Using satellite data time series to investigate phenological characteristics of invasive aquatic plant species across gradients

Invasive aquatic plants, or macrophytes, are a threat to shallow aquatic ecosystems by outcompeting native species and causing considerable ecological and economic harm. This study examines two widely distributed species in the Northern Hemisphere: Nelumbo nucifera (sacred lotus, native to East Asia) and Ludwigia hexapetala (water primrose, native to Central and South America), comparing their phenological traits and productivity across different environmental gradients: native vs. non-native ranges and different climatic regions. Sentinel-2 satellite data covering years from 2017 to 2022 were used to generate time series for Water Adjusted Vegetation Index (WAVI), a proxy for canopy density and biomass, at seven study sites: Mantua lakes and Lake Varese (humid subtropical climate, non-native range for both species), Lake Fangzheng, Lake Bayangdian, and Lake Xuanwu (respectively humid continental, cold semi-arid, and humid subtropical climate, native range for N. nucifera), Lake Grand-Lieu and Santa Rosa Lagoon (respectively temperate oceanic and warm-summer Mediterranean climate, non-native range for L. hexapetala). Seasonal dynamics parameters (phenological metrics and productivity) were extracted from WAVI time series, and their meteo-climatic and environmental drivers were analysed using parametric models (GAMs). The results indicate that N. nucifera exhibits higher productivity in non-native sites compared to the native ones, while in the subtropical native sites, the growing season starts earlier than in the non-native sites. For L. hexapetala, meteo-climatic factors were found to be the main drivers of its phenology, especially temperature and solar radiation. As this approach can be easily extended in terms of spatio-temporal scales and to other macrophyte species, using operational data and available archives, it can benefit studies on the variability of the eco-physiological characteristics of invasive macrophyte species under climate change scenarios that may guide the management and restoration of aquatic ecosystems.