Seasonal variability in temperature, salinity, dissolved oxygen and nutrients content
in the San Juan Channel in Fall, 2007
By Theadora Tolkin
Pelagic Ecosystem Function Apprenticeship,
Friday Harbor Labs, University of Washington
Abstract
The San Juan Archipelago forms the boundary through which all exchange flow between
the Strait of Georgia and the Strait of Juan de Fuca is constricted. The goal of this project
was to describe the variability, as well as mechanisms driving the variability, of
temperature, salinity, dissolved oxygen and nutrient content within and directly south of
San Juan Channel between September and November 2007. A Seabird Seacat SBE-19
CTD profiler was used to measure temperature and Salinity at two sites, North Station
and South Station, in and around San Juan Channel on six cruises. Discrete-depth
samples taken with a Niskin rosette were analyzed for dissolved oxygen and nutrient
content. On two additional cruises, two additional sites at the far south end of San Juan
Channel, Cattle Pass North and Cattle Pass South, were sampled for temperature and
salinity over the course of a tidal cycle to assess the dynamics of tidal mixing on either
side of a sill and constriction there. At North Station, waters were found to be wellmixed and generally less dense than at South Station. Temperature and nutrient content
decreased over the season, while dissolved oxygen content increased. South Station was
more strongly stratified, with surface waters showing a similar trend to North Station, and
deep waters remaining consistently colder, more saline, less oxygenated and higher in
nutrients. Winds played the dominant role in shaping the structure of water
characteristics at North Station, while tides were more important at South Station.
Introduction
The San Juan Archipelago forms the boundary between the Strait of Georgia and
the Strait of Juan de Fuca (Fig. 1). The Strait of Georgia is a large semi-enclosed estuary
bounded by the B.C. mainland and Vancouver Island, fed primarily by the Fraser River,
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and connected to the Pacific Ocean by the Strait of Juan de Fuca. Waters in the Strait of
Georgia bear a brackish, relatively warm signature in the Fall whereas waters in the Strait
of Juan de Fuca are colder and more saline (Masson 2006, Thomson 1981, Waldichuk
1957). Redfield (1950) characterized five water masses typically found mixing in the San
Juan Archipelago. These water masses each have a discrete origin (Fig. 2), and can be
identified by their signature ranges of temperature and salinity (Table 1).
Oceanographers plot temperature against salinity to identify which water masses are
present in a given sample. Water masses of different origins tend to stratify, with colder,
saltier, and therefore denser water masses layering deeper than warmer, less salty water
masses.
Density stratification is a key metric used by oceanographers to characterize their
study sites. Newton (2002) characterized density stratification at sites throughout the
inland waters of Washington, with results ranging from strong and persistent to weak and
infrequent stratification. A strongly stratified water column is one in which very little
mixing between two or more water masses has occurred. Distinct water masses will mix
in response to sufficient physical forcing such as winds (Goodrich 1987) or turbulence
caused by bathymetric features obstructing current flow (Valle-Levinson and Atkinson
1999). The character of density stratification at a site, together with information about
water masses based on temperature and salinity measurements there, can tell us much
about how various external forcing mechanisms are acting on local water masses to bring
them to a site. A combination of tidal energy and abundant discharge from the Fraser
River drive estuarine exchange with coastal shelf water through the Strait of Juan de Fuca
(LeBlond 1991, Waldichuk 1957). Complex bathymetry in the San Juan Archipelago,
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through which all exchange flow between the Straits of Georgia and Juan de Fuca is
constricted, creates an environment of strong mixing.
The Pelagic Ecosystem Function apprenticeship at the University of Washington's
Friday Harbor Laboratories was initiated in 2005 to investigate physical oceanography
and faunal distribution within and around San Juan Channel during the Fall months. The
oceanographic results of these investigations have shown a weakly stratified (according
to methods developed in Newton, 2002) water column within the channel, though the
properties borne by that mixed water column vary on a meteorological, and seasonal
basis (Siple, 2006). Contrastingly, waters just 5km south of the channel have consistently
been shown to be more frequently and strongly stratified, with temperature and salinity
patterns in the upper 30m comparable to the patterns found within-channel. Typically,
there is little variability in the much colder and more saline bottom waters there (Siple,
2006). Despite general adherence to these patterns, apprenticeship data has displayed a
large amount of variance in the range of values for all oceanographic parameters
(temperature, salinity, dissolved O2 content, fluorescence and nutrients) from one year to
the next. In order to reduce variance and distinguish between observations caused by
stochastic processes and those that may be due to larger-scale climatic oscillations such
as the Pacific Decadal Oscillation and El Nino Southern Oscillation (Doo 2005), more
years of observations through changing global climatic conditions is needed.
It was the goal of this project to describe the variability, as well as mechanisms
driving the variability, of temperature, salinity, dissolved oxygen and nutrient content in
San Juan Channel between September and November 2007. One objective was to sample
for all variables at two sites in San Juan Channel on weekly cruises over that study
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period. The second objective was to monitor changes only in temperature and salinity
over the course of a tidal cycle at the most bathymetrically complex point in San Juan
Channel: Cattle Pass.
Methods
North and South Site Characterization
To monitor seasonal changes in the water quality, we sampled at “North Station,”
(4835.00’N, 12302.50’W) and “South Station,” ( 4825.20’N, 12256.60’W) (Fig. ).
These two sites were each sampled on 9/27, 10/4, 10/17, 10/23, 10/30/07 and 11/14/07
with the sample time for each station separated by approximately four hours. A Seabird
Seacat SBE-19 CTD (Conductivity, Temperature, Depth) scanner was used to sample for
temperature and salinity. A Niskin rosette was used to collect water samples at six
discrete depths between the bottom and the surface to be analyzed in the lab for dissolved
oxygen and nutrients content. Nutrient samples were sent to [what] lab at the University
of Washington and analyzed for phosphate, silicate, nitrate, nitrite and ammonium.
Dissolved oxygen content was measured according to the Carpenter modification of the
Winkler titration method (Codispodi 1988), using a Beckman Dosimat microburet, and
used to calibrate dissolved oxygen measurements taken by the CTD (Fig. 3). CTD data
was processed using Seabird software, and plotted using SigmaPlot.
Tidal Dynamics at Cattle Pass
To observe the patterns of tidal mixing around Cattle Pass, we sampled for
temperature and salinity using a Seabird Seacat SBE-19 CTD at “Cattle Pass North,” (48
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28.47N, 122 57.54W) and “Cattle Pass South,” (48 25.83N and 122 56.41W) (Fig. 4).
These two sites were each sampled twice a day on 10/25 with the time for each cast
separated by about two hours. On 11/7, Cattle Pass North was sampled twice, and Cattle
Pass South was sampled only once.
Use of External Parameters
Tide height data were taken from the NOAA station #9449880 at Friday Harbor,
WA. The maximum and minimum tide heights for the 12 hours previous to the time of
sampling were used to calculate tidal exchange relevant to each cast.
Wind speed and direction as well as air temperature was taken from NOAA
NDBC buoy #46088 at New Dungeness. Prevailing wind direction was plotted in a polar
histogram using Matlab 7.0. The winds for the 48 hours previous to the date of sampling
were used for this. Average wind speed for the same period was also calculated and
included in our analysis.
Sea surface temperature for the Pacific Ocean at the mouth of the Strait of Juan de
Fuca was taken from NOAA NDBC buoy #46087 at Neah Bay. Temperatures for each
24-hour period between September 25 and November 14 were averaged to create a single
data point for each day, and plotted in Sigma Plot.
Fraser River discharge and temperature were taken from Environment Canada
station #08MF040 above Texas Creek, BC.
Results
North and South Station
Density Stratification
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North and South Stations display similar patterns of density stratification over time
(Fig. 5). However, on any given day, the depth and slope of the pycnocline vary greatly
between stations.
Table 2 shows how the density stratification results of this study fit
into the context of the inland waters as studied by Newton (2002).
North Station
The waters at North Station were weakly stratified on all cruises (Fig. 6), with an
average Δσ-τ value of 0.42. The least stratified day was October 23. with a Δσ-t of 0.13.
The most stratified day was October 17, with a Δσ-t of 0.88. The total density range for
all cruises was between 23.08 σ-t, which occurred on the surface on October 30 and 24.1
σ-t, which occurred on the bottom on October 4. October 30 and October 4 were the least
dense and most dense days, respectively.
South Station
The waters at South Station were moderately and persistently stratified, with an
average Δσ-τ value of 1.63 (Fig. 7). The least stratified day was October 23, with a Δσ-τ
of 1.24. The most stratified day was September 27, with a Δσ-τ of 2.03. The total
density range for all cruises was between 23.52 σ-τ at the surface on September 27 and
25.65 σ-τ at depth on October 4. The days of least and greatest average surface (top
20m) density were September 27 and October 4, respectively. Those densities were
23.65 σ-τ on September 27 and 23.98 σ-τ on October 4. The days of least and greatest
average density at depth (to 20m above the bottom) were October 30 and October 4,
respectively. Those densities were 23.34 σ-t on October 30 and 24.04 σ-t on October 4.
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Temperature, Salinity, Dissolved Oxygen
North Station
Water temperature at North Station went from an average of 10.07C on September
27 to 8.97C on November 14. The temperature of the water sampled on the intervening
dates was an average of 9.59C, with all temperatures falling within a range of 0.14C
(Fig. 8). Salinity was variable and showed no seasonal trend. The freshest day was
October 30, with an average salinity of 30.14 PSU. The Saltiest day was October 4, with
an average salinity of 31.17 PSU. The total range of salinity was between 29.92 PSU on
the surface on October 30 to 31.21 PSU at the bottom on October 4 (Fig. 9). On a plot of
temperature vs. salinity, all waters sampled fell within the range of Type D&C Mixed
water according to Redfield (1950) (Fig. 10).
Measured dissolved oxygen content increased steadily, from an average of 4.83
μg/L on September 27 to 6.04 μg/L on November 14 (Fig. 11). Percent saturation levels
also increased over the season, from 52.01% on September 27 to 63.67% on November
14. The exception to this was between October 23 and October 30, average measured
dissolved oxygen content decreased from 5.96 μg/L to 5.95 μg/L and percent saturation
decreased from 63.62% to 63.29%.
South Station
Average water temperature for the surface waters (top 20m) at South Station
decreased steadily over the course of all six cruises, from 10.15C on September 27 to
8.87C on November 14. Average water temperatures at depth (bottom 20m) ranged
from 8.97C on October 23 to 8.13C on October 4, but did not show a seasonal trend
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(Fig. 12). Average surface salinity also showed no seasonal trend, ranging between 30.81
PSU on September 27 and 31.17 PSU on October 4. Salinity at depth ranged between
31.99 PSU on October 23 and 32.92 PSU on October 4, showing no seasonal trend (Fig.
13). On a plot of temperature vs. salinity, waters towards the surface fell within the
range of water Types D&C mixed, while deeper waters tended to fall within water Types
C, or B (Fig. 13)
Dissolved oxygen content of the surface waters generally increased over the course
of the season, from 5.32 μg/L on September 27 to 7.33μg/L on November 14, with the
exception of a marked drop from 6.79 μg/L on October 23 to 6.21 μg/L on October 30
(Fig. 14). Percent saturation of the surface waters showed the same trend. Neither
dissolved oxygen content nor percent saturation levels of the deep waters showed any
seasonal trend. The minimum dissolved oxygen content occurred on October 4, at 3.93
μg/L. Maximum dissolved oxygen content occurred on October 23, at 5.4 μg/L.
Nutrients
Data for these nutrients was only available for the three cruises September 27October 17.
North Station
Phosphate concentrations increased over the course of the first three cruises, with
greater concentrations at depth. Silicate concentrations increased at depth and near the
surface, but did not show an increase in the mid-water. Nitrate concentrations increased
over the study period, with greater overall concentrations at depth. Nitrite concentrations
decreased over the study period. Ammonium concentrations were very low, and
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decreased to trace quantities over the course of the study period (Fig. 15 a-d)
South Station
Phosphate concentrations were found only at depth at the beginning of the study
period, but shoaled over time. Silicate concentrations increased during the study period
only in the surface and deep waters, but remained low in the mid-water. Nitrate
concentrations began the study period high only at depth, but shoaled over time. Nitrite
concentrations were high at the surface only in the beginning of the season, lower at
depth, and decreased at all depths over time (Fig. 16 a-d).
North and South Tidal
October 25
North Tidal
Two samples, taken from North Cattle Pass at 0830h and 1415h showed a very
narrow range of temperature and salinity, and were extremely well-mixed, with a Δσ-τ of
0.03 and 0.02, respectively. The morning cast, at 0830h, had an average temperature of
9.49C and a range of 0.01C. The average salinity of the water sampled at 0830 was
30.94 PSU, with a range of 0.04 PSU. The average temperature of the sample taken at
1415h was also 9.49C, but with a range of 0.02C. The average salinity of the same
water was 31.02 PSU, with a range of 0.03PSU. Tide height at 0830h was 1.24m on a
slow ebb, with a current speed of -2.86 kts. Tide height at 1415h was 1.89m on a slow
flood, with a current speed of 2.62 kts (Fig. 17).
South Tidal
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Two samples taken at South Cattle pass at 1030h and 1225h showed wider ranges
of temperature and salinity, with Δσ-τ of 0.11 and 0.79, respectively. The waters
sampled at 1030h had an average temperature of 9.49C and a range of 0.03C. The
average salinity of those waters was 30.95 PSU, with a range of 0.14 PSU. Waters
sampled at 1225h had an average temperature of 9.49C, with a range of 0.37C. The
average salinity of those waters was 31.06 PSU, with a range of 0.94 PSU. Tide height at
1030h was 1m at the end of a slow ebb with a current speed of -1.25 kts. Tide height at
1225h was 1.3m at the beginning of a slow flood with a current speed of 2.04 kts (Fig.
18).
November 7
North Tidal
Two samples, taken from North Cattle Pass at 1250h and 1630h exhibited varying
but overlapping ranges of temperature and salinity values, and stratification values of
0.76 Δσ-τ and 0.35 Δσ-τ, respectively. The temperature of the water at 1250h ranged
from 9.28C at the surface to 8.87C at depth. Salinity at that time ranged from 30.51
PSU at the surface to 31.4 PSU at depth. The temperature of the water at 1630h ranged
from 9.24C at the surface to 9.05C at depth. Salinity at that time ranged from 30.56
PSU at the surface to 30.98 PSU at depth. Tide height at 1250h was 1.95m, at the end of
a slow flood, with a current speed of 1.73 kts. Tide height at 1630h was 1.73m, in the
middle of a fast ebb, with a current speed of -2.31 kts (Fig. 19).
South Tidal
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One sample was taken at this location on the south side of Cattle Pass at 1445h.
Temperature ranged from 9.23C at the surface to 9.05C at the bottom. Salinity ranged
from 30.56 PSU on the surface to 32.97 PSU at the bottom. The stratification index was
2.07 Δσ-τ. Tide height at this time was 2.06m, at slack high tide, with a current speed of
-0.12 kts (Fig. 20).
Discussion
Water Masses
Waters at North and South Stations were found to possess different basic
characteristics as well as divergent responses to external influences such as winds, tides,
oceanic input as a result of coastal upwelling, and freshwater input.
North Station was more well-mixed than South Station on every date sampled.
Even in this mixed water column, density stratification was such that surface (10m) and
bottom (10m) characteristics can be considered separately, and each associated with
distinct external forcing mechanisms. Figure 21 shows the patterns of temperature
change in the North Station surface and deep waters, as well as the change in air
temperature at New Dungeness, and in the temperature of the waters of the Fraser River
and of the sea surface at Neah Bay. Correlating any of North Station patterns to the
oceanic or atmospheric patterns with statistical significance is difficult because the speed
at which water is brought to our study site is unknown. However, the temperature of the
surface waters, which in temperature and salinity are most closely related to water mass
of type D (originating in the surface waters of the Strait of Juan de Fuca and the
intermediate waters of the Strait of Georgia) bear a stronger resemblance to the pattern of
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change of air temperature and Fraser River temperature than did the deep waters over the
season. Deep water temperatures, which are more closely related to water mass of type C
(originating in the Pacific Ocean surface zones) bear a stronger resemblance to the
change in temperature of sea surface temperature at Neah Bay, which is located in the
Pacific Ocean at the mouth of the Strait of Juan de Fuca.
At Neah Bay, sea surface temperature fluctuates based on patterns of upwelling
on the outer coast. Upwelling as a result of southerly winds along the outer coast would
bring oceanic deep waters, which are colder, more saline, and nutrient-rich (Garvine
1971) into the San Juan Archipelago system. Downwelling results in the input of waters
characteristic of the ocean surface. These are warmer, relatively fresher and nutrientpoor waters (Beer 1997). A period of strong downwelling began on September 30,
lasting until October 24, during which time was seen an increase in sea surface
temperature at Neah Bay, as well as in the bottom waters of North Station (c.f. Fig. 21).
The effects of mixing between the surface and deep waters on temperature at
North Station reflects the influences of both atmospheric cooling, and warming as a result
of oceanic downwelling on the outer coast. The maintenance of an almost constant
temperature for the four cruises between October 4 and October 30 is likely a result of the
mixing of increasingly colder estuarine water with increasingly warmer oceanic surface
water. But because salinity did not change progressively during the study period, it can be
concluded that there is variability in the relative influences of waters from the Pacific
Ocean and the Strait of Georgia/Strait of Juan de Fuca on North Station, and the causes of
this variability merit investigation.
Factors determining the relative presences of Pacific Ocean vs. Georgia Strait/Strait
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of Juan de Fuca water, show influence from both tides and winds. It is known that winds
from the north, coincident with the neap tide enhance the volume of lower σ-t water
advected into the San Juan Archipelago (Griffin and Leblond 1990). However, these
conditions did not occur during our study period. During this season, southerly winds
dominated (Fig. 22). Days of northerly winds were not coincident with neap tides.
Nonetheless, surface (10m) salinity did correlate with wind direction, with the saltiest
days occurring on days of the most southerly winds (c.f. Fig. 21). This could be a result
of the plume of the Fraser River being pushed towards our study site by those more
northerly winds. More investigation is required to determine mechanisms driving the
structural details of stratification throughout the water column at North Station.
The stratified waters at South Station possess waters of two distinct origins. On the
surface, all temperature and salinity fall within the range of "Mixed C and D" as
described by Redfield (1950): the surface waters at South Station are the product of
mixing of Pacific Ocean surface water, and water originating in the Strait of Juan de
Fuca. Deeper waters fall within a range indicating mixing between Pacific Ocean middepth water, and Juan de Fuca-Georgia Strait waters (Fig. 21). The signals from other
variables are consistent with this: South Station surface waters are also well-oxygenated
and have lower nutrients concentrations (c.f. Fig. 16), which is characteristic of surfaceoriginating water masses. Deep waters at South Station bear the typical deeper oceanic
signature of low temperature, high salinity, low oxygen, and high nutrients
concentrations.
The pattern of cooling in the surface waters at South Station is different from that at
North Station, indicating that although the same water masses are mixing at both stations,
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the mechanisms driving the mixing are different for each. The steady decrease in
temperature over the season indicates a more atmospheric effect on temperature.
On the
other hand, a consistently higher and less variable salinity signal in the South Station
surface waters than at North Station indicates that there is more of an oceanic influence
here. Pacific Ocean mid-depth waters may be exerting an influence here, even though
the plot of Temperature vs. Salinity for the surface does not fall within category B. It is
possible that as incoming Pacific Ocean surface water was becoming warmer, and mixing
to a relatively constant temperature with waters from the Strait of Georgia/Strait of Juan
de Fuca over the month of October, the presence of colder Pacific Ocean mid-depth water
in the South Station deep layer influenced the South Station surface waters, bringing
temperature down progressively.
We would expect then that the observations of lower surface salinity at South
Station would coincide with when the presence Pacific Ocean intermediate water was not
as strong in the deeper layer. This happened on two days, September 27 and October 23.
The surface layer extended to the greatest depths on those days, and the waters at the
bottom did not bear as strong of a resemblance to Pacific Ocean intermediate water in
their temperature and salinity characteristics. These two days exhibited the weakest tidal
exchanges (Fig. 23), indicating that tides play a role in bringing Pacific Ocean mid-depth
water into the San Juan Archipelago, and are an important factor in structuring the waters
at South Station. More investigation will be required to determine whether the influence
is stronger on the daily or monthly tidal cycle.
Dissolved Oxygen
Physical and biological factors control the amount of dissolved oxygen in water.
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Temperature and salinity affect the solubility of water, with colder, saltier water able to
hold more oxygen at saturation level than warmer, fresher water (Beer 1997). Activity of
photosynthetic organisms increases the amount of dissolved oxygen in water, due to the
release of oxygen into the organisms' surroundings by photosynthesis. Dissolved oxygen
is removed from water with respiration by both photosynthetic and heterotrophic
organisms. These processes are operating independently (Kemp and Boynton 1980).
Winds also play an important role in the infusion of water with dissolved oxygen.
Oxygen is always diffusing over the air-water boundary and further down through the
water column. Winds intensify this diffusion by creating breaking surface waves that
aerate the water and dissolve atmospheric gasses into it (Beer 1997).
Because the abundance of photosynthetic organisms remained fairly constant
during the study period (Pennington 2007), and photosynthetically active radiation (PAR)
decreased (Fig. ), it cannot be considered that photosynthetic activity was responsible for
the increase in dissolved oxygen content observed over the course of the season.
Ammonium concentrations, which are directly related to herbivory and thus can be a
proxy for a portion of the respiratory processes occurring to remove dissolved oxygen
from the water, decreased after the first cruise date. It may be hypothesized that a
decrease in overall respiration over the course of the season was in part responsible for
the increase in dissolved oxygen, but this can not be tested becausee we did not measure
respiration directly. The steady increase in dissolved oxygen content and percent
saturation over the course of the season at North Station and in the South Station surface
waters could also be related to the increase in wind speed and duration during the study
period (Fig. 24). A final factor in the increase of dissolved oxygen content and percent
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saturation could be changing character of the Pacific Ocean surface water at our study
sites as a result of downwelling off the coast. Though the rates of this input could not be
measured precisely, it is known that coastal downwelling brings larger quantities of
oxygen-rich open-ocean surface water to coastal areas (Beer 1997).
Tidal Dynamics at Cattle Pass
Some preliminary information was gathered regarding the way tidal exchange
within one day, and tide height affect the input of oceanic water into San Juan Channel
around Cattle Pass. On two cruises, water was sampled on the north and south sides of
Cattle Pass over part of a tidal cycle. It was found that water on both sides of Cattle Pass
was well-mixed on ebb tides, while the flood tide brought denser water to both sides of
Cattle Pass. Flood tide and slack high waters south of Cattle Pass were more stratified
than the flood tide waters north of Cattle Pass. Flood tide waters north of Cattle Pass
were more stratified than ebb tide waters, with the day of stronger stratification,
November 7, corresponding to the weaker tidal exchanges.
Strong tides disrupt density structure on the spatial scale of a few kilometers, and
the temporal scale of a few hours. This is consistent with observations in LeBlond
(1991), describing the effects of spring vs. neap tides on the flow of oceanic water into
Strait of Georgia deep and intermediate waters. In that study, it was observed that
weaker tides left density stratification intact through the San Juan Archipelago, creating
the necessary potential energy to foster a buoyancy current of dense oceanic water
flowing beneath less dense Georgia Strait water into the deep and intermediate zones of
the Strait of Georgia. This was in contrast to what was observed at South Station, where
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the weakest tides on a fortnightly scale displayed the lowest degrees of density
stratification. While this project was not designed to investigate the development of
inter-strait buoyancy currents through San Juan Channel, it is possible that the tidal
dynamics observed around Cattle Pass are related to this phenomenon. The limited time
and sample size available to this study reduce our ability to further understand inter-strait
tidal dynamics. A study of similar design over a longer portion of a single year would
lead to valuable insights regarding the dynamics of estuarine exchange and deep-water
return flow through San Juan Channel, which is necessary to understanding the
variability and mechanisms behind habitat characteristics in the pelagic zone here.
Acknowledgements
This research would not have been possible without the generous contributions of Friday
Harbor Laboratories, the University of Washington and the Washington Research
Foundation. I would also like to thank Jessica Gill and Randy Jones for their assistance
with data collection aboard the R. V. Centennial, as well as the captains of the ship, Dr.
Dennis Willows, Don English, Mark Anderson and Dr. David Duggins, and Craig Staude
and Alan Cairns for their tireless patience and technical support. I must also extend
gratitude to my family, Vannimal and Dad. Finally I would like to thank my wonderful
teaching staff: Dr. Jan Newton, Dr. Sandra Parker-Stetter, Dr. Breck Tyler and Jen
Nomura for the time and energy they spent challenging me and supporting me to become
a better researcher and scientist.
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