Effect of nutrient loading on biogeochemical and microbial processes in a New England salt marsh

Coastal marshes represent an important transitional zone between uplands and estuaries. One important function of marshes is to assimilate nutrient inputs from uplands, thus providing a buffer for anthropogenic nutrient loads. We examined the effects of nitrogen (N) and phosphorus (P) fertilization on biogeochemical and microbial processes during the summer growing season in a Spartina patens (Aiton (Muhl.)) marsh in the Narragansett Bay National Estuarine Research Reserve on Prudence Island (RI). Quadruplicate 1 m² plots were fertilized with N and P additions, N-only, P-only, or no additions. N-only addition significantly stimulated bacterial production and increased pore water NH₄⁺ and NO₃⁻ concentrations. Denitrification rates ranged from 0 to 8 mmol m⁻² day⁻¹. Fertilization had no apparent effect on soil oxygen consumption or denitrification measured in the summer in intact cores due to high core-to-core variation. P fertilization led to increased pore water dissolved inorganic phosphorus (DIP) concentrations and increased DIP release from soils. In contrast the control and N-only treatments had significant DIP uptake across the soil-water interface. The results suggest that in the summer fertilization has no apparent effect on denitrification rates, stimulates bacterial productivity, enhances pore water nutrient concentrations and alters some nutrient fluxes across the marsh surface.     

Salt stress limitation of seedling recruitment in a salt marsh plant community

Summary Seedling recruitment in salt marsh plant communities is generally precluded in dense vegetation by competition from adults, but is also relatively rare in disturbance-generated bare space. We examined the constraints on seedling recruitment in New England salt marsh bare patches. Under typical bare patch conditions seed germination is severely limited by high substrate salinities. We examined the germination requirements of common high marsh plants and found that except for one notably patch-dependent fugitive species, the germination of high marsh plants is strongly inhibited by the high soil salinities routinely encountered in natural bare patches. Watering high marsh soil in the greenhouse to alleviate salt stress resulted in the emergence of up to 600 seedlings/225 cm². The vast majority of this seed bank consisted of Juncus gerardi, the only common high marsh plant with high seed set. We tested the hypothesis that salt stress limits seedling contributions to marsh patch secondary succession in the field. Watering bare patches with fresh water partially alleviated patch soil salinities and dramatically increased both the emergence and survival of seedlings. Our results show that seedling recruitment by high marsh perennial turfs is limited by high soil salinities and that consequently their population dynamics are determined primarily by clonal growth processes. In contrast, populations of patch-dependent fugitive marsh plants which cannot colonize vegetatively are likely governed by spatially and temporally unpredictable windows of low salinities in bare patches.     

Consumer pressure and seed set in a salt marsh perennial plant community

Summary Seed predation can be an important determinant of plant success, but has received little attention in wetland plant communities. Here, we examine the role of flower and seed predators in limiting the seed production of the dominant perennial plants in a salt marsh plant community. Of the four perennial investigated, direct ovule loss to consumers ranged from 51 to 80%, resulting in seed set reductions ranging from 50% to over 20-fold. Most losses were due to generalist grazing by the grasshopper, Conocephalus spartinae. More species-specific losses were inflicted by planthoppers, and microlepidopteran and dipteran larval seed parasites.Insect abundance and consumer pressure on flowers and seeds increased over the early summer, peaked in the middle of July, and declined through August, and this temporal pattern was reflected in the natural consumer damage incurred by each of the marsh perennials. Juncus gerardi flowers earlier than other marsh perennials and largely escapes heavy consumer losses. Spartina patens and Distichlis spicata flower in the middle of the summer during the peak consumer activity and incur extremely heavy seed losses. Spartina alterniflora flowers late in the summer as consumer pressure is subsiding, which appears to minimize its seed loss. In addition to destroying seeds directly, consumers also markedly reduce the frequency and affect the timing of sexual expression in these plants. In particular, predation drastically reduces the frequency of male flowers, which could lead to pollen limitation of seed set.Intense flower and seed predation on these marsh perennials may be an important determinant of the success of marsh plant populations as well as a potent selective force on their flowering phenologies and reproductive effort.     

Modeling productivity of a salt marsh

A model of primary productivity in a salt marsh is developed and compared to a regression analysis study of data showing dependence of growth on growing season, mean tidal height, and average monthly temperatures for several grass species.     

Small-scale pattern: an evaluation of techniques with an application to salt marsh vegetation

This paper examines the outcome of four different procedures for quantifying small-scale association patterns in vegetation, with respect to data from Church Island salt marsh, Menai Bridge, N. Wales. A main aim is the evaluation of two plot-less techniques, proposed by Stowe & Wade (1979). The first examines frequencies of juxtaposition, the second the frequencies of target species in the regions of occurrence of other species. The results of these analyses are compared with those of two plot-based techniques. A Reciprocal averaging ordination is also employed to describe medium-scale distributional pattern within the data set.Of the two plot-less methods, the species-region method is found to be more useful, particularly because it is capable of detecting asymmetric relationships. When using the recommended collapsed data format the plot-less methods operate at an inconsistent spatial scale approximating to the mean run or clump size of species along the sampling transects. This allows detection of sociological patterns but may obscure environmental relationships. The small-scale relationships depicted by the plot-less methods are in cases in accord with the plot-based results at a range of scales, in other cases they differ: in each circumstance providing insights into the nature of interspecific relationships.Inter-specific pattern in salt marsh vegetation has rarely been studied. The methodology employed here shows that relationships do exist at the small scale, and may allow hypothesis generation with respect to their causality and their role in the dynamics of the marsh system.     

Arbuscular mycorrhizal fungal propagules in a salt marsh

The tolerance of indigenous arbuscular mycorrhizal fungi (AMF) to stressful soil conditions and the relative contribution of spores of these fungi to plant colonization were examined in a Portuguese salt marsh. Glomus geosporum is dominant in this salt marsh. Using tetrazolium as a vital stain, a high proportion of field-collected spores were found to be metabolically active at all sampling dates. Spore germination tests showed that salt marsh spores were not affected by increasing levels of salinity, in contrast to two non-marsh spore isolates, and had a significantly higher ability to germinate under increased levels of salinity (20) than in the absence of or at low salinity (10). Germination of salt marsh spores was not affected by soil water levels above field capacity, in contrast to one of the two non-marsh spore isolates. For the evaluation of infectivity, a bioassay was established with undisturbed soil cores (containing all types of AM fungal propagules) and soil cores containing only spores as AM fungal propagules. Different types of propagules were able to initiate and to expand the root colonization of a native plant species, but spores were slower than mycelium and/or root fragments in colonizing host roots. The AM fungal adaptation shown by this study may explain the maintenance of AMF in salt marshes.     

Seasonal changes in the microbial community of a salt marsh, measured by phospholipid fatty acid analysis

Microbial activity within the environment can have distinct geochemicaleffects, and so changes in a microbial community structure can result ingeochemical change. We examined seasonal changes in both the microbialcommunityand the geochemistry of an inter-tidal salt marsh in north-west England tocharacterise biogeochemical processes occurring at this site.Phospholipid fatty acid (PLFA) analysis of sediment samples collected atmonthly intervals was used to measure seasonal changes in microbial biomass andcommunity structure. The PLFA data were analysed using multivariate techniques(Ward's method and the Mahalanobis distance metric), and we show that the useofthe Mahalanobis distance metric improves the statistical analysis by providingdetailed information on the reasons samples cluster together and identifyingthedistinguishing features between the separate clusters. Five clusters of likesamples were defined, showing differences in the community structure over thecourse of a year.At all times, the microbial community was dominated by PLFA associated withaerobic bacteria, but this was most pronounced in summer (August). Theabundanceof branched fatty acids, a measure of the biomass of anaerobes, started toincrease later in the year than did those associated with aerobes and thefungalbiomarker 18:26 showed a brief late-summer peak.The salt marsh remained mildly oxic throughout the year despite the increase inmicrobial respiration, suggested by the large increases in the abundance ofPLFA, in the warmer months. The conditions therefore remained most favourablefor aerobic species throughout the year, explaining their continual dominanceatthis site. However, as the abundance of PLFA synthesised by anaerobesincreased,increases in dissolved Mn concentrations were observed, which we suggest weredue to anaerobic respiration of Mn(IV) to Mn(II). Overall, the geochemicalconditions were consistent with the microbial community structure and changeswithin it.     

Dimethyl sulfide metabolism in salt marsh sediments

Abstract Anoxic sediment slurries prepared from Spartina salt marsh soils contained dimethyl sulfide (DMS) at concentrations ranging from 1 to 10 μM. DMS was produced in slurries over the initial 1–24 h incubation. After the initial period of production, DMS decreased to undetectable levels and methane thiol (MSH) was produced. Inhibition of methanogenesis caused a 20% decrease in the rate of DMS consumption, while inhibition of sulfate reduction caused a 80% decrease in DMS consumption. When sulfate reduction and methanogenesis were simultaneously inhibited, DMS did not decrease. DMS contributed about 28% to the methane production rate, while DMS probably contributed only 1% or less to the sulfate reduction rate. Incubation of the sediment slurries under an atmosphere of air resulted in similar DMS consumption compared to anaerobic incubations, but MSH and CH₄ were not evolved.
Sediments from the marsh released significant quantities of DMS when treated with cold alkali, indicating that potentially significant sources of DMS existed in the sediments. Values of base-hydrolyzable DMS as high as 190 μmol per liter of sediment were observed near the sediment surface, and values always decreased with depth in the sediment. Simple flux experiments with small intact sediment cores, showed that DMS was emitted from the marsh surface when cores were injected with glutaraldehyde or molybdate and 2-bromoethanesulfonate (BES), but nit when cores were left uninhibited. These results showed that DMS was readily metabolized by microbes in marsh sediments and that this metabolism may be responsible for reducing the emission of DMS from the marsh surface.     

Biological consequences of ice rafting in a New England salt marsh community

Ice rafting of salt marsh peat is a recurrent phenomenon in north temperate regions. This process was simulated in a northern New England salt marsh to test several hypotheses concerning the effects of peat transport from high to low intertidal heights on the growth and mortality of key sessile organisms: the ribbed mussel Geukensia demissa (Dillwyn), the fucoid alga Fucus vesiculosus L. var. spiralis (Farlow) and the cordgrass Spartina alterniflora (Loisel.). Growth rates increased when Geukensia and Fucus were transported to the lower intertidal; however, Spartina died when similarly transported. Predation pressure (primarily from Carcinus maenus L.) on Geukensia was greater when it was rafted to the lower intertidal zone than in the upper intertidal habitat and was size specific; mussels >3.5cm reached a size-escape from crab predation.A winter survey of dislodged mussels revealed that 72% of the mussels collected were dead and 86% had been overgrown by large Fucus plants, >2.5 × the natural frequency of Fucus overgrowth (32%). In marsh habitats where hard substratum is rare, 91% of the Fucus were growing on Geukensia. A dislodgement experiment showed that a significantly greater percentage of Geukensia was dislodged after ice-out when Fucus was attached to the shell than those mussels without Fucus overgrowth. In the spring, a population survey conducted in the salt marsh examined densities, biomass and population structure of Geukensia, as well as densities, percent cover and biomass of Fucus. Values obtained in the foremarsh were compared to those from the peat islands recently rafted to the tidal flats. Both biomass and densities of Geukensia were similar in the two areas; however, the size-frequency distributions of the mussels were different. Since fewer large mussels, Fucus and Fucus-overgrown mussels were found on the newly transported peat islands, this pattern appears to reflect dislodgement of larger Geukensia by attached algae during ice transport. Two ice-related sources of mortality were identified for Geukensia: (1) Fucus overgrowth acted as a vector for mussel dislodgement and was an indirect source of mortality; and (2) ice crushing was a direct source of mortality for non-overgrown mussels.     

Biotechnological potential of North Sea salt marsh plants—a review of traditional knowledge

Extreme environments are characterised by wide variations in physical factors. Tidal flats and their adjacent salt marshes along the North Sea coast experience this type of variability on a daily, seasonal and annual basis. Plants and animals living in these extreme environments have to adapt to these, at times rapid, changes. This can be done by developing specific physiological responses which often encompass the synthesis of unusual chemicals. A number of salt marsh plants have traditionally been used for medical, nutritional or even industrial purposes. Here, examples are presented for these plants.     

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.     

Short-term sedimentation in Tagus estuary,Portugal: the influence of salt marsh plants

Short-term sediment deposition was studied at four salt marsh areas in the Tagus estuary. In areas covered with Sarcocornia perennis, Sarcocornia fruticosa, Halimione portulacoides and Spartina maritima and also in the non-vegetated areas, sedimentation was measured as the monthly accumulation of sediments on nylon filters anchored on the soil surface, from August 2000 to May 2001. Our experiments were used also to determine the influence of the different plant species in vertical accretion rates. Short-term sedimentation rates (from 2.8 to 272.3 g m⁻² d⁻¹) did show significant differences when the four salt marshes studied in the Tagus estuary were compared to each others. Salt marshes closer to the sediment sources had higher sedimentation rates. Our results suggest that the salt marsh type and surface cover may provide small-scale variations in sedimentation and also that sediment deposition values do change according to the position of the different plant species within the salt marsh. Sedimentation is an essential factor in salt marsh vertical accretion studies and our investigation may provide support to help forecast the adaptative response of the Tagus estuary wetlands to future sea level rise.     

To identify impacts in variable systems using anomalous changes: a salt marsh example

A method is proposed to identify impacts of habitat modification in cases where it is difficult to site experimental and control samples. This problem occurs especially in heterogeneous systems, but may pose difficulties in any field experimental situation. The method is relevant to the situation where treated (modified) and untreated sites are spread over a range of habitat types. Types of change are identified and compared to treatments. If a specific change type is associated with a particular treatment then it is likely that the change is causally related to the treatment. There are five stages in the analysis. First, the classes or states of the sample sites, over a period of time, are identified (by numerical classification). Second, for each sample site, the sequence of states is listed. Third, transition matrices are made for each sample site to show the changes which have occurred. Fourth, the transition matrices are classified, to identify types of change. Finally, we use the Chisquared test to indicate whether the treated and untreated sites are associated with particular types of change. As an example, we refer to habitat modification to manage salt-marsh mosquitoes and we evaluate impacts on the environment mainly through changes to the vegetation. We consider that the method has potential to identify changes in heterogeneous systems even though little change was identified in the particular salt marsh studied.     

Screening for bacterial-fungal associations in a south-eastern US salt marsh using pre-established fungal monocultures

Both bacteria and fungi play critical roles in decomposition processes in many natural environments, yet only rarely have they been studied as an integrated community. We examined whether physical associations exist between individual bacterial and fungal species that co-occur on decaying smooth cordgrass, Spartina alterniflora, in a south-eastern US salt marsh. Fungal-pervaded decaying Spartina was used as "bait" for potential bacterial associates. The bundles (infiltrated with one of three dominant fungal members of the decomposer assemblage, or an autoclaved control) were placed in a salt marsh and collected biweekly for 6 weeks during the first experiment (late summer 2002), and weekly for 3 weeks during the second experiment (early summer 2003). Terminal-restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA genes was used to track colonization by bacterial taxa in association with the established fungal species. T-RFLP analysis of 18S-to-28S internal transcribed spacer (ITS) regions was used to monitor changes in fungal communities once bundles had been placed in the field. Results from both years were nearly identical, and showed that invasion by fungi other than the bait species was slow, resulting in a virtual fungal monoculture for several weeks into the experiments. Surprisingly, bacterial communities were unaffected by the identity of the fungal bait. Regardless of the fungal species, and even in the absence of prior fungal colonization, bacterial 16S rRNA profiles were remarkably similar. These results suggest that few species-specific associations, either positive or negative, exist between bacterial and fungal members of the Spartina decomposer community during initial colonization.     

Salt marsh plant-microorganism interaction in the presence of mixed contamination

In this study we investigated under laboratory conditions, the combined effect of the presence of two contaminants (Cu and PAHs) and plants (Halimione portulacoides) on a salt marsh microbial community, in terms of genetic structure, abundance and capacity to remove those contaminants. Plants changed the microbial community structure (evaluated by ARISA), but only in the treatments without PAHs. Also, in the presence of plants, Cu displayed lower toxicity (ToxScreen test) than in its absence. Nevertheless, the presence of plants interfered with the degradation (decreasing it up to 45%) of higher molecular weight PAHs by sediment microorganisms. This was in agreement with the lower microbial numbers (quantified by DAPI) observed in the presence of the plants, pointing for a competition between plants and microorganisms for nutrients. These results are new for salt marshes, and highlight the need for fertilization in order to obtain optimal effects of rhizoremediation for this type of compounds.     

Power analysis to determine sample size for monitoring vegetation change in salt marsh habitats

Numerous initiatives are underway throughout New England and elsewhere to quantify salt marsh vegetation change, mostly in response to habitat restoration, sea level rise, and nutrient enrichment. To detect temporal changes in vegetation at a marsh or to compare vegetation among different marshes with a degree of statistical certainty an adequate sample size is required. Based on sampling 1 m² vegetation plots from 11 New England salt marsh data sets, we conducted a power analysis to determine the minimum number of samples that were necessary to detect change between vegetation communities. Statistical power was determined for sample sizes of 5, 10, 15, and 20 vegetation plots at an alpha level of 0.05. Detection of subtle differences between vegetation data sets (e.g., comparing vegetation in the same marsh over two consecutive years) can be accomplished using a sample size of 20 plots with a reasonable probability of detecting a difference when one truly exists. With a lower sample size, and thus lower power, there is an increased probability of not detecting a difference when one exists (e.g., Type II error). However, if investigators expect to detect major changes in vegetation (e.g., such as those between an un-impacted and a highly impacted marsh) then a sample size of 5, 10, or 15 plots may be appropriate while still maintaining adequate power. Due to the relative ease of collecting vegetation data, we suggest a minimum sample size of 20 randomly located 1 m² plots when developing monitoring designs to detect vegetation community change of salt marshes. The sample size of 20 plots per New England salt marsh is appropriate regardless of marsh size or permanency (permanent or non-permanent) of the plots.     

Disturbance of salt marsh vegetation by wrack mats in Great Sippewissett Marsh

To quantify disturbance to salt marsh vegetation, and to test the notion that disturbance and species richness are related, we studied disturbance of vegetation by 195 wrack mats that had become stranded over Great Sippewissett Marsh. The mats varied in area, thickness, residence time, and elevation of stranding. Mats that were large, that had residence times of 3–4 months, and were stranded at higher elevations produced the most damage, ranging from total eradication to a decrease in shoot density of underlying vegatation. Thickness had no effect. About 70% of mats caused no damage, and of those that did, damage was usually 50% or less of the area beneath the mat. Plant species were differentially distributed along the tidal excursion within the marsh, and there were more species (8–9) high in the intertidal range than lower (4–6 species). Vertical distribution of species, species richness, and rate of annual disturbance were not well correlated, although highest and lowest disturbance rates corresponded to highest and lowest species richness. Most of the increases in species richness, however, occurred over a very small range of disturbance, so narrow that it seemed unlikely that disturbance affected richness. It seems more likely that factors other than disturbance rate control vertical position of species in salt marshes. It also appears that the effect of disturbance by wrack is less important than has been claimed.     

Effects of environmental context on the susceptibility of Atriplex patula to attack by herbivorous beetles

The susceptibility of plants to attack by insect herbivores often depends on local environmental conditions. This study documents variation in herbivore damage by the chrysomelid beetle Erynephala maritima to the annual forb Atriplex patula in two microhabitats within New England salt marshes: bare patches and dense matrix vegetation. Environmental conditions within bare patches differ from those within matrix vegetation in a number of ways. Bare patches are characterized by the absence of perennial grasses and rushes (matrix vegetation) and greater levels of physical stress, and are rapidly colonized by the fugitive annual, Salicornia europaea, a second host plant of these beetles. Surveys of herbivore damage across three marshes revealed that A. patula in bare patches had a greater proportion of leaves damaged by beetles than those within matrix vegetation. Presence or absence of matrix vegetation and presence or absence of S. europaea were experimentally manipulated to determine the proximate cause of this pattern. The presence of S. europaea significantly increased the susceptibility of A. patula to herbivory in experimental plots. Both the extent of herbivore damage to plants and the proportion of plants damaged through time were greater in treatments with S. europaea than in controls, regardless of the presence or absence of matrix vegetation. Plants in S. europaea addition treatments were also less likely to survive to reproduction. Decreased survival appears to result from increased herbivory, suggesting that the negative effect of S. europaea on A. patula is mediated indirectly through shared insect herbivores. These results support the hypothesis that indirect interactions between alternative host plants, mediated by insect herbivores, can be important in natural communities. Received: 9 January 1999 / Accepted: 29 April 1999     

Activity and growth efficiency of heterotrophic bacteria in a salt marsh (Ria de Aveiro, Portugal)

Bacterial utilization of monomers is recognized as an important step in the biogeochemical cycling of organic matter. In this study we have compared the heterotrophic activity of bacterial communities from different micro-habitats within a salt marsh environment (Ria de Aveiro, Portugal) in order to establish spatial patterns of bacterial abundance, monomer turnover rates (Tr) and bacterial growth efficiency (BGE). Differences in bacterial abundance and activity could be found between distinct plant rhizospheres. BGE tended to be lower at Halimione portulacoides banks, when compared to Sarcocornia perennis subsp. perennis banks which, on the contrary, showed the highest bacterial densities. Experiments of amendment of natural samples with organic and inorganic supplements indicated that salt marsh bacteria are not strongly regulated by salinity but the increased availability of labile organic matter causes a significant metabolic shift towards mineralization.