Trophic state evolution in a subtropical reservoir over 34 years in response to different management procedures

Despite their importance for water management, long-term studies on trophic state are relatively scarce in subtropical reservoirs. We analyzed total phosphorus (TP), total nitrogen (TN) and chlorophyll a (Chl a) concentrations in the Billings Reservoir (Brazil) over time: Phase 1 (1977–1992, Tietê River water was pumped to Billings to increase energy generation, 100 m³ s⁻¹); Phase 2 (1992–2007, Tietê water was conveyed to Billings only in special cases for flood avoidance, 8 m³ s⁻¹); and Phase 3 (2007–2010, besides flood control, Billings received Tietê water treated by an in situ flotation system, 13 m³ s⁻¹). We compared our results with data from 12 reservoirs to evaluate current (2005–2009) enrichment conditions. Phosphorus and nitrogen concentrations decreased (p < 0.05, MANOVA) from Phase 1 to 2 and were stable thereafter. TN/TP ratios increment (1977–2010) indicated shift from N- to P-limitation in the reservoir, affecting the phytoplankton. Nutrient levels in Billings are currently between the expected concentrations in mesotrophic and eutrophic reservoirs (0.03 mg L⁻¹ < TP < 0.42 mg L⁻¹, 0.8 mg L⁻¹ < TN < 7.6 mg L⁻¹) and Chl a concentrations exceeded 34 μg L⁻¹, median for the eutrophic waterbodies from the dataset. Although water quality in Billings has improved, nutrient inputs from Tietê River pumping episodes, diffuse and internal sources are still favoring biomass accrual and compromising water uses.