Divergent geographic patterns and functional characteristics: Subtle mapping for ponds in the Yangtze River Delta Region

Pond water surfaces (PWS) possess diverse functional types, such as aquaculture, agriculture-water supplement, and ecosystem regulation. However, existing research often treats PWS as a homogeneous aquatic ecosystem; the absence of a comprehensive PWS classification system hinders ours understanding of PWS background characteristics and is detrimental to watershed management. Here, a comprehensive classification system of PWS, including fish aquaculture ponds (FAP), shrimp and crab aquaculture ponds (SCAP), natural ponds (NP), and landscaping ponds (LP) was proposed from remote sensing perspectives. Additionally, interpretation rules were standardized from multi-features including spectrum, shape, topography, and surrounding geographical environments. Subsequently, refined spatiotemporal data product of PWS in the Yangtze River Delta from 2016 to 2022 was generated using Sentinel-2 images with 10 m spatial resolutions. The results indicate that: (1) The spatiotemporal changes exhibited three stages, i.e., “declining – stable – recovery.” The area of PWS decreased from 5186.52 km² to 4920.90 km²in 2016-2017, stabilized at approximately 4500 km² in 2019-2021, and then rebounded to 4834.12 km² in 2022. (2) Regarding different PWS functional types, significant differences were demonstrated in terms of area, surrounding environment, and spatiotemporal changes. Firstly, FAP dominated in terms of area, accounting for 47.81% of the total. Secondly, FAP was widely around rivers and lakes. At the same time, SCAP was concentrated around lakes or along the coast, LP was primarily found in urban areas, and NP was predominantly found in rural areas and mountainous regions; Thirdly, NP decreased as land remediation work progressed continuously, while LP increased due to policy support for urban renewal. Changes in aquaculture were more complex, experiencing a sharp decline from 2016 to 2020 due to reduced market demand but rebounded in 2021-2022 with supportive policies. In summary, the system and data products developed in this study reveal the diverse relationships of "functional type-geographical environment-driving factor" regarding PWS, implicating appropriate planning for aquatic ecosystem.