A trophic state index for tropical/subtropical reservoirs (TSItsr)

Trophic state is an important property of the aquatic ecosystems as it reflects the anthropogenic influence on water quality and the ecological functioning of rivers, lakes and reservoirs. Trophic state indexes provide an insight on how nutrient and light availability controls phytoplankton development. We analyzed data on total phosphorus (TP, N = 931), chlorophyll a (Chl a, N = 848) and Secchi disk depth (SDD, N = 204) monitored in 18 tropical/subtropical reservoirs from 1996 to 2009 by the Environmental Protection Agency of São Paulo State (Brazil) in a bimonthly basis. Through linear regression with paired data on “TP × Chl a” and “Chl a × SDD”, we proposed a new trophic state index for tropical/subtropical reservoirs (TSItsr). Based on the annual geometric mean concentrations of TP and Chl a, we also assessed the risk of occurrence of individual episodes (e.g. Chl a ≥ 30 μg/L or TP ≥ 50 μg/L) within six categories: ultraoligotrophic (U), oligotrophic (O), mesotrophic (M), eutrophic (E), supereutrophic (S) and hypereutrophic (H). The upper boundaries (as annual geometric means) are (μg/L): 15.9 (U), 23.8 (O), 36.7 (M), 63.7 (E) and 77.6 (S) for TP and 2.0 (U), 3.9 (O), 10.0 (M), 20.2 (E) and 27.1 (S) for Chl a. The lower boundaries for the hypereutrophic state were (μg/L) 77.7 (TP) and 27.2 (Chl a). Comparisons with criteria available in the literature suggested that trophic state limits established for temperate systems are not suitable for tropical/subtropical reservoirs and may overestimate their enrichment condition. Restrictions of the TSItsr are discussed in light of the limiting-nutrient concept, the spatial and temporal water quality variability and the use of Chl a as an indicator of phytoplankton density and biomass. The TSItsr may aid in reservoirs management as a starting point for analyzing data on water quality in the tropics/subtropics since this issue is of paramount importance worldwide.