**1. Introduction**

The mulberry-dyke-fish pond ecosystem, developed by ancient Chinese farmers 2500 years ago with complex irrigation and drainage design, is an artificial eco-agriculture system which are mostly found in the Yangtze River Delta and Pearl River Delta (PRD). Conducive to the cultivation of mulberry-dyke trees, silk rearing, fish and poultry farming, the ecosystem plays an important role in energy circulation and the ecological environment protection [1]. Compared to other agricultural systems, this system has better economic and ecological performance, with advantages in regulation of droughts and floods, stable high outputs, and easy operation [2]. Currently, this traditional agriculture system is believed

**Citation:** Zhang, W.; Cheng, Z.; Qiu, J.; Park, E.; Ran, L.; Xie, X.; Yang, X. Spatiotemporal Changes in Mulberry-Dyke-Fish Ponds in the Guangdong-Hong Kong-Macao Greater Bay Area over the Past 40 Years. *Water* **2021**, *13*, 2953. https://doi.org/10.3390/w13212953

Academic Editor: Athanasios Loukas

Received: 16 August 2021 Accepted: 17 October 2021 Published: 20 October 2021

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under a crisis of extinction caused by the outflow of rural population and the fast expansion of cities and towns, especially in the GBA, which now is one of the most prosperous regions in China. One of the manifestations is that, as part of the GBA, the Pearl River Delta (PRD) region is one of China's most important urban agglomerations with the fastest urbanization rate. Profited from the implementation of China's reform and opening-up policy since 1978, the area of built-up land in the PRD reached 434,570 hm<sup>2</sup> from 1988 to 1998 alone and enlarged by 1.5 times in a ten-year period. This process undoubtedly occupied ponds [1], making them more fragile because of the associated polluted water [3,4]. Moreover, views about the shrinking in the ponds could be summarized into two aspects. First are policy changes. The current average annual growth rate of national agricultural production needs to be maintained at 4.6% to meet the food needs of 22% of the global population, while the output by the ponds is not comparable with large-scale modern agriculture. The traditional pond ecosystem is thus being abandoned by farmers. Second is rural nonpoint and mini-point source pollution. Pollutants from widely used synthetic fertilizers cause various effects to the pond ecosystem, exacerbating material and energy flows within the ecosystem, thus weakening its ecosystem service. Compared with other large waterbodies, ponds are less capable of pollutant dilution, leading them to be abandoned. Under these circumstances, there have been increasing awareness to implement conservation policies for the pond ecosystem preservation. Measures such as numerical assessments, pond inventory mapping and pond ecosystem monitoring through IoT-based devices have been applied [5].

Previous studies on the ponds in the GBA mainly focused on its ecological functions, such as applications of energy theories to make synthetical and quantitative analyses based on energy structure and indices [1]. Their historical development, agricultural heritage and landscape patterns were also investigated. The pond ecosystem, as an important part of the agricultural heritage systems, has prominent agricultural heritage values for enriching production diversity and biological diversity in the GBA [6]. It is often recognized as a reflection of the harmonious coexistence of man and nature, demonstrating circular economy and ecological civilization ideas in China [7]. However, the current unclear status about the ponds has restricted these investigations.

For the monitoring of temporal and spatial changes in fish ponds, a combination of Landsat images from 2000, 2005, and 2015 has been employed to analyze them in the Foshan City in western GBA, accompanied by a similar study in Foshan, to analyze the spatial pattern changes in 1988, 1998, and 2006 [8,9]. Located in the central part of GBA, Zhongshan City has also been studied about the dynamics of its fish ponds using Landsat images in 1990, 2000 and 2013 [10]. Regarding the whole GBA, Keyhole images from 1964 and 1976 were combined with Landsat images from 1988, 2000 and 2012 to detect its ponds' spatial changes [10,11]. The dynamics in fish ponds have been almost released in this study, ye<sup>t</sup> the time-series was still relatively short in comparison with such a long-term (more than 50 years) urbanization process in the GBA. As a result, previous studies did not fully reflect the spatiotemporal changes at a relatively complete scale. The long-term spatial dynamics found in the fish ponds is still unclear.

To respond to the research gap, based on the Landsat images obtained in the period 1986–2019, this study conducted the investigated the long-term spatiotemporal changes in fish ponds and their landscape dynamics to reveal fish ponds' historical development in the GBA. The study results will be valuable for a more comprehensive understanding of the water ecosystem dynamic development in the GBA in past decades. In addition, influencing factors were analyzed to provide an accurate reference for decision making on pond ecosystem restoration and conservation.

### **2. Data and Methods**
