An Ecotoxicological Approach to Assessing the Impact of Tanning Industry Effluent on River Health Mwinyikione Mwinyihija, Andy Meharg, Julian Dawson, Norval J.C. Strachan, Ken Killham School of Biological Sciences, Plant and Soil Sciences, University of Aberdeen, St. Machar Drive, Cruickshank Building, Aberdeen, AB24 3UU, U.K. Received: 28 February 2005/Accepted: 27 July 2005 Abstract. A study was conducted to investigate the sediment health and water quality of the River Sagana, Kenya, as impacted by the local tanning industry. Chemical analysis identified the main chemical pollutants (pentachlorophenols and chromium) while a bioassay addressed pollutant bio- availability. The bioassay, exploiting the luminescence re- sponse of a lux marked bacterial biosensor, was coupled to a dehydrogenase and Dapnia magna test to determine toxicity effects on sediments. Results highlighted the toxicity of the tannery effluent to the sediments at the point of discharge (64% of control bioluminescence) with gradual improvement downstream. There was a significant increase in dehydroge- nase downstream, with the enzyme activity attaining a peak at 600 m, also indicating a gradual reduction of toxicity. Biological oxygen demand (19.56 mg L )1 ) dissolved oxygen (3.97 mg L )1 ) and high lethal dose value (85%)of D. magna also confirmed an initial stress at the point of discharge and recovery downstream. Optical density of surface water demonstrated an increase in suspended particulates and col- our after the discharge point, eventually decreasing beyond 400 m. In conclusion, the study highlighted the importance of understanding the biogeochemistry of river systems impacted by industries discharging effluent into them and the invalu- able role of a biosensor-based ecotoxicological approach to address effluent hazards, particularly in relation to river sediments. The demand for clean water as a resource has risen with the increasing urbanisation (domestic supplies and recreation), industrialisation, and agricultural intensification of the last century (Baumgartner 1996; Sweeting 1994). Ironically, these activities are the major contributors to pollution in riverine environments (SEPA 1999). Rivers have the inherent capacity to dilute and detoxify the substances discharged into them and to recover their original nutrient and oxygen levels. Recently, many rivers have become overloaded with pollutants and may lack the capacity to recover from increasing levels of pollu- tants (Milner 1994). There has been an increasing trend towards the use of bio- monitoring to assess river water quality (SEPA 1999). The analysis of a recognised set of water chemistry parameters is also a commonly employed method of assessing river water quality. An adequate monitoring system should assess both the effects and the distribution of pollutants (Moriarty 1999). Reliance on measurement of pollutants alone, without also assessing biological effects, ignores potential problems (Price 1978). The activity of certain enzymes such as coenzymes F 420, hydrogenase, dehydrogenase (DHA), and adenosine tri- phosphate (ATP) may serve as indicators of these biological effects (Nybroe et al. 1992; Le Bihan and Lessard 1998; Goel et al. 1998). Chemicalmethodshavebeentraditionallyusedtodetermine total concentrations of pollutants, and biologically linked measurements have been used to assess the bioavailable fractions of the pollutants. The study involved the use of lux- based biosensors (Escherichia coli HB101pUCD607), a dehydrogenase activity test, and a set of water chemistry parameters to investigate river health at the study site. The integration of both chemical and biological approaches to underpin ecotoxicity testing is essential. The biosensors are able to indicate bioavailability of the contaminants such as heavy metals (Paton et al. 1997). A lux- based biosensor responds to the presence of toxic compounds with a decline in light production, reporting the effect of the toxicant on bacterial metabolic activity (Meikle et al. 1992). They can integrate environmental factors such as pH, redox potential, exchangeable cations, and biological activity that affect the bioavailability of pollutants (Paton et al. 1997). Dehydrogenase is an oxidoreductase soil enzyme and is extracellular, often relatively stable, and can persist for ex- tended periods, thereby providing a longer-term perspective than measurements involving extant organisms alone. The impact of pollutants on soil health has been addressed through the measurement of enzyme activity (Killham and Staddon 2002). DHA is measured generally by adding a tetrazolium salt, such as triphenyltetrazolium chloride (TTC) or 2-(p-iodophenyl))3-(p nitrophenyl)-5-phenyl tetrazolium Correspondenceto: Dr.MwinyikioneMwinyihija; email: m_mwinyi@ hotmail.co.uk Arch. Environ. Contam. Toxicol. 50, 316–324 (2006) DOI: 10.1007/s00244-005-1049-9