Date: Mon, 10 Feb 97 10:43

Here's my last report from NBP97-1, the Process-2 cruise. In summary, we spent the last week re-visiting the Big 'O' and Minke, and then hunted up some low-nitrate water in the pack-ice (which according to Steve's '90 data should be high-iron water as well). We've just finished another short visit to Minke, mapped to the north (where Alex and Sylvia did one last MOCNESS), swung by 'O', and we are now heading on into McMurdo to arrive in the late morning of the 11th. Production and abundances of phytoplankton at 'O' continued its decline, with production declining by almost an order of magnitude since the first visit, but biomass (chlorophyll) decreasing by only a factor of two or three ( and the centroid of the maximum has gone deeper). Dave Caron still finds healthy-looking Phaeo in sampling from the zodiac, so there is a hint that part of the population can find its way to the near-surface. We intend to follow up on that in our data analysis. And the Phaeo continue to release significant amounts of DOC-14. (At other stations where we don't find Phaeocystis, release is negligible.) After 'O', we stopped at Minke for a brief time (to ensure enough processing time for samples collected there), and then went NW into the ice. We were looking for low-nitrate (= high-iron) water, and Capt. Joe picked a great spot, parking next to a large floe. We did CTD, TM, and hydro casts, started up some N15 productivity experiments, put out the in situ productivity array (for CO2, O2, O18 and 14C), and then went to collect ice and ice bottom samples on a floe using the ship's zodiac. It was a brilliant day. There was a clear blue sky; the mountains and glaciers provided the backdrop, while large tiles of pure white pack-ice slowly floated by. A couple of emperor penguins arrived, scooting on their bellies over the floe to see what's what. We retreived the in situ array the next day. Captain Joe used the ship to nudge some ice floes out of the way so we could pick up the spar and tag line with a boat hook. The PAR loggers showed that beneath the ice, irradiance drops to only 25 umols photons per meter squared per second, which corresponds to Mike H.'s measurements of the irradiance marking the onset of maximal photosynthesis, generally, for the phytoplankton in the Ross Sea. Perhaps they are adapted to the ambient conditions down here (however see below). Corey got a very nice PAR profile with the TM rosette (in the open water), showing an increasing attenuation coefficient at the depth of the fluorescence maximum. Like other stations, most of the pigment is beneath the depth of 10% of surface irradiance. The Hydro team measured 6 uM nitrate just beneath the ice, going down to 2 a little deeper. Silicate was around 50. These are the numbers we were looking for. Rob and Sasha took some samples, and said their fluorescence measurements were showing the same values as healthy (iron-replete) lab populations. Dennis and Rachel say that DOC values for the pack ice were fairly typical. pCO2, however, was as low as we've seen, at 130 ppm, according to Colm and John G. Colm reported net production rates of about 3 uM C per day from the in situ experiment (at 50% surface light), with chlorophyll at about 1 ug per liter at the surface. When Joe Orchardo completed the titrations for this station he found an unusual profile. Net respiration (instead of net production), occurred at the depth just beneath the pack ice, and increasing to about 3 uM deeper down. One the way out of the pack-ice, Alex and Sylvia did a MOCNESS, and collected relatively small numbers of pelagic zooplankton. Jeff and Billy ("Probing the Depths") completed their 90th CTD cast today. And Corey gets the "Golden Dipper" award for his 50th Trace- Metal-Clean Rosette cast. Alex and Sylvia did 29 MOCNESS tows (and 16 Ring Net), and Carol, Mike H., Joe O., Cara, Colm, Ann-Maree and I did 10 in situ productivity experiments. Using Buzz' and J.P.'s newly-designed mini-corer, Herb and Billy retreived two sediment cores from the pack-ice station and Minke. Importantly, for all these operations, we had an equal number of deployments and recoveries. Dave K. and Grieg did 21 bacterial production experiments. Dave C. and Darcy completed about 8 grazing experiments and a bunch more bacterivory experiments, getting the science-party record for time spent at Antarctic the ship's cold lab, processing samples. Ann-Maree says we're up to nearly 700 chlorophyll analyses. Bill and Julian have literally 1000's of N15 analyses ahead of them. Joe O. said he filled something like 350 flasks for O18 analysis. Dennis and Rachel, Colm and John G., and Mary Jo did about 30 DOC/TOC, CO2, and SPM profiles (surface to bottom), respectively. Rob and Sasha's laser operated nearly continuously throughout the cruise. Lary and Alan completed about a dozen pump casts and an equal number of slurpers, getting over 200 samples, and did all the preliminary Thorium counting. Mike H. said he ran 3300 samples through the liquid scintillation counter. Steve and Mike G. did two iron-enrichment experiments, each lasting 10-12 days, and provided daily samples for the Hydro Team (Doug, Joe J., Erik, Calvin, and Joe D.), for Mike H. (PvsE), and Bill and Julian (N15). Aside from all the SPM sampling, Mary Jo will completed editing her video of the cruise the other night. We had a great time at the sneak preview last week, and today she broadcast it over the network so people could view and copy it from their cabins' videotape players. Dennis continued his carbon budget calculations (see my previous weeklies), and has nearly completed all 28 stations. He calculates that the percent of net community production remaining in the upper 250 m of the water column ranges from 25% (at the Ross Ice Shelf) to about 70% at the first stop at the Big 'O'. We see a steady decline in this percentage at 'O', such that on the second stop, it is down to 60%, and the third time at 50%. The broader temporal characteristics of the decline will be interesting to evaluate further in light of all the other measurements made at 'O'. After the last stop at Minke, we pretty much closed up shop, so everyone could process and pack, while we continued to collect surface property data. But at that northern station, Alex and Sylvia got in one more MOCNESS. A few hours before arriving there, we went through an optical front (a sharp drop in the absorption coefficients measured by Mary Jo's AC-3), and pCO2 went up to 240 ppm. The meso- zooplankton community was also different with more carnivorous forms, lots of copepods, and fortunately for Alex and Sylvia, not much green stuff. It's paper-work time. Janet is gently guiding us down the joyful path of shipping, warehousing, hand-carrying, permits, excess baggage, and weights and cubes. All told, I think we made some significant advances on this cruise. First, unlike what many of us had thought earlier, iron seems to be important to Ross Sea productivity during the growth season. The evidence for this comes from Steve and Mike's enrichment experiments and from Sasha and Rob's single-cell fluorescence data. To a first approximation, then, the reason for the large residual concentrations of the major nutrients is that iron is the limiting nutrient. It will be much more complicated than this because of food-web recycling, physical processes, etc., but Ken, your hypothesis about iron is holding up. Second, Dennis' carbon budgets, Lary and Alan's Thorium disequilibria, and Dave K.'s and Grieg's bacterial production numbers suggest that the pathways of much of the production is to the bacteria and the microbial "loop", and sedimentation to the bottom. Third, I think we have uncovered some interesting biological features about the microbial food-web and Phaeocystis. Dave C. and Darcy have found something like a mutualism between bacteria and Phaeocystis, and the extracellular release provides a mechanism. Fourth, we have a pretty good idea of the spatial extent of the pCO2 "drawdown" area using the surface mapping system operated by John G. and Colm. Together with the system being operated on the previous ROAVERRS cruise and with Process cruises 1 and 3, we may even be able to estimate the seasonal and areal extent of the seasonal production cycle in the Ross Sea. The above are in the context of all the other JGOFS operations, such as excellent hydrographic data, surface property maps, nutrient data, productivity data (N15, O2, TCO2, C14, O18), nutrient kinetics, optical data, TOC and DOC, and mesozooplankton distributions. Also we've appreciated the excellent ASA support, from the CTD hardware and the Trace-Metal-Clean Rosette, to all the lab equipment and supplies, to waste handling, the computer network, ET support, and deck operation help, and finally to the zodiac. We also thank Captain Joe and his mates and crew for making our stay aboard the Palmer most pleasant and enjoyable, and for making the trip so memorable. We had a good cruise. John