Changing Salt Marsh Landscape And Its Ecosystem Service At The Yangtze Estuary

During the past decades, the wetlands in the Yangtze estuary have undergone severe changes owing to the influences of natural factors (such as sea level rising, runoff and sediment changes in the Yangtze River, and vegetation succession) and anthropic factors (including reclamation, accretion promotion, deep-water navigation channel maintenance and harbor construction). Based on field investigation, remotely sensed images were classified and corrected for the periods of1980,1990,2000and2010at the Yangtze estuarine wetlands. In combination with existing research results, the impact of salt marsh change on wildlife habitats, water purification, carbon storage, sediment accumulation, wave attenuation of the Yangtze estuarine wetlands were evaluated. The results showed that:1. A large area of natural wetlands had been changed to unnantural wetlands. The area of salt marsh in the Yangtze esturine decreased.The total area of the Yangtze estuarine wetlands changed from525km2in1980to500km2,668km2and1080km2in1990,2000and2010, respectively, with the1980seawall as the landward boundary. The area ratio of the natural wetlands and unnatural wetlands is100:0,82:18,54:46and45:55in1980,1990,2000and2010, respectively. The area of unnatural wetlands increased dramatically.The area ratios of Phragmites/Spartina/Scirpus were27:0:73,77:0:23,47:13:40and39:37:24in1980,1990,2000and2010, respectively, with the total area of salt marsh vegetation changed from205km2to207km2,181km2and161km2.The total length of shoreline increased by56km from1980to2010. The length ratio of the natural shoreline and unnatural shoreline is83:17,74:26,61:39and32:68in1980,1990,2000and2010, respectively. The length of natural shoreline decreased by146km, and the length of unnatural shoreline increased by203km.2. The habitat suitability for Charadriiformes (shorebirds) and Anseriformes (waterfowls) in the Yangtze estuarine wetlands both showed decreased trends.The results indicated that the area percentages of core habitats decreased by56.6%and19.4%for Charadriiformes and Anseriformes, respectively. The area of core habitats decreased by136km2for Charadriiformes and increased by232km2for Anseriformes. The total area of secondary habitats, edge habitats and unsuitable habitats increased by682km2and314km2for Charadriiformes and Anseriformes, respectively. The comprehensive habitat suitability index (H) for Charadriiformes decreased from0.92in1980to0.53in2010and that for Anseriformes decreased from0.89in1980to0.70in2010. Habitat suitability for Charadriiformes decreased more rapidly than that for Anseriformes.3. The capacity of nitrogen and phosphorus retention in the Yangtze estuarine wetlands increased.The N retention capacity increased from711-4039t in1980to5138-9306t in2010and the P retention capacity increased from65-409t in1980to886-1228t in2010. The potential removal capacity of N by the three dominant salt marsh vegetation species increased by800-1100t, and that of P increased by about60-70t during the last thirty years. But the percentage of N and P retention capacity by the salt marsh declined by44-70%and62-84%, respectively. The contribution from Scirpus and Phragmites in the retention capacity declined. In contrast, the contribution from exotic species Spartina alterniflora increased by about20%.4. The ability of promoting carbon storage and sediment accumulation at the tidal salt marsh of the Yangtze estuary increased, while the ability of wave attenuation decreased.During the past decades, the ability of carbon storage of Yangtze estuarine wetlands increased from27×1041in1980to38×104t in2010. The carbon storage of salt marsh vegetation were16×104t,35×104t,25×104t and27×104t in1980,1990,2000and2010, respectively. The contribution to carbon storage of vegetation in the form of Phragmites/Spartina/Scirpus were72:0:28,96:0:4,72:19:9and49:47:4in the four stages. The carbon storage of surface soil were11×104t,18×104t,13×104t and11×104t in1980,1990,2000and2010, respectively, with contribution ratios of Phragmites/Spartina/Scirpus changed from53:0:47to91:0:9,70:10:20and58:31:11.The ability of promoting sediment accumulation in the Yangtze estuarine wetlands increased. The total amount of suspended particulate matter adhered by vegetation increased gradually from8302t in1980to18479t in2010. The ratio of suspended particulate matter adhered by Phragmites/Spartina/Scirpus were43:0:57,87:0:13,43:39:18and22:71:7in1980,1990,2000and2010, respectively. The amounts of sediment accumulation were10.15×106m3,7.39×106m3,9.17×106 m3and10.51×106m3in the four stages, with the contribution of Phragmites/Spartina/Scirpus changed from16:0:84to64:0:36,27:27:46and18:62:20, respectively.The ability of wave attenuation by salt marsh decreased gradually since1990. Under normal tides, the length of seawall without enough vegetation protection from the seaward increased from117km in1980to261km in2010. The length of seawall without enough vegetation protection under storm surge increased from271km in1980to336km in2010.Overall, the area of salt marsh decreased and the vegetation structure changed in the Yangtze estuarine wetlands from1980to2010. Meanwhile, the capacities of carbon storage, sediment accumulation and water purification increased, while the functions for wave attenuation and wildlife habitats declined. These results would provide scientific basis for the trade-off amony different ecosystem services when protecting and utilizing estuarine wetlands.