Dynamics of mangrove forests in Shenzhen Bay in response to natural and anthropogenic factors from 1988 to 2017

Global mangrove forests have exhibited distinct changes in past decades owing to diverse environmental gradients and regional anthropogenic activities. Previous studies have mainly reported alarming mangrove loss. However, few studies have focused on the process of mangrove expansion, and its driving factors remain unclear. By integrating medium-resolution Landsat surface reflectance products and meteorological, hydrological and bathymetric data, the current study investigates the spatiotemporal changes of mangrove forests in Shenzhen Inner Bay (SZIB) of China over the past thirty years, and the relationship between mangrove changes and natural and anthropogenic factors was also identified. The results reveal that the area of mangrove forests in the SZIB increased at a rapid rate of 8.9 ha/yr despite sea level rise, reaching 527.1 ha in 2017. No statistically significant correlation between mangrove area and winter temperature and precipitation was found, whereas a significant positive correlation occurred between mangrove area and sea level rise, in contrast to the common consensus. Human activities affected the SZIB mangroves in two distinct ways. On the one hand, the area of mangrove deforestation occurred on 23.5 ha and 6.8 ha in the Futian and Mai Po reserves, respectively, with reforestation campaigns only compensating for 5.6 ha of the mangrove area lost in the Futian reserve. On the other hand, the reduced tidal prism due to the implementation of the Shenzhen Bay Land Reclamation Project (SZBLRP) resulted in significant deposition in the SZIB, which counterbalanced the negative effect of sea level rise, contributing to 209.9 ha of mangrove expansion by providing suitable accommodation space and accounting for 78.4% of the total mangrove gain. Moreover, the morphological adjustments in the SZIB caused by the Shenzhen River Regulation Project (SZRRP) also had a significant influence on the mangrove extent and distribution, with the mangrove expansion area of 22.7 ha being contributed by the SZRRP, accounting for 8.5% of the total mangrove expansion. The findings of the current study indicate that with the increased sediment supply and accretion rate induced by anthropogenic activities, mangrove forests with limited land retreat space can still expand in the face of sea level rise.