Falmouth Field Course 2008

Group 7

 

CONTENTS

Introduction

Equipment

Nutrients

Offshore

Geophysics

Estuarine

Conclusion

References

 

Charlie Smillie   David Aldridge  Alison Armstrong  Trystan Colwyn-Thomas   Christos-Moritz Loukas

Mike Matson      Penelope Pickers      Ashley Neve      Kathryn Weir      Max Tam

 

 

Introduction

 

The Fal estuary is a ria located in Cornwall on the South-West coast of England which formed at the end of the last glacial period, approximately 10 000 years ago. The main water body within the estuary is known as Carrick Roads, and has a maximum depth of 33m near the mouth; it is the 3rd largest natural harbour in the world. The estuary is fed by 6 tributaries and 28 small creeks but typically has low riverine inputs making it tidally dominated. The tidal range within the estuary is dependent upon location, with the upper river region subject to mesotidal conditions and the lower river region macrotidal conditions. The maximum range on a spring tide is 5.3m with tidal currents typically less than 2 knots.

The catchment of the Fal is predominantly rural with significant nutrient input from agricultural run-off of arable and dairy farmland. Another anthropogenic source of nutrients into the estuary is the sewage treatment works associated with the principal urban centres of Falmouth and Truro. Consequently, some parts of the estuary display elevated nitrogen and phosphorus concentrations which lead to algal blooms, dissolved oxygen sags, and turbidity, which are all symptomatic of eutrophication (Langston et al. 2006). Harmful algal blooms have been observed in the upper Fal Estuary/ Truro River on several occasions, such as in 1995-1996, when a ‘red tide’ of the dinoflagellate Alexandrium tamarense produced paralytic shellfish poisoning (PSP) toxins. Harmful blooms linked to nutrient enrichment continue to occur in the Fal and may also originate in other parts of the system (Langston et al. 2006).

The Fal estuary is recognized as a Special Area of Conservation (cSAC) to protect several important species and habitats which reside within the region, such as the seagrass Zostera, and Maerl. Maerl is a calcareous algae which forms a matrix structure on the seafloor and provides an ideal living environment for many juvenille species, such as lobsters. Unfortunately, mining and processing of metalliferous deposits has lead to major impacts on the biota and sediments of the Fal system since the bronze age, with serious detrimental consequences. During the period of mining activity much of the remobilized metal was deposited in Restronguet Creek; as a result the sediments of the estuary are some of the most heavily polluted with metals in the UK (Pirrie et al. 2003). There is also evidence that some metals have been transported to other parts of the system, particularly to the adjacent creeks on the western side such as the Mylor and Pill creeks, and to the upper Fal (Warwick et al. 1998). Although the last mine was abandoned in 1991, metals (such as Arsenic, Copper, Zinc, Cadmium and Iron) are frequently flushed into the estuary from Restronguet Creek and Carnon Valley after heavy rainfall. Secondary sources of Copper and Zinc originate from the outfall at Falmouth Dockyard and are indicated by increased concentrations in the water column. Sporadic events of elevated Zinc concentrations in the upper Fal may originate from a variety of sources, including local sewage discharges and urban run-off (Langston et al. 2006).

Tributyltin (TBT) compounds used as antifouling paints on ships are also a source of contamination within the estuary. This compound leads to a phenomenon known as imposex in the common dog whelk Nucella lapillus, resulting in female organisms developing male characteristics (Spooner et al. 1991). The use of TBT was banned on small vessels in 1987, however a ban on larger vessels has only applied since 1st January 2008; TBT concentrations in the water column are now low, due to the short half life of TBT in open waters, but high concentrations still persist in the sediments.

The aim of this field course was to develop an understanding of the physical, biological, chemical and geological processes in the Fal estuary by analysing data collected on three surveys using the equipment and resources detailed below. Surveys were conducted in the estuary and in offshore locations to obtain a wide range of data. Click here to see a map of survey areas.

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Equipment and Resources

Equipment

 

CTD & Rosette (Fig 1.2)

A CTD (Conductivity, Temperature and Depth profiler) is used to indicate the vertical structure of the water column by measuring temperature, salinity, and depth. The CTD probe and a Fluorometer can be mounted on sampling devices such as the Rosette, which allows for strategic water sampling on the up cast using Niskin bottles, which can be closed by a messenger at predetermined depths.

 

Figure 1.2 CTD and Rosette

 

ADCP

The ADCP (Acoustic Doppler Current Profiler) uses a sound principle known as the Doppler effect to gather measurements on water current speed, direction, and also acoustic backscatter. The data collected can also be applied mathematically with the Richardson number to indicate the stability of the water column.

 

 

Secchi Disc (Fig 1.3)

A Secchi Disc can be used to determine the light attenuation coefficient (k) and the depth of the euphotic zone. The euphotic zone is approximately 3 times the Secchi depth (the depth at which the Secchi disc can no longer be seen in the water from the surface).

 

 

 

Figure 1.3 Secchi Disc

 

YSI Probe

The YSI probe is a multi-probe sensor, which collects data for temperature, salinity, depth, dissolved oxygen, chlorophyll and turbidity. The measurements can be taken at set time intervals as it is lowered slowly through the water column.

 

Sidescan Sonar (Fig 1.5)

The sidescan sonar creates a detailed image of bathymetric features and types of sediment by emitting ultrasonic sound waves ranging from 100 to 500kHz; higher frequencies create more detailed resolution but less range. The sidescan sonar can be mounted on the vessel hull or towed behind in a tow-fish; the latter option is more preferable as it reduces the effect of boat movement and waves.

 

Figure 1.4 Van Veen Grab

 Van Veen Grab (Fig 1.3)

The Van Veen is a simple and rugged type of grab, with long arms that give good leverage to close its jaws. A ‘bite’ of sediment can be taken from the seafloor; usually the grab collects well-defined surface areas which can be analyzed in the lab.

 

Figure 1.5 Deploying the Sidescan Sonar on Xplorer

 

Zooplankton Net (Fig 1.6)

 

A vertical closing zooplankton net (mesh size 200μm, opening area 60cm) can be used to collect samples for taxonomic identification. In most cases, two sections of the water column are sampled depending on the distribution of chlorophyll as determined by the CTD. Once the net reaches the required depth, it is raised as necessary and closed using a messenger. The sample is stored in a plastic bottle and preserved with formalin.

 

 

 

Figure 1.6 200µm Plankton Net

 

Vessels

 

RV Callista

The RV Callista is a 20m long offshore research vessel which can reach a maximum speed of 19 knots and carries up to thirty people. The stern deck has three deployment points with a lifting capacity of 4 tonnes. Dry and wet laboratories onboard enable in situ analysis of samples, and bow thrusters assist manoeuvrability.

 

 

RV Conway

The RV Bill Conway is a 12m long inshore sampling vessel, commonly used for river and estuarine surveying. It has a maximum speed of 14 knots and is able to carry 14 passengers.

 

 

Ocean Adventure RIB

The Ocean Adventure RIB is a 7m long inflatable vessel which can carry 6 people and has a maximum speed of 35 knots. Due to a low draft, the RIB is able to sample in shallow estuarine regions.

 

Xplorer

The Xplorer is a 12m long geophysical survey vessel which can carry 14 people and has a maximum speed of 25 knots. Onboard, there is a hydraulic crane for lifting heavy equipment.

 

For more information on the RV Callista, Bill Conway and Ocean Adventure, click here.

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Nutrient Analysis

 

Phosphate

A stock standard solution of 15 μmol of phosphate per litre was prepared and used to create calibration standards of 50, 100, 200, 500 and 1000μl. A mixed reagent was prepared consisting of 20% Ammonium Molybdate, 50% Sulphuric Acid, 20% Ascorbic Acid, and 10% Potassium Antimonyl Tartrate. 1ml of mixed reagent was added to all the calibration standards and to 10ml of each of the samples collected on the Callista. After one hour, the samples were measured in a spectrophotometer set at 882nm, and the phosphate concentrations were calculated. 

Chlorophyll

6ml of acetone was added to the samples which were then stored over-night in a refrigerator. This released the chlorophyll from the phytoplankton cells which was then measured using a fluorometer. The chlorophyll concentration was determined using the following equation:

 

 

Dissolved Silicon

The method used to determine the dissolved silicon concentration is outlined in Parsons et al. (1984). A blue complex was created by adding a molybdate and a reducing solution to the water samples, the synthesized solutions of known silicon concentration, and the blanks. The absorbance of the final products was then measured using a spectrophotometer. The concentration of the silicon in the seawater samples was calculated once the calibration curve was determined from the artificial silicon solutions.

Dissolved Oxygen

The dissolved oxygen was calculated using the Winkler method as suggested by Grassoff et al. (1983).

Nitrate

Nitrate was analysed using flow injection analysis, based on the method outlined by Johnson and Petty (1983). The standards used were 1, 5 and 10 µmol/L.

Phytoplankton Taxonomic Identification Method

The samples collected were transferred to 100ml measuring cylinders and left over night so that the phytoplankton would settle to the bottom. A vacuum pump with a curved pipette was used to remove the top 90ml of sample leaving the bottom 10ml, ensuring that there was no phytoplankton re-suspension during the process. The 10ml samples were observed under a microscope and the phytoplankton were counted and identified using a Sedgewick-Rafter Chamber.

Zooplankton Taxonomic Identification Method

The samples were mixed to ensure an even distribution of zooplankton, a 10ml sample was taken from the original sample and viewed on a Bogorov Chamber under a microscope with the aid of guide books. The numbers of individual species were counted and recorded.

 

Figures 2.1 and 2.2 Identifying the Plankton Net samples in the laboratory
 

 

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Offshore Sampling

Introduction

N.B. All locations are quoted relative to WGS 1984 and all times are in GMT.

  • Date: 3rd July 2008
  • Time: 08:00 - 16:00 GMT
  • Weather Conditions: sunny, cloud cover 1/8th. Wind approximately 14 knots.                                                                                                         

    Figure 3.1 Map showing location of study (click to enlarge)

     

  • Tides:

05:00 GMT        high tide        5.0m

11:42 GMT        low tide          0.9m

17:19 GMT        high tide        5.4m

  • Vessel: Callista
  • Aim: To establish the location of the frontal system near Black Head (49' 59.888N, 005' 06.379W) and determine the spatial variations of the stability of the water column, nutrients, and the chlorophyll maximum across the front .

 

  • Responsibilities:

    PSO

    Michael

    Dry Lab

    Kathryn

    Wet Lab

    Charlie, Christos and Penelope

    Deployment Team

    Trystan, Alison and David

    Scribe

    Max

Method

The original plan of the survey was to collect data between Black Rock and approximately 20 nautical miles offshore on a bearing of 135°, but due to adverse weather conditions, this idea had to be aborted. Instead, Callista travelled southwest  from Black Rock (50' 08.345N, 005' 01.543W) to Black Head (49' 59.888N, 005' 06.379W) and then offshore on a bearing of 125° to stations 3, 4 and 5.

At each station, the following equipment was deployed:

  • A rosette frame carrying:
    • A CTD to measure temperature and salinity with depth.
    • A fluorometer to measure changing fluorescence with depth.
    • Niskin bottles used to collect water samples for nutrient, oxygen, and chlorophyll analysis.
  • A zooplankton net (mesh size 200μm, opening area 50cm) to collect samples at varying depths for taxonomic identification.
  • A secchi disc to calculate the attenuation coefficient and the depth of the euphotic zone.

In addition to this, an ADCP attached to the hull of the boat was used to measure change in backscatter, current velocity, and current direction with depth for three transects between the following stations:

  • Station 2 - Station 3 moving across the front from mixed to more stratified water.
  • Station  3 - Station 4 moving from stratified water further offshore.
  • Station 3 - Station 2 moving back over the front to reconfirm the front location at a later time.

Please Note: Unfortunately, due to a malfunction with the CTD, it was only possible to take a surface sample at Station 5.

Safety Considerations

All members of the research team were required to wear life jackets at all times. Great care was taken when handling and during deployment of equipment (CTD etc). Those working on the rear deck were secured by hook straps when handling the CTD. Ropes were also used to steady the CTD in the deployment and retrieval process to avoid damaging the equipment during rough conditions.

Water sample processing

  • Nutrients: water samples collected from niskin bottles were filtered through glass fiber filters (GFF) to remove phytoplankton and zooplankton, and were stored in glass bottles (100ml in total).
  • Silicate: 60ml of water sample was filtered through a GFF from each niskin bottle and was stored in a plastic sample bottle. This removed the risk of silica contamination from using  glass bottles.
  • Oxygen: great care was taken not to unintentionally alter oxygen levels in samples taken. Using a rubber dispensing tube, samples were decanted straight from niskin bottles into glass stopper bottles to reduce contact with the air.  Manganous chloride and alkaline iodide were added to each sample to fix the samples for later lab analysis (Winkler method).
  • Chlorophyll: GFF used in above methods were stored in acetone solution for later lab analysis. Two filters were stored per niskin bottle sample for comparison.
  • Phytoplankton: 100ml of  unfiltered water sample from each niskin bottle was treated with 1ml of lugols solution (to fix samples) which were stored in glass bottles for later lab analysis.

N.B. All locations are quoted relative to WGS 1984 and all times are in GMT.

Figures 3.2, 3.3 and 3.4 Onboard Callista

Results

(click images to enlarge)

Figure 3.5 Phytoplankton - Station1

Figure 3.6 Phytoplankton - Station 2

Figure 3.7 Phytoplankton - Station 3

Figure 3.10 Phytoplankton Data

Figure 3.11 Zooplankton Data

 

 

 

 

 

 

 

 

 

 

Figure 3.14 Nutrients - Station 1

Figure 3.15 Nutrients - Station 2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.19 Station 1 - Oxygen Saturation

 

Figure 3.20 Station 2 - Oxygen Saturation

 

 

 

 

 

 

 

 

 

Figure 3.23 Temperature - All Stations

Figure 3.24 Chlorophyll - All Stations

 

 

 

 

 

 

 

 

Figure 3.26 ADCP Data - Transect from Station 2 to Station 3

Figure 3.28 ADCP Data - Transect from Station 3 to Station 4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Phytoplankton Taxonomy

Diatoms were the dominant phytoplankton group in ten out of the twelve samples taken, and were the only group found ubiquitously in all the samples. Dinoflagellates were dominant at the surface waters of Station 3 (figure 3.7) and at 15m at Station 4 (figure 3.8). Ciliates were present in half the samples and generally contributed little to the total phytoplankton population. Total phytoplankton counts were greatest (111,000 cells l-1) at the surface at Station 2 (figure 3.6) and lowest (3010 cells l-1) at the surface at Station 1 (figure 3.5); the average number of cells from all the samples is 47600 cells l-1. The abundance of diatoms was generally high at all depths whereas dinoflagellates tended to be more abundant at shallower depths and absent from the deepest samples. Due to errors with the CTD, only a surface sample could be collected at Station 5 (figure 3.9); this sample was dominated by diatoms.

Zooplankton

A large range of zooplankton orders were present in all the samples (figure 3.10). Out of nine samples taken, seven were dominated by Copepods (figure 3.12), whereas at Stations 2 (5 - 0m) and 4 (10 - 0m) hydromedusae (figure 3.13) were dominant; nevertheless hydromedusae made up a large proportion of the populations of seven of the samples. Other orders making up a significant percentage of the populations were Siphonophorae (Stations 1 and 4) and Echinoderm larvae (figure 3.11) at Station 2 between 5 - 0m. The greatest abundance of zooplankton was observed at Stations 4 and 5 (21300 and 24875m-1); the lowest abundances were observed at Stations 2 and 3 (10601 and 10097m-1). With the exception of Station 2, zooplankton were more abundant at depth.

When considering the distribution and abundance of zooplankton at the different stations sampled, the Shannon Wiener Index was used to calculate the level of spread of numbers between zooplankton groups sampled. Comparisons were made between different stations and different depths within a single station. For calculated values, see figure 3.10.

Station 1, located at the mouth of the estuary, shows the lowest value indicating lowest diversity of zooplankton species amongst the 5 stations, with a smaller number of zooplankton groups dominating.  Station 2 shows little variation between the two sample depths though this is not significant. Station 3 shows the greatest difference in index values between depths, with a higher value of diversity found in the deeper sample (28 - 15m). This value (1.97) was also the highest of all the stations and closest to Hmax, suggesting greatest similarity in the number of individuals between groups of zooplankton counted. Station 4 has almost identical values at both sample depths and station 5, located at the front, shows a slightly higher index value in the surface sample, again indicating greater similarity in numbers between zooplankton groups.

(click images to enlarge)

Figure 3.8 Phytoplankton - Station 4

Figure 3.9 Phytoplankton - Station 5

Figure 3.12 Copepod

Figure 3.13 Hydromedusae Larvae

 

 

 

 

 

 

 

 

 

Figure 3.16 Nutrients - Station 3

Figure 3.17 Nutrients - Station 4

Figure 3.18 Nutrients - Station 5

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.21 Station 3 - Oxygen Saturation

Figure 3.22 Station 4 - Oxygen Saturation

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.25 Salinity - All Stations

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.27 ADCP Data - Transect from Station 3 to Station 2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Nutrients and Chlorophyll

During sampling at the final station (Station 5) technical difficulties with the CTD and Rosette sampler did not allow for depth interval water samples to be obtained. The data discussed in this section therefore applies to stations 1,2,3 and 4; a surface water sample was obtained from Station 5 to allow some comparisons to be drawn.

Chlorophyll

Chlorophyll is an indirect measurement of phytoplankton and was indicated by fluorescence and also by the acetone extraction method using the water samples collected. The findings show Station 1 (figures 3.14 and 3.24) and Station 2 (figures 3.15 and 3.24) to be well mixed with chlorophyll values being homogenous with depth. As the stations distanted from the shore, stratification gave way to chlorophyll maximums particularly in Station 3 (figures 3.16 and 3.24) and Station 4 (figures 3.17 and 3.24). These chlorophyll maximums are found at the thermocline at approximately 20m depth, where nutrient injections across the thermocline and still moderate levels of light allow preferable living conditions for phytoplankton.

Nitrate

Station 1 (figure 3.14) and 2 (figure 3.15) were both fairly well mixed stations and showed a smaller range in nitrate concentrations with depth compared to Stations 3 (figure 3.16) and 4 (figure 3.17), which were stratified and showed the highest concentrations taken from samples collected below the thermocline. At Station 1, nitrate concentration in both the surface sample and at 14m depth was low with the surface sample measuring 0.4µmolL-1 and the 14m sample measuring 1µmolL-1. At Station 2 three samples were taken with the concentration found at the surface measuring 2.6 µM, the lowest value at 5m measuring 1.4 µmolL-1 increasing by 0.3 µmolL-1 at the third, deepest sample site. Both Stations 3 and 4 show a depletion of nitrate in the water on and above the thermocline. This was exemplified at station three where the difference between the deep water sample and the thermocline sample was 3.4 µmolL-1.

Dissolved Silicon

For Station 1 (figure 3.14) the surface silicon value was high measuring 7.5 µmolL-1 due to the freshwater outflow from the Fal Esturary. The concentration in the deeper sample taken in the chlorophyll band is low measuring 0.6 µmolL-1. Station 2 (figure 3.15) is not affected by a major input of freshwater and therefore a high concentration does not occur in the surface. A high value of 11 µmolL-1 does occur in the 5m sample, however this is unexpected and is thought to be an anomalous value. Ignoring this value the concentration at both the surface and bottom of the profile is low, measuring below 2.5 µmolL-1. Both Stations 3 (figure 3.16) and 4 (figure 3.17) showed lower concentration values above and on the thermocline compared to deeper in the water column. This was displayed in Station 4 where the surface value was 0.9 µmolL-1 and was 1.8 µmolL-1 at 50m.

Phosphate

High concentrations of phosphate, 2.4 µmolL-1 and 1.6µmolL-1, are found respectively in the surface waters at Station 1 (figure 3.14) and 2 (figure 3.15). This is likely due to an increased influence of freshwater from the Fal Estuary for Station 1 and the increased proximity to the land and hence surface runoff for station 2. At Stations 3 (figure 3.16) and 4 (figure 3.17) the water column is stratified, since the stations are located further offshore in deeper waters where there is less friction with the seabed (hence less turbulence and a more stable, stratified water column). The distance from any land source explains why phosphate levels are much lower when compared to dissolved silicon and nitrate; an increase in concentration was found in samples below the thermocline for silicon and nitrogen, but not for phosphate. It is important to consider that phosphate samples are easily contaminated, which could explain some of the anomalous values, for example a higher concentration of 1.2µmolL-1 found at 20m at Station 4. Generally, the range of concentrations is small across the entire data set.

 

Oxygen Saturation

During sampling at the final station (Station 5) technical difficulties with the CTD and Rosette sampler did not allow for depth interval water samples to be obtained. The data discussed in this section therefore applies to Stations 1,2,3 and 4; a surface water sample was obtained from Station 5 to allow some comparisons to be drawn.

Oxygen saturation has been established from samples taken from the Niskin bottles with 2-3 values been attained from all stations except for Station 5 due to faulty equipment. A more continuous profile and duplicate samples would be preferable in order to draw more accurate conclusions.  In general however, the surface values of oxygen tend to be saturated or supersaturated with an increase in oxygen absorbance due to upper layer turbulence. Phytoplankton photosynthesis also leads to addition of oxygen to the water column and therefore it is the balance between the rate of respiration and photosynthesis that determines the saturation state of the water. 

Station 1 (figure 3.19) and Station 2 (figure 3.20)

The oxygen saturation values correlate well with the temperature and chlorophyll profiles remaining fairly constant down through the water column. For station 1 both readings are above 100% and so are supersaturated with oxygen being added to the water through phytoplankton photosynthesis. For station two the values are not supersaturated but are just below 100% ranging from 97.4 - 99.8%. Station two may be slightly under-saturated compared to station 1 as the fluorescence values and hence phytoplankton number is lower however a definite explanation cannot be reached due to the unreliable nature of the data.

Station 3 (figure 3.21) and 4 (figure 3.22)

At station three and four the water column is stratified with the chlorophyll maximum occurring in the thermocline, at approximately 20m. This corresponds well with the dissolved oxygen data which remains supersaturated at the surface and at 20m indicating therefore that the rate of photosynthesis is greater than that of respiration at these depths. However, the data collected below the thermocline for both stations is undersaturated, for example at station 3 it is 90% at 60m, therefore more oxygen is being used for respiration than is being produced through photosynthesis.   

 

 

Temperature at all Stations

The temperature profiles (figure 3.23) between stations display the expected relationship, with more stratified waters located in the offshore stations (Station 3 and Station 4). This is due to a reduction in turbulent kinetic energy, which is driven by internal waves from sea-bed friction (see figure below). Therefore the deeper waters have less vertical mixing processes taking place so the temperature can become more statically stable.

Figure 3.20 Schematic Diagram of a Frontal System

 

 

Salinity at all Stations

The salinity profiles of all stations (figure 3.25) display a similar behaviour with depth, where the salinity values closely fluctuate around 35.2. The exception to this is Station 1, which is located near a freshwater source which is less dense and overlies the denser saline sea waters. ( Anomolies in data for station 5 must be considered, where an uncharacteristic spike in readings is seen at ~25m)

Chlorophyll

Above the thermocline, chlorophyll levels are relatively low, most likely due to nutrient limitation (especially in stratified waters), even though there is an abundance of available light. The chlorophyll profiles for all stations (figure 3.24) show a maximum at approximately 10 - 20 metres depth, with some variation between stations. This indicates an optimum depth where there are sufficient sources of both light and nutrients (replenished from depth) to allow for a maximum in primary production. Below the thermocline, chlorophyll concentrations diminish with depth due to a decrease in irradiance, even though nutrient abundance increases (i.e. the phytoplankton are light limited).

 

ADCP

Station 2 to Station 3 (figure 3.26)

The transect between station 2 and 3 shows the progressive transformation from mixed water to a frontal system. On the right of the picture (from station 2 to about 135m away from station 2), the water column is mixed, due to the low backscatter values throughout the water column; higher backscatter at the surface layer is caused by bubbles and turbulence. The velocity of the water currents increases dramatically at the front and on the stratified side of the front.

Station 3 to Station 2 (figure 3.27)

The front can be distinguished by observing the sharp changes in current direction and velocity. The southwest flowing current is the remainder of the ebb tide flow from the English Channel and the eastward incoming flow is the beginning of the flood tide coming up from the Atlantic. 

Velocity decreases below the thermocline in the stratified waters of the offshore regions beyond the front, where average current velocity reaches values as low as 0.026m/s. The inshore regions, where water is more mixed, have significantly higher velocity values (up to 1.104m/s), and tend to reach several minima (usually <0.375m/s) in lower depths, and near the opposing flow. Just above the seabed, current velocities are high, occasionally exceeding 2m/s.

The higher backscatter values in near-surface waters suggest the presence of zooplankton populations. Beyond the front, the main band of backscatter (and therefore zooplankton) can be found between the surface and 25m; this corresponds to the waters overlying the thermocline. As the thermocline weakens inshore (due to increased mixing) backscatter, and hence the zooplankton population, is more evenly distributed throughout the water column. 

Transect between station 3 and 4 (figure 3.28)

The backscatter values generally peak at around 20 – 30m, resulting from high zooplankton populations; this is an indication of the depth of the thermocline in the water column. There is a depletion of nutrients in the stratified layer; consequently, the plankton are positioned just above the thermocline to obtain nutrients from the mixed waters below whilst still recieving sufficient light for photosynthesis.

 

Richardson Number

The Richardson Number (Ri) for Stations 2 and 3 was calculated to show the balance between the stabilizing effect of the density gradient and the destabilizing effect of the current shear. Calculation of Ri required the velocity data from the ADCP as well as the density data from the CTD from a specific location. This meant that the calculation of Ri was restricted to Stations 2 and 3.

The Richardson Number indicates water column stability as follows:

When below 0.25 – gravitational unstable, overturning occurs

When between 0.25 and 1 – shear flow instability develops

When above 1 – the flow is stable and no mixing occurs between layers       

Station 2 shows clearly that there are two well-mixed water masses: from the surface to 10m, and from 12m to 25m. There is an increase in stability at approximately 10m below the surface. This suggests that the water column at this station is generally well-mixed with some stratification.

Richardson Number graph for Station 2

At Station 3, there are some very large values for Ri, therefore a logarithmic scale was used in order to show the variation more clearly. On the logarithmic scale, 1 equals 1 for Ri, and -0.6 equals 0.25.

The fluctuation of Ri for Station 3 was large and frequent. Apart from the stable layer at the top 5m of the water column, the rest of the water column above the thermocline was well-mixed. The thermocline spanned from approximately 16m to 40m (see figure 3.16). The Richardson Number also varied significantly within this section, affecting stability accordingly and sugggestin that the stratification of the water column was not fully developed and mixing was still occurring to some extent.

Richardson Number graph for Station 3

The water column at Station 4 was fully stratified as shown by the temperature difference between the upper and lower layers, and a distinct thermocline on the CTD profile. The Richardson Number profile shows a substantial stable layer at approximately 15m which corresponds to the thermocline. The Ri profile suggests that the water mass above the thermocline was well mixed; the lower layer of the water column was also well mixed in general, but there were some of stable layers developing. A strong stable layer was also present at approximately 42m.

Richardson Number graph for Station 4

Discussion

At Stations 3 and 4 nutrients were generally lower above the thermocline, with concentrations increasing below the thermocline. However, at Stations 1 and 2, excluding higher nutrient concentrations at the surface due to greater freshwater influence, nutrient concentrations did not increase significantly with depth. The nutrient data relates well to the fluorescence profiles and hence distribution of chlorophyll, indicating that when plankton populations are large, nutrient concentrations are low. At Stations 3 and 4, phytoplankton are trapped above the thermocline as it acts as a barrier to mixing; hence nutrients are gradually depleted in this layer. In the mixed water, phytoplankton are evenly distributed in the water column and nutrient concentrations remain more constant with depth.  

Diatoms were found to be the most dominant phytoplankton group at the mixed stations (1 and 2) and also in the surface sample taken from Station 5 at the front. However, in the stratified waters of Stations 3 and 4, dinoflagellates were the dominant group. This is the expected result as diatoms are non-motile and fast growing, preferring mixed waters which enable them to remain in the euphotic zone. Dinoflagellates, however, are more suited to stratified water conditions, since they have lower nutrient requirements and their motility enables them to remain in the euphotic zone. Copepods were the dominant zooplankton order with the greatest abundance found at the front itself and at the stratified stations beyond; this pattern was also observed for total zooplankton abundance. It would be expected that the highest concentrations of zooplankton would occur at the tidal front coinciding with enhanced primary production in this region (Munk, 1993). However, there is also evidence that tidal fronts are important areas of ‘larval retention’ due to their stability. According to Sinclair & Iles (1985), larvae are distributed in these regions in spite of and not because of the food resources there.

In general, the euphotic zone deepens with distance from the coast, although the figures obtained from the CTD data are considerably larger at Stations 3 and 4 than those calculated from the Secchi Disc depth. Increased light attenuation occurs in the shallower coastal waters where there is more mixing and larger amounts of suspended sediment. The deeper euphotic zone in the stratified water allows the chlorophyll  maximum to occur at the thermocline, where episodic nutrient injections create sustainable growing conditions.

Station Depth of Euphotic from Secchi Disc Data (m) Attenuation Coefficient (k) from Secchi Disc Data (m¯¹) Depth of Euphotic Zone from CTD Data (m)
1 13.5 0.32 >16.2
2 19.5 0.22 20.2
3 21.0 0.21 33.4
4 24.0 0.18 46.5
5 22.5 0.19 26.8

The chlorophyll values obtained from the acetone extraction method show a correlation with the fluorometer readings (Figures 3.14 - 3.18), particularly at Station 3 (Figure 3.16) where the chlorophyll maximum found at  approximately 20m depth coincides with high chlorophyll values (~2.2μgL-1). The oxygen saturation also closely follows the fluorometer readings (Figures 3.19 - 3.22) with an increase in oxygen at the chlorophyll maxima.

Observations of the frontal system located south of Blackhead made by this study correlate with previous work completed by the Western Channel Observatory. The variation in physical conditions either side of the front, such as light availability, turbulence and temperature, have strong influences on the chemical and biological characteristics of the water column. As predicted, the water column at Stations 1 and 2 (mixed side of the front) was much more turbulent than on the stratified side of the front (Stations 3 and 4). Consequently, nutrient concentrations at Stations 1 and 2 were much higher in the surface waters than those at Stations 3 and 4, due to nutrient replenishment by upwelling from below the thermocline. Nutrient concentrations at the front (Station 5) were hard to determine, due to faulty equipment, but the surface samples obtained have nutrient concentrations which are similar to Stations 1 and 2. This is probably due to mixing at the stratified/mixed water interface by turbulent eddies which results from interfacial friction. Differences in current velocity across the front can be clearly seen in figure 3.27; the variation is typical of a frontal system and clearly defines the mixed and stratified waters.

It is important to remember that all of the conclusions detailed above are subject to change due to variations in tidal and weather conditions.

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Geophysics

Introduction

N.B. All locations are quoted relative to WGS 1984 and all times are in GMT.

  • Date: 7/7/08
  • Time:
  • Weather Conditions: Cloud cover 7/8, westerly winds approximately 20mph, rainy spells and some sunny intervals.                                                                                                                                                                                                              

    Figure 4.1 Map showing survey transects (click to enlarge)

     

  • Tides at Helford River:                                              

08:25 GMT        High tide        4.9m                                               

14:43 GMT        Low tide          0.7m

20:30 GMT        High tide        5.1m

  • Vessel: Xplorer
  • Aim: Looking for benthic features and habitats at the mouth of the Helford River.

 

  • Responsibilities:

PSO (i.e. The Boss)

Ashley

Side Scan Sonar

Michael

Track Plot

Trystan

Grab Samples Identification and Recording

Charlie, David, Max and Alison

Photographer

Christos

Observer and Position Scribe Kathryn
Scribe and TS Probe Penelope

Method

The “tow fish” was deployed behind Xplorer at (long, lat) and towed for 4 transect lines which ran parallel to each other near to the mouth of the Helford river. Each line was 2km long and the swath width was 150m. The transect lines overlapped each other by ….m ensuring there were no gaps between the lines. While the side scan sonar was running an observer recorded buoys and boats in the area which may have affected the results. Positions of interesting features were also recorded so that grab samples could be collected after the transect lines were completed. Grabs were collected at four sites using a Van-Veen grab, which was deployed at the same time as a camera to support the grab data.  

Time (GMT) Detail Latitude Longitude
10:30 Start of Line GR7 50° 05.571 N 005° 05.140 W
10:45 End of Line GR7 50° 06.372 N 005° 04.142 W
10:49 Start of Line GR8 50° 06.404 N 005° 04.206 W
11:03 End of Line GR8 50° 05.595 N 005° 05.216 W
11:06 Start of Line GR9 50° 05.632 N 005° 05.285 W
11:19 End of Line GR9 50° 06.445 N 005° 04.290 W
11:23 Start of Line GR10 50° 06.504 N 005° 04.333 W
11:36 End of Line GR10 50° 05.661 N 005° 05.345 W

 

Figures 4.2, 4.3 and 4.4 Using the Equipment on Explorer

 

 

 

Results

 

Geophysics Analysis

Site 1

Site one is comprised of a drape of finer sediment overlying courser sediment, lying perpendicular to the track plots (figure 5.1). The flow direction of the drape is unclear as there is evidence to support both north-westerly and south-easterly flow. A thinning line of coarser sediment is seen extending through the drape, this could be due to an obstacle creating a shadow zone behind it. The long length of this ‘shadow zone’ suggests that the drape is a region of fast flow. The fast flow is further supported by a lack of bedforms in both the drape and surrounding coarser sediment, suggesting an area of upper flat beds according to the bed form classification system relating mean flow velocity to mean sediment size (Allen, 1982).  The drape is funnelled between two areas of bedrock and then fans out again suggesting north-easterly flow. However, south-easterly flow is supported by the presence lobes, located near the point 11:36:39 on the side scan map.          

Site 2


Site 2 exhibits complex patterns of high and low backscatter suggesting a region of rock (figure 5.5). The strike of his rock corresponds to the strike of the headland suggesting that over time the headland has been eroded back (Goode, 1990).

Site 3A

  • Located between 10:43:15 and 10:44:15

  • Coarse sediment with megaripples of heights 0.22-0.57m and wavelength 5.28m.

Site 3B

  • Located between 11:18:15 and 11:18:45  

  • Coarse sediment with megaripples found of heights 0.287-0.385m and lengths 2.083-2.616m.

Megaripples are found in both site 3A and 3B (figure 5.2) due to the Helford river outflow creating high energy currents, which are amplified by the current being diffracted by the neighbouring rock mass. During our survey the winds were westerly; winds from this direction are funnelled through the Helford River Basin creating surface waves and a more turbulent water column. Evidence of wave-formed ripples is seen from bifurcation. Moving from site 3A to 3B some energy is dissipated and the megaripples decrease slightly in size.        

Site 3C

  • Located between 10:54:15 and 11:14:15.

  • Coarse sediment with ripples found of heights 0.02-0.049m and lengths of 1.825m.

Site 3C is a channel of coarse sediment found between two rocky bedforms (figure 5.4), which likely follows an eroded line of weakness following the strike of the surrounding bedrock. Ripples are found in the narrower section of the channel but as the channel width increases this bedform is gradually lost.  The loss of ripples suggests a decrease in energy likely due to the current being spread over a larger area.

Figure 5.1 Fine Sediment Channel (Site 1) Figure 5.2 Megaripples  (Sites 3A and 3B) Figure 5.3 Seabed Classification Map Figure 5.4 Channel of Coarse Sediment (Site 3C) Figure 5.5 Bedrock     (Site 2)

 

 

Click on above images to enlarge

Van Veen Grabs

The grab sites were predetermined by the sidescan sonar imagery at four locations. The sites chosen were areas which would provide a respectable grab, however grab sites 1 and 2, were not very successful with little material obtained. The sites were high in algal growth, which was perceived on the sonagraph as a suitable grab location. The overall area consisted of the Devonian slate of the Portscatho Formation. The sediments were coarse to medium grained with shell fragments present. The grabs also showed an abundance of maerl in  the samples, particularly in grab 4 (Figure 5.9)

 

Grab Site 1 (Figure 5.6):

Location - 50o05.831 N  05o05.117W

Time -11:55

Sea bed composition – Video revealed that the sea bed composition consisted mainly of the Devonian slate rocks with finer sand grains in between the gaps (Figure 5.6). This made it difficult to collect a good sample, with the two grabs taken only managing to retrieve large rock fragments. However, live maerl was found present on approximately 5% of the rock, with some dead species also retrieved.

Fauna & Flora - Laminaria digitata, Bryozoa sp., Crustacea (Pagarus bernhardus, Porcellana platycheles), Gastropoda (Gibbula sp.), Polychaete worm sp., Ascidian sp., live Maerl.

 

Grab site 2 (Figure 5.7):

Location - 50o06.083 N  05o04.714W

Time- 12:20

Sea bed composition – Video revealed fine sediment with a few larger rock fragments. Three grabs were attempted but rocks jammed the jaws of the grab, therefore, fine sediment was lost. It is a possible that sediment is compacted, which therefore resulted in poor penetration of grab into sea bed. Live maerl was present on approximately 3% of rock surface, with dead maerl also present.

Fauna & Flora – Leptochitonidae, Bryozoa sp., live Maerl, Marthasterias glacialis (observed from video camera).

 

Grab site 3 (Figure 5.8):

Location - 50o06.177 N  05o04.364W

Time-12: 43

Sea bed composition – Video revealed sea bed consisting predominantly of Maerl. This was confirmed with the grab, which consisted of approximately 80% Maerl with the rest of the sample consisting of small rocks and bionic material such as small shell fragments.

Fauna & Flora – Bivalve, Polychaete sp., Nematoda sp. live Maerl.

 

Grab site 4 (Figure 5.9):

Location - 50o06.377 N  05o04.353W

Time- 13:02

Sea bed composition – Video revealed sea bed consisting predominantly of maerl. Grab sample confirms this and is very similar in composition to the sample from grab site 3 with the sample consisting of approximately 70% maerl, approximately 10% intact bivalve shells, and the rest consisting of small rocks and bionic material such as shell fragments.

Fauna & Flora – Bivalve, Polychaete sp., Porcellana platycheles, live Maerl, Asterias rubens (observed from video camera).

 

Video transect

  Latitude Longitude Time (GMT)
Start Location (Figure 5.10) 50o 06.245 N

005o 04.617 W

01:20
End Location (Figure 5.11) 50o 06.172 N 005o 04.438 W 01:37

Sea bed composition – The beginning of the transect is predominantly rocky (FIGURE) progressing to finer sediment approximately 1/3 of the way through. This fine sediment persists for approximately 10-15m before the substratum returns to a rocky composition for the second third of the transect. The final third consists of fine sediment with outcrops of rock (FIGURE).

Flora and Fauna – Rhodophyceae, Chlorophyceae, Phaeophyceae, Echinidae, Asteriidae, Demospongiae

Figure 5.6 Grab Site 1 Figure 5.7 Grab Site 2 Figure 5.8 Grab Site 3 Figure 5.9 Grab Site 4 Figure 5.10 Start of Video Transect Figure 5.11 End of Video Transect

 

 

Click on above images to enlarge

 

Discussion

The results from the sidescan sonar clearly display the bathymetry of the surveyed area. They can be divided into three main sections according to their sediment types and bed forms. Section 1 is comprised of a drape of finer sediment overlying coarser sediment. A narrowing line of coarse sediment and the lack of bedform formation on the fine sediment suggest that the flow is fast.

Section 2 has a rocky sea bed and the strike direction on the rock matches the strike direction of the headland. Section 3 is an area of erosion of the adjacent rocky region, and is covered by coarse sediment. Megaripples in section 3A and 3B are formed by the high energy outflow from the Helford River and from diffraction by the headland. Section 3C is a channel of coarse sediment caused by the erosion of a line of weakness between two rocky bedforms.

The results of the Sidescan sonar are supported by the grab samples and the seafloor video clips. The sea bed is mainly composed of Devonian slate rocks with finer sand grains in between the gaps. Maerl is the predominate plant species in this region. Bivalves and Gastropods are the most common macrofauna.

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Estuarine Sampling

 
Introduction

N.B. All locations are quoted relative to WGS 1984 and all times are in GMT.

Figure 6.1 (Above left) Map of Bill Conway  Sampling Locations

Figure 6.2 (Above right) Map of RIB Sampling Locations

(Click to enlarge images)

  • Date: 10.07.08
  • Time: 0800-1300
  • Weather Conditions: Cumulus clouds, 8 octants. Wind speed approximately 8-10ms¯¹.
  • Tides:

                       0429     Low    1.4m

                       1024     High    4.5m

                       1644     Low     1.7m

  • Vessels: Bill Conway and Ocean Adventure RIB.
  • Aim: To investigate spatial variations in nutrients, temperature, salinity, chlorophyll and plankton populations within the Fal Estuary.

 

  • Responsibilities:

    PSO

    Trystan

     

    Bill Conway

     

    Dry Lab

    Kathryn

    Wet Lab

    Penelope and Dave

    Deployment Team

    Ashley and Mike

    Scribe

    Alison

    RIB

    Chris, Charlie, and Max

Method

Both the Bill Conway and the RIB where used to collect data, with the RIB being used to sample to shallower regions in the upper reaches of the estuary.  

On the Bill Conway four transects where taken using the ADCP. Along each transect the following data was also collected:

  • Transect 1: Location - Black Rock (for locations of transects see figures 6.1 and 6.2). At Station 1, a CTD profile was recorded and water samples where collected at three depths using Niskin Bottles mounted on a Rosette.  A zooplankton net was also towed at 2 knots for 1 minute. 

  • Transect 2: Location - north of Restronguet Creek (above Carick Carlys Rock). Surface water samples, taken from the deck wash, were collected at the beginning and end of the transect line, with the third sample taken in the at Station 2 in the deep channel region where a CTD profile was also recorded.  

  • Transect 3: Location - Turnaware Point. Surface samples where taken at the beginning and end of the transect line. At Station 3, as for Station 1, a CTD profile was recorded, three water samples were taken; a zooplankton net was also used.

  • Transect 4: Location - Penarrow Point to Messack Point. Station 4 was located in the deep channel region where a CTD profile was recorded and water samples where collected at three depths.  

Note:

  • Faulty equipment: The CTD mounted on the Rosette was broken and so a YSI was used to measure salinity, temperature, and depth, and the data was recorded from a hand held monitor. 
  • A reduced number of water samples were collected due to reduced time available for collecting data.
  • Unfortunately, due to a problem with transferring data from the boat, the ADCP data was not able to be analysed in time to be displayed on this webpage.

The Latitude and Longitude of each transect and station is shown below:

Transect 1: 50° 08.472 N, 005° 01.112 W to 50° 08.649 N, 005° 02.442 W                           Station 1: 50° 08.657 N, 005° 01.606 W

Transect 2: 50° 11.716 N, 005° 03.293 W to 50° 11.762 N, 005° 01.850 W                          Station 2: 50° 11.737 N, 005° 02.747 W

Transect 3: 50° 12.270 N, 005° 02.393 W to 50° 12.240 N, 005° 02.134 W                           Station 3: 50° 12.223 N, 005° 02.376 W

Transect 4: 50° 10.487 N, 005° 02.527 W to 50° 10.899 N, 005° 01.529 W                           Station 4: 50° 10.785 N, 005° 01.711 W

The RIB collected the following data from four stations:

  • A temperature and Salinity profile.
  • A surface water sample using a Niskin Bottle.
  • At Stations 1 and 2, before and after the mussel beds, a zooplankton net was towed at 1 knot for 3 minutes. 

The Latitude and Longitude of each RIB station is shown below:

  • Station 1: 50° 12.560 N, 005° 01.671 W
  • Station 2: 50° 12.957 N, 005° 01.667 W
  • Station 3: 50° 13.718 N, 005° 00.951 W
  • Station 4: 50° 14.706 N, 005° 01.374 W

Safety Considerations

All members of the research team were required to wear life jackets at all times. Great care was taken when handling and during deployment of the equipment (CTD etc).

Water sample processing

For all the samples collected in the Niskin Bottles, the processing procedure was as explained for Callista. For samples collected from the surface using the deck wash, oxygen analysis was not carried out as the oxygen content of the sample would be altered due to increased exposure to air. 

Figure 6.3 A Front in the Estuary Figure 6.4 Using the CTD Figure 6.5 Langmuir Circulation
 

 

 

Results

(Click on Images to Enlarge)

Figure 6.6 Phytoplankton

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6.8 Nitrate Mixing Diagram

 

 

 

 

 

 

 

Figure 6.11 Nutrients - Conway, Station 1

Figure 6.12 Nutrients -Conway, Station 2

 

 

 

Figure 6.15 Nutrients - RIB, Station 1

Figure 6.16 Nutrients - RIB, Station 2

 

 

 

 

 

 

 

 

 

 

Figure 6.19 Oxygen - Conway, Station 1

Figure 6.20 Oxygen - Conway, Station 3

Figure 6.22 Oxygen - RIB, Station 1

 

 

 

 

 

Figure 6.25 Temperature - All Stations

 

 

 

 

 

 

 

 

 

 

 

 

 

Phytoplankton

N.B The sample collected from Station 2 contained no phytoplankton, which is highly unlikely looking at the other results, and so it can be assumed that there was an error in the collection or processing of this data; therefore Station 2 phytoplankton data are not included in the results.

Diatoms were the most abundant group, with 421 cells found in total, and were present at every station. The largest number of diatoms (121 cells l-1) was found by the RIB Team at Station 3 (figure 6.6). Ciliates were the least abundant, with only 14 cells found in all the samples collected. Dinoflagellates are more abundant, with a total of 39 cells, although they are only found in four of the samples. As all of the samples were collected from the surface, it is not possible to see how phytoplankton varies with depth in the estuary.  

Zooplankton

The Station 1 sample was taken from the mouth of the estuary at the boundary between Carrick Roads and the offshore region (50o 08.657 N, 005o 01.606 W). At this site the greatest numbers of zooplankton were seen from all inshore samples. Dominant species include Siphonophores, which are more characteristic of benthic conditions, and copepods. (figure 6.7)

Station 3 signifies the northern limit of Carrick Roads, where the Truro river meets Carrick roads (50o 12.223 N, 005o 02.376 W). Zooplankton sampled at this site were reduced in number and slightly different in composition to Station 1. Copepods remained the dominant order along with Cirripede larvae (barnacles); numbers of Siphonophores were greatly reduced (figure 6.7).

Samples X and Y were taken from the Ocean Adventure RIB. Total numbers counted at both of these sites were greatly reduced when compared to Stations 1 and 3. Sample Y was sampled south of the mussel farm on Truro river (50o 12.560 N, 005o 01.671 W). Although much smaller in numbers, proportions of Cirripede larvae and Copepod larvae were similar to those of Station 3. Siphonophore numbers were again reduced (figure 6.7). Sample X was taken north of the mussel farm (50o 12.957 N, 005o 01.668 W). The composition at Y shows a relatively even spread in numbers of each order compared to other sites, and this is also seen in the high Shannon index value. Copepods and Cirripede larvae remain two of the dominant orders present (figure 6.7).

(Click on Images to Enlarge)

Figure 6.7 Zooplankton

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6.9 Phosphate Mixing Diagram

Figure 6.10 Silica Mixing Diagram

 

 

 

 

 

Figure 6.13 Nutrients - Conway, Station 3

Figure 6.14 Nutrients - Conway, Station 4

 

 

 

 

Figure 6.17 Nutrients - RIB, Station 3

Figure 6.18 Nutrients - RIB, Station 4

 

 

 

 

 

 

 

 

 

 

Figure 6.21 Oxygen - Conway, Station 4

Figure 6.23 Oxygen - RIB, Station 2

Figure 6.24 Oxygen - RIB, Station 3

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6.26 Diagram displaying transects

 

Figure 6.27 Temperature/ Salinity Graph

 

Estuarine Mixing Diagrams

Concentrations for dissolved phosphate, nitrate and silica were plotted against salinity (figures 6.8 - 6.10). By comparing these values to the theoretical dilution line (TDL), representing conservative behaviour, the chemical behaviour of these nutrients can be examined within the Fal estuary.

Dissolved nitrate values lie close to the TDL indicating conservative behaviour at salinities over 24. One value (175.97 µmol/L at 30.95 salinity) is much higher than would be expected and is possibly due to contamination of the sample, or high localized concentrations of nitrate from anthropogenic or natural sources.

All but one of the values for silica lies above the TDL indicating non-conservative addition of silica in the Fal estuary at salinities greater than 24.

Phosphate values show the greatest divergence from the TDL. The behaviour is non-conservative and shows evidence of large amounts of addition at salinities greater than 24. Two of the values, at salinities of 30.40 and 25.60, are extremely low (<0.001 µmol/L). These values are so much lower than all other values that they may be considered anomalous due to errors involved in collection of the samples, or in chemical analysis. However, it is still possible that these values may represent localised removal by the biota.

The highest concentrations 0f silica and nitrate occur at the riverine end-member and decrease with increasing salinities. The highest values of phosphate are found at higher salinities and decrease towards the riverine end-member.

 

 

Nutrients, Salinity and Chlorophyll

Salinity

Station 1 (Figure 6.11) clearly shows a well-mixed profile, where a salinity value of 35 is maintained throughout the water column down to 7m. Stations 2, 3 and 4 (Figures 6.12, 6.13 and 6.14) give evidence of some stratification, with less saline water found at the surface in comparison to the bottom waters. Continuing to the stations visited by the RIB, stronger stratification takes place, where two layers of water can be distinguished with a saline layer lower in the water column (>32) and a fresher body of water at the surface (<28) (Figures 6.15, 6.16, 6.17 and 6.18).

Nitrate

Station 1 (figure 6.11) is relatively well-mixed throughout the water column. The nitrate data collected supports this pattern, with low concentrations (~12.6µmol/L), which remain almost constant with depth. Towards the head of the Fal estuary there is a steady increase in nitrate concentration. This is due to the formation of a two-layered water column with a lower saline layer and an upper fresh layer, where nitrate reaches higher concentration values closer to the surface. At RIB Station 3 (figure 6.24) concentrations decrease slightly (23µmol/L at 1m), which is a result from the absence of freshwater input from the Fal river. The highest nitrate concentrations were detected at RIB Station 4 (170 - 175 µmol/L at 1m) and several minima were found at Stations 1 to 4 (approximately 3 - 6µmol/L), this is likely to be due to a point source created from anthropogenic inputs.

Phosphate                                                                                                

For the results collected from Conway, phosphate behaves similarly to nitrate in the water column. For the results collected from the RIB stations, the concentrations were higher at Station 1 and 2 (figures 6.15 and 6.16) ~1.3 µmol/L. Station 3 (Figure 6.17) had a concentration of ~0.9µmol/L, and the concentration at Station 4 (figure 6.18) was similar to the results from Conway at ~0.5µmol/L. RIB Stations 1 and 2 (figure 6.15 and 6.16) had considerably high values, confirming that there was a point source of pollution nearby. 

Dissolved Silicon

At Station 1 (figure 6.11), dissolved silicon has a constant concentration (~2µmol/L) throughout the water column. At Station 3 (figure 6.13) there is a negative linear relationship in the water column with the highest concentration at the surface 11µmol/L then 7µmol/L at 3m and then 2µmol/L at 8m depth. Station 4 (figure 6.14) is shown to be similar, with lower concentrations found in the deeper waters. The concentrations are lower in the deeper water as this is more diluted with low concentrated oceanic water. The samples collected from the RIB have higher values than the samples from Conway, where Station 2 and Station 4 (figures 6.16 and 6.18) have the highest dissolved silicon values of ~28µmol\L.

Chlorophyll

The chlorophyll values vary spatially in accordance with the nutrient supply. The largest values discovered were at RIB Station 2 (figure 6.16), where values reached more than 4μg/L. This coincided with a particularly high nutrient supply, which was most likely encouraged by anthropogenic inputs. In the vertical column, chlorophyll values showed little change with depth. This was due to the restrictions in equipment, with the majority of samples within the euphotic zone (figure 6.26). The estuarine region studied is also influenced by strong turbulent mixing generated by the tidal flows, which maintains vertical uniformity throughout the water column.

 

 

Dissolved oxygen analysis

In order to assess the accuracy of the dissolved oxygen measurement technique, duplicates of four samples were taken. On average the standard deviations of the four duplicates ranged from 0.11 - 2.86% and therefore the data can be considered as having a high degree of accuracy.

Considering the surface samples taken from each station, there is a significant decrease of 15% in oxygen saturation from the mouth to the upper stretches (figures 6.19 and 6.24). This is expected since freshwater has a greater supply of nutrients, which promotes increased amounts of heterotrophic nutrition and increases oxygen consumption. Overall the values recorded in the estuary at this time are generally quite low due to recent high rainfall and therefore greater surface runoff of agricultural fertilizers leading to eutrophication.

For Conway Stations 1, 3, and 4, water samples were taken vertically through the water column (figures 6.19 - 6.21). Due to complications with the equipment, samples could only be taken to a maximum depth of 8m, therefore the resulting data does not give a good representation of the whole water column. For the three stations (Stations 1, 2 & 3) the surface saturation at 1m was 2 - 4% higher than the deeper saturation values, due to closer proximity with the atmosphere and  increased trapping of air bubbles as a result of wave action.  With the exception of the saturation value at 8m for Station 3, oxygen values do not show significant changes vertically through the water column. The chlorophyll values also remain relatively constant through the water column, which corresponds well with the oxygen data where a peak in chlorophyll would produce a higher oxygen saturation value. An anomaly was found at Station 3 at 8m depth (figure 6.20), where a saturation value was 107%. Although an increase may be expected in this region due to greater seawater influence at this depth, this value is thought to be too high.

Temperature (figure 6.25)

The temperature structure of the water column is a very good illustration of how the estuary is influenced by the input of river water and the tide. The structure of the temperature profile varies significantly and depends on the status of the tide and location of where the measurement is taken. In July, the temperature of the river is higher than the sea water, therefore the water temperature at the upper part of the estuary should be higher than the temperature at the mouth of the estuary. Our results have confirmed this and such a trend is clearly displayed by the decrease in temperature with increasing proximity to the sea.

The temperature profile also varies significantly at different times within the tide cycle. During the flood tide, the influx of seawater dominates the structure of the temperature profile in the Fal Estuary. The temperature of the water column is homogeneous at the mouth of the estuary (Station 1) on the flood tide. The temperature declines gently with depth in a linear manner in the middle and upper part of the estuary (see Stations 2 and 3). During the ebb tide, the temperature structure of the water column is dominated by river flow, and the warm and less dense river water flows on top of the colder, denser seawater. This arrangement of water masses causes stratification and a clear thermocline can be seen (Station 4 and RIB Station 4).

Transects

The data within this section could be further improved by ADCP data, however due to technical difficulties this data was unable to be retrieved.

The aim of the transects was to discover an anti-cyclonic flow, which is a residual flow occurring as a result of the tidal and river flows. The technique was to use a T/S surface probe to measure surface temperature and salinity values along the transect every few minutes. The findings (figure 6.26) displayed eastern bound oceanic water and western bound river water. This is indicated in the graphs (figure 6.26) with the western side of the estuary characterised by cold, saline, oceanic water and the eastern side characterised by warm, fresher, estuarine water. Although the temperature and salinity variations are minimal, there is moderate change with both parameters along the transect, which indicates that this physical process is occurring. 

Temperature/Salinity Graph

The Temperature against Salinity diagram shows clearly how the temperature changes at different parts of the estuary. It suggests that temperature is higher at lower salinities, and lower at higher salinities, i.e., the temperature of the river water is higher than the sea water. This diagram also shows how the mixing process changes the temperature of the water column.

 

Discussion

As displayed in figure 6.28, there is a clear trend between the depth of the euphotic zone and distance from the mouth of the estuary; the euphotic zone becomes shallower towards the head of the estuary. This is most likely due to increased suspended sediment input from the River Fal, and from increased turbulence which results from the shear between the incoming river water and the underlying seawater.

Figure 6.28 Depth of Euphotic Zone Calculated from Secchi Disc Depths

Station Secchi Depth (m) Euphotic Zone (m)
Conway 1 4.5 13.5
Conway 2 2.4 7.2
Conway 3 2.0 6.0
Conway 4 3.0 9.0

The data collected in this study defines a clear salinity structure which is typical of partially mixed estuaries. The results show the presence of a fresh, less dense layer overlying a more saline, denser layer; the difference between the two layers became more enhanced towards the head of the estuary due to reduced turbulence and mixing. Nutrient levels were higher near the head of the estuary, since the river water is the main source of new nutrients into the system. Overall, both nitrate and silica displayed conservative behaviour within the estuarine system, although it is important to note that the majority of samples were collected in relatively high salinity conditions due to tidal limitations; a more balanced data set may yield alternative results. Phosphate displayed non-conservative behaviour, showing significant addition of dissolved phosphate at relatively high salinities. Additions of phosphate to the estuary are probably a combination of point sources, such as the sewage treatment works at Malpas and the mussel farm south of King Harry Passage, and diffuse sources such as run-off from the surrounding farmland. The relatively small additions of silica to the estuary are probably a result of weathering of silica containing rocks in the Fal drainage basin. The addition of these two nutrients into the estuary was most likely exacerbated by the heavy rainfall experienced in the area prior to the collection of the samples.

Variation in physical conditions found at each sampling site affect the biology in different ways. Zooplankton populations are significantly reduced within the estuary when compared with previous offshore data. Fewer orders of zooplankton were found mainly due to a reduction in salinity and an increase in physical mixing due to tidal influences, however, zooplankton samples were only taken from the surface during the inshore investigation. Surface waters are generally lower in salinity, especially in the RIB samples, and may not be representative of the whole water column at any one site. Samples show variation in tolerance to fresher conditions between orders. Many orders such as Hydromedusae, Chatognathae, and Polychaete larvae were found to be abundant in offshore waters, but are virtually non existent in estuarine samples. Some orders such as Siphonophorae are found at all sites within the estuary, although greatly reduced in numbers, indicating some tolerance to salinity changes. In general, numbers of zooplankton are greatly reduced in riverine samples, but certain orders such as Cirripede larvae dominate the zooplankton composition. This is due to their adaptation to tolerate a larger range of salinities, a characteristic which is expected in coastal and estuarine regions (where typically you will find Cirripedes (Barnacles). In all cases data has been displayed graphically, and for each site, as with offshore samples, a Shannon index number has been calculated (figures 6.6 and 6.7). This number shows diversity relative to Hmax and a measure of evenness in numbers between orders. Most notably copepods are found to be one of the more dominant orders at all locations. The order copepod includes a large number of species, and so results may be representative of different species adapted to different physical and chemical conditions within the estuary.

The dominance of the estuarine surface waters by diatoms is to be expected as diatoms are best adapted to growing in mixed conditions, as found in an estuarine environment, where tidal mixing prevents the thermal stratification of the water column. Although the presence of dinoflagellates in the well mixed lower regions of the estuary conflicts with the preferred growing conditions of this functional group, it is likely they entered the estuary from the offshore environment on the incoming tide (Stations 1, 3 and 4). The highest abundance of dinoflagellates occurs furthest upstream where tidal mixing is reduced (6000 cells m-1). The phytoplankton were identified as Alexandrium, blooms of which are typically caused by high nutrient concentrations typical of low salinities; the extremely low phosphate concentrations here, therefore, probably reflect the rapid uptake by dinoflagellates.

Limitations include lack of data at low salinities, over the full tidal cycle from high to low water, and between springs and neap tidal cycles. The lowest salinity sampled was 24.80. Consequently, there is no data available to interpret the chemical behaviour of nitrate, phosphate or dissolved silica between the riverine end-member and this salinity. Samples were taken between 0825 and 1137; high water was at 1024. Ideally, samples would have been taken after high water in order to eliminate the effects of the incoming tide on nutrient distributions within the estuary. It would also be useful to observe how nutrient concentrations vary temporally between low and high water and between springs and neaps.

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Conclusion

 

Our surveys of the Fal led to findings of pollution sources contributing to higher nutrients in the water column. Fronts were found in the offshore region and in the estuary, however, due to problems with the ADCP data from the estuary, analysis of this could not occur. In Offshore, nutrients varied on either side of the front, with high concentrations in the inshore mixed region and low concentrations in the offshore stratified region.

In geophysical analysis the grabs taken from most sites did not yield as much as expected, when previously looking at the side scan sonar and camera. So without the use of the camera, it could have lead to inaccurate conclusions from the grab data retrieved. The dataset showed conclusive bathymetric features such as drapes, mega-ripples, and a line of weakness creating a channel lined with fine sediment on the sea floor. 

In biology the relative abundance of phytoplankton functional groups varied offshore and in the estuary due to the degree of stratification. The position of the front was determined by the degree of mixing, which determined the presence of nutrients. Data collected from zooplankton samples has shown some relevant relationships between distribution, biomass, which is relevant to chemical and physical boundaries. By far the greatest biomass was found offshore, which then decreased as you head landward with lowering salinities into the estuary. Relative abundance and dominance of certain orders also changes with salinity due to varying tolerance between the orders. However, reliability of data could be questionable due to limited number of samples, number of counts per sample, and identification of zooplankton by order. Greater sampling resolution, temporally and spatially, with classification of phytoplankton and zooplankton down to species level would further increase confidence in the results. This would then increase the understanding of the biology sampled in relation to its chemical and physical environment (i.e tidal influence on movement of species and its effect on the distribution over a time scale).

The degree and type of mixing processes played an important role in controlling all of the parameters measured. The mixing observed was primarily tidal, however wind and wave mixing cannot be excluded. In the offshore region the mixing processes were controlled by bottom friction with the seabed. This influenced the strength and position of the observed front. In the estuarine region, the ebb and flood of the tide created mixing between the saline and riverine waters. There was also a residual anti-cyclonic flow shown to be present, which exists in the absence of tidal flows.

The data collected in this study was accurate in determining how physical, chemical, geological and biological processes interact and vary spatially. However, due to the small area studied conclusions made cannot be taken as a representative for the whole Fal region.

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References

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Grasshoff, K; Ehrhardt, M & Kremling, K. (1983). Methods of Seawater Analysis, Verlag Chemie, Weinheim.

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

Langston, W.J; Chesman, B.S; Burt, G.R; Taylor, M; Covey, R; Cunningham, N; Jonas, P & Hawkins, S.J. (2006). Characterisation of the European Marine Sites in South West England: the Fal and Helford candidate Special Area of Conservation (cSAC). Hydrobiologia. 555: 321-333.

Munk, P. (1993). Differential growth of larval sprat Sprattus sprattus across a tidal front in the eastern North Sea. Marine Ecology Progress Series. 99: 17-27.

Parsons, T.R; Maita, Y & Lalli, C.M. (1984). A Manual of Chemical and Biological Methods for Seawater Analysis, P. 173. Pergamon pres, Oxford.

Pirrie, D; Power, M.R; Rollinson, G; Camm, G.S; Hughes, S.H; Butcher, A.R & Hughes, P. (2003). The spatial distribution and source of arsenic, copper, tin and zinc within the surface sediments of the Fal estuary, Cornwall, UK. Sedimentology. 50: 579-595.

Sinclair, M; Iles, T.D. (1985). Atlantic herring (Clupea harengus) distributions in the Gulf of Maine-Scotian Shelf area in relation to oceanographic features. Can. J. Fish. Aquat. Sci. 46: 113-124

Spooner, N; Gibbs, P.E; Bryan, G.W & Goad, L.J. (1991). The effect of tributyltin upon steroid titres in the female dogwhelk, Nucella lapillus, and the development of imposex, Marine Environmental Research. 32: 37-49.

Warwick, R.M; Langston, W.J; Somerfield, P.J; Harris, J.R.W; Pope, N.D; Burt, G.R & Chesman, B.S. (1998). Wheal Jane Minewater Project. Consultancy studies 1996 – 1999. Final biological assessment Environment Agency, 91 pp.

 

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