Secondary impacts of eutrophication control activities in shallow lakes: Lessons from aquatic macrophyte dynamics in Lake Taihu from 2000 to 2015

Aquatic macrophytes in lakes and rivers are highly sensitive to changes in water quality and ecological conditions. Satellite remote sensing of aquatic macrophyte distributions can map changes in freshwater ecosystem dynamics. However, macrophyte mapping by remote sensing is challenging in lakes dominated by algal blooms or with high turbidity. In this study, we mapped the composition and distribution of emergent/floating and submerged macrophytes in the large turbid and eutrophic Lake Taihu, China. We used a novel classification method and 16 y of MODIS images to map these data. The inter-annual trends in macrophyte distribution and cover show complex dynamics in this heavily-managed lake. The area occupied by emergent/floating macrophytes increased from 39.3 km(2) in 2000 to 90.3 km(2) in 2015 (a 230% increase), whereas the area dominated by submerged macrophytes decreased from 404.6 km(2) in 2001 to 167.5 km(2) in 2015 (a 59% decrease). We also used the annual date of initial macrophyte occurrence and growth period to examine temporal variation in species composition and biodiversity of macrophytes. Aquatic macrophytes in different parts of the lake responded differently to temporal trends in water quality (Secchi disk and nutrient concentrations), hydrology (water level), and meteorological data (daily temperature, precipitation, and h of sunlight). Changes in lake transparency occurred over the study period, reducing the availability of underwater solar irradiance. Light availability was the most influential local environmental factor for macrophyte dynamics. We also found that eutrophication control projects indirectly reduced lake ecosystem quality in some bays. These control projects included water transfers from the Yangtze River (dilution), aquaculture management actions, macrophyte removal, and dredging. Our findings suggest the species composition and area covered by aquatic macrophytes in this shallow lake are driven by multiple mechanisms. These results highlight that efforts to manage the increase in algal-dominated lake areas require a more complete understanding of the direct and indirect impacts of management activities. We also show that satellite-based remote sensing measurements of macrophyte spatial and temporal dynamics provide a useful indicator to evaluate the impacts of these activities.