Created Salt Marsh Research Articles

Can biodiversity of preexisting and created salt marshes match across scales? An assessment from microbes to predators

AbstractCoastal wetlands are rapidly disappearing worldwide due to a variety of processes, including climate change and flood control. The rate of loss in the Mississippi River Delta is among the highest in the world and billions of dollars have been allocated to build and restore coastal wetlands. A key question guiding assessment is whether created coastal salt marshes have similar biodiversity to preexisting, reference marshes. However, the numerous biodiversity metrics used to make these determinations are typically scale dependent and often conflicting. Here, we applied ecological theory to compare the diversity of different assemblages (surface and below‐surface soil microbes, plants, macroinfauna, spiders, and on‐marsh and off‐marsh nekton) between two created marshes (4–6 years old) and four reference marshes. We also quantified the scale‐dependent effects of species abundance distribution, aggregation, and density on richness differences and explored differences in species composition. Total, between‐sample, and within‐sample diversity (γ, β, and α, respectively) were not consistently lower at created marshes. Richness decomposition varied greatly among assemblages and marshes (e.g., soil microbes showed high equitability and α diversity, but plant diversity was restricted to a few dominant species with high aggregation). However, species abundance distribution, aggregation, and density patterns were not directly associated with differences between created and reference marshes. One exception was considerably lower density for macroinfauna at one of the created marshes, which was drier because of being at a higher elevation and having coarser substrate compared with the other marshes. The community compositions of created marshes were more dissimilar than reference marshes for microbe and macroinfauna assemblages. However, differences were small, particularly for microbes. Together, our results suggest generally similar taxonomic diversity and composition between created and reference marshes. This provides support for the creation of marsh habitat as tools for the maintenance and restoration of coastal biodiversity. However, caution is needed when creating marshes because specific building and restoration plans may lead to different colonization patterns.

Nitrate reduction capacity is limited by belowground plant recovery in a 32‐year‐old created salt marsh

Human activities have decreased global salt marsh surface area with a subsequent loss in the ecosystem functions they provide. The creation of marshes in terrestrial systems has been used to mitigate this loss in marsh cover. Although these constructed marshes may rapidly recover ecosystem structure, biogeochemical processes may be slow to recover. We compared denitrification and dissimilatory nitrate reduction to ammonium (DNRA) rates between a 32‐year‐old excavation‐created salt marsh (CON‐2) and a nearby natural reference salt marsh (NAT) to assess the recovery of ecosystem function. These process rates were measured at 5 cm increments to a depth of 25 cm to assess how plant rooting depth and organic matter accumulation impact N‐cycling. We found that, for both marshes, denitrification and DNRA declined with depth with the highest rates occurring in the top 10 cm. In both systems, N‐retention by DNRA accounted for upwards of 75% of nitrate reduction, but denitrification and DNRA rates were nearly 2× and 3× higher in NAT than CON‐2, respectively. Organic matter was 6× lower in CON‐2, likely due to limited plant belowground biomass production. However, there was no response to glucose additions, suggesting that the microbial functional community, not substrate limitation, limited nitrate reduction recovery. Response ratios showed that denitrification in CON‐2 recovered in surficial sediments where belowground biomass was highest, even though biomass recovery was minimal. This indicates that although recovery of ecosystem function was constrained, it occurred on a faster trajectory than that of ecosystem structure.

Carbon Storage Increases with Site Age as Created Salt Marshes Transition to Mangrove Forests in Tampa Bay, Florida (USA)

Coastal wetlands can sequester large amounts of organic carbon (OC), providing an additional motivation for the preservation and restoration of these ecosystems. In Tampa Bay (Florida, USA), created coastal wetlands are initially planted with Spartina spp., but nearly all sites naturally transition into mangrove forests. It was hypothesized that carbon storage in the created wetlands would increase with site age due to the accumulation of soil organic carbon and replacement of salt marsh vegetation with mangrove forests. Mature, mangrove-dominated sites had higher total organic carbon stocks (138.7 ± 13.8 Mg C ha−1) than middle-aged transitional sites (85.6 ± 25.5 Mg C ha−1) or young salt marshes (34.5 ± 7.7 Mg C ha−1). Mature sites consisted of tall trees (> 130 cm tall) and scarce salt marsh vegetation. Transitional sites contained mangrove scrubs (30–130 cm tall) and seedlings (< 30 cm tall) while still supporting salt marsh vegetation; younger sites were dominated by salt marsh vegetation and had no trees. Belowground OC constituted the greatest carbon pool (59.4% of the total OC stock), but belowground OC stocks were not significantly different among the site age classes, suggesting that aboveground OC stocks drove the difference in total OC stocks. The total carbon accumulation rate, including both aboveground and belowground OC, was 4.7 Mg C ha−1 year−1 across the 26-year chronosequence. This study has demonstrated that carbon storage in created coastal wetlands is correlated to wetland age, indicating that these ecosystems have the potential to become significant sources of OC storage.

How do zooplankton respond to coastal wetland restoration? the case of newly created salt marsh lagoons in La Pletera (NE Catalonia)

espanolSe analizaron los cambios en la composicion del zooplancton en algunas lagunas de nueva creacion y ya existentes de La Pletera, una marisma confinada en la costa del Mediterraneo, localizada en los humedales del Baix Ter (NE Peninsula Iberica) y sujeta a un proyecto de restauracion Life (Life Pletera). Se muestreo el zooplancton de tres nuevas lagunas inmedia-tamente despues de su creacion (1 o 2 meses despues) y se comparo con el de tres ya existentes; dos lagunas naturales y una creada catorce anos antes en un proyecto de restauracion previo. Los muestreos incluyeron siete meses antes de la primera conexion hidrica superficial, cuando el proceso de inundacion durante los temporales de mar conecto por primera vez las nuevas lagunas con las ya existentes, y los siete meses posteriores. Los resultados mostraron que la concentracion de nutrien-tes y materia organica fue mas baja en las nuevas lagunas que en las viejas, sugiriendo un proceso de acumulacion en estos ecosistemas, probablemente relacionada con los largos periodos de confinamiento que normalmente siguen a los eventos de inundacion en este tipo de lagunas. Por otro lado, no se encontro diferencias significativas en la comunidad zooplanctonica entre lagunas viejas y nuevas. El patron temporal del zooplancton estuvo fuertemente afectado por el patron temporal hidrolo-gico de inundacion-confinamiento, el cual no difiere entre lagunas viejas y nuevas. Los copepodos calanoides, principalmente Eurytemora velox, dominaron durante el invierno, mientras que el rotifero Brachionus plicatilis domino en verano coincidien-do con el mayor grado de confinamiento. Las variables ambientales y la conexion hidrica explican un 31 % de la variabilidad del zooplancton. La elevada facilidad de dispersion entre lagunas ubicadas muy cerca y facilmente conectadas durante los eventos de inundacion, la eclosion del banco de huevos existente en las lagunas restauradas y la respuesta rapida de las especies zooplanctonicas a los cambios en las condiciones ambientales (aunque con algun grado de retraso), podria explicar la falta de diferencias significativas entre la composicion del zooplancton en las lagunas viejas y nuevas. EnglishWe analysed the changes in the zooplankton composition in several new and old lagoons in La Pletera, a confined Mediterrane-an coastal salt marsh located in the Baix Ter wetlands (NE Iberian Peninsula) that was subjected to a restoration project (Life Pletera). We sampled the zooplankton from three new lagoons immediately after their creation (1 and 2 months after), and we compared the zooplankton with those of three existing lagoons, two natural lagoons and one lagoon created fourteen years ago in a previous restoration project. The sampling included seven months before the first hydrological connection to the surface waters, when flooding during a sea storm connected the new lagoons with the existing lagoons for the first time, and seven months after. The results showed that the nutrient and organic matter concentrations were lower in the new lagoons than in the old lagoons, suggesting accumulation processes in these ecosystems, which were probably related to the long periods of confinement that typically follow flooding events in this type of lagoons. However, we did not find significant differences in the zooplankton community between the old and new lagoons. The seasonal variation in the zooplankton communities was strongly affected by the seasonal hydrological flooding – confinement pattern, which did not differ between the old and new lagoons. Calanoid copepods, mainly Eurytemora velox, dominated during winter, while the rotifer Brachionus plicatilisdominated in summer, coinciding with the highest degree of confinement. The environmental variables and the hydric connec-tion explained 31 % of the zooplankton variability. The high connectivity of the system once it flooded aided dispersal, the hatching of the egg banks existing in the restored lagoons and the fast response of the zooplankton species to changes in the environmental conditions (although with some degree of delay) might explain the lack of significant differences between the zooplankton composition in the old and new lagoons

The interactive effects of created salt marsh substrate type, hydrology, and nutrient regime on Spartina alterniflora and Avicennia germinans productivity and soil development

Recent salt marsh and barrier island restoration efforts in the northern Gulf of Mexico have focused on optimizing self-sustaining attributes of restored marshes to provide maximum habitat value and storm protection to vulnerable coastal communities. Salt marshes in this region are dominated by Spartina alterniflora and Avicennia germinans, two species that are valued for their ability to stabilize soils in intertidal salt marshes. We conducted a controlled greenhouse study to investigate the influences of substrate type, nutrient level, and marsh elevation on the growth and biomass allocation of S. alterniflora and A. germinans, and the consequent effects on soil development and stability. S. alterniflora exhibited optimal growth and survival at the lowest elevation (− 15 cm below the water surface) and was sensitive to high soil salinities at higher elevations (+ 15 cm above the water surface). A. germinans performed best at intermediate elevations but was negatively affected by prolonged inundation at lower elevations. We found that although there was not a strong effect of substrate type on plant growth, the development of stressful conditions due to the use of suboptimal materials would likely be exacerbated by placing the soil at extreme elevations. Soil shear strength was significantly higher in experimental units containing either S. alterniflora or A. germinans compared to unvegetated soils, suggesting that plants effectively contribute to soil strength in newly placed soils of restored marshes. As marsh vegetation plays a critical role in stabilizing shorelines, salt marsh restoration efforts in the northern Gulf of Mexico and other storm impacted coasts should be designed at optimal elevations to facilitate the establishment and growth of key marsh species.

Seasonal variation in the quality of dissolved and particulate organic matter exchanged between a salt marsh and its adjacent estuary

AbstractFluorescence was used to examine the quality of dissolved and particulate organic matter (DOM and POM) exchanging between a tidal creek in a created salt marsh and its adjacent estuary in eastern North Carolina, USA. Samples from the creek were collected hourly over four tidal cycles in May, July, August, and October 2011. Absorbance and fluorescence of chromophoric DOM (CDOM) and of base‐extracted POM (BEPOM) served as the tracers for organic matter quality while dissolved organic carbon (DOC) and base‐extracted particulate organic carbon (BEPOC) were used to compute fluxes. Fluorescence was modeled using parallel factor analysis (PARAFAC) and principle components analysis (PCA) of the PARAFAC results. Of nine PARAFAC components (C) modeled, C3 represented recalcitrant DOM and C4 represented fresher soil‐derived source DOM. Component 1 represented detrital POM, and C6 represented planktonic POM. Based on mass balance, recalcitrant DOC export was 86 g C m−2 yr−1 and labile DOC export was 49 g C m−2 yr−1; no planktonic DOC was exported. The marsh also exported 41 g C m−2 yr−1 of detrital terrestrial POC, which likely originated from lands adjacent to the North River estuary. Planktonic POC export from the marsh was 6 g C m−2 yr−1. Assuming the exported organic matter was oxidized to CO2 and scaled up to global salt marsh area, respiration of salt marsh DOC and POC transported to estuaries could amount to a global CO2 flux of 11 Tg C yr−1, roughly 4% of the recently estimated CO2 release for marshes and estuaries globally.

Limited Vegetation Development on a Created Salt Marsh Associated with Over-Consolidated Sediments and Lack of Topographic Heterogeneity

Restored salt marshes frequently lack the full range of plant communities present on reference marshes, with upper marsh species underrepresented. This often results from sites being too low in the tidal frame and/or poorly drained with anoxic sediments. A managed coastal realignment scheme at Abbotts Hall, Essex, UK, has oxic sediments at elevations at which upper marsh communities would be expected. But 7 years after flooding, it continued to be dominated by pioneer communities, with substantial proportions of bare ground, so other factors must hinder vegetation development at these elevations. The poorly vegetated areas had high sediment shear strength, low water and organic carbon content and very flat topography. These characteristics occur frequently on the upper parts of created marshes. Experimental work is required to establish causal links with the ecological differences, but other studies have also reported that reduced plant β-diversity and lower usage by fish are associated with topographic uniformity. Uniformity also leads to very different visual appearance from natural marshes. On the upper intertidal, sediment deposition rate are slow, water velocities are low and erosive forces are weak. So, topographic heterogeneity cannot develop naturally, even if creeks have been excavated. Without active management, these conditions will persist indefinitely.

Benthic food-web succession in a developing salt marsh

Ecological succession has long been a focal point for research, and knowledge of underlying mechanisms is required if scientists and managers are to successfully promote recov- ery of ecosystem function following disturbance. We addressed the influence of bottom-up pro- cesses on successional assemblage shifts in salt marshes, ecosystems with strong physical gradi- ents, and how these shifts were reflected in the trophic characteristics of benthic fauna. We tracked the temporal development of infaunal community structure and food-web interactions in a young, created salt marsh and an adjacent natural marsh in Mission Bay, California, USA (1996−2003). Macro faunal community succession in created Spartina foliosa habitats occurred rapidly, with infaunal densities reaching 70% of those in the natural marsh after 1 yr. Community composition shifted from initial dominance of insect larvae (surface-feeding microalgivores) to increased dominance of oligo chaetes (subsurface-feeding detritivores) within the first 7 yr. Iso- topic labeling of microalgae, N2-fixing cyanobacteria, S. foliosa and bacteria revealed direct links (or absence thereof) between these basal food sources and specific consumer groups. In combina- tion with the compositional changes in the macroinvertebrate fauna, the trophic patterns indi- cated an increase in food-web complexity over time, reflecting resource-driven marsh succession. Natural abundance stable isotope ratios of salt marsh consumers (infaunal and epifaunal macroin- vertebrates, and fish) initially reflected distinctions in trophic structure between the created and natural marsh, but these diminished during successional development. Our findings suggest that changing resource availability is one of the important drivers of succession in benthic communi- ties of restored wetlands in Southern California.

Effects of desiccation and salinity on the outbreak of a green tide of Ulva pertusa in a created salt marsh along the coast of Osaka Bay, Japan

Field surveys and laboratory experiments were conducted to examine the effect of desiccation and salinity on the outbreak of a green tide of Ulva pertusa at Osaka Nanko bird sanctuary. Reduction of biomass of Ulva spp. was observed at stations where the exposure rate to air was from 30 to 40%. In addition, the exposure rate of 30–40% to air showed no negative impacts on the biomass of benthic microalgae, infauna and non-motile epibenthos. Laboratory experiments revealed that photosynthetic activity of U. pertusa decreased when exposed to air for 4–7 h at 25–35 °C. Salinity decreases from 30 to 25 or 20 accompanied with exposure to air drastically reduced the rate of photosynthesis of this species. These results suggest the possibility of controlling a green tide of U. pertusa without serious physico-ecological damage to benthic microalgae, infauna and non-motile epibenthos by a combination of exposure to air with low salinity.

Inundation Frequency Determines the Post-Pioneer Successional Pathway in a Newly Created Salt Marsh

The effect of inundation frequency on plant community composition, species turnover, total and growth form cover, species richness, and abundance of individual species was investigated in a newly created salt marsh (Belgium) with a gradient of inundation frequencies from 0.01% to 100%. Cover of all plant species was estimated in 119 permanent 2 × 2 m plots along seven randomly chosen transects perpendicular to the main inundation gradient in 2003, 2005, and 2007. Plant composition change clearly varied along the inundation frequency gradient. The cover of annual species increased at a higher rate at higher inundation frequencies, while cover of perennials increased at higher rate at lower inundation frequencies. Species richness and the abundance of most species increased over time, indicating general absence of competitive exclusion among species. Conversely, the abundance and frequency of Atriplex spp., Chenopodium spp., and Salsola kali strongly decreased over time, indicating their early successional character. Frequent inundations hampered plant species turnover because of the low number of species which can tolerate that environmental condition. The appearance of communities dominated by Elymus athericus or Salicornia procumbens strongly increased over time, leading to a stronger separation of plant communities and an appearance of typical salt-marsh zonation.

Marine fungal diversity: a comparison of natural and created salt marshes of the north-central Gulf of Mexico

Marine fungal communities of created salt marshes of differing ages were compared with those of two reference natural salt marshes. Marine fungi occurring on the lower 30 cm of salt marsh plants Spartina alterniflora and Juncus roemerianus were inventoried with morphological and molecular methods (ITS T-RFLP analysis) to determine fungal species richness, relative frequency of occurrence and ascomata density. The resulting profiles revealed similar fungal communities in natural salt marshes and created salt marshes 3 y old and older with a 1.5 y old created marsh showing less fungal colonization. A 26 y old created salt marsh consistently exhibited the highest fungal species richness. Ascomata density of the dominant fungal species on each host was significantly higher in natural marshes than in created marshes at all three sampling dates. This study indicates marine fungal saprotroph communities are present in these manmade coastal salt marshes as early as 1 y after marsh creation. The lower regions of both plant hosts were dominated by a small number of marine ascomycete species consistent with those species previously reported from salt marshes of the East Coast of USA.

Created versus natural wetlands: Avian communities in Virginia salt marshes

Permits to destroy wetlands often require the creation of the same type of wetland elsewhere. An assumption underlying this practice is that such created wetlands will replace the ecological functions lost when the developed wetland was destroyed. Part of this ecological function is providing habitat for wildlife, including, in coastal areas, a suite of bird species tied to salt marshes for some portion of their life cycle. We tested the hypothesis that created wetlands provide habitat for the avian communities lost when wetlands are destroyed by comparing the breeding and wintering birds on 11 small created salt marshes with those on 11 natural reference salt marshes that were carefully matched for size and surrounding land cover. We found that, during the breeding season, created salt marshes had lower avian abundance and richness than reference salt marshes. In particular, wetland-dependent species were poorly represented on created wetlands. On the other hand, bird use outside of the breeding season and use by an important salt marsh obligate species, the clapper rail (Rallus longirostris), did not differ. Created wetlands that we surveyed failed to completely replicate the bird and plant communities that we observed on nearby natural reference salt marshes, raising the question of whether current mitigation policies that encourage wetland creation should continue without further investigation into the success of such wetlands at recreating wildlife habitat.

Ecosystem gas exchange across a created salt marsh chronosequence

Salt marshes created on dredge spoil were compared to natural marshes to evaluate the capacity of created marshes to perform carbon cycle functions. Several carbon cycle attributes were measured in eight created Spartina alterniflora Loisel salt marshes that ranged from one to 28 years, each paired with a nearby mature natural reference marsh. The attributes measured included gross primary production, respiration, net ecosystem exchange, potential microbial respiration (CH4 and CO2), and aboveground biomass. In situ exchange rates of CO2 and plant biomass in created marshes met or exceeded those of reference marshes in three to four years. There was some evidence that ecosystem gas exchange in created marshes developed slightly faster than aboveground biomass production. Soil carbon mineralization per gram carbon was generally higher in the created marshes than reference marshes, suggesting higher carbon quality and/or nutrient availability in the created marshes. However, carbon mineralization rates per gram soil were relatively low in the created marshes due to lower soil organic matter content. With proper construction, we suggest most major carbon fluxes can be established in created salt marshes in less than five years.

Prescribed fire and cutting as tools for reducing woody plant succession in a created salt marsh

This paper reports on efforts to reduce woody successional growth by the native shrub Iva frutescens L. in a created salt marsh by using prescribed fire and cutting. Experimental treatments included a winter burn, cutting plants at ground level, and a combination burn-and-cut treatment, with replicate plots of each. Iva frutescens proved to be extremely hardy, with zero mortality following the cutting, burning, or combination treatment; similar levels of regrowth were observed for all treatments. Individual shrub response, however, was found to be related to initial plant size, ground water level and salinity, and two fire characteristics (total heating >60°C and total heat index >60°C). Fire severity, sediment nutrient concentrations, and other abiotic factors had no observable effects.

Co-occurrence of habitat-modifying invertebrates: effects on structural and functional properties of a created salt marsh

The roles of co-occurring herbivores that modify habitat structure and ecosystem processes have seldom been examined in manipulative experiments or explored in early successional communities. In a created marsh in southern California (USA), we tested the individual and combined effects of two epibenthic invertebrates on nutrient and biomass pools, community structure, and physical habitat features. We manipulated snail (Cerithidea californica) and crab (Pachygrapsus crassipes) presence in field enclosures planted with pickleweed (Salicornia virginica) at elevations matching the plant's lower extent in an adjacent natural marsh. In the 4-month experiment, C. californica altered habitat structure by reducing sediment surface heterogeneity and shear strength (a measure of sediment stability) markedly throughout the enclosures. Both invertebrates had strong negative effects on a group of correlated sediment physicochemical characteristics, including nitrogen and organic matter concentrations and soil moisture. In addition, both invertebrates greatly reduced benthic chlorophyll a, a proxy for biomass of microphytobenthos. Compared to controls, macroalgal cover was up to sixfold lower with crabs present, while snails increased cover at low elevations of enclosures. Unexpectedly, macroalgal cover was eliminated with both species present, perhaps through P. crassipes consumption of larger thalli and C. californica reduction in cover of recruits. Neither species influenced the S. virginica canopy (quantified with an index of branch length and number); however, at the lower elevation of enclosures, the two species together negatively impacted the plant canopy. The two invertebrates' modifications to our experimental marshes led to distinct suites of biotic and physicochemical features depending on their presence or co-occurrence, with the latter producing several unexpected results. We propose that the roles and interactions of habitat-modifying fauna deserve further attention, particularly in the context of efforts to conserve and restore the processes found in natural systems.

Avian Use of Natural and Created Salt Marsh in Texas, USA

Abstract Bird use of three created salt marsh islands, constructed from dredged material near the Aransas National Wildlife Refuge, Texas, USA, was compared with that of natural marsh to provide feedback prior to additional marsh construction. Habitat associations of four bird groups (shorebirds, perching birds, wading birds, and gulls and terns) were similar in all sites, but relative contributions of each group to total avian abundance differed. Differences in site-use by birds were largely explained by differences in ratios of available habitat types, which were products of their geomorphic designs. Created marsh designs differed substantially from the natural marsh, where the unvegetated exposed-substrate and shallow-water habitats preferred by waterbirds were prominent features. The oldest created site (four years old) differed most from the natural marsh. Intertidal areas in the site were almost completely overgrown by vegetation, resulting in dominance of the avian assemblage by perching birds (esp...

Integrated management of artificially created salt marshes in the Wadden Sea of Schleswig-Holstein, Germany

In the about 10,000 km2 large Wadden Sea 353 km2 of salt marshes exist. The great majority of these are artificial, i.e. developed through accretion enhancement techniques. Salt marshes in the Wadden Sea have a high coastal defence value as well as ecological significance. High salt marshes in front of sea walls reduce the wave-impact on the dikes and, therewith, the danger of breaching. At the same time, the salt marshes have a high ecological value, e.g., as resting places for migratory birds. In consequence, for coastal defence, salt marshes should be created and/or maintained. For nature protection, natural dynamic processes (incl. erosion) should prevail. To anticipate this possible conflict among coastal defence and environmental administration, in Schleswig-Holstein an interdepartmental working group established a salt marsh management concept. For information of the interested parties and for monitoring of the concept, an integrated salt marsh board was established, consisting of representatives from public administration and local interest groups (NGO's, dike and water boards, municipalities). Five years after implementation of the concept, this board established a first progress report. Although the concept was, in the beginning, met with great scepticism, five years of practical experience demonstrate that it functions well. The salt marshes are stable or growing, i.e. the management measures are successful. As a planning instrument, the concept is unambiguous, i.e. provides clear guidelines on how to manage the salt marshes. Finally, as an instrument of participation, the salt marsh board is valued positively by all members.

NATURAL AND MANIPULATED SOURCES OF HETEROGENEITY CONTROLLING EARLY FAUNAL DEVELOPMENT OF A SALT MARSH

Ecosystem recovery following wetland restoration offers exceptional opportunities to study system structure, function, and successional processes in salt marshes. This study used observations of natural variation and large-scale manipulative experiments to test the influence of vascular vegetation and soil organic matter on the rate and trajectory of macrofaunal recovery in a southern California created salt marsh, the Crown Point Mitigation Site. During the first three years following marsh establishment, macrofaunal density and species richness recovered rapidly within the Spartina foliosa (cordgrass) zone; densities in the created marsh were 50% of those in the natural marsh after 16 mo and 97% after 28 mo. However, the early successional assemblage had a lower proportion of tubificid and enchytraeid oligochaetes, and a higher proportion of chironomids and other insect larvae than did the mature natural marsh. Most of the colonizers arrived by rafting on sea grass and algae rather than by larval dispersal. Initial planting of S. foliosa had no influence on macrofaunal recovery, perhaps because of variable transplant survival. However, subsequently, both positive and negative correlations were observed between densities of some macrofaunal taxa and shoot densities of S. foliosa or Salicornia spp. (pickleweed). Salinity and measures of soil organics (belowground biomass, combustible organic matter, and chlorophyll a) also were correlated with macrofaunal densities and taxon richness. Of four added soil amendments (kelp, alfalfa, peat, and Milorganite), Milorganite (a sewage product) and kelp both promoted macrofaunal colonization during year 1, but effects were short lived. The most significant sources of heterogeneity in the recovering marsh were associated with site history and climate variation. Faunal recovery was most rapid in highly localized, organic-rich marsh sediments that were remnants of the historical wetland. Elevated sea level during the 1998 El Nino corresponded with similarity of macrofaunal communities in the created and natural marshes. The large spatial scale and multi-year duration of this study revealed that natural sources of spatial and temporal heterogeneity may exert stronger influence on faunal succession in California wetlands than manipulation of vegetation or soil properties.

NEKTON IN GULF COAST WETLANDS: FINE-SCALE DISTRIBUTIONS, LANDSCAPE PATTERNS, AND RESTORATION IMPLICATIONS

Many decapod crustaceans and fishes are common inhabitants of flooded salt marshes in the northwestern Gulf of Mexico, but spatial distributions are uneven, and population sizes are difficult to estimate. We measured fine-scale (1–10 m) distributions of nekton on the vegetated marsh surface using enclosure samplers in Galveston Bay, Texas, and used these patterns to estimate population size. Natant decapod crustaceans were abundant in the marsh; densities of juvenile brown shrimp Farfantepenaeus aztecus, white shrimp Litopenaeus setiferus, and blue crabs Callinectes sapidus were highest 1 m from the water's edge and declined rapidly to 10 m from the edge. We developed regression models to describe these fine-scale density patterns and validated the models with independent data on density distributions from two other marsh systems. We used a Geographic Information System to transfer the density models to a natural marsh landscape; the highly fragmented Elmgrove Point marsh was composed of shallow nonvegetated bottom (37.4% of the area) and Spartina alterniflora vegetation (62.6%) with ∼15% of the vegetated area within 1 m of the marsh–water interface. We estimated that this 437-ha salt marsh complex supported populations of 16.2 million brown shrimp, 15.5 million white shrimp, and 11.3 million blue crabs. We divided the marsh complex into 39 sectors and examined relationships between nekton populations and landscape-scale patterns of marsh fragmentation. The amount of edge and the population estimates in a sector increased consistently with the amount of water up to ∼20–25%. Nekton population declines were not apparent until the late stages of marsh disintegration (>70% open water). We also used our fine-scale density models to simulate the effects of adding creeks to a 1-ha created salt marsh of solid vegetation. For shrimp, 1–2 creeks/ha (or a comparable amount of edge) were required to reach populations equivalent to shallow nonvegetated bottom, and up to 6 creeks/ha were required to reach populations similar to those estimated for the natural marsh complex. Simulated populations of blue crabs reached levels in the natural marsh complex with the addition of fewer creeks (2–3 creeks/ha).

The regulation of ecosystem functions by ecotypic variation in the dominant plant: a Spartina alterniflora salt‐marsh case study

Summary Genetic differences among populations of a keystone species may affect ecosystem functional properties. We tested this by planting Spartina alterniflora from different geographical regions in a newly created salt marsh in Delaware, USA. Spartina alterniflora plants from morphologically distinct short‐form (back marsh) populations were originally collected from Massachusetts (41°34′ N), Delaware (38°47′ N), and Georgia (31°25′ N) in the USA and vegetatively propagated for 6 years in a salt water‐irrigated common garden in Delaware before transfer to a newly created salt marsh. The magnitude of the expression of marsh functions in the created marsh, measured over 5 years, remained distinct in patches of each ecotype. End of season aerial biomass, below‐ground biomass, root and rhizome distribution, canopy height, stem density, and carbohydrate reserves were closer to values reported for the plants’ native sites than to those typical of Delaware. Thus, many of the plant features characteristic of particular latitudes appear to be under genetic control. Such ecotypic differentiation influences ecosystem function through keystone resource and keystone modifier activities. Respiration of the microbial community associated with either dead shoots or the soil varied with plant ecotype in the created wetland and the patterns reflected those reported for their native sites. High edaphic respiration under the Massachusetts ecotype was correlated with the high percentage of sugar in the rhizomes. Edaphic chlorophyll was greater under the canopies of the Massachusetts and Delaware ecotypes than under the Georgia canopy and exhibited a relationship similar to that of algal production rates reported for the native sites. Larval fish were most abundant in pit traps in the Massachusetts ecotype.