Publication Date:
2015-09-01
Description:
Planktonic foraminifera were collected from an oceanic
front off Baja California, Mexico, during April and
May, 1965, in connection with studies of the physical
oceanography of the front. Four major water masses
were present: Southern Surface Water at approximately
0-50 m, Northern Surface Water, forming a submerged
intermediate layer between about 150-50 m, Southern
Deep Water below 150 m, and Northern Deep Water
deeper than 250 m. Planktonic organisms smaller than
2 mm were concentrated in the surface waters, suggesting
that food for foraminifera was most abundant there.
Organisms larger than 2 mm, considered a measure of
potential predators of foraminifera, were abundant in
both surface and intermediate waters.
Most foraminiferal concentrations were from 1 to 100
specimens per m :J, with the largest concentrations in
Southern Surface Water above the front and in deep
water along the front. Lowest concentrations were in
intermediate water, except in the frontal mixing zone,
and at depths below 450 m. Empty shell concentrations
were about one-tenth of associated living concentrations.
Possible errors of concentration estimates were assessed
by comparing paired net and paired tow results.
Seventy percent of these estimates appear to be precise
within a factor of 1.3. The error introduced by patchiness
probably is much larger.
Four foraminiferal assemblages are recognized: ( 1 )
Southern Surface Water assemblage, (2) widespread
species with southern affinity which apparently tolerate
the intermediate water, ( 3) species brought in with the
submerged northern water, and ( 4) the assemblage inhabiting
the deep waters.
The estimated average minimum flux of empty shells
was approximately 6% of the living standing crop I day
by volume. The relative empty shell output was greater
than this for many intermediate water species, and less
for species restricted to southern and to deep water. The
intermediate layer contributed approximately one-half
of the empty shell flux, where specimens with small terminal
chambers (kummerforms) were abundant. The
sediment produced in the front contained about 50%
kummerforms, but the total standing crop of living foraminifera
contained only about 10%.
The tongue of advected intermediate water may have
represented an unfavorable habitat for foraminifera,
where northern species were submerged and possibly deprived
of food or otherwise impeded in their normal
growth. Southern species also may have been displaced
from their normal habitat by mixing processes. These
displacements are suggested as one cause for the formation
of small terminal chambers in specimens inhabiting
intermediate depths. Empty shells apparently arise
through reproduction, stress from displacement, and predation,
with predation being the least important mechanism.
Type:
Article
,
PeerReviewed
Format:
text
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