Net Primary Productivity And Its Fate Of Halophila Beccarii In Pearl Bay,Guangxi

Halophila beccarii is a common seagrass species in the southern part of China,with a distribution in the coastal intertidal zone of the Asia-Pacific region and Tanzania and Kenya in Africa.In this study,the main carbon bugdet processes（net primary productivity,detritus export and herbivory）of Halophila beccarii were quantified by means of tie-line marking and in situ sequestration.The results of the study showed that:（1）Regarding the mean net primary productivity per unit area of leaves,spring(97.12±13.57 g m^-2yr^-1（dry weight,same below）)>winter(38.29±12.30 g m^-2yr^-1)>summer(28.12±6.20 g m^-2yr^-1).Net primary productivity per unit area of petiole was also,spring(33.99±4.75 g m^-2yr^-1)>summer(17.03±3.72 g m^-2yr^-1)>winter(14.55±4.67 g m^-2yr^-1).The mean net primary productivity of the above-ground fraction was 80.77±9.87 g m^-2yr^-1,characterized by high spring(131.12±18.31 g m^-2yr^-1)and low summer(45.15±9.92 g m^-2yr^-1).The mean net primary productivity of the below-ground fraction was 124.32±11.61 g m^-2yr^-1,with productivity in spring(204.57±18.97 g m^-2yr^-1)>winter(95.05±19.37 g m^-2yr^-1)>summer(55.68±10.54 g m^-2yr^-1).（2）Based on the carbon content of the tissues of each part of Halophila beccarii in the study area,the seasonal average net primary productivity was calculated as 51.01 g C m^-2yr^-1.The seasonal average net primary productivity was highest in spring(87.18 g C m^-2yr^-1),followed by winter(46.81 g C m^-2yr^-1)and lowest in summer(25.25 g C m^-2yr^-1).The net primary productivity of the above-ground fraction net primary productivity was highest in spring(35.64 g C m^-2yr^-1),followed by winter(16.62 g C m^-2yr^-1)and lowest in summer(12.28 g C m^-2yr^-1).The net primary productivity of the below-ground fraction net primary productivity was highest in spring(51.54 g C m^-2yr^-1),followed by winter(30.19 g C m^-2yr^-1)and lowest in summer(12.97 g C m^-2yr^-1).（3）Seasonal mean detritus output was 20.48 g C m^-2yr^-1,and comparisons between seasons were spring(33.78 g C m^-2yr^-1)>winter(16.12 g C m^-2yr^-1)>summer(11.54 g C m^-2yr^-1).The detritus drifted away from the seagrass beds under hydrodynamic action,spring(4.35 g C m^-2yr^-1)>summer(1.40 g C m^-2yr^-1)>winter(1.22 g C m^-2yr^-1).The average herbivory across seasons was 1.03 g C m^-2yr^-1,with 1.86 g C m^-2yr^-1in spring,0.74 g C m^-2yr^-1yr^-1in summer and a minimum of only0.50 g C m^-2yr^-1in winter.（4）95.21%of the net primary productivity of the above-ground fraction was exported as carbon in the form of detritus,and 4.79%entered the food web as feeding by herbivores.Of this,11.33%of the detritus drifted away from the seagrass beds under hydrodynamic action and88.67%was decomposed or stored in the marine sediments.In spring,5.22%of the net primary productivity of the aboveground component entered the food web through herbivory and 94.78%was exported as detritus,of which 12.88%drifted off the seagrass bed and 87.12%was decomposed or stored in the marine sediment.6.03%of the net primary productivity of the summer above-ground component entered the food web by herbivory,and 93.97%became detritus.Of this,12.13%of the detritus drifted off the seagrass beds and 87.87%was decomposed or stored in the marine sediments.In winter,3.01%of the net primary productivity of the above-ground component entered the food web through herbivory and 96.99%was exported as detritus.Of this,7.57%of the detritus drifted away from the seagrass beds under hydrodynamic action and 92.43%was decomposed or stored in marine sediments.This study is the first to quantify the carbon budget between net primary productivity and detritus export and herbivory of Halophila beccarii in Pearl Bay,Guangxi,which will provide basic data for the carbon cycle of seagrass ecosystems in southern China.