Interactions between soil physicochemistry and belowground biomass production in a freshwater tidal marsh

Plant and Soil - The success of tidal freshwater wetland restoration is typically gauged by the re-establishment of characteristics found in reference marshes. Although plant species composition...     

Syntrophic-archaeal associations in a nutrient-impacted freshwater marsh

AIMS: Evaluation of the composition, distribution and activities of syntrophic bacteria and methanogens in soils from eutrophic and low nutrient regions of a freshwater marsh, and to compare these results with those obtained from a similar study in the Florida Everglades. METHODS AND RESULTS: Culture dependent and independent approaches were employed to study consortia of syntrophs and methanogens in a freshwater marsh. Methanogenesis from butyrate oxidation was fourfold higher in microcosms containing soil from eutrophic regions of the marsh than from low nutrient regions. Propionate was oxidized in eutrophic microcosms at lower rates than butyrate and with lower yields of methane. Sequence analysis of 16S rRNA gene clone libraries from DNA extracted from microcosms and soils revealed differences such that the dominant restriction fragment length polymorphism (RFLP) phylotypes (representing 82-88% of clone libraries) from eutrophic soils clustered with fatty acid oxidizing Syntrophomonas spp. The four dominant RFLP phylotypes (representing 11-24%) from microcosms containing soils from low nutrient regions were sequenced, and clustered with micro-organisms having the potential for fermentative and syntrophic metabolism. Archaeal 16S rRNA sequence analysis showed that methanogens from eutrophic regions were from diverse families, including Methanomicrobiaceae, Methanosarcinaceae, and Methanocorpusculaceae, but clone libraries from low nutrient soils revealed only members of Methanosarcinaceae. CONCLUSIONS: These findings indicate that syntroph-methanogen consortia differed with nutrient levels in a freshwater marsh. SIGNIFICANCE AND IMPACT OF THE STUDY: This is one of few studies addressing the distribution of fatty acid consuming-hydrogen producing bacteria (syntrophs) and their methanogenic partners in wetland soils, and the effects of eutrophication on the ecology these groups.     

淡水湿地种子库的小尺度空间格局

以长江中下游淡水湿地湖里沼泽为对象,研究了种子库在小尺度空间范围的水平分布格局。在沼泽中选择地表植被分布比较均一的16m×4m大小的方形区域,按1m间隔获取64个内径7·8cm、深5cm的圆柱状土样,通过幼苗萌发法鉴定每个土样的种子库组成。采用2个格局指数(离散系数和Lloyd平均拥挤指数)以及Moran空间自相关系数分析了种子库中优势种的空间分布格局。结果显示种子库由17个物种组成,多年生的锐棱荸荠(Eleocharisacutangula)和龙师草(E·tetraqueter)是种子库中密度最大的物种。两个格局指数显示7个分布频率大于10%的物种的种子全部为聚集分布。MoransI统计分析显示其中只有3个物种为显著的正的空间自相关,表明这3个物种的种子斑块大小超过了目前的两个取样单位之间的距离(1m),而其它4个物种的种子斑块则小于这个尺度。     

Vegetation and seed bank dynamics in a tidal freshwater marsh

Questions: What are the feedbacks among the seed bank, parent vegetation, and landscape structure that control plant species turnover in space and time in a tidal freshwater marsh? How can these feedbacks be used to better understand marsh community dynamics and to establish restoration practices that seek to restore vegetation diversity of this important and widely distributed ecosystem? Location: Potomac River, Virginia, United States (15 km south of Washington, DC). Methods: We sampled the seed bank and standing vegetation in a tidal freshwater marsh and explored similarities between seed bank and vegetation composition through space and time. We then investigated marsh surface elevation, distance to nearest tidal channels, and life history of component species as potential explanations for the observed vegetation patterns. Results: The composition of individual plots changed considerably from year to year; however, the composition at broader spatial (whole marsh) and temporal (4 years) scales was relatively stable. Species composition of the seed bank was dissimilar to both the previous and current year's standing vegetation, and similarity to standing vegetation was particularly low in plots dominated by annual species. Landscape structure and life history characteristics of individual species best explained the spatiotemporal variability in marsh vegetation. Conclusions: Restoration designs should be landscape‐dependent and explicitly incorporate spatially structured elements such as elevation gradients to maximize community diversity in reconstructed tidal freshwater marshes. Optimal designs include areas of high seed input, areas of high species turnover, as well as other areas of greater stability.     

Vegetation and primary production in a floating freshwater marsh in Louisiana

The freshwater floating marsh surrounding Lake Boeuf, LA, is a diverse plant community dominated by Panicum hemitomon Schult. (maidencane), with the ferns Thelypteris palustris Schott. and Osmunda regalis L. and the perennial vines Vigna luteola (Jacq.) Benth. and Polygonum sagittatum L. Using the method of Lomnicki et al., we determined net aboveground primary production to be 1960 g m⁻²y⁻¹ dry weight. The turnover rate, based on the mean longevity of tagged maidencane culms, was 1.21 crops per year.     

The seed bank in a subtropical freshwater marsh: implications for wetland restoration

Seed bank samples were collected from Huli Marsh, a subtropical shallow water mountainous marsh in Hunan Province, South China. Core samples were divided into upper and lower layers (each 5 cm in depth) and allowed to germinate in three water levels (0, 5 and 10 cm) over a 4-month period. A total of 51 species germinated and the mean density was 9211 ± 7188 seedlings m⁻². In the top 5 cm 41 species and 5747 ± 5111 seedlings m⁻² germinated, whereas 40 species and 3464 ± 3363 seedlings m⁻² did so from 5–10 cm. Germinated seedling density was significantly higher in the upper layer, largely due to differences in eight species. With increasing experimental water depth, less seedlings germinated: respectively, 9788 ± 7157 m⁻², 2050 ± 2412 m⁻² and 1978 ± 2616 m⁻², of 44, 21 and 19 species, submerged under 0, 5 or 10 cm. Seven species could emerge only in 0 water level. Vallisneria natans occurred only in 5 cm water, whereas Ottelia alismoides occurred in 10 cm water. In the vegetation survey of the marsh, 25 species were recorded, which was less than half of the species recorded in the seed bank. The top 10 dominants in the standing vegetation, accounting for 89% of vegetation abundance, represented only 10% in the seed bank. Twenty germinated species that also occurred in the standing vegetation accounted for 56% of the total seed bank. Our observed number of species germinating from a Chinese wetland seed bank is within the range observed elsewhere in the northern hemisphere (15–113 species).     

Scale-dependent inhibition drives regular tussock spacing in a freshwater marsh

Regular spatial patterning is common in nature, and various mechanisms of self-organization have been proposed to explain regular patterning. We report on regular spatial patterning in Carex stricta in a freshwater wetland and investigate the applicability of theoretical models that explain regular patterning based on inhibition, facilitation, or interaction between the two. Spectral analysis of aerial photographs revealed that tussocks were regularly spaced at an average distance of 60 cm. Photosynthetically active radiation varied significantly with distance from the tussock and was lowest at intermediate distance from the tussock center (15-40 cm). Using transplants to assay growth conditions, we found that C. stricta grew well in all distance classes with and without natural C. stricta biomass, except at intermediate distances when buried in C. stricta wrack. Our experimental results reveal that C. stricta inhibits its growth in a scale-dependent manner: inhibition was found to peak at intermediate distance from the tussock. We compared three alternative models to examine potential mechanisms driving regularity and found that, similar to our experimental results, scale-dependent inhibition provides the best explanation for the observed regular tussock spacing. Our study underlines the importance of scale-dependent feedback in the formation of regular spatial patterning in ecosystems.     

Microbial communities in contrasting freshwater marsh microhabitats

Heterotrophic microorganisms are widely recognized as crucial components of ecosystems; yet information on their community structure and dynamics in benthic freshwater habitats is notably scarce. Using denaturing gradient gel electrophoresis (DGGE), we determined the composition of bacterial and fungal communities in a freshwater marsh over four seasons. DGGE revealed diverse bacterial communities in four contrasting microhabitats. The greatest compositional differences emerged between water-column and surface-associated bacteria, although communities associated with sediment also differed from those on plant litter and epiphytic biofilms. Sequences of bacterial clones derived from DGGE bands belonged to the Alphaproteobacteria (31%), Actinobacteria (19%) and Bacteriodetes (19%). Betaproteobacteria were notably absent. Fungal clones obtained from leaf litter were mainly Ascomycota , but two members of the Basidiomycota were also identified. Overall, habitat type was the most important factor explaining variation in bacterial communities among samples, whereas temporal patterns in community composition were less pronounced in spite of large seasonal variation in environmental conditions such as temperature. The observed differences among bacterial communities in different microhabitats were not caused by random variation, but rather appeared to be determined by habitat characteristics, as evidenced by largely congruent community profiles of replicate samples taken at 10–100 m distances within the marsh.     

Benthic bacterial and fungal productivity and carbon turnover in a freshwater marsh

Heterotrophic bacteria and fungi are widely recognized as crucial mediators of carbon, nutrient, and energy flow in ecosystems, yet information on their total annual production in benthic habitats is lacking. To assess the significance of annual microbial production in a structurally complex system, we measured production rates of bacteria and fungi over an annual cycle in four aerobic habitats of a littoral freshwater marsh. Production rates of fungi in plant litter were substantial (0.2 to 2.4 mg C g(-1) C) but were clearly outweighed by those of bacteria (2.6 to 18.8 mg C g(-1) C) throughout the year. This indicates that bacteria represent the most actively growing microorganisms on marsh plant litter in submerged conditions, a finding that contrasts strikingly with results from both standing dead shoots of marsh plants and submerged plant litter decaying in streams. Concomitant measurements of microbial respiration (1.5 to 15.3 mg C-CO2 g(-1) of plant litter C day(-1)) point to high microbial growth efficiencies on the plant litter, averaging 45.5%. The submerged plant litter layer together with the thin aerobic sediment layer underneath (average depth of 5 mm) contributed the bulk of microbial production per square meter of marsh surface (99%), whereas bacterial production in the marsh water column and epiphytic biofilms was negligible. The magnitude of the combined production in these compartments (approximately 1,490 g C m(-2) year(-1)) highlights the importance of carbon flows through microbial biomass, to the extent that even massive primary productivity of the marsh plants (603 g C m(-2) year(-1)) and subsidiary carbon sources (approximately 330 g C m(-2) year(-1)) were insufficient to meet the microbial carbon demand. These findings suggest that littoral freshwater marshes are genuine hot spots of aerobic microbial carbon transformations, which may act as net organic carbon importers from adjacent systems and, in turn, emit large amounts of CO2 (here, approximately 870 g C m(-2) year(-1)) into the atmosphere.     

Ten-year seed bank and vegetation dynamics of a tidal freshwater marsh

A decade-long examination was made of recruitment and establishment in a tidal freshwater high marsh along the Delaware River. Over the 10 yr of the study, seed bank, field seedlings, and vegetation showed variable patterns and significant year-to-year fluctuations. Patterns of each species were unique, perhaps the result of specific germination and/or establishment requirements and seedling morphology. For a given species there was little correlation among seed bank, seedling, and vegetation patterns, and germination success did not guarantee establishment. Species diversity showed significant year-to-year fluctuation, but there was no trend, and perennials did not change in importance during the 10-yr period. Because four annual species (Bidens laevis, Cuscuta gronovii, Impatiens capensis, and Polygonum arifolium) composed over 90% of the seed bank and field seedlings, and 58-89% ( = 70 ± 4) of the cover, community dynamics were dependent on seedling recruitment. For a given species life history stages (seed bank, field seedlings, and vegetation) were not predictable over the temporal scale of a decade. It is predicted, however, that if hydrology remains unchanged, the same suite of species will persist. The importance of the parasite Cuscuta gronovii is noteworthy.     

Recovery of freshwater marsh vegetation after a saltwater intrusion event

Greenhouse mesocosms of freshwater marsh vegetation were exposed to a simulated saltwater intrusion event followed by a recovery period during which water levels and interstitial water salinity were adjusted over a range of conditions. Virtually all above-ground vegetation, including the three dominant species, Sagittaria lancifolia L., Leersia oryzoides (L.) Swartz, and Panicum hemitomon Schultes, was killed by the initial saltwater intrusion event. P. hemitomon did not recover, but S. lancifolia and L. oryzoides, as well as many of the other species initially present, exhibited some ability to recover depending on post-saltwater intrusion conditions. Increasingly harsh recovery conditions (for freshwater marsh vegetation), including more reduced soil conditions, higher interstitial salinities, and higher interstitial sulfide concentrations were associated with decreased live above-ground biomass and species richness. The effect of elevated salinity on vegetative recovery became more pronounced under flooded conditions. This experiment illustrates that the response of a freshwater marsh community to the long-term disturbance effect of a transient saltwater intrusion event will be strongly influenced by post-intrusion salinity and water levels.     

Niche separation of ammonia-oxidizing bacteria across a tidal freshwater marsh

Like many functional groups or guilds of microorganisms, the group of ammonia-oxidizing bacteria (AOB) consists of a number of physiologically different species or lineages. These physiological differences suggest niche differentiation among these bacteria depending on the environmental conditions. Species of AOB might be adapted to different zones in the flooding gradient of a tidal marsh. This issue has been studied by sampling sediments from different sites and depths within a tidal freshwater marsh along the river Scheldt near the village of Appels in Belgium. Samples were taken in February, April, July and October 1998. Communities of AOB in the sediment were analysed on the basis of the 16S rRNA gene by application of polymerase chain reaction in combination with denaturing gradient gel electrophoresis (DGGE). In addition, moisture content and concentrations of ammonium and nitrate were determined as well as the potential ammonia-oxidizing activities. Six different DGGE bands belonging to the beta-subclass of the Proteobacteria were observed across the marsh. The community composition of AOB was determined by the elevation in the flooding gradient as well as by the sampling depth. The presence of plants was less important for the community composition of AOB. DGGE bands affiliated with the Nitrosospira lineage were mostly found in the upper part of the marsh and in the deeper layers of the sediment. Two of the three DGGE bands related to the Nitrosomonas oligotropha lineage were more broadly distributed over the marsh, but were predominantly found in the upper layers of the sediment. Members of the environmental Nitrosomonas lineage 5 were predominantly detected in the deeper layers in the lower parts of the marsh. Potential driving factors for niche differentiation are discussed.     

Distribution and diversity of Gallionella-like neutrophilic iron oxidizers in a tidal freshwater marsh

Microbial iron oxidation is an integral part of the iron redox cycle in wetlands. Nonetheless, relatively little is known about the composition and ecology of iron-oxidizing communities in the soils and sediments of wetlands. In this study, sediment cores were collected across a freshwater tidal marsh in order to characterize the iron-oxidizing bacteria (FeOB) and to link their distributions to the geochemical properties of the sediments. We applied recently designed 16S rRNA primers targeting Gallionella-related FeOB by using a nested PCR-denaturing gradient gel electrophoresis (DGGE) approach combined with a novel quantitative PCR (qPCR) assay. Gallionella-related FeOB were detected in most of the samples. The diversity and abundance of the putative FeOB were generally higher in the upper 5 to 12 cm of sediment than in deeper sediment and higher in samples collected in April than in those collected in July and October. Oxygen supply by macrofauna appears to be a major force in controlling the spatial and temporal variations in FeOB communities. The higher abundance of Gallionella-related FeOB in April coincided with elevated concentrations of extractable Fe(III) in the sediments. Despite this coincidence, the distributions of FeOB did not exhibit a simple relationship to the redox zonation inferred from the geochemical depth profiles.     

The sedimentary flux of nutrients at a Delaware salt marsh site: A geochemical perspective

A basic geochemical approach has been used to study the diagenetic cycling and sediment-water exchange rates of essential nutrients at a site in a Delaware salt marsh. The pore water nutrients Si (OH)₄, NH₄ ⁺, PO₄ ^–3, and NO₃ ^– were analyzed and compared seasonally, and potential diffusive fluxes were calculated from the corresponding pore water concentration gradients. Concurrent direct flux measurements were also made using closed chambers deployed on the salt marsh surface under ambient and controlled conditions. The differences in these two approaches allows for estimation of nutrient production and consumption in the root zone and at the salt marsh surface. Silicate and phosphate appear to be primarily geochemically controlled, while the nitrogen species undergo dynamic seasonal redox fluctuations resulting from microbial mediation (e.g., nitrification and denitrification). Specifically:     

Ecological re-engineering of a freshwater impoundment for salt marsh restoration in a hypertidal system

The purpose of this paper was to examine the vegetative, sedimentary, nekton and hydrologic response to ecological re-engineering of a freshwater impoundment in the Upper Bay of Fundy. The dyke was breached in five locations and one channel initiated to connect the river to the borrow pit behind the dyke. This triggered significant self-organization within the restoration site. Existing channels (e.g. borrow pit) were incorporated within the newly excavated and developing creek system, increasing the hydraulic connectivity within the marsh and increasing fish habitat. Vegetation colonization, primarily by Spartina alterniflora, was rapid with almost 100% coverage by the end of the third year. Species associated with high marsh communities such as Juncus gerardii, Scirpus robustus, Ranunculus cymbalaria and Puccinellia maritima were present in increasing abundance post-restoration at the restoration site. The constructed channel eroded downward 2.3 m and the head of the channel retreated 35 m in response to the increased tidal prism. In the year immediately following the breach (2006), the surface of the marsh was unconsolidated and rates of change in surface elevation measured at RSET stations ranged from ?0.7 (±0.1) to 1.7 (±0.2) cm yr^?1 (±1SE). Measurements between years are highly variable. By year 3 the rate of surface elevation change decreased to a more moderate but variable mean of 0.3 (±0.6) cm yr^?1 with the marker horizons recording mean accretion rates of 0.7 cm yr^?1. This implies subsurface consolidation as the sediments dewatered or organic matter decomposed and vegetation became more established. Fish density decreased after restoration, however, remained higher than at the reference site for most years.     

Effect of an aquatic herbicide on sediment nutrient flux in a freshwater marsh

Three concentrations of the herbicide simazine were added to in situ macrophyte-free enclosures with and without sediment contact. Changes in the concentrations of total ammonia, total reactive phosphorus, and silicon were monitored, and net sediment flux was calculated from the difference in nutrient concentration between bottomed and unbottomed enclosures. Rates of sediment release for all three nutrients were unaltered by 0.1 mg · l^–1 simazine in relation to a control, whereas rates were increased proportionally at 1.0 and 5.0 mg · l^–1. These results suggest that increases in dissolved nutrients commonly observed following herbicide treatment of shallow waters may not be attributable solely to macrophyte decay, byt may also involve a complex interaction of biotic and abiotic sediment nutrient exchange processes.Contribution Number 103 from the University of Manitoba Field Station, Delta Marsh, Canada     

The influence of light and nutrients on buoyancy, filament aggregation and flotation of Anabaena circinalis

At Chaffey Dam, New South Wales, Australia, Anabaena circinalis filaments accumulated at the surface as diurnal surface layer thermal stratification developed. Previously buoyant, homogeneously distributed colonies accumulated in the top 2 m, but a proportion lost buoyancy. Similarly, a percentage of A.circinalis suspended in bottles lost buoyancy at depths experiencing >30% surface irradiance (Io). Nutrient addition reduced the proportion of filaments that lost buoyancy following a full day of high irradiance. The greatest axial linear dimension (GALD) was measured for A.circinalis deployed in bottles at three depths in the reservoir. GALD increased in samples exposed to 1 and 30% Io by the following day. The rank order of GALD from smallest to largest grouped samples exposed to 70, 30 and 1% Io, suggesting that increasing GALD is a function of irradiance. The increased GALD of biomass units was attributed to aggregation of filaments in low light. The enlargement of biomass units increased the mean floating velocity, supporting the theory that filament aggregation may be a strategy, utilized by light-limited filaments, to increase light exposure. High irradiance increased the carbohydrate content of cells and decreased the floating velocity of filaments.      

Effects of hydrology on the movements of a large-bodied predator in a managed freshwater marsh

Hydrobiologia - Wetlands are dynamic environments where aquatic organisms are affected by both predictable and unpredictable changes in hydrology. Understanding how abundant large-bodied predators...     

Nitrogen addition does not reduce belowground competition in a salt marsh clonal plant community in Mississippi (USA)

Previous studies have suggested that belowground competition for nutrients influences plant zonation in salt marshes. In this study, I tested the hypothesis that competition for nitrogen structured a clonal plant community in a nitrogen-limited salt marsh in coastal Mississippi, USA. In contrast to most previous field studies that have investigated mechanisms of competition, I examined clonal growth responses of established genets of a nitrogen-demanding low-intertidal species (Spartina alterniflora) to nitrogen addition and the removal of a nitrogen-conserving high-intertidal species (Juncus roemerianus). Nitrogen addition stimulated clonal invasion of the Juncus zone by Spartina but did not reduce the significant competitive effects of Juncus on Spartina. Simulated Juncus shade did not reduce invasion of the Juncus zone by Spartina, indicating that belowground competition reduced clonal invasion. In the last year of the study, the border shifted unexpectedly towards the Spartina zone, resulting in competitive displacement of Spartina by Juncus. Nitrogen addition did not prevent or slow this displacement, further contradicting the nitrogen competition hypothesis. Although growth rates were much more strongly limited by nitrogen in Spartina than in Juncus, nitrogen addition did not cause the displacement of Juncus by Spartina after three growing seasons. I conclude that zonation of Spartina and Juncus is maintained by preemption of space and greater tolerance of low nitrogen supplies by Juncus in the high marsh. These results contrast sharply with findings of reduced belowground competition with nutrient addition in previous studies and highlight the important role of nutrient-mediated competition for space between clonal plants.     

Contributions of three subsystems of a freshwater marsh to total bacterial secondary productivity

Rates of bacterial production were measured in the water column, on the surface of plant detritus, and in the surface sediments of a freshwater marsh in the Okefenokee Swamp, Georgia, USA. Bacterioplankton production rates were not correlated with several measures of quantity and quality of dissolved organic matter, including an index of the relative importance of vascular plant derivatives. Bacterioplankton productivity was high (mean: 63 g C liter^–1 day^–1) compared with rates reported for other aquatic ecosystems. Somewhat paradoxically, bacterial productivity on plant detritus (mean: 13 g C g^–1 day^–1) and sediments (mean: 15 g C g^–1 day^–1) was low relative to other locations. On an a real basis, total bacterial productivity in this marsh ecosystem averaged 22 mg C m^–2 day^–1, based on sample dates in May 1990 and February 1991. Marsh sediments supported the bulk of the production, accounting for 46% (May) and 88% (February) of the total. The remainder was contributed approximately equally by bacteria in the water column and on accumulated stores of plant detritus. Send offprint requests to: M. A. Moran.