Geomorphology and Species Interactions Control Facilitation Cascades in a Salt Marsh Ecosystem

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Responses of Salt Marsh Plant Rhizosphere Diazotroph Assemblages to Changes in Marsh Elevation,Edaphic Conditions and Plant Host Species

An important source of new nitrogen in salt marsh ecosystems is microbial diazotrophy (nitrogen fixation). The diazotroph assemblages associated with the rhizospheres (sediment directly affected by the roots) of salt marsh plants are highly diverse, somewhat stable, and consist mainly of novel organisms. In Crab Haul Creek Basin, North Inlet, SC, the distribution of plant types into discrete zones is dictated by relatively minor differences in marsh elevation and it was hypothesized that the biotic and abiotic properties of the plant zones would also dictate the composition of the rhizosphere diazotroph assemblages. Over a period of 1 year, rhizosphere sediments were collected from monotypic stands of the black needlerush, Juncus roemerianus, the common pickleweed, Salicornia virginica, the short and tall growth forms of the smooth cordgrass Spartina alterniflora, and a mixed zone of co-occurring S. virginica and short form, S. alterniflora. DNA was extracted, purified and nifH sequences PCR amplified for denaturing gradient gel electrophoresis (DGGE) analysis to determine the composition of the diazotroph assemblages. The diazotroph assemblages were strongly influenced by season, abiotic environmental parameters and plant host. Sediment chemistry and nitrogen fixation activity were also significantly influenced by seasonal changes. DGGE bands that significantly affected seasonal and zone specific clustering were identified and most of these sequences were from novel diazotrophs, unaffiliated with any previously described organisms. At least one third of the recovered nifH sequences were from a diverse assemblage of Chlorobia, and γ-, α-, β- and δ-Proteobacteria. Diazotrophs that occurred throughout the growing season and among all zones (frequently detected) were also mostly novel. These significant sequences indicated that diazotrophs driving the structure of the assemblages were diverse, versatile, and some were ubiquitous while others were seasonally responsive. Several ubiquitous sequences were closely related to sequences of actively N₂ fixing diazotrophs previously recovered from this system. These sequences from ubiquitous and versatile organisms likely indicate the diazotrophs in these rhizosphere assemblages that significantly contribute to ecosystem function.     

Microbiological and Physical Properties of Salt Marsh and Microecosystem Sediments

Sediments from natural marsh and four microecosystem (MES) tanks were compared with reference to ATP, carbon content, and physical properties. Mean values for bacterial numbers and ATP were about twice as high in the MES, carbon content was slightly higher in the artificial systems, and the sediment of the MES was enriched for larger grain sizes and depleted of silt and clay. In the natural marsh sediment, ATP was strongly related (Pearson's correlation coefficient) to granulometric (or physical) factors, whereas ATP was related to biological factors in the MES. Approximately 61% of the variation in ATP concentrations in the marsh sediments could be accounted for by the parameters measured. Only half that amount could be explained on the basis of those same variables in the MES. Furthermore, one of the four tanks was significantly different from the other three in terms of ATP concentration, and an anomaly in respect to silt-clay content was demonstrated in another. Careful control and some type of equilibrating procedure will be necessary before the MES can be employed as replicate units.     

Microbial Community Composition and Denitrifying Enzyme Activities in Salt Marsh Sediments

Denitrifying microbial communities and denitrification in salt marsh sediments may be affected by many factors, including environmental conditions, nutrient availability, and levels of pollutants. The objective of this study was to examine how microbial community composition and denitrification enzyme activities (DEA) at a California salt marsh with high nutrient loading vary with such factors. Sediments were sampled from three elevations, each with different inundation and vegetation patterns, across 12 stations representing various salinity and nutrient conditions. Analyses included determination of cell abundance, total and denitrifier community compositions (by terminal restriction fragment length polymorphism), DEA, nutrients, and eluted metals. Total bacterial (16S rRNA) and denitrifier (nirS) community compositions and DEA were analyzed for their relationships to environmental variables and metal concentrations via multivariate direct gradient and regression analyses, respectively. Community composition and DEA were highly variable within the dynamic salt marsh system, but each was strongly affected by elevation (i.e., degree of inundation) and carbon content as well as by selected metals. Carbon content was highly related to elevation, and the relationships between DEA and carbon content were found to be elevation specific when evaluated across the entire marsh. There were also lateral gradients in the marsh, as evidenced by an even stronger association between community composition and elevation for a marsh subsystem. Lastly, though correlated with similar environmental factors and selected metals, denitrifier community composition and function appeared uncoupled in the marsh.     

Microbial Aldicarb Transformation in Aquifer, Lake, and Salt Marsh Sediments

The microbial transformation of [N-methyl-(sup14)C]aldicarb, a carbamate pesticide, occurred in aquifer, lake, and salt marsh sediments. Microbial degradation of aldicarb took place within 21 days in aquifer sediments from sites previously exposed to aldicarb (Jamesport, Long Island, N.Y.) but did not occur in sediments which were not previously exposed (Connetquot State Park, Long Island, N.Y.). At the Jamesport sites, higher aldicarb transformation rates occurred in deep, anoxic sediments than in shallow, oxic sediments. There was a significant negative relationship (P < 0.05) between transformation rates and ambient dissolved O(inf2) levels. Aldicarb hydrolysis rates in Jamesport sediments were 10- to 1,000-fold lower than rates previously reported for soils. In addition, aldicarb degradation rates were not significantly correlated with measurements of bacterial activity and density previously determined in the same sediments. Substantially higher aldicarb degradation rates were found in anoxic lake and salt marsh than in aquifer sediments. Furthermore, we investigated the anaerobic microbial processes involved in aldicarb transformation by adding organic substrates (acetate, glucose), an alternative electron acceptor (nitrate), and microbial inhibitors (molybdate, 2-bromoethanesulfonic acid) to anoxic aquifer, lake, and salt marsh sediments. The results suggest that a methanogenic consortium was important in aldicarb transformation or in the use of aldicarb-derived products such as methylamine. In addition, microbial aldicarb transformation proceeded via different pathways under oxic and anoxic conditions. In the presence of O(inf2), aldicarb transformation was mainly via an oxidation pathway, while in the absence of O(inf2), degradation took place through a hydrolytic pathway (including the formation of methylamine precursors). Under anoxic conditions, therefore, aldicarb can be transformed by microbial consortia to yield products which can be of direct benefit to natural populations of methanogens present in sediments.     

Quantification of Ammonia-Oxidizing Bacteria and Factors Controlling Nitrification in Salt Marsh Sediments

To elucidate the geomicrobiological factors controlling nitrification in salt marsh sediments, a comprehensive approach involving sediment geochemistry, process rate measurements, and quantification of the genetic potential for nitrification was applied to three contrasting salt marsh habitats: areas colonized by the tall (TS) or short (SS) form of Spartina alterniflora and unvegetated creek banks (CBs). Nitrification and denitrification potential rates were strongly correlated with one another and with macrofaunal burrow abundance, indicating that coupled nitrification-denitrification was enhanced by macrofaunal burrowing activity. Ammonia monooxygenase (amoA) gene copy numbers were used to estimate the ammonia-oxidizing bacterial population size (5.6 × 10⁴ to 1.3 × 10⁶ g of wet sediment⁻¹), which correlated with nitrification potentials and was 1 order of magnitude higher for TS and CB than for SS. TS and CB sediments also had higher Fe(III) content, higher Fe(III)-to-total reduced sulfur ratios, higher Fe(III) reduction rates, and lower dissolved sulfides than SS sediments. Iron(III) content and reduction rates were positively correlated with nitrification and denitrification potential and amoA gene copy number. Laboratory slurry incubations supported field data, confirming that increased amounts of Fe(III) relieved sulfide inhibition of nitrification. We propose that macrofaunal burrowing and high concentrations of Fe(III) stimulate nitrifying bacterial populations, and thus may increase nitrogen removal through coupled nitrification-denitrification in salt marsh sediments.     

Modification of Sediments and Macrofauna by an Invasive Marsh Plant

Invasive grasses have recently altered salt marsh ecosystems throughout the northern hemisphere. On the eastern seaboard of the USA, Phragmites australis has invaded both brackish and salt marsh habitats. Phragmites australis influence on sediments and fauna was investigated along a salinity and invasion-age gradient in marshes of the lower Connecticut River estuary. Typical salinities were about 19–24 ppt in Site I, 9–10 ppt in Site II and 5–7 ppt in Site III. Strongest effects were evident in the least saline settings (II and III) where Phragmites has been present the longest and exists in monoculture. Limited influence was evident in the most saline region (I) where Phragmites and native salt marsh plants co-occur. The vegetation within Phragmites stands in tidal regions of the Connecticut River generally exhibits taller, but less dense shoots, higher above-ground biomass, and lower below-ground biomass than does the un-invaded marsh flora. There were lower sediment organic content, greater litter accumulation and higher sediment chlorophyll a concentrations in Phragmites- invaded than un-invaded marsh habitat. Epifaunal gastropods (Succinea wilsoni and Stagnicola catascopium) were less abundant in habitats where Phragmites had invaded than in un-invaded marsh habitat. Macro-infaunal densities were lower in the Phragmites-invaded than un-invaded habitats at the two least saline sites (II and III). Phragmites stands supported more podurid insects, sabellid polychaetes, and peracarid crustaceans, fewer arachnids, midges, tubificid and enchytraeid oligochaetes, and greater habitat-wide taxon richness as measured by rarefaction, than did the un-invaded stands. The magnitude and significance of the compositional differences varied with season and with site; differences were generally greatest at the oldest, least saline sites (II and III) and during May, when faunal densities were higher than in September. However, experimental design and the 1-year study period precluded clear separation of salinity, age, and seasonal effects. Although structural effects of Phragmites on salt marsh faunas are evident, further investigation is required to determine the consequences of these effects for ecosystem function. This revised version was published online in July 2006 with corrections to the Cover Date.     

Distribution and Diversity of Archaeal and Bacterial Ammonia Oxidizers in Salt Marsh Sediments

Diversity and abundance of ammonia-oxidizing Betaproteobacteria (β-AOB) and archaea (AOA) were investigated in a New England salt marsh at sites dominated by short or tall Spartina alterniflora (SAS and SAT sites, respectively) or Spartina patens (SP site). AOA amoA gene richness was higher than β-AOB amoA richness at SAT and SP, but AOA and β-AOB richness were similar at SAS. β-AOB amoA clone libraries were composed exclusively of Nitrosospira-like amoA genes. AOA amoA genes at SAT and SP were equally distributed between the water column/sediment and soil/sediment clades, while AOA amoA sequences at SAS were primarily affiliated with the water column/sediment clade. At all three site types, AOA were always more abundant than β-AOB based on quantitative PCR of amoA genes. At some sites, we detected 10⁹ AOA amoA gene copies g of sediment⁻¹. Ratios of AOA to β-AOB varied over 2 orders of magnitude among sites and sampling dates. Nevertheless, abundances of AOA and β-AOB amoA genes were highly correlated. Abundance of 16S rRNA genes affiliated with Nitrosopumilus maritimus, Crenarchaeota group I.1b, and pSL12 were positively correlated with AOA amoA abundance, but ratios of amoA to 16S rRNA genes varied among sites. We also observed a significant effect of pH on AOA abundance and a significant salinity effect on both AOA and β-ΑΟΒ abundance. Our results expand the distribution of AOA to salt marshes, and the high numbers of AOA at some sites suggest that salt marsh sediments serve as an important habitat for AOA.Nitrification, the sequential oxidation of ammonia to nitrite and nitrate, is a critical step in the nitrogen cycle and is mediated by a suite of phylogenetically and physiologically distinct microorganisms. The recent discovery of ammonia oxidation among Archaea (17, 38) has led to a dramatic shift in the current model of nitrification and to new questions of niche differentiation between putative ammonia-oxidizing Archaea (AOA) and the more-well-studied ammonia-oxidizing Betaproteobacteria (β-AOB). Based on surveys of 16S rRNA genes and archaeal amoA genes, it is evident that AOA occupy a wide range of niches (10), suggesting a physiologically diverse group of Archaea. Additionally, in studies where AOA and β-AOB were both targeted, AOA were typically more abundant than their bacterial counterparts (19, 21, 42). However, there are reports of β-AOB outnumbering AOA in estuarine systems (6, 33), suggesting a possible shift in competitive dominance under certain conditions.Patterns of β-AOB diversity in estuaries have been well characterized and appear to be regulated by similar mechanisms within geographically disparate systems (4, 11, 32). However, AOA distribution and their role in nitrification relative to β-AOB remain to be determined. A few studies have begun to address this question in different estuaries, but no unifying patterns or mechanisms have emerged. Although β-AOB have been well studied along estuarine salinity gradients (1, 3, 4, 7, 11, 13, 22, 33, 39) and recent studies have begun to address AOA in estuaries (1, 6, 22, 32, 33), few have investigated β-AOB in salt marshes (9), and none has included AOA.In this study, we investigated the distribution and abundance of AOA and β-AOB based on the distribution and abundance of amoA genes in salt marsh sediments dominated by different types of vegetation. Although we equate the presence of archaeal amoA genes with the genetic potential to oxidize ammonia, we acknowledge the possibility that all Archaea that have amoA genes may not all represent functional ammonia oxidizers. Vegetation patterns of New England salt marshes are strongly correlated with marsh elevation and are controlled by a combination of interspecific competition and tolerance to physico-chemical stress (28). The dominant grasses of New England salt marshes are Spartina alterniflora and Spartina patens, which typically grow as pure stands. S. alterniflora is found in two phenotypically distinct but genetically identical forms, a tall and a short growth form (34). The tall S. alterniflora grows to heights of 1 to 2 m and is typically found at the edges of the marsh and along creek banks (SAT sites), while the short-form S. alterniflora may reach heights of only 30 cm and is found in sites (SAS sites) slightly higher on the marsh where soil drainage is limited and conditions are more reduced compared to SAT sites (14). Conversely, S. patens, due to its lower tolerance of salt and more reduced conditions, is found in sites (SP sites) highest on the marsh, in areas that receive less flooding (5). Because the marsh is subjected to daily tidal fluctuations, most sites experience periods of anoxia, the degree of which depends on the marsh elevation. We hypothesized that ammonia-oxidizing communities in areas dominated by different marsh grasses would reflect the different edaphic conditions associated with each type of grass, due to differences in vertical zonation in the marsh.     

Populations of Methane-Producing Bacteria and In Vitro Methanogenesis in Salt Marsh and Estuarine Sediments

Most probable numbers (MPNs) of methanogens in various salt marsh and estuarine sediments were determined with an anaerobic, habitat-simulating culture medium with 80% H₂ plus 20% CO₂ as substrate. Average MPNs for the short Spartina (SS) marsh sediments of Sapelo Island, Ga., were maximal at the 5- to 7-cm depth (1.2 × 10⁷/g of dry sediment). Populations decreased to approximately 880/g of dry sediment at the 34- to 36-cm depth. There was no significant difference between summer and winter populations. In tall Spartina (TS) marsh sediments, average populations were maximal (1.2 × 10⁶/g of dry sediment) in the upper 0- to 2-cm zone; populations from the 5- to 36-cm zones were similar (average of 9 × 10⁴/g of dry sediment). Methanogenic populations for TS sediments of James Island Creek marsh, Charleston, S.C., were similar (average of 3 × 10⁶/g of dry sediment) for all depths tested (0 to 22 cm), which was comparable to the trend observed for TS sediments at Sapelo Island, Ga. Sediment grab samples collected along a transect of James Island Creek and its adjacent Spartina marsh had MPNs that were approximately 20 times greater for the region of Spartina growth (average of 10⁶/g of dry sediment) compared with the channel (approximately 5 × 10⁴ methanogens per g of dry sediment). A similar trend was found at Pawley's Island marsh, S.C., but populations were approximately one order of magnitude lower. In vitro rates of methanogenesis with SS sediments incubated under 80% H₂-20% CO₂ showed that the 5- to 7-cm region exhibited maximal activity (51 nmol of CH₄ g⁻¹ h⁻¹), which was greater than rates for sediments above and below this depth. SS sediment samples (5 to 7 cm) incubated under 100% N₂ and supplemented with formate exhibited rates of methanogenesis similar to those generated by samples under 80% H₂-20% CO₂. Replacing the N₂ atmosphere with H₂ resulted in an eightfold decrease in the rate of methanogenesis. In vitro methanogenic activity by TS salt marsh sediments, incubated under 80% H₂-20% CO₂, was similar for all depths tested (0 to 22 cm). TS sediment samples (0 to 7 cm) supplemented with formate and incubated under 100% N₂ had greater rates of methanogenesis compared with unsupplemented samples.     

Diversity, Composition, and Geographical Distribution of Microbial Communities in California Salt Marsh Sediments

The Pacific Estuarine Ecosystem Indicators Research Consortium seeks to develop bioindicators of toxicant-induced stress and bioavailability for wetland biota. Within this framework, the effects of environmental and pollutant variables on microbial communities were studied at different spatial scales over a 2-year period. Six salt marshes along the California coastline were characterized using phospholipid fatty acid (PLFA) analysis and terminal restriction fragment length polymorphism (TRFLP) analysis. Additionally, 27 metals, six currently used pesticides, total polychlorinated biphenyls and polycyclic aromatic hydrocarbons, chlordanes, nonachlors, dichlorodiphenyldichloroethane, and dichlorodiphenyldichloroethylene were analyzed. Sampling was performed over large (between salt marshes), medium (stations within a marsh), and small (different channel depths) spatial scales. Regression and ordination analysis suggested that the spatial variation in microbial communities exceeded the variation attributable to pollutants. PLFA analysis and TRFLP canonical correspondence analysis (CCA) explained 74 and 43% of the variation, respectively, and both methods attributed 34% of the variation to tidal cycles, marsh, year, and latitude. After accounting for spatial variation using partial CCA, we found that metals had a greater effect on microbial community composition than organic pollutants had. Organic carbon and nitrogen contents were positively correlated with PLFA biomass, whereas total metal concentrations were positively correlated with biomass and diversity. Higher concentrations of heavy metals were negatively correlated with branched PLFAs and positively correlated with methyl- and cyclo-substituted PLFAs. The strong relationships observed between pollutant concentrations and some of the microbial indicators indicated the potential for using microbial community analyses in assessments of the ecosystem health of salt marshes.     

Microbiological and Geochemical Characterization of Microbial Fe(III) Reduction in Salt Marsh Sediments

Population densities of anaerobic Fe(III)-reducing bacteria (FeRB) and aerobic heterotrophs were inversely correlated in the surficial (0-2 cm) layers of Sapelo Island, Georgia, salt marsh sediments. In surficial sediments where densities of aerobic heterotrophs were low, the density of culturable FeRB correlated positively with the concentration of amorphous Fe(III) oxyhydroxides extractable by ascorbate. High FeRB densities and a decrease with depth of ascorbate-extractable Fe(III) were observed in the upper 6 cm of a tidal creek core. Culturable sulfate-reducing bacteria (SRB) and SRB-targeted rRNA signals were also detected in the upper 6-cm depth. The disappearance of FeRB below 6 cm, however, coincided with a large increase in the abundance of SRB. Thus, when FeRB are not limited by the availability of readily reducible amorphous Fe(III) oxyhydroxides, FeRB may outcompete SRB for growth substrates. Shewanella putrefaciens- and Geobacteraceae-targeted rRNA signals were at or below detection limits in all sediment samples, indicating that these FeRB are not predominant members of the active FeRB populations. The ubiquitous presence of FeRB at the sites studied challenges the traditional view that dissimilatory Fe(III) reduction is not an important pathway of organic carbon oxidation in salt marsh sediments.     

Structural Interactions among Epilithic Cyanobacteria and Heterotrophic Microorganisms in Roman Hypogea

Abstract Phototrophic microbial communities present in the Roman Catacombs were characterized and different species of terrestrial epilithic cyanobacteria were found to occur as dominant organisms. Eucapsis, Leptolyngbya, Scytonema, and Fischerella were the most frequently encountered cyanobacterial taxa, while a few species of green algae and the diatom Diadesmis gallica occurred in minor amounts. Streptomyces strains, a few genera of eubacteria, and to a lesser extent fungi were always present in the same microhabitats and contributed to the deterioration of stone surfaces. The combined use of light and electron microscopy evidenced the structural relationships among rod-shaped or filamentous bacteria and cyanobacterial cells, as well as the presence of polysaccharide capsules and sheaths, and of mineral precipitates on S. julianum filaments. The significance of the intimate association among the microorganisms was discussed in relation to the damage caused by the growth of biological patinas on stone surfaces. Received: 17 February 1999; Accepted: 19 May 1999     

Coevolution in Multispecific Interactions among Free-Living Species

Ecological interactions among species are the backbone of biodiversity. Interactions take a tremendous variety of forms in nature and have pervasive consequences for the population dynamics and evolution of species. A persistent challenge in evolutionary biology has been to understand how coevolution has produced complex webs of interacting species, where a large number of species interact through mutual dependences (e.g., mutualisms) or influences (e.g., predator–prey interactions in food webs). Recent work on megadiverse species assemblages in ecological communities has uncovered interesting repeated patterns that emerge in these complex networks of multispecies interactions. They include the presence of a core of super- generalists, proper patterns of interaction (that resemble nested chinese boxes), and multiple modules that act as the basic blocks of the complex network. The structure of multispecies interactions resembles other complex networks and is central to understanding its evolution and the consequences of species losses for the persistence of the whole network. These patterns suggest both precise ways on how coevolution goes on beyond simple pairwise interactions and scales up to whole communities.     

Biological Interactions and the Realized Niche of Euplotes vannus from the Salt Marsh Aufwuchs

SYNOPSIS. Euplotes vannus , a hypotrich ciliate. grows well over broad ranges of temperature and salinity. It requires higher densities of food (> 1 × 10⁴ cells/ml) for rapid reproduction than do the other herbivores, the foraminiferan Al-logromia laticollaris (> 1 × 10² cells/ml), and the nematode Chromadorina germanica (∼ 1 × 10³ cells/ml), to which it was compared. If food levels were initially very high (∼ 1 × 10⁸ cells/ml) the ciliates reproduced rapidly and consumed the algae faster than it could reproduce. Some balance between the algae and the ciliates was achieved at initial algal concentrations of ∼ 1 × 10⁵ cells/ml. In microcosm experiments at 25 C with equal numbers of C. germanica and A. laticollaris. E. vannus proved to be a very poor competitor; reaching only 20% of control levels when grow with C. germanica and only 13% when cultured with A. laticollaris . It was a better competitor in 2-species microcosms, at lower temperatures, and when its ratio to the other species was initially higher.
The experimental evidence suggests that E. vannus is best adapted to being a migrating initial colonizer of fresh algal blooms.     

Nitrogen Addition Could Shift Plant Community Composition in a Restored California Salt Marsh

At a salt marsh restoration site, fertilizer trials to improve height growth of Spartina foliosa (a C₄ perennial grass that can reach 140 cm) appeared to favor Salicornia bigelovii (an annual C₃ succulent under 40 cm tall) where the two species co-occurred on the marsh plain. This observation prompted a field experiment to examine the potential for nitrogen (N) addition to shift community composition. Without N addition, total stem length and stem density of S. foliosa did not respond to the presence or absence of S. bigelovii. But where N was added, S. foliosa growth increased only where S. bigelovii was removed from plots. S. bigelovii responded strongly to fertilizer, with mean heights matching those of S. foliosa and 600% increases in biomass, branching, and seed production (to more than 1 million seeds/m²). Soil N also increased seasonally where S. bigelovii was present, suggesting that this species may aid accumulation of N at restoration sites with poor soils. S. foliosa growth is greatest at lower elevations along tidal creeks where it occurs alone. Beyond creek edges, where S. bigelovii and other potential competitors occur, S. foliosa is unlikely to grow tall even with N addition. Thus, there is little point in trying to force mixed-species stands to provide tall S. foliosa for nesting by an endangered bird, Rallus longirostris levipes (the Light-footed Clapper Rail). A marsh construction design that maximizes tidal creek edges is thus recommended when restoration goals include providing habitat for clapper rails.     

Arylsulfatase Activity in Salt Marsh Soils

The presence of arylsulfatase(s) was confirmed in salt marsh soils. The temperatures of maximum activity and inactivation, the pH range over which the enzyme was active, and the K_m values were similar to those of soil enzymes. Unlike soil arylsulfatases, however, the salt marsh enzymes do not appear to be repressed by sulfate. It is postulated that these enzymes may be necessary for the initiation of arylsulfate ester metabolism.     

Spatial and Temporal Assessment of Diazotroph Assemblage Composition in Vegetated Salt Marsh Sediments Using Denaturing Gradient Gel Electrophoresis Analysis

Abstract Diazotroph assemblage compositions were assessed in rhizosphere sediments from the tall and short form Spartina alterniflora growth zones over an annual cycle. Sediment cores were collected for DNA extraction and nitrogenase (acetylene reduction) activity assays, and porewater samples were analyzed for several chemical parameters in March, June, September, and December 1997. These data were collected to determine if within- or between-zone differences in the diazotroph assemblage composition correlated with differences in key environmental variables or acetylene reduction activity. Acetylene reduction rates differed between zones and within a zone over an annual period. Soluble sulfide concentrations were higher in the short form S. alterniflora zone on all dates except those in June and differed within both zones on different sample dates. nifH sequences were recovered from rhizosphere sediment DNA by PCR amplification using nifH specific primers. These amplimers were analyzed using denaturing gradient gel electrophoresis (DGGE), and the resulting patterns were compared by neural network and linear discriminant analyses. Ten prominent amplimers, four of which were apparent heteroduplexes, were observed. DGGE banding profiles showed minor differences among sampling dates and between sample zones, but the overall banding pattern was remarkably consistent. This reflects overall similarity between the amplifiable diazotroph assemblages in the tall and short S. alterniflora growth zones and substantial seasonal stability in assemblage composition. Received: 2 March 1999; Accepted: 4 May 1999