Salt Marsh to Tidal Flat
The shift from a salt marsh to either a tidal flat or subtidal flat generates a loss of significant ecosystem services such as pollution filtration, storm protection, and fisheries enhancement. This regime shift is primarily driven by the rate of sea level rise and the rate of sediment delivery. Transitions to consumer control either through the overharvesting of predators or the introduction of invasive/ exotic species can also contribute to this regime shift. It has long been recognized that salt marshes have the capacity to regulate their platform elevation in response to rises in the sea level through a series of non-linear biophysical feedback mechanisms. However, thresholds exists in the rate of sea level rise (RSLR) and the rate of sediment delivery, where upon the mechanisms that effectively control the platform elevations are no longer able to keep up with sea level rise. Effective management options largely depend on the regional variables of the system. These options range from the reintroduction of top predators and removal of invasive/ exotic species to coordinated dam releases to provide necessary sediment pulses.
Key direct drivers
- Vegetation conversion and habitat fragmentation
- Infrastructure development
- Species introduction or removal
- Global climate change
- Land use impacts are primarily off-site (e.g. dead zones)
- Marine & coastal
Key Ecosystem Processes
- Soil formation
- Primary production
- Nutrient cycling
- Fuel and fiber crops
- Climate regulation
- Water purification
- Regulation of soil erosion
- Natural hazard regulation
- Aesthetic values
- Knowledge and educational values
- Livelihoods and economic activity
- Cultural, aesthetic and recreational values
Typical spatial scale
Typical time scale
- Contemporary observations
Confidence: Existence of RS
- Contested – Reasonable evidence both for and against the existence of RS
Confidence: Mechanism underlying RS
- Well established – Wide agreement on the underlying mechanism
Coastal ecosystems of the intertidal zone located in temperate/ mid-latitudes can shift between a salt marsh and either a tidal flat or subtidal flat. The alternate regimes (tidal flat or subtidal flat) vary regionally based on a number of different factors including tidal basin topography, climate, hydrology, vegetation, and sediment loading. The alternate regimes are:
This regime is largely dominated by the growth and presence of Spartina alterniflora (Smooth Cordgrass) along the east coast of North America and Spartina anglica (Common Cordgrass) in continental northern Europe, halophytes adapted to saline and water logged environments (Marani et al. 2010). The high density, largely monoculture systems exist in the intertidal zone, experiencing daily inundation. While generally found between mean sea level and mean high tide, the range of Spartina and the optimum level for the marsh platform varies regionally based on tidal range, nutrient loading, vegetation, sediment loading, and climate (McKee & Patrick 1988; Morris et al. 2002).
Tidal Flat/ Subtidal Flat
This regime is void of vegetation. Instead, it is characterized by microbial food chains, dominated by benthic macroorganisms and benthic microalgae (Alongi 1998). Existing below mean sea level (MSL), the surface is only exposed during the lowest tides (Fagherazzi et al. 2006; Defina et al. 2007). On the other hand, subtidal flats are permanently submerged.
Drivers and causes of the regime shift
Shift from Salt Marsh to Tidal Flat/ Subtidal Flat
One of the main drivers resulting in a shift from a salt marsh to tidal flat/subtidal flat is an increasing rate of sea level rise. The direct impact of climate change on an increasing rate of sea level rise and the resulting impact on marine coastal systems has been well established (UNEP 2006). Broadly speaking, mean global sea level changes through the dual processes of thermal expansion and shifts in the hydrologic budget (amount of water in the oceans vs. other reservoirs – i.e. atmosphere, glaciers, ice caps, ice sheets & terrestrial reservoirs). It has long been recognized that salt marshes have the capacity to regulate their platform elevation in response to rises in the sea level through a series of non-linear biophysical feedback mechanisms (Murray et al. 2008; Kirwan et al. 2010). However, a threshold exists in the rate of sea level rise (RSLR), where upon the mechanisms that effectively control the platform elevations are no longer able to keep up (Kirwan et al. 2010).
Another direct driver leading to a shift from a salt marsh to either a tidal flat or subtidal flat is changes in sediment delivery. Changes in the delivery rates have been linked to land use management and change throughout the watershed/ catchment basin (Pasternack et al. 2001; UNEP 2006; Kirwan et al. 2011). Reductions in the delivery rates of sediment from upstream inhibit the rate of soil accretion and the effective response of marsh platform elevation to rising sea level.
Furthermore, the tidal flat/ subtidal flat regime can be caused by an increase in consumer control – i.e. unchecked populations of herbivores (e.g. snails, crabs) govern ecosystem productivity and structure. Such an increase can result from either the overharvesting of predators or through the introduction of invasive and/or exotic species. In addition, it has been suggested that nitrogen loading and the weakening of plant defenses (increased salt stress due to climatic extremes) have also contributed to the shift to consumer control and resulting cascading effects leading to the tidal flat/ subtidal flat regime (Bertness & Silliman 2008).
How the regime shift works
Shift from Salt Marsh to Tidal Flat/ Subtidal Flat
The salt marsh regime is sustained by a process of soil accretion that maintains the marsh platform elevation above mean sea level under normal rates of sea level rise (Kirwan et al. 2010). The effective maintenance of the marsh platform elevation and the rate of soil accretion are dependent on a variety of mechanisms and biophysical feedback loops. As sea level rises, the amount and frequency of inundation increases. Increased inundation results in an increase in water depth in the salt marsh and in turn a decrease in flow strength. The decreased flow strength results in an increase in sedimentation. This in turn contributes to an increase in vertical accretion and a higher marsh platform elevation. The higher platform elevation then reduces the amount and frequency of inundation ensuring that the marsh does not get flooded and avoiding a die off of the vegetation.
An increase in inundation also contributes to the growth of vegetation. This increase in vegetative biomass also contributes to an increase in vertical accretion through biostabilization, wave dampening, sediment trapping, organic deposition, and below ground organic production (Fagherazzi et al. 2006; Kirwan et al. 2011). Vertical accretion, as highlighted above contributes to a higher marsh platform elevation, which in turn reduces the amount of inundation ensuring that the marsh does not get flooded. It is important to note that both processes necessitate an effective sediment supply from upstream in the watershed.
However, when the rate of sea level rise increases beyond a certain threshold, or there is a decline in the sedimentation rate or vegetation cover, the salt marsh may shift to a tidal flat. When the rate of sea level rise exceeds the rate of soil accretion, the marsh will become inundated and lead to the drowning and die off of vegetation. The loss of vegetation can lead to further cascading effects through two mechanisms. The first is that the loss of vegetation leads to an increase in erosion of the marsh platform, which further inhibits the growth of vegetation. The second is through a change in soil conditions. The loss of vegetation can result in an increase in soil salinity, which in turn inhibits recolonization and further contributes to the loss of vegetation (Bertness & Silliman 2008).
Just as the rate of sea level rise is not a constant, neither is the sediment delivery rate. As a result, key thresholds regarding the RSLR are directly linked to the suspended sediment concentration. As the sediment load decreases due to changes in upstream land use practices and/or the establishment of dams, the rate of soil accretion decreases, and the critical RSLR at which the marsh shifts to a tidal flat will be reduced.
The rate of sea level rise and suspended sediment concentration also influence key processes occurring at the margins of the marsh through their influence on the tidal flat water depth which contributes to the expansion/reduction of the marsh platform at the marsh boundary (Mariotti et al. 2010). Two different feedback loops directly impact the water depth on the tidal flat. The Wave Factor  at the marsh boundary increases with the tidal flat depth. The increase in the Wave Factor Boundary results in an increase in marsh boundary erosion. Increased erosion increases the sediment supply, which in turn contributes to a decrease in the tidal flat depth (Mariotti et al. 2010). In this manner, it serves as a balancing feedback loop. However, an increase in the tidal flat depth simultaneously contributes to an increased Erosion Factor . This increased EF results in an increase in erosion of the tidal flat, in turn increasing the tidal flat depth. In this manner, it serves as an amplifying feedback loop. Whichever feedback loop is stronger dictates the global character of this process (Mariotti et al. 2010). Thus the RSLR and the suspended sediment concentration contribute directly to these competing feedback loops as both directly influence the tidal flat depth.
Once a tidal flat/ subtidal flat is established, it is maintained by the higher rates of erosion associated with the lack of vegetation and the continued inundation, so that bottom elevation remains below mean sea level (Defina et al. 2007). Even if the rate of sea-level rise drops again to previous levels the tidal flat may be maintained by these processes. The critical rate of sea-level rise at which a shift from tidal flat to salt marsh occurs is therefore lower than the threshold at which the shift from salt marsh to tidal flat occurs. However, increased sedimentation due to a pulse in sediment supply, decreased tidal velocities, or protection by new spits and barrier islands may provide conditions for recolonization of Spartina, a rise in the platform level, and an eventual shift to a salt marsh regime (Defina et al. 2007).  Mariotti et al. (2010) calculate the Wave Factor at the marsh Boundary by taking into account a number of variables including: wave power, number of marsh boundary elements, boundary length, time, wind speed, wind direction.  Mariotti et al. (2010) calculate the Erosion Factor by taking into account a number of variables including: current bottom shear stresses, wave bottom shear stresses, wind direction, wind speed & total area of the tidal flat.
Impacts on ecosystem services and human well-being
Shift from Salt Marsh to Tidal Flat/ Subtidal Flat
Both salt marshes and tidal flats/ subtidal flats provide significant ecosystem services (UNEP 2006). In many cases, they provide similar services but in slightly different ways. For example, while salt marshes contribute to food through serving as nurseries for certain fish species, tidal flats provide important habitat for certain mollusks and crabs (UNEP 2006). However, it is worth noting that the shift from a salt marsh to either a tidal flat or subtidal flat generates a loss of significant ecosystem services in the form of pollution filtration, storm protection, and fisheries enhancement (Gedan et al. 2009). In addition, the loss of these particular ecosystem services draw attention to the impact on and connection to human wellbeing in the form of livelihoods (decrease in fisheries), health (loss of pollution filtration), and climate change vulnerability (decrease in adaptive capacity and loss of storm protection).
Addressing rates of sea level rise requires an international commitment to addressing climate change. In addition, the time frame for the global climate and in turn RSLR to respond to new policies necessitates a long-term outlook. However, there are a number of management actions or interventions that can be implemented on a more local/regional scale to effectively address issues directly related to sediment delivery rates and the process of consumer control. Mudd (2011) suggest that for those watersheds that have been impounded, scheduled dam releases could be coordinated to supply a sediment pulse to encourage marsh establishment and expansion. Restoration practices that reduce/ remove invasive/exotic species and/or the reestablishment of predator populations could effectively shift the community structure in such a manner that reduces the consumer control and allows for the establishment of a salt marsh (Gedan et al. 2009).
Salt marshes exist within a heterogeneous coastal zone that is rapidly becoming human dominated - one third of the human population lives in coastal areas (UNEP 2006). This situation imposes certain constraints that must also be taken into consideration when thinking about the effective management and restoration of salt marshes. The long-term ability of a salt marsh to regulate its height in response to increased rates of sea level rise necessitates the flow of energy and materials into and out of the system. Tidal restrictions due to roads, railroad bridges, and their associated culverts can drastically change the dynamics in the system and have led to shifts in community composition (Gedan et al. 2009). A second key issue is the encroachment of development right up to the marsh's upland boundary. Such encroachment does not leave room for the marsh platform to migrate inland as it responds to increased rates of sea level rise. If the full suite of such pressures and drivers operating at various spatial and temporal scales both within and adjacent to the system are not taken into account, the long term resilience of the system may be compromised, and efforts at restoration may be undermined.
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