Situation Report: NBP 97-3, 21 April, 1997. 78S, 178W, Emperor Station.

NBP 97-3, US JGOFS AESOPS Process cruise 3, occupied major
biogeochemical stations Orca (76.5S, 178W), Minke (76.5S, 169E) and
Emperor (78S, 178W) during the past week. We are observing the decline
of the Ross Sea primary production system and intensive heterotrophic
activity throughout the region. At Station Orca, the mixed layer was
about 100 m, and we suspected ML deepening via brine rejection. This
seemed to be confirmed when we arrived at Minke and saw Z(m)
approaching 200 m. Moreover the water column at this station was
distinguished by a vertically uniform ammonium (NH4+) concentration of
2.46 uM throughout -- an enormous standing stock. This must be oxidized
to NO3 by nitrifiers over the winter.  In general, we continue to be
surprised by the amount of spatial (both horizontal and vertical)
variability, and levels of heterotrophic stocks in the study area.

The major nutrients (NO3, PO4, SiO4, DIC) were depleted in the mixed
layer at all stations, and NH4 was enhanced. DIC and TALK showed abrupt
20-55 uM changes at the base of the mixed layer. The ratio of DIC:NO3
in the surface layer at Orca was about 8.5 (courtesy Team Hydro and the
U Miami group), indicating biological maintenance of the surface
deficits, even though primary production is low. Kristi Hanson reports
that Chlorophyll a  concentrations were 0.1, 0.05 and 0.03 ug/l at Sei,
Orca and Minke, respectively. However, the standing stock at Minke was
distributed over 200 m and was comparable to Orca. Zanna Chase reports
that in situ C14 primary production at Sei and Orca were 6.3 and 1.7
mMolC/m2/d. Not high, but respectable for low biomass and a 5-6 hr
day.  Mike Hiscock's (Duke) P vs E experiments suggested the
phytoplankton community was saturating production rates at very low
irradiance --  just 5 uEinsteins/m2/sec, providing strong evidence of
photoadaptation in the Antarctic twilight. The day is 3 hr long at 78S
today.

This week we highlight the work of the small, but efficient Oxygen
Productivity Group from URI, supervised by Mary-Lynn Dickson. Gross
primary production, net community production, and respiration rate
measurements were made at stations Sei, Orca and Minke from O2 changes
in in situ quartz bottles.  Net community production rates (integrated
to the 1% light level) at Sei, Orca and pseudo-Minke were 50, 23, and
61 mmol O2/m2/24hrs, respectively. Assuming a photosynthetic quotient
of 1.4, production in terms of carbon was estimated to be 36, 17, and
44 mmol C/m2/24hrs for each of those stations.  Dark respiration rates
have been negligible and may reflect the extremely low biomass levels
encountered. However, the contrast between the carbon and oxygen-based
rates seem to indicate intensive community respiration, consistent with
the high NH4 values we've seen.  At stations Sei and Orca all the net
community carbon production can be accounted for from the disappearance
of nitrate in the bottles. At station pseudo-Minke, where extremely
high ammonium concentrations were observed, nitrate assimilation only
accounted for 60% of net production.

DOC (Peltzer/Kirchlechner, WHOI/Otago) showed slight surface
enrichments of 3-6 uMC and some vertical structure above a uniform deep
water concentration of 41 uM at all stations. Bacterioplankton
ecologists Alison Sanford (Horn Point) and Dawn Castle, working most of
the day inside their refrigerated rad van (the cold hot van), observed
very low bacterial production rates at Sei, Orca and Minke, with
Thymidine and Leucine incorporation rates averaging 0.05 and 0.5
pM/l/hr, respectively.  However, these low rates may represent a higher
percentage of the low primary production values than we've seen on
earlier cruises.

The mesozooplankton group (Nordhausen/Zhong/Jordan, SIO/Bama) continue
to capture large hauls of copepods and a few krill and other predators,
and are performing feeding and excretion experiments at each station.
Herbivorous copepods, such as Calanoides acutus (C4, C5), were only
found near the bottom and appear to be at the beginning of diapause.
Carnivorous (Euchaeta) as well as omnivorous copepods (Metridia C5 and
females) appear active with high NH4 excretion rates. Microzooplankton
hunters Mark Dennett and Ee Lin Lim (WHOI) continue the elusive search
for measurable rates of herbivory and bacterivory, performing laborious
dilution experiments at each major station. nanoplankton abundances
appear to be comparable to the Process-1 cruise.

Bill Cochlan's 15N Warriors (Big Joe Hayden, USC and Jay Wheeler, UGA)
continue to battle the elements at the deck incubators, making nitrogen
productivity measurements. Dave Hirschberg and Ellen Roosen deployed
the Thorium pumps twice at each of  the major stations -- results next
time.

We are staying on schedule and are receiving outstanding support from
CTD techs Billy Sweet and Mark Cook, marine techs Buzzy Scott and Mindy
Kayl, and lab techs Rhonda Kelly and Corey Peterson. The Trace Metal
Free Rosette continues to defy the cold, bringing 'em back alive and
uncontaminated for the rate measurements.

The big excitement for the week was our dramatic approach to Emperor
Station near the Ross Ice Shelf. We cruised along the ice barrier in
the early morning sun (at 1100), and crossed the mouth of a large gulf
or crack in the shelf. The sudden onset of meter-thick ice prevented
Captain Joe from probing the mysteries of this 12-mile long crack. A
high noon attempt to take a group photo was aborted by -33C
temperatures. You couldn't identify anybody anyhow. During the week a
vigorous campaign was conducted to name the station formerly known as
RIS. After contentious and searching debate, the election commission
declared Emperor the winner. The name represents the entry of a new
species into the AESOPS pantheon. Allegations of vote fraud were made,
but a recount confirmed that choice.

We were disappointed to learn that tight scheduling prevented regaining
some of the time we lost in port, but still look forward to occupying
all the major stations twice. Results so far are unexpected and very
interesting. Like the Emperors, we roam the pack ice and laugh at the
cold. More next week.

Hugh Ducklow
Chief Sci., NBP 97-3