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Group 8 - Rhys
Williams, Sophie Brookes, Nicholas Curzon, Katie Firman, Emma Heywood, Richard Latham
and James Merrix. |
1.Contents |
2. Introduction
This year’s Plymouth field course ran from 28th
June until the 14th July. During this period
Group 8 investigated the estuary as a whole, from around 0
salinity to open sea. We were looking to see how the three main
scientific disciplines, Biology, Chemistry and Physics,
changed with their location within the estuary and if and how
they affected each other.
Plymouth
is located at the convergence of several river systems, the
Tamar, the Lyner, the Plym and the Tavy. We will be
concentrating our efforts on looking at the Tamar estuary
and Plymouth Sound. The Tamar estuary is meso/macro tidal
estuary with a tidal range of 4.7 metres at springs (Dyer,
1997). Although it can be unwise to classify any estuary as
either totally well mixed or partially mixed, as many
estuaries experience periods within their tidal cycle of
being both fully mixed, partially mixed or salt wedges, the
Tamar estuary is generally regarded as a partially mixed
estuary.
Plymouth Sound, and the
Tamar estuary especially, have been extensively used for
naval operations, and therefore many docks, piers, wharfs
and breakwaters have been built over the last half century.
The estuary has also been extensively dredged creating many
channels and deep water anchorages for the large Naval ships
and cross channel ferries that have to come in and out of
the estuary. Plymouth is a relatively large city and as such
could put a lot of stress on the estuary and its waters, to
such an extent that the chemistry, physics or biology of the
waters could be changed. We will hope to find out whether
these anthropogenic inputs have affected the estuarine
system.
It
is intended to investigate what species are present in the
estuary and how their distribution varies along its length.
Additionally, are their distributions related to changes in
physical properties such as stratification and mixing, or
related to changes in the chemical composition of the water
along the length of the estuary. The variation in various
chemical components such as nitrates, phosphates and
silicates will be measured in order to determine the
behaviour of these pollutants. It will be determined what
type of estuary the Tamar is by examining the mixing and
stratification processes in and around the estuary. Looking
at the different current velocities magnitudes and
directions will also give an indication of changes in flow
and mixing in the estuary. By examining the geophysical
processes an indication of the underlying bathymetry will be
obtained.
The emphasis of
investigation is how the estuary works as a whole, without
focusing on one particular area of science, as this would
not tell the whole story, with Oceanography being considered to
be one of the most interdisciplinary subjects around.
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3. Methodology
Research/Survey vessels
In the investigation of the waters of
Plymouth sound and the Tamar estuary, three vessels were
used, each for a different sampling area, as shown on the
map (click to enlarge).
1. RIBs - The Rigid Inflatable 'Ocean
Adventure' and small vessel 'Coastal Research'
were used for upper estuary sampling. They are small
and manoeuvrable and therefore could be taken into shallower
water further up the Tamar. Due to their size, however,
sampling was limited and no large instruments could be used,
so sampling was limited to collection for chemical and
plankton analysis, with a small CTD multiprobe.
2. Estuarine vessel - 'RV
Bill Conway'. This is a approximately 25ft oceanographic
research vessel, that was used for sampling the middle and
lower estuary area. It was equipped with a variety of
instruments and so was an ideal base for a wide range of
sampling. Data was collected in the form of samples for
chemical analysis from a rosette sampler, ADCP data, and CTD
multiprobe data on a computer, and plankton trawls could
also be undertaken.
3. Offshore vessel - 'Bonito'.
This is a large tug-type boat, which could be taken several
miles offshore. It was equipped with similar instruments to
the Bill Conway, and in addition, a crane that allowed
zooplankton sampling to be undertaken vertically.
Conductivity,
Temperature and Depth probe (CTD)
The CTD probe measures conductivity
(relates to salinity), temperature and depth. It is attached
to a rosette sampler with Niskin sampling bottles attached,
and the unit is deployed while stationary and the CTD
information is relayed back to the boat in real time and
plotted on a computer. It can therefore can be used to help choose where to
take water samples, collected in the Niskin bottles. The CTD
also has a fluorometer that sends out a UV
pulse which excites the phytoplankton. They then emit red
light that can be picked up by the fluorometer’s sensor, so the
greater the amount of phytoplankton, the higher the reading
from the fluorometer will be. Another probe attached is a
transmissometer, which has a light source and a
light detector, sending out a beam of light which is then
reflected off a mirror back to the detector, making the
total distance. This is useful for the measurement of the in
situ clarity of the water, rather than amount of light at a
particular depth, and can therefore be used to help identify
the presence of a phytoplankton bloom. The amount of light
at particular depths is measured using a light meter, a
final sensor on the CTD, though unfortunately this was not
in operation during the data collection period.
As the CTD
needs to be lowered into the water when the vessel is
stationary we needed another method of measuring temperature
and salinity whilst underway. On the offshore vessel,
Bonito, there was a fixed TS probe about 0.5m under the
surface of the water, allowing us to identify any interesting changes whilst under
way, and then make an informed choice about where to drop
the CTD.
Water sampling
Water
samples were taken on all three vessels, using remotely
operated Niskin bottles on rosette samplers at different
depths on Bonito and Bill Conway (estuarine and offshore)
and a surface Niskin bottle on the Ribs. A CTD probe
attached to the rosette allowed analysis of the water column
as the rosette was lowered via a computer. The
closing of the bottles was controlled via this computer
system.
For
collection of these samples, the same methods were carried
out on each vessel. From the Niskin water sample, be this at
different depths (as on Bonito, the offshore vessel, and
Bill Conway, the estuarine vessel) or from surface samples
(from the RIB samples), a volume of water was decanted into
a container for ease of sampling. If dissolved oxygen was to
measured, this was carried out before any water was removed
from the Niskin bottles, to prevent any contact with air. In
this case a tube attached to the Niskin bottle allowed
direct collection of the water, with minimal contact with
the air, into the glass Dissolved Oxygen bottles. These were
then treated with 1ml each of alkaline iodide and manganese
chloride, added just below the meniscus to expel the
contaminated surface layer of the bottle, which fixed the
oxygen content (the Winkler method, Grasshoff et al., 1999) and the bottles were
then stored underwater, so further prevent any contamination. The offshore
and estuarine vessels collected more dissolved oxygen
samples than the RIB due to better facilities to do so, and
the RIB T/S probe also read oxygen concentration directly,
so the Oxygen samples collected on this vessel served as
effective calibration comparisons.
From the
decanted water sample from the Niskin bottles a 60ml syringe
of water was collected, having first been flushed. A glass
fibre filter was then flushed, and any sample bottles also
flushed, all with at least 10ml of the sample water. 60ml of
the sample was filtered into a glass bottle, for phosphate
and nitrate analysis, and a further 30ml into a plastic
bottle, for silicate analysis (Parsons et al., 1984, Johnson
et al., 1983). Glass was not used for the silicate samples
due to the possibility of contamination by silica-based
glass. As the syringes used were 60ml volume, at least two
glass fibre filters were used. In the case of the RIBs, as
the sampling sites were so far up the river, where suspended
sediment was high, the filters became clogged easily and so
more were used as less volume could be filtered through each
one. Two filters from each Niskin sample were kept for
chlorophyll analysis. These were removed and transferred, on
board the boats, into acetone solution, in test tubes, and
stored in cool boxes until they were analysed in the
laboratories. The amount of water filtered through each
sample was noted, as this varied, as further up the estuary
there was more suspended sediment, meaning the filters
became clogged more quickly. In the laboratory a flourometer
measured the chlorophyll concentration in the acetone
solutions (Parsons et al., 1984). This could then be
converted into a figure of chlorophyll concentration in
micrograms per litre in the seawater sample, when combined
with the information on how much water had passed through
each filter.
Plankton
sampling
A 200 micron net was deployed to collect samples of
zooplankton. This can be done either vertically or horizontally. A
horizontal trawl involves towing the net behind the boat at a steady
speed and for known time. This method is quite easy to do and can
produce good results. However the depth that the net is trawled at
is more difficult to control. This is where the vertical trawl comes
in. The net is lowered down through the water column to the desired
lower depth. It is then raised up to the desired higher depth (note not always
the surface) and a messenger is sent down to close the net, thereby
achieving a discrete sample though the water column. To look at phytoplankton, water
samples were taken at specific locations, in the case of the RIBs,
or drained from the Niskin bottles attached to the CTD rosette, as with Bonito and Bill
Conway. This water sample was then placed into a brown
glass bottle (to keep out light) which was pre-treated with Lugols
solution. The Lugols solution is brown in colour and is used to
stain the phytoplankton for easier detection. To identify both types
of plankton, light microscopes were used. Water samples were placed in
either Bogorov slides in the case of the zooplankton, or in
Sedgwick-Rafter trays for the phytoplankton
identification. For quantitative analysis the different numbers
of species were counted in the relevant microscope slides; the
numbers were then multiplied
up to give measures of plankton numbers
in the water column.
Acoustic Doppler
Current Profiler (ADCP)
The ADCP is
another vital piece of equipment that is used on board. Its
primary function is to measure current velocities and
directions in all three axes. It uses the principal of
Doppler shift to measure the current velocities. When the
pulse sent out by the transducer is reflected by a
particular ‘packet’ of water, the wavelength of the
returning pulse will depend on the speed and direction of
the water packet it reflects from. If the packet is moving
away from the sensor, then the wavelength of the return
pulse will be lengthened, called red shift. Conversely, if the
packet of water is moving towards the sensor then the return
pulse will be ‘blue shifted’ and its wavelength will be
shortened. This wavelength measurement and return times
measured by the unit give current velocities and direction
throughout the water column, even when the vessel is moving.
The ADCP was used to measure transects in certain areas and
give information when stationary and collecting samples. It
can also measure backscatter; this was used in stationary
positions and gave an indicator off dense zooplankton areas,
which increase the backscatter measured by the ADCP, and
helped in the decision of which depths to collect
zooplankton samples via the net.
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4. Upper Estuary |
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figure 4.1
Phytoplankton numbers in the upper estuary.
figure 4.3
Phosphate mixing diagram.
figure 4.5
Zooplankton species and their relative abundance. |
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For the purpose of this
description, the upper estuary is defined as those areas of water that were
sampled whilst on the RIBs, 'Ocean Adventure' and 'Coastal Research' on 7th
July 2005. This area is located between the Tamar Bridge and Calstock.
Plymouth
sound and the Tamar estuary were sampled for the following
chemical components:
1. Phosphate
concentration – this may indicate pollution in an
estuary, in particular detergent pollution around built up
areas or sewage works. Phosphate is a nutrient for
phytoplankton, so higher levels often indicate increased
phytoplankton numbers. Therefore it is expected that high
levels may be found near to sewage outfalls and perhaps near the city of
Plymouth, so further up the estuary. Low levels are expected
offshore, where effects from the pollution may have been
diluted. Thermal stratification during the summer months
offshore could cause phytoplankton blooms which can strip
surface water layers of nutrients.
2. Nitrate
concentration – this may indicate pollution from
farming areas, as nitrates are a major component in
fertilizers, and a high nitrate presence in estuarine water
could indicate runoff from nearby farms. Again it is a
nutrient for phytoplankton, so increased levels could
increase phytoplankton biomass. Higher levels are therefore
expected further up the estuary, especially around farmland.
3. Silicate
concentration – this can also indicate a degree of
pollution. Silica also forms the frustule shell for diatoms,
a phyla of phytoplankton, so in areas were diatom biomass is
high, Silicate concentration can be found to be lower. Again
low concentrations are expected offshore
4. Dissolved
Oxygen – saturation percentage measures
relative saturation of the water according to in situ
temperatures and depths, so how much is available for
zooplankton respiration, for example.
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figure 4.2
Silicate mixing diagram.
figure 4.4
Chlorophyll and oxygen concentrations along the estuary.
figure 4.6
Satellite images showing the demise of the phytoplankton
bloom in the English channel. |
By looking at the
distribution of the biological species in the upper estuary it will give a
good indication of the chemical processes that are occurring. This has
particular relevance when considering the nitrates and phosphates, which are
caught in a negative feedback loop with the plankton. The more nutrients
there are, the more phytoplankton can grow, but this diminishes the nutrient
concentration which in turn results in a drop in phytoplankton abundance.
There is little data on the physical characteristics of the upper estuary as
we could not deploy larger instruments, such as rosette samplers from the
RIBs.
Salinity in the upper
estuary ranges from 0 up to approximately 34 at high tide at Tamar bridge,
though dependent on the tidal state. Due to
the shallow depth of the water and the speed of the current, the river is
well mixed so there is not a huge variation in chemical and physical
properties vertically in the water column. This, coupled with the difficulty
in accurately deploying instruments from the ribs, means that only the
surface layer need be sampled.
A look at the phytoplankton
distribution in the upper estuary (figure
4.1) shows that there are large numbers of diatoms at around a salinity
of 20
(just over 150,000m-3). This rapidly drops by salinity of 22.5
(50,000m-3), but remains fairly constant after that. The
dinoflagellates are also abundant at around a salinity of 20 (70,000m-3)
and like the diatoms, fall rapidly in number. However, unlike the diatoms,
they fluctuate in number after that from about 15,000m-3 to
around 65,000m-3. If we compare this with the silicate mixing
diagram (figure
4.2), we see that the
high numbers of diatoms correspond with the bottom of the steepest drop in
silicate concentration. After 21 salinity, the rate of decrease in silicate
concentration is less. This suggests that less silicon is being taken out of
the water column for use in diatom thecae which would result in fewer
diatoms present further downstream. The drop
in diatom numbers could also explain the rise in dinoflagellate numbers as
there will be less competition for light and nutrients, allowing them to
grow and reproduce more rapidly.
Another possible explanation
for the rise in dinoflagellates towards the middle estuary, is the abundance
of phosphates (figure
4.3). At low
salinities, the phosphate concentrations rapidly diminish, which may be due
to the very high numbers of diatoms. However, at salinities between around
10 and 20, the uptake of phosphates by phytoplankton drops, and the graph
levels out. After a salinity of 21, the concentrations start falling again. This
coincides with the higher numbers of dinoflagellates. It is likely that
diatoms are a ‘stronger’ competitor for nutrients etc. than dinoflagellates.
However, when silicate becomes a limiting factor for them (at around 21
salinity - as described above), the dinoflagellates have access to more
nutrients and can increase in number.
Oxygen saturation (figure
4.4) is a measure of how much oxygen is available in the water, as it is
related to temperature and salinity. A higher saturation therefore means
more oxygen available for respiration, by zooplankton or bacteria. It would
therefore be expected that higher chlorophyll concentrations would have
higher reciprocating oxygen levels, due to the oxygen produced in
photosynthesis.
In this case the higher
oxygen saturation is best explained by the effect of physical changes on
oxygen levels. As you travel further down the estuary, there is a marked
temperature change, and colder water will hold oxygen better hence higher
levels. There is also increased mixing in lower areas of the estuary, and
oxygen can increase in surface layers. As most of the data was collected in
surface layers on the RIBS, up to salinity 29, it could be expected that the
higher values are due to mixing.
Zooplankton,
such as copepods, follow a similar trend with numbers of around 80million individuals m-3
~1.5 miles north of the Tamar bridge, down to only 18million m-3
at Tamar bridge (figure
4.5). Their
distribution could be explained by the abundance of the phytoplankton,
especially the diatoms which are a major food source. The likely reason for
there being so many more zooplankton than phytoplankton is that early July
is around the zooplankton bloom time after the phytoplankton bloom in
spring/early summer has faded (figure
4.6).
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5. Middle to Lower Estuary |
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figure 5.1
Phytoplankton distributions at estuarine stations
figure 5.3
CTD probe data at St. John's lake, station 6
figure 5.5
Zooplankton distributions at Cargreen
figure 5.7
Zooplankton distributions at Mountbatten
breakwater
figure 5.9
ADCP Plot, station 7. |
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In this section we are defining the
middle to lower estuary and the processes that occur in the area. Data
collected for the middle to lower estuary was collected on the Bill Conway
boat trip, the area that we sampled being from the Tamar Bridge to the
breakwater located in Plymouth Sound.
In order to understand the processes
that are occurring we can look at the phytoplankton and zooplankton that are
situated in the area, as this will indicate chemical and physical processes
that are occurring in the water. From this information we can then look at
factors such as the nitrate, phosphate and silica levels or the dissolved
oxygen content and see how they affect the distribution of the phytoplankton
and zooplankton.
The middle to lower estuary is greatly
affected by the saline water from the sea, although there is still a slight
influence of the freshwater from the river. We expect to see slight changes
in salinity here, and as this is the area where the freshwater and the sea
water meet, there may be a large difference in the species and numbers of
phytoplankton and zooplankton that we observe. As the middle to lower
estuary is generally a well mixed area, due to the proximity to the sea and
tidal influence, it would be expected that there is a greater concentration
of diatoms than dinoflagellates where the freshwater influence is greater,
as diatoms generally inhabit more turbulent waters. However as you move
further towards the seaward side of the estuary it would expected that
dinoflagellate numbers increase, as they generally inhabit more stratified
waters, which are found more offshore.
To measure these factors we used an
ADCP, a CTD probe and a plankton net whilst onboard the Bill Conway. To
collect zooplankton samples we deployed the plankton net at the top of the
sampling area by the Tamar bridge and at the bottom by the breakwater. To
collect the phytoplankton we collected water samples, which were taken from
Niskin bottles on the rosette sampler.
See methodology.
As the area that we were sampling is quite highly mixed, the stations we
chose to sample were close to external influences such as freshwater inputs
from rivers or nitrate and phosphates from farm land or sewage pipes.
Station 7 was located just north of the
Tamar Bridge near a sewage outfall, which could have increased the nitrates
and phosphates present. This station was the furthest north of all the
stations and therefore would have been the site most affected by the
freshwater input from the Tamar. The Secchi disk reading was taken as 1.1m;
therefore the euphotic zone was about 3.3m deep (figure
5.2) Figure
5.1 shows that at this station there were very high numbers
of chaetoceros sp. and Rhizosolenia delicatula, both diatoms.
There are some dinoflagellates present, though in very small numbers.
Station 6 was located by Saint John’s
lake which not only provides a freshwater input to the system but is also
located next to a sewage works. The sample was taken close to a scum line
and large areas of surrounding farmland. The Secchi disk reading was taken
as 1.6m; therefore the euphotic zone was about 4.8 m deep. (figure
5.3)
Figure
5.1 shows that at station 6 there were very low
concentrations of phytoplankton. The 3 species present were two diatoms,
Rhizosolenia delicatula and chaetoceros sp. There was a
dinoflagellate present; Ceratium Furcus.
Station 4 was located near Sutton
harbour and Mountbatten breakwater. It had
freshwater inputs from both the Tamar and Cattewater rivers, which could
result in a high number of diatoms. Close to the sampling site there was a
lot of farmland and sheep, and the site was also located on a scum line.
These factors could have increased the nitrates and phosphates in the water
which in turn would affect the number of phytoplankton and zooplankton
present.
Figure 5.1 shows that the dominant species here was
chaetoceros sp., other species present were Rhizosolenia delicatula,
Nitzschia sp. and Guinardia flaccida. There were also some
dinoflagellates such as Karenia mikimotoi and a small number of
Eucampia sp.
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figure 5.2
CTD probe data at station 7
figure 5.4
CTD probe data for breakwater, station 2
figure 5.6
Zooplankton distribution at Saltash
figure 5.8
ADCP Profile, station 4 |
Station 2, the final station was located by the Breakwater on the estuarine side. It was in
quite a deep area as the sampling site was located in area that had been
dredged to 12m in 1995. The light penetration at this site was also quite
high; the Secchi disk was recorded as 5.3m, therefore the euphotic zone was
15.9m deep. This reading, however, does not fully represent the actual
euphotic depth as it assumes that the profile is the same all the way down
(figure
5.4). Figure
5.1 shows that at station 2 the dominant species present is
Karenia mikimotoi there are also some Rhizosolenia delicatula.
At this station there were a lot less diatoms present and dinoflagellates
were the predominant species.
The zooplankton data is similar to the
phytoplankton data; where there are more phytoplankton, more zooplankton are
found as they feed on them.
Figure 5.5
represents the zooplankton samples that group 7 collected near Cargreen.
This was the site located furthest up the estuary; therefore it is the
sample that would be the most affected by the freshwater from the river.
There are large numbers of copepods here. These feed on ciliates, a type of
phytoplankton, which could be expected to be found here. There are also some
crustacean larvae.
Figure
5.6
represents the zooplankton collected at station 7, near Saltash. This is
just north of the Tamar bridge, which is further south than Cargreen;
therefore it would be expected to be affected by both the freshwater and the
seawater. At this station the two dominant species are copepods and
hydrozoans, with some cirripede nauplii. This is possibly as hydrozoans are
not self-propelling, and therefore move according to water movements. At
this stage in the estuary there is a lot of turbulent mixing due to strong
flows and meanders in the river, this can be seen in
figure 5.3, where the variation
in salinity is not very great, only varying 1 salinity unit from surface to
depth, indicating mixing has occurred. Therefore more hydrozoans could become trapped in
turbulent flow. The variation in water temperature would increase this
turbulent flow.
Figure 5.7 shows the zooplankton distribution at the station near the
mountbatten breakwater. It has large numbers of copepods, a dominant
species, which follows a possible phytoplankton bloom (see
Upper estuary).
There is a variety of species here, representing more seaward water. There
are also still a fairly large proportion of hydrozoan. As the station was
behind the breakwater and in the lee of the prevailing wind, there could be
increased turbulence here, which again could trap hydrozoans.
The ADCP data that was collected shows
that in some places there is notable shear in the water column. This
means that two water masses are moving in different directions, causing
considerable friction. It is likely that a CTD probe drop would show a
considerable change in the physical and chemical attributes of the water
column at the shear point, with relatively stable conditions above and
below, likely resulting in two distinct plankton communities. The following
ADCP plots correspond to stations 4 and 7. Station 7 is located by the
Tamar Bridge and station 4 by Sutton Harbour. Station 4 corresponds with a
stable body of water; this can be seen by the calculated Richardson number
of 0.401. If a body of water has a Richardson number close to 0 or below
the water column is very unstable, however if it has a Richardson number of
0.25 or above it is considered to be stable. Station 7 has a Richardson
number of 0.0059; this indicates that it is very unstable and therefore well
mixed. This is illustrated in
figure 5.8 and
figure 5.9.
figure 5.8 is a copy of the ADCP plot retrieved from Station
4, in this image there is clearly one layer of water flowing on top of
another, which signifies that there is very little mixing occurring.
figure 5.9 is a copy of the ADCP plot retrieved from Station 7. In this
image there is no visible layers, unlike Station 4, this signifies that the
water column is very well mixed.
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6. Offshore |
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figure 6.1
Chemical depth profiles for offshore station 1
figure 6.3
ADCP profile showing backscatter in offshore waters
figure 6.4b
CTD profile for offshore station 7
figure 6.6
Zooplankton species for station 3 from 6m to surface
figure 6.8
Phytoplankton numbers at station7
figure 6.10
Zooplankton species for station 7 from 28m
to 16m |
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When out on Bonito, on 3rd
July 2005, the aim was to locate the position of
the front where the two different water masses (estuarine water and
sea water) meet. We were expecting to see a change in phytoplankton
phyla distribution over the two water masses and the mixed water
area where they met. Diatoms are found in well mixed, unstratified
water; this would suggest they were to be found on the estuarine
side of the front and in turbulent mixed areas, whereas
Dinoflagellates are found in more stably stratified water columns;
this suggests they would be found on the seaward side of the front,
where seasonal thermal stratification is expected to occur in July,
when the field course was conducted. The mixing of the two water
masses causes deep water to upwell, where colder, nutrient rich
waters flow up into the water column around the front, so it could
be expected to benefit both types of plankton around the front area.
The resulting phytoplankton distributions would affect the
zooplankton distribution. Zooplankton feed on phytoplankton and thus an increase in
phytoplankton, for example a bloom around a front area, could cause
an increased zooplankton layer and concentration.
Station 1 (50°
20.135N, 004° 09.377W), located
inside the breakwater was chosen because it was expected to give a
profile typical of estuarine water., before any effects of the front could
be observed. The nutrient profile (figure
6.1)
for this station, shows nitrate, phosphate,
oxygen and silicate are depleted at the chlorophyll maximum of 14m. This
shows that phytoplankton are blooming at 14m, which corresponds well with
the euphotic zone depth of 14.85m. Silicate is also at its lowest
concentration between 2-4m, indicating uptake of silica by a large diatom
bloom at this depth.
figure
6.2, a histogram showing phytoplankton counts and species distribution,
confirms that diatoms are the main component of the bloom at Station 1, with
highest numbers at 2-4m and the bloom extending down to 15m.
By continually towing the ADCP
throughout the survey, changes in the average backscatter could be observed.
At station 3 (50°
19.077N, 004° 10.690W), it could be seen that an increase in
backscatter occurred at the chlorophyll maximum of 9 metres, this is shown
in
figure
6.3. This was thought to be
indicative of the high plankton numbers found at fronts due to upwelling
waters. The CTD was deployed at each site and the profiles generated, up to
and including station 3, show temperature and salinity stratification down
through the water column, as shown in
figure
6.4a.
After site 3 the profiles show uniform salinity with depth and strong
temperature stratification, as shown in
figure
6.4b.
This is typical of an offshore profile where there is no estuarine influence
on salinity. This is further evidence supporting the presence of the front
located at site 3.
The meeting of warm, less saline
estuarine water and warm, more saline seawater at tidal fronts causes
deepwater upwelling of cold, saline water. The deepwater upwelling is
nutrient rich, causing phytoplankton to thrive. Mixing of the 2 water masses
at fronts causes visible differences in surface roughness on either side of
the front. Diatoms prefer the turbulent, rougher estuarine side of the front
and dinoflagellates prefer the calmer, more stratified offshore side.
figure
6.5, shows a strong, diatom
dominated bloom in the surface waters of station 3 with a smaller bloom at
the base of the euphotic zone. The small numbers of dinoflagellates indicate
that site 3 is located on the estuarine side of the font where the surface
waters are more turbulent.
High numbers of phytoplankton at fronts
are reflected by an increase in zooplankton numbers.
figure
6.6 shows high numbers of
zooplankton at 10-16m which corresponds to the depth at which phytoplankton
bloom. Numbers of chaetognaths in the zooplankton are high at this site in
comparison to site 1 where no chaetognaths were found (See
figure
6.7). This is an important sign that
the front has been reached as chaetognaths are indicators of offshore
waters.
After locating the front, profiles were
taken at several sites on the offshore side. The sites were aimed at showing
the water column with no influence from estuarine water, so that a
comparison could be made between estuarine, offshore and frontal conditions.
The phytoplankton populations for station 7 (50°
15.996N, 004°
12.698W) shown in
figure
6.8 show
phytoplankton blooms occurring down to 50m. This shows that the offshore
waters are much clearer than on the estuarine side of the front. The
phytoplankton population at station 7 is still dominated by diatoms however
the dinoflagellate population is greater in the surface waters than at the
estuarine or frontal sites. This could be due to dinoflagellates preferring
the more stratified waters offshore. The nutrient profiles for station 7,
shown in
figure
6.9, show the chlorophyll
maximum to be between 5-20m. This corresponds with depletion of nitrate,
phosphate and silicate at this depth layer.
figure
6.10 shows high zooplankton numbers at 16-28 meters, just below
the chlorophyll maximum. This is where zooplankton typically feed, below the
main phytoplankton bloom. Chaetognath numbers are again high at this depth
which is indicative of an offshore water sample. |
|
figure 6.2
Phytoplankton distributions for
offshore station 1
figure 6.4a
CTD profile for offshore station 3
figure 6.5
Phytoplankton numbers at different depths for
station 2
figure 6.7
Zooplankton species for station 1 from 3m to surface
figure 6.9
Chemical depth profiles for station 7 |
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7. Overview |
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figure 7.1
Nitrate mixing diagram
figure 7.3
Silicate mixing diagram
figure 7.5
CTD profile, breakwater
figure 7.7
Chlorophyll concentration along estuary
figure 7.9
Zooplankton species along the estuary |
|
An overview…..the estuary as a whole
This section aims to
look at the estuary as a whole, from as close to zero salinity as possible
to the open waters off shore. This section will look at the biological
aspects of the estuary, how the phytoplankton and zooplankton species change
with salinity, a proxy for distance from the estuary head. The chemical and
physical properties of the water column change with salinity and so this
will be shown and discussed. Mixing diagrams, a plot of concentration
against salinity.
Chemical changes
-
At low salinities the
concentration of nitrate is high, falling away as the salinity increases
and the waters become diluted. (Figure
7.1) Nitrate concentrations are higher
in the low salinity waters, due to surface run-off waters flowing over
the surrounding agricultural land which is rich in nitrate, from
fertilizers and manure. However the points do not lie on the TDL, they
lie slightly below it, suggesting that the nitrate is being removed from
the water along the length of the estuary therefore nitrate is
non-conservative. Nitrate is a required nutrient for phytoplankton
growth, therefore likely that the phytoplankton in the water column
along the estuary are removing the nitrate.
Figure
7.8 shows the number of phytoplankton
present, suggesting that nutrient uptake, by the phytoplankton could be
likely cause of the nitrate depletion
-
Figure
7.2 shows how phosphate concentration
varies with salinity. The water has higher concentrations of phosphate
at lower salinities and lower concentrations at higher salinities. Up
until a salinity of 20 the points lie below the TDL, as the salinity
rises, the points cross the TDL and then lie above it. This is
demonstrating non-conservative behaviour: there is phosphate uptake at
salinities less than 20, and addition to the water at salinities above
20. The removal of phosphate is due to the uptake by
phytoplankton, as phosphate is a primary nutrient that they require, for
growth and development.
-
The salinity was 20 at 12, (50o26.321N/004o11.925),
about 1.3km upstream of the sewage works that service Plymouth. The
water sample was taken at 12:49 GMT with low tide for the same day at
12:24 GMT. The pollutants are released into the water column on the ebb
tide to try and remove as much material as possible from the estuary,
with the next incoming tide bringing some of the pollutant laden water
back into the estuary. Pollutant gets carried up past the sewage works
by the process of entrainment; tidally driven, and is what is being
detected and hence why there is an apparent input of phosphate. The peak
of the phosphate input coincides with the location of the sewage works,
who have installed a tertiary processing plant, which reduces pollution
into the estuary, though phosphate pollution remains, probably due to
the detergent runoff into the estuary.
-
Figure
7.3 shows how Silicate concentrations
varies with salinity. The maximum is at low salinities. The source of
silicate is land based material, such as clays, muds, sands, and rock.
The concentration decreases along the estuary, showing non-conservative
behaviour as the points lie below the TDL, suggesting that silicate is
being removed from the water column. Phytoplankton, in particular
diatoms, will remove silicate, as they need it for forming their silica
shells. Figure
7.8 shows that the diatom is
the dominant species of plankton in the upper estuary, which could
explain the removal of silica from the water.
|
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figure 7.2
Phosphate mixing diagram
figure 7.4
CTD profile, upstream of Tamar
bridge
figure 7.6
CTD profile, offshore
figure 7.8
Phytoplankton numbers along the estuary |
CTD Profiles
-
Figure 7.4 is a CTD profile for the middle part of the estuary,
slightly upstream of The Tamar Bridge. This shows a low salinity layer
of water at the surface; this is where the less dense river water is
flowing over the wedge of salt water below. This low salinity section
will be enhance following the recent high rain fall and associated
surface run-off
-
There is a well mixed section of
water below the first halocline, and the salt wedge below the second
halocline. The well mixed section has been mixed by advection, due to
the top layer moving out to sea, driven by the river flow, and the lower
layer moving upstream, driven by the tide.
-
The temperature profile reflects
this salinity stratification with warm less dense water at the surface
and cool dense water at the bottom. Warm water comes from the surface
run-off and the cool water comes from the incoming sea water. There is
then a well mixed layer where the warm surface water has been mixed with
the cool layer below creating a mid temperature range layer in the
middle.
-
The fluorometery peak is located
on the main thermocline, as this provide the best balance of light and
nutrients, which is where phytoplankton growths are therefore often
found, 20-30% of the total water column primary production in due to
localised growth in the thermocline (Revelante and Gillmartin, 1973)
-
Figure
7.5 is a CTD profile by the break water. By this point in
the estuary the water column is showing signs of being well mixed, this
is shown by the more linear increase and decrease in salinity and
temperature respectively. The fluorometery readings show a peak at
between 6 and 12m, and only a slight decrease below this depth. With a
Secchi Depth of 6.3m, making the euphotic zone 18.9m, the availability
light is not critical explaining the small decrease in fluorometery.
-
The expected thermocline that
should normally be present at this time of year is not visible, this is
because of the period of bad weather that has been affecting the water
in the period before sampling. The high winds have deepen the well mixed layer and broken down
the stratification, creating a gradual decrease in temperature from the
surface down through the water column:
'Wind mixing events occurring in an otherwise
stratified water column may temporarily enhance the vertical flux of
nutrients across the pycnocline to the surface layer' (Klein and Coste,
1984)
-
The Fluorometery reading shows
that the peak is at about 18m, with very little phytoplankton in the top
10m. The peak is located where the thermocline would have been before it
was broken down by the bad weather. The lack of plankton in the top 10m
is due to the lack of nutrients which will have been removed by the
plankton in the early stages of their bloom. The Secchi depth for
Station 7 is 11.6m which equates to a euphotic zone depth of about
36.6m, this reflected by the fluorometery readings, which drop off below
this depth.
Hard at work!!
Biological changes
-
Figure
7.7,
is a graph showing chlorophyll concentration along the estuary. Chlorophyll
concentration is an index of phytoplankton biomass which is easy to measure.
This graph shows chlorophyll measurements, to be highest in the upper
estuary, decreasing moving offshore. This corresponds with concentrations of
nutrients, discussed earlier, which are highest at the head of the estuary,
decreasing with increasing salinity. The nutrient profiles for phosphate and
silica both show removal in the upper estuary which is indicative of high
phytoplankton concentrations in this region, confirmed by high chlorophyll
concentrations.
-
The phytoplankton numbers and species distributions
along the estuary can be compared by looking at
figure 7.8. This figure
relates to 3 stations, one in the upper estuary, one in the middle estuary
and one offshore. Numbers of phytoplankton in the upper estuary appear to be
very low from this graph, considering the high chlorophyll concentrations
shown in
figure 7.7. There are many possible reasons for this unusual
result. Firstly, only 1ml of phytoplankton were counted from each sample
site and the results multiplied up for the whole sample. With no replicates
taken, it is possible that the 1ml counted was not an accurate
representation of the sample, causing an unusually low result. Another
possibility is that the high rainfall levels at the time of sampling could
have caused the surface layer sampled to be mainly rainwater and so plankton
deficient.
-
Figure
7.8 clearly shows the dominance of diatoms in
the estuary compared to dinoflagellates. As diatoms are the first of the
phytoplankton to bloom followed by dinoflagellates, it could be that the
sampling period was still within the period of diatom bloom. However, at the
offshore station the diatom numbers decrease and dinoflagellate numbers
increase. This could be due to the high numbers of dinoflagellates typically
found on the offshore side of tidal fronts, as was found whilst sampling on
Bonito.
-
The zooplankton numbers and species distributions along
the estuary are illustrated by
figure 7.9. This graph shows zooplankton
numbers are at their highest in the upper estuary decreasing moving
offshore. This is to be expected as zooplankton feed on phytoplankton which
are shown to be high in numbers in the upper estuary due to the high
chlorophyll concentration.
8. Geology/Geophysics
The geology site visited, Renney Point, located between Heybrook Bay and Plymouth Sound, on 2nd July displayed two different types of geology. The first being a sequence of bedding planes alternating between sandstone and oil bearing shale and the second, an eroded cliff face displaying seven bedding planes composed of sandstone with varying grain and clast sizes. The material from the cliff was deposited over the eroded bedding planes of the oil bearing shale
from 11-0 ka BP associated with the end of the last ice age as suggested by the environment it was deposited in. However, the cliff material is not as consolidated as the rock type below so has eroded at a quicker rate as has a higher erodibility which has exposed the underlying bed rock creating a wave cut platform which extends out into the sea by approximately 250m. |
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Sandstone/oil bearing shale. The bedding planes form a wave cut platform which extends out into the sea by around 250m and are only exposed at low tide with the exception of a few high points composed of a more resistant rock type. The reason for analyzing this area of rock is to better understand the geology of Plymouth which is a drowned river valley as these bedding planes extend right across Plymouth sound.
To test the characteristics of the bedding planes, we firstly took the bearing of the strike of several of the bedding planes to see the trend, using a compass with a built in spirit level. The spirit level was set so the compass was flat running along the strike plane and a bearing was recorded using the ‘right hand thumb rule’. The bearings of the bed at this location varied between 224˚ and
234˚. |
figure 8.1 shows the dextral tear fault running at a bearing of 110˚. |
figure 8.2 shows the recumbent bedding plains and antiform structure. |
|
Once this process was complete the dip angle of the bed was taken using the clinometer which was built into the compass.
The compass is turned on it side and pointed down the steepest part of the bedding plane, then the angle can be taken. The angles which the bedding plane dipped at all varied depending on where they were taken due to the presence of a recumbent antiform,
dips of between 30˚ and 72˚ were recorded.
Two major geological structure were present at this location, the first being the recumbent antiform
and the second being a dextral strike-slip fault, both of which
could have been caused during periods of compression. The recumbent antiform produced beds which have been overturned and dipping at the steepest angles of 64˚ and 72˚ as shown by the geological map of the area. The hinge of the fold was also identified as well as the apparent plunge which varied between location due to varying rates of erosion and bedding thicknesses but was generally found to be between 10˚-20˚ with a bearing of 224˚.
The dextral strike-slip fault recorded was shown to be running along a bearing of 110˚ which is perpendicular to the strike of the bedding planes and running in the
assumed direction of the compressional forces. As a
result the fault had moved ~5m displacing the antiform hinge,
which was offset on either side of the fault by this distance. Other noticeable features along the fold were tension cracks appearing in the direction of the strike plane along the top of the fold and also secondary cracks appearing in the general direction of the fault plane due to the compressional forces, this is shown on the compass rose diagram |
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Bedding plane 7 is slightly darker than
bed 6 below as soil particles produced by biological
activity has leached down through the sediment into the
bedding plane. White shell fragments are identifiable in the
sediment which means this area was possibly flooded at the
time of deposition, closer analysis would enable us to
distinguish the type of shell fish and there habitat, either
shallow marine or brackish waters. |
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Bedding plane 6 shows a change in
colour
from a red to beige, there is more sand grains present in
this bedding plane which measures around 40cm in thickness.
The matrix is also more sand dominated. A possible source
could be a mature beach or the infilling of a river
channel.
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Bedding plane 5 is similar to bedding
plane 1 as is matrix supported; clasts are deposited in a
lateral fashion with noticeable imbrication structures.
Clasts vary in size between 2 and 20cm in diameter and are
noticeably different in colour
suggesting they are a different rock type which has
travelled
a further distance and known as a derived clast. There two
distinct bands within the bed which are noticeable due to
the fact that there is some graded bedding present. This bed
is approximately 1.5m thick and could have been created by
mud flows at the end of the ice age, where increased
precipitation and lack of anchoring vegetation increased
soil erosion. |
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Bedding plane 4 is 15 to 20cm thick and
is composed of a fine grained mud, although most of it is of
an orange appearance; occasional green patches occur
suggesting there are different types of materials and
minerals present causing this bed to react to the atmosphere. Fine sands are present within the mud
which suggests a possible rising sea level bringing in fine
sands. |
|
Bedding plane 3 is 30 cm thick and
formed from material which is semi rounded and with an
average diameter of 3 to 4mm, these small clasts are
supported by a fine grained mud matrix like that of bedding
plane 2. It is a laterally discontinuous bed which pinches
out like those formed by a braided river system were rapid
changes reduce accommodation space and material is deposited
in a short series of bars. Again it could also be a sub
aerial environment. |
|
Bedding plane 2 is again around 30cm
thick and composed of a red material but the rock fabric is
very different suggesting it was laid in a different energy
environment. The material is a very fine grained mud with no
angular clasts, such material is only deposited in low
energy environments so potential environments in which it
was formed would be a delta, the inside of a river meander,
sub marine or sub aerial. |
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The first bedding plane at the bottom
of the cliff creates an unconformity as it overlays the
older more resistant bed rock. Bedding plane 1 is composed
of mainly a red material suggesting that it was deposited in
an oxidizing environment. Large angular clasts are present
with an average diameter of 15cm and are aligned
horizontally with some evidence of imbrication. Due to the
size and angularity of the clasts, its possible to say that
this material has not travelled far from its source as would
be much smaller and more rounded if it were to have
travelled
a great distance. The main body of this bedding plane is
around 30cm thick and is clast supported, these
characteristics suggest that the material was deposited in a
high energy environment like that of a mud or debris flow. ext |
Sidescan SONAR
The Sidescan SONAR sampling technique was used to survey the sea
bed of Wembury Bay to observe the geological structures present
and to identify the different types of sediments in this area
and the fashion in which they were deposited. Another aim of
this investigation was to try and observe any faulting present
and if possible, to correlate this information with that
collected and Renney point.
The Sidescan fish Figure 8.10
was towed behind the Catamaran ‘Natwest II’ at a depth of around
1 meters and speed of 4 knots, sampling at a frequency of 500
KHz. The swath of the scan was 150m but parallel transects were
taken at 100m intervals to allow for overlap. Two perpendicular
tie lines were also taken to show added detail that may have
been missed due to the angle of returns from the initial scans.
The information was transferred through the data cable to an on
board computer which gave a visual print out of the sea bed and
also recorded the GPS readings and times when the fish was
sampling. The image produced by the sidescan was then used to
interpret the different types of sea bed in the area, as
sediment types can be compared using the strength of the return.
Stronger returns would indicate coarser material than a weak
return.
To
get an idea of what type of sediment each return corresponded
to, a grab sample would be taken. A Van Veen grab (figure
8.11) was used to sample
the sediment type, the locations of these grabs was determined
after the SONAR survey was completed, so that grabs would be
taken in areas with different strength returns. This would allow
us to draw conclusions about other sediment types. The transect
print outs were then taken back to the labs were they were
analysed and plotted onto a map which shows the main features to
scale (figure 8.12.)
The map produced shows rock in purple, coarse sands and shingle
in orange and fine sands and mud in yellow. Initial findings,
showed that there were several gullies on the sea bed. These are
believed to be paleo river channels, created during the last ice
when the sea levels were much lower. These channels have
subsequently been infilled since the sea level has risen. There
are two different types of sediments as shown by the Sidescan
sonar and proven by the grab results.
Grab one was taken over a river channel filled with what
appeared to be finer sediment as shown by the sonar images. This
was proven to be correct as fine grained sand was collected, and
100% of it passed through the 1mm sieve. Some small shell
fragments were identifiable and also some very fine clay
particles were present as shown by the sticky texture which
caused it to form blobs on the 1mm sieve. This is caused by the
electro chemical charges found around clay particles which bind
them together. One small polychaete worm was found in this
sample (figure 8.13)
Grab two and three were taken at similar sites, however the
initial grab hit a rocky surface indicated by the fact that it
retrieved a species of Kelp known as Laminaria digitata.
Found living on the specimen of kelp was two types of organisms,
Hydrozoa and Porifera. Also collected was a starfish with a
spiny appearance (figure 8.14) it
was later identified to be Marthasteias glacianis a
typical species found living in rocky habitats.
Grab three, seen in figure 8.15, retrieved a coarser sediment which was not as well
sorted as the sediment from grab one. Large grains
greater than 2mm in size made up 40% of the sample, another 40%
was composed of grains between 1-2mm in diameter and the
remaining 20% was made up of fines less than 1mm in diameter.
Some small well rounded pebbles ranging from 1-5cm in size were
also collected within the sample; the well rounded appearance
suggests that they have travelled a greater distance from their
source than the other sediments in this area. Various Polychaetes were found within this sample suggesting it was
taken from an area with a higher diversity than the previous
sediment grab locations.
Sample site four retrieved a grab containing a layered sediment
(figure 8.16) which was preserved within the grab, the surface
layer was made up of fine sediments and was around 2cm thick. It
was composed of similar sediment to that taken from grab one, it
was hardened; thixotrophic, with again a sticky nature. Below
the fine layer was coarse gravel similar to that found in grab
3, particles ranged in diameter up to 1cm but 60% of the sample
contained particles of 1-2mm in size.
The tie lines taken after the sampling allowed us to match
features on the print outs but more importantly enabled us to
view the bed forms from a different angle. This revealed two new
types of ripple structures which were not displayed on the other
transects. Ripples found in both locations were asymmetric an
bifurcating suggesting the predominant cause was wave action,
they were also orientated in a south westerly fashion which is
the direction of the prevailing wind so it is possible to say
that the predominant wave direction in this area is wind driven.
Although the ripples were caused by the same driving forces,
some characteristics were different at the two sites sampled. In
one location the wave lengths were 1.8m with a height of 0.4m
and a lateral extent of 8-90m which was the width of the
channel. At the other location, they were smaller in scale
measuring 0.9m in wavelength with a height of 0.2m and a lateral
extent of 60m (not the entire channel width). Although the
ripples were only seen on the tie line transects, it is likely
that these features were present throughout the surveyed area
but were not seen due to the fact that the other transects ran
parallel to the bedforms.
The reason for this change in ripple size can be explained when
looking at the map and the prevailing wind and wave direction in
relation to the rock structures. The current flow over the rock
structures is altered in different locations depending on the
rock size and orientation giving a range in bedform
characteristics. The same principle can also be applied when
interpreting the reason for the deposition of the finer
sediments on the leeward side of the rocks in relation to the
wave action. This is an area were there is less energy for
mixing which enables the deposition of the finer sediments in
the water column. Another explanation of there northern trend
could be that they are the result of the riverine inputs which
occur to the north of the location sampled. Figure
8.17 shows the different types of returns produced, the
wood grain texture shows areas of rock, the darker matte areas
show returns from gravel and the lighter matte areas show
returns from finer sediments.
Visible from the sonar print out were fractures in the rock with
similar orientations to those found at
Renney point. It is possible to say that they were formed
in the same period of compression, evidence for this is that
there is also a dextral strike slip fault visible on the print
outs with a bearing of 335° which is a secondary fault angle
similar to those found on Renney rocks. This fault is again
dextral with an apparent horizontal displacement of 15m.
Although the fault is visible in certain locations, it is not
possible to track the fault across the entire sequence as
erosion and sedimentation has taken place within the fault.
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9. Conclusion
Despite much of the data
collected resulting in charts and graphs that agreed with our
expectations, there are many reasons to be wary of the
information. Firstly, when recording our position, we had
several GPS and time systems on board the boats, and these
rarely agreed exactly. The scribe recording all necessary data
at each time sometimes had as many as 6 different parameters to
write down, and it was hard to read and write up all of them
before they changed, especially when the latitude and longitude
consist of up to 8 numerals each.
Chemical sampling also had
problems as it had to be done on the boat which was rolling on
the swell resulting in the possibility of contamination of the
sample, or not providing the exact amount of fixing chemicals to
some of the samples (such as Manganese Chloride in the Oxygen
samples). However, we generally had good weather so this was not
an especially significant factor. On the boats we also had the
problem of drift. This had the most influence in deeper water at
the mouth of the estuary and during the geophysics in Wembury
Bay where the CTD and grabs would not be below the boat when at
the bottom and so their exact position is impossible to
determine without transponders. The boat would also drift up to
150m from the start point of the CTD deployment to the point at
which the last water sample was collected. In shallower waters
the lay-back was not very significant and drift was not very far
as the CTD spent far less time under water.
Once back in the
laboratory, due to the repetitive nature of some of the
analysis, it was easy to lose concentration and small mistakes
were made. The data was kept, but if it caused any anomalous
results, they could be discarded. With time constraints, it was
impossible to take many replicate samples as it would take far
too long to process in the lab, so we sampled a large amount of
the estuary at the expense of some accuracy. However, had we
sampled accurately at the expense of investigating some of the
estuary, it would have been much harder to generate a good
overall idea of the process contained therein. The most
significant aspect affected by this is probably the plankton
counts as when looking at a sample that we’d just collected,
before adding the formalin, several fish larvae were seen.
However, none showed up in the later analysis due to only
sampling 10ml out of the ˝ Liter container. From this it is
possible to infer that the phytoplankton samples were even more
likely to be erroneous as only between ˝ and 1 ml was analysed
from a 60ml sample.
Despite these potential
errors, we believe we have gathered a lot of accurate data, and
have been able to gain a good understanding of the workings of
the estuary as a whole. We have been able to see how all of the
areas of science interact with each other, and that how
virtually any change, no matter how small, can have a large
impact on another aspect of the estuary. We also believe we
could come back and focus in on one particular aspect of the
estuary, using our knowledge gained in the last two weeks to
achieve good results.
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10. References
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Grasshoff, K., Kremling K., and Ehrhardt, M. 1999. 'Methods
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Holligan, P.M. et al., 1984. Photosynthesis, respiration and
nitrogen supply of plankton populations in stratified,
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Johnson, K. and Petty, R.L. 1983. 'Determination of nitrate
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KiŘrboe,
T., 1993. Turbulence, Phytoplankton Cell Size, and the
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Morris, A.W., 1981. Nutrient Distributions in an Estuary:
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Morris, A.W., 1982. Chemical Variability in the Tamar
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Parsons, T.R., Maita, Y., and Lalli C. 1984. 'A manual of
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Pingree, R.D. and Griffiths, D.K., 1978. Tidal Fronts on the
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Pingree, R.D. et al., 1978. The Effects of Vertical
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Pingree, R.D. et al., 1977, The Influence of Biological
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Pingree, R.D. and Mardell, G.T., 1985, Solitary Internal
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Pond, D. et al., 1996, Environmental and Nutritional Factors
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Sharples, J. et al., 2001, Phytoplankton Distribution and
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Disclaimer: The views expressed on this
website do not necessarily reflect the views of the
University of Southampton. |
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