Plymouth Field Course 2005

Group 8

Group 8 - Rhys Williams, Sophie Brookes, Nicholas Curzon, Katie Firman, Emma Heywood, Richard Latham and James Merrix.


1.Contents

Upper Estuary Middle to Lower Estuary Offshore Overview
Introduction Methodology Geology/Geophysics Conclusion

2. Introduction

This year’s Plymouth field course ran from 28th June until the 14th July. During this period Group 8 investigated the estuary as a whole, from around 0 salinity to open sea. We were looking to see how the three main scientific disciplines, Biology, Chemistry and Physics, changed with their location within the estuary and if and how they affected each other.

Plymouth is located at the convergence of several river systems, the Tamar, the Lyner, the Plym and the Tavy. We will be concentrating our efforts on looking at the Tamar estuary and Plymouth Sound. The Tamar estuary  is meso/macro tidal estuary with a tidal range of 4.7 metres at springs (Dyer, 1997). Although it can be unwise to classify any estuary as either totally well mixed or partially mixed, as many estuaries experience periods within their tidal cycle of being both fully mixed, partially mixed or salt wedges, the Tamar estuary is generally regarded as a partially mixed estuary.

Plymouth Sound, and the Tamar estuary especially, have been extensively used for naval operations, and therefore many docks, piers, wharfs and breakwaters have been built over the last half century. The estuary has also been extensively dredged creating many channels and deep water anchorages for the large Naval ships and cross channel ferries that have to come in and out of the estuary. Plymouth is a relatively large city and as such could put a lot of stress on the estuary and its waters, to such an extent that the chemistry, physics or biology of the waters could be changed. We will hope to find out whether these anthropogenic inputs have affected the estuarine system.

It is intended to investigate what species are present in the estuary and how their distribution varies along its length. Additionally, are their distributions related to changes in physical properties such as stratification and mixing, or related to changes in the chemical composition of the water along the length of the estuary. The variation in various chemical components such as nitrates, phosphates and silicates will be measured in order to determine the behaviour of these pollutants. It will be determined what type of estuary the Tamar is by examining the mixing and stratification processes in and around the estuary. Looking at the different current velocities magnitudes and directions will also give an indication of changes in flow and mixing in the estuary. By examining the geophysical processes an indication of the underlying bathymetry will be obtained.

The emphasis of investigation is how the estuary works as a whole, without focusing on one particular area of science, as this would not tell the whole story, with Oceanography being considered to be one of the most interdisciplinary subjects around.

 

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3. Methodology

Research/Survey vessels

In the investigation of the waters of Plymouth sound and the Tamar estuary, three vessels were used, each for a different sampling area, as shown on the map (click to enlarge).

 

1.    RIBs - The Rigid Inflatable 'Ocean Adventure' and small vessel 'Coastal Research' were used for upper estuary sampling. They are small and manoeuvrable and therefore could be taken into shallower water further up the Tamar. Due to their size, however, sampling was limited and no large instruments could be used, so sampling was limited to collection for chemical and plankton analysis, with a small CTD multiprobe.

2.    Estuarine vessel - 'RV Bill Conway'. This is a approximately 25ft oceanographic research vessel, that was used for sampling the middle and lower estuary area. It was equipped with a variety of instruments and so was an ideal base for a wide range of sampling. Data was collected in the form of samples for chemical analysis from a rosette sampler, ADCP data, and CTD multiprobe data on a computer, and plankton trawls could also be undertaken.

3.    Offshore vessel - 'Bonito'. This is a large tug-type boat, which could be taken several miles offshore. It was equipped with similar instruments to the Bill Conway, and in addition, a crane that allowed zooplankton sampling to be undertaken vertically.

Conductivity, Temperature and Depth probe (CTD)

The CTD probe measures conductivity (relates to salinity), temperature and depth. It is attached to a rosette sampler with Niskin sampling bottles attached, and the unit is deployed while stationary and the CTD information is relayed back to the boat in real time and plotted on a computer. It can therefore can be used to help choose where to take water samples, collected in the Niskin bottles. The CTD also has a fluorometer that sends out a UV pulse which excites the phytoplankton. They then emit red light that can be picked up by the fluorometer’s sensor, so the greater the amount of phytoplankton, the higher the reading from the fluorometer will be. Another probe attached is a transmissometer,  which has a light source and a light detector, sending out a beam of light which is then reflected off a mirror back to the detector, making the total distance. This is useful for the measurement of the in situ clarity of the water, rather than amount of light at a particular depth, and can therefore be used to help identify the presence of a phytoplankton bloom. The amount of light at particular depths is measured using a light meter, a final sensor on the CTD, though unfortunately this was not in operation during the data collection period.

As the CTD needs to be lowered into the water when the vessel is stationary we needed another method of measuring temperature and salinity whilst underway. On the offshore vessel, Bonito, there was a fixed TS probe about 0.5m under the surface of the water, allowing us to identify any interesting changes whilst under way, and then make an informed choice about where to drop the CTD.

Water sampling

 

Water samples were taken on all three vessels, using remotely operated Niskin bottles on rosette samplers at different depths on Bonito and Bill Conway (estuarine and offshore) and a surface Niskin bottle on the Ribs. A CTD probe attached to the rosette allowed analysis of the water column as the rosette was lowered via a computer. The closing of the bottles was controlled via this computer system. For collection of these samples, the same methods were carried out on each vessel. From the Niskin water sample, be this at different depths (as on Bonito, the offshore vessel, and Bill Conway, the estuarine vessel) or from surface samples (from the RIB samples), a volume of water was decanted into a container for ease of sampling. If dissolved oxygen was to measured, this was carried out before any water was removed from the Niskin bottles, to prevent any contact with air. In this case a tube attached to the Niskin bottle allowed direct collection of the water, with minimal contact with the air, into the glass Dissolved Oxygen bottles. These were then treated with 1ml each of alkaline iodide and manganese chloride, added just below the meniscus to expel the contaminated surface layer of the bottle, which fixed the oxygen content (the Winkler method, Grasshoff et al., 1999) and the bottles were then stored underwater, so further prevent any contamination. The offshore and estuarine vessels collected more dissolved oxygen samples than the RIB due to better facilities to do so, and the RIB T/S probe also read oxygen concentration directly, so the Oxygen samples collected on this vessel served as effective calibration comparisons.

From the decanted water sample from the Niskin bottles a 60ml syringe of water was collected, having first been flushed. A glass fibre filter was then flushed, and any sample bottles also flushed, all with at least 10ml of the sample water. 60ml of the sample was filtered into a glass bottle, for phosphate and nitrate analysis, and a further 30ml into a plastic bottle, for silicate analysis (Parsons et al., 1984, Johnson et al., 1983). Glass was not used for the silicate samples due to the possibility of contamination by silica-based glass. As the syringes used were 60ml volume, at least two glass fibre filters were used. In the case of the RIBs, as the sampling sites were so far up the river, where suspended sediment was high, the filters became clogged easily and so more were used as less volume could be filtered through each one. Two filters from each Niskin sample were kept for chlorophyll analysis. These were removed and transferred, on board the boats, into acetone solution, in test tubes, and stored in cool boxes until they were analysed in the laboratories. The amount of water filtered through each sample was noted, as this varied, as further up the estuary there was more suspended sediment, meaning the filters became clogged more quickly. In the laboratory a flourometer measured the chlorophyll concentration in the acetone solutions (Parsons et al., 1984). This could then be converted into a figure of chlorophyll concentration in micrograms per litre in the seawater sample, when combined with the information on how much water had passed through each filter.

 

 

Plankton sampling

A 200 micron net was deployed to collect samples of zooplankton. This can be done either vertically or horizontally. A horizontal trawl involves towing the net behind the boat at a steady speed and for known time. This method is quite easy to do and can produce good results. However the depth that the net is trawled at is more difficult to control. This is where the vertical trawl comes in. The net is lowered down through the water column to the desired lower depth. It is then raised up to the desired higher depth (note not always the surface) and a messenger is sent down to close the net, thereby achieving a discrete sample though the water column. To look at phytoplankton, water samples were taken at specific locations, in the case of the RIBs, or drained from the Niskin bottles attached to the CTD rosette, as with Bonito and Bill Conway. This water sample was then placed into a brown glass bottle (to keep out light) which was pre-treated with Lugols solution. The Lugols solution is brown in colour and is used to stain the phytoplankton for easier detection. To identify both types of plankton, light microscopes were used. Water samples were placed in either Bogorov slides in the case of the zooplankton, or in Sedgwick-Rafter trays for the phytoplankton identification. For quantitative analysis the different numbers of species were counted in the relevant microscope slides; the numbers were then multiplied up to give measures of plankton numbers in the water column.

 

 

 

Acoustic Doppler Current Profiler (ADCP)

The ADCP is another vital piece of equipment that is used on board. Its primary function is to measure current velocities and directions in all three axes. It uses the principal of Doppler shift to measure the current velocities. When the pulse sent out by the transducer is reflected by a particular ‘packet’ of water, the wavelength of the returning pulse will depend on the speed and direction of the water packet it reflects from. If the packet is moving away from the sensor, then the wavelength of the return pulse will be lengthened, called red shift. Conversely, if the packet of water is moving towards the sensor then the return pulse will be ‘blue shifted’ and its wavelength will be shortened. This wavelength measurement and return times measured by the unit give current velocities and direction throughout the water column, even when the vessel is moving. The ADCP was used to measure transects in certain areas and give information when stationary and collecting samples. It can also measure backscatter; this was used in stationary positions and gave an indicator off dense zooplankton areas, which increase the backscatter measured by the ADCP, and helped in the decision of which depths to collect zooplankton samples via the net.

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4. Upper Estuary


figure 4.1
Phytoplankton numbers in the upper estuary.


figure 4.3
Phosphate mixing diagram.


figure 4.5
Zooplankton species and their relative abundance.

For the purpose of this description, the upper estuary is defined as those areas of water that were sampled whilst on the RIBs, 'Ocean Adventure' and 'Coastal Research' on 7th July 2005. This area is located between the Tamar Bridge and Calstock.

Plymouth sound and the Tamar estuary were sampled for the following chemical components:

1.                 Phosphate concentration – this may indicate pollution in an estuary, in particular detergent pollution around built up areas or sewage works. Phosphate is a nutrient for phytoplankton, so higher levels often indicate increased phytoplankton numbers. Therefore it is expected that high levels may be found near to sewage outfalls and perhaps near the city of Plymouth, so further up the estuary. Low levels are expected offshore, where effects from the pollution may have been diluted. Thermal stratification during the summer months offshore could cause phytoplankton blooms which can strip surface water layers of nutrients.

2.                 Nitrate concentration – this may indicate pollution from farming areas, as nitrates are a major component in fertilizers, and a high nitrate presence in estuarine water could indicate runoff from nearby farms. Again it is a nutrient for phytoplankton, so increased levels could increase phytoplankton biomass. Higher levels are therefore expected further up the estuary, especially around farmland.

3.                 Silicate concentration – this can also indicate a degree of pollution. Silica also forms the frustule shell for diatoms, a phyla of phytoplankton, so in areas were diatom biomass is high, Silicate concentration can be found to be lower. Again low concentrations are expected offshore

4.                 Dissolved Oxygen –  saturation percentage measures relative saturation of the water according to in situ temperatures and depths, so how much is available for zooplankton respiration, for example.


figure 4.2
Silicate mixing diagram.


figure 4.4
Chlorophyll and oxygen concentrations along the estuary.


figure 4.6
Satellite images showing the demise of the phytoplankton bloom in the English channel.

By looking at the distribution of the biological species in the upper estuary it will give a good indication of the chemical processes that are occurring. This has particular relevance when considering the nitrates and phosphates, which are caught in a negative feedback loop with the plankton. The more nutrients there are, the more phytoplankton can grow, but this diminishes the nutrient concentration which in turn results in a drop in phytoplankton abundance. There is little data on the physical characteristics of the upper estuary as we could not deploy larger instruments, such as rosette samplers from the RIBs.

Salinity in the upper estuary ranges from 0 up to approximately 34 at high tide at Tamar bridge, though dependent on the tidal state. Due to the shallow depth of the water and the speed of the current, the river is well mixed so there is not a huge variation in chemical and physical properties vertically in the water column. This, coupled with the difficulty in accurately deploying instruments from the ribs, means that only the surface layer need be sampled.

A look at the phytoplankton distribution in the upper estuary (figure 4.1) shows that there are large numbers of diatoms at around a salinity of 20 (just over 150,000m-3). This rapidly drops by salinity of 22.5 (50,000m-3), but remains fairly constant after that. The dinoflagellates are also abundant at around a salinity of 20 (70,000m-3) and like the diatoms, fall rapidly in number. However, unlike the diatoms, they fluctuate in number after that from about 15,000m-3 to around 65,000m-3. If we compare this with the silicate mixing diagram (figure 4.2), we see that the high numbers of diatoms correspond with the bottom of the steepest drop in silicate concentration. After 21 salinity, the rate of decrease in silicate concentration is less. This suggests that less silicon is being taken out of the water column for use in diatom thecae which would result in fewer diatoms present further downstream. The drop in diatom numbers could also explain the rise in dinoflagellate numbers as there will be less competition for light and nutrients, allowing them to grow and reproduce more rapidly.

Another possible explanation for the rise in dinoflagellates towards the middle estuary, is the abundance of phosphates (figure 4.3). At low salinities, the phosphate concentrations rapidly diminish, which may be due to the very high numbers of diatoms. However, at salinities between around 10 and 20, the uptake of phosphates by phytoplankton drops, and the graph levels out. After a salinity of 21, the concentrations start falling again. This coincides with the higher numbers of dinoflagellates. It is likely that diatoms are a ‘stronger’ competitor for nutrients etc. than dinoflagellates. However, when silicate becomes a limiting factor for them (at around 21 salinity - as described above), the dinoflagellates have access to more nutrients and can increase in number.

Oxygen saturation (figure 4.4) is a measure of how much oxygen is available in the water, as it is related to temperature and salinity. A higher saturation therefore means more oxygen available for respiration, by zooplankton or bacteria. It would therefore be expected that higher chlorophyll concentrations would have higher reciprocating oxygen levels, due to the oxygen produced in photosynthesis.

In this case the higher oxygen saturation is best explained by the effect of physical changes on oxygen levels. As you travel further down the estuary, there is a marked temperature change, and colder water will hold oxygen better hence higher levels. There is also increased mixing in lower areas of the estuary, and oxygen can increase in surface layers. As most of the data was collected in surface layers on the RIBS, up to salinity 29, it could be expected that the higher values are due to mixing.

Zooplankton, such as copepods, follow a similar trend with numbers of around 80million individuals m-3 ~1.5 miles north of the Tamar bridge, down to only 18million m-3 at Tamar bridge (figure 4.5). Their distribution could be explained by the abundance of the phytoplankton, especially the diatoms which are a major food source. The likely reason for there being so many more zooplankton than phytoplankton is that early July is around the zooplankton bloom time after the phytoplankton bloom in spring/early summer has faded (figure 4.6).

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5. Middle to Lower Estuary


figure 5.1
Phytoplankton distributions at estuarine stations


figure 5.3
CTD probe data at St. John's lake, station 6


figure 5.5
Zooplankton distributions at Cargreen

 


figure 5.7
Zooplankton distributions at Mountbatten breakwater


figure 5.9
ADCP Plot, station 7.

In this section we are defining the middle to lower estuary and the processes that occur in the area.  Data collected for the middle to lower estuary was collected on the Bill Conway boat trip, the area that we sampled being from the Tamar Bridge to the breakwater located in Plymouth Sound.

In order to understand the processes that are occurring we can look at the phytoplankton and zooplankton that are situated in the area, as this will indicate chemical and physical processes that are occurring in the water.  From this information we can then look at factors such as the nitrate, phosphate and silica levels or the dissolved oxygen content and see how they affect the distribution of the phytoplankton and zooplankton.

The middle to lower estuary is greatly affected by the saline water from the sea, although there is still a slight influence of the freshwater from the river.  We expect to see slight changes in salinity here, and as this is the area where the freshwater and the sea water meet, there may be a large difference in the species and numbers of phytoplankton and zooplankton that we observe.  As the middle to lower estuary is generally a well mixed area, due to the proximity to the sea and tidal influence, it would be expected that there is a greater concentration of diatoms than dinoflagellates where the freshwater influence is greater, as diatoms generally inhabit more turbulent waters. However as you move further towards the seaward side of the estuary it would expected that dinoflagellate numbers increase, as they generally inhabit more stratified waters, which are found more offshore.

To measure these factors we used an ADCP, a CTD probe and a plankton net whilst onboard the Bill Conway.  To collect zooplankton samples we deployed the plankton net at the top of the sampling area by the Tamar bridge and at the bottom by the breakwater.  To collect the phytoplankton we collected water samples, which were taken from Niskin bottles on the rosette sampler. See methodology. As the area that we were sampling is quite highly mixed, the stations we chose to sample were close to external influences such as freshwater inputs from rivers or nitrate and phosphates from farm land or sewage pipes.

Station 7 was located just north of the Tamar Bridge near a sewage outfall, which could have increased the nitrates and phosphates present.  This station was the furthest north of all the stations and therefore would have been the site most affected by the freshwater input from the Tamar.  The Secchi disk reading was taken as 1.1m; therefore the euphotic zone was about 3.3m deep  (figure 5.2 Figure 5.1 shows that at this station there were very high numbers of chaetoceros sp. and Rhizosolenia delicatula, both diatoms.  There are some dinoflagellates present, though in very small numbers.

Station 6 was located by Saint John’s lake which not only provides a freshwater input to the system but is also located next to a sewage works.  The sample was taken close to a scum line and large areas of surrounding farmland.  The Secchi disk reading was taken as 1.6m; therefore the euphotic zone was about 4.8 m deep. (figure 5.3)  Figure 5.1 shows that at station 6 there were very low concentrations of phytoplankton.  The 3 species present were two diatoms, Rhizosolenia delicatula and chaetoceros sp.  There was a dinoflagellate present; Ceratium Furcus.

Station 4 was located near Sutton harbour and Mountbatten breakwater.  It had freshwater inputs from both the Tamar and Cattewater rivers, which could result in a high number of diatoms.  Close to the sampling site there was a lot of farmland and sheep, and the site was also located on a scum line.  These factors could have increased the nitrates and phosphates in the water which in turn would affect the number of phytoplankton and zooplankton present.  Figure 5.1 shows that the dominant species here was chaetoceros sp., other species present were Rhizosolenia delicatula, Nitzschia sp. and Guinardia flaccida. There were also some dinoflagellates such as Karenia mikimotoi and a small number of Eucampia sp.


figure 5.2
CTD probe data at station 7


figure 5.4
CTD probe data for breakwater, station 2


figure 5.6
Zooplankton distribution at Saltash

 


figure 5.8
ADCP Profile, station 4

Station 2, the final station was located by the Breakwater on the estuarine side.  It was in quite a deep area as the sampling site was located in area that had been dredged to 12m in 1995.  The light penetration at this site was also quite high; the Secchi disk was recorded as 5.3m, therefore the euphotic zone was 15.9m deep.  This reading, however, does not fully represent the actual euphotic depth as it assumes that the profile is the same all the way down  (figure 5.4).  Figure 5.1 shows that at station 2 the dominant species present is Karenia mikimotoi there are also some Rhizosolenia delicatula.  At this station there were a lot less diatoms present and dinoflagellates were the predominant species.

The zooplankton data is similar to the phytoplankton data; where there are more phytoplankton, more zooplankton are found as they feed on them. Figure 5.5 represents the zooplankton samples that group 7 collected near Cargreen.  This was the site located furthest up the estuary; therefore it is the sample that would be the most affected by the freshwater from the river. There are large numbers of copepods here. These feed on ciliates, a type of phytoplankton, which could be expected to be found here. There are also some crustacean larvae.

Figure 5.6 represents the zooplankton collected at station 7, near Saltash.  This is just north of the Tamar bridge, which is further south than Cargreen; therefore it would be expected to be affected by both the freshwater and the seawater.  At this station the two dominant species are copepods and hydrozoans, with some cirripede nauplii. This is possibly as hydrozoans are not self-propelling, and therefore move according to water movements. At this stage in the estuary there is a lot of turbulent mixing due to strong flows and meanders in the river, this can be seen in figure 5.3, where the variation in salinity is not very great, only varying 1 salinity unit from surface to depth, indicating mixing has occurred. Therefore more hydrozoans could become trapped in turbulent flow. The variation in water temperature would increase this turbulent flow.

Figure 5.7 shows the zooplankton distribution at the station near the mountbatten breakwater. It has large numbers of copepods, a dominant species, which follows a possible phytoplankton bloom (see Upper estuary). There is a variety of species here, representing more seaward water. There are also still a fairly large proportion of hydrozoan. As the station was behind the breakwater and in the lee of the prevailing wind, there could be increased turbulence here, which again could trap hydrozoans.

The ADCP data that was collected shows that in some places there is notable shear in the water column. This means that two water masses are moving in different directions, causing considerable friction. It is likely that a CTD probe drop would show a considerable change in the physical and chemical attributes of the water column at the shear point, with relatively stable conditions above and below, likely resulting in two distinct plankton communities.  The following ADCP plots correspond to stations 4 and 7.  Station 7 is located by the Tamar Bridge and station 4 by Sutton Harbour.  Station 4 corresponds with a stable body of water; this can be seen by the calculated Richardson number of 0.401.  If a body of water has a Richardson number close to 0 or below the water column is very unstable, however if it has a Richardson number of 0.25 or above it is considered to be stable.  Station 7 has a Richardson number of 0.0059; this indicates that it is very unstable and therefore well mixed.  This is illustrated in figure 5.8 and figure 5.9 figure 5.8 is a copy of the ADCP plot retrieved from Station 4, in this image there is clearly one layer of water flowing on top of another, which signifies that there is very little mixing occurring.  figure 5.9 is a copy of the ADCP plot retrieved from Station 7.  In this image there is no visible layers, unlike Station 4, this signifies that the water column is very well mixed. 

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6. Offshore


figure 6.1
Chemical depth profiles for offshore station 1


figure 6.3
ADCP profile showing backscatter in offshore waters


figure 6.4b
CTD profile for offshore station 7
 


figure 6.6
Zooplankton species for station 3 from 6m to surface


figure 6.8
Phytoplankton numbers at station7


figure 6.10
Zooplankton species for station 7 from 28m to 16m

When out on Bonito, on 3rd July 2005, the aim was to locate the position of the front where the two different water masses (estuarine water and sea water) meet. We were expecting to see a change in phytoplankton phyla distribution over the two water masses and the mixed water area where they met. Diatoms are found in well mixed, unstratified water; this would suggest they were to be found on the estuarine side of the front and in turbulent mixed areas, whereas Dinoflagellates are found in more stably stratified water columns; this suggests they would be found on the seaward side of the front, where seasonal thermal stratification is expected to occur in July, when the field course was conducted. The mixing of the two water masses causes deep water to upwell, where colder, nutrient rich waters flow up into the water column around the front, so it could be expected to benefit both types of plankton around the front area.

The resulting phytoplankton distributions would affect the zooplankton distribution. Zooplankton feed on phytoplankton and thus an increase in phytoplankton, for example a bloom around a front area, could cause an increased zooplankton layer and concentration.

Station 1 (50° 20.135N, 004° 09.377W), located inside the breakwater was chosen because it was expected to give a profile typical of estuarine water., before any effects of the front could be observed. The nutrient profile (figure 6.1) for this station, shows nitrate, phosphate, oxygen and silicate are depleted at the chlorophyll maximum of 14m. This shows that phytoplankton are blooming at 14m, which corresponds well with the euphotic zone depth of 14.85m. Silicate is also at its lowest concentration between 2-4m, indicating uptake of silica by a large diatom bloom at this depth. figure 6.2, a histogram showing phytoplankton counts and species distribution, confirms that diatoms are the main component of the bloom at Station 1, with highest numbers at 2-4m and the bloom extending down to 15m.

By continually towing the ADCP throughout the survey, changes in the average backscatter could be observed. At station 3 (50° 19.077N, 004° 10.690W), it could be seen that an increase in backscatter occurred at the chlorophyll maximum of 9 metres, this is shown in figure 6.3. This was thought to be indicative of the high plankton numbers found at fronts due to upwelling waters. The CTD was deployed at each site and the profiles generated, up to and including station 3, show temperature and salinity stratification down through the water column, as shown in figure 6.4a. After site 3 the profiles show uniform salinity with depth and strong temperature stratification, as shown in figure 6.4b. This is typical of an offshore profile where there is no estuarine influence on salinity. This is further evidence supporting the presence of the front located at site 3.

The meeting of warm, less saline estuarine water and warm, more saline seawater at tidal fronts causes deepwater upwelling of cold, saline water. The deepwater upwelling is nutrient rich, causing phytoplankton to thrive. Mixing of the 2 water masses at fronts causes visible differences in surface roughness on either side of the front. Diatoms prefer the turbulent, rougher estuarine side of the front and dinoflagellates prefer the calmer, more stratified offshore side. figure 6.5, shows a strong, diatom dominated bloom in the surface waters of station 3 with a smaller bloom at the base of the euphotic zone. The small numbers of dinoflagellates indicate that site 3 is located on the estuarine side of the font where the surface waters are more turbulent.

High numbers of phytoplankton at fronts are reflected by an increase in zooplankton numbers. figure 6.6 shows high numbers of zooplankton at 10-16m which corresponds to the depth at which phytoplankton bloom. Numbers of chaetognaths in the zooplankton are high at this site in comparison to site 1 where no chaetognaths were found (See figure 6.7). This is an important sign that the front has been reached as chaetognaths are indicators of offshore waters.

After locating the front, profiles were taken at several sites on the offshore side. The sites were aimed at showing the water column with no influence from estuarine water, so that a comparison could be made between estuarine, offshore and frontal conditions. The phytoplankton populations for station 7 (50° 15.996N, 004° 12.698W) shown in figure 6.8 show phytoplankton blooms occurring down to 50m. This shows that the offshore waters are much clearer than on the estuarine side of the front. The phytoplankton population at station 7 is still dominated by diatoms however the dinoflagellate population is greater in the surface waters than at the estuarine or frontal sites. This could be due to dinoflagellates preferring the more stratified waters offshore.  The nutrient profiles for station 7, shown in figure 6.9, show the chlorophyll maximum to be between 5-20m. This corresponds with depletion of nitrate, phosphate and silicate at this depth layer. figure 6.10 shows high zooplankton numbers at 16-28 meters, just below the chlorophyll maximum. This is where zooplankton typically feed, below the main phytoplankton bloom. Chaetognath numbers are again high at this depth which is indicative of an offshore water sample.


figure 6.2
Phytoplankton distributions for offshore station 1


figure 6.4a
CTD profile for offshore station 3


figure 6.5
Phytoplankton numbers at different depths for station 2
 


figure 6.7
Zooplankton species for station 1 from 3m to surface


figure 6.9
Chemical depth profiles for station 7

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7. Overview


figure 7.1
Nitrate mixing diagram


figure 7.3
Silicate mixing diagram


figure 7.5
CTD profile, breakwater

 


figure 7.7
Chlorophyll concentration along estuary

 


figure 7.9
Zooplankton species along the estuary

An overview…..the estuary as a whole

This section aims to look at the estuary as a whole, from as close to zero salinity as possible to the open waters off shore. This section will look at the biological aspects of the estuary, how the phytoplankton and zooplankton species change with salinity, a proxy for distance from the estuary head. The chemical and physical properties of the water column change with salinity and so this will be shown and discussed. Mixing diagrams, a plot of concentration against salinity.

Chemical changes

  • At low salinities the concentration of nitrate is high, falling away as the salinity increases and the waters become diluted. (Figure 7.1) Nitrate concentrations are higher in the low salinity waters, due to surface run-off waters flowing over the surrounding agricultural land which is rich in nitrate, from fertilizers and manure.  However the points do not lie on the TDL, they lie slightly below it, suggesting that the nitrate is being removed from the water along the length of the estuary therefore nitrate is non-conservative. Nitrate is a required nutrient for phytoplankton growth, therefore likely that the phytoplankton in the water column along the estuary are removing the nitrate. Figure 7.8 shows the number of phytoplankton present, suggesting that nutrient uptake, by the phytoplankton could be likely cause of the nitrate depletion

  • Figure 7.2 shows how phosphate concentration varies with salinity. The water has higher concentrations of phosphate at lower salinities and lower concentrations at higher salinities. Up until a salinity of 20 the points lie below the TDL, as the salinity rises, the points cross the TDL and then lie above it. This is demonstrating non-conservative behaviour: there is phosphate uptake at salinities less than 20, and addition to the water at salinities above 20. The removal of phosphate is due  to the uptake by phytoplankton, as phosphate is a primary nutrient that they require, for growth and development.

  • The salinity was 20 at 12, (50o26.321N/004o11.925), about 1.3km upstream of the sewage works that service Plymouth. The water sample was taken at 12:49 GMT with low tide for the same day at 12:24 GMT. The pollutants are released into the water column on the ebb tide to try and remove as much material as possible from the estuary, with the next incoming tide bringing some of the pollutant laden water back into the estuary. Pollutant gets carried up past the sewage works by the process of entrainment; tidally driven, and is what is being detected and hence why there is an apparent input of phosphate. The peak of the phosphate input coincides with the location of the sewage works, who have installed a tertiary processing plant, which reduces pollution into the estuary, though phosphate pollution remains, probably due to the detergent runoff into the estuary.

  • Figure 7.3 shows how Silicate concentrations varies with salinity. The maximum is at low salinities. The source of silicate is land based material, such as clays, muds, sands, and rock. The concentration decreases along the estuary, showing non-conservative behaviour as the points lie below the TDL, suggesting that silicate is being removed from the water column. Phytoplankton, in particular diatoms, will remove silicate, as they need it for forming their silica shells. Figure 7.8 shows that the diatom is the dominant species of plankton in the upper estuary, which could explain the removal of silica from the water.


figure 7.2
Phosphate mixing diagram


figure 7.4
CTD profile, upstream of Tamar bridge


figure 7.6
CTD profile, offshore

 


figure 7.8
Phytoplankton numbers along the estuary

CTD Profiles

  • Figure 7.4 is a CTD profile for the middle part of the estuary, slightly upstream of The Tamar Bridge. This shows a low salinity layer of water at the surface; this is where the less dense river water is flowing over the wedge of salt water below. This low salinity section will be enhance following the recent high rain fall and associated surface run-off

  • There is a well mixed section of water below the first halocline, and the salt wedge below the second halocline. The well mixed section has been mixed by advection, due to the top layer moving out to sea, driven by the river flow, and the lower layer moving upstream, driven by the tide.

  • The temperature profile reflects this salinity stratification with warm less dense water at the surface and cool dense water at the bottom. Warm water comes from the surface run-off and the cool water comes from the incoming sea water. There is then a well mixed layer where the warm surface water has been mixed with the cool layer below creating a mid temperature range layer in the middle.

  • The fluorometery peak is located on the main thermocline, as this provide the best balance of light and nutrients, which is where phytoplankton growths are therefore often found, 20-30% of the total water column primary production in due to localised growth in the thermocline (Revelante and Gillmartin, 1973)

  • Figure 7.5 is a CTD profile by the break water. By this point in the estuary the water column is showing signs of being well mixed, this is shown by the more linear increase and decrease in salinity and temperature respectively.  The fluorometery readings show a peak at between 6 and 12m, and only a slight decrease below this depth. With a Secchi Depth of 6.3m, making the euphotic zone 18.9m, the availability light is not critical explaining the small decrease in fluorometery.

  • Figure 7.6 is a CTD profile for Offshore at station 7. When off-shore the salinity variations between the top and bottom of the water column have become much smaller as the influence of the fresher estuarine waters lessens.

  • The expected thermocline that should normally be present at this time of year is not visible, this is because of the period of bad weather that has been affecting the water in the period before sampling. The high winds have deepen the well mixed layer and broken down the stratification, creating a gradual decrease in temperature from the surface down through the water column:

'Wind mixing events occurring in an otherwise stratified water column may temporarily enhance the vertical flux of nutrients across the pycnocline to the surface layer' (Klein and Coste, 1984)

  • The Fluorometery reading shows that the peak is at about 18m, with very little phytoplankton in the top 10m. The peak is located where the thermocline would have been before it was broken down by the bad weather. The lack of plankton in the top 10m is due to the lack of nutrients which will have been removed by the plankton in the early stages of their bloom. The Secchi depth for Station 7 is 11.6m which equates to a euphotic zone depth of about 36.6m, this reflected by the fluorometery readings, which drop off below this depth.

Hard at work!!

Biological changes

  •  Figure 7.7, is a graph showing chlorophyll concentration along the estuary. Chlorophyll concentration is an index of phytoplankton biomass which is easy to measure. This graph shows chlorophyll measurements, to be highest in the upper estuary, decreasing moving offshore. This corresponds with concentrations of nutrients, discussed earlier, which are highest at the head of the estuary, decreasing with increasing salinity. The nutrient profiles for phosphate and silica both show removal in the upper estuary which is indicative of high phytoplankton concentrations in this region, confirmed by high chlorophyll concentrations.

  • The phytoplankton numbers and species distributions along the estuary can be compared by looking at figure 7.8. This figure relates to 3 stations, one in the upper estuary, one in the middle estuary and one offshore. Numbers of phytoplankton in the upper estuary appear to be very low from this graph, considering the high chlorophyll concentrations shown in figure 7.7. There are many possible reasons for this unusual result. Firstly, only 1ml of phytoplankton were counted from each sample site and the results multiplied up for the whole sample. With no replicates taken, it is possible that the 1ml counted was not an accurate representation of the sample, causing an unusually low result. Another possibility is that the high rainfall levels at the time of sampling could have caused the surface layer sampled to be mainly rainwater and so plankton deficient.

  • Figure 7.8 clearly shows the dominance of diatoms in the estuary compared to dinoflagellates. As diatoms are the first of the phytoplankton to bloom followed by dinoflagellates, it could be that the sampling period was still within the period of diatom bloom. However, at the offshore station the diatom numbers decrease and dinoflagellate numbers increase. This could be due to the high numbers of dinoflagellates typically found on the offshore side of tidal fronts, as was found whilst sampling on Bonito.

  • The zooplankton numbers and species distributions along the estuary are illustrated by figure 7.9. This graph shows zooplankton numbers are at their highest in the upper estuary decreasing moving offshore. This is to be expected as zooplankton feed on phytoplankton which are shown to be high in numbers in the upper estuary due to the high chlorophyll concentration.

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8. Geology/Geophysics

The geology site visited, Renney Point, located between Heybrook Bay and Plymouth Sound, on 2nd July displayed two different types of geology. The first being a sequence of bedding planes alternating between sandstone and oil bearing shale and the second, an eroded cliff face displaying seven bedding planes composed of sandstone with varying grain and clast sizes. The material from the cliff was deposited over the eroded bedding planes of the oil bearing shale from 11-0 ka BP associated with the end of the last ice age as suggested by the environment it was deposited in. However, the cliff material is not as consolidated as the rock type below so has eroded at a quicker rate as has a higher erodibility which has exposed the underlying bed rock creating a wave cut platform which extends out into the sea by approximately 250m.

Sandstone/oil bearing shale. The bedding planes form a wave cut platform which extends out into the sea by around 250m and are only exposed at low tide with the exception of a few high points composed of a more resistant rock type. The reason for analyzing this area of rock is to better understand the geology of Plymouth which is a drowned river valley as these bedding planes extend right across Plymouth sound.

To test the characteristics of the bedding planes, we firstly took the bearing of the strike of several of the bedding planes to see the trend, using a compass with a built in spirit level. The spirit level was set so the compass was flat running along the strike plane and a bearing was recorded using the ‘right hand thumb rule’. The bearings of the bed at this location varied between 224˚ and 234˚.

figure 8.1 shows the dextral tear fault running at a bearing of 110˚. figure 8.2 shows the recumbent bedding plains and antiform structure.

Once this process was complete the dip angle of the bed was taken using the clinometer which was built into the compass. The compass is turned on it side and pointed down the steepest part of the bedding plane, then the angle can be taken. The angles which the bedding plane dipped at all varied depending on where they were taken due to the presence of a recumbent antiform, dips of between 30˚ and 72˚ were recorded.

Two major geological structure were present at this location, the first being the recumbent antiform and the second being a dextral strike-slip fault, both of which could have been caused during periods of compression. The recumbent antiform produced beds which have been overturned and dipping at the steepest angles of 64˚ and 72˚ as shown by the geological map of the area. The hinge of the fold was also identified as well as the apparent plunge which varied between location due to varying rates of erosion and bedding thicknesses but was generally found to be between 10˚-20˚ with a bearing of 224˚.

The dextral strike-slip fault recorded was shown to be running along a bearing of 110˚ which is perpendicular to the strike of the bedding planes and running in the assumed direction of the compressional forces. As a result the fault had moved ~5m displacing the antiform hinge, which was offset on either side of the fault by this distance. Other noticeable features along the fold were tension cracks appearing in the direction of the strike plane along the top of the fold and also secondary cracks appearing in the general direction of the fault plane due to the compressional forces, this is shown on the compass rose diagram

Bedding plane 7 is slightly darker than bed 6 below as soil particles produced by biological activity has leached down through the sediment into the bedding plane. White shell fragments are identifiable in the sediment which means this area was possibly flooded at the time of deposition, closer analysis would enable us to distinguish the type of shell fish and there habitat, either shallow marine or brackish waters.

Bedding plane 6 shows a change in colour from a red to beige, there is more sand grains present in this bedding plane which measures around 40cm in thickness. The matrix is also more sand dominated. A possible source could be a mature beach or the infilling of a river channel. 

Bedding plane 5 is similar to bedding plane 1 as is matrix supported; clasts are deposited in a lateral fashion with noticeable imbrication structures. Clasts vary in size between 2 and 20cm in diameter and are noticeably different in colour suggesting they are a different rock type which has travelled a further distance and known as a derived clast. There two distinct bands within the bed which are noticeable due to the fact that there is some graded bedding present. This bed is approximately 1.5m thick and could have been created by mud flows at the end of the ice age, where increased precipitation and lack of anchoring vegetation increased soil erosion.  

Bedding plane 4 is 15 to 20cm thick and is composed of a fine grained mud, although most of it is of an orange appearance; occasional green patches occur suggesting there are different types of materials and minerals present causing this bed to react to the atmosphere. Fine sands are present within the mud which suggests a possible rising sea level bringing in fine sands.

Bedding plane 3 is 30 cm thick and formed from material which is semi rounded and with an average diameter of 3 to 4mm, these small clasts are supported by a fine grained mud matrix like that of bedding plane 2. It is a laterally discontinuous bed which pinches out like those formed by a braided river system were rapid changes reduce accommodation space and material is deposited in a short series of bars. Again it could also be a sub aerial environment.

Bedding plane 2 is again around 30cm thick and composed of a red material but the rock fabric is very different suggesting it was laid in a different energy environment. The material is a very fine grained mud with no angular clasts, such material is only deposited in low energy environments so potential environments in which it was formed would be a delta, the inside of a river meander, sub marine or sub aerial. 

The first bedding plane at the bottom of the cliff creates an unconformity as it overlays the older more resistant bed rock. Bedding plane 1 is composed of mainly a red material suggesting that it was deposited in an oxidizing environment. Large angular clasts are present with an average diameter of 15cm and are aligned horizontally with some evidence of imbrication. Due to the size and angularity of the clasts, its possible to say that this material has not travelled far from its source as would be much smaller and more rounded if it were to have travelled a great distance. The main body of this bedding plane is around 30cm thick and is clast supported, these characteristics suggest that the material was deposited in a high energy environment like that of a mud or debris flow. ext

 

Sidescan SONAR

The Sidescan SONAR sampling technique was used to survey the sea bed of Wembury Bay to observe the geological structures present and to identify the different types of sediments in this area and the fashion in which they were deposited. Another aim of this investigation was to try and observe any faulting present and if possible, to correlate this information with that collected and Renney point.

The Sidescan fish Figure 8.10 was towed behind the Catamaran ‘Natwest II’ at a depth of around 1 meters and speed of 4 knots, sampling at a frequency of 500 KHz. The swath of the scan was 150m but parallel transects were taken at 100m intervals to allow for overlap. Two perpendicular tie lines were also taken to show added detail that may have been missed due to the angle of returns from the initial scans. The information was transferred through the data cable to an on board computer which gave a visual print out of the sea bed and also recorded the GPS readings and times when the fish was sampling. The image produced by the sidescan was then used to interpret the different types of sea bed in the area, as sediment types can be compared using the strength of the return. Stronger returns would indicate coarser material than a weak return.

To get an idea of what type of sediment each return corresponded to, a grab sample would be taken. A Van Veen grab (figure 8.11) was used to sample the sediment type, the locations of these grabs was determined after the SONAR survey was completed, so that grabs would be taken in areas with different strength returns. This would allow us to draw conclusions about other sediment types.  The transect print outs were then taken back to the labs were they were analysed and plotted onto a map which shows the main features to scale (figure 8.12.)

The map produced shows rock in purple, coarse sands and shingle in orange and fine sands and mud in yellow. Initial findings, showed that there were several gullies on the sea bed. These are believed to be paleo river channels, created during the last ice when the sea levels were much lower. These channels have subsequently been infilled since the sea level has risen. There are two different types of sediments as shown by the Sidescan sonar and proven by the grab results.

Grab one was taken over a river channel filled with what appeared to be finer sediment as shown by the sonar images. This was proven to be correct as fine grained sand was collected, and 100% of it passed through the 1mm sieve. Some small shell fragments were identifiable and also some very fine clay particles were present as shown by the sticky texture which caused it to form blobs on the 1mm sieve. This is caused by the electro chemical charges found around clay particles which bind them together. One small polychaete worm was found in this sample (figure 8.13)

Grab two and three were taken at similar sites, however the initial grab hit a rocky surface indicated by the fact that it retrieved a species of Kelp known as Laminaria digitata. Found living on the specimen of kelp was two types of organisms, Hydrozoa and Porifera. Also collected was a starfish with a spiny appearance (figure 8.14) it was later identified to be Marthasteias glacianis a typical species found living in rocky habitats.

Grab three, seen in figure 8.15, retrieved a coarser sediment which was not as well sorted as the sediment from grab one. Large grains greater than 2mm in size made up 40% of the sample, another 40% was composed of grains between 1-2mm in diameter and the remaining 20% was made up of fines less than 1mm in diameter. Some small well rounded pebbles ranging from 1-5cm in size were also collected within the sample; the well rounded appearance suggests that they have travelled a greater distance from their source than the other sediments in this area. Various Polychaetes were found within this sample suggesting it was taken from an area with a higher diversity than the previous sediment grab locations.

Sample site four retrieved a grab containing a layered sediment (figure 8.16) which was preserved within the grab, the surface layer was made up of fine sediments and was around 2cm thick. It was composed of similar sediment to that taken from grab one, it was hardened; thixotrophic, with again a sticky nature. Below the fine layer was coarse gravel similar to that found in grab 3, particles ranged in diameter up to 1cm but 60% of the sample contained particles of 1-2mm in size.

The tie lines taken after the sampling allowed us to match features on the print outs but more importantly enabled us to view the bed forms from a different angle. This revealed two new types of ripple structures which were not displayed on the other transects. Ripples found in both locations were asymmetric an bifurcating suggesting the predominant cause was wave action, they were also orientated in a south westerly fashion which is the direction of the prevailing wind so it is possible to say that the predominant wave direction in this area is wind driven. Although the ripples were caused by the same driving forces, some characteristics were different at the two sites sampled. In one location the wave lengths were 1.8m with a height of 0.4m and a lateral extent of 8-90m which was the width of the channel. At the other location, they were smaller in scale measuring 0.9m in wavelength with a height of 0.2m and a lateral extent of 60m (not the entire channel width). Although the ripples were only seen on the tie line transects, it is likely that these features were present throughout the surveyed area but were not seen due to the fact that the other transects ran parallel to the bedforms.

The reason for this change in ripple size can be explained when looking at the map and the prevailing wind and wave direction in relation to the rock structures. The current flow over the rock structures is altered in different locations depending on the rock size and orientation giving a range in bedform characteristics. The same principle can also be applied when interpreting the reason for the deposition of the finer sediments on the leeward side of the rocks in relation to the wave action. This is an area were there is less energy for mixing which enables the deposition of the finer sediments in the water column. Another explanation of there northern trend could be that they are the result of the riverine inputs which occur to the north of the location sampled. Figure 8.17 shows the different types of returns produced, the wood grain texture shows areas of rock, the darker matte areas show returns from gravel and the lighter matte areas show returns from finer sediments.

 

Visible from the sonar print out were fractures in the rock with similar orientations to those found at Renney point. It is possible to say that they were formed in the same period of compression, evidence for this is that there is also a dextral strike slip fault visible on the print outs with a bearing of 335° which is a secondary fault angle similar to those found on Renney rocks. This fault is again dextral with an apparent horizontal displacement of 15m. Although the fault is visible in certain locations, it is not possible to track the fault across the entire sequence as erosion and sedimentation has taken place within the fault.

 

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9. Conclusion

Despite much of the data collected resulting in charts and graphs that agreed with our expectations, there are many reasons to be wary of the information. Firstly, when recording our position, we had several GPS and time systems on board the boats, and these rarely agreed exactly. The scribe recording all necessary data at each time sometimes had as many as 6 different parameters to write down, and it was hard to read and write up all of them before they changed, especially when the latitude and longitude consist of up to 8 numerals each.

Chemical sampling also had problems as it had to be done on the boat which was rolling on the swell resulting in the possibility of contamination of the sample, or not providing the exact amount of fixing chemicals to some of the samples (such as Manganese Chloride in the Oxygen samples). However, we generally had good weather so this was not an especially significant factor. On the boats we also had the problem of drift. This had the most influence in deeper water at the mouth of the estuary and during the geophysics in Wembury Bay where the CTD and grabs would not be below the boat when at the bottom and so their exact position is impossible to determine without transponders. The boat would also drift up to 150m from the start point of the CTD deployment to the point at which the last water sample was collected. In shallower waters the lay-back was not very significant and drift was not very far as the CTD spent far less time under water.

Once back in the laboratory, due to the repetitive nature of some of the analysis, it was easy to lose concentration and small mistakes were made. The data was kept, but if it caused any anomalous results, they could be discarded. With time constraints, it was impossible to take many replicate samples as it would take far too long to process in the lab, so we sampled a large amount of the estuary at the expense of some accuracy. However, had we sampled accurately at the expense of investigating some of the estuary, it would have been much harder to generate a good overall idea of the process contained therein. The most significant aspect affected by this is probably the plankton counts as when looking at a sample that we’d just collected, before adding the formalin, several fish larvae were seen. However, none showed up in the later analysis due to only sampling 10ml out of the ˝ Liter container. From this it is possible to infer that the phytoplankton samples were even more likely to be erroneous as only between ˝ and 1 ml was analysed from a 60ml sample.

Despite these potential errors, we believe we have gathered a lot of accurate data, and have been able to gain a good understanding of the workings of the estuary as a whole. We have been able to see how all of the areas of science interact with each other, and that how virtually any change, no matter how small, can have a large impact on another aspect of the estuary. We also believe we could come back and focus in on one particular aspect of the estuary, using our knowledge gained in the last two weeks to achieve good results.

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10. References

  • Dyer, K.R., 1997. Estuaries: A physical introduction 2nd ed. Wiley and Sons, West Sussex.
  • Grasshoff, K., Kremling K., and Ehrhardt, M. 1999. 'Methods of seawater analysis'. 3rd Ed. Wiley-VCH.
  • Holligan, P.M. et al., 1984. Photosynthesis, respiration and nitrogen supply of plankton populations in stratified, frontal and tidally mixed shelf waters. Marine Ecology – Progress Series. 17, 201 -213

  • Johnson, K. and Petty, R.L. 1983. 'Determination of nitrate and nitrite in seawater by flow injection analysis'. Limnology and Oceanography 28:1260-1266.
  • KiŘrboe, T., 1993. Turbulence, Phytoplankton Cell Size, and the Structure of Pelagic Food Webs. Advances in Marine Biology. 29, 1 – 72

  • Morris, A.W., 1981. Nutrient Distributions in an Estuary: Evidence of Chemical Precipitation of Dissolved Silicate and Phosphate. Estuarine, Coastal and Shelf Science. 12, 205 – 216

  • Morris, A.W., 1982. Chemical Variability in the Tamar Estuary, South-west England.  Estuarine, Coastal and Shelf Science. 14, 649 – 661

  • Parsons, T.R., Maita, Y., and Lalli C. 1984. 'A manual of chemical and biological methods for seawater analysis'. 173 p. Pergamon.
  • Pingree, R.D. and Griffiths, D.K., 1978. Tidal Fronts on the Shelf Seas Around the British Isles. Journal of Geophysical Research. 83(9),  4615 – 4621

  • Pingree, R.D. et al., 1978. The Effects of Vertical Stability on Phytoplankton Distributions in the Summer on the Northwest European Shelf. Deep Sea Research. 25, 1011 – 1028

  • Pingree, R.D. et al., 1976. The Influence of Physical Stability on Spring, Summer and Autumn Phytoplankton Blooms in the Celtic Sea. J. Mar. Biol. Ass. UK., 56, 845 – 873

  • Pingree, R.D. and Maddock, L., 1985, Rotary Currents and Residual Circulation Around Banks and Islands. Deep Sea Research. 32(8), 929 – 947

  • Pingree, R.D. et al., 1977, The Influence of Biological Activity and Physical Stability in Determining the Chemical Distributions of Inorganic Phosphate, Silicate and Nitrate. J. Mar. Biol. Ass. UK. 57, 1065 – 1073

  • Pingree, R.D. and Mardell, G.T., 1985, Solitary Internal Waves in the Celtic Sea. Prog. Oceanog. 14, 431 – 441

  • Pond, D. et al., 1996, Environmental and Nutritional Factors Determining Seasonal Variability in the Fecundity and Egg Viability of Calanus helgolandicus in Coastal Waters off Plymouth, UK. Marine Ecology Progress Series. 143, 45 – 63

  • Sharples, J. et al., 2001, Phytoplankton Distribution and Survival in the Thermocline. Limnol. Oceanogr. 46(3) 486 – 496

  • Tattersall, G.R. et al., 2003, Suspended Sediment Concentrations in the Tamar Estuary. Estuarine Coastal and Shelf Science. 57 679 – 688

 

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