The Impacts Of Environmental Change On The Biogeographic Patterns Of Rocky Shore Macrobenthos Along China's Coast

The Yangtze River delta,one of the most thriving and prosperous economic zone in China,is also a heavily populated metropolitan region.To cope with the intractable challenges such as soaring land demands,coastal erosion,sea level rise and other threats,an extensive range of hard-substrate seawalls have been constructed in this area.Seawalls are believed to be able to substantially change the original bottom conditions and landscape structures,while simultaneously providing suitable habitats for many species typically found on rocky shores.These hard-substrate seawalls,serving as stepping stones,may possibly accelerate the pole-ward shifts in geographic ranges of rocky shore species,and alter their biogeographic patterns in the scenario of global warming.An integrated approach,including community ecological survey,phylogeographic analysis,physical oceanographic simulation,metabarcoding and physiological study,was carried out to decipher the impacts of environmental change on the biogeographic patterns of rocky shore macrobenthos along China coast.This study ultimately aimed to enlighten researches on the potential effects of the hard-substrate seawalls on the structure of intertidal communities,the larval dispersal processes,distributions and population genetic structures of rocky shore species,along with the role of climate change in the expansion and population connectivity of rocky shore species along China's coastline.The hard-substrate seawalls were observed to facilitate the northern expansion of the intertidal species from southern natural rocky shores.The results of a series of field surveys on rocky shore macrobenthos communities on seawalls and adjacent natural rocky shores field surveys conducted from 2013 to 2017 clearly indicated high similarity between the community structures of southern natural rocky shores and the hard-substrate seawalls which were south of 33°N.This result also served as evidence that the species on the seawalls south of latitude 33°N may originate from southern natural rocky shores.Nerita yoldii and Crassostrea sikamea are two southern hard-shore species and their historical northern distributional range edges lied around 30°N decades ago.With the construction of seawalls,their northern distribution range limits had moved further north to 33-34°N.The seawalls changed the original bottom condition and provided suitable habitats for southern rocky shore species.Subsequently,seawalls functioned in favour of the northern expansion of certain rocky shore macrobenthos from southern natural rocky shores,and further reconstructed the biogeographic patterns of rocky shore macrobenthos.The hard-substrate seawalls also harbored distinct rocky shore macrobenthos and had realtivel low species richness from the natural rocky shores.The analysis of a diversity showed lower species coverage,species richness and species diversity of macrobenthic communities on the seawalls compared with their counterparts on natural rocky shores.Different species assemblages were also found on the seawalls and natural rocky shores.This leads to our hypothesis that seawalls may not have the whole ecological function as natural rocky shores,and usually result in the decline of biodiversity and ecological value compared to natural habitats.Habitat heterogeneity probably accounted for the relatively higher ? diversity in the communities of hard-substrate seawalls along Jiangsu province.Hence,it seems crucial to increase the habitat heterogeneity of hard-substrate seawalls if further artificial infrastructures are inevitably to be implemented on the shores,on the basis of enhancing biodiversity and ecological values on these seawalls.The hard-substrate seawalls promoted the expanadjacent sion of Fistulobalanus albicostatus from southern and northern natural rocky shores.Significant genetic differences between the southern group and northern group were observed based on the analysis of COI gene structure of F.albicostatus,however,the geographical distances between the boundaries of southern group and northern group had narrowed from hundreds krns to only 40 kms after the contructions of hard-substrate seawalls.This demonstrated that hard-substrate seawalls had a positive role in the expansion of southern group and northern group.Notably that southern haplotypes was observed to move across the Yangtze River estuary,which was traditionally regarded as the phylogeographic break for F.albicostatus.In the context of climate change,the seawalls along the Yangtze River delta act as corridors for the gene flow between southern and northern populations of rocky shore macrobenthos.Population connectivity between southern and northern populations of rocky shore species can be improved as well amid the construction of hard-substrate seawalls.In spring and summer,planktonic larvae of some rocky shore species from southern rocky shores could go across the Yangtze River estuary and exist at the coast of Jiangsu province,as clearly indicated from the results of larval dispersal simulations.Furthermore,the existing seawalls in this region possiblly act as suitable habitats for the settlement of these larvae.Larvae from individuals on the seawalls on Yangte River delta can possibly distribute further north to the northward in spring and summer.Thus,seawalls play as a role of stepping stones for the larval communication between the communities of southern and northern rocky shores,enhancing the population connectivity of macrobenthos from these two shores.Research based on metabarcoding technique with COI gene as the biomarker to identify the larval compositions in adjacent seawaters around the Yangtze River estuary,further elucidate the ongoing tendency of the southern species and population moving further north which was previously showed from the results of larval dispersal simulations.Extreme-high temperature(34.5?)on the rocky shore and heavy rainfall(>50 mm/h)would remarkably decrease the cumulative number of larvae hatching per unit weight of egg ribbon of Siphonaria japonica.The increase of extremely hot days and changes of in the coastline along China had negative effects on the population dynamics and larval recruitments of Siphonaria japonica.The robust salinity and temperature tolerance of larvae would be beneficial to larval survival and dispersal in different envirorunent of adjacent sea and contribute dramatically to the communication and expansion of Siphonaria japonica.In summary,the hard-substrate seawalls also had a key role in changes in the distributions of rocky shore macrobenthos and reshaping their biogeographic pattern.The constructed seawalls,as stepping stones,also positively affected larval communication between the communities of southern and northern rocky shores,as a result,population connectivity was significantly improved between the macrobenthos from different rocky shores concerned.These seawalls will further impact on the biogeographic patterns of rocky shore macrobenthos along China's coast in the face of global climate change to come.Overall,this project is critical to better understanding of the potential impacts in distribution of rocky shore species under the influences of both climate change and human activity,and provides technical support to predict future changes in the biogeographic patterns of rocky shore macrobenthos.