From Back Left: Martha Valiadi, Olly Way, Neil Olsen, Catherine Jarvis, Holly Drake, Nick Dade. Front: Sven Gosden, Tom Simpson.

 

 

 
Foreword    
Offshore

  Introduction

  Method

  Results

 
 

 

Estuary

 

 

  RIBs

 

  Bill Conway

  Introduction

  Method

  Results

  Introduction

  Method

Geophysics

  Geofield

  Geophysics

  Results

 

 

 

 

Abstract

A two week fieldcourse was undertaken by Oceanography and Marine Biology students from the University of Southampton. The object of the course was to familiarise students with genuine field research methods as they undertook a comprehensive survey of the Tamar estuary and coastal area of Plymouth U.K. analyzing the region in terms of its physical, biological and chemical and geological properties.

A variety of methods were used to acquire a dataset which was then analysed. Samples and measurements were taken from 5 different boats and land based measurements were also employed.

Results confirmed existing knowledge to the effect that the Tamar is a partially mixed estuary. The collective data indicated that offshore water column structure is thermally controlled while the estuarine water column structure is salinity controlled.

Chemical analyses of water samples in the estuary identified non-conservative mixing of Silicate due to removal by diatoms, whereas Nitrate showed conservative mixing. Phosphate appeared to follow a conservative mixing trend though points were very scattered around the theoretical dilution line - showing frequent removal and addition of Phosphate from the estuarine system.

The phytoplankton community in the estuary was dominated by diatoms.

The offshore area is characterized by thermal stratification during the summer months leading to distinct thermoclines and deep chlorophyll maxima. Nutrients were severely depleted with values close to or below detection limits. The phytoplankton community was composed mainly of diatoms but other groups were also identified. Zooplankton abundances varied and no clear correlations could be made.

The geological characteristics of the area were also evaluated in terms of local topographical features and sedimentary environments. The main findings were a fault which runs through Renney rocks and to the South East of the Breakwater; the sediments in Plymouth Sound primarily originate from the river Tamar.

 

Introduction

The Tamar Estuary lies on the border between Devon and Cornwall on the southern coast of England. The estuary system is a large marine inlet on the English Channel coast comprising the estuaries of the rivers Tamar, Lynher and Tavy which collectively drain an extensive part of Devon and Cornwall. The Tamar river and its tributaries provide the main input of fresh water into the estuary complex, and form a ria (drowned river valley) with Plymouth lying on the eastern shore. The broader lower reaches of the rivers form extensive tidal mud-flats bordered by saltmarsh communities1

The upper part of the Tamar and Lynher estuaries also include a very well developed estuarine gradient which has not been modified by the construction of locks or weirs. Consequently, they exhibit one of the finest examples of salinity graded communities in the UK2 and are a site of scientific interest and conservation.

 

The Breakwater

Towards the mouth of the estuary there is a man made landmark called the ‘Breakwater.’ This structure was made approximately during the 16th century and was made to reduce the tidal eddys and reduce the damage of episodic events such as storms which could cause floods to the harbour or structural damage.

The ‘Breakwater’ also has an effect on the tidal flow and water currents near to the mouth of the estuary. This is due to the ‘Breakwater’ being made of rock so it is an impermeable barrier and water cannot flow through it so it has to go around it. As the tide comes in from the western side of the ‘Breakwater’ it propagates up and into the estuary and leaves it from the eastern side of the ‘Breakwater.’ The effect of this is to cause the Northern side of the ‘Breakwater’ to have  lower energy currents compared to that of the Southern side of the ‘Breakwater.’ The van Veen grab samples done on the Southern and Northern side further enhance this idea as the size of the grain samples from the Northern side were much smaller than the grain sizes from the Southern side. But there can be differences to this general rule as sediments with a stickier composition require more erosion and transport and therefore are found in higher velocity environments.

 

 

 

 

Introduction

The sampling strategy employed, was to take an onshore-offshore section from the breakwater to station L4 - sampling 5 stations. Due to poor weather conditions and sea-sickness there was only time to sample 4.  

Map of stations surveyed

Station 1                    Station 2                    Station 3                    Station 4

 

Date/Time: 30 June 2005 commencing at 0900GMT Weather conditions

Wind speed/dir: low winds and showers

Cloud cover: 8/8

Tides: HW: 12:30 GMT (4.59m above chart datum)

            LW:  18:50 GMT (1.88m above chart datum)

On the 30th June 2005 commencing at 0900 GMT, we boarded the Bonito research support vessel to undertake an offshore survey from Plymouth Breakwater (50° 20.113 'N, 004° 09.304 'W) to L4(50° 15.969 'N, 004° 13.821 'W).

 In coastal waters off Plymouth there are very low nutrient levels in the summer especially in the upper part of the water column as they have been depleted mostly in the spring (Pingree et al., 1977). This limits primary production and so results in the phytoplankton moving deeper where nutrients are more abundant, so deep chlorophyll maxima occur in the summer months, usually in the regional thermocline, as some thermal stratification is typical during this period (Rodriguez et al., 2000).The thermocline structure is largely determined by abiotic factors, such as depth and stratification of the water column (affected by wind stress and temperature). As stated by Sverdrup, 1953, “the degree of vertical mixing, largely determines the availability of light energy and inorganic nutrients of phytoplankton growth”.

The thermocline structure has a significant impact on the biological distribution of microscopic organisms, so it was important to correlate the location and composition of the plankton community with the available nutrient concentrations in the water column.

Aims

The aim of the offshore data collection was to determine the vertical mixing processes in the waters off Plymouth.  Therefore it will be possible to establish both the indirect and direct effects on the structure and functional properties of plankton communities in the coastal waters of the western English Channel.

Limitations

One of the major problems encountered in this practical was the bad weather conditions on the day of sampling. This caused a large amount of noise in our CTD data which had to be filtered out thus decreasing the amount of data available to work with. At station 4 CTD data was only collected for half the water column below 14m due to technical problems with the CTD device. The bad weather conditions also did not allow us to take many zooplankton samples as it was difficult to deploy the net.  Therefore there is no clear picture of how the zooplankton varies in the water column.

One further limitation is that the sampled chlorophyll a data in this practical could not be used as the profiles obtained were abnormal. This could have been due to leakage while filtering giving low chlorophyll values or even errors during the lab analysis.

Another problem was the phytoplankton abundance in most stations may be very underestimated or may even have a different community composition, since according to Rodriguez et al. (2000), in July the most important phytoplankton in the chlorophyll maxima were smaller than 5µm. These could not have been counted under our microscope as they would be impossible to see under a x10000 magnification. Also, colonies of diatoms, which were the most abundant, were recorded as a single cell leading to further underestimation of the phytoplankton abundance. 

Silicate profiles were not included for stations 1 and 2 as only one measurement of silica was made at each station at 4m and 25m respectively. 

Values for nitrate and phosphate were either close to or below the detection limit and so the uncertainties with these concentrations are increased. The nutrient profiles recorded could vary to a large degree from what was recorded.

 

Method

 

Four stations were sampled and the vertical temperature structure within the water column was determined at each station, to establish the possible presence and significance of a thermocline.  The same methods were used at each site:

Using a CTD rosette (Conductivity Temperature Depth) recorder, it was possible to build a virtual image of the water column profile.  Care must be taken when lowering the CTD rosette into the water as the swell offshore is likely to make the vessel extremely unsteady, causing the CTD to swing whilst in the air, so hard hats and protective footwear must be worn to prevent injury. 

The CTD allowed water samples to be taken at specific depth intervals.  The water samples were collected via the Niskin Bottles attached, which were triggered by an acoustic signal to release the pin holding the bottle open.  This enabled us to collect large water samples which we used to take oxygen, silicon, phosphate and nitrate and phytoplankton samples to be processed in the labs.

Phosphate and Nitrate:  The filter should be flushed with 10ml of sample water to ensure no loose glass fibre is introduced to the sample.  This is used to rinse each sample bottle.  100ml of filtered sea water was collected, so this equates to two 60ml syringes, each having used 10ml to flush the bottles.  These were then stored and sampled in a brown glass bottle.

Silicon:  The samples must be collected in plastic bottles to prevent contamination, which may occur if collected in glass bottles.  These were rinsed twice with 10ml of filtered seawater to again prevent contamination.  100ml of water was then filtered.  All samples were then stored in a cool box and transferred to a fridge as soon as possible.

Chlorophyll:  The water sample; 50-100ml was filtered through a GFC filter of 0.7µm pore size.  The total volumes of filtered water were noted, and the collection bottles were labelled relative to the site and depth.  The filters used were placed into 10ml of 90% acetone filled test tubes using forceps to prevent additional contamination, they were then shaken well and left to extract before analysis the next day.

Oxygen:  This was always taken first to prevent the introduction of atmospheric oxygen into the sample, which could otherwise misrepresent the collected results.  The oxygen bottles are flushed through carefully with the sample water to ensure they are free from bubbles, then two reagents were added; 1ml manganese chloride to fix the oxygen, followed by 1ml of alkaline iodide, before replacing the lid from 45° and storing upright in water.

 

Adding manganese chloride to oxygen sample bottle

 

Zooplankton:  Zooplankton from the water column were collected using a net with a mesh size of 200µm, which was deployed from the vessel using a hydraulic winch.  Involved members must be wearing protective clothing and hard hats.  Once recovered the mesh must be washed from the outside to ensure all the organic matter accumulates in the cod end.  A litre sample was collected from the cod end and put into a labelled container.  To preserve the sample, 20ml of 10% formalin must then be added, ensuring plastic gloves and goggles are worn.

 The ADCP (Acoustic Current Doppler Profiler) enabled us to correlate specific features recognized in the CTD data.  It was used to establish current profiles within the water column and distributions of microorganisms by reflectance strength. 

The ADCP was used to record transect data collected between each site, and reset to collect another series once in position.

 

Results

Station 1

Station 1 was inshore of the breakwater and therefore reasonably sheltered, there were frequent showers, winds were low. The water depth was ~16m . We fired 2 niskin bottles at this site, one at 14.5m and one at 4m. We took one zooplankton net from 12m to surface.
 

 

   

Figure 1a

Graph to show temperature profile at Station 1 and distribution of plankton in the water column

Figure 1b

Graph to show concentrations of Nitrate, Phosphate and fluorescence against depth.

At station 1 there is a generally well mixed water column with no clear thermocline (Fig. 1a). The chlorophyll maximum occurs at approximately 2m depth (Fig.1b) which could be a result of riverine water input, as this station is in the outer part of the Tamar estuary (Holligan et al.,1984).   The riverine water would have a higher nutrient load.  This correlates with the    %O2 saturation profile being higher at the surface (Fig. 1c), implying the presence of a large number of photosynthetic organisms, and then decreasing with depth.

At approximately 4m depth the fluorescence decreases and increases again at approximately 6m depth (Fig.1a). Since our nitrate profile of the water column only begins at 4m it is impossible to estimate the nitrate concentration at the chlorophyll maximum. This would account for the fact that there is a constant decrease of nitrate in the water column coinciding with an increase in fluorescence at 6m depth (Fig.1b). The fact that nitrate does not increase in the lower water column could be the result of the uncertainty at such low levels, causing a false value. The phosphate concentration in the water column was consistently below the detection limit and so is negligible (Fig.1b).

The composition of the phytoplankton community was mainly dominated by diatoms at both sampled depths of 4m and 14.5m, followed by dinoflagellates and then by ciliates, where the dinoflagellates increased with depth and the ciliates decreased with depth (Fig. 1a).  The total phytoplankton abundance measured at these same depths was calculated to be 20833cells/L at 4m and 10000cells/L at 14.5m showing a considerable decrease (Fig. 5). A zooplankton sample was also taken from 12-0 m showing a fairly low abundance of zooplankton at this station. This could be due to the small sizes of diatoms, which cannot be easily consumed by mesozooplankton.

 

Figure 1c

Graph to represent the percent of Oxygen Saturation at Stations 1 and 2.

 

Station 2

Station 2 was the furthest station offshore at location L4 (50° 15.969 'N, 004° 13.821 'W). At this position winds felt stronger and there were still showers - swell here was greater. Niskin bottles were fired at 5m 25m and 45m. Zooplankton nets were hauled at 25m - 18m and at 18m to the surface.
     

Figure 2a

Graph to show temperature profile at Station 2 and distribution of plankton in the water column

Figure 2b

Graph to show concentrations of Nitrate, Phosphate and fluorescence against depth. 

At station 2 (L4) there is a well defined thermocline between 10m and 30m depth (Fig.2a). The top layer of the water column between 0m and 8m depth is very well mixed with a constant temperature of 16.1°C, probably due to high wind stress causing turbulence (Pingree and Holligan, 1975) and therefore mixing, on that day. The chlorophyll maximum here was observed at approximately 22m depth (Fig. 2b), in the thermocline, as it would be expected in an offshore area such as this.  The % O2 saturation profile shows oversaturation at this depth of approximately 107% (Fig. 2c). The fact that the % O2 saturation is higher above the chlorophyll maximum could be the result of high mixing at the near surface layer allowing for diffusion of O2 from the atmosphere into the water column.

Regarding the nutrients, the nitrate profile shows nitrate levels which are below detection limit up to the depth of the chlorophyll maximum. The nitrate then increases with depth as the chlorophyll decreases (Fig. 2b). The phosphate profile shows a slight increase at 25m, but as this value is close to the detection limit, it therefore has a high uncertainty.

In Fig. 5 it is clear that the phytoplankton abundance varies in the same way as chlorophyll with a maximum abundance of 21500 cells/L at a depth of 25m. The phytoplankton community is dominated by diatoms throughout the water column, at 25m and 45m they are the only phytoplankton present. At 5m depth the phytoplankton are mainly dominated by diatoms, though photosynthetic ciliates and dinoflagellates are also present (Fig. 2a). Two zooplankton samples were also taken rising from 25-18m and 18-0m unexpectedly showing a decrease with depth. This could be due to the fact that only diatoms are present in the chlorophyll maximum and so they cannot be preyed on easily by the zooplankton because of their small size.

 

Figure 2c

Graph to represent the percent of Oxygen Saturation at Stations 1 and 2.

 

Station 3

Station 3 was at location  (50° 16.940 'N, 004° 11.700 'W). This station was further towards the shore, winds were still strong and showers continued - there was still quite a large swell. Niskin bottles were fired at 14m 21m and 45m. Zooplankton nets were hauled at 18m - 7m and at 7m to the surface.

Figure 3a

Graph to show temperature profile at Station 3 and distribution of plankton in the water column.

Figure 3b

Graph to show concentrations of Nitrate, Phosphate and fluorescence against depth.

At station 3 there is a generally well mixed water column with no clear thermocline (Fig.3a). The near surface layer of the water column between 0m and 8m depth is very well mixed with a constant temperature of 16.1° just as in station 2. The chlorophyll maximum occurred at approximately 18m depth (Fig. 3b) in agreement with the %O2 saturation profile and the silicate profile (Fig.3c).  The oversaturation of oxygen in the water column and the depletion of silicate at approximately 21m. The nitrate profile also shows nitrate depletion at 21m close to the chlorophyll maximum and it increases above and below it, measurements at 14m and 45m are the highest ones recorded in our four stations (Fig. 3b). The increase above it could be due to vertical mixing processes bringing in more nitrate (Holligan et al., 1984).

The vertical profile of the phytoplankton abundance (Fig.5) roughly agrees with the chlorophyll profile (Fig. 3b) with the highest abundance of 22333 cells/L being recorded at 14m and then decreasing with depth. This could be due the fact that a phytoplankton sample was not taken at the chlorophyll maximum but just before and after it. 

The phytoplankton community was dominated by diatoms with only tiny amounts of photosynthetic ciliates and dinoflagellates at 14m. At 21m there is a more diverse community with a high abundance of ciliates compared to other stations, followed by the dinoflagellates. The diatoms are dominant here as well but have the lowest % contribution to the community that has been recorded in this practical (Fig.1a). Two zooplankton samples were collected from 7-0m and 18-7m from which the abundance was calculated to be 23591 and 2250 cells/L respectively.

 

Figure 3c

Graph to represent the percent of Oxygen Saturation and Silicate Concentration at Station 3.

 

Station 4

Station 4 was at location  (50° 18'.339N, 004° 09'.904W). This station was again further towards the shore, winds were still strong but rain had stopped - there was still quite a large swell. Niskin bottles were fired at the surface, 16m and 29m depth. A Zooplankton net was hauled at 18m to surface.

Figure 4a

Graph to show temperature profile at Station 4 and distribution of plankton in the water column.

Figure 4b

Graph to show concentrations of Nitrate, Phosphate as and fluorescence against depth.

At station 4 the CTD data is available for only the lower half of the water column due to technical problems with the CTD. The fluoresence peaks at approximately 17m (Fig. 4b) in agreement with the phytoplankton abundance profile which also has a maximum value of 34000 cells/L at 16m (Fig. 5). This shows that the chlorophyll maximum is actually at that depth and not above 14m where there is no CTD data available. The temperature structure shows a well mixed water column below 14m (Fig.4a) but it is impossible to say if there is a thermocline starting at a shallower depth, due to the technical errors mentioned above.

Nutrient profiles for silicate (Fig. 4c) and phosphate (Fig.4b) agree with the chlorophyll maximum at the depth of 14m.  The nitrate profile at this station shows values below detection limit throughout the water column (Fig.4b).  Phosphate has high value of 0.06 µmol/L at the surface, compared to other stations, which then decreases with depth until 16m where it reaches values below the detection limit (Fig.4b). The value of 0.06 µmol/L for the phosphate may in reality be an overestimate since it is still close to the detection limit therefore having a high uncertainty.  The silicate profile shows a decrease at 16m showing depletion at that depth (Fig. 4c). Furthermore, the %O2 saturation profile shows an increase at 16m, (Fig. 4c), which is also in good agreement with the fact that phytoplankton are more abundant at this depth (Fig.5).

The phytoplankton community at this station is largely dominated by diatoms with small abundances of dinoflagellates throughout the whole water column.  However photosynthetic ciliates were only found in the chlorophyll maximum (Fig. 4a).

The largest abundances of phytoplankton were found at this station with a maximum of 34000cells/L and a minimum at the surface of 26000 cells/L (Fig. 5). This could explain the large depletion of nitrate and the silicate. A zooplankton sample was only taken at 18-0m showing a small abundance of 1404cells/L probably due to the small sizes of the diatoms.

 

Figure 4c

Graph to represent the percent of Oxygen Saturation and Silicate Concentration at Station 4.

Figure 5

Graph to show phytoplankton abundance over 4 sites

 

ADCP Data
 Click here to see a chart showing Site locations and paths of ADCP files.
ADCP profile for station 2ADCP data can be evaluated in conjunction with the observation records of Zooplankton abundance. The profile (right) is from file 1002 (shown in the chart above) and shows data collected while moving to and away from station 3.

Group1002r.000 Potential feature noted in Backscatter a ~7m depth, from start of track to 760m, suggests patchy plankton. Also at 1900-3000m along 8m depth contour. More backscatter (indicated by rising dB levels) may be related to turbid layer just above seafloor. Very noisy at surface (waves), deeper mixing at ~400m, 1200m, and 1920m along track.

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The aim of the boat work associated with the Bill Conway and the R.I.B. work was to develop an understanding of how the Tamar estuary acts as a transition zone between freshwater input and the coastal sea.

The areas covered by each practical session are shown here:

                                                                                                             

River Tamar transects 1, 2, 3                             River Tamar transects 4, 5, 6                                       River Tamar transects 5, 6, 7

 

Estuarine R.I.B.s

 

 
The aim of the R.I.B.'s work was to collect primary data from a range of salinities from close to zero to the Tamar Bridge (the limit of our sampling area). Observations were made of physical properties: Salinity and Temperature using a T.S probe; chemical properties, Nitrate, Phosphate and Silicate from water samples, dissolved oxygen from the probe; and biological features chlorophyll and phytoplankton/zooplankton cell counts from bottle samples.

In order to calibrate the instruments; a sample for absolute salinity and absolute oxygen were taken to compare the probe readings with the actual values.

 

RIBs - Method

 

Water samples were taken at salinity intervals of 2 from near 0 salinity (1.49) to a salinity of 32 (around the River Tamar bridge). Phosphate, Nitrate, Silica and Chlorophyll 'a' samples were taken for later analysis in the lab at 2 unit salinity intervals. Using the same practical techniques as before. A T, S probe was used to record the salinity temperature pH and % oxygen saturation of the water. Dissolved oxygen samples were taken, using the same method as before, at salinities of 18, 24, 26, 28 and 32. 100ml water samples were taken and placed in lugols iodine for phytoplankton analysis. A zooplankton net was deployed from a stationary mooring at salinity 18.  The duration of deployment and the current flow was recorded in order to work out phytoplankton abundance by finding the amount of water which passed through the net. 

 

Estuaries - Bill Conway
The aim of the Bill Conway practical was to determine the differences in physical characteristics of the main water body, caused by the tidal flow progressing upstream in the estuary. The Conway left the Mayflower Marina at 09:14 BST (08:14 GMT) to undergo a water profiling survey of Plymouth Sound, surrounding estuary and further up river to the Tamar bridge.  The data collected is complementary to the R.I.B.s data - and the 2 data sets are combined in order to gain a complete picture of the Estuary. 

 

Bill Conway - Method

A series of locations to obtain ADCP transects were thought out, based on a general understanding of inputs which may affect the physical characteristics of the local water body.  The first station was at the breakwater where the initial transect using the ADCP was taken.  This helped to gain an idea of how the water column was behaving and an understanding of the current velocity magnitude, and direction of flow.  This was then used to determine the best site to lower a CTD for a vertical profile.  To avoid damage whilst the boat was moored, the ADCP was connected to the starboard side of the Conway.

 Once this information was gathered the CTD was lowered as on the Bonito.  Water samples were collected to determine nutrient levels, dissolved oxygen levels and chlorophyll concentration, at varying depths. The same process was then repeated heading upstream in the estuary (see map for all sites samples were taken). In addition to the above, at stations 4 & 7 a zooplankton net was deployed and at stations 3, 4, 5 & 6 phytoplankton samples were gathered for further study in the labs.  See Bonito Methods for safety issues, deployment methods and sampling techniques including; chlorophyll, zooplankton, phytoplankton, salinity and nutrients. 

A Secchi disk was lowered into the water column until it was not visible.  Once this depth was recorded it was multiplied by three to ascertain the depth of the euphotic zone.  The length of the cord was recorded to get the optical depth.

 

Estuarine Results

 

Nutrients

Figure 5

Estuarine mixing diagram for Nitrate

 

Figure 6

Estuarine mixing diagram for Phosphate

 

Figure 7

Estuarine mixing diagram for Silicate

The Nitrate Estuarine Mixing Diagrams demonstrates conservative mixing. i.e there is no net removal or addition of NO3 over the estuarine system. 

The Phosphate mixing diagram shows more "scattering" around the theoretical dilution line than the Nitrate - on which the points are fairly tight around the line. 

A possible explanation for this is that there are are several inputs to the Tamar from Sewage Treatment plants (see map above), these are now required to treat their outputs to reduce Nitrate but there is no control over amount of Phosphate pumped into the river.  

Other inputs of NO3 and PO4 would be run off from farmland (marked on the map) - though the weather had been quite dry in the days before our sampling so there would not be an excessive run off.

The estuarine mixing diagram for silicate shows non conservative behaviour. Silicate is being removed from the estuarine system. This is most probably due to the large population of diatoms observed. Our phytoplankton samples contained only diatoms - which would be utilising the silica to create protective frustules.

 

Transect Data

Figure 8

ADCP profile at station 1b.  (50° 20.249 'N, 004°09.844 'W)

The ADCP profile for station 1b shows the difference between velocity magnitude of the two layers in the water column.  The backscatter profile shows where there is a layer of zooplankton occurring near the chlorophyll maximum.  This is reflected by a Ri number of 1.05 indicating the layers are stable and stratified.  

Figure 9

ADCP profile at station 7 (50° 24.577'N, 004° 12.235'W)

The ADCP profile for station 7 shows that the velocity magnitude in the water column is uniform. The backscatter profile indicates a strong definitive layer of zooplankton around the chlorophyll maximum. This is reflected by a Ri number of 1.23, indicating a stable water column. 

 

CTD data

Figure 10

T, S profile for station 1 (breakwater)

Figure 11

T, S profile for station 4 Trevol channel

The CTD data collected from Bill Conway and the RIBs allows us to examine the structure and stratification over the length of the Tamar estuary. Furthest from river source at the Breakwater, temperature throughout the water column varies by only ~0.8degrees C and Salinity by 0.4 units. Where as at the Tamar Bridge temperature changes by ~1.7 degrees C over 10m and salinity by 2.1 units over 10m.

This increase in stratification and development of halocline/thermocline seen in the profiles up river, is resultant of the freshwater river input flowing over incoming sea water. The Tamar estuary is classified as partially mixed - which means that while the 2 layers (fresh and sea) are maintained, there is turbulence between the 2 layers, and advective processes causing some mixing and slight reduction in halocline/thermocline.

 

Figure 12

T, S profile for station 5, HMS drake

Figure 13

T, S profile for station 7, Tamar Bridge

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Geofield

Date Friday 1st July
Weather Overcast with poor visibilty
Sampling Techniques Dip and Strike

Orientation bearings of a large fold

Cliff cross section

Introduction The group headed to Heybrook Bay on the afternoon of Friday 1st July to study the geology of the area. Dips and strikes were recorded over the rocks at the shore line. A major antiform fold line cut by a large fault line was found and the orientation of these features was recorded. Fractures caused by the fold were plotted on a Rose diagram, which is shown below, to show how these fractures relate to the fold. A section of the cliff line was examined in cross section and each of the beds described.
Bed description  
Bed 7. This is the final bed to have been deposited, and is approximately 50cm thick. This is a sandy, silty sediment layer with white shell fragments in this light brown sandy matrix. The presence of shells in the sediment shows that the sea level used to be higher than it is at present.
Bed 6. This bed is approximately 70cm. It is beige in colour due to a much higher sand content, the grain size of this layer is larger than the lower layers. this influx of sand into the sediment may be due to the drowning  of a river valley caused by a rise in sea level.
Bed 5. This is the largest of the beds, at around 200cm (2m). It contains a large range of clasts, with the larger ones being arranged horizontally by the energy of the flow. These clasts are supported by a matrix of sand and silt. This bed is thought to have been deposited by a high energy mass movement such as a mud flow. This mudflow was probably caused by the melting of glaciers as the climate warmed. This excess water coupled with the lack of vegetation, characteristic of a glacial environment, enables sediment to be eroded easily. This is then transported downstream and then deposited as this large bed.
Bed 4. This is a smaller bed layer approximately 20cm thick. There are no clasts in this layer, however it does contain the first signs of sand in the sediment. The overall grain size of this layer is bigger than that of the beds below it. The presence of sand shows that a beach had now formed, this is an indication of sea level rise.
Bed 3. This layer is approximately 30 cm thick and contains clasts which are, on average, smaller than 1cm. These clasts are supported in a red/brown, fine grained matrix. As you move along the cliff face this bed pinches out, this is known as 'laterally discontinuous'. The smaller clasts found in this layer shows that they were deposited in a low energy environment, eg. a stream or braided river system.
Bed 2. This is a 30cm layer of very fine grained red clay. The red colour is caused by the presence of oxidised iron in the sediment. There are no clasts in this layer and so it is completely matrix supported. This layer was probably deposited in a low energy environment with calm waters, eg. submarine/offshore or a delta environment.
Bed 1. This is the lowest and oldest sedimentary bed exposed on this cliff face. It is approximately 70cm thick and contains angular to sub angular clasts, around 10cm in size. It is clast supported layer as the clasts are not completely surrounded by a matrix. The large size of the clasts suggests that they were transported in a high velocity/energy flow. These clasts are thought to be from a local source as their sub angular shape suggests that they have not been transported far. The clasts have been arranged horizontally by the water flow, this is called 'imbrication'.
Folding observed

The geofield work showed that there is a fold structure in the sedimentary bedforms at Renney Rocks. This is a convex fold and seems to fold back on its self, therefore this fold can be described as a recumbent, antiform fold. The fold is plunging into the sea at an angle of 70.  The diagram below shows these features:

Figure 14

Diagram to show plunging fold

Figure 15

Rose diagram showing main fault and fractures

Also shown by the geofield fieldwork is a dextral fault line on a bearing of 296, this fault line is also shown on the sidescan sonar with a similar orientation. On the sidescan sonar the fault line plunges in a SSE direction and disappears as it passes through the grid transect. The fault line cuts through the fold line at Renney Rocks, displacing the folding beds 5-6m. This fault line is shown on the sidescan sonar chart to have a displacement of 10.8m.  

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Geophysics

Date/Time: 07/07/05 Depart marina 09:00 BST Weather conditions: Fair to overcast

Cloud cover: 6/8

Tide Times  HW:  06:10GMT 4.89m above chart datum

                   LW: 12:20GMT 1.42m above chart datum

Introduction

Sidescan sonar is a marine geophysical technique, making use of acoustic surveying, to image the seafloor. Pulses of sound are shot sub-horizontally from a towed transducer and the strength and travel time of the reflection off relief or objects is recorded and processed. The result is the production of an image in shades of grey that shows any objects, dips, mounts and even the roughness of the seafloor and therefore its lithology.

This technique is widely used to map the seafloor, to inspect ship channels for obstructions and even to find shipwrecks, dredges and anything else projecting above the seafloor.

A major advantage of this technique is that as it uses sound instead of light, it can be used in murky or black water. One of its disadvantages though, is that it projects a beam only to the side of the       towpath so no image of what is exactly underneath it can be obtained.                                                   The fish being deployed

Aims 

i)  To find and record the fault line across the inshore bay waters, and to establish any offsets which may occur.

ii)  To record the sediment type via sidescan sonar retrieval and sediment grabs.

iii) Examine any variation in sediment type and physical make-up both inside and outside of the breakwater.

 

Method

Using the sidescan sonar ‘fish’, it was possible to follow a transect line on either side of the breakwater to establish the variation of sediment compositions and formations of the seabed sediment, which occurs due to tidal and wave energy.

A multiple transect survey was then worked out and followed where the fault line was believed to run across the bay.  These images were correlated in the lab, and the scales corrected due to the varying speed and angle of the beam from the ‘fish’.  On the original images, the darker shades represent a strong reflection, so therefore are made of more solid matter.

From the scaled images, a plot was recorded and the seabed type found, faults could then be established and channels routed through the rocky outcrops defined.

By taking a grab sample either side of the breakwater, it had the potential to indicate the benthic life and composition of the sediment.  A grab sample was taken at one location either side of the breakwater, both offshore and inshore of it.  Particular care was taken not to allow the tidal drift to push the vessel into the breakwater when on the offshore edge, and when on the inshore edge, care had to be taken to avoid being moved by the North Westerly winds.   Once the grab is deployed from the stern, the vessel cannot be manoeuvred so position must be correct beforehand. 

Two grab samples were taken, one North and one South of the breakwater (the first grab failed - due to the jaws being jammed by a cobble - and so was repeated). A description of the grabs can be viewed in the results section.

Operating the grab

Results

 

The line and grid transect undertaken on the geophysical practical has enabled us to create a geological map showing the different bedforms of the sea bed. This map is shown here

Figure 16a

Geological map of area surveyed by sidescan

The sidescan sonar showed that there are three main geologies in the area. These are shown in the key to be soft, small grained sediment eg. Silt/mud, medium to coarse grained sands and bedrock.

Two Van Veen grab samples were taken in front and behind the breakwater, these are both marked on the map.

Grab North is located behind the breakwater and contained dark, fine grained, soft, sticky sediment. There was a thin oxidized layer, approximately 3mm thick, on the surface of the sample. Then below this is a sticky, anoxic, fine grained sediment.

Grab South is located outside the breakwater. This grab contained a vastly different sediment sample than Grab North. It contained a much greater range in grain size of sediment. The overall composition of this sample was 5-10% <1mm, 5-10% 1-2mm and the remaining sediment being between 2mm-10cm.

The difference in sediment types between the two grabs is predominantly due to the effect of ocean currents and how they differ in each grab location. Grab North is behind the breakwater, therefore the water will be less turbulent here and have a lower velocity. The fine grained sediment will settle out in this low energy environment and deposit on the sea floor, while larger sediments will be deposited at higher velocities. This is shown by the ‘Grab South’ data, as it is in front of the breakwater and has high velocity, turbulent waters due to wave influence. This high energy environment will transport any fine grained sediment away and deposit larger grained sediments.

There is evidence however that goes against this velocity theory. The photo on the right shows an area of finer grained sediment amongst the coarser grained, rippled sands. This is because the finer grained sediment has a sticky composition, and so requires more energy to erode and remove this sediment.

 

Figure 16b

Geological map key

 

Figure 17

Photo of sidescan data showing sediment irregularities. 

The sidescan sonar data shows ripples on the sea bed, appearing as shadow zones. There are two types of ripples shown in the sediment, these are asymmetrical and symmetrical. The diagram on the right shows these two different wave types and their respective shadow zones.

Figure 18

Photo showing types of ripples

The sidescan sonar showed some possible Palio river channels towards the eastern side of the estuary. These are shown on the geological map. These palio river channels occur when deeper channels in the bed rock get filled in with sediment, these then appear as a different colour on the sidescan sonar. This is shown on the right.

Figure 19

Photo showing possible source for theoretical paeleo-channel

These possible palio river channels can be theoretically linked with small river channels still present on the landscape. This is shown on the map insert on the right.

Figure 20

Photo of side scan data showing possible paeleo-channel

The sidescan sonar also identified a dextral fault with a displacement of 10.8m on a bearing of 2960. This is on the same bearing as the fault observed at Renney Point, suggesting that these two faults are part of the same fault line.

Figure 21

Photo of side scan data showing fault line

 

Grab data

Grab Time (GMT) Coordinates Sediment Type Benthic
1 11:23 50˚ 20.0 ‘N, 004˚ 09.4 ‘W Note: This grab failed as a large cobble was jammed between the jaws, this grab was repeated
South 11:28 50˚ 19.9 ‘N, 004˚ 09.4 ‘W

5-10% <1mm Grain size.

5-10% 1-2mm Grain size.

Some small shell fragments.

Rest 2mm-10cm.

Flora attached to larger sediment fragments. 

Sea squirts on 12cm rock fragment.

Calcitic worm formations on larger fragments (pomatoceros). 
North 11:52 50˚ 20.1 ‘N, 04˚ 09.3 ‘W Very Fine, silty sediment composition.  Thin oxidized layer on sediment surface, approximately 3mm thick on sample.  Below this, anoxic sediment continues.

Taratella shells : 7, all originally inhabited, but often found with hermit crabs.

Annelid  (ragworm): 3.

Brittle star : 1.

 Boomer

Since the group finished the sidescan survey in good time it was possible to use the boomer. The boomer works by creating a high voltage electrical pulse which passes over a metal plate - causing it to vibrate. The boomer is able to determine features such as faults below the sea bed. The data which the boomer collected was not available for group 5 to process, so a couple of screen shots were taken, which showed the paeleochannel demonstrating the types of features the boomer is able to observe. 

 

Screen shot of the boomer data - showing the paeleochannel

 

 

 

General Safety Issues

Deployment of instruments from the vessel requires the skipper be notified and ensure lines, ropes and wires are kept clear of whilst on deck.  When deploying heavy instruments, correct lifting procedures must be followed to reduce the risk of injury.  During the operation of the Boomer, high currents flow, so care must be taken handling the wires, and ensuring the tow lines of all instruments is secured.

 

References

Grabemann I., Uncles R.J., Krause G., Stephens J.A., (1997), Behaviour of Turbidity Maxima in the Tamar (U.K.) and Weser (F.R.G.) Estuaries, Estuarine, Coastal and Shelf Science, 45, 235-246pp. 

Holligan P.M., Williams P.J.leB., Purdie D., Harris R.P., (1984), Photosynthesis, respiration and nitrogen supply of plankton populations in stratified, frontal and tidally mixed shelf waters, Marine Ecology Progress Series, 17, 201-213 pp.

Pingree R.D., Maddock L., Butler E.I., (1977), The influence of biological activity and physical stability in determining the chemical distributions of inorganic phosphate, silicate and nitrate, Journal of the Marine Biological Association, 57, 1065-1073 pp.

Pingree R D., Pugh P.R., Holligan P.M., Foster G.R., (1975), Summer phytoplankton blooms and red tides along tidal fronts in the approaches to the English Channel, Nature, 258, 672-676.

Rodriguez F., Fernandez E., Head R. N., Harbour D.S., Bratbak G., Heldal M., Harris R.P., (2000), Temporal variability of viruses, bacteria, phytoplankton and zooplankton in the Western English Channel off Plymouth, Journal of the Marine Biological Association U.K, 80, 575-586 pp.

Sverdrup H.U., (1953), On conditions for the vernal blooming of phytoplankton, Journal du Conseil International pour l’ Exploration de la mer, 18. 287-295 pp.

1: (http://www.jncc.gov.uk/default.aspx?page=2033)

2: (http://www.ukmarinesac.org.uk/pdfs/casestudy-plymouth.pdf)

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Appendix 1: Additional information on the turbidity maximum in Plymouth Sound and Estuaries

Previous studies of the Tamar Estuary in the UK have revealed that this estuary exhibits strong turbidity maxima in the low salinity regions. This estuarine turbidity maximum is a dynamic feature and its generation and maintenance is known to be a result of complex interactions between the tidal dynamics, gravitational circulation and erosion of fine sediment (Dyer, 1988). Over a spring-neap tide the strong suspended particulate matter levels will be greater than during neap tides due to the greater energy caused by the spring tides, which result in being able to suspend more particles.

In the Tamar estuary there is a temporary occurrence of a pronounced turbidity maximum up-channel of the freshwater-saltwater interface is of importance (Grabermann et al 1997). This can occur after long periods of low river discharge during the summer mainly during spring tides. The turbidity maximums location will be largely determined by the tidal asymmetry effects, which can lead to sediment being transported up the estuary towards the head. This also corresponds with the high salinity of up to 30 as far up as the Tamar Bridge. High salinity readings become associated with this area of the Tamar due to the macrotidal range which has high energy associated with it and will bring large volumes of salt water from the estuary mouth. During the summer months this can become further enhanced by the decrease in freshwater flow and the increase in temperature and thus evaporation.

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Disclaimer:  The views expressed on this page are based on our own research and are not necessarily the opinions of Southampton University.