bornman et al 2016

South African Journal of Botany xxx (2016) xxx–xxx

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South African Journal of Botany

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Relative sea-level rise and the potential for subsidence of the SwartkopsEstuary 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 Africab Department of Botany, Nelson Mandela Metropolitan University, Port Elizabeth, South Africac South African Navy Hydrographic Office, Silvermine, Cape Town, South Africad Department for Biodiversity and Conservation Biology, University of the Western Cape, Cape Town, South Africa

⁎ Corresponding author at: South African EnvironmElwandle Coastal Node, PO Box 77000, NMMU Bird StreeSouth Africa.

http://dx.doi.org/10.1016/j.sajb.2016.05.0030254-6299/© 2016 SAAB. Published by Elsevier B.V. All rig

Please cite this article as: Bornman, T.G., etmarshes, South Africa, South African Journal

a b s t r a c t
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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 anelevation gradient and how this structure has changed over the past seven decades, what the primary driversof 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 1.82mm·year−1 over the past 36 years, with a short-termtrend 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. Themain environmental drivers influencing salt marsh distribution were soil moisture and elevation. Elevationdictates 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 surfaceelevation is keeping pace (2.98± 2.34mm·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 interpretedwith caution though because of the short time-series (RSET and RSL) and the high likelihood that the currentratio 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 changeSpartinaAccretionSediment elevationRSETTide gauge

1. Introduction

The projected changes in the rate of sea-level rise will have signifi-cant impacts on the coastal zone, displacing ecosystems, alteringgeomorphological configurations and their associated sedimentdynamics, 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. Theseeconomically valuable ecosystems are highly productive and provide anumber of important functions such as flood and storm abatement,water quality maintenance, carbon and nutrient sequestration, nurseryareas for economically important fisheries, organic matter supply andconservation (Nicholls et al., 1999; Zedler and Kercher, 2005; Costanzaet al., 2014; Kulawardhana et al., 2015; Wigand et al., 2015).

If thesewetlands are to survive risingwater levels, theymust be ableto accrete at a rate such that surface elevation gain is sufficient to offsetthe rate of sea-level rise (Cahoon et al., 1995; Chmura et al., 2001;

ental Observation Network,t Campus, Port Elizabeth, 6031,

hts reserved.

al., Relative sea-level rise andof Botany (2016), http://dx.d

Morris et al., 2002; Van Goor et al., 2003). A number of studies haveshown that coastal salt marshes are able to accrete at a rate equal toor 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 thiscentury coastal marsh sustainability will largely depend on whether ornot marshes can keep pace with accelerated sea-level rise and anincrease 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, ismeasured in the coastal zone with tide gauges that are then correctedfor land movement and changes in the gravitational field to determinerelative 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 (Kirwanet al., 2016). It is very likely that the rate of global mean sea-level riseduring the 21st century will exceed the rate observed at present dueto increases in ocean thermal expansion and loss of mass from glaciersand ice sheets (Church et al., 2011; Church et al., 2013). Global

the potential for subsidence of the Swartkops Estuary intertidal saltoi.org/10.1016/j.sajb.2016.05.003

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2 T.G. Bornman et al. / South African Journal of Botany xxx (2016) xxx–xxx

projections of sea-level rise (SLR) approach 1 m and a rate of20 mm·year−1 by 2100 (Nicholls et al., 2011; IPCC, 2013; Kirwanet al., 2016) and it is anticipated that sea-level will continue to rise forcenturies, despite mitigation measures to reduce greenhouse gasemissions (Church et al., 2013).

Although there is no doubt that the global mean sea-level (GMSL) isincreasing, it is not changinguniformly around theworld (Gregory et al.,2012; Kemp et al., 2015). Relative sea-level (RSL) change can differsignificantly from GMSL because of variability in sea surface height,land movement, coastal geomorphology and bathymetry (Slangenet al., 2012; Church et al., 2013). The largest contribution to relativesea-level in South Africa is as a result of land ice melt and global meanthermal expansion of the oceans (Slangen et al., 2012). Mather et al.(2009) determined that the regional eustatic sea-level rise in PortElizabeth 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 associatedwith global change. Other impacts include an increase in the frequencyand magnitude of storm surges, extreme climatic events (floods anddroughts), reduced mean rainfall and river flow, and increased anthro-pogenic stressors (development and pollution). Climate change hasresulted in a significant increase in themagnitude and return frequencyof sea-level extremes and it is likely that this will increase by an order ofmagnitude during the 21st century (Church et al., 2013; IPCC, 2013;Spencer et al., 2015; Wigand et al., 2015). There is a high likelihood ofan increase in the swell and wave height in the Southern Ocean(Church et al., 2013; Spencer et al., 2015), resulting in an increase inthe frequency and intensity of winter (July–August) storms along thecoast of South Africa (Theron and Rossouw, 2008; Theron et al., 2010).Port Elizabeth was identified as one of six areas in South Africavulnerable to sea-level rise and an increase in storm surges (Theronand Rossouw, 2008). In situ and modelled data at Cape St Francis(100 km to the west of Port Elizabeth) indicate a possible 17% increasein significant wave height as a result of climate change, increasing theheight of a storm surge swell with a return period of 1 year from6.7 m to 7.8 m (PRDW, 2009).

Saltmarsh formation andpersistence 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 increasedinundation, and limited available terrestrial/ecotone habitat for saltmarsh migration (Valentim et al., 2013; Schile et al., 2014; Smith andLee, 2015; Snedden et al., 2015; Veldkornet et al., 2015). Under risingsea-levels, estuarine basins will either become inundated, silt-up orreach a dynamic equilibrium condition when the marsh accretes at arate equal to the rate of relative sea-level rise (RSLR) (Thorne et al.,2014; Carrasco et al., 2016; Kirwan et al., 2016). If marshes remainspatially intact as sea-levels rise the marshes have the capacity tobecome 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 thatmay improve sediment accretion rates are nutrient enrichment ofestuarine waters, increased temperatures and elevated atmosphericCO2 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 inSouth Africa in terms of biodiversity and conservation importance(Turpie et al., 2002; Turpie and Clark, 2007). Colloty et al. (2001) appliedamodified botanical importance rating to Swartkops Estuarywhich alsoconsidered species richness, community type rarity and functionalimportance, increasing the ranking to 4th overall in a country wideassessment. The intertidal salt marsh habitat of the Swartkops Estuary

Please cite this article as: Bornman, T.G., et al., Relative sea-level rise andmarshes, South Africa, South African Journal of Botany (2016), http://dx.d

covers 165 ha (Van Niekerk and Turpie, 2011), while development hasaccounted for the loss of 87.5% of the supratidal salt marsh, with onlyfive ha remaining (Colloty et al., 2000; Van Niekerk and Turpie, 2011).

This study addresses the question of how themicro-tidal SwartkopsEstuary will adapt to increases in relative sea-level associated withglobal climate change. The null hypothesis that the Swartkops Estuaryintertidal salt marsh will be able to accrete sediment at a rate equal toor higher than the rate of current and predicted RSLR was tested bydetermining 1) how the estuarine habitat has responded to changeover the past seven decades, 2) the main physical drivers of the saltmarsh 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 EasternCape Province of South Africa (Fig. 1). Mean annual runoff estimatesare 75–85 × 106 m3 (Hill et al., 1974; Middleton et al., 1981) producedby 636 mm of mean annual rainfall (Reddering and Esterhuizen,1981). The 1354 km2 catchment is characterised by numerous smallimpoundments that have a limited influence on the total runoff to theestuary (Baird et al., 1986). The permanently open estuary opens intothe Indian Ocean in Algoa Bay and is considered an urbanised estuaryas it falls within the Nelson Mandela Bay Metropolitan area. TheSwartkops Estuary is influenced by a semi-diurnal tide with a verticaltidal range of 1.8 m (micro-tidal) and tidal variations that can be lessthan 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 distributionwas digitised usingArcGIS™ 10.3.1. (ESRI®) from 1939 (courtesy of the Department ofSurveys and Mapping) and 2007 aerial images (courtesy of the NelsonMandela Bay Metropolitan Municipality), updated with 2012 SPOT 6and 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 limitedextent of the 1939 aerial photograph. Fine scale horizontal mapping ofthe vegetation was partly done in the field using a GPS device andArcPad® (version 7.0) software. Nine permanent transects (three eachin the lower, middle and upper reaches) were established in the saltmarsh area in order to capture vegetation, elevation and soil physico-chemical variability throughout the estuary (Fig. 1). The percentagevegetation cover was determined using a 1 m2 quadrat every 5 m oneither side and four random quadrats every 20 m along the transectsduring field trips in February 2009, July 2009, February 2010 and July2010. Three replicate soil samples from the top 20 cm were collectedin up to four vegetation zones along each transect during the foursampling trips. Soil characteristicswere determined using the followingmethods: Soil moisture content (Gardner, 1965), organic content(Briggs, 1977); electrical conductivity (The Non-Affiliated Soil AnalysesWorking Committee, 1990) using an YSI 30 M/10 FT hand heldconductivity metre, pH (Black, 1965), redox potential (The Non-Affiliated Soil Analyses Working Committee, 1990) using a MetrohmAG9101 electrode; and particle size (Day, 1965) using the hydrometermethod. GPS coordinates for each transect and sediment sampling siteis provided in the supplementary material.

2.3. Relative sea-level rise

Hourly tidal heights (cm) above the chart datum were provided forthe port of Port Elizabeth by the South African Navy Hydrograhic Office(SANHO) from 1978 to 2014 (Fig. 1). The incorporation of the chartdatum allows for the modelling of sea-level (SL) rise relative to theland, and we therefore use the term relative sea-level (RSL) to denote



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

these data. The RSL rise/trend (RSLR) per annum was calculatedbyfitting aGeneralised LinearModel (GLS) to the time series ofmonthlyRSLs. Although monthly data effectively smooth out the short-periodcomponents 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 orderautoregressive (AR1) term as:

yi ¼ bti þmj þ ρ1 yi−1−bti−1−mj−1� �þ εi

where yi are themonthly RSLs, b is the trend (slope), ti is the timevector,mj are the 12 monthly values that account for the seasonal cycle, ρ1 isthe 1st order (lag 1) autoregressive coefficient that accounts for auto-correlation, and εi is the residual time series. The order of theautoregressive component was found by examining Auto CorrelationFunctions (ACF) and Partial ACFs (PACF) of the residuals after fitting aGLS that does not account for autocorrelation.


2.4. Sediment and wetland elevation

Nine permanent Rod Surface Elevation Table (RSET) stations wereestablished using the method detailed by Cahoon et al. (2002) (Fig. 1).Station 9 was lost halfway through the study and only data from theremaining eight will be reported on. The lower intertidal Spartinamaritima zone (low marsh) was selected as the zone for each stationas this would be the first intertidal salt marsh area exposed to risingsea-levels. S. maritima is also well known for trapping sediment(Leonard and Croft, 2006; Chelaifa et al., 2010; Kirwan et al., 2016).The RSET stationswere surveyed in tomean sea-level (0.01mm 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) inFebruary 2010, but the Feldspar, calcium carbonate shell fragmentsand later quartz granules, did not persist as a result of river floods andstorm surges. RSLR at the RSET stationswere calculated using themeth-od proposed by Cahoon (2015): RSLRwet = MLVw − RSLR; where



Table 1Comparison of area coverage of vegetation and habitat units in 1939 and 2012.

2012 (ha) 1939 (ha) Difference (ha)

Estuarine water 146.46 105.60 40.86Floodplain salt marsh 63.66 70.07 −6.41Intertidal salt marsh 60.70

�143.41 −22.65

Spartina maritima 60.06Supratidal salt marsh 96.80 127.04 −30.24Mudbanks 80.96 77.55 3.41Sandbanks 48.99 146.81 −97.82Zostera capensis 44.70 24.77 19.93Development 143.39 25.12 118.27Total (excl. development) 602.33 695.25 −92.92


RSLRwet is the relative sea-level rise at thewetland,MLVw is thewetlandsurface elevation trend from the RSET measurements and RSLR is therelative sea-level rise at the port of Port Elizabeth tide gauge. GPScoordinates for each RSET station is provided in the supplementarymaterial.

2.5. Statistical analyses

The species and environmental data were analysed using CANOCOfor Windows (version 5.04, Ter Braak and Šmilauer, 2012). CCA wasused to obtain an ordination of the vegetation data constrained byenvironmental 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 pairwisemultiple comparison on mean ranks test) were run using SigmaPlot12.0 (version 12.2.0.45), Systat Software Inc. The statistical analysis forthe 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 changedconsiderably 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, althoughmost of the development was concentrat-ed in the terrestrial and ecotone areas adjacent to the estuary. Thelargest loss in habitat was a 98 ha reduction in sandbank area. In turn,the estuarine open water area increased by 41 ha, despite both mapsdigitised during a low tide period. The floodplain, supratidal and inter-tidal salt marsh decreased in area coverage and only the submergedmacrophyte, Zostera capensis, increased its distribution. Floodplain andsupratidal salt marsh area decreased between the Settlers Bridge andthe Swartkops Village bridges (Fig. 2B). Some terrestrial areas abovethe three bridges (road and rail) at the Swartkops Village were convert-ed into supratidal and floodplain salt marsh areas. The intertidal saltmarsh and S. maritima community were mapped separately in 2012(Table 1), but was indistinguishable in the 1939 black and white aerialphotograph.

3.2. Vegetation and soil analyses

The first canonical axis (horizontal) described 54% of the variation inspecies-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

Fig. 2. A. Vegetation and habitat map of the Swartkops Estuary in 1939


positively with organic content (0.24) and moisture content (0.70).The second canonical (axis vertical) described 68% of the variation inspecies-environment relation and elevation (−0.87) was negativelycorrelated with this axis. The terrestrial fringe species, Lycium cinereum(Lyc_cine) and Tetragonia decumbens (Tet_decu), were associated withhigh elevation as they occurred in the floodplain above the supratidalzone (Fig. 3). Intertidal species such as S. maritima (Spa_mari) wereassociated with sediment with a high moisture, organic, clay and siltcontent, comparatively lower salinity concentration and a lower thanaverage elevation (Fig. 3). The dominant supratidal and floodplainfringe species, Sarcocornia pillansii (Sar_pill), occurred in sandy areaswith a lowermoisture and organic content (Fig. 3). Surface areas devoidof vegetation (Bare in Fig. 3) were largely restricted to very lowelevations 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 portof Port Elizabeth indicates a RSLR rate of 1.82 mm·year−1 from 1978to 2014 (Fig. 4; Table 2). Sea-level data is inherently variable andthe RSLR for the study period (2009–2014) indicate a rate of7.48 mm·year−1, while for the preceding three decades the rate was2.22 mm·year−1 (Fig. 4; Table 2).

3.4. Rod set elevation tables

The wetland surface elevation change over the past six years fromthe 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 MSLthan any of the other RSET stations (Fig. 5). RSET stations 1, 5, 6, 7 and

. B. Vegetation and habitat map of the Swartkops Estuary in 2012.



Fig. 3. CCA ordination of the nine transects in the lower, middle and upper reaches of theSwartkops 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; otherspecies abbreviations provided in the supplementary material; MC = moisture content;OC = organic content; RP = redox potential; EC = electrical conductivity).

Table 2Relative sea-level rise as measured at the port of Port Elizabeth.

Date rangeRate ± SE(mm·year−1) t p-value

1978–2008 2.22 ± 0.64 2.10 b0.051978–2014 1.82 ± 0.49 3.70 b0.00012009–2014 7.48 ± 3.56 2.10 b0.05


8 showed a significant (p b 0.05; n = 72) increase in elevation overthe study period, although not necessarily in consecutive years. RSET 3had a negative surface elevation trend, starting in 2011, the sameperiod that RSET 1 suddenly increased in elevation (Fig. 5). RSET 2 and4 showed no significant (p N 0.05; n = 72) change in elevation overtime.

Table 3 presents the wetland elevation change trend (VLMw) asmeasured by the RSETs, the RSLR rate (tide gauge record) for 1978–2014 and 2009–2014 (study period) and the resultant wetland relativesea-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 acceleratedRSLR of 7.48 mm·year−1, only RSET stations 1 and 7 show an elevationrate surplus. RSET station 3 shows the highest elevation rate deficitunder both RSLR rates. RSET stations 2, 3 and 6 are not keeping pacewith historical sea-level rise.

Fig. 4.Monthly mean relative sea-level (RSL) at t


4. Discussion

4.1. Vegetation and habitat distribution

Vegetation distribution in estuarine systems along the south andwest coast of South Africa typically comprise four community types,i.e. reeds and sedges, supratidal salt marsh, intertidal salt marsh andsubtidal (submerged) macrophyte beds (Coetzee et al., 1997). All fourof these occur in the Swartkops Estuary, although the reeds and sedgesare largely restricted to the upper reaches (outside of themapping areashown in Fig. 2). The 1939 vegetation map was produced using a blackand white aerial image as a reference, which made it impossible to de-termine the extent of S. maritima. Pierce (1982) reported though thatS. maritima specimens from the Swartkops Estuary were identified in1887, so it is apparent that it was present prior to 1939. Floodplainsalt marsh, characterised by supratidal salt marsh interspersed withhalophytic ecotone and terrestrial species, was mapped as a separatecommunity 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 beenlost since 1939. The loss of supratidal andfloodplain saltmarshwas sim-ilar to the results recorded by Colloty et al. (2000), i.e. 35 ha, but the lossof intertidal marsh was less than half of the 50 ha they calculated as theentire estuary was not considered in this assessment. The intertidal saltmarsh habitat of the Swartkops Estuary covers 165 ha (VanNiekerk andTurpie, 2011). The intertidal area determined by our study was slightlyless than this figure because of the limited extent of our mapping area.Although development has taken up ~118 ha of the land on andsurrounding the Swartkops Estuary, supratidal and floodplain saltmarsh still account for 96.80 ha and 63.66 ha respectively. Submergedmacrophyte beds (mostly Z. capensis) increased their distribution by~20 ha. However their cover and distribution is dynamic with completeremoval reported following large floods in late 1984. Prior to this Talbotand 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.

he port of Port Elizabeth from 1978 to 2014.



Fig. 5. Change in wetland surface elevation at eight RSET stations over the study period.


The largest obvious changes since 1939, other than increased devel-opment, was the 98 ha reduction in sandbank area with a concomitantincrease in estuarine water area of 41 ha. The area coverage of thesehabitats undergo dramatic changes depending on tidal range, floodsand storm surges and the water level at the time the images weretaken could differ by more than 1 m. However, the replacement ofexposed sandbanks by open water over the past seven decades appearsto 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 ofreduced 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 thesandbanks, as well as the increase in water column eutrophication(Adams et al., under review), was in all likelihood responsible for theincrease in Z. capensis area cover over time. The sediment dynamics ofthe catchment and estuary were well described by Hill et al. (1974);Erasmus et al. (1980); Middleton et al. (1981); Reddering andEsterhuizen (1981) and Fromme (1988). Hill et al. (1974) attributedthe relatively minor fluvial silt deposition in Swartkops River to thelack of large scale agriculture in the catchment. These sediments aredeposited 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 tideis the dominant source of sediment load into the lower and middlereaches of the estuary (upper extent of the map in Fig. 2) (Redderingand Esterhuizen, 1981).

In the upper reaches, above the Swartkops Village bridges, most ofthe supratidal area was replaced by floodplain salt marsh, indicatingan increase in elevation or, more likely, a reduction in the reach of thetidal water onto the salt marsh. Reddering and Esterhuizen (1981)predicted that the supratidal salt marsh will eventually be elevatedabove the spring high tide level, converting them to floodplain saltmarsh. A possible explanation for these changes could be the sedimenttrap created by the construction of the bridges near the Swartkops

Table 3Relative sea-level rise at the eight RSET stations (RSLRwet) under two RSLR rates.

1978–2014

RSET VLMw

(mm·year−1)RSLR (mm·year−1)

1 8.98 1.822 −0.81 1.823 −10.74 1.824 4.43 1.825 5.56 1.826 0.65 1.827 9.6 1.828 6.19 1.82


Village. Reddering and Esterhuizen (1988) suggest that most of thesedimentation, especially of fines, occurring in the Swartkops Estuaryis freshwater flood driven. The embankments of the railway and roadbridges have created a constriction to flow during flood conditions,resulting in the deposition of suspended sediments upstream of thebridges (Reddering and Esterhuizen, 1988). The mapping study alsoindicated that large sections of floodplain salt marsh upstream and ad-jacent to the road and rail embankments have been converted tosupratidal salt marsh, probably because of damming of water upstreamof the channel constriction.

Most of the supratidal habitat in the middle and lower reaches hasbeen replaced by S. maritima, intertidal salt marsh and mudbanks. Thisis contrary to what Reddering and Esterhuizen (1981) predicted, sug-gesting that the influence of the Settlers Bridge would act as a sedimenttrap behind it, similar to the Swartkops Village bridges, raising the levelof the marsh. The change in salt marsh community, however, indicateincreased flooding in the lower reaches (more intertidal habitat) withincreased deposition taking place against the seaward side of theSwartkops Village road and railway embankments and on leveesbordering the main channel (increased supratidal habitat). Belliardet al. (2016) found that monospecific vegetation, such as S. maritima,typically grows in the low-lying marsh interior, thus supportingsalt 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 settleout in the subtidal channels, rather than on the salt marsh. A reductionin fine sediment reaching the lower reaches (because of depositionabove the Swartkops Village bridges) and a reduction in marine sedi-ment ingress across the berm (because of the Settlers Bridge roadembankment) would have resulted in an overall reduction in sedimentinput to the salt marshes in the lower andmiddle reaches over the pastseven decades.

The landwardmargin of the entire study area depicted in Fig. 2B hasbeen developed and the salt marsh habitat is bordered on all sides byhard structures such as roads, artificial embankments, railway lines,housing developments and industrial areas. These developments willprohibit the migration of the salt marsh into upland areas (Carrascoet 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 maylead to substantial reductions in productivity and organic matteraccretion (Snedden et al., 2015). Canonical Correspondence Analysisidentified three distinct communities, i.e. a floodplain, supratidal andintertidal/subtidal community. The following elevation ranges werecalculated for these communities from the transect data: subtidal =b0.5 m above mean sea-level (AMSL), intertidal = 0.3–2 m AMSL, andsupratidal/floodplain = 1.8–2.5 m AMSL. The most important driversdetermining the species distribution in the Swartkops Estuary were

2009–2014

RSLRwet

(mm·year−1)RSLR (mm·year−1) RSLRwet

(mm·year−1)

−7.16 7.48 −1.52.63 7.48 8.29

12.56 7.48 18.22−2.61 7.48 3.05−3.74 7.48 1.92

1.17 7.48 6.83−7.78 7.48 −2.12−4.37 7.48 1.29




soil moisture content and elevation. Elevation has been identified as animportant 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 sedimentsurfacewill be regulated, mostly through sediment accretion, to reach anew equilibriumwith themean sea-level (Morris et al., 2002). This willdepend on the import and deposition of large volumes of sedimentwhich may not necessarily be available in the sediment load of theSwartkops River or in Algoa Bay. As the elevation changes vertically,so toowill the saltmarsh communities have to change their distributionhorizontally along the elevation gradient. Intertidal species, such asS.maritima and Triglochin spp., were associatedwith lower than averageelevations and sediment with a high moisture, organic, clay and siltcontent. RSLR may therefore significantly impact the S. maritimadynamics 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 contributingfactor in intertidal elevation as surmised from the GIS maps. Themajor-ity of banks in the vicinity of S. maritimaweremuddy (high silt content)rather than sandy. Sand is however the main sediment fraction in thesupratidal marshes.

Sediment salinitywas not identified as a key driver in the SwartkopsEstuary, primarily because of the low values recorded during this study(maximum of 18.74). Water column salinity of the Swartkops Estuarycan be highly variable and is dependent on rainfall and river flow(Baird et al., 1986). Marais (1975) and Grindley (1985) reported hyper-salinewater column conditions between 1969 and 1972, reaching 42 inthe upper reaches, due to low rainfall and high evaporation rates (Bairdet al., 1986). It is expected that stormwater runoff and treated seweragereturn flow would have increased several fold over the past threedecades, thereby ensuring amore constant freshwater input, despite in-creased abstraction from impoundments in the catchment. Enrichmentof the Swartkops Estuary tidal waters (Adams et al., under review)should improve the productivity of the estuarine vegetation that inturn may result in increased sediment accretion rates that may in turncompensate 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 includechanges resulting from the vertical motion of both the land andthe sea surface. Analyses of the mean monthly tide gauge data fromthe 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 ofthe length of the time series used. Analyses of RSL from 1978 to 2008produced 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 longerdatasets, the RSLR for the study period (2009 to 2014) was 7.48 ±3.56 mm·year−1. This significantly increased rate is not an indicationof accelerated RSLR, but rather a result of the short time series used tocalculate the rate. This is particularly evident in Fig. 5 where the RSLRtrend of the last 6 years only approaches the four decade trend line in2014. Although our study was not interested in eustatic sea-level rise,Mather et al. (2009) calculated vertical crustal movement at Port Eliza-beth at+0.66mm·year−1, resulting in a regional eustatic sea-level rise(corrected for crustal movement and barometric change) of between3.55 and 3.75 mm·year−1 (Mather et al., 2009).

The RSLR for Port Elizabeth is currently below the global tide gaugeRSLR rate of 2.8 ± 0.8 mm·year−1 (Church and White, 2011; Churchet al., 2013; IPCC, 2013). The global estimated rate of sea-level riseusing satellite data is higher at 3.2 ± 0.4 mm·year−1 (Church andWhite, 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 tidalvariability in the Swartkops Estuary closely follows that of the port ofPort Elizabeth, although the levels are amplified in the estuary, probablybecause Cape Recife and the harbour structures protect the tide gaugefrom wave set-up. The estuary would therefore be exposed to greatersea-level variability andwould 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 RSETshowed a significant decline in elevation over the study period. Therewas no spatial pattern in the surface elevation change and individualRSET stations rather responded to local geomorphological changes,e.g. erosion of the channel bank at RSET 3 and sand deposition at RSET7 as a result of the natural meandering of the tidal channels. Thorneet al. (2014) determined that proximity to a sediment source was themost important factor determining whether an area increased in eleva-tion or not and that accretion processes must be considered whenforecasting salt marsh accretion rates, especially in urbanised estuaries.The mean wetland surface elevation trend (VLMw) for the SwartkopsEstuary was 2.98 ± 2.34 mm·year−1. Kirwan et al. (2016) foundthe mean rate of elevation change for intertidal marshes to be6.9 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 ±1.33mm·year−1. Despite the negative elevation trend at two of the sta-tions, themeanVLMwwas still higher than historic RSLR, indicating thatoverall the S. maritima marshes are keeping pace with sea-level rise.Kulawardhana et al. (2015) reported that marsh loss or changes inVLMw may often be due to an insufficient mineral sediment supplyand vertical accretion rate rather than directly from RSLR.

To compare the Swartkops Estuarywetland elevation trendwith thesea-level trend, one could either use the historic sea-level trend(1.82mm·year−1) or the short-term sea-level trend (7.48mm·year−1)over the study period (Cahoon, 2015). Although the confidence in themore variable shorter-term sea-level trend is lower, both were com-pared to the wetland elevation trend to determine if 1) the wetland iskeeping pace with historical sea-level rise and 2) what the response ofthe wetland would be should the short-term RSLR trend continue. Thewetland surface elevation trend (VLMw) at the historic RSLR of1.82 mm·year−1 resulted in a RSLRwet where five of the eight RSETstations showed an elevation rate surplus. At the short-term RSLR rateof 7.48 mm·year−1, the RSLRwet resulted in only two RSET stations(one in the lower reaches and one in the upper reaches) experiencingan elevation rate surplus. In the other six RSET stations the sea-levelwas becoming higher relative to the salt marsh surface. Kirwan et al.(2016) found that less than 5% of salt marshes studied around theworld were being submerged by RSLR, but that this figure could changeshould RSLR accelerate in future. The elevation dynamics in saltmarshesare regulated by vertical accretion over longer time periods (Rogerset al., 2013), but the RSLRwet and study period RSLR dataset for theSwartkops Estuary is still so short that the trends are influenced byshort temporal scale oceanographic and hydrological events.

All predictions indicate a significant increase in the magnitude andreturn frequency of sea-level extremes off the coast of South Africa(Theron and Rossouw, 2008; PRDW, 2009; Theron et al., 2010; Churchet al., 2013; IPCC, 2013; Spencer et al., 2015; Wigand et al., 2015). Inaddition, significant changes in land use/land cover in urban settingswill amplify storm surge within estuaries (Bilskie et al., 2014; Primeet al., 2015; Yang et al., 2015). However, it appears that storms havelittle impact on the longer-term elevation dynamics within stable saltmarsh habitats (Rogers et al., 2013; Spencer et al., 2015) because theflow is dampened across the marsh and storm-induced sedimentationcould in fact stabilise coastal marshes in the short term to assist in




copingwith sea-level rise (Baustian andMendelssohn, 2015). However,Raposa et al. (2015) recorded a community shift in response to extremewater levels as salt marshes are replaced by lower elevation species andeventually converted into mudflats/sandbanks. Storm surges along thecoast of South Africa can be severe and Zhang et al. (1995) recordedlarge scale erosion of estuarine banks in the adjacent Gamtoos Estuaryduring a storm surge (wave height N6.5 m and sea level N2.5 m in theport of Port Elizabeth) in 1992. Trapping sediments from storm surgeswill create a positive feedback mechanism to extend S. maritima area(Pierce, 1982), as long as there is an adequate sediment supply andthe 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 inmean rainfall and runoff (Arnell, 1999; Clark et al., 2000; Tadrosset 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 andbase flow will not only alter the sediment input to the estuary, butwill also impact on saline intrusion (Prandle and Lane, 2015), therebyinfluencing salt marsh zonation patterns. The shallow, micro-tidal andflood-tide dominant nature of South African estuaries makes themmore susceptible to the loss of fines as the main sediment source isthe sea (Schumann, 2013).

5. Conclusion

RSLR threatens low-lying coastal ecosystems, human communitiesand infrastructure on a global scale (Pethick, 2001; Neumann et al.,2015; Spencer et al., 2015). The intertidal salt marshes of the SwartkopsEstuary 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 saltmarsh to accelerated sea-level rise is unknown, and will be difficult topredict since most of the ecosystem drivers and response variables arechanging with RSLR, e.g. freshwater inflow, storm surges, sedimentsupply, water quality, productivity and CO2 concentration. Kirwanet al. (2016) is however of the opinion that most salt marshes will beable to keep pacewith accelerated sea-level rise due to inlandmigrationof the marshes and through biophysical feedback processes that willaccelerate sediment accretion. The majority of South African estuariesoccupy drowned river valleys that offer limited upland area intowhich to migrate and those estuaries that do have low-lying adjacenthabitat have mostly been developed, creating a physical barrier topotential 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 enablingupland salt marsh migration (removal of hard structures) (Wigandet al., 2015). Observations of RSLR and the response of coastal ecosys-tems remains a critical area of socially-relevant scientific research toinform 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 beyondthe Swartkops, Kromme and Knysna estuaries, specifically up the eastcoast to include mangrove habitat, and that appropriate material formarker horizons are found so that the influence of subsurface processescan 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 thefield and in the laboratory. The research was funded by the Marine


Living Resources Fund (MCM2007073100018) of the Department ofEnvironmental Affairs (DEA, Oceans and Coasts) and the Departmentof Agriculture, Forestry and Fisheries (DAFF). The RSET stations andtransects form part of the Long-Term Ecological Research array of theSouth African Environmental Observation Network (SAEON) of theNational Research Foundation (NRF). TheDepartment of Botany, NelsonMandela Metropolitan University, are thanked for providing logisticalsupport. Relative sea-level data was provided by the South AfricanNavy 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.

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