Rivers to reservoirs: hydrological drivers control reservoir function by affecting the abundance of submerged and floating macrophytes

Lakes and reservoirs are important sites for biogeochemical cycling on both regional and global scales. Shallow lakes often have higher coverage of submerged aquatic vegetation (SAV) because of increased light penetration to the sediment as well as stronger interactions between the sediment and the water column. These biotic and abiotic interactions can strongly affect nutrient cycling. This study evaluated how hydrologically driven changes in SAV coverage affected nutrient processing within a relatively shallow reservoir. To assess these effects, a comprehensive water quality sampling network was established that quantified nutrient concentrations in the inflows and outflow of the lake. Annual vegetation surveys quantified the spatial coverage of SAV. Annual inflow was significantly lower in the first year of the study compared to the following 2 years. Consequently, SAV coverage was also highest during the lowest flows in the first year and was lower in the following 2 years when flows were greater. NO3-N concentrations were also lowest within Hydrilla beds and in the outflow during the growing season of the first year. Our results suggest that hydrological variation was the main driving variable of SAV coverage, and that the extent of SAV coverage strongly controlled nutrient processing at the whole-reservoir scale.