Remote-sensing monitoring and analysis of China intertidal zone changes based on tidal correction

Abstract

Intertidal zones are located in the transitional area of sea and land and are important to biodiversity protection, social and economic development, and global environmental changes. However, due to the irregular dynamics of the tides and the regular local time of satellite transit, it is challenging to monitor the extent of intertidal zones using only satellite remote-sensing methods. Until now, there has been no complete dataset of Chinas coastal intertidal zones and knowledge about their distribution is limited. In the work described in this paper, we first used 156 Landsat Thematic Mapper/Operational Land Imager images covering the entire coastal area of China in 1995 and 2015 to extract the instantaneous shoreline water boundaries. Then, by combining the tidal level information at the transit of the satellite, we simulated the average high- and low-tide lines and determined the spatial and temporal distribution of intertidal zones by the geographic information system method. At the same time, the land-use types across the intertidal zones were manually interpreted based on high-resolution images from Google Earth. On this basis, the spatial and temporal characteristics of intertidal zone changes across Chinas coastal areas and their driving factors were researched. The results follow. (1) In 2015, the total area of the intertidal zone in China was approximately 14070 km2, of which Fujian Province (18%) was the most distributed in the intertidal zone, followed by Liaoning Province (15%), Shandong Province (14%), Jiangsu Province (12%), and Guangdong Province (11%); the smallest area was distributed in Hainan Province (2%). (2) From 1995 to 2015, the national intertidal zone area decreased by 1375 km2, approximately 8.9%. The intertidal zone area of different provinces and cities showed different increasing and decreasing trends. The largest reduction of intertidal zones was in Zhejiang Province, with an area decrease of 1189 km2. The intertidal zone area of Jiangsu Province was greatly reduced, by which its first position of having the most intertidal zone area of China in 1995 decreased to the fourth in 2015. The intertidal zones in Fujian Province exhibited the greatest increase, reaching 453 km2. The area of intertidal zones between the ports of Shanghai Luchao and Zhejiang Jintang exhibited the greatest decrease, reaching 56.5%. The area of intertidal zones between Fujian Shuiao and Anhaiwan estuaries increased the most, reaching 37.28%, an area of 445.59 km2. (3) The trend of development and utilization of intertidal zones continues to increase. In 1995, the coastal intertidal zone area occupied was 3490 km2, and by 2015, it had reached 4581 km2. The type of development and utilization of intertidal zones changed from agricultural sea (41.8%) to marine aquaculture; the proportion of urban construction occupied also reached 13%. At the same time, the intertidal zone area in the cofferdam (under construction/unknown use) continued to grow, from 18.1% in 1995 to 29% in 2015. The intertidal zone showing the greatest level of development and utilization among the coastal provinces is Jiangsu Province, with an occupied area of 1646 km2, accounting for 35.9% of the developed and utilized national intertidal zone, with the main types of utilization being agricultural sea and mariculture. The other provinces that developed intertidal zones are Guangdong (13.7%), Shandong (12.5%), Liaoning (10.9%), and Shanghai (10.3%). The main development and utilization types of intertidal zones are urban construction and mariculture, with a large number of intertidal zones presently under development. (4) The squeezing effect caused by the combination of artificial and natural factors have caused the reduction of intertidal zones in China. Human development mainly includes marine aquaculture, industrial development, port expansion, urban construction, and reclamation. And the actual high-tide line has moved towards the sea by artificial intertidal zone development. The velocity is even higher than that of low-tide line, resulting in the reduction of intertidal zones. Natural factors mainly include rising sea level and other factors, such as changes in the sediment concentration of rivers entering the sea and coastal hydrodynamic forces, which are also important factors affecting the change of coastal intertidal zones. These factors contribute to the drift of some intertidal zones towards the sea and the expansion of the area. The contradiction between economic development and coastal ecological environmental protection is still outstanding.