1. Relative sea-level rise and the potential for subsidence of the Swartkops
Estuary intertidal salt marshes, South Africa
T.G. Bornman a,b,
⁎, J. Schmidt b
, J.B. Adams b
, A.N. Mfikili a,b
, R.E. Farre c
, A.J. Smit d
a
South African Environmental Observation Network, Elwandle Coastal Node, Port Elizabeth, South Africa
b
Department of Botany, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa
c
South African Navy Hydrographic Office, Silvermine, Cape Town, South Africa
d
Department for Biodiversity and Conservation Biology, University of the Western Cape, Cape Town, South Africa
a b s t r a c ta r t i c l e i n f o
Available online xxxx
Edited by T Riddin
Salt marshes are highly productive and biologically diverse coastal wetlands that are threatened by rising sea-
level. Salt marsh habitats within the Swartkops Estuary were examined to determine their structure along an
elevation gradient and how this structure has changed over the past seven decades, what the primary drivers
of this structure were and whether the salt marsh surface is stable, rising or declining relative to current and fu-
ture sea-level rise. Relative sea-level has been rising by 1.82 mm·year−1
over the past 36 years, with a short-term
trend of 7.48 mm·year−1
measured during the study period. GIS analyses showed that during the last 70 years,
losses of floodplain, intertidal and supratidal salt marsh are mainly attributed to developmental pressure. The
main environmental drivers influencing salt marsh distribution were soil moisture and elevation. Elevation
dictates tidal inundation periodicity and frequency, and thus acts to influence all edaphic factors influencing veg-
etation distribution. Rod Surface Elevation Table results for the past six years indicate that the salt marsh surface
elevation is keeping pace (2.98 ± 2.34 mm·year−1
) with historic relative sea-level rise (RSLR), but at an acceler-
ated RSLR, only two of the eight RSET stations show an elevation rate surplus. These results should be interpreted
with caution though because of the short time-series (RSET and RSL) and the high likelihood that the current
ratio of sediment elevation change will be accelerated in response to the increased sea-level rise.
© 2016 SAAB. Published by Elsevier B.V. All rights reserved.
Keywords:
Climate change
Spartina
Accretion
Sediment elevation
RSET
Tide gauge
1. Introduction
The projected changes in the rate of sea-level rise will have signifi-
cant impacts on the coastal zone, displacing ecosystems, altering
geomorphological configurations and their associated sediment
dynamics, and increasing the vulnerability of social infrastructure.
Coastal wetlands (collectively comprising salt marshes, mangroves,
coastal seeps, peritidal stromatolites, intertidal and supratidal areas)
could experience substantial losses as a result of sea-level rise. These
economically valuable ecosystems are highly productive and provide a
number of important functions such as flood and storm abatement,
water quality maintenance, carbon and nutrient sequestration, nursery
areas for economically important fisheries, organic matter supply and
conservation (Nicholls et al., 1999; Zedler and Kercher, 2005; Costanza
et al., 2014; Kulawardhana et al., 2015; Wigand et al., 2015).
If these wetlands are to survive rising water levels, they must be able
to accrete at a rate such that surface elevation gain is sufficient to offset
the rate of sea-level rise (Cahoon et al., 1995; Chmura et al., 2001;
Morris et al., 2002; Van Goor et al., 2003). A number of studies have
shown that coastal salt marshes are able to accrete at a rate equal to
or exceeding the historical rate of sea-level rise (Plater and Kirby,
2006; Goodman et al., 2007; Madsen et al., 2007; Kirwan et al., 2016).
Salt marshes have long adapted to changing sea-levels but during this
century coastal marsh sustainability will largely depend on whether or
not marshes can keep pace with accelerated sea-level rise and an
increase in storm frequency and magnitude (Simas et al., 2001;
Baustian and Mendelssohn, 2015; Raposa et al., 2015).
The height of the ocean surface at any given location, or sea-level, is
measured in the coastal zone with tide gauges that are then corrected
for land movement and changes in the gravitational field to determine
relative sea-level rise (RSLR) (Gregory et al., 2012; Slangen et al.,
2012; Church et al., 2013). Global sea-levels have risen between 0.17–
0.21 m during 1901–2010 with a mean rate of 1.7–2.3 mm·year−1
since 1971 (Church et al., 2013; IPCC, 2013). Between 1993 and 2010,
the sea-levels rose at 3.2 mm·year−1
(Church and White, 2011;
Church et al., 2013), increasing to 3.7 mm·year−1
at present (Kirwan
et al., 2016). It is very likely that the rate of global mean sea-level rise
during the 21st century will exceed the rate observed at present due
to increases in ocean thermal expansion and loss of mass from glaciers
and ice sheets (Church et al., 2011; Church et al., 2013). Global
South African Journal of Botany xxx (2016) xxx–xxx
⁎ Corresponding author at: South African Environmental Observation Network,
Elwandle Coastal Node, PO Box 77000, NMMU Bird Street Campus, Port Elizabeth, 6031,
South Africa.
SAJB-01515; No of Pages 10
http://dx.doi.org/10.1016/j.sajb.2016.05.003
0254-6299/© 2016 SAAB. Published by Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
South African Journal of Botany
journal homepage: www.elsevier.com/locate/sajb
Please cite this article as: Bornman, T.G., et al., Relative sea-level rise and the potential for subsidence of the Swartkops Estuary intertidal salt
marshes, South Africa, South African Journal of Botany (2016), http://dx.doi.org/10.1016/j.sajb.2016.05.003

2. projections of sea-level rise (SLR) approach 1 m and a rate of
20 mm·year−1
by 2100 (Nicholls et al., 2011; IPCC, 2013; Kirwan
et al., 2016) and it is anticipated that sea-level will continue to rise for
centuries, despite mitigation measures to reduce greenhouse gas
emissions (Church et al., 2013).
Although there is no doubt that the global mean sea-level (GMSL) is
increasing, it is not changing uniformly around the world (Gregory et al.,
2012; Kemp et al., 2015). Relative sea-level (RSL) change can differ
significantly from GMSL because of variability in sea surface height,
land movement, coastal geomorphology and bathymetry (Slangen
et al., 2012; Church et al., 2013). The largest contribution to relative
sea-level in South Africa is as a result of land ice melt and global mean
thermal expansion of the oceans (Slangen et al., 2012). Mather et al.
(2009) determined that the regional eustatic sea-level rise in Port
Elizabeth to be between 3.55 and 3.75 mm·year−1
(Mather et al.,
2009), very similar to the current global sea-level rise rate.
The rise in sea-level is not the only threat to salt marshes associated
with global change. Other impacts include an increase in the frequency
and magnitude of storm surges, extreme climatic events (floods and
droughts), reduced mean rainfall and river flow, and increased anthro-
pogenic stressors (development and pollution). Climate change has
resulted in a significant increase in the magnitude and return frequency
of sea-level extremes and it is likely that this will increase by an order of
magnitude during the 21st century (Church et al., 2013; IPCC, 2013;
Spencer et al., 2015; Wigand et al., 2015). There is a high likelihood of
an increase in the swell and wave height in the Southern Ocean
(Church et al., 2013; Spencer et al., 2015), resulting in an increase in
the frequency and intensity of winter (July–August) storms along the
coast of South Africa (Theron and Rossouw, 2008; Theron et al., 2010).
Port Elizabeth was identified as one of six areas in South Africa
vulnerable to sea-level rise and an increase in storm surges (Theron
and Rossouw, 2008). In situ and modelled data at Cape St Francis
(100 km to the west of Port Elizabeth) indicate a possible 17% increase
in significant wave height as a result of climate change, increasing the
height of a storm surge swell with a return period of 1 year from
6.7 m to 7.8 m (PRDW, 2009).
Salt marsh formation and persistence is a balance between accretion,
subsidence, organic matter input, below and above ground biomass,
mineral sediment input, erosion, tidal inundation and sea-level rise
(Cahoon et al., 1995, 1999, 2000; Cahoon, 2006; Mudd et al., 2009;
Townend et al., 2011; Thorne et al., 2014; Kulawardhana et al., 2015;
Raposa et al., 2015; Belliard et al., 2016; Kirwan et al., 2016). Uncer-
tainties exist about salt marsh resilience to accelerated sea-level rise,
reduced sediment supply, reduced plant productivity under increased
inundation, and limited available terrestrial/ecotone habitat for salt
marsh migration (Valentim et al., 2013; Schile et al., 2014; Smith and
Lee, 2015; Snedden et al., 2015; Veldkornet et al., 2015). Under rising
sea-levels, estuarine basins will either become inundated, silt-up or
reach a dynamic equilibrium condition when the marsh accretes at a
rate equal to the rate of relative sea-level rise (RSLR) (Thorne et al.,
2014; Carrasco et al., 2016; Kirwan et al., 2016). If marshes remain
spatially intact as sea-levels rise the marshes have the capacity to
become even greater carbon sinks due to increased organic accumula-
tion (Mudd et al., 2009; Townend et al., 2011; Hill and Anisfeld, 2015;
Kulawardhana et al., 2015). A consequence of global change that
may improve sediment accretion rates are nutrient enrichment of
estuarine waters, increased temperatures and elevated atmospheric
CO2 that will enhance the growth and productivity of salt marsh species
(Langley et al., 2009; Fox et al., 2012; Kirwan et al., 2016).
The Swartkops Estuary is ranked as the 11th most important in
South Africa in terms of biodiversity and conservation importance
(Turpie et al., 2002; Turpie and Clark, 2007). Colloty et al. (2001) applied
a modified botanical importance rating to Swartkops Estuary which also
considered species richness, community type rarity and functional
importance, increasing the ranking to 4th overall in a country wide
assessment. The intertidal salt marsh habitat of the Swartkops Estuary
covers 165 ha (Van Niekerk and Turpie, 2011), while development has
accounted for the loss of 87.5% of the supratidal salt marsh, with only
five ha remaining (Colloty et al., 2000; Van Niekerk and Turpie, 2011).
This study addresses the question of how the micro-tidal Swartkops
Estuary will adapt to increases in relative sea-level associated with
global climate change. The null hypothesis that the Swartkops Estuary
intertidal salt marsh will be able to accrete sediment at a rate equal to
or higher than the rate of current and predicted RSLR was tested by
determining 1) how the estuarine habitat has responded to change
over the past seven decades, 2) the main physical drivers of the salt
marsh and 3) the response of sediment elevation to RSLR.
2. Materials and methods
2.1. Study site
The Swartkops Estuary is located in the warm temperate Eastern
Cape Province of South Africa (Fig. 1). Mean annual runoff estimates
are 75–85 × 106
m3
(Hill et al., 1974; Middleton et al., 1981) produced
by 636 mm of mean annual rainfall (Reddering and Esterhuizen,
1981). The 1354 km2
catchment is characterised by numerous small
impoundments that have a limited influence on the total runoff to the
estuary (Baird et al., 1986). The permanently open estuary opens into
the Indian Ocean in Algoa Bay and is considered an urbanised estuary
as it falls within the Nelson Mandela Bay Metropolitan area. The
Swartkops Estuary is influenced by a semi-diurnal tide with a vertical
tidal range of 1.8 m (micro-tidal) and tidal variations that can be less
than 0.5 m during neap tides and over 2.0 m during spring tides
(Schumann, 2013).
2.2. Vegetation and sediment
The estuarine vegetation and habitat distribution was digitised using
ArcGIS™ 10.3.1. (ESRI®) from 1939 (courtesy of the Department of
Surveys and Mapping) and 2007 aerial images (courtesy of the Nelson
Mandela Bay Metropolitan Municipality), updated with 2012 SPOT 6
and Google Earth (V 7.1.5.1557; DigitalGlobe 2013; 09/26/2013) imag-
ery. Only a 1050 ha area could be compared because of the limited
extent of the 1939 aerial photograph. Fine scale horizontal mapping of
the vegetation was partly done in the field using a GPS device and
ArcPad® (version 7.0) software. Nine permanent transects (three each
in the lower, middle and upper reaches) were established in the salt
marsh area in order to capture vegetation, elevation and soil physico-
chemical variability throughout the estuary (Fig. 1). The percentage
vegetation cover was determined using a 1 m2
quadrat every 5 m on
either side and four random quadrats every 20 m along the transects
during field trips in February 2009, July 2009, February 2010 and July
2010. Three replicate soil samples from the top 20 cm were collected
in up to four vegetation zones along each transect during the four
sampling trips. Soil characteristics were determined using the following
methods: Soil moisture content (Gardner, 1965), organic content
(Briggs, 1977); electrical conductivity (The Non-Affiliated Soil Analyses
Working Committee, 1990) using an YSI 30 M/10 FT hand held
conductivity metre, pH (Black, 1965), redox potential (The Non-
Affiliated Soil Analyses Working Committee, 1990) using a Metrohm
AG9101 electrode; and particle size (Day, 1965) using the hydrometer
method. GPS coordinates for each transect and sediment sampling site
is provided in the supplementary material.
2.3. Relative sea-level rise
Hourly tidal heights (cm) above the chart datum were provided for
the port of Port Elizabeth by the South African Navy Hydrograhic Office
(SANHO) from 1978 to 2014 (Fig. 1). The incorporation of the chart
datum allows for the modelling of sea-level (SL) rise relative to the
land, and we therefore use the term relative sea-level (RSL) to denote
2 T.G. Bornman et al. / South African Journal of Botany xxx (2016) xxx–xxx
Please cite this article as: Bornman, T.G., et al., Relative sea-level rise and the potential for subsidence of the Swartkops Estuary intertidal salt
marshes, South Africa, South African Journal of Botany (2016), http://dx.doi.org/10.1016/j.sajb.2016.05.003

3. these data. The RSL rise/trend (RSLR) per annum was calculated
by fitting a Generalised Linear Model (GLS) to the time series of monthly
RSLs. Although monthly data effectively smooth out the short-period
components of tidal harmonics, the mean seasonal cycle and inter-
annual variability are retained. To account for the residual autocorrela-
tion we modelled the variance component using a 1st order
autoregressive (AR1) term as:
yi ¼ bti þ mj þ ρ1 yi−1−bti−1−mj−1
À Á
þ εi
where yi are the monthly RSLs, b is the trend (slope), ti is the time vector,
mj are the 12 monthly values that account for the seasonal cycle, ρ1 is
the 1st order (lag 1) autoregressive coefficient that accounts for auto-
correlation, and εi is the residual time series. The order of the
autoregressive component was found by examining Auto Correlation
Functions (ACF) and Partial ACFs (PACF) of the residuals after fitting a
GLS that does not account for autocorrelation.
2.4. Sediment and wetland elevation
Nine permanent Rod Surface Elevation Table (RSET) stations were
established using the method detailed by Cahoon et al. (2002) (Fig. 1).
Station 9 was lost halfway through the study and only data from the
remaining eight will be reported on. The lower intertidal Spartina
maritima zone (low marsh) was selected as the zone for each station
as this would be the first intertidal salt marsh area exposed to rising
sea-levels. S. maritima is also well known for trapping sediment
(Leonard and Croft, 2006; Chelaifa et al., 2010; Kirwan et al., 2016).
The RSET stations were surveyed in to mean sea-level (0.01 mm accura-
cy). RSET stations were sampled in July 2009, February 2010, July 2010,
September 2011, March 2013, April 2014 and May 2015. Marker hori-
zons were established at each station (Cahoon et al., 2002, 2006) in
February 2010, but the Feldspar, calcium carbonate shell fragments
and later quartz granules, did not persist as a result of river floods and
storm surges. RSLR at the RSET stations were calculated using the meth-
od proposed by Cahoon (2015): RSLRwet = MLVw − RSLR; where
Fig. 1. Study site map showing the location of the Swartkops Estuary (Triangles = RSET stations; thick lines = vegetation and sediment transects).
3T.G. Bornman et al. / South African Journal of Botany xxx (2016) xxx–xxx
Please cite this article as: Bornman, T.G., et al., Relative sea-level rise and the potential for subsidence of the Swartkops Estuary intertidal salt
marshes, South Africa, South African Journal of Botany (2016), http://dx.doi.org/10.1016/j.sajb.2016.05.003

4. RSLRwet is the relative sea-level rise at the wetland, MLVw is the wetland
surface elevation trend from the RSET measurements and RSLR is the
relative sea-level rise at the port of Port Elizabeth tide gauge. GPS
coordinates for each RSET station is provided in the supplementary
material.
2.5. Statistical analyses
The species and environmental data were analysed using CANOCO
for Windows (version 5.04, Ter Braak and Šmilauer, 2012). CCA was
used to obtain an ordination of the vegetation data constrained by
environmental variables. Monte Carlo permutation tests (999 permuta-
tions) were performed to assess the significance of the canonical axes.
One Way Repeated Measures Analysis of Variance (Tukey pairwise
multiple comparison on mean ranks test) were run using SigmaPlot
12.0 (version 12.2.0.45), Systat Software Inc. The statistical analysis for
the RSLR calculation was done in R 3.2.2 (R Core Team, 2015).
3. Results
3.1. Vegetation and habitat distribution
The area cover of the different vegetation and habitat units changed
considerably from 1939 to 2012 (Fig. 2; Table 1). Development (primar-
ily housing, roads, railways and industry) was responsible for altering
~118 ha of the area, although most of the development was concentrat-
ed in the terrestrial and ecotone areas adjacent to the estuary. The
largest loss in habitat was a 98 ha reduction in sandbank area. In turn,
the estuarine open water area increased by 41 ha, despite both maps
digitised during a low tide period. The floodplain, supratidal and inter-
tidal salt marsh decreased in area coverage and only the submerged
macrophyte, Zostera capensis, increased its distribution. Floodplain and
supratidal salt marsh area decreased between the Settlers Bridge and
the Swartkops Village bridges (Fig. 2B). Some terrestrial areas above
the three bridges (road and rail) at the Swartkops Village were convert-
ed into supratidal and floodplain salt marsh areas. The intertidal salt
marsh and S. maritima community were mapped separately in 2012
(Table 1), but was indistinguishable in the 1939 black and white aerial
photograph.
3.2. Vegetation and soil analyses
The first canonical axis (horizontal) described 54% of the variation in
species-environment relation and the overall low eigenvalues demon-
strate relatively poor relationships (Table 4, Supplementary material).
This axis was negatively correlated with elevation (−0.35) and
positively with organic content (0.24) and moisture content (0.70).
The second canonical (axis vertical) described 68% of the variation in
species-environment relation and elevation (−0.87) was negatively
correlated with this axis. The terrestrial fringe species, Lycium cinereum
(Lyc_cine) and Tetragonia decumbens (Tet_decu), were associated with
high elevation as they occurred in the floodplain above the supratidal
zone (Fig. 3). Intertidal species such as S. maritima (Spa_mari) were
associated with sediment with a high moisture, organic, clay and silt
content, comparatively lower salinity concentration and a lower than
average elevation (Fig. 3). The dominant supratidal and floodplain
fringe species, Sarcocornia pillansii (Sar_pill), occurred in sandy areas
with a lower moisture and organic content (Fig. 3). Surface areas devoid
of vegetation (Bare in Fig. 3) were largely restricted to very low
elevations and sediment with a high silt content, i.e. mudbanks.
3.3. Relative Sea-level rise
Monthly mean relative sea-level data from a tide gauge in the port
of Port Elizabeth indicates a RSLR rate of 1.82 mm·year−1
from 1978
to 2014 (Fig. 4; Table 2). Sea-level data is inherently variable and
the RSLR for the study period (2009–2014) indicate a rate of
7.48 mm·year−1
, while for the preceding three decades the rate was
2.22 mm·year−1
(Fig. 4; Table 2).
3.4. Rod set elevation tables
The wetland surface elevation change over the past six years from
the eight RSET stations in the Swartkops Estuary is shown in Fig. 5.
RSET 1 in the lower reaches was elevated significantly (p b 0.05; n =
72) higher above mean sea-level (MSL) than the other RSET stations.
Similarly, RSET 7 maintained a significantly lower elevation above MSL
than any of the other RSET stations (Fig. 5). RSET stations 1, 5, 6, 7 and
Fig. 2. A. Vegetation and habitat map of the Swartkops Estuary in 1939. B. Vegetation and habitat map of the Swartkops Estuary in 2012.
Table 1
Comparison of area coverage of vegetation and habitat units in 1939 and 2012.
2012 (ha) 1939 (ha) Difference (ha)
Estuarine water 146.46 105.60 40.86
Floodplain salt marsh 63.66 70.07 −6.41
Intertidal salt marsh 60.70
'
143.41 −22.65
Spartina maritima 60.06
Supratidal salt marsh 96.80 127.04 −30.24
Mudbanks 80.96 77.55 3.41
Sandbanks 48.99 146.81 −97.82
Zostera capensis 44.70 24.77 19.93
Development 143.39 25.12 118.27
Total (excl. development) 602.33 695.25 −92.92
4 T.G. Bornman et al. / South African Journal of Botany xxx (2016) xxx–xxx
Please cite this article as: Bornman, T.G., et al., Relative sea-level rise and the potential for subsidence of the Swartkops Estuary intertidal salt
marshes, South Africa, South African Journal of Botany (2016), http://dx.doi.org/10.1016/j.sajb.2016.05.003

5. 8 showed a significant (p b 0.05; n = 72) increase in elevation over
the study period, although not necessarily in consecutive years. RSET 3
had a negative surface elevation trend, starting in 2011, the same
period that RSET 1 suddenly increased in elevation (Fig. 5). RSET 2 and
4 showed no significant (p N 0.05; n = 72) change in elevation over
time.
Table 3 presents the wetland elevation change trend (VLMw) as
measured by the RSETs, the RSLR rate (tide gauge record) for 1978–
2014 and 2009–2014 (study period) and the resultant wetland relative
sea-level rise rate (RSLRwet) for both periods. A negative RSLRwet trend
(RSLR b VLMw) indicates an elevation rate surplus. RSET stations 1, 4,
5, 7 and 8 therefore have a higher elevation surface in relation to sea-
level if a RSLR of 1.82 mm·year−1
is considered. At an accelerated
RSLR of 7.48 mm·year−1
, only RSET stations 1 and 7 show an elevation
rate surplus. RSET station 3 shows the highest elevation rate deficit
under both RSLR rates. RSET stations 2, 3 and 6 are not keeping pace
with historical sea-level rise.
4. Discussion
4.1. Vegetation and habitat distribution
Vegetation distribution in estuarine systems along the south and
west coast of South Africa typically comprise four community types,
i.e. reeds and sedges, supratidal salt marsh, intertidal salt marsh and
subtidal (submerged) macrophyte beds (Coetzee et al., 1997). All four
of these occur in the Swartkops Estuary, although the reeds and sedges
are largely restricted to the upper reaches (outside of the mapping area
shown in Fig. 2). The 1939 vegetation map was produced using a black
and white aerial image as a reference, which made it impossible to de-
termine the extent of S. maritima. Pierce (1982) reported though that
S. maritima specimens from the Swartkops Estuary were identified in
1887, so it is apparent that it was present prior to 1939. Floodplain
salt marsh, characterised by supratidal salt marsh interspersed with
halophytic ecotone and terrestrial species, was mapped as a separate
community in this study because of the potential habitat it could pro-
vide to the tidal marshes as sea-level rise.
Our analyses showed that 30.24, 22.65 and 6.41 ha of supratidal,
intertidal (including S. maritima) and floodplain salt marsh have been
lost since 1939. The loss of supratidal and floodplain salt marsh was sim-
ilar to the results recorded by Colloty et al. (2000), i.e. 35 ha, but the loss
of intertidal marsh was less than half of the 50 ha they calculated as the
entire estuary was not considered in this assessment. The intertidal salt
marsh habitat of the Swartkops Estuary covers 165 ha (Van Niekerk and
Turpie, 2011). The intertidal area determined by our study was slightly
less than this figure because of the limited extent of our mapping area.
Although development has taken up ~118 ha of the land on and
surrounding the Swartkops Estuary, supratidal and floodplain salt
marsh still account for 96.80 ha and 63.66 ha respectively. Submerged
macrophyte beds (mostly Z. capensis) increased their distribution by
~20 ha. However their cover and distribution is dynamic with complete
removal reported following large floods in late 1984. Prior to this Talbot
and Bate (1987) reported a cover of 16.1 ha in the summer of 1981.
Aerial photographs assessed in 1996 showed a cover of 12.5 ha
(Colloty et al., 2000) compared to the 44.7 ha measured in this study.
Fig. 3. CCA ordination of the nine transects in the lower, middle and upper reaches of the
Swartkops Estuary over four sampling periods (Stars = floodplain community; up-
triangle = supratidal community and down-triangle = intertidal/subtidal community.
Abbreviations: Spa_mari = Spartina maritima and Zos_cape = Zostera capensis; other
species abbreviations provided in the supplementary material; MC = moisture content;
OC = organic content; RP = redox potential; EC = electrical conductivity).
Fig. 4. Monthly mean relative sea-level (RSL) at the port of Port Elizabeth from 1978 to 2014.
Table 2
Relative sea-level rise as measured at the port of Port Elizabeth.
Date range
Rate ± SE
(mm·year−1
) t p-value
1978–2008 2.22 ± 0.64 2.10 b0.05
1978–2014 1.82 ± 0.49 3.70 b0.0001
2009–2014 7.48 ± 3.56 2.10 b0.05
5T.G. Bornman et al. / South African Journal of Botany xxx (2016) xxx–xxx
Please cite this article as: Bornman, T.G., et al., Relative sea-level rise and the potential for subsidence of the Swartkops Estuary intertidal salt
marshes, South Africa, South African Journal of Botany (2016), http://dx.doi.org/10.1016/j.sajb.2016.05.003

6. The largest obvious changes since 1939, other than increased devel-
opment, was the 98 ha reduction in sandbank area with a concomitant
increase in estuarine water area of 41 ha. The area coverage of these
habitats undergo dramatic changes depending on tidal range, floods
and storm surges and the water level at the time the images were
taken could differ by more than 1 m. However, the replacement of
exposed sandbanks by open water over the past seven decades appears
to be a general trend in South African estuaries along the south coast.
This is most likely as a result of a shallowing of estuaries because of
reduced freshwater inflow and floods to scour the channels deeper
(due to increased freshwater abstraction and the construction of in-
channel dams) and constriction of channel flow (through road and rail-
way bridges, jetties and stabilised banks). The reduced mobility of the
sandbanks, as well as the increase in water column eutrophication
(Adams et al., under review), was in all likelihood responsible for the
increase in Z. capensis area cover over time. The sediment dynamics of
the catchment and estuary were well described by Hill et al. (1974);
Erasmus et al. (1980); Middleton et al. (1981); Reddering and
Esterhuizen (1981) and Fromme (1988). Hill et al. (1974) attributed
the relatively minor fluvial silt deposition in Swartkops River to the
lack of large scale agriculture in the catchment. These sediments are
deposited in the upper estuary during flood episodes and generally dis-
tributed according to tidal velocity slowing up the estuary and are typ-
ically not flushed into Algoa Bay. Marine sand swept in on the flood tide
is the dominant source of sediment load into the lower and middle
reaches of the estuary (upper extent of the map in Fig. 2) (Reddering
and Esterhuizen, 1981).
In the upper reaches, above the Swartkops Village bridges, most of
the supratidal area was replaced by floodplain salt marsh, indicating
an increase in elevation or, more likely, a reduction in the reach of the
tidal water onto the salt marsh. Reddering and Esterhuizen (1981)
predicted that the supratidal salt marsh will eventually be elevated
above the spring high tide level, converting them to floodplain salt
marsh. A possible explanation for these changes could be the sediment
trap created by the construction of the bridges near the Swartkops
Village. Reddering and Esterhuizen (1988) suggest that most of the
sedimentation, especially of fines, occurring in the Swartkops Estuary
is freshwater flood driven. The embankments of the railway and road
bridges have created a constriction to flow during flood conditions,
resulting in the deposition of suspended sediments upstream of the
bridges (Reddering and Esterhuizen, 1988). The mapping study also
indicated that large sections of floodplain salt marsh upstream and ad-
jacent to the road and rail embankments have been converted to
supratidal salt marsh, probably because of damming of water upstream
of the channel constriction.
Most of the supratidal habitat in the middle and lower reaches has
been replaced by S. maritima, intertidal salt marsh and mudbanks. This
is contrary to what Reddering and Esterhuizen (1981) predicted, sug-
gesting that the influence of the Settlers Bridge would act as a sediment
trap behind it, similar to the Swartkops Village bridges, raising the level
of the marsh. The change in salt marsh community, however, indicate
increased flooding in the lower reaches (more intertidal habitat) with
increased deposition taking place against the seaward side of the
Swartkops Village road and railway embankments and on levees
bordering the main channel (increased supratidal habitat). Belliard
et al. (2016) found that monospecific vegetation, such as S. maritima,
typically grows in the low-lying marsh interior, thus supporting
salt marsh sedimentation, but does not colonize the high elevated chan-
nel levees. The Swartkops Estuary is characterised by flood-tide domi-
nant currents that is responsible for the deposition of marine sand
(Reddering and Esterhuizen, 1981; Schumann, 2013) that will settle
out in the subtidal channels, rather than on the salt marsh. A reduction
in fine sediment reaching the lower reaches (because of deposition
above the Swartkops Village bridges) and a reduction in marine sedi-
ment ingress across the berm (because of the Settlers Bridge road
embankment) would have resulted in an overall reduction in sediment
input to the salt marshes in the lower and middle reaches over the past
seven decades.
The landward margin of the entire study area depicted in Fig. 2B has
been developed and the salt marsh habitat is bordered on all sides by
hard structures such as roads, artificial embankments, railway lines,
housing developments and industrial areas. These developments will
prohibit the migration of the salt marsh into upland areas (Carrasco
et al., 2016).
4.2. Salt marsh dynamics
Schile et al. (2014) highlighted the importance of including vegeta-
tion responses to sea-level rise as subtle increases in sea-level may
lead to substantial reductions in productivity and organic matter
accretion (Snedden et al., 2015). Canonical Correspondence Analysis
identified three distinct communities, i.e. a floodplain, supratidal and
intertidal/subtidal community. The following elevation ranges were
calculated for these communities from the transect data: subtidal =
b0.5 m above mean sea-level (AMSL), intertidal = 0.3–2 m AMSL, and
supratidal/floodplain = 1.8–2.5 m AMSL. The most important drivers
determining the species distribution in the Swartkops Estuary were
Fig. 5. Change in wetland surface elevation at eight RSET stations over the study period.
Table 3
Relative sea-level rise at the eight RSET stations (RSLRwet) under two RSLR rates.
1978–2014 2009–2014
RSET VLMw
(mm·year−1
)
RSLR (mm·year−1
) RSLRwet
(mm·year−1
)
RSLR (mm·year−1
) RSLRwet
(mm·year−1
)
1 8.98 1.82 −7.16 7.48 −1.5
2 −0.81 1.82 2.63 7.48 8.29
3 −10.74 1.82 12.56 7.48 18.22
4 4.43 1.82 −2.61 7.48 3.05
5 5.56 1.82 −3.74 7.48 1.92
6 0.65 1.82 1.17 7.48 6.83
7 9.6 1.82 −7.78 7.48 −2.12
8 6.19 1.82 −4.37 7.48 1.29
6 T.G. Bornman et al. / South African Journal of Botany xxx (2016) xxx–xxx
Please cite this article as: Bornman, T.G., et al., Relative sea-level rise and the potential for subsidence of the Swartkops Estuary intertidal salt
marshes, South Africa, South African Journal of Botany (2016), http://dx.doi.org/10.1016/j.sajb.2016.05.003

7. soil moisture content and elevation. Elevation has been identified as an
important factor in determining the distribution of salt marsh vegeta-
tion (Kuhn and Zedler, 1997; Noe and Zedler, 2001; Bornman et al.,
2008; Engels, 2010). As the sea-levels rise, the elevation of the sediment
surface will be regulated, mostly through sediment accretion, to reach a
new equilibrium with the mean sea-level (Morris et al., 2002). This will
depend on the import and deposition of large volumes of sediment
which may not necessarily be available in the sediment load of the
Swartkops River or in Algoa Bay. As the elevation changes vertically,
so too will the salt marsh communities have to change their distribution
horizontally along the elevation gradient. Intertidal species, such as
S. maritima and Triglochin spp., were associated with lower than average
elevations and sediment with a high moisture, organic, clay and silt
content. RSLR may therefore significantly impact the S. maritima
dynamics and stability (Valentim et al., 2013; Smith and Lee, 2015),
especially if it results in the erosion of fines from the marsh surface.
Interestingly, it would appear that sand is not the primary contributing
factor in intertidal elevation as surmised from the GIS maps. The major-
ity of banks in the vicinity of S. maritima were muddy (high silt content)
rather than sandy. Sand is however the main sediment fraction in the
supratidal marshes.
Sediment salinity was not identified as a key driver in the Swartkops
Estuary, primarily because of the low values recorded during this study
(maximum of 18.74). Water column salinity of the Swartkops Estuary
can be highly variable and is dependent on rainfall and river flow
(Baird et al., 1986). Marais (1975) and Grindley (1985) reported hyper-
saline water column conditions between 1969 and 1972, reaching 42 in
the upper reaches, due to low rainfall and high evaporation rates (Baird
et al., 1986). It is expected that stormwater runoff and treated sewerage
return flow would have increased several fold over the past three
decades, thereby ensuring a more constant freshwater input, despite in-
creased abstraction from impoundments in the catchment. Enrichment
of the Swartkops Estuary tidal waters (Adams et al., under review)
should improve the productivity of the estuarine vegetation that in
turn may result in increased sediment accretion rates that may in turn
compensate for accelerated rates of sea-level rise (Fox et al., 2012).
4.3. Relative Sea-level rise
Tide gauges measure relative sea-level, and therefore they include
changes resulting from the vertical motion of both the land and
the sea surface. Analyses of the mean monthly tide gauge data from
the port of Port Elizabeth from 1978 to 2014 produced a relative sea-
level rise rate of 1.82 ± 0.49 mm·year−1
. Mather et al. (2009) deter-
mined that the annual sea-level trend for Port Elizabeth to be 2.97 ±
1.38 mm·year−1
. The difference in the two rates may not only be be-
cause of different methods and datasets used, but rather because of
the length of the time series used. Analyses of RSL from 1978 to 2008
produced a RSLR of 2.22 ± 0.64 mm·year−1
, closer to the rate calculat-
ed by Mather et al. (2009). To further highlight the need for longer
datasets, the RSLR for the study period (2009 to 2014) was 7.48 ±
3.56 mm·year−1
. This significantly increased rate is not an indication
of accelerated RSLR, but rather a result of the short time series used to
calculate the rate. This is particularly evident in Fig. 5 where the RSLR
trend of the last 6 years only approaches the four decade trend line in
2014. Although our study was not interested in eustatic sea-level rise,
Mather et al. (2009) calculated vertical crustal movement at Port Eliza-
beth at +0.66 mm·year−1
, resulting in a regional eustatic sea-level rise
(corrected for crustal movement and barometric change) of between
3.55 and 3.75 mm·year−1
(Mather et al., 2009).
The RSLR for Port Elizabeth is currently below the global tide gauge
RSLR rate of 2.8 ± 0.8 mm·year−1
(Church and White, 2011; Church
et al., 2013; IPCC, 2013). The global estimated rate of sea-level rise
using satellite data is higher at 3.2 ± 0.4 mm·year−1
(Church and
White, 2011), increasing recently to 3.7 mm·year−1
(Kirwan et al.,
2016), but the satellite data cannot be accurately applied to the coastal
zone because of land movement, geomorphology and bathymetry
(Church et al., 2013). Schumann (2013) determined that the tidal
variability in the Swartkops Estuary closely follows that of the port of
Port Elizabeth, although the levels are amplified in the estuary, probably
because Cape Recife and the harbour structures protect the tide gauge
from wave set-up. The estuary would therefore be exposed to greater
sea-level variability and would be especially vulnerable to storm surges.
4.4. Relative sea-level rise at the wetland (RSLRwet)
Five of the eight RSET stations showed a significant increase in wet-
land surface elevation, two showed no significant change and one RSET
showed a significant decline in elevation over the study period. There
was no spatial pattern in the surface elevation change and individual
RSET stations rather responded to local geomorphological changes,
e.g. erosion of the channel bank at RSET 3 and sand deposition at RSET
7 as a result of the natural meandering of the tidal channels. Thorne
et al. (2014) determined that proximity to a sediment source was the
most important factor determining whether an area increased in eleva-
tion or not and that accretion processes must be considered when
forecasting salt marsh accretion rates, especially in urbanised estuaries.
The mean wetland surface elevation trend (VLMw) for the Swartkops
Estuary was 2.98 ± 2.34 mm·year−1
. Kirwan et al. (2016) found
the mean rate of elevation change for intertidal marshes to be
6.9 mm·year−1
. The relatively low mean VLMw in the Swartkops Estu-
ary is largely because of natural erosion processes at two RSET stations.
Without those two stations the mean VLMw increases to 5.90 ±
1.33 mm·year−1
. Despite the negative elevation trend at two of the sta-
tions, the mean VLMw was still higher than historic RSLR, indicating that
overall the S. maritima marshes are keeping pace with sea-level rise.
Kulawardhana et al. (2015) reported that marsh loss or changes in
VLMw may often be due to an insufficient mineral sediment supply
and vertical accretion rate rather than directly from RSLR.
To compare the Swartkops Estuary wetland elevation trend with the
sea-level trend, one could either use the historic sea-level trend
(1.82 mm·year−1
) or the short-term sea-level trend (7.48 mm·year−1
)
over the study period (Cahoon, 2015). Although the confidence in the
more variable shorter-term sea-level trend is lower, both were com-
pared to the wetland elevation trend to determine if 1) the wetland is
keeping pace with historical sea-level rise and 2) what the response of
the wetland would be should the short-term RSLR trend continue. The
wetland surface elevation trend (VLMw) at the historic RSLR of
1.82 mm·year−1
resulted in a RSLRwet where five of the eight RSET
stations showed an elevation rate surplus. At the short-term RSLR rate
of 7.48 mm·year−1
, the RSLRwet resulted in only two RSET stations
(one in the lower reaches and one in the upper reaches) experiencing
an elevation rate surplus. In the other six RSET stations the sea-level
was becoming higher relative to the salt marsh surface. Kirwan et al.
(2016) found that less than 5% of salt marshes studied around the
world were being submerged by RSLR, but that this figure could change
should RSLR accelerate in future. The elevation dynamics in salt marshes
are regulated by vertical accretion over longer time periods (Rogers
et al., 2013), but the RSLRwet and study period RSLR dataset for the
Swartkops Estuary is still so short that the trends are influenced by
short temporal scale oceanographic and hydrological events.
All predictions indicate a significant increase in the magnitude and
return frequency of sea-level extremes off the coast of South Africa
(Theron and Rossouw, 2008; PRDW, 2009; Theron et al., 2010; Church
et al., 2013; IPCC, 2013; Spencer et al., 2015; Wigand et al., 2015). In
addition, significant changes in land use/land cover in urban settings
will amplify storm surge within estuaries (Bilskie et al., 2014; Prime
et al., 2015; Yang et al., 2015). However, it appears that storms have
little impact on the longer-term elevation dynamics within stable salt
marsh habitats (Rogers et al., 2013; Spencer et al., 2015) because the
flow is dampened across the marsh and storm-induced sedimentation
could in fact stabilise coastal marshes in the short term to assist in
7T.G. Bornman et al. / South African Journal of Botany xxx (2016) xxx–xxx
Please cite this article as: Bornman, T.G., et al., Relative sea-level rise and the potential for subsidence of the Swartkops Estuary intertidal salt
marshes, South Africa, South African Journal of Botany (2016), http://dx.doi.org/10.1016/j.sajb.2016.05.003

8. coping with sea-level rise (Baustian and Mendelssohn, 2015). However,
Raposa et al. (2015) recorded a community shift in response to extreme
water levels as salt marshes are replaced by lower elevation species and
eventually converted into mudflats/sandbanks. Storm surges along the
coast of South Africa can be severe and Zhang et al. (1995) recorded
large scale erosion of estuarine banks in the adjacent Gamtoos Estuary
during a storm surge (wave height N6.5 m and sea level N2.5 m in the
port of Port Elizabeth) in 1992. Trapping sediments from storm surges
will create a positive feedback mechanism to extend S. maritima area
(Pierce, 1982), as long as there is an adequate sediment supply and
the wave set-up is dampened within the estuary.
An even greater threat to South African estuaries is altered freshwa-
ter inflow. The southern Cape coast is likely to experience decreases in
mean rainfall and runoff (Arnell, 1999; Clark et al., 2000; Tadross
et al., 2011; DEA, 2013; MacKellar et al., 2014; Ziervogel et al., 2014)
with higher frequencies of flooding and drought events projected
(Mason et al., 1999; Ziervogel et al., 2014). A decrease in rainfall and
base flow will not only alter the sediment input to the estuary, but
will also impact on saline intrusion (Prandle and Lane, 2015), thereby
influencing salt marsh zonation patterns. The shallow, micro-tidal and
flood-tide dominant nature of South African estuaries makes them
more susceptible to the loss of fines as the main sediment source is
the sea (Schumann, 2013).
5. Conclusion
RSLR threatens low-lying coastal ecosystems, human communities
and infrastructure on a global scale (Pethick, 2001; Neumann et al.,
2015; Spencer et al., 2015). The intertidal salt marshes of the Swartkops
Estuary have been able to accrete sediment at a higher rate than histor-
ical sea-level rise. However, projections of future RSLR rates are uncer-
tain, with continued concern that large increases in the 21st century
(0.5–2 m) are probable (Nicholls et al., 2011). The response of the salt
marsh to accelerated sea-level rise is unknown, and will be difficult to
predict since most of the ecosystem drivers and response variables are
changing with RSLR, e.g. freshwater inflow, storm surges, sediment
supply, water quality, productivity and CO2 concentration. Kirwan
et al. (2016) is however of the opinion that most salt marshes will be
able to keep pace with accelerated sea-level rise due to inland migration
of the marshes and through biophysical feedback processes that will
accelerate sediment accretion. The majority of South African estuaries
occupy drowned river valleys that offer limited upland area into
which to migrate and those estuaries that do have low-lying adjacent
habitat have mostly been developed, creating a physical barrier to
potential migration. Countering the potential loss in estuarine ecosys-
tem services will require maximisation of estuarine area, sediment sup-
ply and the establishment of upstream and lateral conservation areas.
Extreme RSLR adaptive management may eventually include the resto-
ration of salt marsh drainage (through the road and railway berms), in-
creasing marsh elevation (supply of additional sediment) and enabling
upland salt marsh migration (removal of hard structures) (Wigand
et al., 2015). Observations of RSLR and the response of coastal ecosys-
tems remains a critical area of socially-relevant scientific research to
inform long-term adaptive coastal management (Nicholls et al., 2011;
Carrasco et al., 2016; Kemp et al., 2015; Prime et al., 2015). It is impera-
tive though that the network of RSET instruments be expanded beyond
the Swartkops, Kromme and Knysna estuaries, specifically up the east
coast to include mangrove habitat, and that appropriate material for
marker horizons are found so that the influence of subsurface processes
can be distinguished from the surface processes.
Acknowledgements
The authors wish to thank Mr. Bevan O'Reilley, Mr. Reanetsi Pohlo,
Ms. Ntando Mndela and Mr. Olwethu Duna for their assistance in the
field and in the laboratory. The research was funded by the Marine
Living Resources Fund (MCM2007073100018) of the Department of
Environmental Affairs (DEA, Oceans and Coasts) and the Department
of Agriculture, Forestry and Fisheries (DAFF). The RSET stations and
transects form part of the Long-Term Ecological Research array of the
South African Environmental Observation Network (SAEON) of the
National Research Foundation (NRF). The Department of Botany, Nelson
Mandela Metropolitan University, are thanked for providing logistical
support. Relative sea-level data was provided by the South African
Navy Hydrographic Office (SANHO).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.sajb.2016.05.003.
References
Adams, J.B., Pretorius, L., Snow, G.C., 2016. Deterioration in the water quality of an
urbanised estuary with recommendations for improvement. Water SA (under
review).
Arnell, N.W., 1999. Climate change and global water resources. Global Environmental
Change 9, 31–49.
Baird, D., Hanekom, N.M., Grindley, J.R., 1986. Estuaries of the Cape. In: Heydorn, A.E.F.,
Grindley, J.R. (Eds.), Part II synopses of available information on individual systems.
Report No. 23: Swartkops. CSIR Research Report 422, pp. 1–83.
Baustian, J.J., Mendelssohn, I.A., 2015. Hurricane-induced sedimentation improves marsh
resilience and vegetation vigor under high rates of relative sea level rise. Wetlands
35, 795–802.
Belliard, J.-P., Di Marco, N., Carniello, L., Toffolon, M., 2016. Sediment and vegetation spa-
tial dynamics facing sea-level rise in microtidal salt marshes: insights from an
ecogeomorphic model. Advances in Water Resources. http://dx.doi.org/10.1016/j.
advwatres.2015.11.020.
Bilskie, M.V., Hagen, S.C., Medeiros, S.C., Passeri, D.L., 2014. Dynamics of sea level rise and
coastal flooding on a changing landscape. Geophysical Research Letters 41, 927–934.
Black, C.B., 1965. Methods of Soil Analysis. American Society of Agronomy, Madison,
Wisconsin.
Bornman, T.G., Adams, J.B., Bate, G.C., 2008. Environmental factors controlling the vegeta-
tion zonation patterns and distribution of vegetation types in the Olifants Estuary,
South Africa. South African Journal of Botany 74, 685–695.
Briggs, D., 1977. Sources and Methods in Geography. Butterworths, London.
Cahoon, D.R., 2006. A review of major storm impacts on coastal wetland elevation.
Estuaries and Coasts 29, 889–898.
Cahoon, D.R., 2015. Estimating relative sea-level rise and submergence potential at a
coastal wetland. Estuaries and Coasts 38, 1077–1084.
Cahoon, D.R., Reed, D., Day, J., 1995. Estimating shallow subsidence in microtidal salt
marshes of the southeastern United States: Kaye and Barghoorn revisited. Marine Ge-
ology 128, 1–9.
Cahoon, D.R., Day, J.W., Reed, D., 1999. The influence of surface and shallow subsurface
soil processes on wetland elevation: a synthesis. Current Topics in Wetland Biogeo-
chemistry 3, 72–88.
Cahoon, D.R., Marin, P.E., Black, B.K., Lynch, J.C., 2000. A method for measuring vertical
accretion, elevation, and compaction of soft, shallow-water sediments. Journal of Sed-
imentary Research 70, 1250–1253.
Cahoon, D.R., Lynch, J.C., Perez, B.C., Segura, B., Holland, R.D., Stelley, C., Stephenson, G.,
Hensel, P., 2002. High-precision measurements of wetland sediment elevation: II.
The rod surface elevation table. Journal of Sedimentary Research 72, 734–739.
Cahoon, D., Hensel, P., Spencer, T., Reed, D.J., McKee, K.L., Saintilan, N., 2006. Coastal
wetland vulnerability to relative sea-level rise: wetland elevation trends and process
controls. In: Verhoeven, J.T.A., Beltman, B., Bobbink, R., Whigham, D.F. (Eds.),
Ecological StudiesWetlands and Natural Resource Management Vol. 190. Springer-
Verlag, Berlin, pp. 271–292.
Carrasco, A.R., Ferreira, Ó., Roelvink, D., 2016. Coastal lagoons and rising sea level: a re-
view. Earth-Science Reviews 154, 356–368.
Chelaifa, H., Mahē, F., Ainouche, M., 2010. Transcriptome divergence between the
hexaploid salt-marsh sister species Spartina maritima and Spartina alterniflora
(Poaceae). Molecular Ecology 2010, 1–14.
Chmura, G.L., Helmer, L.L., Beecher, C.B., Sunderland, E.M., 2001. Historical rates of salt
marsh accretion on the outer bay of Fundy. Canadian Journal of Earth Science 38,
1081–1092.
Church, J.A., White, N.J., 2011. Sea-level rise from the late 19th to the early 21st century.
Surveys in Geophysics 32, 585–602.
Church, J.A., White, N.J., Konikow, L.F., Domingues, C.M., Cogley, J.G., Rignot, E., Gregory,
J.M., van den Broeke, M.R., Monaghan, A.J., Velicogna, I., 2011. Revisiting the earth's
sea-level and energy budgets from 1961 to 2008. Geophysical Research Letters 38,
L18601. http://dx.doi.org/10.1029/2011GL048794.
Church, J.A., Clark, P.U., Cazenave, A., Gregory, J.M., Jevrejeva, S., Levermann, A., Merrifield,
M.A., Milne, G.A., Nerem, R.S., Nunn, P.D., Payne, A.J., Pfeffer, W.T., Stammer, D.,
Unnikrishnan, A.S., 2013. Sea level change. In: Stocker, T.F., Qin, D., Plattner, G.-K.,
Tignor, M., Allen, S.K., Boschung, J., Nauels, A., Xia, Y., Bex, V., Midgley, P.M. (Eds.),
Climate Change 2013: The Physical Science Basis. Contribution of Working Group I
8 T.G. Bornman et al. / South African Journal of Botany xxx (2016) xxx–xxx
Please cite this article as: Bornman, T.G., et al., Relative sea-level rise and the potential for subsidence of the Swartkops Estuary intertidal salt
marshes, South Africa, South African Journal of Botany (2016), http://dx.doi.org/10.1016/j.sajb.2016.05.003

9. to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.
Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
Clark, B.M., Steffani, N.C., Young, S., Richardson, A.J., Lombard, A.T., 2000. The effects of cli-
mate change on marine biodiversity in South Africa. Report prepared for the
Foundation for Research Development. South African Country Study on Climate
Change. Vulnerability and Adaptation Assessment, Marine Biodiversity Section,
Pretoria.
Coetzee, J.C., Adams, J.B., Bate, G.C., 1997. A botanical importance rating of selected cape
estuaries. Water SA 23, 81–93.
Colloty, B.M., Adams, J.B., Bate, G.C., 2000. The use of a botanical importance rating to
assess changes in the flora in the Swartkops Estuary over time. Water SA 26,
171–180.
Colloty, B.M., Adams, J.B., Bate, G.C., 2001. The botanical importance rating of the estuaries
in former Ciskei/Transkei. WRC Report No. 160/01 (146 pp.).
Core Team, R., 2015. R: A Language and Environment for Statistical Computing. R
Foundation for Statistical Computing, Vienna, Austria (URL https://www.R-project.
org/).
Costanza, R., de Groot, R., Sutton, P., van der Ploeg, S., Anderson, S.J., Kubiszewski, I.,
Farber, S., Turner, R.K., 2014. Changes in the global value of ecosystem services. Global
Environmental Change 26, 152–158.
Day, P.R., 1965. Particle fractionation and particle-size analysis. In: Black, C.A., Evans, D.D.,
White, J.L., Ensminger, L.E., Clark, F.E., Dinauer, R.C. (Eds.), Methods of Soil
AnalysisPart I. Agronomy Vol. 9. American Society of Agronomy, Inc., Madison, Wis-
consin, pp. 545–566.
DEA (Department of Environmental Affairs), 2013. Long-term adaptation scenarios flag-
ship research programme (LTAS) for South Africa. Climate Trends and Scenarios for
South Africa. Pretoria, South Africa. LTAS Phase 1, Technical Report (No. 1 of 6)
(132 pp.).
Engels, J.G., 2010. Drivers of Marsh Plant Zonation and Diversity Patterns along Estuarine
Stress Gradients PhD Thesis University of Hamburg (95 pp.).
Erasmus, T., Watling, R.J., Emmerson, W., Reddering, J.S.V., 1980. South East Cape monitor-
ing programme. Report Series. Report No. 1 (April 1979–April 1980). Univeristy of
Port Elizabeth, pp. 38–48.
Fox, L., Valiela, I., Kinney, E.L., 2012. Vegetation cover and elevation in long-term experi-
mental nutrient-enrichment plots in Great Sippewissett Salt Marsh, Cape Cod, Massa-
chusetts: implications for eutrophication and sea level rise. Estuaries and Coasts 35,
445–458.
Fromme, G.A.W., 1988. Dynamics of the Swartkops Estuary. CSIR Report EMA-T 8804 (30
pp.).
Gardner, W.H., 1965. Soil water content. In: Black, C.A., Evans, D.D., White, J.L., Ensminger,
L.E., Clark, F.E., Dinauer, R.C. (Eds.), Methods of Soil AnalysisPart I. Agronomy Vol. 9.
American Society of Agronomy, Inc., Madison, Wisconsin, pp. 82–83.
Goodman, J.E., Wood, M.E., Gehrels, W.R., 2007. A 17-yr record of sediment accretion in
the salt marshes of Maine (USA). Marine Geology 242, 109–121.
Gregory, J.M., White, N.J., Church, J.A., Bierkens, M.F.P., Box, J.E., Van Den Broeke, M.R.,
Cogley, J.G., Fettweis, X., Hanna, E., Huybrechts, P., Konikow, L.F., Leclercq, P.W.,
Marzeion, B., Oerlemans, J., Tamisiea, M.E., Wada, Y., Wake, L.M., Van De Wal,
R.S.W., 2012. Twentieth-century global-mean sea level rise: is the whole greater
than the sum of the parts? Journal of Climate 26, 4476–4499.
Grindley, J.R., 1985. Estauries of the Cape. Part 11: synopses of available information on
individual systems. Report No. 30: Knysna CMS 13. CSIR Research Report 429,
pp. 1–81.
Hill, T.D., Anisfeld, S.C., 2015. Coastal wetland response to sea level rise in Connecticut and
New York. Estuarine, Coastal and Shelf Science 163, 185–193.
Hill, Kaplan, Scott, Partners, 1974. Environmental study, Swartkops River basin. Part 2;
Volume 6: Technical Data Report: Meteorology. Pepared for the City Engineer, Port
Elizabeth (25 pp.).
IPCC, 2013. In: Stocker, T.F., Qin, D., Plattner, G.-K., Tignor, M., Allen, S.K., Boschung, J.,
Nauels, A., Xia, Y., Bex, V., Midgley, P.M. (Eds.), Climate Change 2013: The Physical
Science Basis. Contribution of Working Group I to the Fifth Assessment Report of
the Intergovernmental Panel on Climate Change. Cambridge University Press,
Cambridge, United Kingdom and New York, NY, USA. http://dx.doi.org/10.1017/
CBO9781107415324 (1535 pp.).
Kemp, A.C., Dutton, A., Raymo, M.E., 2015. Paleo constraints on future sea-level rise.
Current Climate Change Reports 1, pp. 205–215.
Kirwan, M.L., Temmerman, S., Skeehan, E.E., Guntenspergen, G.R., Fagherazzi, S., 2016.
Overestimation of marsh vulnerability to sea level rise. Nature Climate Change 6,
253–260.
Kuhn, N.L., Zedler, J.B., 1997. Differential effects of salinity and soil saturation on native
and exotic plants of a coastal salt marsh. Estuaries 20, 391–403.
Kulawardhana, R.W., Feagin, R.A., Popescu, S.C., Boutton, T.W., Yeager, K.M., Bianchi, T.S.,
2015. The role of elevation, relative sea-level history and vegetation transition in de-
termining carbon distribution in Spartina alterniflora dominated salt marshes. Estua-
rine, Coastal and Shelf Science 154, 48–57.
Langley, J.A., McKee, K.L., Cahoon, D.R., Cherry, J.A., Megonigal, J.P., 2009. Elevated CO2
stimulates marsh elevation gain, counterbalancing sea-level rise. Proceedings of the
National Academy of Sciences of the United States 106, 6182–6186.
Leonard, L.A., Croft, A.L., 2006. The effect of standing biomass on flow velocity and turbu-
lence in Spartina alterniflora canopies. Estuarine Coastal and Shelf Science 69,
325–336.
MacKellar, N., New, M., Jack, C., 2014. Observed and modelled trends in rainfall and tem-
perature for South Africa: 1960–2010. South African Journal of Science 110, 1–13.
http://dx.doi.org/10.1590/sajs.2014/20130353.
Madsen, A.T., Murray, A.S., Andersen, T.J., Pejrup, M., 2007. Temporal changes of accretion
rates on an estuarine salt marsh during the late Holocene — reflection of local sea
level changes? The Wadden Sea, Denmark. Marine Geology 242, 221–233.
Marais, J.F.K., 1975. Comparative Studies on the Nutrition and and Ecology of Mullet in the
Swartkops Estuary PhD Thesis University of Port Elizabeth.
Mason, S.J., Waylen, P.R., Mimmack, G.M., Rajaratnam, B., Harrison, J.M., 1999. Changes in
extreme rainfall events in South Africa. Climatic Change 4, 249–257.
Mather, A.A., Garland, G.G., Stretch, D.D., 2009. Southern African sea levels: corrections,
influences and trends. African Journal of Marine Science 31, 145–156.
Middleton, B.J., Lorentz, S.A., Pitman, W.V., Midgley, D.C., 1981. Surface water resources of
South Africa, vol. V, drainage regions MNPQRST, the eastern Cape. Report No. 12/81,
Hydrological Research Unit. Univ. of the Witwatersrand, Johannesburg, South Africa.
Morris, J.T., Sundareshwar, P.V., Nietch, C.T., Kjerfve, B., Cahoon, D.R., 2002. Responses of
coastal wetlands to rising sea-level. Ecology 83, 2869–2877.
Mudd, S.M., Howell, S.M., Morris, J.T., 2009. Impact of dynamic feedbacks between
sedimentation, sea-level rise, and biomass production on near-surface marsh
stratigraphy and carbon accumulation. Estuarine, Coastal and Shelf Science 82,
377–389.
Neumann, B., Vafeidis, A.T., Zimmermann, J., Nicholls, R.J., 2015. Future coastal population
growth and exposure to sea-level rise and coastal flooding — a global assessment.
PloS One 10, e0118571. http://dx.doi.org/10.1371/journal.pone.0118571.
Nicholls, R.J., Hoozemans, F.M.J., Marchand, M., 1999. Increasing flood risk and wetland
losses due to global sea-level rise: regional and global analyses. Global Environmental
Change 9, S69–S87.
Nicholls, R.J., Marinova, N., Lowe, J.A., Brown, S., Vellinga, P., De Gusmão, D., Hinkel, J., Tol,
R.S.J., 2011. Sea-level rise and its possible impacts given a “beyond 4 °C world” in the
twenty-first century. Philosophical Transactions of the Royal Society 369, 161–181.
http://dx.doi.org/10.1098/rsta.2010.0291.
Noe, G.B., Zedler, J.B., 2001. Spatio-temporal variation of salt marsh seedling
establishment in relation to the abiotic and biotic environment. Journal of Vegetation
Science 12, 61–74.
Pethick, J., 2001. Coastal management and sea-level rise. Catena 42, 307–322.
Pierce, S., 1982. What is Spartina doing in our estuaries? South African Journal of Science
78, 229–230.
Plater, A., Kirby, J., 2006. The potential for perimarine wetlands as an ecohydrological and
phytotechnological management tool in the Guadiana estuary, Portugal. Estuarine,
Coastal and Shelf Science 70, 98–108.
Prandle, D., Lane, A., 2015. Sensitivity of estuaries to sea level rise: vulnerability indices.
Estuarine, Coastal and Shelf Science 160, 60–68.
PRDW, 2009. Eskom nuclear sites site safety reports numerical modelling of coastal
processes, Thyspunt. Report No. 1010/2/101. Prestedge Retief Dresner Wijnberg
(Pty) Ltd Consulting Port and Coastal Engineers, Cape Town.
Prime, T., Brown, J.M., Plater, A.J., 2015. Physical and economic impacts of sea-level
rise and low probability flooding events on coastal communities. PloS One 10,
e0117030. http://dx.doi.org/10.1371/journal.pone.0117030.
Raposa, K.B., Weber, R.L.J., Ekberg, M.C., Ferguson, W., 2015. Vegetation dynamics in
Rhode Island salt marshes during a period of accelerating sea level rise and extreme
sea level events. Estuaries and Coasts. http://dx.doi.org/10.1007/s12237-015-0018-4.
Reddering, J.S.V., Esterhuizen, K., 1981. The sedimentary ecology of the Swartkops
Estuary. Rosie Report 1. Geology Department, University of Port Elizabeth (89 pp.).
Reddering, J.S.V., Esterhuizen, K., 1988. The Swartkops Estuary: physical description and
history. In: Baird, D., Marais, J.F.K., Martin, A.P. (Eds.), The Swartkops Estuary South
African National Scientific Programmes Report No. 156, pp. 41–56.
Rogers, K., Saintilan, N., Howe, A.J., Rodríguez, J.F., 2013. Sedimentation, elevation and
marsh evolution in a southeastern Australian estuary during changing climatic
conditions. Estuarine, Coastal and Shelf Science 133, 172–181.
Schile, L.M., Callaway, J.C., Morris, J.T., Stralberg, D., Parker, V.T., Kelly, M., 2014. Modeling
tidal marsh distribution with sea-level rise: evaluating the role of vegetation,
sediment, and upland habitat in marsh resiliency. PloS One 9, e88760. http://dx.doi.
org/10.1371/journal.pone.0088760.
Schumann, E.H., 2013. Sea level variability in South African estuaries. South African Jour-
nal of Science 109. http://dx.doi.org/10.1590/sajs.2013/1332.
Simas, T., Nunes, J.P., Ferreira, J.G., 2001. Effects of global climate change on coastal salt
marshes. Ecological Modelling 139, 1–15.
Slangen, A.B.A., Katsman, C.A., Van de Wal, R.S.W., Vermeersen, L.L.A., Riva, R.E.M., 2012.
Towards regional projections of twenty-first century sea-level change based on
IPCC SRES scenarios. Climate Dynamics 38, 1191–1209.
Smith, S.M., Lee, K.D., 2015. The influence of prolonged flooding on the growth of Spartina
alterniflora in Cape Cod (Massachusetts, USA). Aquatic Botany 127, 53–56.
Snedden, G.A., Cretini, K., Patton, B., 2015. Inundation and salinity impacts to above- and
belowground productivity in Spartina patens and Spartina alterniflora in the
Mississippi River deltaic plain: implications for using river diversions as restoration
tools. Ecological Engineering 81, 133–139.
Spencer, T., Brooks, S.M., Evans, B.R., Tempest, J.A., Möller, I., 2015. Southern North Sea
stormsurge event of 5 December 2013: water levels, waves and coastal impacts.
Earth-Science Reviews 146, 120–145.
Tadross, M., Davis, C., Engelbrecht, F., Joubert, A., Archer van Garderen, E., 2011. Chapter 3:
regional scenarios of future climate change over southern Africa. In: Davis, C.L. (Ed.),
Climate Risk and Vulnerability: A Handbook for Southern Africa. Council for Scientific
and Industrial Research, Pretoria, South Africa, pp. 28–50.
Talbot, M.M.B., Bate, G.C., 1987. The distribution and biomass of the seagrass Zostera
capensis in a warm temperate estuary. Botanica Marina 30, 91–99.
Ter Braak, C.J.F., Šmilauer, P., 2012. Canoco Reference Manual and user's Guide: Software
for Ordination, Version 5.0. Microcomputer Power, Ithaca, USA (496 pp.).
The Non-Affiliated Soil Analyses Working Committee, 1990. Handbook of Standard
Soil Testing Methods for Advisory Purposes. Soil Science Society of South Africa,
Pretoria.
Theron, A., Rossouw, M., 2008. Analysis of potential coastal zone climate change impacts
and possible response options in the southern African region. Proceedings from
9T.G. Bornman et al. / South African Journal of Botany xxx (2016) xxx–xxx
Please cite this article as: Bornman, T.G., et al., Relative sea-level rise and the potential for subsidence of the Swartkops Estuary intertidal salt
marshes, South Africa, South African Journal of Botany (2016), http://dx.doi.org/10.1016/j.sajb.2016.05.003

10. the Science Real and Relevant: 2nd CSIR Biennial Conference (www.conference.csir.
co.za).
Theron, A., Rossouw, M., Barwell, L., Maherry, A., Diedericks, G., de Wet, P., 2010.
Quantification of risks to coastal areas and development: wave run-up and erosion.
Proceeding of the Science Real and Relevant Conference. Reference: NE20-PA-F
(www.conference.csir.co.za).
Thorne, K.M., Elliott-Fisk, D.L., Wylie, G.D., Perry, W.M., Takekawa, J.Y., 2014. Importance
of biogeomorphic and spatial properties in assessing a tidal salt marsh vulnerability
to sea-level rise. Estuaries and Coasts 37, 941–951.
Townend, I., Fletcher, C., Knappen, M., Rossington, K., 2011. A review of salt marsh
dynamics. Water and Environment Journal 25, 477–488.
Turpie, J., Clark, B., 2007. Development of a conservation plan for temperate South African
estuaries on the basis of biodiversity importance, ecosystem health and economic
costs and benefits. Final Report. C.A.P.E. Regional Estuarine Management Programme
(125 pp.).
Turpie, J.K., Adams, J.B., Joubert, A., Harrison, T.D., Colloty, B.M., Maree, R.C., Whitfield, A.K.,
Wooldridge, T.H., Lamberth, S.J., Taljaard, S., Van Niekerk, L., 2002. Assessment of the
conservation priority status of South African estuaries for use in management and
water allocation. Water SA 28, 191–206.
Valentim, J.M., Vaz, N., Silva, H., Duarte, B., Caçador, I., Dias, J.M., 2013. Tagus Estuary and
Ria de Aveiro salt marsh dynamics and the impact of sea level rise. Estuarine, Coastal
and Shelf Science 130, 138–151.
Van Goor, M.A., Zitman, T.J., Wang, Z.B., Stive, M.J.F., 2003. Impact of sea-level rise on the
morphological equilibrium state of tidal inlets. Marine Geology 202, 211–227.
South African national biodiversity assessment 2011: technical report. In: Van Niekerk, L.,
Turpie, J.K. (Eds.), Volume 3: Estuary Component. CSIR Report NumberCSIR/NRE/
ECOS/ER/2011/0045/B. Council for Scientific and Industrial Research, Stellenbosch
(294 pp.).
Veldkornet, D.A., Adams, J.B., Potts, A.J., 2015. Where do you draw the line? Determining
the transition thresholds between estuarine salt marshes and terrestrial vegetation.
South African Journal of Botany 101, 153–159.
Wigand, C., Ardito, T., Chaffee, C., Ferguson, W., Paton, S., Raposa, K., Vandemoer, C.,
Watson, E., 2015. A climate change adaptation strategy for management of coastal
marsh systems. Estuaries and Coasts. http://dx.doi.org/10.1007/s12237-015-0003-y.
Yang, Z., Wang, T., Voisin, N., Copping, A., 2015. Estuarine response to river flow and sea-
level rise under future climate change and human development. Estuarine, Coastal
and Shelf Science 156, 19–30.
Zedler, J.B., Kercher, S., 2005. Wetland resources: status, trends, ecosystem services, and
restorability. Annual Review of Environmental Resources 30, 39–74.
Zhang, P., Schumann, E.H., Shone, R.W., 1995. Effects of a storm surge coincident with a
high spring tide in the Gamtoos estuary, May 1992. South African Journal of Science
91, 412–414.
Ziervogel, G., New, M., Archer van Garderen, E., Midgley, G., Taylor, A., Hamann, R., Stuart-
Hill, S., Myers, J., Warburton, M., 2014. Climate change impacts and adaptation in
South Africa. WIREs Climate Change 5, 605–620.
10 T.G. Bornman et al. / South African Journal of Botany xxx (2016) xxx–xxx
Please cite this article as: Bornman, T.G., et al., Relative sea-level rise and the potential for subsidence of the Swartkops Estuary intertidal salt
marshes, South Africa, South African Journal of Botany (2016), http://dx.doi.org/10.1016/j.sajb.2016.05.003