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