Mesozooplankton Biomass Documentation: JGOFS Arabian Sea
PIs: Karen Wishner (URI), Marcia Gowing (UCSC)
Data Base Organizer and Chief Technician: Celia Gelfman
Assistants: Chelsea Donovan, Heidi Franklin, Maureen Koneval, Lauren
Lankau, John Maccario, Jennifer Saltzman, Dorothee Schreiber, Cynthia
Venn, Kathleen Vignes.
Samples were collected with a double 1 m2 MOCNESS (two 1 m2
MOCNESS systems side-by-side), a multiple opening-closing net
system with environmental sensors and control of the nets from
shipboard (Wiebe et al. 1976, 1985). The nets were 153 micron
mesh with a 1 m2 mouth opening (when towed at a 45 degree
angle). The net was towed usually at 1.5 - 2.5 kts behind the
ship. Wire was payed out at 10 - 25 m/min and hauled in at 5 -
20 m/min. Environmental data from the MOCNESS included time,
depth, temperature (Sea-Bird SBE 3), salinity (Sea-Bird SBE 4),
light transmission (SeaTech 25 cm beam transmissometer), and
oxygen (Sea-Bird SBE 13). The volume filtered through each net
was determined by the MOCNESS program, taking into account flow
past the system (measured with a modified TSK flowmeter mounted
on the net frame) and the towing angle. In some cases, the
volume filtered per time from trouble-free portions of a tow
was extrapolated to time intervals with electronic problems.
Usually 16 discrete samples were collected in an oblique haul
from about 1000 m to the surface. Typically a day and a night
tow were taken at each of the six long stations during four
seasonally-spaced process cruises (TN043, TN045, TN050,
TN054). Occasional additional tows and samples to deeper
depths were also done. During the monsoon cruise (TN050),
there were often problems with knotted or ripped nets because
of the rough seas; data from this cruise should be treated with
caution. Tow times and geographic locations can be found in the
cruise event log.
Cod ends were placed on ice immediately after retrieval. Nets
were hosed down with filtered (nominally 2 micron) seawater.
For most day tows, the entire sample was preserved in 4%
borate-buffered formaldehyde. For most night tows, the samples
were split in a NMFS-style (flat-bottomed) plankton splitter.
Half the sample was preserved as above for displacement
volumes, wet weights, and distributions; one quarter (or the
entire remaining half for very small samples and for surface
samples) was set aside in a refrigerator for dry weight and CHN
subsampling several hours later; and one quarter (when
available) was preserved in paraformaldehyde for electron
microscopy of zooplankton gut contents.
For dry weights, the sample was size-fractionated through a
sieve series (2000, 1000, 505, 202 micron) and each size
fraction was resuspended in a known volume of filtered
seawater. Aliquots were taken with a Stempel piston pipette,
filtered onto combusted pre-weighed GF/D 25 mm glass fiber
filters, rinsed with distilled water, loosely wrapped in
aluminum foil, placed in an oven (60 degrees C) for several
days, and stored in a desiccator for shipment home. Two
replicates were done for each size fraction. Four blanks
(aliquots of filtered seawater treated as above) were done for
each tow. In the lab, filters were dried in an oven (60
degrees C) for 8 - 10 hr, weighed for dry weight on a Cahn
microbalance, ashed in a muffle furnace (500 degrees C) for 4
hr, and weighed for ash. For dry weights, the pre-cruise filter
weight and the mean dry weight blank for the cruise (a single
value for each cruise) were subtracted from the measured
post-cruise dry weights. The ash-free dry weight was calculated
as the post-cruise dry weight minus the post-cruise ash weight
and the cruise mean ash weight blank. For carbon values, a
conversion factor from the literature was used [log (dry
weight) = 0.499 + 0.991 (log carbon)] (Wiebe et al. 1975, Wiebe
1988). The mean of the 2 replicate filters from each sample
was used. In cases where no measurements were made and in cases
where the weight of a filter with sample was less than the
blank and pre-cruise filter weight, the data were recorded as
ND (no data). Carbon values from CHN measurements were also
obtained by Roman from some of these same samples in the upper
200 m on cruises TN043, TN045, and TN050.
Displacement volumes and wet weights were measured several
months after the cruise on preserved samples. Either the
entire sample or a half split was used. Large singular
organisms (large fish or jellyfish) were removed beforehand.
Displacement volumes were done with no size fractionation. Wet
weights were done on the large (> 2 mm) and small (< 2 mm) size
fractions after separation by sieving.
Bongo tows were taken at many of the hydro and intermediate
stations, as well as the long stations. Wishner's bongo frame
(used on TN043, TN045, and the beginning of TN050) had mouth
openings of 61 cm, used one net with 153 micron mesh and one
with 335 micron mesh, and had a General Oceanics counter
flowmeter in each mouth opening. A Wildlife Systems electronic
time-depth recorder was attached to the frame. Oblique tows
from the starboard side were done to about 200 m depth at about
1.5 - 2 kts. Wire was payed out at 30 m/min and hauled in at
20 m/min. Half of each sample was preserved for displacement
volume and distributional measurements, while the other half
was used for dry weight, ash-free dry weight, and carbon
analyses (and CHN analyses by Roman on the 153 micron mesh net
sample) as described above.
In the Zooplankton Biomass--MOCNESS and Bongo files, data are
presented as mmoles of carbon for each size fraction and for
the sum of all size fractions, totaled within the indicated
depth range (per m2). For some MOCNESS tows, depth intervals
with missing data were represented as the mean of the biomass
in the samples immediately above and below in order to
calculate a value for the total depth range.
References
Wiebe, P.H. 1988. Functional regression equations for zooplankton
displacement volume, wet weight, dry weight, and carbon: a
correction. Fish. Bull. U.S. 86:833-835.
Wiebe, P.H., S.H. Boyd, and J.L. Cox. 1975. Relationships between
zooplankton displacement volume, wet weight, dry weight, and carbon.
Fish. Bull. U.S. 73:777-786.
Wiebe, P.H., K.H. Burt, S.H. Boyd, and A.W. Morton. 1976. A multiple
opening-closing net and environmental sensing system for sampling
zooplankton. J. Mar. Res. 34:312-326.
Wiebe, P.H., A.W. Morton, A.M. Bradley, J.E. Craddock, T.J. Cowles,
V.A. Barber, R.H. Backus, and G.R. Flierl. 1985. New developments in
the MOCNESS, an apparatus for sampling zooplankton and micronekton.
Mar. Biol. 87:313-323.