Physicochemical and diatom trophic state indexes: A complementary approach for improving water sustainability in a high Andean urban stream

Abstract

Discharge of untreated wastewater into freshwater ecosystems increases nutrient concentrations, causing eutrophication and demanding more intensive monitoring and control activities, particularly in developing areas. Two approaches are commonly used for assessing trophic states of rivers: (1) physicochemical trophic state indexes (TSIs), and (2) trophic indexes based on bioindicators, mainly periphytic diatoms (TDIs). Even when these two approaches seem to be very different, they can be complementary under certain circumstances. This is the case with Río Chili (Arequipa, Peru), a shallow regulated river used for multi-purpose activities, but which is highly polluted due to the discharge of municipal wastewater. The present study assessed the suitability of different TSIs and TDIs by processing data from historical water quality registers and recent monitoring, including periphytic diatom sampling. TDIs were compared with TSIs applied to both recent and historical records. Results indicated that TSIs can be easily obtained from measurements of phosphorus concentrations, but they are less sensitive and resulted in a high degree of homogeneity among the classification of trophic conditions along the urban path of the river. Alternatively, TDIs showed higher precision and sensitivity, reporting detailed classifications of the sampling points. TDIs suggested that Río Chili presented conditions that varied from mesotrophic to eutrophic as consequence of wastewater discharges and soil occupation. A routine use of TDIs with occasional assessment by physicochemical TSIs may contribute to water quality sustainability by informing managers of the effects of organic and phosphorus pollution on eutrophication at a lower cost.