The Interaction of Two Coastal Plumes and its Effect on the Transport
of Alexandrium Fundyense
by
Christie L. Wood
B.S. Mathematics
Massachusetts Institute of Technology, 2005
B.S. Earth, Atmospheric and Planetary Sciences
Massachusetts Institute of Technology, 2005
Submitted in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
and the
WOODS HOLE OCEANOGRAPHIC INSTITUTION
September 2007
C Christie L. Wood. All rights reserved.
The author hereby grants to MIT permission to reproduce and to distribute publicly paper
and electronic copies of this thesis document in whole or in part in any medium now
known or hereafter created.
Signature of Author:
Certified by
,
Joint Program in Physical Oceanography
Massachusetts Institute of Technology
Woods Hole Oceanographic Institution
September 2007
C
Glenn R. Flierl
Professor of Oceanography
Thesis Supervisor
Accepted by
4
At affaele Ferrari
OF TEOHNOLOGYV
OCT 222007
LIBRARIES
MASSACgraphy
Chairman, Joint Committee for Physical Oceanographv
Massachusetts Institute of Technology
Woods Hole Oceanographic Institution
ARCHVES
The Interaction of Two Coastal Plumes and its Effect on the Transport
of Alexandrium fundyense
by
Christie L. Wood
Submitted to the Massachusetts Institute of Technology and the Woods Hole
Oceanographic Institution in partial fulfillment of the requirements for the degree of
Master of Science
Abstract
Harmful algal blooms (HABs) of A.fundyense, more commonly known as "red
tides", are a serious economic and public health concern in the Gulf of Maine. Until
recently, there was very little known about the mechanisms regulating the observed
spatial and temporal distributions of A.fundyense in this region. In the beginning of this
work a review of previous research on A.fundyense and the mechanisms controlling their
spatial and temporal distributions in the Gulf of Maine is presented. One of the major
conclusions that can be drawn from previous work is that a thorough understanding of the
interactions between river plumes is essential to our understanding of this problem. The
rest of this thesis intends to contribute to the understanding of these plume interactions
and their effect on the transport of A.fundyense. Mixing between two interacting river
plumes with various buoyancies is investigated through laboratory experiments. These
experiments indicate that under these idealized conditions, there was little mixing
between the plumes after their initial interaction. A numerical model is used to explore
the effects of river mouth size and flux variations on the interaction between two plumes.
It is shown that based on river mouth geometry and flow rates the effect of the southern
plume on the path of the northern plume can be predicted. In the final section a simple
NP model is coupled with the physical model to explore the possible effects of river
plume interaction on the distribution of A. fundyense. Based on our modeled results, it
appears as if the southern river under certain conditions could temporarily act as a shield
preventing A.fundyense from reaching the coast but that this was not a permanent state.
Thesis Supervisor: Glenn R. Flierl
Title: Professor of Oceanography, Massachusetts Institute of Technology
Acknowledgements
I am very grateful to have had the opportunity to study physical oceanography in
the MIT/WHOI Joint Program. I would first like to thank my thesis advisor, Glenn Flierl,
for his guidance and for allowing me plenty of research freedom. Because of him my
research experience in the joint program a positive one. I would like to thank Dennis
McGillicuddy for introducing me to the Red Tide issue in the Gulf of Maine, for allowing
me to join him on his research cruise OC425 and for all of his additional advice.
There are several people who made the experimental section of this work
possible. First and foremost I would like to thank Claudia Cenedese for advising me on
this section of my thesis and giving me access to the geophysical fluid dynamics lab at
WHOI. I would like to also thank Valentina for her throrough preliminary studies which
served as a guide for my study. I would also like to thank Keith Bradley who was always
willing to help me in the lab and whose unwavering pleasant demeanor made me look
forward to work every day.
I would also like to thank several people who I worked with on research not
included in my thesis but whose time and effort significantly contributed to my
education. Jim Lerczak introduced me to the Regional ocean Modelling System (ROMS)
and to estuarine studies. I spent a summer working with Amy Bower on eddies in the Red
Sea. Larry Pratt introduced me to the fascinating world of hydraulics. I would like to
thank him for his guidance on my class project and for being my advisor during my
summer student fellowship. I would also like to thank Carl Wunsch for having the
seminar on ocean observation and for all of his help and advice when I was in tough
situations.
There were many other people in the Joint Program who I would like to thank. I
would especially like to thank the following staff: Mary Ellif, Ronni Schwartz, Carol
Sprague, Marsha Gomes, Julia Westwater and Laishona Vitalli. I would also like to thank
my wonderful classmates in both the joint program and in paoc: Peter Sugimura, Evgeny
Logvinov, Jinbo Wang, Andrew Barton, Martha Buckley, Eunjee Lee and Scott Stransky.
I special thanks to Evgeny for getting me through the more difficult times and to Jinbo
for being the best officemate any one could ask for. My friends in the physical
oceanography department for all of their advise and support: Stephanie Waterman,
Tatiana Rykova, Katie Silverthorne and Jessica Benthuysen. A special thanks to my three
best friends in the joint program, Whitney Krey, Christine Mingione and Colleen Petrik,
who were always available for a drink at the Kidd and other stress relieving activities. I
would also like to thank my other friends in the joint program: Kevin Cockrell, Kate
D'Epagnier, Caleb Mills, David Stuebe and Matt Jackson.
I would also like to thank several of my family members for all of their
encouragement and support. A special thanks to my mother and father for their
continuous support and encouragement. I'd like to thank my grandfather (known to some
as the 'Old Fisherman') who was always eager to discuss my research and share his
knowledge of the ocean with me. I would also like to thank my roommate Karen Keller
and my kitten Nami who always seem to be at home ready to cheer me up.
Contents
1
Introduction
9
2
Background
13
3
2.1
Life Cycle ofAlexandriumfundyense ...............................
2.2
Alexandriumfundyense Cyst Distribution ...............
2.3
The Gulf of Maine Coastal Current .......................................
17
2.4
The "Plume Advection Hypothesis" ................................
19
2.5
The "Cyst Source" Hypothesis ....................................
22
2.6
Possible Mechanisms for Entrainment .............................
24
2.7
Summary .......................
....
.....................
13
..........
.
15
....... 27
Laboratory Experiments
29
3.1
Apparatus..........................................................
30
3.2
Procedure ....................................................
32
3.3
Results ..............
. ....
........
................
........
3.3.1
Summary of Experiments ....................................
3.3.2
Vertical Plume Structure ..
33
33
..................................... 35
3.3.3
3.4
37
Mixing ...............................................
42
Summary and Discussion .......................................
43
4. Physical Numerical Model
4.1
Model Formulation .........................
.................. 44
4.2
Base Case ................................
.................. 46
4.3
Variations in River Mouth Geometry ...........
.................. 52
4.4
Variations in River Fluxes ....................
.................. 58
4.5
Summary and Discussion .................
.. ................... 66
65
5. Coupled Biological Physical Model
5.1
Biological Model Formulation .................................
65
5.2
Coupled Biological-Physical Model ..............................
68
5.4
5.2.1
Model Setup ...............................
5.2.2
Results .................
............................
Summary and Discussion .......................................
............
68
69
78
Chapter 1
Introduction
Harmful algal blooms (HABs), more commonly known as "red tides", are a
serious economic and public health concern. In the Gulf of Maine the most serious
problem associated with HABs is paralytic shellfish poisoning (PSP). PSP, a potentially
fatal neurological disorder, is caused by human ingestion of shellfish (e.g., mussels,
clams, oysters and scallops) that have consumed the toxic dinoflagellate Alexandrium
fundyense. The onset of symptoms is rapid and, in the most severe cases, PSP results in
respiratory arrest within 24 hours of consumption of the toxic shellfish. There is no
known antidote for PSP, thus making blooms of A. fundyense a major threat to public
health. The Journal of Shellfish Research tried to emphasize the seriousness of the red
tide problem to their readers through a somewhat comical cartoon on one of their covers
(fig. 1-1). Here they show a bloom of A. fundyense as an ominous dark mass staring at the
shellfish who, out of fear of becoming toxic, have sought refuge on the shore. In response
to the threat of red tide, regional monitoring programs have been established to sample
coastal shellfish beds regularly and test for PSP toxins. It has been demonstrated that
toxicity in the mussel Mytilus edulis is a good indicator of the presence of A. fundyense
cells (Shumway et al., 1988). Discovery of PSP toxins leads to shellfish bed closures,
causing a serious economic hardship to the coastal fishing industries.
Figure 1-1. A cartoon from the cover of Journal of Shellfish Research Vol. 7, No.4
Blooms of the toxic dinoflagellate A. fundyense are a recurring feature in the Gulf
of Maine. Following an outbreak in Canada in 1957, five monitoring stations were
established in the coastal Maine area and were expanded to 119 sampling stations after a
year of high toxicity in 1974 (Shumway et al., 1988). Blooms of A. fundyense have
occurred every year since 1958 along the southern coast of Maine between the months of
May and October. Blooms began appearing along the northern coast of Massachusetts in
1972 and have occurred every year since, excluding 1987 (Shumway et al., 1988).
Anderson (1997) reviewed studies of the bloom dynamics with focus on various
subregions in the GOM. Observations show that peak toxicity in the western GOM
occurs early in the bloom season (May-June) and is less severe than peak toxicity in the
eastern GOM, which occurs later in the bloom season (July-August). It has also been
noted that the Penobscot Bay region, which separates the eastern and western GOM, and
is sometimes referred to as the "PSP sandwich" (Shumway et al., 1988), is usually devoid
of PSP.
Until recently, there was very little known about the mechanisms regulating the
observed spatial and temporal distributions of A. fundyense in the Gulf of Maine. There
were several studies that focused on sub-regions of the Gulf of Maine (e.g. Anderson and
Keafer, 1985; Franks and Anderson, 1992a; Franks and Anderson, 1992b; Martin and
White, 1988; White and Lewis, 1982). The 1997-2001 Ecology and Oceanography of
Harmful Algal Blooms-Gulf of Maine (ECOHAB-GOM) project greatly added to the
understanding of the interconnections between the regional bloom dynamics of A.
fundyense in the Gulf of Maine. In the second chapter of this thesis I present a review of
previous research on A. fundyense and the mechanisms controlling their spatial and
temporal distributions in the Gulf of Maine. Much of the research discussed in this
section was part of the ECOHAB-GOM project. One of the major conclusions that can be
drawn from previous work is that a thorough understanding of the dynamics of river
plumes and interactions of river plumes is essential to our understanding of this problem.
The rest of this thesis intends to contribute to the understanding of these plume
interactions. In Chapter 3, mixing between two interacting river plumes with various
buoyancies is investigated through laboratory experiments. In chapter 4 a numerical
model is used to explore the effects of river mouth size and flux variations on the
interaction between two plumes. In both Chapter 3 and 4 the possible effects on the
transport of A. fundyense are discussed and in Chapter 5 a simple NP model is coupled
with the physical model to explore more explicitly the possible effects of river plume
interaction on the distribution of A. fundyense.
Chapter 2
Background
2.1 Life Cycle of Alexandriumfundyense
Anderson (1998) describes the life cycle of Alexandrium species (figure 2-1). The
life cycle involves both sexual and asexual reproduction. During asexual reproduction,
division by binary fission yields vegetative motile cells, which contribute to the
development of blooms. Sexual reproduction begins with the formation of gametes,
which fuse to form swimming zygotes, which then become dormant resting cysts. The
species Alexandrium fundyense also has another resting stage called a "temporary cyst",
which is a result of a sudden shift to unfavorable conditions. However, the following
discussion of A. fundyense cysts refers to the dormant resting cysts, which are a
mandatory stage in the life cycle and critical to our understanding of the population
dynamics of this species.
There are several factors that regulate how long a cell will remain as a dormant
resting cyst. Internally, there is a mandatory maturation period (Anderson, 1980) and an
endogenous annual clock (Anderson and Keafer, 1987). In addition, mature cysts will
remain in this resting state (known as "quiescence") while conditions in the overlying
waters are unfavorable for growth. There are several external factors which have been
found to control the germination of mature A. fundyense cysts. If the temperature of the
overlying water is above or below a range that allows germination, the cells will remain
quiescent (Anderson, 1998; Anderson et al., 2005b). Light (Anderson et al., 1987;
Anderson et al., 2005b) and oxygen (Anderson et al., 1987) also affect the rate of
germination. If overlying water conditions remain unfavorable for germination, or if cells
are buried deep in the anoxic sediments, the cysts can remain quiescent for years. How
long cysts can live is difficult to determine; however, Keafer et al. (1992) suggest that the
half life ofAlexandriumfundyense cysts in anoxic sediments is approximately five years.
Once germination occurs, the overlying water column is inoculated with cells and, as
previously mentioned, the cells begin to divide via binary fission creating a bloom.
Clearly, the location of cyst accumulations in the surface sediments (known as
"seedbeds") is important in determining the location of the resulting blooms.
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Figure 2-1. Life cycle diagram for Alexandrium fundyense The labeled stages are: (1)
motile vegetative cell; (2) temporary cyst; (3) "female" and "male" gametes; (4) fusing
gametes; (5) swimming zygote; (6) resting cyst; (7&8) motile germinated cell; (9) pair of
vegetative cells after division by binary fission (Anderson, 1998).
2.2 Alexandrium fundyense Cyst Distribution
Anderson et al. (2005b) produced the first Gulf-wide survey showing the
abundance of cysts in bottom sediments (fig. 2-2). The data used for this map came from
multiple surveys (White and Lewis, 1982; Martin and Wildish, 1994; J. Martin,
unpublished data; Anderson et al., 2005b); however, most of the samples from between
the Bay of Fundy and the western extreme of the sampling domain are from the 1997
ECOHAB-GOM survey. The cyst distribution map clearly shows three distinct zones of
high cyst concentration. One of these seedbeds is located near Grand Manan Island at the
mouth of the Bay of Fundy, where maximum abundance is close to 2000 cysts /cm3
(Anderson et al., 2005b). There is evidence that the Bay of Fundy cyst bed is a persistent
feature (Anderson, 2005b; Martin and Wildish, 1994; J.Martin, unpublished data). This
could be explained by the eddy system that lies above this seedbed and is able to retain a
fraction of the vegetative cells produced by germination from this seedbed, resulting in
high cell concentrations in the overlying waters (Martin and White, 1988). Anderson et
al. (2005b) propose that this area serves as an "incubator" for the region, with many of
these retained cells depositing new cysts at the end of the bloom season to replenish the
underlying seedbeds. In the cyst map by Anderson et al. (2005b) there were also high
concentrations of cysts offshore of both Penobscot and Casco Bays. Anderson et al.
(2005b) observed interannual variability in cyst abundance at these locations, which is
attributed to interannual variations in cyst deposition. The circulation in the Gulf of
Maine suggests possible links between the location and interannual variability of these
seedbeds and the seedbed in the Bay of Fundy.
45
cysts/cm
600
500
44
400
300
-J
43
200
42
100
0
-71
-70
-69
-68
Longitude
-67
-66
-65
Figure 2-2. Distribution and abundance ofAlexandriumfundyense cysts in the
surface sediments (top cm) of the Gulf of Maine(Anderson et al. 2005b).
3
2.3 The Gulf of Maine Coastal Current
The Gulf of Maine (GOM) is a mid-latitude marginal sea bounded by New
England and southeastern Canada. The general circulation of the GOM is cyclonic
(Bigelow, 1927). The major cyclonic gyre in this region is centered on the Jordan Basin
and is sometimes referred to as the Jordan Basin Gyre (Pettigrew et al, 1998). Another
dominant feature of the circulation is a complex coastal current system that flows from
the gulf coast of Nova Scotia to Massachusetts (Brooks, 1985). The Gulf of Maine
Coastal Current (GMCC) is highly variable and is thought to consist of several branches
(Pettigrew et al., 1998).
The two main branches of the GMCC are referred to as the Eastern Maine Coastal
Current (EMCC) and the Western Maine Coastal Current (WMCC). The EMCC extends
from the mouth of the Bay of Fundy along the coast of Maine to Penobscot Bay. A major
portion of the EMCC turns offshore and contributes to the cyclonic circulation of the
basin and another portion continues southwestward to join with the WMCC (Pettigrew et
al., 1998 ). The EMCC could be split into two parts (Keafer et al., 2005b): a low-salinity,
coastally trapped buoyant current originating from upstream low-salinity waters from the
St. John river, and a more offshore branch which originates from Scotian shelf waters.
The WMCC extends from the Penobscot Bay to Cape Cod, Massachusetts. The WMCC
is augmented by river outflow from the Kennebec, Androscoggin, Saco and Merrimack
rivers. This low-salinity water is sometimes referred to as the "plume" (Anderson et al.,
2005a). The WMCC is therefore also made of two distinct water masses: the plume and a
more offshore component. There is a branch point in the WMCC near Cape Ann,
Massachusetts, where some of the current enters Massachusetts Bay and the rest travels
along the eastern edge of Stellwagen Bank. The latter segment undergoes another
bifurcation at Cape Cod with some of the current extending south towards Nantucket and
the other portion which travels to and around Georges Bank . Figure 2-3 shows a
schematic of the circulation in the GOM as described by Keafer et al. (2005b), which
builds upon previous schematics, such as those by Bigelow (1927) and Brooks (1985).
44
43
42
41
-71
-70
-69
-68
-67
-66
Figure 2-3. General near-surface circulation of the Gulf of Maine (Keafer et al., 2005b).
The degree of connection between the two main branches of the Gulf of Maine
Coastal Current appears highly variable (Pettigrew et al., 1998). Pettigrew et al. (2005)
investigated the GMCC from 1988 to 2001 using extensive hydrographic surveys, current
meter moorings, tracked drifters and satellite thermal imagery. The degree to which the
EMCC veered offshore or joined the WMCC during the summer months varied greatly
during the three year study. In 1998, almost all of the EMCC was directed offshore,
whereas there was a nearly continuous throughflow between the EMCC and WMCC in
2000. Keafer et al. (2005b) suggest that part of the inshore branch of the EMCC may
flow directly into the plume of the WMCC. They refer to this continuum of fresh water as
the "inside track" or the Gulf of Maine Coastal Plume (GOMCP). A study by Geyer et al.
(2004) shows that the volume of freshwater transport by the plume in the western Gulf of
Maine exceeds the local riverine inflow of fresh water by 30%, suggesting a significant
contribution from the St. John and further supporting the existence of a GOMCP.
2.4 The "Plume Advection Hypothesis"
The complex and highly variable hydrography of the Gulf of Maine and the
complicated population dynamics of A. fundyense make the study of the bloom dynamics
of this dinoflagellate very challenging. Based on observations of toxicity at five stations
in the western Gulf of Maine (see figure 2-4) over three bloom seasons, Franks and
Anderson (1992a) showed that toxic shellfish outbreaks typically showed a north-tosouth progression and A. fundyense cells were predominately observed in the low-salinity
waters (highest concentrations were located in waters < 31.5 psu). A peak in river
discharge of the Androscoggin and Kennebec rivers prior to the detection of toxicity
suggests that these rivers are the source of this fresh water. This led Franks and Anderson
(1992a) to hypothesize that this annual southward progression of toxicity is a result of
alongshore advection of Alexandriumfundyense in the coastally trapped buoyant plume
formed by river discharge (this hypothesis is often referred to as the "plume advection
hypothesis").
Figure 2-4. The station locations for the study by Franks and Anderson (1992a).
Franks and Anderson (1992a) also suggested that alongshore winds had an
important effect on the motion of the plume. It was observed that downwelling-favorable
(southwestward) winds increased the speed of the plume and held the plume to the coast
increasing the intensity and alongshore extent of toxicity. Upwelling-favorable
(northeastward) winds decreased the speed of the plume and forced the plume offshore
potentially causing it to separate from the coast (see figure 2-5). Separation would result
in a decrease in toxicity of intertidal shellfish. In addition to wind direction, the volume
of discharge from the Kennebec and Androscoggin rivers was also shown to have a
significant impact on plume velocities and, in years of high river discharge, it was
observed that toxicity patterns were relatively independent of the wind patterns, whereas,
in years of low river discharge, the wind had a greater influence (Franks and Anderson,
1992a).
N
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Figure 2-5. Surface (left) and vertical across-shelf (right) plots of salinity distribution of
a buoyant coastal plume subject to no wind stress (top), downwelling-favorable wind
stress (middle) and upwelling-favorable wind stress (bottom) (Franks and Anderson,
1992a).
Franks and Anderson (1992b) further tested the plume-advection hypothesis of
Franks and Anderson (1992a) using historical records of shellfish toxicity, river outflow
and windstress from 1979 to 1989. As predicted by the plume-advection hypothesis,
every toxic outbreak in Massachusetts was preceded by both an outbreak in Maine and an
increase in the discharge of the Androscoggin River. The observations from 1985 present
the only contradiction to the north-south progression of the "plume advection
hypothesis", showing almost simultaneous outbreaks of toxicity over a wide section of
the coast.
2.5 The "Cyst Source" Hypothesis
There were several issues that were left unresolved by Franks and Anderson
(1992a and 1992b) most notably the origin of the A. fundyense cells. Franks and
Anderson (1992a) suggest that the cells enter the plume in the western gulf of Maine near
the mouth of the Kennebec River, but did not suggest a source for these cells. Anderson
et al. (2005a) confirmed the general elements of the plume advection hypothesis and
refined this conceptual model by defining two potential source populations: those
originating from the offshore benthic cyst populations (McGillicuddy et al., 2003) and
those being transported to the WMCC from the EMCC (Townsend et al., 2001).
Several studies provide evidence that the EMCC acts as an important pathway for
A. fundyense cells to enter the Western Gulf of Maine. Several surveys by Martin and
White (1988) indicate westward penetration of high cell densities from the "incubator"
region east of Grand Manan Island into the EMCC and Townsend et al. (2001) note that
the highest cell concentrations in the Gulf of Maine are observed in the cold, nutrient-rich
waters of the EMCC. Anderson et al. (2005b) suggest that some of the cells transported
in the EMCC become entrained in the western GOM waters, where they may cause
immediate toxicity, while others are deflected offshore. As previously mentioned, the
degree to which the EMCC flows into the western GOM is highly variable and this could
have a significant impact on the intensity of toxicity. Leursen et al. (2005) show that low
toxicity in the western GOM occurred when a strong front developed, also referred to as a
period where the "door is closed" (Leursen, 2001), and, in years when a weak front
develops, the "door is open" and high toxicity was observed in the western GOM.
Leursen et al. (2005) conclude that the EMCC has a significant effect on the intensity of
toxicity in the western GOM either by advection of nutrient-rich water or by direct
transport of cells into the WMCC. Keafer et al. (2005b) observed the highest abundance
of A. fundyense within the fresher waters of the EMCC, which originate from the outflow
of the St. John that flows over the seedbed east of Grand Manan Island. They therefore
suggest that cells could be transported directly from the EMCC into the WMCC via the
GOMCP described earlier. The GOMCP can be partially deflected offshore due to
upwelling-favorable wind stress, thus adding another source of variability to the
distribution of A. fundyense blooms.
There is evidence that offshore cyst beds also play an important role in blooms of
A. fundyense in the western Gulf of Maine. A study by McGillicuddy et al. (2003)
suggests that, during upwelling-favorable winds, the river plume in the western GOM
thins and can extend far enough offshore to be over the seedbeds observed near Casco
Bay and Penobscot Bay, allowing the possible entrainment of these cells into the plume.
With subsequent downwelling-favorable winds, the plume would return to the coast
transporting the cells to the shellfish beds. A study by Stock et al. (2005) further supports
the importance of offshore cyst beds. Results from their coupled physical-biological
model suggest that the cysts germinated from offshore seedbeds could account for the
observed timing and magnitude of A.fundyense blooms in the spring in the western
GOM. This mechanism could also explain the near simultaneous outbreaks of toxicity
observed in the western Gulf of Maine in 1985 by Franks and Anderson (1992b).
There is an observed west-to-east shift in the center of mass of vegetative cells
that cannot be explained solely by the east-to-west transport of the EMCC and the plume
of the WMCC. Townsend et al. (2001) hypothesized that the A. fundyense cells that enter
the EMCC do not initially flourish due to the deep mixing and turbulence, which limits
the amount of light reaching the cells. However, as the water reaches the west, the water
becomes more stratified which is more favorable for growth. McGillicuddy et al. (2005)
present results from a coupled physical-biological model that suggest another possible
explanation for the west-to-east trend of toxicity. Early in the season they suggest that
temperature could be the major limiting factor in the eastern Gulf of Maine. Higher
concentrations of vegetative cells are observed in the western gulf of Maine primarily due
to higher temperatures and cumulative impact of cysts being transported from the two
major cyst beds located in the Bay of Fundy and offshore of Casco and Penobscot Bays.
During the transition from spring to summer the western Gulf of Maine becomes nutrient
depleted. Nutrient limitation results in the induction of sexuality in A.fundyense, which
leads to the formation of resting cysts (Anderson and Lindquist, 1985). The cyst
accumulations offshore of Penobscot and Casco Bays observed by Anderson et al.
(2005b) are found in the general area where the model predicts nutrient limitiation will
occur. Although the western Gulf of Maine becomes nutrient-limited, growth of
vegetative cells still occurs in the nutrient-rich eastern Gulf of Maine.
2.6 Possible mechanisms for entrainment
Horizontal ocean currents are several orders of magnitude greater than swimming
speeds of plankton; therefore plankton can be considered passive tracers in a lateral
context. In contrast, ocean currents are considerably weaker in the vertical, and thus
plankton, with typical swimming speeds of meters to hundreds of meters per day, are able
to change their vertical position (Hetland et al., 2002). Dinoflagellates like Alexandrium
fundyense are light limited and it is therefore beneficial for them to be able to change
their vertical position. There are several possible mechanisms by which the cells could
enter the plume from the EMCC or from offshore, and they all depend on light-seeking
swimming behavior of Alexandrium fundyense.
The analysis of density surfaces presented in Keafer et al. (2005b) suggests that
cells that are located beneath the surface mixed layer (>10m depth) can be transported
directly beneath the thin river plume and enter the plume via vertical light-seeking
swimming behavior. In another study (Keafer et al., 2005a), temperature and salinity
analysis is used to infer that surface populations at the outer edge of the Penobscot plume
can be subducted underneath the low salinity Kennebec plume and the A. fundyense cells
could enter the plume via vertical light-seeking swimming.
The previous mechanism involves the interaction of two buoyant coastal currents.
In contrast, modeled experiments by McGillicuddy et al. (2003) and Hetland et al. (2002)
suggest mechanisms by which cells may be entrained into a river plume undergoing
offshore and onshore excursions as a result of varying wind conditions. Hetland et al.
(2002) proposed the "frog tongue" hypothesis, which establishes a range of upward
swimming velocities that would allow plankton to enter a plume during upwelling
favorable conditions and then be transported towards the coast during subsequent
downwelling events. In order for plankton to enter a plume, they must swim with a
vertical velocity wp which satisfies
H ,,plume, / T < w, < KI/ H,,,i
where Hp is the thickness of the plume, Hil,,, is the thickness of the mixed layer (where
H,,,x Hp,,,,e - 0(10) ), T is the time between onshore and offshore extremes of the
plume, and K is the magnitude of mixing in the mixed layer (Hetland et al., 2002). This
inequality indicates that the plankton must swim slow enough so that they will be evenly
distributed in the mixed layer and can therefore be subducted under the plume as it moves
offshore. When the plankton are beneath the plume they must swim quickly enough to
enter the plume before it begins to return to the coast. A diagram of this model is seen in
figure 2-6.
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I
D
~
(-
OT
t·c-
-- -
- -
- -
-
- -
- -
Figure 2-6. A cartoon illustrating the various stages in the entrainment of a portion of an
offshore plankton patch. The squiggly, straight horizontal and circular arrows, represent
swimming, Ekman transport velocity and turbulent mixing respectively. The dark grey
represents the plankton patch (Hetland et al., 2002).
A CROSS-ISOBATH TRANSPORT MECHANISM
FOR INITIATION OFALEXANDRIUM BLOOMS
UPWELLING
,·
--------
DOWNWELLING
I`~-,.
-., ,,~.r
c~r r
Figure 2-7. Schematic of a proposed mechanism for the entrainment of A. fundyense
cells originating in offshore cyst beds. During upwelling favorable conditions the plume
thins and extends offshore above the cyst beds where newly germinated light-seeking
cells swimming towards the surface can enter the plume. During downwelling favorable
conditions these cells are transported towards shore in the plume. (McGillicuddy et al.,
2003)
McGillicuddy et al. (2003) used a coupled three-dimensional biological-physical
model to investigate how germinated cells from offshore cyst beds could contribute to
nearshore blooms. The model results suggest that under upwelling conditions a plume
may thin and extend far enough offshore to be above the observed cyst beds. This would
allow the newly germinated cysts to be entrained in the plume as they swim vertically
towards the light in order to begin vegetative growth. As the winds become downwellingfavorable the plume moves onshore and thickens thereby exposing the coast to these
toxic cells from offshore. A schematic of this process is shown in figure 2-7.
2.7 Summary
From the research presented, it is clear that the gulf-wide circulation, particularly
the dynamics of the buoyant coastal plumes, and cyst bed locations play an important role
in the distribution of A. fundyense blooms in the Gulf of Maine. Within the Gulf of
Maine, A. fundyense cells are transported within these buoyant coastal plumes which are
part of the two major coastal currents: the EMCC and the WMCC. There is evidence that
these cells originate from the cyst beds in the Bay of Fundy and offshore of Penobscot
and Casco Bays. The EMCC transports these cells from the Bay of Fundy to Penobscot
Bay where it splits into two different branches, transporting cells directly into the western
GOM, where they can become entrained in the WMCC and transported into the western
GOM , or they can be deflected offshore where they become encysted to form the
offshore seedbeds. The germinated cells from these offshore cyst beds can be entrained in
the plume of the WMCC during an upwelling favorable wind event and transported back
to shore where they can cause toxicity in the shellfish beds during a subsequent
downwelling event. The one-way path of A. fundyense cells in the Gulf of Maine suggests
that the self-seeding cyst population in the Bay of Fundy is the cause of persistent
occurrences of toxicity along the coast and that the GMCC is its main means of
transportation. Therefore, in order to understand the variability in the distribution of A.
fundyense , we need to understand the dynamics of river plume interactions that feed this
coastal current.
Chapter 3
Laboratory Experiments
Consider a case where one river plume, carrying A. fundyense, travels along the
coast and comes into contact with another river plume. The first plume could go above,
around or be subducted beneath the second plume. If the first plume goes above the
second plume, then it is possible that the A. fundyense will reach the coast and be
consumed by shellfish. If the plume goes around or is subducted beneath the second
plume the A. fundyense will need to enter the second plume in order to gain access to the
coast. Two possible ways of doing this are through mixing between the two plumes and
through swimming. In this chapter I explore the first possibility by trying to quantify
mixing in a series of laboratory experiments modeling the interaction of two riverplumes
with various densities and flow rates.
3.1 Apparatus
Laboratory experiments were conducted to investigate the interaction between
two coastal plumes. The experimental setup was configured to represent a northern and
southern plume flowing along a vertical wall above a flat bottom.
The experiments were conducted in a cylindrical tank with a diameter of 2.1 m, a
height of 0.45 m and a flat bottom. The tank was rotated counter-clockwise, to simulate
the northern hemisphere, with a fixed rotation rate of f=1. The tank was filled with ocean
water (p = 1.022 g / cm3 ) to a depth of approximately 15 cm.
The two rivers were simulated by pumping buoyant water at a constant rate at two
different locations on the side of the tank. The top view and side views of the
experimental set up can be seen in figure 3-1 and figure 3-2 respectively. The sources
were two pipes with diameters of 1.5 cm located 116 cm apart. In the figure, the northern
river is labeled with an N and the southern river is labeled with an S. To minimize
excessive mixing at the source, foam was wrapped around the end of the pipe.
A conductivity probe was positioned 59 cm away from the southern plume
(labeled with a P in the fig 2.2.1 and fig. 2.2.2). The probe was fixed to the side of the
tank so that it could be moved vertically and perpendicular to the wall of the tank.
Ile
"P
rr·
I
t-:~
I.
,· I
it6
Ii
i
/
S
z
/
· `P
Figure 3-1 Top view of tank set up. N and S indicate the locations of the northern and
southern plumes respectively. P indicates the location of the probe when closest to the
wall. (Graphic made by Valentina)
c
o
cn
a
I:
i:
ii:
ii
i:
i·
ii
210
Figure 3-2 Side view of tank set up. Dimensions are in cm.
3.2 Procedure
The two source waters were created by mixing sea water and fresh water. The
densities of these two fluids were determined with a model DMA58 Anton Paar
densitometer which has an accuracy of 10-5 g/cm 3.
The tank was rotated counter-clockwise at a rotation rate of f =1. After the
ambient fluid reached solid body rotation the two sources were turned on. After the
northern current came into contact with the southern current, a profile was taken near the
wall. The frequency of each profile was 1 point per 0.01 cm and only the downward
profiles were used.
Six mixtures of known densities ranging from that of fresh water to that of ocean
water were made. Their densities were first measured with the densitometer and then with
the conductivity probe. The approximate analytical relationship between voltage and
density is the following second order equation
V = aL
where p* = p -Po,
+a2
+ a3
(3.1)
po is the freshwater density and p ( g/cm3 ) is the density which
corresponds to the voltage V. Using this equation we can find the coefficients which best
fit the density and voltage measurements.
Solving equation 3.1 for density we find
2
P = Po
+
-(a 2 + a - 4ai(a
a+
2a(3.2)3
-V))
(3.2)
Using this equation and the calibration coefficients, voltage profiles were converted to
density profiles.
3.3 Results
3.3.1 Summary of Experiments
Six variations of the experiment were done. In each one the densities and flow
rates of each river were changed. A summary of the experiments can be seen in table 3-1.
The table shows the densities (p) of the ocean water and fresh water which were mixed to
make the north and south river waters, the density of the tank water, the flow rate of the
northern plume (Qn) and the flow rate of the southern plume (Qs). The desired or ideal
density, reduced gravity and buoyancy flux for the northern river (pn, g'n, Bn) and the
southern river (ps, g's, Bs) are listed. The reduced gravity is defined as
g'g
(po-p)
o + p)/2
where Po is the density of the tank water. The buoyancy flux is defined as
B = g'Q.
In addition, the experimental values for the density, reduced gravity, buoyancy flux and
the error between the ideal and experimental value of g' for each river are listed.
Experiment
p ocean water
(g/cc)
p fresh water (g/cc)
p tank water (g/cc)
1.021
0.999
1.022
Qn (cc/s)
g'n ideal (g/cc)
Pn ideal (g/cc)
Bn ideal
Pn exp (g/cc)
g'n exp (g/cc)
Bn exp
5
1
1.021
5
1.021
0.950
4.75
Qs (cc/s)
g's ideal (g/cc)
Ps ideal (g/cc)
Bs ideal
Ps exp (g/cc)
g's exp (g/cc)
Bs exp
10
5
1.016
50
1.016
4.934
49.34
1
2
3
1.022
1.022
0.998
0.999
1.021
1.021
Northern Plume
10
10
10
5
1.011
1.016
100
50
1.011
1.016
10.111
4.984
101.11
49.84
Southern Plume
10
10
5
5
1.016
1.016
50
50
1.016
1.016
5.216
4.984
52.16
49.84
4
5
6
1.022
0.998
1.020
1.022
0.998
1.022
1.022
0.998
1.020
10
25
0.995
250
0.998
21.456
214.56
20
25
0.996
500
0.998
22.819
456.38
20
25
0.995
500
0.998
21.456
429.12
10
5
1.015
50
1.015
4.929
49.29
10
5
1.017
50
1.016
5.164
51.64
5
5
1.015
25
1.015
4.929
24.645
Table 3-1 Summary of experiments
3.3.2 Vertical Plume Structure
The density profile taken closest to the wall of the tank is shown in figure 3-3 for
each experiment. Each profile was filtered using a boxcar filter of 30 increments. Plotted
on top of the profile are the experimental densities of the northern plume, southern plume
and the ambient tank water indicated by red, blue, and green lines respectively. The sharp
gradient in density seen near the top of several of the profiles is a result of the probe
coming out of the water.
In these profiles we see various vertical structures. In experiment 1 there appears
to be southern plume water overlying northern plume water with a mixed layer between
these two water masses and between the northern plume water and the tank water. In the
second experiment the southern plume and the northern plume have the same densities
and, as would be expected, there is a single layer of both northern and southern plume
water with a shallow mixed layer between it and the ambient tank water. In the third
experiment we see the opposite of what was observed in experiment 1. There is a
northern plume water overlying southern plume water. The fifth experiment shows a
similar structure to the third. The difference between the measured densities of the three
water masses and the densities of each layer in the profile could be due to problems
during probe calibration. The fourth experiment has a slightly different structure. From
the profile it appears that there is a mixed layer with a linearly decaying density overlying
a layer of southern plume water. A similar structure is observed in the sixth experiment
however, the density profile in the upper layer is not quite linear. In both cases it is
possible that the probe did not come out of the water and may have missed a thin layer of
northern plume water that may have existed at the surface of the water.
Expedment
1
II
I
I
F
l
l
I
Ki
I
I
S
I
I
I
I
I
2
Experiment
I
I
I!
~I
I.
0.995 1
I I
_
I
It
lu
Ii
*
I
I.
I
.005 1.01 1.015102
I
1.02
Experiment
3
0.R
1
*
Ji *I
I
I
10051.01 1.0151.02 1.0
Expement 4
-2
4
EI
0~5
1
1.0051.01 1.015
Expeiment
5
Experment 6
25
35
Figure 3-3 Density profiles from each experiment taken closest to the wall of the tank.
Density is in g / cm3 and the y-axis represents the distance from the top of the profile in cm.
3.3.3 Mixing
To determine the amount of mixing occurring between the plumes after interaction the
Richardson number was calculated for each experiment. The Richardson number is defined as
- g
Ri = h,,,
where h, is the measured thickness of the mixed layer between the northern and southern plumes
and
Ugx
=ghpx
is the geostophic velocity calculated for each plume and
h
2Qxf
is the theoretical depth of each plume at the side of the tank. It is usually assumed that for Ri << 1
mixing is important. The values for h,, h,,, u, and Ri for each plume for all six experiments can be
found in table 3-2.
Exp
1
2
3
4
5
6
hm (cm)
1
0
0.55
1.75
0.9
1.5
hpn (cm)
3.24
2
1.41
4.48
1.32
1.37
hps (cm)
2.01
2
1.96
2.01
1.97
1.42
ugn (cm/s)
1.76
3.16
3.77
2.11
5.5
5.41
Ugs (cm/s)
3.15
3.16
3.2
3.15
3.19
2.65
Ri
2.04
0
8.14
6.4
2.98
3.25
hm* (cm)
0.5
0
0.07
0.27
0.3
0.46
Table 3-2 Summary of important parameters used to estimate the importance of mixing between the
northern and southern plumes.
In the second experiment the two plumes had the same buoyancy flux after interaction the
plumes are indistinguishable from each other and therefore no Richardson number can be calculated.
In all of the other experiments the Richardson number is considerably greater than one. To see how
sensitive the Richardson number is to changes in the measured value of mixed layer thickness, the
mixed layer thickness for a Richardson number of 1,
h,,, = abs
•
-
)
was calculated. In figure 3-4 the upper and lower bounds of the measured mixed layer are indicated
by green lines. The upper and lower bounds based on the mixed layer thickness predicted for cases
in which the Richardson number is equal to one are represented by red lines. As can be seen for all
experiments the mixed layer thickness for the case of Ri = 1 is considerably smaller than the value
measured and from the shape of the profiles it is not possible to define the mixed layer using those
bounds. Assuming that our equations are appropriate for plume interactions we can infer that mixing
is relatively unimportant between the southern and northern plume.
EpeqOrimnt
1
fll
I
I
I
I
I
I
K
*1
I
I
1
1
1
0
1
1
' I
1.19 1.• 10I 1.0
1.018
1016 1.017
101
Expeime
4
Eqeiment 3
S
i
I
I
1
I
" I
I ·I
401
mh
2W
2M
a-
a
1.018
1.02
1.0I
4 1. . 1.01
1. 0141.014
1.0161.0181.02
1.014
1.011,012
Expefiment6
2W-
I
-
I
Exp
I 5
I
n
I
I
I
I
I
I
I
I
I
I
401
2W0
2M
n
0.U
-I
1
I
1.05
p1
I
1.01 1015
1.02
I
I
1 21 1
I
I
I
I
1.022
1014 116 1018 1.02
1 1 01
Figure 3-4 Comparison of measured mixed layer thickness between the northern and southern
plumes and the mixed layer thickness predicted for a Richardson number of one.
The same process was repeated for the mixed layer between the lower plume (northern or
southern depending on the experiment) and the ambient water. Since the ambient water was
motionless the equations for the Richardson number and mixed layer thickness for Ri = 1 simplify to
the following
Ri =
h,g'
2
Ug
h,,. =abs 9
where h,,,,
h, and ug are all calculated for the lower plume. The values for these parameters can be
found in table 3-3. In all of the other experiments the Richardson number is less than one.
Experiment
1
2
3
4
5
6
hm (cm)
0.6
0.9
1
0.65
0.7
0.65
hp (cm)
2.01
2
1.41
4.48
1.32
1.37
ug (cm/s)
3.15
3.16
3.77
2.11
5.5
5.41
Ri
0.3
0.45
0.71
0.15
0.53
0.48
hm* (cm)
2.01
2
1.41
4.48
1.32
1.37
Table 3-3 Summary of important parameters used to estimate the importance of mixing between the
lower plume and the ambient water.
In figure 3-5 the upper and lower bounds of the measured mixed layer between the lower
plume and the ambient water are indicated by green lines. The upper and lower bounds based on the
mixed layer thickness predicted for cases in which the Richardson number is equal to one are
represented by red lines. As can be seen for all experiments the mixed layer thickness for the case of
Ri = 1 is considerably larger than the value measured which leads to the conclusion that most of the
mixing occurs between the plumes and the ambient water.
Eperinmel
Experient2
II
I
-
I
I
II
I
I
I
I
U1014
1.01
1.01
1.018
1.I .I
1016 1.011.018
1,014
1.015
1019 1.02
Eqeero 3
Eperiment4
fill
I
I
I
I
I
I
I
I
I
I
I
IL-
MO
t
2M
0
I
I
I
I
II
1,011.012
1.014
1.016
1.0181.02 1.
I
I 1
1. 1.04
1 l. 1.01
1.012
1.014
1.016
1.018
1.02
Expeiment5
Experiment6
imi----__
[
I
I
i
I
LI
0.99 1
I
1.5
1.01 1.0151.02
1.02 1.0
4
1. 11 1.012 1.014
1 .0161018
1.02
1022
Figure 3-5 Comparison of measured mixed layer thickness between the northern and southern
plumes and the mixed layer thickness predicted for a Richardson number of one.
3.4 Summary and Discussion
In this chapter I try to quantify mixing between two river plumes through a series
of laboratory experiments modeling the interaction of two plumes with various densities
and flow rates. Based on these experiments it appears that much of the mixing occurs
between the plumes and the ambient water and that mixing is relatively unimportant
between the southern and northern plume. However, the profiles in these experiments
were taken downstream of the southern source and it is possible that mixing could be
occurring upon the initial interaction of the two plumes and has subsided by the time they
reach the location where the profile was taken. Therefore, from these experiments we can
conclude that, under these idealized conditions, if the A. fundyense did not enter the
second plume near the source they would have to find another way to enter the second
plume. One possibility is through their light seeking swimming behavior.
Chapter 4
Physical Model
Again consider a case where one river plume, carrying A. fundyense, travels along
the coast and comes into contact with a southern river plume without A. fundyense. This
time, the first plume is denser than the second and is therefore not likely to go above the
second plume but will probably go beneath it or around it. It is possible for A. fundyense
to enter the second plume, and then gain access to the coastal shellfish beds, through
vertical swimming. However, for this to be effective the first plume must go beneath the
second plume. If the northern plume is unable to go beneath the southern plume then the
coast might be protected from the A. fundyense and an outbreak of red tide. Using a series
of numerical model runs I investigate the effects of the southern plume on the path of the
northern plume and attempt to specify under what conditions the northern plume will be
subducted beneath the southern plume.
4.1 Model Formulation
In this work a simple 2 '/2 layer model is used to study the interaction of two river
plumes propagating along the coast in the direction of Kelvin wave propagation. Here the
basic model formulation is described. In the following equations the subscript s denotes
the top layer which originates from the southern river, n denotes the middle layer which
originates from the northern river, and 0 denotes the bottom layer or the ambient ocean
water. We assume that the pressure in the fluid is hydrostatic and, within each layer, the
horizontal velocity is independent of depth. The pressure within each layer can then be
written as
+h -z)
+h,
p, =gp (h,
p, = gpsh, + gp,(h, + ho - z)
Po = gPsh, + gph,+gPo (ho - z)
where p,, Px and hx are the pressure, density and thickness of the xth layer
respectively. Assuming that there is no horizontal pressure gradient in the bottom layer
the depth of the bottom layer can be written as
ho
=
_
h -
h,
Po
Po
Using this new definition for ho the equations for pressure can be simplified to
PS
Po - Ps hi
p,
PI, p h+ Po - P
Pn
Pn
Po - P
PO
h-z
Po
Po = -gz
If we set p, = (1- 6,)Po and p, = (1- 6,)po, the first two equations become
s= 6Sgh, + 6,gh, - gz
PS
g
hS +6h
16n
z
1-6n
with the Boussinesq approximation, the second equation simplifies to
P-
gs,h +gS,h, - z
Pn
The horizontal pressure gradients can therefore be written as
1
-Vp,
Ps
1
=
IVp,
gbsVh, + g6,Vh,
g6,Vh, + g6,Vh,
Pn
For the model we can now use the Bernoulli form of the momentum equations
U + (f +
x u = -VB
at
wheref is the coriolis parameter,
av
ax
au
ay
and the Bernoulli function is
B=P
p
(+1 2 +2)
2
The mass conservation equations for the model are
_w
VVW:F-=
0.
h
The model domain consists of a 25km x 300km rectangular basin. Fresher water
is discharged uniformly (in y and z) at the coastline from two rectangular river mouths, of
length L, and depth hr,, centered at a distance of 296km and 256km from the southern
end of the domain. The base case was run at both a .25km resolution in x and y with a 1
minute time step and .5km resolution in x and y with a 2 minute time step. No qualitative
differences were evident so for the rest of the runs the lower resolution was used.
All model runs presented neglect the influence of tides, winds and bottom friction.
Mixing is also neglected in the numerical model partially due to the experimental results
described in the previous chapter but also to keep the model as simple as possible. The
coriolis parameter fis set to 10- 4 s-' and the ambient ocean water remains motionless and
3.
maintains a constant density po of 1020 kg / m'm
In section 4.2 the base case and initial analysis is presented. In section 4.3 the
effect of the river mouth geometry is investigated. In these runs all parameters remained
constant except the river mouth lengths and depths. The following ranges were used for
both river mouths: depth of the river mouths 2 m < h, < 14 m , length of the river
mouths 1 km < Lr < 7 km . In section 4.4 the effect of varying river fluxes is investigated
by keeping everything constant except the flux of each river which was varied within the
range 1250 m3 / s < Q < 10,000 m 3 / s.
4.2 Base Case
As a base case (run I in table 4-1) we consider two almost identical river inflows.
Both have a river mouth 2 m deep and 1 km wide and a flux of 1250 m3 / s. The
difference between the two rivers is their densities. The density of the northern river ( p,)
is 1015 kg / m3 and the density of the southern river ( p,) is 1010 kg / m3 .The difference
in density was necessary to create the 2 ½2 layer system, as described, with the northern
plume being the lower layer.
The thickness of the northern plume (h , ), thickness of the southern plume (hs)
and the total depth of river input (h, + h, ) after a period of 15 days can be seen in figure
4-1. Both river outflows initially form bulges and then turn right to form a coastal
current. After 15 days two bulges have formed which are wider and deeper than the
downstream coastal current which forms to the south of the southern bulge. It is also
evident from these plots that the southern bulge is diverting a significant portion of the
northern plume away from the coast.
The horizontal velocities for the northern and southern plumes can be seen in
figure 4-2 and figure 4-3 respectively. As seen in the figures, the coastal current is
unidirectional with southward velocities of approximately 5 cm/s in the northern fluid
and 10 cm/s for the southern fluid. The bulge regions are anticyclonic and appear to be in
cyclostrophic balance. The maximum velocities in the bulge regions are 10 cm/s for the
northern bulge and 25 cm/s for the southern.
0
5
10 15
x(km)
20
5
10 15
x(km)
20
0
5 10 15 29
x(km)
Figure 4-1 The thickness of the northern plume (h,), thickness of the southern plume
(h, ) and the total depth of river input (h,, + h, ) after a period of 30 days are shown from
left to right (m) for the base case.
w1
06
05
0.4
03
02
01
.15
0l
~-
-
-
--
"
Figure 4-2 Depth (m) and horizontal velocities (m/s) for the northern plume.
06
105
04
-0.05
03
-0.1
0.2
01
0
.25
01
.3
111
0.36
10
20
30
40
50
Figure 4-3 Depth (m) and horizontal velocities (m/s) for the southern plume.
In Fong & Geyer (2002) it was discovered that in the absence of an ambient
current the bulge would continue to expand. In this model the rivers are flowing into a
motionless ocean so it wouldn't be unreasonable to assume that each bulge would
continue to grow in the absence of an ambient alongshore current. A growing southern
bulge could significantly affect the distribution of any pollutants or organisms originating
in the northern plume. As previously mentioned, in this base case the southern bulge
seems to divert the northern plume away from the coast. If the plume continued to grow it
is possible that it could shield more of the coast from the northern plume and anything it
may be transporting. In figures 4-4 and 4-5 the depth of the northern and southern plumes
as they vary in the cross shore direction at the latitude of their respective river mouths can
be seen at one day intervals for 30 days. (Note that in figure 4-4 the plume seems to
thicken which is probably due to a wall effect). As can be seen the depth of the northern
bulge continues to vary whereas the depth of the southern plume begins to converge. It
appears that when the northern plume comes into contact with the southern plume it acts
to control the growth of the southern bulge like the ambient current controlled the single
plume bulge in the model used by Fong and Geyer (2002). So at least in this base case, it
seems that that the southern bulge would shield the same section from the northern plume
over time.
I
I
I
Northern Plume
I
•
'
I
-0.2
E
~-----~--c7-.
~--c*
--L
-~---e
--
-0.4
--
:s
E
,
c-
`-·=--~-sz---.
-~c~C~C14~
'4-;;ts
-0.6
E
-0.8
E
-1.2
-
-1.4
-
-1.6
-
-1.8
-
I
I
I
I
--
--
I
I
I
I
I
5
10
15
20
25
30
35
40
45
-
0
50
x (km)
Figure 4-4 Change in the width (cross shore direction) of the northern bulge region over
a thirty day period
Southern Plume
.- b
u
-0.2
-0.4
-0.6
-0.8
-1
-1.2
-1.4
-1.6
-1 R
0
5
10
15
20
25
x (km)
30
35
40
45
50
Figure 4-5 Change in the width (cross shore direction) of the southern bulge region over
a thirty day period.
4.3 Variations in River Mouth Geometry
As seen in the base case the southern bulge can affect the path of the northern
plume as it progresses along the coast. Fong and Geyer (2002) found that the shape of the
bulge in the single plume was found to depend mostly on the velocity of the river inflow
and the width of the river mouth. A large velocity and narrow river mouth resulted in a
bulge which extended farther offshore. In this set of numerical experiments the width Lr
and depth h, of both river mouths is varied in an attempt to alter the horizontal size of
both the northern and southern bulges. The fluxes of both rivers remain the same for all
runs (Q,,,
Q= 1250
m3 / s ) . By holding the fluxes constant the velocities of the rivers
will increase as river mouth size decreases. Both an increase in river outflow velocity and
a decrease in river mouth size should act to increase the horizontal extent of the bulge.
However, it is not certain a priori whether variations in river mouth size will have an
effect on horizontal bulge size in the two plume case, especially in regards to the southern
bulge which seems to be affected by the northern plume. In this set of experiments I
attempt to discover if variations in river mouth size have any effect on bulge size and the
effect of these variations on the path of the northern plume. A summary of the
experiments can be found in table 4-1. All numerical runs were analyzed after a period of
thirty days.
Run
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
hrn (m)
2
2
2
2
2
2
2
4
6
8
10
12
14
14
14
14
14
14
14
hrs (m)
2
4
6
8
10
12
14
14
14
14
14
14
14
12
10
8
6
4
2
Ln
(km)
1
1
1
1
1
1
1
2
3
4
5
6
7
7
7
7
7
7
7
Ls
(km)
1
2
3
4
5
6
7
7
7
7
7
7
7
6
5
4
3
2
1
un (mis)
0.63
0.63
0.63
0.63
0.63
0.63
0.63
0.16
0.07
0.04
0.03
0.02
0.01
0.01
0.01
0.01
0.01
0.01
0.01
us (m/s)
0.63
0.16
0.07
0.04
0.03
0.02
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.02
0.03
0.04
0.07
0.16
0.63
Rn
6.250
6.250
6.250
6.250
6.250
6.250
6.250
0.781
0.231
0.098
0.050
0.029
0.018
0.018
0.018
0.018
0.018
0.018
0.018
Rs
6.250
0.781
0.231
0.098
0.050
0.029
0.018
0.018
0.018
0.018
0.018
0.018
0.018
0.029
0.050
0.098
0.231
0.781
6.250
Rn/Rs
1.0
8.0
27.0
64.0
125.0
216.0
343.0
42.9
12.7
5.4
2.7
1.6
1.0
0.6
0.4
0.2
0.1
0.0
0.0
Table 4-1 Variations in river mouth geometry
Looking at plots of the northern plume thickness (h,), the thickness of the
southern plume ( h, ) and the total depth of river input (h, + h, ) for all of the runs there
appears to be two different qualitative results. For the majority of cases (runs 1-17) the
northern plume detached from the coast near the location of the southern bulge. In runs
18 and 19 the northern plume remained connected. The plots of the northern plume
thickness ( h, ), the thickness of the southern plume (h, ) and the total depth of river input
( h, + h, ) for run 17, representing the first case, and run 18, representing the second case,
can be seen in figures 4-6 and 4-7 respectively. In figure 4-7, it can be seen the maximum
depth of the southern plume is approximately 0.6 m; this is probably the reason that the
northern plume was able to remain attached to the coast and is not a very realistic case.
1.5
6E
>1
0
5
10
15
x*m)
20
5
10
15
x(km)
20
0 5
10 15 20
x(m)
Figure 4-6 The thickness of the northern plume (h,,), thickness of the southern plume
(h,) and the total depth of river input (h,, + h, ) after a period of 30 days are shown from
left to right (m) for run 17.
U
5
10
15
x(m)
2J
5
10
15
x(km)
J
U 5
I
15
I
J2
x(km)
Figure 4-7 The thickness of the northern plume (h,), thickness of the southern plume
(h, ) and the total depth of river input (h, + h, ) after a period of 30 days are shown from
left to right (m) for run 18.
The Rossby number, defined as
R =
Ur
fZr
where u,. is the velocity at the river mouth, has been found to best characterize the shape
of the bulge in the case of the single plume (Fong and Geyer, 2002) so, in an attempt to
quantitatively describe the effect of variations in river mouth geometry on the path of the
northern plume, the Rossby numbers were estimated for each river mouth. Since the size
of both river mouths was varied the ratio of the northern Rossby number and the southern
Rossby number was also calculated. To estimate the extent to which the northern plume
is deflected away from shore, the location of the center of mass of the northern plume at
the latitude of the southern river mouth is calculated. The distance between the coast and
the center of mass of the northern plume for various Rossby number ratios is shown in
the three plots in figure 4-8. The middle graph shows runs which all had an R, of
approximately 0.018. variations in R, under these conditions did not seem to alter the
location of the center of mass of the northern plume. The top graph shows runs which all
had an R, of 6.25 but varying R, values. For lower R, values which indicate a low source
velocity and a large river mouth, the Rossby ratio is higher and the distance between the
center of mass of the northern plume and the coast increases. The bottom graph shows
runs which all had an R, of 0.018 but varying R, values. Variations in R,, gave similar
results as those described for the runs with an R, of 6.25. It appears that for lower R,
values the southern bulge does not extend as far off shore but becomes thicker and it
therefore seems that the thickness of the southern bulge has an impact on the path of the
northern plume.
Rn=6.25
5•'
t
23.
-
23 1
22.
E 22.5-
21.521 -
20.50
50
100
150
200
250
300
350
400
250
300
350
400
RnlRs
Rs=0.018
B.
*
20 E
m 10
50
0
50
100
150
200
RnlRs
Rn=0.018
.
20-
E
'4*
15 -
C
! 10 5-
0
0
0.2
0.4
0.6
0.8
1
1.2
RnlRs
Figure 4-8 Comparison between the ratio of Rossby numbers for both river mouths and
the distance between the coast and the center of mass of the northern plume near the
southern river mouth
4.4 Variations in River Fluxes
The following numerical experiments were used to explore the effect of variations
in both southern and northern river fluxes on the path of the northern plume. In all of
these runs the southern and northern river mouths are 3 km wide and 6 m deep. The flux
of the northern and southern rivers vary within the range of 1250 m3 I s < Q < 10,000
m3 / s . The variation results in a change in velocity at the river mouths and therefore a
change in the Rossby numbers for each river plume. A summary of the experiments can
be found in table 4-2. Since the model has periodic boundary conditions, when the
southern river reaches the southern end of the domain it will then reappear at the northern
end. Due to the greater fluxes in these runs, the southern plume reached the southern end
of the domain in a much shorter time period so all of these numerical runs were analyzed
after a period of only twenty days. Some wrap-around still occurred, but we hope did not
impact the results.
Run
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Qn
1250
1250
1250
1250
2500
2500
5000
5000
7500
7500
7500
7500
10000
10000
Qs
2500
5000
7500
10000
1250
7500
1250
7500
1250
2500
5000
10000
1250
7500
un (mis)
0.07
0.07
0.07
0.07
0.14
0.14
0.28
0.28
0.42
0.42
0.42
0.42
0.56
0.56
us (mls)
0.14
0.28
0.42
0.56
0.07
0.42
0.07
0.42
0.07
0.14
0.28
0.56
0.07
0.42
Rn
0.231
0.231
0.231
0.231
0.463
0.463
0.926
0.926
1.389
1.389
1.389
1.389
1.852
1.852
Table 4-2 Variations in river fluxes
Rs
0.463
0.926
1.389
1.852
0.231
1.389
0.231
1.389
0.231
0.463
0.926
1.852
0.231
1.389
RnlRs
0.5
0.3
0.2
0.1
2.0
0.3
4.0
0.7
6.0
3.0
1.5
0.8
8.0
1.3
In terms of whether the northern plume separates from the coast due to the
southern bulge, all of the runs fit into three categories based on their Rossby number
ratio. In all of the runs with R, / Rs < 0.6 the northern plume separated from the coast at
the location of the southern bulge. In runs with R, / Rs> 1 the northern plume was able to
pass directly under the southern plume. For the two cases where Rn / Rs was between
these values part of the plume was able to go under the southern bulge but most of the
plume went around the bulge. Plots of the northern plume thickness (h,), the thickness of
the southern plume (hs) and the total depth of river input (hn + h, ) for runs 6, 8, and 10
are shown in figure 4-9, 4-10 and 4-11 respectively. Run 6 is an example of the first
regime with Rn / R,<0.6, run 8 is an example of a case where 0.6 <R,, / R,<1 and run 10
is an example of a case where R, / R,> 1.
0
5
10 15
x(km)
20
5
10
15
x(km)
M0
0 5 10 15 20
x(km)
Figure 4-9 The thickness of the northern plume (h,, ), thickness of the southern plume
( hs ) and the total depth of river input (h,, + hs ) after a period of 30 days are shown from
left to right (m) for run 6.
0
5
10
15
20
5
10
15
20
0
5 10 15 20
Figure 4-10 The thickness of the northern plume (h, ), thickness of the southern plume
( h ) and the total depth of river input (h,, + h, ) after a period of 30 days are shown from
left to right (m) for run 8.
0
5
10
15
x~n)
20
5
10
15
I(km)
20
0
5
10 15 20
x(km)
Figure 4-11 The thickness of the northern plume (h,), thickness of the southern plume
(h, ) and the total depth of river input (h, + hs ) after a period of 30 days are shown from
left to right (m) for run 10.
To quantitatively compare the effect of flux on the path of the northern plume we
again compare the Rossby ratio of each run to the distance from the coast to the center of
mass of the northern plume at the latitude of the southern river mouth (figure 4-12). For
small Rossby ratios the distance of the center of mass of the northern plume from the
coast increases quickly as the ratio approaches zero. For Rossby ratios greater than one
the distance is approximately 12 and 18 km.
0
2
4
6
8
10
Rn/Rs
Figure 4-12 Comparison between the ratio of Rossby numbers for both river mouths and
the distance between the coast and the center of mass of the northern plume near the
southern river mouth
4.5 Summary and Discussion
Using a series of numerical model runs I explored the effects of the southern
plume on the path of the northern plume. In the first set of model runs the northern plume
was almost always diverted from the coast, however, the distance it went offshore varied.
It appears that when the southern river mouth is large the southern bulge does not extend
as far off shore but becomes thicker and the center of mass of the northern plume was
farther offshore. It therefore seems that the thickness of the southern bulge has an impact
on the path of the northern plume. In the runs where the river fluxes were varied it was
shown that R, / R, > 1 the northern plume was able to pass directly under the southern
plume and for R, / RS < 0.6 the northern plume separated from the coast at the location of
the southern bulge. For the two cases where R, / R, was between these values, part of the
plume was able to go under the southern bulge but most of the plume went around the
bulge. Under these idealized conditions, we can therefore determine if and how far the
northern plume will be diverted from the coast based on river mouth geometry and
knowledge of the flow rates of each river. As previously mentioned, if the northern plume
containing A. fundyense was diverted to the east by the southern plume the section of
coast that did not come in contact with the northern plume could be protected from an
outbreak of red tide.
Chapter 5
Coupled Biological-Physical Model
5.1 Biological Model formulation
Here I describe the simple N-P biological model the will be coupled with the
physical model to illustrate the possible effects of plume interaction on the transport of
Alexandrium fundyense. The following system of coupled differential equations describes
the interaction between the phytoplankton A. fundyense (P) and the nutrients (N) which
they consume. The subscripts n and s are used to indicate whether the phytoplankton is in
the northern or southern plumes respectively.
d h, N, +V -(u,h,N, - Kh, VN, )= -h, .uptake, - K'(N, - N,, )- '(N, - N, )
dt
d h P +V (u,h,P,- h VP,)- h,(uptake, - dP, )+wPo - wP
dt
d hsN +V (ush,N, - KhVN,)= -h, -uptake, - K'(N, - N)
dt
_h, P +V -(ush,Pý. - Kh,VP )= h, (uptake, -dP, )+ wP,
dt
where h is the plume thickness, u is the plume velocity, Kis the horizontal diffusivity, K'
is the mixing across the base of the layer, d is the death rate of A. fundyense, w is the
upwards swimming speed of the phytoplankton, P0 is the concentration of A. fundyense at
the source and uptake is the rate at which A. fundyense consumes the nutrients in the
plume and is defined as
uptake, =
NX +ks
where y is the maximum uptake rate and ks is the half saturation constant.
To compare the effects of the death rate and vertical swimming speed on the
concentration of phytoplankton in each plume we simplify the steady state model
equations to the following
w
dP, = -(P-P/')
h
W
P,= P,
(assuming the layer thicknesses are equal). We can rewrite these equations so that the
phytoplankton concentrations in each plume are a function only of Po and w/dh.
w( 1
S dh
1 + (w /dh))o
P- dA l+(w /dh) o
A plot of P, and P, can be seen in figure 5-1. The concentrations are equal when the
swimming rate w/h is equal to the death rate in each plume. For w/dh to increase, either
the swimming speed of the phytoplankton must increase or the thickness of each plume
must decrease. In either case, the rate of transfer of phytoplankton from the northern into
the southern plume increases. So when w/dh increases P, goes to the limit Po whereas
P, continues to increase.
25 -
I
I
I
0.5
I
1
I
20
15
10
50s
0
I
1,5
I
2
I
25
wldh
Figure 5-1 Phytoplankton concentration for the northern plume (red) and the southern
plume (blue) as they vary with w/dh.
5.2 Coupled Biological-Physical Model
5.2.1 Model Set up
For the coupled biological-physical model the physical model from chapter 4 was
used with a slightly larger domain (32 km x 512km) and a 0.5km resolution in x and y
with a 150 second time step. A seedbed of phytoplankton (P0 = 1 mmol / m3 ) was located
near 5 km offshore of the northern river mouth in a Gaussian patch with a half width of 3
km. The two rivers supply high nutrients (5 mmol / m3 ) but no phytoplankton. The
following constants were used
Po= 1 mmol/m
3
N O= 2 mmoll/ m 3
K=50 m2 /s
K'= 0.005 Iday
w = 0.1 m/day
d = 1 /day
p = 2/day
ks = 0.1 mmol/m
3
Two runs from section 4.4 were chosen to be coupled with the biological model. Run 6
which had
Q,
= 2500 and Q, = 7500 was an example of the first regime with
R, / R, <0.6 where the northern plume separated from the coast at the location of the
southern bulge. Run 10 which had
Q~
= 7500 and Q, = 2500 was an example of a case
where R, / R, > 1 and the northern plume was able to pass directly under the southern
plume.
5.2.2 Results
The first case, with Q, = 2500 and Qs = 7500, was run for a period of 20 days.
In figures 5-3 through 5-6 the plume depths, amount of nutrients (hN) and amount of
phytoplankton (hP) can be seen for both the northern and southern plumes for a period of
4, 7, 9 and 12 days respectively. As mentioned previously this is a case where the
northern plume separated from the coast at the location of the southern bulge. After 4
days phytoplankton can be seen in both the northern and southern plumes. In the northern
plume they are located just north of the southern river mouth and in the southern plume
they appear to be congregating around the outside edge of the southern bulge. After 7
days the location of the phytoplankton in the northern plume seems to have remained the
same. In the southern plume, however, they are no longer located around the outer edge
of the bulge but are now at the coast south of the southern river bulge. The source for
these appears to be upward swimming from the northern plume, followed by nutrient
fueled growth in the population. After 9 days the group of phytoplankton in the southern
plume has progressed south where there were nutrients which had been transported by the
southern plume previously but appear to have now been mostly consumed by the
phytoplankton. After 12 days it appears that, due to nutrient limitation, the phytoplankton
could no longer survive downstream of the southern river mouth and most are found
closer to the southern river bulge.
hn'Nn
hn
hn*PPn
1.6
16
2.5
1.4
'14
1.2
2
12
10
1.5
0.8
)
0.6
05
0.2
u
lu
mU
x(km)
Y
M 1U
x(10)
x(k)
hs'Ns
hs'Ns
hs'PPs
hs'PPs
fli0,01
lOB
0.08
1.5
).07
20
107
0.[]6
0.06
is5
0.05
O.O4
0.04
10
003
15
0.03
0.02
0.02
3.5
0
Y
x(km)
x(km)
x(km)
Figure 5-3 River plume depth (m), nutrient and phytoplankton amounts for the northern
and southern plumes for the first case after a period of 4 days.
hn"Nn
hn'PPn
i
14
25
7
12
6
2
10
5
1.5
8
6
4
05
2
u
AU ;J U
lu
0
x(km)
hs
10 20 30
x(km)
x(knm)
hs"Ns
··- · ·-
hs*PPs
· ·- · ·-
4
14
3.5
1.2
3
15
25
.06
E
r
2
r
0.6
10
15
0.4
5
0.5
I.I
S - ,
x(km)
U
lu
x(km)
,
3•u
U
U
zu
j
x(km)
Figure 5-4 River plume depth (m), nutrient and phytoplankton amounts for the northern
and southern plumes for the first case after a period of 7 days.
IU
hn'PPn
hnMNn
U
x(km)
0
x(km)
IU
L
J1
u
lu
U
x(kTm)
x (km)
hs*Ns
hs*PPs
10
20
x(km)
x(km)
Figure 5-5 River plume depth (m), nutrient and phytoplankton amounts for the northern
and southern plumes for the first case after a period of 9 days.
hnt Nn
hn
hn'PPn
16
16
3
5
14
45
2.5
12
2
4
3.5
10
3
i
1.5
2.5
6C
2
1
1.5
1
2
0.5
m
U
U
1U
AU
x(km)
x(kIn)
x(km)
hs
hs'Ns
hs*PPs
JU
20
8I
3,
1.5
7
16
3
3.5
14
6
12
5
10
4
2
I
3
15
6
I
2
3
0.5
n
0
x(kn)
x(ým)
x(km)
Figure 5-6 River plume depth (m), nutrient and phytoplankton amounts for the northern
and southern plumes for the first case after a period of 12 days.
The second case, which had Q, = 7500 and Q, = 2500 so that R, / R, > 1, was also run
for a period of 20 days. In figures 5-7 through 5-10 the plume depths, amount of nutrients
and amount of phytoplankton can be seen for both the northern and southern plumes for a
period of 4, 6, 10 and 14 days respectively. As previously mentioned this is a case where
the northern plume was able to pass directly under the southern plume. After a period of
4 days the phytoplankton in the northern plume are directly under the source of the
southern plume. The phytoplankton in the southern plume are located near the nutrients
in the northern plume. After 6 days phytoplankton patches in both plumes have been
advected south along the coast. There seem to be two areas of high nutrients in the
southern plume, one near the southern river mouth and one slightly downstream. The
phytoplankton patch in the southern plume is located near the more southern nutrient
patch whereas the northern phytoplankton patch is near the region of high nutrients next
to the southern river mouth. After both 10 and 14 days the nutrients in both plumes are
located relatively near the source and there are relatively little nutrients downstream. So,
as can be expected, the phytoplankton patches in both plumes are now located closer to
the river mouths.
hn
hn'Nn
hn*PPn
30
30
5
4.5
g.9
0.9a
El.8
0.8
25
25
•.7
0.7
4
20
3.5
3.8
0.6
20
3
0.5
E
?i.
15
25
0.4
0.4
2
0.3
0.3
10
Q.3
0.2
1
S
5
0.1
0.5
U u
Ai
U
U
U
x(kem)
hs
IU
lu
£U
Axi A
x(kýn)
x(knm)
hs'Ns
hs*PPs
0
- --
1.25
2.5
2.5
16
1.2
14
12
2.15
1.5
10
8
4
0.5
2
u iU zu JU
x(kn)
x(km)
0n
x(km)
Figure 5-7 River plume depth (m), nutrient and phytoplankton amounts for the northern
and southern plumes for the second case after a period of 4 days.
hn*PPn
hn'Nn
30
5
3.5
4.5
25
3
3.5
2.5
3
)
2
2.5
2
1.5
1
5
1
0.5
0.5
In
n
in
u
nn
I
lu ur
vU
W
ru
x(IqnW)x(m)
x(•n)
X(m)
(•n)
xhsNs
.5
hs'Ns
hs'PPs
x(ten)
hs*PPs
.8
1.6
12
1.4
10
1.2
1.5
I
1
8
)
3.13
0.8
6
1
+.G
0.6
4
0.4
0.4
2
nu
x(km)
x(km)
x(km)
Figure 5-8 River plume depth (m), nutrient and phytoplankton amounts for the northern
and southern plumes for the second case after a period of 6 days.
0
hn'PPn
x(km)
x(km)
hs*Ns
hs*PPs
10 20 30
x(km)
u
hn'Nn
IU za
x(km)
U
IU
/U
x(km)
U
1U
LU
U
x(km)
Figure 5-9 River plume depth (m), nutrient and phytoplankton amounts for the northern
and southern plumes for the second case after a period of 10 days.
hn*PPn
hn'Nn
hn
25
25
5
B
7
20
20
6
4
3
)
15
5
i
4
10
2
3
2
5
1
n
w
x(hn)
x(n)
x(On)
hs
hs*Ns
hs*PPs
12
4
10
I
.5
3
6o
2
1
3.5
30
20
10
0
x(km)
V
lu
fu
x(km)
JJ
0
x(km)
Figure 5-10 River plume depth (m), nutrient and phytoplankton amounts for the northern
and southern plumes for the second case after a period of 14 days.
78
5.3 Summary and Conclusions
Here we presented a coupled biological-physical model. It was used to test the
effects of river fluxes on the distribution of phytoplankton in a two plume system. In the
first case which, had
Q, = 2500
and Q, = 7500 and R, / R, <0.6, the northern plume
separated from the coast at the location of the southern bulge. It was thought that this
could possibly prevent the phytoplankton from reaching the coast at the south of the
southern river mouth. After a period of days, results from the coupled biologicalphysical model showed that in fact, the phytoplankton in the northern plume were north
of the southern river mouth and the phytoplankton which swam up into the southern
plume were located on the very edge of the southern bulge and not near the coast.
However, after 7 days the phytoplankton patch in the southern river was located along the
coast near the southern bulge. So it appears as if the southern bulge was able to
temporarily prevent the phytoplankton from reaching the coast, however, this state could
not be maintained. In the second case which had
Q,,
= 7500 and
Q, = 2500 and R, / R,>
1, the northern plume was able to pass directly under the southern plume. In this
circumstance the phytoplankton patches remained along the coast and their position along
the coast seemed to vary according to nutrient levels in the plumes.
A major concern when dealing with A. fundyense in the Gulf of Maine is whether
it is able to get to shore where it can be consumed by shellfish. The purpose of much of
this study was to investigate if a southern plume could prevent phytoplankton from the
northern plume from reaching the coast. Based on our modeled results, it appears as if the
southern river under certain conditions could temporarily shield the coast from A.
fundyense but that this was not a permanent state.
Bibliography
Anderson, D.M., 1980. Effects of temperature conditioning on development and
germination of Gonyaulax tamarensis(Dinophyceae) hypnozygote. Journal of
Phycology. 16, 166-172.
Anderson, D.M., 1997. Bloom dynamics of toxic Alexandrium species in coastal waters.
Limnology and Oceanography. 42, 1009-1022.
Anderson, D.M., 1998. Physiology and bloom dynamics of toxic Alexandrium species,
with emphasis on life cycle transitions. In: Anderson, D.M., A.D. Cembella and
G.M. Hallegraeff (eds.), Physiological Ecology of Harmful Algal Blooms.
Springer, Berlin, pp. 29-48.
Anderson, D.M. and N.L. Lindquist, 1985. Time-course measurements of phosphorous
depletion and cyst formation in the dinoflagellate Gonyaulax tamarensisLebour.
Journal of Experimental Marine Biology and Ecology. 86(1), 1-13.
Anderson, D.M., B.A. Keafer, 1987. An endogenous annual clock in the toxic marine
dinoflagellate Gonyaulax tamarensis.Nature. 325, 616-617.
Anderson, D.M., C.D. Taylor, E.V. Ambrust, 1987. The effects of darkness and
anaerobiosis on dinoflagellate cyst germination. Limnology and Oceanography
32, 340-351.
Anderson, D.M., B.A. Keafer, W.R. Geyer, R.P. Signell, T.C. Loder, 2005a. Toxic
Alexandrium blooms in the western Gulf of Maine: the "plume advection
hypothesis" revisited. Limnology and Oceaongraphy. 50, 328-345.
Anderson, D.M., C.A. Stock, B.A. Keafer, A.B. Nelson, B. Thomson, D.J. McGillicuddy,
M. Keller, P.A. Matrai, and J. Martin, 2005b. Alexandriumfundyense cyst
dynamics in the Gulf of Maine. Deep Sea Research. 52. 2522-2542.
Bigelow, H.B., 1927. Physical Oceanography of the Gulf of Maine. Fisheries Bulletin.
40. 511-1027.
Brooks, D.A., 1985. Vernal Circulation in the Gulf of Maine. Journal of Geophysical
Research, 90, 4687-4705.
Fong, D.A., and W.R. Geyer, 2002. The alongshore transport of freshwater in a
surface-trapped river plume. J Phys. Oceanogr., 32, 957-972.
Franks, P.J. S. and D.M. Anderson, 1992a. Alongshore transport of a toxic phytoplankton
bloom in a buoyancy current: Alexandrium tamarense in the Gulf of Maine.
Marine Biology, 112, 153-164.
Franks, P.J.S. and D.M. Anderson, 1992b. Toxic phytoplankton blooms in the
southwestern Gulf of Maine: testing hypotheses of physical control using
historical data. Marine Biology. 112, 165-174.
Garcia Berdeal, I., B.M. Hickey, and M. Kawase, 2002. Influence of wind stress and
ambient flow on a high discharge river plume.
J Geophys. Res., 107, 3130,
doi: 10. 1029/2001 JC000932.
Garvine, R.W., 1999. Penetration of buoyant coastal discharge onto the continental shelf:
a numerical model experiment. J. Phys. Oceanogr., 29, 1892-1909.
Garvine, R.W., 2001. The impact of model configuration in studies of buoyant coastal
discharge. J Mar. Res., 59, 193-225.
Geyer, W.R., R.P. Signell, D.A. Fong, J. Wang, D.M. Anderson, B.A. Keafer, 2004. The
freshwater transport and dynamics of the western Maine coastal current. Deep Sea
Research. 24, 1339-1357.
Griffiths, R.W. and E.J. Hopfinger, 1983.
Gravity currents moving along a lateral
boundary in a rotating frame. J. FluidMech., 134, 357-399.
Hetland, R.D., D. J. McGillicuddy, and R. P. Signell, 2002. Cross-frontal entrainment of
plankton into a buoyant plume: The frog tongue mechanism. Journal of Marine
Research. 60: 763-777.
Hetland, R.D., 2005. Relating river plume structure to vertical mixing. J Phys.
Oceanogr., 35, 1667-1688.
Keafer, B.A., K.O. Buesseler, D.M. Anderson, 1992. Burial of living dinoflagellate
cysts in estuarine and nearshore sediments. Marine Micropaleontology 20, 147161.
Keafer, B.A., J.H. Churchill, and D.M. Anderson, 2005a. Blooms of the toxic
dinoflagellate, Alexandrium fundyense in the Casco Bay region of the western
Gulf of Maine: Advection from offshore source populations and interactions with
the Kennebec River plume. Deep Sea Research. 52. 2631-2655.
Keafer, B.A., J.H. Churchill, D.M. McGillicuddy, and D.M. Anderson, 2005b. Bloom
development and transport of the toxic Alexandriumfundyense populations within
a coastal plume in the Gulf of Maine. Deep Sea Research. 52. 2674-2697.
Leursen, R.M., 2001. Relationships between oceanographic satellite data and
Alexandrium distributions in the Gulf of Maine. Masters Thesis, University of
Maine, Orono, Maine. p.91.
Leursen, R.M., A.C. Thomas and J.Hurst. 2005. Relationships between satellite-measured
thermal features and Alexandrium-imposed toxicity in the Gulf of Maine. Deep
Sea Research. 52, 2656-2673.
Martin, J.L., and A. White, 1988. Distribution and abundance of the toxic dinoflagellate
Gonyaulax excavate in the Bay of Fundy. Canadian Journal of Fisheries and
Aquatic Sciences 45, 1968-1975.
Martin, J.L., and D.J. Wildish, 1994. Temporal and spatial dynamics of Alexandrium
cysts during 1981-1984 and 1992 in the Bay of Fundy. In: Forbes, R. (Ed.),
Proceedings of the Fourth Canadian workshop on Harmful Marine Algae.
Canadian Journal of Fisheries and Aquatic Sciences. p. 2016.
McGillicuddy, D.J., R.P. Signell, C.A. Stock, B.A. Keafer, M.D. Keller, R.D. Hetland,
and D.M. Anderson, 2003. A mechanism for offshore initiation of harmful algal
blooms in the coastal Gulf of Maine. Journal of Plankton Research. 25: 11311138.
McGillicuddy, D.J., D.M. Anderson, D.R. Lynch, and D.W. Townsend, 2005.
Mechanisms regulating large-scale seasonal fluctuations in Alexandrium
fundyense populations in the Gulf of Maine: Results from a physical-biological
model. Deep Sea Research. 52. 2698-2714.
O'Donnel, J., 1990. The formation and fate of a river plume: A numerical model. J
Phys. Oceanogr., 20, 551-569.
Pettigrew, N.R., D.W. Townsend, H. Xue, J.P. Wallinga, P.J. Brickley, R.D. Hetland,
1998. Observations of the eastern Maine Coastal Current and its offshore
extensions in 1994. Journal of Geophysical Research 103, 623-639.
Pettigrew, N.R., J.H. Churchill, C.D. Janzen, L.J. Mangum, R.P. Signell, A.C. Thomas,
D.W. Townsend, J.P. Wallinga, and H. Xue, 2005. The kinematic and
hydrographic structure of the Gulf of Maine Coastal Current. Deep Sea Research.
52. 2369-2391.
Shumway, S., S. Sherman-Caswell, J.W. Hurst, 1988. Paralytic Shellfish poisoning in
Maine: monitoring a monster. Journal of Shellfish Research, 7. 643-652.
Stock, C.A., D.J. McGillicuddy, A.R. Solow and D.M. Anderson, 2005. Evaulauting
hypotheses for the initiation and development ofAlexandrium fundyense blooms
in the western Gulf of Maine using a coupled physical-biological model. Deep
Sea Research. 52, 2715-2745.
Townsend, D.W., N.R. Pettigrew, A.C. Thomas, 2001. Offshore blooms of the red tide
dinoflagellate Alexandrium spp., in the Gulf of Maine. Continental Shelf
Research. 21, 347-369.
White, A.W., and C.M. Lewis, 1982. Resting cysts of the toxic, red tide dinoflagellate
Gonyaulax excavate in Bay of Fundy sediments. Canadian Journal of Fisheries
and Aquatic Sciences. 39, 1185-1194.
Yankovsky, A.E. and D.C. Chapman, 1997. A simple theory for the fate of buoyant
coastal discharges. J. Phys. Oceanogr., 27, 1386-1401.