Date: Tue, 04 Feb 97 05:00

Subject: Chief Scientist's report #3

We've finished our second run of the 76-30 line. It turned out to be good to stick an extra long station in the middle (at 'O'), not only because it was interesting, but because it gave everyone a bit of a break. The Alphabet-Soup stations come fast and furious. By Orca, like the plankton themselves, we were pretty well depleted. Now, we will do more underway mapping and CTD stations, hit the big sites one more time, go into the pack ice to try to see high-iron conditions, and finally, a bit more underway to allow people to pack up and process samples and to fill in our map of surface properties. In regard to the latter, the surface pCO2 system operated by John Goddard has proved extremely useful in providing an indication of the spatial extent of the bloom area. The second time through the line, fluorescence is down by 40%, the water is clearer by about 20% in beam transmission, and nutrients are up. The biomass pattern along the line is fairly stable, so I think we can argue that we are sampling a temporal trend. There is always advection (of course, of course), but the advection would have to be such to replace the water we sampled the first time with nearly an exact replica (albeit with lower biomass-indicator concentrations). A plot of the density structure along the line shows that 'O' is in the middle of acyclonic feature which may help explain the biomass variability along the line. Kent Chen and Andy Archer prepared some postscript files of fluorescence and NH3 that we e-mailed to NSF which illustrate the changes. My supposition about DOC release being responsible for the discrepancy between C14 estimates of production and both CO2 and O2 that occurred at 'O' turned out to be true. We are seeing _significant_ release, and it is highly light-dependent. We're waiting on the counter to give Carol Knudson and I all the numbers (I can't believe the number of samples in there), so I'm not prepared to give a firm estimate, but my guess is it will be 20-40% of particulate uptake. The extracellular release only seems to be important in Phaeocystis waters. Dave Caron is continuing his experiments in trying to understand the relationship of Phaeo to bacteria, and the release numbers may be important in interpreting his data. Sasha Chekalyuk and Rob Olson are getting fluorescence data suggesting that the Phaeo is not photosynthetically active. Altogether, we believe we are witnessing the response of a plankton ecosystem to iron limitation. And it will be a great story to tell when we get all the pieces of the puzzle together. Dennis Hansell has now produced rough carbon budgets for all the long stations. He computes the fraction of biomass still resident in the surface layers, and these values range from 22% at the Ross Ice Shelf (probably the oldest seasonally) to 50-60% at Minke and 'O'. Lary Ball and Alan Fleer now have pump casts at many of the stations, and they are finding substantial Thorium disequilibria. Mary Jo Richardson has continued to get everyone on camera describing their work. She has started editing, to produce a 60 min videotape, and will be an excellent documentary for the cruise. Alex Gonzalez and Sylvia Pinca thought that their nets had misfired on their last tow at Orca, but when a succeeding tow did the same thing... They got large green aggregates in nets opened for the increment 50-150 m. This could indicate that one fate of the surface produced material is to aggregate and sink. There is not enough stuff there to show up on the fluorescence trace as anything but noise, but it may be significant to the carbon flux. Bill Cochlan and Julian Herndon continue to collect voluminous samples for N15 analysis ashore. They've got something like 1500 N15 uptake samples, 250 samples taken to measure regeneration and DON release, and 500 ammonia and urea analyzes. They size fractionate at every station and depth. They've also done a couple of nutrient kinetics experiments to get the important half-saturation constants for N-uptake. We just completed a return trip to the Ross Ice Shelf station. We were welcomed by a calm seas, a sunny sky, with a "sun dog" (like a rainbow on either side of the sun), some great 'bergs, snow petrels and skuas, and Minke whales spouting in the distance. We took a break, and while Capt. Joe did some nice ship manuevers, Buzz Scott and Herb Baker took many for zodiac rides to see the ship from a different vantage point, with the Ice Shelf as backdrop. Mike Gordon also collected ice samples. We're on the way to our fourth visit to the Big 'O'. It's getting colder but we've got clear skies and calm seas. More next week. John Marra, Chief Scientist, Process-2