Situation Report 3: NBP 97-3, AESOPS Process Cruise 3. 28 April, 1997

Since last report we completed a busy, nonstop week of sampling, 
finishing  long occupations of Stations Emperor, Orca-2. Big-O, and 
Minke-2. In addition we ran the S-P-O-Z-E-A transect (that's AEZOPS 
running to the west), carrying out deep CTD's, productivity casts and 
MOCNESS tows at all stations. We have been continuously in pack ice 0.3 
to over 1 meter thick since 11 April, throughout our study area.   This 
has been a period of intense cold with daily temperatures ranging from a 
balmy -20C at Orca-2 to -29 on Emperor Station. It has also been a time 
of dwindling daylight, culminating in a dreary overcast day at Minke-2 on 
27 April, with just 1 hr, 57 min. of sunshine. This was the last day of 
sunlight along the AEZOPS line until August. Biogeochemical processes 
during this summer to winter transition continue to surprise and 
fascinate. 

Repeat occupations of Stations Orca and Minke over 8-9 days allowed us to 
investigate trends associated with the onset of winter. The mixed layer 
at Orca, initially ca. 50-60 m (delta 0.01 sigma-theta criterion), 
deepened to 120 m by 21 April. NO3 increased slightly in the mixed layer 
at Orca, as expected from the mixing, but was still substantially below 
the deeper values (24-25 vs 29-30 uM). Ammonium stayed roughly constant 
at about 1.0 uM at this station. Total inorganic carbon (DIC) decreased 
from 2203 to 2195 uM between our two occupations at Orca, suggesting 
continued biological influence on the carbon system at this site. At 
Station Minke, we were all struck by the deep mixed layer and high NH4 
seen on our first visit (190 m, 2.5 uM). At Minke-2 just completed, with 
data hot off the analyzers, the mixed layer increased to 217 m, and NH4 
stayed high at 2.3 uM. DIC increased from 2112 to 2119 uM, consistent 
with ML deepening and a slight increase in NO3 here. These data come from 
CTD jocks Mark Cook and Billy Sweet; Team Hydro --  Dennis Guffy, Calvin 
Mordy, Ron Patrick, Howard Rutherford and Jim Schmitt; and PI Frank 
Millero's Carbon Force of Chris Edwards, Jamie Goen, Davey Purkerson and 
Mary Roche. 

Primary production declined with time within and between stations. The 
C14 Swat Team supplied by PI's Dick Barber, John Marra and Walker Smith 
(Zanna Chase, Kristi Hanson, Mike Hiscock and Beth Jordan) completed 
successful in situ production deployments in the ice  at all stations 
except The Big O, where active pressure ridging and high winds 
discouraged that day's attempt. Primary productivity sampling has been 
slowed down by a forced furlough of the TM Rosette system, following a 
burst hydraulic line, which sprayed oil around the deployment area (in 35 
knot winds at -29C). Fortunately the rosette was inside and is still 
clean, but we suspended use to keep it that way. The PP groups (C14, N15 
and O18) are now doubling up on the CTD rosette system. 

Mary-Lynn Dickson (working with co-PI Mike Bender, URI), saw the 
transition from net oxygen production (net autotrophy) to net respiration 
(net heterotrophy) in the water column at station Orca over the past 2 
weeks. Water column oxygen metabolism changed from +23 to -64 mmol 
O2/m2/d at Orca. This latter value represents a respiration rate 
equivalent to -58 mmol C/m2/d. It will be interesting to identify the 
carbon sources supporting this respiration. At station Minke 2 there was  
net O2 production throughout the euphotic zone but the rate was reduced 
from +61 to +32 mmol O2/m2/d. Station Emperor had a net production of +59 
mmol O2/m2/d (42 mmol C/m2/d), similar to production rates measured at 
stations Sei 1 and Minke 1. The lowest net production found so far on the 
cruise was measured  at The Big O on the SPOZEA  line. There, oxygen 
production was 10 mmol/m2/d or 7.5 mmol C/m2/d. 

Carbon-14 is an optimistic tracer which always gives positive numbers. 
C14-ers Zanna Chase (LDEO) and Mike Hiscock (Duke) found that water 
column in situ primary production rates at Orca declined from 1.7 to 0.5 
mmol C/m2/d and were below 1.0 at all recent stations. The maximum 
productivity is at the surface.  C uptake drops off rapidly with depth, 
and below about 30m there is no difference between light and dark 
bottles.  Production per unit biomass averaged 2 mgC/mg Chl_a/d, about 
the same or a little higher than during Process-2. Hiscock's P vs E 
experiments continue to show dramatic evidence of adaptation to low light 
levels. More on Mike's work next week. The phytoplankton are dying by 
inches (or millimoles) but they're taking advantage of every last photon 
to be had. 

Chlorophyll concentrations determined by Kristi Hanson declined at both 
stations and are now about 0.02 - 0.03 ug/l. The standing stocks stayed 
constant at 5-6 mg Chl/m2, and the declines in concentration were 
probably due to the mixing we're seeing. 

The Thorium pumpers (Dave Hirschberg and Ellen Roosen, WHOI) completed 
the slow process of counting the Orca-1 samples and report a slight 
TH-234 deficit in the upper water column, indicating that particle export 
is active in the region. The export might be driven by high 
mesozooplankton stocks, which MOCNESS-ers Walter Nordhausen and Xuefeng 
Zhong (SIO) are catching throughout the study area. For comparison, there 
was no Th-234 deficit on Process-1, and a larger deficit on P-2.

Van dwellers and microbiologists Dawn Castle (UDEL) and Alison Sanford 
(HPEL) obtained beautifully resolved vertical profiles of very low 
bacterial production rates and estimated glucose and amino acid turnover 
rates averaging over 100 days at all major stations. Although abundance 
is still high, the bacteria seem limited by low food availability rather 
than temperature per se. Interestingly, activity has been consistently 
highest at the bottom of the water column (300 - 800 m) at all stations. 
This finding is interesting because DOC, the presumed bacterial food 
source, is slightly elevated in the upper water column. 

This week's focus is on the microzooplankton group of Mark Dennett and 
Eelin Lim (WHOI), out here working for PI's Dave Caron (WHOI) and Darcy 
Lonsdale (SUNY-SB). Mark and Eelin stand somewhat dazed in the glare of 
the spotlight, because they're tired. They have completed large scale, 
three-day-long dilution experiments at all seven major stations so far, 
in just 15 days. These laborious and time consuming experiments use up to 
87 bottles per sample, all to derive just two quantities at a single 
depth: the rates of phytoplankton growth, and their ingestion by 
microzooplankton. The latter are the zooplankton small enough (mostly < 
200 um), and numerous enough to be captured reliably by Niskin bottles. 
Mark and Eelin report that both herbivory and bacterivory through Station 
Orca-2 have been undetectable -- another sign of winter, and valuable 
data for specifying and modeling the annual cycles. But it's a lot of 
work, just to get -- zero. Undaunted, Mark and Eelin plan a final 
dilution party at Station Sei-2 this Wednesday. 

We are now underway toward our final stop at Station Sei before heading 
north. Several groups will be sampling underway and we'll stop to obtain 
Thorium calibration data and deep DOC concentrations at 3-degree 
intervals as we leave the Antarctic continental shelf. Rhonda Kelly, Rich 
Iszard-Crowley, Campbell Buzz Scott and Jon Alberts deployed the first of 
two ice drifter buoys for the Australians this afternoon under clear 
skies in meter-thick ice at  75 36.55S, 172 53.22E.  A long final sunset  
gave lucky sky watchers an extended view of the Green Flash. It's better 
here than in the tropics. 

Hugh Ducklow
Chief Scientist, NBP 97-3