Orthophosphate analysis (modified MAGIC procedure)

Thierry MOUTIN

Soluble Reactive Phosphate (SRP) were analysed on board according to Murphy & Riley (1962) after a x6 concentration following the MAGnesium Induced Coprecipitation procedure (Karl et al. 1992). All chemicals were of reagent grade quality and solutions were prepared using deionized Milli-Q water. Sample bottles were washed before use with 10% HCl. To avoid contamination, only one bottle (polycarbonate, 60 ml) was used for sampling, centrifugation, precipitate treatment and analysis. Spectrophotometric analysis was performed with a CECIL CE 1011 spectrophotometer using a special cell with long optical length (100 mm) and small volume (10 ml).

First results indicate surface (10-15 m) SRP concentration of 12.3 nM (SD=4.7, N=50). This is above the blank value determined using natrium hydroxyde and hydrochloric acid as for the samples treatment (b_k=3.9 nM, SD=1.8) but is not consistent with orthophosphate concentration obtained from turnover times as proposed by Thingstad et al. (1999). Turnover time (T=[o-P]/V, [o-P] is the natural orthophosphate concentration and V the total uptake of orthophosphate) was obtained from ³³P experiment (Thingstad et al., 1993). The total required uptake may be indirectly obtained from bacterial production taking a C:P ratio of 50 and primary production taking a C:P ratio of 106. With considering V=V_Br+V_Pr and the turnovertime, it is possible to determine o-P. Following this procedure, surface (10-15 m) phosphate concentration varied from 3 near the strait of Gibraltar to 0.2 nM in the ionian sea. The difference of about 10 nM observed between this calculation and the MAGIC method may be explained either by DOP hydrolysis during the acid treatment or by the fact that the real blank value for seawater could not be obtained. Whatever the explanation, it seems reasonable to consider as a reference the value obtained at surface from the turnovertime. This latter value is then taken as the " blank " for the determination of o-P concentration with the MAGIC procedure.

This intricate procedure seems to be the only one which can provide a reasonable picture of the distribution of o-P concentration in surface waters of the Mediterranean Sea. A new detection limit of 5 nM seems to be reasonable. This concentration deapen from the strait of Gibraltar (about 10 m) to the M IO sîte in the ionian sea (about 90 m).

 

---

DATA  SET

 

		Phosphate PROSOPE
		Modified MAGIC procedure
CTD	bouteille	profondeur en m	[PO4] nM
9	20	11
9	18	16	486
9	16	21	420
9	14	26	429
9	12	31	415
9	10	42	561
9	8	52
9	6	61	702
9	4	76	797
9	2	92	745
CTD	bouteille	profondeur en m	[PO4] nM
11	20	5	0
11	18	11	3
11	16	20
11	14	29	10
11	12	38	14
11	10	49	25
11	8	59	57
11	6	80	207
11	4	100	290
11	1	150	356
CTD	bouteille	profondeur en m	[PO4] nM
14	20	5
14	18	15	2
14	16	29	15
14	14	40	39
14	12	45	66
14	10	60	225
14	8	69	250
14	6	90	176
14	4	110	210
14	1	150	303
CTD	bouteille	profondeur en m	[PO4] nM
17	20	5	0
17	18	10	1
17	16	25	6
17	14	40	42
17	12	55	156
17	10	70	185
17	8	80	79
17	6	94	28
17	4	109	37
17	1	151	138
CTD	bouteille	profondeur en m	[PO4] nM
20	20	7	0
20	18	13	0
20	16	27	0
20	14	41	4
20	12	61	21
20	10	77	52
20	8	90	115
20	6	105	148
20	4	122	133
20	1	150	164
CTD	bouteille	profondeur en m	[PO4] nM
23	20	6	0
23	18	11	0
23	16	23	0
23	14	40	0
23	12	55	11
23	10	65	20
23	8	80	58
23	6	95	59
23	4	120	122
23	1	150	127
CTD	bouteille	profondeur en m	[PO4] nM
26	20	6	0
26	18	10	0
26	16	29
26	14	50	1
26	12	70	0
26	10	85	0
26	8	98	6
26	6	115	8
26	4	140	39
26	1	170	62
CTD	bouteille	profondeur en m	[PO4] nM
33	20	4	0
33	18	14	0
33	16	30	0
33	13	50	2
33	10	70	3
33	8	90	5
33	6	100	9
33	4	131	14
33	2	150	25
33	1	200	83
CTD	bouteille	profondeur en m	[PO4] nM
44	20	7	0
44	18	17	0
44	16	32	1
44	14	50
44	12	70	3
44	10	90	4
44	8	109	6
44	6	130	14
44	4	150	30
44	2	200	93
CTD	bouteille	profondeur en m	[PO4] nM
60	20	7	0
60	18	17	0
60	16	32	0
60	14	50	0
60	12	75	0
60	10	90	0
60	8	110	2
60	6	130	21
60	4	150	27
60	2	200	97
CTD	bouteille	profondeur en m	[PO4] nM
63	20	4	0
63	18	14	0
63	16	30	0
63	14	50	0
63	12	70	0
63	10	90	0
63	8	110	17
63	6	130	33
63	4	150	46
63	2	200	77
CTD	bouteille	profondeur en m	[PO4] nM
68	20	5	4
68	18	15	0
68	16	30	0
68	14	50	0
68	12	71	10
68	10	90	12
68	8	110	44
68	6	130	103
68	4	150	133
68	2	200	180
CTD	bouteille	profondeur en m	[PO4] nM
70	20	5	0
70	18	15	0
70	16	30	0
70	14	50	4
70	12	64	2
70	10	75	15
70	8	90	46
70	6	110	103
70	4	129	142
70	2	150	173
CTD	bouteille	profondeur en m	[PO4] nM
73	20	4	0
73	18	15	0
73	16	25	6
73	14	40	8
73	12	45	3
73	10	60	96
73	8	75	225
73	6	90	251
73	4	110	250
73	2	151	298
CTD	bouteille	profondeur en m	[PO4] nM
78	20	4	4
78	18	9	0
78	12	29	5
78	10	39
78	8	47	14
78	6	60	19
78	4	70	150
78	3	90	248
78	2	110	250
78	1	150	313
CTD	bouteille	profondeur en m	[PO4] nM
96	20	13	0
96	17	32	0
96	15	40	0
96	10	45	4
96	8	55	40
96	7	60	39
96	6	65	105
96	5	70	177
96	4	74	224
96	3	90	258
96	2	110	282
96	1	150	293
CTD	bouteille	profondeur en m	[PO4] nM
104	20	15	0
104	17	30	0
104	15	40
104	14	45
104	12	55	0
104	11	60	21
104	10	65	31
104	9	70	84
104	8	75	94
104	7	80	132
104	5	90	234
104	4	100	256
104	3	110	287
104	2	130	309
104	1	150	315