R . Gaudy et al. J. Exp. Mar. Biol. Ecol. 247 2000 51 –65
53
2. Methods
Samples were collected on several occasions in October 1994 from two stations located in the Berre lagoon and in the Eastern part of the Gulf of Fos, respectively, in
front of each outlet of the channel connecting these two areas Fig. 1. Zooplankton was sampled by several oblique hauls with a WP2 net UNESCO, 1968, screened through a
coarse mesh silk 350 mm then transported to the laboratory in cold boxes filled with 30 l of surface water from each habitat. The experiments started about 6–8 h later.
Fig. 1. Map of the investigated region and location of the two sampling stations black dots. A Site of the Berre Lagoon; B site of the Gulf of Fos.
54 R
. Gaudy et al. J. Exp. Mar. Biol. Ecol. 247 2000 51 –65
2.1. Respiration and ammonia excretion Six successive experiments were carried out. In each of them, several sets of equal
numbers of adult females were prepared. This number varied from 10 to 50 in Acartia clausi and 100 to 200 in Acartia tonsa according to the abundance of the species in the
samples. Animals were incubated for 24 h in 125-ml flasks filled with 0.45-mm filtered water adjusted at three different salinities 15, 25 and 35‰. The flasks were maintained
in the dark, at three different temperatures 10, 15 and 208C. For each temperature– salinity combination, test flasks three with Acartia clausi, three with A
. tonsa and one control flasks without copepods were prepared. As non significant difference appeared
between the results of the six successive experiments ANOVA test, P , 0.05 for a given temperature–salinity combination, the data were pooled for statistical treatment,
giving a maximum of 18 data points range 12–18 per species, for each of the nine temperature–salinity combinations. At the end of the incubation period, the water
oxygen concentration of each flask was measured with an YSI model 57 oxymeter compensated for salinity variation. Samples of 25 ml were withdrawn from the different
flasks for ammonia concentration measurement according to the Koroleff 1969 colorimetric method. Respiration and excretion rates were calculated from differences in
oxygen and ammonia concentration between controls and test flasks, taking into account the number of individuals in each flask and the incubation time. They were expressed as
specific rates, using an average dry weight of 10 mg for A
. clausi and 5 mg for A. tonsa Cervetto, 1995.
2.2. Feeding experiments In each experiment, 40–70 Acartia clausi females from the Gulf of Fos and 50–200
Acartia tonsa females from the Berre lagoon were incubated in 300-ml flasks for 24 h. Under each experimental condition, three to four test flasks with copepods and two to
three control flasks without copepods were prepared and placed on a vertically rotating wheel 1 rpm, to avoid particles settlement. The experiments were run in the dark, at
188C. The food medium consisted in a non-axenic culture of Dunaliella tertiolecta
. The characteristics of the incubation water differed according to the type of experiment as
explained below. Five experiments were completed to study the effect of salinity. The water from each
sampling station was filtered on Millipore 0.45 mm and enriched with a part of the Dunaliella culture adjusted to obtain a similar food concentration in the successive
experiments range 0.9–1.14 ppm for experiments 1–4, except in the last experiment where the concentration were lower 0.41–0.43 ppm. The salinity ranged from 13 to
15‰ for the Berre brackish station and 30 to 40‰ for the Fos marine station, according to the dates of sampling. In the first experiment, Acartia tonsa
, the only species present at that time, was incubated in low and high salinity water. In the four following
experiments, crossed incubations were prepared with A . clausi and A. tonsa, each
species being incubated in low salinity or high salinity water. Two set of experiments were carried out to study the effect of food density series A
and B. Different volumes of the algae culture stock were added to filtered water, at
R . Gaudy et al. J. Exp. Mar. Biol. Ecol. 247 2000 51 –65
55
16–20‰ salinity an intermediate value between sea and lagoon conditions to obtain four different concentrations of food medium.
A Coulter Counter-type multisizer 256 channels equipped with a 70-mm aperture tube was used to determine particles concentrations. Daily ingestion rates were
calculated from the difference in particles concentration in volume units between control and experimental flasks, taking into account the incubation time and the biomass
of incubated copepods. In the successive experiments, as less than 50 of phytoplankton was cleared 24–44, no food limitation occurred in the incubation flasks.
3. Results
