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Falmouth Field Course 2008
Group 7 |
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CONTENTS
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Charlie Smillie David
Aldridge Alison Armstrong Trystan Colwyn-Thomas
Christos-Moritz Loukas
Mike Matson
Penelope Pickers Ashley Neve
Kathryn Weir Max Tam
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Introduction |
The Fal estuary is a
ria located in Cornwall on the South-West coast of England which
formed at the end of the last glacial period, approximately 10 000
years ago. The main water body within the estuary is known as
Carrick Roads, and has a maximum depth of 33m near the mouth; it is
the 3rd largest natural harbour in the world. The estuary
is fed by 6 tributaries and 28 small creeks but typically has
low riverine inputs making it tidally dominated. The tidal range
within
the estuary is dependent upon location, with the upper river region subject
to mesotidal conditions and the lower river region macrotidal conditions.
The maximum range on a spring tide is 5.3m with tidal currents
typically less than 2 knots.
The catchment of the
Fal is predominantly rural with significant nutrient input from
agricultural run-off of
arable and dairy farmland. Another anthropogenic source of nutrients
into the estuary is the sewage treatment works associated with the principal
urban centres of Falmouth and Truro. Consequently, some parts of the estuary
display elevated nitrogen and phosphorus concentrations which lead
to algal blooms, dissolved oxygen sags, and turbidity, which are all
symptomatic of eutrophication (Langston et al. 2006). Harmful algal
blooms have been observed in the upper Fal Estuary/ Truro River on
several occasions, such as in 1995-1996, when a ‘red tide’ of the dinoflagellate Alexandrium tamarense produced paralytic
shellfish poisoning (PSP) toxins. Harmful blooms linked to nutrient
enrichment continue to occur in the Fal and may also originate in
other parts of the system (Langston et al. 2006).
The Fal estuary is recognized as a Special
Area of Conservation (cSAC) to protect several important species and
habitats which reside within the region, such as the seagrass
Zostera, and Maerl.
Maerl is a calcareous algae which forms a matrix structure on the
seafloor and provides an ideal living environment for many juvenille
species, such as lobsters.
Unfortunately, mining and processing of metalliferous deposits has
lead to major
impacts on the biota and sediments of the Fal system since the bronze
age, with serious detrimental consequences. During the period of mining activity much of the remobilized metal was deposited in Restronguet
Creek; as a result the sediments of the estuary are some of the most heavily polluted with metals
in the UK (Pirrie et al. 2003). There is also evidence that some metals
have been transported to other parts of the system, particularly to
the adjacent creeks on the western side such as the Mylor and Pill
creeks, and to
the upper Fal (Warwick et al. 1998). Although the last mine was
abandoned in 1991, metals (such as Arsenic, Copper, Zinc, Cadmium
and Iron) are frequently flushed into the estuary from Restronguet
Creek and Carnon Valley after heavy rainfall. Secondary sources of
Copper and Zinc originate from the outfall at Falmouth Dockyard and
are indicated by increased concentrations in the water column.
Sporadic events of elevated
Zinc concentrations in the upper Fal may originate from a variety of
sources, including local sewage discharges and urban run-off
(Langston et al. 2006).
Tributyltin (TBT)
compounds used as antifouling paints on ships are also a source of
contamination within
the estuary. This compound leads to a phenomenon known as imposex in
the common dog whelk Nucella lapillus, resulting in female
organisms developing male characteristics (Spooner et al. 1991).
The use of TBT was banned on small vessels in 1987, however a ban on
larger vessels has only applied since 1st January 2008; TBT
concentrations in the water column are now low, due to the short
half life of TBT in open waters, but high concentrations still
persist in the sediments.
The aim of this field
course was to develop an understanding of the physical, biological,
chemical and geological processes in the Fal estuary by analysing data collected on three surveys using the
equipment and resources detailed below. Surveys were conducted in
the estuary and in offshore locations to obtain a wide range of
data. Click here to see
a map of survey areas.
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Equipment
and Resources |
Equipment
CTD & Rosette
(Fig 1.2)
A CTD
(Conductivity, Temperature and Depth profiler) is used to indicate the
vertical structure of the water column by measuring temperature,
salinity, and depth. The CTD probe and a Fluorometer can be mounted on
sampling devices such as the Rosette,
which allows for strategic water sampling on the up cast using Niskin
bottles, which can be closed by a messenger at predetermined depths.
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Figure 1.2 CTD and Rosette
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ADCP
The ADCP
(Acoustic Doppler Current Profiler) uses a sound principle known as the
Doppler effect to gather measurements on water current speed, direction,
and also acoustic backscatter. The data collected can also be applied
mathematically with the Richardson number to indicate the stability of
the water column.
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Secchi Disc (Fig 1.3)
A
Secchi Disc can be used to determine the
light attenuation coefficient (k) and the depth of the euphotic
zone. The euphotic zone is approximately 3 times the Secchi depth (the
depth at which the Secchi disc can no longer be seen in the water
from the surface).
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Figure 1.3 Secchi Disc
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YSI Probe
The YSI
probe is a multi-probe sensor, which collects data for temperature,
salinity, depth, dissolved oxygen, chlorophyll and turbidity. The
measurements can be taken at set time intervals as it is lowered slowly
through the water column.
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Sidescan Sonar (Fig 1.5)
The sidescan
sonar creates a detailed image of bathymetric features and types of
sediment by emitting ultrasonic sound waves ranging from 100 to 500kHz;
higher frequencies create more detailed resolution but less range. The sidescan sonar can be mounted on the vessel hull or towed behind in a
tow-fish; the latter option is more preferable as it reduces the effect
of boat movement and waves.
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Figure 1.4 Van Veen
Grab
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Van Veen Grab (Fig 1.3)
The Van
Veen is a simple and rugged type of grab, with long arms that give good
leverage to close its jaws. A ‘bite’ of sediment can be taken from the
seafloor; usually the grab collects well-defined surface areas which can
be analyzed in the lab.
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Figure 1.5 Deploying
the Sidescan Sonar on Xplorer
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Zooplankton Net (Fig 1.6)
A vertical closing zooplankton net (mesh size 200μm, opening
area 60cm) can be used to collect samples for taxonomic
identification. In most
cases, two sections of the water column are sampled
depending on the distribution of chlorophyll as determined
by the CTD. Once the net reaches the required depth, it is raised
as necessary and closed using a messenger. The sample is
stored in a plastic bottle and preserved with formalin.
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Figure 1.6 200µm Plankton Net
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Vessels
RV Callista
The RV Callista is
a 20m long offshore research vessel which can reach a maximum speed of
19 knots and carries up to thirty people. The stern deck has three
deployment points with a lifting capacity of 4 tonnes. Dry and wet
laboratories onboard enable in situ analysis of samples, and bow
thrusters assist manoeuvrability.
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RV Conway
The RV Bill Conway
is a 12m long inshore sampling vessel, commonly used for river and
estuarine surveying. It has a maximum speed of 14 knots and is able to
carry 14 passengers.
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Ocean Adventure RIB
The Ocean Adventure RIB is a 7m long inflatable
vessel which can carry 6 people and has a maximum speed of 35 knots. Due
to a low draft, the RIB is able to sample in shallow estuarine regions.
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Xplorer
The
Xplorer is a 12m long
geophysical survey vessel which can carry 14 people
and has a maximum speed of 25 knots. Onboard, there is a hydraulic
crane for lifting heavy equipment.
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For more
information on the RV Callista, Bill Conway and Ocean Adventure, click
here.
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Nutrient Analysis |
Phosphate
A stock standard solution of 15 μmol of phosphate
per litre was prepared and used to create calibration standards of 50,
100, 200, 500 and 1000μl. A mixed reagent was prepared consisting of 20%
Ammonium Molybdate, 50% Sulphuric Acid, 20% Ascorbic Acid, and 10%
Potassium Antimonyl Tartrate. 1ml of mixed reagent was added to all the
calibration standards and to 10ml of each of the samples collected on
the Callista. After one hour, the samples were measured in a spectrophotometer
set at 882nm, and the phosphate concentrations were calculated.
Chlorophyll
6ml of acetone was
added to the samples which were then stored over-night in a
refrigerator. This released the chlorophyll from the phytoplankton
cells which was then measured using a fluorometer. The chlorophyll
concentration was determined using the following equation:
Dissolved
Silicon
The method used to
determine the dissolved silicon concentration is outlined in Parsons et
al. (1984). A blue complex was created by adding a molybdate and a
reducing solution to the water samples, the synthesized solutions of
known silicon concentration, and the blanks. The absorbance of the final
products was then measured using a spectrophotometer. The concentration
of the silicon in the seawater samples was calculated once the
calibration curve was determined from the artificial silicon solutions.
Dissolved Oxygen
The dissolved oxygen was calculated using the
Winkler method as suggested by Grassoff et al. (1983).
Nitrate
Nitrate was analysed
using flow injection analysis, based on the method outlined by Johnson
and Petty (1983). The standards used were 1, 5 and 10 µmol/L.
Phytoplankton Taxonomic Identification Method
The samples collected were transferred to 100ml
measuring cylinders and left over night so that the phytoplankton would
settle to the bottom. A vacuum pump with a curved pipette was used to
remove the top 90ml of sample leaving the bottom 10ml, ensuring that
there was no phytoplankton re-suspension during the process. The 10ml
samples were observed under a microscope and the phytoplankton were
counted and identified using a Sedgewick-Rafter Chamber.
Zooplankton Taxonomic Identification Method
The samples were mixed to ensure an even
distribution of zooplankton, a 10ml sample was taken from the original
sample and viewed on a Bogorov Chamber under a microscope with the aid
of guide books. The numbers of individual species were counted and
recorded.
Figures 2.1 and 2.2 Identifying the
Plankton Net samples in the laboratory |
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Offshore
Sampling |
Introduction
N.B. All locations are
quoted relative to WGS 1984 and all times are in GMT.
- Date:
3rd July 2008
- Time:
08:00 - 16:00 GMT
- Weather Conditions: sunny, cloud cover 1/8th. Wind approximately 14
knots.
Figure 3.1 Map showing location of study (click to
enlarge)
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- Tides:
05:00
GMT high tide
5.0m
11:42
GMT low tide
0.9m
17:19
GMT high tide
5.4m
- Vessel:
Callista
- Aim:
To establish the location of the frontal system near Black Head (49'
59.888N, 005' 06.379W)
and determine the spatial variations of the stability of the water
column,
nutrients, and the chlorophyll maximum across the front .
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Responsibilities:
PSO |
Michael |
Dry Lab |
Kathryn |
Wet Lab |
Charlie,
Christos and Penelope |
Deployment
Team |
Trystan,
Alison and David |
Scribe |
Max |
Method
The original
plan of the survey was to collect data between Black Rock and
approximately 20 nautical miles offshore on a bearing of 135°, but due
to adverse weather conditions, this idea had to be aborted. Instead,
Callista travelled southwest from Black Rock (50'
08.345N, 005' 01.543W) to Black Head (49'
59.888N, 005' 06.379W) and then offshore on a bearing of 125° to stations 3, 4 and 5.
At each
station, the following equipment was deployed:
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A rosette frame
carrying:
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A CTD to
measure temperature and salinity with depth.
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A fluorometer to
measure changing fluorescence with depth.
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Niskin bottles
used to collect water samples for nutrient, oxygen, and chlorophyll
analysis.
- A
zooplankton net
(mesh size 200μm, opening area 50cm)
to collect
samples at varying depths for taxonomic identification.
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A secchi
disc to calculate the attenuation coefficient and the depth of the
euphotic zone.
In addition to
this, an ADCP attached to the hull of the boat was used to measure change in
backscatter, current velocity, and current direction with depth
for three transects between the following stations:
- Station 2 - Station
3 moving across the front from mixed to more stratified water.
- Station 3 - Station 4
moving from stratified water further offshore.
- Station 3 - Station 2
moving back over the front to reconfirm the front location at a
later time.
Please
Note: Unfortunately, due to a malfunction with the CTD, it was only
possible to take a surface sample at Station 5.
Safety Considerations
All members of the research team were
required to wear life jackets at all times. Great care was taken when
handling and during deployment of equipment (CTD etc). Those working on
the rear deck were secured by hook straps when handling the CTD. Ropes
were also used to steady the CTD in the deployment and retrieval process
to avoid damaging the equipment during rough conditions.
Water sample processing
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Nutrients:
water samples collected from niskin bottles were filtered through
glass fiber filters (GFF) to remove phytoplankton and zooplankton, and were
stored in glass bottles (100ml in total).
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Silicate: 60ml of
water sample was filtered through a GFF from each niskin bottle and was stored in a plastic sample bottle. This
removed the risk of silica contamination from using glass
bottles.
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Oxygen: great
care was taken not to unintentionally alter oxygen levels in samples
taken. Using a rubber dispensing tube, samples were decanted
straight from niskin bottles into glass stopper bottles to reduce
contact with the air. Manganous chloride and alkaline iodide were
added to each sample to fix the samples for later lab analysis (Winkler
method).
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Chlorophyll: GFF
used in above methods were stored in acetone solution for later lab analysis. Two filters were
stored per niskin bottle sample for comparison.
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Phytoplankton:
100ml of unfiltered water sample from each niskin bottle was
treated with 1ml of lugols solution (to fix samples) which were
stored in glass bottles for later lab analysis.
N.B. All locations are
quoted relative to WGS 1984 and all times are in GMT.
Figures 3.2, 3.3
and 3.4 Onboard Callista
Results |
(click images to
enlarge)
Figure 3.5
Phytoplankton - Station1
Figure 3.6
Phytoplankton - Station 2
Figure 3.7
Phytoplankton - Station 3
Figure 3.10 Phytoplankton Data
Figure 3.11
Zooplankton Data
Figure 3.14
Nutrients - Station 1
Figure 3.15
Nutrients - Station 2
Figure 3.19 Station
1 - Oxygen Saturation
Figure 3.20 Station
2 - Oxygen Saturation
Figure 3.23
Temperature - All Stations
Figure 3.24
Chlorophyll - All Stations
Figure 3.26 ADCP Data - Transect
from Station 2 to Station 3
Figure 3.28 ADCP Data - Transect
from Station 3 to Station 4
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Phytoplankton Taxonomy
Diatoms were the
dominant phytoplankton group in ten out of the twelve
samples taken, and were the only group found
ubiquitously in all the samples. Dinoflagellates
were dominant at the surface waters of Station 3 (figure 3.7) and at 15m
at Station 4 (figure 3.8). Ciliates were present in half
the samples and generally contributed little to the
total phytoplankton population. Total phytoplankton
counts were greatest
(111,000 cells l-1) at the surface at Station 2 (figure 3.6) and lowest (3010
cells l-1) at the surface at Station 1 (figure 3.5); the average number of
cells from all the samples is 47600 cells l-1. The abundance
of diatoms was generally high at all depths whereas dinoflagellates tended
to be more abundant at shallower depths and absent from the deepest
samples. Due to errors with the CTD, only a surface sample could be
collected at Station 5 (figure 3.9); this sample was
dominated by diatoms.
Zooplankton
A large range of zooplankton
orders were present in all the samples (figure 3.10). Out of
nine samples taken, seven were dominated by Copepods
(figure 3.12), whereas at
Stations 2 (5 - 0m) and 4 (10 - 0m) hydromedusae (figure
3.13) were
dominant; nevertheless hydromedusae made up a large proportion of the
populations of seven of the samples. Other orders making up
a significant percentage of the populations were Siphonophorae (Stations 1 and 4) and Echinoderm larvae
(figure 3.11) at Station 2 between 5 - 0m. The greatest abundance of zooplankton
was observed at Stations 4 and 5 (21300 and 24875m-1);
the lowest abundances were observed at Stations 2 and 3
(10601 and 10097m-1). With the exception of
Station 2, zooplankton were more abundant at depth.
When considering the
distribution and abundance of zooplankton at the
different stations sampled, the Shannon Wiener Index was
used to calculate the level of spread of numbers between
zooplankton groups sampled. Comparisons were made
between different stations and different depths within a
single station. For calculated values, see figure 3.10.
Station 1, located at
the mouth of the estuary, shows the
lowest value indicating lowest diversity of zooplankton
species amongst the 5 stations, with a smaller number of
zooplankton groups dominating. Station 2 shows little
variation between the two sample depths though this is
not significant. Station 3 shows the greatest difference
in index values between depths, with a higher value of
diversity found in the deeper sample (28 - 15m). This value (1.97)
was also the highest of all the stations and closest to Hmax, suggesting greatest similarity in the
number of individuals between groups of zooplankton
counted. Station 4 has almost identical
values at both sample depths and station 5, located at
the front, shows a slightly higher index value in the
surface sample, again indicating greater similarity in
numbers between zooplankton groups.
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(click images to
enlarge)
Figure 3.8
Phytoplankton - Station 4
Figure 3.9
Phytoplankton - Station 5
Figure 3.12 Copepod
Figure 3.13
Hydromedusae Larvae
Figure 3.16
Nutrients - Station 3
Figure 3.17
Nutrients - Station 4
Figure 3.18
Nutrients - Station 5
Figure 3.21 Station
3 - Oxygen Saturation
Figure 3.22 Station
4 - Oxygen Saturation
Figure 3.25
Salinity - All Stations
Figure 3.27 ADCP Data - Transect
from Station 3 to Station 2
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Nutrients and Chlorophyll
During sampling at the final
station (Station 5) technical difficulties with the CTD
and Rosette sampler did not allow for depth interval
water samples to be obtained. The data discussed in this
section therefore applies to stations 1,2,3 and 4; a
surface water sample was obtained from Station 5 to
allow some comparisons to be drawn.
Chlorophyll
Chlorophyll is an indirect measurement of phytoplankton
and was indicated by fluorescence and also by the
acetone extraction method using the water samples
collected. The findings show Station 1 (figures 3.14 and
3.24) and Station 2 (figures 3.15 and 3.24) to be well
mixed with chlorophyll values being homogenous with
depth. As the stations distanted from the shore,
stratification gave way to chlorophyll maximums
particularly in Station 3 (figures 3.16 and 3.24) and
Station 4 (figures 3.17 and 3.24). These chlorophyll
maximums are found at the thermocline at approximately
20m depth, where nutrient injections across the
thermocline and still moderate levels of light allow
preferable living conditions for phytoplankton.
Nitrate
Station 1 (figure 3.14) and 2 (figure 3.15) were both fairly well mixed stations and
showed a smaller range in nitrate concentrations with
depth compared to Stations 3 (figure 3.16) and 4 (figure
3.17), which were
stratified and showed the highest concentrations taken
from samples collected below the thermocline. At Station
1, nitrate concentration in both the surface sample and
at 14m depth was low with the surface sample measuring
0.4µmolL-1
and the 14m sample measuring 1µmolL-1.
At Station 2 three samples were taken with the
concentration found at the surface measuring 2.6 µM, the
lowest value at 5m measuring 1.4
µmolL-1
increasing by 0.3
µmolL-1
at the third, deepest sample site. Both Stations 3 and 4
show a depletion of nitrate in the water on and above the thermocline.
This was exemplified at station three where the difference
between the deep water sample and the thermocline sample
was 3.4
µmolL-1.
Dissolved Silicon
For Station 1
(figure
3.14)
the surface
silicon value was high measuring 7.5
µmolL-1 due
to the freshwater outflow from the Fal Esturary. The
concentration in the deeper sample taken in the
chlorophyll band is low measuring 0.6
µmolL-1. Station 2
(figure 3.15)
is not affected by
a major input of freshwater and therefore a high
concentration does not occur in the surface. A high
value of 11
µmolL-1
does occur in the 5m
sample, however this is unexpected and is thought to be
an anomalous value. Ignoring this value the
concentration at both the surface and bottom of the
profile is low, measuring below 2.5
µmolL-1. Both
Stations 3
(figure
3.16) and 4 (figure 3.17)
showed lower concentration values above and on the
thermocline
compared to deeper in the water column.
This
was displayed
in Station 4
where
the surface value was 0.9 µmolL-1 and was 1.8
µmolL-1 at 50m.
Phosphate
High
concentrations of phosphate, 2.4 µmolL-1 and
1.6µmolL-1, are found respectively in the
surface waters at Station 1
(figure
3.14)
and 2
(figure
3.15).
This is likely due to an increased
influence of freshwater from the Fal Estuary for Station
1 and the increased proximity to the land and hence
surface runoff for station 2. At Stations 3
(figure
3.16)
and 4 (figure
3.17)
the water column is stratified,
since the stations are located further offshore in
deeper waters where there is less friction with the
seabed (hence less turbulence and a more stable,
stratified water column). The distance from any land
source explains why phosphate levels are much lower when
compared to dissolved silicon and nitrate; an increase
in concentration was found in samples below the
thermocline for silicon and nitrogen, but not for
phosphate.
It is important to consider that phosphate samples are
easily contaminated, which could explain some of the
anomalous values, for example a higher concentration of
1.2µmolL-1 found at 20m at Station 4.
Generally, the range of concentrations is small across
the entire data set.
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Oxygen
Saturation
During sampling at the final
station (Station 5) technical difficulties with the CTD
and Rosette sampler did not allow for depth interval
water samples to be obtained. The data discussed in this
section therefore applies to Stations 1,2,3 and 4; a
surface water sample was obtained from Station 5 to
allow some comparisons to be drawn.
Oxygen saturation has
been established from samples taken from the Niskin
bottles with 2-3 values been attained from all stations
except for Station 5 due to faulty equipment. A more
continuous profile and duplicate samples would be
preferable in order to draw more accurate conclusions.
In general however, the surface values of oxygen tend
to be saturated or supersaturated with an increase in
oxygen absorbance due to upper layer turbulence.
Phytoplankton photosynthesis also leads to addition of
oxygen to the water column and therefore it is the
balance between the rate of respiration and
photosynthesis that determines the saturation state of
the water.
Station 1 (figure 3.19) and Station 2 (figure 3.20)
The oxygen saturation values correlate well with the
temperature and chlorophyll profiles remaining fairly
constant down through the water column. For station 1
both readings are above 100% and so are supersaturated
with oxygen being added to the water through
phytoplankton photosynthesis. For station two the values
are not supersaturated but are just below 100% ranging
from 97.4 - 99.8%. Station two may be slightly
under-saturated compared to station 1 as the
fluorescence values and hence phytoplankton number is
lower however a definite explanation cannot be reached
due to the unreliable nature of the data.
Station 3 (figure 3.21) and 4 (figure 3.22)
At station three and four the water
column is stratified with the chlorophyll maximum
occurring in the thermocline, at approximately 20m. This
corresponds well with the dissolved oxygen data which
remains supersaturated at the surface and at 20m
indicating therefore that the rate of photosynthesis is
greater than that of respiration at these depths.
However, the data collected below the thermocline for
both stations is undersaturated, for example at station
3 it is 90% at 60m, therefore more oxygen is being used
for respiration than is being produced through
photosynthesis.
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Temperature at all Stations
The temperature
profiles (figure 3.23) between stations display the expected relationship, with
more stratified waters located in the offshore stations (Station 3
and Station 4). This is due to a reduction in turbulent kinetic
energy, which is driven by internal waves from sea-bed friction (see
figure below). Therefore the deeper waters have less vertical mixing
processes taking place so the temperature can become more statically
stable.
Figure 3.20 Schematic Diagram of a Frontal System
Salinity at all Stations
The salinity
profiles of all stations (figure 3.25) display a similar behaviour with depth,
where the salinity values closely fluctuate around 35.2. The
exception to this is Station 1, which is located near a freshwater
source which is less dense and overlies the denser saline sea
waters.
( Anomolies in data
for station 5 must be considered, where an
uncharacteristic spike in readings is seen at ~25m)
Chlorophyll
Above
the thermocline, chlorophyll levels are relatively low,
most likely due to nutrient limitation (especially in
stratified waters), even though there is an abundance of
available light. The chlorophyll profiles for all stations (figure 3.24)
show a maximum at approximately 10 - 20 metres depth,
with some variation between stations. This indicates an
optimum depth where there are sufficient sources of both
light and nutrients (replenished from depth) to allow
for a maximum in primary production. Below the
thermocline, chlorophyll concentrations diminish with
depth due to a decrease in irradiance, even though
nutrient abundance increases (i.e. the phytoplankton are
light limited).
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ADCP
Station
2 to Station 3 (figure 3.26)
The transect between station 2 and 3 shows the
progressive transformation from mixed water to a
frontal system. On the right of the picture (from
station 2 to about 135m away from station 2), the water
column is mixed, due to the low
backscatter values throughout the water column; higher
backscatter at the surface layer is caused by bubbles
and turbulence. The velocity of the water currents
increases dramatically at the front and on the stratified
side of the front.
Station 3 to Station
2
(figure 3.27)
The front can be distinguished by
observing the sharp changes in current direction and
velocity. The southwest flowing current is the remainder
of the ebb tide flow from the English Channel and the
eastward incoming flow is the beginning of the flood
tide coming up from the Atlantic.
Velocity decreases below
the thermocline in the stratified waters of the offshore
regions beyond the front, where average current velocity
reaches values as low as 0.026m/s. The inshore regions,
where water is more mixed, have significantly higher
velocity values (up to 1.104m/s), and tend to reach
several minima (usually <0.375m/s) in lower depths, and
near the opposing flow. Just above the
seabed, current velocities are high, occasionally
exceeding 2m/s.
The higher backscatter values in
near-surface waters suggest the presence of zooplankton
populations. Beyond the front, the
main band of backscatter (and therefore zooplankton) can
be found between the surface and 25m; this corresponds
to the waters overlying the thermocline. As the thermocline weakens inshore
(due to increased mixing)
backscatter, and hence the zooplankton population, is more
evenly distributed throughout the water column.
Transect between station 3 and 4 (figure 3.28)
The backscatter values generally peak at around 20 – 30m,
resulting from high zooplankton populations; this is an indication of the depth of the thermocline in
the water column. There is a
depletion of nutrients in the stratified layer;
consequently, the plankton
are positioned just above the thermocline to obtain
nutrients from the mixed waters below whilst still
recieving sufficient light for photosynthesis.
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Richardson Number
The
Richardson Number (Ri) for Stations 2 and 3 was calculated
to show the balance between the stabilizing effect of the
density gradient and the destabilizing effect of the
current shear. Calculation of Ri required the velocity
data from the ADCP as well as the density data from the CTD from a specific location.
This meant that the calculation of Ri was restricted to
Stations 2 and 3.
The Richardson Number indicates water column
stability as follows:
When below 0.25 – gravitational unstable, overturning occurs
When between 0.25 and 1 – shear flow instability develops
When above 1 – the flow is stable and no mixing occurs
between layers
Station 2 shows clearly that there are two well-mixed
water masses: from the surface to 10m, and from 12m to
25m. There is an increase in stability at approximately
10m below the surface. This suggests that the water
column at this station is generally well-mixed with some
stratification.
Richardson Number
graph for Station 2
At
Station 3, there are some very
large values for Ri, therefore a logarithmic scale was
used in order to show the variation more clearly.
On the logarithmic scale, 1 equals
1 for Ri, and -0.6 equals 0.25.
The fluctuation of Ri for Station 3 was large and
frequent. Apart from the stable layer at the top 5m of
the water column,
the rest of the water column above
the thermocline was well-mixed.
The thermocline spanned from
approximately 16m to 40m (see figure 3.16). The
Richardson Number also varied significantly within this
section, affecting stability accordingly
and
sugggestin that the stratification of the water column
was not fully developed and mixing was still occurring
to some extent.
Richardson Number
graph for Station 3
The water column at Station 4 was fully stratified as shown by the temperature difference between the
upper and lower layers, and a distinct thermocline on the CTD
profile. The Richardson Number profile shows a
substantial stable layer at approximately 15m which
corresponds to the thermocline. The Ri profile
suggests that the water mass above the thermocline was
well mixed; the lower layer of the water column was also
well mixed in general, but there were some of stable
layers developing. A strong stable layer was also
present at approximately 42m.
Richardson Number graph for Station 4
|
Discussion
At Stations
3 and 4 nutrients were generally lower above the thermocline, with
concentrations increasing below the thermocline. However, at
Stations 1 and 2, excluding higher nutrient concentrations at the
surface due to greater freshwater influence, nutrient concentrations
did not increase significantly with depth. The nutrient data relates
well to the fluorescence profiles and hence distribution of
chlorophyll, indicating that when plankton populations are
large, nutrient concentrations are low. At Stations 3 and 4,
phytoplankton are trapped above the thermocline as it acts as a
barrier to mixing; hence nutrients are gradually depleted in this
layer. In the mixed water, phytoplankton are evenly distributed in
the water column and nutrient
concentrations
remain more constant with depth.
Diatoms
were found to be the most dominant phytoplankton
group at the mixed stations (1 and 2) and also in the surface sample
taken from Station 5
at the front. However, in the stratified waters of Stations 3 and 4,
dinoflagellates were the dominant
group. This
is the expected result as diatoms are non-motile and fast growing,
preferring mixed waters which enable them to remain in the euphotic
zone. Dinoflagellates, however, are more suited to stratified water
conditions,
since they have lower nutrient requirements
and their motility enables them to
remain in the euphotic zone. Copepods were the dominant zooplankton order with the greatest
abundance found at the front itself and at the stratified stations
beyond; this pattern was also observed for total zooplankton
abundance.
It would be expected that the highest
concentrations of zooplankton would occur at the tidal front
coinciding with enhanced primary production in this region (Munk,
1993). However, there is also evidence that tidal fronts are
important areas of ‘larval retention’ due to their stability.
According to Sinclair & Iles (1985), larvae are distributed in these
regions in spite of and not because of the food resources there.
In general,
the euphotic zone deepens with distance from the coast, although the
figures obtained from the CTD data are considerably larger at
Stations 3 and 4 than those calculated from the Secchi Disc depth.
Increased light attenuation occurs in the shallower coastal waters
where there is more mixing and larger amounts of suspended sediment.
The deeper euphotic zone in the stratified water allows the
chlorophyll maximum to occur at the thermocline, where episodic
nutrient injections create sustainable growing conditions.
Station |
Depth of Euphotic from
Secchi Disc Data (m) |
Attenuation Coefficient
(k) from Secchi Disc Data (m¯¹) |
Depth of Euphotic Zone from CTD Data (m) |
1 |
13.5 |
0.32 |
>16.2 |
2 |
19.5 |
0.22 |
20.2 |
3 |
21.0 |
0.21 |
33.4 |
4 |
24.0 |
0.18 |
46.5 |
5 |
22.5 |
0.19 |
26.8 |
The
chlorophyll values obtained from the acetone extraction method show
a correlation with the fluorometer readings (Figures 3.14 - 3.18),
particularly at Station 3 (Figure 3.16) where the chlorophyll
maximum found at approximately 20m depth coincides with high
chlorophyll values (~2.2μgL-1). The oxygen saturation
also closely follows the fluorometer readings (Figures 3.19 - 3.22)
with an increase in oxygen at the chlorophyll maxima.
Observations
of the frontal system located south of Blackhead made by this study
correlate with previous work completed by the
Western Channel Observatory.
The variation in physical conditions either side of the
front, such as light availability, turbulence and temperature, have
strong influences on the chemical and biological characteristics of
the water column. As predicted, the water column at Stations 1 and 2
(mixed side of the front) was much more turbulent than on the
stratified side of the front (Stations 3 and 4). Consequently,
nutrient concentrations at Stations 1 and 2 were much higher in the
surface waters than those at Stations 3 and 4, due to nutrient
replenishment by upwelling from below the thermocline. Nutrient
concentrations at the front (Station 5) were hard to determine, due
to faulty equipment, but the surface samples obtained have nutrient
concentrations which are similar to Stations 1 and 2. This is
probably due to mixing at the stratified/mixed water interface by
turbulent eddies which results from interfacial friction.
Differences in current velocity across the front can be clearly seen
in figure 3.27; the variation is typical of a frontal system and
clearly defines the mixed and stratified waters.
It is important to remember that all of the
conclusions detailed above are subject to change due to variations
in tidal and weather conditions.
|
Geophysics |
Introduction
N.B. All locations are
quoted relative to WGS 1984 and all times are in GMT.
-
Date:
7/7/08
-
Time:
-
Weather
Conditions: Cloud cover 7/8, westerly winds approximately 20mph,
rainy spells and some sunny intervals.
Figure 4.1 Map showing survey transects
(click to enlarge)
|
-
Tides at
Helford River:
08:25
GMT High tide 4.9m
14:43
GMT Low tide 0.7m
20:30
GMT High tide 5.1m
-
Vessel:
Xplorer
-
Aim:
Looking for benthic features and habitats at the mouth of the
Helford River.
PSO
(i.e. The Boss) |
Ashley |
Side
Scan Sonar |
Michael |
Track Plot |
Trystan |
Grab
Samples Identification and Recording |
Charlie, David, Max and Alison |
Photographer |
Christos |
Observer and Position Scribe |
Kathryn |
Scribe and TS Probe |
Penelope |
Method
The “tow
fish” was deployed behind Xplorer at (long, lat) and towed for 4
transect lines which ran parallel to each other near to the mouth of
the Helford river. Each line was 2km long and the swath width was
150m. The transect lines overlapped each other by ….m ensuring there
were no gaps between the lines. While the side scan sonar was
running an observer recorded buoys and boats in the area which may
have affected the results. Positions of interesting features were
also recorded so that grab samples could be collected after the
transect lines were completed. Grabs were collected at four sites
using a Van-Veen grab, which was deployed at the same time as a
camera to support the grab data.
Time (GMT) |
Detail |
Latitude |
Longitude |
10:30 |
Start of Line GR7 |
50°
05.571 N |
005° 05.140 W |
10:45 |
End
of Line GR7 |
50°
06.372 N |
005° 04.142 W |
10:49 |
Start of Line GR8 |
50°
06.404 N |
005° 04.206 W |
11:03 |
End
of Line GR8 |
50°
05.595 N |
005° 05.216 W |
11:06 |
Start of Line GR9 |
50°
05.632 N |
005° 05.285 W |
11:19 |
End
of Line GR9 |
50°
06.445 N |
005° 04.290 W |
11:23 |
Start of Line GR10 |
50°
06.504 N |
005° 04.333 W |
11:36 |
End
of Line GR10 |
50°
05.661 N |
005° 05.345 W |
Figures 4.2, 4.3 and 4.4 Using the Equipment on Explorer |
|
|
|
Results |
Geophysics Analysis
Site 1
Site one is
comprised of a drape of finer sediment overlying courser
sediment, lying perpendicular to the track plots (figure
5.1). The flow direction of the drape is unclear as
there is evidence to support both north-westerly and
south-easterly flow. A thinning line of coarser sediment
is seen extending through the drape, this could be due
to an obstacle creating a shadow zone behind it. The
long length of this ‘shadow zone’ suggests that the
drape is a region of fast flow. The fast flow is further
supported by a lack of bedforms in both the drape and
surrounding coarser sediment, suggesting an area of
upper flat beds according to the bed form classification
system relating mean flow velocity to mean sediment size
(Allen, 1982). The drape is funnelled between two areas
of bedrock and then fans out again suggesting
north-easterly flow. However, south-easterly flow is
supported by the presence lobes, located near the point
11:36:39 on the side scan map.
Site 2
Site 2 exhibits complex patterns of high and low
backscatter suggesting a region of rock (figure 5.5).
The strike of his rock corresponds to the strike of the
headland suggesting that over time the headland has been
eroded back (Goode, 1990).
Site 3A
Site 3B
Megaripples are
found in both site 3A and 3B (figure 5.2) due to the
Helford river outflow creating high energy currents,
which are amplified by the current being diffracted by
the neighbouring rock mass. During our survey the winds
were westerly; winds from this direction are funnelled
through the Helford River Basin creating surface waves
and a more turbulent water column. Evidence of
wave-formed ripples is seen from bifurcation. Moving
from site 3A to 3B some energy is dissipated and the
megaripples decrease slightly in size.
Site 3C
Site 3C is a
channel of coarse sediment found between two rocky
bedforms (figure 5.4), which likely follows an eroded
line of weakness following the strike of the surrounding
bedrock. Ripples are found in the narrower section of
the channel but as the channel width increases this
bedform is gradually lost. The loss of ripples suggests
a decrease in energy likely due to the current being
spread over a larger area.
Figure 5.1
Fine Sediment Channel (Site 1) |
Figure 5.2
Megaripples (Sites 3A and 3B) |
Figure 5.3
Seabed Classification Map |
Figure 5.4
Channel of Coarse Sediment (Site 3C) |
Figure 5.5
Bedrock (Site 2) |
|
|
|
|
|
Click on above images to enlarge
Van Veen Grabs
The grab sites were predetermined by the sidescan sonar
imagery at four locations. The sites chosen were areas
which would provide a respectable grab, however grab
sites 1 and 2, were not very successful with little
material obtained. The sites were high in algal growth,
which was perceived on the sonagraph as a suitable grab
location. The overall area consisted of the Devonian
slate of the Portscatho Formation. The sediments were
coarse to medium grained with shell fragments present.
The grabs also showed an abundance of maerl in the
samples, particularly in grab 4 (Figure 5.9)
Grab Site
1 (Figure 5.6):
Location
- 50o05.831 N 05o05.117W
Time
-11:55
Sea bed
composition
– Video revealed that the sea bed composition consisted
mainly of the Devonian slate rocks with finer sand
grains in between the gaps (Figure 5.6). This made
it difficult to collect a good sample, with the two
grabs taken only managing to retrieve large rock
fragments. However, live maerl was found present on
approximately 5% of the rock, with some dead species
also retrieved.
Fauna &
Flora
- Laminaria digitata, Bryozoa sp., Crustacea (Pagarus
bernhardus, Porcellana platycheles), Gastropoda (Gibbula
sp.), Polychaete worm sp., Ascidian sp., live Maerl.
Grab site
2 (Figure 5.7):
Location
- 50o06.083 N 05o04.714W
Time-
12:20
Sea bed
composition
– Video revealed fine sediment with a few larger rock
fragments. Three grabs were attempted but rocks jammed
the jaws of the grab, therefore, fine sediment was lost.
It is a possible that sediment is compacted, which
therefore resulted in poor penetration of grab into sea
bed. Live maerl was present on approximately 3% of rock
surface, with dead maerl also present.
Fauna &
Flora
– Leptochitonidae, Bryozoa sp., live Maerl,
Marthasterias glacialis (observed from video
camera).
Grab site
3 (Figure 5.8):
Location
- 50o06.177 N 05o04.364W
Time-12:
43
Sea bed
composition –
Video revealed sea bed consisting predominantly of Maerl.
This was confirmed with the grab, which consisted of
approximately 80% Maerl with the rest of the sample
consisting of small rocks and bionic material such as
small shell fragments.
Fauna &
Flora
– Bivalve, Polychaete sp., Nematoda sp. live Maerl.
Grab site
4 (Figure 5.9):
Location -
50o06.377 N 05o04.353W
Time-
13:02
Sea bed
composition –
Video revealed sea bed consisting predominantly of maerl.
Grab sample confirms this and is very similar in
composition to the sample from grab site 3 with the
sample consisting of approximately 70% maerl,
approximately 10% intact bivalve shells, and the rest
consisting of small rocks and bionic material such as
shell fragments.
Fauna &
Flora
– Bivalve, Polychaete sp., Porcellana platycheles,
live Maerl, Asterias rubens (observed from video
camera).
Video
transect
|
Latitude |
Longitude |
Time (GMT) |
Start
Location (Figure 5.10) |
50o 06.245 N |
005o 04.617 W |
01:20 |
End Location
(Figure 5.11) |
50o 06.172 N |
005o 04.438 W |
01:37 |
Sea bed
composition –
The beginning of the transect is predominantly rocky
(FIGURE) progressing to finer sediment approximately 1/3
of the way through. This fine sediment persists for
approximately 10-15m before the substratum returns to a
rocky composition for the second third of the transect.
The final third consists of fine sediment with outcrops
of rock (FIGURE).
Flora and
Fauna –
Rhodophyceae, Chlorophyceae, Phaeophyceae, Echinidae,
Asteriidae, Demospongiae
Click on above images to enlarge
|
Discussion
The
results from the sidescan sonar clearly display the bathymetry of
the surveyed area. They can be divided into three main sections
according to their sediment types and bed forms. Section 1 is
comprised of a drape of finer sediment overlying coarser sediment. A
narrowing line of coarse sediment and the lack of bedform formation
on the fine sediment suggest that the flow is fast.
Section 2 has a rocky sea bed and the strike direction on the rock
matches the strike direction of the headland. Section 3 is an area
of erosion of the adjacent rocky region, and is covered by coarse
sediment. Megaripples in section 3A and 3B are formed by the high
energy outflow from the Helford River and from diffraction by the
headland. Section 3C is a channel of coarse sediment caused by the
erosion of a line of weakness between two rocky bedforms.
The
results of the Sidescan sonar are supported by the grab samples and
the seafloor video clips. The sea bed is mainly composed of Devonian
slate rocks with finer sand grains in between the gaps. Maerl is the
predominate plant species in this region. Bivalves and Gastropods
are the most common macrofauna.
|
Estuarine
Sampling |
Introduction
N.B. All locations are
quoted relative to WGS 1984 and all times are in GMT.
Figure 6.1
(Above left) Map of Bill Conway Sampling Locations
Figure 6.2
(Above right) Map of RIB Sampling Locations
(Click to enlarge
images) |
- Date:
10.07.08
- Time:
0800-1300
-
Weather Conditions: Cumulus clouds, 8 octants. Wind speed
approximately 8-10ms¯¹.
- Tides:
0429 Low 1.4m
1024 High 4.5m
1644 Low 1.7m
- Vessels:
Bill Conway and Ocean Adventure RIB.
- Aim:
To investigate spatial variations in nutrients, temperature,
salinity, chlorophyll and plankton populations within the Fal
Estuary.
- Responsibilities:
PSO |
Trystan |
Bill Conway |
Dry Lab |
Kathryn |
Wet Lab |
Penelope and Dave |
Deployment Team |
Ashley and Mike |
Scribe |
Alison |
RIB |
Chris, Charlie, and Max |
Method
Both the Bill Conway and the
RIB where used to collect data, with the RIB being used to sample to
shallower regions in the upper reaches of the estuary.
On the Bill Conway four
transects where taken using the ADCP. Along each transect the
following data was also collected:
-
Transect 1: Location -
Black Rock (for locations of transects see figures 6.1 and 6.2). At
Station 1, a CTD profile was recorded and water
samples where collected at three depths using Niskin Bottles mounted
on a Rosette. A zooplankton net was also towed at 2 knots for 1
minute.
-
Transect 2: Location -
north of Restronguet Creek (above Carick Carlys Rock). Surface water
samples, taken from the deck wash, were collected at the beginning and end of the transect line,
with the third sample taken in the at Station 2 in the deep channel
region where a CTD profile was also recorded.
-
Transect 3: Location -
Turnaware Point. Surface samples where taken at the beginning
and end of the transect line. At Station 3, as for Station 1, a CTD
profile was recorded, three water samples were taken; a
zooplankton net was also used.
-
Transect 4: Location -
Penarrow Point to Messack Point. Station 4 was located in the
deep channel region where a CTD profile was recorded and water
samples where collected at three depths.
Note:
- Faulty equipment: The CTD
mounted on the Rosette was broken and so a YSI was used to measure
salinity, temperature, and depth, and the data was recorded from a
hand held monitor.
- A reduced number of water
samples were collected due to reduced time available for collecting
data.
- Unfortunately, due to
a problem with transferring data from the boat, the ADCP data
was not able to be analysed in time to be displayed on this
webpage.
The Latitude and Longitude of
each transect and station is shown below:
Transect 1: 50°
08.472 N, 005° 01.112 W to 50° 08.649 N, 005° 02.442 W
Station 1: 50° 08.657 N, 005° 01.606 W
Transect 2: 50°
11.716 N, 005° 03.293 W to 50° 11.762 N, 005° 01.850 W
Station 2: 50° 11.737 N, 005° 02.747 W
Transect 3: 50°
12.270 N, 005° 02.393 W to 50° 12.240 N, 005° 02.134 W
Station 3: 50° 12.223 N, 005° 02.376 W
Transect 4: 50°
10.487 N, 005° 02.527 W to 50° 10.899 N, 005° 01.529 W
Station 4: 50° 10.785 N, 005° 01.711 W
The RIB collected the following
data from four stations:
- A temperature and Salinity
profile.
- A surface water sample
using a Niskin Bottle.
- At Stations 1 and 2,
before and after the mussel beds, a zooplankton net was towed at
1 knot for 3 minutes.
The Latitude and Longitude
of each RIB station is shown below:
- Station 1: 50° 12.560
N, 005° 01.671 W
- Station 2: 50° 12.957
N, 005° 01.667 W
- Station 3: 50° 13.718
N, 005° 00.951 W
- Station 4: 50° 14.706
N, 005° 01.374 W
Safety Considerations
All members of the research team were
required to wear life jackets at all times. Great care was taken when
handling and during deployment of the equipment (CTD etc).
Water sample processing
For all the samples collected in the
Niskin Bottles, the processing procedure was as explained for Callista.
For samples collected from the
surface using the deck wash, oxygen analysis was not carried out as the
oxygen content of the sample would be altered due to increased exposure
to air.
Figure 6.3 A Front in the Estuary |
Figure 6.4 Using the CTD |
Figure 6.5 Langmuir Circulation |
|
|
|
Results |
(Click on Images to Enlarge)
Figure 6.6 Phytoplankton
Figure 6.8 Nitrate
Mixing Diagram
Figure 6.11 Nutrients - Conway, Station 1
Figure 6.12 Nutrients -Conway, Station 2
Figure 6.15 Nutrients - RIB, Station 1
Figure 6.16 Nutrients - RIB, Station 2
Figure 6.19 Oxygen - Conway, Station 1
Figure 6.20 Oxygen - Conway, Station 3
Figure 6.22 Oxygen - RIB, Station 1
Figure 6.25
Temperature - All Stations
|
Phytoplankton
N.B The
sample collected from Station 2 contained no
phytoplankton, which is highly unlikely looking at the
other results, and so it can be assumed that there was
an error in the collection or processing of this data;
therefore Station 2 phytoplankton data are not included
in the results.
Diatoms
were the most abundant group, with 421 cells found in
total, and were present at every station. The largest
number of diatoms (121
cells l-1)
was found by the RIB Team at Station 3 (figure 6.6).
Ciliates were the least abundant, with only 14 cells
found in all the samples collected. Dinoflagellates are
more abundant, with a total of 39 cells, although they
are only found in four of the samples. As all of the
samples were collected from the surface, it is not
possible to see how phytoplankton varies with depth in
the estuary.
Zooplankton
The Station 1 sample was taken from the mouth of the
estuary at the boundary between Carrick Roads and the
offshore region (50o 08.657 N, 005o
01.606 W). At this site the greatest numbers of
zooplankton were seen from all inshore samples. Dominant
species include Siphonophores, which are more
characteristic of benthic conditions, and copepods.
(figure 6.7)
Station 3 signifies the northern limit of Carrick Roads,
where the Truro river meets Carrick roads (50o
12.223 N, 005o 02.376 W). Zooplankton
sampled at this site were reduced in number and slightly
different in composition to Station 1. Copepods remained
the dominant order along with Cirripede larvae
(barnacles); numbers of Siphonophores were greatly
reduced (figure 6.7).
Samples X and Y were taken from the Ocean Adventure RIB.
Total numbers counted at both of these sites were
greatly reduced when compared to Stations 1 and 3.
Sample Y was sampled south of the mussel farm on Truro
river (50o 12.560 N, 005o 01.671
W). Although much smaller in numbers, proportions of
Cirripede larvae and Copepod larvae were similar to
those of Station 3. Siphonophore numbers were again
reduced (figure 6.7). Sample X was taken north of the
mussel farm (50o 12.957 N, 005o
01.668 W). The composition at Y shows a relatively even
spread in numbers of each order compared to other sites,
and this is also seen in the high Shannon index value.
Copepods and Cirripede larvae remain two of the dominant
orders present (figure 6.7).
|
(Click on Images to Enlarge)
Figure 6.7 Zooplankton
Figure 6.9 Phosphate Mixing Diagram
Figure 6.10 Silica Mixing Diagram
Figure 6.13 Nutrients - Conway, Station 3
Figure 6.14 Nutrients - Conway, Station 4
Figure 6.17 Nutrients - RIB, Station 3
Figure 6.18 Nutrients - RIB, Station 4
Figure 6.21 Oxygen - Conway, Station 4
Figure 6.23 Oxygen - RIB, Station 2
Figure 6.24 Oxygen - RIB, Station 3
Figure 6.26 Diagram displaying transects
Figure 6.27
Temperature/ Salinity Graph
|
Estuarine Mixing
Diagrams
Concentrations for
dissolved phosphate, nitrate and silica were plotted
against salinity (figures 6.8 - 6.10). By comparing these values to
the theoretical dilution line (TDL), representing
conservative behaviour, the chemical behaviour of these
nutrients can be examined within the Fal estuary.
Dissolved nitrate
values lie close to the TDL indicating conservative behaviour
at salinities over 24. One value (175.97 µmol/L at 30.95
salinity) is much higher than would be expected and is
possibly due to contamination of the sample, or high
localized concentrations of nitrate from anthropogenic
or natural sources.
All but one of the
values for silica lies above the TDL indicating
non-conservative addition of silica in the Fal estuary
at salinities greater than 24.
Phosphate values show
the greatest divergence from the TDL. The behaviour is
non-conservative and shows evidence of large amounts of
addition at salinities greater than 24. Two of the
values, at salinities of 30.40 and 25.60, are extremely
low (<0.001 µmol/L). These values are so much lower than
all other values that they may be considered anomalous
due to errors involved in collection of the samples, or
in chemical analysis. However, it is still possible that
these values may represent localised removal by the
biota.
The highest
concentrations 0f silica and nitrate occur at the
riverine end-member and decrease with increasing
salinities. The highest values of phosphate are found at
higher salinities and decrease towards
the riverine end-member.
|
Nutrients, Salinity and Chlorophyll
Salinity
Station 1 (Figure 6.11)
clearly shows a well-mixed profile, where a salinity
value of 35 is maintained throughout the water column
down to 7m. Stations 2, 3 and 4 (Figures 6.12, 6.13 and
6.14) give evidence of some stratification, with less
saline water found at the surface in comparison to the
bottom waters. Continuing to the stations visited by the
RIB, stronger stratification takes place, where two
layers of water can be distinguished with a saline layer
lower in the water column (>32) and a fresher body of
water at the surface (<28) (Figures 6.15, 6.16, 6.17 and
6.18).
Nitrate
Station 1 (figure 6.11)
is relatively well-mixed throughout the water column.
The nitrate data collected supports this pattern, with
low concentrations (~12.6µmol/L), which remain almost
constant with depth. Towards the head of the Fal estuary
there is a steady increase in nitrate concentration.
This is due to the formation of a two-layered water
column with a lower saline layer and an upper fresh
layer, where nitrate reaches higher concentration values
closer to the surface. At RIB Station 3 (figure 6.24)
concentrations decrease slightly (23µmol/L at 1m), which
is a result from the absence of freshwater input from
the Fal river. The highest nitrate concentrations were
detected at RIB Station 4 (170 - 175 µmol/L at 1m) and
several minima were found at Stations 1 to 4
(approximately 3 - 6µmol/L), this is likely to be due to
a point source created from anthropogenic inputs.
Phosphate
For the
results collected from Conway, phosphate behaves
similarly to nitrate in the water column. For the
results collected from the RIB stations, the
concentrations were higher at Station 1 and 2 (figures
6.15 and 6.16) ~1.3 µmol/L. Station 3 (Figure 6.17) had
a concentration of ~0.9µmol/L,
and the concentration at Station 4 (figure 6.18)
was similar to the results from Conway at ~0.5µmol/L.
RIB Stations 1 and 2
(figure 6.15 and 6.16) had considerably high values,
confirming that there was a point source of pollution
nearby.
Dissolved
Silicon
At Station
1 (figure 6.11), dissolved silicon has a constant
concentration (~2µmol/L)
throughout the water column. At Station 3 (figure 6.13)
there is a negative linear relationship in the water
column with the highest concentration at the surface 11µmol/L
then 7µmol/L
at 3m and then 2µmol/L
at 8m depth. Station 4 (figure 6.14) is shown to be
similar, with lower concentrations found in the deeper
waters. The concentrations are lower in the deeper water
as this is more diluted with low concentrated oceanic
water. The samples collected from the RIB have higher
values than the samples from Conway, where Station 2 and
Station 4 (figures 6.16 and 6.18) have the highest
dissolved silicon values of ~28µmol\L.
Chlorophyll
The
chlorophyll values vary spatially in accordance with the
nutrient supply. The largest values discovered were at
RIB Station 2 (figure 6.16), where values reached more
than 4μg/L. This coincided with a particularly high
nutrient supply, which was most likely encouraged by
anthropogenic inputs. In the vertical column,
chlorophyll values showed little change with depth. This
was due to the restrictions in equipment, with the
majority of samples within the euphotic zone (figure
6.26). The estuarine region studied is also influenced
by strong turbulent mixing generated by the tidal flows,
which maintains vertical uniformity throughout the water
column.
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Dissolved oxygen analysis
In order to
assess the accuracy of the dissolved oxygen measurement technique, duplicates of
four samples were taken. On average the standard deviations of the four
duplicates ranged from 0.11 - 2.86% and therefore the data can be considered as
having a high degree of accuracy.
Considering the
surface samples taken from each station, there is a significant decrease of 15%
in oxygen saturation from the mouth to the upper stretches (figures 6.19 and
6.24). This is expected since freshwater has a greater supply of nutrients,
which promotes increased amounts of heterotrophic nutrition and increases oxygen
consumption. Overall the values recorded in the estuary at this time are
generally quite low due to recent high rainfall and therefore greater surface
runoff of agricultural fertilizers leading to eutrophication.
For Conway
Stations 1, 3, and 4, water samples were taken vertically through the water
column (figures 6.19 - 6.21). Due to complications with the equipment, samples
could only be taken to a maximum depth of 8m, therefore the resulting data does
not give a good representation of the whole water column. For the three stations
(Stations 1, 2 & 3) the surface saturation at 1m was 2 - 4% higher than the
deeper saturation values, due to closer proximity with the atmosphere and
increased trapping of air bubbles as a result of wave action. With the
exception of the saturation value at 8m for Station 3, oxygen values do not show
significant changes vertically through the water column. The chlorophyll values
also remain relatively constant through the water column, which corresponds well
with the oxygen data where a peak in chlorophyll would produce a higher oxygen
saturation value. An anomaly was found at Station 3 at 8m depth (figure 6.20),
where a saturation value was 107%. Although an increase may be expected in this
region due to greater seawater influence at this depth, this value is thought to
be too high.
Temperature (figure 6.25)
The temperature
structure of the water column is a very good illustration of how the estuary is
influenced by the input of river water and the tide. The structure of the
temperature profile varies significantly and depends on the status of the tide
and location of where the measurement is taken. In July, the temperature of the
river is higher than the sea water, therefore the water temperature at the upper
part of the estuary should be higher than the temperature at the mouth of the
estuary. Our results have confirmed this and such a trend is clearly displayed by
the decrease in temperature with increasing proximity to the sea.
The temperature
profile also varies significantly at different times within the tide cycle.
During the flood tide, the influx of seawater dominates the structure of the
temperature profile in the Fal Estuary. The temperature of the water column is
homogeneous at the mouth of the estuary (Station 1) on the flood tide. The
temperature declines gently with depth in a linear manner in the middle and
upper part of the estuary (see Stations 2 and 3). During the ebb tide, the
temperature structure of the water column is dominated by river flow, and the
warm and less dense river water flows on top of the colder, denser seawater.
This arrangement of water masses causes stratification and a clear thermocline
can be seen (Station 4 and RIB Station 4).
Transects
The data within this
section could be further improved by ADCP data, however
due to technical difficulties this data was unable to be retrieved.
The aim of the
transects was to discover an anti-cyclonic flow, which
is a residual flow occurring as a result of the tidal
and river flows. The technique was to use a T/S surface
probe to measure surface temperature and salinity values along the transect every
few minutes. The findings (figure 6.26) displayed eastern bound
oceanic water and western bound river water. This is
indicated in the graphs (figure 6.26) with the western side of the estuary
characterised by cold, saline, oceanic water and the eastern
side characterised by warm, fresher, estuarine water.
Although the temperature and salinity variations are minimal,
there is moderate change with both parameters along the
transect, which indicates that this physical process is
occurring.
Temperature/Salinity Graph
The Temperature
against Salinity diagram shows clearly how the
temperature changes at different parts of the estuary.
It suggests that temperature is higher at lower
salinities, and lower at higher salinities, i.e., the
temperature of the river water is higher than the sea
water. This diagram also shows how the mixing process
changes the temperature of the water column.
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Discussion
As
displayed in figure 6.28, there is a clear trend between the depth
of the euphotic zone and distance from the mouth of the estuary; the
euphotic zone becomes shallower towards the head of the estuary.
This is most likely due to increased suspended sediment input from
the River Fal, and from increased turbulence which results from the
shear between the incoming river water and the underlying seawater.
Figure 6.28
Depth of Euphotic Zone Calculated from Secchi Disc Depths
Station |
Secchi Depth (m) |
Euphotic Zone (m) |
Conway 1 |
4.5 |
13.5 |
Conway 2 |
2.4 |
7.2 |
Conway 3 |
2.0 |
6.0 |
Conway 4 |
3.0 |
9.0 |
The data
collected in this study defines a clear salinity structure which is
typical of partially mixed estuaries. The results show the presence
of a fresh, less dense layer overlying a more saline, denser layer;
the difference between the two layers became more enhanced towards
the head of the estuary due to reduced turbulence and mixing.
Nutrient levels were higher near the head of the estuary, since the
river water is the main source of new nutrients into the system.
Overall, both nitrate and silica displayed conservative behaviour
within the estuarine system, although it is important to note that
the majority of samples were collected in relatively high salinity
conditions due to tidal limitations; a more balanced data set may
yield alternative results. Phosphate displayed non-conservative
behaviour, showing significant addition of dissolved phosphate at
relatively high salinities.
Additions of phosphate to the estuary are probably a combination of
point sources, such as the sewage treatment works at Malpas and the
mussel farm south of King Harry Passage, and diffuse sources such as
run-off from the surrounding farmland. The relatively small
additions of silica to the estuary are probably a result of
weathering of silica containing rocks in the Fal drainage basin. The
addition of these two nutrients into the estuary was most likely
exacerbated by the heavy rainfall experienced in the area prior to
the collection of the samples.
Variation in physical conditions
found at each sampling site affect the biology in different ways.
Zooplankton populations are significantly reduced within the estuary
when compared with previous offshore data. Fewer orders of
zooplankton were found mainly due to a reduction in salinity and an
increase in physical mixing due to tidal influences, however,
zooplankton samples were only taken from the surface during the
inshore investigation. Surface waters are generally lower in
salinity, especially in the RIB samples, and may not be
representative of the whole water column at any one site. Samples
show variation in tolerance to fresher conditions between orders.
Many orders such as Hydromedusae, Chatognathae, and Polychaete
larvae were found to be abundant in offshore waters, but are
virtually non existent in estuarine samples. Some orders such as
Siphonophorae are found at all sites within the estuary, although
greatly reduced in numbers, indicating some tolerance to salinity
changes. In general, numbers of zooplankton are greatly reduced in
riverine samples, but certain orders such as Cirripede larvae
dominate the zooplankton composition. This is due to their
adaptation to tolerate a larger range of salinities, a
characteristic which is expected in coastal and estuarine regions (where
typically you will find Cirripedes (Barnacles). In all
cases data has been displayed graphically, and for each site, as
with offshore samples, a Shannon index number has been calculated
(figures 6.6 and 6.7). This number shows diversity relative to Hmax
and a measure of evenness in numbers between orders. Most notably copepods are found
to be one of the more dominant orders at all locations. The order
copepod includes a large number of species, and so results may be
representative of different species adapted to different physical
and chemical conditions within the estuary.
The dominance of the
estuarine surface waters by diatoms is to be expected as diatoms are
best adapted to growing in mixed conditions, as found in an
estuarine environment, where tidal mixing prevents the thermal
stratification of the water column. Although the presence of
dinoflagellates in the well mixed lower regions of the estuary
conflicts with the preferred growing conditions of this functional
group, it is likely they entered the estuary from the offshore
environment on the incoming tide (Stations 1, 3 and 4). The highest
abundance of dinoflagellates occurs furthest upstream where tidal
mixing is reduced (6000 cells m-1). The phytoplankton
were identified as Alexandrium, blooms of which are typically
caused by high nutrient concentrations typical of low salinities;
the extremely low phosphate concentrations here, therefore, probably
reflect the rapid uptake by dinoflagellates.
Limitations include
lack of data at low salinities, over the full tidal cycle from high
to low water, and between springs and neap tidal cycles. The lowest
salinity sampled was 24.80. Consequently, there is no data available
to interpret the chemical behaviour of nitrate, phosphate or
dissolved silica between the riverine end-member and this salinity.
Samples were taken between 0825 and 1137; high water was at 1024.
Ideally, samples would have been taken after high water in order to
eliminate the effects of the incoming tide on nutrient distributions
within the estuary. It would also be useful to observe how nutrient
concentrations vary temporally between low and high water and
between springs and neaps.
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Conclusion |
Our surveys
of the Fal led to findings of pollution sources contributing to
higher nutrients in the water column. Fronts were found in the
offshore region and in the estuary, however, due to problems with
the ADCP data from the estuary, analysis of this could not occur. In
Offshore, nutrients varied on either side of the front, with high
concentrations in the inshore mixed region and low concentrations in
the offshore stratified region.
In
geophysical analysis the grabs taken from most sites did not yield
as much as expected, when previously looking at the side scan sonar
and camera. So without the use of the camera, it could have lead to
inaccurate conclusions from the grab data retrieved. The dataset
showed conclusive bathymetric features such as drapes, mega-ripples,
and a line of weakness creating a channel lined with fine sediment
on the sea floor.
In biology
the relative abundance of phytoplankton functional groups varied
offshore and in the estuary due to the degree of stratification. The
position of the front was determined by the degree of mixing, which
determined the presence of nutrients. Data collected from
zooplankton samples has shown some relevant relationships between
distribution, biomass, which is relevant to chemical and physical
boundaries. By far the greatest biomass was found offshore, which
then decreased as you head landward with lowering salinities into
the estuary. Relative abundance and dominance of certain orders also
changes with salinity due to varying tolerance between the orders.
However, reliability of data could be questionable due to limited
number of samples, number of counts per sample, and identification
of zooplankton by order. Greater sampling resolution, temporally and
spatially, with classification of phytoplankton and zooplankton down
to species level would further increase confidence in the results.
This would then increase the understanding of the biology sampled in
relation to its chemical and physical environment (i.e tidal
influence on movement of species and its effect on the distribution
over a time scale).
The
degree and type of mixing processes played an important role in
controlling all of the parameters measured. The mixing observed was
primarily tidal, however wind and wave mixing cannot be excluded. In
the offshore region the mixing processes were controlled by bottom
friction with the seabed. This influenced the strength and position
of the observed front. In the estuarine region, the ebb and flood of
the tide created mixing between the saline and riverine waters.
There was also a residual anti-cyclonic flow shown to be present,
which exists in the absence of tidal flows.
The data
collected in this study was accurate in determining how physical,
chemical, geological and biological processes interact and vary
spatially. However, due to the small area studied conclusions made
cannot be taken as a representative for the whole Fal region.
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