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
3.1. Respiration Respiration rates were maximum at the highest temperature tested 208C but the
effect of temperature varied according to species and salinities Table 1. In Acartia clausi, respiration increased regularly with temperature. In A
. tonsa this was observed
only at the salinity of 35‰, while at 15 and 25‰, respiration was least at the intermediate temperature. Q
values derived from 10 and 208C rates were similar in
10
both species at each salinity condition Table 2. The salinity influence at a given temperature was marked only in A
. tonsa, in which the respiration rate obtained at the
highest salinity largely exceeded those obtained at 15 and 25‰. In A . clausi, for each
temperature condition, respiration rates were slightly higher at the lowest salinity and lower at the upper salinity, a tendency which is opposite to the results obtained in A
. tonsa
. The ANOVA performed on total data showed that temperature and salinity had
significant effects on respiration rate in A . tonsa only Table 3.
3.2. Excretion In both species, ammonia excretion rates showed a small range of variation between
10 and 158C but increased markedly at the highest temperature tested Table 1. The salinity variation had no marked effect on the ammonia excretion rates in A
. clausi
, at any temperature. In A. tonsa, the excretion rates were relatively homogeneous at low and medium salinity for a given temperature but decreased 10 and 158C or
increased 208C at 35‰.
The excretory Q of A
. tonsa was more than three times higher at 35‰ than at the
10
other salinities Table 2. This contrasted with the relative stability of Q values
10
observed at the different salinities in A . clausi.
The ANOVA performed on total data shows that, in both species, temperature had a significant effect on ammonia excretion, contrary to salinity Table 3.
3.3. O:N ratio The O:N atomic ratios were calculated from the average values of respiration and
56 R
. Gaudy et al. J. Exp. Mar. Biol. Ecol. 247 2000 51 –65 Table 1
Respiration, ammonia excretion and O:N at different combinations of temperature and salinity Species
Temperature 8C Salinity ‰
15 25
35
23 21
21
Respiration 10
ml O h Ind
6S.M.; n 5 number of data
2
Acartia tonsa 10
5.4360.57 4.7060.96
6.3861.56 n 5 18
n 5 18 n 5 18
15 3.7960.55
3.6860.90 7.5761.80
n 5 18 n 5 18
n 5 18 20
8.1561.09 9.5661.78
14.1862.63 n 5 18
n 5 18 n 5 18
Acartia clausi 10
6.5561.22 5.2760.96
5.8161.78 n 5 16
n 5 16 n 5 14
15 7.8461.66
6.3262.15 6.3162.20
n 5 15 n 5 14
n 5 14 20
12.6363.29 11.8861.66
11.3162.07 n 5 16
n 5 16 n 5 15
24 21
21
Excretion 10
mAtg N ? NH h Ind
6S.M.; n 5 number of data
4
Acartia tonsa 10
5.8061.08 7.6361.58
2.2760.72 n 5 17
n 5 18 n 5 13
15 5.9861.35
7.7762.68 3.0160.80
n 5 11 n 5 18
n 5 16 20
7.0460.38 10.3662.50
10.8862.41 n 5 18
n 5 18 n 5 15
Acartia clausi 10
2.8260.66 1.7860.41
2.4860.62 n 5 16
n 5 15 n 5 13
15 3.0260.56
2.7660.70 3.4360.64
n 5 12 n 5 12
n 5 17 20
6.2561.59 4.7061.69
5.6561.74 n 5 15
n 5 16 n 5 14
O:N atomic ratio, from respiration and ammonia excretion
Acartia tonsa 10
8.36 5.5
25.09 15
5.66 4.23
22.45 20
10.34 8.24
11.64 Acartia clausi
10 20.74
26.43 20.92
15 28.69
20.45 24.28
20 18.04
13.58 17.87
Table 2 Values of Q
calculated between 10 and 208C of respiration and excretion at different salinities
10
Function Species
Salinity ‰
15 25
35 Respiration
Acartia tonsa 1.5
2.03 2.22
Acartia clausi 1.93
2.25 1.94
Excretion Acartia tonsa
1.21 1.36
4.79 Acartia clausi
2.21 3.64
2.28
R . Gaudy et al. J. Exp. Mar. Biol. Ecol. 247 2000 51 –65
57 Table 3
Results of ANOVA on temperature and salinity effects on respiration and excretion rates Species
Source of Sum of
df Mean
F P value
variation squares
square Respiration
a 26
Acartia tonsa Temperature
0.0245 2
0.0122 15.09
1.04 10 Salinity
0.0076 2
0.0038 4.68
0.011 Interaction
0.0017 4
0.0004 0.53
0.713 Within
0.1244 153
0.0008 Total
0.1583 161
Acartia clausi Temperature
0.0402 2
0.0201 2.32
0.101 Salinity
0.0054 2
0.0027 0.31
0.731 Interaction
0.0667 4
0.0166 1.92
0.108 Within
1.3254 153
0.0086 Total
1.4379 161
Excretion
26 27
Acartia tonsa Temperature
1.65 10 2
8.29 10 6.08
0.003
27 27
Salinity 6.68 10
2 3.34 10
2.45 0.089
27 27
Interaction 8.66 10
4 2.17 10
1.58 0.179
25 27
Within 2.09 10
153 1.36 10
25
Total 2.41 10
161
26 27
25
Acartia clausi Temperature
3.63 10 2
1.82 10 12.59
1.22 10
27 27
Salinity 2.26 10
2 1.13 10
0.78 0.46
27 27
Interaction 3.19 10
4 7.97 10
0.55 0.69
25 27
Within 1.65 10
108 1.44 10
25
Total 1.98 10
116
a
Significant values of F are in bold.
ammonia excretion at each temperature–salinity combination. The values ranged from 13.6 to 28.69 in Acartia clausi and from 4.2 to 25.09 in A
. tonsa Table 1. Except the
high values recorded at 35‰ for 10 and 158C, O:N ratios were considerably lower in A
. tonsa than in A. clausi. Considering total data, the difference between species was highly significant F 5 32.6, P , 0.00002.
3.4. Food ingestion The specific ingestion rates calculated in the salinity experiments are reported on
Table 4. The results of experiments 2–5 in which both species were present were pooled in two groups of data corresponding to low salinity , 16‰ and high salinity
. 30‰ conditions. ANOVA performed on these grouped data indicated that specific daily ingestion did not differ significantly according to species or salinity.
In the two food density experiments, ingestion increased with food concentration in both species, but the ANOVA showed that the effect of concentration upon ingestion was
significant only for series A Table 5. As salinity had no significant effect on food intake, results of food concentration and
salinity experiments were pooled to calculate the relationship between ingestion and food concentration. This relationship was similar in both species, without any tendency
58 R
. Gaudy et al. J. Exp. Mar. Biol. Ecol. 247 2000 51 –65 Table 4
Effect of the salinity on food ingestion and result of the ANOVA Exp. no.
Salinity Concentration
Ingestion
3 21
21
range ppm
mm h ng DW animal
A . clausi
A . tonsa
1
, 16‰
1.05 nd
6.18 2
1.14 6.81
6.29 3
1.01 3.29
3.63 4
1.03 9.07
2.10 5
0.43 4.80
3.37 1
. 30‰
1.21 nd
10.80 2
1.02 6.97
7.05 3
1.03 4.10
4.71 4
0.89 6.66
6.54 5
0.41 1.48
2.39 ANOVA
Source of variation F
P value Salinity
0.01 0.95
Species 0.62
0.45 Interaction
1.24 0.29
Table 5 Ingestion of food at different food concentrations and result of the ANOVA test
Series Food
Ingestion
3 21
21
concentration mm h
ng DW animal ppm
A . clausi
A . tonsa
A 0.96
2.29 2.30
1.09 2.08
4.43 1.25
9.52 7.32
1.75 17.37
20.46 B
0.91 3.07
5.31 1.23
4.85 5.24
1.42 7.02
10.07 1.50
5.90 11.70
Source of variation F
P value ANOVA Series A
Concentration 10.41
0.04 Species
4.44 0.12 ns
ANOVA Series B Concentration
4.43 0.13 ns
Species 6.51
0.08 ns Significant; ns, not significant.
R . Gaudy et al. J. Exp. Mar. Biol. Ecol. 247 2000 51 –65
59
Fig. 2. Relationship between the food ingestion in volume units and the particles concentration in Acartia clausi and Acartia tonsa.
to saturation of ingestion in the limits of our experimental concentrations Fig. 2. The curves were linearized using a log
transformation of the ingestion values. The
10
correlation coefficients r between log ingestion and concentration were 0.72 in A
.
10 3
21
clausi and 0.80 in A . tonsa. Regression equations between ingestion I log
mm h ng
10 21
DW animal and food concentration C ppm were I 5 0.57 log
C 1 4.07 in A .
10
clausi and I 5 0.59 log C 1 3.78 in A tonsa
. The slopes and the intercepts of the two
10
regressions were not significantly different at P , 0.05 ANCOVA test.
4. Discussion
