Vol. 53 - Alaska Resources Library and Information Services

Bongo samples were usually split no further than 1/8, and were never
split to less than 1/16, of the original sample. Such splits place the
lower level of detection in sub-samples at about 8-16 animals of each
taxon per 200-300 m 3 average tow, assuming perfectly uniform distribution
of the larvae in the splitting process.
Early in the season, the
zooplankton community is not well developed, so splits were usually not
necessary.
Under these conditions the entire sample is examined and the
probable lower level of detection for reporting becomes about 1 in 250 m³
(assuming an average tow), or roughly 5 per 1000 m³ or 30 per 100 m².
MOCNESS nets filtered about three times more water per meter of
depth than the Bongo tows (200-300 m 3 per 20 m depth interval for
MOCNESS compared with 200-300 m 3 per 60 m for Bongo nets).
However, our
share of PROBES MOCNESS samples was 20% of the original (the splits were
done on board ship), so we always examined plankton from a smaller volume
of water with MOCNESS samples than Bongo samples.
Theoretically, MOCNESS
samples should be less capable of detecting low larval abundance than
Bongo samples.
In addition, the finer mesh of the MOCNESS nets usually
necessitated more sub-sampling, but not early in the season (April and
early May) at early stages in the development of the zooplankton
community.
Since MOCNESS nets had a finer mesh, they were more prone to net
clogging than the larger mesh Bongo nets.
Clogging may be caused by
phytoplankton as well as zooplankton, and reduces the actual amount of
seawater filtered. The analytical problem created by this condition is
compounded by the location of the flow-metering device.
Unlike the
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