National Oceanography Centre,
Southampton |
Falmouth
Field Course 2009
Group 7
|
|
Charles Annett -
Emelie Brodrick - Tom Horton - Pippa Knight - Ben Libby - Suzie Plumb -
Stef Rowland - Carl Zammit
|
|
The Fal Estuary is located on the South coast of
Cornwall, England. The shoreline has a length of 127km making it
England’s largest estuary and the third largest natural harbour
in the world. The estuary mouth lies between Pendennis Point and St Anthony Head and extends to Tresillian, 18km
inland. The estuary is macrotidal, with a spring tide of
5.3 m at Falmouth, however it is mesotidal at Truro, with a
spring tide of 3.5 m. The total area of the estuary (2482 ha)
comprises important subtidal (1736 ha), intertidal mudflat (653
ha) and saltmarsh (93 ha) environments. It is a ria
characterised by a deep meandering channel with depths of up to
34m – the deepest in England. The channel meanders northwards
becoming narrower and shallower (as shallow as 5m near King Harry
Reach) and broad, shallow platforms ranging between 0.3 and 4.6
m deep flank both sides of the channel.
Since the 16th century, the Fal has
been a natural refuge for boats and great of importance
for national and international trade. Tailings from metal mines
caused environmental problems in the western tributaries, to the
east, in the St. Austell mining district, tin works and china clay
mining, caused similar problems as the Fal was
used for transport and waste disposal. Other metals such
as zinc
and cadmium have entered the Fal due to agriculture, these
releases, as well as the input of sewage from local towns and
industry, make the Fal one of the most polluted estuaries in
England. Although mining has now ceased in the region, industry
such as boat building, fishing and cargo import/export are still
thriving, currently the largest business is offering mooring
to boats of all sizes, local or visiting. |
Fig. 1.1 - Overview of the Fal Estuary |
|
Fig. 1.2 - Tributary of
Restronguet Creek - Runoff from Wheal Jane Tin mine |
The Fal estuary has been designated a Special
area of Conservation (SAC), and a Site of Special of Scientific
Interest (SSSI) by the European Environment Council. This is
largely due to the extensive beds of a rare species of coralline
algae, maerl, which provides an important habitat for many other
organisms, also dead maerl harvested for plant fertiliser.
The complex biological, chemical and physical
processes occurring in the region make the Fal estuary an area
of major oceanographic interest and our aim is to carry out a
scientific study of the marine environment between the 1st
and the 11th July 2009, to increase our
understanding of the relationships and processes that occur
there. This information can then be passed on to agencies such
as Natural England to help them better manage the region. |
|
|
RV Callista
Range |
400nM |
Length |
19.75m |
Beam |
7.40m |
Draft |
1.80m |
Top Speed |
15knt |
Passengers |
30 max |
|
|
|
Fig. 2.1 - RV Callista |
Scientific and on deck equipment:
-
Capstan – 1.5 tonne limit
-
Side Davits – 2 x 100kg
limit
-
A-Frame and winch – 4 tonne lift capacity
-
Closing zooplankton net
-
Secchi disk
-
Hull mounted RDI Workhorse ADCP – 600 kHz
-
Seabird 21 Thermosalinograph
-
CTD + rosette + mounted fluorometer + Niskin
bottles
-
Vessel logger running TECHSAS logging
software.
Uses:
Callista is
used primarily as an offshore research vessel. It is fully
equipped with on board wet and dry labs and is the largest
research vessel used. |
RV Xplorer
Range |
100nM |
Length |
11.88m |
Beam |
5.20m |
Draft |
1.20m |
Top Speed |
25knt |
Passengers |
12 + Crew |
|
|
|
Fig. 2.2 - RV
Xplorer |
Scientific and on deck equipment:
-
Capstan – 0.5 tonne and a hydraulic crane
-
Bathymetric Surveying side scan sonar and
boomer
-
Van Veen Grab
-
Genoa 11c chart plotter
-
Furuno GP36 DGPS
Uses:
Xplorer is
used for the geophysical and estuarine parts of the course. It is a fully
equipped survey vessel with a sidescan sonar, grabbing
equipment, ADCP and CTD.
|
|
Ocean Adventurer
RIB
Range |
100nM |
Length |
7.00m |
Beam |
2.55m |
Draft |
0.50m |
Top Speed |
3.5knt |
Passengers |
6 + Crew |
|
|
|
Fig. 2.3 - Ocean Adventurer RIB |
Scientific and on deck equipment:
-
Hull – Ribtech 700
-
Simrad SDGPS, depth
sounder and chart plotter
-
Temperature / Salinity probes
-
Secchi Disk
-
Phyto / Zooplankton
nets
Uses:
Ocean Adventurer is smaller and more versatile,
allowing it to travel further up the estuary and tributaries, for sample collecting. |
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CTD and Rosette
Fig. 3.1 - CTD on Rosette |
The CTD is deployed into the water
column to measure conductivity (salinity) and temperature
against depth. It is attached to a rosette, thus allowing room for
sampling bottles (Niskin bottles) and other equipment, such as a fluorometer and a
transmissometer. These readings are then all relayed back to the
onboard computer, logged into a data file and saved for analysis
on land. |
|
Niskin Bottle
Fig. 3.2 - Niskin Bottle |
Niskin bottles (fig 3.2) are used to
take water samples from known depths in the water column. They
can be attached to a rosette and closed electronically by a
sending a signal from the onboard computer. A bottle can also be
deployed on a hydroline, using a messenger sent down the line
to close the bottle. On smaller vessels, the Niskin bottle can
sample the surface waters whilst being held by hand. |
|
ADCP
|
The Acoustic Doppler Current
Profiler is used to measure water current velocities and
directions over the whole water column. The ADCP emits known
acoustic signals over a range of angles, which are then
reflected by the water particles. Depending on whether the
particle is moving towards or away from the ADCP, the frequency
will be either higher or lower than the original transmission. |
|
Secchi Disk
Fig. 3.3 - Secchi Disk |
The Secchi Disk is a simple, but
effective piece of equipment, as shown in fig. 3.4, it is
comprised of
a circular disk divided into segments of black and white. This
is then attached to a rope with metre markings on
it. The disk is then lowered into the water, to the point at
which it just goes out of sight. The depth from the surface down
the string is known as the Secchi Disk depth. In order to
calculate the depth of the 1% light level, or euphotic depth, the
Secchi Disk
depth is multipled by three.
|
|
Zooplankton Nets
Fig. 3.4 - Zooplankton Net |
The zooplankton net
is comprised of a conical net with a collecting bottle attached to
the end of it. Another form of the same net is the zooplankton
closing net. This has a depth gauge attached to the top of the net,
which is used to determine when the messenger should be used to
close the net. The first of these nets is commonly used in
horizontal sampling, whereas the latter is used vertically. |
|
Sidescan Sonar
Fig. 3.5 - Sidescan Towfish |
The
sidescan sonar is commonly used to map the seabed. The
towed sidescan unit, known as the towfish, emits
acoustic pulses over a range of angles towards the
seafloor. The time interval and strength of the
reflected pulse are combined to give a real-time image
of the seabed. Features which protrude out of the seabed
provide stronger reflection than those which are
submerged into the seabed. It is this variation in
strong and weak areas which can be interpreted to give an image
of the seabed. |
|
DGPS Max
|
The DGPS Max is used for
geophysical surveys, as it is accurate to within one metre.
|
|
Van Veen Grab
Fig. 3.6 - Van Veen Grab |
The Van Veen
Grab is
used in bottom truthing. It is lowered through the water
column, and once it hits the seabed, its latch opens,
and thus captures a sample of the sea floor when it is
pulled back up. |
|
Video Camera
Fig. 3.7 - Video Camera |
The use of a video camera in
geophysical surveys is useful for providing a picture of the
seabed for both grab samples (ensuring the site is suitable) and
checking sidescan results. The camera is usually attached to a
cable, with a weight attached to the bottom. This is lowered slowly through the water column, with a
live feed to an on board television screen. There is also a ring
of lights around the camera for seeing below the euphotic zone;
however, this usually distorts colours. |
|
T/S Probe or
YSI Probe
Fig. 3.8 -
YSI
Probe |
The T/S
Probe can be used to monitor changes in water salinity
and temperature. This can be done by placing the probe
into a bucket that is constantly being filled by an
on-board pump, which is collecting water from the
surface. Alternatively, the YSI Probe can be manually
lowered through the water column, providing depth,
temperature, salinity, dissolved oxygen, pH and other
parameters. |
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Thursday 2nd of July
2009
Tidal Information
(GMT) |
HW |
0032 |
4.4m |
LW |
0707 |
1.8m |
HW |
1312 |
4.4m |
LW |
1938 |
1.9m |
|
Positions
for the day:
PSO: Pippa
On
Deck: Ben, Charles, Tom
Labs: Carl, Emelie, Stef
Computer:
Suzie
Log Book:
Ben and Suzie
|
Weather: showers, complete cloud
cover, 3-6m/s wind 140° |
Aim:
To interpret and understand the
biological, chemical and physical characteristics and
relationships seen in the immediate offshore environment near
the Fal estuary.
Readings were taken from our start
point at Black Rock, to the Lizard Point, with one station the other
side of the Lizard Point. Since the tide was flooding, the main
eddies caused by the headland are assumed to be east of this point. The
station to the west of Lizard Point will give an indication of the pre
headland water column structure. Measurements in the more
sheltered Falmouth Bay area will give an indication of the structure of the water column
in this environment. |
Equipment Used:
ADCP
CTD
Secchi Disk
Vertical Closing Net - 200μm
mesh and 60cm diameter
Wet Lab - equipped with: glass bottles for
nitrate and phosphate, phytoplankton and O2 samples; plastic bottles for silicon samples;
and
plastic tubes for chlorophyll samples.
|
Method:
En route to Lizard Point on RV Callista, 5 stations were
sampled and two further points were used for the ADCP:
Table 1 - Positions
of Stations/Points |
Station or Point |
Latitude |
Longitude |
Location |
1 |
50°08.669 N |
005°01.338 W |
Black Rock |
2 |
50°04.557 N |
004°59.770 W |
The Wrigglers |
3 |
49°59.084 N |
005°03.695 W |
One mile SE of Black Head |
4 |
49°56.053 N |
005°11.250 W |
Two miles SE of Lizard Point |
5 |
49°55.618 N |
005°16.788 W |
Two miles SW of Lizard Point |
A |
49°56.99 N |
005°10.29 W |
East of Lizard
Point |
B |
49°59.540 N |
005°05.296W |
The
Manacles |
|
Fig 4.1 - Map of Transects
Click map to
enlarge |
Between the stations,
as well as at each station, the ADCP was used to record the
structure of the water column. The ADCP was also used to plot two
transects on the return journey. These were between station 5
and point A, and point A and point B.
At each of the
stations, the CTD was deployed to within approximately 2m of the
seabed. From the data acquired, appropriate water sample depths were
chosen. These comprised of a deep, medium and shallow water sample.
Once the CTD was back on board ship, samples for dissolved
oxygen, silicon concentration, nitrate and phosphate concentration,
phytoplankton content and chlorophyll concentration were taken from
each depth, and appropriate fixing reagents were added. A Secchi Disk reading was also taken. The CTD and the ADCP readings on
the computers were used to determine depth ranges for the zooplankton sampling. |
ADCP
Transect 1 - From station 1 to station 2 (0856 - 0922) |
Fig. 4.2a - Backscatter for
Transect 1
Fig. 4.2b - Direction for Transect
1
|
The
first 1000m of transect 1 shows a two layer system, with
the top layer reaching velocities of about 0.25m/s in a
north-north-westerly direction, and the bottom layer
flowing easterly at around 0.01 – 0.05m/s. Once away
from the mouth of the estuary, the water velocity is
mainly homogenous with values between around 0.05 –
0.1m/s. Across the transect, the direction changes from
north-west, through west, to south. High backscatter
values occur at depth (75 – 82dB) which is likely to be
caused by high suspended bed load from the estuary. |
Fig. 4.2c - Velocity for Transect 1 |
Transect 2 - From station 2 to station 3 (1012 - 1058) |
Fig. 4.3a - Backscatter for
Transect 2
Fig. 4.3b - Direction for Transect
2 |
The
flow in transect 2 shows a well defined two-layer
structure. The surface water appears to flow in a
south-south-easterly direction changing to
south-westerly along the transect, which may indicate
the influence of the headland. There appears to be
similar flow in the bottom layer, but in the opposite
direction. At Black Head, the flow starts in a
north-easterly direction, which changes to
west-north-west at Wrigglers. Fig 4.3c shows a
transition in flow speed between the top and bottom
layers. In the top layer, velocity fluctuates between
0.2-0.25m/s, and in the bottom layer the range is
between 0.01 and 0.10m/s. There is high backscatter
regions along the transect (75 – 78dB) which could be
due to either high turbulence or suspended bed loads in
the bands nearer the bottom. |
Fig. 4.3c - Velocity for Transect 2 |
Transect 3 - From station 3 to station 4 (1136 - 1220) |
Fig. 4.4a - Backscatter for
Transect 3
Fig. 4.4b - Direction for
Transect 3 |
Fig.
4.4b appears to show a largely homogenous water column
flowing in a north-easterly direction around the
headland with a layer of water coming southwards down
the coast around Black Head. Due to the different
direction and velocity properties of the two bodies of
water, when they meet they create flow velocity and
direction gradients, which is likely to cause shear and
mixing. There is an area of high backscatter near the
headland (75 to 80dB) which may have been caused by
turbulence created as the water passes the headland.
|
Fig. 4.4c - Velocity for
Transect 3 |
Transect 4 - From station
4 to station 5 (1256 - 1327) |
Fig. 4.5a - Backscatter for
Transect 4
Fig. 4.5b - Direction for Transect
4
|
Towards
Lizard Point, fig. 4.5c shows a large amount of fast
flowing water (0.5 – 0.8m/s) mixing through the water
column, indicating eddying around Lizard Point and the
seafloor outcrop. This assumption is supported by the
decrease in velocities of the water column to around
0.35 – 0.45m/s once the point has been rounded. There is
a small fast flowing region towards the middle of the
water column (around 0.55m/s) which corresponds to an
area of high backscatter (75 – 80dB) possibly indicating
turbulence, as opposed to a phytoplankton bloom. |
Fig. 4.5c - Velocity for Transect 4 |
Transect 5 - From station
5 to point A (1357 - 1432) |
Fig. 4.6a - Backscatter for
Transect 5
Fig. 4.6b - Direction for Transect
5 |
As
transect 5 was taken travelling eastwards around Lizard
Point, it is similar to transect 4 in that it shows a
south-easterly flow around the headland. However, past
the headland there appear to be lower velocities (0.35 –
0.55m/s) due to the slacking of the tide. The surface
layer follows the coastline, in a north-easterly
direction. However, it is possible that due to the
decrease in velocity at depth, the flow is turning back
on itself and creating small eddies. There is an area of
high backscatter (75 – 80dB) in the surface waters and
at depth after the headland, which may correlate to the
change in flow velocities. |
Fig. 4.6c - Velocity for Transect 5
|
Transect 6 - From point A
to point B (1433 - 1507) |
Fig. 4.7a - Backscatter for
Transect 6
Fig. 4.7b - Direction for Transect
6
|
In
transect 6 there appear to be two main bodies of water.
The first is moving southerly, as it comes round the
headland, and expands up to the surface layers, with a
velocity between 0 and 0.15m/s. The second body of water
is towards the end of the transect, where interference
from The Manacles may have caused the formation of
strong eddies (0.45m/s), and thus explain the change in
direction from northerly to south-easterly. High
backscatter (75 – 83dB) can be seen where the eddies are
forming, and also where the water rounding the headland
is being forced up through the water column. |
Fig. 4.7c - Velocity for Transect 6 |
|
CTD
- T/S Profiles and Richardson Numbers
Fig. 4.8a - T/S Profile for Station
1 |
Station 1 (fig. 4.8a)
At
Station 1 the temperature profile shows a gradual
decrease with increasing depth from 17.83°C at the
surface to 15.78°C at depth. This gradual decrease
indicates that there is no defined thermocline, which is
characteristic of a well-mixed water column. However
analysis with Richardson Numbers indicates a developing thermocline at 7m, which prevents intense mixing above
this depth. They also indicate a turbulent (Ri <0.25) boundary
layer below 20m. The salinity profile shows a slight
increase with depth (0.14), which could be due to the
influence of fresh water inputs at the surface and the
mixing characteristics of the water column. |
Fig. 4.8f - Richardson Numbers for
Station 1 |
Fig. 4.8b - T/S Profile for Station
2 |
Station 2 (fig. 4.8b)
As at
station 1, there appears to be no obvious thermocline,
which is characteristic of a well mixed water column.
The temperature decreases steadily from 17.65°C to
11.98°C. This is in the same ratio as the decrease seen
at station 1 (approximately 2°C drop every 15m).
However, with further analysis, Richardson Numbers show
that the structure of the water column is in fact very
complex, with three developing thermoclines at 10, 25
and 36m, which provide a stabilising element, preventing
total homogeneity of the water column. Salinity shows a
small increase from 34.56 to 35.22. This is caused by
the denser, more saline water sinking below the slightly
fresher water. |
Fig. 4.8g - Richardson Numbers for
Station 2 |
Fig. 4.8c - T/S Profile for Station
3 |
Station 3 (fig. 4.8c)
At
Station 3 the salinity profile shows an overall change
of less than 0.1 over 70m (although minor fluctuations
are present). This minor change is again characteristic
of a well-mixed water column. Richardson numbers however
show that the water column shows a definite structure,
with mixing occurring above 22m, and between 40m and the
bottom, whereas there is a stable layer from 20 – 40m
which is indicated in the T/S profile as well as by the
Richardson Number plot (Fig. 4.8h). The change in
temperature still does not show a fully defined thermocline, however a strong gradient between
approximately 21m (16.5°C)
and 36m (12.2°C)
is present. This indicates a partially stratified water
column, as the temperature past 36m only changes
slightly (12.2°C
at 36m – 11.8°C
at 72m). |
Fig. 4.8h - Richardson Numbers for
Station 3 |
Fig. 4.8d - T/S Profile for Station
4 |
Station 4 (fig. 4.8d)
Station 4 is located near Lizard Point, a large
headland. The influence of this feature can be seen in
the structure of the water column, the erratic nature of
the data collected indicates large differences between
water layers and hence a high degree of mixing, this is
also indicated by the Richardson Numbers in the surface
layers (Ri <0.25), which show that the layer is
turbulent. There is no defined thermocline, but a net
temperature change from 16.86°C at the surface to
12.29°C
at depth (not steady as the measurements are quite
varied) and a salinity gradient from 34.90 to
35.19 respectively. Developing thermoclines are however
present at 10, 40 and 60m which provide a minor
stabilizing effect on the otherwise well mixed water
column. |
Fig. 4.8i - Richardson Numbers for
Station 4 |
Fig. 4.8e - T/S Profile for Station
5 |
Station 5 (fig. 4.8e)
Station 5 is located on the western side of Lizard
Point, the data show a steadier decrease in temperature
than at station 4 from 17.40°C at the surface to 12.95°C
at depth with the data points showing less deviation.
The salinity profile shows a small change from 34.8 at
the surface to 35.2 at depth, this small change (0.4)
over >50m again indicates the well-mixed nature of the
water column. Richardson number analysis (Fig. 4.8j)
shows a turbulent surface 5m (Ri <0.25) leading to a
thermocline providing a large stabilising element from 5
– 10m, below this however is a large transitionary layer
(Ri >0.25 & <1), indicating a degree of mixing but also
a degree of stratification. |
Fig. 4.8j - Richardson Numbers for
Station 5 |
|
Nutrients
Station 1 (fig. 4.9a)
At Black Rock, chlorophyll concentrations
are universally low. Throughout the vertical profile,
the concentration does not go above 1.5μg/L, with most
values between 1.1μg/L and 1.3μg/L. Although chlorophyll
concentrations are low, phytoplankton cell numbers
(mainly dinoflagellates) are high at 10.6m (see
fig. 4.1a)
which may suggest a bloom without a corresponding
chlorophyll peak. This dinoflagellate bloom may also
explain the low nitrate and phosphate levels at this
depth. The data also show the presence of a surface
water layer, with nitrate and phosphate concentrations
being much higher in surface layer compared with deeper
concentrations, whereas silicon concentrations are
slightly lower in the surface layer than at depth. As
station 1 was situated within the mouth of the estuary,
it is likely that the surface water has been influenced
by riverine inputs, explaining the high nitrate and
phosphate concentrations; however,
fig. 4.8a
shows no riverine input. Lower levels of silicon in the
surface layer may have been caused by a diatom bloom
further up the estuary. |
Fig. 4.9a - Nutrient and
Chlorophyll Plot for Station 1 |
Station 2 (fig. 4.9b)
Chlorophyll concentrations at station
2 are low throughout the profile, though generally
higher than at station 1. The high chlorophyll
concentrations at the surface (>2μg/L) may be anomalies,
as only 2 readings from the CTD gave this high
concentration. Similarly, only one point at 51m reads
1.9μg/L. If these values are not taken into account,
chlorophyll fluctuates between 1.1 and 1.3μg/L through
out the water column. The nutrients, nitrate and phosphate,
show opposite trends, with nitrate higher at the surface
than at depth, and phosphate higher at depth than at the
surface. Silicon is approximately seven times more
concentrated at depth than at the surface, even though
the highest concentration is 0.0066μg/L at 51.4m. |
Fig. 4.9b - Nutrient and
Chlorophyll Plot for Station 2 |
Station 3 (fig. 4.9c)
At station 3 there is a very prominent peak in
chlorophyll at 9m. At 12μg/L, it is six times
the level of chlorophyll in the water both above and
below the peak, and over 12 times the level of the
chlorophyll at the bottom of the profile. From figure
4.10c, it is clear that this bloom consists entirely of dinoflagellates at ~1300 cells/ml. Interestingly,
phosphate and silicon concentrations appear to be
slightly higher at 9m, along with the chlorophyll peak,
rather than depleted in this layer. Nitrate levels were
undetectable throughout the water column, which may
have been due to complete utilization by the
phytoplankton. |
Fig. 4.9c - Nutrient and
Chlorophyll Plot for Station 3 |
Station 4 (fig. 4.9d)
The levels of chlorophyll at station 4
are relatively uniform at about 1μg/L throughout the
water column. Nutrient levels are very low in the
euphotic zone, preventing a chlorophyll peak. Silicon
and phosphate concentrations show an increase with
depth, reaching 0.004μg/L and 0.2μg/L, respectively, at
64.2m. Similarly, nitrate is 1.36μg/L at 64.2m, and
undetectable in surface water. |
Fig. 4.9d - Nutrient and
Chlorophyll Plot for Station 4 |
Station 5 (fig. 4.9e)
The final station shows a small
chlorophyll peak above the background level of 0.8 to
1μg/L. Here, phytoplankton are possibly utilizing the nutrients
mixed up from depth by tidal mixing. The chlorophyll
maximum (~2μg/L) is at 12m and reaches background levels
of below 1μg/L by 25m. Nitrate and phosphate are
undetectable or zero around the chlorophyll peak.
Nitrate is detectable again at 42m at 0.4μg/L.
Silicon
is measurable at depth, although it is low at about
0.001μg/L. The low levels of silicon at the surface may be due to the
spring diatom bloom. As these diatoms have died, their
sinking skeletons may have undergone dissolution, and
therefore put some silicon back into the deeper water. |
Fig. 4.9e - Nutrient and
Chlorophyll Plot for Station 5 |
|
Oxygen
Table 2 - Oxygen
Saturation Results for Offshore |
Station No. |
Depth (m) |
Temp. (°C) |
Salinity |
O2
% Saturation |
1 |
27.0 |
16.1 |
35.2 |
106.3 |
10.7 |
17.0 |
35.1 |
109.1 |
17.9 |
17.9 |
35.0 |
109.3 |
2 |
51.4 |
12.0 |
35.2 |
86.3 |
23.5 |
14.8 |
35.1 |
102.7 |
9.8 |
17.3 |
35.1 |
109.4 |
3 |
24.6 |
15.5 |
35.1 |
127.0 |
9.6 |
17.9 |
35.2 |
113.8 |
3.7 |
18.2 |
35.2 |
111.8 |
4 |
64.2 |
12.3 |
35.2 |
96.1 |
30.4 |
14.9 |
35.2 |
100.1 |
6.7 |
15.6 |
35.1 |
99.4 |
5 |
42.2 |
13.4 |
35.2 |
94.7 |
16.5 |
15.5 |
35.2 |
82.9 |
4.3 |
17.3 |
35.2 |
107.4 |
|
Oxygen analysis was
carried out, using the Winkler method, to investigate
the amount of oxygen dissolved in the Fal and
surrounding offshore waters.
At the first station, we
found supersaturated oxygen levels. This correlates well
to the high number of phytoplankton we collected at that
site. Areas of high primary production usually portray
high levels of oxygen during daylight, due to the
process of photosynthesis. This pattern was observed
again at site
3, where we found the highest levels of both oxygen
saturation and primary production. This is also the site
where oxygen levels were lowest at surface compared to depth – probably due to an
increased activity of zooplankton .
Oxygen is taken out of the
water by respiration and decomposition. In our deepest
sample at site 2, there is a large depletion of oxygen.
This is due to the lack of photosynthetic organisms
operating at this depth.
Oxygen levels at site 5
are low compared to the other sites, except at the
surface. Diffusion from the air would be high due to the
high turbulence of the waves, due to an increase in surface area
allows, Decomposition and
respiration (especially through zooplankton) found at
lower depths reduces oxygen concentrations. |
Site 4 appears to be the most
stable in terms of oxygen saturation levels. It is
possible that the amount
of consumed oxygen is almost evenly balanced by
processes that give off oxygen at all the depths
sampled. Also note the lower levels of oxygen in areas
with less temperature. Colder waters are less capable of
holding gaseous material.
|
|
Plankton
Table 3 - Secchi
Disk Depths at Offshore Stations |
Station |
Secchi Disk Depth
(m) |
Euphotic Depth (m) |
Attenuation Coef.
(k) |
1 |
8.0 |
24.0 |
0.18 |
2 |
7.5 |
22.5 |
0.19 |
3 |
7.5 |
22.5 |
0.19 |
4 |
8.0 |
24.0 |
0.18 |
5 |
8.0 |
24.0 |
0.18 |
|
Phytoplankton
Fig. 4.10a - Phytoplankton at
Station 1 |
Fig. 4.10b - Phytoplankton at
Station 2 |
Fig. 4.10c - Phytoplankton at
Station 3 |
Fig. 4.10d - Phytoplankton at
Station4 |
Fig. 4.10e - Phytoplankton at
Station 5 |
Figures 4.10a-e show that at this time
of year in the Fal estuary the phytoplankton population is
largely dominated by dinoflagellates, particularly of the
species Karenia mikimotoi (fig. 4.11), a harmful invasive species
first described in Japan in 1935. The first large red tide bloom
in the English Channel occurred in 2003 spreading around the
world in ship’s ballast water (Daugjberg et al, 2000).
There are a few diatoms present as well, but these tend to
dominate in spring and autumn rather than early July (Rodriguez
et al, 2000) because of reduced silicon levels, (see
figures 4.9a-e). Silicon is an important nutrient for diatoms as they
use it to secrete in the form of opal frustules around their
cell. Cilliates were present in small numbers but flagellates
were absent from all stations except station 5 at a depth of 16.5m.
The largest abundance of
phytoplankton were found at intermediate depths of around 7 to
20m. The surface waters contain few cells, most probably due to
depleted nutrient levels (see figures 4.9a-e). At depths of more
than 20m, the light levels are too low to support
photosynthesis and so therefore, fewer phytoplankton are found
at these depths. Normally at this time of year in the Western English
Channel, irradiance is high and waters are stratified so that
the surface mixed layer is above the compensation depth. This
provides ideal conditions for phytoplankton growth and blooms
occur throughout the summer. Surface waters become depleted of
nutrients and so the dinoflagellates are found at a chlorophyll
maximum on the thermocline where there are more nutrients.
Station 3 had the largest number
of cells per ml with a maximum of 13100 cells per ml, all of
which were dinoflagellates. Station 4 had the lowest abundances
of phytoplankton, reaching a peak of only 900 cells per ml at
6.7m.
|
Fig. 4.11 -
Karenia Mikimotoi |
Zooplankton
The zooplankton samples were
concentrated down to 500ml of water and a 2ml sub sample of this was
analysed under a microscope. Table 4 shows the
depths sampled at each station.
Table 4 -
Zooplankton Net Depths |
Station |
Depth
of Tow |
1 |
13 - 7m |
2 |
30 - 0m |
3 |
12 - 7m |
28 - 18m |
4 |
30 - 0m |
5 |
20 - 7m |
The figures 4.11a to
4.11g
show that the zooplankton are mainly composed of
hydromedusae and echinoderm larvae.
|
Fig. 4.11a - Key
for Zooplankton Graphs
|
Station 1 was
sampled between 13 and 17m and is
dominated by cnidarians with a concentration of
1179.2 hydromedusae per m3 seawater.
Cirripedes, polychaetes and decapods were also
abundant. |
Fig. 4.11b -
Zooplankton at Station 1
|
Station 2
sampled the top 30m of the water column and it was
found that echinoderm larvae dominated here with an
average of 736.9 animals per m3 of water.
There were also many hydromedusae (486.3 per m3)
and polychaetes (221.1 per m3).
|
Fig. 4.11c -
Zooplankton at Station 2
|
Station 3 had
two samples, one shallow and one deep. The shallow
sample showed a wide range of taxa dominated by
echinoderm larvae (693.9 per m3),
hydromedusae (517.0 per m3) and copepoda
(258.5 per m3). The deep sample had
smaller numbers of individuals in most taxa, for
example copepods. However the sample was largely
dominated by extremely high concentrations of
hydromedusae reaching 4333.2 individuals per m3.
|
Fig. 4.11d -
Zooplankton at Station 3 Deep
Fig. 4.11e -
Zooplankton at Station 3 Shallow
|
Station 4 was
another sample of the top 30m of water. It contained
fewer cnidarians and echinoderms than previous
stations and in fact, cirripede larvae were the most
abundant although they were only in concentrations
of 206.3 per m3. This station was found
to have unusually low numbers of zooplankton.
|
Fig. 4.11f -
Zooplankton at Station 4
|
Station 5 shows
the largest echinoderm larvae concentration (1292.2
per m3) of all the stations. Copepods,
chaetognaths and hydromedusae are also present. |
Fig. 4.11g -
Zooplankton at Station 5
|
|
Discussion
The
investigation was aimed at interpreting and better understanding
the biological, chemical and physical characteristics and
relationships seen in the immediate offshore area near the Fal
Estuary. ADCP data relayed information showing increased
turbulence around coastal headlands. An increase in turbulence
results in mixing throughout the water column. Firstly, wave
action will increase the water surface area allowing greater diffusion between the air and surface waters.
This is seen mostly at station 5, where the water in the upper
layer is supersaturated with oxygen, whereas the samples beneath
show greater amounts of oxygen depletion. As well as gaseous
exchanges, turbulence also plays a key role in biological,
ecological and chemical relationships. Turbulence mixes the water column, meaning there is less stratification. A less
stratified water column makes it more difficult for
phytoplankton populations to grow because they are constantly
being mixed throughout the euphotic zone and below this, where
photosynthesis is not possible. A well mixed water
column also means salinity will be very similar throughout. Low
concentrations of nitrate, phosphate and silicon were found in
the euphotic zone (top 20-25m), probably resulting from phytoplankton
consumption. Concentrations of the nutrients are likely to have increased below
this depth because less phytoplankton were found there. Karenia
mikimotoi, a dinoflagellate, was the dominant phytoplankton
species found at all stations. Although they do not use silicon
to a large extent, levels remain low most likely due to a
previous spring diatom bloom. Oxygen levels in the surface layer
were mainly observed to be supersaturated due to the presence of
photosynthetic organisms, but this is only likely to have an
impact during
daylight hours, due to low irradiance at night. Zooplankton
numbers correlate strongly to phytoplankton concentrations. The
main taxa found in this region were hydromedusae and echinoderm
larvae which are characteristic of low nutrient environments. |
|
|
Monday 6th of July 2009
Tidal Information
(GMT) |
HW |
0015 |
4.6m |
LW |
1101 |
1.4m |
HW |
1632 |
4.9m |
LW |
2325 |
1.4m |
|
Positions
for the day:
PSO: Carl (the boss)
On
Deck: Ben, Emelie, Stef, Suzie, Tom
Scribes: Charles, Emelie,
Pippa
Sonar Monitor: Ben and Charles
Video Monitor: Charles
|
Weather:
West or South-West, force 5-7, increasing to 8 at times,
veering northwards later, sea state rough, squally showers,
visibility moderate or good. |
Aim:
To survey the seabed in the bay
off Swanpool, Falmouth, whilst also using the Van Veen Grab to
bottom truth the sidescan data.
|
Equipment used:
Sidescan Sonar
DGPS Max
Video Camera
Van Veen Grab
Hydropro Computer Package
|
Method:
In the bay of Swanpool, 6
transects were made on RV Xplorer:
Table 5 - Start
and End of Transects |
|
Start |
End |
Transect |
Latitude |
Longitude |
Latitude |
Longitude |
1 |
50°08.747 N |
005°03.200 W |
50°08.201 N |
005°04.404 W |
2 |
50°08.174 N |
005°04.295 W |
50°08.705 N |
005°03.156 W |
3 |
50°08.660 N |
005°03.101 W |
50°08.124 N |
005°04.277 W |
4 |
50°08.070 N |
005°04.244 W |
50°08.607 N |
005°03.072 W |
5 |
50°08.575 N |
005°03.003 W |
50°07.986 N |
005°04.277 W |
6 |
50°07.977 N |
005°04.147 W |
50°08.513 N |
005°02.936 W |
|
Fig. 5.1 -
Sidescan Sonar Transects and Grab Sites
Click to map
to enlarge |
Along each of these transects, a
sidescan sonar towfish was used to map the seabed. Using the
data from the sidescan, grab sites were chosen to bottom truth
the results. Granular
material produces specific sedimentary bedforms depending on the
level of hydrodynamic forcing it is exposed to, these bedforms
are then visible on the sidescan sonar trace.
All of this data was then used to provide
a description and interpretation of the seabed in the area.
|
Grab Sites
Grab Site 1
Time: 1016 GMT
Latitude: 50°08.482N
Longitude: 005°03.112W
Sediment:
When sieved approximately 70% of
the sediment had a particle size of greater than
2mm, 20% between 1 and 2mm and 10% less than
1mm. Sediment could possibly be a very fine
gravel, but with increased grain size due to the
large volume of dead mearl, overlying a muddy sediment. Live:Dead
maerl ratio is 1:1. Large clumps of live maerl
may stop bedforms from forming.
Biology:
The proportions of live maerl
seem to be indicative of the equitability and
diversity of species in the benthic habitat. Due
to this the black brittle star (Ophiocoma
nigra) and the Banded Carpet Shell (Paphia
rhomboids) are largely dominant. There is
also a high proportion of dead shells from which
the Small Queen Scallop (Aequipecten
opercularis) and Qahog (Mercenaria
mercenaria) can be identified. These species
are all indicative of a coarse gravely sediment.
|
Fig. 5.2
- Grab Samples from Site 1 |
|
Grab Site 2
Time: 1029 GMT
Latitude: 50°08.595N
Longitude: 005°03.532W
Sediment:
When sieved approximately 90% of
the sediment had a particle size of greater than
2mm and only 10% less than 1mm. The sediment had
a high proporition of calcareous sediment with a
Live:Dead maerl ratio of 1:19. Distinct bedforms could be defined possibly due to wave
action, since they show bification.
Biology:
The Banded Carpet shell (Paphia
rhomboid) is abundant (2 small, 3 medium and
3 large identified) indicating these bivalves
are a highly resilient species able to colonise
a high stress environment near to the shore.
|
Fig. 5.3
- Grab Sample from Site 2 |
|
Grab Site 3
Time: 1046 GMT
Latitude: 50°08.356N
Longitude: 005°03.747W
Sediment:
When sieved approximately 97% of
the sediment had a particle size greater than
2mm, 2% between 1 and 2mm and 1% less than 1mm.
The sediment could be defined as very fine
gravel sized sediment with small traces of mud however the
majority of these particles are calcareous
deposits with a Live:Dead maerl ratio of 3:17.
Bedforms were again identified. The wavelength
of these bedforms may affect the representation of the
area as it could be sampled in a peak or a
trough.
Biology:
The biota is again influenced by
the Live:Dead maerl ratio. More biota was found
than in grab 2 with the Black Brittle Star (Ophiocoma
nigra) again present (2 individuals) along
with 3 unidentifiable Polychaete worms and 2
Banded Carpet Shells (Paphia rhomboids).
A Juvenile fish was also caught in the grab.
|
Fig. 5.4
- Grab Sample from Site 3 |
|
Grab Site 4
Time: 1104 GMT
Latitude: 50°08.130N
Longitude: 005°03.815W
Sediment:
When sieved approximately 98% of
the sediment had a particle size of greater than
2mm with only 2% between 1 and 2mm. The sediment
could be defined as very coarse gravel sized
sediment but the
majority of sediment defining this grain size is
dead maerl. The Live:Dead maerl ratio is 1:19.
Biology:
The biota was similar to grab
site 2 with 2 polychates identified as well as 2
live Banded Carpet Shells (Paphia rhomboids).
There was also a high proportion of dead
bivalve shells and other detritus. Shells
accumulate in the troughs of the bedforms and so
the troughs
are richer in biogenic materials.
|
Fig. 5.5
- Grab Sample from Site 4 |
|
|
Sidescan
The
majority of the seafloor investigated was shown to be
transverse sinuous in-phase bificated megaripples. These
megaripples were typically around 1.3m high and 0.6m
long. They are small symmetrical low energy current
ripples. As 2-D megaripples form perpendicular to the
flow direction, this shows that the waves were coming from the
South East, and travelling North Westerly. The megaripples found varied slightly in dimensions due to
localised differing flow speeds, but typically they are
formed in areas of turbulent rough flow and intermediate
flow speeds.
As we
approached the coastline to the North, the water became
shallower and there were less distinctive bedforms
found. This resulted in two distinct areas of flat
homogenous seabed. These were to the North East and the
West of the bay. In the North East, the lack of bedforms
is explained by the effect of the headland at Pendennis
Point. This headland acts as a shelter to the benthic
environment, which is therefore exposed to slower
wave velocities. The area of homogenous flatbed to the West is
the result of the shape of the coastline in the bay.
Areas
of rocky outcrop were found nearest to the cliff
boundary in the far East and West of the bay, as well a
smaller rocky outcrop to the South of the
study area.
Finally, one area of the rock to the North West, where
the depth was the shallowest, was found to support a
population of non-calcareous macroalgae.
|
Fig. 5.6 - Section of Sidescan
Trace |
|
Discussion
Over the surveyed area the main
sediment could be defined as fine gravel, which is mainly
due to calcareous deposits of dead maerl. The dominant particle
size is greater than 2mm with traces of fine mud at some sites.
A large area of transverse sinuous in phase megaripples was
identified in the centre of the bay with areas of homogeneous
bed and rocks in western and eastern areas. The biota identified
from the grab samples was a reflection of the ratio of live
to dead maerl. The dominant species are Black
Brittle Stars (Ophiocoma nigra) and Banded Carpet Shells
(Paphia rhomboids) with a large amount of dead shells
found at most sites. Some unidentifiable polychaete worms were
found in grabs 3 and 4.
|
|
|
Thursday 9th of July 2009
Tidal Information
(GMT) |
HW |
0041 |
1.2m |
LW |
0615 |
4.8m |
HW |
1254 |
1.3m |
LW |
1826 |
5.1m |
|
Positions
for the day:
PSO: Tom
On
Deck: Ben and Charles
Labs: Carl, Emelie and
Stef
Computer: Pippa and
Suzie
Log Book: Pippa
|
Weather:
mostly overcast with sunny intervals, max
temperature 20°C, visibility very good, wind direction NNW,
speed 13-14 mph, gusts of up to 26-28 mph |
Aim:
To analyse how
the Fal estuary changes for certain physical, chemical and
biological parameters from the source, where
the salinity is 0, to the estuary mouth near Black Rock, where
the salinity approaches 35. This will develop an understanding
of how the estuary acts as a transition zone between the
freshwater river and the start of the coastal waters.
|
Equipment Used:
T/S and YSI Probes
CTD
Secchi Disk
Zooplankton Net
Lab Equipment (glass bottles for
nitrate and phosphate, phytoplankton and O2 samples; plastic bottles for
silicon samples;
and
plastic tubes for chlorophyll samples)
|
Method:
Starting in the upper estuary, on
Ocean Adventurer RIB, four stations were sampled. Once on Xplorer, a further
four stations were sampled and four transects were
made across the estuary. Between stations 1 and 2, and along
transect 4, a zooplankton net was also towed. The positions of
these have been tabulated below:
Table 6 -
Stations in Upper and Lower Estuary |
Station |
Latitude |
Longitude |
Upper Est. 1 |
50°14.699 N |
005°01.383 W |
Upper Est. 2 |
50°13.711 N |
005°00.946 W |
Upper Est. 3 |
50°12.957 N |
005°01.663 W |
Upper Est. 4 |
50°12.566 N |
005°01.677 W |
Lower Est. 1 |
50°12.438 N |
005°01.840 W |
Lower Est. 2 |
50°10.838 N |
005°01.731 W |
Lower Est. 3 |
50°09.691 N |
005°02.201 W |
Lower Est. 4 |
50°08.496 N |
005°01.462 W |
Table 7 - Transect Start and End Points |
|
Start |
End |
Location |
Latitude |
Longitude |
Latitude |
Longitude |
TRAN 1 |
50°12.391 N |
005°01.793
W |
50°12.494 N |
005°01.856 W |
TRAN 2 |
50°10.864 N |
005°01.572 W |
50°10.481 N |
005°02.495 W |
TRAN 3 |
50°09.697 N |
005°03.186 W |
50°09.681 N |
005°01.804 W |
TRAN 4 |
50°08.431 N |
005°01.113 W |
50°08.618 N |
005°02.447 W |
|
Fig. 6.1 -
Stations in Upper and Lower Estuary and Transects
Click map to
enlarge |
Table 8 - Locations of Zooplankton Trawls |
|
Start |
End |
Trawl |
Latitude |
Longitude |
Latitude |
Longitude |
A |
50°12.264
N |
005°02.298
W |
50°12.992 W |
005°02.498 W |
B |
50°08.438 N |
005°01.190 W |
50°08.469 W |
005°01.448 W |
|
|
Samples in the
upper estuary were taken at the surface for nitrate, phosphate,
silicon and chlorophyll. At station 4, a phytoplankton samples
was also taken. A YSI Probe was also used to measure salinity,
percentage saturation of dissolved oxygen, temperature and pH against depth through the water
column.
In the lower
estuary, a CTD rosette was deployed at each station to measure
temperature, salinity and turbidity against depth. Samples for
nitrate, phosphate, silicon, chlorophyll, percentage saturation
of dissolved oxygen and
phytoplankton were also taken at approximately 1m from the
benthos, at the 1% light level
(determined by using the Secchi Disk) and at the surface.
|
ADCP
Transect 1
- East to West (1137 - 1140) |
Fig. 6.2a - Backscatter for
Transect 1
Fig. 6.2b - Direction for Transect
1 |
The flow direction and
velocity contour plots (figures 6.2b and 6.2c
respectively) for transect 1 appear to show a layer of
faster southwards moving water (0.3m/s), with a layer of
slower (0 – 0.13m/s) well-mixed water below. The faster,
lighter water at the surface is likely to be a
layer of riverine influenced water moving southwards
with the ebbing tide, whereas the deeper water is as more dense,
probably due to the influence of the tidal flow, thus
forming an eddy structure. The backscatter contour plot
(Fig. 6.2a) shows a fairly uniform readout with no high surface readings,
indicating little or no primary production in the
region.
|
Fig. 6.2c - Velocity for Transect 1 |
Transect 2 - East to
West (1226 - 1239) |
Fig. 6.3a - Backscatter for
Transect 2
Fig. 6.3b - Direction for Transect
2 |
The direction contour plot (Fig.
6.3a) for transect 2, moving east to west from Messack
Pt to Penarrow Pt, shows the ebbing tide
propagating southwards through the main channel (6 – 15m
travelling at 180°
to the transect). A region of deeper, probably well-mixed water
can also be seen moving perpendicular to the
ebbing tide, likely to be due to the slack water in the
deeper contours being mixed by the shear between the
above layer of tidal flow. This consequently causes an
eddy structure. The flow velocities (Fig. 6.3c)
throughout the water column are fairly uniform (0 –
0.125 m/s) with a stationary boundary layer (15 – 17.5m)
separating the two regions, moving in opposite
directions. The backscatter contour plot again shows
fairly uniform levels (60 – 65dB). However, there are
some regions of high levels towards the end of the
transect, which are due to bottom noise in the shallower
water.
|
Fig. 6.3c - Velocity for Transect 2 |
Transect 3 - West to
East (1254 - 1304) |
Fig. 6.4a - Backscatter for
Transect 3
Fig. 6.4b - Direction for Transect
3 |
Conducted 15 minutes after
low water, the flow direction and velocity contour plots
(Figs. 6.4b & 6.4c) for transect 3,
show the tide in its slack water state. Flow velocities
and directions throughout the water column are erratic,
showing sharp changes over small areas with flow
velocities not exceeding 0.125m/s and generally
averaging lower than 0.06m/s. The backscatter contour
plot (Fig. 6.4a) is once again fairly uniform with
surface values averaging above 65dB, indicating higher
levels of primary production in these areas, however no
areas of especially high backscatter levels can be
identified.
|
Fig. 6.4c - Velocity for Transect 3 |
Transect 4 - East to
West (1334 -1355) |
Fig. 6.5a - Backscatter for
Transect 4
Fig. 6.5b - Direction for Transect
4 |
The flow direction and velocity
contour plots (Figs. 6.5b & 6.5c) for
transect 4 show the flood tide propagating
North (90°
from the ship track). However, two regions of flow can
be identified. Water of a high flow velocity (0.25 –
0.4m/s) can be seen at the far end of the transect (800m
– 1574m), possibly caused by the tidal stream flowing
fast around Pendennis Pt and the shallow water causing
higher flow velocities. Another region of flow can be
identified in the main channel, this region is slower (0
– 0.125m/s), possibly due to the greater depth further
off Pendennis Pt coupled with a similar tidal stream
velocity. The backscatter contour plot (Fig. 6.5a) is
again generally uniform, although a region of high
backscatter (80 – 100dB) between 0 – 10m at approximately
1300m can be seen along the track. This could be due to
an algae bloom.
|
Fig. 6.5c - Velocity for Transect 4 |
|
CTD
Upper
Estuary Station 1 |
|
Fig. 6.5a - T/S Profile at Upper
Estuary Station 1 |
Temperature and salinity
both show a smooth gradient in relation to depth,
with an overall change of 0.52°C
and 0.30 respectively. As with previous stations, the
absence of a thermocline is typical of a well-mixed
water column. |
|
Upper Estuary Station 2 |
|
Fig. 6.5b - T/S Profile at Upper
Estuary Station 2 |
The measurements for
temperature at this station agree with the patterns seen
previously, with no distinct thermocline, rather a
steady decline from 16.1°C
to 15.3°C.
Again, this suggests that the water is well-mixed.
Salinity has a very shallow gradient, increasing by only
0.73 over 9m.
|
|
Upper Estuary Station 3 |
|
Fig. 6.5c - T/S Profile at Upper
Estuary Station 3 |
At this station
temperature changes relatively little with depth, with
an overall decrease of 0.38°C.
Salinity continues to demonstrate a minimal variation of
0.30. Thus this station seems to follow the same pattern
as previous stations. |
|
Upper Estuary Station 4 |
|
Fig. 6.5d - T/S Profile at Upper
Estuary Station 4 |
Apart from the small spike
in temperature at 0.5m, which may be an anomalous
reading, the measurements exhibit a smooth decrease with
depth, from 15.66°C
to 15.00°C.
Salinity, as with previous stations, shows a nominal
change of 0.41. As previously stated, the absence of a
distinct thermocline suggests a well-mixed water column. |
|
Lower Estuary Station 1 |
|
Fig. 6.5e - T/S Profile at Lower
Estuary Station 1 |
Temperature shows a
gradual change with depth, from 15.92ºC to 15.09ºC, with
no defined thermocline. Salinity also demonstrates
little change with depth, with a total increase of 0.61.
Both of these patterns are characteristic of a
well-mixed water column. However further analysis using
Richardson Numbers (Fig. 6.5f) shows a layer of laminar
water near the surface (0 – 10m, Ri > 1) separated by a
thermocline at 8-9m from a turbulent (Ri <0.25) boundary
layer below 11m. |
Fig. 6.5f - Richardson Numbers for
Lower Estuary Station 1 |
Lower Estuary Station 2 |
|
Fig. 6.5g - T/S Profile at Lower
Estuary Station 2 |
As before there is
evidence of a well-mixed water column, as there is no
definite thermocline or halocline. Temperature displays
a steady negative gradient with depth, changing from
14.95ºC at the surface to 13.96ºC at depth. Salinity
changes by just 1.5 across 25m. With analysis using
Richardson numbers, again a laminar layer of water (Ri
>1) is may be present from the surface to 16m, which is
separated from a turbulent layer (Ri <0.25) below 18m by
a developing thermocline at 17m. |
Fig. 6.5h - Richardson Numbers for
Lower Estuary Station 2 |
Lower Estuary Station 3 |
|
Fig. 6.5i - T/S Profile at Lower
Estuary Station 3 |
Although the temperature readings at
this station show more variation than previous stations,
there is still no evidence of stratification from the
T/S profiles. Temperature changes from 14.92ºC to
13.74ºC, and salinity increases from 34.99 to 35.23.
Richardson Numbers show that there are in fact three
developing thermoclines at 5, 10 and 17m. These
thermoclines provide a stabilizing element and prevent
intense mixing above this depth, but not a large enough
element to induce stratification. |
Fig. 6.5j - Richardson Numbers for
Lower Estuary Station 3 |
Lower Estuary Station 4 |
|
Fig. 6.5k - T/S Profile at Lower
Estuary Station 4 |
This station follows the
same pattern as all the previous stations. The
temperature changes from 14.46ºC to 13.55ºC, and
salinity changes by 0.18, suggesting a well-mixed water
column. A trend can be seen on progression through these
four stations – moving towards the mouth of the estuary,
the temperature decreases and salinity increases, most
likely due to tidal forces mixing the freshwater (river)
and saline (sea) water masses. Richardson Numbers show
that the water column is split into two layers. A
laminar (Ri >1) layer from the surface to 10m, and a
turbulent (Ri <0.25) layer beneath this. |
Fig. 6.5l - Richardson Numbers for
Lower Estuary Station 4 |
|
Nutrients
An Estuarine mixing diagram is
used to asses whether a nutrient behaves in a conservative or non-conservative
way throughout an estuary. The diagram is a plot of the
concentration of the constituent against a conservative element
that can be used as a tracer. In almost all estuarine mixing
diagrams this tracer is salinity. Riverine and seaward end
members are identified as the furthest up river point with the
least salinity and the most seaward point with the most salinity
respectively. A Theoretical Dilution Line (TDL) is then drawn
between the two. Points are plotted on the graph and deviation
from the line is used to determine conservative or
non-conservative behaviour. Conservative behaviour is when the
points plot close to the TDL and the concentration is determined
by the mixing of the riverine and seaward end members.
Non-conservative behaviour is when the points plot off the TDL
and the constituent is being added or removed from the estuary
due to biological uptake, addition from pore waters or other
processes. If the points are above the TDL then the constituent
is being added, if it is below then the constituent is being
removed.
Nitrate |
Fig. 6.6a - Nitrate Mixing Diagram |
Figure 6.6a is an
estuarine mixing diagram for nitrate. Riverine end
members are plotted at very low salinities and high
concentrations. The values recorded in the estuary
however are at much higher salinities and lower
concentrations. Therefore when all these values are
plotted there is a small cluster of points due to the
large salinity and concentration range. It is impossible
to determine conservative or non-conservative behaviour
from this figure. Therefore the riverine end members are
not plotted on figure 6.6b below and the riverine end
member is taken as the lowest salinity value from the
RIB stations. The lowest salinity RIB station is also
taken as the riverine end member for the estuarine
mixing diagrams for phosphate and silicon.
|
Fig. 6.6b - Nitrate Mixing Diagram
Using Lowest Salinity |
Figure 6.6b suggests that nitrate could be behaving
non-conservatively in the estuary. All of the RIB points
and the majority of the Xplorer points are plotted below
the TDL suggesting there is a removal of nitrate in the
estuary.
|
Phosphate |
Fig. 6.6c -
Phosphate Mixing Diagram at Lowest Salinity |
Figure 6.6c is a mixing
diagram for phosphate. The figure suggests reasonably
uniform concentrations down the estuary. One value
however has a much higher concentration of 2.55µmol/L
suggesting that phosphate is being added at this point.
This location is station 3 on the rib near to the King
Harry ferry so could possibly be an anthropogenic
source.
|
Silicon |
Fig. 6.6d -
Silicon Mixing Diagram at Lower Salinities |
Figure 6.6d is a mixing
diagram for silicon. The figure suggests that there is
possible removal of silicon as all points apart from one
are plotted below the TDL. The main process of silicon
removal is likely to be biological uptake by diatoms.
This removal of silicon may coincide with a higher
concentration of diatom cells. |
The analysis of the three nutrients
suggests that phytoplankton are utilising nitrate in the higher
salinity waters completely. Therefore nitrate is the limiting
nutrient in the estuary, and phosphate levels remain
un-depleted. Analysis of the collected phytoplankton samples
(figure 6.8a) shows that a significant proportion of the
phytoplankton are diatoms, which also utilise silicon, as well
as nitrate and phosphate. As silicon levels are lowest in the
higher salinity water, this suggests that the type of
phytoplankton depleting the water of nitrate are diatoms.
|
Oxygen
Table 9 - Oxygen
Saturation Results for Estuary |
Station Number |
Depth (m) |
Temperature (°C) |
Salinity |
O2 %
Saturation |
Lower Estuary 1 |
1.0 |
16.02 |
33.89 |
|
7.5 |
15.28 |
34.38 |
98.9 |
12.3 |
15.08 |
34.54 |
104.5 |
Lower Estuary 2 |
1.3 |
14.92 |
34.83 |
102.0 |
13.5 |
14.36 |
35.08 |
96.8 |
24.0 |
13.97 |
35.18 |
94.7 |
Lower Estuary 3 |
1.0 |
14.92 |
34.96 |
107.0 |
16.5 |
14.36 |
35.17 |
|
28.0 |
13.97 |
35.23 |
94.9 |
Lower Estuary 4 |
1.0 |
14.67 |
35.06 |
|
18.0 |
13.59 |
35.25 |
93.9 |
22.4 |
13.57 |
35.26 |
94.1 |
Upper Estuary 1 |
1.0 |
16.71 |
32.90 |
100.8 |
1.0 |
16.71 |
32.90 |
101.0 |
1.0 |
16.71 |
32.90 |
110.3 |
|
An oxygen analysis was
carried out, using the Winkler method, to investigate
the amount of oxygen dissolved in the Fal and its
tributaries. Unfortunately it should be noted some of
the samples were exposed to the air and therefore gave
no reliable results (left blank in table 9).
Stations 1 and 4 show more saturated levels of oxygen
deeper in the water column. In contrast,
stations 2 and 3 show supersaturated levels of
oxygen in the surface layers, and less saturated in
the deeper layers. Measurements from the RIB, the
furthest point up the estuary, show supersaturated levels of oxygen in the top
layer.
It would therefore be most likely that the
unknown quantities of the surface layers at station 1
and station 4 would be supersaturated. This suggests
that primary production is active in the top layers of
the euphotic zone. Phytoplankton levels were found to be
high at these stations in the surface layers, and it is the
photosynthetic processes of phytoplankton that releases
oxygen into the water. In the lower layers most readings
are below 100% saturation. Reasons for this could include high
zooplankton levels (consuming oxygen through
respiration), and decomposition of material by bacteria
in mid to lower layers in the water column.
|
These data and the results relate well
and suggest the same as the oxygen data collected
offshore, building up a picture of the Fal estuary and
the nearby sea. Primary production supersaturates the
surface layers, leaving oxygen depleted areas where
there are high levels of decomposition and respiratory
activity. |
|
Chlorophyll
Chlorophyll samples were taken
down the estuary and can be used as an indicator for primary
production.
Table
10
- Secchi Disk Data from Estuary |
Station |
Secchi Disk Depth (m) |
Euphotic Depth (m) |
Attenuation Coef. (k) |
Upper Estuary 1 |
2.0 |
6.0 |
0.72 |
Upper Estuary 2 |
2.5 |
7.5 |
0.58 |
Upper Estuary 3 |
1.5 |
4.5 |
0.96 |
Upper Estuary 4 |
1.5 |
4.5 |
0.96 |
Lower Estuary 1 |
2.5 |
7.5 |
0.58 |
Lower Estuary 2 |
4.5 |
13.5 |
0.32 |
Lower Estuary 3 |
5.5 |
16.5 |
0.26 |
Lower Estuary 4 |
6.5 |
19.5 |
0.22 |
6.7a - Surface Chlorophyll in Upper
Estuary |
Figure 6.7a shows the surface chlorophyll samples taken
from the RIB. Rib 1 is taken from furthest up river with
rib 4 taken nearest Carrick Roads. The samples show
little variation with the highest values at rib site 3
and the lowest values at rib site 2. All values lay
between 4.27 and 5.03 µg/l. These values are low but the
depth at these stations was shallow (less than 6m) and
therefore suspended material in the water caused the euphotic zone to be very shallow (see table
10).
|
6.7b - Chlorophyll Against Depth in
Lower Estuary |
Figure 6.7b shows the chlorophyll samples from the Niskin
bottles from Xplorer plotted against depth. These
samples were fixed with acetone and then analysed in the
laboratory using a fluorometer. Station 1, 3 and 4 show
a chlorophyll minimum at depth, whereas station 2 shows a
chlorophyll maximum. The highest concentration of
chlorophyll is at station 1 at depth, and the lowest
concentrations are found at station 4. These data appear
inconsistent and it is difficult to find a trend in
the results through the estuary. |
6.7c - Flourometer Readings Against
Depth in Lower Estuary |
Figure 6.7c shows the fluorometer readings (representing
chlorophyll) from the CTD against depth. This figure
shows that the highest readings are found at station 1
which is the site furthest up river. Stations 2, 3 and 4
show similar concentrations to each other with values
between 0.1 and 0.4. None of the stations show any
distinct variation with depth.
These
results are inconsistent with results obtained from the
Niskin samples in figure 6.7b.
|
|
Plankton
Phytoplankton
Figure 6.8a shows the
population composition of phytoplankton
in the Fal estuary. RIB station 4 is at
the top of the estuary, the furthest
location from the mouth. Station 1 is a
little nearer the mouth, followed by
station 2 and 3 respectively and station
4 is at the mouth of the estuary. See
map Figure 6.1. |
Fig. 6.8a
- Phytoplankton at Upper Estuary Station 4 and
Lower Estuary Stations |
Fig. 6.8b -
Chaetoceros sp. |
Diatoms dominated the
phytoplankton population at stations 1,
2 and 3 but as we approached the sea at
station 4, dinoflagellates started to
dominate, as in all the offshore
stations. Surprisingly the rib station 4
(which was the furthest location from
the mouth) was found to have an almost
even number of dinoflagellates and
diatoms. Looking at the results of
stations 1 – 4, it would seem that
dinoflagellates flourish offshore
whereas diatoms grow well in the
estuary, however the values for rib
station 4 contradict this hypothesis. |
Station 2 had a much
higher total number of diatoms than any
of the other stations. The dominant
diatom group present were Chaetoceros
sp. and the main species of
dinoflagellate were Karenia mikimotoi. |
Fig. 6.9 -
Zooplankton at Locations A and B |
Fig. 6.9 shows the population
distributions of zooplankton sampled along trawls A and
B. Trawl A had an average of 38.2 animals per m3
whereas trawl B had over three times more with an
average of 126.2 individuals per m3. As well
as having more animals per m3, trawl B had a
more diverse array of taxa. Trawl A is dominated by
cirripede larvae and there were a small number of
hydromedusae, copepods and polychaete larvae present
too. Trawl B contained mainly copepods, cirripede and
gastropod larvae. There were also decapod and polychaete
larvae, copepod nauplii and hydromedusae present.
Neither of these trawls was as diverse in
terms of taxa when compared to the stations offshore and
they completely lacked cladocera, mysidacea,
chaetognatha, siphonophores, ctenophores, appendicularia,
fish larvae and fish eggs, and most interestingly,
echinoderm larvae, which were abundant in all the
offshore stations. Also, the total numbers of animals
per m3 in the estuary were considerably lower
than offshore values, where total numbers ranged from
721.5 to 2958.5 animals per m3. The effect of
the riverine input, nutrients and salinity may
account for these large differences in taxa compositions
between the estuary and offshore zooplankton. |
|
Discussion
The
investigation was carried out with the aim of interpreting the
biological, chemical and physical characteristics and
relationships seen in the Fal estuary and its tributaries. This
will give a better understanding of how the Fal estuary acts as
a transition zone between the freshwater river and the saline
coastal waters.
Physically,
the study area of the estuary was saline with little salinity
variation. Salinities ranged from around 35 / 36 near the mouth,
to 32 further up the estuary. This indicates that the Fal is a
well mixed estuary. A mixed area is usually typified by a
relatively high turbulence. The oxygen analysis of the upper
level of the water column (especially in the top 1m) shows
supersaturation. This suggests a turbulent flow and increased
wave action, as an increase in surface area of waves means more
oxygen will enter the surface through diffusion. Turbulence
increased towards the mouth of the estuary due to a lack of
sheltered environments. Mixing in the Fal is also aided by the
mesotidal characteristics of the estuary.
In terms of
the biology in the estuary, samples from the surface waters in
the upper estuary showed a dominance of diatoms (Chaetoceros
spp) whereas dinoflagellates were more abundant at station 4
nearer the mouth of the estuary. This may be due to physical or
chemical differences in properties (eg. salinity or nutrients)
of the water in different areas of the estuary. Conditions at
stations further towards the mouth will be similar to those of
the sea whereas stations nearer the head of the estuary will be
more influenced by riverine inputs. The largest counts of
phytoplankton were found at station 2; however the CTD
fluorometer read the largest values for chlorophyll at station
1. Taking the laboratory method to be the most accurate, this
shows that the CTD method may not be entirely efficient at
measuring chlorophyll. Also chlorophyll is only a proxy for
biomass. Sometimes many small phytoplankton are present with
little chlorophyll between them, thus low biomass is inferred,
when in fact cell numbers are quite high, though biomass remains
low.
Zooplankton
samples showed less diverse populations and fewer numbers of
individuals throughout the estuary than the samples taken
offshore. Most animals found in an estuary are marine in origin,
rather than freshwater so it may be that these zooplankton are
more abundant nearer the mouth at location B (as seen in the
results) due to better tolerance to the conditions there than at
location A. This may also explain why species diversity is
higher at the mouth and higher still offshore.
Throughout the
estuary, nitrate appears to show non conservative behaviour. It
had been removed from the system, possibly by phytoplankton.
Towards the seaward end nitrate is being totally used up by the
phytoplankton. As this is the first nutrient to be depleted, it
is the limiting factor in the estuary. Phosphate shows relative
conservative behaviour, and also very little spread in the range
of concentrations found. Silicon acts non-conservatively toward
the sea, suggesting that the phytoplankton that are using the
nutrients are diatoms, as they also require silicon.
|
|
|
|
Through the offshore, geophysical
and estuary studies, a picture of the Fal estuary has been
obtained.
The estuary appears to be a
characteristic well mixed estuary, mixed by tidal mixing and
wave action, with relatively low differences in temperature and
salinity through the water column. Further offshore, the
temperature varies much more through the water column, and
salinity varies less. Offshore, Lizard Point headland generated
a lot of shear, further increasing turbulence and mixing in the
water column.
Plankton biota assessments suggest
that diatoms dominate in the upper estuary and dinoflagellates
become more prevalent in the lower estuary and dominate
offshore. In both the estuary and offshore, nitrate appeared to
be the limiting nutrient, with noticeable reductions in silicon.
Offshore phosphate was also reduced, but in the estuary levels
were maintained , possibly due to anthropogenic sources.
Zooplankton are less diverse in the estuary compared to
offshore.
Chlorophyll levels were almost
universally low, related to a lack of the limiting nutrient
nitrate, though a couple of peaks were identified in the
offshore data.
The geophysical survey suggested
that the prevalent benthos was fragmented dead maerl, with a low
percentage of live maerl and broken shells. Dominante biota were
Black Brittle Stars and Banded Carpet Shells with a few
unidentifiable polycheate worms. Wave action creates sinuous
megaripples.
|
|
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|
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