| Summary: | SOIREE Sediment Traps - Water column POC and PON results<br<br> <b>METHOD</b><br> Two litre samples filtered through preweighed, precombusted (450°C for 4 hours) GF/F filters;<br> assumed negligible carbonate so samples non-acidified; whole filters analysed on CHN analyser<br> <br> Corrected values were calculated based on a filtered volume of 2 litres, rather than 500 ml<br> as reported for the initial values<br> <br> <b>NOTES</b><br> 1. The initial and OUTSIDE PATCH water column integrated POC values from SOIREE are higher<br> than expected for Southern Ocean HNLC waters. For example, unpublished data provided by<br> Wilford Gardner (USA) from the US JGOFS AESOPS transect, show that between 52-72°S 170°W<br> integrated POC standing stocks (0-50 m) in summer ranged from 3002-14413 mgC/m2 in Dec 1997<br> and 3603-10209 mgC/m2 in Jan 1998 SOIREE POC (MLD integrated) range from 4500-6200 mgC/m2<br> outside the iron-fertilised patch which were not substantially different from that measured<br> inside the patch during the SOIREE bloom (5100-8500 mgC/m2)<br> <br> 2. There is no obvious indication from either sampling or analytical techniques that the POC<br> values are artificially inflated due to sample handling, etc.<br> <br> 3. Charette & Buesseler (2000, G3) suggest that a previous export event must have occurred prior<br> to SOIREE due to a substantial 234Th deficit relative to 238U, which might explain the SOIREE POC<br> results. Since there is no indication from daily SeaWiFS images of a surface bloom from Nov 1998-Jan 1999<br> (C. Law pers. comm., 1999), nor any \"memory\" of such an event in the pCO2 data (Watson et al., 2000, Nature),<br> Charette & Buesseler (2000) suggested that such a bloom is likely to have been a subsurface feature,<br> or that there was a significant delay in export<br> <br> 4. \"Live\" biological carbon in HNLC waters outside the patch represented a significant or similar<br> amount of the organic carbon that was measured in the water column, ranging from 78-100% of the<br> MLD-integrated POC (see Table A below). If we assume that the water column samples would have sampled<br> only the phytoplankton and the microzooplankton (and not heterotrophic bacteria and mesozooplankton)<br> then this proportion decreases to 40-60% but is still relatively substantial.<br> <br> 5. POC values inside and outside the SOIREE patch were similar despite 6-fold and 3-fold increases in<br> chlorophyll a concentrations and phytoplankton abundance, respectively, inside the iron-fertilised patch<br> (Boyd et al., 2000, Nature)<br> <br> 6. Therefore, we conclude that perhaps there was a significant POC component in the water column at<br> the time that the SOIREE site was occupied, representing relict organic material derived from a previous<br> bloom/export event.<br> <br> 7. Given the uncertainties, however, it is wise to treat the interpretation of the above POC data with caution.<br> <br> <pre> <b>Table A</b> 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 </pre>
|
|---|