Date: Tue, 30 Dec 1997 15:34:16 -0500
4th Weekly Progress Report R/V ROGER REVELLE December 30, 1997 The fourth week of the Process 1 cruise continued to be extraordinarily productive. The weather has been variable: on Dec. 27 the wind blew over 35 knots all day while on Dec. 29 it was at 8 knots or less. We spent about 12 hrs hove to due to weather on Dec. 27; otherwise, we have not had much weather related downtime. We have completed a section of 11 "long" stations between 53'00"S and 64'40"S and 7 "CTD only" stations. A dense CTD section across the subsurface Polar Front at about 60'50"S showed temperature at 100 m going from +1.0 degC to -1.0 degC in 15 nm. Close to the ice at 64'10"S, -1.0 degC water was at the surface and down to about 120 m. This cold water was present as a subsurface layer (40 to 140 m) for about 200 nm northward from 64'10"S to 60'50"S. Overlayering the cold layer was a shallow layer of low salinity water (33.5 to 33.7 pss) that had a temperature of about 0 degC. For 200 nm, from 60'50"S to 64'40"S, there was, therefore, exceptional stability in the euphotic zone as well as high nutrients. The mean incident PAR on our long stations was 38 E/m2/d, not very different from other JGOFS Process Studies. Nitrate in the surface was about 20 microM from 54'20"S to the pack ice at 64'40"S. In contrast, silicate went from about 1 microM at 54'20"S to over 50 microM at 64'40"S. Unraveling the physics and biology that maintain a steep meridional silicate gradient and constant nitrate will keep us occupied for a while. Biomass as indicated by chlorophyll was high by oceanic standards, the highest being 2.5 mg Chl/m3. (Recall that in IronEx-2, the Fe-enhanced maximum was 2.4 mg Chl/m3.) Over the zone of the steep silicate gradient mean chlorophyll at the 25% light depth was 1.68 mg Chl/m3. By any ocean standard this 200-nm zone is a high biomass region. Primary production over the steep silicate gradient varied from about 640 to 1260 mg C/m2/d. Next to the ice the values were all greater than 1000 mg C/m2/d and the station at 64'10"S was the highest at 1260. The maximum chlorophyll-specific production rate (Pbmax) was relatively low - about 25 mg C/mgChl/d - but considering that the temperature was +1.0 to -1.0 degC, that's not surprising. Oxygen production indicates that net community production was about 65% of the 14C estimated production. Si and N production numbers require post-cruise processing, but will be of great interest to the interpretation of the high Si, high export and high biomass situation. Macrograzing appeared to be up to 8X higher at the ice edge (50 fecal pellets/copepod/day) compared to farther north (6 fecal pellets/copepod/day). Krill appeared in the upper 50 m at night and were not found in the tows. As expected, these high diatom waters support high concentrations of macrograzers. Consistent with these results, thorium measurements indicate that export is extremely high in the productive stations close to the ice edge where silicate was very high. The dilution experiment estimates of gross growth phytoplankton rates were less than 0.2 d-1 close to the ice and around 0.4 d-1 in the warmer water to the north. Growth usually exceeded grazing. At a Phaeocystis dominated station, net O2 production was 120% of the 14C particulate production, indicating lots of DOC production. Maximum values for bacterial production between 53'S and the ice edge (64'10"S) ranged from 0.33 - 0.95 mg C/m3/day (leucine-based) and 0.68 - 1.53 mg C/m3/day (thymidine-based). The leucine-based peak production value at the northern edge of the polar front (60'54"S) was 0.95 mg C/m3/day), about 2X that within the front at 61'40"S. At the ice edge, production reached a maximum of 0.59 mg C/m3/day at 20 m depth and fell dramatically to 0.01 mg C/m3/day at 40 m in the cold subsurface layer. One of the joys of the JGOFS cruises is that a great deal of data processing is done on the cruise, so we have a clear sense of what we've found. However, don't get too wedded to the particular numbers we are reporting here. All will be revised. Process 1 participants had a nice Christmas at sea. The crew and scientific party had good food and warm fellowship. The cribbage competition was really the biggest social event. Many skillful and crafty cribbage players fell by the wayside as the Process 1 tournament reached its climax. Process 1 has begun its final station, a re-occupation of Station 1 at 53'S. We will send one more report just before reaching port. Regards, Dick Barber, Chief Scientist