National Oceanography Centre, Southampton

Falmouth Field Course 2009

Group 7

Charles Annett - Emelie Brodrick - Tom Horton - Pippa Knight - Ben Libby - Suzie Plumb - Stef Rowland - Carl Zammit

IntroductionVesselsEquipmentOffshoreGeophysicsEstuarineConclusionsReferences

 

Introduction

 

The Fal Estuary is located on the South coast of Cornwall, England. The shoreline has a length of 127km making it England’s largest estuary and the third largest natural harbour in the world. The estuary mouth lies between Pendennis Point and St Anthony Head and extends to Tresillian, 18km inland. The estuary is macrotidal, with a spring tide of 5.3 m at Falmouth, however it is mesotidal at Truro, with a spring tide of 3.5 m. The total area of the estuary (2482 ha) comprises important subtidal (1736 ha), intertidal mudflat (653 ha) and saltmarsh (93 ha) environments. It is a ria characterised by a deep meandering channel with depths of up to 34m – the deepest in England. The channel meanders northwards becoming narrower and shallower (as shallow as 5m near King Harry Reach) and broad, shallow platforms ranging between 0.3 and 4.6 m deep flank both sides of the channel.

Since the 16th century, the Fal has been a natural refuge for boats and great of importance for national and international trade. Tailings from metal mines caused environmental problems in the western tributaries, to the east, in the St. Austell mining district, tin works and china clay mining, caused similar problems as the Fal was used for transport and waste disposal. Other metals such as zinc and cadmium have entered the Fal due to agriculture, these releases, as well as the input of sewage from local towns and industry, make the Fal one of the most polluted estuaries in England. Although mining has now ceased in the region, industry such as boat building, fishing and cargo import/export are still thriving, currently the largest business is offering mooring to boats of all sizes, local or visiting.

  

Fig. 1.1 - Overview of the Fal Estuary

 

Fig. 1.2 - Tributary of Restronguet Creek - Runoff from Wheal Jane Tin mine

The Fal estuary has been designated a Special area of Conservation (SAC), and a Site of Special of Scientific Interest (SSSI) by the European Environment Council. This is largely due to the extensive beds of a rare species of coralline algae, maerl, which provides an important habitat for many other organisms, also dead maerl harvested for plant fertiliser.

The complex biological, chemical and physical processes occurring in the region make the Fal estuary an area of major oceanographic interest and our aim is to carry out a scientific study of the marine environment between the 1st and the 11th July 2009, to increase our understanding of the relationships and processes that occur there. This information can then be passed on to agencies such as Natural England to help them better manage the region.

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Vessels

RV Callista

 

Range

400nM

Length

19.75m

Beam

7.40m

Draft

1.80m

Top Speed

15knt

Passengers

30 max

 

         Fig. 2.1 - RV Callista

Scientific and on deck equipment:

  • Capstan – 1.5 tonne limit
  • Side Davits – 2 x 100kg limit
  • A-Frame and winch – 4 tonne lift capacity
  • Closing zooplankton net
  • Secchi disk
  • Hull mounted RDI Workhorse ADCP – 600 kHz
  • Seabird 21 Thermosalinograph
  • CTD + rosette + mounted fluorometer + Niskin bottles
  • Vessel logger running TECHSAS logging software.

Uses:

Callista is used primarily as an offshore research vessel. It is fully equipped with on board wet and dry labs and is the largest research vessel used.

RV Xplorer

 

Range

100nM

Length

11.88m

Beam

5.20m

Draft

1.20m

Top Speed

25knt

Passengers

12 + Crew

 

Fig. 2.2 - RV Xplorer

Scientific and on deck equipment:

  • Capstan – 0.5 tonne and a hydraulic crane
  • Bathymetric Surveying side scan sonar and boomer
  • Van Veen Grab
  • Genoa 11c chart plotter
  • Furuno GP36 DGPS

Uses:

Xplorer is used for the geophysical and estuarine parts of the course. It is a fully equipped survey vessel with a sidescan sonar, grabbing equipment, ADCP and CTD.

 

 

Ocean Adventurer RIB

 

Range

100nM

Length

7.00m

Beam

2.55m

Draft

0.50m

Top Speed

3.5knt

Passengers

6 + Crew

 

Fig. 2.3 - Ocean Adventurer RIB

Scientific and on deck equipment:

  • Hull – Ribtech 700
  • Simrad SDGPS, depth sounder and chart plotter
  • Temperature / Salinity probes
  • Secchi Disk
  • Phyto / Zooplankton nets

Uses:

Ocean Adventurer is smaller and more versatile, allowing it to travel further up the estuary and tributaries, for sample collecting.

 

OffshoreGeophysicsEstuarine

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Equipment

CTD and Rosette

Fig. 3.1 - CTD on Rosette

The CTD is deployed into the water column to measure conductivity (salinity) and temperature against depth. It is attached to a rosette, thus allowing room for sampling bottles (Niskin bottles) and other equipment, such as a fluorometer and a transmissometer. These readings are then all relayed back to the onboard computer, logged into a data file and saved for analysis on land.

Niskin Bottle

Fig. 3.2 - Niskin Bottle

Niskin bottles (fig 3.2) are used to take water samples from known depths in the water column. They can be attached to a rosette and closed electronically by a sending a signal from the onboard computer. A bottle can also be deployed on a hydroline, using a messenger sent down the line to close the bottle. On smaller vessels, the Niskin bottle can sample the surface waters whilst being held by hand.

ADCP
 

The Acoustic Doppler Current Profiler is used to measure water current velocities and directions over the whole water column. The ADCP emits known acoustic signals over a range of angles, which are then reflected by the water particles. Depending on whether the particle is moving towards or away from the ADCP, the frequency will be either higher or lower than the original transmission.

Secchi Disk

Fig. 3.3 - Secchi Disk

The Secchi Disk is a simple, but effective piece of equipment, as shown in fig. 3.4, it is comprised of a circular disk divided into segments of black and white. This is then attached to a rope with metre markings on it. The disk is then lowered into the water, to the point at which it just goes out of sight. The depth from the surface down the string is known as the Secchi Disk depth. In order to calculate the depth of the 1% light level, or euphotic depth, the Secchi Disk depth is multipled by three.

Zooplankton Nets

Fig. 3.4 - Zooplankton Net

The zooplankton net is comprised of a conical net with a collecting bottle attached to the end of it. Another form of the same net is the zooplankton closing net. This has a depth gauge attached to the top of the net, which is used to determine when the messenger should be used to close the net. The first of these nets is commonly used in horizontal sampling, whereas the latter is used vertically.

Sidescan Sonar

Fig. 3.5 - Sidescan Towfish

The sidescan sonar is commonly used to map the seabed. The towed sidescan unit, known as the towfish, emits acoustic pulses over a range of angles towards the seafloor. The time interval and strength of the reflected pulse are combined to give a real-time image of the seabed. Features which protrude out of the seabed provide stronger reflection than those which are submerged into the seabed. It is this variation in strong and weak areas which can be interpreted to give an image of the seabed.

DGPS Max
  The DGPS Max is used for geophysical surveys, as it is accurate to within one metre.

 

Van Veen Grab

Fig. 3.6 - Van Veen Grab

The Van Veen Grab is used in bottom truthing. It is lowered through the water column, and once it hits the seabed, its latch opens, and thus captures a sample of the sea floor when it is pulled back up.

Video Camera

Fig. 3.7 - Video Camera

The use of a video camera in geophysical surveys is useful for providing a picture of the seabed for both grab samples (ensuring the site is suitable) and checking sidescan results. The camera is usually attached to a cable, with a weight attached to the bottom. This is lowered slowly through the water column, with a live feed to an on board television screen. There is also a ring of lights around the camera for seeing below the euphotic zone; however, this usually distorts colours.

T/S Probe or YSI Probe

Fig. 3.8 - YSI Probe

The T/S Probe can be used to monitor changes in water salinity and temperature. This can be done by placing the probe into a bucket that is constantly being filled by an on-board pump, which is collecting water from the surface. Alternatively, the YSI Probe can be manually lowered through the water column, providing depth, temperature, salinity, dissolved oxygen, pH and other parameters.

OffshoreGeophysicsEstuarine

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Offshore Boat

Thursday 2nd of July 2009

ADCPCTDNutrientsOxygenPlanktonDiscussion

Tidal Information  (GMT)

HW

0032

4.4m

LW

0707

1.8m

HW

1312

4.4m

LW

1938

1.9m

 

 Positions for the day:

 PSO: Pippa

 On Deck: Ben, Charles, Tom

 Labs: Carl, Emelie, Stef

 Computer: Suzie

 Log Book: Ben and Suzie

 

Weather: showers, complete cloud cover, 3-6m/s wind 140°

 

Aim:

To interpret and understand the biological, chemical and physical characteristics and relationships seen in the immediate offshore environment near the Fal estuary.

 

Readings were taken from our start point at Black Rock, to the Lizard Point, with one station the other side of the Lizard Point. Since the tide was flooding, the main eddies caused by the headland are assumed to be east of this point. The station to the west of Lizard Point will give an indication of the pre headland water column structure. Measurements in the more sheltered Falmouth Bay area will give an indication of the structure of the water column in this environment.

Equipment Used:

ADCP

CTD

Secchi Disk

Vertical Closing Net - 200μm mesh and 60cm diameter

Wet Lab - equipped with: glass bottles for nitrate and phosphate, phytoplankton and O2 samples; plastic bottles for silicon samples; and plastic tubes for chlorophyll samples.

 

Equipment

 

Method:

En route to Lizard Point on RV Callista, 5 stations were sampled and two further points were used for the ADCP:

Vessels

 
Table 1 - Positions of Stations/Points
Station or Point Latitude Longitude Location
1 50°08.669 N 005°01.338 W Black Rock
2 50°04.557 N 004°59.770 W The Wrigglers
3 49°59.084 N 005°03.695 W One mile SE of Black Head
4 49°56.053 N 005°11.250 W Two miles SE of Lizard Point
5 49°55.618 N 005°16.788 W Two miles SW of Lizard Point
A 49°56.99 N 005°10.29 W East of Lizard Point
B 49°59.540 N 005°05.296W The Manacles

Fig 4.1 - Map of Transects

Click map to enlarge

Between the stations, as well as at each station, the ADCP was used to record the structure of the water column. The ADCP was also used to plot two transects on the return journey. These were between station 5 and point A, and point A and point B.

At each of the stations, the CTD was deployed to within approximately 2m of the seabed. From the data acquired, appropriate water sample depths were chosen. These comprised of a deep, medium and shallow water sample. Once the CTD was back on board ship, samples for dissolved oxygen, silicon concentration, nitrate and phosphate concentration, phytoplankton content and chlorophyll concentration were taken from each depth, and appropriate fixing reagents were added. A Secchi Disk reading was also taken. The CTD and the ADCP readings on the computers were used to determine depth ranges for the zooplankton sampling.

 

ADCP

Transect 1 - From station 1 to station 2 (0856 - 0922)

Fig. 4.2a - Backscatter for Transect 1

Fig. 4.2b - Direction for Transect 1

The first 1000m of transect 1 shows a two layer system, with the top layer reaching velocities of about 0.25m/s in a north-north-westerly direction, and the bottom layer flowing easterly at around 0.01 – 0.05m/s. Once away from the mouth of the estuary, the water velocity is mainly homogenous with values between around 0.05 – 0.1m/s.  Across the transect, the direction changes from north-west, through west, to south. High backscatter values occur at depth (75 – 82dB) which is likely to be caused by high suspended bed load from the estuary.

Fig. 4.2c - Velocity for Transect 1

Transect 2 - From station 2 to station 3 (1012 - 1058)

Fig. 4.3a - Backscatter for Transect 2

Fig. 4.3b - Direction for Transect 2

The flow in transect 2 shows a well defined two-layer structure. The surface water appears to flow in a south-south-easterly direction changing to south-westerly along the transect, which may indicate the influence of the headland. There appears to be similar flow in the bottom layer, but in the opposite direction. At Black Head, the flow starts in a north-easterly direction, which changes to west-north-west at Wrigglers. Fig 4.3c shows a transition in flow speed between the top and bottom layers. In the top layer, velocity fluctuates between 0.2-0.25m/s, and in the bottom layer the range is between 0.01 and 0.10m/s. There is high backscatter regions along the transect (75 – 78dB) which could be due to either high turbulence or suspended bed loads in the bands nearer the bottom.

Fig. 4.3c - Velocity for Transect 2

Transect 3 - From station 3 to station 4 (1136 - 1220)

Fig. 4.4a - Backscatter for Transect 3

Fig. 4.4b - Direction for Transect 3

Fig. 4.4b appears to show a largely homogenous water column flowing in a north-easterly direction around the headland with a layer of water coming southwards down the coast around Black Head. Due to the different direction and velocity properties of the two bodies of water, when they meet they create flow velocity and direction gradients, which is likely to cause shear and mixing. There is an area of high backscatter near the headland (75 to 80dB) which may have been caused by turbulence created as the water passes the headland.

Fig. 4.4c - Velocity for Transect 3

 

Transect 4 - From station 4 to station 5 (1256 - 1327)

Fig. 4.5a - Backscatter for Transect 4

Fig. 4.5b - Direction for Transect 4

Towards Lizard Point, fig. 4.5c shows a large amount of fast flowing water (0.5 – 0.8m/s) mixing through the water column, indicating eddying around Lizard Point and the seafloor outcrop. This assumption is supported by the decrease in velocities of the water column to around 0.35 – 0.45m/s once the point has been rounded. There is a small fast flowing region towards the middle of the water column (around 0.55m/s) which corresponds to an area of high backscatter (75 – 80dB) possibly indicating turbulence, as opposed to a phytoplankton bloom.

Fig. 4.5c - Velocity for Transect 4

Transect 5 - From station 5 to point A (1357 - 1432)

Fig. 4.6a - Backscatter for Transect 5

Fig. 4.6b - Direction for Transect 5

As transect 5 was taken travelling eastwards around Lizard Point, it is similar to transect 4 in that it shows a south-easterly flow around the headland. However, past the headland there appear to be lower velocities (0.35 – 0.55m/s) due to the slacking of the tide. The surface layer follows the coastline, in a north-easterly direction. However, it is possible that due to the decrease in velocity at depth, the flow is turning back on itself and creating small eddies. There is an area of high backscatter (75 – 80dB) in the surface waters and at depth after the headland, which may correlate to the change in flow velocities.

Fig. 4.6c - Velocity for Transect 5

 

Transect 6 - From point A to point B (1433 - 1507)

Fig. 4.7a - Backscatter for Transect 6

Fig. 4.7b - Direction for Transect 6

In transect 6 there appear to be two main bodies of water. The first is moving southerly, as it comes round the headland, and expands up to the surface layers, with a velocity between 0 and 0.15m/s. The second body of water is towards the end of the transect, where interference from The Manacles may have caused the formation of strong eddies (0.45m/s), and thus explain the change in direction from northerly to south-easterly. High backscatter (75 – 83dB) can be seen where the eddies are forming, and also where the water rounding the headland is being forced up through the water column.

Fig. 4.7c - Velocity for Transect 6

 

CTD - T/S Profiles and Richardson Numbers

Fig. 4.8a - T/S Profile for Station 1

Station 1 (fig. 4.8a)

At Station 1 the temperature profile shows a gradual decrease with increasing depth from 17.83°C at the surface to 15.78°C at depth. This gradual decrease indicates that there is no defined thermocline, which is characteristic of a well-mixed water column. However analysis with Richardson Numbers indicates a developing thermocline at 7m, which prevents intense mixing above this depth. They also indicate a turbulent (Ri <0.25) boundary layer below 20m. The salinity profile shows a slight increase with depth (0.14), which could be due to the influence of fresh water inputs at the surface and the mixing characteristics of the water column.

Fig. 4.8f - Richardson Numbers for Station 1

Fig. 4.8b - T/S Profile for Station 2

Station 2 (fig. 4.8b)

As at station 1, there appears to be no obvious thermocline, which is characteristic of a well mixed water column. The temperature decreases steadily from 17.65°C to 11.98°C. This is in the same ratio as the decrease seen at station 1 (approximately 2°C drop every 15m). However, with further analysis, Richardson Numbers show that the structure of the water column is in fact very complex, with three developing thermoclines at 10, 25 and 36m, which provide a stabilising element, preventing total homogeneity of the water column. Salinity shows a small increase from 34.56 to 35.22. This is caused by the denser, more saline water sinking below the slightly fresher water.

Fig. 4.8g - Richardson Numbers for Station 2

Fig. 4.8c - T/S Profile for Station 3

Station 3 (fig. 4.8c)

At Station 3 the salinity profile shows an overall change of less than 0.1 over 70m (although minor fluctuations are present). This minor change is again characteristic of a well-mixed water column. Richardson numbers however show that the water column shows a definite structure, with mixing occurring above 22m, and between 40m and the bottom, whereas there is a stable layer from 20 – 40m which is indicated in the T/S profile as well as by the Richardson Number plot (Fig. 4.8h). The change in temperature still does not show a fully defined thermocline, however a strong gradient between approximately 21m (16.5°C) and 36m (12.2°C) is present. This indicates a partially stratified water column, as the temperature past 36m only changes slightly (12.2°C at 36m – 11.8°C at 72m).

Fig. 4.8h - Richardson Numbers for Station 3

Fig. 4.8d - T/S Profile for Station 4

Station 4 (fig. 4.8d)

Station 4 is located near Lizard Point, a large headland. The influence of this feature can be seen in the structure of the water column, the erratic nature of the data collected indicates large differences between water layers and hence a high degree of mixing, this is also indicated by the Richardson Numbers in the surface layers (Ri <0.25), which show that the layer is turbulent. There is no defined thermocline, but a net temperature change from 16.86°C at the surface to 12.29°C at depth (not steady as the measurements are quite varied)  and a salinity gradient from 34.90 to 35.19 respectively. Developing thermoclines are however present at 10, 40 and 60m which provide a minor stabilizing effect on the otherwise well mixed water column.

Fig. 4.8i - Richardson Numbers for Station 4

Fig. 4.8e - T/S Profile for Station 5

Station 5 (fig. 4.8e)

Station 5 is located on the western side of Lizard Point, the data show a steadier decrease in temperature than at station 4 from 17.40°C at the surface to 12.95°C at depth with the data points showing less deviation. The salinity profile shows a small change from 34.8 at the surface to 35.2 at depth, this small change (0.4) over >50m again indicates the well-mixed nature of the water column. Richardson number analysis (Fig. 4.8j) shows a turbulent surface 5m (Ri <0.25) leading to a thermocline providing a large stabilising element from 5 – 10m, below this however is a large transitionary layer (Ri >0.25 & <1), indicating a degree of mixing but also a degree of stratification.

Fig. 4.8j - Richardson Numbers for Station 5

Nutrients

 

Station 1 (fig. 4.9a)

At Black Rock, chlorophyll concentrations are universally low. Throughout the vertical profile, the concentration does not go above 1.5μg/L, with most values between 1.1μg/L and 1.3μg/L. Although chlorophyll concentrations are low, phytoplankton cell numbers (mainly dinoflagellates) are high at 10.6m (see fig. 4.1a) which may suggest a bloom without a corresponding chlorophyll peak. This dinoflagellate bloom may also explain the low nitrate and phosphate levels at this depth. The data also show the presence of a surface water layer, with nitrate and phosphate concentrations being much higher in surface layer compared with deeper concentrations, whereas silicon concentrations are slightly lower in the surface layer than at depth. As station 1 was situated within the mouth of the estuary, it is likely that the surface water has been influenced by riverine inputs, explaining the high nitrate and phosphate concentrations; however, fig. 4.8a shows no riverine input. Lower levels of silicon in the surface layer may have been caused by a diatom bloom further up the estuary.

Fig. 4.9a - Nutrient and Chlorophyll Plot for Station 1

Station 2 (fig. 4.9b)

Chlorophyll concentrations at station 2 are low throughout the profile, though generally higher than at station 1. The high chlorophyll concentrations at the surface (>2μg/L) may be anomalies, as only 2 readings from the CTD gave this high concentration. Similarly, only one point at 51m reads 1.9μg/L. If these values are not taken into account, chlorophyll fluctuates between 1.1 and 1.3μg/L through out the water column. The nutrients, nitrate and phosphate, show opposite trends, with nitrate higher at the surface than at depth, and phosphate higher at depth than at the surface. Silicon is approximately seven times more concentrated at depth than at the surface, even though the highest concentration is 0.0066μg/L at 51.4m.

Fig. 4.9b - Nutrient and Chlorophyll Plot for Station 2

Station 3 (fig. 4.9c)

At station 3 there is a very prominent peak in chlorophyll at 9m. At 12μg/L, it is six times the level of chlorophyll in the water both above and below the peak, and over 12 times the level of the chlorophyll at the bottom of the profile. From figure 4.10c, it is clear that this bloom consists entirely of dinoflagellates at ~1300 cells/ml. Interestingly, phosphate and silicon concentrations appear to be slightly higher at 9m, along with the chlorophyll peak, rather than depleted in this layer. Nitrate levels were undetectable throughout the water column, which may have been due to complete utilization by the phytoplankton.

Fig. 4.9c - Nutrient and Chlorophyll Plot for Station 3

Station 4 (fig. 4.9d)

The levels of chlorophyll at station 4 are relatively uniform at about 1μg/L throughout the water column. Nutrient levels are very low in the euphotic zone, preventing a chlorophyll peak. Silicon and phosphate concentrations show an increase with depth, reaching 0.004μg/L and 0.2μg/L, respectively, at 64.2m. Similarly, nitrate is 1.36μg/L at 64.2m, and undetectable in surface water.

Fig. 4.9d - Nutrient and Chlorophyll Plot for Station 4

Station 5 (fig. 4.9e)

The final station shows a small chlorophyll peak above the background level of 0.8 to 1μg/L. Here, phytoplankton are possibly utilizing the nutrients mixed up from depth by tidal mixing. The chlorophyll maximum (~2μg/L) is at 12m and reaches background levels of below 1μg/L by 25m. Nitrate and phosphate are undetectable or zero around the chlorophyll peak. Nitrate is detectable again at 42m at 0.4μg/L. Silicon is measurable at depth, although it is low at about 0.001μg/L. The low levels of silicon at the surface may be due to the spring diatom bloom. As these diatoms have died, their sinking skeletons may have undergone dissolution, and therefore put some silicon back into the deeper water.    

Fig. 4.9e - Nutrient and Chlorophyll Plot for Station 5

 

Oxygen

Table 2 - Oxygen Saturation Results for Offshore
Station No. Depth (m) Temp. (°C) Salinity O2 % Saturation
1 27.0 16.1 35.2 106.3
10.7 17.0 35.1 109.1
17.9 17.9 35.0 109.3
2 51.4 12.0 35.2 86.3
23.5 14.8 35.1 102.7
9.8 17.3 35.1 109.4
3 24.6 15.5 35.1 127.0
9.6 17.9 35.2 113.8
3.7 18.2 35.2 111.8
4 64.2 12.3 35.2 96.1
30.4 14.9 35.2 100.1
6.7 15.6 35.1 99.4
5 42.2 13.4 35.2 94.7
16.5 15.5 35.2 82.9
4.3 17.3 35.2 107.4

Oxygen analysis was carried out, using the Winkler method, to investigate the amount of oxygen dissolved in the Fal and surrounding offshore waters.

At the first station, we found supersaturated oxygen levels. This correlates well to the high number of phytoplankton we collected at that site. Areas of high primary production usually portray high levels of oxygen during daylight, due to the process of photosynthesis. This pattern was observed again at site 3, where we found the highest levels of both oxygen saturation and primary production. This is also the site where oxygen levels were lowest at surface compared to depth – probably due to an increased activity of zooplankton .

Oxygen is taken out of the water by respiration and decomposition. In our deepest sample at site 2, there is a large depletion of oxygen. This is due to the lack of photosynthetic organisms operating at this depth.

Oxygen levels at site 5 are low compared to the other sites, except at the surface. Diffusion from the air would be high due to the high turbulence of the waves, due to an increase in surface area allows, Decomposition and respiration (especially through zooplankton) found at lower depths reduces oxygen concentrations.

Site 4 appears to be the most stable in terms of oxygen saturation levels. It is possible that the amount of consumed oxygen is almost evenly balanced by processes that give off oxygen at all the depths sampled. Also note the lower levels of oxygen in areas with less temperature. Colder waters are less capable of holding gaseous material.

 

Plankton

Table 3 - Secchi Disk Depths at Offshore Stations
Station Secchi Disk Depth (m) Euphotic Depth (m) Attenuation Coef. (k)
1 8.0 24.0 0.18
2 7.5 22.5 0.19
3 7.5 22.5 0.19
4 8.0 24.0 0.18
5 8.0 24.0 0.18

Phytoplankton

Fig. 4.10a - Phytoplankton at Station 1

Fig. 4.10b - Phytoplankton at Station 2

Fig. 4.10c - Phytoplankton at Station 3

Fig. 4.10d - Phytoplankton at Station4

Fig. 4.10e - Phytoplankton at Station 5

Figures 4.10a-e show that at this time of year in the Fal estuary the phytoplankton population is largely dominated by dinoflagellates, particularly of the species Karenia mikimotoi (fig. 4.11), a harmful invasive species first described in Japan in 1935. The first large red tide bloom in the English Channel occurred in 2003 spreading around the world in ship’s ballast water (Daugjberg et al, 2000). There are a few diatoms present as well, but these tend to dominate in spring and autumn rather than early July (Rodriguez et al, 2000) because of reduced silicon levels, (see figures 4.9a-e). Silicon is an important nutrient for diatoms as they use it to secrete in the form of opal frustules around their cell. Cilliates were present in small numbers but flagellates were absent from all stations except station 5 at a depth of 16.5m.

The largest abundance of phytoplankton were found at intermediate depths of around 7 to 20m. The surface waters contain few cells, most probably due to depleted nutrient levels (see figures 4.9a-e). At depths of more than 20m, the light levels are too low to support photosynthesis and so therefore, fewer phytoplankton are found at these depths. Normally at this time of year in the Western English Channel, irradiance is high and waters are stratified so that the surface mixed layer is above the compensation depth. This provides ideal conditions for phytoplankton growth and blooms occur throughout the summer. Surface waters become depleted of nutrients and so the dinoflagellates are found at a chlorophyll maximum on the thermocline where there are more nutrients.

Station 3 had the largest number of cells per ml with a maximum of 13100 cells per ml, all of which were dinoflagellates. Station 4 had the lowest abundances of phytoplankton, reaching a peak of only 900 cells per ml at 6.7m.

Fig. 4.11 - Karenia Mikimotoi

Zooplankton

The zooplankton samples were concentrated down to 500ml of water and a 2ml sub sample of this was analysed under a microscope. Table 4 shows the depths sampled at each station.

Table 4 - Zooplankton Net Depths

Station

Depth of Tow
1 13 - 7m
2 30 - 0m
3 12 - 7m
28 - 18m
4 30 - 0m
5 20 - 7m

 

The figures 4.11a to 4.11g show that the zooplankton are mainly composed of hydromedusae and echinoderm larvae.

 

Fig. 4.11a - Key for Zooplankton Graphs

 

Station 1 was sampled between 13 and 17m and is dominated by cnidarians with a concentration of 1179.2 hydromedusae per m3 seawater. Cirripedes, polychaetes and decapods were also abundant.

Fig. 4.11b - Zooplankton at Station 1

 

 

 

Station 2 sampled the top 30m of the water column and it was found that echinoderm larvae dominated here with an average of 736.9 animals per m3 of water. There were also many hydromedusae (486.3 per m3) and polychaetes (221.1 per m3).

 

 

Fig. 4.11c - Zooplankton at Station 2

 

Station 3 had two samples, one shallow and one deep. The shallow sample showed a wide range of taxa dominated by echinoderm larvae (693.9 per m3), hydromedusae (517.0 per m3) and copepoda (258.5 per m3). The deep sample had smaller numbers of individuals in most taxa, for example copepods. However the sample was largely dominated by extremely high concentrations of hydromedusae reaching 4333.2 individuals per m3.

Fig. 4.11d - Zooplankton at Station 3 Deep

Fig. 4.11e - Zooplankton at Station 3 Shallow

 

Station 4 was another sample of the top 30m of water. It contained fewer cnidarians and echinoderms than previous stations and in fact, cirripede larvae were the most abundant although they were only in concentrations of 206.3 per m3. This station was found to have unusually low numbers of zooplankton.

Fig. 4.11f - Zooplankton at Station 4

 

Station 5 shows the largest echinoderm larvae concentration (1292.2 per m3) of all the stations. Copepods, chaetognaths and hydromedusae are also present.

Fig. 4.11g - Zooplankton at Station 5

 

Discussion

The investigation was aimed at interpreting and better understanding the biological, chemical and physical characteristics and relationships seen in the immediate offshore area near the Fal Estuary. ADCP data relayed information showing increased turbulence around coastal headlands. An increase in turbulence results in mixing throughout the water column. Firstly, wave action will increase the water surface area allowing greater diffusion between the air and surface waters. This is seen mostly at station 5, where the water in the upper layer is supersaturated with oxygen, whereas the samples beneath show greater amounts of oxygen depletion. As well as gaseous exchanges, turbulence also plays a key role in biological, ecological and chemical relationships. Turbulence mixes the water column, meaning there is less stratification. A less stratified water column makes it more difficult for phytoplankton populations to grow because they are constantly being mixed throughout the euphotic zone and below this, where photosynthesis is not possible. A well mixed water column also means salinity will be very similar throughout. Low concentrations of nitrate, phosphate and silicon were found in the euphotic zone (top 20-25m), probably resulting from phytoplankton consumption. Concentrations of the nutrients are likely to have increased below this depth because less phytoplankton were found there. Karenia mikimotoi, a dinoflagellate, was the dominant phytoplankton species found at all stations. Although they do not use silicon to a large extent, levels remain low most likely due to a previous spring diatom bloom. Oxygen levels in the surface layer were mainly observed to be supersaturated due to the presence of photosynthetic organisms, but this is only likely to have an impact during daylight hours, due to low irradiance at night. Zooplankton numbers correlate strongly to phytoplankton concentrations. The main taxa found in this region were hydromedusae and echinoderm larvae which are characteristic of low nutrient environments.

 

 

 

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Geophysics Boat

Monday 6th of July 2009

Grab SitesSidescanDiscussion

 

Tidal Information  (GMT)

HW

0015 4.6m

LW

1101 1.4m

HW

1632 4.9m

LW

2325 1.4m

 

Positions for the day:

PSO: Carl (the boss)

On Deck: Ben, Emelie, Stef, Suzie, Tom

Scribes: Charles, Emelie, Pippa

Sonar Monitor: Ben and Charles

Video Monitor: Charles

 

Weather: West or South-West, force 5-7, increasing to 8 at times, veering northwards later, sea state rough, squally showers, visibility moderate or good.

 

Aim:

To survey the seabed in the bay off Swanpool, Falmouth, whilst also using the Van Veen Grab to bottom truth the sidescan data.

 

Equipment used:

Sidescan Sonar

DGPS Max

Video Camera

Van Veen Grab

Hydropro Computer Package

 

Equipment

Method:

In the bay of Swanpool, 6 transects were made on RV Xplorer:

Vessels

 

 

Table 5 - Start and End of Transects
 

Start

End

Transect Latitude Longitude Latitude Longitude
1 50°08.747 N 005°03.200 W 50°08.201 N 005°04.404 W
2 50°08.174 N 005°04.295 W 50°08.705 N 005°03.156 W
3 50°08.660 N 005°03.101 W 50°08.124 N 005°04.277 W
4 50°08.070 N 005°04.244 W 50°08.607 N 005°03.072 W
5 50°08.575 N 005°03.003 W 50°07.986 N 005°04.277 W
6 50°07.977 N 005°04.147 W 50°08.513 N 005°02.936 W

Fig. 5.1 - Sidescan Sonar Transects and Grab Sites

Click to map to enlarge

 

Along each of these transects, a sidescan sonar towfish was used to map the seabed. Using the data from the sidescan, grab sites were chosen to bottom truth the results. Granular material produces specific sedimentary bedforms depending on the level of hydrodynamic forcing it is exposed to, these bedforms are then visible on the sidescan sonar trace. All of this data was then used to provide a description and interpretation of the seabed in the area.

 

Grab Sites

Grab Site 1

Time: 1016 GMT

Latitude: 50°08.482N  Longitude: 005°03.112W

Sediment:

When sieved approximately 70% of the sediment had a particle size of greater than 2mm, 20% between 1 and 2mm and 10% less than 1mm. Sediment could possibly be a very fine gravel, but with increased grain size due to the large volume of dead mearl, overlying a muddy sediment. Live:Dead maerl ratio is 1:1. Large clumps of live maerl may stop bedforms from forming.

Biology:

The proportions of live maerl seem to be indicative of the equitability and diversity of species in the benthic habitat. Due to this the black brittle star (Ophiocoma nigra) and the Banded Carpet Shell (Paphia rhomboids) are largely dominant. There is also a high proportion of dead shells from which the Small Queen Scallop (Aequipecten opercularis) and Qahog (Mercenaria mercenaria) can be identified. These species are all indicative of a coarse gravely sediment.

 

Fig. 5.2 - Grab Samples from Site 1

Grab Site 2

Time: 1029 GMT

Latitude: 50°08.595N  Longitude: 005°03.532W

Sediment:

When sieved approximately 90% of the sediment had a particle size of greater than 2mm and only 10% less than 1mm. The sediment had a high proporition of calcareous sediment with a Live:Dead maerl ratio of 1:19. Distinct bedforms could be defined possibly due to wave action, since they show bification.

Biology:

The Banded Carpet shell (Paphia rhomboid) is abundant (2 small, 3 medium and 3 large identified) indicating these bivalves are a highly resilient species able to colonise a high stress environment near to the shore.

 

Fig. 5.3 - Grab Sample from Site 2

Grab Site 3

Time: 1046 GMT

Latitude: 50°08.356N  Longitude: 005°03.747W

Sediment:

When sieved approximately 97%  of the sediment had a particle size greater than 2mm, 2% between 1 and 2mm and 1% less than 1mm. The sediment could be defined as very fine gravel sized sediment with small traces of mud however the majority of these particles are calcareous deposits with a Live:Dead maerl ratio of 3:17. Bedforms were again identified. The wavelength of these bedforms may affect the representation of the area as it could be sampled in a peak or a trough.

Biology:

The biota is again influenced by the Live:Dead maerl ratio. More biota was found than in grab 2 with the Black Brittle Star (Ophiocoma nigra) again present (2 individuals) along with 3 unidentifiable Polychaete worms and 2 Banded Carpet Shells (Paphia rhomboids). A Juvenile fish was also caught in the grab.

 

Fig. 5.4 - Grab Sample from Site 3

Grab Site 4

Time:  1104 GMT

Latitude: 50°08.130N  Longitude: 005°03.815W

Sediment:

When sieved approximately 98% of the sediment had a particle size of greater than 2mm with only 2% between 1 and 2mm. The sediment could be defined as very coarse gravel sized sediment but the majority of sediment defining this grain size is dead maerl. The Live:Dead maerl ratio is 1:19.

Biology: 

The biota was similar to grab site 2 with 2 polychates identified as well as 2 live Banded Carpet Shells (Paphia rhomboids). There was also a high proportion of dead bivalve shells and other detritus. Shells accumulate in the troughs of the bedforms and so the troughs are richer in biogenic materials.

 

Fig. 5.5 - Grab Sample from Site 4

Sidescan

The majority of the seafloor investigated was shown to be transverse sinuous in-phase bificated megaripples.  These megaripples were typically around 1.3m high and 0.6m long. They are small symmetrical low energy current ripples. As 2-D megaripples form perpendicular to the flow direction, this shows that the waves were coming from the South East, and travelling North Westerly. The megaripples found varied slightly in dimensions due to localised differing flow speeds, but typically they are formed in areas of turbulent rough flow and intermediate flow speeds.

As we approached the coastline to the North, the water became shallower and there were less distinctive bedforms found. This resulted in two distinct areas of flat homogenous seabed. These were to the North East and the West of the bay. In the North East, the lack of bedforms is explained by the effect of the headland at Pendennis Point. This headland acts as a shelter to the benthic environment, which is therefore exposed to slower wave velocities. The area of homogenous flatbed to the West is the result of the shape of the coastline in the bay.

Areas of rocky outcrop were found nearest to the cliff boundary in the far East and West of the bay, as well a smaller rocky outcrop to the South of the study area. Finally, one area of the rock to the North West, where the depth was the shallowest, was found to support a population of non-calcareous macroalgae.

 

Fig. 5.6 - Section of Sidescan Trace

Discussion

Over the surveyed area the main sediment could be defined as fine gravel, which is mainly due to calcareous deposits of dead maerl. The dominant particle size is greater than 2mm with traces of fine mud at some sites.  A large area of transverse sinuous in phase megaripples was identified in the centre of the bay with areas of homogeneous bed and rocks in western and eastern areas. The biota identified from the grab samples was a reflection of the ratio of live to dead maerl. The dominant species are Black Brittle Stars (Ophiocoma nigra) and Banded Carpet Shells (Paphia rhomboids) with a large amount of dead shells found at most sites. Some unidentifiable polychaete worms were found in grabs 3 and 4.

 

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Estuarine Boat

Thursday 9th of July 2009

ADCPCTDNutrientsOxygenChlorophyllPlanktonDiscussion

 

Tidal Information  (GMT)

HW

0041 1.2m

LW

0615 4.8m

HW

1254 1.3m

LW

1826 5.1m

 

 Positions for the day:

 PSO: Tom

 On Deck: Ben and Charles

 Labs: Carl, Emelie and Stef

 Computer: Pippa and Suzie

 Log Book: Pippa

 

Weather: mostly overcast with sunny intervals, max temperature 20°C, visibility very good, wind direction NNW, speed 13-14 mph, gusts of up to 26-28 mph

 

Aim:

To analyse how the Fal estuary changes for certain physical, chemical and biological parameters from the source, where the salinity is 0, to the estuary mouth near Black Rock, where the salinity approaches 35. This will develop an understanding of how the estuary acts as a transition zone between the freshwater river and the start of the coastal waters.

 

Equipment Used:

T/S and YSI Probes

CTD

Secchi Disk

Zooplankton Net

Lab Equipment (glass bottles for nitrate and phosphate, phytoplankton and O2 samples; plastic bottles for silicon samples; and plastic tubes for chlorophyll samples)

 

Equipment

Method:

Starting in the upper estuary, on Ocean Adventurer RIB, four stations were sampled. Once on Xplorer, a further four stations were sampled and four transects were made across the estuary. Between stations 1 and 2, and along transect 4, a zooplankton net was also towed. The positions of these have been tabulated below:

Vessels

 

Table 6 - Stations in Upper and Lower Estuary
Station Latitude Longitude
Upper Est. 1 50°14.699 N 005°01.383 W
Upper Est. 2 50°13.711 N 005°00.946 W
Upper Est. 3 50°12.957 N 005°01.663 W
Upper Est. 4 50°12.566 N 005°01.677 W
Lower Est. 1 50°12.438 N 005°01.840 W
Lower Est. 2 50°10.838 N 005°01.731 W
Lower Est. 3 50°09.691 N 005°02.201 W
Lower Est. 4 50°08.496 N 005°01.462 W

 

Table 7 - Transect Start and End Points
  Start End

Location

Latitude Longitude Latitude Longitude
TRAN 1 50°12.391 N 005°01.793 W 50°12.494 N 005°01.856 W
TRAN 2 50°10.864 N 005°01.572 W 50°10.481 N 005°02.495 W
TRAN 3 50°09.697 N 005°03.186 W 50°09.681 N 005°01.804 W
TRAN 4 50°08.431 N 005°01.113 W 50°08.618 N 005°02.447 W

 

Fig. 6.1 - Stations in Upper and Lower Estuary and Transects

Click map to enlarge

Table 8 - Locations of Zooplankton Trawls
  Start End
Trawl Latitude Longitude Latitude Longitude
A 50°12.264 N 005°02.298 W 50°12.992 W 005°02.498 W
B 50°08.438 N 005°01.190 W 50°08.469 W 005°01.448 W
 

Samples in the upper estuary were taken at the surface for nitrate, phosphate, silicon and chlorophyll. At station 4, a phytoplankton samples was also taken. A YSI Probe was also used to measure salinity, percentage saturation of dissolved oxygen, temperature and pH against depth through the water column.

In the lower estuary, a CTD rosette was deployed at each station to measure temperature, salinity and turbidity against depth. Samples for nitrate, phosphate, silicon, chlorophyll, percentage saturation of dissolved oxygen and phytoplankton were also taken at approximately 1m from the benthos, at the 1% light level (determined by using the Secchi Disk) and at the surface.

 

ADCP

 

Transect 1 - East to West (1137 - 1140)

Fig. 6.2a - Backscatter for Transect 1

Fig. 6.2b - Direction for Transect 1

The flow direction and velocity contour plots (figures 6.2b and 6.2c respectively) for transect 1 appear to show a layer of faster southwards moving water (0.3m/s), with a layer of slower (0 – 0.13m/s) well-mixed water below. The faster, lighter water at the surface is likely to be a layer of riverine influenced water moving southwards with the ebbing tide, whereas the deeper water is as more dense, probably due to the influence of the tidal flow, thus forming an eddy structure. The backscatter contour plot (Fig. 6.2a) shows a fairly uniform readout with no high surface readings, indicating little or no primary production in the region.

 

Fig. 6.2c - Velocity for Transect 1

Transect 2 - East to West (1226 - 1239)

Fig. 6.3a - Backscatter for Transect 2

Fig. 6.3b - Direction for Transect 2

The direction contour plot (Fig. 6.3a) for transect 2,  moving east to west from Messack Pt to Penarrow Pt, shows the ebbing tide propagating southwards through the main channel (6 – 15m travelling at 180° to the transect). A region of deeper, probably well-mixed water can also be seen moving perpendicular to the ebbing tide, likely to be due to the slack water in the deeper contours being mixed by the shear between the above layer of tidal flow. This consequently causes an eddy structure.  The flow velocities (Fig. 6.3c) throughout the water column are fairly uniform (0 – 0.125 m/s) with a stationary boundary layer (15 – 17.5m) separating the two regions, moving in opposite directions. The backscatter contour plot again shows fairly uniform levels (60 – 65dB). However, there are some regions of high levels towards the end of the transect, which are due to bottom noise in the shallower water.

 

Fig. 6.3c - Velocity for Transect 2

Transect 3 - West to East (1254 - 1304)

Fig. 6.4a - Backscatter for Transect 3

Fig. 6.4b - Direction for Transect 3

Conducted 15 minutes after low water, the flow direction and velocity contour plots (Figs. 6.4b & 6.4c) for transect 3, show the tide in its slack water state. Flow velocities and directions throughout the water column are erratic, showing sharp changes over small areas with flow velocities not exceeding 0.125m/s and generally averaging lower than 0.06m/s. The backscatter contour plot (Fig. 6.4a) is once again fairly uniform with surface values averaging above 65dB, indicating higher levels of primary production in these areas, however no areas of especially high backscatter levels can be identified.

 

Fig. 6.4c - Velocity for Transect 3

Transect 4 - East to West (1334 -1355)

Fig. 6.5a - Backscatter for Transect 4

Fig. 6.5b - Direction for Transect 4

The flow direction and velocity contour plots (Figs. 6.5b & 6.5c) for transect 4 show the flood tide propagating North (90° from the ship track). However, two regions of flow can be identified. Water of a high flow velocity (0.25 – 0.4m/s) can be seen at the far end of the transect (800m – 1574m), possibly caused by the tidal stream flowing fast around Pendennis Pt and the shallow water causing higher flow velocities. Another region of flow can be identified in the main channel, this region is slower (0 – 0.125m/s), possibly due to the greater depth further off Pendennis Pt coupled with a similar tidal stream velocity. The backscatter contour plot (Fig. 6.5a) is again generally uniform, although a region of high backscatter (80 – 100dB) between 0 – 10m at approximately 1300m can be seen along the track. This could be due to an algae bloom.

 

Fig. 6.5c - Velocity for Transect 4

CTD

 

Upper Estuary Station 1

 

Fig. 6.5a - T/S Profile at Upper Estuary Station 1

Temperature and salinity both show a smooth gradient in relation to depth, with an overall change of 0.52°C and 0.30 respectively. As with previous stations, the absence of a thermocline is typical of a well-mixed water column.

 

Upper Estuary Station 2

 

Fig. 6.5b - T/S Profile at Upper Estuary Station 2

The measurements for temperature at this station agree with the patterns seen previously, with no distinct thermocline, rather a steady decline from 16.1°C to 15.3°C. Again, this suggests that the water is well-mixed. Salinity has a very shallow gradient, increasing by only 0.73 over 9m.

 
 

Upper Estuary Station 3

 

Fig. 6.5c - T/S Profile at Upper Estuary Station 3

At this station temperature changes relatively little with depth, with an overall decrease of 0.38°C. Salinity continues to demonstrate a minimal variation of 0.30. Thus this station seems to follow the same pattern as previous stations.

 

Upper Estuary Station 4

 

Fig. 6.5d - T/S Profile at Upper Estuary Station 4

Apart from the small spike in temperature at 0.5m, which may be an anomalous reading, the measurements exhibit a smooth decrease with depth, from 15.66°C to 15.00°C. Salinity, as with previous stations, shows a nominal change of 0.41. As previously stated, the absence of a distinct thermocline suggests a well-mixed water column.

 

Lower Estuary Station 1

 

Fig. 6.5e - T/S Profile at Lower Estuary Station 1

Temperature shows a gradual change with depth, from 15.92ºC to 15.09ºC, with no defined thermocline. Salinity also demonstrates little change with depth, with a total increase of 0.61. Both of these patterns are characteristic of a well-mixed water column. However further analysis using Richardson Numbers (Fig. 6.5f) shows a layer of laminar water near the surface (0 – 10m, Ri > 1) separated by a thermocline at 8-9m from a turbulent (Ri <0.25) boundary layer below 11m.

Fig. 6.5f - Richardson Numbers for Lower Estuary Station 1

Lower Estuary Station 2

 

Fig. 6.5g - T/S Profile at Lower Estuary Station 2

As before there is evidence of a well-mixed water column, as there is no definite thermocline or halocline. Temperature displays a steady negative gradient with depth, changing from 14.95ºC at the surface to 13.96ºC at depth. Salinity changes by just 1.5 across 25m. With analysis using Richardson numbers, again a laminar layer of water (Ri >1) is may be present from the surface to 16m, which is separated from a turbulent layer (Ri <0.25) below 18m by a developing thermocline at 17m.

Fig. 6.5h - Richardson Numbers for Lower Estuary Station 2

Lower Estuary Station 3

 

Fig. 6.5i - T/S Profile at Lower Estuary Station 3

Although the temperature readings at this station show more variation than previous stations, there is still no evidence of stratification from the T/S profiles. Temperature changes from 14.92ºC to 13.74ºC, and salinity increases from 34.99 to 35.23. Richardson Numbers show that there are in fact three developing thermoclines at 5, 10 and 17m. These thermoclines provide a stabilizing element and prevent intense mixing above this depth, but not a large enough element to induce stratification.

Fig. 6.5j - Richardson Numbers for Lower Estuary Station 3

Lower Estuary Station 4

 

Fig. 6.5k - T/S Profile at Lower Estuary Station 4

This station follows the same pattern as all the previous stations. The temperature changes from 14.46ºC to 13.55ºC, and salinity changes by 0.18, suggesting a well-mixed water column. A trend can be seen on progression through these four stations – moving towards the mouth of the estuary, the temperature decreases and salinity increases, most likely due to tidal forces mixing the freshwater (river) and saline (sea) water masses. Richardson Numbers show that the water column is split into two layers. A laminar (Ri >1) layer from the surface to 10m, and a turbulent (Ri <0.25) layer beneath this.

Fig. 6.5l - Richardson Numbers for Lower Estuary Station 4

 

Nutrients

An Estuarine mixing diagram is used to asses whether a nutrient behaves in a conservative or non-conservative way throughout an estuary. The diagram is a plot of the concentration of the constituent against a conservative element that can be used as a tracer. In almost all estuarine mixing diagrams this tracer is salinity. Riverine and seaward end members are identified as the furthest up river point with the least salinity and the most seaward point with the most salinity respectively. A Theoretical Dilution Line (TDL) is then drawn between the two. Points are plotted on the graph and deviation from the line is used to determine conservative or non-conservative behaviour. Conservative behaviour is when the points plot close to the TDL and the concentration is determined by the mixing of the riverine and seaward end members. Non-conservative behaviour is when the points plot off the TDL and the constituent is being added or removed from the estuary due to biological uptake, addition from pore waters or other processes. If the points are above the TDL then the constituent is being added, if it is below then the constituent is being removed.

 

Nitrate

Fig. 6.6a - Nitrate Mixing Diagram

Figure 6.6a is an estuarine mixing diagram for nitrate. Riverine end members are plotted at very low salinities and high concentrations. The values recorded in the estuary however are at much higher salinities and lower concentrations. Therefore when all these values are plotted there is a small cluster of points due to the large salinity and concentration range. It is impossible to determine conservative or non-conservative behaviour from this figure. Therefore the riverine end members are not plotted on figure 6.6b below and the riverine end member is taken as the lowest salinity value from the RIB stations. The lowest salinity RIB station is also taken as the riverine end member for the estuarine mixing diagrams for phosphate and silicon.

 

Fig. 6.6b - Nitrate Mixing Diagram Using Lowest Salinity

Figure 6.6b suggests that nitrate could be behaving non-conservatively in the estuary. All of the RIB points and the majority of the Xplorer points are plotted below the TDL suggesting there is a removal of nitrate in the estuary.

 

 

Phosphate

Fig. 6.6c - Phosphate Mixing Diagram at Lowest Salinity

Figure 6.6c is a mixing diagram for phosphate. The figure suggests reasonably uniform concentrations down the estuary. One value however has a much higher concentration of 2.55µmol/L suggesting that phosphate is being added at this point. This location is station 3 on the rib near to the King Harry ferry so could possibly be an anthropogenic source. 

 

 

Silicon

Fig. 6.6d - Silicon Mixing Diagram at Lower Salinities

Figure 6.6d is a mixing diagram for silicon. The figure suggests that there is possible removal of silicon as all points apart from one are plotted below the TDL. The main process of silicon removal is likely to be biological uptake by diatoms. This removal of silicon may coincide with a higher concentration of diatom cells.

The analysis of the three nutrients suggests that phytoplankton are utilising nitrate in the higher salinity waters completely. Therefore nitrate is the limiting nutrient in the estuary, and phosphate levels remain un-depleted. Analysis of the collected phytoplankton samples (figure 6.8a) shows that a significant proportion of the phytoplankton are diatoms, which also utilise silicon, as well as nitrate and phosphate. As silicon levels are lowest in the higher salinity water, this suggests that the type of phytoplankton depleting the water of nitrate are diatoms.

 

Oxygen
Table 9 - Oxygen Saturation Results for Estuary
Station Number Depth (m) Temperature (°C) Salinity O2 % Saturation
Lower Estuary 1 1.0 16.02 33.89  
7.5 15.28 34.38 98.9
12.3 15.08 34.54 104.5
Lower Estuary 2 1.3 14.92 34.83 102.0
13.5 14.36 35.08 96.8
24.0 13.97 35.18 94.7
Lower Estuary 3 1.0 14.92 34.96 107.0
16.5 14.36 35.17  
28.0 13.97 35.23 94.9
Lower Estuary 4 1.0 14.67 35.06  
18.0 13.59 35.25 93.9
22.4 13.57 35.26 94.1
Upper Estuary 1 1.0 16.71 32.90 100.8
1.0 16.71 32.90 101.0
1.0 16.71 32.90 110.3

An oxygen analysis was carried out, using the Winkler method, to investigate the amount of oxygen dissolved in the Fal and its tributaries. Unfortunately it should be noted some of the samples were exposed to the air and therefore gave no reliable results (left blank in table 9).

Stations 1 and 4 show more saturated levels of oxygen deeper in the water column.  In contrast, stations 2 and 3 show supersaturated levels of oxygen in the surface layers, and less saturated in the deeper layers. Measurements from the RIB, the furthest point up the estuary, show supersaturated levels of oxygen in the top layer.

             It would therefore be most likely that the unknown quantities of the surface layers at station 1 and station 4 would be supersaturated. This suggests that primary production is active in the top layers of the euphotic zone. Phytoplankton levels were found to be high at these stations in the surface layers, and it is the photosynthetic processes of phytoplankton that releases oxygen into the water. In the lower layers most readings are below 100% saturation. Reasons for this could include high zooplankton levels (consuming oxygen through respiration), and decomposition of material by bacteria in mid  to lower layers in the water column.

These data and the results relate well and suggest the same as the oxygen data collected offshore, building up a picture of the Fal estuary and the nearby sea. Primary production supersaturates the surface layers, leaving oxygen depleted areas where there are high levels of decomposition and respiratory activity.

 

 

Chlorophyll

Chlorophyll samples were taken down the estuary and can be used as an indicator for primary production.

Table 10 - Secchi Disk Data from Estuary

Station Secchi Disk Depth (m) Euphotic Depth (m) Attenuation Coef. (k)
Upper Estuary 1 2.0 6.0 0.72
Upper Estuary 2 2.5 7.5 0.58
Upper Estuary 3 1.5 4.5 0.96
Upper Estuary 4 1.5 4.5 0.96
Lower Estuary 1 2.5 7.5 0.58
Lower Estuary 2 4.5 13.5 0.32
Lower Estuary 3 5.5 16.5 0.26
Lower Estuary 4 6.5 19.5 0.22

 

6.7a - Surface Chlorophyll in Upper Estuary

Figure 6.7a shows the surface chlorophyll samples taken from the RIB. Rib 1 is taken from furthest up river with rib 4 taken nearest Carrick Roads. The samples show little variation with the highest values at rib site 3 and the lowest values at rib site 2.  All values lay between 4.27 and 5.03 µg/l. These values are low but the depth at these stations was shallow (less than 6m) and therefore suspended material in the water caused the euphotic zone to be very shallow (see table 10).

6.7b - Chlorophyll Against Depth in Lower Estuary

Figure 6.7b shows the chlorophyll samples from the Niskin bottles from Xplorer plotted against depth. These samples were fixed with acetone and then analysed in the laboratory using a fluorometer. Station 1, 3 and 4 show a chlorophyll minimum at depth, whereas station 2 shows a chlorophyll maximum. The highest concentration of chlorophyll is at station 1 at depth, and the lowest concentrations are found at station 4.  These data appear inconsistent and it is difficult to find a trend in the results through the estuary.

6.7c - Flourometer Readings Against Depth in Lower Estuary

Figure 6.7c shows the fluorometer readings (representing chlorophyll) from the CTD against depth. This figure shows that the highest readings are found at station 1 which is the site furthest up river. Stations 2, 3 and 4 show similar concentrations to each other with values between 0.1 and 0.4. None of the stations show any distinct variation with depth.

These results are inconsistent with results obtained from the Niskin samples in figure 6.7b.

 

 

Plankton

Phytoplankton

Figure 6.8a shows the population composition of phytoplankton in the Fal estuary. RIB station 4 is at the top of the estuary, the furthest location from the mouth. Station 1 is a little nearer the mouth, followed by station 2 and 3 respectively and station 4 is at the mouth of the estuary. See map Figure 6.1.

Fig. 6.8a - Phytoplankton at Upper Estuary Station 4 and Lower Estuary Stations

Fig. 6.8b - Chaetoceros sp.

Diatoms dominated the phytoplankton population at stations 1, 2 and 3 but as we approached the sea at station 4, dinoflagellates started to dominate, as in all the offshore stations. Surprisingly the rib station 4 (which was the furthest location from the mouth) was found to have an almost even number of dinoflagellates and diatoms. Looking at the results of stations 1 – 4, it would seem that dinoflagellates flourish offshore whereas diatoms grow well in the estuary, however the values for rib station 4 contradict this hypothesis.

Station 2 had a much higher total number of diatoms than any of the other stations. The dominant diatom group present were Chaetoceros sp. and the main species of dinoflagellate were Karenia mikimotoi.

 
Zooplankton

Fig. 6.9 - Zooplankton at Locations A and B

Fig. 6.9 shows the population distributions of zooplankton sampled along trawls A and B. Trawl A had an average of 38.2 animals per m3 whereas trawl B had over three times more with an average of 126.2 individuals per m3. As well as having more animals per m3, trawl B had a more diverse array of taxa. Trawl A is dominated by cirripede larvae and there were a small number of hydromedusae, copepods and polychaete larvae present too. Trawl B contained mainly copepods, cirripede and gastropod larvae. There were also decapod and polychaete larvae, copepod nauplii and hydromedusae present.

Neither of these trawls was as diverse in terms of taxa when compared to the stations offshore and they completely lacked cladocera, mysidacea, chaetognatha, siphonophores, ctenophores, appendicularia, fish larvae and fish eggs, and most interestingly, echinoderm larvae, which were abundant in all the offshore stations. Also, the total numbers of animals per m3 in the estuary were considerably lower than offshore values, where total numbers ranged from 721.5 to 2958.5 animals per m3. The effect of the riverine input, nutrients and salinity may account for these large differences in taxa compositions between the estuary and offshore zooplankton.

Discussion

The investigation was carried out with the aim of interpreting the biological, chemical and physical characteristics and relationships seen in the Fal estuary and its tributaries. This will give a better understanding of how the Fal estuary acts as a transition zone between the freshwater river and the saline coastal waters.

Physically, the study area of the estuary was saline with little salinity variation. Salinities ranged from around 35 / 36 near the mouth, to 32 further up the estuary. This indicates that the Fal is a well mixed estuary. A mixed area is usually typified by a relatively high turbulence. The oxygen analysis of the upper level of the water column (especially in the top 1m) shows supersaturation. This suggests a turbulent flow and increased wave action, as an increase in surface area of waves means more oxygen will enter the surface through diffusion. Turbulence increased towards the mouth of the estuary due to a lack of sheltered environments. Mixing in the Fal is also aided by the mesotidal characteristics of the estuary.

In terms of the biology in the estuary, samples from the surface waters in the upper estuary showed a dominance of diatoms (Chaetoceros spp) whereas dinoflagellates were more abundant at station 4 nearer the mouth of the estuary. This may be due to physical or chemical differences in properties (eg. salinity or nutrients) of the water in different areas of the estuary. Conditions at stations further towards the mouth will be similar to those of the sea whereas stations nearer the head of the estuary will be more influenced by riverine inputs. The largest counts of phytoplankton were found at station 2; however the CTD fluorometer read the largest values for chlorophyll at station 1. Taking the laboratory method to be the most accurate, this shows that the CTD method may not be entirely efficient at measuring chlorophyll. Also chlorophyll is only a proxy for biomass. Sometimes many small phytoplankton are present with little chlorophyll between them, thus low biomass is inferred, when in fact cell numbers are quite high, though biomass remains low.

Zooplankton samples showed less diverse populations and fewer numbers of individuals throughout the estuary than the samples taken offshore. Most animals found in an estuary are marine in origin, rather than freshwater so it may be that these zooplankton are more abundant nearer the mouth at location B (as seen in the results) due to better tolerance to the conditions there than at location A. This may also explain why species diversity is higher at the mouth and higher still offshore.

Throughout the estuary, nitrate appears to show non conservative behaviour. It had been removed from the system, possibly by phytoplankton. Towards the seaward end nitrate is being totally used up by the phytoplankton. As this is the first nutrient to be depleted, it is the limiting factor in the estuary. Phosphate shows relative conservative behaviour, and also very little spread in the range of concentrations found. Silicon acts non-conservatively toward the sea, suggesting that the phytoplankton that are using the nutrients are diatoms, as they also require silicon.

 

 

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Conclusions

Through the offshore, geophysical and estuary studies, a picture of the Fal estuary has been obtained.

The estuary appears to be a characteristic well mixed estuary, mixed by tidal mixing and wave action, with relatively low differences in temperature and salinity through the water column. Further offshore, the temperature varies much more through the water column, and salinity varies less. Offshore, Lizard Point headland generated a lot of shear, further increasing turbulence and mixing in the water column.

Plankton biota assessments suggest that diatoms dominate in the upper estuary and dinoflagellates become more prevalent in the lower estuary and dominate offshore. In both the estuary and offshore, nitrate appeared to be the limiting nutrient, with noticeable reductions in silicon. Offshore phosphate was also reduced, but in the estuary levels were maintained , possibly due to anthropogenic sources. Zooplankton are less diverse in the estuary compared to offshore.

Chlorophyll levels were almost universally low, related to a lack of the limiting nutrient nitrate, though a couple of peaks were identified in the offshore data.

The geophysical survey suggested that the prevalent benthos was fragmented dead maerl, with a low percentage of live maerl and broken shells. Dominante biota were Black Brittle Stars and Banded Carpet Shells with a few unidentifiable polycheate worms. Wave action creates sinuous megaripples.

 

 

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References

Daugbjerg, N., Hansen, G., Larsen, J. and Moestrup, Ø. 2000. Phylogeny of some of the major genera of dinoflagellates based on ultrastructure and partial LSU r DNA sequence data, including the erection of three new genera of unarmoured dinoflagellates. Phycologia 39, 302 – 317.

DEFRA home web page, http://www.defra.gov.uk/marine/index.htm, the Marine Environment and Fisheries, front page, most recently updated Nov 2008.

Grasshoff, k., K. Krenling, M. Ehrhardt (1999). Method of Seawater analysis. 3rd ed. Wiley – VCH.

Location of the Fal Estuary, University of Exeter / projects in Exeter, http://projects.exeter.ac.uk/geomincentre/estuary/Main/loc.htm, page created May 2001.

Rodriguez, F., Fernandez, E., Head, R. N., Harbour, D. S., Bratbak, G., Heldal, M. and Harris, R. P. 2000. Temporal variability of viruses, bacteria, phytoplankton and zooplankton in the western English Channel off Plymouth. Journal of the Marine Biological Association of the United Kingdom. 80, 575 – 586.

Weiss, R. (1970). “The solubility of Nitrogen, Oxygen, and Argon in Water and Seawater” Deep Sea Res. 17 : 721-35.

Wentworth, CK, 1922, A scale of grade and class terms for clastic sediments: The Journal of Geology, v.30, p. 377-392.

www.waterontheweb.org (accessed on July 4th 2009)

 

 

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