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WELCOME to Group Three's Plymouth Website


Figure 1 - Group 3. Left to right - Natalie Silverthorn,
Kimberley Bridge, Sarah Karran, Andy Bailey,
Tom Atkinson, Haydn Cooke and Luke Bartrop
 

QUICK REFERENCE - Click on the boats to view the practical write-ups
 

RIBs	BILL CONWAY	NATWEST II	BONITO
Ocean Adventure Coastal Research

Introduction
 
The main aim of the field course was to study the river Tamar (see Fig.2), it's estuary and convergence with tributaries and the surrounding coastal area to build an overall view of the processes dominating each zone and the transition between them. The macrotidal, partially-mixed Tamar is an excellent estuary to study because a salinity track from 0 to 34 can be made easily by small-boat, enabling a detailed analysis of the changes involved along the track. Easy access can also be gained to Plymouth Sound and the surrounding coastal area, making survey relatively easy for a student field course!

 

Figure 2 - Map of Tamar estuary

The field course was undertaken over a 2 week period, with 4 boat practicals and associated laboratory and data sessions following each one. The boat practicals and an introductory 'Geofield' practical are shown in the website below, with corresponding initial findings in the form of results and analysis. Unfortunately, the resolution of some of the thumbnails is not exceptional, but by clicking on them and opening a new window, they should become clearer!

The RIB, Bill Conway and Bonito practicals involved using similar instrumentation and equipment. A guide to each set of apparatus, as well as the Sidescan sonar from Natwest II, is shown below. The introduction to each practical lists which equipment was used in each case.


INSTRUMENT GUIDE

SECCHI DISK A disk with black and white segments used to calculate the 1% light level depth using the 'Secchi depth' - the depth at which the contrast is no longer visible Figure 3 - The dog ran off with our secchi disk	TEMPERATURE – SALINITY PROBE A handheld device used to determine temperature and salinity ( pH and dissolved oxygen saturation % can also be measured by it). It can be lowered through the water column to give values for different depths. Figure 4 - TS Probe
CONDUCTIVITY, TEMPERATURE & DEPTH PROBE (CTD) Measures temperature and conductivity. Water flowing through the conductivity cell is used to calculate salinity. Data is logged via computer. (See figure 6).	ACOUSTIC DOPPLER CURRENT PROFILER (ADCP) Determines current velocity and direction by measuring change in doppler shift, backscatter created by particulate material is also measured. Figure 5 - ADCP
NISKIN BOTTLE  A water bottle triggered by a remote source if mounted on a rosette or a messenger on a line.   Figure 6 - CTD rosette sampler with niskin bottles attatched to the frame.	PLANKTON NET Used for collecting zooplankton samples. Mesh size used in this case was 200 µm. The net can either be trawled or simply lowered to a specified depth for a water column sample Figure 7 - plankton net being deployed
MINIBAT   A towed vehicle containing a CTD. It is able to collect a relatively large amount of data over a larger area compared to a CTD mounted on a rosette. Figure 8 - minibat

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Day 1 - 01.07.05 - 1300-1530GMT
GEOFIELD

INTRODUCTION

AIM

The aim of the practical was to study the bedding, folds and fractures of the rocks at Renney Point as a precursor to the Geophysical survey on Natwest II.


REPORT

A brief geology survey was carried out to measure the strike and dip of the rock structures. The strike is defined as the bearing along the horizontal line. The true dip is found to be the bearing 90 degrees to the strike. From the National Geological Survey it can be seen that the main rock found in this area is Lower Devonian.

The main feature found at Renney Point is an Antiform fold; this is a fold in the rock forming an inverse ‘u-shape’. This particular formation is not perfectly symmetrical and could have been caused due to the rock being pushed up against a more resistant rock. The top of the fold, like many other similar structures, has been eroded away causing overturned bedding to be viewed at the sides of the antiform. Overturned bedding is categorised as rock beds that are past the vertical so they are pointing the opposite way to the other surrounding beds.

There is a large fault within the rock structure facing out to Renney Rocks with a bearing of 128^o. Once looking at the location of the vertical rock strata it can be seen that there has been a shift in the two sides of five metres. This is likely to have been caused by a couple of separate tectonic events, as the force required to move the rock that sort of distance would have to be sizable. The dip of the overall structure is 8^o.  

Along with the main fault line there are also many dominant fractures which occur around the same bearing as the main fault line (see fig. 9). These occur because the rock is weaker around the fault line so are more likely to break producing dominant fractures. In addition to these are secondary fractures which occur at around 90^o to the main fault line.

An analysis of the Cliff face adjacent to Renney Point concluded that there were seven clear separate layers making up the cliff. The seven layers are labelled to the right of figure 10 and annotated in the text below.

Figure 9 - orientation of primary and secondary faults	Figure 10 - profile of the cliff adjacent to Renney Point

1. Subaerial. Clasts angular ~ 10cm Ø. High velocity flow of post glacial debris due to ice melt in middle Britain 17000 ybp.
2. Subaerial. Fine grained mud. Iron rich due to an oxidising environment, such as a delta.
3. Subaerial. Matrix supported by laterally discontinuous bed. Small clasts spaced apart ~ 1cm Ø.
4. See number 2.
5. Subaerial. Larger grained substrate with similarly orientated layered clasts. Laterally extensive. ~ 12,000 ybp earth closest to sun in its elliptical orbit. Described as a stage in the Milankovitch cycle, resulting in melting of glaciers during climate maximum ~8000 ybp. Fast moving mud flows and little vegetation gave rise to the orientation of the clasts.
6. Submarine. Sand grains present resulting from possible sea level rise. Tendency of sand to fill river channels.
7. Submarine. Shell fragments visible in sandy substrate.
    

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Day 2 - 02.07.05 - 0800-1530GMT
ESTUARINE RIBs PRACTICAL
'Ocean Adventure' and 'Coastal Research'

INTRODUCTION

AIM

The aim of the RIBs practical was to build a profile of the nutrient characteristics of the Tamar, from it's riverine source (0.2 salinity) down to the 34 salinity water found by the Tamar bridge. Plankton samples were taken over this area to be able to explain the nutrient profiles and subsequent estuarine mixing diagrams.  
 

METHOD

Using calculations from the tidal curve and secondary port data , we estimated the earliest time of arrival at Cotehele Quay (due to the depth of river restricting boat draught) as being approximately 0940GMT. We arrived at Cotehele Quay and, because there was enough river depth and a flooding tide, decided to progress upriver further to Calstock (Station 1). Stations can be seen in figure 11.

The two boats used were 'Ocean Adventure' and 'Coastal Research'.  The group was split into two teams of four and three on the respective boats.  We chose to sample data downstream from Calstock due to the flooding tide in the estuary and therefore we would be sampling against the flow of the water ensuring data was not collected from the same volume of water at different locations on the estuary. 
            

Figure 12 - Natalie, Kim and Tom on the Coastal Research	Figure 13 - Ocean Adventure

The initial sample was taken at 1130GMT.  Data for salinity, temperature, pH and dissolved O2 percentage were recorded using the T–S probe.  A water sample was taken from each location from which a filtered volume of 60ml was contained in a plastic bottle (it was important to use plastic rather than glass in this case to avoid silica contamination within the samples).  The filter was stored in test tubes with a small amount of acetone to use for later phytoplankton observation.  Transmittance was recorded at various locations using the Secchi disk method.

The sampling locations were chosen by salinity rather than geographical location taking a salinity range from, 0 to a value of 32 using an interval of every 2 salinity units.  To maximise time efficiency the two RIBs sampled every other location in a leap frog pattern.  Oxygen samples were taken at randomised locations to give an unbiased representation of the river profile.  These samples were taken by hand using a Niskin bottle. A glass bottle was then filled with the water from the Niskin and allowed to overflow to the same volume to ensure no extra oxygen entered it. The bottle was then sealed and stored submerged in water to avoid contamination.  One sample was taken with the Plankton Net (fig. 14). This was done at a mooring buoy on the estuary so as to allow the RIB to turn off it’s engines thus eliminating the possibility of damage to the boat and/or plankton net during sampling. The flow of the estuarine water was great enough on its own to obtain a sample.

The nutrient, oxygen and chlorophyll samples taken were then analysed using the standard techniques which are referenced below:

Manual chlorophyll, dissolved Phosphate and Silicon:

Parsons T. R. Maita Y. and Lalli C. (1984) “ A manual of chemical and biological methods for seawater analysis” 173 p. Pergamon.

 Dissolved oxygen:

Grasshoff, K., K. Kremling, and M. Ehrhardt. (1999). Methods of seawater analysis. 3rd ed. Wiley-VCH.

Nitrate by Flow injection analysis:

Johnson K. and Petty R.L.(1983)  “Determination of nitrate and nitrite in seawater by flow injection analysis”.  Limnology and Oceanography 28 1260-1266.

RESULTS AND ANALYSIS

Plankton

Mesozooplankton:  A 420 second tow was taken using the plankton net, the total population count of the sample was 757393392.  63% of the sample was copepods, 28% Cairripede nauplii and the remaining 9% was made up of Hydrozoans, Bryozoa, Chaetognath, Cirripede cyprid, Echinoderm Larvae and Gastropod Larvae.   
 

Phytoplankton:  When comparing the phytoplankton population sizes with data from dissolved PO4 conc. (figure 19) and dissolved NO3 conc. (figure 18) .  A spike in nutrient levels can be seen at a salinity of approximately 15.  A phytoplankton sample was taken at station 9 which had a salinity of 15.52.  This sample consisted predominantly of diatoms (68%) and dinoflagellates (32%).  Station 9 is situated immediately after a tributary to the river Tamar which could explain the spike in nutrient levels resulting in a diatom bloom.  At station 14 (sal 24.13) dinoflagellates and diatoms make up relatively even proportions of the plankton sample with 42% and 58% respectively.  High plankton levels results in a decrease in nutrient levels especially NO3 conc. PO4 remains relatively high.  The pie charts show that dinoflagellate populations are relatively high at stations 9 and 14 and then drop at the subsequent stations.  The chart showing PO4 concentration shows two drops in PO4 concentration after stations 9 and 14 where dinoflagellate populations were high.  Diatom populations remain high throughout suggesting that dinoflagellates use more PO4 than diatoms.  Total populations begin to decrease again after station 15 which as the Secchi disk results shows is unlikely to be due to a decrease in the eupotic zone but is most likely to be due to nutrient limitations.  From the samples taken Ciliates were only found at station 15 this could be because of the salinity range ciliates exist in or more likely it is because station 15 had the highest population of all the samples and ciliates were still only counted for a small percentage and therefore it is possible that they were present in all samples but only showed up at station 15 due to the higher population size.  The high population numbers at stations 14 and 15 can be seen on the chart for dissolved oxygen which shows spikes in levels at these locations most likely caused as a result of increased phytoplankton photosynthesis adding dissolved oxygen to the water.

Nutrients

ESTUARINE MIXING DIAGRAMS

Estuarine mixing diagrams (EMD) were produced to determine the behaviour of nitrate, phosphate and silica down the estuary. Silica, as shown in Figure 17, shows a non-conservative addition in the upper estuary, salinities 0 to 13, and then a continual removal in the lower part of the estuary, salinities 14 to 33. The reasons for this addition and removal cannot be determined directly from the EMD although explanations may be inferred from the charts and observations on the field day. Silica sources are typically from the riverine end member opposed to the seaward end member. The addition of silica in the upper Tamar estuary may be caused from the large amount of agricultural land surrounding the estuary, with this in mind it may be suggested that the high concentrations of silica may be caused by run-off of agricultural fertilizers and other chemicals that typically have high silica content. In the lower part of the estuary where there is continual removal of silica, a number of reasons may be suggested, including the removal of silica by phytoplankton; more specifically diatoms that take up high amounts of silica, which is used in test formation. The concentration of silica in lower parts of the estuary may be possibly described as being conservative; this then suggests that silica concentration in higher salinities may also be effected by dilution in the large amount of marine water.

Nitrate concentrations, as shown in Figure 18, show a non-conservative addition near the riverine end member, at salinities of 0 to 7, in the rest of the estuary nitrate displays what may be described as a consistent removal the further you progress down the estuary towards the seaward end member. Again, reasons for this behaviour cannot be inferred directly from the EMD although possible causes of the trends could include the input of agricultural chemicals from surrounding farmland in the lower salinity areas in the upper estuary. In the rest of the estuary, in the salinity range of 8 to 33, the consistent removal of nitrate with the increasing salinity that may be due to the removal of nitrate by increasing populations of phytoplankton, for instance, at station 9 with a salinity of 15.5 the plankton sample collected contained a total population of 1.8x10⁷, whereas further towards the seaward end member at station 18 with salinity of 32.2 a total population of 3.6x10⁷. This shows a two-fold increase in phytoplankton population numbers, which may be associated with the consistent removal of nitrate as salinity increases towards the seaward end member.

 

Phosphate concentrations, as shown in Figure 19, show a non-conservative removal towards the riverine end member up to a salinity value of 25 and then non-conservative addition in the salinity range of 25 to 33. The reasons for this non-conservative removal and addition cannot be directly inferred from the EMD although reasons do become clear when the biology and surrounding land usages are considered, for example, in the upper estuary, in the salinity range of 0 to 25, phosphate removal could be due to the uptake of phosphate by phytoplankton species. The non-conservative addition in the more saline regions, salinity range of 25 to 33 could be due to input of sewage outfall from the sewage processing plant at Henn Point, which discharges into the Tamar Estuary south of Coombe Bay. Sewage is discharged on the ebb tide and although sampling was completed on the flooding tide, the phosphate concentrations are elevated enough to suggest that some of the phosphate remained in the estuary after the turning of the tide so creating an appearance of non-conservative addition in this region.

 

 DATA

A full set of data can be found at Group3/Ribs020705

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Day 5  - 05.07.05 - 0800-1630GMT
'BONITO' OFFSHORE PRACTICAL

INTRODUCTION

 

AIM

The aim of the offshore fieldwork was to address the following question: 

‘How do vertical mixing processes in the waters off Plymouth affect, directly and indirectly, the structure and functional properties of plankton communities?’


METHOD

The original plan for the day was to sample an initial station behind the breakwater in Plymouth Sound (50˚ 20.129’N 004˚ 09.290’W) for a time series collected at the same site by all 12 groups. We were then going to head East and collect samples at set stations in Bigbury Bay, at the mouths of the rivers Yealm and Elbe. While on route to the Bigbury Bay stations, we were going to tow 'MiniBAT', a device which collects CTD, Fluorometer and Transmissometer data whilst on the move, allowing for a large area of CTD data collection in a relatively short time. Unfortunately on the day, the sea state proved too rough (with large swells) which meant all sampling had to be done within the confines of the breakwater and up the river Tamar as far north as the Tamar Bridge (50º 24 480N 004º 12 270W). Trying to deploy the CTD rosette and miniBAT in the swells outside the breakwater would have proved too dangerous. Sample sites that were used can be seen in figures 20 and 21.

Figure 22 - Haydn and Tom outside in the rain on Bonito	Figure 23 - Been burning the candle at both ends Anthony?

Below are the references for the lab procedures which were undertaken to analyse the samples collected:
Manual chlorophyll, dissolved Phosphate and Silicon:

Parsons T. R. Maita Y. and Lalli C. (1984) “ A manual of chemical and biological methods for seawater analysis” 173 p. Pergamon. 

Dissolved oxygen:

Grasshoff, K., K. Kremling, and M. Ehrhardt. (1999). Methods of seawater analysis. 3rd ed. Wiley-VCH. 

Nitrate by Flow injection analysis:

Johnson K. and Petty R.L.(1983)  “Determination of nitrate and nitrite in seawater by flow injection analysis”.  Limnology and Oceanography 28 1260-1266.


RESULTS AND ANALYSIS

Plankton

In offshore systems the abundance of zoo and phytoplankton is largely controlled by the vertical structure of the water. Surface waters tend to be well lit and nutrient poor, while the darker, deeper waters are nutrient rich. As well as this there may well be temperature stratification in the form of seasonal thermoclines. As was described above the adverse weather conditions prevented any offshore sampling on the day, so the sampling was taken from the breakwater in Plymouth Sound up to the Tamar Bridge. This radically altered the kind of data which was collected as estuarine systems are controlled by tides and salinity gradients, so these factors also must be considered when describing zoo and phytoplankton distribution. Also the limiting factors related to nutrients offshore do not apply, the River Tamar and its tributaries provide an excess of nutrients from agriculture, industrial and domestic sources. In the previous few weeks before the survey was undertaken there were high levels of precipitation, this would too add to the available nutrients in the system.
 

Figure 24 - phytoplankton diversity for all stations	Figure 25 - zooplankton abundance for all stations

The first station was taken north of the breakwater (see map), from the histogram of phytoplankton abundance (Fig.24) it can be seen that the diatom population remains relatively stable as you go down the water column. The number of ciliates seems to increase as depth increases; this suggests that they may prefer deeper water. The nutrient data also backs up these findings (Figs. 31 to 35) the levels of NOз are at a low level (for the estuary); this suggests that the dinoflagellates and ciliates are removing it from the system. The zooplankton abundance charts (Fig.26) display a change in species composition from the surface down to 10m; this could be related to the presence of both a thermocline and halocline, which form physical barriers that block vertical migration. The total individual counts of both phytoplankton and zooplankton (Figs.24+25) show a correlation, when zooplankton numbers are low the phytoplankton have a higher abundance, this is due to the predator/prey relationship between the two groups.

Figure 26 - zooplankton diversity for station 1a	Figure 27 - zooplankton diversity for station 2

Station 2 was sampled at Barn Pool (see fig 21), the diatoms there show a high dominance in the population (Fig. 27) at around 60%, this can also be seen in the silica levels (Fig. 32), which are low due to the diatom uptake.  A thermocline at around 10m (Fig. 39) may be blocking the downward migration of ciliates, which show a rise in population down to the themocline, but are far less abundant at higher depths. The zooplankton counts are dominated by Cirripede’s, who seem to prefer the less saline conditions from the abundance charts (Fig. 27). There is also a high number of zooplankton residing at this station, with the phytoplankton showing a lower abundance (Fig. 25).

 

Figure 28 - zooplankton diversity for station 3	Figure 29 - zooplankton diversity for station 4

Station 3 at the mouth to Saint Johns Lake displays a thermo and halocline at 4m, high surface levels of silica (Fig. 33) can account for the lower dominance of diatoms. But below the clines the silica levels are lower and the diatoms have greater dominance. Nutrient output from the lake (Fig. 33) provides the needed food source for the dinoflagellates and ciliates to become more abundant. The zooplankton count taken at 4m-surface was dominated by Cirripede’s and Copepods (Fig. 28), but the population is much smaller than at station 2 (Fig. 27), so therefore the numbers of phytoplankton cells is higher.

 

Figure 30 - zooplankton diversity for station 5

Station 4, just south of the Tamar Bridge displayed lower levels of nutrients (Fig.34); this is due to the high numbers of phytoplankton at this site (Fig. 29). The diatom species display around a 90% dominance in the population (Fig. 29), which accounts for the low levels of silica. These high phytoplankton numbers have a dramatic effect on the number of zooplankton present (Fig. 25), which is very low compared to samples further seaward. The diversity of the zooplankton all seems to be related to the salinity as there were only 2 species found at this station with Cirripede’s accounting for the vast majority of the population (Fig. 29).

Station 5 was sampled at the Mayflower Marina and high concentration of nutrients were found, (Fig.35) especially silica which would explain the lower dominance of diatoms and the increased numbers of dinoflagellates and ciliates (Fig.30). Again there is a relationship between the numbers of phytoplankton and zooplankton, with zooplankton numbers being higher the phytoplankton show a decline. The zooplankton population at this site shows a higher diversity than at other sites sampled, this could be due to the sheltered conditions at the site and the water column is well-mixed (Fig. 35) from 1m down. There is a surface thermocline, which is due to the calm wave conditions and sunny weather on the sample day.


Nutrients

 

STATION 1 The diatom population remains relatively stable through the water column at this site.  However, as photosynthesis cannot occur below the critical depth, there is a continual decrease in chlorophyll a concentration (from ~18.8 to ~10.3 µg/L) – shown at every station.  An increase in Dissolved Oxygen is seen between 8m and 13m (~88 to ~101%). It is most likely that this increase is seen because there is a peak in phytoplankton at this depth. The increase in phytoplankton numbers gives rise to an increase in photosynthetic production of which oxygen is a product. Nitrate concentrations increase beyond 8m depth after the chlorophyll maximum. An initial increase is seen in both the silica (~1.13 to ~1.52 µmol/L) and phosphate(~0.68 to ~1.12 µmol/L) concentrations from 0-8m after which a decrease is seen (silica ~1.52 to ~1.09 µmol/L, phosphate ~1.12 to ~0.67 µmol/L).

Figure 31 - Vertical profile of nutrients, dissolved oxygen saturation % and chlorophyll a for station 1

STATION 2 We might assume that the upper layer is representative of the euphotic zone and that a decrease in both average chlorophyll a concentration (~17.8 to ~10.2µg/L ) and % dissolved oxygen saturation (~102.6 to ~101.05%) is a result of decreasing light levels below the surface hence less phytoplankton.  As the abundance of phytoplankton decreases, so does the level of nutrient uptake explaining an increase in levels of nitrate (~2.63 to ~3.08 µmol/L) and phosphate (~0.84 to ~0.658 µmol/L).  The continued decrease in silica (~1.75 to 1.23µmol/L) concentration may be due to uptake by siliceous diatoms as necessary for growth and not photosynthesis. The lower layer contains fewer synthesising phytoplankton and thus less oxygen is produced.

Figure 32 - Vertical profile of nutrients, dissolved oxygen saturation % and chlorophyll a for station 2

STATION 3 Dissolved silica and nitrate concentration decreases ( Silica~3.68 to ~2.30 µmol/L, nitrate ~4.445 to ~4.400 µmol/L) down through the water column. This nutrient uptake is usually representative of phytoplankton, although a decrease is also seen in chlorophyll a concentration (~25 to ~9.9 µg/L) and dissolved oxygen %  (~102.75 to ~102.2%). Dissolved phosphate concentration increases (~0.730 to ~0.750 µmol/L ) this could be due to the sewage out fall at Henn Point, which discharges into the Tamar Estuary south of Coombe Bay (North of station 3) or farm land run off.

Figure 33 - Vertical profile of nutrients, dissolved oxygen saturation % and chlorophyll a for station 3

STATION 4 A decrease in chlorophyll a is seen (~5.0 to ~2.4 µg/L). This explains the increases seen in all the nutrients with depth, (nitrate ~7.33 to ~7.44 µmol/L, phosphate ~0.770 to ~0.790 µmol/L, silica ~2.03 to ~5.1 µmol/L). As expected due to a reduction in photosynthesis a decrease in dissolved oxygen saturation was observed (~102.5 to ~102.05%). These lower values of phytoplankton could be due to turbulence and disturbance near the bridge from all the traffic that passes under it.

Figure 34 - Vertical profile of nutrients, dissolved oxygen saturation % and chlorophyll a for station 4

STATION 5 An increase in dissolved oxygen saturation % is seen (~103.105 to ~103.350), but unlike at station 1 it is not accompanied by an increase in chlorophyll a concentration (~27 to ~13.8 µg/L). There is an increase in nitrate (~1.18 to ~1.30 µmol/L) and silica (~1.58 to ~26 µmol/L) concentrations with depth, this is easily explained by the decrease in chlorophyll a concentration. Phosphate decreases with depth from ~0.730 to ~0.700 µmol/L. The decrease in phytoplankton with depth could be due to a thermocline or pollution from the marina.

Figure 35 - Vertical profile of nutrients, dissolved oxygen saturation % and chlorophyll a for station 5

ADCP Profiles

 

Figures 36 and 37 show a section of ADCP transect taken from an area mid-channel in the East of Barnpool. A strong eddy is found to be present in this area, with an intense area of maximum velocity of 0.5ms^-1 moving southwards in the deeper part of the water column at a depth of 17m to 30m, with a reduced velocity of 0 ms^-1 to 0.125ms^-1 moving northwest to north nearer the surface at a depth of 0m to 17m. This transect was taken from south to north, so the higher intensity area may be said to be further south than the area of lower intensity. This is due to the ebbing tide forcing water out of the Tamar Estuary through the Narrows and Barnpool, with some water being forced out through –The Bridge while the rest is being forced through Drake Channel, to the North of Drake’s Island. This forcing causes water to move at a greater velocity to move around the outside of the corner causing an eddy to occur, which was picked up on the ADCP transects.

MiniBAT

The Minibat was deployed from by the breakwater (50°20.382N, 004°09.334W) at 1130GMT, towed to Barnpool (50°21.359N, 004°10.021W) and stopped profiling at 1217 GMT. The Minibat was deployed a number of times throughout the sampling period, although the data was useable it was decided that the first run provided the most useful data in relation to the rest of data collected. The data collected by the Minibat complements and enhances the data supplied by the CTD profiles at these areas and it provides an insight to the water characteristics between the stations. The data has been processed to give figures 40 and 41, which shows the vertical structure of the water column for both salinity and temperature.           

            

The CTD profile for station one (figure 38) show a strong thermocline at 8m (14.7°C to 15.1°C  at 12m), the Minibat data also shows a strong thermocline around 8m depth (varying over the run), although the strength of the thermocline is more pronounced in the CTD profile due to scaling. Figure 40 shows the change from the cooler more saline water at station one, 15.4°C to the warmer fresher water at station two, 15.9°C. The difference in water temperature is also seen on the CTD profiles for stations one and two (figure 39).

The CTD profile for station one shows a halocline at 8m (34.65 to 35.5 at 12m), this is not as visible in figure 41,  using the Minibat data. Figure 41 shows a salt wedge (Salinity 32.10 to 31.60) between stations one and two. The warmer less dense freshwater is seen in the surface waters by station two, as expected, because the station is further into the estuary and closer to the freshwater inputs. The cooler, denser more saline water ( salinity 34.65) is seen at the surface by station one at the breakwater, because it is close to the seaward end member. As the profile moves up the estuary closer to the riverine end member the colder saline water sinks below the less dense fresher water ( salinity 22 from the CTD profile). This is seen excellently in figure 41, which complements the CTD profiles for stations one and two, that the salinity is higher at the breakwater than by Barnpool.
 

DATA

A full set of data for the Offshore Practical can be found at: Group3/Offshore/050705

 

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Figure 38 - Vertical profile of temperature and salinity at station 1

Figure 39 -Vertical profile of temperature and salinity at station 2

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Day 9 - 09.07.05 - 0800-1500GMT
'BILL CONWAY' PRACTICAL

INTRODUCTION

 

WEATHER CONDITIONS	TIDE TIMES	SAMPLING LOCATIONS	INSTRUMENTS USED
GENERAL Fine and Dry (0/8 cloud); Wind - S F1;  Air  Temperature - 24°; Sea State - Slight.	DEVONPORT (GMT) HW 0718, 4.8m; 1927, 5.1m; LW 1333, 1.4m	Plymouth Sound (50 20 230N  004 08 620W) to Tamar Bridge (50 24 480N 004 12 270W) Figure 42 - Chart of Plymouth sound with locations of sampling sites Figure 43 - Chart of the narrows to the Tamar Bridge with locations of sampling sites	- ADCP - Rosette with CTD and     Niskin Bottles -  Plankton Net -  Secchi Disk -  Sampling kit (containers, filters, measuring devices etc.) -  Boat equipment (GPS/VHF/medical & safety equipment)

     
AIMS

The aim of the Bill Conway estuarine boat work was to develop an understanding of how the Tamar estuary acts as a transition zone between the freshwater input and the coastal sea, as a follow up to the RIBs practical.  This was possible by analysis of the physical, chemical and biological environments in the lower part of the estuary.  The data collected was combined with that collected by the RIBs in the upper estuary to try and provide a holistic view of the processes occurring from Calstock to Plymouth Breakwater. Sampling stations can be seen on figures 42 and 43.

METHOD

The instruments listed above were used to complement each other for data collection.
 

Figure 44 - Andy with the CTD	Figure 45 - PSO overseeing his workers

A series of horizontal ADCP transects allowing calculation of the Richardson Number were conducted.  A plankton net was trawled behind ‘Bill Conway’ at both the first and last station to provide a sample for comparison with backscatter data and the sampled transferred to lugols solution.  Using a niskin bottle rosette attached to the CTD, surface samples were taken for analysis of the phosphate, nitrate, dissolved silicon and chlorophyll concentrations, which allowed for assessment of the potential conservative or non-conservative behaviour of the nutrients.  The oxygen content of the water samples was also measured. 

 

The Conway CTD was calibrated with the RIB CTDs before sampling began, to ensure that the data collected by each vessel could be compared and valid conclusions drawn for the entire area of estuary sampled.
 

Below are the references for the lab procedures which were undertaken to analyse the samples collected:
 

Manual chlorophyll, dissolved Phosphate and Silicon:

Parsons T. R. Maita Y. and Lalli C. (1984) “ A manual of chemical and biological methods for seawater analysis” 173 p. Pergamon. 

Dissolved oxygen:

Grasshoff, K., K. Kremling, and M. Ehrhardt. (1999). Methods of seawater analysis. 3rd ed. Wiley-VCH. 

Nitrate by Flow injection analysis:

Johnson K. and Petty R.L.(1983)  “Determination of nitrate and nitrite in seawater by flow injection analysis”.  Limnology and Oceanography 28 1260-1266.

RESULTS AND ANALYSIS

Plankton

In estuaries the main factor controlling the abundance and diversity of the resident plant and animal populations is the salinity gradient, from freshwater to seawater conditions. As well as salinity, phytoplankton populations are affected by the amount of available nutrients present in the system. There is a high input of these nutrients into estuarine systems from industry, agriculture and domestic sources, so these areas display high levels of phytoplankton primary production and nutrient limitation becomes largely irrelevant. This production provides the base of the food web for other species to populate the system e.g. zooplankton. The data which our group collected on the Bill Conway sampling day has been merged with Group 2’s Ribs data for the same day so that a broader picture of how these changes in salinity and nutrient availability affect the phytoplankton and zooplankton populations. 

Figure 49 - total zooplankton population numbers	Figure 50 - total phytoplankton population numbers

At low salinities on the River Tamar there are high concentrations of nutrients (Figs. 55 to 58), which decrease as the salinity increases and as you move seaward the abundance of phytoplankton increases (Fig.50). This is because the phytoplankton use up these nutrients to reproduce, so from the phytoplankton abundance graph (Fig. 50) a general increasing trend can be seen. However this isn’t a perfect correlation as the graph shows, at a salinity of around 28.8 there were high concentrations of phytoplankton cells, which suggests a bloom at this location. This higher cell concentration was also evident in the nitrate concentration graph (Fig. 56), where at the same salinity an uncharacteristic low concentration can be seen. This is because the higher number of phytoplankton in the area needed more available nitrate to reproduce so the nutrient was used up. This fact is also supported by the number of zooplankton that were present at that location. The zoo and phytoplankton display a predator/prey relationship, but there is also competition for space and in this location the levels of zooplankton are very low (Fig. 49) because the phytoplankton cell concentration was at a high level.
 

Figure 51 - estuarine phytoplankton diversity	Figure 52 - zooplankton diversity at salinity 20.6

Looking at the phytoplankton diversity charts (Figs. 52 to 54) a change in population structure was evident, at the more saline, seaward end of the estuary diatoms tended to dominate the community. At a salinity of 28.8 there was further evidence of the ‘bloom’ mentioned above, there was a change in dominance from diatoms to dinoflagellates, who use nitrate as there source of nutrition. There was also a change in zooplankton diversity as the salinity decreases up the river. At a salinity of 35.2 (Fig. 54) there was a varied number of different species present, with copepods dominating the population. There was a less diverse population at a salinity of 28.8 (Fig. 53), but no species seemed to dominate the community. At the last zooplankton trawl, salinity 20.6, the population was almost entirely dominated by Copepod nauplii  with a 98% share of the community, which may suggest that this species is more adapted to the freshwater conditions.

Figure 53 - zooplankton diversity at salinity 28.8	Figure 54 - zooplankton diversity at salinity 35.2

Nutrients
 

Dissolved phosphate, as shown in Figure 55, displays a typically non-conservative behaviour with variation in the concentrations of dissolved phosphate in respect to the theoretical dilution line (TDL). The reasons for these variations cannot be inferred directly from the estuarine mixing diagram, although hypotheses can be made. In this case variations may be attributed to the alternate variations in the concentration of chlorophyll a, such that when the concentration of chlorophyll a is reduced, the respective concentration of dissolved phosphate is elevated, in respect to the TDL. This pattern is true in salinity range of 0 to 25. In salinities above 25, the dominant process controlling the concentration of dissolved phosphate in the estuary changes, in this area the elevated concentrations of dissolved phosphate could be caused by a sewage outfall located at  Henn Point. This may cause elevated concentrations of phosphate due to the fact that both nitrate and silica may be easily removed from the sewage although phosphate is more difficult to remove and so is often discharged in high concentrations into the estuary.

Dissolved nitrate, as shown in Figure 56, displays a consistent removal in the concentration of dissolved nitrate over the whole of the salinity spectrum with a range of 0 to 35, typical of non-conservative behaviour. The reasons for this consistent removal of dissolved nitrate cannot be directly attributed to the estuarine mixing diagrams, although again hypotheses can be made. The consistent removal of dissolved nitrate may be attributed to the removal of this nutrient by phytoplankton because concentrations of phytoplankton remain at a relatively high level throughout the entire salinity range. At the salinity range of 26 to 35 there is a strong removal of dissolved nitrate below the levels of removal observed in the upper estuary, this may be due to the increasing distance of the inputs of dissolved nitrate to these salinities.

Dissolved silica, as shown in Figure 57, shows non-conservative behaviour throughout the entire estuarine system from zero salinity to salinity 35. The reasons for this behaviour cannot be directly inferred from the mixing diagram, although it may be hypothesized that the reasons for this removal may be attributed to the uptake of dissolved silica by diatoms, which are present in high numbers throughout the entire estuarine system. This phytoplankton species take up high concentrations of dissolved silica for test formation. Dissolved silica may be input into the estuary from mainly riverine and anthropogenic sources such as the runoff of agricultural fertilizers into the estuary that contain high concentrations of dissolved silica.

Dissolved oxygen, as shown in Figure 58, shows low saturations (~100%) in the upper estuary to a salinity of 8, after which the saturation of dissolved oxygen increases to a maximum concentration of 123% at a salinity of 22 and then decreases to ~100% upon reaching the seawater end member. The reasons for this cannot be directly assumed from the figure, although it may be hypothesized that the period in which the estuary was sampled may be after the spring phytoplankton bloom. This would mean that the bacteria in the water column that decompose the dead phytoplankton would use up the dissolved oxygen, known as Biological Oxygen Demand (BOD). Further down the estuary towards the seawater end member the BOD is decreased due to reduced concentrations in chlorophyll a and therefore phytoplankton, causing elevated saturations of dissolved oxygen to occur.

The Richardson Number

The Richardson number is a measure of the stability of the water column. In Figure 59 the Richardson number allows the determination of the stability of the water column of Station 4 in the Narrows, initially on an ebbing tide at 1048GMT and returning to a similar location in the Narrows at Station 4a on the flooding tide at 1407GMT. This stability may be determined by the size of the Richardson number such that if Ri<0.25 the water is prone to mixing because it only takes a low amount of energy to move water from one depth to another. If Ri>1 then the water is not prone to mixing as a high amount of energy is required to move water from one depth to another, so much so that mixing is unlikely to occur and the water is stable in its arrangement. In Figure 59 the scale only reaches 5, although many values are greater than 5, because the lines for Ri=0.25 and Ri=1 may be added and variations in stability profiles may then be viewed more accurately.

 

            A comparison of the stability of the water column may be drawn on the basis of the profiles of Richardson numbers. Such that on the ebbing tide, Station 4, the water is mostly stable in its arrangement, although becomes unstable between the depths of 3.5m to 6m and between 15m and the base of the profile. This may be related to the temperature salinity profile produced by the CTD at this station, as shown in Figure 60, such that the water becomes unstable during the thermocline and halocline at a depth of 3.5m to 6m, also the deeper instability may be caused by the variations in temperature and salinity as shown on the temperature salinity profile for this station. At station 4a, which was taken on the flooding tide, the stability profiles vary, such that instability is seen between depths of 3m and 4m and at a depth of 17m to the base of the profile. This may again be related to the temperature salinity profile for station 4a, Figure 61, such that the instability in the upper water column may be due to alternation of slightly higher and lower salinity water within this layer. At the deeper depths, the instability may be caused by the slight variations in both temperature and salinity. The differences in stability profiles may be associated to the differences in tidal state in which the profiles were taken.

 

ADCP Profiles

STATION 1 - Transect From Picklecombe Point to Ramscliff Point

The current velocity along the first transect is relatively slow - ranging from 0ms^-1 to 0.250ms^-1. There is an area to the centre of the velocity profile which shows a water body that is stationary. This could be caused by the shadow of Plymouth Breakwater. The velocity direction diagram shows that the vast majority of the water is moving southward, out of the estuary; this is due to the ebbing tide. Towards the east of the transect there is an area which shows the water moving almost in the opposite direction to the main southerly flow. This is caused by eddying water produced when water has been drawn around the headland, changing its direction before being turned again by the ebbing tide. 
 

STATION 2 - Transect from Mount Batten Breakwater to Royal Citadel

The velocity of the water is relatively high towards the edge of the Mount Batten Breakwater, travelling at around   0.3ms^-1. The direction of the water towards Royal Citadel is to the west which could be due to the input of water from the River Plym and water that has entered the river area from the Estuary and has changed its direction with the out going river water. The Backscatter plot shows that there are high amounts of particulate in the water, particularly in the centre. This is due to the amount brought in by the River Plym as well as the marina traffic which stirs up particulate in the water column.
 

STATION 3 - Transect for the Narrows - Wilderness Point to Devil's Point

The Direction of the flow in this section of the river is very much uniform; with all of the water moving with the ebbing tide. The interesting aspect of this site is the fact that the velocity of the water is a great deal higher on the eastern side of the channel than the west. This could be due to bend in the estuary prior to the transect.

Upriver, at station 4, the tidal current continued with a velocity of 0.6m/s directly downriver, with little influence from the West Lakes. Where 'Looking Glass' protruded near Station 5 (transect to the Navy Dock), the current reverses and there was slight eddying. At Carew Point (Station 6), the current velocity was greatest mid-channel, at the deepest part. There is evidence of the river input from the river Lynther towards the south of the channel (left on the plot) where the direction of the flow is eastward. There is a layer of sediment to be seen at around 4 metres.



DATA

A full set of data for the Bill Conway practical can be found at 'group3/conway090705'

 

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Day 12 - 12.07.05 - 0800-1500GMT
'NATWEST II' SURVEY PRACTICAL

INTRODUCTION

 

WEATHER CONDITIONS	TIDE TIMES	SAMPLING LOCATIONS	INSTRUMENTS USED
Fine and Dry (0/8 cloud); Wind - 0900GMT - S F1 backing SE F3 by 1000GMT  Air  Temperature - 24°; Sea State - Slight.	DEVONPORT (GMT) HW 0909, 4.6m LW 1504, 1.7m	Outside Plymouth Breakwater Start point: (50 20 400N 004 10 900W) Figure 65-Chart of area surveyed on Natwest II	Sidescan Sonar fish Modified VanVeen grab

AIM

The aim of the geophysical survey was to observe the readings from a sidescan sonar fish towed along a series of transects (created by the group) to understand how it measures seafloor composition. From this, the aim was to find varying composition sites within the sampling area and take a series of grabs to observe the real sediment types corresponding to the sonar readout. A chart of the sample area was then made to show understanding of the sonar survey.

METHOD

 

Calibration of the sidescan equipment was carried out at 08:41 GMT at co-ordinates 50˚20.063N and 004˚11.490W.  The ‘fish’ was deployed at 08:55 GMT and transects were taken southwest of the breakwater. Line 1 started at 09:00:24 co-ordinates 50˚20.376N and 004˚10.846W. Line 1 stopped at 09:25:09 co-ordinates 50˚19.589N and 004˚07.994W. Three more consecutive lines were scanned at 100m intervals.

 

Figure 66 - Sidescan sonar fish	Figure 67 - Van veen grab

Once the sidescan data had been observed three locations were chosen for grabs.  These were chosen to give samples in differing sediment types. Grab 1 was taken at 11:09:12 co-ordinates 50˚20.141N and 004˚10.445W. Grab 2 was taken at 11:31:06 co-ordinates 50˚20.107N and 004˚10.353W. Grab 3 was taken at 11:49:50 co-ordinates 50˚19.899N and 004˚09.225W. Each grab sample was sieved to determine sediment size, sorting and biological presence. A fourth grab was taken by recommendation of the staff onboard at 11:56:25 co-ordinates 50˚20.104N and 004˚09.437W.

Figure 70 - discharge of fine sediment from the filter	Figure 71 - another member of staff asleep!!!

RESULTS AND ANALYSIS
 

Figure 72 - chart displaying the sediment types observed by the sidescan
 

Once the raw data of the 4 transects had been processed and placed in a large graph, the formation of the sea bed from south of Plymouth breakwater to Cawsands Bay in the west can be seen.

The most abundant substance found was sand which can mostly be found within the main channel to the west of Plymouth sound. The far western point of Plymouth sound can be seen as bedrock at 246500, 50500 on the graph. This sand was found to be completely flat, this is due to the high velocity of the water and the fact that the channel has previously been dredged, removing any chance of ripples.

Sand was found with two types of ripple at the breakwater and in Cawsands Bay. The breakwater has sand with parallel ripples formed by tide action; these are parallel due to the movement of water in one plane. Sand in Cawsands Bay has bifurcating ripples caused by wave action.

Grab one, taken within the main channel, brought up very well sorted shell rich gravel. Grab two taken south of the breakwater, found much of the same. Grab 3 failed to sample the sediment. Grab 4 was taken north of the breakwater and found well sorted mud with benthic organisms present. Also an anoxic layer was found at this site.

DATA

A full set of data can be found at Group3/Geophysics120705


Overall Conclusions
 

From the 30^th June to 14^th July 2005 Southampton University Oceanography undergraduates surveyed and sampled the river Tamar estuary and surrounding coastal waters. Unfortunately due to adverse weather conditions we were unable to travel further than the breakwater on the offshore boat. As a result of this our data is restricted to within the estuary, although we were fortunate enough to get 300m past the breakwater when on the Nat West II.

Due to the time of year the survey was carried out the end of a spring bloom was evident in the samples collected on all of the estuarine boats (Bonito included as we sampled the estuary). A general trend was observed in the amount of phytoplankton collected at the seaward end being higher than that at the riverine end member. Zooplankton populations correlated with the phytoplankton populations due to their predator prey relationship, a peak being followed by a trough, alternately.

As expected the nutrients followed the trend that they were higher at the riverine end than the seaward end member, this is due to the input from the river and due to heavy rainfall. These high nutrient levels along with previous hot weather could be a cause for high phytoplankton abundance (a mini bloom) thus high zooplankton levels.  Non conservative removal of silica was evident this was due to the high diatom numbers dominating the phytoplankton population. All other nutrients behaved conservatively.

The physical properties of the estuary were considered from zero salinity in the upper reaches at Calstock to the Breakwater. In the initial part of the field course most of the physical properties were controlled by the halocline due to the unusually wet and windy conditions creating a more well mixed water column, rather than the expected partially mixed estuary, with a small halocline due to the input of fresher water from a variety of riverine sources including the River Tamar, River Tavy and the River Lynher in the Tamar Estuary and the River Plym in Plymouth Sound. The development of a thermocline was seen after the weather improved somewhat and this lead to differing physical characteristics of the water column that were more controlled by a thermocline rather than halocline. The velocity of the water within the Tamar estuary was seen to increase when travelling around the meanders due to the centrifugal forces that apply; these forces also allowed the setting up of significant eddies in this system, in areas such as Barnpool in the Tamar Estuary and south of the Royal Corinthian Yacht Club in the River Plym, creating both mixing and instability.

AND FINALLY . . .

We would like to say a special thank you to Anthony and Simon for all they help and hard work over the past two weeks ( offshore boat excluded, see figure 23). We'd also like to thank all the members of staff and PhDs. A massive shout out to all the boat crews who made all the hard work seem like fun, especially Bob and his bananas.


 

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