Plymouth Field Course - 2005

Group 10

Back row left to right: Stuart, Lauren, Dave, Andre, Angela, Paul, Simon

 Front row: Lottie

            Introduction        Aims & Objectives        Itinerary        Practicals

            Summary        Pictures        References        Useful Links


Introduction

   

The Tamar estuary is a meso-macro- tidal, flood dominated estuary separating the two counties of Cornwall and Devon at Plymouth, on the south west coast of the UK (Figure 1a) . There are two large rivers joining and influencing processes within the estuary called the Tavy and the Lynher.  The estuary is approximately 31 km long from Plymouth Sound to the limit of the salinity influence at Weir Head, with an average river discharge of 22m3s-1       

 

Over the 14 day period between 29th June and 13th July data was collected giving an insight into the biological, chemical and physical aspects of the upper and lower reaches of the estuary, together with offshore areas and a geophysical survey of the area. The aim was to gain a comprehensive understanding of how the Tamar estuarine system works.

Figure 1a Area map Click to view an enlarged, detailed version


Aims and Objectives

Our aim is to study the physical, chemical, biological and geophysical aspects of Plymouth Sound and its environments. The studied area will be between zero salinity on the Tamar River and offshore sampling station E1 located close to the Eddystone Lighthouse (Figure 1a). This will be carried out over a period of two weeks and will help us, as a group and as individuals, to better understand the factors controlling and influencing all parts of the areas oceanography as well as to grow more confident in sampling and measuring them. This will require multiple skills including working as a team as well as academic performance.


Itinerary

30/6/05 - Geophysics Field

1/7/05 -   Estuarine Boat

4/7/05 -   Geophysics Boat

8/7/05 -   Estuarine RIBS

11/7/05 - Offshore Boat

 

Back To The Top


Practicals

Estuarine Boat   Geo Field   Geophysics Boat    Estuarine RIB's    Offshore Boat


Estuarine Boat

Location: Tamar bridge, to Breakwater, Plymouth sound

Weather:   Rain, 8/8 cloud cover, NW wind

  

How the Biology, Physics and Chemistry changes in the lower reaches of the River Tamar

The chemical physical and biological data collected from the 'Bill Conway' was between the Tamar bridge and the breakwater in Plymouth Sound (Figure 1a). The equipment used included a CTD, ADCP, Niskin bottles, a plankton net and a surface water pump. This was used to acquire the data required to illustrate how the composition of the Tamar varies chemically, biologically and physically along a series of transects in relation to distance downstream. This would then be compared to data sets from offshore and further up the river.

Chemistry

The aim was to obtain data that would describe oxygen, silica, nitrate and phosphate distributions both vertically through the water column and spatially within the estuary. Sixteen samples were collected from 5 different locations (see appendix 1) and immediately fixed then stored and transported with the glass bottles submerged in water.

 

 

 

 

 

Figure 2a shows three quite different patterns of oxygen saturation throughout depth. Station 1, beside the Tamar Bridge, shows that there is a lower saturation of oxygen in the surface waters than at depth in the vertical profile. This could be due to a sewage outfall to the north of the Tamar Bridge, upstream of station 1 on the Eastern banks of the Tamar. At station 3, mid way between Tamar Bridge and The Sound, there is  high oxygen saturation of the surface waters due to a high riverine influx from the River Lynher.  There is a large difference between the oxygen saturation in the surface waters and midway through the profile at 6 metres, this is due to a salt-wedge produced by the incoming tide at the time of sampling. Station 4, closer to The Sound, shows a more typical estuarine oxygen depth profile, with a high surface water saturation, slightly decreasing in the first 6 metres, before a major decrease in saturation of about 4% in the next 4 metres. This is due to a high light attenuation in the top 6 metres, before a major decrease in light penetration, preventing phytoplankton photosynthesis at depth.

Figure 2b. Silica theoretical dilution in the River Tamar.

The estuarine mixing diagram suggests that there is removal of silicon, particularly at salinities of 10-20%.  The removal of silicon is likely to be biological, with removal by diatom species which have been found in high concentration in the estuary.  At higher salinities at our end point there is evidence of addition, due to anthropogenic sources such as inputs from sewage works and rainwater runoff from surrounding farmland, with many points clustering around the same area.  (Morris et al. 1981) found that the silicate-salinity relationships invariably indicated an appreciable loss of dissolved silicate from solution within the estuary.   

Figure 2c. Nitrate theoretical dilution in the River Tamar.

There is no significant evidence of non-conservative or conservative behaviour of  nitrate concentrations within the Tamar estuary (Figure 1c). There are small variations in the concentrations around the theoretical dilution line but these are not significant enough to suggest non-conservative behaviour.  Nitrate concentrations are therefore dependent on salinity and are not greatly affected by biological processes or anthropogenic inputs to the estuary.

 

 

Figure 2d. Phosphate concentration down the River Tamar.

The artificial dilution line shows that there is removal of phosphate down the estuary.  If the theoretical dilution line is used then only slight removal can be seen.  Removal would occur through uptake by biological organisms for growth.  In general the phosphate concentrations follow a different distribution around the theoretical dilution line than that of silicon and nitrate. (Morris et al.1981) found that tributary and anthropogenic inputs to the lower 10km of the Tamar Estuary had a major influence on the distribution of phosphate throughout the estuary but exerted only minor localized effects on the distributions of nitrate and silicate.

 

Biology

Phytoplankton samples were taken at seven different sites, five upstream of the Tamar Bridge by the ribs group and two by group 10 aboard 'Bill Conway', one just below the Tamar Bridge  and one behind Plymouth Breakwater (Figure 1a). The samples were taken from the surface water at each station. Figure 2e shows that the concentration of phytoplankton overall tends to decrease with distance downstream, with the highest concentration of 195,000 cells per litre found at station 2 and the lowest concentration of 15,000 cells per litre at the final station with an estimated distance of 5nm between the two stations. The phytoplankton concentrations decrease at every station after 2 apart from at station 6 where the River Lynher joins the Tamar, producing a sharp increase of 40,000 cells per litre. This is due to the input of nutrients by the River Lynher, providing the phytoplankton with an increased food source, especially as there had been a period of high rainfall for 3 days prior to sampling. Diatoms are the dominant group in the estuary with the Chaetoceros genus found in the greatest abundance. Downstream, from station 5 onwards ciliates begin to appear, this may be due to the increase in salinity found in the estuary the further downstream that is travelled, together with the increase in zooplankton abundance found downstream, acting as a limiting factor. 

Figure 2e. Phytoplankton concentrations and species composition in the River Tamar. Click to view an enlarged, detailed version

    From the two pie charts representing zooplankton species compositions at the two different stations (Figure 2f) it is possible to see that a clear difference in the species found. The predominant species found at the Tamar bridge, where the salinity was 29.4, were the Calanoid copepod group, numerically the most dominant organisms amongst zooplankton communities in British waters (Todd et al. 1996),with the Nauplii copepod  segment also representing a large segment. The large abundance of hydrozoans in the sample taken from the breakwater, where a salinity of 35 was recorded allows us to make the assumption that species abundance is dependent on salinity. Hydrozoans are usually found in high numbers during the summer months in saline waters, probably in this case feeding on a blooming phytoplankton community. There is also a large segment representing the Calanoid copepod  here, confirming the dominance of this group in Plymouth Sound. Chaetognath and copepod nauplii abundance appears to be similar at both sites.  

 

Figure 2f. Species composition of zooplankton at the Tamar bridge and Plymouth Breakwater.

Physics

Transmissometer - station 1 (Close to the Tamar Bridge)

This profile shows a decrease in turbidity with depth. There appears to be an upper 2 m deep layer of higher turbidity. This over lays the less turbid region beneath. This may be due to increased particulate matter in a less dense riverine flow overlaying the denser saline water. The steep gradient turbidity at 2m suggests that the water column is stratified at this location. As this station is near the Tamar Bridge turbidity may have been created by the motion of water circulating around the pillar structures.

This profile indicates greater light transmission than in the upper estuary. There is no obvious stratification at this location. Light transmission through the water column appears reduce linearly with depth suggesting that the water is not stratified.  This may be due to increased wind mixing.

 

 

 

 

Figure 2h. Chlorophyll depth profile at station 1 and 7.Click to view an enlarged, detailed version

 

 

Figure 2g. Transmissometer depth profile at station 1 and 7.Click to view an enlarged, detailed version

Chlorophyll fluorometer data from station 1, at the Tamar Bridge, shows a 2m deep layer of increased chlorophyll concentration. This layer corresponds with the region of increased turbidity.  The decrease in chlorophyll concentration from this depth throughout the water column may be due to attenuation reducing the photosynthetically available light. Station 7 shows a generally lower chlorophyll concentration than at station 1 reducing approximately linearly with depth throughout the water column.  No stratification is evident at this station 

 

The ADCP transects indicate a reduction in current with distance downstream.  Currents become slower and change from a 0.5m s-1 maximum at the Tamar Bridge (Figure 2i) to a maximum of 0.3m s-1 in The Sound (Figure 2k). Water masses within the estuary with different velocities become less clearly  distinguishable indicating increased mixing of the water column.   This is particularly obvious when comparing images from station 1 (Figure 2i) and station 7 (Figure 2k). The calculated Richardson number for the site is 5.01 indicating a highly stratified water column.                                                                                                               

 

 

 

 

Figure 2i  ADCP profile Station 1Click to view an enlarged, detailed version

 

 

 

Figure 2j ADCP profile Station 4Click to view an enlarged, detailed version

Figure 2j is the ADCP transect from station 4 the location where the out flow from St John's Lake mudflats enters the Tamar. The transect was taken across the river from South to North, the current changes direction towards the North of the transect. This change in velocity may be due to the two water masses meeting and mixing.  The Richardson number for this channel was calculated to be 0.140, this shows that the flow in this location is turbulent, probably due to the volume of water that must pass though a single channel to drain the mudflats in each tidal cycle. This is supported by the CTD profiles below.

 

 

Figure 2k ADCP profile Station 7Click to view an enlarged, detailed version

Figure 2k shows the ADCP transect taken across the narrows. This is a very dynamic area with large volumes of water flowing through a deep narrow channel. The transect shows fast velocities in the central and deeper areas of the channel, compared to lower velocities on the outer edge of the channel where friction with the bedrock reduces the speed of flow.

Click to view an enlarged, detailed version

Station 1.  Temperature in the upper 2 m decreases from 18.3°C to 17.9°C. The salinity profile shows a 2m deep layer in which salinity increases from 27.5 to 30.75. Below this layer, in the remaining 13 m of the water column, both salinity and temperature decrease at relatively constant rate.  These profiles suggest riverine water overlays a more saline water body indicating stratification of the water column at this station.

Click to view an enlarged, detailed version

Station 4.   This station is situated at the confluence of the outflow from St. John’s Lake and the river Tamar. The profiles indicate a well mixed water column due to the dynamic environment (see Figure 10, ADCP profile). This is supported by the Richardson number, 0.14, calculated for this section of the estuary which indicates turbulent flow.

Click to view an enlarged, detailed version

Station 7.   Profiles show an increase in salinity and decrease in temperature from station 1. There is no obvious stratification at this station although the temperature gradient of change becomes shallower below 5 m.  These profiles appear to indicate a well mixed water column as reflected by the Richardson number, 0.210. This station was situated at the Western Channel. The tide had just begun to flood and the restricted flow induced turbulent mixing

Back To The Top


Geo-Field

Location: Renney Point, Heybrook Bay, Plymouth, England

Weather: Intermittent rain, 8/8 cloud cover, moderate wind

 

The aim of this practical was to gain information about the strike and dip of the rock formations at Renney Point.  This was done using a compass clinometer.  Dip was measured by placing the compass clinometer along the plane of the rock and reading the value off the small black arrow.  Strike was measured by placing the compass clinometer along the horizontal plane and taking a bearing using the right-hand rule.  One fault line was selected and the bearings of the associated fractures were taken and plotted on a compass rose. Photographs were also taken of the area. These were drawn onto a map.

 

Vegetation and soil with several shells and small pebbles probably transported by sea birds and deposited after feeding, unlikely to have been submerged.
More modern soil formed by general deposition of material. A gradient in colour indicates the transition between top vegetated soil and older deposited material.
Matrix supported section, therefore material is not fluvially derived and was likely to have settled after land slide or slope failure. This could have been caused by the lack of vegetation during the climatic optimum (6 000 – 8 000 Years Before Present). Clasts are locally derived.
Slightly composite sediments probably deposited by small streams. Traces of quartz visible.

Fluvial deposits, gravel and fines washed away. Lag deposits.

  Bedrock.

Figure 3a Cross section

An exposed cross-section of a cliff was drawn in a field sketch to enable us to see what may have caused the exposed rocks that we had been studying. The layers within this cross-section and their cause of formation were identified (Figure 3a). Faults and fractures were also identified and bearings taken.  

Found within the rock formations was an antiform and the fold was recumbent. This caused many fractures. A right lateral fault was found in another area this fault had been displaced by approximately 5m.  The hinge lines were exposed and visible.

Back To The Top


Geophysics Boat

Nat West II  

Location: Whitsand Bay, Plymouth sound Breakwater

Weather: Intermittent heavy rain, 6/8 cloud cover SW moderate wind

 

The aims of the geophysics boat practical are to conduct a bathymetric survey and produce sidescan images of the James Egan Layne and the Scylla wrecks in Whitsand Bay and a section of the seafloor to the north of the breakwater in Plymouth Sound and to collect grab samples of the seafloor to assist with interpretation of sidescan images and to determine if different sediment types contained different species of organisms.

A towed sidescan sonar fish (Geo Acoustics Ltd) was deployed from the survey vessel Nat West II. In Whitsand Bay images were obtained of the James Egan Layne and the Scylla from 2 transects (Table 1).                 

Transect  Latitude  Longitude
1 start 50º19’.6N 4º15’.3W

2  end

50º20’.5N 4º15’.1W

 Table 1     Transect positions of scan 1    

Due to rough seas it was decided that any further survey work beyond the Plymouth breakwater should not be undertaken.  

An area to the north of the breakwater, approximately from Queens Grounds to the west to Duke Rock to the east, was surveyed in 4 transects.  (Table 2)  

Transect  Latitude  Longitude
1 start 50º20’.355N 4º10’.186W
   end 50º20’.326N 4º08’.192W
4 start 50º20’.500N 4º08’.200W

   end

50º20’.500N 4º10’.300W
Sample No. Latitude Longitude
1 50º20’.318N 4º10’.327W
2 50º20’.393N 4º09’.172W
3 50º20’.500N 4º08’.200W
4 50º20’.400N 4º09’.100W

Van Veen grab samples of sediment were taken at 4 locations. (Table 3)  

     

Table 3      Location of grab samples

Table 2     Transect positions of scan 2   

Results and interpretation  

Figure 4a. Plot of sidescan sonar trace of scan 2. Click to view an enlarged, detailed version

Sidescan sonar traces were produced of both sets of transects. The sidescan trace indicated the presence of a fault the dip and orientation of which was sufficiently similar to that of the fault surveyed  during  the on-shore geological survey carried out 4 days previously for us to investigate further.

Figure 4b: Position of major fault (green) and the disjointed fractures (red) resulting from the geological lateral movement. The blue triangle roughly represents the area over which we did our calculations to determine the offset.

To determine the offset a triangle was drawn over the data and lengths recorded. These values were then converted into real measurements using the scale determined from the side scan data of 1cm to 5.7m. Using basic trigonometry, the Sin rule, the size of the desired side of the triangle was determined to be 5.5m.

 

 

 

 

Figure 4c: The devised right angled triangle used to calculate the offset. Not to proportion.

However, the fault angles between both sites did not match in their bearings. On-shore: 5m offset and 280º bearing. Off-shore: 5.5m offset and 335º bearing. This meant that the offset distance was sufficiently close to support the hypothesis that they were the same fault however the differing bearings did not. Observing the compass rose diagram for the offshore two main fault directions are seen, with the main fault at the 280º bearing but with the secondary fault centred at 330º.What this in suggests is that the pressure upon the same fault, on-shore and offshore have resulted differing main faults like figure 3 illustrates.

 

 

 

 

 

 

 

 

Figure 4d: The two differing main faults as the result of pressure. The arrows indicate the pressure direction, the red line the main fault off-shore and the blue the main fault on-shore.

A possible explanation for the differing main faults may be the weakening effect that the fold found on-shore may have had on the surrounding rocks.

Areas of bedrock were indicated to the western side of the Western Channel. Within these areas were regions of medium to coarse sediment forming asymmetric ripples.  These suggest that the flow in this area is unidirectional. (See Figure 4a)  To the eastern side of the Eastern Channel bedrock was indicated with fine sediments found to the west of this region. Behind the eastern end of the breakwater was evidence of coarse sediments laying over finer sediments which indicated the motion of back eddies.  Towards the centre of the breakwater drag marks from anchor chains were apparent. (See Figure 4a) 

Grab No. Image Description

1. Depth: 9 m

Sediment grain size: 95%  > 2 mm 5% 1-2 mm

No layering of sediment was observed.  The sediment comprised a coarse grained mixture of red/brown sand and broken shells.  The shell fragments included Mercenaria mercenaria, Littorina, Nucella and Ostrea.  Organisms present were 2 tube worms, a shrimp (species unknown), a scallop, annelids and 2 crabs.  

2   Depth: 15 m     

Sediment grain size: 1-2 mm

No layering of sediment was observed.  The sediment was finer than in the first grab sample.  It comprised muds and small red/brown rock fragments.  Organisms present were a Eupagarus spp, tube worms, polychaetes, 2 Ophiuria, turettella and copepods.

3   Depth 13 m         

  Sediment grain size: < 1 mm

The sediment comprised black mud and silt.  No rocks or pebbles were present although many turettella shells were found.  Organisms present were Ophiuria, tube worms and Eupagarus spp

4  Depth 16 m  

Sediment grain size:  98% < 1 mm2% 1-2 mm

Prior to opening the grab, water containing finer sediments was lost.  Layering was observed within the sample.  A lighter coloured upper layer overlay a black, anoxic lower layer. Organisms present were Ophiuria, turettella, polychaetes, Eupagarus spp.  Preserved terrestrial material (beech mast and leaves) were also present in the anoxic layer.  

Conclusion

The obtained side scan sonar from Whitsand Bay show the two wrecks and, from the detail seen, information about the state of the wreck and the degrees of changing sedimentation on and around it. When compared to side scan sonars of the same site from previous years (not included) it can be seen that they have had little effect on the surrounding sedimentation though the ships themselves continue to be slowly broken down.

The findings seen at the breakwater confirm the tidal signature of Plymouth Sound where the stronger flood tide flows in through the Western Channel and the ebb tide flows out through the Eastern channel, this is seen by the type and structure of sediment and its pattern of deposition. This is confirmed by the grab samples which indicate that the area to the west of the breakwater has coarser sediment and the anoxic layer is deeper than the depth of the grab whereas the sample taken from the area north of the centre of the breakwater consists of fines and is consistent with a region of low water flow. The benthic biota reflects the sediment configurations.

Sub-bottom Profile

A sub-bottom profile produced by a boomer revealed a paleo-channel within Plymouth Sound. (Figures 4e and 4f)

A -    Present Sea Bed Level.

B -     Recent Infilling.

C -     Lowered sea level due to mini-ice-age around 500 years ago, this held a lot of the precipitation on land as ice (isotacy) and therefore decreasing fluvial output.

D -    Infilling (6 000 – 3 000 Years Before Present) occurring during the Holocene maximum when the Northern Hemisphere is at its closest to the sun in a 20 000 year cycle, this is a period of increased temperature and precipitation causing sea levels to rise around 20m and therefore fluvial deposition to increase.

E -     Period of cooler climate lowers sea level and river flow creates a channel in the deposited material of the Ria (~6 000 YBP).

F -      Deposition / Infilling after last major ice age (18 000 – 6 000 YBP).

G -     River valley during last ice age (~20 000 YBP) when sea levels were around 125m lower than present day due to the isotacy caused especially by the Antarctic and Greenland Ice-sheets. The shape of the valley suggests that this profile shows a river bending to the east as it flows out of the page to the sea.

H -    Bedrock.  

 

Figure 4e    Profile of paleo-channel.

 

Figure 4f Time line for paleo-channel.

 

 

 

Back To The Top


Estuarine RIBS

Location: Calstock to Tamar Bridge on the River Tamar.

Weather: Dry, 7/8 cloud cover with sunny spells, NW light wind.

Biology

Phytoplankton

As expected there was a clear change in phytoplankton dominance in the upper Tamar estuary between salinities 12 and 28. The constant artificial nutrient sources from surface run-off, especially from anthropogenic land use, have led to ciliate dominance in the salinity range of 12-18. Ciliates have a high optimum nutrient level therefore, considering the salinity-nutrient relationship as shown on figure 4a, they are expected to be found within areas of low salinities. However the possibility that these are in fact benthic ciliates that have been re-suspended into the water column cannot be discarded. As the salinities increase and nutrient values decrease downstream diatoms become the dominant phytoplankton group, especially Chaetoceros sp. This group were found initially at salinity 18 in low numbers and then dominated in the following samples. In this area nutrient levels were still relatively high and ciliates were found throughout, but the differing nutrient optimum between the species promotes more diatom growth. Other factors such as lower turbulence and increased light penetration also favour diatom dominance. Nitzshia sp., although not dominant, were found throughout the salinity spectrum demonstrating high tolerance and widespread habitat. Krenia mikimotoi was the only dinoflagellate found within the samples and at salinities of 18 and 26 at relatively low numbers.

Figure 4a Bar graph to show phytoplankton abundance. Click to view an enlarged, detailed version

Unfortunately, due to an error in the flow meter recording when taking a plankton sample we were unable to gain reliable data for analysis of zooplankton numbers above the Tamar Bridge .

Physics

Flocculation is the agglomeration of destabilized particles into microfloc and subsequently into bulky floccules called floc which can settle out of the water column. The addition of a reagent called a flocculant or a flocculant aid may promote the formation of the floc. On the Tamar flocculation of silicone creates the turbidity maximum, and the flocculant aid is salinity. Flocculation occurs at a salinity of 2  and re-dissolves before a salinity of 6, where the suspended material is mostly particulate sediment, which will settle in The Sound (Figure 4c).  This turbidity maximum is reflected by the Secchi disk depths, which are as little as 12cm in flocculated middle estuary. The high amount of matter in the water will lead to light being attenuated very fast, leading to minimum benthic plant life, as well as limiting the amounts of free floating phytoplankton and therefore affecting the amounts of nutrients that can potentially built up in the estuarine waters. (Figure 4b)

 

 

 

Figure 4b Seechi depth against salinity. Click to view an enlarged, detailed version

Figure 4c Turbidity maximum

 

 

Chemistry

Nitrate  

There is a clear negative correlation when comparing nitrate concentration to salinity in the River Tamar estuarine environment (Figure 4d). There is a section where non-conservative behaviour may be present but as most of the points are close to the TDL (Theoretical Dilution Line) it  cannot be described as non-conservative. The outliers at salinities 2 and 4 could suggest non-conservative behaviour but do not follow the trend of the rest of the points. As salinity increases, nitrate concentration decreases significantly. At a salinity of 27.5 the nitrate concentration is approximately 1/20th of the nitrate concentration at a salinity of 1. Removal of nitrate by phytoplankton occurs progressively down the estuary.

 

 

Figure 4d Nitrate concentrations Click to view an enlarged, detailed version 

Phosphate  

Again, there is a clear negative correlation between the nutrient, phosphate, and salinity. (Figure 4e) There are indications of non-conservative behaviour but as with nitrate, the points are too close to the TDL to be deemed non-conservative. The values of phosphate are approximately 1.8µmol/l at a salinity of 1, at a salinity of 34 the phosphate value is approximately 0.1µmol/l, a value approximately 18 times smaller at the seaward end. This suggests removal of phosphate by phytoplankton  towards the saltwater end of the estuary. 

 

 

Figure 4e Phosphate concentration. Click to view an enlarged, detailed version

Silicon

From the graph it can be seen that silicon has a non-conservative relationship with salinity (Figure 4f).  At low salinity silicon concentration is high and at higher salinities the silicon concentration is lower.  At the riverine end the silicon levels are almost 40 times higher than the concentrations found at the higher salinities.  This relationship shows that there is removal of silicon in the estuary.  There are high levels of removal between salinities of 10 and 20.  The removal of silicon at salinities of 2-6 is caused by flocculation of the particulate matter in the estuary.  When a salinity of 6 is reached the silicon returns to being in solution and the removal which occurs at salinities after this is due to the uptake of silicon by diatoms.   

Figure 4f Silicon concentrations. Click to view an enlarged, detailed version

There is a clear positive correlation between salinity and dissolved oxygen concentrations in the Tamar Estuary as illustrated in Figure 4g. There is a slight decrease in dissolved oxygen at lower salinities but this could be explained by the fact that upstream at the freshwater end where we took the first measurements, there is a sewage outfall. It is also possible that these results could be erroneous. If these points were not included a stronger correlation may be apparent.   

This positive correlation can be attributed to the wind driven mixing which occurs at the surface of the water column. The salinity relates to the position of sampling in the estuary, therefore as salinity increases the samples are from more seaward stations. Wind driven mixing becomes a more significant influence on oxygen saturation, the temperature of water also has an impact on oxygen saturation. Other factors which could contribute to oxygen saturation may be salinity and phytoplankton abundance.  

 

Figure 4g Salinity against dissolved oxygen concentrations for the Tamar Estuary. Click to view an enlarged, detailed version

Back To The Top


Offshore Boat

Location: Eddystone rocks, Plymouth sound

Weather: Sunny, 0/8 cloud, WNW v. light wind.

Biology

Phytoplankton

Figure 5a shows the distribution and abundance of phytoplankton groups across the thermocline at three stations sampled during the offshore study. The Breakwater station was taken just inside Plymouth breakwater at the mouth of the estuary, L4 station is approximately 2km south south-west of Plymouth breakwater and it is a further 1.6km in the same direction to Eddystone Rocks . From this data it is possible to describe how the distribution and abundance of phytoplankton changes with distance offshore in comparison to within Plymouth Sound.

There is a clear increase in the numbers of Rhizosolenia setigera with distance offshore, and at both the L4 station and at Eddystone Rocks this group dominates the sample in terms of overall percentage. This may indicate a lower optimum nutrient level and a better adaptation to a marine environment. Increased light penetration and a deeper water column should also promote its growth. On the other hand Rhizosolenia delicatula seems to be less successful  further away from the estuary. Although dominant within the breakwater its numbers are quickly reduced at L4 and then absent from Eddystone Rocks . Other species, such as Nitzschia and Rhizosolenia stolterfothii, show constant quantities throughout the sites. This reflects a high degree of tolerance and adaptation to considerably different light, nutrient and temperature gradients especially for Nitzschia as it has been present through every sample since salinity is 0.

Figure 5a Phytoplankton distribution and abundance. Click to view an enlarged, detailed version

Zooplankton

Figure 5b illustrates the contrasting zooplankton group types and overall abundance above (25m) and below (45-28m) the thermocline at offshore station L4. The first obvious difference is the greater abundance (over 3 times) of individuals found above the thermocline. This is due to the presence of phytoplankton, zooplankton's major food source, being in the euphotic zone above the thermocline. This food availability allows a greater number of  zooplankton groups, especially filter feeders such as Siphonophores and Appendicularians, to thrive within this area. The sampled individuals below the thermocline should be equally dependant on euphotic zone resources and therefore probably migrate diurnally within the water column. As expected Copepod calanoid was the dominant group at both of the sampled depths.

Figure 5b Zooplankton groups and abundances' at two depths above and below the thermocline at station L4.Click to view an enlarged, detailed version

Figure 5c shows the contrast between zooplankton populations within the euphotic zones in the sheltered estuarine area behind the breakwater and the offshore site at the Eddystone Rocks. The larger zooplankton numbers found behind the breakwater can be accounted for by the high nutrient levels found within the estuary, combined with the more compressed water column and the coastal features that enhance larval settlement. This reflects the expected more productive coastal waters, especially with fluvial inputs, in contrast with an offshore site. Interestingly the overall number of copepod calanoid, dominant throughout all sites, was approximately the same. This indicates the increased number of niches and competition found behind the breakwater, contrasting with the stable, low diversity community found offshore. There are a relatively large number of hydrozoans at the breakwater site; this could be due to individuals being trapped behind the breakwater by the ebbing tide.

Figure 5c Zooplankton groups and abundances' at the breakwater station and Eddystone Rocks. Click to view an enlarged, detailed version  

Chemistry

Phosphate, Silicon and Nitrate

Station 1- This plot shows silica concentration increases with depth.. This may be due to increased diatom abundance above this depth utilizing the silica present.  This can be related to the fluorimetery plot from the CTD data which indicates peak fluorescence occurs at approximately 7m. Phosphate also increases but nitrate concentrations appear to decrease suggesting nitrate is being utilized.

Station 2 - These plots show reduced levels of all 3 nutrients between 20-30m.  This coincides with the chlorophyll maxima shown on the CTD plots from this station. Therefore this suggests that nutrients are being removed by phytoplankton at this depth.  Below this an increase in concentrations is apparent

Station 3 - The plot indicates silica and nitrate both increase at approximately 20m which corresponds with the base of the chlorophyll maxima shown on the CTD data.

 

Station 4 - Silica appears to increase in concentration in the upper 10m then the rate of increase reduces below this depth. Phosphate also shows lowest concentration at 10m.  This is the depth of the chlorophyll maxima indicated on the CTD plot suggesting that phytoplankton are utilizing these nutrients

Oxygen

Figure 5d shows a plot of oxygen saturation. Station 1, in Plymouth Sound indicates undersaturation to a depth of 10m which may be due to mixing. Sites 2 and 4 show a steep reduction in oxygen concentration to the depth of the chlorophyll maxima beneath which the rate of decrease declines. Site 3 is the station at which the water column was mixed by the turbulence produced by the Eddystone Rocks and shows no influence of the phytoplankton. The over-saturation indicated in the plots is probably due to wind mixing and the drawn-down of atmospheric oxygen in the surface layer.  The plot does not show what would be anticipated as the highest concentrations of oxygen would be expected to be found at the same depth as the chlorophyll maxima however this is not the case consequently interpretation has not been  possible.

 

Figure 5d Oxygen saturation plot. Click to view an enlarged, detailed version

 

Physics

Figure 5e CTD Station 1. Click to view an enlarged, detailed version

Site 1(Figure 5e).  This plot shows a similar structure to that of station 7 of the estuarine data – also taken in Plymouth Sound.  There is no evident stratification - indicating a well mixed environment.

Figure 5f CTD Station2. Click to view an enlarged, detailed version

Site 2. (L4)(Figure 5f)  A weak thermocline is evident beneath which there is a chlorophyll peak. This may be due to a wind mixed layer at the surface.

Figure 5g CTD Station 3. Click to view an enlarged, detailed version

Site 3 and 4.(Figures 5g and h)  The tide was ebbing during the sampling period consequently it was anticipated that the water column on the western side of Eddystone Rocks would show evidence of turbulence whereas the water column on the eastern side would be unaffected.  This is illustrated in these two plots which shows the eastern water column to be more strongly stratified than that to the west.. The thermocline to the west is weaker than that to the east.  The chlorophyll maxima in both plots were found to be below the thermocline. This may be due to wind mixing at the surface preventing a maxima forming in the upper layer.  Light intensity at the surface may also be too great for phytoplankton to form a maxima in this region.

Figure 5h CTD Station 4. Click to view an enlarged, detailed version

 

Figure 5i ADCP velocity Profile 1, East of Eddystone Rocks. Click to view an enlarged, detailed version Two ADCP profiles were taken on both sides (upstream East and downstream West) of the Eddystone Rocks aiming to identify the rocks effect on water flow. Figure 5i shows quite a layered uniform flow, except on the southern end of the transect. Here the flow appears to be distorted and reduced. The tide was ebbing from the east and therefore this pattern may have been caused by the physical barrier posed by the bathymetry. The transect on Fig 5j was taken in the opposite direction and therefore the image is reversed. Considering this mirror image we can see the opposite flow pattern to the previous transect. The flow seems to be enhanced in the same area where it was previously reduced probably due to the water enveloping the rocks. It is then apparent that the rocks caused disturbances on either side, reducing flow upstream and increasing it downstream. 

Figure 5j ADCP velocity Profile 2, West of Eddystone Rocks. Click to view an enlarged, detailed version

Back To The Top


Summary

An estuary by definition is “a semi enclosed body of water with free connection to the open sea and with in which sea water is measurable diluted with fresh water derived from land drainage” ( Prichard , 1963). The Tamar River flows from Bodmin Moor and enters the sea at Plymouth. This flow of water, controlled by the laws of physics, transports a multitude of chemical nutrients, which support a plethora of biota. The aim of our investigation was to better understand all factors in this system through chemical, biological and physical monitoring of the estuary between 0 salinity to the offshore land mark, the Eddystone Rocks .    

Our findings show various changes as the water progresses down stream.

At low salinity, sampled by RIBs, the marine input is minimal, therefore nutrient concentrations are high and though the volumes of sediment suspended in the water prevent utilisation by phytoplankton, instead the nutrients are taken up by extensive reed beds on the banks – this is shown in the lower estuary compared to conservative nutrient readings of the upper estuary. When the water reaches the salinity 2 isohaline, the salt acts as a floccuatant aid and a lot of the nutrients come out of solution and become particulate, this creates the turbidity maximum and forces much of the sediment to be deposited creating mudflats. This is especially apparent in the silicon concentrations. At around salinity 4 the flocs re-dissolve and are available for use by the phytoplankton which can now survive in the clearer water due to the increased photic zone. In lower salinities the dominant phytoplankton are generally cilliates as they have high optimum nutrient level and therefore can out compete species that prefer higher salinities. At this point the flow is stratified though many areas are turbulent due to back eddying caused by the topography.

As the marine influence increases the freshwater tends to flow above the denser saltwater. This is reflected in the oxygen concentration profiles where the salt intrusion is seen to be deeper  than the oxygen rich riverine water. The nutrient content of the mid estuary (below the Tamar Bridge) shows concentrations slightly lower than those of conservative behaviour due to removal by high concentrations of phytoplankton - which are mostly diatoms in this region of the estuary. The increased amounts of phytoplankton attract zooplankton. Many of these are meroplankton taking advantages of the estuary’s shelter or holoplankton  which thrive in the region forming the basis of an increasing food web. As the marine influence increases and tidal flow becomes more dominant the stratification is broken down and the flow becomes more turbulent making available the nutrients below the euphotic zone.

Once the water flows out into Plymouth Sound it is fully mixed with respect to salinity, however a thermocline becomes apparent, this is reflected in the euphotic zone and therefore affects the nutrient profile as phytoplankton utilise the available nutrients, and the zooplankton follow.

Offshore this pattern becomes more defined as the flow becomes more laminar due to the deeper water. However, at Eddystone the raised topography disturbs the flow and the thermocline is temporarily broken down, this mixing provides a new nutrient input and a patch of increased primary production can be seen at times.

Back To The Top


Pictures

           

     Group Photo                   P.S.O.                     The Winch Wench


References

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

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

Morris, A.W., Bale, A.J., Howland, R.J.M. (1981) "Nutrient Distributions in an Estuary: Evidence of Chemical Precipitation of Dissolved Silicate and Phosphate". Estuarine, Coastal and Shelf Science 12 205-216.

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

Todd, C.P., Laverack, M.S. and Boxall,G.A, (1996), "Coastal Marine Zooplankton", Cambridge University Press, UK, p106

 


Useful Links

http://www.pml.ac.uk/

http://www.soes.soton.ac.uk/

http://www.pml.ac.uk/l4

http://www.ndbc.noaa.gov

http://www.plymouth.ac.uk/pages/view.asp?page=7708


Back to the top!