| Salt marshes, mangroves and seagrass become the most commonly wetlands in estuary by the effects of tide and salt water from oceanic ecosystem. Their community structure and basic ecological processes were intensively documented in the past few decades. Further up the estuary, however, the often extensive tracts of tidal freshwater marshes are not so well researched, especialy for the Yangtze River estuary which is one of the largest estuaries in the world. In this area, wetlands are greatly affected by human and economic activities.Investigations in this study were carried out at the typical tidal freshwater marsh Chongxi wetland at the west end of Chongming Island, Yangtze River estuary during 2010 and 2012, which is dominated by Phragmites australis (common reed). At this site, the stands of P. australis have been cut annualy between November to December for non-food commodity production. Dynamics of net photosynthesis rate, aboveground, belowground and litter fall biomass and structural characteristics of P. australis were mearsured and analyzed, respectively. The contribution of P. australis to crab which is the dominant benthic macro-benthic invertebrates was investigated by feeding experiments in laboratory. Based on the monitoring and observational data from field surveys and published information, a carbon cycle model was developed by using the software STELLA. The model was applied to estimate the organic carbon contribution from tidal freshwater marshes to surrounding estuarine and oceanic ecosystems, and to pretend the effects of global climate change.The main results of this study were summarized as follows:(1)The curves of net photosynthetic rate of P. australis displayed a double-peak pattern. The peak values and corresponding occurring time were different between different months. The maximum net photosynthetic rate occurred at 11:00 in August with the value of (16.2±1.3) μmol·m-2s-1. The photosynthetically active radiation (PAR) was the dominant factor affecting the instantaneous photosynthetic rate of the leaves of P. australis. The other factors followed in descending order were vapor pressure deficit (VPD), transpiration rate (Tr), air temperature (T), relative humidity (RH), stomatal conductance (Gs) and environmental carbon dioxide concentration (Pca).(2) The dynamic curve of total living biomass (aboveground and belowground together) of P. australis was tipical S-shape. The total biomass gradually increased in March and decreased in late September. The dynamics of aboveground and belowground biomass could be divided into three stages:aboveground increase and belowground decrease (from March to May), both aboveground and belowground increase (from June to September) and then decrease synchronously (October and November). From March to November, the root-shoot ratio ranged from 0.89 to 19.19.The biomasses of leaves, sheath and stems increased gradually from late March until leaves and sheath got the highest values in August, and stem had the highest value of (3 193.0±93.9) g m-2 in September. The biomass of panicle showed an increasing tendency, which bloomed in August. Both dynamic curves of living rhizome and root biomass were S-shape. In late September, the total biomass of common reed reached the highest value of (9 351.0±381.5) g m-2. The results indicated the aboveground accumulation and production of P. australis in the Chongxi wetland were relatively higher than other estuarine and coastal regions in China.(3) The fallen litter biomass of P. australis occured at the end of July, and it reached the maximum value of (99.8±13.2) g m-2 in late August. The accumulated annual biomass of fallen litter was about 702 g m-2, of which 432.2 g m-2 is fallen leaves, which corresponds to a proportion as high as 68.7%. The biomass of fallen sheaths and stems were 133.6 and 133.0 g m-2, respectively, which correspond to proportions as 19.0% and 18.9%. The fallen panicle biomass was only 3.8 g m-2 and have a relative proportion of 0.5%. The belowground biomass of dead organs (dead rhizomes and roots together) got the highest value of (837.9±62.7) g m-2 in May, and the lowest value of (260.7±28.8) g m-2 in November.(4) For living plant organs, Zizania latifolia showed the highest biomass contribution to both female and male sesarmid crabs, of which the contribution of Z. latifolia leaves for female Chiromantes dehaani had the maximum value of (0.185±0.022) g DW ind-1d-1. Except for female Sesarmops sinensis, the biomass contributions of living plant organs to sesarmid crabs are Z. latifolia, P. australis and Scirpus triqueter sorted by biomass in descending order. For plant litter, P. australis had very high values of biomass contribution to both female and male sesarmid crabs, of which the contribution of P. australis leaves for female C. dehaani had the maximum value of (0.206 ± 0.017) g DW ind-1d-1. The biomass contributions of plant litter to sesarmid crabs are P. australis, Z. latifolia and Scirpus triqueter which sorted by biomass in descending order. Water content, carbon content, nitrogen content and carbon-nitrogen ratio of plant organs are not the dominant factors of food preferences of the two sesarmid crabs at the site in this study.(5) The accumulated annual contribution of organic carbon in conditions of plant shoots are harvested in winter is 891 g C m-2, of which 612 g C m-2 flows out of the wetland directly as plant residue, which corresponds to a proportion as high as 68.7%. The carbon exported as POC and DOC together correspond to the remaining 279 g C m-2 and have a relative proportion of 31.3%. The total organic carbon loss continually increases after a short decrease at the start of April of 2010 and retains a high value from mid-July to mid-November. Thereafter, the organic carbon loss rapidly decreases to approximately zero because the aboveground plant organs were harvested at the site.(6) The carbon concentration in the aboveground organs and belowground plant organs, and accumulated annual loss of organic carbon correspond to the temperature, and are predicted to decrease when the sea level rises; therefore, a rise in sea level indicates negative effects. The accumulated annual organic carbon contribution to the surrounding estuarine and oceanic ecosystems is predicted to increase 309 g C m-2 with increases of temperature by 4.5 ℃ and sea level by 1 m. Therefore, wetlands are predicted to export additional organic carbon under global warming conditions. |
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