Mutualism between ribbed mussels and cordgrass
enhances salt marsh nitrogen removal
DONNA MARIE BILKOVIC, MOLLY M. MITCHELL, ROBERT E. ISDELL, MATTHEW SCHLIEP, AND ASHLEY R. SMYTH1
Virginia Institute of Marine Science, College of William & Mary, P.O. Box 1346, Gloucester Point, Virginia 23062 USA
Citation: Bilkovic, D. M., M. M. Mitchell, R. E. Isdell, M. Schliep, and A. R. Smyth. 2017. Mutualism between ribbed
mussels and cordgrass enhances salt marsh nitrogen removal. Ecosphere 8(4):e01795. 10.1002/ecs2.1795
Abstract. Salt marsh ecosystems have declined globally and are increasingly threatened by erosion, sea
level rise, and urban development. These highly productive, physically demanding ecosystems are populated by core species groups that often have strong trophic interactions with implications for ecosystem
function and service provision. Positive interactions occur between ribbed mussels (Geukensia demissa) and
cordgrass (Spartina alterniflora). Mussels transfer particulate nitrogen from the water column to the marsh
sediments, which stimulates cordgrass growth, and cordgrass provides predator and/or heat stress refuge
for mussels. Here, we test mussel facilitation of two functions in salt marshes that relate to N removal:
microbial denitrification and water filtration. Microcosm experiments revealed that the highest rates of
N2 production and nitrification occurred when mussels were present with marsh vegetation, suggesting
that mussels enhanced coupling of the nitrification–denitrification. Surveys spanning the York River
Estuary, Chesapeake Bay, showed that the highest densities of mussels occurred in the first meter for all
marsh types with mainstem fringing (1207 265 mussels/m2) being the most densely populated. The
mussel population was estimated to be ~197 million animals with a water filtration potential of 90–135
million L/hr. Erosion simulation models demonstrated that suitable marsh habitat for ribbed mussels along
the York River Estuary would be reduced by 11.8% after 50 years. This reduction in mussel habitat resulted
in a projected 15% reduction in ribbed mussel abundance and filtration capacity. Denitrification potential
was reduced in conjunction with projected marsh loss (35,536 m2) by 205 g N/hr, a 16% reduction. Because
of the predominant occurrence of ribbed mussels at the marsh seaward edge and because the highest proportional loss will occur for fringing marshes (20%), shoreline management practices that restore or create
fringing marsh may help offset these projected losses.
Key words: biogeochemistry; denitrification; ecosystem functions; fringing marsh; Geukensia demissa; nitrogen; ribbed
mussels; salt marsh; Spartina; wetland.
Received 10 February 2017; accepted 20 March 2017. Corresponding Editor: Wyatt F. Cross.
Copyright: © 2017 Bilkovic et al. This is an open access article under the terms of the Creative Commons Attribution
License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
1
Present address: Kansas Biological Survey, University of Kansas, 2101 Constant Avenue, Lawrence, Kansas 66047 USA.
E-mail: donnab@vims.edu
INTRODUCTION
(Barbier et al. 2011). The latter is critical to maintain water quality and is a management goal with
significant societal investment. Salt marshes have
long been recognized to provide numerous
ecosystem services, but their position at the land–
water interface places them at high risk from multiple stressors including erosion, sea level rise,
and development (Kennish 2001). Significant global declines in salt marshes have occurred over
Humans rely on ecosystems to provide a range
of services fundamental to their well-being (Costanza et al. 1997). Global declines in estuarine and
coastal ecosystems (marsh, seagrass, oyster reefs)
have been linked to significant loss of viable fisheries, nursery habitat provision, and water filtration services provided by suspension feeders
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BILKOVIC ET AL.
higher than many other bivalves (including
Brachidontes exustus, Spisula solidissima, and
Mercenaria mercenaria) (Riisgard 1988). Ribbed
mussels and oysters preferentially selected the
same algae species under controlled experimental
conditions (Espinosa et al. 2008). However,
G. demissa are able to retain particles of a smaller
size, such as small-sized bacteria, compared to
C. virginica and Mytilus edulis (Wright et al. 1982).
Bacteria in free suspension have been shown to
contribute 25.8% to the metabolic carbon requirements of marsh mussels (Langdon and Newell
1990) and may be important contributors to the
carbon and nitrogen budgets of mussels. Other
food sources for ribbed mussels include
microzooplankton (Langdon and Newell 1990,
Lonsdale et al. 2009), Spartina (plant) detritus
(Peterson et al. 1985, Kreeger et al. 1988, Langdon and Newell 1990), microphytobenthos, phytoplankton, and protists (Kemp et al. 1990,
Newell and Krambeck 1995, Kreeger and Newell
1996). Particle retention efficiency of ribbed mussels is also high with 100% retention of 4- to 5-lm
or larger particles and about 70% retention of
2-lm particles. In comparison, oysters retain
100% of 4- to 5-lm or larger particles and about
50% of 2-lm particles (Riisgard 1988). In general,
the filtration capacity, which reflects filtration rate
and particle retention efficiency, of ribbed mussels is relatively high among marine bivalves.
Ribbed mussels transfer particulate nitrogen
(N) from the water column to the marsh sediments through filtration. About half of the nitrogen from filtered suspended particles is ingested
by the mussel and about half of the ingested
nitrogen is excreted as ammonium (NH4+; Jordan
and Valiela 1982), a form of nitrogen that is used
for primary production. Through biodeposition,
the mussels can “fertilize” marsh plants and have
been shown to stimulate the growth of cordgrass
(Bertness 1984). Alternatively, mussel presence
may enhance sediment microbial processes. For
example, nitrification, the microbial oxidization of
ammonium to nitrite and then nitrate, both
bioavailable forms of N, may be enhanced in the
presence of mussels because of the increased
ammonium availability associated with excretion
and mineralization of biodeposits. Further, under
anaerobic conditions, the produced nitrate can be
reduced to gaseous nitrogen, dinitrogen (N2), and
nitrous oxide (N2O), during denitrification, a
the past century (Kennish 2001), a trend that is
not likely to be reversed. Growing coastal populations (Small and Nicholls 2003) and rising seas
(e.g., Boon and Mitchell 2015) will likely lead to
increased shoreline armoring and subsequent loss
of wetland habitats and ecosystem function (e.g.,
Bilkovic and Roggero 2008, Peterson and Lowe
2009 and references within, Bulleri and Chapman
2010, Dugan et al. 2011, Dethier et al. 2016).
Ribbed mussels (Geukensia demissa) and cordgrass (Spartina alterniflora) have a mutualistic relationship which can enhance multiple ecosystem
functions (Angelini et al. 2015). Ribbed mussels
remove large amounts of particulate organic
material comprising algae, detritus, and bacteria
(Gosner 1971) from overlying waters through filtration and consumption. A portion of this material is transferred to the sediments as biodeposits,
where it can fuel microbial processes, including
denitrification. Additionally, by actively manipulating their habitat, mussels help to stabilize the
marsh. Specifically, mussels tend to aggregate
around cordgrass stems (Nielsen and Franz 1995),
stimulate marsh plant root and rhizome growth
with biodeposits, and bind sediment, which
increases marsh height, stabilizes the marsh, and
reduces erosion (Bertness 1984). In turn, highdensity clumps of marsh plants serve as predator
and/or heat stress refuge for mussels. Because
ribbed mussels are the predominant intertidal
bivalve species (Kuenzler 1961) found throughout
salt and brackish marsh systems along the Atlantic Coast and into the Gulf of Mexico, and have a
high filtration capacity, they are capable of mediating ecosystem services at local and broader estuary scales. Human and climate change-induced
salt marsh loss places ribbed mussel ecosystem
service provision at risk even before it is fully
quantified. Accordingly, a better understanding of
their facilitation of marsh function can inform the
prioritization of wetland mitigation, conservation,
and restoration efforts, with the intent of maintaining and enhancing ecosystem services.
Filtration rates of suspension feeders vary
depending on factors such as the species, size of
the individual, velocity of the water, and water
temperature (Rice 2001, Comeau et al. 2008). In
addition, for bivalves in the intertidal zone, filtration is restricted to periods of submergence. Filtration rates of G. demissa can be similar to those
of the eastern oyster (Crassostrea virginica) and
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compared to intertidal and subtidal flats (Piehler
and Smyth 2011).
To investigate the potential for ribbed mussel,
G. demissa, facilitation of nitrogen cycling and thus
water quality enhancement, we characterized
G. demissa contribution to two functions in salt
marshes that relate to N removal: microbial denitrification and water filtration. We conducted a
continuous flow microcosm experiment to determine the influence of G. demissa on nitrogen processes in salt marshes. We used extensive
population surveys and literature-derived filtration rates to estimate total contribution to water
processing rates within a sub-estuary of the Chesapeake Bay. Our experiment was designed to
investigate the effects of the individual species
(cordgrass, mussel) as well as their combined
effects on nitrogen cycling. To examine the implications of ongoing wetlands loss from erosion for
ribbed mussel-mediated denitrification and water
filtration, we modeled future (50 years from now)
marsh extent and ribbed mussel abundance and
distribution along the York River. Understanding
the current and projected future capacity for mussels to process water and nutrients can inform
marsh creation and conservation efforts, and
microbial process that is an important N removal
mechanism in coastal systems (Fig. 1). Complete
denitrification leads to N2, an inert form of nitrogen only available for primary production by
nitrogen fixation. In contrast, incomplete denitrification to N2O contributes to harmful greenhouse
gases. Mussels facilitate sediment nitrogen cycling
by transferring and concentrating nitrogen and
carbon from tidal water to the marsh sediment
through feeding and byssal thread production
and decomposition (Bertness 1984). The organic
matter and nitrogen transferred by mussels then
become available for microbial metabolism. Mussel feeding activity and bioturbation may also
oxygenate the sediments, which could enhance
rates of coupled nitrification–denitrification as
has been shown for other macrofauna including
mollusks (Laverock et al. 2011). In this sense,
mussels are mediators of the removal and recycling of nitrogen from aquatic ecosystems,
although the magnitude of their contribution is
yet uncertain. However, efforts to understand the
movement of nitrogen through marine ecosystems have revealed habitat-specific differences in
denitrification with higher rates found in the
structured habitats of oyster reefs and marshes
Nitrogen gas (N2)
Atmospheric N,
Runoff,
Fertilizer
Filter feeding
Phytoplankton
Organic
nitrogen
Aerobic
Anaerobic
Ammonium
(NH4+)
Ammonium
(NH4+)
Nitrite
(NO2–)
Nitrate
(NO3–)
Nitrate (NO3–)
N2O, N2
Fig. 1. Conceptualization of the role of ribbed mussel in mediating the marsh nitrogen cycle. Some symbols
courtesy of the Integration and Application Network (ian.umces.edu/symbols/), University of Maryland Center
for Environmental Studies.
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BILKOVIC ET AL.
systems in Chesapeake Bay and generally representative of conditions encountered throughout
the Bay and similar estuaries (Reay and Moore
2009). The York River Estuary is a brackish system approximately 64 km long and begins at the
confluence of the Mattaponi and Pamunkey rivers (Fig. 2). It possesses a wide range of salinities
from approximately 20 ppt near the mouth of the
river, to 0 ppt several kilometers upriver of the
deepen our comprehension of the mutualistic
relationship between ribbed mussel and cordgrass
originally explored by Bertness (1984).
MATERIALS AND METHODS
Study area
This study was conducted in the York River
Estuary, Virginia, one of five major tributary
Fig. 2. Ribbed mussel sampling sites along the York River Estuary, Virginia. Potential mussel–marsh habitat
area was estimated on the basis of suitable mussel habitat and observed mussel distribution patterns (salt marsh
habitat within 4 m of the marsh–estuary interface and with salinity >8).
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22, and 24 h, preserved with 200 lL of 50%
ZnCl2, and stored underwater, below the collection temperature prior to analysis for dissolved
gases. Dissolved N2, Ar, and O2 concentrations
were measured using a Blazers Prisma QME 200
quadrupole mass spectrometer (MIMS; Kana
et al. 1994). Water samples (25 mL) for dissolved
inorganic nitrogen (DIN) analysis were also collected and immediately filtered through 0.45-lm
Whatman polyethersulfone filter and frozen until
analysis. Filtrate was analyzed with a Lachat
Quick-Chem 8000 (Lachat Instruments, Milwaukee, Wisconsin, USA) automated ion analyzer for
combined nitrate and nitrite (NOx), and ammonium (NH4+).
Fluxes of N2, NH4+, and NOx were calculated
following methodology described in Smyth et al.
(2013), and based on the difference between concentrations leaving and entering the microcosm,
flow rate, and surface area of the microcosm. A
positive flux represents production in excess of
consumption, and a negative flux is demand in
excess of consumption within the microcosm. N2
and O2 fluxes were calculated using the ratio
with Ar (Kana et al. 1994, Ensign et al. 2008).
With this technique, a net positive N2 flux
indicates denitrification dominates, while a net
negative N2 flux indicates nitrogen fixation dominates. Denitrification efficiency, the percent of
the total benthic DIN efflux into the water column that is N2, describes the portion of nitrogen
that is remineralized relative to removal through
denitrification. Denitrification efficiency was calculated using the following equation (Eyre and
Ferguson 2002):
confluence. Annual salinity distribution is correlated with freshwater river discharge (Sisson
et al. 1997). Mean tidal range near the mouth of
the York River is 0.7 m and increases to 1.1 m in
the upper tidal freshwater reaches of the Mattaponi River. The estuary supports a wide range
of habitats, from freshwater swamps to tidal
freshwater marshes to salt marshes, and the
watershed is dominated by forested (61%) and
agricultural (19%) land use (Reay 2009).
Mussel effects on nitrogen cycling in marshes
Continuous flow microcosm (10 cm diameter
and 35 cm height) experiments were used to
determine nitrogen fluxes (Smyth et al. 2013) for
treatments with and without ribbed mussels. Nine
intact sediment cores (10 cm deep) were collected
during low tide on 7 July 2014 from a Spartina
alterniflora marsh on Whittaker Creek in Gloucester Point, VA (37.333580° N, 76.436667° W). The
experimental design consisted of four treatments
with three replicates each: (1) mussels only,
(2) mussels + sediment, (3) mussels + marsh vegetation (S. alterniflora) + sediment, and (4) marsh
vegetation + sediment. The mussel-only treatment consisted of one large mussel (mean DW
[SD]: 0.75 0.06 g) in each replicate and the
other treatments with mussels consisted of three
live mussels (mean DW [SD] per core: 1.6 0.6 g)
in each replicate, with one exception, a mussel +
sediment replicate was found post-experiment to
have only one live mussel (0.9 g). Unfiltered York
River water (18 ppt) was used as replacement
water and held in a reservoir for the continuous
flow incubations. Microcosms were incubated in
an environmental chamber at 26°C, the same temperature as the collection site, under dark conditions to prevent bubble formation that would
interfere with dissolved gas measurements. Each
microcosm was capped with a gas and water-tight
top, which had an inflow and outflow port. Water
from the reservoir was pulled through the microcosms at a flow rate of 2 mL/min. Two lines,
which flowed directly from the reservoir into sample vials (bypass lines), were used to test the quality of water being pumped into the microcosm.
The microcosms were pre-incubated for approximately 18 h to reach steady state. Incubations
then lasted for an additional 24 h. Three 15 mL
samples of water were collected from each microcosm’s outflow line and the bypass lines at 18, 20,
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N
N2 flux=ðN
DIN þ N
N2 fluxÞ 100 (1)
An efficiency greater than 50% indicates that
more mineralized nitrogen is being removed
through denitrification than recycled (nitrogen
sink); however, if the efficiency is less than 50%,
nitrogen recycled back to the water column is
greater than nitrogen removal (nitrogen source).
Mass balance equations were used to estimate
the proportion of denitrification that was coupled to nitrate production from nitrification as:
DNFc = DNFt + x, where DNFc is coupled nitrification–denitrification, DNFt is the total net positive N2 flux, and x is the measured nitrate flux.
Only positive nitrate fluxes and positive N2
fluxes were used in the calculation (Gonzalez
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BILKOVIC ET AL.
et al. 2013). We used one-way ANOVA with
Tukey’s honestly significant difference procedure
to examine differences in fluxes, calculated nitrification rates, percent of denitrification that is
coupled to nitrification, and denitrification efficiency among all treatments (JMP v.11.2.0). Percent of denitrification coupled to nitrification
was arcsine-transformed prior to statistical analysis to normalize data.
CI ¼
dry soft tissue wt ðgÞ 1000
internal shell cavity capacity ðgÞ
(2)
Within each quadrat, we counted marsh plant
stems and determined the mean tallest height of
five stems for each species present. We used a
hand-held YSI sonde to record dissolved oxygen,
salinity, conductivity, pH, turbidity, water temperature, and chlorophyll a in the waters near the
marsh edge at each transect, 0.3 m above the
bottom.
We calculated mussel abundance for each
marsh within each 1-m interval from the marsh
edge and then estimated the average abundance
per interval for each marsh type: mainstem fringing, mainstem extensive, tidal creek fringing, and
tidal creek extensive. We calculated the potential
total area of marsh habitat per each 1-m interval
available to mussels along the York River (constrained by areas with salinity >8, and within 4 m
of the marsh–estuary edge) using wetlands spatial
data (CCRM-VIMS Tidal Marsh Inventory 2013)
in ArcGIS 10.1. We then estimated the total water
processing rate (L/h) for mussels in the York River
using mean density of mussels per hectare based
on marsh type, total hectares of available suitable
marsh habitat, previously estimated ribbed mussel clearance rates from June to October (1.6–
2.4 Lh 1gDW 1, Galimany et al. 2013), and the
average dry weight biomass of mussels.
We used generalized linear models to assess
the main effect of marsh type, and covariate factors of distance from marsh edge, and S. alterniflora stem density on the abundance of ribbed
mussels. We examined adult and new recruit
(<15 mm) mussels separately. We applied a
log-linear Poisson regression model and post
hoc pairwise multiple comparisons of factors
using the packages “GLM” and “phia” in R
(R Development Core Team 2011). If a covariate
had a significant effect on mussel density, linear
regression analyses were used to determine the
percent of variation in mussel density that the
covariate explained (R2 value). We compared
condition indices among marsh types using the
Kruskal-Wallis rank sum test followed by the
Mann-Whitney U test for multiple comparisons.
We determined the relationship between total
mussel (shell and tissue) dry weight (g) and shell
volume (L 9 W 9 H mm) with linear regression
analyses (JMP 10.0.2).
Mussel abundance and distribution
We conducted mussel surveys at 20 S. alterniflora-dominated marshes on the York River
within ribbed mussel salinity preferences (~8–30
ppt) during the summer (June–July 2013; Fig. 2).
Marshes were categorized as fringing (narrow
bands of cordgrass along the shoreline) or
embayed/extensive (wider meadow marshes in
embayments; henceforth extensive) and 10
marshes were randomly selected from each category. We extracted tidal marsh information
(extent and type) from a recent tidal marsh
inventory (2009) developed using the highresolution basemap imagery and field surveys
(Mitchell et al. 2011). Marshes were further distinguished as being positioned on the mainstem
of the York River (n = 13) or within primary tidal
creeks of the York River (n = 7). We determined
mussel recruit (<15 mm) and adult density
within 0.25-m2 quadrats along six replicate transects placed at 5-m intervals along the shore.
Transects ran perpendicular to the shore from the
edge of the marsh to the high marsh habitat.
Because mussel density varies with tidal elevation (Bertness 1984), we placed four quadrats
along each transect at 1-m intervals from the
marsh–estuary edge representing distances of
0–1, 1–2, 2–3, and 3–4 m (preliminary sampling
showed that the vast majority (>99%) of mussels
are found within 4 m). We collected a representative subsample of mussels (n = 10) from two
quadrats in two transects in each marsh (40 animals in total) to document the size and biomass
distribution of the population. Each mussel was
then measured (L 9 W 9 H, digital calipers,
0.1 mm), shucked to dry tissue and shell separately (60°C for 48 h), and ashed (550°C for 4 h)
to determine ash-free dry matter (AFDM). We
estimated a bivalve condition index (CI) based
on shell weight (Crosby and Gale 1990) for each
animal (Eq. 2).
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rates (SE) were significantly higher (410.9
146.1 lmol Nm 2h 1) in the “whole ecosystem”
treatment which included mussels + vegetation + sediment than in the treatment with ribbed
mussels alone (58.2 17.9 lmol Nm 2h 1;
one-way ANOVA, F3,8 = 7.45, P < 0.05; Fig. 3).
The marsh vegetation + sediment and mussel +
sediment treatments had intermediate net
denitrification (208.4 44.2 and 251.3 79.9
lmol Nm 2h 1, respectively), which were not
significantly different from each other or the other
treatments (a = 0.05).
There was an uptake of NOx for the marsh vegetation + sediment and mussel + sediment treatments, but efflux in the whole ecosystem
We estimated the potential loss of marsh habitat due to erosion over the next 50 years by spatially adjusting the leading edge of marshes
inland by the current erosion rates (m/yr; VIMS
Shoreline Studies Program 2012, Chesapeake Bay
EPR (1937–2009) Shoreline Change, http://web.
vims.edu/physical/research/shoreline/GISData/Fle
n 2014)
xviewer/SSP_for_web/; Rodrıguez-Caldero
multiplied by 50 years. The subset of the marshes
previously identified as having the potential for
sea level rise-driven inland migration (i.e., no barriers to migration such as shoreline armoring, low
elevation) by Bilkovic et al. (2009) were adjusted
inland. Potential mussel habitat was identified as
the first 4 m of the projected future marsh extent
from the wetted edge, and was divided into four
1-m intervals and grouped by marsh type. We
calculated the future mussel mean (95% CI) abundance (number/m2) and biomass (g/m2) as measured in the field study for each marsh type and
distance interval 9 the area of marsh in each of
those categories. We made the assumption that
competition for space is limiting abundance and
growth in the crowded, high-density, leading
marsh edge (Stiven and Kuenzler 1979) and that
given a smaller area of available future habitat
mussels would not increase their densities.
Percent change in future mussel water processing potential was then calculated the same way
as described above, with future mussel abundance and biomass substituted for the present
estimates. Percent change in denitrification
potential was derived using estimates of mean
N2 fluxes for the whole ecosystem treatment
(mussels + marsh + sediment; 410.9 146.1 N2
flux (lmol Nm 2hr 1)) and projected loss of
marsh area (m2) from erosion. We calculated the
change in denitrification (lmol N/hr) that might
occur with a change in the first 2 m of mainstem
marsh area where mussel densities were the
most abundant and similar or more dense than
experimental conditions (
x ¼ 423 mussels/m2 in
the whole ecosystem treatment compared to natural densities (mainstem extensive, 408 mussels/
m2; mainstem fringing, 789 mussels/m2).
Fig. 3. Net denitrification (N2 efflux) occurred in all
treatments; however, the highest rates were observed
in the whole ecosystem treatment that included mussels, marsh vegetation, and marsh sediment possibly
because of enhanced coupling of the nitrification–denitrification cycle when ribbed mussels are present in
the marsh.
RESULTS
Mussel effects on nitrogen cycling in marshes
All treatments had a net positive N2 flux, indicating net denitrification. Mean net denitrification
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Mussel abundance and distribution—
York River Estuary
treatment and the treatment with mussels alone.
Mean NOx fluxes (SE) were significantly greater
in the “whole ecosystem” (85.1 8.2 lmol
m 2h 1) and mussels alone (59.4 7.1 lmol
m 2h 1) treatments than in the marsh vegetation + sediment ( 26.7 6.8 lmol m 2h 1) and
mussel + sediment ( 30.7 12.8 lmol m 2h 1)
treatments (F3,8 = 47.5, P < 0.001). There was an
efflux of NH4+ from all treatments, although the
efflux from marsh vegetation + sediment and
mussel + sediment treatments was negligible
(Fig. 3). NH4+ efflux was highest in the treatment
with mussels alone which may be from mineralization of biodeposits and/or direct excretion from
the mussel, followed by the whole ecosystem
treatment (F3,8 = 44.5, P < 0.001). Net DIN (sum
of NH4 and NOx) fluxes were generally positive
for treatments with mussels (Fig. 3), indicating a
net source of inorganic nitrogen to the water
column. Two exceptions were observed in the
mussels + sediment treatment, when net DIN
fluxes were negative (mean: 38.5 8.8 lmol
m 2h 1), indicating a net sink of inorganic nitrogen. The nitrification rate was significantly higher
for the whole ecosystem treatment compared to
all other treatments (one-way ANOVA, F3,8 =
11.1, P = 0.003; Table 1). The mussel-only treatment had a lower denitrification efficiency
because of the higher rates of both ammonium
and nitrate production (Table 1). Whole ecosystem treatments had intermediate levels of denitrification efficiency (>50%) because of an increase
in nitrogen regeneration either directly from the
mussels or from mineralization of biodeposits
(one-way ANOVA, F3,8 = 92.2, P < 0.001). The
proportion of denitrification that was coupled to
nitrate production from nitrification was highest
for mussel-only and whole ecosystem treatments (one-way ANOVA, F3,8 = 14.0, P = 0.002;
Table 1).
Mussel abundance was highly variable among
marsh types and position; fringing marshes along
the mainstem of the estuary possessed the highest
average number of animals (adjusted mean: 204
mussels/m2) followed by mainstem-extensive (122
mussels/m2) and creek-fringing (21 mussels/m2)
marshes. Ribbed mussels were present at 18 of the
20 marshes, absent in two tidal creek-extensive
marshes. When present in tidal creek-extensive
marshes, mussels were sparse with only eight total
animals observed. Mussel density increased with
S. alterniflora density (F1, 440 = 143.5, P < 0.001,
R2 = 0.24) and decreased with increasing distance
into the marsh from the seaward edge (F1, 478 =
74.2, P < 0.001, R2 = 0.13). The highest mean densities occurred in the first meter for all marsh types
with mainstem-fringing (1207 265 mussels/m2)
and mainstem-extensive (630 152 mussels/m2)
marshes being the most dense (Table 2). Over 85%
of the animals were found in the first 2 m from
the marsh edge for every marsh type. Adult and
recruit mussels followed similar patterns of distribution among and within marshes (Table 3).
Generally, mussels in mainstem-fringing marshes were the most abundant and in the best condition compared to other marshes. Mussels in
mainstem-fringing marshes had significantly
higher CI values (113.2 2.6) than those in mainstem-extensive (101.3 2.1) or creek-fringing
(103.5 4.1) marshes (Kruskal-Wallis, P < 0.001).
Condition indices could not be estimated for tidal
creek-extensive marshes because of the scarcity of
animals. Although they were smaller in number,
mussels in tidal creek-fringing marshes had the
highest average biomass (0.7 g dry weight of tissue) compared to other marsh types (0.2 g DW)
(Kruskal-Wallis, P < 0.001). Mussels in fringing
marshes within tidal creeks had a broader size
Table 1. Calculated nitrification rates, percent of denitrification that is coupled to nitrification, and denitrification
efficiency (the percent benthic efflux that is N2) for each treatment, mean (SE) for each treatment.
Treatment
Mussels
Marsh
Mussels & sediment
Mussels, marsh, & sediment
Calculated nitrification
rate (lmol Nm 2hr 1)
Percent denitrification
coupled to nitrification
b
a
100% (0.00)
87.36% (0.85)b
86.79% (6.80)b
100% (0.00)a
117.60 (14.14)
181.76 (37.51)b
220.62 (47.37)b
495.99 (78.35)a
Denitrification efficiency
14.14% (1.08)c
98.68% (0.71)a
96.54% (2.18)a
65.12% (7.81)b
Note: Significant (a < 0.05) differences among treatments for each estimate are denoted by different superscript letters.
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Table 2. Mean mussel density (number of mussels/m2 [SE]) by marsh type and 1-m increment distances from the
marsh edge.
Distance (m) from marsh edge moving landwards
Marsh type
0–1
1–2
2–3
3–4
Creek extensive
Creek fringing
Mainstem extensive
Mainstem fringing and marsh island
0
167 (57)
630 (152)
1207 (265)
0.3 (0.2)
34 (17)
185 (55)
371 (125)
0
11 (7)
71 (20)
252 (126)
0
8 (6)
59 (23)
16 (7)
distribution than mainstem marshes (Fig. 4). The
overall relationship between mussel dry weight
and shell volume (inclusive of all marsh types)
was strongly correlated positively (R2 = 0.97,
n = 324, Eq. 3).
(mean: 90–135 million L/hr) on the basis of
observed biomass and potential clearance rates
(Galimany et al. 2013).
DWðgÞ ¼ 0:44209306 þ 0:00021836
Our marsh erosion simulations (Table 4,
Fig. 5) indicated that suitable marsh habitat for
ribbed mussels along the York River would be
reduced by 11.8% from 390 to 343 ha after
50 years of erosion and sea level rise. Of that
11.8% overall change, ~20% of suitable fringing
marsh habitat was lost and ~11% of suitable
extensive marsh habitat was lost. These losses
were fairly similar for marshes occurring in
either tidal creeks or along the mainstem estuary
(Table 4). This reduction in mussel habitat
3
ðshell volume, mm Þ
Projected mussel abundance and distribution—
York River Estuary
(3)
We estimated that there is approximately
390 ha of marsh habitat suitable for ribbed mussel occupancy along the York River. The mussel
population on the York was estimated to be
~197 million animals (range: 83–313 million,
95% CI; Appendix S1: Table S1). The water filtration potential of mussels on the York River is
between 35 and 218 million liters per hour
Table 3. Analysis of deviance of the effects of marsh type, distance in marsh from seaward edge, Spartina alterniflora density (stems/m2), and the interactions on the abundance (mussels/m2) of ribbed mussel adults and
recruits.
Model terms
Adult mussels—density
NULL
Type
Distance
Spartina
Type : Distance
Type : Spartina
Distance : Spartina
Type : Distance:Spartina
Recruit-sized mussels—density
NULL
Type
Distance
Spartina
Type : Distance
Type : Spartina
Distance : Spartina
Type : Distance : Spartina
df
Deviance
Residual df
Residual deviance
P(Chi)
3
1
1
3
3
1
3
73,118
80,065
11,844
198
8761
3362
924
479
476
475
474
471
468
467
464
365,179
292,061
211,996
200,152
199,955
191,194
187,832
186,908
<0.001
<0.001
<0.001
<0.001
<0.001
<0.001
<0.001
3
1
1
3
3
1
3
15,528
22,116
5552
219
3224
23
91
479
476
475
474
471
468
467
464
93,074
77,546
55,430
49,877
49,658
46,434
46,411
46,320
<0.001
<0.001
<0.001
<0.001
<0.001
<0.001
<0.001
Note: Models are fitted sequentially, and a X2 test is used to test for significance.
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30
Tidal creek fringing
Mainstem embayed
Mainstem fringing
Total mussel dry weight (g)
25
20
15
10
5
0
R2 = 0.97, n = 324
0
20,000
40,000
60,000
80,000
100,000
120,000
Shell volume (mm3)
Fig. 4. Relationship between ribbed mussel dry weight and shell volume. Mussels in fringing marshes within
tidal creeks had a broader size distribution than mainstem marshes. The overall relationship between mussel dry
weight and shell volume (inclusive of all marsh types) was strongly correlated (R2 = 0.965, n = 324). Line and
confidence of prediction (95% CI) is shown as an expanded shaded area.
resulted in a projected 15% reduction in ribbed
mussel abundance from 197 million mussels to
167 million mussels (range: 71–263 million, 95%
CI). Future filtration capacity was similarly
reduced by 13.9–15.3% (Appendix S1: Table S1)
as a result of decreased mussel biomass.
Denitrification was reduced in conjunction with
projected marsh area loss (35,536 m2) by 205 g
N/hr, a 16% reduction (Table 5).
DISCUSSION
Mussel–Marsh nitrogen dynamics
Interactions between ribbed mussels and cordgrass can have positive effects on water quality
and modify nutrient cycling. The increased flux
of material from the water column to the sediment as a result of mussel filtration not only
cleans the water, but also can stimulate microbial
activity. Our results showed the highest rates of
denitrification (net N2 production) and nitrification for treatments with marsh vegetation and
mussels, suggesting the possibility of enhanced
coupling of nitrification–denitrification when
ribbed mussels are present in the marsh. This is
further supported in mass balance estimates of
Table 4. Estimated percent change in marsh habitat
area from erosion after 50 years by distance from
the marsh edge and marsh type.
Distance (m) from marsh edge
Marsh type
Tidal creek
Extensive
Fringing
Mainstem estuary
Extensive
Fringing
Total
0–1
1–2
2–3
3–4
7.2
7.0
8.3
13.4
8.8
15.5
7.9
12.6
11.3
25.0
17.7
11.4
20.7
13.2
12.6
11.3
27.1
18.7
11.2
22.3
13.3
12.5
11.5
25.5
20.1
14.5
22.9
13.3
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Total
11.1
10.2
20.6
17.4
11.4
20.2
11.8
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Fig. 5. Projected change in marsh habitat in the next 50 years was estimated on the basis of current erosion
rates. Changes were greatest along the mainstem estuary where erosion rates were highest and barriers to inland
migration are extensive.
100% of denitrification being coupled with nitrification when mussels and cordgrass were
together. The mechanism for the enhanced rates
of denitrification in the whole ecosystem treatments may be due to some combination of the
following (1) increased organic nitrogen provided by the mussels, (2) tightly coupled
sediment oxygenated–deoxygenated root zones
mediated by the plants and mussels increasing
nitrification (Jordan and Valiela 1982, Howes
et al. 1986, Laverock et al. 2011), and (3)
increased availability of labile carbon compounds into the surrounding soil from plant
exudes, which have been shown to increase local
Table 5. Change in denitrification potential was determined from estimated future mainstem marsh area loss.
Marsh type
Mainstem-extensive marshes
Mainstem-fringing marshes
All mainstem marshes
Marsh area
loss (m2)
7426
28,109
35,536
Denitrification
potential loss (%)
10.1
18.1
15.5
Denitrification potential
loss (g Nh 1)
43
162
205
Annual lost denitrification
potential (g Nm 2yr 1)
6
23
29
Notes: The analysis considered only the first 2 m of the marsh where mussel densities were the most abundant and similar
or more dense than experimental conditions (x ¼ 423 mussels/m2 in the whole ecosystem treatment compared to natural densities (mainstem extensive: 408 mussels/m2, mainstem fringing: 789 mussels/m2). Annual loss in denitrification was conservatively estimated for 7 months of activity, which would be inclusive of the marsh plant growing period.
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nitrogenase activity (Boyle and Patriquin 1981)
by serving as a carbon source for denitrifiers. The
mussels and cordgrass modify the availability
resources to the microbial community as well as
the physical environment and in turn affect rates
of nitrogen cycling processes.
Mussels and marsh plants may have a facultative mutualistic relationship where cordgrass
provides particulate nitrogen for mussels to filter
and the added ammonium from mussel excretion in a nitrogen-limited marsh may increase
cordgrass growth and result in better structural
habitat for the mussels (Jordan and Valiela 1982,
Bertness 1984). For a New England salt marsh–
estuarine ecosystem, Jordan and Valiela (1982)
estimated that ribbed mussels filtered 1.8 times
the particulate nitrogen exported from the marsh
by tidal flushing with slightly more than half of
the nitrogen absorbed excreted as ammonium.
The position of ribbed mussels at the sediment–
water interface likely affords an added benefit of
promoting the retention of nitrogen in the marsh
for use by cordgrass as opposed to being
released in the water column for phytoplankton
use, which may lead to nuisance algal blooms or
hypoxia events.
The observed patterns in N2 fluxes suggest that
the whole ecosystem has more nitrogen removal
than the individual pieces. Partial treatments
(mussel + sediment; marsh + sediment) that
included sediment both enhanced N2 relative to
the mussel alone. In both cases, there was an
increase in the importance of water column nitrate
for denitrification, as indicated by a decrease in
nitrification. It is likely that denitrification in these
treatments is limited by nitrate because there is
ample organic carbon available from the primary
producers, from the sediment community, and
from the mussel biodeposits. The lack of difference in nitrogen fluxes between the mussel + sediment and marsh + sediment treatments suggests
that these species individually function similarly
in terms of their ability to modify the nitrogen
cycle. The whole ecosystem treatment with both
species had the highest rate of denitrification.
Conditions in this treatment are just right for denitrification where there is plenty of carbon from the
marsh, the sediments, and the mussels and there
is an ample supply of nitrate due to increased
nitrification. However, if the roots increase O2, the
presence of O2 may limit denitrification, which is
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an anaerobic process. Overall, having both mutualistic species in a system enhanced the ecosystem
service of denitrification.
Dense assemblages of bivalves are major components in the recycling of nutrients in estuaries
because of their ability to move material from the
water column to the sediment. Intertidal dense
mussel beds in two Dutch estuaries had very fast
turnover rates for chlorophyll a and ammonium
(3 wk or less) that exceeded those rates for individual organisms and were similar to rates
observed for intertidal oyster reefs in South Carolina (Dame et al. 1991). The enhanced denitrification in marshes with mussels present
combined with the relatively dense ribbed mussel assemblages in York River marshes also suggests that they are a major contributor to N
cycling on a system level. Most ribbed mussels
tend to settle on aggregates of adult mussels
around the stems of S. alterniflora (Nielsen and
Franz 1995) and can reach densities of 2000–3000
in New England and 10,000 in Jamaica Bay, New
York, per m2 (Kuenzler 1961, Lent 1969, Stiven
and Kuenzler 1979, Bertness and Grosholz 1985,
Lin 1989, Franz 1997, 2001). Similarly, mussel
densities of 3000–4000 were regularly observed
along the York River with a few sites reaching
5000–8000 animals/m2. These observed York
River abundances translate to ~56,000 kg of mussel biomass and mean clearance rates of 90–
135 million L/hr (using mean filtration estimates
from June to October, Galimany et al. 2013). By
comparison, the historically low current oyster
population on the York River was estimated to
be able to filter 109 million L/hr during peak
summer months (June–August), an 85% decline
in filtration capacity since ~1900 (Zu Ermgassen
et al. 2013) and near the mid-range of estimates
for ribbed mussel filtering potential. Using the
volume of the York River (796,920,000 m3) and
residence time of 11 days, the proportion of the
estuary that could be filtered by mussels within
its residence time would be ~1.5–2.3%, which is
similar to the estimated % volume filtered by
present-day oysters on the York (Zu Ermgassen
et al. 2013). This assumes that all water is available for filtration, which is not entirely valid
owing to potential water access limitations from
stratification or spatial positioning in the estuary
(Pomeroy et al. 2006). More precisely, only tidal
water exposed to the marsh will be available for
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BILKOVIC ET AL.
filtration by ribbed mussels, but mussels can
potentially filter all of the water in the marsh
during a tidal cycle (Jordan and Valiela 1982).
This in turn will limit export of particulates into
the greater estuary, serving to reduce turbidity
and enhance water quality. Moreover, ribbed
mussels are filtering nearshore and intertidal
waters complementing oyster filtration of deeper
bottom waters, leading to improved localized
water quality in multiple estuarine habitats.
reef-building mollusks, ribbed mussels form
large aggregates integrated into the sediment
matrix and anchored to the marsh grass. Biodeposits from ribbed mussels contribute to sediment accumulation and introduction of new
material to the sediment surface in the marsh
directly (Smith and Frey 1985). For oyster reefs,
biodeposit accumulation occurs mostly on the
sediment surface and is integrated into deeper
sediment over time (Rodriguez et al. 2014). Mussel waste products are likely more accessible to
bacteria which reside deeper in the sediment as
compared to the oysters, where biodeposits are
more likely to settle on the surface. This difference in the quality of organic matter, as well as
where and how biodeposits accumulate, could
alter sediment nitrogen cycling processes such as
denitrification as well as other fluxes such as
NOx and NH4+.
Comparison of N2 fluxes to other bivalve studies
Oysters and mussels are two of the most abundant species in coastal ecosystems, providing
water quality benefits due to filtration. Net N2
fluxes measured in this study fall within the
range of N2 fluxes found in oyster reef ecosystems, which are highly variable based on site and
seasonal differences (Kellogg et al. 2014 and
references therein). We observed net positive N2
fluxes for our treatments, indicating that
denitrification was occurring in excess of nitrogen fixation. Although each treatment was net
denitrifying, the presence of mussels increased
denitrification in the salt marsh ecosystem by
about 200 lmol Nm 2hr 1. Net N2 fluxes from
the mussel + marsh treatment were similar to
fringing intertidal oyster reefs in North Carolina
(Piehler and Smyth 2011). Although denitrification rates were similar, ammonium fluxes were
higher in the mussel + marsh treatment, likely
due to the presence of mussels inside the chamber which are a direct source of ammonium. In
experiments looking at the direct effects of individual oysters on sediment nitrogen cycling, the
individual organisms had the highest rate of N2
production, and this rate was higher than what
was observed for the individual mussel; however, when oysters and sediment were combined,
N2 production decreased (Smyth et al. 2013),
while for the mussel, the presence of the sediment or marsh grass led to an increase in N2 production compared to when the mussel was
alone. This would suggest that there is an interactive effect between the mussel and sediment
microbial community that is absent from the oysters. Differences in particle retention, filtration
capacity, and life history between the two
bivalves (Riisgard 1988) can contribute to the
observed differences as can differences in
biofilms or gut microbiomes. While oysters are
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Conserving and enhancing cordgrass–mussel
mutualism for ecosystem services
The spatial distribution of ribbed mussel populations within a marsh clearly indicates the significance of the immediate marsh edge habitat.
The highest densities of mussels were observed
within narrow fringing marshes and within the
first meter of the marsh. Likely, the availability of
food items and accessibility of the habitat during
larval settlement periods contributed to the high
densities observed in fringing environments. In
tidal creek habitats, mussels were fewer in number, but larger in size, which may suggest that
predation pressure is lessened in those marsh settings or growth/maturation is delayed due to
shorter feeding periods. These patterns are consistent with other research noting although there
were less abundant mussels higher on shore,
mussel lifespan tends to increase with increasing
marsh elevation. Some mussels in the higher
tidal zones reach 15 years or older, while mussels on the marsh edge tend to be around 6 or
7 years old (Lutz and Castagna 1980, Brousseau
1982, Bertness and Grosholz 1985, Franz 2001).
In addition, contributing to the differences in
population structure along an elevation gradient,
mussels that are farther inland from the marsh
edge tend to grow slower as a result of shorter
submergence and feeding time, which can delay
maturation an additional year compared to the
mussels along the edge of the marsh.
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BILKOVIC ET AL.
high bank height and are thus highly susceptible
to loss from erosion, sea level rise, and human
development (Bilkovic et al. 2009).
Our results suggest that the restoration and
conservation of even narrow, fringing marshes
inhabited by ribbed mussels have the potential to
improve water quality and perhaps alleviate
localized eutrophication. This is of particular
importance both ecologically and economically.
For example, in Chesapeake Bay, a total maximum daily load (TMDL) for key pollutants has
been established by the U.S. Environmental Protection Agency to restore clean water. The TMDL
requires a 25% reduction in nitrogen, 24% reduction in phosphorus, and 20% reduction in sediment, a costly endeavor to implement (Wainger
2012). Therefore, identifying management activities that will enhance the cost-effectiveness of the
TMDL is a high priority.
Ribbed mussels have been described as salt
marsh keystone species that enhance multifunctionality (Angelini et al. 2015). The presence of
mussel aggregates helps to sustain high levels of
multiple marsh functions including decomposition, primary production, water infiltration, and
soil accretion. Our results indicate that ribbed
mussel–cordgrass mutualism also enhances
water quality functions—filtration and denitrification—at the land–water interface, a zone experiencing intense human–natural interactions.
Shoreline management strategies should encourage the conservation or creation of marsh habitat
that supports ribbed mussel populations. The
ecological role mussels and other bivalves may
play in mediating eutrophication and providing
ecosystem services under varying and changing
environmental conditions remains an important
area for research.
The value of narrow fringing marshes is often
overlooked, despite evidence that these marshes
are able to perform many of the desired ecosystem
services provided by more extensive meadow
marshes, including wave attenuation (Knutson
et al. 1982, Shepard et al. 2011), fish and invertebrate utilization (Minello et al. 1994, Peterson and
Turner 1994, Micheli and Peterson 1999, Currin
et al. 2008), sediment trapping (Neubauer et al.
2002), and groundwater nitrate removal (Tobias
et al. 2001). This study further supports the value
of fringing marsh for water quality enhancement
mediated by the dominant marsh bivalve, ribbed
mussels. In many settings, fringing marshes are
highly vulnerable to erosion and sea level rise
because of the presence of barriers to their landward migration such as shoreline armoring and
residential or urban infrastructure. The narrow
fringing marshes in such a setting will likely be
lost first. This study indicated that the highest proportional loss will occur for fringing marshes
(20%), while extensive marshes were projected to
experience 11% loss. This may result in a potential
loss of 15% filtration capacity and 16% denitrification by ribbed mussels in the York River. Our estimates of future marsh loss, and associated
ecosystem service loss, are likely conservative
because we based erosion rates on historic changes
and did not incorporate erosion exacerbated by
sea level rise or marsh drowning. The loss of fringing mainstem estuary marshes may also compromise habitat connectivity across the greater
seascape. Marshes in connected seascapes may be
subsidized by surrounding marsh habitats (e.g.,
mussel larval source) and ensure the sustainability
of mussel populations, while those in highly fragmented seascapes may suffer the effects of isolation. Habitat fragmentation has been linked with
shifts in biodiversity, loss of habitat-specific
sensitive or functionally important species, and
isolation of populations when connectivity is
diminished (Kareiva and Wennergren 1995, Fahrig
2003, Thrush et al. 2008), but estuarine systems
have been far less studied than in terrestrial systems even though estuaries and coasts have experienced substantial habitat loss and fragmentation
(e.g., Lotze et al. 2006). Anticipated significant
marsh loss is not limited to the York River Estuary;
in Virginia tidal waters of Chesapeake Bay, ~38%
of marshes will be unable to migrate because of
adjacent developed lands, armored shores, and/or
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ACKNOWLEDGMENTS
This study was supported by National Science
Foundation (Grant Number 1600131), NSF Women in
Science and Engineering (WISE), and the David H.
Smith Conservation Research Postdoctoral Fellowship
(A.R.S). We thank our colleagues at the Center for
Coastal Resources Management, Virginia Institute of
Marine Science, for field and laboratory support. We
also thank Hunter Walker, Iris Anderson, and BK Song
for laboratory assistance. This paper is Contribution
No. 3622 of the Virginia Institute of Marine Science,
College of William & Mary.
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BILKOVIC ET AL.
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