cleveland bay report
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James Cook University *
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2006
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Date
Jan 9, 2024
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docx
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Salinity: Measuring Salinity and Residence Across Cleveland Bay
1.
For each site calculate the depth averaged salinity:
Station 1:
35.53 ppt
Station 2:
35.50 ppt
Station 3:
35.51 ppt
Station 4:
35.49 ppt
Station 5:
35.44 ppt
Station 6:
35.49 ppt
Station 7:
35.55 ppt
Station 8:
35.25 ppt
Station 9:
35.37 ppt
Station 10:
35.28 ppt
Station 11:
35.50 ppt
Station 12:
35.59 ppt
Station 13:
35.52 ppt
Station 14:
35.75 ppt
The depth – averaged salinity was calculated by removing the first ten and last ten salinity
measurements at each station as these measurements were taken at surface depths (~1m-
10m) which can have extremely variable salinity measurements that produce unreliability
due to excessive surface mixing. Figure 1 shows the depth – averaged salinity calculated
from the remaining salinity measurements provided by the CTD cast.
2.
On a map of Cleveland Bay, write the salinity of each profile station and draw
contours on by hand.
The latitude and longitude co – ordinates were taken from the CTD data and confirmed using
the metadata for each station. Using ArcGIS Pro, stations were placed, and corresponding
salinities were inputted. Finally, contour lines were hand drawn showing salinity trends in
Cleveland Bay.
3.
Calculate the residence time for different parts of Cleveland Bay.
E
= 0.00467 m/day
S
(cs)
= 35.28 ppt
h
= depth (m)
S
(x)
= depth – averaged salinity (ppt)
Time In
Time Out
Station
Salinity (ppt)
Depth (m)
Flushing Time (days)
Figure 1: Depth – Averaged salinity for 14 stations across Cleveland Bay.
N
Figure 2: Salinity of Cleveland Bay
Figure 3:
Equation for Flushing Time
8:31:15
8:32:12
Station 1
35.53
5.69
8.68
10:00:00
10:00:40
Station 4
35.49
5.17
6.54
14:54:45
14:59:12
Station 9
35.37
12.48
6.69
13:31:38
1:34:30
Station 12
35.59
8.43
15.84
12:54:52
12:55:55
Station 14
35.75
4.41
12.66
The flushing or residence
time of a system is the
approximate time a “parcel”
of water will remain in the
system. Figure 3 shows the model (used for the Coral Sea and so was manipulated to be
used for Cleveland Bay, where the Coral Sea Salinity (S
cs
) was replaced with the furthest
offshore value (Station 10) of 35.28 ppt. It can be noted it was not the lowest salinity value
as expected from previous studies but was used to maintain consistency across studies and
allow comparison. Furthermore, there is limited research into the evaporation rates of
Cleveland Bay, so estimates of Coral Sea Evaporation provided in Wang et al. (2007) were
used by taking the average and then converted from mm/month to m/day to produce the
constant estimate of approximately 0.00467 m/day.
4.
Comment on the salinity features in the bay. Are there any anomalies? What are
the explanations for these?
Figure 1 shows there is variability of salinity across Cleveland Bay which can be expected due
to the dynamic nature of ocean systems and water movement. Figure 2 provides imagery of
the trend of decreasing salinity across from west to east towards the coral sea. It can be
noted that on the day the measurements were taken, nearby dredging and rainfall
increasing run off from nearby rivers of Ross River, Alligator Creek and Crocodile Creek
resulted in amplified sediment content in the bay, evident with the high turbid waters
documented on the day. This increased sediment can cause an increase in salinity in the bay
in comparison to the bordering Coral Sea. The morning south – easterly 18km/hr and
afternoon 40km/hr easterly winds allowed lower salinity waters from the Coral Sea in the
east to be pushed into the bay. Thus, the trend of increasing salinity moving west across
Cleveland Bay as lower salinity waters from the Coral Sea were pushed into the Bay.
The depth – averaged salinity calculations in Figure 1 show an anomaly high salinity at
Station 14 of 35.75ppt in comparison to neighbouring Station 1 with a salinity measurement
of 35.53ppt. The field data shows that Station 1 salinity was measured at 8:31am when
evaporation is low whereas Station 14 salinity was measured at 12:54pm at which
evaporation rates are at its highest. High evaporation removes water while the dissolved
salts remain causing increased salinity until mixing or rainfall restores salinity levels as well
as nearby dredging increasing sediment concentration and thus salinity in the bay making
more of an effect in the afternoon as sediment is dispersed. Another explanation of lower
salinity at Station 1 than Station 14 is the latter precipitation on the day and night prior to
Figure 4: Red circles around the stations with
which the flushing time was calculated.
Table 1: Table showing times of salinity measurements, salinity, depth, and flushing time for stations across Cleveland Bay.
Figure 5: Scatter graph showing salinity and flushing time of 5 stations across Cleveland Bay with trendline
and R
2
value showing
correlation.
6.00
7.00
8.00
9.00
10.00 11.00 12.00 13.00 14.00 15.00 16.00
35.1
35.2
35.3
35.4
35.5
35.6
35.7
35.8
R² = 0.5
Salinity and Flushing Times of Cleveland Bay
Flushing Time (Days)
Salinity (ppt)
investigation which can induce lower salinity while simultaneously increasing the output of
freshwater from nearby freshwater waterways generating lower salinity water near this
output zone located close to Ross River.
Table 1 shows the flushing times calculated for 5 of the 14 stations across Cleveland
Bay to gain an understanding of the variation in flushing times across the bay. Although close
in proximity, Station 14 and Station 1 have a variation in flushing time from 12.66 days to
8.68 days, respectively, due to the high variation in salinity calculated for these sites. Figure 5
shows a positive correlation between depth – averaged salinity and flushing time, confirmed
by a positive R – Squared value. As salinity increases, the flushing time increases as the
water becomes denser, remaining within the bay for longer periods. An average of these
flushing times gives a value of approximately 10 days for Cleveland Bay. In comparison to
other studies of the Coral Sea that found approximated flushing times of 38 to 45 days, the
low flushing time for Cleveland Bay can be assumed due to the smaller area and volume of
the bay, large river inputs of Ross River, Alligator Creek and Crocodile Creek as well as salinity
variation due to sediment input that increase water movement in and out of the bay
allowing for a faster flushing time than the Coral Sea.
5.
Submit your field notes.
-
Study was conducted during a neap tidal zone where the moon was at a waning
crescent phase.
-
Noticeably large swell on the day with high winds in the afternoon which can
implicate the reliability of some measurements, especially pinpointing location using
compass.
Morning: South – East winds at approximately 18 km/hr
Afternoon: Easterly winds at approximately 40 km/hr.
-
The water had extremely high turbidity and was very murky which can be explained
by the nearby dredging as well as recent rain increases sediment input from Ross
River into the bay.
-
Noted that there was recent rain the day prior to fieldwork as well as the early
morning of.
(7th of October = 1.2mm, 8th of October = 2.8mm)
-
Temperature was approximately 23
o
C in the morning and 27.78
o
C.
Flushing Time
(Days)
Longitude
Latitude
Depth
(m)
Depth - Averaged Salinity (ppt)
Station 8
n/a
146.986047
-19.174639
12.42
35.25
Station 9
6.69
146.952464
-19.161173
12.48
35.37
Station 4
6.54
146.946047
-19.230983
5.17
35.49
Station 2
n/a
146.885648
-19.24711
5.44
35.50
Station 10
15.84
146.922056
-19.15298
12.35
35.28
Station 5
n/a
146.971103
-19.217856
4.88
35.44
Station 7
n/a
147.009771
-19.185381
4.31
35.55
Station 3
n/a
146.910266
-19.244599
5.64
35.51
Station 6
n/a
146.994743
-19.202196
4.11
35.49
Station 11
n/a
146.891198
9.30
35.50
Station 1
8.68
146.844196
-19.237865
5.69
35.53
Station 13
n/a
146.865285
-19.223843
6.65
35.52
Station 12
n/a
146.883224
-19.202017
8.43
35.59
Station 14
12.66
146.83328
-19.236264
4.41
35.75
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6.
References:
Wang, Y., Ridd, P. V., Heron, M. L., Stieglitz, T. C., & Orpin, A.R. (2007) Flushing time of solutes and
pollutants in the central Great Barrier Reef lagoon, Australia.
Marine and Freshwater Research,
58
(8)
.
778-791.
https://doi.org/10.1071/MF06148
You, C., Jia, C., & Pan, G. (2010). Effect of salinity and sediment characteristics on the sorption and
desorption of perfluorooctane sulfonate at sediment – water interface.
Environmental Pollution,
158
(5). 1343-1347.
https://doi.org/10.1016/j.envpol.2010.01.009