Introduction to the Falmouth Estuary

 Fig 1. Overview map of locations sampled

Falmouth Estuary is located on the southern Cornish coast and forms the large and sheltered natural harbor of Carrick Roads, a drowned valley at the junction of seven rivers.

The Fal Estuary is a drowned river or 'ria'. The deep water channel which winds its way upstream to Truro dates back to the last Ice Age when the sea level was much lower than at present. It is the third largest natural harbor in the world and the area known as Carrick Roads extends four miles from Black Rock to Turnaware Point with nowhere being less than one mile wide. Falmouth, the principal Port of Cornwall, lies at the mouth of the Fal Estuary on the South West coast.

The Fal estuarine system is characterized by a mean spring tidal range of 5m and tidal currents reaching 2 knots. Prevailing winds come from the South-west, mean surface water temperatures in summer are approximately 16°C and salinities in the open coastal waters surrounding the Fal are in the region of 35.

Extensive sea grass (Zostera) beds and the Coralline algae (Maerl) beds exist in the Fal estuary and have contributed to the classification of the estuary as a Special Area of Conservation.

The Fal region has been subjected to a history of pollution events due to heavy mining activity in the estuaries catchment area, along with domestic sewage effluent discharge. Localized effluent input continues in Carnon Valley, along with input from the County Adit, and there was a notable discharge of mining effluent (containing high heavy metal concentrations) into Restonguet Creek during the 1990’s. This has lead to increased metal levels detected within organisms in the Fal and some genetic adaptation of organisms to survive in the more polluted locations.

The combination of metal pollution and increased nutrient inputs has also led to the occurrence of red tide events (post 1995) due to blooms of the dinoflagellate Alexandrium minutum, resulting in paralytic Shellfish poisoning.

R/V Callista

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Fig 2. Map showing offshore sample locations

Fig 4. Backscatter at Carrick Roads

Fig 5. Velocity Magnitutde for Carrick Roads

Figure 7. The velocity magnitude at the offshore station

Figure 8. The velocity direction at the offshore station

Figure 10.  The velocity magnitude at St Anthony Head

Figure 11. The velocity direction at St Anthony Head

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R/V GreyBear

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Geophysics - R/V Grey Bear

Aim:

The aim of the boat work was to develop an understanding of the underwater area East of Falmouth Estuary taking into consideration the bedforms, rock type, sediment type and marine species present at the sites.

Objectives:

To meet this aim we used the following objectives:

• To carry out sufficient transects to give an accurate diagrammatic representation of the bottom structure of the area East of Falmouth Estuary.

• To identify the marine species present at the different grab sites.

• To identify the sediment/rock type along the transects, aided by the deployment of grabs.

• To identify any bedform formations in the sediment.

Key Outcomes:

Bed form ripples were identified close to the shoreline, existing in a cohesive, bioclastic/stone dominated sediment.
Rich biodiversity existed within the cohesive sediment and areas of bedrock had a far lower biodiversity.
Homogenous areas identified originally as soft sediment in the side scan record, turned out to be kelp forests existing primarily in deep fissures between the shallower bedrock.
Large expanses of solid bedrock (part of the Devonian Portscatho Formation) were identified which run offshore in a South-Easterly direction with the strike of the bedrock consistent with that of the strata on land.
 

Planning:

As the area East of Falmouth Estuary had not previously been studied it was decided to carry out 5 transects measuring 2km in length.  The sidescan sonar results will then be used to determine appropriate locations for the van veen grab.  Stations that are presumed to be appropriate are those that display characteristics of soft sediment and not rock substrata.  The strategy can therefore, be adapted depending upon the sidescan sonar results.

Locations:

Equipment:

• Sidescan sonar

• Van Veen grab

• R/V Grey Bear

• Stacked sieves

• Literature for interpreting the sidescan sonar results

• Literature to aid in the identification of marine organisms

Sampling:

In order to understand the geology of the area the sidescan sonar was towed as a fish 1m below the surface of the water and 1m behind the stern of the Grey Bear.  The fish was towed at 3knots along 5 transects.  A paper print out of the sidescan sonar was produced, as the transects were carried out, and analysed for potential grab sites, i.e. those with a finer sediment type.  Once appropriate sites were identified a Van Veen grab was deployed and observations were made upon the material brought back up.

Once back in the lab the sidescan sonar was further analysed to determine the sediment type, the size of the ripples present in the sediment and the direction of the strike of the rock.  A map was then produced of the features observed.

Complications: There was difficulty in selecting the grab sites as the majority of the area surveyed was rock.

Results:

Bedform Ripples Off St. Anthony Head

The ripples illustrated at 50°083N 005°00.7W were on the sidescan sonar plots are a result of shallow water waves.  The waves are defined by:

 d < λ / 20                   where  d: depth                                                             

                                                λ: wavelength

 It can be shown that the wavelength of the waves which caused the ripples were of a wavelength great enough to cause an influence on the sea bed.  Such bedforms are characteristic of storm waves.  For these bedforms to remain unscathed, the tidal influence in the area must have little effect on the seabed.  Admiralty charts illustrated that the tidal waves in the area of these bedforms propagate perpendicular to the coast line, hence currents are unable to remain for long enough to reshape the seabed.

Site 1

Position: 30330N, 184754E

Time: 12:10:43 AST

Water depth: 14m

The plan was to sample on soft sediment identified as an area of uniform backscatter from the side scan record. However the sampling position may have differed from the expected position causing us to sample on bedrock.

Sediment: No sediment collected at site due to sampling on solid bedrock.

Biology: Collected Kelp from bedrock in an area of possible extensive kelp forests.

Site 2

Position: 30449N, 184638E

Time: 12:21:08 AST

Water depth: 10m

Again the plan was to sample on an area of uniform backscatter identified as soft sediment. This was at the boundary between rock and weed (R.Wd) and stone with broken shells (St.bkSh) as shown on nautical chart of Falmouth harbour (5602.5). Again the desired sample wasn’t attained and the sample was believed to have been taken on solid bedrock.

Sediment: no sediment collected due to sampling on solid bedrock.

Biology:

• Several large Kelp;

• An amphipod (fig 30);

• American Hard Shell Clam (Mercenaria mercenaria);

• Calcareous algae, Maerl.

Site 3

Position: 30635 N, 184648W

Time: 12:48:48 AST

Water depth: 13m

The expected sediment sample was stone with broken shells (St.bkSh) as identified by previous sediment grabs in the location noted on nautical charts of the area. The area sampled was shown as a uniform area of backscatter, initially interpreted as an area of soft sediment.

Sediment: 20% of the sediment sample was <500µm and 75% was >1mm. The remaining sediment consisted of stone and broken shells as expected.

Biology:

•  Molluscs- Auger shell (Turritella communis)

     Lepidochtuna cinereus

                       Peppery Furrow Shell (Sciobicularia plana)

                       Limpet

• White Rag worm;

• Nematode;

• Juvenile common shore crab (Carcinus meanes)

Site 4

Position: 30629N, 185005E

Time: 13:28:19 AST

Water depth: 12.8m

The plan was to sample on sediment identified in the side scan record as uniform backscatter. However sampling again collected only kelp and it was believed sampling had taken place on solid bedrock.

Sediment: No sediment collected at site due to sampling on solid bedrock.

Biology: Collected Kelp from bedrock in an area of possible extensive kelp forests. Also identified was Palmaria Palmata which is a type of seaweed.

Site 5

Position: 30455N, 185138E

Time: 13:40:40 AST

Water depth: 10m

The plan was to sample again on sediment identified in the side scan record through an area of uniform backscatter. However sampling took place on rock and weed as before.

Sediment: No sediment collected at site due to sampling on solid bedrock.

Biology:

• Kelp;

• Molluscs;

• Lamimaria Hyperorea Seaweed;

• Bryazoans found on Kelp (Membranipora membranacea)

Conclusion:

Our geophysics survey showed that most of the seabed we surveyed was found to be hard rock, with some smaller areas of softer sediments found in between the areas of bedrock. The softer sediments, which showed bedforms and ripples tended to be found closer to the shoreline.

 Our grabs showed that areas which we originally thought were softer sediments were often kelp forests. These were shown by  homogenous areas of backscatter on the side-scan record.

Four out of five of our grabs showed areas of hard rock with maerl and kelp thriving. Our other grab showed an area of softer sediment found closer to the shore. This grab showed a high diversity of benthic species surviving in cohesive sediment, consisting of stone and bioclastics, along with a finer grain sand.

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Ocean Adventure

Coastal Research

Estuary - RIBs

Weather: 5/8 cloud cover, slight SW breeze, calm water.

Tides: (GMT)

0800 1.94

1400 4.38

2030 1.96

Aim:

To investigate the salinity change in the upper reaches of the estuary, with turbidity, dissolved oxygen content, plankton numbers, nutrient concentration and chlorophyll concentration as investigative factors.

Objectives:

The following objectives were used to accomplish our aim:

•  To sample stations at set salinity intervals to compare the data with the salinity gradient up the river.

• To assess the non-conservative or conservative mixing of dissolved chemicals at different points in the estuary.
• To establish a point of turbidity maxima.

Planning:

The upper reaches of the estuary were assumed to be typical of the fresh/salt water mixing interface with salinity near 0 being attainable at the top of the estuary. Sampling at set salinity intervals was a practical plan as it accounts for differences in chemical concentration at all stages in the river, (as the mixing interface may be over a short distance which geographical sampling could miss out).

After initial sampling it was realised that this mixing interface was in the very upper reaches of the estuary, (a flooding tide and lack of freshwater input were attributing factors) so geographical sampling sites were set downstream of Truro instead to test for any changes in the investigative factors at a more or less constant salinity ~ 30.

Locations:

Equipment:

Two shallow water vessels, a Rib and a Dory, each having a YSI probe for salinity, depth and light attenuation data collection. Each vessel was equipped with a Secchi disk and a Niskin bottle, and equipment for holding different chemical samples. This included glass bottles for nitrate, phosphate and oxygen sampling, plastic bottles for silicate sampling and test tubes for storage of the glass fibre filters which were used for analysis of chlorophyll concentration.

A plankton net was carried on board the Dory for 2 trawls at desired points along the estuary.

Key Outcomes: 

• Addition of silicate and removal of nitrate are taking place within the estuary, with a gradient to lower nutrient concentrations moving upstream.
• A large Copepoda zooplankton population were found to be present at both stations. Phytoplankton populations at all stations primarily contained Rhizosolenia, with large populations of N.Longissima also present.
• Population numbers of phytoplankton and zooplankton were higher further upstream (station B) than compared to stations located at higher salinities downstream (station A).
• A physically stable water column was present with stronger thermal stratification at stations closer to the estuary mouth (those stations with a higher salinity).

Sampling:

At each geographical station a depth profile was taken with the YSI probe measuring Salinity, Temperature, Dissolved Organic Matter and pH a 0.5m intervals.

A Secchi Disk was used to find the light attenuation depth at each station.

At the stern of the vessels, chemical samples were taken by collecting  a large amount of water from the surface and passing 50ml through a glass microfibre filter for both nutrients, (phosphate and nitrate sample) and silicate. The samples were refrigerated and the microfibre filters were stored in test tubes for chlorophyll analysis back at the laboratory.

Dissolved oxygen content was measured only once on each vessel, (equipment allowing) at stations A and B3. These were collected under the surface layer by taking a horizontal sample with the Niskin Bottle. Once prepared, they were stored submerged in a container of sea water.

Aboard the Dory two plankton trawls were deployed at Stations A and B which were preserved with formalin for analysis back at the laboratory.

Complications:

The original plan was to sample at salinity intervals from the sea, going up the river with increasing freshness. However, on the day of sampling, the tides meant that the sea water followed us up the river and so salinity wasn’t decreasing. The plan then changed to sample at geographical locations down the river. The stations are placed in the following order down the river: B2, B3, B4, B, B5, B6, C, B1, B7, A, D. Station B1 and B2 were sampled before we changed plan, and hence before the tide got up the river. This explains the lower salinities recorded for these two stations, and also why the salinities don’t necessarily increase as you progress down the river to the sea.

Results:

Zooplankton

Figure 28 displays the % of total abundance of which zooplankton identified in the taxonomic identification, contribute to the total zooplankton abundance at each sampling station. The most abundant zooplankton species were Copepoda, which contributed ~58% of total zooplankton abundance at both stations A and B. The water sample from station A also contained 27% Cirripedia larvae, with station B containing 18% of the same zooplankton species.

Station B also contained ~10% Gastropod larvae, with station A containing ~3% of the same species. Hydromedusae were also present at both stations, as were Copepoda nauplii. Both species contributed ~5% to total zooplankton abundance at station B and <2% at station A. Additionally Polychaeta larvae and Bryozoa were identified at station A, contributing <1% to total zooplankton abundance in both cases.

Phytoplankton

From the total % abundance graphs for phytoplankton (Fig. 39) it can be seen that station C has the highest amount of Rhizosolenia with about 64%, compared to station A and B of which had percentages of 13 and 34. The dominant species found at station A was Thalasiocira which contributed 46% to the total abundance. This species was not found at any other site. Guinardia floccida was found at station A in small amounts (1%) and station C with about 5%. One of the few species found at all 3 sites was Chaetoceros sp stations B and C had relatively low amounts, only 10% whereas station A had a large proportion of its abundance made up by this species (34%). Nitzchia had low was in very low abundance up the river it was only found at station A and B and it was >1%. Ceratrium was only found at station B and in very small amounts. The main contribution to station B was N.Longissima making up 51% of the total abundance; it was also found in the water column at station C but in much smaller quantities. The species Rarenia mikimotoi and Mesodinium rubrum were both found at station A but in small amounts >2%. At station B Eucampia sp and Thalasiovia made up very small % abundance. It was found that at station C Coscinodiscus contributed 12% to the abundance and the final few percentages were Eucampia sp and Thalassiosira.

Figure 40- graph comparing phytoplankton and zooplankton numbers.

At station A phytoplankton and zooplankton numbers were lower than at station B, station A had a phytoplankton number of 127900 cells/l whereas station B had a phytoplankton number of 626000 cells/l. Phytoplankton might be limiting the population and numbers of zooplankton because phytoplankton are zooplankton’s primary food source. At station B the larger phytoplankton population is able to support a larger zooplankton population.

Salinity Profiles

Figure 41- Salinity profile for upper estuary.

The profile shows salinities increasing with depth at all stations. The fresher riverine input from the Fal is less dense and hence sits above the more saline water. The salinities are typically lower at stations further up the estuary due to increased freshwater input and decreased mixing. It is well mixed at depth, indicated by the consistent salinity, despite varied surface salinities.

Figure 42- Salinity profile for the lower estuary 

The profile shows increasing salinities with depth at all stations. There is a fresher layer overlying a denser saline layer. The salinities for this profile are higher than those of the upper estuary, as the fresh water input of the Fal is less pronounced, and increased mixing with seawater increases salinity. Station B6 shows a text book surface fresh layer with a peak in freshness at approx. one meter.

Figure 43- Temperature profile for the upper estuary

The profile shows temperature decreases with depth. Surface temperatures are elevated by increased solar radiation and reduced wind stress which leads to the formation of a seasonal thermocline in the summer months.

Figure 44- Temperature profile for the lower estuary

The profile shows temperature generally decreases with depth. This is due to the decreasing solar radiation. Station B6, concurrent with the surface fresh layer (shown in figure 2), shows a surface layer temperature peak. The lower estuary appears to fairly less well mixed at depth than the upper.

Oxygen and pH

NB: Two RIBS were used to collect the data. Ocean Adventure sampled station B1, B2, B3, B4, B5, B6 and B7. Coastal Research sampled stations A, B, C and D. The probes were not calibrated. Groups that sampled before us in the week experienced varied data between the probes used on the different boats, and carried out calibration work in the lab to check the reliability of the probes and hence the data. The results show that the probes were too different and hence the data cannot be quantitatively trusted. The trends however can be observed and should give a fair representation of what was going on.

Figure 45- Dissolved oxygen profile for the upper estuary 

The profile shows in general oxygen decreases with depth. This could illustrate a phytoplankton layer with net respiration shown by the oxygen decrease. The water column appears well mixed at depth, with a fairly consistent oxygen level. With the elevated riverine input, turbidity levels would be useful as sediment input would limit light penetration, and hence it cannot be assumed that the shallowness of the water assures light penetration to depths for biological activity. 

Figure 46- Dissolved oxygen profile for the lower estuary

The profile shows oxygen levels decreasing with depth, possibly due to phytoplankton activity, although nutrient levels will be needed to collaborate this.

Figure 47- pH profile for the upper estuary 

The profile shows pH is fairly consistent with depth in the upper estuary. Station B shows an increase with depth.

Figure 48- pH profile for the lower estuary  

The profile shows pH is fairly consistent with depth in the lower estuary

Richardson number = 0.696

This means the water mass is relatively stable, however it has come from an unstable source.

Secchi Disk

The depth of the euphotic zone generally increases as you move out to sea. The sediment in the river will decrease the euphotic depth, as the increased turbidity absorbs and scatters the light, thus decreasing the depth to which it penetrates. Stratification at the lower end of the estuary will aid light penetration to depth, as illustrated by the deeper euphotic zones.

Conclusion:

Estuarine mixing diagrams from the estuary for nitrate, phosphate and silicate indicate that the nutrients are behaving conservatively with regards to salinity. There is no indication that addition or removal is occurring, however as nutrient samples were only collected at a limited range of salinities this is not conclusive. Nutrient concentrations display a gradient from the lower estuary toward the head with increasing concentrations moving up stream.  

Elevated nutrient concentrations in the upper estuary are supporting a large plankton population. This consists of both a phytoplankton and zooplankton population with the zooplankton grazing the phytoplankton. The gradient of nutrients identified through water column samples is also evident through the distribution of phytoplankton through the estuary, with the large phytoplankton populations coinciding with high nutrient concentrations. 

Temperature and salinity profiles in the estuary indicate a typical partially mixed estuary type (2 on the Hansen-rattary classification diagram) exists in the estuary with a deepening thermocline moving toward the estuary mouth. Stable stratification appears to exist at all of the station but of varying degrees of stability. Profiles of the estuarine pH indicate increasing alkalinity moving toward the estuary mouth, although the difference in the pH between the lowest station and the station closest to the estuary head is minimal (<1 pH). 

Due to sampling taking place as salinity moved up the estuary this brief study may be far from the true picture. Future work should concentrate on mapping nutrient distributions over many salinities and covering multiple tidal conditions to provide a more accurate view of the Fal estuary.

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Fig 37. Map showing stations sampled in the estuary by the RIBs

Fig 38. Zooplankton Abundance

Fig 39. Phytoplankton abundance

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R/V Bill Conway

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Estuary - R/V Bill Conway

Weather: sunny, 0/8 cloud cover, breezy

Tides: (GMT)

 0720 5.19m

1350 0.71m

1940 5.54m

Aim:

To assess the extent of vertical mixing processes in the estuarine waters off Falmouth and to collect information on nutrient concentrations and plankton numbers in said areas.

Objectives:

 To meet this aim we used the following objectives:

• To sample at sufficient stations to give an accurate representation of the estuarine conditions in the Fal estuary system.

• To quantify vertical mixing processes at work in the estuary, through measurements of various water column parameters (such as density).

• To quantify and identify the plankton communities present (phytoplankton and zooplankton).

Key Findings:

• Addition of nitrate and phosphate is occurring in the lower estuary.
• A gradient of oxygen distribution exists between the lower and upper estuary stations, with a mussel farm responsible for a sharp fall in oxygen parameters downstream of the farm location.

Planning:

In order to achieve a comprehensive understanding of the estuarine system in Falmouth Harbour the following plan was devised. Transects will be carried out across the river at various points. These transects will look at the currents present in each location. Further studies on the physical, chemical and biological structure will be carried out using a CTD will niskin bottles attached.

Locations:

See map ->

Equipment:

• CTD attached to a 12 Rosette frame, using 6, 2l Niskin Bottles and attached Flourometer & Transmissometer

• Towing Plankton Net (200µm mesh size, 60cm diameter)

• Acoustic Doppler Current Profiler (ADCP)

• Seechi Disk

• Light Profiling Attenuation Meter

• Preservatives (i.e. acetone) for zooplankton, phytoplankton, chlorophyll

• Sufficient number of sampling bottles (glass and plastic)

Sampling:

At each sampling station a depth profile was taken with the CTD probe measuring Salinity and Temperature

A Secchi Disk was used to find the light attenuation depth at each station.

Chemical samples were taken by collecting a water from desired depths, (decided by analysing the CTD and ADCP outputs after a single deployment). and 50ml of sample water was passed through a glass microfibre filter for both nutrients, (phosphate and nitrate sample) and silicate. The samples were refrigerated and the microfibre filters were stored in test tubes for chlorophyll analysis back at the laboratory.

Dissolved oxygen content was measured at each station using sample waters from each depth. Once prepared, they were stored submerged in a container of sea water.

Zooplankton trawls is carried out at the sea-end member and the estuarine end-member for 2 minutes at 1.5 knots.

At station 6 the equipment, including temperature probe and salinity probe was calibrated with that of the ribs.

Complications:

There are no known complications when collecting data for this investigation.

Results:

Chemical

The oxygen saturation within the estuary varies strongly between stations (Fig.50) with a gradient from lower saturation at the estuary mouth (~112% saturation) to higher oxygen saturations moving upstream (127-134% at the stations closest to the estuary head). There is a sharp fall in oxygen stations moving downstream from station 6 (~134%) to station 5 (~114%) with the stations lying upstream and downstream of a mussel farm located in the estuary channel.

 The oxygen concentrations (Fig.51) reveal a similar picture with a gradient from low oxygen concentration at the estuary mouth (~285 µmol l^-1) to higher oxygen concentrations at the upper stations (~326 µmol l^-1). There is again a sharp change in oxygen concentrations between stations 5 and 6 with a change of 44 µmol l^-1 between the stations.

The estuarine mixing diagram for nitrate (Fig.52) indicates addition of the nutrient is taking place in the estuary between a salinity range of 5-30. At higher salinities it appears that the nutrient is then behaving conservatively. Addition of phosphate also appears to be taking place (Fig.53) within the estuary, whereas silicate (Fig.54) is behaving conservatively with regards to salinity.

Biological Analysis

Zooplankton

Figure 55 displays the % of total abundance of which zooplankton identified in the taxonomic identification, contribute to the total zooplankton abundance at each sampling station. The most abundant zooplankton species were Copepoda, which contributed ~42% of total zooplankton abundance at station 1  and 61% at 2. The water sample from station 7 also contained 32% Cirripedia larvae, with station 1 containing 18% of the same zooplankton species.

Station 1 also contained ~12% Gastropod larvae, whilst station 7 contained none of the same species. Hydromedusae were also present at both stations, as were Cladocera. Additionally Appendicularia larvae were identified at station 1, contributing ~8% to total zooplankton abundance.

Phytoplankton

From the total % abundance graphs for phytoplankton (Fig. 56) it can be seen that for all stations the most abundant phytoplankton is Rhizosolenia delicatula. This phytoplankton is most abundant at station 4 where it contributes to 64% of the station’s phytoplankton. At stations 1 and 3 Rhizosolenia delicatula contribute ~50% of total abundance and at station 7 they contribute the least to total abundance at ~31%. The dominant species found at station 7 is Chaetoceros which contributed ~33% to the total abundance. This species was found at the other sites but at slightly lower % of total abundance ~15%. Rhizosolenia setigera was found at stations 1,4 and 7 contributing ~6% to total abundance. This species was found in negligible amounts at station 3. The species Karenia mikimotoi was found at all 4 stations, contributing ~11% at station 1, ~5% at station 3, ~9% at station 5 and ~6% at station 7. Thalassiosira rotula was also found at all four stations contributing the largest percentage of ~11% at station 7 and ~5% at the other 3 stations. The species Eucampia was only found at station 7, representing 6% of the total abundance. This may be because this species favours slightly fresher lower salinities.

At station 7 higher phytoplankton levels are seen compared with station 1. At station 7 there are 3540000 phytoplankton cells/L, which is over double the number seen at station 1 where there are 1420000 phytoplankton cells/L.

Lower zooplankton levels are seen at station 7 compared with station 1. At station 1, 265 m^-3 compared with 209m^-3 at station 7.

The elevated zooplankton levels seen at station 1 accompanied by the lower phytoplankton levels could be due to the large zooplankton population cropping the phytoplankton population. The area around station 1 at the time of sampling could be in the later stages of a bloom, where phytoplankton numbers are now decreasing and zooplankton numbers are still large due to the lag behind the phytoplankton decline after the bloom. At station 7 a slightly smaller zooplankton population exists accompanied by larger amounts of phytoplankton.

Station 1 (Fig 57) indicates that higher nitrate concentrations are present in deeper water but that the phytoplankton population is only present in the upper (surface) layer. This shallow phytoplankton population corresponds to the shallow secchi disk observation, and is possibly as a result of high turbidity in these waters (during the data collection sea conditions were stormy, resulting in a well mixed water column). Fig 58 and Fig 59 demonstrate a positive relationship between nitrate and chlorophyll, indicating higher chlorophyll concentrations coincide with high nutrient concentrations.

Physical:

The ADCP and the CTD were two instruments used for physical data collection aboard the Bill Conway.  These instruments allowed the analysis of the estuarine dynamics with respect to water salinity, temperature, density, flow velocity magnitude and direction.  There is a noticeable trend up the estuary of flow direction and velocity associated with the temperature and salinity of the water.  At the mouth of the estuary, the water column was generally well mixed with uniform velocity magnitude and direction. A seasonal thermocline was present at station 1, as seen in Figure 60.  The Richardson number for this location at the thermocline was 0.17.  Hence, the water column is mixed however not enough to cause the breakdown of the thermocline.  The mean water velocity was 0.125m/s.  Backscatter between 10m and 15m decreased to 60dB, indicating the presence of a thermocline and a dominant zooplankton population.

As the vessel progressed up the estuary, the presence of fresh water became more dominant, a non uniform flow becomes increasingly influential.  The non uniform flow below 15m water depth was overlaid by a well mixed water with a velocity ranging between 0.125m/s - 0.250m/s. Station 2 had a higher velocity of 0.5m/s on the water surface, the water column mean water velocity was 0.250m/s.  The Richardson number was calculated as 0.02, indicating the dominance of shear in the water column.  Thus the presence of a thermocline at 14m was minimal (Figure 61).  Figure 3 illustrates the use of the ADCP for identification of a thermocline using the presence of the ADCP backscatter.  Station 3 showed a well mixed (Figure 62) water column, a thermocline was present at 12m, where the salinity was 34.94psu (Figure 63).  At station 3, the Richardson number was 0.0007 indicating large amounts of turbulent mixing.  Figure 64 shows the increased velocity in the estuarine deeper channels.  The figure shows that the water velocity is lower in the shallower water, as a result of increased shear and mixing, thus resulting in a change to water direction (Figure 65).  The water column at station 4 was well mixed.  The temperature decreased by 0.5°C over a depth of 9m and the salinity increased by 0.21psu indicating a well mixed system (Figure 66)  This is supported by the Richardson number of 0.04. At station 5, the surface water also is less saline, hence as the vessel progressed up the estuary, the river’s influence is becoming increasingly dominant (Figure 67).  The ADCP transect of the river at station 5, shows us the velocity direction and magnitude and the progression of the fluvial and saline water (Figure 68).   Station 6 and 7 also show the characteristics of the fresh water layer over the denser, saline water. The water between 0m and 3.5 m decreased by 0.07°C and the salinity increased by 0.04°C. Below this mixed layer was a thermocline and halocline 6m deep.  The bottom 3m had a salinity of 33.9psu and a temperature of 17°C (Figure 69).

The progression of the salt water wedge up the estuary is dominant in determining analysis of the processes and relationship existing in the estuary.

Conclusion:

The distribution of the 3 major estuarine nutrients indicates that nutrient addition is taking place between salinity ranges of 20-30. This suggests that the addition is taking place in the lower estuary in more saline water. The oxygen samples have that a sharp fall in both oxygen saturation and oxygen concentration is occurring between stations 5 and 6. The location of a mussel farm between these stations suggests that the shellfish are responsible for the fall in oxygen parameters, due to uptake of the oxygen during respiration of the mussels. The gradient from high oxygen saturation and concentration to lower values moving towards the estuary mouth is typical of partially mixed estuaries such as the Fal.

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Main Findings:

We found within the Fal estuary system that the vertical density structure of the water column was effected by the input of freshwater from the River Fal. Moving offshore we found a strong seasonal thermocline, with a large phytoplankton andzooplankton population thriving.

Website Links:

http://www.metoffice.gov.uk/weather/charts/index.html

Weather information

http://www.cornwall-online.co.uk/carrick/falmouth.htm

Background information on the area

http://www.swenvo.org.uk/environment/estuaries.asp

Southwest Observatory Environment- Information on estuaries

http://www.mba.ac.uk/NMBL/publications/osspub/occasionalpub2.htm

National Marine Biological Laboratory- See for publication on heavy metals in Fal estuary.

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Further Investigations

Callista 

Future investigations would be to sample:

1. Further offshore: We would expect to see a stronger thermocline, with no mixing unless initiated by a storm. We would expect to see the plankton sitting around the thermocline, as this area is optimum for light and nutrients. Depleted surface nutrient levels would indicate phytoplankton activity, and then a build up of nutrients stuck beneath the thermocline would show the biological pump and succession.
2. Looking for a tidal front:  We would look for the boundary between the stratified Atlantic and the mixed Western Channel: by sampling at intervals along the south coast, droping the CTD at stations to analyse stratification. The front would show a difference in plankton distribution, nutrient levels and irradiance, indicating the two water masses.   

Grey Bear 

Future investigations would be to sample:

1. For shipwrecks: Shipwrecks are noted on admiralty charts. The side scan sonar would show shadowed areas, possible shaped as a ship!! This would be an interesting area to investigate, further looking into the biology and chemistry around the wreck as it transforms into an artificial reef. Grabs can be taken of the sediment in and around it, and analysis combined with data from the Bill Conway would build a picture of the area.
2. The Harbours: We would expect to see areas of dredging which would be more interesting to analyse than the rock and kelp we saw. Grabs would also contain more sediment, and hence a better insight into the marine benthos could be achieved!

Bill Conway and RIBS 

Future investigations would be to sample:

1. On Shipwrecks: Shipwrecks identified by the Grey Bear, or from charts, would be investigated. Looking at the water column above the wreck, with nutrient levels etc, would allow an overview of the shipwrecks efficiency as a reef. ADCP profiles would show currents around the reef and hence an insight into the physical protection it provides and therefore the often use as a nursery ground for young fish.
2. Restronguet Creek: This creek is reknowned for its heavy metal input to the estuary, and hence sampling up the creek would enable us to quantitavely asses the input. Sampling for heavy metals would be added to the usual nitrate, phosphate, silicate, oyygen and chlorophyll analysis.
3. The Harbours: A political/ecological issue in this area is the use of TBT to stop the hull of ships rusting. It has already been banned on ships, 25m, and by 2008 on all boats . Although efficient at limiting rust, it is proven to cause imposex in dogwhelks and affects mussels, and hence the economy in this area from mussel farming. Sampling in the harbours, (Mylor and Falmouth), and comparing phytoplankton levels, nutrients and physical parameters (light, temperature, salinity), will allow us to investigate the extent to which TBT impairs the habitat.
4. Further up to Truro: Due to tide implications, we couldn’t sample up as far to Truro as we had wanted. We would therefore pre plan and start in Truro sampling down the river next time.  

All views expressed in these pages are those of the authors of this site, and are not necessarily those of the University Of Southampton, the National Oceanography Centre or the School of Ocean and Earth Sciences.