IntroductionBoatsEquipmentOffshoreGeophysicsEstuarineConclusionsReferences





 





The Fal is the third deepest natural harbour in the world.
It contains 12 square miles of sheltered navigatable water and is the most polluted estuary in the United Kingdom.
Underlying geology is Carnmellis granite to the North and metamorphic rocks to the west.
Since 1995 Red tides have occurred by a combination of metal and nutrient inputs. Benthic habitats include important 4500 year old Maerl deposits.

         Estuaries are partially contained water bodies with connections to both the sea and fresh water inputs. They can be tidally dominated and have a conservative gradient of mixing between saline and fresh water along the length of the estuary. These two different water masses cause very specific and complex biological, chemical, physical and geophysical processes.

      The Fal estuary is situated in the British Isles on the South West coast of England. This area accounts for around 31% of the entire English coastline and includes many European designations of SACs and SPAs designed to protect the wide variety of ecosystems and species, many rare, inhabiting them. The Fal has been subjected to extreme anthropogenic inputs from metal mining and tourism, as well as dredging activities, oil and release of anti-fouling and sewage.

      Formation of the Fal occurred at the end of the last ice age when the sea level rose and is now classified as a Ria, or drowned river valley. The Fal itself is one of the deepest natural ria inlets in the world and with relatively low fresh water input despite its 6 tributaries and is classified as partially to well mixed, with a varying meso-macrotidal range which ranges from springs at 4.6m and neaps at 2.2m.

       Over this 2 week field course we aim to examine the processes and characteristics of the Fal estuary and surrounding coastline to interpret the differences between the contrasting conditions.


 

RV Callista
Scientific equipment:
ADCP
CTD and Rosette
Digital thermosalinograph
Fluorometer
Transmissometer
Offshore research vessel equipped with both wet and dry laboratory facilities and “A” frame with associated winch for a maximum of 4 tonnes lifting.
Max Speed: 14-15 knots
Range: 400nm
Passengers: 30 max
Draft: 1.8m

MV Xplorer
A coastal research vessel with the bridge level with the aft deck. The stern deck has a large working area with a hydraulic capstan crane for lifting heavy equipment.
Max Speed: 25 knots
Passengers: 12 max
Draft: 1.2m

Ocean adventure RIB
Equipment:
YSI multi probe
Niskin bottles
Secchi disk
Temperature Salinity probe (TS probe)
Max speed 35 knots
Passengers: 6 max
Draft: 0.5m


CTD Rosette
Used to measure conductivity temperature and depth. Also equipped with a fluorometer, transmissometer and niskin bottles (which were closed remotely from onboard using a computer at chosen depths). The rosette is lowered down the water column using a crane and winch system. It is connected to a computer which logs the data.

Van Veen Grab
A 0.5m3 grab used for collecting sediment from the sea bed. Care has to be taken into consideration when measuring sediment profiles due to compaction and collapse as well as loss of fine sediment due to an imperfect seal allowing leakage.

Sidescan sonar
A device used image the seabed to determine bedforms and the substrate.
A towfish is attached to a line and towed behind the vessel. Sound pulses are emitted at set intervals and the backscatter is recorded by the tow fish's transceiver. A trace is produced based on the strength of the backscatter and is interpreted to infer the bathymetry and artefacts on the sea floor.

YSI multi probe 
Used to record salinity, temperature, dissolved oxygen % saturation and pH throughout the water column. The probe is lowered by hand and the data either recorded onto the computer or logged by hand.

 

Zooplankton closing nets
A net with a mesh size of 200µm used for a vertical profile of the zooplankton species present.

 

TS probe
Measures the temperature and salinity using a thermistor / platinum thermometer. Used for a preview of the water column, especially during the offshore practical when looking for the tidal front.

 


Google image of offshore routes including station locations

●Tides and Weather

Time (GMT) Tide Height Weather
06.02 Low 1.7m Sea state: calm/slight. Cloud cover 2/8 octants at departure, wind 135°, 6.2 knots
12.12 High 4.4m
18.27 Low 1.8m

●Equipment

CTD Rosette (Temperature, Salinity, Pressure) with 4 x Niskin bottles (xL) flourometer and transmissometer
ADCP and WinRiver software
Closing Zooplankton Net (200 and 60µm) with 60cm diameter
Secchi Disk - used to calculate the Secci depth and light attenuation
Plastic bottles for silicate samples
Plastic tubing for collecting dissolved oxygen samples from the Niksin bottles
NaviFish - used to visualise the charts in WGS24
Glass bottles for storing nitrate, phosphate and phytoplankton samples
Reagents - dissolved oxygen: Manganese II Sulphate and Potassium Iodide
Syringes and Glass Fibre Filters - for Nutrient and Chlorophyll filtrate
Cool Box - Storage of water samples

●Station Location

Station Number Longitude (W) Latitude (N)
Black Rock 005°01.42 50°08.62
2 004°45.025 50°05.01
3 004°40.04 50°16.59
4 004°39.34 50°17.88
5 004°38.65 50°19.38
6 004°38.51 50°19.67
7 004°47.170 50°11.19
8 004°52.136 50°09.362

Secchi disk deployment  CTD Deployment  Sampling for dissolved oxygen from the Niskin bottle

Date: 01/02/09        Departure: 7.50GMT              Return: 16.30 GMT          PSO: Vicky Leader

●Background: Planktonic community structure and abundance is primarily controlled by the physical and chemical conditions in the surface waters of the ocean. These properties are affected by vertical mixing in the water column which can be subjected to thermal stratification if conditions (depending on water depth, tidal mixing and surface insolation) permit.

●Aims: To collect water samples at pre-determined stations and perform chemical, physical and biological analysis in order to locate the position of the tidal front. To determine how vertical mixing processes in the waters off Falmouth affect, directly or indirectly, the structural and functional properties of plankton communities.

● Sampling Route: The route taken (shown on the left) was chosen with knowledge of the previous group's data in the effort to locate the tidal front, whilst expanding the area over which the sampling was taken place. This would aim in the completion of the offshore dataset when other groups' data had been collated. Starting at Black Rock, which will be sampled by every group and therefore act as a control and a view as to changes in the water column over the two weeks, the decision was made not to travel so far offshore as the front had been discovered relatively close to shore the day before. The sampling strategy included staying near the coast east to Falmouth in the effort to ascertain the spatial position of the front. The aim was to cross the front several times from station 5 onwards in order to gain several vertical profiles demonstrating the changes in stratification either side of the front.

● Sampling Procedures
Ship board chemical sampling at stations 1,2,3, and 7 from Niskin bottles.

Dissolved Oxygen Sampling

Samples were taken for calculating dissolved oxygen as soon as the Niskin Bottles were recovered from the rosette. Small samples were decanted from the Niskin bottles via plastic tubing and then transferred to the wet lab where they were fixed with 1ml of Manganese chloride solution and 1ml of Alkaline iodide solution. Storage in a cool box submerged in water to prevent atmospheric oxygen contaminating the samples.

Nutrient Sampling
Plastic syringes and glass filter fibres were used to filter samples from the Niskin bottles: the filtrate was divided into the glass bottle for nitrate and phosphate analysis, and a plastic bottle for silicon analysis. Both filters were retained and added to acetone for cholophyll analysis.

  • Chlorophyll samples: place filter in tubes containing 90% acetone. Stored in fridge.
  • Phytoplankton samples: water from Niskin bottles poured into brown glass containing preservative (10% formalin)
  • Zooplankton trawl using a closed net and closed at differing depths. Samples stored in plastic bottles containing 10% formalin.
  • Secchi Disk deployed at every station to determine light attenuation coefficient and depth of euphotic zone.
  • ADCP in use for the entire trip recording flow rates and backscatter.

● Experimental Procedures
Chlorophyll -
Analysed using a fluorometer (10-AU turner designs), filters were preserved in 6ml acetone over night for extraction and then placed into the fluorometer. Total concentration of chlorophyll per litre was calculated using: value of fluormeter x (volume of acetone / volume of sample).
Oxygen - Analysed using the Winkler method as outlined by Grasshoff et al 1999.
Nitrate - Analysis using a spectrophotometer using the flow injection technique as in Johnson and Petty, 1983.
Phosphate and Silicate - Using a spectrophotometer as in Parsons and Lalli, 1984.
N.B. During these experiments 4 random replicates were taken to check for errors and accuracy.
Phytoplankton - Water samples were stained using lugols to identify major taxonomic groups. 1ml of each sample was pipetted into a Cedric Rafter cell where 10x20 grids were viewed under the microscope, which represented 0.2ml of the solution. To convert these results to cells per litre3, the counts were multiplied by 500. Magnification: 40x to locate and 100x to identify.
Zooplankton - 2ml of each sample was transferred to a Boggonoff plate where the major taxonomic groups were identified using Larink and Westheide's "Coastal Plankton". Using the tally, the total cells in the 500ml sampling bottle was calculated which was then divided by the total volume of water sampled (determined by the diameter and length of the zooplankton net).

Table showing the Attenuation coefficients (k) at each station

Station Secci Depth (m) Euphotic Zone (m) Attenuation Coefficient, k
1 11 33 0.13
2 9.5 28.5 0.15
3 8.5 25.5 0.17
4 5.5 16.5 0.26
5 5.5 16.5 0.26
6 3.5 10.5 0.41
7 6.5 19.5 0.22
8 11.5 34.5 0.13

Results

Stacked pie-chart of Zooplankton Abundance
Figure 1 - Phytoplankton abundance at all stations

Stacked pie-chart of Phytoplankton Abundance
Figure 2 - Zooplankton abundance at all stations.

Phytoplankton analysis
At a depth of 30.94m at station 1 there were no phytoplankton identified (Fig 1). This is almost certainly due to error in the handling/measuring of the sample as fluorescence values are similar to the sample at 11.5m and chlorophyll values are much higher. At a depth of 11.5 metres the sample is dominated by Rhizosolenia alata and Karenia mikimotoi. Phytoplankton abundance is 6000cells L-1.
Station 2 data shows dominance of Karenia mikimotoi. At all three stations, K. mikimotoi dominates, with other species only being present, in low abundance, at15.98m. Such a high presence of dinoflagellates can be explained by nutrients being too low for diatom species to dominate.  The phytoplankton abundance decreases with depth, mainly due to lower irradiance. Station 3 shows a completely different species composition to station 2, at both 3.5m and 28.5m the diatom Chaetoceros dominates accounting for 45% of the sample from 3.5m and 70% at 28.5m.There is deep mixing at station 3, making nutrients available higher up in the euphotic zone. There are no K. mikimotoi at station 3; this can be attributed to the Chaetoceros competitively excluding K. mikimotoi when nutrients, especially silicon, are high.

Zooplankton analysis
Station 1 has the lowest overall abundance of zooplankton at 1733m-3. Hydromedusae are dominant at station 1, with abundance of 627m-3. Hydromedusae dominate although echinoderm larvae and cirripedia larvae are also in relatively high abundance (Fig 2).
At depths of 17.9m – 8.0m, at station 2, hydromedusae are dominant, accounting for 69% of the total sample. This is the highest abundance of hydromedusae from all the trawls with 2366 m-3. The highest overall abundance of zooplankton is found at site 2 between 28.0m and 22.0m, with 3824 m-3. This is likely to be due to the strongly stratified conditions being favourable for phytoplankton growth which the zooplankton graze.
At station 3, the shallower trawl between 15.3 m and the surface shows high zooplankton abundance, dominated by hydromedusae but also with a high proportion of echinoderm larvae. At a depth of 24.6m – 19.9m, the data shows a diverse sample, although abundance is one of the lowest at 2367 m-3. A high diversity could be due to a high presence of phytoplankton, meaning food resources aren’t limiting.

CTD and Nutrient Analysis
Station 1 - There is a gradual decrease in temperature of 1.4°C exhibiting possible stratification (Fig 3). The discrete dissolved oxygen, nutrient, and chlorophyll data show little change throughout the water column suggesting a low presence of phytoplankton 
Station 2 - A thermocline is present between 20 and 24m, this shows that there are stratified waters in the top few metres of the water column (Fig 4). These stratified waters have resulted in a chlorophyll maxima at 14m, with a fluorescence value of 24V. The presence of a thermocline at 22m causes a decrease in dissolved oxygen due to respiration.
Station 3 - Station three has a weak stratification from the surface to depth throughout the entire water column (Fig 5). Dissolved oxygen decreases with depth, due to respiration from organisms present in the water column at depth, but is not as pronounced as station 1 and 2 due to mixing up to the surface layer. The phosphate concentration increases at depths 3.5m, 21.5m and 28.5m.Silicon concentration also increases at these depths, by 1.1µM. The fluorescence and discrete chlorophyll data show that there is a chlorophyll maximum between 20m and 22m depth of 2.06V, compare to 1.05V at the surface, which is due to the higher nutrient concentrations at depth.

Station 7 – A thermocline is present at 21m depth with a chlorophyll maximum below the thermocline at 35m which is supported by the discrete chlorophyll data (Fig 6). Nitrate, phosphate and silica increase in concentration below the thermocline due to utilisation during the spring phytoplankton blooms in the surface layer. Dissolved oxygen saturation declines with depth due to a lack of mixing of the water column up to the surface layer due to the thermocline

Station 1 Vertical Profile                         Station 2 Vertical Profile                     Station 3 Vertical Profile                      Station 7 Vertical Profile
Figure 3 - Vertical profile at station 1.    Figure 4 - Vertical profile at station 2.    Figure 5 - Vertical profile at station 3.    Figure 6 - Vertical profile at station 7.
ADCP
Figure 7 (taken after station 7) shows the presence of a eddy. These eddies are formed from contrasting water currents at the Oodman point, along the 50m contour line. There was a westerly surface current, influenced by the outgoing tide, which circled the headland from the East. The magnitude of this Westerly current was 0.15 m/s, whereas the deeper easterly current, at approximately 25m, was propagating with a magnitude of 0.015 m/s. It was also noticed that the top 15m of the water column was stratified minimising the eddies influence of the surface water. Due to its size, tidal influence and proximity to the coastline it is hard to predict the time scale and overall influence of the eddy on the biota. Larger eddies, such as those found in the Gulf Stream, can be modelled and accurately predicted as to their behaviour and temporal variability.

The backscatter image produced from the ADCP at station 7 shows distinct patchiness throughout the entire water column (Fig 8). The backscatter can be seen in three layers, 10m, 25m, and 45m suggesting three different layers of plankton.
Patchiness of phytoplankton can be caused from the following:
1. Spatial changes in the physical conditions.
2. Water turbulence
3. Grazing from a high abundance of zooplankton.
4. Localised reproduction.
 The chlorophyll maximum as measured from the CTD scan corresponds with the ADCP backscatter.
      

Richardson's number
Richardson number expresses the relative importance of static stability and relative stability using the following equation (Stewart, 2005),

If the Richardson’s number is greater than 0.25 then the water column is stratified, if below the velocity shear enhances mixing. The Richardson’s number was calculated for the vertical profile of the following stations 1-7. Due to the small boundary between stratified and mixed water masses it is hard to determine the magnitude and accurate use of the Richardson’s number. The Richardson’s number is also an indication of the differing magnitudes and directions of the currents throughout the water column.

The vertical profile of station 1 was mostly mixed except at three depths, 4, 21, and 30m (Fig 9). As mentioned in the CTD analysis, there is a prominent thermocline present at 20m at station 2. The Richardson number shows that there is mixing taking place just above this thermocline (Circa -12) with some variation above and below this point. At station 3, according to the analysis of the temperature/salinity data we concluded that station three was the approximate position of the tidal front (Fig 10). Station 7 exhibited water column mixing in the first 20m of water whereas just beneath the thermocline there are stratified waters (approximately 20-30m).  Below this there is variation between stratified and turbulent waters.

 

 



Figure 7 - Eddy's present after station 7.
Backscatter from ADCP at station 7 showing zooplankton and phytoplankton distributions
Figure 8 - Backscatter at station 7.
 Graph of Richardson number wih depth at station 1
Figure 9 - Richardson's number at station 1.
Graph of Richardson number with depth at station 2 
Figure 10 - Richardson's number at station 3.

  
Figure 11 - Velocity versus depth at station 2

Velocity with Depth

Station 2 -
The magnitude of the flow velocity at the surface (0.17 m/s) decreases marginally to a depth of 18 m and peaks again to a magnitude of 0.10 m/s at 22 m (Fig 11). This second peak corresponds to the chlorophyll maximum and the position of the thermocline. The flow then decreases uniformly to 60 m depth.
Station 7 -
As with all the previous stations the surface water velocity is again not majorly influenced by wind stress (fig 12). There is a maximum flow of 0.20 m/s at 10 m depth decreasing to a minimum of 0.10 m/s at 50 m. The flooding tide is the biggest influence on the flow velocity at this station.

        
Figure 12 - Velocity versus depth at station 2.
 

CTD and Secci disk light data comparison
The r2 value between the CTD and Secci Disk 1% light depth comparison is 0.62. This show the two instruments are relatively accurate. It is noted accuracy will always be negligible between the two measurements as the CTD is an electronic value and secci disk is a crude estimate. Surface irradiance may have been changed during each measurement affecting the attenuation coefficient.

 


Figure 13 - Light comparisons
Conclusions

Fronts are areas of larger than average horizontal gradients of water parameters for example, temperature and salinity. Differences in stratified and turbulent water aid in identifying these fronts. Stratification and mixing depend on the tidal stream velocity and the depth of the water column. Using the CTD data we were then able to differentiate, in situ, the likely position of the tidal front on the day (see Richardson graphs). The tidal front influences the abundance of certain species of phytoplankton and zooplankton. This is used as an indicator of the relative position of the front throughout the year.

From the CTD data we expected station 3 to be the location of the front. Station 1 showed mixed water (see graph) displaying less vertical variation. Station 2, ten nautical miles offshore, displayed stratified waters which were reflected from the thermocline on the CTD data. We therefore expected to find an intermediate between the two extremes, station 3.

As evidenced from the CTD data and the Richardson numbers there is no clear indication of the existence of a distinctive tidal front.  The CTD data suggested the region of a front was present at Station 3, due to neither distinct stratified water, nor definite mixed water, and so was decided as the correct area to sample.  However, when attempting to locate the coastal side of the front, we were not successful and ended up in estuarine waters.  This would suggest, along with the erroneous Richardson data, that a distinctive front was not found.



Date: 04/07/09        Departure: 07.30GMT           Return: 12.15GMT               PSO: Keiron Roberts

Introduction
A geophysical survey of a section of the Fal estuary was conducted using sidescan sonar aboard the Xplorer in order to determine the bathymetry of the area and produce a benthic habitat map. The Fal is a particularly interesting area to survey with distinctive ecosystems including extensive rare maerl beds. The survey consisted of four parallel transects crossing the deep central channel and a single transect which penetrated St. Mawes Harbour. A Van Veen grab was used at specific sites in order to characterise the sediment and benthos as well as an underwater camera.   

Aims

  • To collect grabs from designated sites in order to identify species and determine sediment composition. Also to use an underwater camera to visualise the benthos.
  • To perform sidescan transects of the area to provide an overview of the bathymetric structure of the seafloor and to distinguish bed formations and areas of interest.
  • To gather and collect data and produce a geophysical and biological benthic map of the surveyed area.

Sampling Route
An initial line was taken experimentally which entered St. Mawes Harbour, but with a high number of moorings and increasingly shallow water as the Harbour approached it made the sidescan difficult to interpret. Four parallel transects, situated across the entrance to St. Mawes Harbour and between Black Rock and the mainland, were surveyed which crossed over the deep central channel in the Fal estuary (see google images to the right).

● Tides and Weather

Time (GMT) Tide Height (m) Weather
02.36 High 4.4 Light rain with cloud cover ranging from 3 -7 octants. The wind speed varied from 10-15Kts at approximately 108° or Easterly. The sea state ranged between 2 – 3.
09.21 Low 1.7
15.04 High 4.6
21.49 Low 1.7

● Equipment

Video Camera – An underwater video camera attached to a line which was lowered into the water column. The image was displayed onto a television screen and the footage recorded onto a DVD. This was ideal for gaining a preview of the seabed before deploying the grab.
Sidescan SONAR – A towfish was attached to a line and towed at a constant speed behind the vessel, emiting a sound pulse at 100kHz. This pulse is emitted and the backscatter is recorded by the towfish’s geoacoustic sidescan transceiver. The data is then processed by hydropro navigation. The Backscatter was interpreted based on the colour, shapes and shadows.
Van Veen grabs – A lightweight, long levered sediment sampler with a 0.5m3 capacity. Once opened and deployed the grab closes after impacting with the seabed. This gives a sample of the seabed from the specific area.

● Track Locations

  Start of Line End of Line
Longitude (W) Latitude (N) Longitude (W) Latitude (N)
1 05°01.34 50º08.58 05°00.30 50º09.18
2 05°01.43 50º08.46 05°02.11 50º09.37
3 05°02.06 50º09.37 05°01.38 50º08.47
4 05°01.34 50º08.49 05°02.02 50º09.39
5 05°01.57 50º08.39 05°01.29 50º08.49

Lithology of the Falmouth District

By looking at the sidescan trace, the lithology of the Falmouth district estuary has been identified as consisting of middle to upper Devonian metamorphic mudstone and sandstone slate with porphyry quartz intrusions overlaid with sandstone turbidities, up to 2m in depth (Fig 22).

Figure 22 - Lithology of Falmouth district.

Sidescan Trace

A benthic map was produced using the sidescan trace with areas of interest mapped as well as bedforms and sediment properties. Geometric corrections and calculations were used to measure certain bedforms such as sandwaves and outcrops of bedrock. Below are artefacts and interesting points from the trace:

Figure 14 - Bedrock Outcrop

Outcrop of bedrock at position 183750mE, 32250mW with a height of 0.85m (Fig 14). Another outcrop of bedrock located at 183800mE, 32950mN was also measured, which had a height of 1.68m.  These rocks have been identified as mid to late Devonian metamorphic mudstone and sandstone slate.


Figure 15 - Hull and wake of Clipper on trace.
Other ship's wakes appeared on the trace at several locations and this figure shows the disturbance caused by the Clipper Beaune (Fig 15).

The bathymetry of the seafloor affected how the same bedforms appeared on the trace depending on the illumination angle (Fig 16). At point A the tow fish is in water depth of approximately 30m and the outcrop of bedrock appears very dark and defined on the upper trace. Conversely, at point B the tow fish is in roughly 11m of water and the same bedrock on the lower trace appears much lighter with a large shadow (Fig 17). This is due steep angle of inclination dropping into the channel.


Figure 16 - Schematic demonstrating how different illumination angles produce different traces.

Figure 17 - The same rock outcrop apparent as different colours on two traces.

The trace at the start of line 4 exhibited distortion as the fish was deployed too near the surface waters (Fig 18). Our own wake and associated turbulence meant that the electrical pulse could not reach the seafloor and as a result white blanks appeared on the trace.

Figure 18 - Turbulence from our propeller appearing on the trace.

In this excerpt from the sonograph of two parallel tows, there is a clear change in tone with the seafloor appearing darker in the upper trace (Fig 19). This is due to the bathymetry of the seafloor as the depth of the water column increases from left to right as we crossed into the deep channel. The sediment in the deep channel is coarser due to the higher flow velocities that scour the seabed. In the shallow water lining the channel, the sediment is finer as the water velocities are slower which allows fine particles to settle out of the water column.

Figure 19 - Change in sediment properties as the deep channel is traversed.

The benthic map produced from the sidescan trace is shown in figure 20. Points of interest include the seagrass beds and sub-aqueous sand waves which had a height of 0.32m and a wavelength of 2.31m. A contour plot of the tracks crossing the main channel was obtained using the sidescan trace (Fig 21). The depth of the water column was calculated using geometric corrections from the trace in order to determine the bathymetry of the channel. This allows us to make connections between sediment properties and position within the channel.

 

Figure 20 - Benthic map produced Figure 21 - Contour map

Grab Samples

PURPOSE
A Van Veen grab was used to survey the seabed at four different locations along the transects. The locations were decided for the following reasons:
Grab 1 - To sample a suspected weed bed as identified from sidescan sonar line 3.
Grab 2  - To sample suspected course sediment (using the sidescan trace) at the entrance to Saint Mawes Harbour.
Grab 3 - To sample a small patch of ripples located on the sidescan sonar at the end of line 3. In reality, due to the size of the patch and the strong current, we drifted south west into the deep channel and didn’t manage to sample the ripples.
Grab 4 - To sample suspected fine sediment, using sidescan data, for calibration with other sediment types.
Note: A grab was planned at the mouth of the harbour however the video showed a seagrass patch which could not be sampled.

Grab 1

Table 1 - Biota in sample from Grab 1
Common name Genus/species name
Maerl Corallinacease
Sipunculid worms Sipuncula
Polychaete Nereis diversicolor
Shore crab Carcinus maenas
Velvet swimming crab Necora puber
Fan worm Sabella
Red seaweed Furcellaria lumbricalis
Marbled swimming crab Liocarcinus marmoreus
 

  
Figure 22 - Grab 1                 Figure 24 - identifying species

                                                

Location
20m NE Eastern Narrows buoy
Time: 10:27 GMT
Latitude: 50°09.4433N
Longitude: 005°01.8896W
Depth: 12.4m

Findings:
Grab 1 consisted of a healthy maerl bed on fine muddy sediment. There is relatively high faunal diversity, with an abundance of crab species.  It is unusual for there to be a presence of maerl on muddy sediment. Biota found in the sample are displayed in table 1.

Grab 2

Table 2 - Biota in sample from Grab 2
 
Common name Genus/species name
Mearl Corallinacease
Large sunset shell Gari depressa
Amphioxus Amphioxus
Tower shell Turitella communis risso
Long clawed porcelain crab Pisidia longicornios
 

Figure 25 - Grab 2               Figure 26 - Amphiouxus

Location
East of Castle Buoy
Time: 10:50 GMT
Latitude: 50°08.9880N
Longitude: 005°01.55344W
Depth: 6.1m

Findings:
Grab 2 consisted of well sorted coarse sediment, with a high presence of dead maerl, possibly caused by dredging. There was a relatively low abundance of macrofauna compared with other sites. Biota found in grab 2 are shown in table 2.

Grab 3

Table 3 - Biota in sample from Grab 3
 
Common name Genus/species name
Maerl Corallinacease
Polychaete Nephyts caeca fabricius
Sand mason Lanice conchilega
Long clawed porcelain crab Pisidia longicornis

 

 
Figure 27 - Grab 3                  Figure 28 - Live Maerl

 

Location
Time: 11:29 GMT
Latitude: 50°08.8844N
Longitude: 005°01.5679W
Depth: 35m

Findings:
Grab 3 consisted of muddy sediment, which was highly anoxic and had a shallow redox layer. There is a high presence of dead maerl, possibly swept from the original site. Due to the anoxic sediment there is a relatively low abundance of macrofauna. Biota found in grab three are shown in table 3.

Grab 4

Table 4 - Biota in sample from Grab 3
 
Common name Genus/species name
Polychaete Nereis diversicolor 
Red seaweed Plocamium cartilagineun
Brown seaweed Laminaria saccharina
Turban top shell Gibbula magus
Heart urchin Echinocardium cordatum
Isopod Family - Idoteidae
 
Figure 29 - Heart urchin     Figure 30 - Isopod

Time: 11:44 GMT
Latitude: 50°09.4314N
Longitude: 005°02.1326W
Depth: 9.4m

Findings:
Grab 4 was largely composed of dead maerl and course sediment. This sediment was largely biogenic. Notable macrofauna findings include a large heart urchin specimen, and a high abundance of bivalves.Biota found in grab 4 are shown in table 4.

 

Summary

The sidescan data shows that the benthos has predominately coarser sediment to the west in the estuary gradually becoming finer to the east near the shore. Down the transect there is a channel with exposed metamorphic mudstone slate and a slope consisting of finer sediment. On the eastern shore there is a presence of seaweed showing a reduced flow rate. Seagrass is present in Saint Mawes harbour with a finer sediment present either side.

The grabs revealed a varied benthic community with all grabs having maerl present in varying conditions. Grabs 1, 2 and 4 taken at shallow depths have a larger proportion of living organisms in comparison to grab site 3 taken at below 30m depth. This is due to the increased velocity currents moving the dead organisms and sediment out to sea.


Locations of RIB and Xplorer stations Date: 08/07/09            Departure: 07.45 GMT            Return: 17.30 GMT       PSO: Charlee Bennett

Introduction
With both freshwater and saltwater inputs, the Fal estuary acts as a transitory zone between these two physically different water masses. The chemical, physical and biological properties of the estuary will have separate water signatures than its adjacent coastal seas and local estuarine processes will change these water properties within the estuary itself, resulting in a dynamic ecosystem.

Aims

  • To determine the physical structure of the estuary and how it changes from the mouth of the Fal to the offshore region.
  • Use ADCP data to understand how the tides affect conditions within the estuary and calculate the residence time of the estuary.
  • Measure algal biomass in the estuary at different locations and relate to nutrient distributions.
  • Determine the behaviour of nutrients within the estuary, i.e conservative, non-conservative, using estuarine mixing diagrams.

Sampling Route
Each station sampled using the RIB was equally spaced with stable leisure pontoons used as platforms, as this gave precise points where there was little drift. ADCP transects were taken intermittently downstream and at major changes in hydrodynamics of the estuary e.g. mouths of major creeks and rivers. The two extremities of the estuary were chosen at the village Feock (North) and at Black Rock (South).

Sampling Procedures
Sampling was conducted on board the RIB and Xplorer to take advantage of the different capabilities of both vessels. Work conducted on the RIB allowed us to penetrate far north into the estuary where chemical sampling was conducted along with data logging using a YSI probe. ADCP transects were conducted aboard the Xplorer as well as CTD vertical profiling with niskin bottles for chemical sampling. 

Experimental Procedures
Same biological, chemical and physical lab procedures used for offshore.

Station Locations
The location of each station and the start and end positions of ADCP transects are shown on the google images to the right (click to enlarge). 


● Tides and Weather
Time (GMT) Tide Height (m) Weather
00.05 Low 1.3 Cloud cover varied from 4-7 octants. Wind speed was approximately 14Kts in southerly direction. Sunny intervals.
05.36 High 4.8
12.20 Low 1.3
17.48 High 5.3

ADCP transect positions

Results


Figure 31 - Vertical CTD profile at station 3
Figure 32 - Vertical CTD profile at station 4

Figure 33 - Vertical CTD profile at station 5
Vertical Depth profile from the CTD and YSI data
Xplorer stations
Station 3 - There is a thermocline present at 13m with a chlorophyll maximum at 12m (Fig 31). However, according to the constant salinity, the water column is well mixed suggesting that the thermocline is seasonal. As expected the dissolved oxygen concentration decreases with depth.
Station 4 - Both the temperature at salinity remains homogeneous with depth suggesting a well mixed water column (Fig 32). The chlorophyll fluctuates between 0.16µg/L and 0.32µg/L throughout the water column.
Station 5 - Measurements were made at station 5 to quantify the effect of a sewage output in the surrounding area (Fig 33). Compared with other stations, the chlorophyll levels are relatively high and increase with depth, 25-30µg/L. This suggests high levels of phytoplankton explaining the low levels of nutrients. Unfortunately this prevents a correlation to be found between the sewage input and nutrient levels.
Station 6 -
The salinity fluctuates in the top 14m of the water column which is representative of the flooding tide (Fig 34). Station 6 is located at the most seaward end of the estuary exhibiting typical well mixed conditions.

Rib stations

The salinity profile for station one shows a well mixed homogeneous water column which is also seen with the temperature and oxygen profile (Fig 35). Salinity Increases by 0.6 and temperature decreases by 0.4ºC. The Oxygen saturation declines by 8% from surface to depth showing a well mixed water column from depth up to the surface/atmosphere. A chlorophyll maximum exists at 2m depth, which is possibly due to high irradiance levels in the first metre of water.
      Station 3 has a well mixed surface layer with some saline ocean waters being seen at 6m – 7m depth with a drop in temperature of 0.8ºC and an increase of salinity of 0.5 (Fig 36). Oxygen saturation declines with depth and chlorophyll concentrations are high throughout the column in comparison to Rib station 1, which is possibly due to the higher concentrations of phosphate and nitrate in the surface waters compared to Station 1.

Figure 34- Vertical CTD profile at station 6

Figure 35 - Vertical YSI profile at RIB station 1

Figure 36 - Vertical YSI profile at RIB station 3.

Turbidity
To gain an approximate idea of turbidity within the system, a comparison between the euphotic depth (calculated from Secchi data) and true depth can be made.  A relatively shallow euphotic zone could suggest attenuation due to suspended sediment within the water, which would result from turbidity.  With this theory, stations of similar depths but with different euphotic zones can be assessed for turbidity.

Station 3 (lat: 50°10.850, long: 5°01.732) and Station 4 (lat: 50°08.919, long: 5°01.648) both had depths of 28-29 m within the channel, but different locations, with Station 3 further upstream and Station 4 at Black Rock (see Site Plot).  A difference of 3 meters in the euphotic depth suggests more turbidity upstream.

Station 1 (lat: 50°12.183, long: 5°02.491) and Station 5 (lat: 50°09.027, long: 5°02.280) at depths of ~11m, again display increased turbidity upstream.  Station 1 has a euphotic zone four meters above the seafloor, whereas the euphotic zone reaches the bottom depth at Station 5.

As the only sites with similar depths, there is not the satisfactory amount of data as needed to present a firm conclusion.  However, for the region surveyed with the Xplorer vessel it can be assumed that there is greater turbidity upstream.  This could be explained by the high energy input from the river and tributaries carrying suspended particulate matter into the estuary.  This is diluted downstream by the larger volume of the estuary and also by tidal influence.

Attenuation coefficient is highest at RIB 3 (lat: 50°12.183, long 5°01.732) this shows maximum turbidity and therefore is the possible null point of the estuary.

Residence Time of The Fal Estuary
The residence and flushing times of an estuary are two different concepts that are often confused. Flushing time is the time required for the freshwater inflow to equal the amount of freshwater originally present in the estuary and is influenced by the tidal range. It is specific to freshwater (or materials dissolved in it) and represents the transit time through the entire system (e.g., from head of tide to the mouth). Residence time is the average time particles take to escape the estuary. It can be calculated for any type of material and will vary depending on the starting location of the material. The residence time calculated on the 08/07/09 is in relation to a parcel of  water moving through the estuary.

The value obtained was 3.4 days. There is an error of one day on either side of this calculation. The value for V Total was calculated using rough approximations for the estuarine volumetric dimensions. The flushing time of the estuary was obtained using the formula below. A tidal flushing time of 13 hours was obtained. Again there is an estimated error of 3 hours due to the aproximate calculation of the area of the estuary.

Pictures from the Boat work



Figure 37 - Estuarine mixing diagram for Phosphate

Figure 38 - Estuarine mixing diagram for Silicate

Figure 39 - Estuarine mixing diagram for Nitrate
 

TDL Analysis
Theoretical dilution lines are based on 4 general assumptions:
1. There is only one riverine and marine end member.
2. There is no addition of porewaters.
3. The estuary is in steady state.
4. The estuary is constant on a time scale greater than the residence time.

It is already clear that as there are several tributaries entering the Fal estuary including 2 large inputs (The Percuil and the Penryn rivers) close to the mouth, this will affect the TDL and must be taken into account when analysing data. There is a large gap in concentrations as the riverine end member was taken high up the Truro River at salinities ranging from 0-5, however samples could not be taken very far up the river in the rib and as the tide was relatively high at the times of sampling the salinity generally did not get lower than 30. Therefore for all the TDL lines there is a gap in the data ranging from salinities of 5-30 and resulting in an area of unknown concentrations which cannot be commented on.
      Non-conservative behaviour can be due to temporal variability in end member concentrations, additional sources external to the riverine end member or removal or addition through chemical or biological processes.

Phosphate Estuarine Mixing Diagram
Phosphate behaves mainly in a conservative manner in the estuary, with a slight addition to the net source for salinities above 30 (Fig 37). This addition may be due to the 2 large riverine inputs increasing the phosphate concentration, although there may be other reasons which cannot be identified due to the lack of data between salinities of 5-30. There are 2 anomalies from station 4 at a salinity of 33.86 with a phosphate concentration of 1.72µmol, far higher than other concentrations around this salinity. These samples were taken from a pontoon rather than jetties and the pontoon was situated close to a mussel farm therefore anthropogenic inputs may be the cause of the high nitrate levels.  

Silicate Estuarine Mixing Diagram
Silicate demonstrates non-conservative behaviour with extreme addition towards the mouth of the estuary above salinities of 30.6, potentially due to the large riverine inputs (Fig 38). Again the lack of data from the riverine end member to above 30 makes it difficult to determine another cause for the dramatic increase other than the riverine inputs. The silicate addition is far higher than phosphate and nitrate addition which are readily removed by all photosynthetic organisms whereas silicate is only taken up by certain taxa such as Diatoms.

Nitrate Estuarine Mixing Diagram
Nitrate behaves conservatively at the mouth of the estuary, plotting close to the TDL (Fig 39). There were however several points plotted that were found to be below the detectable limit close to the seawater end member. This is likely to be due to the low resolution of the spectrophotometer using the flow injection technique and because the peaks drawn were so small that they were indistinguishable.

Phytoplankton and Zooplankton  Analysis


Figure 40 - Google image of zooplankton trawls.
Figure 41 - Phytoplankton abundance
Figure 42 - Zooplankton abundance

Phytoplankton analysis
The most notable feature of the phytoplankton abundance measured in the estuary is the extremely high abundance of Chaetoceros at station 1 (Figs 40 and 41). The data showed there to be 651500 Chaetoceros cells L-1. Other species including Karenia mikimotoi are present, however Chaetoceros dominate the sample. The high phytoplankton abundance in station 1 can be explained by relatively high nutrient concentrations giving ideal conditions for plankton growth. The abundance of phytoplankton cells present decreases towards the marine end of the estuary due to lower input of nutrients from the riverine source.
       Station 4 is slightly further down the estuary and shows a Chaetoceros dominant composition of phytoplankton at 14.5m. The diversity of station 4 seems to be higher than at all other stations, with Alexandrium, Karenia mikimotoi, and Polykrikos accounting for 36% of the sample.

Zooplankton analysis
The trawl for station 1 encountered some problems, the most significant was due to the water column being relatively shallow, a lot of suspended matter was captured in the sample net  resulting in not all matter being filtered  into the sampling bottle.
      The overall zooplankton abundance is very different between the two samples, station 1 has a total abundance of 31 m-3, and station 2 has a total of 204 m-3 (Fig 42). These numbers aren’t exact due to the crude method of calculating the total volume of water sampled, especially as the flowmeter on the plankton net wasn’t working. Comparisons between the two can still be made.
       Along with having a higher abundance of zooplankton at station 4, the diversity is also greater; this is likely to be due to a higher input of coastal meroplankton, coupled with more favourable conditions approaching full seawater salinity. The two groups present in highest abundance are the Copepoda and Cladocera, with values of 61 m-3 and 59 m-3.
       Station 1, is dominated by gastropod larvae, with an abundance of 17m-3. Other zooplankton are present in low abundance, with values from 1m-3 to 4m-3. This lower abundance could be explained by conditions being more stressful towards the riverine end of the estuary, resulting in fewer species being adapted to the abiotic conditions.
       The zooplankton in the estuary are in lower abundance but more diverse than that from further offshore (Station 2, offshore data) which was mainly dominated by Hydromedusae. Total abundance for offshore zooplankton peaked at 3824m-3, whereas in the estuary it only reached 204m-3.The higher coastal diversity is due to a higher input of meroplankton. The lower abundance of zooplankton in the estuary could be due to flushing from the riverine input along with more stressful conditions being present.


Figure 43 - ADCP data for transect 1

Figure 44 - ADCP data for transect 2

Figure 45 - ADCP data for transect 3
 

ADCP Analysis


Figure 46 - ADCP data for transect transect 4

Figure 47 - Equation used to calculate the coriolis displacement.

Analysis
The strong northerly flooding tide follows the deep channel up the estuary in each transect. The highest velocity is always seen in the channel and mid estuary and is lowest on the fringes. This is due to energy conservation by flowing through the estuary in the straightest line possible. The saline water mass sinks to the lowest possible depth (in the channel) due to the density of the seawater (Figs 43, 44 and 45).
      River influences can affect the flow rates but due to the strong spring tide the southerly river currents are very low. On the westerly side (near Falmouth harbour) of transect  4, the current turns southerly due to the Penryn River input (Fig 46). During transect 3 there is a strong southerly change in current direction, again on the western side of the estuary. However, at this time there is no obvious river input and could therefore be due to the backflow of the tide, as there is also a small backflow on the eastern side of the transect.
      The flooding tide is strongest on western side of the estuary with some backwash on the eastern side. This could be due to the coriolis effect. At this latitude and with an average tidal current of 0.5 m/s the Easterly change in velocity is 0.59 m/s. Over the tidal time period this is roughly a displacement of 6.4km, which is greater than the width of the estuary so that the coriolis can be observed (Fig 47).

Conclusion
The ADCP data show a strong flooding spring tide which follows the channel on Eastern side of the estuary due to energy conservation. The backwash mainly occurs on the westerly side of the estuary, this could be due to the coriolis effect. Due to the low riverine inputs and due to the season and size of the rivers, riverine influences are very small and any southerly direction in current is most likely due to backwash with the exception of Penryn River. 


Figure 48 - Vertical salinity profile

The Salinity profile of the Fal Estuary
The vertical salinity plot for the Fal estuary shows a well to partially mixed estuary throughout from coordinates 50°14.699  005°01.383 (Malpass) to 50°08.62  005°01.42 (Black Rock)(Fig 48). There is a fresh water influence in the surface waters particularly at the mouth of tributaries and creeks, which is seen on the east coast predominately due to the tidal influence on the west coast of the Fal estuary. A high salinity at depth is caused by the greater density of oceanic waters compared to fresh water inputs. The high salinity up the river is due to sampling occurring at high water going to low water, compared to further down the estuary to Black rock where the tide went from low/slack water to high water. At Turnaware point, 4500m from the first station at Malpass, the estuary becomes narrow and shallow causing the higher salinity ocean waters to mix with the less saline surface waters, creating a higher surface salinity. This surface salinity is seen on the surface salinity contour plot and also exhibits the fresh water influences from the rivers and creeks to the east of Falmouth (Fig 49).

 


Figure 49 - Horizontal salinity profile

Conclusions

Using the data recorded, the Fal estuary exhibits large variation depending on the distance from the coastline. According to the CTD and YSI data the estuary exhibited a well-mixed structure with little to no stratification. The salinity also remained uniform with depth with little variation throughout the tidal cycle. This then affected the phytoplankton and zooplankton abundance resulting in an increase in diversity at the river end member and an increase in abundance of species at the mouth. It was difficult to measure the behaviour of the nutrients due to the influence of more than one contributing river. However, both the phosphate and nitrate appeared to behave conservatively and the silicate behaved non-conservatively.


The Fal estuary is a well mixed and tidally dominated area of water affected by physical parameters such as tidal, local weather and anthropogenic inputs. Large riverine inputs result in high concentrations of nutrients which are reflected in the planktonic abundance and biodiversity. The anthropogenic inputs were hard to quantify as no direct inputs were found. Clear stratification was seen offshore due to the presence of a seasonal thermocline. In the adjacent coastal areas, however, mixing was not as evident as the thermocline, potentially due to the calm weather. Nutrient profiles were typical for this time of year and the chlorophyll maxima reflected this. The bathymetric survey found a higher abundance of rare, protected species such as Maerl and seagrass than expected, suggesting the Fal estuary is more productive than previously thought. In order to gain a full overview of the physical, chemical and biological conditions of the Fal estuary, a long term study covering different tidal states and seasonal conditions should be conducted, as well as surveying a wide area.

 

 

 

 

Brown, E., Park, D., Phillips, J., Rothery, D., and Wright, J., 1999, “Waves, Tides and Shallow Water Processes” Second Edition, Buterworth Heinmann, Chapter 6
Flett, J. S. and Hill, J. B., 1946, “Geology of the Lizard and Meneage”, Second Edition, Memoir of the Geological survey of Great Britain, Sheet 359 (England and Wales)
Leveridge, B. E., Holder, M. T. and Goode,  A. J. J.,  1990, “Geology of the country around Falmouth”, Memoir of the British Geological Survey, Sheet 352 (England and Wales)
Mille, C. B.,2004, “Biological Oceanography”, Blackwell Publishing, Chapters 2 and 6
Pickard, G. L. and Emery, W. J., 2003 “Descriptive Physical Oceanography”, Fifth Edition, Butterworth Heinemann, Chapter 8
Steward, R.H., 2005, ‘Introduction to Physical Oceanography’, Dept. Oceanography, Texas A&M University
Web page accessible here http://oceanworld.tamu.edu/resources/ocng_textbook/contents.html, Date cited 7/07/09
Map www.projects.exeter.ac.uk/…/images/creeks.jpj
RIB picture www.soes.soton.ac.uk/.../boats/img/rib-s1.jpg
Webpage Banner www.noc.soton.ac.uk
Photograph of Falmouth www.destinationsouthwest.co.uk/images/main/info_pic_falmouth.jpg

Disclaimer: All the information contained within this project is entirely the work of the members of group 4 (Falmouth field trip 2009) , the ideas and results contained within are independent of the National Oceanography Centre of Southampton.

We would all like to thank our project tutor, Eric Achterburg, for his insightful yet sarcastic inputs. They were invaluable: