Saltmarshes Response to Human Activities on a Prograding Coast Revealed by a Dual-Scale Time-Series Strategy

Abstract

On many prograding coasts, human activities such as alien species introduction and tidal flat reclamation, as countermeasures against land scarcity, are having a major impact on saltmarshes. Despite their capacity for regeneration given ample sediment supply, saltmarshes are likely to disappear if they continue to be impacted by such activities without effective monitoring and protection. However, effective monitoring requires a combination of saltmarsh species mapping and short timespan change analyses, which are difficult to accomplish due to the widespread, rapid, and complex characteristics of saltmarsh replacement in the region. Integrating multi-source remote sensing data with a times-series strategy, this study constructed saltmarsh species maps for the Jiangsu middle coast with 89.4 % overall accuracy (∼16.1 % higher than that achieved by the individual image classification method). Subsequently, we used these maps to explore the interaction between saltmarshes and human activities by a dual-scale analysis: macroscale analysis, aiming at saltmarsh ecosystem wide processes, to reflect the general evolutionary trend for all saltmarsh species, and microscale analysis, focusing on a species-specific process, to reveal the focal response mechanism for Spartina alterniflora. The results show that: (1) In the past 20 years, over 97.6 % (227.72 km2) of the loss of native saltmarshes (Phragmites australis and Suaeda salsa) was caused by tidal flat reclamation, and the invasion of S. alterniflora poses a potential threat to the future well-being of the native saltmarshes. The limited remaining area of the native saltmarshes, together with their long regeneration time, suggests that future land replacement will be dominated by the interaction between S. alterniflora and reclamation. (2) Intertidal reclamation promotes the expansion of S. alterniflora, as revealed by high linear correlations (mainly with R 2 > 0.6) between the lateral expansion rate of S. alterniflora and reclamation. The prerequisite for this correlation was a sufficient width of S. alterniflora (850–1000 m for most of the coasts), and we propose that by maintaining this critical width, S. alterniflora will be capable of relatively rapid invasion after reclamation.