Group 7

Southampton University, Plymouth Field course 2004

Group 7

Rosie Corke, Katrina Horsey, Scott Minns, Polly Hill, Arthur Walmsley, Robert McBarron, Andrew Brighton, Emma Hazard, Stuart Mcluskey.

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Section 1: Introduction

Section 2: Estuarine boat work

Section 3: Offshore boat work

Section 4: Rib boat work

Section 5: Geophysical practical

Section 6: Data Locations

Introduction

The Tamar estuary is a transition zone between the River Tamar and Plymouth Sound. As the estuary is semi-enclosed, with a large freshwater input, it has different physical, biological and chemical characteristics than the coastal sea it flows into. Further out into the English Channel , towards Eddystone Rocks , the waters become thermally stratified during the summer months. The degree of stratification depends on water depth and tidal strength. Vertical mixing in the water column influences the physical and chemical properties of the surface layer of the ocean, which can in turn affect the abundance and distributions of planktonic organisms.

Our objectives during the Plymouth field course are to:

1. Determine the physical structure of the Tamar estuary, and how this changes from the mouth of the Tamar, through Plymouth sound to the offshore region.
2. How the tides affect the conditions within the estuary and the adjacent coastal region through time.
3. Investigate the major nutrient concentrations within the estuary and whether these nutrients behave conservatively or non-conservatively.
4. Measure the levels of algal biomass and see how the estuarine and offshore areas compare. Determine whether the algal biomass correlates with the nutrient distributions in any way.
5. Determine what is controlling primary production in the offshore and estuarine regions. Sample the planktonic communities in both regions and see if they differ in biomass and species composition.
6. Measure the turbidity of the estuary and determine what is causing the turbidity. Compare the light attenuation in the water column in the estuary and the offshore region.

To meet these objectives surveys will be carried out in the offshore region on Terschelling, in the estuary and Plymouth sound on Bill Conway and in the upper Tamar on the small coastal research vessels. A geophysical survey will be undertaken in the estuary to allow us to construct a more completed view of the estuary.

Estuarine Boat Work:

Bill Conway

Several ADCP transects were carried out at points along the estuary and CTD drops done in the middle which also allowed water samples to be taken from depth. Data from group 8 who were on the ribs was also used to allow us to construct an entire picture of the estuary from 0 to 35 salinity. Click to enlarge images.

STATION 1: Plymouth Sound

Date	Time GMT	Location	Tidal Times	Conditions
24/06/04	10:02	50°21.481N 004°09.358W	High Water 0940 GMT Low Water 1550 GMT	5/8 cloud cover, 10 knots westerly wind, wave height 0.5m

An ADCP transect was carried out from Picklecombe Fort to Ramscliffe Point. There was virtually no flow, the profile was homogenous across the transect. The maximum flow at this point is 0 - 0.25ms^-1 and as a consequence current direction was poorly defined.

Figure 2.1. Transmissometer and chlorophyll data from CTD

Transmissometer data shows high voltages indicating clear waters down to 8m (1% light depth as calculated from the secchi disk depth) (Fig 2.1). This correlates with higher chlorophyll concentrations down to this depth. Below 8m tidal resuspension has resulted in more turbid waters causing a decrease in chlorophyll. At 6-8m there is a slight thermocline present, the salinity is fairly constant suggesting the area is well mixed.

STATION 2: The Narrows

Date	Time GMT	Location	Conditions
24/06/04	09:49	50°21.645N 004°10.181W	6/8 cloud cover, 20 knots westerly wind, showers

An ADCP transect was undertaken across the estuary from Devils Point to Wilderness Point. There is virtually no flow and the profile is homogenous across the river. We expected the flow here to be high however the transect had to be carried out during slack water as the boat would not be able to stay on line in the 4knots of water which occurred afterwards. The max flow at this point is 0 - 0.25ms^-1 and as a consequence current direction is poorly defined.

Figure 2.2. Density from CTD profile.

The CTD profile, taken at the mid-point of the transect showed that the water was unstable, this is due to the sudden increase in depth and strong currents passing through the narrows (Fig 2.2) .

STATION 3: St Johns Lake Confluence

Date	Time GMT	Location	Conditions
24/06/04	13:57	50°22.044N 4°11.171W	3/8 cloud cover, 17 knots westerly wind, wave height 0.3m

An ADCP transect was carried out from the swing bridge on the eastern side to St Johns Lake on the western side. A CTD drop was also carried out on a tidal front found along the transect however the data did not save.

Figure 2.3. ADCP results from transect.

Highest velocity was found on the outside of the corner (0.5-0.75ms) as can be seen in fig 2.3 in 10m water depth with low velocities of 0.2-0.3ms in the deep water of the centre channel. This is because the water is accelerated as it travels around the outside of the bend, the water in the deep centre channel is slow moving but has a very large volumetric discharge compared to the fast flow on the outside of the bend.

STATION 4: Opposite Basin number 5

Date	Time GMT	Location	Conditions
24/06/04	11:29	50°23.500N 4°11.667W	4/8 cloud cover, 10 knots westerly wind, wave height 0.3m

Figure 2.4. Salinity and temperature variation with depth.

A front was noticed at this location and so a CTD drop was carried out revealing a major change in salinity and temperature between 3 and 5m, a very well developed pycnocline can be seen (Fig 2.4)

An ADCP transect was carried out however this was lost due to technical difficulties.

STATION 5: Lyhner Confluence

Date	Time GMT	Location	Conditions
24/06/04	12:45	50°23.980N 4°12.331W	7/8 Cloud cover, 16knot North Westerly wind.

Figure 2.5. First ADCP transect of Lynher confluence.

Here three ADCP transects were carried out from Henn point to Carew point to Kinterbury point to look at the current variations as the 2 rivers meet as they did a front was seen. (Fig 2.7) The highest current speeds are found on the eastern part of the estuary (0.5ms^-1) on the inside of the river bend. Usually highest flow is on the outside of a river bend, probably due to the displacement of the water in the River Tamar to the east by the input of water from the confluence of the Lynher on this turn from the west. The western edge of this transect has a small current flow at about 0.1-0.2ms^-1, the general flow here is downstream. (Fig 2.5 ) This area is shallow and the slow speed is probably due to the water input of the Lynher acting as a barrier preventing much downstream movement of the western Tamar River and so forcing the flow of the Tamar to concentrate on the eastern edge.

Figure 2.6. Second ADCP transect of Lynher confluence.

The second transect (Fig 2.6) showed a water speed of 0.4-0.5ms^-1 in the surface of the centre channel of this transect. There was very small current flow in the deep part of the centre channel and the shallow edges. It is likely to be due to the large saline outflow of the Tamar preventing the less saline water of the Lynher forcing its way out into the estuary in any other region other than a rapid outflow of buoyant fresh water at the surface. Unfortunately no CTD was taken here but we would expect it to show find a thin layer of fresher water in the surface layer just after the confluence of the Tamar and the Lynher. Figure 2.8 shows the frontal region of the confluence of the Tamar and the
Lynher at the surface as a thin strip of foam.

Figure 2.7. Third ADCP transect of Lynher confluence.

The third transect shows virtually no flow on the western edge, with high flows (0.5-0.75ms^-1) concentrated on the eastern side of the centre channel of the estuary (Fig 2.7).

Figure 2.8. Frontal region at the Lynher Confluence

STATION 6: Tamar Road Bridge

Date	Time GMT	Location	Conditions
24/06/04	12:26	50°24.514N 004°12.325W	Raining with 7/8 cloud cover and 9 knots of wind.

Figure 2.9. ADCP transect at Tamar Bridge

A transect was taken north of the road bridge to avoid any effects the pillars may have on the water column structure as we were on an ebbing tide. The current was found to be strongest (Fig 2.9) in the western central channel (0.5 – 0.75ms^-1) which corresponds well with the tidal diamond data. Lowest suspended particulate matter (SPM) is located in the area of highest flow, due to its high flushing rate (this is taken from backscatter data).

Figure 2.10. Transmissometer data from CTD profile.

The CTD was deployed in the centre of this transect, upstream of the bridge to allow for drifting downstream towards the bridge due to the tide. The transmissometer data (Fig 2.10) from this CTD drop shows these to be the most turbid waters. This is probably due to resuspension of the bedload by the eddies which form around the bridge supports.

Secchi Disk

Secchi disks were also taken at each station, with the exception of Station 2, the Narrows , to determine the depth of the Euphotic zone. (Table 2.1) In addition coloured filters were used at station 3 to determine how far different wavelengths penetrate. (Table 2.2) From this it can be seen that the depth of the euphotic zone decreases the further up the estuary.

Table 2.1. Secchi depths and 1% light depth from stations.

Station Number	Secchi Depth	1% Light Depth
1	3	9.59
3	1.96	6.31
4	1.6	6.06
5	1.67	5.35
6	1.46	4.65

Table 2.2. Filter results from Secchi disk at Station 3.

Filter Colour	Secchi Depth (m)	% Light Depth (m)
None	1.96	6.31
Red	1.96	6.31
Blue	1.72	5.38
Green	2.18	6.98

Nutrient Data and Profiles

The water samples were analysed for nutrient concentrations so that a Theoretical Dilution Line (TDL) could be constructed from the riverine and coastal end members to represent the behaviour of these nutrients throughout the estuary. A theoretical dilution line represents the concentration of nutrients that would occur if there was no addition or uptake of nutrients. From comparing our nutrient concentrations to the TDL it can be determined if the nutrient is behaving conservatively or non-conservatively (uptake or addition).

	Nitrate (Figure 2.11) shows conservative behaviour along the estuary, its main addition is through run-off so the concentration is gradually diluted down the river.
Figure 2.11. Nitrate concentration with salinity
	Silica (Figure 2.12) shows clear non-conservative distribution, the most likely cause of the removal is by diatoms. There were not many diatoms found present in the sample however a problem with the iodine used with the samples means it is likely a number were overlooked.
Figure 2.12. Silicate concentration with salinity
	The phosphate (Figure 2.13) data does not follow either a conservative or non conservative distribution very closely, this is because unlike silica and nitrate who's main addition to the estuary comes through run-off, phosphate has many diffuse inputs including rainwater and sewage.
Figure 2.13. Phosphate concentration with salinity

Biology

Oxygen data

Unfortunately the oxygen data collected on this survey was found to be unsuitable once analysed. Future research would involve samples being collected at the same depth as phytoplankton or if more collection bottles were available vertical profiles of oxygen concentrations taken at each station. Samples taken from the surface where a saturation of ~100% is expected could be used as a standard measure to which other samples could be compared.

Oxygen saturation can give an indication of the processes occurring within the water column, for example, phytoplankton photosynthesis and respiration. Saturation values above that at the surface would indicate high populations of phytoplankton, whereas saturation below would indicate that there is more respiration occurring than photosynthesis. This may be a result of smaller populations of phytoplankton and possibly higher numbers of zooplankton present.

Phytoplankton abundance can also be indicated by chlorophyll and nutrient concentration. Despite the lack of phytoplankton cells found in our samples, the presence of chlorophyll indicates that there were phytoplankton cells there and the uptake of silica shown by the mixing diagram suggests that diatoms are the dominant type of phytoplankton cells. Judging by the data from other groups who did find phytoplankton cells, there may have been a problem during the analysis of our samples. The phytoplankton samples were taken at depths below the pycnocline where we would expect to find large populations of phytoplankton and perhaps those cells present were too small to be identified. When sampling for phytoplankton in the future, shallower water samples should be collected. In an estuary the euphotic zone is shallow due to the dynamic environment causing high turbidity so samples should ideally be taken from the upper 4m.

Zooplankton Tow Results

Figure 2.13. Zooplankton taxa from Station 2.

Sample 4 (Fig 2.13) was taken at the Lynher River confluence and shows a high diversity of zooplankton taxa dominated by copepods (48%).

Figure 2.14. Relative proportions of 4 most dominant zooplankton taxa at each station.

Sample 13 (Fig 2.14) taken from fresh water up the Tamar River was found to contain only copepods.

Estuaries are dynamic environments and therefore are home to few species of zooplankton. High levels of physiological stress means organisms must adapt to survive, which is energetically expensive and so few species survive. In sea water there is a much more stable salinity and a high gene pool from which new species may evolve. In fresh water there are few species present due to a small gene pool and therefore species die out.

Offshore Boat Work:

INTRODUCTION

Aim: To collect water samples around the thermocline at a variety of stations offshore to determine how the vertical mixing processes affect the structure of the phytoplankton communities. These samples were used to measure the nutrient, chlorophyll, dissolved oxygen, phytoplankton and zooplankton concentrations above and below the thermocline. Below are some pictures taken during the sampling:

Station 1	Station 2	Station 2 Drop 2	Station 3	Station 4	Station 5
50'20.327N 4'09.195W	50'02.084N 4'22.458W	50'01.989N 4'23.545W	50'06.970N 4'17.438W	50'40.621N 4'15.726W	50'10.523 N 4'16.294W
0830 GMT	1027 GMT	1050 GMT	1325 GMT	1427 GMT	1507 GMT

Station 1 was inside the breakwater in Plymouth sound. From here Terschelling proceeded offshore to Station 2 which was offshore sampling station E1. Here the CTD showed a deep chlorophyll maximum. Station 3 was halfway between E1 and Eddystone Rocks to see how far the deep chlorophyll maximum seen at station 2 extended. Stations 4 and 5 were at Eddystone Rocks, with one station on the lee side and one station on the weather side to investigate the effect of the rocks on the water column stratification.

DESCRIPTION OF FINDINGS

At all sites sampled there was a thermocline present , Figure 3.1 shows the CTD data from station 3, half way between E1 and Eddystone Rocks. A deep chlorophyll maximum, indicated by a change in fluorimetry, can be seen at about 23m - just below the thermocline. Figure 3.2 also shows the deep chlorophyll maximum as actual concentration of chlorophyll. This deep chlorophyll maximum will form here as conditions are suitable for phytoplankton growth. Below the thermocline light levels may not be sufficient for phytoplankton growth although at this site the 1% light depth is calculated at 39.9m from the secchi disk reading so this may not be the case here. Above the thermocline phytoplankton growth will be limited by the availability of mayor nutrients. Vertical profiles of the major nutrients can be seen in Figures 3.3, 3.4 and 3.5. It can be seen that the concentration of nutrients above the thermocline are very low and will limit phytoplankton growth and production. Below the thermocline nutrient concentrations increase but are still relatively low.


Figure 3.1 Vertical CTD profile taken at station 3.	Figure 3.2 Graph displaying the deep chlorophyll maximum, taken at station 3.	Figure 3.3 Graph of Silica plotted against depth taken at station 2.


Figure 3.4 Graph of nitrate plotted against depth taken at station 2.	Figure 3.5 Graph of phosphate plotted against depth taken at station 2.

ADCP data at station E1 showed absorbance between 15 and 25 metres depth were there should have been an increase in backscatter as the CTD shows very high concentrations of chlorophyll (Fig 3.6). It was later discovered that the ADCP cable was broken and this may be the cause of our unusual results.

Figure 3.6 ADCP backscatter profile taken at station 2.

The zooplankton and phytoplankton communities were also analysed from the samples. All our zooplankton samples show a large proportion of Noctiluca. This is a dinoflagellate that is both heterotrophic and autotrophic and can therefore utilize nutrients or ingest other plankton. This means it is not reliant on nutrient in the water column to grow and reproduce. As can be seen from the nutrient profiles the levels of major nutrients are low at most stations. The nutrient levels are probably low due to utilization early on in the summer by diatom blooms. As the season progresses the succession of plankton shifts from diatoms to dinoflagellates that can proliferate as the light levels continue to increase.

Figure 3.7 Phytoplankton taxa from station 2.

Figure 3.7 shows dominance of dinoflagellates Peridinium spp. in the phytoplankton samples collected at Station 2 and Figure 3.8 shows dominance by Noctiluca as found in our zooplankton samples from the same sample site. However the size of noctiluca means that it will be retained in the net more so than other smaller plankton so the numbers in the sample may be disproportionate to the actual numbers in the sea water.

Figure 3.8 Zooplankton taxa taken at station 2.

A polychaete worm from a zooplankton sample taken from station 2.

The phytoplankton community structure may also change above and below the thermocline. Samples were taken at station 3 using the CTD data seen in Figure 3.1 as a guide to where the thermocline was located. Figure 3.9, 3.10 and 3.11 show the proportions of the main groups at different depths. Diatoms dominate the community at the surface and at, to a lesser extent at 12m. At the surface and at 12m diatoms dominate the community. This is to be expected as they are quick to utilize the nutrients the diffuse up over the thermocline. Dinoflagellates dominate the community below the thermocline, perhaps as these species are less efficient at nutrient uptake and so are restricted to the lower part of the euphotic zone.


Figure 3.9. Phytoplankton taxa at surface, station 3.

Figure 3.10. Phytoplankton taxa at 12m, station 3.

Figure 3.11. Phytoplankton taxa at 50m, station 3.

COMPARISON TO GROUP 3.

As our group was only one of two groups to make it to E1 it was decided to compare the two groups data and see if there had been any changes between the two sampling dates. The chlorophyll maximum moved from 25-30m on 29th June to 15-20m on 4th July due to the change in the depth of thermocline. The stormy weather encountered between the two sampling dates, mixed the water column breaking down the thermocline which is now beginning to re-establish. The reduced solar radiation means that the thermocline is taking longer to reach the former depths.

Rib Boat Work:

INTRODUCTION:

The work took place on the 01/06/2004. Aim of this practical was to map the physical, chemical and biological process up the river, and allow us to compare to areas further down the estuary. Water samples were collected at salinity intervals along the river to be analysed for nutrients and phytoplankton.

Nutrients and Biology

Silica, phosphate and nitrates all showed almost identical dilution graphs as were found by group 8 during their RIB day.

Figure 4.1 Chlorophyll trend along the river.

The general trend of chlorophyll concentration can be seen in Figure 4.1. This shows the chlorophyll concentration decreasing as salinity increases. However when looking at our cell counts there are some diversions from this trend. Sites 4, by the road bridge, and 6, near Weir Quay Sailing Club have low cell counts. This could be due to low nutrients concentrations as a result of phytoplankton bloom earlier in the season.

Figure 4.2. Overall cell counts for each site sampled

Figure 4.2 shows the overall cell counts for each site sampled with the proportions of the main taxa indicated. Sites 1,2 and 3 were collected by Group 8 on Bill Conway and do not show much variation in either species diversity or cell numbers. This may be because all of these sites were between 30 and 34 salinity thus there was not a very significant change in the environmental parameters and so the same species will dominate at each site– diatoms in this case. Sites 4 to 7 were collected by group7 in the RIBs. At site 4 there is a significant decrease in cell numbers but although diatoms still dominate the relative proportions of dinoflagellates and ciliates increase. The drop in cell numbers could be due to a phytoplankton bloom earlier in the year utilizing most of the nutrients. The rise in ciliates could be due to a small input of nutrients from an external source as these are opportunistic group of phytoplankton. Site 5 has a much higher cell count of 123 cells per ml- this site was located at the mouth of the River Tavy so the increase in cell numbers may reflect an input of nutrients to the water column by the river. Diatoms dominate at this site, and as diatoms are efficient at utilizing available nutrients and forming blooms, our sample seem consistent with a nutrient input at this point. At site 6 cell numbers decrease to 26 cells per ml and ciliates dominate the taxa. Dinoflagellates also increase in proportion although overall cell numbers are lower. There is some agricultural land surrounding this station which could cause an input of inorganic nutrients through run off of fertilizers and other substances but this is unlikely to be significant as the cell numbers are low. Site 7 shows an increase in cell numbers and dominance by diatoms and ciliates with no dinoflagellates. This could be due to counting errors or simply that there were no dinoflagellates present in our sample.

Zooplankton diversity increases as salinity increases, as can be seen in Figures 4.3 and 4.4


Figure 4.3 Zooplankton sample taken by the RIBs		Figure 4.4 Zooplankton sample taken aboard the Bill Conway

At the less saline sample sites the community is dominated by copepods and mysids (Figure 4.3) with a few larvae present. As salinity increases the number of meroplankton increases, including copepod larvae, cirripede cyprids and decapod larvae. (Figure 4.4) Other taxonomic groups also appear such as gastropods, echinoderms and polychaetes. This change in diversity can be related to gradients in the physical environment. As the estuarine environment is highly dynamic with a steep salinity gradients that only well adapted, specialised organisms can tolerate. Therefore species diversity is low. As the environment becomes more marine and stable a wider range of organisms and their larvae can tolerate the conditions thus species diversity increases. However the sample of zooplankton may not accurately reflect the community from which it was sampled. Using a net with a fixed mesh size will only collect a certain size fraction of the zooplankton community. This means that the sample collected may not necessarily represent the community accurately. Our trawl was only 3 minutes long and so a relatively small sample was taken and this may cause some inaccuracies.

Geophysical practical work:

Natwest II

INTRODUCTION:

Date	Time GMT	Location	Survey	Conditions
27/06/04	09:01	50°21.835N 00 4°10.864W	Side scan sonar	Sunny, 10 knot wind

Figure 5.1. Side scan sonar of the Dock wall.

On leaving the marina a line was taken at 09:01 GMT travelling north from 50°21.835N, 4°10.864W. To survey the edges of the dock wall, shown in Figure 5.1. Three different types of construction can be seen in the trace. From information given it is known this are

1. Solid front made of concrete, used in the navel docks.

2. Concrete pillars with relatively large gaps in between

3. Wooden pillars with smaller gaps in between, used for non military sections.

Some side swipe can be seen on the trace from the solid concrete walls.

This survey highlights the advantages of side scan sonar. It is a useful, and relatively cheap, tool for surveying large areas in a short space of time and can detect different substrates and bedforms which will be discussed later. It can differentiate between substrates and therefore highlight area for more detailed investigation by divers or a 3D chirp system for example. However it cannot fully identify types of bedform or substrate without prior information available. Therefore should not be used for a survey where there is no previous knowledge of the area.

A detailed survey was undertaken travelling north from 50°24.342N, 4°12.293W at 1127 GMT. This line passed by the road bridge over the estuary (Fig 5.2)

Only the middle bridge pillar can be seen on the trace as the two outer ones are built on outcrops of rock already present in the area. This provides a solid base for the pillars. It can be seen on the trace, as the solid rock has a strong return.

To the North of bridge there is mud on left, sand on right it is likely this is due to differential current flows and friction at the edge of the river leading to deposition of fine sediments, more coarse material in the centre of the channel where flow is faster (same amount of friction but more water)

Figure 5.2. Side scan sonar of transect at Bridge.

The middle pillar of the road bridge causes eddying of the water flow; this promotes mixing in the water column which will disturb any bedforms present. As a result it can be seen from the trace there are no bedforms north and south of the bridge supports for 75m North and 20m South. As the influence of the bridge lessens the sand waves re-appear both sides. To the north of the bridge the wavelength is 2.4m and they carried on for 220m. To the south of the bridge the wavelength is 1.6m and they occur over 100m. The difference in wavelengths of the sand ripples to the North and South of the bridge could be due to a change in current speed and/or depth of the water column. The waves are all in the direction of tidal and are therefore caused by the current flow.

The crests of the sand ripples show up as darker areas on the trace as they reflect the sound and the troughs a lighter colour as there is less sound available to return. The wavelength can be measured using the crest to crest distance. Some of the sand ripples bifocate which indicate the asymmetry in the flow causing the ripples. The ripples to the North of the bridge showed bifocation and also to a lesser extent so did the ripples to the west of the middle pillar. From the trace it can be determined that the tide flooding up the river has more influence on the sand ripples than the ebbing tide.

Different types of substrate can be identified by the strength of return from the side scan. Fine sediments with a small grain size absorb the sound and so show up as pale areas of little return. Larger grain sediments such as sand reflect more of the sound as they are less compact and have more surface area to reflect the acoustic pulse. These show up as darker, mottled areas. Hard substrates such as the bridge pillar reflect almost all of the acoustic pulse. The tow we were using was able to map a maximum of 75m either side.

The mooring lines show how accuracy can be compromised if decisions are made on side scan only, we know the moorings were all in straight lines along the river however due to the boat going at a slight angle they appeared on our sidescan sonar to be angles. If we had no prior knowledge we would have assumed this to be they way they were.

Grabs:

Figure 5.3. Van veen grab.

For grab sampling we used a Van Veen grab, as shown in figure 4.

1) 50°23.491N, 4°12.45 W, 1219 GMT. (slack water) A sediment sample was taken using a Van veen Grab. This area, just south of the confluence with the River Lynher, is sheltered with small craft moorings on the Western side of the estuary. The sediment was black and anoxic, with fine homogenous grains. There was no epifauna or infauna present but there were some empty worm casts running through the sample. The water column was 9m deep.

2) 50°24.171N, 4°12.409W, 1236 GMT (slack water) A second grab was taken, north of the Lynher confluence at a racing buoy south of the road bridge, on the western side of the estuary. The sediment here was darker and anoxic but an oxic layer of approximately 10cm deep could be seen. The anoxic layer was more compacted than the oxic layer but both had fine grains, homogenous throughout the sample. There was no evidence of any epifauna or infauna. The water column was 11m deep.

3) 50°24.583N, 4°12.127°W. 1245 GMT (slack water) The final grab was taken north of the road bridge on the eastern side of the river near the pontoon. The sediment sample was much lighter in colour, the grain size was coarser, with shells from slipper limpets, mussels, gastropods and other bivalve shells mixed into the sediment. However there was no live fauna except one small crab 2cm long.

The samples from the grab can be used to calibrate the side scan sonar trace. The sediment grain size determines the strength of reflection of the sound as previously explained. This means that the shade of the trace can be matched up visually with the grain size of the sediment sampled from the same area. Therefore it is possible to determine the approximate grain size of the sediment using the trace without the need to take a grab sample.

Due to technical difficulties, some of the navigational data for the side scan traces were unavailable, so Group 4's plot was used as they did a transect in the same area. Therefore some of the isometric plot is based on well informed assumptions.

Section 6: data locations

Below are the locations for all the data files collected and processed during our fieldwork at Plymouth:

Bill Conway
Data type	File type	Data location
CTD data	Excel spreadsheet and sigma plot graphs	Group7/Estuary/Physical/Processed/CTD/
CTD data	.RAW	Group7/Estuary/Physical/Raw/CTD
ADCP data	Winriver	Group7/Estuary/Physical/Raw/ADCP
Nitrate	Excel	Group7/Estuary/Chemical/Processed
Phosphate	Excel	Group7/Estuary/Chemical/Processed
Silicate	Excel	Group7/Estuary/Chemical/Processed
DO2		No useful data collected
Chlorophyll	Excel	Group7/Estuary/Biology/Processed
Zooplankton	Excel	Group7/Estuary/Biology/Processed
Phytoplankton		No useful data \collected

Natwest II Geophysics
Data type	File type	Data location
Navigational data	Excel spreadsheet	Group7/Geophysical Boat/Processed/ Bridge nav data.xls

Small Boats
Data type	File type	Data location
Nitrate	Excel	Group7/Small Boats/Chemical/Processed
Phosphate	Excel	Group7/Small Boats/Chemical/Processed
Silicate	Excel	Group7/Small Boats/Chemical/Processed
Zooplankton	Excel	Group7/Small Boats/Biology/Processed
Phytoplankton	Excel	Group7/Small Boats/Biology/Processed

Offshore Boat Terschelling
Data type	File type	Data location
CTD data	Excel spreadsheet and sigma plot graphs	Group7/ Offshore boat /Physical/Processed/CTD/
CTD data	.RAW	Group7/ Offshore boat /Physical/Raw/CTD
ADCP data	Winriver	Group7/ Offshore boat /Physical/Raw/ADCP
Nitrate	Excel	Group7/ Offshore boat /Chemical/Processed
Phosphate	Excel	Group7/ Offshore boat /Chemical/Processed
Silicate	Excel	Group7/ Offshore boat /Chemical/Processed
DO2	Excel	Group7/ Offshore boat /Chemical/Processed
Chlorophyll	Excel	Group7/ Offshore boat /Biology/Processed
Zooplankton	Excel	Group7/ Offshore boat /Biology/Processed
Phytoplankton	Excel	Group7/ Offshore boat /Biology/Processed