Dataset: SedimentTraps - POC PON Water Column

SOIREE Sediment Traps - Water column POC and PON results METHOD
Two litre samples filtered through preweighed, precombusted (450°C for 4 hours) GF/F filters;
assumed negligible carbonate so samples non-acidified; whole filters analysed on CHN analyser

Corrected values were calculated based on a filtered volume of 2 litres, rather than 500 ml
as reported for the initial values

NOTES
1. The initial and OUTSIDE PATCH water column integrated POC values from SOIREE are higher
than expected for Southern Ocean HNLC waters. For example, unpublished data provided by
Wilford Gardner (USA) from the US JGOFS AESOPS transect, show that between 52-72°S 170°W
integrated POC standing stocks (0-50 m) in summer ranged from 3002-14413 mgC/m2 in Dec 1997
and 3603-10209 mgC/m2 in Jan 1998 SOIREE POC (MLD integrated) range from 4500-6200 mgC/m2
outside the iron-fertilised patch which were not substantially different from that measured
inside the patch during the SOIREE bloom (5100-8500 mgC/m2)

2. There is no obvious indication from either sampling or analytical techniques that the POC
values are artificially inflated due to sample handling, etc.

3. Charette & Buesseler (2000, G3) suggest that a previous export event must have occurred prior
to SOIREE due to a substantial 234Th deficit relative to 238U, which might explain the SOIREE POC
results. Since there is no indication from daily SeaWiFS images of a surface bloom from Nov 1998-Jan 1999
(C. Law pers. comm., 1999), nor any "memory" of such an event in the pCO2 data (Watson et al., 2000, Nature),
Charette & Buesseler (2000) suggested that such a bloom is likely to have been a subsurface feature,
or that there was a significant delay in export

4. "Live" biological carbon in HNLC waters outside the patch represented a significant or similar
amount of the organic carbon that was measured in the water column, ranging from 78-100% of the
MLD-integrated POC (see Table A below). If we assume that the water column samples would have sampled
only the phytoplankton and the microzooplankton (and not heterotrophic bacteria and mesozooplankton)
then this proportion decreases to 40-60% but is still relatively substantial.

5. POC values inside and outside the SOIREE patch were similar despite 6-fold and 3-fold increases in
chlorophyll a concentrations and phytoplankton abundance, respectively, inside the iron-fertilised patch
(Boyd et al., 2000, Nature)

6. Therefore, we conclude that perhaps there was a significant POC component in the water column at
the time that the SOIREE site was occupied, representing relict organic material derived from a previous
bloom/export event.

7. Given the uncertainties, however, it is wise to treat the interpretation of the above POC data with caution.
 

Table A				mixed layer (0-65 m)						
				col integrated POC 						
		OUT stations only 	mg C m-2				mg C m-2		%			%
									
		STATION	phyto	hbact	microzoo	mesozoo 	total 	POC water column	Difference (ALL)	Diff (phyto + micro only)
12-Feb-99	T1141/1	1595	504	772		1600		4471	5615			79.6			42.2
16-Feb-99	T1152/1	2023	346	620		1600		4589	5881			78.0			44.9
18-Feb-99	T1157/1	1595	290	616		1600		4101	5157			79.5			42.9
21-Feb-99	T1168/1	1885	374	706		1600		4565	4564			100.0			56.8