Dataset: ox18
Deployment: AII-119-4

   PI:              Michael Bender and J. Kiddon
   of:              University of Rhode Island
   dataset:         Gross and Integrated Gross Oxygen Productivity
   dates:           April 29, 1989 to May 7, 1989
   location:        N: 46.8  S: 46.4  W: -20.2  E: -19 
   project/cruise:  North Atlantic Bloom Experiment/Atlantis II 119, leg 4
   ship:            Atlantis II
 
   sta=station number, each day cycles a new station number
   gross_integ_prod=Integrated O-18 Gross O2 Production,
         units=mmoles o2/m2/day
   depth=sample depth, units=meters
   gross_o2=O-18 Gross O2 Production, units=umoles O2/L/14hr
 

Methodology: J. Kiddon, M.L. Bender; URI O-18 Gross O2 Production

units: umoles O2/L/14 hr

North Atlantic Bloom Experiment

Atlantis II Cruise 119 Leg 4

Seawater samples were spiked with H2O enriched in O-18, such that the
18/16 oxygen mass ratio in the sample water was about twice the natural
level. Gross production during a 14 hour bottle incubation generates O2
with the same enriched ratio, thereby enhancing the 18/16 ratio in the large
original dissolved O2 pool. The tagged O2 mixes with the large O2 pool before
being respired, thereby minimizing changes in the isotopic composition of
the measured O2 pool associated with respiration. Thus, the O-18 enrichment
of the dissolved O2 is taken to be a proportional measure of gross production.



Water samples were drawn before dawn, spiked with 0.2 ml of H2O(18) and
incubated in 100 ml quartz bottles for 14 daylight hrs at the depth of collection.
A drifting buoy was used for sample deployment. The incubated samples, as
well as unincubated samples from each depth were processed to strip and
collect dissolved gases. This stripping process was accomplished by introducing
approximately 50 ml. of seawater into an evacuated two chamber container;
the degassed water remained in a lower chamber and the stripped gases rose
to an upper glass ampoule. The ampoule was flame sealed and returned to
the lab where the 18/16 ratio of the dissolved O2 was measured with an isotope
ratio mass spectrometer as the per mil difference relative to a laboratory
standard (referred to as 'del' measurements).



The gross O2 production, denoted [O2]p, was calculated as: [O2]p = {[(del)f
- (del)i]/[(del)p - (del)f]}*[O2]i (eqn 1) The parameters (del)i and (del)f
are the 'del' values of the dissolved O2 measured respectively before and
after incubation. (del)p is the isotopic composition of the O2 produced
during the incubation, calculated knowing the volumes (V) of the H2O(18)
spike and the sample, the mole fraction of O-18 in the spike (0.980) and
the mole fraction of O-18 in natural seawater (0.002). That is, (del)p =
{[(X)incub/(X)ref] - 1}*1000, where (X)incub is the mole fraction of O-18
in the spiked sample water, itself calculated as: (Vspike/Vbottle)*0.978
+ 0.002; and (X)ref is the separately determined O-18 mole fraction in the
laboratory standard. [O2]i in equation 1 is the initial O2 concentration,
determined via Winkler titration by the Oceanographic Data facility. Equation
1 may be derived from a more intuitive equation which expresses the final
isotopic composition (del)f as a weighted average of the isotopic compositions
of the initial O2 and the O2 added during production: (del)f = {[O2]i*(del)i
+ [O2]p*(del)p}/{[O2]i + [O2]p}.

Integrated gross O2 production

units: mmoles O2/m2/day

Leg 4



Integrated values of productivity were calculated using the histogram
method. The euphotic region (surface to the 1% light level, Knudson et al.)
was divided into intervals of uniform productivity associated with O-18
gross production measurements. The summation interval was defined as the
depth interval bounded by the two mid points of three adjacent sampling
depths. For example, if depths 4, 12, 20 and 30 meters were sampled, the
productivity measured at 20m was taken to represent the interval 16 to 25
meters. The productivity of the shallowest sample represents the interval
from the surface to the mid point with the next deepest sample, i.e., from
0 to 8 meters. The deepest sample represents the interval from the last
mid point to the 1% light level.

Net O2 Production

units: umoles O2/L/24 hr

Leg 4

Net O2 production was determined as the difference in the measured dissolved
O2 concentrations of sea water, measured before and after a 24 hour light/dark
incubation by Winkler titrations. High precision in the Winkler determinations,
+/- 0.1% umole O2/L, was achieved both by using an automated titrator (Radiometer,
model ABU93) and by averaging four replicate measurements for each water
sample.



Eight replicate water samples were drawn into quartz bottles from a Go-Flo
flask containing water collected from the euphotic region before dawn (sample
volumes about 100 ml, known to 0.01ml). Winkler titrations were performed
on four of the replicates immediately, and the results averaged to establish
the initial O2 concentration. The remaining four samples were incubated
for 14 daylight hours at the depth of collection (attached to a drifting
buoy), and further incubated for 10 night hours in a darkened, ship board
incubator, maintained at constant temperature by flowing surface water.
Winkler titrations were then performed on the incubated samples and the
results averaged. Net production was calculated as the difference between
the final and initial O2 concentrations.

Integrated net O2 production

units: mmoles O2/m2/day

Leg 4

Integrated values of net O2 productivity were calculated using the histogram
method. The summation intervals were defined as described above for the
integrated gross production, e.g., with boundaries set midway between sampled
depths. Productivity was integrated over the euphotic region, to the 1%
light level (Knudson et al.). In cases where data was sparse (stations 19
and 20 and, in general, for depths near the 1% light level), the net productivity
was augmented using computed values of respiration rates and extrapolated
gross O2 productivities: net prod = gross prod - 24 * resp rate.

Reference:

Knudson, C. W.S. Chamberlin and J. Marra (1989)

Primary production and irradiance data for U.S. JGOFS (Leg 4) Atlantis
II (Cruise 112.4) Technical report LDGO-89.4. Lamont-Doherty Geological
Observatory, Palisades, N.Y.