ORIGINAL ARTICLE

Establishing relative sea level trends where a coast lacksa long term tide gauge

Joanna Ellison & Pippa Strickland

Received: 31 August 2013 /Accepted: 29 November 2013# Springer Science+Business Media Dordrecht 2013

Abstract Vulnerability assessment of coastal areas to projected sea level rise requires incor-poration of historic trends in relative sea level change as an exposure factor. Most shorelines ofdeveloping countries lack long term tide gauges, such as the Pacific Islands region, which areespecially vulnerable to climate change impacts. This study has the objective of demonstratinghow long-term relative sea level trends can be derived from proxy records, on the tectonicallyunstable main island of Fiji. At Tikina Wai on the western coast, while elevations of presentmangrove zones of Rhizophora stylosa, Rhizophora samoensis and Bruguiera gymnorrhizawere <1.2 m around mean sea level, sediment cores down to 3 m showed mangroveoccurrence meters lower than they can grow today. Pollen analysis identified past locationsof these mangrove species zones, and the present day elevations of the species were used toreconstruct past sea levels. Results of this study showed that relative sea-level has been slowlyrising for the last several centuries at about 2.1 mm a−1, yet mangrove communities haveremained resilient with nearly equivalent net sedimentation rates, though with some zoneretreat landwards. With such local subsidence, the Tikina Wai district is more exposed to futuresea level rise projections than stable coastal areas elsewhere, with additional exposure inhaving a micro-tidal range. Adaptation actions identified to address this risk include enhance-ment of sedimentation under mangrove communities through coastal and catchment planningto remove obstructions to sediment supply, reducing non-climate stresses to increase organicproduction, and replanting of degraded areas. Such information on relative sea level trends canbe used to identify where adaptation resources are best concentrated.

Keywords Adaptation . Climatechange .Mangrovecommunities .Pacific Islands .Sea level rise .

Vulnerability assessment

1 Introduction

Vulnerability is the potential to be harmed by a combination of exposure and sensitivity tostresses, and is reduced by the capacity to adapt to those stresses (Adger et al. 2007; Mertzet al. 2009). Exposure refers to the character, magnitude, and rate of change that a species or

Mitig Adapt Strateg Glob ChangeDOI 10.1007/s11027-013-9534-3

J. Ellison (*) : P. StricklandUniversity of Tasmania, Launceston, Tasmania, Australiae-mail: Joanna.Ellison@utas.edu.au

system is likely to experience, and assessments of vulnerability of ecosystems to climatechange impacts have focussed on climatic modelling of temperature and precipitation changesto evaluate exposure (Dixon et al. 2003; Leemans and Eickhout 2004; Vos et al. 2008;Klausmeyer and Shaw 2009). For mangrove communities, climate warming is however likelyto be mostly beneficial, increasing mangrove productivity and biodiversity (Nicholls et al.2007; Gilman et al. 2008). By contrast, the effects of relative sea-level rise are likely to benegative or even severely detrimental tomangrove ecosystems (Gilman et al. 2008; Krauss et al.2010), hence the inclusion of relative sea-level trends for such coastal sites is vital for theirvulnerability assessment and adaptation planning. Recent global mean sea level rise projectionsvary with scenarios of anthropogenic forcings across ranges of 0.26–0.98 m by 2100 (IPCC2013), which are higher than previous projections of 0.18–0.59 m by 2099 (1.5–9.7 mm a−1)(IPCC 2007). However relative sea level trends at different coastlines will depart significantlyfrom global mean sea level rates owing to uplift or subsidence (Meehl et al. 2007).

Changes in sea level can result from variation in the volume of ocean water or adjustmentmovement of the land, continental shelf or ocean floor (Fig. 1), and a coastal location willexperience relative sea-level change or stability owing to a combination of these factors. Somecoastal areas experience long term relative sea level rise owing to tectonic subsidence,sediment compaction or fluid extraction (Syvitski et al. 2009; Nicholls and Cazanave 2010),to which will be added projected global sea-level rise caused by expanding oceans and icemelt. Coastlines with subsidence such Bangladesh (Karim and Mimura 2008) and ChesapeakeBay (Arenstam Gibbons and Nicholls 2006) are more exposed, while those with tectonic upliftare less exposed (Nicholls and Lowe 2004; Nicholls and Cazanave 2010). Such relative sealevel trends are measured by long term tide gauges (Bindoff et al. 2007), however mostcoastlines in the developing world lack such records.

The Pacific Islands are predicted to highly vulnerable to global climate change (Nichollsand Mimura 1998; Nicholls and Lowe 2004; IPCC 2007; Voccia 2012), because of highexposure and low adaptive capacity. On these coastlines, sedimentary sheltered areas aredominated by mangrove communities which provide coastal accretion, stability and protectionvalues, as well as a food resource for local communities (Ellison 2009a). Direct humanpressure has resulted in significant mangrove Rhizophora area losses in the last few decades(Spalding et al. 2010; Giri et al. 2011), to which sea-level rise impacts will add further pressure.

Fig. 1 Causes of relative sea-level change in mangrove areas

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Measurement of present day sea-level change is by two different techniques, tide gaugesand satellite altimetry (Bindoff et al. 2007). Tide gauges directly show sea level variations withrespect to the coastal land on which they lie, which is the information needed for a coastalclimate change vulnerability assessment. A record of over 19 years is needed to average outcycles in tidal amplitudes and phases (Pugh 1987), and a record of at least 30 years to show atrend within acceptable confidence limits (Emery and Aubrey 1991). For developing states onsmall islands, sea level rise represents the most severe consequence of climate change (Voccia2012). However, there are few long term tidal records in the Pacific Islands region (Mitchellet al. 2000; Church et al. 2006), and, owing to tectonic settings, some of those that do exist areonly locally relevant.

The term mangrove refers to both the intertidal forest communities of low latitude shores, aswell as the constituent species (Tomlinson 1994; Spalding et al. 2010). Given that mangrovecommunities are restricted to within the tidal range (Boto and Bunt 1981; Wolanski et al. 1992;Ellison 2009b), their fossil records have been used to reconstruct past sea-levels. Belowcanopy mangrove pollen deposition has been shown in many studies to reflect local specieszonation, particularly using percentage abundance results as opposed to pollen concentration(Muller 1964; Spackman et al. 1966; Woodroffe et al. 1985; Grindrod 1985, 1988; Behlinget al. 2001, 2004; Ellison 2005). Distribution of pollen in surface mud of a mangroveecosystem has showed that there is a high proportion of the genus Rhizophora in andimmediately adjacent to the Rhizophora zone (Muller 1959; Wijmstra 1969; Grindrod 1985,1988; Woodroffe et al. 1985; Behling et al. 2001; Ellison 2005), which can be used as a sea-level indicator. Rhizophora is a dominant genus in the majority of mangrove ecosystems of theworld (Tomlinson 1994). Elevations of modern mangrove communities can be used tointerpret an accurate sea-level reconstruction from stratigraphic mangrove pollen patterns(Ellison 1989, 1993, 2005; Behling et al. 2001).

Vulnerability assessments to sea level rise impacts in many cases are not able to includeconsideration of relative sea level trends of coastlines (Gravelle and Mimura 2008; Louckset al. 2010), as such information is not available. Coastal managers require site specificinformation on relative changes in sea level to determine vulnerability (Suresh Babu et al.2012), in order to prioritise options in adaptation planning. This study demonstrates how long-term relative sea level trends of a coastline without a long term tide gauge can be derived fromproxy records. Such information will allow better assessment of the vulnerability of coastlinesto global sea level rise (Nicholls et al. 2007).

2 Study area

Fiji consists of about 844 islands and islets, of which 106 are inhabited, located between thelatitudes of 15o30′ and 20o 30′S, and around the 180o meridian. The total land area is18,272 km2, and 87 % of this occurs in the two largest islands, Viti Levu and Vanua Levu.Larger islands are of volcanic origin, and smaller islands derived from calcareous deposits andlimestone (Ellison 2009a). All islands are surrounded by fringing and barrier reefs, and Fiji hasthe third most extensive mangrove area in the Pacific Island region.

The Fijian islands occur across a complex plate boundary between the obliquely convergingPacific and Indo-Australian mega plates, and Viti Levu is located on a micro-plate betweenthese (Nunn 1998). The island is believed to have domed throughout the Quaternary(Dickenson 1967; Nunn 1998), at rates fast enough to affect recent relative sea-level (Nunnand Peltier 2001), with the south coast uplifting and the north coast subsiding (Miyata 1984;Nunn 1990, 1991; Nunn and Peltier 2001), and the west coast likely slowly subsiding (Nunn

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1998). Other islands are either rising or subsiding (Nunn and Peltier 2001), exemplifying howdirect application of global sea-level projections is inappropriate in regions such as Fiji.

Viti Levu is a high volcanic island, with the highest peak rising to 1,324 m and an area of10,389 km2 (Nunn 1998). Fiji’s climate is tropical and influenced by the South East Tradescausing an orographic influence on rainfall distribution across the higher islands of Viti Levuand Vanua Levu, with up to 6,000 mm a−1 in the windward mountains and leeward rain-shadows of about 2,000 mm a−1 (Fiji Meteorological Service 2013). The El Nino SouthernOscillation (ENSO) creates climate variations every 2–5 years, particularly affecting winds andprecipitation (Meehl 1996).

The network of 12 high resolution SEAFRAME (Sea Level Fine Resolution AcousticMeasuring Equipment) tide gauges have been established across the Pacific since the early1990s (Mitchell et al. 2000), including one at Lautoka in Northern Viti Levu installed inOctober 1992. These records are too short for a reliable long-term estimate of change in meansea level (Pugh 1987; Emery and Aubrey 1991; Aung et al. 2011), and have been showingstrong effects of El Nino on records so far (Aung et al. 2011). The 18.9 years Lautoka recordshowed a rate of relative sea level rise of 4.9 mm a−1 (Aung et al. 2011). There is a longer termtide gauge at Suva, where a 27 year record to 2001 showed a rate of relative sea-level rise of6.7 mm a−1 (Church et al. 2006). This record however has gaps in the sea-level data, and thecharacter of the record is somewhat different from nearly locations, calling into question thedatum reliability of this gauge (Church et al. 2006). The Lautoka record is also affected byvertical tectonic movement (Aung et al. 2011).

Fiji has a total mangrove area of 425 km2, and 7 native mangrove species and one hybrid(Ellison 2009a). Three species are the most common, Rhizophora stylosa and Rhizophorasamoensis both growing in deeper water relative to Bruguiera gymnorrhiza, which occurs moreto landward. R. stylosa and R. samoensis have crossed to create the sterile hybrid Rhizophora xselala. Other less common mangrove species include Xylocarpus granatum, Lumnitzeralittorea, Excoecaria agallocha, and Heritiera littoralis (Smith 1981; Pillai 1990), which allgrow close to the dry land margin. The mangrove fern Acrostichum auruem is also widespread.

The Lomawai mangrove area (18o02.20′S, 177o17.23′E) is part of the Tikina Wai district onthe leeward side of Viti Levu, with an annual rainfall of c. 2,000 mm (Fiji MeteorologicalService 2013). This drier climate causes extensive salt flats in the upper intertidal zone, whichare rare elsewhere in Fiji and have cultural significance for traditional salt making (Fiu et al.2010). The majority of local people are semi-subsistence farmers and fishermen dependent onland and marine resources for food security and income, including the district’s 441 ha ofmangrove resources. Mangrove wood is harvested for firewood and construction materials,and fish and crabs are a major source of protein. Three mangrove reserves are regularlysurveyed by appointed Monitors reporting to a Marine Resource Management Committee withrepresentatives from each local village (Ellison 2009a), and a Mangrove Area ManagementPlan provides a broad guide by which decision makers can ensure the sustainable use of thearea to meet local needs for food, income and cultural maintenance. The district has under-taken a vulnerability assessment for climate change impacts on the mangrove resources (Fiuet al. 2010) in order to prioritise options in adaption planning, and requested information onrelative sea-level trends of their coastline.

3 Methodology

Three cores were collected from the Lomawai mangrove area (Fig. 2), along a shore-perpendicular transect of 1.4 km from sea to land, using Hiller and Russian Peat sampler

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sidewall corers that ensure no vertical compaction (Moore et al. 1991). This sample designreduces the influence of local land based edge effects, and maximises the record of influence ofnon-local factors such as climate and sea-level (Ellison 2008). Coring sediment to reconstructpast environments is research guided by assumptions such as Walther’s Law ofUniformitarianism, which states that “The various deposits of the same facies areas andsimilarly the sum of the rocks of different facies areas are formed beside each other in space,though in cross-section we see them lying on top of each other” (Middleton 1973, p. 979). Theguidance that this gives to core based research is that one core is representative of a basin,which is why stratigraphic studies use limited replication unless they are looking for finerdetails of basin sedimentary evolution (Ellison 2008). The principle also guides that sedimen-tary units get older with increasing depth.

Core LW1 was recovered from 10 m inside the seaward edge of the mangrove area inRhizophora stylosa forest, core LW2 was recovered from the centre of the mangrove areaadjacent to a creek, in mixed Bruguiera/Rhizophora forest, and LW3 400 m from the landwardmargin of Rhizophora adjacent to salt flats (Fig. 2). The even surface mud level was taken asthe top of each core (0 cm depth), and elevations of core surfaces and the mangrove zonalboundaries were surveyed relative to MSL (mean sea level) by a differential GlobalPositioning System and automatic levels to benchmarks in the village recording MSL datumheight. Cores were sub-sampled at 0.1 m intervals and sample contamination was prevented bywashing outside of the corer before opening, and dismantling and washing it before recoveringthe next lower section. The color of stratigraphic units was determined by comparison withMunsell Soil Charts, and texture determined by feel analysis (Thien 1979).

Pollen concentration was carried out using standard techniques (Faegri and Iverson 1989)modified for resistant mangrove sediments (Ellison 2008), and to each sample a knownnumber of exotic spores were added to allow the determination of pollen concentration persediment volume. Pollen was identified by comparison with a reference collection andpublished descriptions (Weng et al. 2007). Other palynomorphs such as fungal spores,

Fig. 2 Map of the Lomawai mangrove area showing vegetation zone distributions and location of coring sites

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microforaminifera, dinoflagellates and chlorophyllaceae were excluded from the count.Results were transferred into pollen diagrams using Tilia and Tiliagraph (Grimm 1988),showing the relative representation of each taxon as a percentage of the total pollen sumwhich includes mangrove taxa, non-mangrove trees and shrubs, ferns, herbs and aquatics. Theremainder of each sample was analysed for percent organic matter content by loss-on-ignitionat 550 °C for 2 h (Bengtsson and Enell 1986). Four radiocarbon dates were obtained from thecores, one from each core at about a depth of 1 m to determine recent net sedimentation rates,and one from a lower level in the seaward core to indicate long term relative sea level trends.Age determinations were carried out by Accelerator Mass Spectrometry (AMS) radiocarbondating, with acid wash pretreatments and δ13C determination, which is a measure of the ratioof the Carbon (C) stable isotopes of 12C and 13C that allows identification of the sourceenvironment of the sediment. Calibration of radiocarbon dates to conventional years wascarried out using the Pretoria Calibration procedure (Talma and Vogel 1993; Vogel et al. 1993).

4 Results

The elevation survey results showed an elevational range of 1.17±0.01 m for mangrovecommunities in the Lomawai mangrove area (Table 1) across a 1.4 km transect, with a groundlevel gradient of 0.07 %. The mangrove seaward edge of Rhizophora was around 0.34 mbelow MSL, and Rhizophora dominated from this elevation up to where Bruguiera occurredmixed with Rhizophora above 0.23 m above MSL. The landward Rhizophora boundary withsalt flats was at 0.66 m above MSL. Rhizophora occurs across this whole range from R. stylosaat the seaward edge, R. samoensis inside this, and stunted R. samoensis and R. selala at higherelevations, while Bruguiera grows in a narrower range from 0.23 to 0.83 m above MSL.

Sediments have accumulated over time along the study transect, as shown by the dates anddeep sediment accumulation under the present mangrove area, with cores LW1 and LW2 coresnot reaching basal rock (Fig. 3). Sediment was predominantly fine grained silts and clays, withshallow sand lenses indicative of storms or tsunamis. Organic content was about 10 % atdepth, increasing to around 20 % towards the surface in seaward cores. However, sedimentremained relatively inorganic nearer the surface at the landward core LW3 (Fig. 4).

Radiocarbon results are shown in Table 2, including calibrated years for each radiocarbondate, and net sedimentation rates calculated from depths above each date. The radiocarbondates shown relative to elevation of the three cores (Fig. 3) show increasing age with depthbetween all three cores. The 93–98 cm date from LW1 is younger relative to the other twocores, and higher sedimentation rates (Table 2) may reflect disturbance within the mangrovesediment, such as bioturbation. Recent crab mounds were observed at the site, in which case

Table 1 Elevations of mangrovezone boundaries and core surfacelevels at Lomawai mangroveswamp relative to MSL Datum,with an estimated survey errorof ±0.01 m

Location Elevation (m)

Upper Bruguiera boundary 0.83

Saltflat/Rhizophora boundary 0.66

Rhizophora/Bruguiera boundary 0.23

Rhizophora seaward edge −0.34LW1 core surface elevation 0.22

LW2 core surface elevation 0.23

LW3 core surface elevation 0.73

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bioturbation can introduce modern carbon to lower levels of sediment, so causing netsedimentation rates to over-estimated. Other core sites did not show recent bioturbation, withnet sedimentation rates at the other two cores of 1.1–2.0 mm−1.

Pollen diagrams of the cores (Figs. 5, 6 and 7) show the relative representation of eachtaxon recorded in samples down each core as a percentage of the total pollen sum, whichincludes mangrove taxa, non-mangrove trees, shrubs, herbs, aquatics and ferns, and excludesother palynomorphs from invertebrates and lower plants. Surface samples in each core showpollen assemblages found under the present community, and lower levels represent commu-nities present back through time. The line at −0.36 m in Fig. 3 shows the lowest elevationmangrove communities can grow at the present time.

In the seaward core LW1 mangrove pollen was dominant at 70–95 % of the total sumthroughout the 3 m core with a combination of Rhizophora, Bruguiera gymnorrhiza,Excoecaria agallocha, Heritiera littoralis and Xylocarpus granatum found (Fig. 5).Rhizophora dominated for the top 0.95 m with 50–80 % abundance, then a sharp transitionat 95–100 cm to 60 % domination by Bruguiera pollen, which fluctuates but tends to increasein domination with depth to 3 m. Pollen concentrations through the core were between 20,000to 55,000 grains cm−3 with a few outliers at around 80,000 grains cm−3 (Fig. 5). There was ageneral decrease in percentage organics with depth from around 30 % to 8 % with one outlierof 50 % organic content found at 0.3 m depth (Fig. 4). The lowest depth relative to this corethat mangrove communities can grow today is 0.56 m (Table 1, Fig. 3).

Fig. 3 Stratigraphy of cores across the Lomawai mangrove transect (vertical exaggeration=× 3.75), andcalibrated radiocarbon dates. Elevations have been adjusted to MSL datum, and the horizontal line at −0.36 mis the lower limit of mangrove habitats today

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In the central core LW2 mangrove pollen of 70–80 % of the pollen sum dominated theupper 1 m, and 60–80 % 1.0–2.6 m (Fig. 6). Both Rhizophora and Bruguiera are presentthoughout this depth, with increasing presence of Heritiera littoralis at lower levels. Pollenconcentrations through the core were between 30,000 to 60,000 grains cm−3 with an outlier at1.3 m depth, and there was a gradual increase in percent organic content with depth (Fig. 4).Below 2.8 m there is an increase in proportions of pollen of dryland trees and shrubs combined

Fig. 4 Results from percent organic matter determination in Lomawai cores

Table 2 AMS radiocarbon dating results from Lomawai cores, with calibrated radiocarbon dating ages and netsedimentation rates calculated above each date. Ages were calibrated using the Pretoria Calibration Procedure for shortlived samples (Talma and Vogel 1993; Vogel et al. 1993, recommended by the Beta Analytic Radiocarbon Laboratory)

Core Beta samplecode

Depth (m) δ13C 14C age(year BP)

Conventionalage (cal yr BP)

2 Sigma calibration(cal years AD)

Net sedimentationrate (mm a−1)

LW1 260030 0.93–0.98 −26.8 150±40 120±40 1670–1780 2.8–4.2

1790–1960 4.3–19.4

LW1 260031 1.93–1.98 −27.3 1110±40 1070±40 890–1030 1.7–2.0

LW2 260032 0.93–0.96 −27.0 540±40 510±40 1330–1340 1.4

1400–1450 1.2–1.7

LW3 260033 0.63–0.71 −25.7 440±40 430±40 1420–1500 1.1–1.3

1600–1610 1.6–1.7

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with a decrease in mangrove pollen (Fig. 6). The lowest depth relative to this core thatmangrove communities can grow today is 0.57 m (Table 1, Fig. 3).

In the landward core LW3 Rhizophora dominated at the top of the core with mangrovepollen at 60–100 % of the total sum for the top 1.4 m before falling to around 20 % at 2.3 m(Fig. 7). Pollen concentrations were between 45,000 to 12,000 grains cm−3, generally decreas-ing with depth, and with a couple of outlying higher concentrations at 0.4 m and 1.3 m depth(Fig. 7). Percent organic content was found to be lower than in more seaward LW1 and LW2cores, ranging from 3 to 15 %, and did not have the decreasing trend with depth found in theother two cores at Lomawai (Fig. 4). The lowest depth relative to this core that mangrovecommunities can grow today is 1.07 m (Table 1, Fig. 3).

Fig. 5 Lomawai Core LW1 stratigraphy and pollen diagram, showing pollen percentage of most frequent taxa,sums of ecological groups, pollen concentrations and calibrated radiocarbon dates

Fig. 6 Lomawai Core LW2 stratigraphy and pollen diagram, showing pollen percentage of most frequent taxa,sums of ecological groups, pollen concentrations and calibrated radiocarbon dates

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5 Discussion

Surface levels of each pollen diagram show the modern pollen assemblage for the mangrovevegetation type, which can be used to help interpret assemblages at lower depths. The seawardcore (Fig. 5) was in Rhizophora forest, with a few Bruguiera trees adjacent, and close to a sandridge dominated byCocos nucifera. The pollen assemblage in surface mud had 75% Rhizophorapollen, 20 % Bruguiera pollen and 15 % Cocos pollen, with a concentration of about 40,000grains cm−3. The lowest level at which the lowest elevation mangrove species Rhizophora cangrow in current sea levels is 0.56 m depth on Fig. 5 (Table 1, Fig. 3) and yet little change occursbelow that. If this mangrove area was prograding laterally in stable sea level, then at 0.56 m depththe mangrove pollen proportion would fall to 50 % or less and pollen concentration would alsodecrease, as shown for the Cameroon coast of central Africa (Ellison and Zouh 2012).

Pollen diagrams from all three Lomawai cores rather indicate the presence of mangrovecommunities at lower levels than today throughout the last few centuries, with the percentageof mangrove pollen consistently around 80% alongwith high pollen concentrations. In core LW1this extends to at least 3 m depth (Fig. 5), in core LW2 to 2.6 m depth, and in core LW3 to 1.5 mdepth (Fig. 7). These depths are up to 2 m below where mangrove communities can grow today(Fig. 3), and results of high percentage of mangrove pollen (Muller 1959; Bartlet and Barghoorn1973; Cohen and Spackman 1977) (Figs. 5, 6 and 7) and with δ13C levels typical of mangrovesediment (Woodroffe 1981; Ellison 1993, 2005) (Table 2) all show that the core sites were withina mangrove forest throughout this deposition period of up to 1,000 years. Mangrove communitiesin these past few centuries were able to grow at deeper elevations than at present, showing that thesite has experienced relative sea-level rise over the last 1,000 years. Suchmangrove pollen recordsextending to lower than present mangrove forest elevations have been found elsewhere (Bartletand Barghoorn 1973; Grindrod 1985, 1988; Ellison 2005), in each case interpreted as risingrelative sea level over time. Subsiding coasts such as southern New Guinea show mangrovepollen of c. 80 % consistently dominant up to 6 m depth in a location with a <3 m tidal range(Ellison 2005), and results from Lomawai show mangrove pollen of c. 80 % consistentlydominant up to 3 m depth, in a location with a 1.5 m tidal range (Gravelle and Mimura 2008).

Fig. 7 Lomawai Core LW3 stratigraphy and pollen diagram, showing pollen percentage of most frequent taxa,sums of ecological groups, pollen concentrations and calibrated radiocarbon dates

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The seaward edge core (Fig. 5) has the most distinctive transition with the more landwardmangrove species Bruguiera dominating the lower part of the core with 40–60 % of the pollensum. At 100 cm from the top of the core, dated at around 120 years calibrated years before present,Bruguiera suddenly declines, and is replaced by a seaward community ofRhizophorawith around60 % of the pollen sum, indicating sea-level rise and the landward migration of the mangroveswamp. The central core (LW2) (Fig. 6) shows a gradual transition from Rhizophora to Bruguieradominated forest over the core period, but the record is less clear than the other two cores probablybecause it is located to a creek edge (Fig. 2) that may have migrated over time. The landward core(Fig. 7) on the edge of the salt flat (Fig. 2) has a distinct transition from a dry land tree dominantcommunity in the lower half of the core to a mangrove dominated community in the upper half,again indicative of landward migration of the mangrove swamp in response to subsidence.

Bruguiera grows today in the elevation range of 0.23–0.86 m aboveMSL, and pollen records(Fig. 5) show that the 1.93–1.98 level in the seaward core (1.71–1.76 m below MSL datum)formed in a Bruguiera forest, with similar relative abundances to the surface modern analoguesof core LW2 (Fig. 6). Using the Bruguiera elevation today to position to former sea level, thecalibrated date of 1,070±40 formedwhen sea-level was 2.28±0.34m below currentMSL datum.The error is derived from both the date error and the elevation outer limits. Dividing this depth bytime gives a rate of relative sea-level rise of 2.1±0.4 mm a−1. This result of relative sea-level riseconcurs with the interpretation of tectonic subsidence previously found on this coastline (Nunn1998; Nunn and Peltier 2001), probably also associated with sediment accumulation on the broadshelf offshore (Nunn and Peltier 2001), this demonstrated by the deep sediment deposits found inthis study. These replicated results of mangrove community presence at depths below today(Figs. 5, 6 and 7) indicate that sedimentary accretion has beenmostly able to keep upwith the rateof sea-level rise during the last 1,000 years, with gradual landward migration of the mangroveswamp (Figs. 5 and 7). A similar pollen record has been shown from southern West Papua withrelative sea-level rise as a result of tectonic subsidence (Ellison 2005) and gradual landwardmigration of mangrove zones, contrasting with pollen records from Northern Australia whichrather show sea-level stabilisation about 6,000 years ago followed by seaward progradation ofmangrove zones without elevation change (Woodroffe et al. 1985; Grindrod 1988).

In the mangrove area, net recent sedimentation rates from cores LW2 and LW3 are shownto be around 1.1–2.0 mm a−1 (Table 2), and the seaward core indicating more rapid sedimen-tation and mixing. In estuaries of southern West Papua net mangrove sedimentation rates of0.6–1.5 mm a−1 occurred (Ellison 2005) hence rates at Lomawai are similar. Use of surfaceelevation tables and associated techniques can allow quantification of the different contribu-tions to mangrove sediment accretion such as organic detritus from the mangrove communi-ties, mineral sediment from river discharge, and soil volume change and compaction (Cahoonet al. 2002, 2006; McKee et al. 2007; Krauss et al. 2010). For a vulnerability assessment thenet vertical accretion that is the consequence of all of these is the sensitivity factor, and suchrates can be calculated based on the average for the longer term record, though do not indicatevariation during different time periods within this (McKee et al. 2007).

Results of this study therefore demonstrate that sea-level has been slowly rising on theTikina Wai coastline of western Viti Levu for the last several centuries, with inorganicdominated net sedimentation rates of 1.1–2.0 mm a−1 allowing mangrove communities to stayin position but not quite keeping up, so causing landward migration of mangrove zones overtime. This is shown by high proportions of mangrove pollen in stratigraphy at depths metersbelow where mangrove communities currently occur, along with high pollen concentrations inall stratigraphy and δ13C results typical of mangrove sediment. Hence this area of Viti Levuwith a long-term relative sea-level rise rate of about 2.1 mm a−1 is more exposed to global sealevel rise than coastal areas elsewhere that are stable. In conditions where relative sea-level rise

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exceeds sediment accretion rates, mangrove communities die back from the seaward edge andretreat landward, following the shift in intertidal range (Gilman et al. 2008), as demonstratedfrom the extensive coastal swamps of southern West Papua (Ellison 2005, 2009b). Sedimentsupply determines mangrove ability to keep up with relative sea-level rise, and is most criticalto reduce the exposure of subsiding coasts (Nicholls and Cazanave 2010).

6 Vulnerability assessment

Relative sea-level trends at coastal sites are an important component of a vulnerabilityassessment with respect to climate change, but are difficult to quantify at remote sites(Fuentes et al. 2011) or those without a long term tide gauge (Al-Jeneid et al. 2008). Thisstudy has demonstrated that where the coastline lacks a long term tide gauge, relative sea-leveltrends can be reconstructed from pollen analysis of mangrove stratigraphy in order tocontribute to a vulnerability assessment.

Projections of the IPCC 4th Assessment were of global eustatic sea level rise of 0.18–0.59 m by 2099 (1.5–9.7 mm a−1) (IPCC 2007), and the 5th Assessment projects up to 0.98 mby 2081–2100 relative to 1986–2005 sea levels (IPCC 2013). Results from this study showthat these global rates are the minimum applicable to the Tikina Wai coastline as a futureexposure factor. Adding the long term local subsidence component of 2.1 mm a−1 would give ahigher local sea-level rise projection, such as 3.6–11.8 mm a−1 compared to global projectionsof 1.5–9.7 mm a−1 (IPCC 2007). This study has shown that the long term net sedimentationrates in the Lomawai mangrove communities are mostly between 1.1 and 2.0 mm a−1 thoughhigher at the seaward edge, hence projected rates of sea-level rise are in excess of mangrovenet sedimentation rates. Over the last few centuries this mangrove coastline has shownadaptive capacity to a relative sea level rise of 2.1 mm a−1, but it will likely show increasedexposure to increased rates of rising sea-level, that may result in dieback.

As in all microtidal areas, the elevation ranges where mangrove communities grow aresmaller than those for macrotidal areas. Table 1 shows the elevational range of mangrovecommunities at Tikina Wai to be less than 1.2 m, and individual species less than this. Withoutnet sedimentation and with relative sea-level rise, the inland migration of these inundationhabitats across the low gradient intertidal slope (Fig. 2) would involve relocation of the currentmangrove seaward edge to more than half way towards the current landward edge, and resultin loss of the salt flats which have cultural significance (Fiu et al. 2010). This can be mitigatedby prioritizing several adaptation actions that maintain and enhance sedimentation.

Substrate elevation change is the net consequence of the site sediment budget, with a rangeof biophysical sources. At a range of mangrove communities in Belize and Florida (McKee2011) found that elevation change rates varied from −5.4 to +10.9 mm a−1 with mangrove rootcontributions a major source. At Lomawai, inorganic content results (Fig. 4) show mangrovesediments to be dominated by inorganic input, which is highest in the landward LW3 locationindicating catchment sources. Other sediment sources include longshore transport, gains fromoffshore of sediment derived from coral reefs and calcareous seagrass beds, and autochthonousinput from mangrove productivity. Major losses include longshore transport down-coast,mangrove litter and sediment lost offshore and erosion. Erosion can be enhanced by higher-energy conditions such as boat wakes, which also tend to affect the seaward edge and creekmargin mangrove communities, which are more vulnerable to sea level rise.

At sites identified to be vulnerable to relative sea level rise, adaptation planning shouldtherefore prioritise sedimentation rates and surface elevation increase. Root mat growth hasbeen found to be a major contributor to surface elevation gain and vertical development is

Mitig Adapt Strateg Glob Change

enhanced when mangrove communities are more productive (McKee 2011), but elevationlosses occur where plant growth is low (McKee et al. 2007). Increased peat retention insediment also contributes to climate change mitigation, and organic content results (Fig. 4)show a relatively low organic contribution to net sedimentation. Reducing non-climatestressors on mangrove communities such as disturbance will enhance their productivity. InTikina Wai this has only recently been improved through monitoring of resource use, andimposition of social controls by the Resource Management Committee.

Replanting of degraded areas also enhanced mangrove productivity and seedlings promotesedimentation from root mat development, causing sediment surface elevation gain underdensely replanted mangrove communities at both high and low tidal sites (Huxham et al.2010). Seedling density also enhances accretion rates by providing friction to tidal watermovement to promote sediment flocculation and settling (Huxham et al. 2010; Kumara et al.2010). The Tikina Wai district has undertaken planting of mangrove seedlings in degradedareas, as well as along disturbed creek margins and on offshore sandbanks.

In summary, the following actions promote mangrove sedimentation:

1. Reduction of non-climate stressors, such as human impacts, to improve health andcondition of the existing mangrove forest

2. Rehabilitation of degraded mangrove areas, particularly sections that are eroding, as denseseedlings enhance accretion

3. Coastal zone planning to remove obstructions to sediment supply. This includes removalor redesign of coastal structures that interrupt longshore drift or enhance reflective waveaction

4. Influencing river dam design and operation to maintain fluvial sediment supply to themangrove area

5. Prohibition of sediment removal or dredging from areas that are a source of sediment tomangrove areas

6. Reduction and control of boat wakes close to mangrove areas and margins.

Active enhancement of mangrove sediment accretion rates, such as by use of coastalstructures, has been shown to be successful in mangrove restoration along an eroding coastlinein Malaysia (Hashim et al. 2010; Kamali et al. 2010; Tamin et al. 2011). Another possibility isthe beneficial use of dredge spoils, which could augment mangrove sediment elevation (Lewis1990) but which would need to avoid excessive or sudden sediment deposition that can killmangrove species (Ellison 1998). An accidental dredge spill onto an offshore tidal flatoccurred in King Bay, Western Australia, and sediment transported into the mangrovecommunities by tides provided a 1 to 2 mm deposition (Semeniuk 1994) without causingnegative effects. Such active adaptation actions are an area for future research, particularly atmore vulnerable subsiding coastlines.

7 Conclusions

Sea level rise is the greatest threat to mangrove communities of all the consequences of globalwarming, and detailed information on relative sea level rise is necessary for effective adapta-tion strategies. Given that many mangrove coastlines lack long term tide gauges, this study hasshown how disciplines of palaeoecology can be directly used in vulnerability assessment andcontribute to site specific adaptation planning. Other techniques of past sea level reconstructionusing precise indicators such as coral microatolls and wave cut notches (Miyata et al. 1990;Nunn and Peltier 2001) can also be so utilised. Information on relative sea level trends on

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subsidence/uplift in the Pacific islands and elsewhere would be a way of identifying whereresources could be concentrated for mitigating sea-level rise.

Rising sea-levels are expected to alter the position, area, structure, species composition andhealth of mangrove communities. The mangrove communities of Lomawai have shownresilience to past relative sea level rise, largely maintaining position during sea-level rise ofthe last 1,000 years at least. There has been slow landward migration which demonstrates thatthey will be further impacted by increased rates of relative sea-level rise unless adaptation thatreduced exposure is prioritised. Vulnerability of mangrove communities and response to sea-level rise will also be influenced by anthropogenic disturbances and such stresses, whichreduce the resilience of the ecosystem to adapt to sea-level rise. Growing populations in thePacific Island region are likely to increase resource pressure and pollutant impacts onmangrove communities in the future. The key to continued resilience of such mangrove areasis site specific vulnerability assessment to identify the exposures and sensitivities of eachsystem, and using results to prioritize adaptation planning that reduce these as part of a targetedstrategy. Local communities such as Tikina Wai are the best level to carry out such plans inmangrove areas that support resource-dependent people, facilitated by accessible technical andstakeholder support, useful legislation and associated procedures, and adequate resourcing.

Acknowledgments This research was funded the United Nations Environment Program (UNEP) GlobalEnvironment Facility (GEF) project “Coastal resilience to climate change: Developing a generalizable methodfor assessing vulnerability and adaptation of mangroves and associated ecosystems” awarded to the WorldWildlife Fund (WWF US). Research was facilitated by Monifa Fiu and Francis Areki of the WWF South PacificProgram Office, and we thank the communities of Tikina Wai villages for their hospitality, assistance duringfieldwork and support of the research. Brigid Morrison and Rob Anders of the University of Tasmania alsohelped during fieldwork, Kesho Sharma of the Fiji Lands Department assisted with surveys to benchmarks, andMichael Helman drew the Figures. We are grateful for the comments of two anonymous reviewers who allowedimprovements to the manuscript.

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