Situation Report 4. NBP 97-3 AESOPS Process-3. 08 May, 1997

We are currently heading north toward Port Lyttelton, NZ, in 55 South
latitude and 40 - 50 knot winds. Science operations were completed with
deep CTD casts at 71, 68 and 65 South, and deep Thorium-234 pumping at
65 South early on 06 May. Since last report we completed a long
occupation at Station Sei (74S, 176E), including an in situ
productivity deployment and off-ship ice sampling. Overall we made 7
successful in situ deployments, 59 CTD's,  23 Trace Metal Rosette
casts, and 13 MOCNESS tows. Ice samples were collected at all major
stations.

Station Sei was occupied on 13-14 April and again on 29 April - 01 May,
enabling us to look at trends over a 2-week period, during which time
the day length decreased from 7.9 to 3.7 hr. The surface layer was
nearly 0.1 salinity units fresher on the second occupation, suggesting
advection during the sampling interval. PO4 and NO3 both increased but
were still depleted relative to deep water. A subsurface NH4 peak
deepened but still reached to 1.7 uM. The deep DOC maxima seen earlier
have been erased by deep mixing. DOC concentrations in the deep water
are consistent with earlier observations made on Process-1 and -2,
while the present surface values are higher than on P-1 but lower than
on P-2, when peak surface enrichments were seen.

Since the repeat of station Sei, the CO2 group made full profiles at
the 3 deep stations and is sampling every 0.25 degree underway through
the APFZ. As expected, surface values of DIC and TAlk decreased with
each 3 degrees North by approximately 10 uMol/kg. Initial trends
underway are similar to the previous transect although values of DIC
have increased slightly varying from 4 to 9 uMol/kg with higher deltas
occurring in the higher latitudes.  Similar increases in surface pCO2
are also observed.  These findings are consistent with lower biological
activity and the onset of winter conditions.

Chlorophyll concentrations stayed in the 0.02 - 0.03 range along the
SPOZEA transect, but were higher at Station Sei-2  (0.05 - 0.06 ug/l
compared with 0.10 - 0.14 at Sei-1). There was no measurable herbivory,
so the decreases in Chl were due to deep mixing.

Primary production continued the trends reported earlier.  We saw the
highest integrated productivity (75.1 mgC/m2/d -- on deck incubations)
at Sei 1. PP decreased to 22 and 7 mgC/m2/d for on-deck and in situ
incubations, respectively at Sei-2.  This all fits nicely with the
diminishing photoperiod.  The on deck incubations at Sei-2 were  still
relatively high compared to other stations.  Comparison of in-situ with
on-deck incubations at Sei-2 and Minke-2  continued to prove the
obvious- there's less light under a meter of ice than on the helo-deck.
Net oxygen production production at Sei-2 was 13 mmol O2/m2/d, compared
to 50 mmol O2/m2/d measured during our first occupation.

This week's focus is on Mike Hiscock. He's been on all 3 process
cruises so far.  Mikey  put a lot of effort into carrying out diel
series of P vs E experiments when he wasn't playing poker or
maintaining a demanding social schedule ;-). While some samples are
still counting, the results available at press time suggest marked
phytoplankton adaptation to low light intensity, with photosynthesis
saturating at  just a few uEins/m2/sec. and photoinhibition above 50
uEins. The phytoplankton were likely capable of carrying on
photosynthesis even in the predawn and post-sunset twilight periods.
This is very different from the characteristics seen on Process 2.  The
Pb max was normally around 100 to 200 uEin with little if any
photoinhibition on the earlier cruise.

At Sei 2 the C-14, O-18 and N-15 groups (The Canadian Club) joined
forces in a time-course experiment.  They incubated 3m water in the
on-deck clear incubator for 8, 16 and 24 hours, and measured changes in
tracer incorporation, bulk nutrients, and DOC.  The incubators
miraculously withstood their final challenge of  60-80 knot winds and
-25C  while the ship stopped in a total whiteout.  C14 incorporation
was  half of what it was the (sunny) day before.  The first 8 hours
encompassed the photoperiod, and there was  no difference in C-14
incorporation between 8, 16 and 24 hour incubations.  This suggests
there isn't much respiration of incorporated 14C during the night,
which was one of our explanations for the discrepancy between O and
C-based productivity measurements.

Bacterial respiration might provide another answer for the
discrepancy.  Although the bacterial production (BP) was very low in
absolute terms, averaging just 2-5 mgC/m2/d, it was still a substantial
fraction of the C-14 primary production: 23 to 67% depending on the
method used to estimate the BP  (thymidine vs leucine incorporation).
Using the bacterial respiration to production ratio (ca. 5) established
on earlier NBP cruises, we estimate that bacterial respiration was
about the same order as the C14 PP. There was measurable bacterivory
(about 0.1 of the stock per day) at stations Orca-2, Minke-2 and Sei-2
(finally!).

With this cruise, US JGOFS completes its first season of observations
in the Ross Sea. In spite of the vagaries of a 4-year planning process
and ship scheduling, various avoidable and unavoidable delays and the
challenging weather and sea conditions, we were successful in capturing
the first stirrings of the Phaeocystis bloom in October and the winding
down of primary production in the waning daylight of April. In between
we saw the bloom pass from  its peak into a slow decline in
January-February. The strong CO2 drawdown seen  in the summer has
weakened but persisted throughout our April observations. Next season
the picture should be completed with observations of the bloom takeoff
on the Process-4 cruise.

We survived the loss of the video system in the 02 lounge, incubator
freezeups and the hydraulic blowout, the eruption in cabin 133 and the
photo session at the Ice Shelf. We have many people to thank and
remember. Captain Joe Borkowski and his officers (Robert, Vladimir and
Tom) and crew got us there and back again,  kept us on time and on
station, and answered emergency calls with speed and grace. Ernest
Stelly and his galley crew (Marta, Nora and Nestor) kept us gorged on
cookies, rice and beans and homemade salsa (and the science party
favorite -- Cajun pizza). The ASA gang headed by MPC Jon Alberts worked
above and beyond the call of duty: thanks for everything, Rhonda and
Corey, Mindy and Buzz, Paul, Kris and Rich. We couldn't have done it
without any of you.

That's it until next fall -- er, spring.

Hugh Ducklow
Chief Scientist, NBP 97-3.