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. Gaudy et al. J. Exp. Mar. Biol. Ecol. 247 2000 51 –65
1. Introduction
The pelagic copepods Acartia clausi and A . tonsa characterize many coastal or
estuarine environments where they can reach high population densities. In Atlantic temperate regions, they sometimes co-exist at the same time Lee and McAlice, 1979
but, most frequently, A . tonsa dominates during summer and A. clausi during winter
Deevey, 1948; Conover, 1956; Heinle, 1966; Jeffries, 1967, which contributes to reduce food competition.
In the Marseilles region French Mediterranean coast, the zooplankton of the Gulf of Fos is dominated by A
. clausi Blanc et al., 1975; Benon et al., 1976, while in the adjacent eutrophic brackish lagoon, the Etang de Berre, it is mainly composed of Acartia
tonsa Gaudy, 1989. These two areas are connected by a channel where currents allow the transportation of the pelagic fauna in both directions Gaudy, 1986. The local
populations of A . clausi and A. tonsa are challenged by highly variable conditions of
temperature, salinity or food conditions. During a previous study on the distribution of these two Acartia species we observed that each Acartia population remained largely
dominant in its respective habitat during most of the year, but did not maintain themselves in the adjacent pelagic ecosystem despite water transfer through the channel
Cervetto, 1995. Among the possible factors which could explain this separation, the strong salinity gradient existing between the Gulf of Fos average salinity 35‰ and the
Etang de Berre average salinity 15‰ was considered. Experimental studies on the immediate and short term mortality of the two species in relation to salinity variations
Cervetto et al., 1995, 1999 demonstrated that adult and copepodites of both species displayed a large degree of tolerance to rapid salinity change. Tester and Turner 1991
demonstrated that nauplii of A
. tonsa had low tolerance to salinity particularly above
25‰. As the long term maintenance of animal populations depends on the success of their reproduction, it is advantageous that the part of energy devoted to ovogenesis
would be as large as possible. Considering the energy balance equation, as adult copepods do not moult, energy available for reproduction is directly related to food
acquisition and inversely to metabolic expenditures Kiørboe et al., 1985. Thus it is important to know to what extent metabolism is affected by differences in the
temperature and salinity conditions of marine and brackish habitats. According to
¨ Paffenhofer and Stearns 1988, the restriction of Acartia tonsa to nearshore or estuarine
waters would be due to the impossibility to complete its food ration at the lower food concentration met in the open sea. In the literature, many works concern the ecophysiol-
ogy of A . clausi or A. tonsa, but few are devoted to a direct comparison of the
physiology of the two species in view of explaining their difference of distribution Conover, 1956; Anraku, 1964 and no data is available for the more salty and less
productive waters of the Mediterranean region. The aim of this paper is to analyze the variations of respiration and excretion rates in
Acartia clausi and A . tonsa under different salinity and temperature combinations in the
range of their Mediterranean habitat. The effect of salinity and food concentration on their feeding rates is also investigated. These results are discussed in relation with the
separate distribution of the two species in the coastal environment.
R . Gaudy et al. J. Exp. Mar. Biol. Ecol. 247 2000 51 –65
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2. Methods
