Biological consequences of current-topography interactions at Cobb Seamount
Date
2018-07-12
Authors
Dower, John F.
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Abstract
Shallow oceanic seamounts have long been known to
support rich nektonic stocks. However, the mechanism by
which this occurs has never been satisfactorily explained.
This thesis examines the role of current-topography
interactions in the planktonic community at Cobb Seamount, a
shallow seamount 500km west of Vancouver Island.
Current-topography interactions at seamounts give rise
to a variety of flow phenomena, the two most important being
(i) the formation of closed clockwise vortices, known as
Taylor cones, and (ii) isopycnal doming of the density field
near the topography. Since the 1950's the classical
explanation for the high biological productivity of shallow
seamounts has been based on the notion that (i) nutrientrich
water upwells over seamounts, promoting enhanced
primary production and that (ii) a Taylor cone then traps
and concentrates this primary production over the seamount.
This classical explanation further suggests that energy is
transmitted from the phytoplankton to zooplankton stocks,
and leads to an accumulation of zooplankton near the
seamount, which acts as the food source to support seamount
fish. This thesis challenges the validity of this
mechanism, based on extensive physical and biological
sampling carried out during three cruises to Cobb Seamount
in the summers of 1990, '91 and '92: isopycnal doming and
Taylor cone recirculations both occur at Cobb, but the
Taylor cone does not penetrate close enough to the surface
to trap plankton.
Nevertheless, phytoplankton stocks are consistently
high near Cobb, with local chlorophyll concentrations at
least twice as high as background levels. These regions of
high chlorophyll concentration map closely with areas where
isopycnal surfaces dome upward by as much as 30m. These
data provide the first evidence that high phytoplankton
stocks may be permanent features near shallow seamounts.
Spatial patterns in the mesozooplankton community
composition are examined using the Percent Similarity Index.
Based on simple straight-line separation between samples,
community composition around Cobb changes only slightly over
distances of up to 150km. When samples are compared on the
basis of relative distance to the seamount, however, it is
seen that proximity to the seamount is a better predictor of
community variability. Between-sample resemblance is found
to be lower among samples within 30km of Cobb. This pattern
may be caused by (i) predation by seamount fish or (ii)
behavioural responses causing the zooplankton to avoid the
seamount.
A simple four-compartment ecosystem model is used to
address the questions of (i) how a persistent high
phytoplankton stock can be maintained over a seamount in the
absence of a trapping mechanism, and (ii) whether seamount
fish stocks rely on autochthonous energy sources. The model
shows that persistent high phytoplankton stocks are caused
primarily by the improved light conditions experienced by
the phytoplankton as they dome over the seamount. Depending
on the degree of nutrient limitation, the addition of
nutrients to the near-surface waters via doming may also be
important. The model demonstrates that phytoplankton stocks
show almost no response to predation on zooplankton by
seamount fish.
Together, the field data and the ecosystem model show
that while current-topography interactions do contribute to
the maintenance of high-biomass communities at shallow
seamounts, the classical bottom-up enrichment/retention
mechanism does not apply to Cobb. Additionally, this work
suggests that seamount fish stocks rely on allochthonous
energy sources. Rather than a long chain that begins with
phytoplankton and ends with rich nektonic stocks, the
"seamount effect" near Cobb is the result of a wide
assortment of physical-biological interactions. Different
organisms operating at several levels of the food web "feel"
the influence of the seamount in different ways that are
only loosely and occasionally coupled.
Description
Keywords
Nekton, British Columbia, Marine biology, Seamounts, Submarine topography, Cobb Seamount