Verh. Internat. Verein. Limnol. 27 1–4 Stuttgart, December 2000 ©2000 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart 0368-0770/00/0027-01 $1.00 Feeding activity of Gammarus fossarum (Crustacea: Amphipoda) in acidic and low mineralized streams Olivier J. Dangles and François A. Guérold Introduction This paper is a part of an ongoing investigation into the impact of acidification on the functioning of the headwater forested systems in the Vosges Mountains (Northeastern France). Because of poorly buffered surface waters (due to granitic and sandstone under- lying rocks), this region is very sensitive to acid rain- fall (MASSABUAU et al. 1987, P ROBST et al. 1990). Consequently, numerous streams cannot support fish populations (Salmo trutta fario) and have impov- erished benthic macroinvertebrate communities (GUÉROLD et al 1993, 1995). Several studies have demonstrated that one of the most striking conse- quences of running water acidification, is the disap- pearance of crustaceans (S UTCLIFFE & CARRICK 1973, FRIBERG et al. 1980, ENGBLOM & LINGBELL 1984). In the Vosges Mountains, as demonstrated by pre- vious field studies including more than 80 streams (GUÉROLD 1992, GUÉROLD et al. 1997), Gammarus fossarum commonly occurs in circumneutral streams, but is eradicated from acidified freshwaters. Low pH and associated factors (aluminum, low calcium con- tent) are known to cause stress-related mortality (HARGEBY 1990) and inhibit growth (HARGEBY & P ETERSEN 1988), apparently because of osmotic imbalance (SUTCLIFFE 1978, V ANGENECHTEN et al. 1989). As a shredder, feeding primarily on detrital material, Gammarus fossarum is a key species involved in the incorporation of the energy from allochthonous inputs into the freshwater food web (MALTBY 1992). In this context, acidic stress, through reducing shredder abundance and diversity or feeding activity, could have considerable conse- quences for the functioning of the system (MALTBY 1994). For example, alteration in leaf breakdown and resulting accumulation of leaf litter have usually been observed in acidic streams (KIMMEL et al. 1985, MEEGAN et al. 1996, DANGLES & GUÉROLD, 1998). In this sense, feeding activity appears to be an inter- esting parameter which could be used to make infer- ences about potential changes in ecosystem pro- cesses. The aim of this study is to investigate the impact of stream acidification on the feeding activity of Gammarus , via the direct toxicity of the water and the indirect alteration of the quality of the food, notably its conditioning by microbes. Study area The experiment was conducted in April 1998, in three headwater streams draining the granitic catch- ment of the Vosges Mountains. Streams were chosen in order to include an acidified stream (Rouge- Rupt), a slightly acidified stream (Grands-Clos) and a circumneutral stream (Tihay). Water temperature, pH, and conductivity were measured twice a day in the field. In addition, water samples were collected each day in order to measure Cl – , NO 3 – ,SO 4 2– , Ca 2+ , Na + , Mg 2+ , K + , alkalinity and total aluminum. Table 1 gives the main chemical characteristics of the three studied streams. Rouge-Rupt was charac- terized by low pH (4.9), low conductivity (16 μS cm –1 ), negative alkalinity (–3 μEq L –1 ), low cal- cium content (0.9 mg L –1 ) and relatively high total aluminum concentration (203 μg L –1 ). Grands-Clos showed less acidic conditions – mean pH: 6.0; con- ductivity: 19 μS cm –1 ; alkalinity: 27 μEq L –1 ; calcium content: 1.4 mg L –1 ; and total aluminum: 80 μg L –1 . Tihay was typical of waters draining granitic rock, with a pH of 6.7; conductivity: 35 μS cm –1 ; alkalin- ity: 86 μEq L –1 ; calcium content: 3.3 mg L –1 ; and total aluminum: 27 μg L –1 . The latter stream was considered as a reference. The water chemistry of the three streams remained relatively constant through- out the experiment which lasted for 6 days. Materials and methods Beech leaves (Fagus sylvatica) were collected after abscission in the fall of 1997. Two types of condi- tioning were then tested in two streams located in the same area. A sample of leaves was conditioned in an highly acidic stream (Gentil Sapin Brook – mean pH: 4.68; mean alkalinity: –19 μEq L –1 ; mean total