Study Of Certain Ecological Relationships Between Kandelia Candel Vegetation And Common Macro-benthic Fauna In Jiulongjiang Estuary

1.Changes of macro-benthic faunal community with stand age of rehabilitated Kandeliacandel mangrove in Jiulongjiang EstuaryDistribution properties of macro-benthic faunal communities were compared amongrehabilitated Kandelia candel forests of different ages in Jiulongjiang Estuary,China.FromJuly 2005 to July 2006,seasonal surveys were conducted within four representative forests:K4,K7,K19 and K43,with ages of 4,7,19 and 43 years,respectively.Two non-vegetatedflats (NF1 and NF2) were set as the references.Numbers of faunal species in rehabilitatedforests were higher than NF1 and NF2,while younger forests (K4 and K7) generally hadmore macro-benthic fauna species than older ones (K19 and K43).Among the total 22species recorded,Uca arcuata,Metaplax elegans,Phascolosoma esculenta and Littorariamelanostoma were dominant.Macro-benthic faunal community structures were differentamong the four rehabilitated K.candel forests.The dominant species in younger forests wasM.elegans,while U.arcuata had a higher abundance than M.elegans in non-vegetation flatsand older forests.Gastropods were abundant in younger forests.Distribution properties ofdominant species differed among rehabilitated K.candel forests.U.arcuata had the largestdensity in non-vegetated flats,and was infrequent in younger forests.M.elegans had highabundances in younger forests.High abundance of P.esculenta was found in older forests,while L.melanostoma was more abundant in younger forests.Since both macro-benthicfaunal community and vegetation were similar in K19 and K43,rehabilitated K.candelmangrove was considered steady 19 years after being planted.That is,in the case ofmacro-benthic fauna and vegetation,a K.candel mangrove may evolve to a mature state at astand age of about 20 years.2.Leaf consumption by Sesarmaplicata in a mangrove forest at Jiulongjiang EstuaryFeeding ecology of Sesarma plicata,the most abundant crab species in a mangrove forestdominated by Kandelia candel at Jiulongjiang Estuary,China,was investigated through field and laboratory experiments.Feeding preference and consumption rates were determined onmature,senescent and decomposed leaves of Kandelia candel,Bruguiera gymnorrhiza andAegiceras corniculatum.In the laboratory,S.plicata preferred leaves of K.candel over thoseof B.gymnorrhiza and A.corniculatum,and consumed significantly more decomposed leavesthan mature and senescent ones,irrespective of crab size.Field experiments with limitedpower failed to reveal detectable species preferences despite more consumption of K.candel,but decomposed leaves of each species were again preferred.Leaf characteristics associatedwith preference changed with plant species and leaf state.Low tannins and high watercontent characterized the preferred state of leaves.Species preference was significantly andnegatively related to crude fibers and C:N ratios for mature leaves,and crude fiber forsenescent leaves,but significantly and positively related to water content for decomposedleaves.Leaf consumption rates averaged for all leaf categories from laboratory no-choicefeeding experiments were 0.101,0.055 and 0.017 gDW ind^-1 d^-1 for large,medium and smallcrabs,respectively.In this forest,mean density ofS.plicata was 20.5 ind m^-2 as assessed by amanual catching method.Leaf litter removal rate during neap tide days by sesarmid crabswas about 1.33 gDW m^-2 d^-1 in April 2006.The leaves removed by crabs were grazed on thesediment surface or taken into crab burrows,shredded and stored before being eaten.3.Seasonal dynamics of leaf litter removal by crabs in a Kandelia candel mangrove forest inJiulongjiang EstuarySeasonal variability of leaf litter removal by crabs was observed from May 2006 to April2007 in a Kandelia candel mangrove forest in Jiulongjiang Estuary,China.Monthly averagequantities of leaf fall ranged 0.85～3.86 gDW m^-2 d^-1,with high values in May,August,October and November.The whole-year leaf fall was 6.48 t ha^-1 a^-1 (1.81 gDW m^-2 d^-1).Standing stock of leaf litter on the forest floor was 7.78 gDW m^-2 averaged from the wholeyear's data,with the lowest value in December (1.23 gDW m^-2) and the highest in April(16.18 gDW m^-2).Annually averaged removal rate of leaf litter by crabs was 0.59 gDW m^-2d^-1.High seasonal variability was observed on removal rates of leaf litter by crabs.The valuesin the winter months (December,January and February) were 0.07～0.09 gDW m^-2d^-1,much lower than those in other months with values of 0.59～1.18 gDW m^-2d^-1.Annually averagedpercentage of leaf fall removed by crabs was 33%,with the highest values in September(reached 76%) and the lowest values in winter months (December,January and February).During the whole year,the estimated tidal export of leaf litter was 444.07 gDW m^-2,accounting for about 67% of the leaf fall production.In leaf litter removed by crabs,a largeproportion was buried by crabs,and only 12% was consumed by crabs on the forest flood.Leaf litter removal rate,consumption rate on the forest floor,percentages of leaf fall andstanding stock removed on the forest floor were significantly positively correlated with airtemperature.4.Change of soil property induced by foraging of mangrove crab Sesarmea plicata onmangrove leafThe effects of mangrove crab Sesarmea plicata on soil chemical properties and enzymeactivities were quantified in laboratory,with emphasis on crab activity and foraging Kandeliacandel leaf.Crab activity has no significant effect on soil OC,TN and available phosphoruscontents,but significantly increased soil NO₃^--N content from 131.44μg·g^-1 in the control to194.79μg·g^-1.Soil urease activities were hampered while sucrase activity was enhanced bycrab activity.Foraging K.candel leaf by S.plicata presented different effects on soilproperties to crab activity.Forging K.candel leaf significantly increased the soil OC,TKNcontents,and soil enzyme activities except invertase and urease activities were significantlyaugmented during the foraging.Significant correlations were found among soil chemicalproperties and enzyme activities.By forging 1 g K.candle leaf,154.94 mg OC and 10.26 mgTN contents were accumulated in soil by crab.Removing crab feces material from soilweakened foraging effect on soil properties,indicating a medium role of nutrient retention inmangrove soil.