Spatiotemporal Distribution Of Mangrove Molluscs Under Sea-level Rise

Eighty-one percent of Chinese mangrove is located at the outside of seawalls now due to the large-scale construction of seawalls.Under the background of global sea level rise(SLR),Seawall and rising sea level compress the living space of mangrove,threatening its existence.Molluscs are an important part of the mangrove ecosystem,an understanding of the relationship between the spatial distribution of molluscs in mangrove and tidal inundation is required for effective ecosystem management and conservation,which is significant for the prediction of the response of mangrove ecosystems under global climate change.The distributions of mangrove flora and fauna are greatly affected by tidal inundation,and surface elevation is a reliable quantitative indicator of the effects of tidal inundation.Most recent studies have focused exclusively on the quantitative relationships between mangrove-plant zonation and surface elevation,neglecting mangrove molluscs.In this study,we measured surface elevation in a typical mangrove forest of subtropical inner bay wetland in Zhenzhu Bay(Guangxi,China),using a real-time kinematic global positioning system(Real Time Kinematic GPS unit,RTK).Combined with traditional quarterly quadrats survey of different life-form molluscs,the influences of surface elevation on the spatial distributions of mangrove molluscs was studied.In addition,structural equation model(SEM)was used to study the effects and pathways of tidal inundation on molluscs with different life-forms.Rod Surface Elevation Table(RSET)and two measuring method of surface accretion were used to predict the responses of mangrove mollusc communities to different rates of SLR,and evaluated the vulnerability of mangrove molluscs outside the seawalls to SLR.The main results and conclusions of this study are as follows:(1)The spatial distributions of mangrove plants and molluscs showed distinct patterns of zonation related to surface elevation,species composition of community exhibited obvious correlation with surface elevation,different species tended to inhabit different niches along the elevation gradient and the niche overlap between different species was high,independent of season.(2)Several significant correlations were identified among mangrove species.A significant negative correlation was found between the distribution of Aegiceras corniculatum and Bruguiera gymnorhiza,while a significant positive correlation was found between the distribution of Excoecaria agallocha and Lumnitzera racemosa.The distribution of some molluscs was also significantly correlated with certain mangrove plants.The distribution of Littoraria melanostoma was significantly positively correlated with A.corniculatum,while the distribution of L.melanostoma was significantly negatively correlated with B.gymnorhiza.The distribution of Indoaustriella plicifera and some epifaunal molluscs like vioacea were significantly positively correlated with Avicennia marina.(3)Compared with flooding time and other abiotic drivers,biotic drivers had the most significant direct effects on mangrove molluscs.The increase in the abundance of mangrove plants significantly increased the abundance of arboreal molluscs while significantly reducing the abundance of epifaunal and infaunal molluscs,and reducing the species and functional diversity of different life-forms of molluscs.The increase in the species diversity of mangrove plants significantly reduced the abundance of arboreal molluscs,and at the same time significantly increased the abundance of epifaunal and infaunal molluscs,and significantly increased the species diversity and functional diversity of different life forms of molluscs.Infaunal molluscs were mainly affected by the abundance of mangrove plants,while epifaunal molluscs were more significantly affected by the species diversity of mangrove plants.(4)Tidal inundation affected the abundance,species diversity and functional diversity of mangrove molluscs directly and indirectly by modulating abiotic and biotic drivers.Molluscs with different life-forms had different effects and pathways of being affected by tidal inundation.The increase in flooding time directly increased the abundance and diversity of arboreal molluscs and the abundance of epifaunal molluscs,but for the diversity of epifaunal molluscs and the abundance and diversity of infaunal molluscs,the direct influence of the flooding time was not significant,and mainly exerted an indirect effect through abiotic and biotic drivers.The abundance of arboreal molluscs was extremely sensitive to changes in flooding time,because the increase in flooding time would increase the abundance of arboreal molluscs through direct and indirect effects.For the species diversity and functional diversity of arboreal molluscs and the abundance of epifaunal molluscs,the direct and indirect effects of flooding time were opposite.For the abundance and diversity of infaunal molluscs,the indirect effect of the flooding time through abiotic and biotic drivers was opposite.The various ways of tidal flooding on molluscs restricted the influence of flooding time on the community structure of molluscs except for the abundance of arboreal molluscs.(5)The surface elevation changes in mangrove forests at Zhenzhu Bay were 4.769.61 mm a-1,experienced a significant increase in surface elevation.The two monitoring methods both show that the surface elevation changes were proportional to the surface accretion rate.Surface elevation change decreased exponentially with increasing ground elevation,the monitoring data of the plastic-mesh tile methods was smaller than the marker horizon.(6)We established a dynamic surface elevation changes model based on different sea level rise rates.Our model showed that at the current rate of sea level rise(2 mm a-1),the surface elevation of mangrove will increase by 2100,and the mangrove will expand toward the sea.Tidal inundation and inter-species competition may drive mollusc toward the seaward intertidal zone.The high-intertidal area of the mangrove was more vulnerable to the rapid rise of sea level.Under the SLR of 4.57-5.20 mm a-1,the surface elevation of the high-intertidal area will decrease.When the SLR reaches 6.88-8.14 mm a-1,the surface elevation of all mangrove tidal flats will decrease and mollusc will migrate toward the landward intertidal zone,the area of mangrove will decrease.Under the SLR of 16 mm a-1,the width of simulated section will be reduced to about 17.0-29.6%in 2020.Except for marine species such as Littoraria scabra on the sea edge,the abundance of most molluscs will decrease significantly with the increase in the expected elevation of the distribution,especially the Cerithidea ornata and Cassidula nucleus on the land edge,which abundance will decrease by 1%in 2020 or even disappears.Rapid SLR and landward barriers(such as seawalls,roads and mountains)will cause coastal squeeze to mangrove,leading to the loss of molluscs in high-intertidal area and the intrusion of molluscs from seaward areas to the landward areas,changing the original mollusc community structure and reducing the overall species diversity.This study quantifies the relationship between mangrove molluscs and surface elevations,systematically and comprehensively assesses the effects and pathways of tidal inundation on mangrove molluscs with different life-forms,and reveals the response of different life-forms of mollusks to tidal inundation.The vulnerability of mangrove to different rates of sea level rise has been assessed,providing a basis for predicting the impact of global climate change on mangrove ecosystems.