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Plymouth 2004 Group 6Southampton University field course to Plymouth 23rd June-8th July 2004. All timings are in Greenwich Mean Time (GMT) All tides referenced to Devonport, Plymouth ContentsGeneral IntroductionGeophysics Practical - Estuarine Practical - Offshore Practical - Ribs Practical - ReferencesGeneral ConclusionGroup 6Dave Bolton - Alex Cargo - Wendy Crichton - Claire Fletcher - Elizabeth Grant - James Knights - Bradley Lewington - Michael Plant - Angus Roberts - Simon Washington General IntroductionThe aim of this field course was to study the geophysics, chemistry, biology and physics of the estuarine, coastal and offshore waters around Plymouth, and combine the results to provide a detailed overview of the complete system. Estuarine work was carried out on the R. V. Bill Conway and RIBs, from the breakwater in Plymouth Sound to the lower limit of the freshwater in the River Tamar. The offshore work took place on the M.V. Terschelling, from the breakwater to Eddystone Rocks, and the geophysical surveys of the estuary and Sound on the catamaran Natwest II. The
Tamar estuary is located in South West England, a temperate region. It is
classified as a drowned river valley (Tattersall et. al. 2003) and a
partially mixed estuary (Uncles & Lewis, 2001). The Tamar is mesotidal, with
a mean tidal range of 3.5m and an annual average discharge of 27m3s-1.
The estuary is 31.7km long and consists of the main river and the Rivers Tavy
and Lynher, which feed into the Tamar in the lower reaches. The combined rivers
flow into Plymouth Sound through the Narrows (a restricted, deep channel) before
entering the English Channel (Tattersall et. al. 2003). Geophysics Boat Practical
IntroductionAt 0805 GMT, the Natwest II left the Mayflower International Marina with 10 students, two scientists and two crew members. First going into Plymouth Sound, at 0820 the West Vanguard Buoy was passed and the first tow commenced up the Hamoaze Channel into the River Tamar. The preliminary tow was charted to travel up the eastern bank of the Tamar, past the Naval Dockyards and finish approximately ½ mile past the Tamar and Royal Albert Bridges (50º 24.4562N, 4º12.2514W). During the tow, the Natwest II remained within 40-50 metres of the eastern bank, to include the dock walls, the entrance to Mayflower International Marina, as well as three frigates and one submarine stationed at the Naval docks. At 1005 the boat turned and began a return survey down the western bank. At 1025, the boat entered the River Lynher to survey, after assessing the weather conditions. This continued until 1034, when the water depth became too shallow. The fish (side scan sonar) was recovered at 1040, and track lines for the first major survey finalised. The area was chosen because the preliminary tow indicated a change in sediment type and possible features. Tracklines were planned across the estuary (west to east) above the bridge, but this was unrealistic due to fast currents, so tracklines along a north-south orientation were carried out. The first trackline traced the following route: 50º24.550 N 4º12.180W to 50º24.550N 4º12.600W. This trackline was surveyed once using 500kHz frequency between 1145 and 1200, and again using 100kHz between 1203 and 1216. A survey of Plymouth Sound was conducted between 1302
and 1337, but showed a homogenous seabed and did not merit further
investigation. A Van Veen grab (Fig. 1 & 3) (0.3m3 capacity) sample (50º 20.060N,
4º 09.804W) at
Data AnalysisA Sidescan Survey was conducted from 50o 21.8697 N, 4o 10.8663W up the east bank of the Tamar river. This area consists mainly of the military dock and is dredged to 8.5m in the south and 9.1m further north.
Two different pillar sections were compared, as annotated on the Group 6 isometric plots.
There was a 100m long Portuguese frigate (F332) moored at the HMS Drake dock no. 5 and a 90m long submarine at 50 o 23.3096N, 4 o 11. 5202W. Between the two ships, a single section of ripples 90m x 50m was recorded. The ripples had an average wavelength of 11.9m and were slightly curved. Tracks north of the Tamar BridgeThe eastern side of the plot shows areas of sand waves/ripples, with none present on the western side. It also shows the channel is deeper on the western side, suggesting a faster flow, and therefore higher levels of suspension and lower rates of deposition. This is supported by tidal diamond information from the admiralty charts, which shows tidal streams to be greater above the bridge than in surrounding areas. Most of the area sampled was behind Ernsettle pier, which may have disrupted flow and caused velocity to decrease (low suspension, high deposition). If sampling was carried out above the pier, sand ripples may not be present. Sand waves present north of
Tamar bridge were found to be transverse sinuous and out of phase.
The sediment was likely to be > 0.3mm.
The area between the Tamar
Bridge and the River Lynher
There was increased flow around
the bridge due to constriction of the River Tamar through a narrow channel,
which lead to scouring of the bed and deepening of the channel (Fig. 6). The sediment
surrounding the bridge returned a strong acoustic response indicating hard/smooth rock,
providing a stable foundation for the bridge. Further south, harder sediment
gave way to softer, more absorbent sediment with extensive bedforms in
transition. The dominant bedforms were linear, with crestlines perpendicular to
water flow. The average wavelength was approximately 8m. They were numerous and found over a
large area, indicating a stable flow regime. South of the softer sediment, where
the River Lynher meets the River Tamar, more features are found. Two distinctive
sediments are present: one harder/smoother, and one softer/ finer. The
most southern part of the track consisted of more uniform sediment, giving a stronger
response. Advantages and Disadvantages of High and Low Frequencies
ConclusionA repeated track, north of the
Tamar
All data for this section can be found in: Group 6 > Geophysics > Processed Estuarine Boat Practical
IntroductionOn
Station 1 was used for the calibration of the T-S probes on the RIB and therefore was not analysed further. Fig. 12 Location of CTD profiles and ADCP transects
CTD AnalysisIntroductionCTD profiles were taken at 9 locations along the River Tamar and in Plymouth Sound. Only 6 profiles have been studied in detail (see below) because of time restrictions and because these showed the most interesting features. CTD Stations 4, 5 & 6 (Fig. 12 & 13)Temperature A thermocline was present at all stations with approximately the same gradient. The surface mixed layer was clearly defined in stations 5 and 6, with a depth ranging from 2-5m. Station 4 showed no mixed layer but a large mass of cool, denser water below 10m. Stations 5 and 6 did not cover the whole water column depth and therefore the deep layer cannot be easily seen. The surface temperatures varied between the stations with a range of 0.5°C. Generally, the thermocline varied by 1.7 °C over 9m, with the deep water mass having a temperature of 14.7°C. In stations 5 and 6, river water overlies more saline water, in a layer 2-3m thick. A well-defined halocline was seen, with salinity varying by 2 units over 8m. Station 4 showed a deeper layer with a salinity of 34. This corresponds to the cool water mass seen in the temperature profile. Again at station 4, no surface layer was seen and the halocline extended to the surface. This may be the highest point up the estuary where the halocline meets the surface. Further investigation would be required to confirm this. The fluorescence data showed a chlorophyll (and therefore phytoplankton) maximum at approximately 5 metres. This corresponded to the base of the surface mixed layer. The levels of chlorophyll then dramatically decrease, with the halocline reaching much lower levels in the deeper layer.
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Fig. 13 Plots of Temperature, Salinity, Transmission and Fluorescence for CTD stations 4, 5 & 6 |
Fig.14 Plots of Temperature, Salinity, Transmission and Fluorescence for CTD stations 8, 9 & 10 |
Sampling from Bill Conway and the rigid inflatables, nutrient levels detected were notably lower than expected at this time of year. However, the general fluctuations and inter-relationships can be accounted for and explained.
Fig. 15 Careful preparation of filter for collecting water samples |
Fig. 16 Plankton net deployment at first tow station |
Fig. 17 ADCP after removal from water for steaming back to dock |
Fig. 18 Oxygen sample fixed with Winkler reagent |
Three
ADCP transects were taken at the point where the River Lynher joins the Tamar
Estuary, between 1126 and 1135 GMT. This was on a flood
tide, with high water occurring at
The
transects run between: -
1). Bull Point and Henn Point (Fig. 19a Velocity profile, 19b Ship track).
2). Henn Point and Carew Point (Fig. 20a Velocity profile, 20b Ship track).
3). Carew Point and Kinterbury Point (Fig. 21a Velocity profile, 21b Ship track).
Fig. 19a & b Velocity profile and ship track of Bull Point to Henn Point |
Fig. 21a & b Velocity profile and ship track of Carew Point to Kinterbury Point |
Fig. 20a & b Velocity profile and ship track of Henn Point to Carew Point |
Fig.22a & b Velocity profile and ship track of Plymouth Sound |
T
From
here, a parcel of water moved into the centre of the River Lynher (Fig
20a &b) at similar speeds to figures 19a & b, again lying at
intermediate depths in the water column. An eddy in this area is visible due to
the presence of a divergence of the tidal path up the Rivers Lynher and Tamar.
On the velocity profile there is a section of missing data due to the wake of a
passing boat.
As
the tidal parcel continues up the River Tamar (fig 21a & b), the flow remains
fastest on the western side with slower moving water to the east.
All
of the Estuarine Mixing Diagrams
illustrate a scavenged profile with non-conservative behaviour of the major
nutrients. These profiles would usually correspond to a chlorophyll maximum in
the higher salinity waters towards the mouth of the estuary. However, combining
the observations made by both the RIB and Bill Conway groups, it is apparent
that the chlorophyll maximum occurs in a window of 0 to 12 salinity units.
This
apparent anomaly can be explained by the transport of high concentrations of
nutrients into the upper estuary waters as a result of the high levels of
precipitation that occurred prior to the survey.
The fields around the upper Tamar Estuary are used for relatively high
intensity pastoral and market garden farming. Both of these industries utilise
large amounts of fertilisers and pesticides, in order to increase the efficiency
and production of the land. Any precipitation will wash surface deposits and
leach soil deposits of these nutrient high substances into the Tamar Estuary,
acting as the supply for the phytoplankton bloom. The Tamar Estuary is known to
respond quickly to variations in precipitation (Uncles and Lewis, 2001), which
supports the suggestion of recent precipitation being responsible for the high
nutrient concentration. Lance {1968} reported that copepods are able to gain
dominance in brackish waters because the low salinity excludes the more
metabolically active oceanic species. This will have removed any competition for
the copepods and aided the upper estuary bloom.
The
decrease in nutrient concentration down the salinity gradient is related to
mixing and dispersion, and utilisation by phytoplankton populations further down
the estuary. Whilst the chlorophyll maximum is highest towards the head of the
estuary, there is still a significant mass of phytoplankton in the mid to lower
estuary that requires a constant nutrient supply
The data for this section can be found in:
Group 6 > Estuarine > Processed
Fig. 24 The research vessel Terschelling, used for offshore data collection |
Date | Time | Location | Tide | Weather |
1st July 2004 | 0745 - 1645GMT | Offshore Waters of Plymouth Sound and Eddystone Rocks | Low Water: 1030GMT 1.1m
High Water: 1650GMT 5.19m |
Variable cloud cover (2/8 to complete cover). South
westerly winds Force 4, gusting 5. Bright spells. Some showers.
Sea State: 1 -2m swell |
Fig. 25 The CTD on Terschelling |
Fig. 26 The CTD being deployed to 45m |
Fig. 27 Brad taking an oxygen sample. |
Fig.28 The deck, where the CTD and Plankton nets were lowered from |
The Terschelling cruise revealed that the water column was well mixed in the surface layers. Due to the nature of the CTD and the size of the vessel, sampling the very surface layers of the water column was not possible. This means the surface samples therefore came from depths ranging between 1m and 5m
The laboratory analysis of water column chemistry showed that at some stations, nitrate levels were beyond the detection limits of the equipment. These results have been omitted from the plots because the negative values cause false trends (i.e. the plot is forced through zero). This only occurred at station 3 (surface sample) and so does not impact the analyses significantly.
Flow through the plankton net can be calculated in two ways. The first is by finding the area of the net mouth, and multiplying this by the distance towed. For the first plankton tow, this was the method used. For subsequent tows, a flow meter was attached, which produces a much more accurate result. This makes the margin of error between the two methods large but rather than making the results less stable, it goes to show the extent and usefulness of different methods of calculation.
Due to computer and mechanical error, the Minibat equipment was unavailable for use on this survey day. This means that undulating measurements of physical parameters could not be carried out.Station
1was located at 50º 13.108N, 4º 14.988W. The
main thermocline is at a depth of 16m, between 16m and 30m depth the temperature
decreases from 14.5°C to 12°C. From 30m there was a slight decrease from
12°C to 11.75°C at the end depth of 46m. On the fluorescence plot the lowest
values were at the surface, and there was an increase to a maximum at 20m to30m this
area was just below the thermocline which may indicate a phytoplankton bloom in
the nutrient rich water below the thermocline. This agrees with the depth of the
euphotic zone calculated from the secchi disc depth which was approximately 32m.
This bloom may have occured in this region due to sufficient light at depth (euphotic
zone) and mixing of nutrients below the thermocline maybe making the bloom light
limited. Chlorophyll and nitrate data both show concentration peaks at 25m. Silicate displayed a minimum at approximately 20m.
Phosphate concentration increased from the surface down to around 25m
where it showed a maximum, before concentration decreased.
The
ADCP data
from this station clearly shows backscatter from bubbles in the wave zone at the
surface. The thermocline is visible
between 15 and 20m. Higher
backscatter either side of the thermocline could indicate a high density of
phytoplankton reflecting the acoustic signal.
The phytoplankton data from this location indicates that diatoms dominate
the surface waters. This supports
the draw-down seen in silicate concentration from 0 to 20m.
Diatoms are non-motile organisms that need to be continually mixed upwards in
the water column in order to obtain sufficient light and nutrients for growth.
They have a high requirement for silicate in the construction of their
external cell wall, or frustule. A
second layer of backscatter at between 20 and 25m could show a second body of
phytoplankton. Cell counts show that
dinoflagellates dominate at this depth. These
organisms are motile, and thus able maintain their position in the water column
(Pomeroy et al, 1956) .
Occupying the water column at this level allows them to utilise nutrient
resources mixed up from deeper waters.
Diatoms above exploit nutrients in the surface waters.
Phytoplankton appear to be well mixed throughout the water column.
This is likely to be the result of a storm event earlier in the week.
Station
2 was located at 50º09.510N, 4º18.114W
This
station was located at 50º 17.940N, 4º 13.486W.
As at station 2, it seems likely here that the thermocline seen between
~3 and ~12m developed only recently. From
12m downwards, temperature
was constant, indicating a well-mixed water column.
The chlorophyll profile appeared to show maximum phytoplankton growth from
the surface to around 10m. The
fluorescence profile indicated some mixing at the surface, and only resembled
the chlorophyll profile below 10m. Further support for the depth location of the phytoplankton
bloom can be seen in the ADCP
backscatter plot for this station. Here, a layer of distinct scatter is visible
between 0 and 10m. Again, the oxygen
saturation plot may represent mixing, especially in the surface waters. The
nutrient profiles all appear to agree with the chlorophyll plot in placing the
phytoplankton bloom at approximately 10m. Nitrate,
phosphate and silicate concentrations all show a minimum concentration at this depth,
indicating utilisation of nutrients, with some degree of recycling below 10m.
This station was located at 50º
16.392N, 4º 15.045W. The
thermocline here was at approximately 18m.
Phosphate shows a very strong decrease in concentration from the surface
to 11m. It is then stable for around
3m, before increasing from 26 to 35m. The increase
declines, but continues until ~45m. Nitrate
also shows a draw-down, this time from 0 to 10m. It then increases from 10 to
45m. This corresponds to the
phosphate draw-down. Silicate
at station 4 displays a minimum concentration at approximately 15m.
It then increases in concentration to 45m.
The initial decrease in the surface waters reflects the fact that diatoms
dominate at this station, particularly at a depth of 26m. This is also seen in the
backscatter plots from the ADCP
data. The second most dominant
phytoplankton are dinoflagellates. A small number of ciliates were
present. Most organisms were present in the surface 0 to14m. The chlorophyll and oxygen plots also correspond, peaking between
15 and 20m. This correlation
supports the depth and location of the phytoplankton bloom.
Station 5 was located at 50º 19.583N, 4º
11.208W. The thermocline at this
site was judged from CTD and ADCP data to be approximately 9m.
Oxygen levels peaked between 2 and 4m, after a minimum percent saturation
at the surface. Immediately after
the oxygen maximum, levels rapidly decreased between 4 and 10m.
The chlorophyll plot was fairly linear, highest at approximately 2m and lowest at
approximately 11m.
Nitrate shows a pronounced minimum between the surface and 5m.
The maximum concentration was at the shallowest depth (1.1m), suggesting a strong draw-down in nitrate concentration by a biological
population. The vertical silicate
profile shows a minimum peak at a depth of around 7m, before increasing between 7 and
11m. This corresponds with the chlorophyll
plot as diatoms are buoyed up into the well-lit surface waters where they
utilise available silicate. The
draw-down continues to 7m, and recycling begins near the thermocline.
Phosphate levels show a jagged profile, but as there were only 3 data
points collected, no firm conclusions can be drawn.
The CTD data showed that the water body at this site has been completely turned
over, and thus the erratic phosphate profile has been attributed to this.
The
Richardson Number was calculated at each of the five sites, using ADCP data to
find the shear and CTD data to find the change of density with depth. The mean
density of seawater was assumed to be 1026 kg m-3. Figure 29 shows the
Richardson Number at each station.
Fig. 29: Richardson number at each station
Station Number | |
1 | 12.88 |
2 | 16.71 |
3 | 237.4 |
4 | 21.11 |
5 | 3.42 |
The
shear at all of the stations was very low, as there was little change of
velocity with depth. Stations 1, 2 and 4 give intermediate values, indicating a
relatively stable density structure. The lower the
Fig. 30 The ADCP used on Terschelling |
Fig. 31 Deploying the Zooplankton net |
Fig. 32 The Secchi disc, used to estimate the depth of the euphotic zone |
Fig. 33 Lowering the Secchi depth until it disappears |
Depth of the
euphotic zone and attenuation coefficient
Fig. 34: The depth of the euphotic zone and the attenuation coefficient
Station number |
Secchi disk data |
Light sensor data |
|||
Secchi disk depth (m) |
Euphotic zone depth (m) |
Attenuation coefficient (m-1) |
Euphotic zone depth (m) |
Attenuation coefficient (m-1) |
|
1 |
12.3 |
36.9 |
0.117 |
32.78 |
0.1043 |
2 |
12.5 |
37.5 |
0.115 |
41.67 |
0.1065 |
3 |
6.6 |
19.8 |
0.218 |
23.89 |
0.1080 |
4 |
7 |
21 |
0.206 |
32.78 |
0.1092 |
5 |
6 |
18 |
0.240 |
29.17 |
0.1130 |
The data from the secchi disk and the light sensor give
very similar results. Minor differences result from the way the systems work.
The secchi is a very basic tool and is greatly affected by an inhomogeneous
water column. If the surface water is clear and the rest of the column is not,
the secchi disk will give a large euphotic zone depth. The light sensor can
detect a patchy water column, but was located below the water bottles on the CTD
rosette, giving an interrupted view. In this case, the secchi disk is likely to
be more accurate.
The euphotic zone data shows a pattern of increasing depth with increasing distance from the shore. This is to be expected, as the amount of suspended material decreases with distance from shore. Also, the euphotic zone is consistently deeper than the thermocline. The attenuation coefficient decreases with increasing distance from shore. A higher attenuation coefficient means that the water is clear and contains less suspended matter. This is to be expected in offshore waters.
Phytoplankton
were sampled from the CTD bottles and so only a small water volume was measured.
This means that only small chunks of the water column were sampled. This gives a
limited picture to total numbers or population dynamics.
All
sites sampled were dominated by phytoplankton in the upper layers. Other classes
were made up from dinoflagellates and cillicates-small single celled animals.
Diatoms
were dominant in all except the estuarine sample. This has a similar number of
dinoflagellates and diatoms. Dinoflagelates had the largest population at site 1
however they were present over the whole water column at the other sites.
ADCP
data shows a backscatter maximum at 18-25 metres. The bottles that sampled these
areas have large populations with dinoflagellates maximum at site 1 and a diatom
maximum at sight
The
zooplankton samples were carried out using a plankton net trawled at certain
depths. All depths contained similar dominate classes. There were large numbers
of noctiluca, a large diatom in all the samples. The samples also contained
large populations of copepods. The off shore site 1 samples showed the greatest
range of species. The lowest diversity is the site closest to the estuary in
shallow water.
At
site 1 there a number off classes that are only present in the deeper nettings.
Three
of the 4 sites showed a similar number of diatoms at depth and in the surface
layers. Other species however showed an increase in the number of individuals in
the top 20m.
Fig. 36 Zooplankton population structure at each site |
Fig. 35 Phytoplankton population structure at each site |
The
general aim of the investigation was to look at mixing with distance
offshore and its affect on plankton distribution.
The data for this section can be found in:
Group 6 > Offshore > Processed
Date | Time | Location | Tide | Weather |
4/7/2004 | 0900 - 1400GMT | Tamar river Calstock to Tamar bridge | High Water: 0700, 5.43m
Low Water: 1310, 0.68m |
Overcast, cloud cover varied from 4/8 - 8/8, wind approximately 5knots. |
Fig. 37 Ocean Adventure used to sample The Upper Tamar |
Fig. 38 Coastal Research the second boat used |
Fig. 39 Repairing the steering after breakdown up the Tamar estuary |
Fig. 40 The TS probe used on both boats |
Fig. 41 Locations of stations sampled on the RIBs by group 6 on 04-07-04
Station | Location (N) | Location (W) | Time (GMT) | Rib |
1a | 50°29.745N | 4°11.631W | 10:05:00 | blue |
1b | 50°29.888N | 4°13.164W | 10:04:00 | white |
2a | 50°29.321N | 4°13.425W | 10:25:00 | blue |
2b | 50°29.115N | 4°13.167W | 10:19:00 | white |
3a | 50°28.248N | 4°13.588W | 10:49:00 | blue |
3b | 50°27.827N | 4°14.353W | 10:45:00 | white |
4a | 50°27.387N | 4°14.053W | 11:04:00 | blue |
4b | 50°27.369N | 4°13.676W | 10:56:00 | white |
5a | 50°27.593N | 4°13.521W | 11:10:00 | blue |
5b | 50°28.001N | 4°13.431W | 11:07:00 | white |
6a | 50°26.762N | 4°12.339W | 11:56:00 | blue |
6b | 50°27.138N | 4°12.358W | 11:26:00 | white |
7a | 50°25.684N | 4°11.952W | 12:21:00 | blue |
7b | 50°25.885N | 4°11.959W | 11:46:00 | white |
8a | 50°24.862N | 4°12.099W | 12:36:00 | blue |
8b | 50°24.428N | 4°12.252W | 12:35:00 | white |
N.B. Locations and times of stations sampled on R. V. Bill Conway by group 5 on 04-07-04, can be found on their webpage.
Samples
were collected at 15 sites along the River Tamar.
Of these, 9 sites were studied further as they displayed interesting
characteristics. Temperature
profiles have not been included as water column temperature varied by only 0.5°c
throughout the river and no significant trends were found.
Station
2
The
profile was taken in the upper estuary where the water column is predominately
freshwater. Salinity increases
linearly with depth, with values ranging from 1.9 at the surface to 4.3 at 3m.
It is difficult to identify two separate water bodies, suggesting that
gentle mixing between the layers is occurring.
Station
3
A
marked increase in salinity is seen between 1m and 1.5m. A salt wedge can be
identified at this depth, as the denser saline water underlies the fresher
surface water.
Station
4
Salinity
increases with depth until at 2m a decrease of 0.3 units is recorded.
Conclusive interpretation cannot be made without more data points.
Station
6
Salinity
decreases by 9 units in less than 2m. This
rapid increase was noted during sampling, and 2 further profiles were taken at
this point. They showed constant
temperature and similar readings. A possible explanation for this significant
increase is that the salt water moving upstream during the flood tide may have
become trapped. As the tide ebbed, the more saline parcel of water remained in
the same position whilst the parcel of freshwater flowed seawards with the tide.
Although no obvious river features were seen such as an island or inlet,
embayments or sub-tidal shallow areas may have trapped saline water during the
flood.
Other
factors may have influenced this unusual water column structure.
For example high rainfall in the two weeks preceding produced a large
volume of fresh river water with an increased flow speed in the surface layer. Local
geology may also influence the ability of the river bed to retain water.
Station
7
Another
large salinity gradient was also noted at this station.
A salinity of 15.2 at the surface increased to 18.9 at a depth of 2.5m.
This illustrates that the salt tongue is still lying at the base of the water
column. The salinity gradient is not as great as at Station 6 due to an increase
in turbulent mixing between the water bodies as estuary volume increases
seawards.
Station
9
Two
eddies with diameters of approximately 1m were identified, and temperature and
salinity measurements were carried out next to them.
Salinity varied by only 0.41 units in 5.5m, indicating a well mixed water
column. The eddies caused turbulent mixing of the water column, and therefore
the two parcels of water identified upstream cannot be seen at this station.
The
vertical salinity profile for station 11 is similar to station 9. The water
column is a well mixed homogenous body, with a salinity variation of only 0.7
units over 3m.
Silicate
The estuarine mixing diagram for silica shows non-conservative behaviour, with removal throughout the estuary. The position of the riverine end member is open to interpretation, as there was much variation at low salinities. The silica concentrations were higher in freshwater and decreased as the salinity increased. This could be due to the presence of diatoms, phytoplankton which use silica to create frustules. Data from the phytoplankton analysis showed that diatoms dominated at most sites, which supports this idea.
The estuarine mixing diagram for nitrate shows conservative behaviour at salinities below 20, and slight addition above. The lack of removal suggests that large phytoplankton populations have little effect on the levels of nitrate, indicting an excess. The addition could originate from a number of different sources. The land surrounding the river at low salinities was mainly used for agriculture, including cattle farming and intensive market farming. Runoff from the fields into drainage channels that lead into the river may have contained organic fertilizers or animal waste. This effect may have been intensified by the high levels of precipitation over the past few weeks. Human waste from boats may also have caused unusual localized effects.
The estuarine mixing diagram for phosphate shows a great deal of scatter near the riverine end member. At salinities above 20, there is non-conservative behaviour with addition. This addition may have been due to a point source such as a sewage outfall, but none was found in the relevant region on the charts. Other sources of phosphate include domestic sewage and human waste. There is no addition found on the nitrate and silica diagrams, so the source of addition is likely to be a phosphate-rich source. No removal was seen across the estuary despite the presence of phytoplankton. This supports the hypothesis that phosphate is not a limiting factor to phytoplankton.
The graph of chlorophyll against salinity shows a general trend of decreasing concentration with increasing salinity, although there is a great deal of scatter at low salinities. This decrease in chlorophyll is likely to be due to lower phytoplankton numbers in marine environments, caused by lack of nutrients and a deepening of the water column. In a more riverine environment, nutrients are high and non-limiting, and the water column is shallow. Due to this phytoplankton are not mixed past the euphotic zone.
Fig. 46 The secchi disc used on the rib |
Fig. 47 The zooplankton net being brought aboard |
Fig. 48 Zooplankton sample at a salinity of 20. |
Phytoplankton was sampled at various stations from a zero salinity river water end member at the top of the Tamar River to a seawater end member in Plymouth Sound. Diatoms were found to dominate at most sample sites. At two sites (50º 27.827N, 4º 14.353W and 50º 27.138N, 4º 12.358W) sampled by the Coastal Research vessel, a red-coloured dinoflagellate was predominant. However, this organism could not be fully identified due to the amount of terrestrial debris in the water column, which also adhered to the cells themselves. These stations were some 12 salinity units apart. The difficulties in identifying this dinoflagellate mean it is now impossible to say whether the two sites were populated by the same species, or by different members of the same genre. The difference in salinity does, however, suggest that they were different species.
The most dominant diatom (Bacillariophyceae), Chaetoceros, was found near the sea water end member at a salinity of 30.3. The nutrient profiles for the estuary taken at the same time as the phytoplankton samples indicate that silicate is removed along most of the length of the river. This is good supporting evidence for the dominance of diatoms in this area, as diatoms are the only phytoplankton species with a high demand for silicate. Other numerous diatom species collected from the Tamar include Nitzschia longissima and Rhizosolenia setigera.
The data for this section can be found in:
Group 6 > Ribs > Processed
The
four boat practicals carried out between 23rd June and 8th
July gave an insight into the estuarine-offshore system and its interactions.
Although the weather was not ideal, a large amount of data was gathered,
processed and analysed.
Also
studied was the difference between high and low frequency sidescan sonar.
The
estuarine boat practical sampled the water column from above the Tamar Bridge to
Plymouth Sound. CTD and ADCP profiles were taken, along with water samples for
nutrient, oxygen and plankton analysis. Again, certain areas were studied in
detail:
This
data was combined with RIB data further up the estuary.
The
offshore boat practical took data from two stations around the Eddystone Rocks,
two off of the headland and one near the breakwater. The stations were analysed
separately, as weather conditions changed the aim of the practical and the
stations were chosen accordingly. CTD and ADCP profiles were taken, and water
samples collected.
The
RIBs practical took water samples and T-S profiles from Calstock to the Tamar
Bridge along the estuary. Data was combined with that collected from the
estuarine boat on the same day. Estuarine mixing diagrams were created from the
results.
The
fieldtrip has been very successful and a greater understanding has been gained
about the processes occurring in the estuarine-offshore system.
Lance,
J. {1963}. The salinity tolerance of some estuarine planktonic copepods.
Limnology and Oceanography (8(4)) pp 440-449.