Determining critical light and hydrologic conditions for macrophyte presence in a large shallow lake: The ratio of euphotic depth to water depth

[Display omitted] •Determined the critical light threshold for the survival of macrophytes using the index of Zeu/WD.•Evaluated the potential relationship between Zeu/WD and the presence frequency and coverage proportion of macrophytes.•Determined the potential areas for the recovery of macrophytes in Lake Taihu using the Zeu/WD derived by MODIS images. Light determines macrophyte distribution, community composition and biomass in shallow lakes. Therefore, it is vital to determine the critical underwater light climate thresholds for macrophyte degradation and recovery. In this study, we first proposed a novel index, defined as the ratio of euphotic depth (Zeu) to water depth (WD), as a measure of the underwater light supply for macrophytes. The underwater light environment in Lake Taihu (a large, shallow, eutrophic lake) was then characterized based on this index (Zeu/WD) using field measurements collected from 2006 to 2013 (8 years×4 seasons×32 sites). The distribution of the macrophyte presence frequency (MPF, the number of investigations that identified macrophytes divided by the total number of investigations) was greater than 0.70 in Xukou Bay and East Lake Taihu over the 32 investigations, followed by the other sites distributed in East Lake Taihu. The proportion of macrophyte coverage increased with the increase in Zeu/WD. A significant relationship was observed between Zeu/WD and MPF for the 19 sites with macrophytes (r2=0.48, p