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Salt marsh hydrology assessment report
M. Hensel
The role of hydrology and hydrological dynamics in salt marshesis widespread and vital to
nearly all marsh processes and restoration efforts. This report will focus on (1) the
important aspects of hydrology in the formation and maintenance of salt marshes, (2) types
and effects of hydrological disruptions on salt marsh functioning, and (3) the effects of
hydrological restoration on a salt marsh and how to evaluate the effectiveness of
restoration.
Hydrology in salt marsh ecosystems
Salt marshes are formed when slow moving water near a protected shoreline,
barrier island, or estuary leads to the accumulation of fine sediment which is in turn
colonized by halophytic plants (Bertness 1998). In turn, incoming tide water carves
channels and builds the intricate salt marsh creek system (Fagherazzi and Kirwan 2012).
Once initial plants such as the foundation species Spartina alterniflora are present, marshes
grow through self-facilitation by trapping suspended sediments from the water column and
creating organic matter (i.e. peat). The formation and regulation of marsh growth is a
direct product of the amplitude of the tide and the elevation of the marsh, also known as
the hydroperiod (Bertness 1998, Morris et al. 2002, Fagherazzi and Kirwan 2012).
Hydroperiod changes locally and regionally but these variations determine the amount of
sediment and nutrients that are delivered into marshes via tidewater. In turn, tidal
flushing governs fluctuating salinity and oxygen regimes which determines microbial
productivity, influence plant zonation, and supports the whole marsh food web (Figure 1)
(Magenheimer et al. 1996, Mendelssohn and Morris 1999, Crain et al. 2009).
In addition to the hydroperiod, the water table is another important aspect of marsh
hydrology that can affect overall marsh processes. The water table in a marsh can vary
based on level of tidal flushing, tidal restriction, precipitation, or freshwater runoff
(Montalto et al. 2006). Marsh biogeochemistry driven by microbial processes is highly
dependent on the water table. If the water table is too low (e.g. drought, tidal restriction),
oxygen permeates deep into the peat, increasing rates of decomposition. If the water table
is too high (e.g. flooding, outgoing tidal restriction), the waterlogged peat becomes
anaerobic, slowing decomposition as organic matter accumulates (Portnoy 1999).
As outlined in Figure 1, water table and hydroperiod, as well as water flux, are the
important factors that determine hydrology in marshes. Edaphic factors such as salinity,
pH, oxygen and sulfide levels are all highly dependent on these hydrological processes. The
level of these edaphic factors in turn affects marsh community composition and
organization. Marsh biotic structure drives many important ecosystem functions and
services such as productivity, decomposition, and nutrient cycling. Like many coastal
ecosystems, salt marshes contain a number of biotic feedbacks between these aspects of
the model in Figure 1. For example, proper hydrology that supports a healthy plant
community in turn will lead to marsh growth through peat accretion, hummock formation,
evapotranspiration and decomposition induced marsh platform subsidence.
Effects of hydrological disturbance on salt marsh ecosystems
Human impacts are widespread drivers of change in marshes throughout the world
(Silliman et al. 2009). Besides habitat destruction, restriction or elimination of tidal flow is
the largest human disturbance to salt marsh ecosystems, especially in New England
(Bertness et al. 2002, Crain et al. 2009). The most common hydrological changes to
marshes are from mosquito ditching and the building of dikes, roads, or tidal gates which
restrict the amount and frequency of tidal flooding into creeks and onto the marsh
platform. Because of the extensive amount of ditching and diking done in New England
marshes (i.e. 30% of Maine marshes are tidally restricted and 95% of New England
marshes are ditched (Gedan et al. 2009)), researchers have a wide range of examples of the
effects of hydrological disturbance on salt marsh processes. Due to the frequency of these
hydrological disruptions and the fact that hydrological restoration is the most common
type of marsh restoration, most of the effects of the disruption and restoration of tidal flow
into marshes should be outlined in the other sections in this report. Below is briefly
outlined some of the effects of tidal flow restriction on salt marsh ecosystems
Restricting or blocking tidal flow from dikes, tide gates, or culverts, alters the
hydroperiod of a marsh, and in turn can negatively affect the soil biogeochemistry,
vegetation structure, and animal community (see Figure 1). Some of these impacts, such as
the lack of sediment delivery or waterlogged peat, cause decreases in the productivity of
the foundation species Spartina alterniflora (Portnoy 1999) and reduce the efficiency of
sulfide reducing microbes (Portnoy and Giblin 1997). Also, tidal restriction resulting in
desalination of a marsh, often combined with an increase in terrestrial nutrient runoff, can
cause a marsh to shift from one dominated by S. alterniflora to dominated by the invasive
species Phragmites australis (Silliman and Bertness 2004, Smith et al. 2009). The shift in
vegetation from Spartina to Phragmites causes a subsequent crash of the marsh food web
(Warren et al. 2002, Silliman and Bertness 2004, Smith et al. 2009) (but see (Burdick and
Dionne 1996) which suggests fish may not respond this way).
In summary, as figure 1 indicates, changes in marsh hydrology can affect many
ecological outputs from local marsh ecosystems. Altered salinity and nutrient delivery, as
well as changes in the water table, affect marsh plant communities, which in turn cascades
up the whole food web.
Evaluating the effects of hydrological restoration in a salt marsh
One advantage of the centuries of human impacts on salt marshes is that managers
and researchers have a number of case studies on the effectiveness of restoring salt
marshes. The first step in a successful project is the actual restoration of tidal flow.
Common tidal restorations include either complete flow restoration or an improvement on
earlier tidal restriction. Outlined in Warren et al 2002, some of the common restoration
events include the removal of tidal gates, installation of self regulating tidal gates, resizing
of culverts, outlet/channel dredging or cleaning, and the removal of filled channels (Table
1).
Once tidal flow is restored, there are a number of predicted responses from the
biological community in the marsh (Table 2). Timing of recovery depends on a number of
factors, including length and extent of disturbance, and the type of restoration (Zedler
2000). Specifically, once flow is restored, managers can expect rebounding in the water
table, salinity of the marsh platform, and subsequent change in marsh vegetation (i.e.
return to Spartina from Phragmites) (Burdick and Dionne 1996). As the plant community
returns to its natural state, invertebrate, fish, and bird communities will rebound (Warren
et al. 2002, Gedan et al. 2009), but the timing of these events varies from marsh to marsh
(Roman et al. 1995, Burdick and Dionne 1996). The relationship between time since initial
disturbance and expected rate of recovery after restoration is currently unknown, but this
may vary predictably with degree of hydrological restoration and the state of the prerestored marsh.
To evaluate the effectiveness of marsh restoration, researchers and managers need
to implement a number of monitoring protocols to understand how hydrology and the
marsh community respond to tidal flow restoration. Neckles et al 2012 outline a number of
ways to measure the success of marsh restoration loosely related to the functions and
services desired by managers. First, to understand how hydrological restoration has
affected the way that tides flood the marsh, it is important to monitor tidal heights and the
elevation of the marsh relative to tidal heights. Although tidal buoys and reports are often
accurate, the exact amount of marsh platform flooding can only be determined by direct,
local measurements such as a contour map. Measuring the tidal signal and elevation
should be done over at least a month to see how flooding changes across multiple moon
phases. Some of the best methods for this monitoring are through the use of continuous
water level recorders. Also, to further evaluate the effectiveness of restoration on
hydrological dynamics, managers should monitor the changes in tidal creeks thought
permanent cross section profiles of said creeks. Water table depth should also be
measured through permanent groundwater wells installed within the marsh and can be
used to determine if the water table is responding to restoration. Finally, water quality
parameters in the main channel and within porewater should be monitored to determine if
changes in dissolved oxygen, salinity, temperature, and pH have corresponded with the
restoration efforts.
An important aspect of marsh restoration that is often ignored is the changes in
marsh functioning and services before and after restoration takes place. A pre-restoration
marsh, despite often dominated by the highly productive and deep rooting Phragmites, still
provides a number of ecosystem services that are important to humans. Because of its high
growth rate, Phragmites most likely is an important carbon sequesterer and shoreline
protector. In fact, some marsh restorations in highly nutrient rich areas (e.g. around a
coastal airport) may seek out Phragmites as opposed to Spartina marshes. Based on this,
managers and researchers should try to quantify changes in ecosystem functions and
services throughout the restoration process. As mentioned in this report, plant and animal
communities are different in restored and disturbed marshes, but how service provision
changes with time since restoration is unknown. One would expect that, as a restoration
progressed, some services might be lost or reduced (e.g. runoff filtration, shoreline
protection) while others are increased (e.g. aesthetic value of marshfront property,
recreation from bird watching). As well as implementing the monitoring protocols
outlined in the first part of this section, managers should closely measure functions and
services before, during, and after marsh hydrological restoration efforts.
HYDROLOGY
Tidal inundation
Water Table level
Water flux
Biotic feedbacks
Peat accretion
Hummock formation
Evapotranspiration
Decomposition-induced subsidence
EDAPHIC FACTORS
Oxygen/ Sulfides
Salinity
pH
Biotic Feedbacks
Shading
Evapotranspiration
Oxygenation of rhizosphere
BIOTIC STRUCTURE
Species Composition
Zonation
Biotic Feedbacks
Species interactions
ECOSYSTEM PROCESSES
Nutrient cycling
Productivity
Diversity
Decomposition
Figure 1 from Crain et al. 2009). Edaphic factors in a marsh are driven by hydrological
processes such as tidal inundation and water table. These factors in turn affect the biotic
structure of the marsh which has important feedbacks with the overall marsh ecosystem
and hydrology. Important marsh ecosystem processes that drive marsh services are the
output of this model, indicating that all of these processes are tightly connected
Table 1, from Warren et al 2002. Six of the most common marsh restoration approaches
used in Connecticut over the last 45 years. Note that rarely is tidal flow completely
restored, as tide-gate removal and culvert resizing are the most common restoration
techniques.
Table 2 from Burdict et al 1997. Summary of responses in salt marsh functions to tidal
restriction and restoration in a Maine andba New Hampshire marsh. In these examples,
marsh elevation, water table, salinity, plant cover and community composition responded
positively to marsh restoration. Here, fish consumers did not have a significant increase in
abundance in response to restoration (Figure 9 in Burdict et al 1997)
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