DOI: 10.5061/dryad.pg4f4qrr9
title: Predicting changes in molluscan spatial distributions in mangrove forests in response to sea-level rise

description: Molluscs are an important component of the mangrove ecosystem, and the
 vertical distributions of molluscan species in this ecosystem are
 primarily dictated by tidal inundation. Thus, sea-level rise (SLR) may
 have profound effects on mangrove mollusc communities. Here, we used
 dynamic empirical models, based on measurements of surface elevation
 change, sediment accretion, and molluscan zonation patterns, to predict
 changes in molluscan spatial distributions in response to different
 sea-level rise rates in the mangrove forests of Zhenzhu Bay (Guangxi,
 China). The change in surface elevation was 4.76–9.61 mm yr−1 during the
 study period (2016–2020), and the magnitude of surface-elevation change
 decreased exponentially as original surface elevation increased. Based on
 our model results, we predicted that mangrove molluscs might successfully
 adapt to a low rate of SLR (2.00–4.57 mm yr−1) by 2100, with molluscs
 moving seaward and those in the lower intertidal zones expanding into
 newly available zones. However, as SLR rate increased (4.57–8.14 mm yr−1),
 our models predicted that surface elevations would decrease beginning in
 the high intertidal zones and gradually spread to the low intertidal
 zones. Finally, at high rates of SLR (8.14–16.00 mm yr−1), surface
 elevations were predicted to decrease across the elevation gradient, with
 molluscs moving landward and species in higher intertidal zones blocked by
 landward barriers. Tidal inundation and the consequent increases in
 interspecific competition and predation pressure were predicted to
 threaten the survival of many molluscan groups in higher intertidal zones,
 especially arboreal and infaunal molluscs at the landward edge of the
 mangroves, resulting in a substantial reduction in the abundance of
 original species on the landward edge. Thus, future efforts to conserve
 mangrove floral and faunal diversity should prioritize species restricted
 to landward mangrove areas and protect potential species habitats.
description: Six transects were drawn from the seaward forest
 edge to the landward forest edge, and elevation was measured in 5–10 m
 horizontal intervals along each transect using a Global Navigation
 Satellite System-Real Time Kinematic GPS unit (G970 GNSS RTK, UniStrong
 Inc.) with a vertical precision of 15 mm (Fig. 1a). Measured elevations
 were converted to elevations above/below local mean sea level, which was
 determined based on control points located 3 km from the study area (Fig.
 1a). To sample along the entire elevation gradient, a total of 36 sampling
 sites were established at 25 cm intervals between −15 and 150 cm
 elevation; there was no sampling site at 135 cm as there was a gap in the
 mangrove forest at this elevation.From April 2017 to January
 2018, molluscs were sampled quarterly at each of the 36 sampling sites
 during low tide. To collect molluscs as comprehensively as possible, we
 sampled the arboreal, epifaunal, and infaunal molluscan communities within
 the mangrove forest at each site. Arboreal molluscs attached to the
 mangrove trunks, leaves, and prop roots were collected by hand in three
 randomly placed quadrats (5 × 5 m; 10 m apart) at each sampling site. To
 collect epifaunal molluscs, five quadrats (1 × 1 m; 5 m apart) were
 randomly placed at each sampling site, and all epifaunal molluscs on the
 sediment surfaces within each quadrat were collected. To collect infaunal
 molluscs, one quadrat (0.25 × 0.25 m) was randomly placed in each
 epifaunal quadrat. The sediment in each infaunal quadrat was collected to
 a depth of 30 cm and sieved through a 1 mm mesh to obtain infaunal
 molluscs.  From April 2017
 to January 2018, molluscs were sampled quarterly at each of the 36
 sampling sites during low tide. To collect molluscs as comprehensively as
 possible, we sampled the arboreal, epifaunal, and infaunal molluscan
 communities within the mangrove forest at each site. Arboreal molluscs
 attached to the mangrove trunks, leaves, and prop roots were collected by
 hand in three randomly placed quadrats (5 × 5 m; 10 m apart) at each
 sampling site. To collect epifaunal molluscs, five quadrats (1 × 1 m; 5 m
 apart) were randomly placed at each sampling site, and all epifaunal
 molluscs on the sediment surfaces within each quadrat were collected. To
 collect infaunal molluscs, one quadrat (0.25 × 0.25 m) was randomly placed
 in each epifaunal quadrat. The sediment in each infaunal quadrat was
 collected to a depth of 30 cm and sieved through a 1 mm mesh to obtain
 infaunal molluscs.  Changes in
 surface elevation were recorded using nine RSET instruments (Cahoon et
 al., 2002) at 3–12 month intervals between July 2016 and August 2020,
 comprising a total of eight sets of measurements; the total period
 assessed was 49 months. The nine RSET benchmarks were established in July
 2015 and distributed among the <em>A. corniculatum</em>,
 <em>K. obovata</em>, and <em>B. gymnorhiza</em>
 communities along the SJ transect at depths of 5–6 m (Fig. 1b). Each RSET
 instrument consisted of a deep benchmark and a measuring arm. During RSET
 installation, a deep benchmark was established at each measurement
 location by driving stainless steel rods (15 mm in diameter) into the soil
 profile until refusal and then fixing the rods in place with cement. A
 machine-notched pipe was connected to the top of the stainless-steel rod
 to hold the measuring arm. When it was time to take a reading, the
 measuring arm was attached to the benchmark and levelled. Nine fiberglass
 pins were inserted into the measuring arm and lowered onto the soil
 surface, and the distance from the arm to the top of each pin was
 measured. These nine distance measurements were repeated in four
 directions, yielding 36 total measurements, which were combined to
 generate an average reading per sampling site. Sediment
 accretion was measured using the
 marker-horizon method around both the RSET benchmarks and the transect
 sampling sites. Marker horizons around RSET benchmarks were deployed in
 July 2016 and were sampled at 3–12 month intervals until December 2018,
 comprising a total of six sets of measurements; the total period assessed
 was 29 months. Marker horizons around the transect sampling sites were
 deployed in July 2017 and were sampled at 3–6 month intervals until August
 2019, comprising a total of four sets of measurement; the total period
 assessed was 25 months. Three feldspar markers (50 cm × 50 cm) were
 deployed around each sampling site or RSET benchmark. At each sampling
 time, one cubic soil core was taken at an undisturbed location on each
 marker horizon, and the depth of the sediment above the marker horizon at
 three positions in each core was measured to give an average reading per
 marker horizon. The mean depth across the three cores was used to
 represent sediment accretion at each sampling site or RSET
 benchmark.
description: Abbreviation:<em> Nn, Nassarius nodifer; Ns, Nassarius sinarus;
 Bz, Batillaria zonalis; Mc, Merisca capsoides; Cc, Cerithidea cingulata;
 Cf, Clithon faba; Ls, Littoraria scabra; Ny, Nerita yoldi; Bm, Batillaria
 multiformis; Pp, Patelloida pygmaea; Cd, Cerithidea djadjariensis; Pc,
 Pinguitellina cycladifomis; La, Laternula anatina; Gc, Glauconome
 chinensis; Ea, Enigmonia aenigmatica; Ab, Assiminea brevicula; Lp,
 Littoraria pallescens; Lm, Littoraria melanostoma; Is, Indoaustriella
 scarlatoi; Iq, Iravadia quadrasi; Os, Onchidium struma; Al, Assiminea
 latericea; Md, Mainwaringia dantaae; Lo, Laemodonta octanfracta; Lar,
 Littoraria ardouiniana; Ss, Salinator sanchezi; Isp, Iravadia sp.; Ip,
 Indoaustriella plicifera; Cl, Cerithidea largillierti; Ts, Terebralia
 sulcata; Cm, Cerithidea microptera; Pa, Pharella acutidens; Nv, Neritina
 violacea; Sj, Stenothyra japonica; Ala, Allochroa layardi; Isa, Iracadia
 sakaguchii; Gco, Geloina coaxans; Co, Cerithidea ornata; Cn, Cassidula
 nucleus</em>