Plymouth 2004 - Group 2


From top left going clockwise:

Ruth Lawford, Clara Bolton, Richard Marsh, Kathryn Carder, Emily Venables, Phil Dix, Graham Davies, Amelie Meyer

 

Table of contents

 

Regional setting -

 

Bill Conway  -

26/06/04 - Chemical and biological sampling of the Tamar estuary
Introduction

Results and discussion

Conclusions

Nat West II -

29/06/04 - Sidescan sonar of James Egan Layne
Introduction

Results and Discussion

Conclusions

Ribs - 03/07/04 - Chemical and biological sampling of the upper Tamar estuary
Introduction

Results and Discussion

Conclusions

Terschelling - 06/07/04- Offshore and coastal chemical, biological and physical sampling
Introduction

Results and Discussion

Conclusions

 

Appendix -

 

 

 

 

Regional setting:

 

 

 

Plymouth Sound and many other estuaries on the south-west coast of England, are classified as ria estuaries, thus implying that they are drowned river valleys created by land subsidence, a rise in sea level or a combination of both; forming deep narrow well-defined channels with a significant marine influence. The high salinity and shelter of rias generally lead to a high biodiversity compared with normal estuarine systems of lower salinity regimes. The River Tamar is the main freshwater input draining into Plymouth Sound, its main tributaries being the Tavy and the Lynher. The Tamar Valley is the largest estuarine system in the south west, supporting approximately 400,000 people within its catchment area. The well-developed salinity gradient allows for a high diversity of habitats and communities in this area, including extensive sub-littoral sandbanks, mudflats, salt marshes and reed beds. Geologically, the headlands at the entrance to Plymouth Sound consist principally of Lower Devonian Slates, whereas around Plymouth Hoe, Middle Devonian Limestone formed from warm water coral reefs is found. Land use in the area is a mixture of residential, commercial, industrial and green space. The unique environment of Plymouth Sound is currently protected under the European Special area of Conservation programme. 

Chart section illustrating the main areas sampled by each research vessel

 

 

Bill Conway

 

  

The Bill Conway

The Bill Conway is a coastal vessel (length 11.74m) designed for scientific use and can operate at sea up to 60 miles from a safe haven. This boat was used to sample the Tamar estuary from its seaward extremity near Plymouth Breakwater (position: 50 20.236N, 04 08.314W) to an upstream station near Cargreen (position: 50 26.366N, 004 11.911). The vessel is equipped with a frame and winch system, an ADCP profiler, GPS, chart plotter and a number of instruments. These include a rosette system with a CTD sensor, 5 go-flo sampling bottles, a fluorometer and a transmissometer. On the 26/06/2004, a number of stations in the middle section of the Tamar estuary were sampled for chemical and biological parameters and ADCP profiling was carried out on a number of relevant transects. On this day, weather conditions were relatively poor with 8/8 cloud cover, heavy rain and winds at speeds of 12-15 knots in a south to south-westerly direction. High tide occurred at 11.30 GMT. The Tamar estuary is considered partly mixed in its lower reaches and transitional or well-mixed in the upper reaches (Uncles et al. 1985).

     

 

 

Aim:  

 

   

To develop an understanding of how the Tamar estuary acts as a transitional zone between the source of the River Tamar and the adjacent coastal sea.

   

   

Objectives:  

              

Sample locations 

               

    

      

Results and analysis:

              

                  

 

 

   Figure 1:  Theoretical dilution line for phosphate

 

Figure 2:   Phosphate concentration versus salinity                    

Values above the Theoretical Dissolution line indicate addition of phosphate to the system; values below the theoretical line indicate removal from the system

 

Figure 3a: Silica concentration versus salinity in the Tamar estuary.

 

 

 

 

           Figure 3b: Nitrate concentration versus salinity in the Tamar estuary.

 

 

 

 

Analysis of silica concentrations of water samples taken showed a decrease from a maximum of approximately 80 ΅mol/L at zero salinity to almost 0 ΅mol/L at salinity 35 (Figure 3a). The vertical silica profiles taken at intervals between Cargreen and the breakwater display similar trends. Surface concentrations are generally high,  increasing to a maximum at 4m depth (CTD 3) or 7m depth (CTD 6 and 9) before decreasing again to a minimum at the deepest point. Dissolved silica originates from the chemical weathering of rocks and is exported to the estuary via riverine systems. Concentrations of silica are high near the water surface due to the stratification structure of the estuary which dictates that less dense fresh river water overlies denser inflowing seawater. However, maximum concentrations were observed at around 4m depth due to high levels of uptake by diatoms in the surface few metres of the euphotic zone. At the Narrows (CTD 6) concentrations were slightly lower and the profile showed a maximum at depth. 

The principal sources of nitrate and phosphate to an estuarine system are anthropogenic ones, primarily land run-off from agricultural areas with nutrients originating from fertilizers and pesticides. Land use around the sampled areas was primarily crop and dairy farming, and the heavy rain experienced prior to sampling is likely to have increased the flux of nutrients washed into the river. Nutrient concentrations decreased downstream due to dilution by nutrient-poor incoming tidal seawater. 

 

 

 

 

 

  

  Figure 4: Chlorophyll concentration versus salinity in Tamar estuary

The chlorophyll results exhibit justifiable trends, with highest concentrations found up river where nutrient levels are highest. This allows high phytoplankton productivity providing that there are no other limiting factors. Also, sites upstream are less influenced by turbidity due to mixing within the tidal cycle, and thus flushing occurs to a lesser extent than in areas such as the Sound. Hence chlorophyll levels accumulate within phytoplankton populations. This is further supported by the fact that the lowest concentration (CTD site 6) is found in the area known as the Narrows, which experiences strong, fast currents at all  times bar slack water. It is therefore not expected that phytoplankton would be abundant here, and thus chlorophyll levels are low. The vertical profiles also support the standard hypothesis that light decreases through the water column with depth, with an exponential decrease in chlorophyll production down to the maximum depth at which 1% of surface light can penetrate. This is the euphotic zone, defined approximately as 3 times the secchi disk depth. For example, CTD station 4 has a euphotic zone depth of 6.75m. Below this depth, you would only expect to see low levels of chlorophyll due to mixing within the water column.

The relationship between phytoplankton and chlorophyll is not as clear as would be expected. The ratios of phytoplankton to chlorophyll concentration suggest highest chlorophyll concentrations at lowest phytoplankton levels, for example at CTD station 7 where phytoplankton concentration was 21000 cells/L and average chlorophyll concentration was 2.15΅g/L. At CTD station 1, the phytoplankton concentration is 4000 cells/L, with an average chlorophyll concentration of 4.87΅g/L.

              

 

   

Figure 5: Zooplankton group distributions

-Zooplankton (Figure 5): Three zooplankton trawls were carried out in the Tamar estuary. Trawl 1 was taken at CTD station 1.

  • tow duration = 3 minutes

  • tow distance = 180 m

  • net diameter = 50 cm

  • mesh size = 200 ΅m). 

The four main groups of zooplankton present were copepods (34 %), medusae (15 %), siphonophores (11 %) and dinoflagellates (8 %). 

  Trawl 2 was taken at CTD station 9 (specifications same as for Trawl 1). The four main groups present were copepods (47 %), medusae (11 %) and hydrozoans & gastropods (6 %).The dominant species in both trawls were copepods and medusae. Copepods are the predominant form of crustaceans found in the plankton community, with the order Calinoida being the most abundant. All copepods are holoplanktonic, meaning they are permanently present in the water column throughout the year. It is therefore not unlikely to find them as the primary zooplanktonic species.  Medusae are conspicuous, common inhabitants of both the open sea and coastal waters. Some species are holoplanktonic, but some are also present as meroplankton during larval stages. 

A third trawl, taken by group 3, showed the presence of five main zooplankton groups: copepods (34 %), cerripede nauplius & amphipods (22 %) and fish eggs & gastropods (11%).

 

           
 

-Phytoplankton: Phytoplankton samples were taken at eight different stations, four upstream sampled from the RIBs by group 3 and the remainder from the Bill Conway in the lower reaches of the Tamar estuary. The stations were distributed between Station 1 (Cotehele Quay) down to the Station 8 (East Breakwater). The samples were taken from between 2m and 8m depth. No correlation was found between chlorophyll a concentrations and the concentration of phytoplankton cells. Figure 6 shows that diatoms are present in high numbers in the river, especially upstream and at the confluence of the rivers Tamar and Lynher. At stations closer to the breakwater,  higher percentages of dinoflagellates are present. This shows the impact of salinity and turbulence on the phytoplankton community. Turbulence is the dominant parameter in the river as far as the Narrows. At Old Limekilns, ciliates dominate the phytoplankton assemblage, this may be due to the presence of a nearby sewage outfall introducing nutrients specific to this species into the river system. Diatoms that require turbulent flow to remain suspended in the water column are present in high numbers at the confluence of the two rivers, whereas other phytoplankton species (flagellates and ciliates) are flushed down the river by this turbulence. Downstream of the Narrows, salinity is the dominant parameter controlling phytoplankton distribution. Dinoflagellates are abundant in this section of the estuary as they require constant saline conditions to survive. Diatoms were found in low numbers in this area, possibly due to the occurrence of a bloom a few weeks ago which would have depleted diatom-specific nutrients from the water column.

 

           Figure 6: Phytoplankton concentration versus distance down the river Tamar and relative group abundances.

 

 

 

 

Figure 7: ADCP backscatter profile North to South across the mouth of the River Lynher 

A number of ADCP profiles were carried out on various relevant profiles in the estuary. An ADCP backscatter profile taken across the mouth of the Lynher River (Figure 7) was of particular interest due to the presence of a large area of suspended matter on the southern side of the channel. A similar signal was detected on the western side of an ADCP transect carried out across the River Tamar just south of its confluence with the River Lynher (Figure 8). This could be due to the presence of a mass of phytoplankton or by the presence of the thermocline in the water column. However, taking into account the depth of the euphotic zone in this area (secchi disk at this site gave a reading of 2.25m) and turbid conditions in the water – the possibility of this signal being caused by phytoplankton is low. A more likely explanation is the presence of suspended sediment / colloidal matter, due to re-circulating incoming tidal water scouring the bed of the confluence zone.

            Figure 8: ADCP backscatter profile West to East across the river Tamar

             

 

Conclusions:

Main findings:

Ψ      Oxygen concentrations generally decreased with depth due to gaseous exchange at the air-water interface.

Ψ      Phosphate, nitrate and silica concentrations decreased in a seaward direction due to their main sources being of riverine origin and the effect of dilution with seawater. Chlorophyll concentrations understandably followed trends in major nutrient distribution, and were higher in stable less turbid zones.

Ψ      Phytoplankton analyses showed that upstream, diatoms dominated the phytoplankton community (as silica concentration were higher at these sites). At higher salinities downstream, the assemblage was dominated by dinoflagellates, which are less tolerant of salinity variations and require lower concentrations of essential nutrients to prosper.

Ψ      Zooplankton trawls showed that copepods and Medusae dominated the estuarine assemblage.

Ψ      ADCP data collected near the confluence of the rivers Tamar and Lynher showed high concentrations of suspended particulate matter due to scouring of the seabed by tidal currents.

 

 

Data for Bill Conway can be found under file: group2\Boat Data\Estuary Ribs

 

 

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Nat West II

 

 

Introduction:

 

 

 

 

 

 

Nat West II

Using a towfish sidescan sonar (Figure 9) three areas were surveyed. The first was the wreck of the frigate, HMS Scylla which was sunk only three months ago in Whitsand Bay. The area was scanned in a grid of lines parallel and perpendicular to the orientation of the wreck (itself oriented NE to SW) The second survey was only around 200m away, also in Whitsand Bay, and was the WWII wreck of the American Liberty Ship James Egan Layne. Data was collected in the same way and then the towfish was brought aboard to be taken to the third area where a transect of Cawsand Bay was carried out. A single scan was done across the bay and three grabs were taken and analysed within the transect.

 

Figure 9:

The sidescan sonar towfish emits acoustic signals which are reflected back at different strengths depending on the forms on the seabed. It can be operated at two frequencies depending on whether quality or quantity of data is important.

 

 

 

Aim:

 

 

To conduct a detailed survey of several areas in the outer part of  Plymouth Sound and the surrounding coast.

 

 

 

 

Objective:

  • To produce a detailed survey of the wreck of the James Egan Layne in Whitsand Bay.

  • To survey an area of Cawsand Bay to identify different sediment bed types.

  • To obtain some sediment samples within the area as an aid to data interpretation.

 

 

Results and analysis:

  • The James Egan Layne:

 

 

Figure 10: The James Egan Layne

The James Egan Layne was a 400 feet long, 7000 ton American-built British Liberty ship (Figure 10).  In March 1945 it was hit by a torpedo fired by a U-boat, breaking the stern section away from the boat and causing it to sink in 24m of water in Whitsand Bay, west of Plymouth. Today the ship's bows and hull are relatively intact, but other parts (including masts and holds) have been severely damaged by storms. The surrounding seabed is littered with deck equipment and broken ship parts.

 

Figure 11: Sides scan sonar of The James Egan Layne 

 

The sidescan images of the wreck (e.g. Figure 11) were transferred to a track plot of the survey lines, created using  'surfer' and 'corel draw' software. This involves using Pythagoras' theorem to calculate distances of objects from the sidescan, allowing for the water depth. Thus their true horizontal distances from a theoretical point on the seabed below the boat can be derived. The overall outline of the wreck and features are plotted as accurately as possible by taking a position for an individual feature from as many different sonar passes as is feasible. 

 

The plot indicates that the  wreck is oriented NE with her bow towards the shoreline. This is reasonable as attempts were made to try and beach her before she sank. The wreck is mainly intact up to a point approximately 20m before the stern which coincides with the region hit by the torpedo. Here, the stern has broken away leaving the rear holds exposed. There is minimal superstructure remaining and most of the deck plating is missing, most of which is scattered around the stern section. This forms a large debris field on both sides of the ship, especially around the large hole in the starboard side of the ship, just behind the bow.

 

Figure 12a: Track plot for the James Egan Layne, showing the orientation of the wreck, debris and surrounding bedrock.

 

There is a large area of sand ripples stretching to the NE of the bow caused by oscillating tidal currents and by currents being deflected around the wreck. This has resulted in a net removal of sediment from around the base and sides of the wreck (Figure 12a). The removal of this material appears to be causing areas of re-suspended sediment in the water column around the wreck site, which is clear at several points on the sonar images.  

 

A large area of bedrock outcrops NW of the wreck is also very clear on the sonar images, which protrude above the otherwise fairly flat seabed, which consists mainly of muddy sand.

 

 

 

  • Cawsand Bay:

 

Figure 11: Sidescan sonar of bedrock in Cawsand Bay

 

The aim of our survey transect at Cawsand Bay was to investigate the different sediments and bedforms in this area. The Admiralty chart suggested several possible bed types ranging from rock and weed through to broken shingle, gravel, rock outcrops and fine sand. The track route changed from our initial plan due to ships anchored in the bay, but this did not affect the results which were collected.

 

The track started at 12.51.05 GMT and continued until 13.12.45 GMT. 

-Travelling in a NW direction:

 

 

Start of line:   50o20’229N  04o10’658W      Marked gravel and broken shingle.

End of line:    50o20’410N  04o10’810W      Near Hooe Lake Point.

 

 

 

Figure 12: Sidescan sonar of ripples in Cawsand 

Bay

 

 

Start of line    50o20’410N  04o10’740W       Going SE from Hooe Lake point.

End of line:    50o20’320N  04o10’700W        SW of previous starting point

-Long transect across Cawsand Bay in a SW direction:

Start of line :   50o20’230N  04o10’630W        Near to above.

End of line:    50o19’650N  04o11’700W         Marked fine sand and broken shingle.

 

After an initial look at our data it was decided that sediment grabs would be taken for each of the possible sediment types in order to calibrate the sonar data:

 

 

  • Grab 1 : Contained fine grained, dark grey, cohesive, homogenous sediment, which was well sorted and likely to be a sandy mud because individual grains were visible. The grain size ranged between 32 and 500 ΅m, representing a relatively low energy environment. A low abundance and diversity of organisms was present: 2 worms and tubes, 2 bivalves and a gastropod were found.

  • Grab 2 : No sediment was present. Little was retrieved other than rock and seaweed, confirming sidescan data.

  • Grab 3 : Much coarser sediment consisting of shell fragments and gravel, producing bioclastic sand was sampled. The shell fragments were angular and had a vast size range (0.5 mm to 5 cm), with the presence of some whole shells. Colours were varied due the different types of shell present. This represented a high energy environment, with greater turbidity than that associated with the Grab 1 environment. No living benthic fauna was found.

 

Side scan Sonar Analysis:

 

Figure 13: Track plot of the Cawsand Bay sidescan sonar

 

 

 

 

 

 

 

 

Figure 13 is a track plot of our route with major features marked to scale. Distances measured on the raw data printout are slant heights from the towfish, therefore real distances and heights on the seabed were obtained using Pythagoras’ theorem, trigonometry and the water depth at that particular point. The scale from the raw data print out was found to be 4.92m to every centimetre.

 

 

 

 

 

 

Figure 14: Generalised 3D depth velocity grain size diagram showing the relationship among bed phases and grain size for a variety of flow velocities and flow depths. (Diagram from Sam Boggs Jr, 1987. Principles of sedimentology and stratigraphy, Macmillan Publishing Company NY 784pp.)

The raw sidescan data clearly shows 3 main bed types:

  1. The first, at the NE corner of Cawsand Bay appears to be bedrock (Figure 11) with fairly consistent height as in general the shadow zones are small and regular. At the edge of this zone is a particularly large outcrop which we calculated to be 1.6m high.

  2. The second major area is distinguished by clear ripple features (Figure 12) although care was taken during interpretation due to turning of the boat at this point. With hindsight another track in this general area would have been useful to widen the area of analysis. Ripple types ranged from transverse straight swept and transverse sinuous in phase to much more irregular and asymmetrical formations. An accurate measurement of height could not be taken because two ripples of differing heights in phase would produce the same low return area on the image at these small scales. The wavelength of the ripples was found to be in the order of 1.1m. However, smaller wavelengths were present. The ripples appear to be formed in a sheltered area behind rock outcrops and are oriented parallel to the shoreline. Figure 14 was used to categorise the bedforms. In general the water depth in this area was about 14m and the third sediment grab yielded grain sizes in the region of 2mm. According to the diagram the bedforms would be referred to as ripples if flow velocity exceeded 50cms-1. Our survey took place at one hour before high water near to spring phase. Therefore from the closest tidal diamond a surface tidal stream of 0.9knots or 45cms-1 is expected. Referring to the ADCP 8 data collected from the Bill Conway, flow velocities of up to 75cms-1 were recorded in the middle to lower water column suggesting that our bedforms would be categorized as either megaripples or as very small sand waves. It is assumed that these are a few centimetres in height and change regularly with tidal current fluctuations.

  3. The third area consists of fine sandy mud with rocky outcrops of heights ranging up to 1.5m situated at the NE end of the track. There is also a small area of coarser grained sediment halfway along this track.

 

 

 

Limitations of map plotting from raw side-scan sonar printouts:

 

A number of sources of error were identified whilst plotting the route taken by Nat West II and the seabed location of the wreck of James Egan Layne:

1. The distance in metres between the position of the GPS aerial and the position of the fish being towed, referred to as ‘layback’ (Maximum error ~6m).

2. Horizontal and vertical fluctuations in the position of the fish being towed due to swell and movement of the sampling vessel (maximum error 1m and 2m respectively).

3. Accuracy of the Global Positioning System used. In the case of Nat West II, the maximum error could have been 10m, but average error equals 2-3m.

4. Movement of the wreck or other targets on the seabed due to water currents (maximum error 1m).

5. The fact that a number of deck plates on the wreck are missing means that reflections are sometimes coming from inside the boat. This further complicates the process.

The maximum overall error was approximately 20m. This means that the survey lines carried out following the same co-ordinates did not always line up exactly. For this reason the accurate plotting of a complex feature such as a wreck is difficult, and variations in the orientation of the James Egan Layne and in the position of bedforms are inevitable if information from different survey runs is being amalgamated.

Our best images of the wreck were taken whilst trying to locate it and plan our survey transect lines.

 

 

 

 

Conclusion:  

 

 

Main findings:

  • James Egan Layne

  Ψ The orientation of wreck from the plot shows it to be lying in a NE direction, bow toward the shoreline between 15 and 20m of water depending on the state of the tide.

  Ψ Large debris field have formed on both sides of the ship, due to the break up of the ship over time with the action of waves etc.

  Ψ There is a large area of sand ripples stretching across the bow caused by oscillating water flows and the influence of currents being deflected around the

            wreck resulting in scour.

  Ψ A large area of bedrock outcrops NW of the wreck is also very clear on the sonar images.

 

  • Cawsand Bay

  Ψ3 main bed types along the transect:

 

 

1)      Bedrock - NE corner of Cawsand Bay .

2)      Megaripples or very small sandwaves with a wavelength of 1.1 m or less.

3)      Fine sandy mud with rocky outcrops at the NE end of the track a small, with an area of coarser grained sediment halfway along this track.

 

 

Data for Nat West II can be found in file: group2\Boat Data\Geophysics Natwest2

 

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RIB sampling

 

 

 

 

 

 

 

Coastal Research Ocean Adventure

 

 

 

Introduction:

 

 

 

 

 

Ocean Adventure is a 7.4m rigid inflatable boat which is used in company with the dory “Coastal Research” to venture upstream to the lower and ideally zero salinity areas of the River Tamar. Both are inshore vessels, only allowed to be operated up to 3 miles away from harbour. These boats were used to sample the upper Tamar estuary from the Tamar Bridge, so as to overlap with the data collected by the “Bill Conway”, to as near to zero salinity as the low spring tides on 03/06/04 allowed. Both vessels are equipped with a GPS, chart plotter, multi parameter probe and a go-flo sampling bottle.

 

The weather started off with full cloud cover, but during the day became much brighter with intermittent sunshine and a fairly constant 5m/s windspeed which ranged from a south westerly to a north westerly direction.

 

Low water springs was 1311 GMT so we could not be at Cotehele quay any later than 1110 GMT according to secondary port information.

 

Aim:

 

  • To gain an understanding of the characteristics of the lower River Tamar and its input into Plymouth Sound.

 

 

Objective:

 

 

 

·                     To collect physical, biological and chemical data to analyse the upper estuarine environment.

·                     To interpret this data in relation to data collected further downstream and offshore.

 

 

 

 

 

 

 

Sample Locations:

 

 

Results and analysis:

 

 

  • Salinity

 

Figure 15: Salinity-depth profiles at various locations

 

  • Oxygen

 

Salinity-depth profiles were taken using a multi-parameter probe at intervals of approximately 2 throughout the Tamar estuary. All of the profiles (Figure 15) indicate that the estuary is relatively well-mixed, with the exception of stations 9 and 10. These two stations, with surface salinities of 23.0 and 24.42, display a sharp halocline between 0m and 1m with higher salinities at depth due to denser seawater underlying fresher river water.

 

 

 

 

 

Dissolved oxygen profiles (Figure 16) show a decrease in oxygen concentration at stations 2, 3, 11 and 12 and remain almost constant with depth at stations 4, 8, 9 and 10. An increase in oxygen concentration with depth is observed at stations 5, 6 and 7. Station 1 is slightly anomalous; a sharp decrease in oxygen occurs between 0m and 1m after which the concentration increases to a relatively constant 70% dissolved oxygen from 2m depth onwards. Our depth profiles only extend to relatively shallow depths; they are therefore all probably contained within the mixed surface layer, extended by the bad weather conditions prior to sampling. Generally, lower oxygen concentrations were found towards the seaward end of the estuary.

 

 

Figure 16: Oxygen-depth profiles at various locations

 

  • Nutrients

 

Figure 17: Nitrate concentration versus salinity in the Tamar estuary

 

Dissolved nitrate concentrations in the estuary on 03/07/04 (Figure 17) followed the same general trend as the samples taken on Bill Conway and the RIBs on the 26/06/04 (Figure 3b); both showed a decrease in nitrate concentration as salinity increased towards the open sea due to dilution and mixing. Nitrate concentrations in the upper estuary were found to be higher on the 03/07/04 (151.13 ΅mol/L at a salinity of 2) than on the 26/06/04 (119.99 ΅mol/L at a salinity of 2). On the 03/07/04 , sampling in the upper estuary was carried out at low water on a spring tide. For this reason, the total volume of water in the river was very low at the time of sampling, leading to a higher proportion of the total water present being of riverine origin, as the river discharge does not vary according to the tidal cycle. This is a possible explanation for nitrate concentrations being higher at the same salinity during spring tides compared to the data collected on 26/06/04 during a neap tide. Nitrate concentrations at the seaward end of the estuary were similar on both days: 12.84 ΅mol/l at salinity 30 on 26/06/04 and 13.56 ΅mol/l on the 03/07/04 .

Figure 18: Silica concentration versus salinity

 

As in the Conway data collected on 26/06/04 (Figure 3a), the silica concentration within the upper estuary decreases towards the open sea, the minimum value of 2.33 ΅mol/l at a salinity of 34.4. Maximum values (63.03 ΅mols/l) occur at salinity 2.06 (Figure 18). Concentrations are higher in the upper estuary due to the riverine input, and lower further down the estuary due to dilution by silica deficient seawater. The majority of values fall below the theoretical dilution line, indicating removal from the system by factors other than dilution such as consumption by phytoplankton (diatoms in particular – found to be the dominant species with in the estuary). 

Figure 19: Chlorophyll concentrations versus salinity

 

Chlorophyll levels decrease in a seaward direction, from a maximum of 506 ΅g/l at a salinity of 2.06, to a minimum of 29.34 ΅g/l at a salinity of 34.8. This pattern correlates with the distribution of silica and nitrate within the estuary – two essential nutrients for phytoplankton growth. The chlorophyll distribution collected was similar to the data taken on the 26/06/04 , but the correlation was more linear, showing a closer relationship to salinity and thus distance along the estuary.

 

 

Figure 20: Phosphate concentration versus salinity

 

The concentration of phosphate was lowest (0.415 ΅mol/L) at the highest salinity of 34.49. The highest concentration (4.77 ΅mol/L) was found at at a salinity of 7.96 but unexpectedly, this was not the lowest salinity sampled. Both phosphate and nitrate concentrations followed the same trend, showing a reduction in concentration down the estuary. Most of the points roughly followed the theoretical dilution line, apart from at a minor peak at 7.96 salinity. Overall the data from 03/07/04 showed a better conformity to the theoretical dilution line than the phosphate graph from the 26/06/04 (Figure 20).

 
  • Plankton:

 

-Zooplankton

 

Medusa

Dinoflagellate

Decapod larvae

Copepod

 

Mysid

 

Figure 21: Examples of zooplankton identified.

 

 

Figure 22: Zooplankton distribution in the River Tamar.

 

 

 

 

 

Four zooplankton trawls were carried out in the Tamar estuary, Trawl 1 was upstream and Trawl 4 was taken near the estuary mouth. The blue RIB deployed both Trawls 1 and 2, whilst Bill Conway (group 3) deployed trawls 3 and 4.  The zooplankton net used had the following specifications:

  • net diameter = 50 cm

  • mesh size = 200 ΅m

  • tow duration = 3 minutes

Zooplankton abundance was found to be highest at the South Hooe station where around 1120 cells per m3 were recorded. Copepods were the most dominant group, combined with a small number of mysids which are known to favour fresh water conditions. This site was beyond the most riverine phytoplankton sample taken, and therefore can not be linked to phytoplankton populations. Although overall population size decreased down the estuary at Neal Point and Ernesettle Pier, group diversity increased. At these sites, Dinoflagellates and larvae were present, which could be due to a lower copepod concentration  causing reduced biological stress as a result of less competition for food and nutrients. Total population rose again at Smeaton Pass where cerripedes became the dominant group. The overlap of phytoplankton and zooplankton data is too small for any conclusions to be drawn about relationships between them. However, the low zooplankton population at Ernesettle Pier suggests the average phytoplankton population of 125 million cells per m3 is not high enough to sustain a larger zooplankton population.     

     

-Phytoplankton:

Figure 23: Phytoplankton distribution down the River Tamar

 

Total phytoplankton population remained reasonably stable with between 10 and 10.5 x 107 individuals present per m3 throughout the estuary. However, results from The Narrows showed a small decrease in populations whereas there was a significant population increase at Mallard Shoal in the East of Plymouth Sound. The reduced population in the area of the narrows can be partially explained by the increased current speed due to the narrowing of the channel. This leads to increased turbidity, which is the dominant physical parameter in this area. As a result, the flushing of water is increased throughout the full tidal cycle, decreasing for a maximum of an hour at slack water and therefore making it difficult for phytoplankton populations to establish themselves. The significant increase in population at Mallard Shoal is most likely to be due to high nutrient input levels from the River Plym and may include waste contributions from the numerous surrounding marinas, storm drains and both urban and agricultural runoff. This indicates the effect of the chemical control on the organisms. The most prominent species in the samples throughout the estuary was found to be diatoms, explained by the fact that they can make use of all nutrients within the water column, even at low concentrations. Dinoflagellates are also present in the vicinity of Wearde Quay and the Tamar bridge, which could be because they are better adapted to living in lower light conditions and require lower nutrient concentrations than diatoms. The nutrient concentrations at Mallard Shoal were high enough to enable ciliates to colonize.

 

Conclusions:

Main findings:

Ψ      Oxygen concentration generally decreased with increasing salinity within the estuary.

Ψ      Phosphate, nitrate and silica concentrations all decreased as the estuary became more saline, due to their main inputs being of a riverine origin.

Ψ      Chlorophyll concentrations were greater in low salinity water than in the high salinity areas, this corresponded to the trends in the major nutrient distribution.

Ψ      Zooplankton trawls showed that the sample furthest upriver contained mainly dinoflagellates, with the two samples at a medium salinity having copepods as the most abundant group. Cerripedes dominated the zooplankton community in the most saline sample.

Ψ      Phytoplankton analysis showed that diatoms were the dominant group at all sites sampled.  The next most abundant group were ciliates. At Mallard Shoal they still only made up a small proportion of the sample, but the larger than usual number of ciliates could be attributed to the sewage outflow at this location.

 

 

Data for RIBs can be found under file: group2\Boat Data\Estuary Ribs

 

 

 

 

 

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Terschelling

 

 

Terschelling

 

 

 

 

 

 

Introduction:

 

 

Terschelling is a 40 metre long, multi-purpose salvage vessel often used for research purposes in the fields of geology, oceanography and archaeology. It is registered in Gibraltar and capable of berthing 19 persons.

The area east of Plymouth Sound had not been sampled by any groups this year; therefore this is where sampling took place.  The first station was at East Breakwater for calibration purposes, and then the Terschelling travelled on a bearing of 140o until she was 7 nautical miles offshore.  Preceding this, a transect was travelled directly towards the coast in a northerly direction.

L4 is a location 10 nautical miles south west of Plymouth used as a survey station by the Plymouth Marine Laboratory. Physical, chemical and biological data is obtained weekly with a particular emphasis on zooplankton and phytoplankton activity. The station has a 55 metre depth and position 50o15N 04o13’W. Data is available here.

 

 

Figure 24:  Safety feature of Terschelling

 

 

 

 

Aims:

 

  • To investigate the possible presence of a front to the East of Plymouth, in view of a previous group having located one to the West.

  • To obtain offshore nutrient, light, phytoplankton and zooplankton data, and consider their variation with distance from the coastline, and the influence of a possible front within this.

     

 

 

 

 

 

Objectives:

  • To use a CTD to assess changes in the thermocline and to find the main front in the area.

  • To collect vertical zooplankton profiles and chemical data to support our findings.

 

 

 

Sample Locations:

 

Result and analysis:

 

 

 

  • Oxygen:

The oxygen data did not show any trend between stations, however in comparison with the Bill Conway data, values were higher.  For example the maximum value was 337.0 ΅mol/ l, 125 % saturation taken at 45 m from station 4 and the minimum value was 263.7 ΅mol/l, 98.2 % saturation taken at 26 m from station 3. See above.

 

  • CTD:

The general trend observed at all 17 CTD stations showed a rapid decrease in temperature down to the seasonal thermocline, below which temperature remained approximately constant between 12 and 14.5°C depending on the station and its maximum depth. Inshore stations were generally warmer at depth than offshore stations. Also observed at all sampling sites was a low increase in salinity in the surface few metres (rarely more than 0.5 salinity units), below which salinity remained relatively constant at a value between 34 and 35.5.

 

At deeper stations, the thermocline tended to be present at a greater depth. For example, Station 4 (the station furthest offshore) had a water column depth of ~60m and a thermocline depth of ~20m (Appendix (D)). At Station 17, close to the shoreline, the water was ~8m deep and the thermocline was identified at ~3m below the surface (Appendix (Q)). At shallower stations, the thermocline had been pushed towards the surface by bottom turbulence. Mixing in the bottom layer of the water column due to friction with the seabed mixes water down from the surface heated layer and pushes up the thermocline. This is supported by the fact that at Station 17, water temperatures near the bottom of the water column are in the order of 14.3°C (compared to 15°C at the surface). However, at Station 4, temperatures below the thermocline were found to be a relatively constant 12.3°C (compared to 14.6°C at the surface) showing that mixing through the thermocline was minimal at this deeper site.

 

At stations further offshore (e.g. Stations 3, 5, 6 and 7- Appendix (C), (D), (E) and (F) respectively) wind mixing is more intense, creating a deeper well-mixed surface layer and consequently a sharper boundary between the surface warmer layer and the deeper cooler layer below the thermocline, which is itself more stable. Nearer to the shore, the water column was found to be more continuously stratified, indicating ongoing mixing processes at these sites (e.g. Stations 1, 13 and 14 – Appendix (A), (M) and (N)).

 

The presence of low-frequency internal waves with a wave height of approximately 2 to 3m also means that the position and depth of the thermocline fluctuates spatially.

 

The position at which the thermocline emerges at the water surface is termed a front. Fronts separating two water masses with different physical properties are often visible at the water surface with the naked eye, recognizable by the presence of a strip of debris and/ or foam. A front was identified at a number of our CTD stations, namely stations 9, 11, 12 and 13, where the thermocline was particularly shallow ( Appendix (I), (K), (L) and (M) respectively). It is likely that the front observed at different stations constitutes the same patchy physical structure. Fronts observed in the real ocean are rarely linear. At Station 9 (water depth 30m), the presence of a rock outcrop on the seabed may have increased turbulence and mixing and pushed up the thermocline.

 

 

 

  • Secchi disk:

 

The data shows a trend that the lowest limit of the eutrophic zone does increase with distance from the coast as the depth increases (figure X).  At the stations closest to the coast the limit of the eutrophic zone was deep enough to reach the sea floor and this was confirmed with zooplankton trawls (figure Y) and phytoplankton analysis (figure Z). 

The site with the shallowest eutrophic depth (14.4m) was station 1 this is because it was just inside the breakwater in the eastern channel, and so has the sediment and suspended particulate matter from the Tamar and connecting rivers running past it.  This stops light from penetrating as deep as it can offshore where there is less suspended sediment present. 

 

 

  • Nutrients:

 

Figures 25 and 26 show graphs to compare the nutrient depth profiles. The profile for silicate is very similar between the two stations although the concentration generally appears to be higher inshore, possibly due to the fact that light is more of a limiting factor there.

 

 

Figure 25: Nutrient depth profile for station 4

 

 

 

 

 

Figure 26: Nutrient depth profile for station 16

 

 

 

 

  • Plankton:

Figure 27: Copepod Nauplii

Figure 28: Polychaete

Figure 29: Diatom chain
 

 

- Zooplankton:

In total, ten vertical zooplankton trawls were taken at five different stations and a 5ml sample from each bottle was analysed in the laboratory. The surface samples showed a great increase in dinoflagellates offshore. Site three contained almost 6.5 million dinoflagellates in the 15-0m sample and site 4, which was further offshore, showed almost 5 million of the same group in the sample of the same depth. In contrast, the inshore samples which were sites 1 and 16 contained only 500 000 and 400 000 dinoflagellates respectively. At site three, the sample from 10m to 5m which was just through the thermocline contained more appendicularians than the trawl from 15m to the surface. This was an unexpected result as the second trawl should have contained more than the shorter one. However, this emphasises the variability of results between trawls at the same site and this should be considered within the results. The depth of the euphotic zone varied from 15m at the breakwater to 30m at the furthest station offshore. As would be expected, far fewer phytoplankton were present in the samples at depth than those well within the euphotic zone. At some sites, notably site 10, more phytoplankton were present in the lower than the upper part of the euphotic zone. This is easily explained by the fact that nutrients quickly become depleted at the surface due to high productivity in the high light levels.

 

Figure 29: Zooplankton group distribution along the route of the Therschelling on 06/07/04.

 

 

- Phytoplankton:

Total phytoplankton cell counts generally stayed fairly constant, fluctuating between 60 and 110 cells per ml with only one exception. The sample from station 8 was the anomaly containing 280 cells per ml. Initially this was considered to be a result of the station being near shallower water, which could have increased mixing of nutrients within the water column. However, the CTD data showed that there was no weakening of the thermocline here and would suggest that the sample happened to catch an abundance of phytoplankton which was unrelated to other data which was collected. Apart from this, a higher fluctuation was noticed between where the front was first observed and the coast. There was an overall dominance of diatoms within the samples taken below the thermocline. The only significant count of ciliates was at the station furthest offshore. Ciliates would be expected to be present in areas of particularly high nutrient content but this was not the case at station 4. Dinoflagellates composed between Ό and 1/3 of the samples taken at the surface. This could be due to the fact that they may not require as high a nutrient content as diatoms and nutrients were found to be depleted in surface waters.

 

 

Figure 28: Phytoplankton group distribution along the route of the Therschelling on 06/07/04.

 

 

For more information on the biological, chemical and physical coastal research around Plymouth, click here.

 

 

 

Conclusion:

 

Ψ      Considering the results of the seventeen CTD profiles, a fluctuating front was fount to be present in the region of sites 9, 11, 12 and 13, shown by the reduction in depth of thermocline.

Ψ      The phytoplankton samples taken at stations 1, 3, 4, 8, 10 and 16 showed very little variations in total population magnitude, excluding the possible anomalous result for station eight, although it is possible that some of the variation between sites 9 and 16 were caused by the varying thermocline within the frontal region. Group diversity also remained consistent, diatoms being the most dominant group, particularly in deeper water.

Ψ      Zooplankton populations varied between almost 200000 and 9million individuals per m3, although with no apparent pattern or explanation. 

Ψ      Nutrients showed little variation with distance from the coastline, although most dropped in concentration within the thermocline due to the presence of phytoplankton in this region.

 

 

Data for Terschelling can be found under file: group2\Boat Data\Offshore Tershelling

 

 

 

 

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Appendix

 

 

 

Salinity and temperature vertical profiles for the offshore stations. Stations 1 to 17 is figure A to Q respectively.        

 

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Q

Back to CTD interpretation

 

 

 

Sample Locations for Bill Conway:

 

CTD 1   - 50o26’366N 04o11’911W.   Near Cargreen

CTD 2   - 50o24’842N 04o12.208W.  North of Tamar Bridge
CTD 3   - 50o24’282N 04o12’294W.  South of Tamar Bridge
CTD 4   - 50o23’759N 04o12’475W.  Confluence with Lynher
CTD 5   - 50o21’790N 04o11’157W.     Devonport
CTD 6   - 50o21’570N 04o10’109W.     The Narrows
CTD 7   - 50o20’236N 04o08’314W.     East end of breakwater
CTD 8   - 50o20’273N 04o09’043W.     Behind breakwater
CTD 9   - 50o20’360N 04o09’705W.     Western Channel
CTD 10 - 50o23’888N 04o12’659W.   On Tamar north of Lynher River
ADCP 1  - 50o26’361N 04o12’017W to 50o26’381N 04o11’978W. Cargreen
ADCP 2  - 50o24’901N 04o12’447W to 50o24’963N 04o11’920W.  North of Tamar Bridge
ADCP 3   - 50o24’295N 04o12’363W to 50o24’268N 04o12’234W.South of Tamar Bridge
ADCP 4a - 50o23’982N 04o12’399W to 50o23’979N 04o12’646W.Confluence of Tamar and Lynher
ADCP 4b - 50o23’979N 04o12’646W to 50o23’582N 04o12’637W. Confluence of Tamar and Lynher
ADCP 4c - 50o23’582N 04o12’637W to 50o23’770N 04o12’250W. Confluence of Tamar and Lynher
ADCP 5   - 50o21’968N 04o11’072W to 50o21’631N 04o11’298W. West Mud
ADCP 6   - 50o21’583N 04o10’110W to 50o21’500N 04o10’228W. The Narrows
ADCP 7a - 50o23’982N 04o12’399W to 50o23’979N 04o12’646W.Vanguard Bank to drakes
ADCP 7b - 50o21’364N 04o10’000W to 50o21’490N 04o09’867W.Vanguard Bank to drakes
ADCP 8   - 50o20’679N 04o09’463W to 50o20’585N 04o07’780W.Redding Point to Ramscliff Point
ADCP 9a - 50o23’998N 04o12’393W to 50o23’966N 04o12’637W.Confluence of Tamar and Lynher
ADCP 9b - 50o23’966N 04o12’637W to 50o23’588N 04o12’617W.  Confluence of Tamar and Lynher
ADCP 9c - 50o23’588N 04o12’267W to 50o23’702N 04o23'267W.  Confluence of Tamar and Lynher

 

Zooplankton:

 

Trawl 1 - from 50o26’412N 04o11’953W to 50o26’352N 04o11’880W

Trawl 2 - from 50o20’360N 04o09’705W to 50o20’213N 04o09’722W

Trawl 3 - at 50o28’001N 04o13’335W 

 

 

 

 

 

Phytoplankton:

 

          1 - 50o29'746N 04o12'456W    Cotehele Quay

          2 - 50o28'666N 04o13'073W    North Hoe Mine

          3 - 50o28'001N 04o13'385W    South Hooe

          4 - 50o27'959N 04o14'300W    Old Limekilms

          5 - 50o26'366N 04o11'911W    Cargreen

          6 - 50o23'759N 04o12'475W    Confluence of rivers Tamar and Lynher

          7 - 50o21'570N 04o10'109W    The Narrows

          8 - 50o20'236N 04o08'314W    East breakwater

 

 

 

 

 

 

 

 

 

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Sample locations for RIB's

 

Station 1   – 50°24.558 004°12.220 to 50°24.484 004°12.203

Station 2   – 50°24.980 004°12.146 to 50°24.937 004°12.135

Station 3   – 50°26.722 004°12.352 to 50°26.662 004°12.265

Station 4   – 50°27.155 004°12.328 to 50°27.132 004°12.341

Station 5   – 50°27.822 004°12.741 to 50°27.806 004°12.781

Station 6   – 50°28.002 004°13.421 to 50°27.960 004°13.457

Station 7   – 50°27.678 004°13.504

Station 8   – 50°25.232 004°12.197 to 50°25.188 004°12.047

Station 9   – 50°25.236 004°12.103 to 50°25.203 004°12.095

Station 10 – 50°25.313 004°12.114 to 50°25.281 004°12.112

Station 11 – 50°27.364 004°12.388 to 50°27.270 004°12.322

Station 12 – 50°27.887 004°12.782 to 50°27.851 004°12.746

Station 13 – 50°28.000 004°13.385

Station 14 – 50°27.698 004°13.516 to 50°27.748 004°13.517

Station 15 – 50°27.360 004°13.720 to 50°27.411 004°13.639

 

 

Ernesettle Pier

South Tamar Trot

Thorn Point

Salter Mill

Weirquay

South Hooe

Quay Ru

South Tamar Trot 2

South Tamar Trot 3

North Tamar

Clamoack Quay

Hole’s Hole

South Hooe 2

Quay Ru 2

700 m south of Pentillie Quay

 

 

 

 

 

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Sample locations for Terschelling

Station 1 – 50o20'168''N, 004 o08'222''W to

 50 o20'120''N, 044 o08'060''W  

East Breakwater, near Fort Bovisand  

Station 2 - 50 o16'633''N, 004 o05'642''W to

50 o13'950''N, 044 o03'230''W  

 

Station 3 - 50 o14'204''N, 004 o03'017''W to

50 o14'251''N, 004 o02'479''W  

Near West Rutts

Station 4 - 50 o11'299''N, 003 o59'974''W to

50 o11'359N, 003 o59'715''W  

Between NGS East and West  
Station 5 - 50 o12'024''N, 003 o59'856''W    
Station 6 - 50 o12'532''N, 003 o59'884''W    
Station 7 - 50 o13'015''N, 003 o59'993''W    
Station 8 - 50 o13'534''N, 003 o59'977''W East Rutts  
Station 9 - 50 o14'016''N, 004 o00'009''W    

Station 10 - 50 o14'503''N, 004 o00'005''W to

50 o14'498''N, 004 o00'004''W  

 
Station 11 - 50 o15'018''N, 004 o00'024''W    
Station 12 - 50 o15'486''N, 004 o00'000''W   
Station 13 - 50 o16'041''N, 004 o00'004''W    
Station 14 - 50 o16'475''N, 003 o59'908’’W    
Station 15 - 50 o17'509''N, 003 o59'899''W    
Station 16 - 50 o17'960''N, 003 o59'956''W    
Station 17 - 50 o18'011''N, 004 o00'210''W  

 

 

 

References:

Papers and Books

 

  • UNCLES, R.J. et al 1984 Observed Fluxes of Water, Salt and Suspended Sediment in a Partly Mixed Estuary. Estuarine, Coastal and Shelf Science (1985) 20, 147-167.

  • LALLI, M.C. & PARSONS, T.R. 2002 Biological Oceanography An Introduction.

  • BOGGS JR, S. 1987 Generalised 3D depth velocity grain size diagram showing the relationship among bed phases and grain size for a variety of flow velocities and flow depths.  Principles of sedimentology and stratigraphy, Macmillan Publishing Company NY 784pp.

 

Charts

 

Admiralty Leisure Charts

 

  • SC 30 2002 Plymouth Sound and approaches.

 

  • SC 1613 2003 Eddystone Rocks to Berry Head.

 

  • SC 871 2001 Rivers Tamar, Lynher and Tavy.

 

 

Websites