1160 Environmental Toxicology and Chemistry, Vol. 24, No. 5, pp. 1160–1164, 2005  2005 SETAC Printed in the USA 0730-7268/05 $12.00 + .00 INFLUENCE OF DURATION OF EXPOSURE TO THE PYRETHROID FENVALERATE ON SUBLETHAL RESPONSES AND RECOVERY OF DAPHNIA MAGNA STRAUS SEBASTIA ´ N REYNALDI* and MATTHIAS LIESS Department of Chemical Ecotoxicology, UFZ Centre for Environmental Research, Permoserstrasse 15, D-04318 Leipzig, Germany ( Received 26 April 2004; Accepted 1 November 2004) Abstract—This study compares lethal and sublethal responses of Daphnia magna Straus exposed to fenvalerate continuously (21 d) and as a pulse (24 h). Survival was reduced more severely in the continuous- than in the pulse-exposure regime. Complete mortality occurred at 1 g/L for continuous exposure and at 3.2 g/L for pulse exposure. Regarding reproductive endpoints, fenvalerate delayed the age at first reproduction. At the beginning of the reproductive phase (day 10), this delay resulted in a reduction of the neonates per living female at similar concentrations in both exposure regimes (0.3 and 0.1 g/L for continuous and pulse exposure, respectively). The population growth rate was inhibited in continuous and pulse exposure at 0.3 and 0.6 g/ L, respectively. However, the effects of fenvalerate in the pulse exposure were transient. After 21 d, a recovery to values close to the controls occurred with respect to the total neonates per female and the population growth rate over a broad range of concentrations from 0.1 up to 1 g/L. In contrast, no substantial recovery occurred in the continuous-exposure regime. Keywords—Pulse exposure Toxicant Population growth rate Recovery Daphnia magna INTRODUCTION Fenvalerate is characterized by strong sorptive properties and low water solubility; its logarithm to base 10 of the oc- tanol/water partition coefficient (K OW ) is 6.2 [1]. These char- acteristics are expected to reduce its bioavailability in natural environments, resulting in pulse exposures for aquatic organ- isms [1,2]. On other hand, the low water solubility results in bioconcentration into organisms from surrounding matrices such as water [3–5]. Although pyrethroids bioconcentrate in organisms, depuration also occurs [4]. Consequently, it is like- ly that the exposure duration influences the toxicity of fen- valerate to Daphnia magna Straus. The differences between effects of continuous and pulse exposure to pyrethroids on crustaceans have not been well characterized with respect to chronic responses and recovery. Therefore, the main objective of this study was to evaluate these effects by exposing Daphnia magna Straus to fenval- erate, either continuously (21 d) or as a pulse (24 h). The responses of survival, reproduction, and population growth rate were compared at low (sublethal: 0.03–0.3 g/L) and high (lethal: 0.6–3.2 g/L) concentrations of fenvalerate following continuous and pulse exposure. Such concentrations have been reported during the growing season for vegetable crops in surface waters of agricultural areas [6], whereas short- term peak concentrations of 0.8 to 6 g/L were measured in agricultural streams during runoff events [7–9]. Therefore, the concentrations tested in the present work can be considered as environmentally realistic concentrations of fenvalerate. MATERIALS AND METHODS Daphnia magna culture Continuous cultures of Daphnia magna Straus (clone B, Bayer, Monheim, Germany) were maintained at a density of * To whom correspondence may be addressed (sebastian.reynaldi@ufz.de). 10 adults/L in M7-Elendt medium at 20°C  1°C in a light: dark regime of 16:8 h, with light intensity of approximately 15 mol/m 2 /s [10]. The conductivity, pH, and oxygen con- centration were 630 S/cm, 7.4, and 7.15 mg/L, respectively. Once every week a new culture was initiated using 24-h-old neonates from a three-week-old culture. The newly released neonates were discarded, and the medium was renewed three times a week at regular intervals. The animals were fed with a suspension of batch-cultured green microalgae (Desmodes- mus subspicatus), which were cultured in algae medium ac- cording to Grimme and Boardman [11]. The algae were har- vested in exponential growth phase, centrifuged, and the pellet resuspended in Elendt-M7 medium in desired dilutions. The animals were fed three times a week with equivalent daily rations of approximately 0.045 mgC/Daphnia/d for the one- week-old animals and approximately 0.07 mgC/Daphnia/d for the older animals. Fenvalerate exposure and measurement Fenvalerate, (RS)--cyano-3-phenoxybenzyl (RS)-2-(4- chlorophenyl)-3-methylbutyrate (CAS: 51630–58–1), was ob- tained from Riedel-de Hae ¨n, Seelze, Germany (99.9%). A stock solution of 1.25 g/L was prepared using dimethylsulf- oxide ([DMSO], Merck, Darmstadt, Germany, 99.8%). The maximum amount of DMSO in the test medium was 0.0003% (v/v). Exposure consisted of six different nominal concentra- tions for both exposure regimes (control, 0.03, 0.1, 0.3, 0.6, and 1 g/L). For pulse exposure, in addition 3.2 g/L was tested. The animals were fed during exposures in both regimes; the alga suspension was added to the fenvalerate solutions before the Daphnia individuals were introduced. The concen- trations of the test solutions were measured during the first 24 h of exposure for both treatments. Accurate detection was limited to the concentrations 0.6, 1, and 3.2 g/L (n = 3) at t = 1 h. Additionally, a 1 g/L test solution (n = 3) was measured at t = 24 h. Samples were measured at the Institute for Ecological