| Crabs are large benthic animals widely distributed in coastal wetlands,which have certain disturbance effects on the sedimentary environment of wetlands.Crab burrows are unique structures for crabs to adapt to and to transform the environment of wetlands.There is a certain distribution law among those burrows.In addition,crabs can also influence the decomposition,burial and release of biogenic elements in sediments through feeding,transportation,burrowing or other ways,which are often known as "ecosystem engineers" by researchers.In this paper,a field study was established in Fengxian costal wetland,and three different habitat types were selected to study the distribution characteristics of crab burrows and the disturbance effect of crabs.A fixed habitat survey sample was set in the communities of Spartina alterniflora(low-tide zone),Phragmites communis-Spartina alterniflora(middle-tide zone)and Phragmites communis(high tide-zone),respectively.The field survey and seasonal study were carried out from December 2018 to September 2019.The distribution of crab burrows and habitat characteristics were obtained in winter(December),spring(march),summer(June)and autumn(September).On this basis,a field experiment was carried out on the influence of different disturbance processes of crabs on the physical and chemical properties of sediments in areas with intense crab disturbance.In addition,crab and non-crab control experiments were set up in different plant community types to continuously determine the dynamic changes of vegetation characteristics and plant growth characteristics caused by crab disturbance during a growing season.The main results of this paper are summarized as follows:Ⅰ.Distribution characteristics of crab burrows in Fengxian coastal wetland(1)The density of crab burrow in high tide Phragmites communis was significantly higher than that of the others(P<0.05),and reached the highest value of the year in summer.The comparison of mean opening diameter of crab burrows between habitats showed that high tide Phragmites communis habitat was significantly lower than that of low tide Spartina alterniflora habitat and middle tide mixed habitat(P<0.05),and there was no significant difference between the latter two habitats(P>0.05).In autumn,the mean opening diameter of crab burrows were significantly lower than that of other seasons(P<0.05),while there was no significant difference between other seasons(P>0.05).(2)Crab burrow density has significant positive correlations with the elevation of tidal flat,sediment total organic carbon,total nitrogen content,moisture content,conductivity(P < 0.05),and significant negative correlations with vegetation density,the density of buds(P < 0.05),among them,the elevation of tidal flat and total organic carbon content have the highest positive correlation in all factors,vegetation density and bud density are negative correlation factors.Small crab burrows density has more significant correlation factors than large and medium-sized crab burrows.Ⅱ.Disturbance effects of crabs on the physical and chemical properties of sediments(1)The crab burrows can improve the water content of the sediments in the lowtide Spartina alterniflora habitat(P<0.05),and reduce the water content of the sediments in the high-tide Phragmites communis habitat(P<0.05).Crab burrows had significant effects on the sediments within 0-20 cm of total organic carbon,total phosphorus,total nitrogen,ammonium nitrogen and their ratios(P<0.05),but there were differences among seasons,habitats and depths.(2)Crab burrow deposition process can make the bottom of the burrow sediments have significantly higher moisture content,total organic carbon,total nitrogen and ammonium nitrogen content(P<0.05),burrowing process will make the bottom of the burrow sediments transport towards the surface,and make its moisture content,total nitrogen and ammonium nitrogen content decreased significantly(P<0.05),crab feeding-discharge process can significantly reduce the moisture content and total organic carbon content of surface sediments(P<0.05).Ⅲ.Disturbance effects of crabs on vegetation and plant growth characteristics(1)Crab disturbance had no significant effect on vegetation characteristics of the Spartina alterniflora community in each season(P>0.05),but significantly reduced the plant height in low and medium tidal flats in spring and summer(P<0.05),and significantly reduced the chlorophyll content of the plant in low tide flats in autumn(P<0.05).(2)Crab disturbance can significantly reduce the density of new shoots in Phragmites communis community and Phragmites communis-Spartina alterniflora mixed community in spring(P<0.05),and reduce the vegetation coverage of the above two communities in autumn(P<0.05),but it does not bring significant negative effects in summer(P>0.05).And the base diameter of Phragmites communis in high tide in autumn(P<0.05).Overall,this paper reveals the Fengxian coastal wetland crab burrow distribution characteristics,explores the crab burrow and crab activity disturbance in different seasons on sediment physical and chemical properties,vegetation characteristics and the influence of plant growth characteristics.It provides some references for the soil and water conservation of the erosion coastal wetland and the ecological restoration of the Spartina alterniflora invasion wetland. |
