Physical processes and zooplankton distribution in the Great South Channel : observational and numerical studies
Citable URI
https://hdl.handle.net/1912/5624Location
Great South ChannelGulf of Maine
DOI
10.1575/1912/5624Keyword
Marine zooplankton; Marine phytoplankton; Copepoda; Food chains; Marlin (Ship) Cruise; Endeavor (Ship: 1976-) Cruise EN237Abstract
This thesis addresses the question, "How do small-scale physics and biology
combine to produce dense aggregations of certain species of zooplankton in the Great
South Channel (GSC) of the Gulf of Maine?" The thesis consists of three relatively
independent parts: an observational study made while following two right whales as
they fed on dense patches of the copepod Galanus finmarchicus in the northern GSC;
a detailed description of a tightly integrated set of biological and physical observations
made in the GSC by means of a new instrument, the Video Plankton Recorder (VPR);
and a two-dimensional Eulerian numerical model that simulates one way in which a
physical flow field, combined with a biological behavior pattern, may produce dense
plankton patches at a convergent front.
Part I: Data from a wide variety of instruments was combined to produce a coherent
picture of the physical and biological environment near two feeding right whales
observed in June, 1989. Instruments included a CTD (with transmissometer), a
MOCNESS net system, a 150-kHz ADCP, and a towed acoustic plankton profiler
operating at 120 and 200 kHz. Acoustic data were intercalibrated with net-tow
data and with "noise" in the transmissometer signal in order to estimate copepod
abundance in the plankton patches on which the whales were feeding. One of the
whales was observed to reverse course when copepod abundance dropped below about
1.5- 4.5 x 103 copepods/m3
, which is consistent with independent estimates of the
density of copepods necessary for a right whale to gain more energy from the prey it
ingests than it loses to the extra hydrodynamic drag it experiences while feeding.
Part II: The VPR is a towed underwater microscope designed to image plankton
non-invasively with sufficient resolution to obtain information on the spatial distribut
ion of organisms on scales ranging from millimeters to hundreds of kilometers. CTD
instrumentation mounted on the VPR makes it possible to correlate biological and
hydrographic data with great precision. This study reports data from one transect made across the GSC in May, 1992. The data show close correlations between hydrographic
features (such as fronts, plumes and water masses) and broad-scale plankton
distribution. In addition, it was possible to correlate the fine-scale (order tens of meters)
patchiness in plankton distribution with the local stability of the water column
(as indicated by gradient Richardson number). In one case, biological data provided
an aid in determining the origin of one of the observed water masses.
Part III: This chapter presents a two-dimensional Eulerian numerical model that
shows how depth-keeping swimming behavior on the part of an organism, combined
with a convergent flow field at a surface front , can create dense patches of the organism.
In this model a steady-state flow field and vertical diffusivity field are prescribed,
along with the initial distribution of the plankton. The plankton swim vertically with
speeds that depend only on depth, but the form of that depth-dependence may take
into account such factors as the vertical variation in light level or in the concentration
of some prey organism. An analysis of various nondimensional parameters associated
with the model illustrates the roles played in determining the final structure of the
patch by such factors as diffusion, water velocity and details of the animals' swimming
behavior. Output from the model is compared with data taken at a dense plankton
patch observed near a front in the northern Great South Channel in early June, 1989.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1995
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Suggested Citation
Thesis: Epstein, Ari W., "Physical processes and zooplankton distribution in the Great South Channel : observational and numerical studies", 1995-06, DOI:10.1575/1912/5624, https://hdl.handle.net/1912/5624Related items
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