Stress Model - Environ. Sci. Technol. 2005, 39, 4150-4158...

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Evidence for the Stepwise Stress Model: Gambusia holbrooki and Daphnia magna under Acid Mine Drainage and Acidified Reference Water Stress ALMUT GERHARDT,* LUC JANSSENS DE BISTHOVEN, AND AMADEU M. V. SOARES Department of Biology, University of Aveiro, Campus Santiago, 3810-193 Aveiro, Portugal The Stepwise Stress Model (SSM) states that a cascade of regulative behavioral responses with different intrinsic sensitivities and threshold values offers increased behavioral plasticity and thus a wider range of tolerance for environmental changes or pollutant exposures. We tested the SSM with a widely introduced fish (Girard) (Pisces, Poeciliidae) and the standard laboratory test species Straus (Crustacea, Daphniidae). The stress was simulated by short-term exposure to acid mine drainage (AMD) and to acidified reference water (ACID). Recording of behavioral responses with the multispecies freshwater biomonitor (MFB) generated continuous time-dependent dose - response data that were modeled in three-dimensional (3D) surface plots. Both the pH-dependent mortalities and the strong linear correlations between pH and aqueous metals confirmed the toxicity of the AMD and ACID gradients, respectively, for fish and Daphnia, the latter being more sensitive. AMD stress at pH e 5.5 amplified circadian rhythmicity in both species, while ACID stress did so only in G. holbrooki . A behavioral stepwise stress response was found in both species: D. magna decreased locomotion and ventilation (first step) (AMD, ACID), followed by increased ventilation (second step) (AMD). decreased locomotion (first step) (AMD, ACID) and increased ventilation at intermediate pH levels (second step) (AMD). Both species, although from different taxonomic groups and feeding habits, followed the SSM, which might be expanded to a general concept for describing the behavioral responses of aquatic organims to pollution. Stepwise stress responses might be applied in online biomonitors to provide more sensitive and graduated alarm settings, hence optimizing the “early warning” detection of pollution waves. Introduction Exposure time is a crucial mediator of toxicity. However, it is underestimated in risk assessment schemes, which cur- rently rely on the use of fixed end points such as LC x ’s, EC x ’s, and NOECs as the basis of deterministic or probabilistic ecological risk assessment ( 1 ). Alternatively, new approaches concentrate on regression model fittings to toxicity data from different end points ( 1 , 2 ) and consideration of exposure time in acute toxicity (mortality) and bioconcentration kinetics ( 3 - 5 ). The application of the time-to-death concept, described more than 30 years ago ( 6 ), has been reluctant ( 7 ), even though response surfaces of mortality as a function of toxicant concentration and acute exposure time have suc- cessfully been used to predict chronic toxicity of metals in Daphnia magna ( 8 ). Next to prediction of mortality, 3D concentration - response - time surfaces might predict sub- lethal end points (e.g., behavior) as well, taking empirical models (e.g., Stepwise Stress Model, see below) as rationale
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This note was uploaded on 08/10/2011 for the course BIOL 1108L taught by Professor Stanger-hall during the Spring '09 term at University of Georgia Athens.

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Stress Model - Environ. Sci. Technol. 2005, 39, 4150-4158...

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