458
A. Puddu et al
Bacterial numbers were counted with acridine
orange staining (Hobbie et al. 1977) and bacterial production by 3H -leucine incorporation. All
materials in contact with the samples were
washed with 1 N HCI and ultrapure water
(Millipore Milli-Q) prior to use. All the glassware
was heated before use (500 n C), Specific growth
rates (}l d- l ) were estimated from the increase in
bacterial numbers during the exponential
growth phase.
Results
Field Measurements
Variability ranges of DOC and BCPTdR together
with mean values and their respective 95% confidence intervals resulting from all the data in the
upper and bottom layers are summarized in the
Table for the two investigated frontal regions in
each cruise. The overall set of DOC values ranged
from 53 to 281 }lM while in winter cruises it was
53-124 pM. The significance of differences in the
median values among different data groups was
analysed by using the Mann-Whitney Rank Sum
Test. Concentrations of DOC in the upper and
bottom layers were reasonably constant in winter
surveys in the two areas. The differences between
layers were small, but significant (P the south, the data set of DOC in February 1997
was limited to only 4 surface values and the
resulting average value is not representative of
the area; then, it may not be compared with the
other averages derived from much more numerous data sets. The bottom average in winter periods is substantially the same in both years and
areas, 74±2 pM considering all data together, and
can be considered as a threshold value for this
coastal ecosystem. Contrary to winter results,
concentrations of DOC in summer surveys varied in the two investigated years. Mean value differences were significant between layers in the
northern region in both summer cruises
(P (P layers in the two summer cruises differed both in
the north (P while differences between areas were significant
only for surface waters in June 1996 (P Values of BCP from all the cruises in the two
frontal regions varied in the range 0.01-7.1 ]lg C
1-1 h- 1 (Table). The variability in winter periods
was much more limited with values in the range
0.01-1.6]lg C 1-1 h- 1 • In the north. the difference
between surface averages resulting from consecutive winter cruises was not significant, while
bottom averages were significantly different
Table. Summary of dissolved organic carbon (DOC) concentrations and bacterial carbon
production rates (BCP) in the two sampled areas in different cruises, in upper (~ m) and
bottom layers, G.I. is the 95% confidence interval for the mean
DOC (p.M)
Bep (p.gC I-I h-1)
North
South
North
South
June '96
Min-max
74-281 88-219
0.03-1.26 0.02-0.92
n
66
37
30
31
Upper layer mean±C.I.
191±16 153±15
O.60±0.28 0.38±O.11
Bottom layer mean±C.I.
137±13 123±13
O.10±0.02 0.10±O.1l
February '97
Min-max
53-123 84-153
0.03-0.67 0.05-1.61
n
76
4
30
24
Upper layer mean±C.I.
95±8
111
0.23±0.08 0.62±O.38
Bottom layer mean±C.I.
'74±3
O.21±0.1O O.16±O.09
June '97
Min-max
66-162 71-142
0.09-7.09 0.08-0.62
n
81
23
30
30
Upper layer mean±C.I.
123±8 100±27
2.45±1.8 O.36±0.39
Bottom layer meaD±C.I.
81±9
84±1l
O.15±O.05 0.11±0.04
February'98
Min-max
65-124 58-ll5
0.02-1.19 0.01-0.45
n
63
51
30
27
Upper layer mean±C.I.
95±10
83±7
0.37±O.34 O.18±O.10
Bottom layer mean±C.I.
75±5
73±4
O.O3±O.01 O.Ol±O.O04