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Bornman et al 2016

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Bornman et al 2016

  1. 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. 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. 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. 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. 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. 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. 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. 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). 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