Estuaries have long been researched due to their ecological importance.  This is owed to a diverse range of habitats and biodiversity due to spatial and temporal changes in chemical and physical gradients. They are also important transition zones of minerals and materials from terrestrial to marine environments.

The Fal Estuary, Cornwall, is a ria (drowned river-valley) which formed at the end of the last glacial maximum approximately 10,000-17,000 years ago and is one of the largest natural harbours in the world. It measures 30m deep at the mid-channel with 4-5m deep shallow side banks. The major fresh water sources include the Fal and Truro Rivers as well as other smaller tributaries that discharge into the Western English Channel. Surface water temperatures around Falmouth have a summer average of 16^◦C and a winter average of 9^◦C. Estuaries exhibit salinity gradients from freshwater at the head mixing to full seawater salinity at the mouth.  Tidal range varies up the estuary with the mean spring tidal range at the mouth typically 5m and maximum tidal currents typically below 2 knots.  The prevailing wind direction is characteristically south westerly. 

The Fal estuary has a rural water catchment derived from arable and dairy agriculture and reflects the underlying geology: Carnmellis granite and metamorphic rocks.  Mining processing of metalliferous deposits since the Bronze Age to modern day (1992) has led to increase in metal concentrations within the sediments and overlying waters.  Moreover, high inputs of organic matter (sewage) and nutrients have led to eutrophication in the upper Fal. The Fal supports a  wide range of communities including the largest Maerl bed in South West England as well as diverse epifaunal and infaunal communities, described as a Primary Site of Natural Marine Biological Importance by Bishop and Holme (1980).  Furthermore, as a result of the anthropogenic perturbation it has been designated as a Special Area of Conservation (SAC) and defined as ‘Sensitive Areas’ under the Nitrate Directive (91/676/EC).

Technical specification and dimensions:

 R.V. Callista

Length overall: 19.75m
Breadth Max: 7.40m
Draft: 1.8m
Depth midship: 2.85m

Speed: 14-15Kts, range 400nm
Passengers max: 30

“A” frame and associated winch – 4 tonne lifting capacity, 150m x 14mm cable
Hull mounted RDI Workhorse ADCP 600Khz
Fluorometer
Transmissometer
CTD and rosette
Thermistor YSI

Length overall: 11.74m
Beam: 3.96m
Full Load Draft: 1.3m
Freeboard to Working Deck: 1.0m
Gross Tonnage: 8.4 tonnes
Max Speed: 10 knots
Cruising Speed: 9 knots, range 150 nm
Crew: 2 Scientists: 12

Simrad Navigation computer.
DGPS: Trimble Survey
PC: c/w Transas Navi-Fisher vectorised electronic chart system, and Trimbles Hydra Pro survey system.
A-Frame: Stern, 750kg, 3m max height

Length overall: 7.00m
Beam: 2.55m
Full Load Draft: 0.5m
Max Speed: 35 knots
Cruising Speed: 25 knots, range 100 nm
Manning 6 person inc crew.

Hull Ribtec 700
Simrad SDGPS, Depth sounder and Chartplotter

Length overall: 12.00m
Beam: 5.20m
Full Load Draft: 1.20m
Max Speed: 25 knots
Cruising Speed: 18 knots
Manning: 2 crew, 12 scientists

Heila deck crane with winch
Hydraulic capstan - 1 tonne
Geonav 11c chart plotter. Navionics Gold

Van Veen Grab

Plankton Nets & Flow Meter

Towfish

The Van Veen grab is designed to collect samples of the seabed. The contents can then be analysed according to biotic composition and grain size by sieving through various mesh sizes.

These mesh nets have a spacing of 200µm to collect zooplankton in the water column. As the net is retrieved the contents are pushed towards the collecting bottle attached to the bottom of the net. The flow meter is a 3-blade impellor connected to a 5 digit counter. The counter records each revolution of the impellor.

The Towfish is a torpedo shaped Geoacoustic sidescan instrument with a mounted transducer that emits a fan shaped acoustic pulse between 100KHz – 500KHz whilst being towed subsurface behind the boat.  It is used to define bathymetric features.

Niskin bottle

This multi parameter probe can be lowered by a simple cable manually and is used to generate a vertical profile of various parameters, including; temperature, salinity and chlorophyll.

These bottles collect water samples at predetermined depths. Sub-samples can also be taken from the bottles for further analysis. The bottles are attached to either a hydroline or a CTD rosette.

The CTD is a multi parameter platform capable of continuously recording temperature and salinity with depth. Additional instruments that can be attached include; a fluorometer, Niskin bottle(s) and a transmissometer.

This simple black and white circle is lowered by hand into the water column until the colours can no longer be distinguished. The attenuation coefficient and depth of euphotic zone can then be calculated.

This is used to measure the speed and direction of currents within the water column and can be mounted on the hull of a vessel.

An accurate reading of wind speed can be obtained by using this equipment. Simply hold it in front of yourself into the direction of the wind and record off the highest value.

Click to enlarge

                             Methods

Fig 1.1 Chart showing location of transects and sample sites.

1. ADCP transects will be carried out to measure the currents of the water column offshore.

2. Multiple transects will be performed between Black rock (station 1) and Porthkerris (station 3) with a mid station between them (station 2).

3. The triangulation will be repeated to ensure a comparison between tides periods.

4. At the end of each transect the CTD will be deployed to produce a vertical profile of the water column.

5. Niskin bottles attached to the CTD rosette will be used to collect water samples at determined depths. The collected sample can be sub-sampled into appropriate bottles for all nutrients, dissolved O₂, lugol (phytoplankton) and test tubes for chlorophyll.

6. A Secchi disc will be deployed at each CTD site so that the euphotic zone depth can be calculated.

Silicon:

All stations show similar concentrations (below 3μmoll^-1). Station 7, however, shows a marked increase compared with other stations and a maximum of 6.835μmoll^-1 at a depth of 11m.

Fig 1.2 Silicon vertical profile

Phosphate:

All stations have a concentration below 1μmoll^-1 at all depths. These low concentrations are likely to be due to the increased distance from land (compared to our estuarine samples) causing the nutrient input from terrestrial sources to be greatly reduced. The input of recycled nutrients will therefore have a larger affect on the over-all concentrations. The removal of nutrients by phytoplankton and the low nutrient inputs caused the resulting reduced phosphate concentrations.

Fig 1.3 Phosphate vertical profile

Nitrate:

Stations 3 and 7 show a marked decrease in oxygen saturation compared to the other stations, possibly due to their sheltered location behind a headland. Consequently there was a reduction in wave size in the area, and hence a smaller surface area of the sea surface/air interface, possibly accounting for the reduction in % saturation.

Fig 1.4 Nitrate vertical profile

Chlorophyll:

In areas with high chlorophyll concentrations, generally have low values of silicon. This is likely to be due to the fact that diatoms are utilizing the silicon, depleting the concentrations in seawater. For example the highest chlorophyll concentration was found at 6.5m at station 3, where there was a silicon concentration of 2.18µmol/l.  This trend is not displayed at all stations. Change in chlorophyll with nitrate and phosphate levels shows no clear relationship.  There would need to be many more samples and replicates in order to provide an accurate insight into relationship between chlorophyll and nutrients concentrations.

Fig 1.5 O₂ vertical profile

Dissolved O₂:

The nitrate concentrations at all the stations show no evident trends. All values fall between the range of 2.5 to 5.5μmoll^-1. More data is required to make any conclusive summations about changes with depth or between stations.

Fig 1.6 O₂ vertical profile

A clear wind driven surface layer exists at high water (Fig. 1.13) to around 6m. A small halocline (0.5) and thermocline (0.2°c) overly a homogeneous section extending to the sea floor. Fluorescence is greatest in this upper layer and shows a decrease below 6m. Theoretically, shear stresses between these layers are low at slack water leading to a static water column.  Since sampling took place 47min before HW (Fig. 1.13) low stress is assumed, having the tendency to increase over time as tidal velocities increase. This cannot be verified as no ADCP data for this location exists. It would be expected that shear stress and constant strong SW winds would lead to mixing of the two layers and act to increase water mixing depth. The signal at station 5 (Fig 1.14) and 9 (Fig 1.15), whilst noisy, does not show the stratification seen in 1 (Fig. 1.13) as both thermocline and halocline are far less pronounced. The depth of mixing increases to around 13m at profile 9 (Fig 1.15) as slack water is again approached (38 minutes before Low Water). Clearly the ebbing tide overrides the stratification seen at HW.

Overall water temperature increases by around 0.5°c over almost 6 hours, a complete ebbing of tide. The freshwater input signature is weak (0.2 salinity decrease) but present. A small density compensation decrease was seen in raw data but was not plotted here. At this time of year, riverine flux is warmer than the surrounding ocean and to some extent, the effect of this is seen in the data.

Temperature and salinity show homogeneous profiles with depth at all states of the tide indicating a well mixed area influenced more by wind induced mixing. The effect of tidal currents at this station is masked by this and so interpretation is difficult. Potentially, the Helford riverine flux has an effect on this area. This is subject to seasonal and annual variation and so again it is unwise to draw conclusions from just one data set. A chlorophyll maximum is seen to exist at 15m on profile 8 (Fig1.19). The absence of a front and corresponding thermocline together with the small vertical area in which it exists leads to the belief that this data is erroneous and should not be analysed further.

Thermo and haloclines appear symmetrical at profile 3 (Fig 1.20) indicating steady slow mixing at this station. This is essentially homogeneous but becoming cooler and more stratified over the 3 hours between sampling. A small dual layer thermocline develops by the second time of sampling (4h 27min post HW). Fluorescence and salinity do not show corresponding changes indicating that the cooler water is arriving from the immediate area and as such has similar nutrient properties.

Fig 1.29

Time: 08.38 – 08.46

Length of Transect? 1552m

Start Position: 50° 00.369N, 005° 01.816W

End Position: 50° 03.673N, 005° 02.367W

Velocity direction contour plot – The directional contour plot is of a transect across the mouth of the estuary.  The mid depth and bottom waters display a mean northerly flow into the estuary.  This correlates with the movement water due to a flooding tide.  The transect was taken approximately 1hr before high tide 09.46GMT.  Surface waters tend to flow in an in easterly direction, probably a result Ekman transport via strong south-westerly winds.  At the far western end of the transect, the water column displays water flow in a southerly direction, possibly explained by the outflow of riverine waters.

Velocity magnitude contour plot – The velocity of the flow is greatest near the surface of the water, ~ 0.3m\s.  Deeper water is slower, ~0.1m/s, as the tide approaches slack water at high tide.

Average backscatter contour plots found nothing of interest in the mid-depths where we were expecting to find increased phytoplankton/zooplankton concentrations.  High levels of backscatter were found at the surface due to surface waves producing turbulence.

Fig 1.30

Time: 09.45 – 10.05

Length of transect: 3704m

Start Position: 50° 08.332N, 005° 02.502W

End Position: 50° 06.298, 005° 02.642W

Velocity direction contour plot – Transect 2 runs from the mouth of the estuary, in a southerly direction for 3704m.  The north end of the transect shows the average direction of net flow in an easterly direction. The south end of the transect has a net flow in a more north-easterly direction.

Velocity magnitude contour plot – The time of transect was around slack water, high water (09.46GMT) so tidal currents were minimal. Surface waters ahd an average velocity of approximately 0.3m/s, and the deeper water net flow is slower at approximately 0.15m/s.

Fig 1.31

Time: 10.18 – 10.42

Length of transect: 4482m

Start Position: 50° 06.522N, 005° 02.587W

End Position: 50° 04.108N, 005° 03.929W

Velocity direction contour plot – Transect 3 was carried out in a SSW direction, for approximately 4482m and passes across the mouth the River Helford.  The north end of the transect displays a a mean net transport in a north-easterly direction.  Towards the end of the transect, the direction of flow veers northerly, and then north westerly.  These changes in direction may be potential eddying and are mostly likely to be due to the influence of the River Helford and the flooding tide.

Velocity magnitude contour plot – Surface layer currents are faster than mid and bottom layers by ~0.15m/s at the start of the transect.  The end of the transect sees the magnitude of velocity constant throughout the whole water column (0.1m/s).

Average backscatter contour plots were inconclusive, with no areas of interest being highlighted.  An increase in backscatter due sediment suspension on the ebbing tide was expected but this was not found.

Fig 1.32

Time: 11.50 – 12.08

Length of transect: 3614m

Start Position: 50° 04.441N, 005° 03.513W

End Position: 50° 06.454N, 005° 02.194W

Velocity direction contour plot – The south end of the transect shows the deeper water flows in a south-westerly direction, following the contour of the coast.  Shallower water above this body is moving south-easterly.  As the transect moves northwards, the directional flow of the water-column becomes more homogenous and in an easterly direction.  This direction is influenced by the ebbing body of water, moving out of the River Helford.

Velocity magnitude contour plot – The magnitude of the net flow changes across the transect. The south end has a constant velocity throughout the water column (~0.15m/s), and gradually forms two layers as the transect moves northwards.  The surface layers travel ~0.1m/s faster than layers below.  The faster moving surface layer is probably the fresher, riverine water moving offshore on the ebbing tide.

Fig 1.33

Time: 12.37 – 12.53

Length of transect: 3200m

Start Position: 50° 06.505N, 005° 02.168W

End Position: 50° 08.289N, 005° 01.958W

Velocity direction contour plot – The beginning of the transect, in the middle of the bay shows the net transport flowing in an easterly direction. As the transect approaches the mouth of the Fal estuary, the ebbing tide is now evident, with net transport flowing in a southerly direction.  The transect was taken approximately 3hr after high tide (09.46GMT), so the ebbing currents are reaching there maximum velocity.

Velocity magnitude contour plot – The magnitude is constant across the transect, ~0.2m/s.

An interesting area of increased backscatter was found at the beginning of transect 5.  This may be a zooplankton bloom at ~ 15m.  Areas of high zooplankton concentration often indicate phytoplankton blooms.  Zooplankton are migratory so they may be below the thermocline, explaining deeper than usual location.

Fig 1.34

Time: 13.19 – 13.35

Length of transect: 3336m

Start Position: 50° 08.267N, 005° 02.540W

End Position: 50° 06.418N, 005° 02.572W

Velocity direction contour – This transect was very noisy and a definitive direction was difficult to identify.  The average direction of flow was in a southerly direction with the top 5m of water moving in east.

Velocity magnitude contour plot – The magnitude is constant across the transect, ~0.2m/s.

The average backscatter contour plot identified a zooplankton bloom ~3.5hrs earlier.  Due to the effects of the tides, the bloom was not identified in the second transect of the area. 

Fig 1.35

Time: 13.48 – 14.11

Length of transect: 4154m

Start Position: 50° 06.457N, 005° 02.587W

End Position: 50° 04.293N, 005° 03.854W

Velocity direction contour – The water column has split itself into 2 main layers.  The upper layer is travelling in n easterly net flow direction, above the lower layers that move in a westerly direction.  This induces shear at the layer interfaces and potential turbulence, highlighted by a small increase in backscatter at the correlating depth.

Velocity magnitude contour plot – The magnitude is constant across the transect, ~0.2m/s, apart from a small increase in velocity at the surface (0.3m/s).

Fig 1.36

Time: 14.30 – 14.41

Length of transect: 3298m

Start Position: 50° 04.650N, 005° 03.364W

End Position: 50° 06.259N, 005° 02.183W

Velocity direction contour – Transect 8 recorded very noisy data with large gaps due to bubbles.  The surface layers were moving in a south easterly direction and deep water was moving in a south westerly direction.

Velocity magnitude contour plot- Again, the contour plot is very poor due to noisy data.  The average magnitude of velocity is ~0.15m/s.

Fig 1.37

Time: 14.52 – 15.05

Length of transect: 3183m

Start Position: 50° 06.438N, 005° 02.219W

End Position: 50° 08.195N, 005° 01.885W

Velocity direction contour – Transect 8 recorded very noisy data with large gaps due to bubbles.  The surface layers were moving in a southerly across the whole water column.  This agrees with the direction of the ebbing tide ( transect taken 5hrs after high water).

Velocity magnitude contour plot- Again, the contour plot is very poor due to noisy data.  The south end of the transect shows velocity of currents to be 1.5m/s slower than water at the north end of the transect, at the mouth of the estuary. This is due to ebbing water still moving from the estuary.

Time: 09.45 – 10.05

Length of transect: 3704m

Start Position: 50° 08.332N, 005° 02.502W

End Position: 50° 06.298, 005° 02.642W

Average backscatter contour plot – An interesting feature was found in the backscatter contour plot. An increase in backscatter was recorded at ~ 15m depth and could indicate a zooplankton bloom.  At the time we were unable to sample and tow plankton nets so these predictions are only speculative.

Fig 1.39 -Transect 5

Time: 12.37 – 12.53

Length of transect: 3200m

Start Position: 50° 06.505N, 005° 02.168W

End Position: 50° 08.289N, 005° 01.958W

An interesting area of increased backscatter was found at the beginning of transect 5.  This may be a zooplankton bloom at ~ 15m.  Areas of high zooplankton concentration often indicate phytoplankton blooms.  Zooplankton are migratory so they may be below the thermocline, explaining deeper than usual location.

Fig 1.40 Direction 2 and 6

Transect 2 was taken at 09.45 – 10.05 GMT, approximately at high tide (09.46 GMT).  Transect 6 was taken at 13.19 – 13.35 GMT, approximately 3.5hrs after high tide.  It is interesting to see how the directional velocity has changed through the tide.  Surface layers have stayed relatively constant, staying in an easterly/southerly direction.  This may be the fresh water riverine discharge from both the Fal estuary and the River Helford.  The lower surface waters have completely changed direction.  At high water, bottom water net flow was in a northerly direction.  As the tide ebbed, the direction swung around almost 180 degrees, and the mean net transport was in a southerly direction.  The directional velocity become noisier and less uniform, suggesting that eddying may be occurring as the tide ebbs.

Fig 1.41 Direction 3 and 7

Transect 3 was taken at 10.18 – 10.42 GMT, approximately 45 minutes after high tide (09.46 GMT). Transect 7 was taken at 13.48 – 14.11 GMT, approximately 4hrs, after high tide.  There is a change is direction in surface water, moving swinging from a northerly direction 45 minutes after high tide, to a south-easterly direction 4hrs after high tide.  Deeper water also varies with tidal state, swinging from a north-westerly direction 45 minutes after high tide, to a south-westerly direction 4hrs after high tide.  This again displays a basic clear two layer water column structure, depending on density.

Fig 1.42 Direction 4 and 8

Transect 4 was taken at 11.50 – 12.08, approximately 2hrs after high tide (09.46 GMT).  Transect 8 was taken at 14.30 – 14.41, approximately 4.5hrs after high tide.  Surface waters have changed from a north-easterly direction at 11.50 to a south easterly direction and 14.30.  This change in surface water flow direction is due to the ebbing flow of both the Helford River and the Fal Estuary.  Bottom waters have stayed relatively constant, with a constant, west/south-west direction.

Fig 1.43 Direction 5 and 9

Transect 5 was taken 12.37 – 12.53, approximately 3hrs after high tide (09.46 GMT).  Transect 9 was taken at 14.52 – 15.05, approximately 5hrs after high tide.  The northern end of both transects is highly influenced by the outflow of water from the Fal estuary.  At both states of the tide, the movement of water was in a southerly direction.  This means than in this regions, tidal currents dominates the directions of currents in the bay.  The southern end of transect 5 (mid-tide), shows that the directional currents flow in an easterly direction.  As the tide approaches low tide (transect 9), the region is dominated by a southerly flowing current.  This means that the currents in Falmouth bay are most influenced by the Fal estuary’s ebbing water at low tide.

Link to Specifications

1. Data was collected at 5 stations along the River Fal and the River Truro.

2. At each station a water sample was taken at the surface using a hand deployed Niskin bottle. Sub samples were taken for each parameter.

3. Nutrient samples were filtered and divided into glass (nitrate and phosphate) and plastic (silicate) bottles for lab analysis (as above). Chlorophyll trapped on glass fibre filters was dissolved in acetone, the turbidity of this solution was later measured by an in lab fluorometer. Phytoplankton samples (100ml) were placed into glass bottles containing Lugol’s iodine to stain biogenic material. Duplicate O₂ samples were taken at station 1 and 3

4. At each station a vertical profile was taken using a multi-parameter probe, measuring depth, temperature, salinity, dissolved oxygen and pH.

5. The probe was placed into the Niskin bottle at each station in order to obtain the salinity of the water sample.

6. A plankton net was deployed at station 1 and station 4 and trawled for 3minutes. The plankton sample was stored in a plastic bottle for microscopic analysis of zooplankton and phytoplankton.

Fig 2.1 South section of area surveyed

Fig 2.2 North section of area surveyed

Silicon

Silicon concentrations at Conway CTD Site 1 appear uniform with depth most likely due to the strong mixing at the mouth of the Fal, caused by the combination of large waves and spring tides which would produce a well mixed water column. Conway CTD sites 2 and 3 show similar profiles. It is likely that due to limited geographical separation, any hydrodynamic or ecological processes in operation will act similarly. Conway CTD site 4 appears to show an increase of silicon with depth however this could be due to poor resolution as only two sample depths were taken.

Fig 2.3 Silicon vertical profile

Phosphate

Conway CTD site 1 is again homogeneous likely due to mixing at the entrance of the river. Above 4m, Conway CTD 2 and 3 show similar profiles. The large value for the 10m sample in site 2 (>21 µmol l^-1) is likely to be anomalous; a decrease similar to site 3 would be expected considering owing to their close proximity.  

Fig 2.4 Phosphate vertical profile

Nitrate

Conway CTD Station 1 appears to be homogeneous; however owing to the paucity of data between to the surface and 10m samples interpretation is limited. Taking into account the unfavourable conditions (ref), the apparent changes observed in sites 2 and 3 may be due to sampling error. Conway Site 4 shows levels of nitrate much greater than all other sites at a surface concentration of 24µgl^-1.  However, RIB site 4 reported a dramatic decrease in nitrate concentrations between Conway sites 3 and 4, which owing to their close proximity it is likely to be a sampling error.

Fig 2.5 Nitrate vertical profile

Salinity and Temperature

Profiles are homogeneous at all stations indicating a well mixed water column most likely caused by the high winds.

Salinity

Temperature

Chlorophyll

Conway CTD Sites 1, 2 and 3 located in the Fal River appear uniform with depth, however station 4 exhibits a decrease in chlorophyll concentration with depth. RIB station 5 reported the highest surface chlorophyll concentration (>9µgl^-1) of all the sample sites, which is likely due to its location in the Truro River which reported the highest measured nutrient concentrations.

Fig 2.6 Chlorophyll vertical profile

Dissolved O₂

The oxygen depth profile shows a general decrease in oxygen with depth.  This is expected because the biggest source of oxygen is diffused from the atmosphere at the water-air interface.  RIB station 5 shows the lowest oxygen content as well as the sharpest drop in oxygen content.  This is a relatively shallow, close to shore area and the decrease in oxygen may be due to plant matter in the water decomposing.

Fig 2.7 Dissolved O₂ vertical profile

Richardson Number

Analysis of the density profiles in the water column calculated using calibrated salinity, temperature and depth readings taken from the YSI probe onboard the Bill Conway showed intense vertical turbulence and overturning. Therefore meaning any Richardson number calculations are invalid.

Phosphate:

Strongly non-conservative behaviour evident from data collected. [PO₄^3-] at Rib station 5 is over 4 times that which is expected from the TDL. (REF) Whilst continuous addition exists through weathering of rocks, the large fluxes evident in this region are likely be attributed to domestic (washing powders), agricultural (fertiliser runoff). This may explain the high count of 32000 cells l^-1 at RIB station 5. (see Phytoplankton table of results).

Fig 2.8 Phosphate vs. salinity

Nitrate:

Photoautotrophic utilisation by macro and microphytes present in abundance are likely due to the high levels of phosphate.  Nitrate also being a plant nutrient is likely to be depleted in this situation due to the increased intake caused by the rapid phytoplankton population growth.

Fig 2.9 Nitrate vs. saliinty

Silicon:

Remineralisation of diatoms (forming the previous spring bloom event in this part of the estuary) possibly explains the addition of this nutrient to the lower reaches and hence the non-conservative behaviour observed. Since no known external fluxes exist in this area, liberation solely from the water column is probable.

Fig 2.10 Phosphate vs. salinity

Fig 2.16 The velocity magnitude of the flow at transect 2 varies across the transect.  The south end of the transect, on the outside of the bend in the river, is ~0.25 m/s faster than north end of the transect, on the inside of the bend in the river.  This variation in velocity is due to helical flow.

Fig 2.17 Backscatter indicates the amount of activity throughout the transect.  Regions of increased backscatter occur in the surface waters, 0m to ~1.5m depth.  This may be due to wind-induced waves breaking at surface, causing turbidity.  Bottom waters also produce increased backscatter caused by suspended sediments, due to frictional currents flowing along the seabed.

Fig 2.19 The velocity magnitude of the flow at transect 3 varies across the transect.  The north end of the transect, on inside of the bend in the river, is ~0.20 m/s slower than south end of the transect, on the outside of the bend in the river.  This variation in velocity is due to helical flow

Fig 2.20 There are two prominent regions of backscatter near the surface that stand out from the rest of the transect.  One maximum at 2m depth at the north end of transect and a second maximum at 4.5m depth. This may indicate a increased chlorophyll levels due to a phytoplankton bloom.

Fig 2.21 The directional contour plot shows the net flow of water at transect 4 was in a southerly direction across the mouth of the Truro River tributary. The direction of the net flow was homogonous throughout the transect and water column.  Very little variation occurred, due to a very intense ebb flow.  The transect was taken at 10.45GMT, and low water was at 13.21GMT.

Fig 2.22 The velocity magnitude of the flow at transect 4 varies across the transect.  The magnitude of the flow is higher in the centre of the channel (~4m/s), where frictional influences by the sea bed are minimal.  Flow at the edge of the channel is slower (~2.5m/s).

Fig 2.23 The most prominent region of backscatter is located at ensemble number 20411 and at a depth of ~2m. This indicates increased levels of chlorophyll and a potential phytoplankton bloom.

Transect 5 - Length of transect: 126m Start: 50° 13.623N, 005° 00.855W End: 50° 13.557N, 005° 00.840W

Fig 2.24 Transect 5 was across the mouth of a very shallow section in the River Fal.  The south end of the transect clearly shows a westerly net flow and the north end of the transect shows south westerly net flow.

Fig 2.25 The north end of transect 5 has a high magnitude of net flow (~5m/s).  The south end of the transect shows a reduced magnitude in flow velocity.  This may be due to shallower waters and increased friction with the seabed.

Fig 2.26 The middle of the transect shows a patch of increased backscatter. Due to the shallow depth of the water, this could be a region of suspended sediment of a phytoplankton bloom. CTD samples will need to be used to clarify the what is causing this increase in backscatter.

Fig 3.2 Homogenous fine sediment at position 50^o10.6340N, 005^o02.178W

Fig 3.3Depression in sediment (1.37m) at position (50^o10.6340N, 005^o02.5893W).  The depression is likely to be a relic of the underlying geomorphology and not a function of any impact upon the seabed.

Fig 3.5 Location of grab 1 at position 50°09.9107N, 005°02.6096W.

Fig 3.6 Location of grab 2 at position 50°09.6420N, 005°02.9374W .  Shows ripples measuring 0.238m in height and 2.22m in wavelength.  They can be seen to be wave induced as indicated by the bifurcation.

Fig 3.7 Location of grab 3 at position 50°10.1510N, 005°02.6383W.   Showing protrusion of bedrock surrounded by gritty mud with high content of biogenic material, including dead maerl.

It is important to note that when analysing a sonagraph print out, if certain assumptions, as described by Blondel and Murton (1997) are not met then the trace may show artefacts and distortions which can lead to the appearance of apparent features which do not represent any solid physical feature on the seabed. One of the assumption is that the seabed underneath the fish is horizontal with no mean gradient over the swath width. Our data is collected from a transect near the west bank of the Fal estuary so has a slopping seabed which could have lead to distortion of our data. 

Three grabs are not enough to represent the flora and fauna which inhabit the area. In order to obtain a more accurate picture, many more samples would have to be taken and species would have to be counted and identified in more detail.

click to enlarge