Six decades of field observations reveal how anthropogenic pressure changes the coverage and community of submerged aquatic vegetation in a eutrophic lake

Six decades field observation data series on submerged aquatic vegetation (SAV), water level and water quality from Lake Taihu were compiled to reveal the dynamics in coverage and species composition of SAV and their anthropogenic drivers. We found that both SAV species composition and coverage area declined significantly in Lake Taihu during the period, and the increasing nutrient levels and water level as well as decreasing water clarity were responsible for these change trends. Specifically, the decrease in species richness could be particularly well predicted by total nitrogen (TN) and the ratio of water clarity (i.e., Secchi disk depth (SDD)) to water level (WL), contributing 47.3 % and 32.3%, respectively, while the coverage of macrophytes was most strongly related to the water level, accounting for 70.1 % of the variation. A classification tree analysis revealed a threshold of TN of 3.2 mg/L and SDD/WL of 0.14 that caused a shift to a eutrophic low-macrophyte dominated state. Our results highlight that SDD/WL must be improved for SAV recolonization, rather than merely reducing nutrient input and regulating water level. Our findings provide scientific information for lake managers to prevent plant degradation in macrophyte-dominant lakes and facilitate a shift to a macrophyte-dominant state in eutrophic lakes.

Automated summary

The study compiled long-term field observation data on submerged aquatic vegetation (SAV), water level, and water quality from Lake Taihu, and found that the composition and coverage of SAV in the lake have significantly declined in the past six decades. The increase in nutrient levels and water level, as well as the decrease in water clarity, were identified as the main factors driving these changes. Specifically, the decrease in species richness was well predicted by total nitrogen and the ratio of water clarity to water level, while the coverage of macrophytes was strongly related to water level. The study also revealed a threshold level of total nitrogen and the ratio of water clarity to water level that caused a shift to a eutrophic low-macrophyte dominated state. The findings emphasize the importance of improving water clarity in relation to water level for SAV recolonization.