The Intertidal Communities on Dissipa-tive Beaches at Risk for Sea Level Rise—The Relationship of Beach Mor-phodynamics and Species Range.

Sea-level rise is likely to cause significant physical changes to beaches in the higher latitudes, resulting in steeper beaches with larger particle sizes. These physical changes have implications for beach invertebrate communities, which are determined largely by sediment particle size, and hence for ecosystem function. Previous studies have explored the relationships between invertebrate communities and environmental variables such as particle size, beach slope, and exposure to wave action. Yamanaka et al. (2010) quantified the abundance of meiofauna and macrofauna across a range of beaches in the UK. The authors confirmed the predominant role of beach physical factors in determining infaunal species composition on the less wave-dominated beaches typically found over much of the European coastline. The more dissipative beaches, or the flat beaches with finer particles and gentler slopes, had a higher density of organisms, but a smaller range of species richness. If predictions that accelerated sea-level rise<!–[if supportFields]> XE “sea-level rise (SLR)” <![endif]–><!–[if supportFields]><![endif]–> will move beaches towards a more reflective morphodynamic state are correct, this could lead to potential adverse consequences for ecosystem functioning through the declining abundance of benthic organisms between 0.3 and 1mm in size. —Michelle Schulte
Yamanaka, T., Raffaelli, D., White, P.C.L., 2010. Physical determinants of intertidal communities on dissipative beaches: Implications of sea-level rise<!–[if supportFields]> XE “sea-level rise (SLR)” <![endif]–><!–[if supportFields]><![endif]–>. Estuarine, Coastal and Shelf Science 88, 267–278.

The authors utilized various indices of beach morphodynamic state to quantify the physical characteristics of beaches in three different estuarine locations on the east coast of the UK that experience different tidal ranges, slopes, and range of particle size. The three contrasting field sites in the UK are the Humber estuary, the Ythan estuary, and the Firth of Forth. Five or six sampling sites were selected within each locality, restricted to a short area of the outer estuary or coastal site in order to minimize any potentially confounding effects of salinity<!–[if supportFields]> XE “salinity” <![endif]–><!–[if supportFields]><![endif]–>. At each station, a cylindrical core was pushed into the sediment to the depth of 10 cm on a randomly chosen surface to sample macrofauna, meiofauna, and sediment. Macrofauna were separated from sediment using a 500 µm mesh, preserved in 70% ethanol<!–[if supportFields]> XE “ethanol” <![endif]–><!–[if supportFields]><![endif]–>, identified to species level, and counted using a microscope. Meiofauna were separated from sediment using a 64 µm mesh, preserved in ethanol, and stained with Rose Bengal, identified to the lowest possible taxon, and counted. Particle size was determined by dry sieving through a tower of mesh sieves. The slope at each sampling station was calculated by measuring the height and distance of the sample site. The exposure at each beach site was calculated using the index derived from wind velocity, direction, duration, and the effective fetch.
Yamanaka et al. created new indices to determine the morphodynamic state of the beach and the wave energy. A combination of non-metric Multi Dimensional Scaling (NMDS), and an eigenvector-based approach, DCA, was used, in conjunction with cluster analysis to explore the main trends and patterns in the data in terms of physical and biological variables of the sites. In addition, stepwise multiple linear regression was used to explore the relationships of abundance and number of species with morphodynamic state. One-way analyses of variance (ANOVA<!–[if supportFields]> XE “ANOVA” <![endif]–><!–[if supportFields]><![endif]–>) were used to test the importance of each independent variable, and also to test the difference of physical variables between the three areas.
The authors explored the relationships between beach fauna and morphodynamic variables, to test whether more dissipative beaches support a high abundance of macrofauna and meiofauna as well as higher macrofaunal species richness. The authors ask how these relationships may inform our understanding of the impacts of sea-level rise<!–[if supportFields]>XE “sea-level rise (SLR)”<![endif]–><!–[if supportFields]><![endif]–> on benthic community structure and function. They compared the differences in the physical characteristics of each of the beaches. Median particle size was not significantly different between estuaries, but beach slope and wave exposure differed significantly. The Humber had a much higher range of exposures and a shallower beach slope than the Ythan and the Forth.
The fauna within these three sites differed in their composition and abundance. There was more overlap in species composition between the Humber and the Ythan, despite an order of magnitude difference in abundance. For each scenario, the more dissipative beaches contained higher abundances of all fauna. So that dissipative beaches with finer particles and shallow slopes generally support a higher abundance of macrofauna.
However, for species richness, Yamanaka et al. found that less dissipative beaches generally support higher macrofaunal species richness. Both the Ythan and the Humber had lower species richness compared to the Forth, but differed markedly in the numbers of individuals recorded. The Forth had an intermediate number of macrofauna individuals but the most taxa represented. In addition, the authors found that the length of exposure to the sun and the beach slope affect the abundance of small, benthic organisms. There were no clear relationships between diversity indices and beach physical variables.
Yamanaka et al. confirmed the predominant role of beach physical factors in determining infaunal species composition on the less wave-dominated beaches typically found over much of the European coastline. All of the species recorded can be described as deposit feeders, filter feeders, or predators. Past studies illustrate that large polychaetes are disproportionately important for ecosystem processes such as nutrient cycling. Thus, functional diversity and compositional effects rather than species richness, may play an important role in driving ecosystem processes. A greater diversity of large species including polychaete species was found at more sheltered sites on the Ythan and the Forth. If sea-level rise<!–[if supportFields]>XE “sea-level rise (SLR)”<![endif]–><!–[if supportFields]><![endif]–> pushes beaches towards steeper slopes and coarser particles, as indicated in the study by Yamanaka et al., then the abundance of these larger species is likely to decline, with consequent reduction in ecosystem functioning.
In summary, the authors illustrate the validity of the trend that more dissipative beaches have a higher abundance of macrofauna and meiofauna compared to reflective beaches when analyzing less wave-dominated beaches. In addition, the authors suggest that sea-level rise<!–[if supportFields]> XE “sea-level rise (SLR)” <![endif]–><!–[if supportFields]><![endif]–> could have a significant impact on ecosystem functioning in northern temperate beaches, through the effects of changing particle size and wave exposure on benthic species richness and abundance, especially the larger-bodied polychaetes. 

Responses to Plant Communities to Incremental Hydraulic Restoration

The number of tidal restriction projects is growing but the environmental analysis of their effects is inadequate. It is recognized that tidal restrictions reduce soil salinity and change soil chemistry as well as encourage peat subsidence. This leads to the death of native halophytes and the invasion of freshwater plants and exotic species like Phragmites. Moreover, it is increasingly difficult to quantify the effects of tidal restrictions on salt marsh landscapes because evidence suggests that community responses to tidal restoration projects vary immensely depending on the landscape. Smith et al. (2009) investigated the plant composition and cover of a restricted salt marsh after 7 years of incremental tidal restoration. They established 124 randomly assigned transect lines, which were surveyed every two years for the length of the project. After 7 years of restored tidal interaction, the restricted site still had a species composition different than the unrestricted site, but native grass coverage increased by two-thirds and the soil salinity was restored along the natural gradient. Acadia Tucker
Smith, S., Roman, C., James-Pirri, M., Chapman, K., Portnoy, J., Gwilliam, E. 2009. Responses of Plant Communities to Incremental Hydrologic Restoration of a Tide-Restricted Salt Marsh in Southern New England (Massachusetts, U.S.A.). Restoration Ecology 17, 606–618.

Hatches Harbor Marsh in Northern Cape Cod, Massachusetts has a long history of human disturbance. Over 1400 ha of marshland have been tidally restricted by dikes during the last 350 years. The biggest dike project was established in 1930 to reduce the mosquito population and later to protect an airport from storm surges by restricting half of the tidal flow into the marsh. It severely impacted the community structure and function of the salt marsh by a serious decline in soil salinity and the expansion of invasive and freshwater species. Freshwater species expansion was so pervasive in the high marsh that shrubs and tress colonized the area creating a distinctly different habitat than the unrestricted section of the marsh. In 1998, the original 0.6 m culverts protecting the airport were replaced with four 2.1 m culverts complete with adjustable floodgates to control the amount of seawater flowing into the marsh. Seawater was introduced incrementally over a period of seven years by opening the floodgates gradually over time instead of rapidly restoring the complete flow of seawater, because authorities wanted to ensure the safety of the airport runway from tidal inundation.

There were 98 transect lines monitored in the restricted marsh and 26 transect lines monitored in the unrestricted area of the marsh. Incremental increased amounts of seawater were introduced into the restricted salt marsh each year for seven years as the floodgates were opened progressively wider every year. Smith et al. sampled the water level, soil salinity, and chemistry as well as plant composition and cover in 1997, 2002, 2004, and 2006. They used statistical tests to analyze the differences in species composition for the restricted and unrestricted sites between the different years and elevations of the marsh.
After 7 years of restoration, the restricted salt marsh had regained many of the native plant species that inhabited the unrestricted marsh. Both areas were similiar in terms of plant composition along the same salinity and elevation gradients. The tidal height of the restricted section of marsh was 0.26 m in 1997 before restoration efforts (39% lower than the unrestricted area) and increased by 50% in 2006 after the floodgates were fully opened, reaching nearly 63% of the total tidal activity of the unrestricted site. There was no change in soil salinity for the unrestricted site (average salinity was 32 ppt) but there was a dramatic jump in soil salinity for the restricted site each year after 1997. The biggest increase in salinity occurred between 1997 and 2002 during the first reinstatement of tidal activity into the salt marsh and the most significant increase occurred within 90 m of the tidal creek. Sulfide concentrations, from the decomposition of peat, remained lower than the average concentration for New England salt marshes and therefore results were considered inconclusive. However the low concentration of sulfide in the soil did indicate the high permeability and rate of leaching of the sandy peat. Of the vegetation profile recorded in 1997, roughly two-thirds of the freshwater and high marsh shrubs declined after tidal activity was fully restored in 2006.
S. alterniflora expanded much more rapidly than other native grasses. The total coverage of S. alterniflora tripled from 1997 records while S. patens and other low elevation native grasses did not expand as successfully. S. alterniflora is extremely salt tolerant and was able to survive in the lower elevations flooded by tidal seawater and colonize new areas upland. Smith et al. speculated that the low expansion rate of the other native plants suggests that they could not migrate upland fast enough to escape the increase in water level and soil salinity.

The overall cover of Phragmites remained unchanged after tidal restoration but its distribution throughout the salt marsh adjusted to the new hydrology. This invasive plant disappeared from all areas with a higher salinity of 25 ppt by shifting upland and away from the tidal creek. The authors believe that the incremental restoration of tidal activity into the salt marsh allowed for the gradual expansion of Phragmites into areas of salt-killed vegetation. Phragmites was quickly able to invade areas of the marsh that were vacant after salt intolerant plants died off from the increase in soil salinity. The decline of salt intolerant plants is rapid after restoring tidal flow but the growth of native grasses is slower than that of Phragmites. Therefore if the regulation of Phragmites is a priority for restoration projects, the most efficient way to restore a hydraulically isolated salt marsh is to reinstate the maximum tidal flow all at once. This will limit the competitive edge of Phragmites to colonize before the native grasses. 

The Evolution of Salt Marsh Landscape Patterns Helps to Predict Restoration Efforts

The ability to understand why certain environmental factors affect tidal creek stability is important to predict how salt marsh communities will respond to climate change and restoration efforts. Salt marsh function is dependant on the landscape which is chiefly shaped by the interaction between the flow of water and the density of vegetation. However topography, sediment transport, nutrient availability and uptake, sediment accumulation, and groundwater flow are also important yet misunderstood contributing factors to the organization of the land. Salt Marshes have a number of landscape patterns that range from low lying sloughs with sparse vegetation to high marsh ridges with dense vegetation. Larsen and Harvey (2010) developed the Ridge and Slough Automata Landscape model (RASCL) to represent the slow evolution of salt marshes over millennia demonstrating the interaction between vegetation and water flow. Understanding this interaction allows researchers to predict how different disturbances affect the landscape. Appropriate mitigation and restoration efforts can be determined based on the arrangement and specific shape of vegetation patches in regards to the orientation and velocity of flowing water. Acadia Tucker
Larsen, L., Harvey, J. 2010. Modeling of hydroecological feedbacks predicts distinct classes of landscape pattern, process, and restoration potential in shallow aquatic ecosystems. doi:10.1016/j.geomorph.2010.03.015.

RASCL, in particular, reproduced two wetland ecosystems in the Everglades of Florida because it represents both densely vegetated, flow-resistant ridges, and sparsely vegetated sloughs. This makes it applicable to most salt marsh ecosystems that resemble areas with dense vegetation interacting with low-lying, fast flowing creeks. Larson and Harvey used a range of abiotic and biotic parameters such as initial water level, suspended sediment load, soil porosity, and periods of high flow fluxes to account for wet seasons. The model function was based on two dominating feedback loops, signals that either accelerate or inhibit the process of evolution. Sediment-flow feedback is a positive signal that accelerates the build up of vegetation patches by the accumulation of peat and sediment, increasing the density of plants and decreasing the flow of water. Differential peat accretion feedback, a negative signal, prohibits the accumulation of peat and sediment in shallow and deep waters to regulate the coverage of high-density patches of vegetation.
RASCL was run 125 times to show the main geomorphic processes that create different landscape patterns under different parameters. In general, during the early stages of landscape development, patches of vegetation are not permanent and develop gradually. During this time developing patches of vegetation are vulnerable to erosion caused by fast flowing water and insufficient sediment transport. Patches of vegetation only become stable once a certain volume of vegetation (less than 1%) reaches a critical elevation above the surface of the water. As more plants colonize an area, their roots trap sediment and slowly build up a landmass beneath them (positive feedback). Newly formed patches begin to elongate and expand as the flow of water is redirected around them causing sediment to be redistributed on the downstream side of the patch. Peat accumulation and patch elongation stop once the flow of water decreases and the water level becomes too shallow among the patches of vegetation (negative feedback). This further redirects water away from the flow-resistant, dense patches of vegetation and intensifies the flow of water over the edges of sparsely vegetated areas. The increase of flow intensifies erosion and narrow creeks form along the areas of reduced flow resistance, interlaced throughout the patches of dense vegetation. Salt marsh ecosystems stabilize once the rate of erosion equalizes the expansion of new patches.
The model produced three major landscape patterns that stabilized at different points along the evolutionary process. The majority of salt marsh landscapes developed into an area dominated by ridged topography with limited water flow through the dense patches of plants (75%). Other simulations developed into a diverse landscape with ridges of dense vegetation interacting with flowing creeks and shallow sloughs (18%). However, under some of the parameters, the landscape pattern never stabilized and continued to shift as patches of vegetation were continually created and destroyed (7%).
By manipulating the input variables Larsen and Harvey determined that any alteration in the velocity or volume of water entering a salt marsh could severely transform the landscape pattern and thus significantly change the natural function of the ecosystem. If the flow of water is limited it could cause a rapid expansion of densely vegetated patches, while an increase in flow could drown the whole habitat. Larsen and Harvey found that by analyzing the shape and orientation of vegetation patches they could predict the particular disturbances active within a salt marsh and therefore take the corrective measures to restore the damaged ecosystem. For example, the rapid expansion of densely vegetated patches produced by a disruption of flow permanently alters the natural erosion rate, which can be detected by analyzing the shape change of certain vegetation patches. Restoring the normal flow of water cannot reverse this result alone; it can only be reversed by reducing the concentration of vegetation before restoring the natural flow of water. 

Climate Change and the Ecological Role of Foun-dation Species in New England Salt Marshes

Foundation species are important primary producers that promote and prolong the health of an ecosystem because they offer key regulatory functions. Salt marsh communities have several foundation species that nurture other organisms by providing a refuge from predation and shelter from harsh elements. Many salt marsh invertebrates, live near the upper edge of their thermal limits and will be greatly impacted by environmental warming. For this reason, it is increasingly important to understand the exact role of foundation species in alleviating the negative impacts of global climate change. Spartina patens is a New England salt marsh halophyte, a plant adapted to living in saline conditions, that traps sediment in its roots to create a sanctuary for small organisms. It also moderates soil temperature and salinity by providing canopy leaf shade and helps fertilize the soil with oxygen because of its rapid root expansion. Gedan and Bertness (2010) looked at the role of foundation species by studying the response of salt marsh function to warming with and without the presence of S. patens. They concluded that the loss of a foundation species has greater negative impact on healthy salt marsh function than do temperature increases alone.–Acadia Tucker
Gedan, K., Bertness, M., 2010. How will warming affect the salt marsh foundation species Spartina patens and its ecological role? Oecologia 164, 479–487.

The high marsh of the Narragansett Bay National Estuarine Research Reserve in Rhode Island is dominated by S. patens. To examine the effect of increasing temperatures on the high marsh habitat, Gedan and Bertness (2010) manipulated outdoor temperatures and the presence of S. patens in a sampling of 32 plots. Each plot was randomly assigned to one of two plant treatments (removal or control) and one of two temperature treatments (warming chamber or control). The warming chambers, with an average increase of 2.6°C, were constructed from plastic covers and designed to focus sunlight and trap warm air. Herbicide and weed mats were installed to ensure the complete removal of S. patens in the designated treatment areas.
The controlled increase in temperature was not intensive enough to directly harm S. patens. The goal of the warming chamber was to create an environment that challenged the optimal temperature threshold of salt marsh invertebrates and represent a realistic temperature increase predicted for New England by the IPCC. The focus of this experiment was to observe the effect of each treatments’ ability to change the soil salinity, the soil temperature, the air temperature, the presence of oxygen, and the evaporation potential and relative humidity of the test environments. The impacts of each treatment were evaluated by the change in epifaunal composition ¾the abundance of animals living on the marsh bed¾ and the primary productivity of soil algae and S. patens.
Gedan and Bertness (2010) expected that the ecological function of the high marsh would be disrupted the most by climatic stress. However, their results showed that the loss of the foundation species had the most devastating impact. Vegetation removal increased all of the abiotic stress factors while warming alone had a limited impact on the high marsh community. For example, the soil salinity was the highest in areas where the vegetation had been removed because the decrease in canopy cover increased the evaporation tendencies of the soil. As seawater evaporates it leaves behind heavy salt deposits that accumulate in the soil. Conversely, the warming chamber decreased the salinity of the soil because it created a humid environment that lowered the rate of evaporation. The soil temperature increased only in plots (an average of 3.8 °C) where there was vegetation loss and controlled warming, while vegetation removal and warming had no effect on the concentration of oxygen in the soil. Warming also had no effect on algae productivity or the composition of marsh bed organisms.
In comparison, vegetation removal accounted for a 133% increase of algae productivity and a dramatic reduction in the amount of epifaunal organisms, especially crustaceans and gastropods. This shows that warming does not have as significant an effect on high marsh communities as the threats of increased soil salinity, high evaporation rates, and habitat loss. Therefore, a foundation species’ most important ecological role is to alleviate the abiotic conditions listed above instead of regulating the air and soil temperatures. The loss of a foundation species generates more harmful change in a high marsh ecosystem than will temperature increase alone. 

Salt Marsh Vegetation Restoration in the Face of Climate Change

Salt marsh restoration in the age of climate change is facing major decisions in the application of restorative management and rehabilitation techniques. In the past, rehabilitation practices focused on restoring habitats to natural levels of biodiversity. However, given the increasing variability of predicted climate patterns, management efforts must facilitate salt marshes ability to adapt to these varied conditions. Green et al. (2009) compared the difference in vegetative growth between areas under natural and assisted regeneration in salt marsh habitats to understand the colonization patterns of native plant species, with consideration to the effects of tidal flooding, climate change, and changes in topography. Results show that each plant has different colonization preferences and, in healthy salt marshes, a clear vegetation profile can be distinguished. Therefore it is very important to understand the scope of each salt marsh species in order to effectively restore damaged salt marsh environments.¾ Acadia Tucker
Green, J., Reichelt-Brushett, A., Jacobs, S., 2009. Re-establishing a saltmarsh vegetation structure in a changing climate. Ecological Restoration and Management 10, 2030.

Sponsors Lagoon in Brisbane, Australia is largely composed of salt marsh ecosystems that have been disturbed by sand mining, trash dumping, and more recently four-wheel vehicles that have caused major erosion and a decrease in biodiversity. At the start of the experiment, the micro-elevation of eroded patches of salt marsh was restored with native dune sand from the surrounding area. The experiment compared planted restoration sites (assisted) with non-planted restoration (natural) sites to see if seed fertilization alone could rehabilitate a disturbed area. There was a total of four scenarios that were used in this analysis; two plots that remained undisturbed, two plots that were rehabilitated by planting Saltwater Couch (the dominant native specie), two plots that were rehabilitated by soil restoration alone (non-planted), and two plots that remained disturbed and untreated. Green used statistical modeling programs to analyze the percent contributions of each colonizing plant species for each location and to test the similarity between restored sites and undisturbed sites at the end of the three-year test period.
Saltwater Couch successfully colonized each of the restoration sites in the middle to lower elevation range. However, the rapid expansion of Saltwater Couch was only facilitated in areas where supplementary planting occurred. Because this plant can only colonize short distances from existing patches, supplementary planting is needed to restore segregated patches of salt marsh. Non-planted species, such as Sarcacnia and Suerta, also successfully colonized both restoration sites. Although significant Sarcocornia cover was found at each of the restoration sites, results show that its colonization was prohibited by Saltwater Couch competition. Sarcocornia had an increase in seed germination the closer the colonized plants were to their seed source. This projects that the colonization of certain species could be slower or non-existent in areas dominated by Saltwater Couch.
As a result, plant colonization is related to site specific factors as well as each plant species method of reproduction. Management decisions should therefore be made on a site-specific basis depending on how disrupted the salt marsh is.  Highly degraded areas with low elevations and poor soil quality must be rehabilitated through supplementary or assisted planting. In areas that are open to seed dispersal and have a health soil profile, the combination of supplementary planting and natural colonization is the best technique. Reestablishing a healthy vegetation profile is important for salt marsh restoration but it is not enough to counteract the predicted sea level rise. Mitigation should therefore reserve “buffer zones” in higher elevations to allow inundated salt marshes to migrate inland if the need may arise.