The Bill Conway was used for our work in the estuary.

11.74 meters long purpose built research vessel

Simrad SDGPS
Simrad Depth, Speed and Wind and Water Temperature. Chart Plotter .
GPS: Trimble Survey

The Xplorer was used for geophysical surveying with a fully equipped side scan sonar and grabs.

12m x 5.2m x 1.2m Gemini fastcat

Side Scan Sonar and Boomer.
Genoa 11c chart plotter.
Furuno GP36 DGPS
Heila deck crane and winch

The RIB is a smaller and more versatile boat with little draft allowing it to be used to collect samples further up the estuary.

Hull Ribtec 700

 Simrad SDGPS, Depth sounder and Chartplotter

The Callista is primarily an offshore research vessel with fully equipped wet and dry labs.

19.75 meters long twin hull purpose built research vessel

Hull mounted RDI Workhorse ADCP 600Khz
Seabird 21 Thermosalinograph
CTD and rosette with mounted Fluorometer and Transmissometer
Vessel logger – running TECHSAS logging software

“A” frame and associated winch – 4 tonne lifting capacity (Max vert height 3.5m approx)

Figure 1.1: Station 1 CTD and lab results graph (click to enlarge)

Station 1

At station 1 (figure 1.1) chlorophyll is lowest at surface, reaching a maximum of ~2.7µmol/L between 8 and 12m. The thermocline occurs between 5.5 and 7m with a temperature difference of 0.3°C.  Oxygen saturation is lowest either side of the chlorophyll maximum; at the surface and again at 20m. Close to the chlorophyll maximum at 5.5m oxygen saturation is ~106.5%, above and below the chlorophyll maximum the O₂ saturation is approximately 102.5%. Nitrate and phosphate reach their lowest values at around 5.8m, Nitrate concentrations range from ~0.42-1.05µmol/l, Phosphate concentrations range from ~0.185-0.323µmol/l. Silicon appears to increase with depth from 0.5 at the surface to 1.5µmol/l at 20m.

Figure 1.2: Station 2 CTD and lab results graph (click to enlarge)

Station 2

At station 2 (figure 1.2) chlorophyll shows a sharp peak at 22m of ~5.3µmol/L. A thermocline is present from 12 to 25m, covering a temperature range of 1.5ºC. There appears to be a general decrease in O₂ saturation from 112-87.5% between the surface and 45m. Nitrate values range from 0.4 at the surface to 3.3µmol/l at 45m. Phosphate decreases from 0.15 at the surface to 0.68µmol/l at 45m. Silicon increases from trace levels up to a maximum of 3.6µmol/l at depth.

Figure 1.3: Station 3 CTD and lab results graph (click to enlarge)

Station 3

At station 3 (figure 1.3) maximum chlorophyll concentration of 4.7µmol/l occurs at ~21m. The thermocline occurs between 18 and 25m with a temperature range of 2ºC. O₂ saturation appears to be greatest in the first 20m, 103%, and lowest, 90%, at 60m. Nitrate values are at a minimum at 21m, 0.4µmol/l, and greatest above and below the thermocline at ~3.4µmol/l. Phosphate values are also at a minimum at 21m, 0.15µmol/l, above and below the chlorophyll maximum phosphate concentrations are greater at ~0.55µmol/l. Silicon concentrations are ~0.75µmol/l in the surface 25m and at 60m, silicon concentrations are greater at 3.25µmol/l.

Figure 1.4: Station 4 CTD and lab results graphs (click to enlarge)

Station 4

At station 4 (figure 1.4) a clear thermocline with temperature difference of approx.2.6˚C. Chlorophyll is distributed largely over a large depth of the water column, reaching a maximum of 3.5 volts. Surface chlorophyll concs. reach around 2.1volts. Nitrate concentration is highest mid depth around 7m, corresponding to the chlorophyll maximum peak. Phosphate and silicate increase with depth.  Oxygen saturation decreases with depth, with maximum surface values of ~112%. The 1% light intensity is at ~22m depth.

Figure 1.5: Station 7 CTD and lab results graphs (click to enlarge)

Station 7

At station 7 (figure 1.5) the chlorophyll maximum occurs at ~10m and at a concentration 3.45µmol/l. The thermocline is from 5-12m and covers a range of 1.3ºC. O₂ saturation decreases with depth, from a maximum of 117% near the surface to a lowest recorded value 93% at 25m. Nitrate concentrations range from a minimum of 0.6µmol/l at 10m to ~2.0µmol/l above and below the chlorophyll maximum. Phosphate increases with depth from 0.17µmol/l at the surface to 0.38µmol/l at 25m. Silicon concentrations also increase with depth from 0.36µmol/l at the surface to 2µmol/l at 25m.

From the CTD profile data graphs many aspects of the frontal system can be seen. The stations show progressively more thermal stratification from inshore to offshore. Station 7, on the front, has the largest surface chlorophyll concentration, with the fluorometer reading 2.4 Volts. This increased surface chlorophyll concentration may be due to convergence of flow “by concentrating the biomass from the mixed and stratified water on either side” (Sharples & Simpson, 2001).

Overall chlorophyll concentrations of the water column are greatest at the stations close to the front (2, 4 and 7), also the chlorophyll maximum becomes progressively more defined offshore.

Oxygen saturation is noticeably greater at the front (station 7) at 117% compared to 105% in the well mixed waters (Station 1) and 103% in the stratified offshore water of station 3. This may be a direct result of the increased primary productivity at the front. Further explanations for increased oxygen levels could be explained by entrainment from the enhanced wave breaking which occurs at fronts and their interaction with currents. Such conditions increase the generation of gas bubbles and associated downwelling currents create higher oxygen levels further down the water column (Baschek et al, 2006).

Figure 1.6: Zooplankton graphs key

Figure 1.13: Total zooplankton per m³ at each station

Figure 1.7: Staiton 1 depth 4-10 m zooplankton Graph  

Figure 1.8: Station 2 depth 25-20 m zooplankton graph

Figure 1.9: Station 2 depth 21-0 m zooplankton graph

Figure 1.10: Station 3 Depth 28-0 m zooplankton graph  

Figure 1.11: Station 3 depth 15-0 m zooplankton graph

Figure 1.12: Station 4 depth 15-0 zooplankton graph

Figure 1.14: Station 4 depth 3-0 zooplankton graph

Figure 1.15: Bongo Nets at Station 7 zooplankton graph

As seen in figures 1.7-1.15 copepods were the dominant group at every station and depth sampled, with the highest densities being 46250 per m³ at depths between 0-15m at station 4. Copepod nauplii were also abundant at all stations except between 15-0m at station 3, with a maximum of 1450 per m³ found at station 2 between 21-0m.

The Siphonophores, which were most abundant at Station 1, were found in greater concentration in the shallow water (above 20m) but at all stations other than station 1 and station 2 at 25-20m, the Hydromedusae exceeded the Siphonophore densities. Bongo nets were deployed in the surface and showed a taxonomic distribution of zooplankton similar to that of stations 1 and 2, with the groups of highest concentration being the Copepoda, Hydromedusae and Siphonophores.

The highest densities of zooplankton were found at station 2 between depths of 25 and 20m and the lowest at station 3 between 28 and 0m. At all stations, the zooplankton were distributed in the region of water surrounding the chlorophyll maximum and the thermocline.

Through calculation of a Simpsons Diversity index for each of stations 1 to 4, it can be seen that station 4 represented both the highest and the lowest diversity with the highest (of 0.85) being the shallower of the two samples. Overall, the zooplankton diversity was greatest in the samples taken from the shallower water for each site. The likely explanation for this is the greater level of irradiance supporting phytoplankton growth in the shallower water, therefore providing an grazing source for the zooplankton.

Zooplankton graze at and around the chlorophyll maximum where phytoplankton concentrations are highest. Due to the seasonal succession of phytoplankton in temperate waters, the nutrient depleted surface water that is apparent during the summer has resulted in the deepening of the chlorophyll maximum. Henceforth, the zooplankton migrate to the deeper waters around the thermocline and chlorophyll maximum where light levels are still sufficient but nutrient concentrations are higher due to mixing from below (Colebrook, 1979).

Figure 1.16: Biological composition at each station graph 9Click to enlarge)

Figure 1.17: Biological speciation of diatom graph (click to enlarge)

Figure 1. 18: Biological speciation of ciliates graph (click to enlarge)

Figure 1.19 Biological speciation of dinoflagellates graph (click to enlarge)

There is the general compositional trend in the offshore region of high dominance by diatoms over dinoflagellates and ciliates. (Figure 1.16) Dinoflagellates are present in low numbers at most of the sites and ciliates only appear at the more inshore, less stratified sites (station 1 and 7). The obvious reasoning for this is the season. In sea temperature terms it is still relatively early in the summer i.e. the sea has not yet warmed enough to become significantly thermally stratified. This means that the water column is still fairly mixed and on top of this, recent bad weather has increased the mixing processes occurring. Mixing keeps the water temperature lower and the nutrient content of the water higher. These are ideal conditions for diatoms, which grow faster and tend to out-compete dinoflagellates, especially if the limiting nutrient of silica, is available for the diatoms to build their frustule.

If the dinoflagellate numbers are looked at individually, the highest abundance of them can be witnessed at station 3 at the thermocline (21.2m). Station 3 was highly stratified and the thermocline at this location generally contained the most phytoplankton. The higher population of dinoflagellates at this site compared to other sites is due to the stratification. The lower level of mixing action at this site is more accommodating to dinoflagellates because they are motile and can move vertically through the water column, in and out of the nutrient rich waters around the thermocline. Diatoms on the other hand, are immobile so rely heavily on mixing to move them into the nutrient rich waters.

At the more stratified sample sites (2 & 3) there is a more obvious increase in phytoplankton abundance from the surface waters to the thermocline. This is due to the nutrient mixing at stratified sites being restricted by the thermocline and thus fewer phytoplankton existing above it compared to within it, as long as the thermocline is still in the euphotic zone.

Site 4 was situated two miles off shore and also has high phytoplankton abundance just below the thermocline. This site was not as well stratified as site 3 but did appear to experience a certain amount of upwelling. This was marked by a high nutrient level under the thermocline, which would explain the high phytoplankton population here.

The diatom composition (Figure 1.17) was very mixed but the species that appeared most frequently were Rhizosolenia delicatula, Rhizosolenia setigera, Rhizosolenia alata. This composition will be comparable with the estuarine populations.

Station 1

Magnitude

Direction

Magnitudes between 0 and 0.5m/s at this station (fig. 1.20). There is no differentiation between the top water layer and the bottom water layer. There are no visible patterns and this can be attributed to the fact that this data was taken at 09:38 (GMT), which was taken at slack water at 09:45

This data was collected at slack water, this means that no particular flow direction (fig. 1.21)should be found through out the water column. At the beginning of the sample a flow out of bay can be seen at almost 180^o due south. flow direction throughout the remainder of the water column is sporadic and non-uniform.

Figure 1.20: Station 1 ADCP Velocity Magnitude (click to enlarge)

Figure 1.21: Station 1 ADCP Velocity Direction (click to enlarge)

Backscatter

High readings of backscatter (fig. 1.22) can be seen in the surface layer (93-125dB). Below the surface layer (>4.6m), progressive reduction in backscatter (60-76dB) can be observed, with patches of comparably very low backscatter (60dB).

Figure 1.22: Station 1 ADCP Backscatter (click to enlarge)

Station 2

Varies between 0 and 0.3m/s down through the water column. Flow magnitude (fig. 1.22) is low in the upper layer of the water column (0.075-0.150m/s). There is a noticeable increase down into the water column, which peaks at between 20-30m (0.225-0.300m/s). After 30m the velocity decreases again down to the sea bed.

At the surface the flow was found at approximately 45^o to the wind direction (fig. 1.23), which should be expected. This switches at roughly 10m and begins to flow towards the coast with the tide. This flow direction dominates on average for the rest of the water column.

Figure 1.22: Station 2 ADCP Velocity Magnitude (click to enlarge)

Figure 1.23: Station 2 ADCP Velocity Direction (click to enlarge)

At the surface backscatter is high because of the surface turbulence (90-75dB). Below this an area of relatively low backscatter is obvious and this can probably be attributed to a shadow caused by the turbulence. The backscatter remains low down through the water column until roughly 20m. At this point there is an increase to 75dB, just above the thermocline. At the thermocline our data shows a chlorophyll peak, which might represent a high phytoplankton concentration. Below the thermocline another peak of roughly 75dB is found and then it remains low down to the sea floor. The peaks found at 20 and 25m are possibly caused by grazing zooplankton at high concentrations.

Figure 1.24: Station 2 ADCP Backscatter (click to enlarge)

Station 3

Magnitude

Velocity magnitudes (fig. 1.25) measured at sight 3 were, on average relatively low with a total range of 0 – 0.3m. Surface velocities measured at the site were found to be ~0.15m/s and penetrate to a depth of 18m. Velocity magnitude values measured below this depth are relatively low ranging between 0.05 and 0.1m/s.

Flow direction values (fig. 1.26) found at site 3 show a 180 degree rotational change between surface and bottom water. Surface flow oriented at 180 degrees gradually changes with depth to a bottom water flow direction of near 0 degrees. Note this change is gradational and continues through the duration of the ADCP transect . A reverse ekman spiral can also been seen (see further discussion below).

Figure 1.25: Station 3 ADCP Velocity Magnitude (click to enlarge)

Figure 1.26: Station 3 ADCP Velocity Direction (click to enlarge)

Station 3 exhibited backscatter values with a relatively low spectral range (From 65 to 80) (fig 1.27). Two distinct zones of relatively high backscatter values ranging between 71 and 73 were recorded at ~9m and ~21m straddling a thermocline of depth (17-20m). Backscatter readings below the 23 meters are uniformly low exhibiting values of less than 68.   

Figure 1.27: Station 3 ADCP Backscatter (click to enlarge)

Station 4

Magnitude

Overall velocity values are high for the area (fig. 1.28), between 0 and 2m/s. Surface velocity is highest (0.5 – 2m/s) and it decreases down through the water column.

The direction found is in accordance with the flood tide as direction of flow is in towards Falmouth Bay (fig. 1.29).

Figure 1.28: Station 4 ADCP Velocity Magnitude (click to enlarge)

Figure 1.29: Station 4 ADCP Velocity Direction (click to enlarge)

There is backscatter peaks (fig. 1.30) at the surface and at the sea bed, but no significant peaks throughout the water column.

Figure 1.30: Station 4 ADCP Backscatter (click to enlarge)

Station 7

Magnitude

Shows high velocity at the surface (above 0.3m/s), which rapidly decreases down to roughly 8 meters (fig. 1.31) At this point the water column stabilizes and the velocity remains the same (between 0.225 and 0.074 m/s) until it reaches just above the sea bed. At this point velocity increases to more than 0.3m/s. Note, that through the stable section of the water column the velocity does decrease on average, but only by a very small amount.

Station 7 was taken at 14:52 (GMT) and high tide occurred at 15:31 (GMT), this means that the tide was going in. This corresponds with the flow shown on the velocity direction diagram (fig. 1.32) in that the flow occurred at between 360^o and 90^o, flowing through 0^o and this corresponds to the tide flowing into the bay.

Figure 1.31: Station 7 ADCP Velocity Magnitude (click to enlarge)

Figure 1.32: Station 7 ADCP Velocity Direction (click to enlarge)

Shows high backscatter at the surface layer and at the sea bed. Through the water column (fig. 1.33) a band of slightly higher backscatter rising up through the water column and presumably reaching the surface just off the read out. This can be used as an indicator for the front.

Figure 1.33: Station 7 ADCP Backscatter (click to enlarge)

Reverse Ekman Transport at Station 3

At station 3 the ADCP data recorded showed a complete switch in direction down through the water column. Initially Ekman transport was proposed as the cause for this movement. However upon closer inspection we discovered that the net direction of transport was occurring in the opposite direction to that, that would have suggested wind driven Ekman transport. The readings were taken at 11:51 (GMT) on the 2^nd July, 2008 and admiralty charts suggested high tide would be at 15:31 (GMT). This meant that flow direction would be expected at roughly 360^o to the boat into Falmouth bay. However our surface data indicated that the rotational change through the water column was occurring in the opposite direction. We propose that this anomaly has been caused by a process called reverse Ekman transport. In contrast from a wind driven spiral, a reverse Ekman spiral originates from a bottom current flowing in a given direction. The orientation of friction associated with this flow is working in a direction 180 degrees from the original current track. As Coriolis is the mechanism for right sided deflection in the northern hemisphere, the bottom derived spiral will rotate accordingly. However, when viewed from the surface the direction of rotation appears to be left sided.

            Typical Ekman spirals display a diminishing flow velocity with distance from the originating force, in this case the north flowing bottom current. However, velocity magnitude data measured at the site reveal a pattern in opposition to this general trend (Aebecht et al., 1997). In this case surface velocities were measured at slightly higher magnitudes than those measured at depth. The occurrence of this trend can be attributed to a signal lag time between directional changes in bottom flow originating from tidal reversal. In addition, as a reverse ekman spiral initiates at the sea bed, wind conditions will have a relatively large influence on surface current dynamics and can potentially erase most of the Ekman influence.

Figure 1.34: Station 1 Richardson number graph

Figure 1.35: Station 2 Richardson number graph

Figure 1.36: Station 3 Richardson number graph

Station 1(figure 1.34) shows greatest stability in the region of the chlorophyll maximum. There are 2 regions of high instability, one at the surface and one at depth. The peak at the surface occurs at 1.6m and gives a Richardson’s value of 0.0002 and corresponds to where the chlorophyll concentration is lowest. The lowest R_i value observed from any station, therefore indicating greatest instability has a value of 7.5e⁰⁰⁵ at 20m and corresponds to the depths where water temperature and oxygen saturation are lowest.

Stability of the water column at station 2 (figure 1.35) has increased from station 1. Instability increases at depths beyond 26m after which point the chlorophyll concentration declines rapidly, with R_i values ranging from 0.5 to 0.0007. The greatest instability again corresponds to the lowest oxygen saturation value.

Station 3 (figure 1.36) is a very stable water column with the only depths showing instability being at 3m and between 42 and 49m. Both of these zones correlate to the lowest chlorophyll concentrations. Stable Richardson’s values range from 1.5 to 11.6 and the unstable from 0.1 to 0.01.

Station 4 (figure 1.37) has 2 main areas of high instability which correspond to the areas of water surrounding the chlorophyll concentration boundaries. A high Richardson’s value of 333 (which is the highest value observed for any station) is seen at 11.6m is located at a depth equal to the region of the chlorophyll maximum.

Station 7 (figure 1.38) has 2 peaks in stability at depths of 8.6m and 12.5m, with Richardson’s values of 32 and 94 respectively which correspond to the 2 levels of the thermocline that exist at this station and also the 2 levels of chlorophyll peaks.

Figure 1.37: Station 4

Richardson number graph

Figure 1.38: Station 7

Richardson number graph

The aim of our offshore work was to identify a tidal mixing frontal system. Salinity values were homogenous throughout the experiment showing no significant variation with depth and are thus not shown graphically. Our nearest inland location (station 1) near Black Rock exhibited a well mixed water column. Station 3, located 7 miles offshore displayed a highly stratified water column. These stations mark the geographical boundaries on our study. Between these two sites a gradational change from a well mixed water column to highly stratified water column was observed. Based on this observation station 7 was chosen as our final locality and consequently the location of the tidal mixing front.

Data showed us a well mixed water column at station 1, at slack water. This explains the sporadic flow patterns observed by the ADCP. Station 2 showed a velocity direction profile indicating a rotational change with depth. We can speculate that the variations shown come as a result of preferential flow due to coriolis and possible ekman transport in the surface waters. Station 3 shows reverse ekman spiral.  Station 4 & 7 show the flood tide in full flow and so the direction is clearly, on average, flowing into the bay.

At all the stations turbulent weather conditions present are indicated by the high backscatter readings found at the sea surface. The well mixed nature of the water at station 1 shows a uniformly low backscatter throughout the lower portions of the water column. Station 2 shows low backscatter throughout the water column apart from two peaks. These peaks occur just above and below the location of the thermocline. We speculate these appear as a result of zooplankton populations grazing above and below the chlorophyll maximum. At station 3, a similar pattern was observed. Station 7 displays a spatial variation in high backscatter readings illustrated by a general trend moving from a depth of 12m to near the surface at the termination of the transect. This trend is the basis for the assertion that a tidal mixing front existed at this location. The flow magnitude recorded by the ADCP is on average at its highest value at station 4 & 7, which is where the flood tide is in full flow. At the other stations flow magnitude is lower and more sporadic throughout the water column. Specifically, at stations 2 & 3 smalls peaks in backscatter are found in bands down into the water column at varying depths.

The first station was taken at Black rock (lat: 50^o8.48N, long: 005^o1.48W), here we found a well mixed water column. This is indicated by the weak thermocline and undefined chlorophyll maximum. Station 3 (lat: 50^o2.54N, long: 004^o56.38W) was taken at 7.5 miles off the coast, conditions here were highly stratified. The thermocline was strong and the chlorophyll maximum was well defined. Finally on returning closer to the coast, the front was located at station 7 (lat: 50^o6.40N, long: 005^o1.51W). Backscatter readings illustrate this nicely as the front rises to the sea surface indicated by an area of slightly higher values. At the front, intermediate water column conditions were found between station 1 and station 3, where a stratified structure was forming.  

Introduction

Figure 2.1: Map of survey area showing sidescan sonar transect lines (click to enlarge)

A benthic survey of the Fal estuary was carried out onboard the RV Explorer. Six transect lines, each between 1 and 2 miles long were surveyed in the area from East Narrows to Shag Rock (see figure 2.1). A sidescan sonar with a frequency of 100kHz and a total swath width of 150m and a 75m slant range was used to map the sea floor along the transect lines. The resultant sonograph was used to determine grab sample locations. Due to adverse weather conditions in the estuary only 2 of 4 planned grab samples were executed using a Van-Veen grab (grab 1: 50°08.845N, 05°01.137W; grab 2: 50°08.655W, 05°01.164N).

Furthermore, video footage from an underwater camera attached to a CTD rosette was used to confirm benthic habitat boundaries while the boat drifted. The track plot of the video can be seen in figure 2.2. The video data revealed more diverse floral and faunal assemblages than the grab samples.

Aims

• To explore the geophysical features of the Fal estuary and identify typical bedforms, sediment types and benthic habitats.

Equipment

• Towed Side-scan sonar with associated sonograph

• Towed video camera

•  Van-veen grab

Crew

Sonar Deployment: Joe Green            

Sonar Analysis: Allison Martin, Arwen Bargery, Emma Steventon

Grab Deployment: Emma Steventon, Emily Savage

Grab Sample Analyst: Allison Martin, Arwen Bargery, Rosanna von Zweigbergk

Video Master: Joe Green and Doug March

Position coordinator: Doug March, Andrew Ashdown.

Logbook: Marcus Sampson

Method

A total of 6 transects were made between East Narrows and Shag Rock crossing the outlet of Saint Mawes Harbour (see Route Map). The side-scan sonar was carefully lowered into the water at a depth of 7.5m, 10% of the 75m swath range. The length of the rope was 3.76m and the slant was 45°.  The offset was approximately 1.5m. Each transect stretched for approximately 1-2 miles. The printout from the sonograph was continuously monitored and points of interest were recorded as potential grab sampling sites. During each transect the latitude and longitude was recorded every two minutes to allow our position to be recorded.

Ground truthing was performed at two grab sites determined by previous interpretation of interesting areas on the sonogram. A Van-veen grab was deployed and the grab sample was analysed on deck, sieved and photographed. The sediment grain size was determined and the main fauna then identified.

To compliment the side-scan sonar results, a video camera was lowered and video footage of the sea floor was recorded while the vessel drifted for 6 minutes around grab site 2.

Due to bad weather conditions we were unable to deploy the boomer to get a seismic reading.

Figure 2.2: Bathymetric Track Plot showing major seafloor sediment types, features and video track plot.

Figure 2.3: Bathymetric Track Plot Key

Large Scour Marks (Transect 1)

A series of 5 large scour marks parallel to one another can be seen in Transect 1. They have a large, gentle depression with a marked sharp incline at the far end, facing a north-east direction. The scours have an average area of 40m by 30m and the total depth of the depression is relatively small, not exceeding more than a meter. The scours are larger than anchor marks and possibly originate from large-scale sediment movement but it is impossible to interpret with this level of surveying.

Fine sediment and seaweed (Transect 2 and 3)

Traversing transects 2 and 3 is a pale area of diameter 174m by 34.5m. There is a gradual fade in tone suggesting this to be the low reflective value of fine sediment. The surrounding darker shading covers an area of approximately 224m by 57m and is thought to be seaweed amid the maerl bed. However, there are other explanations such as smooth rock exposed by hydrodynamic processes or a scour hole. Exact identification could only be achieved through further investigation.

Lineations (Transects 1, 2 & 3)

Traversing Transects 1, 2 and 3 are large scale linear shallow grooves. The grooves generally trend to the northwest and appear to spread from a single point. They vary in length from 30m to 200m and are spaced approximately 46m apart. The water depth above them is relatively shallow at 1.9m. The grooves depress and then have a short, steep westerly facing slope covering a total area of 0.018km.  It is likely that these lineations are transient and could well be caused by helical currents. This results from the flood tide that causes a secondary, transverse helical current asymmetrical to the main Carrick Roads flow. The helical current movement causes scouring of the sediment to occur.

Bedrock outcroppings, (Transects 5 and 6):

In the Eastern region of the mapping area is a series of high backscatter images of varying size. Lineations observed within these regions can be connected and show a southeast/northwest trend. These areas of high backscatter may represent subdued bedrock outcroppings. The bedrock geology found in and around the Fal estuary is largely structurally controlled and primarily composed of slates. Axial planar cleavage striking to the southwest and dipping at ~50 degrees in the area is dominant over bedding planes, virtually erasing them. The coincident orientations of cleavage planes and areas of high backscatter readings suggest bedrock outcroppings are the most accurate interpretation. In the most North-Eastern region of the mapping area (Transect 6) lunate bedrock outcroppings are seen. The rounded morphology of these outcroppings observed on the sonar is interpreted to be a result of partial covering by sediment due to the hydrodynamics in the area. Widths range from 80 to 105m but the lengths of the features could not be determined as they continued out of the mapping area. Another possible interpretation of these data is that they are a series of benthic macro floral assemblages.

Figure 2.4: Image Grab Sample 1

The sediment in the grab samples (fig. 2.4-2.5) was light grey/brown, moderately sorted and lower fine to lower medium grained muddy silt. Maerl and shell fragments (lithics) were the dominant components in both grabs, with a sheet silicate matrix present and low SiO₂ content (<10%). Lithic fragments are relatively non–spherical and display a range of rounded to sub-angular grains though shell fragments can display sharp angularity at times. Grab sites 1 and 2 varied little with the exception of a marginal decrease in shell material noted at site 2 indicating that the site was slightly more sorted than site 1.

Macro benthic fauna collected from the grab samples were limited to 4 Veneroid bivalves ranging in size between 5 and 20mm.

Figure 2.5: Image Grab Sample 2

Figure 2.6: There is a covering of Maerl, giving the purple and white background  to the area. The colouration of the image is not accurate because the video camera had a light attached that distorted true colour. Thus the Maerl is likely to have been pink in colour, indicating that it was alive. It is also possible to see a species of Rhodophyta (red macro algae) in the background. Scattered in the area are many Macoma clam species (possibly Macoma nasuta). In the foreground, the Macoma clam is open, indicating that it is dead. The clam is a deposit feeder and uses its siphons to draw in sediment from just under the sediment-surface interface.

Figure 2.7: There is a single vertical frond of the kelp Thongweed, Himanthalia elongata, in the centre. Centrally surrounding the kelp is more Maerl and Macoma and to the right of the image there is evidence of sponges, possibly of the family Suberites.

Figure 2.8: In the foreground there is evidence of green macro algae. From what can be seen, this could be Velvet Horn algae (Codium tomentosum). Maerl is also present alongside red macro algae in the background.

Figure 2.9: An arm of a sea star (Asteroidea) is visible in the lower right. It cannot be reliably identified, but we speculate that it is a spiny sea star.

Figure 2.10: the image of the Fanworm species Sabella pavonina is recorded, commonly known as the Peacock Worm. In the live video footage it is possible to see the Fanworm open then close its crown.

Maerl is a member of the red algae Phylum (Rhodophycota). Maerl is common in only a few places around the British Isles, one of which is the Fal region. Identification and classification of the maerl forming algae are difficult to tell apart without extensive experience of classification of coralline algae. The three most common species found around the British Isles are; Lithothamnion corallioides, Lithothamnion glaciale and Phymatolithon calcareum (Jackson, 2007).  Maerl occurs in two main growth forms; firstly as a thin encrusting species on rocks, boulders, pebbles and shells etc and then secondly, as a loose lying algal gravel. It normally occurs in the mid to lower regions of the photic zone, where there is considerable but not excessive water movement (Haward, 1996). The dominant form of maerl seen in our mapping area is in the form of calcareous, unattached, branched nodules. It forms dense beds of algal gravel. The branches tend to be about 4mm in diameter and roughly 15cm long. It was difficult quantify the size of the observed plants using the video camera, as it was not equipped to do so. When alive, the algae has a red to pinkish tinge, and when dead, appears violet with white extremities. These maerl beds dominate the northwest portion of the mapping area and are displayed on side scan sonar images as areas of high but uniform backscatter. The transition between maerl beds and silty sediment found in the grab samples marks the most dramatic feature found in the mapping area and traverses all 6 side scan transects recorded. Video footage confirms this boundary between these substrate types and verifies the hypothesis that maerl can be mapped definitively using side scan sonar. 

The area that was studied displays a clear transitional boundary between dominant substrates which we believe marks the effect of the river input on the physical properties of the benthos. The main feature that was observed was the marked change in sediment type across the linear boundary that runs across all of our transects in a NNE direction. This aligns with the influence of the Percuil River into the main Carrick Roads area. The possible river-influenced sediment, to the west of the boundary, appears to be coarser according to the sonar return. On the east side of the boundary, the area on the sonar is markedly lighter, implying finer sediment. On the east side of the boundary, our two grab samples were collected and confirmed the sediment type as being fine silts and muds.

On the west side of the boundary, the video camera was deployed and observed large expanses of maerl beds. Thus the darker area of back scatter has been influenced by the high return from the calcareous maerl as well as a difference in sediment type.

There were a number of individual features that were observed and have been discussed as separate entities. What is apparent is that the area undergoes a continuous variable weather pattern and hence changing hydrodynamics in the area. Therefore many features are transient and can change from day to day. It is important to note that the area is very active in terms of on-going anthropogenic influences such as the constant shipping, naval and leisure activities occurring in the Fal estuary which impact the benthic structure considerably.  

Introduction

The physics, chemistry and biology of the water column in and around the Fal estuary were investigated on board the RV Bill Conway, and the Ocean Adventure RIB. The Fal Estuary is fed by a large number of rivers, creeks and tributaries. The Turo and Fal River were included in our study directly, whilst other riverine influences were taken into account by examining their impact on the main estuary. Despite this, the Estuary has little freshwater influence being tidally dominated. Furthermore, many frontal systems were found during the survey and their properties were investigated.

Tides

Aim

• To examine the physical, chemical, and biological properties of the Fal estuary and the relationships they exhibit.

• To interpret and explain the relationships seen.

Equipment

• Model F16 a Neil Brown CTD attached to frame

• RDI ADCP

• Secchi Disk

• Duncan and Associates 200μm zooplankton net equipped with Hydrobios

• Anamometer

• Niskin sampling bottles GO

• Trimble GPS Standard Survey

• Yellow Springs model 30M-100FT instrument 30M-100 T/S Probe

• Computers equipped with SOES software

RIB Crew

PSO: Doug March

Logbook: Doug March

Equipment Deployment: Andy Ashdown, Rosanna Von Zweigbergk

Chemists: Andy Ashdown, Rosanna Von Zweigbergk

Conway Crew

PSO: Joe Green

Logbook: Arwen Bargery, Joe Green

Chemists: Allison Martin, Emma Steventon

Computer Master: Alex Bryk, Arwen Bargery

Computer Analyst: Alex Bryk

Secchi Disk: Allison Martin, Joe Green

Equipment Deployment: Emily Savage, Marcus Sampson

Map of transects and stations

Click to enlarge

Method

The team was split in two, with three scientists on the RIB and seven on the Conway. Alongside surveying the Fal estuary, the RIB surveyed the Truro River and the Conway surveyed the mouth of the Fal River, to collect samples of a more freshwater origin. The freshwater input in this area is low, therefore a sample was also collected from the Truro River at an earlier date to establish a riverine end member to calibrate the data. Eleven transects across the estuary and aforementioned rivers were done in total and surface samples were taken at various locations along the transects. Surface samples were also taken at substantial changes in salinity. The CTD and T/S probe were deployed to create a vertical profile of the water column and samples were taken at points of certain interest. Each collected water sample was prepared onboard for processing the following day according to the laboratory methods described above. Data was then analyzed and major trends were investigated.

Sewage

The whole Fal region is heavily enriched in nutrients through anthropogenic influences. The main sources of nutrients to the area are primarily diffuse. The agricultural influence particularly affect the nitrate values. Additionally there is impact from point source local sewage outputs and industrial sources (fig. 3.1). The sewage has most impact on the phosphate levels in the area, whilst the nitrate is more heavily affected by the agricultural sources. The nutrient status of the area is not only affected by the anthropogenic chemical input into the area but also by physical parameters such as the local geology and sediment type, land usage, volume, dilution and flushing rate, rainfall, vertical mixing, and wave exposure (fig. 3.2). These things are almost always individual to the area and highly variable seasonally. This means that not all variations in nutrients can be predicted or explained due to there being so many contributing factors.

Figure 3.2: Estimated sources of estuarine nutrients.

(Langston et al, 2006)

Figure 3.1: Effluence in the Fal Estuary Region: showing some of the larger discharge consents to the Fal estuary.

The open symbols represent sewage discharge and the closed symbols represent heavier outputs of trade consents and miscellaneous sources of effluent (Langston et al, 2006).

We hope that our findings may be of use to any potential developers or conservationists in the Fal estuary who are interested in the biological, chemical and physical aspects of Falmouth waters.

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