Long-term monitoring of total suspended matter concentration in fishponds in the Guangdong-Hong Kong-Macao Greater Bay Area

Excessive total suspended matter (TSM) concentrations can exert a considerable impact on the growth of aquatic organisms in fishponds, representing a significant risk to aquaculture health. This study revised existing unified models using empirical data to develop an optimized TSM retrieval model tailored for the Guangdong-Hong Kong-Macao Greater Bay Area (GBA) (R2 = 0.69, RMSE = 7.78 mg/L, and MAPE = 0.23). Employing top-of-atmosphere (TOA) reflectance data from Landsat satellites, accessed via Google Earth Engine (GEE), the refined model facilitated the generation of TSM datasets for fishponds across the GBA from 1986 to 2019. Over these 34 years, there was a marked decline in TSM levels in fishponds, with an approximate 50% reduction in annual average TSM. This decline was particularly notable in the northern, western, and eastern GBA regions, leading to a spatial distribution characterized by higher TSM concentrations in the central and southern regions and lower concentrations in the peripheral regions. Seasonally, TSM levels in GBA fishponds are significantly higher during spring and summer compared to autumn and winter. Regarding natural factors, wind speed shows a significant positive correlation with long-term TSM fluctuations in these environments (p < 0.01). Stocking density, regulated artificially, emerges as a pivotal factor affecting TSM fluctuations. Specifically, TSM concentrations are elevated during periods of high stocking density in the rapid growth phase, and decrease during the mature and harvesting phases when stocking densities are reduced. Furthermore, fishponds situated in impervious areas exhibit significantly higher TSM concentrations compared to those in cropland or forested areas. The economic costs associated with aquaculture drive variations in stocking densities across different land uses within the GBA, contributing to the observed spatial variations in TSM levels. Given the status of the GBA as one of China's most advanced aquaculture regions, the insights from this study hold substantial value from both economic and ecological viewpoints.