Hydrobiologia 501: 45–58, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 45 Responses of the planktonic diatom Asterionella formosa Hassall to abiotic environmental factors in a reservoir complex (south-eastern France) C. Bertrand 1 , S. Fayolle, E. Franquet & A. Cazaubon UMR CNRS 6116 – IMEP Laboratoire d’Ecologie des Eaux Continentales M´ editerran´ eennes, Facult´ e des Sciences et Techniques de Saint-J´ erôme, Avenue Escadrille Normandie-Niemen 13397, Marseille cedex 20, France Tel: 04-91-28-80-36. Fax: 04-91-28-80-30. E-mail. celine.bertrand@univ.u-3mrs.fr Received 14 January 2002; in revised form 27 February 2003; accepted 23 May 2003 Key words: Mediterranean reservoirs, Asterionella formosa, population dynamics, abiotic interactions Abstract The River Durance and its main tributary, the Verdon, are both highly regulated rivers flowing in south-eastern France. The course of both rivers is interrupted by a series of reservoirs with quite different geographical, morpho- metric, climatic, hydrodynamic and chemical characteristics. The planktonic diatom Asterionella formosa Hassall, which has undesirable cyclic effects from the water management point of view, was studied in this complex of reservoirs located in the Mediterranean region. The results indicate that only the monomictic calcareous reservoirs show a bimodal pattern of Asterionella formosa abundance. The population dynamics of this algal species was found to depend on both the morphometric features and the hydraulic mode of management (retention time) used at each reservoir, as well as on the physical (temperature, suspended matter) and chemical (nitrogen, silica, calcium) characteristics of the water. Introduction With a view to providing the ever-increasing water and energy requirements of human populations, many ef- forts have been made during recent decades to develop rivers by building artificial reservoirs. These means of hydraulic regulation can considerably affect the phys- ical, chemical and biological conditions pertaining in the waters (Thornton et al., 1990). Many hydrobiological studies have been carried out on reservoirs in France (Champeau et al., 1982; Bin Molé et al., 1986; Le Cohu et al., 1994; Ber- thon et al., 1996; Tadonkele Dzatchou, 1999; Leitao & Léglize, 2000). The large complex of artificial reservoirs in south-eastern France was built on two regulated rivers, the Durance and its main tributary, the Verdon. The reservoirs on these two rivers have quite different geographic, morphometric, climatic, hydro- dynamic and chemical characteristics. The respons- ibles for managing the waters in this complex have to find ways of making the water drinkable whenever the planktonic diatom Asterionella formosa Hassall proliferates in some of the reservoirs and enters the drinking water system with which the city of Marseille is supplied. Asterionella formosa is a species which has been commonly found in many temperate lakes. This co- lonial diatom has generally been described as an in- dicator of eutrophic conditions (Hutchinson, 1967; Aleya, 1992; Reynolds, 1998, 2000). It has also been observed in some oligotrophic lakes (Rawson, 1956; Alvarez-Cobelas & Rojo, 1994; Romo, 1998; Interlandi et al., 1999; Negro et al., 2000). The popu- lation dynamics of this species have been studied since the middle of the 20th Century at Lake Windermere (Lund, 1949, 1950). Field prospections and exper- imental data have recently shown that some abiotic factors, such as the temperature and the phosphorus and silica contents of the water, can have considerable effects on the growth of Asterionella formosa (e.g. Reynolds, 1984; Hayakawa et al., 1994). The present study carried out in the Mediterranean region focused on the auto-ecology of Asterionella formosa at the reservoir complex scale. It was pro-