Seasonal patterns of daily net photosynthesis,transpiration and net primary productivity of Juncus roemerianus and Spartina alterniflora in a Georgia salt marsh

Summary Studies of the seasonal CO₂ and water vapor exchange patterns of Juncus roemerianus and Spartina alterniflora were conducted in an undisturbed marsh community on Sapelo Island, Georgia. Daily patterns of net photosynthesis, transpiration, leaf diffusive conductance and water-use efficiency in response to ambient conditions were monitored on intact, in situ plants. Net primary productivity was calculated from the daytime CO₂ fixation totals, nighttime CO₂ loss, leaf standing stock and aboveground to belowground biomass ratios for each plant type.The tall form of S. alterniflora had higher rates of photosynthesis and higher water-use efficiency values which, in conjunction with low respiratory losses and large leaf standing crop, results in high values of net primary productivity. The environmental factors in the marsh which permit these physiological responses occur in less than 10% of the marsh. Overall, the physiological capabilities of the short form of S. alterniflora were reduced in comparison to the tall form, but the combination of environmental factors which determine the physiological responses of this form occur in a much greater portion of the marsh, and the short form of S. alterniflora dominates the Sapelo Island marshes.The response patterns of the C₃ species, J. roemerianus, differed somewhat from the height forms of S. alterniflora. A large, seasonally constant leaf standing crop coupled with moderate rates of photosynthesis resulted in a net primary productivity value which was between the tall and short height forms of S. alterniflora. However, as with the tall S. alterniflora, the environmental conditions under which this high productivity and high water loss rate can be sustained are restricted to specific regions of the environmental gradient in the marsh.Contribution No. 435 from the University of Georgia Marine Institute     

EFFECTS OF SULFIDE ON THE GROWTH OF THREE SALT MARSH HALOPHYTES OF THE SOUTHEASTERN UNITED STATES

A hydroponic culture experiment was performed to ascertain whether sediment soluble sulfide at in situ concentrations plays a role in the determination of height forms of Spartina alterniflora in salt marshes of the United States. Additional experiments were conducted for both Spartina cynosuroides and Borrichia frutescens to determine if sulfide also influences the overall distribution of these species in the marsh. In situ soluble sulfide concentrations ranged from 0.02 mm in creek bank sites up to 3.0 mm in the inner marsh. In culture treatments, both plant height and biomass production of S. alterniflora were inhibited at a sulfide concentration as low as 1.0 mm , strongly suggesting a role for sulfide in the determination of height forms in the marsh. Production of S. cynosuroides was inhibited at high sulfide concentrations. However, over a range of concentrations similar to in situ values, no significant reduction in growth was observed, indicating sulfide was not a primary determinant of growth in stands of S. cynosuroides on Sapelo Island, Georgia. A sulfide concentration of 0.5 mm inhibited production in B. frutescens. In situ sulfide concentrations as high as 0.5 mm were found only in mixed stands of Juncus roemerianus and B. frutescens.     

UNDERGROUND BIOMASS PROFILES AND PRODUCTIVITY IN ATLANTIC COASTAL MARSHES

An underground biomass profile and productivity study involved year-long sampling programs in 18 stands of salt marsh plants in Georgia, Delaware, and Maine. As the result of the monthly or bimonthly marsh coring program three types of underground biomass profiles were found. In the first, the concentration of macro-organic matter (MOM) was uniform with depth; the notable example of this type was creekbank Spartina alterniflora in the southern part of the coast. A second type had a high MOM concentration at the surface which decreased with depth. This, the most common type of profile, was exemplified by Spartina patens, S. alterniflora from the high marsh along the southern coast (Georgia), and creekbank S. alterniflora from the northern part of its range (Maine). The third type of profile was seen where a large rhizome mat developed 15–20 cm below the surface. Spartina cynosuroides and Phragmites communis were typical examples of this type of profile, resulting in a low biomass at the surface, a higher biomass somewhat below the surface, and a low concentration at depth. The annual maxima and minima of MOM biomass were used to calculate annual increments, which can be considered minimum annual production values. These productivity values ranged from a low of 80 g C/m² for creekhead S. alterniflora in Maine to a high of 1690 g C/m² for Juncus gerardi in Maine. The mean for all plant stands was 650 g C/m². Since the average carbon content of the MOM was 35.3%, this corresponds to 1850 g dry weight/m² per year. As a measure of the relative activity of the total pool of macro-organic material in the soil, turnover times were calculated by dividing the total macro-organic matter by the annual increments. Within the MOM pool there are several components with turnover times varying from days to years. The turnover time for the entire pool ranged from 18 months in two Georgia salt marsh plant stands to 224 months for one in Maine. In the two instances where values for a species could be compared between Maine and Georgia, the turnover time was shorter at the more southerly site. These results can probably be attributed to slower microbial decay rates in the cooler climate. In Georgia and Maine where the turnover values for a species were determined for two elevations, the time was shorter at the lower elevation.     

Estimation of the primary productivity of Spartina alterniflora using a canopy model

A comprehensive canopy productivity model was built to study the productivity of a primary salt marsh grass, Spartina alterniflora. in Georgia, USA The canopy model was unique in employing plant demographic data to reconstruct canopy profiles and dynamics, which showed many growth processes that are otherwise difficult to discern in the field By linking canopy dynamics and leaf photosynthesis, the net total primary productivity of S alterniflora m a Georgia salt marsh was estimated to be 1421, 749, and 1441 g C m^-2 yr^-1 for the tall, short, and N-fertilized short populations respectively These estimates are reasonable in terms of the physiological capacity of S alterniflora and well below the range of 3000–4200 g C m^-2 yr^-1 as reported by some recent harvest studies Our detailed analysis suggested the net total productivity of S alterniflora might be greatly overestimated in the past This is mainly because of 1) failure to consider the translocation of photosynthate between aboveground and belowground parts, and 2) possible overestimates of belowground production We estimated the net belowground production to be 872, 397, and 762 g C m^-2 yr^-1 for the tall, short, and N-fertilized populations respectively After receiving nitrogen fertilizer, the net leaf carbon fixation in the short population increased from 1489 to 2487 g C m^-2 yr^-1, and our simulation showed the contribution of elevated leaf N to this increase was small, 21%, compared with that of increased leaf area, 79% Both tall and short populations allocated ca 48-49% of their annual gross leaf carbon fixation to belowground structures Nitrogen enrichment caused more allocation to aboveground parts in the short population, mainly for increasing leaf area The canopy model assumed that there was no leaf photosynthesis under tidal submergence, but if this assumption was relaxed, then leaf carbon fixation might increase 7–13% for different S alterniflora populations Although this research focused only on a salt marsh species, our general approaches, especially the coupling of leaf physiology with the reconstructed canopies, should be applicable to the study of production processes of many other plant populations     

A GROWTH COMPARISON OF SPARTINA ALTERNIFLORA LOISEL. ECOPHENES UNDER AEROBIC AND ANAEROBIC CONDITIONS

Spartina alterniflora Loisel. culms were collected from tall (creekbank), short (highmarsh) and dieback sites in a North Carolina salt marsh and grown in aerobic and anaerobic simulated marsh systems in the greenhouse. There were no significant differences between density, aerial live biomass, height, leaf width or root biomass and sprig source. All variables were significantly different between aeration treatments. Aerated systems had an average of 6.3 times more biomass than the unaerated treatments. There was a significant interaction between sprig source and aeration treatments based on a multivariate analysis of variance representing overall plant performance. These results demonstrated that pioneer sprigs from the dieback sites had an advantage over the other sprigs when exposed to unaerated systems. Tall plants performed better than plants from short or dieback zones in the aerobic systems.     

Phenotypic plasticity of <Emphasis Type="Italic">Spartina alterniflora</Emphasis> and <Emphasis Type="Italic">Phragmites australis</Emphasis> in response to nitrogen addition and intraspecific competition

Phenotypic plasticity of the two salt marsh grasses Spartina alterniflora and Phragmites australis in salt marshes is crucial to their invasive ability, but the importance of phenotypic plasticity, nitrogen levels, and intraspecific competition to the success of the two species is unclear at present. Spartina alterniflora Loisel. is an extensively invasive species that has increased dramatically in distribution and abundance on the Chinese and European coasts, and has had considerable ecological impacts in the regions where it has established. Meanwhile, Phragmites australis Cav., a native salt marsh species on the east coast of China, has replaced the native S. alterniflora in many marshes along the Atlantic Coast of the US. This study determined the effects of nitrogen availability and culm density on the morphology, growth, and biomass allocation traits of Spartina alterniflora and Phragmites australis. A large number of morphological, growth, and biomass parameters were measured, and various derived values (culm: root ratio, specific leaf area, etc.) were calculated, along with an index of phenotypic plasticity. Nitrogen addition significantly affected growth performance and biomass allocation traits of Spartina alterniflora, and culm density significantly affected morphological characteristics in a negative way, especially for Spartina alterniflora. However, there were no significant interactions between nitrogen levels and culm density on the morphological parameters, growth performances parameters, and biomass allocation parameters of the two species. Spartina alterniflora appears to respond more strongly to nitrogen than to culm density and this pattern of phenotypic plasticity appears to offer an expedition for successful invasion and displacement of Phramites australias in China. The implication of this study is that, in response to the environmental changes that are increasing nitrogen levels, the range of Spartina alterniflora is expected to continue to expand on the east coast of China.     

SEASONAL PATTERNS IN THE BELOWGROUND BIOMASS OF SPARTINA ALTERNIFLORA (GRAMINEAE) ACROSS A TIDAL GRADIENT

Belowground biomass of two ht forms of the salt marsh cordgrass, Spartina alterniflora, in a New England salt marsh exhibited a seasonal cycle. Biomass was highest in midsummer with no secondary peak in biomass corresponding with the autumn dieback of aboveground parts. Total annual biomass production and the maximum depth that living tissue penetrated into the substrate decreased with increasing tidal ht. Substrate characteristics (soil aeration, pH, nutrient levels) known to affect aboveground biomass of S. alterniflora also decreased with increasing tidal ht and may similarly affect belowground biomass across the same tidal gradient.     

Effect of the Spartina alterniflora Root-Rhizome System on Salt Marsh Soil Denitrifying Bacteria

Nitrous oxide (N₂O) reductase activity was used as an index of the denitrification potential in salt marsh soils. In a short Spartina alterniflora marsh, the seasonal distribution of N₂O reductase activity indicated a causal relationship between S. alterniflora root-rhizome production and the denitrification potential of the soil system. The relationship was not discerned in samples from a tall S. alterniflora marsh. To further examine the in situ plant-denitrifier interaction in the short S. alterniflora marsh, plots with and without living S. alterniflora were established and analyzed for N₂O reductase activity 5 and 18 months later. In the plots without living Spartina there was a significant reduction in the soil denitrification potential after 18 months, indicating that in the SS marsh the denitrifiers are tightly coupled to the seasonal production of below-ground Spartina macroorganic matter. In plots with intact Spartina, the soil denitrification potential was not altered by NH₄NO₃ or glucose enrichment. However, in plots without living Spartina, there were significant changes in soil N₂O reductase activity, thus indicating that the plants can serve as a “buffer” against this form of pulse perturbation.     

Differential population response of allocation,phenology, and tissue chemistry in <Emphasis Type="Italic">Spartina alterniflora</Emphasis>

Phenotypic variation within species is widespread among salt marsh plants. For Spartina alterniflora, the dominant species of low intertidal wetlands across the Altantic and Gulf coasts of the US, distinct phenological and morphological differences among populations from different latitudes have been found. To determine whether S. alterniflora plants from lower latitudes and those regenerated from Delaware tissue cultures would maintain differences from that of native plants, we conducted a field study in a natural salt marsh in Delaware, US. After two growing seasons, plant height, stem density, above- and belowground biomass, elemental composition, and nutrient resorption were measured. Natural variation in porewater salinity influenced physiological traits of Na⁺/K⁺ ratio regulation and nitrogen resorption efficiency similarly across populations. While plant height exhibited plasticity where populations tended to converge to a similar height, several other traits remained distinct. Delaware plants had a greater rate of rhizome growth than Georgia and Louisiana plants, which correlated with a greater magnitude of fall senescence. If traits such as seasonal translocation are plastic and can change with the length of the growing season, climate warming may alter belowground biomass production of S. alterniflora in wetlands of the mid-Atlantic.     

Formyltetrahydrofolate Synthetase Sequences from Salt Marsh Plant Roots Reveal a Diversity of Acetogenic Bacteria and Other Bacterial Functional Groups

Sixty-two partial formyltetrahydrofolate synthetase (FTHFS) structural gene sequences were recovered from roots of salt marsh plants, including Spartina alterniflora, Salicornia virginica, and Juncus roemerianus. Only S. alterniflora roots yielded sequences grouping with FTHFS sequences from known acetogens. Most other FTHFS or FTHFS-like sequences grouped with those from sulfate-reducing bacteria. Several sequences that grouped with Sphingomonas paucimobilis ligH were also recovered.     

EVALUATION OF PROLINE ACCUMULATION IN THE ADAPTATION OF DIVERSE SPECIES OF MARSH HALOPHYTES TO THE SALINE ENVIRONMENT

Proline accumulation by eight major species of salt marsh halophytes was examined under growth chamber and field conditions. When the plants were exposed to increasing salinities in the growth chamber, they accumulated proline after a threshold salinity had been reached. Three general patterns were apparent. Limonium carolinianum (Walt.) Britt. and Junius roemerianus Scheele began to accumulate proline at 0.25 m NaCl with accumulations up to 63.6 μmoles per gram fresh weight at higher salinities. The C₄ grasses, Spartina alterniflora Loisel., Spartina patens (Aiton) Muhl., and Distichlis spicata (L.) Greene, had threshold salinity levels around 0.5 m NaCl and accumulated proline to 27.4 μmoles per gram fresh weight. The succulents, Salicornia bigelovii Torr., Salicornia virginica L., and Borrichia frutescens (L.) DC, did not accumulate proline until very high salinities (0.7 m) were reached. Water stress imposed by polyethylene glycol instead of NaCl caused similar proline accumulation in the species studied, but to different extents. Field measurements of proline content and soil salinities correlated well with the findings from growth chamber experiments. Rates of proline accumulation and breakdown in L. carolinianum were sufficient for osmotic adjustment by the plant to the changes in interstitial salinity in the marsh. The significance of proline accumulation as an adaptation to the salt marsh environment was species specific. We suggest that proline accumulation is of considerable importance in L. carolinianum and J. roemerianus, important to the C₄ grasses at certain times and in certain locations in the marsh, and of little importance in the succulents.     

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.     

Influence of Mycorrhizal Inoculation, Inundation Period, Salinity, and Phosphorus Availability on the Growth of Two Salt Marsh Grasses, Spartina alterniflora Lois. and Spartina cynosuroides (L.) Roth., in Nursery Systems

Restoration of salt marsh ecosystems is an important concern in the eastern United States to mitigate damage caused by industrial development. Little attention has been directed to the mycorrhizal influence on plantings of salt marsh species to stabilize estuarine sediments and establish cover. In our study, seedlings of two salt marsh grasses, Spartina alterniflora and Spartina cynosuroides, were grown in soil with a commercial, mixed species inoculum of arbuscular mycorrhizal fungi. Plants were grown in experimental “ebb and flow” boxes, simulating three levels of tidal inundation, to which two levels of applied phosphorus (P) and two levels of salinity were imposed. After 2.5 months, S. alterniflora was poorly colonized by arbuscular mycorrhizae, developing only fungal hyphae and no arbuscules, but S. cynosuroides became moderately colonized. Mycorrhizal inoculation marginally improved growth and P and nitrogen (N) content of both plant species at low levels of P supply but significantly increased tillering in both plant species. This factor could be beneficial in enhancing ground cover during restoration procedures. Greater P availability increased the mycorrhizal status of S. cynosuroides and improved P nutrition of both plant species, despite a reduction in the root‐to‐shoot ratio. Increasing salinity reduced mycorrhizal colonization of S. alterniflora but not of S. cynosuroides. Growth and nutrient content of S. alterniflora was improved at higher levels of salinity, but only increased nutrient content in S. cynosuroides. Increased duration of tidal inundation decreased plant growth in both species, but tissue P and N concentrations were highest with the longest time of inundation in both species.     

Lacunal allocation and gas transport capacity in the salt marsh grass Spartina alterniflora

Summary Lacunal allocation as the fraction of the total cross sectional area of leaves, stem bases, rhizomes, and roots was determined in both tall and short growth forms of Spartina alterniflora collected from natural monospecific stands. The results indicate that in both growth forms lacunal allocation is greater in stem bases and rhizomes than in leaves and roots and that tall form plants allocate more of their stem and rhizome to lacunae than short form plants.Measurements made in natural stands of Spartina alterniflora suggest that total lacunal area of the stem base increases with increasing stem diameter and that stem diameter increases with increasing plant height and above-ground biomass. However, the fraction of cross section allocated to lacunae was relatively constant and increased only with the formation of a central lacuna.Experimental manipulations of surface and subsurface water exchange were carried out to test the influence of flooding regime on aerenchyma formation. No significant differences in lacunal allocation were detected between plants grown in flooded (reduced) and drained (oxidized) sediments in either laboratory or field experiments. While aerenchyma formation in Spartina alterniflora may be an adaptation to soil waterlogging/anoxia, our results suggest that lacunal formation is maximized as a normal part of development with allocation constrained structurally by the size of plants in highly organic New England and Mid-Atlantic marshes.The cross sectional area of aerenchyma for gas transport was found to be related to the growth of Spartina alterniflora with stands of short form Spartina alterniflora exhibiting a lower specific gas transport capacity (lacunal area per unit below ground biomass) than tall form plants despite having a similar below-ground biomass supported by a 10 fold higher culm density. The increased specific gas transport capacity in tall vs. short plants may provide a new mechanism to explain the better aeration, higher nutrient uptake rates and lower frequency of anaerobic respiration in roots of tall vs. short Spartina alterniflora.     

Tolerance and avoidance: Two contrasting physiological responses to salt stress in mature marsh halophytes Juncus roemerianus Scheele and Spartina alterniflora Loisel

For most plants, elevated salinities can promote both hyperionic and hyperosmotic stress, often resulting in decreased growth and increased mortality. In previous studies involving plant–water relations, two contrasting physiological mechanisms to water stress have emerged: (i) stress-tolerance, which can be achieved through osmotic adjustment and changes in tissue elasticity, and (ii) stress-avoidance, which restricts further water loss through decreased stomatal conductance and changes in leaf morphology and/or orientation. While these processes have been well characterized in angiosperms during drought, few studies have considered these responses in halophytes during salt-stress. In this study, experimental microcosms were used to evaluate salt-tolerance and salt-avoidance in two contrasting coastal-marsh halophytes, Juncus roemerianus and Spartina alterniflora. In mature S. alterniflora, preacclimated to freshwater, only salt-tolerance mechanisms (osmotic adjustment and increased tissue rigidity) were observed during high salinity conditions. In contrast, physiological modifications observed in mature J. roemerianus involved salt-avoidance through decreased stomatal conductance. These physiological responses are consistent with zonation patterns in these plants, wherein S. alterniflora resides in the lower marsh and must contend with long-term salt exposure and J. roemerianus inhabits the upper reaches of salt-marshes where salinities tend to be lower and where salt-stress often involves transient exposure to high salinities.     

Comparison of Nitrogen Fixation Activity in Tall and Short Spartina alterniflora Salt Marsh Soils

A comparison of the N₂ fixers in the tall Spartina alterniflora and short S. alterniflora marsh soils was investigated. Zero-order kinetics and first-order kinetics of acetylene reduction were used to describe the activity of the N₂ fixers in marsh soil slurries. It was found that the V_max values were approximately 10 times greater for the N₂ fixers in the tall Spartina than in the short Spartina marsh when raffinose was used as the energy source. In addition, the (K_s + S_n) values were approximately 4 to 15 times lower for the N₂ fixers in the tall Spartina than in short Spartina marsh. First-order kinetics of nitrogen fixation for several substrates indicate that the N₂ fixers in the tall Spartina marsh were two to seven times more active than those in the short Spartina marsh. Ammonium chloride (25 μg/ml) did not inhibit nitrogen fixation in the tall Spartina marsh, but there was a 50% inhibition in nitrogen fixation in the short Spartina marsh. On the other hand, sodium nitrate inhibited nitrogen fixation almost 100% at 25 μg/ml in both soil environments. Amino nitrogen (25 to 100 μg/ml) had little or no effect on nitrogen fixation. The results indicate that the N₂ fixers in the tall Spartina marsh were physiologically more responsive to nutrient addition than those in the short Spartina marsh. This difference in the two populations may be related to the difference in daily tidal influence in the respective areas and thus provide another explanation for the enhanced S. alterniflora production in the creek bank soil system.     

Ecological processes shaping Central Patagonian salt marsh landscapes

Plant zonation is one of the most conspicuous ecological features of salt marshes worldwide. In this work we used a combination of field transplant and greenhouse experiments to evaluate the importance of interspecific interactions and physical stress in the determination of the major plant zonation patterns in Central Patagonian salt marshes. There, Spartina alterniflora dominates the low marsh, and Sarcocornia perennis the high marsh. We addressed two questions: (i) What prevents Spartina alterniflora from colonizing the Sarcocornia perennis‐dominated high marsh zone? and (ii) What prevents Sarcocornia perennis from colonizing the Spartina alterniflora‐dominated low marsh zone? Our experimental transplants combined with neighbour exclusion treatments showed that the presence of Sarcocornia perennis negatively affects Spartina alterniflora, preventing it from surviving and/or spreading. Complementary field transplant and greenhouse experiments showed that Sarcocornia perennis did not survive the frequent tidal submersion by approximately 1.5 m of turbid seawater in the Spartina alterniflora zone, but its survival was independent of the presence of Spartina neighbours, and of the strong soil anoxia as well. Our results suggest that Spartina alterniflora is excluded by Sarcocornia perennis towards the low marsh, where frequent and prolonged submersion limit the survival of the latter. We provide and discuss key baseline information to facilitate the future design of ecophysiological experiments designed to accurately identify the exact mechanisms acting in every situation.     

Seasonal Variability of Diazotroph Assemblages Associated with the Rhizosphere of the Salt Marsh Cordgrass, Spartina alterniflora

Nitrogen fixation is the primary N source in the highly productive but N-limited North Inlet, SC, USA salt marsh system. The diverse assemblages of nitrogen-fixing (diazotrophic) bacteria associated with the rhizospheres of the short and tall growth forms of Spartina alterniflora were analyzed at two sites, Crab Haul Creek and Goat Island, which are in different tidal creek drainage systems in this marsh. The sites differed in proximity to the main channel for tidal intrusion and in several edaphic parameters. We hypothesized that either the differing abiotic environmental regimes of the two sites or the variation due to seasonal effects result in differences in the diazotroph assemblage. Rhizosphere samples were collected seasonally during 1999 and 2000. DNA was purified and nifH amplified for denaturing gradient gel electrophoresis (DGGE) analysis of diazotroph assemblage composition. Principal components analysis was used to analyze the binary DGGE band position data. Season strongly influenced assemblage composition and biplots were used to identify bands that significantly affected the seasonal and site-specific clustering. The types of organisms that were most responsive to seasonal or site variability were identified on the basis of DGGE band sequences. Seasonally responsive members of the anaerobic diazotrophs were detected during the winter and postsenescence conditions and may have been responsible for elevated pore water sulfide concentrations. Sequences from a diverse assemblage of Gammaproteobacteria were predominant during growth periods of S. alterniflora. Abiotic environmental parameters strongly influenced both the S. alterniflora and the diazotrophic bacterial assemblages associated with this keystone salt marsh plant species.     

Oligochaeta in Spartina stems: the microdistribution of Enchytraeidae and Tubificidae in a salt marsh,Sapelo Island,USA

The distribution and abundance of Enchytraeidae and Tubificidae in and around Spartina alterniflora plants in a tidal salt marsh on Sapelo Island, Georgia, USA were studied using two different sampling techniques: wet funnel extraction and stem dissection. At least 80% of all worms inhabited leaf sheaths at the bases of S. alterniflora plants, and densities were low in sediment, root and surface debris samples. Oligochaete densities were dependent on the position within the marsh, the height on stems and the stage of sheath decay. Six predominant species were identified and included Marionina appendiculata, Marionina spartinae, Marionina waltersi, Marionina paludis, and Monopylephorus parvus. Individual species were distributed differently on stems and enchytraeids were more common than tubificids on standing-dead and further up S. alterniflora stems. Estimates of oligochaete densities in salt marsh habitats are increased dramatically when the numbers of worms on stems are considered. Possible advantages of the stem microhabitat are discussed in relation to the biology and ecology of oligochaetes.     

Ecological significance of wing polymorphism in Fulgoroidea which inhabit tidal salt marshes

• 1 The vegetation of New Jersey tidal salt marshes is composed primarily of two grasses, Spartina patens, which occupies a narrow elevational zone of high marsh and varies little in structure from site to site, and Spartina alterniflora, an intertidal species which occurs as two spacially separated growth forms (tall and short). Fulgoroids, polymorphic for wing length, inhabit these grasses.
• 2 Populations of the common fulgoroid inhabiting short form S.alterniflora were composed of equal numbers of macropters and brachypters. Mesothoracic wings of brachypters were subequal in length to the abdomen allowing for short but inefficient flights compared to macropters.
• 3 Fulgoroids residing in S.patens produced mostly brachypters having small meso-and vestigial metathoracic wings and were incapable of flight.
• 4 Brachypter density of all fulgoroids was correlated with the seasonal increase in biomass of the host grass, while macropter density was not.
• 5 Peak macropter density occurred shortly before maximum biomass of the host grass was attained.
• 6 The production of macropterous forms was correlated with high levels of crowding incurred during nymphal stages.
• 7 Spatial and temporal variation in the structure of the host grasses and habitat reliability are suggested as important factors dictating the wing-polymorphism strategies of fulgoroids.