Plymouth 2005

Group 11

Danielle Owen, Irli Shijaku, Gareth Rapkin, Richard Rogers, Daniel Stoker,

 Gemma Saunders, Chris Croxson and Rebecca Martin.

 

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  1. Introduction

  2. Geofield

  3. Geophysics

  4. Estuarine

  5. Offshore

  6. Discussion

  7. Conclusions

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Introduction

The Tamar estuary is located in Plymouth, on the south west coast of the UK and consists of the main river, plus two tidal sub-estuaries called the Tavy and the Lynher1.  The system is approximately 31 km long from the boundary with Plymouth Sound to the limit of the salinity intrusion at Weir Head2.  It is a mesotidal/macrotidal, flood dominant estuary with an average river discharge of 22 m3s-13.  Tides are semi-diurnal with a range of 2.2 m at neap tides and 4.7 m at springs3.

Data was collected during a 14 day period from 29th of June 2005 to the 13th of July 2005.  The results presented are a combination of the data gathered from offshore, estuarine and riverine surveys of the area together with geophysical data of the seabed.  The important biological, chemical and physical processes that take place in the estuary were investigated, together with analysis of how these processes interact and influence the system as a whole.  The aim was to gain a basic understanding of how the Tamar estuarine system works. 

Below is a table of the four surveys and critical information about weather, time and tides:

 

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Geofield Survey                                           

The purpose of the geology field trip was to gain a basic understanding of the geomorphology of the coastline surrounding Plymouth Sound in order to assist in interpretation of the side-scan sonar data collected on the geophysical boat survey. The main tasks included calculation of the dip and strike of the rock formations at Renney Point, just east of Plymouth Sound  (048 800N, 249 250E), and analysis of the sediment layers composing a cross section of the cliff face.

The strike was found by taking a bearing of the rock face in the horizontal plane with the compass. The direction of the bearing was determined using the right hand rule technique.  The general direction of the strikes for Renney point were approximately 220o-240o.

The dip was measured along the vertical plane of the rock using the clinometer arrow on the compass.  The freely rotating arrow measures the incline by pointing directly downwards allowing the corresponding angle to be read. The general angle of dip for rocks at Renney point was ~40o, although this reduced to 0° at the crown of the fold.

The rock formations at Renney Point show a distinct asymmetric antiform fold.  The fold was recumbent (folded back upon itself), resulting in many fractures being present along its length.  A distinct fault was observed at the site (with bearing of 290o). This was right-lateral and shows that the fold happened prior to the fault causing significant displacement. 

The structure of the sediment layers of the nearby cliff face were also studied. By looking at the different clast sizes, colouration, orientation of clasts and layer dimensions, the nature of sedimentation was used to make observations of past environments:

These observations have made it is possible to explain what was occurring in the paeleoclimate as different sections of the sediment were laid down:

The Earth moves in an elliptical orbit, thus it is closer to the Sun at some points of the year than others (eccentricity).  Furthermore, the axis of the Earth is not vertical with respect to its orbit and as a result the Northern Hemisphere is sometimes tilted towards the sun (procession). Every 20,000 years eccentricity and procession combine (Milankovich cycle) giving rise to a climate maximum.  In the past, this has lead to glacial melt and a hot and humid environment in the Northern Hemisphere.  The last time this occurred was approximately 8,000 years ago.

Increased water vapour in the atmosphere during a climate maximum gives rise to  amplified rainfall and increased temperatures.  Little vegetation can survive this harsh change in environment and as a result much dies out. Vegetation acts to prevent erosion as sediment is held together by the roots.  The increased rainfall and sensitivity to erosion combine to create mudflows.  This is not a direct effect of the glaciers melting, since glacial melt water did not reach the south coast of England.  Rather, it is the increased rainfall which caused the land movement and hence brought about the mudflow deposits seen in the sediment above.

The sea level was slowly rising and the encroaching waters are indicated by the  presence of medium sand grains.  The point of marine inundation is revealed where shells are found within the sediment. This occurred around 3,000 years ago, at the end of the Flandrian Transgression.

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Geophysics

Introduction

The geophysics survey was aimed at taking the knowledge gained from the Geofield survey and applying it with respect to side-scan sonar to study the geomorphic features of the seabed. The aim was to carry out a seabed and sub-bottom survey using the catamaran Nat West II, in order to determine the composition and geomorphology of the substrata in the selected areas.

 

Methods

A range of equipment was available to us to conduct our survey and a plan for the day was prepared.

Two series of transects were drawn up (fig. 2.1) by reference to chart SC 1613 (2005), with transect frequency set at intervals of 100m.  The first series was in the area of the disused explosive dumping ground and the second in the area of the James Egan Layne and the Scylla shipwrecks in Whitsand Bay. The fish was towed within a few metres of the sea surface throughout both surveys.  The first survey had depths of around 30m while the second was shallower at approximately 12m. The side-scan system picked up some very clear information, including the unmistakable image of the wrecked James Egan Layne. The sonar plot was retained for further processing and interpretation back in the lab.

Once the side-scan surveys were complete, four areas were selected for grab sampling.  This allowed for confirmation of the bottom sediment composition as shown by the side-scan sonar trace. Furthermore, the samples recovered were examined for biological content.

A sub-bottom profile was also undertaken using a boomer along a transect running parallel to the breakwater across Plymouth Sound.

 

Side-scan survey

In the lab, a section of the trace created by the side scan sonar was selected for further analysis.  A surfer plot of Northings and Eastings for the selected area, together with the times at which the side scan sonar swept the area, was created.  Features on the side scan trace were then transferred to the surfer plot in order to create a map of the seabed.

The map created shows several features of interest (Fig. 2.2).  The strength of the return was used to make inferences about the sediment type composing the bottom.  A strong return is caused by hard, rough surfaces and the result is a dark area on the side-scan trace.  A weak return results from soft or smooth surfaces and thus a light patch is seen.  Using this principle, three seabed types were identified on the map; fine sediment, coarse sediment and bedrock. 

The majority of the area is covered with apparently uniform fine sediment. Large bedrock exposure is limited to the south-east of the survey adjacent to Rame Head, although small patches were seen interspersed all over the map. A large area of coarse sediment with small ripples lies to the north-west of the bedrock. A clear boundary separates the coarse sediment from the finer sediment. Furthermore, the orientation of the bands of coarse sediment are directed towards the shore (Fig. 2.3). A possible explanation for this is that as the water currents sweep around the headland to the east they will eddy and as a result bottom sediments in the vicinity tend to be swept towards the shore and then back east towards the headland in a circular motion.

 

 

In addition to changes in the bottom sediments, other phenomena were also plotted on the map, most notably the James Egan Layne Wreck (Fig. 2.4).  The wreck is surrounded by coarse sediments in which deep, large, ripples are visible.  Scour is caused by the presence of the wreck modifying the currents. The ripples could be also be caused by these currents, or by wave action. However the nature of the ripples (e.g. symmetric or asymmetric) cannot be seen from the scan, and further study would be needed.

 

Grabs

By referring to the sidescan plot and the substrata classification on the chart, several areas of the plot were selected for grab sampling using a Van Veen grab.

At grab site #1 there were a number of small bivalves and small polychaetes found. Coarser grains were found at site #3, while at site #4 larger bivalves were found (Fig. 2.5).

 

 

 

Sub-bottom profiling

Using a Boomer, it was decided to do a transect across Plymouth Sound (see map). The boomer was deployed at 50°18.904N, 04°10.826W on the western side of Plymouth Sound and recovered at 50°19.431N, 04°07.608W to the north of Renney Rocks creating a transect of the Sound. The catamaran towed the apparatus at the surface of the water and the hydrophone was towed alongside it.

The Boomer emitted a low-frequency sound pulse (350-5000 Hz) and the return signal was picked up by the hydrophone and recorded by a computer. Once the transect was complete the sub-bottom profile was studied to investigate  possible morphological features beneath the seabed.

As mentioned previously in the Geology section, prior to the Flandrian Transgression which started about 7,000 years ago, the sea level was around 200m below present levels. This meant that the English Channel had not  yet been flooded and would still have been a large river feeding into the Atlantic ocean off the coast of France. By carrying out a boomer profile across Plymouth sound the aim was to try and see the old river channels of the Tamar estuary before prior to sea-level rise.

The profile created is shown in Fig. 2.6. The image has been annotated to make interpretation easier. There appear to have been four previous riverbeds beneath the present seabed. These vary in depth, however, the deepest is shown to be approximately 25-30m below the seabed although it has since been sediment in-filled.

 

 

From analysis of the locations of other sub-bottom profiles conducted across Plymouth Sound, it is possible to draw the approximate route of the old river that used to run through the area before sea-level rise.

 

 

 

 

 

 

 

 

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Estuarine Survey

Introduction

The survey of the Tamar estuary was divided into two areas.  The shallow upper and middle estuary (from 0-32 salinity) was covered by the small boats Coastal Research and Ocean Adventure on the 1st July 2005, while the lower estuary down to its boundary with Plymouth Sound was surveyed using R.V. Bill Conway (32-35 salinity) on the 8th July 2005.

The aim of the survey was to investigate how the Tamar estuary acts as a transition zone between the freshwater input and the coastal sea and to understand further the processes controlling the biology, physics and chemistry of the system.

In the shallower regions upstream (0-32 salinity) the focus was to gather data from surface waters in order to create a horizontal profile of the various parameters (temperature, salinity, chlorophyll, inorganic nutrients, plankton) as the salinity changes from freshwater to seawater (salinity gradient). In the lower parts of the estuary, the primary aim was to look at vertical profiles of the water column and to investigate how these changed over a much more limited range of salinities (32-35 salinity) as well as study the nature of water movement in and out of the estuary.  The survey in the lower estuary also provided information to complete the horizontal profile of the Tamar estuarine system thus providing a comprehensive data set covering the full salinity range of 0-35.

Upper and Middle Estuary

Methods for small boats

A variety of sampling methods were used to gather data. A tidal curve was created to calculate the times that the shallow upper reaches of the estuary would be accessible while maintaining a necessary safety margin of 1.5m depth below the boat.

Sampling was initiated at 0 salinity, which was found to be in the region of Calstock. In order to maximise productivity within the time constraints, the group was split into 4 people per boat. Sampling stations were staggered between the two boats at intervals of 2 units of salinity up to 32 (Fig. 3.1). This allowed boats to remain in close proximity to each other, for safety and radio contact.

 

Results

Physical:

Temperature & Salinity

The horizontal temperature-salinity profile of the upper estuary, produced from surface measurements, is in-line with expectations of a partially mixed estuary3.  Salinity values increased with distance down the estuary.  Surface temperatures remained ~19.5°C in the upper reaches due to riverine input.  When salinity values exceeded 18, the temperature declined to a minimum of 17.5°C as the "colder" influences from the seawater became more important.

Three vertical temperature-salinity profiles were collected along the estuary.  The profiles are typical of those of a partially-mixed estuary3 and illustrate weak thermal and haline stratification. There is very little variation between the profiles, except for the temperature and salinity gradients as described above. 

Chemical:

Phosphate

The laboratory analysis of phosphate involved using a spectrophotometric detector to determine the concentration of phosphate in each of the samples4.

The variation in phosphate concentration in the surface waters of the estuary with respect to salinity is plotted in Fig. 3.2.  The grey line on the graph represents the theoretical dilution line (TDL); the line on which all points should lie if phosphate behaved conservatively.  Points that lie above the line indicate an addition of phosphate to the system, and points below indicate removal.  As all the points lie below the line, phosphate can be said to be behaving non-conservatively. 

Phosphate can exist in either dissolved or particulate states in marine systems and is a required macronutrient for phytoplankton growth. The likely reason for the decrease over the salinity range is increased uptake by phytoplankton.   

 

Nitrate

Nitrate was processed using a flow injection analyser feeding into a spectrophotometric detector5.

The variation in nitrate concentration in the surface waters of the estuary with respect to salinity is plotted in Fig 3.3 and a TDL has been added.  Since most points lie below the line, nitrate can be said to be behaving non-conservatively. 

Like phosphate, nitrate is required by phytoplankton for growth.  The decrease in nitrate concentration with distance down the estuary is again likely to reflect uptake by phytoplankton which are most abundant towards the mouth of the system.

Silicon

Silicon concentration was determined in much the same way to phosphate using a spectrophotometric detector6.

Silicate concentrations in the surface waters of the estuary were plotted against salinity and a TDL drawn (Fig. 3.4).  The results are in-line with the findings of nitrate and phosphate in that silicate is seen to be behaving non-conservatively.  Silicate is required by diatoms for their tests.  Thus, a decrease in silicate concentration is observed with distance down the estuary because of the increase in phytoplankton populations towards the mouth of the system.

Nitrate, phosphate and silicate all appear to be removed from the estuary along the salinity gradient.  As mentioned above the most likely explanation for this is biological removal by phytoplankton.  This result is supported by the  phytoplankton counts which show a peak in abundance at a salinity value of 20.  From 20-30 salinity, phytoplankton abundances are high and hence increased removal of the macronutrients are observed.

Chlorophyll

Fig. 3.5 shows variation in chlorophyll concentration in surface waters with changing salinity.  The plot shows very little change in chlorophyll concentrations with distance down the estuary.  This is unusual because a peak in phytoplankton abundance was observed at salinity 20.  Chlorophyll is usually used as an indicator of phytoplankton biomass and so a peak in chlorophyll at salinity 20 was expected.

 

 

 

 

 

Biological:

Zooplankton

Zooplankton was collected using a plankton trawl net with a mesh size of 200 microns.  Specimens were preserved in formalin and counted/classified back in the lab.

Fig. 3.6 shows the number of cells per m3 of different zooplankton families at different locations along the Tamar estuary. The trawl taken from the rib was at a salinity of 24 and shows that the total zooplankton population was about 110 animals per m3 with juvenile copepods being the most abundant.

At the Tamar bridge the spread between families is fairly even.  Numbers reached >8,000 animals per m3 which is 80 times the value recorded by the ribs further up the estuary. 

The third trawl was conducted at the breakwater.  Here, the number of animals was again very high, with the total population being well over 10,000 animals per m3.  Hydrozoans and Appendicularians were the most common species.

Phytoplankton

Phytoplankton samples were collected at a total of 7 stations throughout the Tamar estuary during the estuarine survey (see table). 

Station 8 was located furthest up the estuary (lowest salinity) while the sample taken at the breakwater was located just inside Plymouth Sound (highest salinity).  Phytoplankton were counted/identified in the lab and a clear difference in total abundance and species composition was observed.

The abundance of phytoplankton cells varied with distance along the estuary (Fig. 3.7).  There was a clear peak at station 11 (~200,000 cells/L), followed by a gradual decrease towards Plymouth Sound.  There are increased nutrient concentrations in the upper reaches of the estuary which could account for the higher phytoplankton counts at station 11.

The species composition of phytoplankton within the estuary was primarily diatoms.  Of these the most abundant genus was Chaetoceros

 

 

 

 

 

 

 

Lower Estuary

Methods

The RV Bill Conway was used to sample the lower estuary, between the Tamar Bridge and the breakwater.

Equipment available included a CTD rosette that measured depth, temperature, salinity, chlorophyll content, light attenuation and held six auto-firing Niskin bottles. The Niskin bottles  were used to collect in-situ water samples for subsequent inorganic nutrient analysis, together with oxygen, chlorophyll and phytoplankton samples. In addition, a hull-mounted Acoustic Doppler Curent Profiler (ADCP) and T-S probe, secchi disc and zooplankton net were utilised.

The survey began at 9:00 GMT, at which time the tide was ebbing.  As a result the decision was made to sample at the breakwater first and then to head back up the estuary towards the Tamar Bridge.  It was hoped that this would avoid following the same water mass down the estuary and would give greater nutrient concentrations in our results. The Tamar Bridge was reached just before slack water, so a further decision was made to do a repeat transect and vertical profile at the station in the narrows in order to compare the physical properties of the water column before and after the tide turned.

          

Physical:

CTD:

Temperature

The profiles for all stations show a decline in temperature with depth with a maximum in the surface 2m (Fig. 3.9).  However, there is some variation in the position and gradient of the main thermocline.  During sampling at stations 1 to 8 the tide was ebbing and station 9 was reached shortly after low water. Temperature is greatest in the surface waters because of solar heating and the influence of warm freshwater which, due to its lower density, lies over the colder, denser seawater. Station 4 shows a steep vertical gradient which indicates that the water column is well mixed. There are varying degrees of mixing at different depths because each station has distinct variations in the volume of riverine input and, due to time differences, CTD measurements were taken at different stages of the ebbing tide.

Salinity

At all stations, salinity is seen to increase with depth. Saline water entering the estuary is dense and thus occurs deeper in the water column, while less saline freshwater is less dense and thus tends to be found towards the surface.  In addition, salinity and temperature are observed to mirror one another.  Hence, haloclines occur at very similar depths to thermoclines.  This leads to stratification of the water column.

Fluorescence

Peaks in fluorescence in the profiles shown in Fig. 3.9 indicate chlorophyll maxima and therefore give a good indication of the location of the main phytoplankton biomass. It is expected that these maxima will be in the top few metres of the euphotic zone where photosynthesis can take place most efficiently, utilising the maximum amount of solar radiation. Peaks can also occur at depth, where a density gradient (stratification) is apparent.  This prevents phytoplankton being mixed between the surface and deeper layers. In well mixed regions a chlorophyll maxima is not usually observed. This is illustrated in the profile for station 4 (the Narrows), where the fluorescence shows no real structure.  Hence the phytoplankton are being mixed throughout the entire water column.

 

ADCP

The ADCP was used to investigate current fluxes at sites of interest along the estuary, such as fresh water inputs and the estuarine outputs either side of the breakwater. The results provided by the ADCP, together with vertical profiles taken at the centre of the transects, allows inferences to be made about the effects of current fluxes on nutrient concentrations as well as temperature and salinity.

Transects taken in the Narrows show that a velocity variation occurs during the ebb tide, particularly at the mid-channel 10 m level and in the shallow margins.

Direction of flow was southwards due to the ebbing tide.  A slower flow over a different direction is visible at the start of the transect.  This is due to friction of the shallow right hand margin of the channel, and is likely to produce eddies.  The left hand margin appeared to have a stronger outflow, although this is contrary to the expected corriolis effect in estuaries.  Further investigation at different tidal states would be required in order to further understand the flow dynamics

The outflow showed a large amount of backscatter above 10 m and below 15m caused by flushing out of particulate matter from the estuary.  The fluorometer reading for station 4 (mid Narrows channel) showed medium concentrations of chlorophyll at 1.58 volts. Transport of suspended sediment is directed down-estuary at both spring and neap tides and sediment is flushed to the sea with fresh water7.

The Narrows was revisited at slack water and comparison plots for velocity magnitude and direction showed the effect of the low tidal outflow at the surface and the start of the inflowing tide at depths between 10 and 20 m. 

While the velocity magnitude is very low at slack water, flow was still occurring, and looking at the velocity direction below, in different directions. The top 5m of the water column was moving almost directly south (180o) indicating the river flow through the Narrows. Between 10m and 20m there was flow almost due north up through the Narrows (360o) and represented the beginning of the flood tide creeping up the estuary. The interface between these layers of current flow was an area of shear turbulence, indicated by the differences in velocity direction occurring between 5-10m. (see below - Richardson Number)

At slack water the backscatter shows clearly that a layer of particulate matter was present at a depth of ~5m.

Comparison with the CTD plot for this station reveals that a chlorophyll max was present at this depth.  A plankton trawl may have revealed that high abundance of phytoplankton and/or zooplankton was evident.

 

Richardson Number

The Richardson number is a measure of the balance between the stabilising buoyancy of the density gradient and the destabilising effect of vertical shear. It is used to determine whether the flow (in the vertical) is turbulent or laminar and, therefore, whether the water mass will overturn or not.

Collection of current velocity data throughout the water column using the ADCP, together with the density values obtained from the CTD, allows calculation of the Richardson number for the stations sampled. This gives an insight into the vertical stability of the water column at these stations and the processes that control the vertical movements of the water column.

The equation for the Richardson number is given below:

 

Ri = g xx dz

                                                         ρ x du2 

 

As an example, the Richardson number for stations 4 and 9 (the narrows) at both ebb and slack water has been calculated.  

STATION 4

          Ri = 9.8 x (1025.156 - 1025.057) x 10.55

                   ((1025.156 + 1025.057)/2)  x  0.3572

 

                                 = 0.07834446

STATION 9                                                                                    

           Ri = 9.8 x (1025.038 - 1025.038) x 10.55

                   ((1025.038 + 1025.038)/2)  x  0.232

 

                                 = 1.23784168

When the tide was in mid ebb at station 4, the Richardson number was <0.25 indicating turbulent flow. When calculating the Richardson number for the same station at slack water (station 9) the Richardson number is >1 indicating laminar flow. The increased tidal flow that is obviously present during the ebb stage of the tide in such a narrow passage exerts more of an effect on the water column than the stabilizing effect of the density gradient, hence turbulent flow. During slack water the density gradient of the water column has a stronger effect than the now decreased tidal flow, increasing stability and resulting in laminar flow.

 

Chemical:

Dissolved Oxygen

Oxygen samples were take from the Niskin bottles on the CTD rosette and an then processed back in the labs8.

Only limited vertical sampling was undertaken, but trends may still be observed in the plots (Fig 3.17).  In general, surface waters seem to be more oxygenated than deeper waters.  This is a result of wind action and the resultant transfer of oxygen across the ocean-atmosphere interface. Conversely, the decreased oxygen levels observed at greater depths is a result of biological activity. Respiration at all depths of the water column removes dissolved oxygen whilst its replacement by photosynthesis occurs primarily near the surface, resulting in a net loss of oxygen at greater depths.

Equipment restraints resulted in some stations not being to sampled enough to make many inferences other than the general trend from surface to bottom. However, general conclusions can be drawn about dissolved oxygen content from station to station. At sites where the CTD displayed an interesting temperature salinity profile more oxygen samples were taken.

Stations 1,2,3,6 and 9 all display similar trends in terms of the profile and the relative values of dissolved oxygen. The obvious exception to this is station 5 where the value at depth is appreciably greater than that of the surface value.  Station 5 is located in the narrows, a bottleneck where high velocity flows scour the seabed to a depth of around 40m.  The water column is highly turbid resulting in vertical mixing.

 

Nutrients

All of the stations sampled in the lower estuary displayed similar nitrate/phosphate/silicon trends with very low concentrations throughout the water column due to uptake by phytoplankton. The only profile that showed any other trend is station 8 (fig. 3.18) which showed a slightly higher phosphate concentration.  This station was located at the Tamar Bridge and had low phytoplankton concentrations.  Hence, phosphate may be replenished by riverine inputs.
 

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Offshore Survey

Introduction

The offshore survey of the coastal waters in and around Plymouth Sound was conducted on July 4th 2005. Bonito (15m survey vessel) was used to sample a number of stations in and around Plymouth Sound.

The aim of this survey was to gather information and improve understanding of the stratification of the water column and subsequent effects on biota distribution.   Variation in stratification of temperature, salinity and nutrients at different stations and their effects on the phytoplankton and zooplankton populations were investigated.

 

Methods

As with the Bill Conway survey, a CTD rosette, ADCP and zooplankton net were utilised to collect samples.

 

The coordinates are shown in the table to the left (sampling stations are shown in Fig. 4.1).  The first station inside the breakwater in Plymouth Sound was used as to test the equipment and sampling techniques.

 

Results

Physical

CTD

The CTD data for the offshore survey is shown in Fig. 4.2. 

Temperature

Each station showed a steady decrease in temperature with increasing depth (Fig. 4.2).  This is due to warmer, less saline water overlying colder, saltier (and therefore more dense) water indicating continued estuarine influence.  A thermocline is present at station 4 at around 15m.  This is caused by incident solar radiation heating the surface layer. 

Salinity

With little variation, the salinity plots for all stations show increasing salinity with depth (Fig. 4.2).  This is strongly suggestive of a river system impacting upon the profile.  The plots are typical of a partially mixed estuary. 

Fluorescence

A fluorometer mounted on the CTD measured the amount of chlorophyll in the water column (Fig 4.2).  With the exception of station 5, an increase in chlorophyll concentration is observed with increasing depth, until a sub surface chlorophyll maxima is reached.  This typically falls in a region of a strong density gradient.  The chlorophyll concentration then tapers off with depth as irradiance decreases.  At station 5 the water column is only about 15m deep so the base of the euphotic zone falls near the sea floor. 

ADCP

A front was crossed at ~ 2000 metres along a transect taken from position 50o21.850 N,  04o 09. 975 W to 50o19.606 N, 04o10.987 W.  This is clearly visible around 2500m, indicated by the increase in velocity along the front boundary.

 

Directional changes were evident.  Any interference from the metal hull of the vessel would be a constant and therefore directional change is a valid indicator.

 

An interesting pattern emerges with respect to backscatter variation.  As the front is approached backscatter is ~ 68 dB.  This then reduces to ~ 60 dB through the front before returning to ~ 68 dB.  A backscatter of ~ 75 dB . was returned close to the headland near Kingsand due to surface roughness of shallow water at this point.

 

Chemical:

Dissolved Oxygen

The dissolved oxygen content of a water mass can be increased by input from the atmosphere across the ocean-atmosphere boundary layer and also as a result of photosynthesis. Similarly, it can be removed from the water mass by plant and animal respiration as well as other chemical processes such as organic waste decomposition.

Dissolved oxygen content is largely affected by biological activity but is also dependant upon temperature, salinity and pressure as well as atmospheric actions, turbidity and waste input.

The dissolved oxygen content, in terms of saturation and concentration, for the offshore stations are shown in Fig. 4.6. Both plots show a decrease in oxygen content with depth, as would be expected with the associated decrease in water temperature and increase in both salinity and pressure. However, at stations 1, 2 and 6 it can be seen that there is an increase in dissolved oxygen from the surface to depth between 5 and 15m before a subsequent decrease down to the sea floor

Ideally oxygen samples would have been taken at many more depths per station in order to draw a clearer profile of behavior. This is particularly true of stations 4 and 5, where there were equipment failures.

The observed oxygen maximum can be attributed to the presence of a thermocline in these areas. Increased photosynthesis at the thermocline coupled with reduced mixing resulted in a build up of dissolved oxygen, and hence, the observed maximum.  Another factor that could contribute to the oxygen maximum near the thermocline, especially near the mouth of an estuary or fluvial outflow, is the input of oxygen-rich river waters that are slightly denser than surface waters and hence sink below them. However, this is more commonly observed in winter when the river water temperature is comparably lower, hence increasing it’s density.

Nutrients

Fig. 4.7 shows the nutrient profiles for samples taken at station 2.  This is characteristic of the profiles generated for the other stations.  There is no significant change in the concentrations of any macronutrients sampled and generally the concentration levels are below the level of detection.  This could be due to nutrient depletion by phytoplankton.

 

 

 

 

 

 

 

Biological:

Phytoplankton

Phytoplankton samples were collected at 4 of the stations visited during the offshore survey.  Data gathered as the CTD was lowered was used to make decisions about which depths to take the samples.  In general, samples were taken at the surface and around the thermocline if a chlorophyll maxima was observed.  Additionally, an extra sample was collected from below the thermocline at some sites (see table). 

The position of the stations is plotted in Fig. 4.1.  Station 4 was located furthest from the estuary while station 6 was located just inside the estuary itself.  An analysis of the phytoplankton species and numbers was conducted in the lab and showed clear differences in total abundance and species composition between the different sites.

 

The abundance of phytoplankton varied dramatically with distance from the estuary.  At station 6 total phytoplankton abundances reached >800,000 cells/L at 1.2m.  However, at station 4 abundances were less than 30,000 cells/L at all depths. 

In addition, analysis of species composition proved interesting.  Diatoms were seen to be the most abundant group at all stations.  Diatoms are usually the dominant group found in the spring bloom in temperate waters since they require high nutrient levels and relatively turbulent conditions. In the Tamar Estuary and surrounding coastal waters the mixed layer is relatively deep and nutrient levels are high, providing ideal conditions for diatoms to thrive. 

Further analysis of the composition of the phytoplankton sampled, showed a dramatic shift in species composition between station 6 (within the estuary) and those located in the coastal waters surrounding Plymouth Sound (stations 3-5).  At station 6, the diatom Chaetoceros was the most abundant, while at station 4 the diatom Rhizosolenia stolterfothii was most abundant.  

 

Zooplankton

The zooplankton population follows the same patterns as the phytoplankton (their primary food source). As with the phytoplankton, greatest population numbers were found at station #5 located in the lower estuary. Here, there were high nutrient levels from riverine and anthropogenic sources resulting in increased phytoplankton population; which will lead to increased zooplankton numbers.

At station #4, two trawls were conducted. One at depth between 22m and 8m. The aim of this was to sample the zooplankton population occurring at the chlorophyll maxima. The second trawl was between 8m and 0 to compare to the two populations. Zooplankton numbers are higher at the chlorophyll maxima where there was a greater phytoplankton population present compared to overlying surface waters, which were stripped of nutrients and so supported a smaller phytoplankton population.

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Conclusions

 

Upon reviewing our data it can be seen that the environmental parameters observed behaved in a fashion characteristic of a temperate, mesotidal/macrotidal, partially mixed estuary. The timing of the field course has conventionally coincided with the mid to late stages of the spring bloom.  The observed factors each have implications upon one another.  

 

Estuarine Physics

The lower reaches of the Tamar comprise a partially mixed estuary, and measurements taken over the field course support this.  The upper estuary shows a salt wedge feature at all states of the tide.  Turbidity associated with sheer forces between water masses and the morphology of the estuary floor creates dynamic water movement further down-estuary (this is especially prevalent at meanders in the river)  This results in a patchy distribution of plankton in the upper estuary. 

Light penetrates to greater depths as you move from the estuarine environment to offshore.   There is a strong correlation with suspended sediment load in the water column.  Spring and neap tidal cycles result in varying suspended sediment levels.  River runoff also has an impact on the suspended sediment levels (Uncles et al. 1985).  At the estuary mouth the depth of the euphotic zone increases rapidly (by a factor of 4), between a salinity of 28 and 32. 

 

Estuarine Chemistry

Nitrate, phosphate, silicate and dissolved oxygen were measured.  Each of these nutrients exhibited similar (non-conservative) behaviour, in surface waters along the estuary. The nutrients are used by phytoplankton and hence limit the primary production of the system above salinities around 20.  Phosphate is known to limit productivity in freshwater ecosystems whereas it is nitrate that limits the biota of marine ecosystems.  Dissolved oxygen concentrations were found to decrease with depth.

  

Estuarine Biology 

Phytoplankton and zooplankton populations exhibit strong differences in species abundance and diversity at the locations surveyed.  A maximum in phytoplankton abundance was observed at a salinity of 20 in the estuarine environment, while the highest abundance was in the near shore environment.  The dominant species changed from Chaetoceros in the estuary to Rhizosolenia in the coastal waters.

The processes taking place in the upper estuary are more readily explained than those of the mid to lower estuary with respect to the data collected. The upper estuary is fluvially dominated and in comparison, the mid to lower estuary is subject to complex interactions of tidal and fluvial flows coupled with increased wind and wave action, creating a much more dynamic system. This is compounded by the fact that some of the sampling methods were insufficient to fully describe and explain the environmental conditions, in some cases only scant conclusions may be drawn. Data from previous years could be accessed to further explain the biological, chemical and physical dynamics of this system over this period. 

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References

Dyer, K.R.  (1997).  Estuaries: A Physical Introduction. 2nd ed.  John Wiley and Sons, London.

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

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

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

Tattersall, G.R., Elliot A.J. and Lynn, N.M.  (2003).  Suspended sediment concentrations in the Tamar estuary.  Estuar. Coast. Shelf Sci., 57: 679-688.

Uncles, R.J. and Stephens, J.A.  (1990).  Computed and observed currents, elevations, and salinity in a branching estuary.  Estuaries, 13: 133-144.

Uncles, R. J., Elliott, R. C. A. and Weston, S. A. (1985) Observed fluxes of water, salt and suspended sediment in a partly mixed estuary. Estuarine, Coastal and Shelf Science. 20. Pp 147-167.

Figures:

Maps: Google map, Digimap

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