Plymouth 2004 Group 6

Southampton University field course to Plymouth 23rd June-8th July 2004.

All timings are in Greenwich Mean Time (GMT) 

All tides referenced to Devonport, Plymouth


Contents

General Introduction

  Geophysics Practical - Estuarine Practical - Offshore Practical - Ribs Practical - References

General Conclusion

Group 6

Dave Bolton - Alex Cargo - Wendy Crichton - Claire Fletcher - Elizabeth Grant - James Knights - Bradley Lewington - Michael Plant - Angus Roberts - Simon Washington 


General Introduction

The aim of this field course was to study the geophysics, chemistry, biology and physics of the estuarine, coastal and offshore waters around Plymouth, and combine the results to provide a detailed overview of the complete system. Estuarine work was carried out on the R. V. Bill Conway and RIBs, from the breakwater in Plymouth Sound to the lower limit of the freshwater in the River Tamar. The offshore work took place on the M.V. Terschelling, from the breakwater to Eddystone Rocks, and the geophysical surveys of the estuary and Sound on the catamaran Natwest II.

The Tamar estuary is located in South West England, a temperate region. It is classified as a drowned river valley (Tattersall et. al. 2003) and a partially mixed estuary (Uncles & Lewis, 2001). The Tamar is mesotidal, with a mean tidal range of 3.5m and an annual average discharge of 27m3s-1. The estuary is 31.7km long and consists of the main river and the Rivers Tavy and Lynher, which feed into the Tamar in the lower reaches. The combined rivers flow into Plymouth Sound through the Narrows (a restricted, deep channel) before entering the English Channel (Tattersall et. al. 2003).  


Geophysics Boat Practical

 
Date Time Location Tide Weather
24/06/2004 0830 - 1500 GMT Tamar River, Lynher River & Plymouth Sound High water 0940 GMT 4.71m

Low water 1550 GMT 1.71m

Blustery with occasional showers. Gusting force 4 to 6. Some bright spells. Pressure 989mb.

Introduction

At 0805 GMT, the Natwest II left the Mayflower International Marina with 10 students, two scientists and two crew members. First going into Plymouth Sound, at 0820 the West Vanguard Buoy was passed and the first tow commenced up the Hamoaze Channel into the River Tamar. The preliminary tow was charted to travel up the eastern bank of the Tamar, past the Naval Dockyards and finish approximately ½ mile past the Tamar and Royal Albert Bridges (50º 24.4562N, 4º12.2514W). During the tow, the Natwest II remained within 40-50 metres of the eastern bank, to include the dock walls, the entrance to Mayflower International Marina, as well as three frigates and one submarine stationed at the Naval docks. At 1005 the boat turned and began a return survey down the western bank. At 1025, the boat entered the River Lynher to survey, after assessing the weather conditions. This continued until 1034, when the water depth became too shallow. The fish (side scan sonar) was recovered at 1040, and track lines for the first major survey finalised. The area was chosen because the preliminary tow indicated a change in sediment type and possible features. Tracklines were planned across the estuary (west to east) above the bridge, but this was unrealistic due to fast currents, so tracklines along a north-south orientation were carried out. The first trackline traced the following route: 50º24.550 N 4º12.180W to 50º24.550N 4º12.600W. This trackline was surveyed once using 500kHz frequency between 1145 and 1200, and again using 100kHz between 1203 and 1216.

A survey of Plymouth Sound was conducted between 1302 and 1337, but showed a homogenous seabed and did not merit further investigation. A Van Veen grab (Fig. 1 & 3) (0.3m3 capacity) sample (50º 20.060N, 4º 09.804W) at 1358 showed a fine, muddy sediment with a one millimetre oxic layer (Fig. 2). The only fauna visible were five gastropods (Fig. 4) and two polychaete worms. Another grab (50º 19.783N,  4º 11.700W) was coarser with a higher proportion of sand, however the sample was incomplete, as the grab was only partially full. Further grabs yielded little sediment and were abandoned.

Fig. 1 The Van Veen grab used on the Natwest II  Fig. 2 A grab sample taken to give an indication of the bottom composition Fig. 3 Careful lowering of the heavy grab used Fig. 4 The Gastropod Turitella found in one of the grab samples

Data Analysis

The Docks

A Sidescan Survey was conducted from 50o 21.8697 N, 4o 10.8663W up the east bank of the Tamar river. This area consists mainly of the military dock and is dredged to 8.5m in the south and 9.1m further north.

  •  Due to the removal of sediment by dredging, there was a lack of bedforms.  Finer, more cohesive sediment may be limited in this area; therefore bedforms, which require such sediment, do not arise.
  •  The presence of a depression at 50o 22.0147 N, 4o 11.1004W is associated with a meander in the estuary.  As the water is forced round, it undercuts the meander causing deposition on the eastern bank.  The depression measures 31.2m by 18.7m, with a depth of 0.98m.

Two different pillar sections were compared, as annotated on the Group 6 isometric plots.

  • The first consisted of 10 concrete pillars 6.7m apart and 10m away from the jetty. They are approximately 2m in diameter.
  • The second set further up consists of 7 rows of wooden pillars 1.8m (~6 feet) apart with 18 in each row spaced at 3.72m apart (~12ft).
  • The difference in spacing and pillar diameter highlights the old and new construction methods. The clarity is limited by the size of the transducer, which also limits the horizontal resolution.

There was a 100m long Portuguese frigate (F332) moored at the HMS Drake dock no. 5 and a 90m long submarine at 50 o 23.3096N, 4 o 11. 5202W. Between the two ships, a single section of ripples 90m x 50m was recorded. The ripples had an average wavelength of 11.9m and were slightly curved.

Tracks north of the Tamar Bridge

The eastern side of the plot shows areas of sand waves/ripples, with none present on the western side.  It also shows the channel is deeper on the western side, suggesting a faster flow, and therefore higher levels of suspension and lower rates of deposition.  This is supported by tidal diamond information from the admiralty charts, which shows tidal streams to be greater above the bridge than in surrounding areas.  Most of the area sampled was behind Ernsettle pier, which may have disrupted flow and caused velocity to decrease (low suspension, high deposition).  If sampling was carried out above the pier, sand ripples may not be present.

Sand waves present north of Tamar bridge were found to be transverse sinuous and out of phase.  The sediment was likely to be > 0.3mm.  

Fig. 5 Sidescan trace showing the wake of a passing vessel

Fig. 6 A pillar of the Tamar Bridge with sandwaves above and an area of scouring upriver creating a deeper hole

The area between the Tamar Bridge and the River Lynher

There was increased flow around the bridge due to constriction of the River Tamar through a narrow channel, which lead to scouring of the bed and deepening of the channel (Fig. 6). The sediment surrounding the bridge returned a strong acoustic response indicating hard/smooth rock, providing a stable foundation for the bridge. Further south, harder sediment gave way to softer, more absorbent sediment with extensive bedforms in transition. The dominant bedforms were linear, with crestlines perpendicular to water flow. The average wavelength was approximately 8m. They were numerous and found over a large area, indicating a stable flow regime. South of the softer sediment, where the River Lynher meets the River Tamar, more features are found. Two distinctive sediments are present: one harder/smoother, and one softer/ finer. The most southern part of the track consisted of more uniform sediment, giving a stronger response.

Advantages and Disadvantages of High and Low Frequencies

  • Higher frequencies gave better resolution and showed more clearly defined features, for example the mooring lines and boats.
  • The 100 kHz frequency was affected by the depth echo sounder, producing regular lines on the trace.
  • Some features are visible on the 100 kHz trace and not on the 500 kHz trace, showing the lower frequency had a wider spectrum.
  • The 100 kHz trace shows a better contrast between certain sediment types.

Sidescan detects the noise created when another vessel passes near to the surveying ship (Fig. 5).  It is important that noises like these are recognised when analysing the trace, so that they can be distinguished from actual bedforms and objects on the sea floor.   

Conclusion

A repeated track, north of the Tamar Bridge, showed the differences between high (500kHz) and low frequency (100Hz) sidescan sonar. Combining the above analysis allows an overview of the estuary to be made. In the north, above the Tamar Bridge, bedforms were found on the eastern side of the estuary. The channel was deeper on the western side and the flow disrupted by Ernsettle pier. Harder rock was present further south, upon which the Tamar Bridge was built. More bedforms resulted from the River Lynher meeting the River Tamar and a transition between two different sediment types was seen in the area. South of the confluence, a more uniform sediment type prevailed. The survey completed near the docks showed man- made features, such as a jetty and pillars. Detail was such that old and new construction styles could be identified. Although some ripples and a depression due to a meander were present, most of the channel was dredged to allow access to the military port.

In conclusion, it can be seen that sidescan sonar has many practical uses in the world of oceanography. It can be used to identify detailed features on the seafloor, as well as larger objects such as warships, depending on the acoustical frequency utilised in the survey.  

All data for this section can be found in:

Group 6 > Geophysics > Processed

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Estuarine Boat Practical

Fig. 7  The Research Vessel Bill Conway

 

 

Date Time Location Tide Weather
27/06/2004 0810 - 1600GMT Tamar River, Lynher River & Plymouth Sound High Water 1230 GMT 4.6m

Low Water 1850 GMT 1.82m

Variable cloud cover (2/8 to complete cover).  South westerly winds Force 4, gusting 5.  Bright spells. Some showers.

Introduction

On the 27th June 2004, Group 6 undertook the estuarine boat practical onboard the Research Vessel Bill Conway (Fig. 7). The cruise was charted to sample the water column from a location north of the Tamar Bridge, down the Tamar Estuary and into Plymouth Sound. This allowed comparisons between mid-estuary and open water/ transitional environments. Before leaving the Marina, a CTD (Conductivity Temperature Depth)  calibration profile was taken and relayed to the RIB group. At each station an ADCP (Acoustic Doppler Current Profiler)  transect of the river was taken to establish variations and volumes of flow. Along the transectCTD profiles were completed (Fig. 8, 10 & 11). Data collected, whilst the CTD was being lowered through the water column, was used to decide on the position of the discrete water samples taken on the return profile. These samples were taken using Niskin bottles attached to the CTD rosette and fired using a remote trigger (Fig. 9). To complement the CTD drops Secchi disk observations were also taken to determine the turbidity of the water column and light attenuation.

Fig. 8 Preparing the CTD for a station.

Fig. 9 Loading the triggers for each bottle

Fig. 10 Careful winching of the CTD

Fig. 11 CTD deployment subsurface

Station 1 was used for the calibration of the T-S probes on the RIB and therefore was not analysed further.

Fig. 12  Location of CTD profiles and ADCP transects

Location Time GMT GPS(North) GPS(West) Description Station
1 1045 50°26.386 4°11.859 Nr River Tavy, North of Bridge 2
2 1105 50°24.362 4°11.859 South of Tamar Bridge 3
3 1155 50°23.606 4°12.513 Carew Pt. to Kinterbury Pt. 4
4 1239 50°24.048 4°12.428 Bull Pt. to Henn Pt. 5
5 1255 50°23.811 4°12.859 Henn Pt. to Carew Pt 6
6 1327 50°21.601 4°10.181 The Narrows 7
7 1416 50°20.217 4°08.159 Plymouth Sound East Side 8
8 1505 50°20.165 4°08.671 Plymouth Sound Centre, Behind Breakwater. 9
9

1525

50°20.105

4°09.751

Plymouth Sound West Side 

10

CTD Analysis

Introduction

CTD profiles were taken at 9 locations along the River Tamar and in Plymouth Sound.  Only 6 profiles have been studied in detail (see below) because of time restrictions and because these showed the most interesting features.  

CTD Stations 4, 5 & 6 (Fig. 12 & 13)

Temperature

A thermocline was present at all stations with approximately the same gradient. The surface mixed layer was clearly defined in stations 5 and 6, with a depth ranging from 2-5m. Station 4 showed no mixed layer but a large mass of cool, denser water below 10m. Stations 5 and 6 did not cover the whole water column depth and therefore the deep layer cannot be easily seen.  The surface temperatures varied between the stations with a range of  0.5°C. Generally, the thermocline varied by 1.7 °C over 9m, with the deep water mass having a temperature of 14.7°C.

Salinity

In stations 5 and 6, river water overlies more saline water, in a layer 2-3m thick.  A well-defined halocline was seen, with salinity varying by 2 units over 8m.  Station 4 showed a deeper layer with a salinity of 34. This corresponds to the cool water mass seen in the temperature profile. Again at station 4, no surface layer was seen and the halocline extended to the surface. This may be the highest point up the estuary where the halocline meets the surface. Further investigation would be required to confirm this.

Transmission

High levels of rainfall caused increased run-off into the rivers, indicating higher suspended particulate material in the fresher surface waters. Transmission voltages were 1.3V in the surface layer and 1.5V in the deeper water.

Fluorescence

The fluorescence data showed a chlorophyll (and therefore phytoplankton) maximum at approximately 5 metres.  This corresponded to the base of the surface mixed layer. The levels of chlorophyll then dramatically decrease, with the halocline reaching much lower levels in the deeper layer. 

CTD Stations  8, 9 & 10 (Fig. 12 & 14)

Temperature

The mixed layer at Station 8 extended to 4m at 14.6°C and to 7m at 14.45°C at Station 9. Below these depths the main thermocline was present. Station 10 was completely mixed to a depth of 12 m. This was the maximum depth sampled at this station, as poor weather prevented further CTD deployment.

Salinity

Salinity profiles followed the temperature profiles and showed the mixed layer extending down to the same depths.

Transmission

The profiles suggest that higher suspended particulate matter may be found in surface waters. Station 8 showed a relatively linear decrease in transmission between the surface and a depth of 6m. Station 9 shows variable transmission, linear with scatter within the first 10m.  Below 10m, there was a great deal of variation in the level of transmission.  Station 10 showed relatively little variation in transmission across the entire vertical profile.

Fluorescence

Fluorescence was low at Station 8, and variable through the water column. Station 9 illustrates a distinct chlorophyll maximum at 5m, within the surface mixed layer. Station 10 varies with depth, showing a reduction in fluorescence in the top 2m of the water column.     

   

Fig. 13 Plots of Temperature, Salinity, Transmission and Fluorescence for CTD stations 4, 5 & 6

Fig.14 Plots of Temperature, Salinity, Transmission and Fluorescence for CTD stations 8, 9 & 10

Nutrient and Chlorophyll Observations (Conway and RIB data)

Sampling from Bill Conway and the rigid inflatables, nutrient levels detected were notably lower than expected at this time of year.  However, the general fluctuations and inter-relationships can be accounted for and explained.

  • Higher phosphate (PO4) and nitrate (NO3) were observed at lower salinities, with a decrease approaching higher salinity.  This could be explained by  removal of nutrients along the sampling route.
  • Silicate appears to be scavenged from lowest to highest salinity.  This is most likely related to the large numbers of diatoms observed throughout the estuary, in the plankton tow samples.  Diatoms have high silica requirements, due to its role in the construction of their frustule.  Thus the non-conservative behaviour of silicate observed at lower salinities could be attributed to high biological demand for this nutrient.
  • Phosphate and nitrate are both high at low salinities, thus it could be concluded that silicate is limiting phytoplankton growth at the top of the River Tamar.
  • Plankton samples showed that dinoflagellate numbers increase at higher salinities (i.e. in Plymouth Sound).  Low phosphate and nitrate concentrations at higher salinities could be due to removal by increasing dinoflagellate populations.  Nitrate is the most conservative of the nutrients examined, but still shows evidence of removal right along the sampling route.
  • The relationship for phosphate is unclear. Along the sampling route, it appeared to show non conservative removal with increasing salinity. This could be due to phytoplankton requirements.  However, above salinity of ~28, there appears to be an input of phosphate which dampens the removal and produces a conservative relationship. As there is more than one end member in the Tamar Estuary, the change in phosphate behaviour could be due to the confluence of the Rivers Tavy and Lynher inputting nutrients. Due to the high levels of rainfall in the days prior to sampling, surface runoff from agricultural sources, e.g. dairy farming (east bank) and market gardening (west bank) 50º 26.500N, 4º 11.900W; and from industrial sources, such as the one at Tamerton Lake (including sewage works), 50º 24.48N, 4º 11.4W, could have lead to the  increase in nutrient levels in the estuary. Another possible explanation is a possible input of phosphate to the estuary,  from sewage outfall pipes, for example, at Kinterbury Creek, 50º 24.15N,  4º 12.10W.
  • Due to the lack of replicates at the end member positions, the error due to sampling cannot be measured. Therefore, a slight error in the end member concentration, would result in a change in the position of the Theoretical Dilution Line (TDL) and thus a change in the interpretation of the results. 
  • Chlorophyll was lower at high salinities.  This could simply be because the phytoplankton population at the seawater end is small. It could also be due to vertical migration of the phytoplankton as they avoid flushing/being carried to inhospitable low salinities.

Oxygen Saturation

The percentage saturation of oxygen throughout the water column at stations 3, 4 and 5 was very similar, with a range of  76-79%. At sites 2 and 7, the range was larger (72-84%) but showed no clear pattern. To be able to investigate the oxygen saturation in the estuary successfully, at least five data points and replicates need to be collected. In some cases, there are only two data points, thus there are not enough to draw a firm conclusion.

 

Fig. 15 Careful preparation of filter for collecting water samples

Fig. 16 Plankton net deployment at first tow station

Fig. 17 ADCP after removal from water for steaming back to dock

Fig. 18 Oxygen sample fixed with Winkler reagent

Acoustic Doppler Current Profiler (ADCP) profiles 

The Tamar Estuary, between the Tamar Bridge and the Naval Docks

Three ADCP transects were taken at the point where the River Lynher joins the Tamar Estuary, between 1126 and 1135 GMT. This was on a flood tide, with high water occurring at 1230.

  The transects run between:  -

1). Bull Point and Henn Point (Fig. 19a Velocity profile, 19b Ship track).

2). Henn Point and Carew Point (Fig. 20a Velocity profile, 20b Ship track).  

3). Carew Point and Kinterbury Point (Fig. 21a Velocity profile, 21b Ship track).   

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Fig. 19a & b Velocity profile and ship track of Bull Point to Henn Point

Fig. 21a & b Velocity profile and ship track of Carew Point to Kinterbury Point

Fig. 20a & b Velocity profile and ship track of Henn Point to Carew Point

Fig.22a & b Velocity profile and ship track of Plymouth Sound

The velocity and ship track profiles show that the flooding tide was moving predominantly up the western side of the River Tamar. This may be due to the Coriolis effect or the fact that this is the outside of the meander in the river channel.  The highest current velocity was in the centre of the channel in terms of vertical profile, due to the reduction of friction in this area (Fig 19a &b). 

From here, a parcel of water moved into the centre of the River Lynher (Fig 20a &b) at similar speeds to figures 19a & b, again lying at intermediate depths in the water column. An eddy in this area is visible due to the presence of a divergence of the tidal path up the Rivers Lynher and Tamar. On the velocity profile there is a section of missing data due to the wake of a passing boat.

As the tidal parcel continues up the River Tamar (fig 21a & b), the flow remains fastest on the western side with slower moving water to the east.

Plymouth Sound (Fig. 22a Velocity Profile, 22b Ship Track).

The transect of the Sound was taken between 1404 and 1416 GMT on an ebb tide. Figures 22a & b show that a greater volume of water was leaving the sound via the eastern channel of the breakwater, shown by the faster rates of flow compared with the western channel. The position of the breakwater is clearly visible on figures 22a & b. Here the flow rates were reduced and angled towards the breakwater channels, as the breakwater constricts the mouth of the estuary and therefore the ebb tidal flow. High wave heights in this area (~80cm) could have been responsible for acoustic interference in the upper section of the water column in the channels. This interference was not present behind the breakwater or in the upper estuary.

Fig. 23 The power failed so a more traditional method of positioning was used

Conclusion

 All of the Estuarine Mixing Diagrams illustrate a scavenged profile with non-conservative behaviour of the major nutrients. These profiles would usually correspond to a chlorophyll maximum in the higher salinity waters towards the mouth of the estuary. However, combining the observations made by both the RIB and Bill Conway groups, it is apparent that the chlorophyll maximum occurs in a window of 0 to 12 salinity units.

This apparent anomaly can be explained by the transport of high concentrations of nutrients into the upper estuary waters as a result of the high levels of precipitation that occurred prior to the survey.  The fields around the upper Tamar Estuary are used for relatively high intensity pastoral and market garden farming. Both of these industries utilise large amounts of fertilisers and pesticides, in order to increase the efficiency and production of the land. Any precipitation will wash surface deposits and leach soil deposits of these nutrient high substances into the Tamar Estuary, acting as the supply for the phytoplankton bloom. The Tamar Estuary is known to respond quickly to variations in precipitation (Uncles and Lewis, 2001), which supports the suggestion of recent precipitation being responsible for the high nutrient concentration. Lance {1968} reported that copepods are able to gain dominance in brackish waters because the low salinity excludes the more metabolically active oceanic species. This will have removed any competition for the copepods and aided the upper estuary bloom.

The decrease in nutrient concentration down the salinity gradient is related to mixing and dispersion, and utilisation by phytoplankton populations further down the estuary. Whilst the chlorophyll maximum is highest towards the head of the estuary, there is still a significant mass of phytoplankton in the mid to lower estuary that requires a constant nutrient supply  

The data for this section can be found in:

Group 6 > Estuarine > Processed

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Offshore Boat Practical

Fig. 24 The research vessel Terschelling, used for offshore data collection

 

Date Time Location Tide Weather
1st July 2004 0745 - 1645GMT Offshore Waters of Plymouth Sound and Eddystone Rocks  Low Water: 1030GMT 1.1m

High Water: 1650GMT 5.19m

Variable cloud cover (2/8 to complete cover).  South westerly winds Force 4, gusting 5.  Bright spells. Some showers.

Sea State: 1 -2m swell

Introduction

A task was undertaken on the 1st July 2004, to survey and sample the water column offshore of Plymouth using the research vessel Terschelling. Five stations were decided upon and surveyed using a number of methods and techniques. The first station was positioned just north of Eddystone Rock (50 °13.108N, 4°14.988W), with sampling commencing at 0940GMT. The water column was profiled using an ADCP, CTD and samples were taken for nutrient, oxygen, chlorophyll and plankton analysis using water sampling bottles attached to the CTD. The bottles were triggered at depths of 3.4m, 20m, 25.5m and 45m. In addition to this, vertical plankton samples were taken at 45m to 30m and 25m to the surface. Finally, a Secchi disk was lowered so the 1% light attenuation depth could be calculated. At this station there was a 1-2m swell, winds of 22 knots and patchy cloud cover. A Minibat was deployed at 1038GMT, however due to technical difficulties had to be recovered and no data was obtained. The second station sampled was 3.2km south of Eddystone Rock (50 °09.510N, 4°18.114W at 1110GMT). The ADCP, CTD and water sampling bottles were deployed, with the bottles triggered at the surface, 10m, 24m and 46m. A vertical plankton sample was taken between 25m to 15m and 15mto the surface, however this time a flow meter was used. The Secchi disk was lowered as before. The third station was 1.6km south of Ranne Head (50 °17.940N, 4°13.486W), which was sampled at 1215GMT. The same method was followed, with water sampling bottles triggered at the surface, 8m, 10m, and 38m. A front was observed on the CTD, so a decision was made to follow this in a South-Westerly direction. Station 4 was located at 50 °16.392N, 4°15.045W, 2.4km south- west of station 3. Sampling commenced at 1400GMT, using the same procedure as before, with discrete water samples collected at the surface, 10.5m, 15.45m, 26m and 45m. The vertical plankton net sampled between 40m to 20m and 18m to the surface. The fifth station was located in Cawsand bay(50 °19.583N, 4°11.208W), and was sampled during a period of rainfall. Sampling commenced at 1440GMT, with bottle samples collected at depths of 1.1m, 3m, 7m and 10.5m.  

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Fig. 25 The CTD on Terschelling

Fig. 26 The CTD being deployed to 45m

Fig. 27 Brad taking an oxygen sample.

Fig.28 The deck, where the CTD and Plankton nets were lowered from 

Sampling error and variability

The Terschelling cruise revealed that the water column was well mixed in the surface layers.  Due to the nature of the CTD and the size of the vessel, sampling the very surface layers of the water column was not possible.  This means the surface samples therefore came from depths ranging between 1m and 5m

The laboratory analysis of water column chemistry showed that at some stations, nitrate levels were beyond the detection limits of the equipment.  These results have been omitted from the plots because the negative values cause false trends (i.e. the plot is forced through zero).  This only occurred at station 3 (surface sample) and so does not impact the analyses significantly.

Flow through the plankton net can be calculated in two ways.  The first is by finding the area of the net mouth, and multiplying this by the distance towed.  For the first plankton tow, this was the method used.  For subsequent tows, a flow meter was attached, which produces a much more accurate result.  This makes the margin of error between the two methods large but rather than making the results less stable, it goes to show the extent and usefulness of different methods of calculation.

Due to computer and mechanical error, the Minibat equipment was unavailable for use on this survey day.  This means that undulating measurements of physical parameters could not be carried out. 

CTD Analysis

Station 1

Station 1was located at 50º 13.108N, 4º 14.988W. The main thermocline is at a depth of 16m, between 16m and 30m depth the temperature decreases from 14.5°C to 12°C. From 30m there was a slight decrease from 12°C to 11.75°C at the end depth of 46m. On the fluorescence plot the lowest values were at the surface, and there was an increase to a maximum at 20m to30m this area was just below the thermocline which may indicate a phytoplankton bloom in the nutrient rich water below the thermocline. This agrees with the depth of the euphotic zone calculated from the secchi disc depth which was approximately 32m. This bloom may have occured in this region due to sufficient light at depth (euphotic zone) and mixing of nutrients below the thermocline maybe making the bloom light limited. Chlorophyll and nitrate data both show concentration peaks at 25m. Silicate displayed a minimum at approximately 20m.  Phosphate concentration increased from the surface down to around 25m where it showed a maximum, before concentration decreased.

The ADCP data from this station clearly shows backscatter from bubbles in the wave zone at the surface.  The thermocline is visible between 15 and 20m.  Higher backscatter either side of the thermocline could indicate a high density of phytoplankton reflecting the acoustic signal.  The phytoplankton data from this location indicates that diatoms dominate the surface waters.  This supports the draw-down seen in silicate concentration from 0 to 20m.  Diatoms are non-motile organisms that need to be continually mixed upwards in the water column in order to obtain sufficient light and nutrients for growth.  They have a high requirement for silicate in the construction of their external cell wall, or frustule.  A second layer of backscatter at between 20 and 25m could show a second body of phytoplankton.  Cell counts show that dinoflagellates dominate at this depth.  These organisms are motile, and thus able maintain their position in the water column (Pomeroy et al, 1956) .  Occupying the water column at this level allows them to utilise nutrient resources mixed up from deeper waters.  Diatoms above exploit nutrients in the surface watersPhytoplankton appear to be well mixed throughout the water column. This is likely to be the result of a storm event earlier in the week.

Station 2

Station 2 was located at 50º09.510N, 4º18.114W further out to sea past Eddystone rocks than station 1.  The main thermocline is at a depth of 10m. Storms a week previous are likely to have mixed the whole water column. At this point the thermocline is slower to reform than at station 1, as station 1 has some sheltering from Eddystone Rocks . There is then a gradual decrease in temperature from 10m to 47m decreasing from 14.5°C to 11.6°C. Chlorophyll was low initially in the surface waters, increasing gradually to a maximum of 1.4mg/l at ~25m.  The fluorescence profile shows a similar trend, peaking between 20 and 30m.  This showed the general depth location of the phytoplankton bloom, which in theory should coincide with the level of maximum chlorophyll. Oxygen saturation displayed a large decrease from the surface to approximately 22m, and then a very slight increase from 22m to the bottom.  This plot is most likely to represent mixing in the upper surface layer of the water column.  This mixing prevents phytoplankton productivity near the surface through turbulent physical processes, and also because light levels may be too high (thus limiting).  Silicate shows a decrease to ~10m before a large increase.  The initial shape of this profile is also likely to be a result of mixing keeping nutrients at the surface high, and preventing biological utilisation.

Station 3

This station was located at 50º 17.940N, 4º 13.486W.  As at station 2, it seems likely here that the thermocline seen between ~3 and ~12m developed only recently.  From 12m downwards, temperature was constant, indicating a well-mixed water column.  The chlorophyll profile appeared to show maximum phytoplankton growth from the surface to around 10m.  The fluorescence profile indicated some mixing at the surface, and only resembled the chlorophyll profile below 10m.  Further support for the depth location of the phytoplankton bloom can be seen in the ADCP backscatter plot for this station. Here, a layer of distinct scatter is visible between 0 and 10m.  Again, the oxygen saturation plot may represent mixing, especially in the surface waters.  The nutrient profiles all appear to agree with the chlorophyll plot in placing the phytoplankton bloom at approximately 10m.  Nitrate, phosphate and silicate concentrations all show a minimum concentration at this depth, indicating utilisation of nutrients, with some degree of recycling below 10m.

Station 4

This station was located at 50º 16.392N, 4º 15.045W.   The thermocline here was at approximately 18m.  Phosphate shows a very strong decrease in concentration from the surface to 11m.  It is then stable for around 3m, before increasing from 26 to 35m.  The increase declines, but continues until ~45m.  Nitrate also shows a draw-down, this time from 0 to 10m. It then increases from 10 to 45m.  This corresponds to the phosphate draw-down.  Silicate at station 4 displays a minimum concentration at approximately 15m.  It then increases in concentration to 45m.  The initial decrease in the surface waters reflects the fact that diatoms dominate at this station, particularly at a depth of 26m. This is also seen in the backscatter plots from the ADCP data.  The second most dominant phytoplankton are dinoflagellates.  A small number of ciliates were present. Most organisms were present in the surface 0 to14m. The chlorophyll and oxygen plots also correspond, peaking between 15 and 20m.  This correlation supports the depth and location of the phytoplankton bloom.

Station 5

Station 5 was located at 50º 19.583N, 4º 11.208W.  The thermocline at this site was judged from CTD and ADCP data to be approximately 9m.  Oxygen levels peaked between 2 and 4m, after a minimum percent saturation at the surface.  Immediately after the oxygen maximum, levels rapidly decreased between 4 and 10m.  The chlorophyll plot was fairly linear, highest at approximately 2m and lowest at approximately 11m.  Nitrate shows a pronounced minimum between the surface and 5m.  The maximum concentration was at the shallowest depth (1.1m), suggesting a strong draw-down in nitrate concentration by a biological population.  The vertical silicate profile shows a minimum peak at a depth of around 7m, before increasing between 7 and 11m.  This corresponds with the chlorophyll plot as diatoms are buoyed up into the well-lit surface waters where they utilise available silicate.  The draw-down continues to 7m, and recycling begins near the thermocline.  Phosphate levels show a jagged profile, but as there were only 3 data points collected, no firm conclusions can be drawn.  The CTD data showed that the water body at this site has been completely turned over, and thus the erratic phosphate profile has been attributed to this.

Richardson Number

The Richardson Number was calculated at each of the five sites, using ADCP data to find the shear and CTD data to find the change of density with depth. The mean density of seawater was assumed to be 1026 kg m-3. Figure 29 shows the Richardson Number at each station.  

Fig. 29: Richardson number at each station

Station Number Richardson Number
1 12.88
2 16.71
3 237.4
4 21.11
5 3.42

The shear at all of the stations was very low, as there was little change of velocity with depth. Stations 1, 2 and 4 give intermediate values, indicating a relatively stable density structure. The lower the Richardson Number, the less energy required to destabilise the water column. Station 1 was therefore less stable than stations 2 and 4. Station 3 had a much higher value, over an order of magnitude larger than the others. This suggests either an extremely stable water column or an error in calculation. The density profile shows the top of the thermocline at the surface, without a surface mixed layer, which may have effected the results. Station 5 had a very low Richardson Number, implying a more unstable water column. The density profile  supports this data, showing a stepped structure. Mixing may have occurred in the water column to make certain parts homogenous, which results in steps. This could be due to the CTD causing turbulence in the water. If this was the case, stepping would be seen in all profiles and is not. Another alternative is that a physical process such as internal waves is the cause, but this would require further investigation to confirm.

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Fig. 30 The ADCP used on Terschelling

Fig. 31 Deploying the Zooplankton net

Fig. 32 The Secchi disc, used to estimate the depth of the euphotic zone 

Fig. 33 Lowering the Secchi depth until it disappears

 

Depth of the euphotic zone and attenuation coefficient

The depth of the euphotic zone and the attenuation coefficient were calculated using secchi disk depth and light sensor data at each station. Figure 34, below, shows the results:

Fig. 34: The depth of the euphotic zone and the attenuation coefficient

Station number

Secchi disk data

Light sensor data

Secchi disk depth (m)

Euphotic zone depth (m)

Attenuation coefficient (m-1)

Euphotic zone depth (m)

Attenuation coefficient (m-1)

1

12.3

36.9

0.117

32.78

0.1043

2

12.5

37.5

0.115

41.67

0.1065

3

6.6

19.8

0.218

23.89

0.1080

4

7

21

0.206

32.78

0.1092

5

6

18

0.240

29.17

0.1130

The data from the secchi disk and the light sensor give very similar results. Minor differences result from the way the systems work. The secchi is a very basic tool and is greatly affected by an inhomogeneous water column. If the surface water is clear and the rest of the column is not, the secchi disk will give a large euphotic zone depth. The light sensor can detect a patchy water column, but was located below the water bottles on the CTD rosette, giving an interrupted view. In this case, the secchi disk is likely to be more accurate.  

The euphotic zone data shows a pattern of increasing depth with increasing distance from the shore. This is to be expected, as the amount of suspended material decreases with distance from shore. Also, the euphotic zone is consistently deeper than the thermocline. The attenuation coefficient decreases with increasing distance from shore. A higher attenuation coefficient means that the water is clear and contains less suspended matter. This is to be expected in offshore waters.

 

Phytoplankton samples (Fig. 35)

Phytoplankton were sampled from the CTD bottles and so only a small water volume was measured. This means that only small chunks of the water column were sampled. This gives a limited picture to total numbers or population dynamics.

All sites sampled were dominated by phytoplankton in the upper layers. Other classes were made up from dinoflagellates and cillicates-small single celled animals.

Diatoms were dominant in all except the estuarine sample. This has a similar number of dinoflagellates and diatoms. Dinoflagelates had the largest population at site 1 however they were present over the whole water column at the other sites.

ADCP data shows a backscatter maximum at 18-25 metres. The bottles that sampled these areas have large populations with dinoflagellates maximum at site 1 and a diatom maximum at sight

 

Zooplankton Samples (Fig. 36)

The zooplankton samples were carried out using a plankton net trawled at certain depths. All depths contained similar dominate classes. There were large numbers of noctiluca, a large diatom in all the samples. The samples also contained large populations of copepods. The off shore site 1 samples showed the greatest range of species. The lowest diversity is the site closest to the estuary in shallow water.

At site 1 there a number off classes that are only present in the deeper nettings.

Three of the 4 sites showed a similar number of diatoms at depth and in the surface layers. Other species however showed an increase in the number of individuals in the top 20m.

 

Fig. 36 Zooplankton population structure at each site

Fig. 35 Phytoplankton population structure at each site 

Conclusion  

The general aim of the investigation was to look at mixing with distance offshore and its affect on plankton distribution. Storms in the past week are likely to have increased mixing. Station 1 showed a phytoplankton bloom below the thermocline at 32m. Diatom blooms were signified by very low concentrations of silicate. The diatoms in particular exploited nutrients at the surface, whereas dinoflagellates dominated the water column. Station 2 fluorescence profiles corresponded with phytoplankton bloom depths. Mixing here may have prevented phytoplankton productivity near the surface, however this may also be the result of high and limiting light levels. Station 3 chlorophyll levels showed maximum phytoplankton between the surface and approximately 10m. After 12m the water column becomes well mixed and homogenous. At station 4, recycling of nutrients below 10m was indicated and a decrease in silicate in the surface water reflected the dominance of diatoms. All stations, with the exception of 5, showed a stable water column reflected in the Richardson numbers. Low shear was seen at all stations.  

The data for this section can be found in:

Group 6 > Offshore > Processed

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Estuarine RIBS

Date Time Location Tide Weather
4/7/2004  0900 - 1400GMT Tamar river Calstock to Tamar bridge  High Water: 0700, 5.43m

Low Water: 1310, 0.68m

Overcast, cloud cover varied from 4/8 - 8/8, wind approximately 5knots.  
 

Introduction

To the complete the practical program for the 2004 field course, Group 6 undertook upper estuarine sampling onboard the vessels ‘Coastal Research’ (Fig. 37) and ‘Ocean Adventure’ (Fig. 36) to establish the extent of the saltwater intrusion into the River Tamar. At 0830 on July 4th, the group left Mayflower International Marina and proceeded up the Tamar River to Calstock. Here the two groups split up with ‘Coastal Research’ returning back down the River taking water samples and vertical profiles at 4 salinity unit intervals from 2 units. The other members onboard ‘Ocean Adventure’ continued past Calstock until the point at which zero salinity was recorded on the CTD probe (50º 29.745N, 4º 11.631W at 1005). Surface water samples and a vertical CTD profile of the water column were taken, before ‘Ocean Adventure’ went back down river sampling every 4 salinity units from the original zero sample site. Sechhi disk observations were also made (Fig. 46). Both groups continued sampling until they reached 28 and 30 salinity units respectively in order to establish an overlap with data collected by the group onboard the R.V. Bill Conway, and create a complete estuarine sample transect. 

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Fig. 37 Ocean Adventure used to sample The Upper Tamar

Fig. 38 Coastal Research the second boat used

Fig. 39 Repairing the steering after breakdown up the Tamar estuary

Fig. 40 The TS probe used on both boats

Fig. 41 Locations of stations sampled on the RIBs by group 6 on 04-07-04

 Station   Location (N)   Location (W)  Time (GMT) Rib
 1a   50°29.745N   4°11.631W  10:05:00 blue
 1b   50°29.888N   4°13.164W  10:04:00 white
 2a   50°29.321N   4°13.425W  10:25:00 blue
 2b   50°29.115N   4°13.167W  10:19:00 white
 3a   50°28.248N   4°13.588W  10:49:00 blue
 3b   50°27.827N   4°14.353W  10:45:00 white
 4a   50°27.387N   4°14.053W  11:04:00 blue
 4b   50°27.369N   4°13.676W  10:56:00 white
 5a   50°27.593N   4°13.521W  11:10:00 blue
 5b   50°28.001N   4°13.431W  11:07:00 white
 6a   50°26.762N   4°12.339W  11:56:00 blue
 6b   50°27.138N   4°12.358W  11:26:00 white
 7a   50°25.684N   4°11.952W  12:21:00 blue
 7b   50°25.885N   4°11.959W  11:46:00 white
 8a   50°24.862N   4°12.099W  12:36:00 blue
 8b   50°24.428N   4°12.252W  12:35:00 white

N.B. Locations and times of stations sampled on R. V. Bill Conway by group 5 on 04-07-04, can be found on their webpage.

T-S Probe Analysis (Fig. 40)

Samples were collected at 15 sites along the River Tamar.  Of these, 9 sites were studied further as they displayed interesting characteristics.  Temperature profiles have not been included as water column temperature varied by only 0.5°c throughout the river and no significant trends were found.  

Station 2

The profile was taken in the upper estuary where the water column is predominately freshwater.  Salinity increases linearly with depth, with values ranging from 1.9 at the surface to 4.3 at 3m.  It is difficult to identify two separate water bodies, suggesting that gentle mixing between the layers is occurring.

Station 3

A marked increase in salinity is seen between 1m and 1.5m. A salt wedge can be identified at this depth, as the denser saline water underlies the fresher surface water.

Station 4

Salinity increases with depth until at 2m a decrease of 0.3 units is recorded.  Conclusive interpretation cannot be made without more data points.

Station 6

Salinity decreases by 9 units in less than 2m.  This rapid increase was noted during sampling, and 2 further profiles were taken at this point.  They showed constant temperature and similar readings. A possible explanation for this significant increase is that the salt water moving upstream during the flood tide may have become trapped. As the tide ebbed, the more saline parcel of water remained in the same position whilst the parcel of freshwater flowed seawards with the tide.  Although no obvious river features were seen such as an island or inlet, embayments or sub-tidal shallow areas may have trapped saline water during the flood.

Other factors may have influenced this unusual water column structure.  For example high rainfall in the two weeks preceding produced a large volume of fresh river water with an increased flow speed in the surface layer.  Local geology may also influence the ability of the river bed to retain water. 

Station 7

Another large salinity gradient was also noted at this station.  A salinity of 15.2 at the surface increased to 18.9 at a depth of 2.5m. This illustrates that the salt tongue is still lying at the base of the water column. The salinity gradient is not as great as at Station 6 due to an increase in turbulent mixing between the water bodies as estuary volume increases seawards.

Station 9

Two eddies with diameters of approximately 1m were identified, and temperature and salinity measurements were carried out next to them.  Salinity varied by only 0.41 units in 5.5m, indicating a well mixed water column. The eddies caused turbulent mixing of the water column, and therefore the two parcels of water identified upstream cannot be seen at this station.

Station 11

The vertical salinity profile for station 11 is similar to station 9. The water column is a well mixed homogenous body, with a salinity variation of only 0.7 units over 3m.

Station 13

This station shows similar characteristics to station 7, although salinity values are higher.

Station 15

Station 15 shows a reasonably well mixed water column with a salinity variation from surface to bottom of only 1.5 units.

Nutrient Analysis

In the following estuarine mixing diagrams, there is variability around the riverine end member. In all cases, the end member is taken to be the lowest salinity. This variation can cause the TDL to move, which may result in a different profile and interpretation. The standard deviation was calculated for six bottles in the silica data set and was found to be low in all cases.

Silicate (Fig. 42)

The estuarine mixing diagram for silica shows non-conservative behaviour, with removal throughout the estuary. The position of the riverine end member is open to interpretation, as there was much variation at low salinities. The silica concentrations were higher in freshwater and decreased as the salinity increased. This could be due to the presence of diatoms, phytoplankton which use silica to create frustules. Data from the phytoplankton analysis showed that diatoms dominated at most sites, which supports this idea.  

 

Fig. 42 Graph to show estuarine mixing diagram for silicate. 

Nitrate (Fig. 43)

The estuarine mixing diagram for nitrate shows conservative behaviour at salinities below 20, and slight addition above. The lack of removal suggests that large phytoplankton populations have little effect on the levels of nitrate, indicting an excess. The addition could originate from a number of different sources. The land surrounding the river at low salinities was mainly used for agriculture, including cattle farming and intensive market farming. Runoff from the fields into drainage channels that lead into the river may have contained organic fertilizers or animal waste. This effect may have been intensified by the high levels of precipitation over the past few weeks. Human waste from boats may also have caused unusual localized effects.

 

Fig. 43 Graph to show estuarine mixing diagram for Nitrate. 

Phosphate (Fig. 44)

The estuarine mixing diagram for phosphate shows a great deal of scatter near the riverine end member. At salinities above 20, there is non-conservative behaviour with addition. This addition may have been due to a point source such as a sewage outfall, but none was found in the relevant region on the charts. Other sources of phosphate include domestic sewage and human waste. There is no addition found on the nitrate and silica diagrams, so the source of addition is likely to be a phosphate-rich source. No removal was seen across the estuary despite the presence of phytoplankton. This supports the hypothesis that phosphate is not a limiting factor to phytoplankton.

 

Fig. 44 Estuarine mixing diagram for phosphate. 

Chlorophyll

The graph of chlorophyll against salinity shows a general trend of decreasing concentration with increasing salinity, although there is a great deal of scatter at low salinities. This decrease in chlorophyll is likely to be due to lower phytoplankton numbers in marine environments, caused by lack of nutrients and a deepening of the water column. In a more riverine environment, nutrients are high and non-limiting, and the water column is shallow. Due to this phytoplankton are not mixed past the euphotic zone.

 

Oxygen (Fig. 45)

Stations 22 and 9 showed the expected rise in oxygen saturation (%) with a decrease in temperature. Station 3, which was shown to be well mixed by the temperature profile, indicated the presence of a phytoplankton bloom by an oxygen maximum peak at 8m. Saturation fell from the surface and increased sharply up to the peak. Stations 1 and 6 were unusual, as oxygen saturation (%) increased proportionally with temperature. This may be due to an error in data handling. The rib data collected further upstream showed lower levels of oxygen saturation (64.0% - 76.2%) when compared to the stations downstream (see above), where levels were a minimum of 85.7%.  

Fig. 45 oxygen saturation in the Tamar estuary. 

 

 

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Fig. 46 The secchi disc used on the rib

Fig. 47 The zooplankton net being brought aboard

Fig. 48 Zooplankton sample at a salinity of 20.

 

RIBs & Conway data: phytoplankton in the River Tamar and Plymouth Sound.

Phytoplankton was sampled at various stations from a zero salinity river water end member at the top of the Tamar River to a seawater end member in Plymouth Sound.  Diatoms were found to dominate at most sample sites.  At two sites (50º 27.827N, 4º 14.353W and 50º 27.138N, 4º 12.358W) sampled by the Coastal Research vessel, a red-coloured dinoflagellate was predominant. However, this organism could not be fully identified due to the amount of terrestrial debris in the water column, which also adhered to the cells themselves.  These stations were some 12 salinity units apart.  The difficulties in identifying this dinoflagellate mean it is now impossible to say whether the two sites were populated by the same species, or by different members of the same genre.  The difference in salinity does, however, suggest that they were different species.

The most dominant diatom (Bacillariophyceae), Chaetoceros, was found near the sea water end member at a salinity of 30.3.  The nutrient profiles for the estuary taken at the same time as the phytoplankton samples indicate that silicate is removed along most of the length of the river.   This is good supporting evidence for the dominance of diatoms in this area, as diatoms are the only phytoplankton species with a high demand for silicate.  Other numerous diatom species collected from the Tamar include Nitzschia longissima and Rhizosolenia setigera.

Zooplankton (Fig. 48): Analysis from Ribs and the Bill Conway

Diversity should be highest at the ecotone point, where riverine conditions meet marine conditions. This is reflected in the data at station 5, which was taken higher up the river and has a lower diversity than station 6.

The zooplankton sample was collected at a salinity of 15.25 along the River Tamar and contained mainly Mysids.  Fish larvae and Cladocerans were the only other species found.

A zooplankton sample (Fig. 48) collected at a salinity of 20.89 showed a much greater diversity. Fish larvae (Fig. 49) were now the predominant species, although high numbers of Copepods naupilii were also counted.  Other species remained in smaller numbers relative to the two dominant species.

A change in taxonomic groups was found, probably due to variation in species tolerance to salinity. Mysids are more characteristic of freshwater species, whereas Copepod naupilii are a more marine group. Fish larvae were found in both situations suggesting a tolerance to wider salinity gradients.  

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Fig. 49 Fish found in zooplankton sample

Conclusion

Vertical salinity profiles along the estuary suggested the presence of a salt tongue at station 6.  This was also seen at station 7.  An explanation for this could be the entrapment of a saline parcel of water upstream when the tide retreated.  Water column measurements were taken near to an eddy at station 9, showing a well mixed water column.  Variation in salinity from the surface to the bottom of each profile was unpredictable.  Station 6 showed the largest change, from 10 salinity units at the surface to 19 at 2m.

Silicate and phosphate both showed non-conservative behaviour along the estuary. Nitrate showed some evidence of addition, which may be a result of local farming and agriculture.  High levels of nitrate in the Tamar estuary could indicate that phytoplankton growth is limited by some other factor. 

Phytoplankton populations were highest closer to the freshwater end member.  Numbers decreased with movement seawards, likely due to nutrient limitation or the increasing dominance of physical processes.  Chaetoceros diatoms dominated near the seawater end member, whereas freshwater appeared to be dominated by an unidentified red dinoflagellate.  

The data for this section can be found in:

Group 6 > Ribs > Processed

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General Conclusion

The four boat practicals carried out between 23rd June and 8th July gave an insight into the estuarine-offshore system and its interactions. Although the weather was not ideal, a large amount of data was gathered, processed and analysed.

The geophysics boat practical surveyed a large area up the River Tamar and in Plymouth Sound. Both man-made and natural features were observed, with special attention paid to three main sections:

  • The Docks
  • Tracks north of the Tamar Bridge
  • The area between the Tamar Bridge and the River Lynher

Also studied was the difference between high and low frequency sidescan sonar.  

The estuarine boat practical sampled the water column from above the Tamar Bridge to Plymouth Sound. CTD and ADCP profiles were taken, along with water samples for nutrient, oxygen and plankton analysis. Again, certain areas were studied in detail:

  • The confluence of Rivers Tamar and Lynher
  • The Plymouth Sound breakwater

This data was combined with RIB data further up the estuary.  

The offshore boat practical took data from two stations around the Eddystone Rocks, two off of the headland and one near the breakwater. The stations were analysed separately, as weather conditions changed the aim of the practical and the stations were chosen accordingly. CTD and ADCP profiles were taken, and water samples collected.  

The RIBs practical took water samples and T-S profiles from Calstock to the Tamar Bridge along the estuary. Data was combined with that collected from the estuarine boat on the same day. Estuarine mixing diagrams were created from the results.  

The fieldtrip has been very successful and a greater understanding has been gained about the processes occurring in the estuarine-offshore system.


References  

Lance, J. {1963}. The salinity tolerance of some estuarine planktonic copepods. Limnology and Oceanography (8(4)) pp 440-449.

 Pomeroy, L.R., Haskin, H.H., Ragotzkie, R. A. {1956}. Observations in Dinoflagellates Blooms. Limnology and Oceanography ((1))pp- 54-60.

 Tattersal, G.R., Elliot, A.J., and Lynn , N.M. , {2003}. Suspended sediment concentrations in the Tamar Estuary. Estuarine, Coastal and Shelf Science (57) pp 678-688.

 Uncles, R.J., and Lewis, R.E. {2001}. The transport of fresh water from river to coastal zone through a temperate estuary. Journal of Sea Research (46) pp 161-175.

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