Distribution Characteristics Of Crab Burrows In The Typical Salt Marshes Of Yangtze Estuary And Their Ecological Effects

Crab burrows have a unique structure,which is essential for ecosystem functions and for the survival and reproduction of crabs in tidal salt marshes.Besides,it is an effective method to estimate crab density by counting the burrows in current researches.In our study,five typical habitats of the salt marshes,including Spartina alterniflora zone,Spartina alterniflora-Phragmites australis zone,Phragmites australis zone,Phacelurus latifolius-Phragmites australis zone and Metasequoia glyptostroboides zone,in the intertidal zone around Chongming Island were choosen to set up the fixed sampling sites.The characteristics of crab burrows and related factors,including vegetation and sediment were surveyed quarterly.Based on these,we chose two typical habitats?Spartina alterniflora zone and Phragmites australis zone?to carry out the field controlled experiments in field.We set up four different treatments to analyze the effects of the crab burrows on the sediments and the vegetations.Simultaneously we did the indoor controlled experiments to get deep insight to reveal such effects through the control of crab density,temperature,and water level,etc.Main results and conclusions of the present study were summarized as follows:1.Distribution characteristics of the crab burrows1)There were significant differences in the density of the crab burrows among the different habitats?P<0.01?,and the differences were basically consistent in the different seasons.In each season,the burrow density appeared to be lowest in S.alterniflora zone and S.alterniflora-P.australis zone and highest in P.latifolius-P.australis zone and M.glyptostroboides zone.The burrow density in P.australis zone was somewhere in between.There were also significant differences in the opening diameter of the crab burrows in the different habitats.In the spring,the opening diameters in the S.alterniflora zone and S.alterniflora-P.australis zone were significantly higher than those in the other three habitats?P<0.05?;In the summer,the opening diameters in the S.alterniflora-P.australis zone,P.australis zone and M.glyptostroboides zone were significantly greater than those in the P.latifolius-P.australis zone?P<0.05?,but the burrow opening diameters in the S.alterniflora zone were not significantly different from the others?P>0.05?.In the autumn,the opening diameters in S.alterniflora zone were significantly smaller than those of the other four habitats?P<0.05?.2)There were different seasonal variations in crab burrow density and opening diameter in the different habitats.The crab burrow density in the S.alterniflora zone and P.australis zone appeared to vary significantly among seasons?P<0.05?,while not in the other three habitats?P>0.05?.The opening diameters didn't show significant seasonal varitations in P.latifolius-P.australis zone and P.australis zone?P>0.05?,but did in the other three habitats?P<0.05?.2.The influencing factors of the crab burrow distribution characteristicsThe influencing factors of the burrow distribution characteristics varied in different research areas.For the whole area,water content,salinity,conductivity,median particle diameter,bulk density,total carbon and total nitrogen content of the sediment were important factors affecting the burrow density.Among which water content and salinity had significant negative corelations to the burrow density?P<0.05?,and the coefficient of correlation were-0.619 and-0.594 respectively.The opening diameter was significantly affected by vegetation factors rather than sediment properties.There were significant negative correlations between the vegetation coverage and the opening diameter of the burrow?P<0.05?,and also between the plant height and the opening diameter of the burrow?P<0.05?.Plant density appeared to have significant positive corelations to the burrow opening diameter.However,to a single habitat,the environmental factors affecting the distribution characteristics of the crab burrows were significantly different among the habitats.The factors of the sediment which had significantly vertical varitions were much more concerned?P<0.05?.3.The ecological effects of the crab burrows1)The effects of crab burrows on the sediment physicochemical properties,including the effects of formation,maintenance of the burrows and also their structures on the sediment.We found out that the crab burrows did not have significant effects on the conductivity,oxidation-reduction potential,pH and total organic carbon content of the sediment?P>0.05?,but did on the water content,bulk density,median particle diameter,total carbon and total nitrogen content of the sediment siginificantly?P<0.05?.The effects of crab burrows were significantly different among the habitats and seasons.The influence of crab burrows on water content,bulk density and median particle diameter of the sediment appeared to be significant in the P.australis zone in autumn and the S.alterniflora zone in summer?P<0.05?.Compared with S.alterniflora zone,the total carbon and total nitrogen content of the sediment were more likely to be significantly affected by crab burrows in P.australis zone?P<0.05?.2)In the field controlled experiment in field,the effects of crab burrows on the release of sediment CO₂ happened to be significant only in the P.australis zone in the spring due to the complicated sedimentary and hydrological condition?P<0.05?.However,the results of indoor experiments showed that the formation and maintenance of the crab burrows and the presence of crab burrow structures could significantly promote the release of CO₂?P<0.05?,and it could increase significantly with the formation and maintenance frequency of burrows?crab density?going up?P<0.05?.The results also showed that the promotion effect of the burrow structure itself was relatively weak.The indoor temperature controlled experiment also showed that the formation and maintenance of crab burrows could significantly promote the release of CO₂?P<0.05?either in high or low temperature,but the effects varied between the different states.The indoor water level controlled experiment found that the crab burrows could significantly promote the release of CO₂ under different water levels?P<0.05?,while such effects gradually decreased as the water level rised.3)The crab burrows had no significant effects on the vegetations in both two habitats?P>0.05?.Significant variations in the plant morphological and physiological parameters,biomass mainly occurred between the different habitats in the same season and among the different seasons in the same habitat.The results of correlations between vegetation indices and environmental factors indicated that plant morphological parameters were related to specific environmental factors in the different habitats.The net photosynthetic rate and transpiration rate in each habitat appeared to have weak correlations with the sediment properties.Overall,the crab burrow distribution characteristics appeared to have significant spatial and temporal differences.The physicochemical properties of sediment were important factors affecting the burrow density.The opening diameter was significantly affected by vegetation factors.The crab burrows changed partial properties of sediment to a certain extent,but it had little effect on the vegetation.It provided a deep insight for the studies on the structure and function of the salt marsh ecosystems,and also the scientific basis for the restoration of the degraded salt marshes.