The use of diatoms (Bacillariophyta) as indicators of water quality in an intermittent, urban and highly polluted river in Bolivia

Abstract

The Rocha River is born at the foot of the Eastern Andes (2685 m asl) and discharges into the Caine River (2527 m asl), after 70 km of crossing the city of Cochabamba and semi-rural surrounding areas. Its basin is inhabited by ca. 1.3 million people dedicated to agriculture, plastic, paper, tannery/leather, poultry industries. The discharge peak is reached in the rainy season (November-March, ca. 3 m3s-1) while during the dry season (April-October) it is only ca. 0.5 m3s-1. The river is contaminated by agricultural, factory and domestic waste. Especially during the dry season, it turns dark green and emits strong sewage odors. The present study intends to provide a monitoring tool for this river, useful for recovery and management practices. Sampling took place during the dry (2 dates) and rainy (1 date) seasons in 2008 at 6 stations in an agricultural/semiurban zone and 3 in urbanized areas. Epilithic samples were obtained by brushing rocks and oxidized in the laboratory with nitric acid to produce permanent slides using Naphrax mounting medium. Microscope identifications at 1000X were made using specialized literature. Water chemistry was measured in situ and in the laboratory. The Shannon diversity index and the Specific Polluosensitivity Index (IPS) were calculated and both species counts and IPS values were analyzed using CCA (Canonical Correspondence Analysis). The diatom community along the river was composed of 276 species and varieties. The most specious genera were Nitzschia, Gomphonema, Navicula, Ulnaria, Pinnularia and Fragilaria, which together comprise 77% of the total diatom community. Species composition varied at each of the 9 stations along the three sampling dates. There is no underlying river or land use typology, but localities associated with agriculture had higher species numbers, while sites associated with urban development had a lower number of taxa. 41% of the taxa were unknown, hindering the calculation of the IPS, which nevertheless shows a low (agricultural sites) and the lowest (urban sites) quality for both dry and rainy seasons. Despite the lower number of taxa used in the index calculation, the categorization correlates well with water chemistry (R2 adjusted=0.45), being temperature, BOD5 and orthophosphates the variables showing the highest correlations with taxa and index values. Thus, diatoms are a useful tool for monitoring the Rocha River and could be applied to other rivers in the semiarid region of the Bolivian Andes.