosidase, β-N-acetylglucosaminidase, phosphatase, and arylsulfatase) and microbial activity (electron transport system activity) were measured across a transect from a upland soil to a wetland sediment in two constructed wetland sites in the USA. Along with the activities, hydrochemistry was determined in inflow and outflow of the wetlands. In both wetlands, the enzyme activities in the sediments were significantly lower than the adjacent upland soils. For hydrochemistry, significant decreases were observed in phosphate and nitrate concentrations in outflow water compared to inflow water. However, there were no significant changes in other anions (F^-, Cl^-, SO ₄ ^2- . For dissolved organic carbon, it seems that the wetlands would be a source rather than a sink. The results suggest that the enzymatic approach represents a valuable method to assess decomposition processes in wetland sediments, and that characteristically low enzyme activities in the sediments may be important in the water quality amelioration function. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effects of in situ drying on sediment–phosphate interactions in sediments from an old wetland

We examined the effects of drying on sediment–P interactions for sediments from a shallow wetland ca. 6000 years old. Sediments from this wetland would have been subjected to numerous drying and wetting episodes during the life of the wetland. The factors affecting potential orthophosphate release were compared in surface sediments that had previously been desiccated for 3 months, surface sediments that had remained inundated and wet sediment 25–30 cm below the surface. All sediments released small amounts of orthophosphate once subjected to anaerobic conditions. Sulfate alone stimulated phosphate release from the surface sediment, irrespective of the previous hydrological status, whereas orthophosphate release from deep sediments was co-limited by carbon and sulfate. Decreases in soluble Fe (II) were measured concurrently with sulfate-stimulated P release, which is consistent with formation of iron sulfides. Similar numbers of culturable sulfate-reducing bacteria were obtained from the wet and dry sediments, their tolerance to desiccation explaining why release could occur after extensive drying of the sediment. Phosphate adsorption isotherms of the sediments showed that sediments from all sites had a relatively low affinity for ortho-phosphate. The adsorption data showed reasonable fit to the Freundlich adsorption isotherm. No difference in the maximum amount of ortho-phosphate uptake was observed if the sediment isotherm experiments were done under an inert atmosphere or, following exposure to air, suggesting that even anoxic sediments were not highly reduced. However, significant variations in the value of the Freundlich constants were observed. The results presented in this study are consistent with the hypothesis that repeated wetting/drying cycles select for bacteria that are tolerant to periods of desiccation and/or oxidation. Furthermore, repeated wetting and drying cycles may result in changes to sediment mineralogy.     

Effects of Water Level on Three Wetlands Soil Seed Banks on the Tibetan Plateau

Background

Although the effect of water level on germination in soil seed banks has been documented in many ecosystems, the mechanism is not fully understood, and to date no empirical studies on this subject exist. Further, no work has been done on the effect of water level on seed banks of drying and saline-alkaline wetlands in alpine areas on the Tibetan Plateau.

Methodology

We examined the effects of water level (0 cm, 5 cm and 10 cm) on seed germination and seedling establishment from soil seed banks at 0–5 cm and 5–10 cm depths in typical, drying, and saline-alkaline wetlands. We also explore the potential role of soil seed bank in restoration of drying and saline-alkaline wetlands.

Principal Findings

Species richness decreased with increase in water level, but there almost no change in seed density. A huge difference exists in species composition of the seed bank among different water levels in all three wetlands, especially between 0 cm and 5 cm and 0 cm and 10 cm. Similarity of species composition between seed bank and plant community was higher in 0 cm water level in drying wetland than in the other two wetlands. The similarity was much higher in 0 cm water level than in 5 cm and 10 cm water levels in all three wetlands. Species composition of the alpine wetland plant community changed significantly after drying and salinization, however, species composition of the seed bank was unchanged regardless of the environment change.

Conclusions/Significance

Water level greatly affects seed bank recruitment and plant community establishment. Further, different water levels in restored habitats are likely to determine its species composition of the plant community. The seed bank is important in restoration of degraded wetlands. Successful restoration of drying and salinization wetlands could depend on the seed bank.     

Colonisation by Dreissena of Great Lakes coastal ecosystems: how suitable are wetlands?

1.  Invasive zebra ( Dreissena polymorpha ) and quagga mussels ( Dreissena bugensis ) have become widespread throughout the Great Lakes basin. However, some types of Great Lakes coastal wetlands may be unsuitable for Dreissena invasion.
2.  To test this observation, artificial substrata were placed in wetlands (with emergent vegetation) and in adjacent open water (without emergent vegetation) habitats in two types of Great Lakes coastal ecosystems: drowned river mouth (DRM) and coastal fringing systems. Wetlands in DRM systems generally have deep organic sediment and limited water movement, whereas coastal fringing wetlands generally have low to moderate amounts of organic sediment and intense wind and wave action.
3.  We did not find a significant difference in Dreissena colonisation between wetlands and adjacent open water habitat in fringing systems. However, Dreissena colonisation was significantly lower in DRM wetlands than in the adjacent open water. We also found significantly lower survival in DRM wetlands than adjacent open water habitats, whereas survival did not differ significantly in coastal fringing wetlands and the adjacent open water.
4.  Our results suggest that vulnerability to Dreissena invasion varied among wetland types with DRM wetlands being less suitable than fringing wetlands. We suggest that colonisation and survival of Dreissena is lower in wetlands with deep organic sediment and less turbulent water.     

Reducing acidic discharges from coastal wetlands in eastern Australia

Much of eastern Australia's coastal lowlands are underlain by Holocene sulfidic sediments. Large areas have been drained for agriculture. Drained, sulfidic sediments oxidize and produce highly acidic discharge (pH<4) with significant impacts on estuarine ecosystems. The rate of production of acid from drained floodplains is between 100 to 300 kg H₂SO₄ /ha/y and hundreds of tonnes of H₂SO₄ can be discharged in a single flood from the floodplain. Generation and export of acidity is controlled by the water balance of the floodplain, the characteristics of the drainage system and the distribution of sulfides. Evapotranspiration by native plants and crops plays a dominant role in the oxidation of sediments in dry periods. In wet periods, upland discharges to floodplains dominate the water balance. Drain spacing and drain depth are critical factors in the export of acidity into coastal streams. Amelioration of acidic outflows requires an understanding of the interaction between chemical and hydrological processes in sulfidic landscapes. Redesign of drainage systems to manage surface waters and reduce drain density with the treatment of drains with lime offer promise for treating acidic discharge and reducing impacts. Reflooding of drained, partially oxidized floodplains with freshwater may not be a panacea because of the large volumes of acid stored in the soil, a lack of labile organic matter in the sediments needed to reduce sulfate and irreversible changes to the soil due to oxidation. Tidal brackish water reflooding of unproductive acidified lowlands offers promise for rehabilitating wetlands. Sulfidic wetlands which are still undrained should remain so unless all acidic discharge can be treated.     

Wetland environmental bioreactor system contributes to the decomposition of cellulose

Recently, numerous species of aquatic invertebrates inhabiting wetlands have been shown to possess endogenous cellulase, following the discovery that termites have cellulase genes encoded in their own genome rather than relying on symbiotic bacteria for decomposing cellulose. Wetlands have been empirically shown to play an important role in the decomposition of land‐originating hard‐to‐degrade polysaccharides such as cellulose. However, the mechanism that connects the cellulase producer and the wetlands remains unknown, which makes it very difficult to evaluate the ecological function of wetlands. Here we found that a macrobenthic bivalve, Corbicula japonica, secretes its cellulase to the wetland sediment. Secreted cellulases are immobilized in the components of the sediment. Moreover, adding cellulose or glucose to C. japonica could trigger its cellulase secretion level. These findings suggest a novel wetland cellulose decomposition mechanism. The decomposition ability of wetlands was previously ascribed only to microbes and/or invertebrates that contain cellulases. Our findings suggest that benthic animals supply wetlands with their enzymes as decomposition agents, while wetland sediments serve as immobilization scaffolds for the enzymes. This system, which was named by us an “environmental bioreactor system,” could provide a key function in wetlands.     

Phytoremediation in Wetland Ecosystems: Progress,Problems, and Potential

Assessing the phytoremediation potential of wetlands is complex due to variable conditions of hydrology, soil/sediment types, plant species diversity, growing season, and water chemistry. Conclusions about long-term phytoremediation potential are further complicated by the process of ecological succession in wetlands. This review of wetlands phytoremediation addresses the role of wetland plants in reducing contaminant loads in water and sediments, including metals; volatile organic compounds (VOC), pesticides, and other organohalogens; TNT and other explosives; and petroleum hydrocarbons and additives. The review focuses on natural wetland conditions and does not attempt to review constructed wetland technologies. Physico-chemical properties of wetlands provide many positive attributes for remediating contaminants. The expansive rhizosphere of wetland herbaceous shrub and tree species provides an enriched culture zone for microbes involved in degradation. Redox conditions in most wetland soil/sediment zones enhance degradation pathways requiring reducing conditions. However, heterogeneity complicates generalizations within and between systems. Wetland phytoremediation studies have mainly involved laboratory microcosm and mesocosm technologies, with the exception of planted poplar communities. Fewer large-scale field studies have addressed remediation actions by natural wetland communities. Laboratory findings are encouraging with regards to phytoextraction and degradation by rhizosphere and plant tissue enzymes. However, the next phase in advancing the acceptance of phytoremediation as a regulatory alternative must demonstrate sustained contaminant removal by intact natural wetland ecosystems.     

Effects of wetting and drying on methane emissions from ephemeral floodplain wetlands in south-eastern Australia

Four approaches were used to assess the effect ofinundation on methane emissions from floodplainwetlands in Victoria, Australia: (i) fieldobservations following natural rainfall events; (ii)experimental manipulation of water levels in smallfloodplain depressions; (iii) experimentalmanipulation of water levels in replicated mesocosms;and (iv) in vitro incubation of floodplainsediment under laboratory conditions. Raftery'sSwamp, a large (150 ha) wetland on the floodplain ofthe Goulburn River, became inundated in June 1993following autumn-winter rainfall. Methane emissionspeaked (1.7 ± 0.05 mmol m^-2 h^-1) somesix months later, and the methane content of sedimentgas bubbles reached 59% v/v, even though the positivesediment redox potentials (176 to 243 mV) indicatedthat sediments were only moderately reducing. Threesmall (< 1 ha) depressions on the floodplains of theRiver Murray and Kiewa River were inundated eithernaturally (by rain and/or overflow from nearby rivers)or artificially by flooding at specific times of year;emissions from these sites were usually negligibleafter flooding in autumn or winter. In contrast, theonset of methane emission was very rapid (within 3 to6 days) after the depressions had been flooded insummer, and the methane content of sediment gasbubbles could then reach 36% v/v. At their peak,emissions from the ephemeral wetlands were similar topeak emissions from permanent wetlands insouth-eastern Australia. Emissions from replicatedwetland mesocosms (4.5 m diameter, 0.9 m deep) werealways very small (<0.2 mmol m^-2 h^-1),regardless of time of flooding, water depths, orseason. In vitro incubation of wetland sedimentunder anaerobic conditions indicated a progressivedecrease in benthic methanogenesis with sedimentdesiccation and exposure to air. Ephemerallyinundated floodplain wetlands may be sites ofsignificant methane emission, especially over thesummer months. Moreover, the survival and rapidreactivation of methanogenic archaea after prolongeddrying of wetland sediments suggests thatmethanogenesis is possible even from re-wettedfloodplain environments that had earlier experiencedan extended dry phase.     

Groundwater shapes sediment biogeochemistry and microbial diversity in a submerged Great Lake sinkhole

For a large part of earth's history, cyanobacterial mats thrived in low‐oxygen conditions, yet our understanding of their ecological functioning is limited. Extant cyanobacterial mats provide windows into the putative functioning of ancient ecosystems, and they continue to mediate biogeochemical transformations and nutrient transport across the sediment–water interface in modern ecosystems. The structure and function of benthic mats are shaped by biogeochemical processes in underlying sediments. A modern cyanobacterial mat system in a submerged sinkhole of Lake Huron (LH) provides a unique opportunity to explore such sediment–mat interactions. In the Middle Island Sinkhole (MIS), seeping groundwater establishes a low‐oxygen, sulfidic environment in which a microbial mat dominated by Phormidium and Planktothrix that is capable of both anoxygenic and oxygenic photosynthesis, as well as chemosynthesis, thrives. We explored the coupled microbial community composition and biogeochemical functioning of organic‐rich, sulfidic sediments underlying the surface mat. Microbial communities were diverse and vertically stratified to 12 cm sediment depth. In contrast to previous studies, which used low‐throughput or shotgun metagenomic approaches, our high‐throughput 16S rRNA gene sequencing approach revealed extensive diversity. This diversity was present within microbial groups, including putative sulfate‐reducing taxa of Deltaproteobacteria, some of which exhibited differential abundance patterns in the mats and with depth in the underlying sediments. The biological and geochemical conditions in the MIS were distinctly different from those in typical LH sediments of comparable depth. We found evidence for active cycling of sulfur, methane, and nutrients leading to high concentrations of sulfide, ammonium, and phosphorus in sediments underlying cyanobacterial mats. Indicators of nutrient availability were significantly related to MIS microbial community composition, while LH communities were also shaped by indicators of subsurface groundwater influence. These results show that interactions between the mats and sediments are crucial for sustaining this hot spot of biological diversity and biogeochemical cycling.     

不同盐度对闽江河口沉积物硝化作用的影响

选择闽江河口鳝鱼滩湿地与道庆洲湿地为对象,采集湿地沉积物在室内进行沉积物硝化培养,分析不同盐度水平对湿地沉积物硝化作用的影响.结果表明:闽江河口湿地沉积物硝化速率普遍较低,鳝鱼滩湿地沉积物最高硝化速率只有0.193 mg·kg^-1·d^-1,而道庆洲湿地沉积物硝化速率最高不超过0.050 mg·kg^-1·d^-1.盐度的升高会抑制闽江河口湿地沉积物的硝化作用.低盐度时(5),硝化速率下降的主要原因是硝化细菌活性受到抑制;随着盐度的升高(10),硝化速率略有上升但仍低于初始值,这是由于随着盐度的升高,盐度对好氧氨化细菌活性的抑制程度有所加强,导致系统产生NH_4^+-N的速率下降,从而造成好氧氨化细菌对表观硝化速率下降的贡献减少.沉积物硝化活性对于盐度的响应存在地域差异.咸水湿地(鳝鱼滩湿地)沉积物中的微生物对盐度的变化适应性较强,从而使该湿地沉积物硝化活性在高盐度条件下仍然较高.淡水湿地(道庆洲湿地)沉积物对盐度变化的适应性较弱,导致其沉积物硝化活性在高盐度条件下低于中等盐度.闽江河口鳝鱼滩与道庆洲的短叶茳芏湿地沉积物硝化作用较低的主要原因是沉积物呈酸性及淹水状态下的缺氧条件弱化了沉积物的硝化作用.两处湿地的硝化速率与硝化活性随时间的变化趋势为先升高后下降,这是由初始NH_4^+-N浓度、氧气含量和反硝化共同作用造成的.