Estuaries and Coasts (2018) 41 (Suppl 1):S180–S196
DOI 10.1007/s12237-017-0254-x

Generalizing Ecological Effects of Shoreline Armoring
Across Soft Sediment Environments
J. E. Dugan 1 & K. A. Emery 1,2 & M. Alber 3 & C. R. Alexander 4 & J. E. Byers 5 &A. M. Gehman 5 & N. McLenaghan 3 & S. E. Sojka 6

Received: 25 October 2016 / Revised: 9 April 2017 / Accepted: 13 April 2017 / Published online: 24 July 2017
# The Author(s) 2017, corrected publication 2019

Abstract Despite its widespread use, the ecological effects ofshoreline armoring are poorly synthesized and difficult to generalize across soft sediment environments and structure types.
We developed a conceptual model that scales predicted ecological effects of shore-parallel armoring based on two axes: engineering purpose of structure (reduce/slow velocities or prevent/stop flow of waves and currents) and hydrodynamic energy(e.g., tides, currents, waves) of soft sediment environments.
We predicted greater ecological impacts for structures intended
to stop as opposed to slow water flow and with increasing hydrodynamic energy of the environment. We evaluated our predictions with a literature review of effects of shoreline armoringfor six possible ecological responses (habitat distribution, species assemblages, trophic structure, nutrient cycling, productivity, and connectivity). The majority of studies were in low-energy environments (51 of 88), and a preponderance addressedchanges in two ecological responses associated with armoring:habitat distribution and species assemblages. Across the 207
armoring effects studied, 71% were significantly negative,
22% were significantly positive, and 7% reported no significant
difference. Ecological responses varied with engineering purpose of structures, with a higher frequency of negative responses
for structures designed to stop water flow within a given hydrodynamic energy level. Comparisons across the hydrodynamic
energy axis were less clear-cut, but negative responses prevailed(>78%) in high-energy environments. These results suggest thatgeneralizations of ecological responses to armoring across a
range of environmental contexts are possible and that the proposed conceptual model is useful for generating predictions of
the direction and relative ecological impacts of shoreline
armoring in soft sediment ecosystems.

Communicated by Marianne Holmer

Keywords Coastal armoring . Shore protection structures .
Salt marsh . Mangrove . Estuary . Beach . Ecosystem function

Electronic supplementary material The online version of this article(doi:10.1007/s12237-017-0254-x) contains supplementary material,
which is available to authorized users.
* J. E. Duganj_dugan@lifesci.ucsb.edu

1Marine Science Institute, University of California, Santa
Barbara, CA, USA

2

Department of Environmental Sciences, University of Virginia,
Charlottesville, VA, USA

3

Department of Marine Sciences, University of Georgia, Athens, GA,
USA

4

Skidaway Institute of Oceanography, University of Georgia,
Savannah, GA, USA

5

Odum School of Ecology, University of Georgia, Athens, GA, USA

6

Randolph College, Lynchburg, VA, USA

Introduction
Soft sedimentary shores composed of mud, sand, and gravel
make up the majority (two thirds) of the world’s coastlines(Reise 2001). Soft sediments are associated with a variety of
ecosystems including beaches, dunes, coastal bluffs, marshes,
estuaries, bays, and inlets (Nordstrom 2000). These areas provide a range of ecosystem functions and services, ranging
from storm protection to wildlife habitat to carbon sequestration (e.g., Snelgrove 1999; Piersma 2009). Human use of theshore is intense, with most of the world’s megacities and morethan 600 million people living in the coastal zone (Neumann
et al. 2015). The coasts are the sites of major cities, ports, andresidential development, and many areas have been altered toEstuaries and Coasts (2018) 41 (Suppl 1):S180–S196

accommodate human activities such as agriculture andcommerce.
Soft sedimentary shores are inherently dynamic, and thishas led to the installation of coastal armoring structures built
for the purpose of protecting upland areas and slowing or
halting erosion and migration of the shoreline (Nordstrom
2000; Rippon 2001; Charlier et al 2005; Griggs2005a, b).
Shoreline armoring is widely used on all types of open andsheltered coasts and is being increasingly applied to soft sediment shores to protect human infrastructure and reduce shoreline retreat (Bulleri and Chapman 2010; Gittman et al. 2015).
The resulting proliferation of shoreline armoring in the second
half of the twentieth century has led to extensive hardening ofcoastlines in many regions (Nordstrom 2000; Airoldi et al.
2005). Thousands of kilometers of armoring are present on
the coasts of Europe and Japan, and up to 60% of the shoreline
has been armored along some urban coasts (Airoldi et al.
2005). In the USA, armoring is also widespread, occupying
12–30% of the total shorelines of individual states andreaching proportions of 50–70% or more along urban coasts(Gittman et al. 2015). Furthermore, the extent of armoring is
expected to increase as a result of expanding coastal populations and cities interacting with growing threats from climatechange, storm surges, and sea level rise.
Armoring of shorelines results in a suite of geomorphic andphysical effects on soft sediment coastal ecosystems (e.g.,
Nordstrom 2014). By fixing shoreline position, armoring constrains possible responses and evolution of soft shores tochanges in sea level and other dynamic coastal processes(Griggs 2010). The most immediate effect of an armoring
structure is placement loss, which is the direct loss of shoreline
habitat resulting from the footprint of the structure itself (e.g.,
Kraus and McDougal 1996). Placement loss can be substantial
in high-energy environments where larger dimensions arenecessary to ensure that the armoring structure is stable. The
presence of armoring along a coast also alters hydrodynamics,
modifying the flow of water and affecting sediment dynamics
of soft shore environments (e.g., Fletcher et al. 1997; Mileset al. 2001; Runyan and Griggs 2003; Martin et al. 2005). The
hardened faces of alongshore structures, such as seawalls andrevetments placed on beaches, reflect wave energy and constrain natural landward migration of the shoreline, generally
leading to the loss of beach area and width as well as flanking
erosion of adjacent shorelines (e.g., Hall and Pilkey 1991;
Griggs 2005a, b, 2010). The geomorphic and erosive processes involved in these shoreline changes have been well described through numerical, laboratory, and field studies(e.g.,Kraus and McDougal 1996; Ruggiero 2010), and coastal
engineers have a fairly good understanding of which aspectsof the physical environment must be considered when
installing shoreline armoring in different coastal settings. For
example, the US Army Corps of Engineers has developed
guidance that can be used to calculate stable sizes for armoring

S181

structures intended for different shorelines (USACE 2002,
Coastal Engineering Manual).
In contrast, the ecological responses to shoreline armoring
have received far less attention and are difficult to generalize
across ecosystems and structure types. Although recent reviews on ecological responses to armoring are valuable andare beginning to address this important gap (e.g., Bulleri and
Chapman 2010; Dugan et al. 2012; Nordstrom 2014; Perkins
et al. 2015; Gittman et al. 2016b), the majority of available
studies have been conducted in a specific ecosystem, precluding a critically needed broader synthesis across soft sedimentecosystems. For example, there is evidence that the presenceof armoring affects water quality (i.e., Bolduc and Afton
2004), habitat connectivity (i.e., Dugan and Hubbard 2006),
and species distributions (i.e., Morley et al. 2012). However,these studies were conducted in a tidal marsh, an open
coast beach, and an estuarine beach, respectively, so theycannot be evaluated across a common framework.
Moreover, one would expect that ecological responses toarmoring would vary depending on the type of structure
installed (e.g., seawalls vs. breakwaters vs. constructed
oyster reefs) and its relative location on the shore profile.
In other words, the ecological effects of armoring are expected to be context dependent based on both the characteristics of the environment and those of the armoring
structure itself.
To address the need for a common synthetic frameworkon ecological effects of armoring, we developed and evaluated a conceptual model that allows more general comparisons of ecological responses of soft sediment coastal
ecosystems to armoring across the spectrum of open to
sheltered shores and a range of different types of armoring
structures. We predicted that ecological effects would intensify as (1) a function of increasing energy at thearmoring structure and (2) with increasing influence ofthe structure in modifying the velocities and flow of waterfrom waves and currents. These formed the two axes of ourconceptual model. To critically evaluate the predictive
power of our conceptual model, we identified a suite ofsix general categories of ecological responses that we expected would be affected by the presence of armoring. We
then conducted a literature review of studies on the ecological effects of a diversity of shore-parallel armoring structures ranging from living shorelines to seawalls across aspectrum of soft sediment environments. We categorized
our literature review results according to the hydrodynamic
energy of the environment and the intended effects of eacharmoring structure on water velocities and flow. We quantified the number and direction of significant ecologicalresponses reported, which enabled us to assess the predictive power of our conceptual model. We also used our
results to identify key data gaps and develop further
hypotheses.

S182

Conceptual Model
Our analysis focused on shore-parallel structures placed ineither the intertidal or the nearshore subtidal zones of thecoast. We included numerous types of shoreline armoring
and coastal defense structures, such as seawalls, revetments,
bulkheads, and breakwaters, as well as sills, constructed oyster reefs, and living shorelines. Living shorelines are highlyvariable in structure and purpose and sometimes incorporate
sills, revetments, plantings, and oyster reefs (see Gittman et al.
2016a). In some manifestations, living shorelines can be indistinguishable from traditional armoring (Pilkey et al. 2012).
For our analysis, as long as they were parallel to shore, living
shoreline studies were included, regardless of the range ofways in which they were designed and constructed.
Although also widespread in a variety of soft sediment environments, our analysis excluded studies of groins, jetties, andother armoring structures built perpendicular to shore.
In order to place this wide variety of armoring structures
into a common framework, we asked two key questions: (1) Is
the engineering purpose of the structure to slow the velocity ofwater flow from waves and tides impinging on a shoreline or
to completely prevent or stop the flow of water to the shoreline? (2) What is the hydrodynamic energy at the structure?
We reasoned that if the purpose of the structure is to stop waterflow and the hydrodynamic energy is high, that would requirea very different type of armoring and cause more pronounced
ecological effects than if the purpose was to slow water flow ina low-hydrodynamic energy setting. Our conceptual model istherefore organized along the axes of the intended effect of thestructure on water flow and the hydrodynamic energy at thestructure, which allows us to broadly categorize armoring
structures as they are applied to different shoreline situations,elevations, and soft sediment environments.
The axis of water flow in the conceptual model can bethought of as a measure of the extent to which water generated
by waves and tides is prevented from moving through or overthe structure to the shoreline. Impermeable structures generally stop or prevent water flow through or over the structure
whereas permeable or low height structures serve to slow water velocity and allow flow through or over the structure toreach the shoreline (Fig. 1, top). The size of the structure isalso a consideration, as taller and longer structures will be
more effective at stopping water flow to the shoreline. At
one end of the spectrum, a seawall or revetment installed toprevent wave and storm surge intrusion is intended to stop
water from reaching upland areas. In more sheltered areas,such as harbors and estuaries, a much smaller bulkhead canoften provide a similar function. Revetments placed on open
coasts designed to stop waves from reaching coastal cliffs,
highways, or buildings tend to be tall and wide but are generally considered less reflective of wave energy than a seawall inthe same setting. Smaller revetments that are more typical of

Estuaries and Coasts (2018) 41 (Suppl 1):S180–S196
Living Shoreline

Slow
Tidal Salt Marsh

Low

Oyster Reef

Revetment

Bulkhead

Water flow
Mangrove

EnergyEstuarine Beach

Seawall

Stop
Ocean Beach

High

Fig. 1 Illustration of gradients in the two axes of influence for theconceptual model of shoreline armoring effects. Top row Engineering
purpose with respect to intended effect of structure on water flow (slowvs. stop). Bottom row Hydrodynamic energy (low to high) at the structure
as determined by the environment

sheltered shorelines, however, can often be somewhat more
permeable and also tend to be less reflective of hydrodynamic
energy than a bulkhead. Shorter structures, such as sills, aregenerally designed to retain sediments and still allow water toflow across or through the structure, serving to reduce waterflow and velocity (Gittman et al. 2014). Again, living shorelines can span a broad range of permeability, size, and purpose
with regard to their intended effects on water flow (i.e.,
Bilkovic et al. 2016; Gittman et al. 2016a).
A second important consideration that determines the typeof armoring structure installed in a particular area is theamount of hydrodynamic energy that reaches and interacts
with the structure. Hydrodynamic energy, broadly defined,
encapsulates several important contributing aspects that affectarmoring structure decisions, including the relative influenceof waves and tides in the environment and the tidal elevationof the structure. In general, marshes and mangroves are lowerenergy environments with tides dominating the hydrodynamic
conditions (tide range/wave height > 3) (Hayes 1979), whereas open coast beaches are high-energy, wave-dominated environments (tide range/wave height = 0.5 to 1) that occupy the
opposite end of the hydrodynamic energy spectrum (Fig. 1,
bottom). In the middle are medium- or mixed-energy shores
influenced by both tides and waves (tide range/wave height = 1
to 3). Hydrodynamic energy also varies within an environment, based on factors such as the tidal height in the profileat which the structure is placed and the role of local influencessuch as boat wakes and fetch. For example, the average hydrodynamic energy at a seawall placed well above mean highwater on an open coast beach will be lower than that of aseawall located below mean sea level on the same shore profile (e.g., Weggel 1988). Although the conceptual model could
theoretically be applied within a single soft sediment habitat(e.g., low vs. high elevation on an estuarine beach), in thisstudy we focused on differences in hydrodynamic energy

Estuaries and Coasts (2018) 41 (Suppl 1):S180–S196

S183

across environments so that we could encompass a broad
range of ecosystem types (from beaches to marshes) in ourevaluation of the conceptual model.
For our analyses, we used the two axes (effects on waterflow and hydrodynamic energy setting) to divide our conceptual model into two categories of intended effects of the structure on water flow to the shoreline (slow or stop water flow)
and three levels of hydrodynamic energy of the environment(low, medium, and high). The resulting six boxes (Fig. 2, labeled 1–3 for the hydrodynamic energy level and a or b for theengineering purpose of slowing or stopping water flow)
allowed us to scale the effects of shore-parallel armoring structures across a range of soft sediment environments and structure types. One result of this categorization is that the range ofpossible combinations of coastal armoring structure and ecosystem is bounded, with some types of structures tending tooccur more prevalently in certain ecosystems. For example,

MediumLow

Hydrodynamic Energy

High

Conceptual ModelBox 3aBox 3be.g. Detachedbreakwaters

e.g. Seawalls
Revetments

Box 2aBox 2be.g. Sills,
Shore-parallel
breakwaterse.g.
Bulkheads,
Seawalls,
Revetments

Box 1aBox 1be.g. Oyster
reefs, Living
shorelines,
Sills

Slow

e.g. Bulkheads

Stop

Effect on Water Flow
Fig. 2 Conceptual model showing predicted ecological impacts in soft
sediment environments across the array of shoreline armoring types used
to either slow or stop water flow (x-axis) and with different hydrodynamic
energy levels at the armoring structure (y-axis). Ecological impacts are
predicted to increase as one moves up and to the right within theparameter space

salt marshes are low in energy, and structures placed there tostop water flow, such as bulkheads (box 1b), are generally
lower in height and require a smaller cross-shore footprint tomaintain structural stability than in an open coast environment(e.g., USACE 2002). Large, detached breakwaters that slow
water flow are found along open coasts (box 3a) or in bays
(box 2a) whereas smaller sills are more prevalent in marsh andestuarine settings (box 1a). Revetments that stop or prevent
water flow to the shoreline, albeit with less direct reflection ofenergy than seawalls or bulkheads, can be found on open coast
beaches (box 3b) as well as lining the shores of estuaries,
harbors, and bays (box 2b).
We used the conceptual model to predict the relative ecological impact of armoring structures given different combinations along the two axes (diagonal arrow in Fig. 2). Alongthe water flow axis, we predict that structures designed to slow
rather than stop water flow will also allow more natural functioning and connectivity of aquatic and terrestrial habitats as
opposed to those designed to stop water flow to the shoreline.
Modeling studies have demonstrated that increased permeability of armoring structures could reduce wave reflection(e.g., Mallayachari and Sundar 1994; Zhu and Chwang
2001; Karim et al. 2009) and overtopping (e.g., Hieu andVinh 2012), both of which could decrease sediment erosion
and alter hydrology, affecting nutrient cycling and water quality. Impermeable barriers that completely prevent water flow
to reach the shoreline will reflect more of the energy fromwaves and tides than those designed to slow velocity but stillallow water flow through the structure to the shoreline. The
hydrodynamic energy of the environment (the vertical axis)
will also affect the design and impact of the armoring structure
both across and within soft sediment environment types.
Armoring structures in high-energy shoreline environments
tend to be larger than those in low-energy environments(USACE 2002), leading to greater placement loss and, therefore, likely greater impacts to habitat and species distributions.
Thus, a structure designed to slow water flow in an environment with low hydrodynamic energy (e.g., a low crested riprap sill in a marsh) would be expected to show the least
amount of ecological impact, whereas one designed to stop
water flow in a high-energy environment (e.g., a seawall on an
open coast beach) would be expected to show the greatest
impact. We did not have an a priori expectation as to whichof these axes would be more important and so predicted ageneral upward increase in ecological impacts commensuratewith intensification of both factors (Fig. 2).
To investigate these predictions for ecological impacts,
we identified six categories of ecological responses that
we expected could be altered by the presence of shoreline armoring in soft sediment ecosystems (see Fig. 3 forexamples of negative responses). These categories aredescribed below, along with the rationale for eachcategory.

S184
Fig. 3 Comparison between
unarmored (a) and armored (b)shorelines, with examples ofeffects for the six ecologicalresponses evaluated in this review(E1 habitat distribution, E2
species assemblage, E3 trophic
structure, E4 nutrient cycling, E5
productivity, E6 connectivity).
Broken ellipses in panel b signify
negative impacts and correspondto the ellipse of the same color inpanel aEstuaries and Coasts (2018) 41 (Suppl 1):S180–S196

a. Unarmored shore

(E4)

(E3)

(E5)

(E6)

High

(E1)
Natu

ral shMidoreline pro

b. Armored shore

(E2)

Low
Decreased waterexchange (E4)
fileFewer trophic
levels (E3)

Decreased
productivity (E5)
Reduced abundance& diversity (E2)
Changes in wrack
deposition (E6)Mid

Loss of high intertidal zone(E1)Altere(E1) Habitat Distribution: The loss or alteration of coastal habitats associated with armoring can directly impact many
functions of soft sediment ecosystems including species
distributions, biodiversity, connectivity, productivity,
food webs, and wildlife support. In addition to the immediate placement loss that occurs when the footprint of anarmoring structure covers a portion of the shore habitat,
over time, the presence of a structure can result in the loss
and alteration of intertidal habitat on its seaward side due
to increased erosion and subsequent conversion to
subtidal habitat (Fig. 3). This can result in a loss of habitatfor intertidal biota and of nesting habitat for birds, fish,and sea turtles. Armoring structures can also result in thealteration of habitat characteristics, such as grain size,
shore profile, and light regime, which can affect species
distributions. By blocking seawater inundation and water
flow, armoring can also result in the loss of intertidal
habitat on the landward side of a structure and its conversion to upland. Armoring may also provide a novel hardsubstratum habitat in an area otherwise devoid of anything but soft sediment, and in some cases, this novel
habitat may also include three-dimensional aspects, suchas the nooks and crannies associated with rock revetments, that increase habitat complexity and provide refuges and microhabitat for some organisms.
(E2) Species Assemblage: The shifts in habitat noted above
and other environmental characteristics of armoring
structures can affect species assemblages, with

d pro

Low

file

consequent implications for biodiversity, abundance,
size structure, and community composition (Fig. 3).
Armoring structures can also potentially support both
native and invasive species that require hard substrates
and may provide stepping stones for their dispersal andspread to new areas.
(E3) Trophic Structure: Shifts in trophic and food web structure associated with armoring follow from shifts in habitat characteristics, species distribution, and productivity (Fig. 3). This can include changes in prey or predator abundance, shifts in diet, and altered complexityand functional redundancy of food webs. This category
includes effects on animals that forage in coastal soft
sediment ecosystems, including birds, fishes, reptiles,
and mammals.
(E4) Nutrient Cycling: Changes in hydrology and sedimentcharacteristics associated with armoring will likely affect microbial communities and biogeochemical cycling with impacts to nutrient cycling, rates (i.e., denitrification), organic matter dynamics, and oxygen
levels (Fig. 3). The presence of the structure may alsointerfere with water exchange across the interface, potentially reducing surface water runoff and associatednutrients from the upland.
(E5) Productivity: Primary production may be affected by
the presence of armoring, particularly if there arechanges in light (through shading) or nutrient availability (Fig. 3). Primary production includes that ofEstuaries and Coasts (2018) 41 (Suppl 1):S180–S196

S185

phytoplankton, macroalgae, vascular plants, andmicrophytobenthos. Secondary production may beaffected as well, either as a consequence of changes
in primary production or changes in habitat and species distribution.
(E6) Connectivity: Armoring can represent a physical barrier that interferes with the exchange and accumulationof organisms, wrack, litter, sediment, and propaguleson the shoreline (Fig. 3). Depending on the locationof the structure, this can prevent or deter the movement
between upland, intertidal, and subtidal areas and affectvital shoreline ecotones at the boundary of land and sea.
Loss of connectivity has implications for many of theother ecological effects described above, such as nutrient cycling, productivity, species assemblages, and trophic structure.

was evaluated, we assessed each box and/or response category
result separately. However, if more than one variable wasmeasured within a particular effect category, it was only
counted once. For example, if a study measured the abundance of multiple species, it was only included once under
species assemblage (E2). In most cases, multiple variables
responded similarly (i.e., there were significant reductions inall species evaluated). In the few cases where there weremixed results, a paper was scored according to the majorityof effects (i.e., if the abundance of four out of five species was
significantly reduced, this was counted as a negative effect).
Effects of armoring on habitat distribution (E1) and species
assemblages (E2) were often reported together (i.e., a change
in habitat was associated with a change in species distribution
or abundance). We separated these effects for our analyses byassigning changes in habitat availability or quality, including
nesting habitat, to E1 and changes in abundance or distribution of organisms to E2.

Methods: Literature Review and Evaluationof Conceptual Model

Results

To evaluate the predictions of our conceptual model, we used
the results of a literature review that focused on the six categories of ecological responses to shoreline armoring (E1–E6)
we identified. We conducted a systematic search of Google
Scholar and Web of Science using key words related toarmoring (breakwater, bulkhead, coastal armoring, coastal
hardening, living shoreline, oyster reef, riprap, revetment, seawall, shoreline armoring, shoreline hardening, sill, impoundment) and environment (beach, estuary, lagoon, mangrove,salt marsh, tidal creek, harbor, river mouth). This was augmented by papers that came to our attention through conferences and other means as we were conducting this effort.
Papers were included in the literature review if they contained
ecological results.
We classified each study in terms of environment and typeof armoring structure to assign it into one of the six boxes inour conceptual model. Due to the limited scope of information
available, we did not further classify studies or study results
based on tidal elevation, size (height and length), submergence regime, or construction material of the armoring structures for our analysis.
We identified which of the six categories of ecologicalresponses were evaluated in each study and whether the effects were significantly positive, significantly negative, or not
significant according to the authors of each paper reviewed.
Examples of negative responses are illustrated in Fig. 3, andadditional examples of positive and negative responses areprovided in Table S1.
In studies where more than one box in the conceptual model was studied or more than one ecological response category

We located a total of 88 studies that evaluated ecological effects of shore-parallel coastal armoring on soft sediment environments (Table S2). The majority of studies (n = 51) wereconducted in low-energy environments: most of these wereconducted in salt marshes and tidal creeks (n = 47) and only
four in mangroves. A total of 24 studies were conducted in
medium-energy systems, including studies in harbors, rivermouths, and estuaries. We located only 13 ecological studies
conducted in high-energy environments, the majority ofwhich were conducted on open coast sandy beaches.
The majority of studies in low-energy environments investigated structures designed to slow water (Box 1a), such as
sills, rather than stop water flow (e.g., bulkheads) (Box 1b)(Fig. 2, Table 1). Box 1a in our conceptual model included
most of the living shoreline and oyster reef studies but also
Table 1 Distribution of studies of ecological effects of shoreline
armoring across the axes of hydrodynamic energy and intended effectof armoring structure on water flow that define the six boxes in theconceptual model
Hydrodynamic energy of environment

Effect on water flow
Slow
a

Stop
b

High: 3Medium: 2

5
13

11
19

Low: 136

24

Note that the total for this table (n = 108) exceeds the number of studies(88) because studies that examined more than one structure type wererepresented in multiple boxes of the conceptual model, as appropriate

S186

Estuaries and Coasts (2018) 41 (Suppl 1):S180–S196

included sill, revetment, and riprap installations in low-energy
environments, whereas Box 1b primarily included studies ofbulkheads and impoundments designed to stop water flow.
Studies in the medium-energy environments were split fairly
evenly between structures designed to slow vs. stop water(Table 1). Those in Box 2a of the model included studies ofdetached breakwaters in harbors and bays, whereas those inBox 2b included studies of seawalls, bulkheads, and shoreline
revetments. For high-energy environments, there were morestudies of structures designed to stop water flow (Table 1).
Box 3a of our model included studies of detached breakwatersthat were mostly conducted along open sandy coastlines whileBox 3b covered studies of seawalls and massive revetments
on open coast sandy beaches.
We identified results that covered all of the six categories of
ecological response variables (E1–E6), indicating a surprisingly wide range of investigations of the ecological impacts
of armoring (Table 2). However, a preponderance of these was
focused on alterations in E2 (species assemblage (94)),
followed by E1 (habitat distribution (57)). There were far fewer studies that evaluated the responses to armoring in regard toE3 (trophic structure (18)), E4 (nutrient cycling (18)), E5 (productivity (13)), and E6 (connectivity (7)). Below, we summarize the results for each of the ecological responses and thencompile the information into an overview of positive and negative effects across all categories. Mixed results were rare andonly reported in three studies. A list of the individual papers
included in this analysis along with their assigned boxes, ecological response variables, and significant effects can be foundin Table S2.
E1: Habitat Distribution
The effects of coastal armoring on habitat distributions andavailability were well represented in the literature review witha total of 57 observations, with studies measuring effects interms of intertidal zone widths and distributions, habitat characteristics (e.g., depth, elevation, slope, and grain size), andnursery and nesting habitat for birds, fish, and sea turtles aswell as the provision of novel hard substrate habitats for epifauna. The majority of these observations were reported for
low-energy environments (33 in salt marsh and tidal creeks
and 1 in mangroves), but results were available for mediumenergy environments (14) and high-energy environments (9).
Similar numbers of studies evaluated armoring structures
placed to slow (30) as opposed to stop (27) water flow for this
response (Fig. 4). Of the 57 observations related to effects onhabitat associated with armoring, 38 were negative, 17 werepositive, and 2 were detected to have no difference.
The large numbers of studies of armoring effects on habitat
distribution was spread among all six boxes of our conceptualmodel (Fig. 4). Where structures were installed to slow waterin low-energy environments, Box 1a, a mix of positive andnegative responses was reported. Multiple studies concluded
that adding constructed oyster reefs, living shorelines, or permeable riprap armoring structures provided significant new
habitat area (e.g., Davis et al. 2002. Piazza et al. 2005,
Swann 2008, Powers et al. 2009, Scyphers et al. 2014,
Gittman et al. 2016a). However, a myriad of negative observations was also reported for Box 1a. Armoring, including
riprap and marsh impoundments, eliminated habitat, reduced
habitat quality, or provided habitat suitable for invasive species (Hendon et al. 2000; Peterson et al. 2000; Boys et al.
2012; Geraldi et al. 2014; Lowe and Peterson 2014; Patrick
et al. 2014, 2016). For Box 1b of our model, where bulkheads
and seawalls were put in place to stop the flow of water in lowenergy environments, all observations were negative except
for one where a bulkhead was reported to provide new habitatfor epifaunal communities (Wong et al. 2011). In all otherstudies of salt marshes and tidal creeks in which the armoring
structure was designed to stop water flow (Box 1b), studies
reported that habitat was lost, habitat quality was reduced, orTable 2 Distribution of study results from the literature review that were reported as significantly positive, negative, and not significant (NS) for eachof the six ecological responses (E1–E6) in each of the six boxes of our conceptual model (1a, 2a, 3a, 1b, 2b, and 3b)
Direction of response

E1: Habitat Distribution
E2: Species Assemblage
E3: Trophic Structure
E4: Nutrient Cycling
E5: Productivity
E6: Connectivity
Total

Box 1aBox 1bBox 2aBox 2bBox 3aBox 3b

+

−

NS

+

−

NS

+

−

NS

+

−

NS

+

−

NS

+

−

ND

10
12
3
1
1
0
27

8
15
2
8
5
1
39

2
4
1
1
1
0
9

1
1
1
1
1
0
5

13
18
4
4
2
1
42

0
0
0
0
1
0
1

4
3
0
0
0
0
7

3
8
0
1
0
1
13

0
1
0
0
0
0
1

1
2
0
0
0
0
3

6
14
3
1
0
3
27

0
0
1
1
0
0
2

1
1
1
0
0
0
3

2
5
0
0
0
0
7

0
0
0
0
0
0
0

0
0
0
0
0
0
0

6
9
2
0
2
1
20

0
1
0
0
0
0
1

Numerous studies reported multiple ecological effects which resulted in the total sample size (n = 207) presented hereEstuaries and Coasts (2018) 41 (Suppl 1):S180–S196

S187

Number of observations

High
MediumLow

Hydrodynamic Energy

E1: Habitat distribution
20
15
10
5
0

Box 3a

1

2

0

20
15
10
5
0

PosNeg NS
20
15
10
5
0

Box 2a4

30

Box 1a10

82

PosNeg NS

600

PosNeg NS
20
15
10
5
0

Box 2b6
10

PosNeg NS

PosNeg NS
20
15
10
5
0

Box 3b

20
15
10
5
0

Box 1b13
10

PosNeg NS

Direction of response

Slow
Stop
Effect on Water Flow
Fig. 4 Ecological effects on habitat distribution (E1) reported in studies
included in our literature review. The histograms correspond to the six
combinations of intended effects of an armoring structure on water flow
and the hydrodynamic energy of the environment represented by theboxes in the conceptual model (Fig. 2). In the histogram for each box,
the number of significantly positive (green), negative (red), and not significant (NS) (gray) observations is plotted

the armoring structures provided habitat for undesirable invasive species (e.g., Bozek and Burdick 2005; Baily and Pearson
2007; Freiss et al. 2008; McPherson 2009; Balouskus andTargett 2012; Gittman et al. 2016b). Similarly, seawalls inmangrove ecosystems resulted in reduced mangrove forest
habitat area (Heatherington and Bishop 2012).
For Box 2a of our model, where structures were put inplace to slow water in medium-energy environments, effects
on habitat were again mixed. Permeable structures such as
riprap structures, oyster reefs, and constructed habitat benches
provided habitat, often by increasing the availability of structurally complex habitat (Toft et al. 2007, 2013; Pister 2009;
Drexler et al. 2014). Other studies, however, found that riprap
structures eliminated soft sediment intertidal and benthic habitat (Sobocinski et al. 2010; Heerhartz et al. 2014; Dethieret al. 2016). For Box 2b, where seawalls and bulkheads
were used to stop water flow, all observations but one were
negative. Although Drexler et al. (2014) found that seawalls
provided habitat for oysters, many other studies found thatthese structures generally reduced habitat (Bilkovic andRoggero 2008; Sobocinski et al. 2010; Heerhartz et al. 2014;
Dethier et al. 2016) unless remedial actions were undertaken
to increase habitat complexity (Browne and Chapman 2011,2014).
For structures installed to slow water in high-energy environments (Box 3a), Martin et al. (2005) concluded that a lowcrested breakwater structure provided new habitat, but others
reported that breakwaters, revetments, and low-crested structures reduced structural complexity and eliminated habitat(Moschella et al. 2005; Vaselli et al. 2008). For Box 3b ofour model, several studies found that seawalls reduced oreliminated intertidal and upper shore and coastal dune habitats
in sandy beach ecosystems (Dugan and Hubbard 2006; Duganet al. 2008; Jaramillo et al. 2012; Rodil et al. 2015). Negative
results were also reported where armoring reduced the qualityof critical beach nesting habitats for sea turtles, a globallythreatened group (Rizkalla and Savage 2011).
When taken together, the majority of the results for habitat
distribution (72%) were negative, particularly for structuresdesigned to stop the flow of water (92% overall), a result inagreement with predictions of our conceptual model. These
negative results were most commonly associated with the lossof habitat area and reduced habitat quality. For structuresintended to slow the flow of water, positive results made up
50% of the results in Box 1a, 57% for Box 2a, and 33% in Box
3a. Most of these were associated with constructed oyster
reefs and living shorelines that provided new habitat for native
species.
E2: Species Assemblages
Effects on species assemblages were the most commonly
documented ecological response to shoreline armoring in
our review, with a total of 94 observations. The majority ofthese were in low-energy habitats (47 in salt marsh andtidal creek ecosystems, 3 in mangroves), with 25 inmedium-energy habitats and only 16 observations inhigh-energy open coast environments. Approximately
equal numbers of studies evaluated structures placed toslow (49) as opposed to stop (45) water flow.
A majority of the significant responses of species assemblages to armoring were considered negative (69), with only
19 reports of positive responses and 6 reports of no significant
differences detected (Fig. 5). When distributed across the boxes of our conceptual model, we found that most of the positive
responses were observed for Box 1a (structures designed toslow water in low-energy environments). Positive results included increases in epiphyte and epifaunal abundance anddiversity on the structures themselves (e.g., Wong et al.
2011; Peters et al. 2015), particularly for oysters (e.g., Piazza
et al. 2005; Powers et al. 2009; Scyphers et al. 2011), as wellas increases in other invertebrates and in fish on living

S188

Estuaries and Coasts (2018) 41 (Suppl 1):S180–S196

E2: Species assemblage

Number of observations

High
MediumLow

Hydrodynamic Energy

Box 3a
20
15
10
5
0

Box 3b

5
1

0

20
15
10
5
0

Box 2aBox 2b

8
3

1

20
15
10
5
0

PosNeg NS

12

14
2

0

PosNeg NS

Box 1bBox 1a
20
15
10
5
0

1

0

PosNeg NS

PosNeg NS
20
15
10
5
0

9

15
4

PosNeg NS

20
15
10
5
0

18

1

0

PosNeg NS

Direction of response

Slow

Stop
Effect on Water Flow
Fig. 5 Ecological effects on species assemblages (E2) reported in studies
included in the literature review. The histograms correspond to the six
combinations of intended effects on water flow and the hydrodynamic
energy of the environment and represented by the boxes in the conceptualmodel (Fig. 2). In the histogram for each box, the number of significantly
positive (green), negative (red), and not significant (NS) (gray) observations is plottedshorelines (Gittman et al. 2016a). However, the majority ofobservations in Box 1a were negative and included decreased
species diversity and/or abundance for a wide range of assemblages including microbial communities (Bernhard et al.
2012), primary producers (e.g., Sturdevant et al. 2002;
O’Connor et al. 2011), infaunal invertebrates (e.g., Peterson
et al. 2000; Seitz et al. 2006; Bilkovic and Mitchell 2013),
nekton and fish (Bilkovic 2011; Boys et al. 2012; Lowe andPeterson 2014, 2015) and waterbirds (Bolduc and Afton
2003). In Box 1b, negative responses to armoring dominatedthe results with decreases in diversity and abundance reported
for mangroves (Anthony and Gratiot 2012; Heatherington andBishop 2012), salt marsh vegetation (Bozek and Burdick2005), invertebrates (Seitz et al. 2006; Lawless and Seitz
2014; Swamy et al. 2002), and nekton and fish (e.g.,
Balouskus and Targett 2016; Lowe and Peterson 2014, 2015).
For medium-energy environments, studies of structures designed to slow water (Box 2a) had mixed results, but again,

negative impacts made up the majority (75%) of the reports,
including impacts on invertebrates (Morley et al. 2012;
Dethier et al. 2016) and on nekton and fish (Scyphers et al.
2015; Torre and Targett 2016). The few positive results wereprimarily associated with epifauna on the armoring structures
themselves (Toft et al. 2013; Drexler et al. 2014) or in one casefish (Toft et al. 2007). For Box 2b, observations were almost
entirely negative, with bulkheads and seawalls, resulting in
reductions in invertebrates (Sobocinski et al. 2010; Rolet
et al. 2015; Dethier et al. 2016; Heerhartz et al. 2016), fish(e.g., Bilkovic and Roggero 2008; Munsch et al. 2014;
Scyphers et al. 2015), and even terrapins (Isdell et al. 2015).
Effects on species distribution were again almost entirely
negative for armoring structures in high-energy environments(Boxes 3a and 3b). Studies classified in Box 3a found that thepresence of armoring depressed invertebrate diversity andabundance (e.g., Moschella et al. 2005; Martins et al. 2009;
Bacchiocchi and Airoldi 2003) and also facilitated invasive
species (Vaselli et al. 2008). The sole positive result for Box
3a was reported for benthic diversity and for fish associatedwith offshore breakwaters (Martin et al. 2005). For Box 3b, allresults were negative including responses by coastal dune
plants (Rodil et al. 2015), infaunal invertebrates (e.g.,
Lucrezi et al. 2010; Jaramillo et al. 2012), and birds (Duganet al. 2008).
Armoring was associated with declines in both species diversity and species abundance across all soft sediment environments and structure types. The majority of observations inall six boxes of our conceptual model were negative, andpositive results were most often associated with structuresdesigned to slow water. Although our synthesis is limited by
the literature available, this outcome is in general agreementwith the predictions of our conceptual model, with negative
results predominating (≥86%) in studies of armoring structures designed to stop water flow at all hydrodynamic energy
levels (Boxes 1b, 2b, and 3b). For the structures designed toslow water flow, positive results made up 39% of the reports
in Box 1a, 25% in Box 2a, and 17% in Box 3a.

E3: Trophic Structure
Trophic structure and food webs were among the least studied
ecological response to armoring found in our review (Table 2).
Studies included in this category evaluated variables such as
the number of trophic categories, prey availability, shifts indiet, and predator abundance, including fish, birds, and marine
mammals. A total of only 18 trophic structure effects wereidentified, with the majority of studies occurring in lowenergy habitats (10 in salt marshes or tidal creeks and 1 inmangroves), 4 in medium-energy habitats and 3 in highenergy beaches. More of these studies evaluated structures
placed to stop (11) as opposed to slow (7) water. A majority

Estuaries and Coasts (2018) 41 (Suppl 1):S180–S196

Box 3aBox 3b
+

-

NS

6%

High

22%

78%

94%

Box 2aBox 2b5%

MediumHydrodynamic Energy

13%
37%
58%
87%

Box 1aBox 1b
6%

12%

Low

of the significant responses were considered negative (11),with only 5 reports of positive responses.
Across the boxes of the conceptual model (Table 2), we
found mixed results for Box 1a: salt marsh habitats with riprap
armoring and sills in the Chesapeake Bay were found to havefewer trophic levels (e.g., Bilkovic and Mitchell 2013).
However, living shorelines (Gittman et al. 2016a) and sillsin North Carolina (Wong et al. 2011) were reported to maintain higher trophic levels and oyster reefs were associated withincreased prey for fishes (Grabowski et al. 2005). For Box 1b,
the presence of bulkheads and levees were found to reduce
prey availability and result in diet shifts for nekton in marshes(e.g., Lowe and Peterson 2015), although Wong et al. (2011)
reported that bulkheads had a positive effect due to epifaunalcolonization. For Box 2b, Munsch et al. (2015) documenteddifferent food availability and consumption by juvenile
salmon adjacent to seawalls, and Jackson et al. (2015) foundthat shorebirds preferred to forage at unarmored sites. There
was only one study in Box 3a: Martin et al. (2005) observed
an increase in the number of trophic groups (fish) near a lowcrested armoring structure. For Box 3b, significantly reduced
diversity (50% lower) and abundance (66% lower) of shorebirds, key intertidal predators, as well as 75% fewer gulls and
86% fewer seabirds were reported on California beaches
where seawalls were present (Dugan and Hubbard 2006;
Dugan et al. 2008).
Although there are relatively few observations available fortrophic structure and food web responses to shore-parallel
armoring, the majority of results (≥75%) in studies ofarmoring structures designed to stop water flow (Boxes 1b,
2b, and 3b) were negative, whereas positive results comprised
half of the results in Box 1a and 100% in Box 3a (Table 2), an
outcome in general agreement with the predictions of ourconceptual model.

S189

43%
45%

Slow

94%

Water Flow

Stop

Fig. 6 Summary of the direction of ecological responses (+/−) of
combined observations for habitat distribution (E1) and species assemblages (E2) reported in studies in the literature review. The pie charts
correspond to the six combinations of intended effects on water flow
and the hydrodynamic energy represented by the boxes in the conceptualmodel (Fig. 2). In the pie chart for each box, the percentage of significantly positive (green), negative (red), and not significant (NS) (gray)observations are plotted

E4: Nutrient Cycling
The effects of coastal armoring on nutrient cycling have notbeen widely documented, with a total of only 18 reports in ourreview (Table 2). The response variables considered in thestudies included nutrient concentrations, rate measurements(i.e., denitrification), organic matter composition, and oxygen
levels. The majority of these observations were studies of lowenergy environments (15 in salt marshes and tidal creeks and 1
in mangroves) with only a few in medium-energy habitats (2)and none for higher-energy open coast environments. Most ofthese observations were for armoring structures placed to slow(11) as opposed to stop (7) water. A majority of the significant
responses to armoring related to nutrient cycling were considered negative (14), with only 2 reports of positive responses.
Across the boxes of our conceptual model, we found thehighest number of observations for Box 1a (Table 2). These
were primarily negative. For example, salt marsh

impoundments were associated with declines in dissolved oxygen levels and salinity (Bolduc and Afton 2004) and lower
rates of nutrient accumulation (Sturdevant et al. 2002).
Several studies documented reductions in sediment organic
matter or total organic carbon and nitrogen in association withdifferent armoring structures (sills, revetments, and impoundments) in salt marshes (e.g., Bryant and Chabreck 1998;
Peterson et al. 2000; Currin et al. 2008; Bilkovic and
Mitchell 2013). In Box 1b, lower organic carbon concentrations were again observed in association with structures(Peterson et al. 2000), and Windham-Myers et al. (2013)
found that the reduction in tidal flushing created by the presence of an impoundment resulted in anoxia and the buildup of
reduced sulfur. Working in a mangrove system, Dick andOsunkoya (2000) found reduced leaf litter decomposition
and greater retention of carbon, nitrogen, and phosphorus on
the landward side of tidal floodgate structures. Invasive algae

S190

growing on bulkheads and revetments in salt marshes wereassociated with increased N2 production and represented thepositive observations reported for Boxes 1a and 1b (Geraldi
et al. 2014). There was only one study for Box 2a, wherein
Morley et al. (2012) saw an increase in temperature atarmored sites, and there was one study for Box 2b,wherein lower sediment organic carbon was observed inassociation with areas with more shoreline armoring in adeveloped river estuary (Partyka and Peterson 2008).
Although there are relatively few observations available for
evaluating the response of nutrient cycling to armoring, themajority of results (>80%) were negative for studies ofarmoring structures designed to either stop or to slow waterin medium-energy environments (Boxes 2a and 2b). The few
positive results were reported only for low-energy environments (Boxes 1a and 1b) (Table 2).
E5: Productivity
The effects of coastal armoring on productivity were not well
represented in the literature review, with a total of only 13
observations (Table 2). These studies measured effects including the primary production of macroalgae, plants, andmicrophytobenthos and the secondary production of higherorganisms. The majority of these observations were reported
for low-energy environments (10 in salt marsh and tidal creeks
and 1 in mangroves) with none in medium-energy environments and only 2 in high-energy beach environments(Table 2). The dominance of primary producers in salt marshes
compared to beaches and other shore types may explain someof this disparity. These studies were balanced between
armoring structures placed to slow (7) as opposed to stop (6)water. For the limited number of studies (13) examining theeffects of shoreline armoring on productivity, the majority of
results (69%) were negative (9), 2 were positive, and 2 were
found to have no change (Table 2). All the positive and nochange results for this ecological response category were observed in low-energy environments (Boxes 1a and 1b).
Although there were relatively few observations ofarmoring effects on productivity, four of the six boxes fromour conceptual model (Table 2) were represented in the literature review. For Box 1a, riprap, sill, and impoundment structures were associated with reduced productivity and biomassof plants and algae (O’Connor et al. 2011; Sturdevant et al.
2002) and reduced cover, growth, and biomass of marsh grass(Spartina spp.) (Currin et al. 2008; Bilkovic and Mitchell2013). Reductions in productivity in salt marsh (Freiss et al.
2008) and mangrove (Heatherington and Bishop 2012) ecosystems were also associated with seawalls (Box 1b).
However, Wong et al. (2011) observed positive responses toarmoring in low-energy habitats, reporting that the presence ofboth sills (Box 1a) and bulkheads (Box 1b) led to greater
secondary production in salt marshes in North Carolina thanEstuaries and Coasts (2018) 41 (Suppl 1):S180–S196

in habitats without the added structure. The single study for a
sandy beach (Rodil et al. 2015) concluded that the presence ofboth revetments and seawalls (Box 3b) limited the growth anddevelopment of coastal strand and dune vegetation.

E6: Connectivity
Despite the importance of connectivity, this was the least documented ecological response in our literature review, with a
total of only seven observations (Table 2). The studies included in this ecological response evaluated effects of armoring on
the exchange of materials, mobile organisms, and propagules
across shore zones and ecosystems. The majority of theseobservations were reported for low- and medium-energy environments, with one in salt marshes and tidal creeks and one
in mangroves, four in medium-energy habitats, and one in ahigh-energy beach environment. Most of these studies evaluated armoring structures placed to stop (five) as opposed toslow (two) water (Table 2). All observations related to effectsof shoreline armoring on connectivity were negative, including those for structures designed to stop or to slow water flow.
Although there were relatively few observations ofarmoring effects on connectivity, five of the six boxes fromour conceptual model (Table 2) were represented. The addition of armoring structures in low-energy environments
to limit the flow of water (Box 1a) restricted the passage offish and crustaceans (Boys et al. 2012). In Box 1b, theaddition of a seawall reduced availability and prevented
movement of propagules in a mangrove ecosystem(Anthony and Gratiot 2012). In medium-energy soft sediment environments, Box 2a, the only study available foundthat riprap armoring significantly limited material transferfrom adjacent marine and terrestrial habitats to the shoreline (Heerhartz et al. 2014). For Box 2b, results were again
negative, indicating that bulkheads and seawalls limited
material transfer and restricted the movement of fish species, including economically valuable salmon (Heerhartz
et al. 2014; Munsch et al. 2014; Heerhartz and Toft
2015). In the one example for a high-energy environment(Box 3b), seawalls were found to eliminate the upper intertidal zones of sandy beaches, thereby reducing materialtransfer and retention in the form of marine macrophyte
drift (Dugan and Hubbard 2006). These results indicate
the presence of an armoring structure can prevent thepassage of organisms, and in many cases it also reduces
the deposition and retention of drift material and key
subsidies, such as macrophyte wrack.
Although only a few studies examined the effects ofshoreline armoring on connectivity, the negative effect ofa loss of connectivity across zones and ecosystems andthe associated habitat fragmentation and restriction oflandward movement was reported in all soft sedimentEstuaries and Coasts (2018) 41 (Suppl 1):S180–S196

environments and in all of the boxes with results represented in our conceptual model.

Summary of Ecological Effects
A total of 207 effects were evaluated across the six boxes of
the conceptual model. They were split fairly evenly between
results of studies of structures designed to slow water (106)and those designed to stop it (101). Although the majority ofstudies were conducted in low-energy environments (75 in
Box 1a and 48 in Box 1b), all boxes of the conceptual model
were represented (Table 2). Across all 207 effects evaluated,
71% were reported to be significantly negative, 22% weresignificantly positive, and only 7% were not significant(Table 2).
Habitat distribution (E1) and species assemblage (E2) were
the two ecological responses with sufficient results to allow
comparison across all six boxes of the conceptual model(Figs. 4 and 5). These two ecological effects were also often
closely related (i.e., when there is a shift in habitat that often
has an effect on species assemblage) due to the important
influence of habitat on species distributions. When we combined the results for these two responses to provide a broader
summary (Fig. 6), the percentage of negative responses reported within each hydrodynamic energy category were greaterfor structures designed to stop water flow (Boxes 1b, 2b, and3b) (16 to 49% greater) than for those designed to slow waterflow (Boxes 1a, 2a, and 3a). The percentage of negative responses reported also increased with increasing hydrodynamic energy for structures designed to slow water flow (45% in
Box 1a, 58% Box 2a, and 78% in Box 3a) but were more
uniformly high for those designed to stop water flow acrossenvironments (94% in Box 1b, 87% in Box 2b, and 94% in
Box 3b) (Fig. 6).
The percentage of positive ecological effects for the
combined results for E1 and E2 were largely the converseof the negative results. Within a given hydrodynamic energy environment, the percentage of positive results weregreater for structures built to slow as opposed to stop or
prevent water flow (Fig. 6). For structures designed to slow
water flow, the percentage of positive ecological effects
clearly declined with increasing hydrodynamic energy,
from 42% in low- to 22% in high-energy environments.
This trend for positive effects was less clear for structuresdesigned to stop water flow as all boxes had a very low
percentage of positive effects, and no positive results werereported in high-energy environments. Many of the positive results reported were from studies of constructed oyster reef and living shoreline structures, although a numberof positive effects we tallied were associated with the colonization of new and novel hard substrate habitats provided by armoring or shoreline protection structures.

S191

Collectively, the combined results for E1 and E2 in Fig. 6
were consistent with our predictions that the ecological effectsof shoreline armoring would be greater for structures designed
to stop as opposed to slow water flow and provide some evidence that ecological effects may intensify with increasinghydrodynamic energy of the environment or setting. They alsosuggest that the purpose of the structure with respect to waterflow has a greater effect on ecological responses than thehydrodynamic energy of the soft sediment environment.

Discussion
As indicated by the number of recent papers in our literature review (as well as the other papers included in thisspecial issue), the ecological effects of shoreline hardening
are receiving increased attention. Placing this informationin the framework of our conceptual model enabled us toscale the ecological effects of shore-parallel armoring andallowed comparisons across a range of soft sediment ecosystems and armoring structures. However, our review revealed major gaps in knowledge and highlighted the factthat existing information on ecological responses toarmoring is unevenly distributed across habitat types anddoes not necessarily cover the range of potential environmental and armoring contexts. We found the majority ofstudies have been conducted in low-energy systems, particularly salt marshes, with much less attention to beaches
and open coast shores. There was also a notable dearth ofstudies in mangrove systems. The distribution of studies
across the various ecological responses were largely focused on changes in habitat and species distribution, leaving crucial gaps in our understanding of how the presenceof shoreline armoring affects key ecological responses ofnutrient cycling, connectivity, productivity, and trophic
structure. Filling these gaps will allow a far more complete
evaluation and synthesis of the ecological responses toshoreline armoring than was possible here.
Despite the gaps in knowledge, the majority of studies in
our literature review reported significantly negative effects ofshoreline armoring in all six categories of ecological responses that we evaluated. Shoreline armoring of a wide arrayof structure types resulted in habitat loss, shifts in species
assemblages and trophic structure, changes in nutrient cycling, reduced productivity, and the loss of connectivity in soft
sediment environments across all boxes of our conceptualmodel. Negative effects of armoring on habitat and connectivity have the potential to trigger impacts in all the other ecological responses we evaluated.
Reported positive effects of armoring were far fewer andwere less evenly distributed across our six ecological response
categories and the boxes of the conceptual model. Althoughlow, the proportion of positive effects reported was generally

S192

higher in the ecological response categories of habitat distribution (E1), species assemblages (E2), and trophic structure(E3) than in the other three categories (E4–E6) (Table 2). Thispattern seems consistent with the colonization and use of thenovel hard substrate habitats provided by armoring structuresin otherwise soft-bottomed ecosystems by a variety of organisms that prefer hard substrates (e.g., Meyer et al. 1997; Daviset al. 2002; Swann 2008; Browne and Chapman 2011).
However, facilitation of species distributions by armoring
structures can be ecologically negative if non-native invasive
species are involved because such species may preferentiallyuse artificial structures as stepping stones, potentially increasing their spread to new areas (e.g. Airoldi et al. 2005, Bulleri
and Airoldi 2005, Tyrell and Byers 2007).
Placing the results from our literature review in the framework of our conceptual model enabled us to coarsely scale theecological effects of armoring and allowed comparisonsacross a range of soft sediment ecosystems and structures.
The percentage of negative responses varied clearly with theintended purpose of armoring structures on water flow, increasing from those designed to slow water flow to those
designed to stop water flow within a given hydrodynamic
energy level. The distribution of results among the six boxes
in our conceptual model was consistent with our predictionthat the ecological effects of shoreline armoring would begreater for structures designed to stop as opposed to slow
water. Although less clear-cut, there was also evidence that
ecological effects may intensify with increasing hydrodynamic energy of the environment. Overall, our results suggest thatthe purpose of the structure with respect to water flow has agreater effect on ecological responses than the hydrodynamic
energy of the soft sediment environment (Table 2, Fig. 6). Thisfinding has potential implications for refining the design andpermeability of armoring structures in ways that can reduce
ecological impacts, particularly in low-energy environments.
One of the limitations of the results reported here is that our
synthesis relies on the reported significance of responses instudies with a wide range of sample sizes. Using effect size,
which takes sample size and variance into account, can provide
a normalized measure that can be more quantitatively compared across studies. The recent paper by Gittman et al.
(2016b), which compared effect sizes for ecological responses
to three armoring structure types (breakwaters, riprap revetments, and seawalls), concluded that greater ecological impacts
on biodiversity and abundance were associated with seawalls
compared to revetments and breakwaters. This result is inagreement with that predicted by our conceptual model for
structures designed to stop vs. slow water flow. However, theirmeta-analysis did not address any possible differences withrespect to the different hydrodynamic energy levels of soft sediment environments affected by armoring. In our review, theecological response of species assemblages (E2) was the only
category with sufficient data to allow comparisons of effect

Estuaries and Coasts (2018) 41 (Suppl 1):S180–S196

sizes across most of our conceptual model (five of six boxes)(see Table S3 for complete results). We found the lowest effectsizes for armoring structures in low-energy environments(Boxes 1a and 1b) with two- to five-fold higher effect sizes inmedium- and high-energy environments, a result that is broadly
consistent with our predictions. These results, along with thoseof Gittman et al. (2016b), suggest that comparing effect sizes
from studies designed to make common measurements acrossall six boxes of our conceptual model could advance synthesis
and allow more general predictions of ecological responses toarmoring across soft sediment ecosystems and structure types.
Another refinement of our conceptual model would be toincorporate quantitative information on permeability and hydrodynamic energy of armoring structures. We divided ourconceptual model into six boxes for heuristic purposes but
recognize that both axes are continuous variables that can bescaled in terms of water flow (i.e., m3 s−1) and energy (i.e.,
kW m−1). This refinement would allow one to focus more
precisely on the hydrodynamic energy at the structure andhow impacts might be influenced by characteristics, such as
tidal elevation of the structure. For example, the lower anarmoring structure is located with respect to high water levels,
the greater the associated physical impacts (Weggel 1988,
Wiegel 2002a, b, c). Our conceptual model would predict
ecological effects to scale similarly with decreasing intertidal
elevation of the structure, which would move it up the hydrodynamic energy axis and consequently magnify the effects itexerts on the coastal ecosystem. This also implies that asexisting armoring structures effectively move lower on the
shore profile with rising sea level, their ecological impacts
would be expected to increase. Considering additional attributes of armoring structures such as size, construction material (e.g., Nordstrom 2014) or the amount of surface area thatis partially or completely submerged would provide fruitfulways to further refine and increase the specificity of the predictions of our conceptual model.
This effort provides a needed first step in generating discussion and motivating synthesis that can lead to a comprehensive
framework for scaling the ecological effects of shoreline
armoring across a range of coastal soft sediment ecosystems.
The conceptual model allowed us to evaluate predictions regarding the direction and relative ecological impacts of shoreparallel armoring structures in different soft sediment environments based on relatively simple criteria. The results of ourliterature search were largely consistent with the predictionsof our conceptual model and suggest that such crossenvironment generalizations are possible and may have implications for balancing the protection of coastal infrastructurewith the conservation of coastal ecosystems. However, ouranalysis also highlights substantial research gaps and the needfor comprehensive studies designed to make systematic comparisons of the ecological effects of shoreline armoring acrossstructure types and environments. The results from these types

Estuaries and Coasts (2018) 41 (Suppl 1):S180–S196

of comprehensive efforts could be useful for assessing the relative ecological costs of various approaches to shoreline
armoring and for informing the development of strategies tominimize their impacts on coastal ecosystems (Nordstrom
2014, 2016). Increasing the ability to generalize ecological responses to shoreline armoring across soft sediment coastal ecosystems and structure types is especially important, given thatthe motivation to build additional armoring in soft sedimentenvironments is expected to continue to increase in response
to sea level rise, coastal development, and other pressures.
Acknowledgements This study was supported by an LTER Network
synthesis working group award from the National Science Foundationand made possible by participants from the Georgia Coastal Ecosystems,
Virginia Coast Reserve, Santa Barbara Coastal, Plum Island Ecosystems,
and Florida Coastal Everglades Long Term Ecological Research (LTER)
projects. We thank Karen McGlathery and the staff of the Virginia Coast
Reserve LTER and the Anheuser-Busch Coastal Research Center,
University of Virginia, and the staff of the Skidaway Institute of
Oceanography, University of Georgia and the Georgia Coastal
Ecosystems LTER for hosting the working group meetings. We are gratefulto the LTER Network Office for the logistical support for the working
group meetings. We gratefully acknowledge the National Science
Foundation’s support of the Long Term Ecological Research programs ofSanta Barbara Coastal LTER (Award No. OCE-1232779), the Georgia
Coastal Ecosystems LTER (Award No. OCE 12-37140), and the Virginia
Coast Reserve LTER (Grant No. 1237733). We thank the National Science
Foundation (Award No. OCE 14-58845) for support during manuscript
preparation. Any opinions, findings, and conclusions or recommendations
expressed in this material are those of the authors and do not necessarilyreflect the views of the National Science Foundation (NSF).

Open Access This article is distributed under the terms of the Creative
Commons Attribution 4.0 International License (http://creativecommons.
org/licences/by/4.0/), which permits use, duplication, adaptation, distribution
and reproduction in any medium or format, as long as you give appropriate
credit to the original author(s) and the source, provide a link to Creative
Commons license and indicate if changes were made.

References
Airoldi, Laura, Marco Abbiati, Michael W. Beck, Stephen J. Hawkins,
Per R. Jonsson, Daniel Martin, Paula S. Moschella, Andreas
Sundelöf, Richard C. Thompson, and Per Åberg. 2005. An ecological perspective on the deployment and design of low-crested andother hard coastal defence structures. Coastal Engineering 52:
1073–1087.
Anthony, Edward J., and Nicolas Gratiot. 2012. Coastal engineering andlarge-scale mangrove destruction in Guyana, South America:
averting an environmental catastrophe in the making. EcologicalEngineering 47: 268–273.
Bacchiocchi, Francesca, and Laura Airoldi. 2003. Distribution and dynamics of epibiota on hard structures for coastal protection.
Estuarine Coastal and Shelf Science 56: 1157–1166.
Baily, Brian, and Alistair W. Pearson. 2007. Change detection mapping
and analysis of salt marsh areas of central southern England fromHurst Castle Spit to Pagham Harbour. Journal of Coastal Research
23: 1549–1564.

S193
Balouskus, Richard G., and Timothy E. Targett. 2012. Egg deposition by
Atlantic Silverside, Menidia menidia: substrate utilization and comparison of natural and altered shoreline type. Estuaries and Coasts
35: 1100–1109.
Balouskus, Richard G., and Timothy E. Targett. 2016. Fish and blue crab
density along a riprap-sill-hardened shoreline: comparisons with
Spartina marsh and riprap. Transactions of the American Fisheries
Society 145: 766–773.
Bernhard, Anne E., David Marshall, and Lazaros Yiannos. 2012.
Increased variability of microbial communities in restored salt
marshes nearly 30 years after tidal flow restoration. Estuaries andCoasts 35: 1049–1059.
Bilkovic, Donna M. 2011. Response of tidal creek fish communities todredging and coastal development pressures in a shallow-water estuary. Estuaries and Coasts 34: 129–147.
Bilkovic, Donna M., and Molly M. Mitchell. 2013. Ecological tradeoffs
of stabilized salt marshes as a shoreline protection strategy: effects ofartificial structures on macrobenthic assemblages. EcologicalEngineering 61: 469–481.
Bilkovic, Donna M., and Molly M. Roggero. 2008. Effects of coastal
development on nearshore estuarine nekton communities. Marine
Ecology Progress Series 358: 27–39.
Bilkovic, Donna M., Molly Mitchell, Pam Mason, and Karen Duhring.
2016. The role of living shorelines as estuarine habitat conservation
strategies. Coastal Management44:3, 161–174.
Bolduc, Francois, and Alan D. Afton. 2003. Effects of structural marsh
management and salinity on invertebrate prey of waterbirds in marsh
ponds during winter on the Gulf Coast Chenier Plain. Wetlands 23:
897–910.
Bolduc, Francois, and Alan D. Afton. 2004. Hydrologic aspects of marsh
ponds during winter on the Gulf Coast Chenier Plain, USA: effectsof structural marsh management. Marine Ecology Progress Series
266: 35–42.
Boys, Craig A., Frederieke J. Kroon, Tim M. Glasby, Kevin Wilkinson.
2012. Improved fish and crustacean passage in tidal creeks following
floodgate remediation. Journal of Applied Ecology 49(1): 223–233.
Bozek, Catherine M., and David M. Burdick. 2005. Impacts of seawalls
on saltmarsh plant communities in the Great Bay Estuary, New
Hampshire USA. Wetlands Ecology and Management 13: 553–568.
Browne, Mark A., and M. Gee Chapman. 2011. Ecologically informed
engineering reduces loss of intertidal biodiversity on artificial shorelines. Environmental Science & Technology 45: 8204–8207.
Browne, Mark A., and M. Gee Chapman. 2014. Mitigating against theloss of species by adding artificial intertidal pools to existing seawalls. Marine Ecology Progress Series 497: 119–129.
Bryant, John C., and Robert H. Chabreck. 1998. Effects of impoundment
on vertical accretion of coastal marsh. Estuaries 21: 416–422.
Bulleri, Fabio, and Laura Airoldi. 2005. Artificial marine structures facilitate the spread of a non-indigenous green alga, Codium fragile ssp.
tomentosoides, in the north Adriatic Sea. Journal of Applied
Ecology 42: 1063–1072.
Bulleri, Fabio, and Maura G. Chapman. 2010. The introduction of coastal
infrastructure as a driver of change in marine environments. Journalof Applied Ecology 47: 26–35.
Charlier, Roger H., Marie Claire P. Chaineux, and Selim Morcos. 2005.
Panorama of the history of coastal protection. Journal of Coastal
Research 21:(1), 79–111.
Currin, Carolyn A., Priscilla C. Delano, and Lexia M. Valdes-Weaver.
2008. Utilization of a citizen monitoring protocol to assess the structure and function of natural and stabilized fringing salt marshes inNorth Carolina. Wetlands Ecology and Management 16: 97–118.
Davis, J.L.D., L.A. Levin, and S.M. Walther. 2002. Artificial armoredshorelines: sites for open-coast species in a southern California
bay. Marine Biology 140: 1249–1262.

S194
Dethier, Megan N., Wendel W. Raymond, Aundrea N. McBride, Jason D.
Toft, Jeffery R. Cordell, Andrea S. Ogston, Sarah M. Heerhartz, andHelen D. Berry. 2016. Multiscale impacts of armoring on Salish Sea
shorelines: evidence for cumulative and threshold effects. Estuarine
Coastal and Shelf Science 175: 106–117.
Dick, Todd M., and Olusegun O. Osunkoya. 2000. Influence of tidal
restriction floodgates on decomposition of mangrove litter. Aquatic
Botany 68: 273–280.
Drexler, Michael, Melanie L. Parker, Stephen P. Geiger, William S.
Arnold, and Pamela Hallock. 2014. Biological assessment of easternoysters (Crassostrea virginica) inhabiting reef, mangrove, seawall,and restoration substrates. Estuaries and Coasts 37: 962–972.
Dugan, Jenifer E., and David M. Hubbard. 2006. Ecological responses to coastal armoring on exposed sandy beaches. Shore& Beach 74: 10–16.
Dugan, J.E., Airoldi, L., Chapman, M.G., Walker, S. and T.A. Schlacher.
2012. Estuarine and coastal structures: environmental effects: a focus on shore and nearshore structures. In: Human-induced problems(uses and abuses), In: Estuaries and coasts, eds. M. Kennish, M.
Elliot, 17–41. Treatise on Estuarine and Coastal Science Vol. 8
Chapter 2, Elsevier.
Dugan, Jenifer E., David M. Hubbard, Iván F. Rodil, David L. Revell, andStephen Schroeter. 2008. Ecological effects of coastal armoring on
sandy beaches. Marine Ecology 29: 160–170.
US Army Corps of Engineers. 2002. Coastal Engineering Manual.
Engineer Manual 1110: 2–1100.
Fletcher, Charles H., Robert A. Mullane and Bruce M. Richmond. 1997.
Beach loss along armored shorelines on Oahu, Hawaiian Islands.
Journal of Coastal Research v.13, no. 1, 209–215.
Freiss, D., I. Möller and T. Spencer. 2008. Case Study: Managed
Realignment and the Re-establishment of Saltmarsh Habitat,
Freiston Shore, Lincolnshire, United Kingdom. In: The Role ofEnvironmental Management and Eco-Engineering in Disaster Risk
Reduction and Climate Change Adaptation, 65–78. Nyon,
Switzerland: ProAct Network Association.
Geraldi, Nathan R., Ashley R. Smyth, Michael F. Piehler, and Charles H.
Peterson. 2014. Artificial substrates enhance non-native macroalga
and N2 production. Biological Invasions 16: 1819–1831.
Gittman, Rachel K., F. Joel Fodrie, Alyssa M. Popowich, Danielle A.
Keller, John F. Bruno, Carolyn A. Currin, Charles H. Peterson,and Michael F. Piehler. 2015. Engineering away our natural defenses: an analysis of shoreline hardening in the US. Frontiers in
Ecology and the Environment 13: 301–307.
Gittman, Rachel K., Charles H. Peterson, Carolyn A. Currin, F. Joel
Fodrie, Michael F. Piehler, and John F. Bruno. 2016a. Living shorelines can enhance the nursery role of threatened estuarine habitats.
Ecological Applications 26: 249–263.
Gittman, Rachel K., Alyssa M. Popowich, John F. Bruno, and Charles H.
Peterson. 2014. Marshes with and without sills protect estuarine
shorelines from erosion better than bulkheads during a category 1
hurricane. Ocean & Coastal Management 102: 94–102.
Gittman, Rachel. K., Stephen B. Scyphers, Carter S. Smith, Isabelle P.
Neylan, and Jonathan H. Grabowski. 2016b. Ecological consequences of shoreline hardening: a meta-analysis. BioScience 66(9): 763-773.
Grabowski, Jonathan H., A. Randall Hughes, David L. Kimbro, andMargaret A. Dolan. 2005. How habitat setting influences restored
oyster reef communities. Ecology 86: 1926–1935.
Griggs, Gary B. 2005a. California’s retreating coastline: where do we gofrom here. In California and the World Ocean 2002, eds. Orville T.
Magoon, Hugh Converse, Brian Baird, Beth Jines, Melissa MillerHenson, 121–125. Reston, Virginia: American Society of Civil
Engineers.
Griggs, Gary B. 2005b. The impacts of coastal armoring. Shore andbeach 73: 13–22.

Estuaries and Coasts (2018) 41 (Suppl 1):S180–S196
Griggs, Gary B. 2010. The effects of armoring shorelines—the California
experience. In Puget Sound Shorelines and the Impacts ofArmoring—Proceedings of a State of the Science Workshop, eds
Hugh Shipman, Megan Dethier, Guy Gelfenbaum, Kurt L Fresh,
RS Dincola, 77–84. Reston, Virginia: US Geological Survey
Scientific Investigations Report 2010–5254.
Hall, Mary J., and Orrin H. Pilkey. 1991. Effects of hard stabilization on
dry beach width for New Jersey. Journal of Coastal Research v. 7,
no. 3, 771–785.
Hayes, Miles O. 1979. Barrier island morphology as a function of tidal
and wave regime. In Barrier Islands, ed. S.P. Leatherman, 1–27.
New York: Academic.
Heatherington, C., and Melanie J. Bishop. 2012. Spatial variation in the
structure of mangrove forests with respect to seawalls. Marine andFreshwater Research 63: 926–933.
Heerhartz, Sarah M., Megan N. Dethier, Jason D. Toft, Jeffery R. Cordell,and Andrea S. Ogston. 2014. Effects of shoreline armoring on beach
wrack subsidies to the nearshore ecotone in an estuarine fjord.
Estuaries and Coasts 37: 1256–1268.
Heerhartz, Sarah M., and Jason D. Toft. 2015. Movement patterns andfeeding behavior of juvenile salmon (Oncorhynchus spp.) alongarmored and unarmored estuarine shorelines. Environmental
Biology of Fishes 98: 1501–1511.
Heerhartz, Sarah M., Jason D. Toft, Jeffery R. Cordell, Megan N. Dethier,and Andrea S. Ogston. 2016. Shoreline armoring in an estuary constrains wrack-associated invertebrate communities. Estuaries andCoasts 39: 171–188.
Hendon, J. Read, Mark S. Peterson, and Bruce H. Comyns. 2000. Spatiotemporal distribution of larval Gobiosoma bosc in waters adjacent tonatural and altered marsh-edge habitats of Mississippi coastal waters. Bulletin of Marine Science 66: 143–156.
Hieu, Phung Dang, and Phan Ngoc Vinh. 2012. Numerical study of wave
overtopping of a seawall supported by porous structures. Applied
Mathematical Modelling 36: 2803–2813.
Isdell, Robert E., Randolph M. Chambers, Donna M. Bilkovic, andMatthias Leu. 2015. Effects of terrestrial–aquatic connectivity on
an estuarine turtle. Diversity and Distributions 21: 643–653.
Jackson, Nancy L., Karl F. Nordstrom, Sherestha Saini, and David R.
Smith. 2015. Influence of configuration of bulkheads on use ofestuarine beaches by horseshoe crabs and foraging shorebirds.
Environmental Earth Sciences 74: 5749–5758.
Jaramillo, Eduardo, Jenifer E. Dugan, David M. Hubbard, Daniel
Melnick, Mario Manzano, Cristian Duarte, Cesar Campos, andRoland Sanchez. 2012. Ecological implications of extreme events:
footprints of the 2010 earthquake along the Chilean coast. PloS One
7: e35348.
Karim, Mohammed Fazlul, Katsutoshi Tanimoto, and Phung Dang Hieu.
2009. Modelling and simulation of wave transformation in porous
structures using VOF based two-phase flow model. Applied
Mathematical Modelling 33: 343–360.
Kraus, Nicholas C., and William G. McDougal. 1996. The effects ofseawalls on the beach: part I, an updated literature review. Journalof Coastal Research v. 12, no. 3, 691–701.
Lawless, Amanda S., and Rochelle D. Seitz. 2014. Effects of shoreline
stabilization and environmental variables on benthic infaunal communities in the Lynnhaven River System of Chesapeake Bay.
Journal of Experimental Marine Biology and Ecology 457: 41–50.
Lowe, Michael R., and Mark S. Peterson. 2014. Effects of coastal urbanization on salt-marsh faunal assemblages in the northern Gulf of
Mexico. Marine and Coastal Fisheries 6: 89–107.
Lowe, Michael R., and Mark S. Peterson. 2015. Body condition andforaging patterns of nekton from salt marsh habitats arrayedalong a gradient of urbanization. Estuaries and Coasts 38:
800–812.
Lucrezi, Serena, Thomas A. Schlacher, and Wayne Robinson. 2010. Can
storms and shore armouring exert additive effects on sandy-beachEstuaries and Coasts (2018) 41 (Suppl 1):S180–S196
habitats and biota? Marine and Freshwater Research 61: 951–962.
Mallayachari, V., and V. Sundar. 1994. Reflection characteristics of permeable seawalls. Coastal Engineering 23: 135–150.
Martin, Daniel, Fabio Bertasi, Marina A. Colangelo, Mindert de Vries,
Matthew Frost, Stephen J. Hawkins, Enrique Macpherson, Paula S.
Moschella, M. Paola Satta, and Richard C. Thompson. 2005.
Ecological impact of coastal defence structures on sediment andmobile fauna: evaluating and forecasting consequences of unavoidable modifications of native habitats. Coastal Engineering 52:
1027–1051.
Martins, Gustavo M., André F. Amaral, Francisco M. Wallenstein, andAna I. Neto. 2009. Influence of a breakwater on nearby rocky
intertidal community structure. Marine Environmental Research 67:
237–245.
McPherson, Martina L. 2009. Adaptation to sea-level rise in North
Carolina. Masters Thesis, Nicholas School of the Environment,
Duke University, Durham: North Carolina.
Meyer, David L., Edward C. Townsend, and Gordon W. Thayer. 1997.
Stabilization and erosion control value of oyster cultch for intertidal
marsh. Restoration Ecology 5: 93–99.
Miles, J. R., Russell, P. E., and D. A. Huntley. 2001. Field measurementsof sediment dynamics in front of a seawall. Journal of Coastal
Research 17:(1), 195–206.
Morley, Sarah A., Jason D. Toft, and Karrie M. Hanson. 2012. Ecologicaleffects of shoreline armoring on intertidal habitats of a Puget Soundurban estuary. Estuaries and Coasts 35: 774–784.
Moschella, P. S., M. Abbiati, P. Aberg, L. Airoldi, L. M. Anderson, F.
Bacchiocchi, F. Bulleri, G. E. Dinesen, M. Frost, E. Gacia, L.
Granhag, P. R. Jonsson, M. P. Satta, A. Sundelof, R.C. Thompson
and S. J. Hawkins. 2005. Low-crested coastal defence structures asartificial habitats for marine life: using ecological criteria in design.
Coastal Engineering 52: 1053-1071.
Munsch, Stuart H., Jeffery R. Cordell, and Jason D. Toft. 2016. Fine-scale
habitat use and behavior of a nearshore fish community: nursery
functions, predation avoidance, and spatiotemporal habitat
partitioning. Marine Ecology Progress Series 557: 1–15.
Munsch, Stuart H., Jeffery R. Cordell, Jason D. Toft, and Erin E. Morgan.
2014. Effects of seawalls and piers on fish assemblages and juvenile
salmon feeding behavior. North American Journal of Fisheries
Management 34: 814–827.
Neumann, Barbara, Athanasios T. Vafeidis, Juliane Zimmermann, andRobert J. Nicholls. 2015. Future coastal population growth and exposure to sea-level rise and coastal flooding—a global assessment.
PloS One 10: e0118571.
Nordstrom, Karl F. 2000. Beaches and dunes of developed coasts.
Cambridge: Cambridge University Press.
Nordstrom, Karl F. 2014. Living with shore protection structures: a review. Estuarine, Coastal and Shelf Science 150: 11–23.
Nordstrom, Karl F. 2016. Facilitating landform migration by removing
shore protection structures: opportunities and constraints.
Environmental Science & Policy. doi:10.1016/j.envsci.2016.07.012.
O’Connor, Mary I., Christy R. Violin, Andrea Anton, Laura M. Ladwig,
and Michael F. Piehler. 2011. Salt marsh stabilization affects algal
primary producers at the marsh edge. Wetlands Ecology andManagement 19: 131–140.
Partyka, M.L., and M.S. Peterson. 2008. Habitat quality and salt-marsh
species assemblages along an anthropogenic estuarine landscape.
Journal of Coastal Research 1570–1581.
Patrick, Christopher J., Donald E. Weller, Xuyong Li, and Micah Ryder.
2014. Effects of shoreline alteration and other stressors on submerged aquatic vegetation in subestuaries of Chesapeake Bay andthe mid-Atlantic coastal bays. Estuaries and Coasts 37: 1516–1531.
Patrick, Christopher J., Donald E. Weller, and Micah Ryder. 2016. The
relationship between shoreline armoring and adjacent submerged
aquatic vegetation in Chesapeake Bay and nearby Atlantic Coastal
Bays. Estuaries and Coasts 39: 158–170.

S195
Perkins, Matthew J., Terence P.T. Ng, David Dudgeon, Timothy C.
Bonebrake, Kenneth M.Y. Leung. 2015. Conserving intertidal habitats: What is the potential of ecological engineering to mitigate
impacts of coastal structures? Estuarine, Coastal and Shelf Science
167: 504–515.
Peters, Joseph R., Lauren A. Yeager, and Craig A. Layman. 2015.
Comparison of fish assemblages in restored and natural mangrove
habitats along an urban shoreline. Bulletin of Marine Science 91:
125–139.
Peterson, M.S., B.H. Comyns, J.R. Hendon, P.J. Bond, and G.A. Duff.
2000. Habitat use by early life-history stages of fishes and crustaceans along a changing estuarine landscape: differences between
natural and altered shoreline sites. Wetlands Ecology andManagement 8: 209–219.
Piazza, Bryan P., Patrick D. Banks, and Megan K. La Peyre. 2005. The
potential for created oyster shell reefs as a sustainable shoreline
protection strategy in Louisiana. Restoration Ecology 13: 499–506.
Piersma, T. 2009. Threats to intertidal soft sediment ecosystems. In:
Water policy in the Netherlands: integrated management in a densely populated delta, eds. Stijn Reinhard, Henk Folmer, 57–69.
Resources for the Future, Washington D.C.
Pister, Benjamin. 2009. Urban marine ecology in southern California: theability of riprap structures to serve as rocky intertidal habitat. Marine
Biology 156: 861–873.
Pilkey, O.H., Young, R., Longo, N., and A. Coburn. 2012. Rethinking
living shorelines. White paper, Program for the Study of Developed
Shorelines, Western Carolina University, March 1, 2012, https://livingshorelinesacademy.org/index.php/resources/literature/item/
pilkey-et-al-2012.
Powers, Sean P., Charles H. Peterson, Jonathan H. Grabowski, andHunter S. Lenihan. 2009. Success of constructed oyster reefs inno-harvest sanctuaries: implications for restoration. Marine
Ecology Progress Series 389: 159–170.
Reise, Karstan. 2001. Ecological comparisons of sedimentary shores.
Ecological Studies Vol 151. Berlin Heidelberg: Springer-Verlag.
Rippon, Stephen. 2001. The transformation of coastal wetlands: exploitation and management of marshland landscapes in North West
Europe during the roman and medieval periods. London: British
Academy. Oxford University Press, Oxford
Rizkalla, Carol E., and Anne Savage. 2011. Impact of seawalls on loggerhead sea turtle (Caretta caretta) nesting and hatching success.
Journal of Coastal Research 27: 166–173.
Rodil, Iván F., Eduardo Jaramillo, David M. Hubbard, Jenifer E. Dugan,
Daniel Melnick, and Carlos Velasquez. 2015. Responses of dune
plant communities to continental uplift from a major earthquake:sudden releases from coastal squeeze. PloS One 10: e0124334.
Rolet, Céline, Nicolas Spilmont, Dominique Davoult, Eric Goberville,
and Christophe Luczak. 2015. Anthropogenic impact onmacrobenthic communities and consequences for shorebirds innorthern France: a complex response. Biological Conservation
184: 396–404.
Ruggiero, Peter. 2010. Impacts of shoreline armoring on sediment dynamics. In: Puget Sound Shorelines and the Impacts of Armoring—
Proceedings of a State of the Science Workshop, May 2009, eds
Hugh Shipman, Megan Dethier, Guy Gelfenbaum, Kurt L Fresh,
RS Dincola, 179–186. Reston, Virginia: US Geological Survey
Scientific Investigations Report 2010-5254.
Runyan, K., and G.B. Griggs. 2003. The effects of armoring seacliffs on
the natural sand supply to the beaches of California. Journal of
Coastal Research v. 19, no. 2, 336–347.
Scyphers, Steven B., J. Steven Picou, Robert D. Brumbaugh, and Sean P.
Powers. 2014. Integrating societal perspectives and values for improved stewardship of a coastal ecosystem engineer. Ecology andSociety 19: 38.

S196
Scyphers, Steven B., Sean P. Powers, and Kenneth L. Heck Jr. 2015.
Ecological value of submerged breakwaters for habitat enhancement
on a residential scale. Environmental Management 55: 383–391.
Scyphers, Steven B., Sean P. Powers, Kenneth L. Heck Jr., and Dorothy
Byron. 2011. Oyster reefs as natural breakwaters mitigate shoreline
loss and facilitate fisheries. PloS One 6: e22396.
Seitz, R.D., R.N. Lipcius, N.H. Olmstead, M.S. Seebo, and D.M.
Lambert. 2006. Influence of shallow-water habitats and shoreline
development on abundance, biomass, and diversity of benthic preyand predators in Chesapeake Bay. Marine Ecology Progress Series
326: 11–27.
Snelgrove, Paul V.R. 1999. Getting to the bottom of marine biodiversity:
sedimentary habitats. Bioscience 49 (2): 129–138.
Sobocinski, Kathryn L., Jeffery R. Cordell, and Charles A. Simenstad.
2010. Effects of shoreline modifications on supratidal macroinvertebrate fauna on Puget Sound, Washington beaches. Estuaries andCoasts 33: 699–711.
Sturdevant, Angela, Christopher B. Craft, and John N. Sacco. 2002.
Ecological functions of an impounded marsh and three natural estuarine marshes along Woodbridge River, NY/NJ Harbor. Urban
Ecosystems 6: 163–181.
Swamy, Varun, Paul E. Fell, Misha Body, Michael B. Keaney, Mawuli K.
Nyaku, Elizabeth C. Mcilvain, and Alice L. Keen. 2002.
Macroinvertebrate and fish populations in a restored impounded salt
marsh 21 years after the reestablishment of tidal flooding.
Environmental Management 29: 516–530.
Swann, LaDon. 2008. The use of living shorelines to mitigate the effectsof storm events on Dauphin Island, Alabama, USA. In: Mitigating
impacts of natural hazards on fishery ecosystems, ed. K. D.
McLaughlin, 47-57. American Fisheries Society, Bethesda MD.
Toft, Jason D., Jeffery R. Cordell, Charles A. Simenstad, and Lia A.
Stamatiou. 2007. Fish distribution, abundance, and behavior alongcity shoreline types in Puget Sound. North American Journal of
Fisheries Management 27: 465–480.

Estuaries and Coasts (2018) 41 (Suppl 1):S180–S196
Toft, Jason D., Andrea S. Ogston, Sarah M. Heerhartz, Jeffery R. Cordell,and Emilie E. Flemer. 2013. Ecological response and physical stability of habitat enhancements along an urban armored shoreline.
Ecological Engineering 57: 97–108.
Torre, Michael P., and Timothy E. Targett. 2016. Nekton assemblages
along riprap-altered shorelines in Delaware Bay, USA: comparisonswith adjacent beach. Marine Ecology Progress Series 548: 209–218.
Tyrell, M.C., and James E. Byers. 2007. Do artificial substrates favornonindigenous fouling species over natives? Journal of
Experimental Marine Biology and Ecology 342: 54–60.
Vaselli, Stefano, Fabio Bulleri, and Lisandro Benedetti-Cecchi.
2008. Hard coastal-defence structures as habitats for nativeand exotic rocky-bottom species. Marine Environmental
Research 66: 395–403.
Weggel, J.R. 1988. Seawalls: the need for research, dimensional considerations and a suggested classification. Journal of Coastal ResearchSpecial Issue 4 29–39.
Weigel, Robert L. 2002a. Seawalls, seacliffs, beachrock: what beach effects? Part 1. Shore & Beach 70: 13–22.
Weigel, Robert L. 2002b. Seawalls, seacliffs, beachrock: what beacheffects? Part 2. Shore & Beach 70: 13–22.
Weigel, Robert L. 2002c. Seawalls, seacliffs, beachrock: what beach effects? Part 3. Shore & Beach 70: 2–14.
Windham-Myers, Lisamarie, Kristen Ward, Mark Marvin-DiPasquale,Jennifer L. Agee, Le H. Kieu, and Evangelos Kakouros. 2013.
Biogeochemical implications of episodic impoundment in a restored
tidal marsh of San Francisco Bay, California. Restoration Ecology
21: 124–132.
Wong, Melisa C., Charles H. Peterson, and Michael F. Piehler. 2011.
Evaluating estuarine habitats using secondary production as a proxyfor food web support. Marine Ecology Progress Series 440: 11–25.
Zhu, Shutang, and Allen T. Chwang. 2001. Investigations on thereflection behaviour of a slotted seawall. Coastal Engineering
43: 93–104.