Phosphate was analysed on board immediately after sampling on a
spectrophotometer (Shimadzu UV 160) with a 10 cm optical path cell
whereas nitrate was determined at the laboratory with an autoanalyser
(Skalar) on frozen samples (- 20°C). Phosphate and nitrate concentrations were determined according to the classical methods described by
Strickland & Parsons (1972) with the respective detection limits of
0.02 and 0.05 pM.
Total chlorophyll a (Chl a) was determined using 0.4 pm Nuclepore
polycarbonate filters (cmises in 1998) or 25 mm Whatman GF/F (Plagia).
Size fractionation was realized on 3 pm Nuclepore filters and 20 pm
Nylon sieve. Filters were frozen and analysed later by acidification fluorometric procedure in 9()'/n acetone (in 1998) or methanol 100
ll /n (in
1999) extracts (Holm-Hansen etal, 1965).
Orthophosphate uptake was measured using
33 PO
+
as a radiotracer.
After addition of 5 uCi and 150 µCi 33 PO
4
to 10 ml (Plagia 3) and
1 000 ml seawater (Plagia 1 and 2) respectively, samples were incubated in an on-board incubator at in situ temperature and at in situ
light (Plagia 1 and 2) or 40% of incident light (Plagia 3). At incubation times from a few minutes to several hours,
33 PO
4
3- uptake was
stopped. Samples were filtered on parallel 0.22 pm Millipore filters
and 1 |um Nuclepore filters within one hour after the end of incubation. They were dried and stored with 4 ml of scintillation cocktail
until counting at the laboratory with a Tri-Carb liquid scintillation
analyser model 1500 (Packard). Measurements in 1998 were similar
but incubation times were too long at Biomet 111 and Pegase. Therefore, results of turn-over time of phosphate correspond to an upper
limit. ()n the contrary, size fractionation of uptake is actual.
Alkaline phosphatase activity (Apa) was determined by the fluorometric
method using methyl-umbelliferyl phosphate (Muf-P) as substrate
(Hoppe, 1983; Ammerman, 1993). Most measures consisted in evaluating the maximum activity) 1 (Vmax) with a saturating substrate final
concentration of 250 |aM. Kinetic parameters were determined using
substrate final concentrations from 0.5 to 250 IUM. Two millilitres of
whole water samples and prefdtered samples on 0.2 pm or 1 pm Nuclepore filters were incubated in duplicate with 50 |al of substrate solution in the dark at in situ temperature during ten hours. Reaction was
stopped and thc samples were frozen (- 20°C). The increase fluorescence
of Muf (excitation at 365 nm and emission at 460 nm) was recorded by
a fluorescence spectrometer (Kontron SFM 25).
All parameters were averaged on a 20 metres depth.
Results and discussion
Large increase of Chl a between Januaty and March 1998 as well as high
values measured at the end of February 1999 for the two types of stations
“Gironde” and “Oceanic” confirm the occurrence of winter phytoplankton blooms within the Gironde plume (fig. 1). These blooms were
121
spectrophotometer (Shimadzu UV 160) with a 10 cm optical path cell
whereas nitrate was determined at the laboratory with an autoanalyser
(Skalar) on frozen samples (- 20°C). Phosphate and nitrate concentrations were determined according to the classical methods described by
Strickland & Parsons (1972) with the respective detection limits of
0.02 and 0.05 pM.
Total chlorophyll a (Chl a) was determined using 0.4 pm Nuclepore
polycarbonate filters (cmises in 1998) or 25 mm Whatman GF/F (Plagia).
Size fractionation was realized on 3 pm Nuclepore filters and 20 pm
Nylon sieve. Filters were frozen and analysed later by acidification fluorometric procedure in 9()'/n acetone (in 1998) or methanol 100
ll /n (in
1999) extracts (Holm-Hansen etal, 1965).
Orthophosphate uptake was measured using
33 PO
+
as a radiotracer.
After addition of 5 uCi and 150 µCi 33 PO
4
to 10 ml (Plagia 3) and
1 000 ml seawater (Plagia 1 and 2) respectively, samples were incubated in an on-board incubator at in situ temperature and at in situ
light (Plagia 1 and 2) or 40% of incident light (Plagia 3). At incubation times from a few minutes to several hours,
33 PO
4
3- uptake was
stopped. Samples were filtered on parallel 0.22 pm Millipore filters
and 1 |um Nuclepore filters within one hour after the end of incubation. They were dried and stored with 4 ml of scintillation cocktail
until counting at the laboratory with a Tri-Carb liquid scintillation
analyser model 1500 (Packard). Measurements in 1998 were similar
but incubation times were too long at Biomet 111 and Pegase. Therefore, results of turn-over time of phosphate correspond to an upper
limit. ()n the contrary, size fractionation of uptake is actual.
Alkaline phosphatase activity (Apa) was determined by the fluorometric
method using methyl-umbelliferyl phosphate (Muf-P) as substrate
(Hoppe, 1983; Ammerman, 1993). Most measures consisted in evaluating the maximum activity) 1 (Vmax) with a saturating substrate final
concentration of 250 |aM. Kinetic parameters were determined using
substrate final concentrations from 0.5 to 250 IUM. Two millilitres of
whole water samples and prefdtered samples on 0.2 pm or 1 pm Nuclepore filters were incubated in duplicate with 50 |al of substrate solution in the dark at in situ temperature during ten hours. Reaction was
stopped and thc samples were frozen (- 20°C). The increase fluorescence
of Muf (excitation at 365 nm and emission at 460 nm) was recorded by
a fluorescence spectrometer (Kontron SFM 25).
All parameters were averaged on a 20 metres depth.
Results and discussion
Large increase of Chl a between Januaty and March 1998 as well as high
values measured at the end of February 1999 for the two types of stations
“Gironde” and “Oceanic” confirm the occurrence of winter phytoplankton blooms within the Gironde plume (fig. 1). These blooms were
121