During the first two weeks of July 2008 Group 3, eleven Oceanographers and Marine Biologists, undertook an investigation into the physical, chemical, and biological processes in the Falmouth area. Falmouth is situated in the South West tip of England in the county of Cornwall and is exposed to prevailing south westerly winds from the Atlantic Ocean.

The estuary is a Ria typified by steep-sided valleys common to the majority of British estuaries. Falmouth is the third deepest natural harbour in the world and it is fed by 6 main rivers the Truro, Tresillian, Pill Creek, Restrongruet Creek, Mylor Creek and Penryn River as well as many smaller creeks and tributaries.¹ However, the volume of freshwater delivered to the estuary is small, suggesting the effect of freshwater on estuarine condition is negligible. The estuary is well-mixed and varies in tidal range, with the main body being predominantly mesotidal (2-4m).

One of the most metal polluted areas in Britain, the Fal has been exposed to high levels of copper, zinc and cadmium mainly through agriculture and mining activity. In the past this has had an adverse effect on the biology leading to the area becoming a Candidate Special Area of Conservation.

The Fal estuary and the offshore region were surveyed using four research vessels, Callista, Explorer, Bill Conway, and Ocean Adventure. As the estuary itself is a dynamic environment varying seasonally, temporally, and spatially a variety of measurements were undertaken.
 

Please browse our website for our findings
from the estuary and offshore area!

RV Callista



Day boat used for sampling both the
Estuary and Offshore

Length: 19.75m
Beam: 7.4m
Draft: 1.8m
Speed: 15 knots
Passengers: 30 max
A-Frame: 4 tonne capacity
Capstan: 1.5tonne pull
Davits: 2x100kg
Range from safe haven: 60mls

RV Bill Conway

Day boat used for sampling the Estuarine Area
 

Length: 11.74m
Beam: 3.96m
Draft: 1.3m
Speed: 9knots
Passengers: 14 including crew
A-Frame: 750kg
Davits: 1x50kg
Trawl Winch: max wire length 70m
Range from safe haven: 60mls

RV Xplorer

Survey boat used to scan the geophysics
of the Helford River area

Length:12.0m
Draft: 1.00m
Speed: 25 knots (max)
Passengers: 12 passengers plus crew
Capstan: Anchor retrieval
Range from safe haven: 60 mls

Ocean Adventure (RIB)

RIB used to sample the upper reaches of the
Estuary and the Fal River

Length: 7.0m
Beam: 2.55m
Draft: 0.5m
Speed: 35knots max
Passengers: 6
Range: 20mls from safe haven

ADCP-Acoustic Doppler Current Profiler

ADCP sends out pulses of sound that are received back as echoes at varying times depending on current properties. Data on current velocity and direction is received via an on-board computer using the program WinRiver Aquirer and is processed further using the program WinRiver Playback. Backscatter readings indicate turbidity and presence of biology within the water column.

CTD – Conductivity, Temperature and Depth Profiler

The CTD gives a vertical profile of the water column. For Bill Conway, the set up used consisted of six GoFlo Bottles used to take water samples loaded on a Rosette with an attached YSI Probe (F105c). The CTD is deployed using a winch with communication with the onboard computer showing depth of the water column.
Both an up-cast and a down-cast were sampled, with the GoFlo Bottles being fired via remote onboard computer, at depths determined from the down-cast.

Zooplankton Net

The net used had an entrance diameter of 0.4m and a mesh size of 200µm. The net is towed behind the vessel and a Hydro Bioskiel counter is used to measure the amount of water flowing through the net. The sample is collected in a vessel attached to the net and the net is rinsed with water from the onboard pump into the collection vessel.

Secchi Disk

The secchi disk is used to determine the depth of the euphotic zone (1% light level) of the water column. The disk, approximately 20-30cm, has alternating black and white panels. The disk is lowered until not longer visible, this depth is multiplied by 3 to get the depth of the euphotic zone. The disk should be operated by the same individual at each station to avoid discrepancies.

Side-Scan Sonar - Geo Acoustics Ltd (159D)

The side scan sonar device, also known as a ‘tow fish’, was towed behind Xplorer and emitted a sound pulse at a frequency of 100kHz. The pulse emitted was reflected off of the seafloor and returned to the towfish, resulting in a black and white image. The darker lines represent stronger reflections, which indicate sediment types such as coarse sand. Lighter lines are often indicative of mud or silt. The side scan image can also show depressions and elevations in the seafloor.

Van Veen Grab

The Van Veen Grab is a lightweight long-levered sediment sampler that takes large samples of soft sediment. The sample size used was 0.5m³.

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RV Bill Conway and Ocean Adventure (RIB) were used to take measurements in the estuary from Malpas to Black Rock.                   

The aims of this section are to:

- Estimate and compare the phytoplankton abundance from surface samples taken
               at stations ranging from Malpas to Black Rock.

- Understand how the physical, chemical and biological parameters change within 
               the estuary and to suggest reasons for the distribution of these parameters, which
               include temperature, salinity, dissolved oxygen, turbidity, and chlorophyll.

- Determine how nutrient levels changed as a result of pollution inputs from both
               point and diffuse sources and biological activity.   
 

Method

RIB

Ocean Adventure started at Malpas, the RIB's upper limit in the estuary, in order to move against tidal flow so that the same body of water was not sampled twice. A depth profile was taken using two separate YSI probes, YSI 6600 is a 5 parameter (Temperature, salinity, chlorophyll, turbidity, oxygen saturation) multiprobe and YSI 650 is a two parameter multiprobe (Temperature, salinity). Water samples were taken to determine the silicon, nitrate, and phosphate concentrations at every location and bottles were collected to determine the oxygen percent saturation at 4 locations. Samples were taken using a hand-held Niskin bottle triggered at a depth of 0.5m. Salinity was monitored continuously with the aim of selecting optimum sampling stations as the RIB travelled down the estuary to Turnaware Point.
 
For the most part, buoys and pontoons were used for convenience, safety, and positioning. A total of 6 stationary samples were taken and one sample was collected in a bottle while the RIB was moving due to a lack of fixed mooring positions. The wrong net was provided so zooplankton samples could not be collected on the RIB. Using bottles filled with Lugols, phytoplankton samples were collected at 4 locations for analysis to determine the phytoplankton abundance and species present at each location.

Conway

RV Bill Conway headed to Black Rock and carried out a transect and a depth profile using an ADCP and a CTD (the set up used consisted of a T/S probe attached to a rosette). Water samples, zooplankton samples, and phytoplankton samples were taken at this location to be analysed in the same manner as the samples collected on the RIB. The vessel then moved to Smuggler’s Cottage, the highest point up the estuary for the vessel. Transects 2, 3 and 4 were carried out to form a triangle to determine the effect of the river inputs in this area. A CTD was deployed at the middle of transect 2 and water samples, zooplankton samples, and phytoplankton samples were taken at this location. The vessel travelled back down the estuary and carried out another 5 transects and CTD depth profiles down to Falmouth Harbour. No samples were taken at CTD stations 3 and 7, however surface samples were taken at 0.5 salinity intervals.

Transect Positioning Data for Bill Conway

Laboratory Methods

Nutrient analysis was subdivided into the analysis of chlorophyll, phosphate, nitrate, dissolved silicon and oxygen concentration.
The chlorophyll was examined by fluorometry³, whilst the phosphate and nitrate⁶ were examined by flow injection analysis spectrophotometry, with nitrite being measured as a representative of nitrate. The silicon was also analysed using flow injection analysis spectrophotometry with the silicon being treated with reagent (ammonia molybdate) and reducing agent (MRR; Metol Sulphite : Oxilic Acid : Sulphuric Acid : MQ Water, at a ratio of 10:6:6:8 respectively). The data was analysed using two separate sets of standards as the first set were unsuitable for the low absorbance samples. The oxygen concentration was calculated using the semi automated Winkler reagent method⁵.
Water samples were also analysed for phytoplankton and zooplankton quantities. Having been left to settle overnight, the phytoplankton samples were then analysed after the sample had been concentrated (9/10 of the volume of the sample was removed leaving the plankton who had settled in the concentrated sample). The zooplankton samples we analysed under a microscope using a Bogorov Counting Chamber containing 5ml of sample.

Physics

Station Data

Station C1

Station C1 shows homogenous salinity, the Richardson’s number shows laminar flow in the top layer with shear created at the thermocline between layers. It then fluctuates between laminar and turbulent in the deep layer, thus creating the underlying well-mixed water body.

Station C4

Station C4 shows a slight thermocline in the upper 4m of the water column, above which laminar flow was present. Where the upper outflowing freshwater layer meets the flooding saltwater, shear is created, lowering the Richardson's number. Below this, the water column is well mixed with temperature and salinity homogenous with depth.

Station C6

Station C6 shows a well mixed water column with salinity and temperature homogenous throughout. Collected nutrient samples suggest they are unvarying in concentration however, samples were limited to 3 depths reducing the ability for reliable analysis.

Discussion

The stations nearer the head of the estuary tend to have pronounced partially mixed structures with strong haloclines occurring at stations R1 and R2 (Figure 4). In comparison, those stations located nearer the mouth, for example C1 and C6 (Figures 1 and 3), demonstrate greater homogeneity in salinities with depth. It can be said therefore, that the Fal estuary is well mixed near the mouth, but becomes progressively more partially mixed near the head. This is probably a result of tidal dominance near the mouth and the impact of the river near the head. It must be noted however, that stations R1 and R2 were only sampled during low tide. Therefore the real effect of the tide cannot be derived from the results. Temperature also follows this structure of increased stratification near the head and homogenous distribution with depth near the mouth.

The ADCP data is illustrated in figure 7 and shows the progressive movement of the tide over the survey. Transect 2 shows the most riverward transect, indicating faster flow at depth. This shows the landward movement of seawater at depth with the seaward river flow above showing a stratified water column. Looking at transect 5, the point where the fast moving flooding tide meets the freshwater is clearly seen as the stratification breaks down, transect 6 showing the well mixed tide on the flood. Transect 1 was taken at Black Rock, the most seaward sampling area and shows a well mixed and slowing moving water column, there is movement in and out of the estuary due to a turbulent eddy caused by Black Rock. This transect was taken at the beginning of the survey where the tide is turning, the slack water clearly seen in this figure.

The current pattern corresponds to the phytoplankton data (Figures 12 and 15), as the water column becomes more mixed, the number of dinoflagellates decreases as mixed conditions are damaging to the flagella whereas the number of diatoms, that are more robust, increases.

Table 1: Secchi Disk results showing depth of euphotic zone as 3x secchi disk depth and the attenuation coefficient (k) - Transect 1 is positioned last as it was sampled at Black Rock same as sample 7

Secchi disk data (Table 1) indicates that the depth of the euphotic zone decreases around the middle estuary. This can be explained by increased confluence of several freshwater inflows at this point increasing the sediment load. This was also the point where the flooding tide met freshwater causing increased turbulence (see Figure 7, ADCP 5). The attenuation coefficient shows a general increase landward, where the increased sediment load and increased numbers of phytoplankton (Figure 12) led to faster attenuation of available light.
 

Chemistry

Nutrient concentrations decrease with increasing salinities. The estuarine mixing diagrams for nutrients in the estuary show mixed results. The mixing diagram for nitrate (Figure 8) shows active removal from the water column, whereas silicon (Figure 9) and phosphate (Figure 10) shows active addition. The addition of both nutrients could be explained by the high levels of precipitation occurring in the area over the period of the survey and preceding days which would increase surface runoff into the estuary. The removal of nitrate can be attributed to the presence of dinoflagellates, particularly Alexandrium (Figures 11 and 16) which has been previously related to fluxes in inorganic nutrients, mainly nitrate (Amniot et al 2001)².

As nutrient samples were only taken at certain depths depending on the data from the downcast, there is little data to explain the vertical distribution of these nutrients (Figures 1-6). Vertical profile 1 from Conway (Figure 1) shows a visible peak in nitrate which is echoed by phosphate below the thermocline, as nutrients are limited above due to biotic and abiotic factors such as stratification and utilisation by phytoplankton. Chlorophyll is reduced at depth due to light attenuation reducing the numbers of phytoplankton.

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Figure 1: Conway Station 1 Vertical Profiles


Figure 2: Conway Station 4 Vertical Profiles


Figure 3: Conway Station 6 Vertical Profiles


Figure 4: RIB Station 2 Vertical Profiles


Figure 5: RIB Station 5 Vertical Profiles


Figure 6: RIB Station 7 Vertical Profiles


Figure 7: Estuarine ADCP data from Bill Conway

Figure 8: Nitrite Estuarine Mixing Diagram

Figure 9: Silicon Estuarine Mixing Diagram


Figure 10: Phosphate Estuarine Mixing Diagram

Aims

On 4 July 2008 group 3 went out on RV Callista to carry out offshore measurements to better understand the chemistry, physics, and biology of the offshore region.

The aims of this section are to:

-         Estimate and compare phytoplankton and zooplankton abundance from samples taken at various depths at stations ranging from Carrick Roads to 3.5 miles offshore to help understand the dynamics of the water column.

-         Determine the position of the front and how physical, chemical and biological parameters change either side of this and with distance from the shore.

-         Understand how the tidal movements affect the water column in Carrick Roads.

-         Compare the results obtained in the offshore region with those of the estuary.
 

Method

The original plan for our day offshore was cut short by an advancing weather system. The Force 5 increasing Force 7 South Easterly winds brought with it a 3 metre swell, scuppering our original plans for offshore sampling.

Our transect instead lead us offshore from Black Rock on a 3 mile ADCP transect run, with a CTD downcast at the start, at the 2 mile point and at 3 mile turnaround, to establish a Vertical Profile. The Niskin Bottles were fired at the surface, middle and bottom of the water column. At each of these stations we also deployed 200µm mesh zooplankton net and the samples preserved in formalin. A Secchi Disk was also deployed and the cloud cover noted.

The initial sampling strategy was to use the running time ADCP data to establish the presence of a tidal front at a location offshore, and sample around it establishing its characteristics and precise orientation.

After re-entering the estuary, a new estuarine sampling strategy was adopted and transects across the estuary were taken. CTD casts were sampled at the channels deepest point and Secchi data was gathered as for the offshore sites. The Zooplankton net samples were completed with a horizontal tow, instead of the vertical lift adopted offshore.

For details of the Laboratory Methods used to analyse the nutrients collected in the samples please click here.

                                                                      


Transect Positioning Data for Callista

Results/Discussion

For the offshore work, the ADCP and vertical profile data have been analysed. The ADCP data corresponds to the continuous transects and the vertical profiles correspond to the CTD profiles taken.

ADCP Data

Looking in detail at the ADCP data (Figures 11) several sections have been picked out giving an overall picture of the water column during the sampling. Figure 11 shows the ADCP data in detail. The first transect taken, travelling between Black Rock and CTD 2, positioned offshore, was taken at the point of an ebbing tide. Tidal diamonds A and B were used to determine the direction of tidal flow. The ship track shows a south westerly flow which supports the data from these tidal diamonds showing two interacting water masses. The ADCP profile shows higher velocity flow at depth close to CTD 2 with the maximum velocities at 30-42m depth at 0.4-0.5ms^-1. At mid-way along the shelf the water velocity begins to increase with a transition from 0.1-0.3 ms^-1; this continues offshore.
The backscatter profile for this transect shows consistently high levels at the surface, around 90dB. This may be indicative of large populations of zooplankton but at this location is more likely to be caused by wave action due to the rough weather. The Richardson number of 0.1 at Black Rock correlates with this data indicating turbulent flow, however backscatter is much lower (67dB) toward Black Rock indicating a well-mixed vertical profile. Less backscatter correlates with the increasing velocity as there will be higher levels of mixing giving smaller populations of zooplankton, but at 4815m offshore, there is a slight increase in backscatter, from values around 69dB to 74dB.  This could indicate the beginning of a frontal system.  This would be more evident if there was less mixing.

Transect 2 looks at the period between CTD 2 and CTD 3 and indicates an increase in velocity from 0.5-0.7ms^-1 during the second 1000m of the transect as it continued offshore. The increased influences of wind and wave could be responsible for this change.
The backscatter profile here shows high levels of backscatter in the surface 4m at values of 86dB, this continues the trend of the previous transect and is likely to be due to wave action. This is the location at which the frontal system was expected to be found, however there is no evidence from either ADCP or CTD data that suggests this to be true; it is likely that due to the worsening sea conditions the front has been broken up.

The ADCP Data looking at Falmouth Docks to Falmouth Bank shows flow at depth on the right hand side of the channel, indicative of the dredged area of Falmouth Docks. This shows an area of velocities of 0.18ms^-1 to 0.2ms^-1 flowing towards the east at 190m from Falmouth Docks at a depth of 2-4m.  This is an area of higher velocity compared with the rest of the transect.  It is located at the edge of the dredged area, where the walls of the harbour have less effect.  At this time, the tide is slack (shown using the tidal diamonds), and therefore the predominant influence on the water flow is by the river input from Penryn River.
Backscatter is lower where there are higher velocities, around 78dB, and the main range of values occurs at the low shelf between 1 and 2m, possibly due to the presence of zooplankton communities.

The final two transects we have chosen to look at in detail are in the area of Falmouth Docks and Carrick Roads. The first, taken between Falmouth Bank and the left side of Carrick Roads shows low velocities in the deep channel of Carrick Roads, the main change in velocity comes from 4m depth downwards, with velocities range between 0.05ms^-1 and 0.1ms^-1.
The higher velocities in the surface 4m are likely to be caused by slack tide conditions and the dominance of wind stress. There are also high levels of backscatter along the shelf on the side of the deep channel that may be caused by large sediment loads entering from Penryn River.
Below 20m in the main channel, backscatter falls from values of 70dB to 65dB.  This reflects the lack of zooplankton communities present in this area.  Secchi disk data in this location gave a euphotic zone limit of 15m.  The zooplankton populations in the area may have been in the euphotic zone at this time as part of vertical migration to feed on the phytoplankton populations that require the euphotic zone for photosynthesis, thus explaining the lack of zooplankton at depth.

Figure 11: ADCP data files for Callista


Figure 12: Station 1 vertical profile


Figure 13: Station 2 vertical profile


Figure 14: Station 3 vertical profile


Figure 15: Station 4 vertical profile


Figure 16: Station 5 vertical profile


Figure 17: Station 6 vertical profile


Figure 18: Station 7 vertical profile

 

The final transect between Falmouth Docks and Carrick Roads showed an area of high surface velocity (0.31-0.4ms^-1 on the outside of the meander at Carrick Roads, beginning approximately 1500m along the transect.  This is due to current acceleration around the meander, which may be caused by surface wind stress funneling through the channel, as well as the start of the flood tide.  There is no correlation between this area and the backscatter profile.
Over the shelf, backscatter values are high (78-88dB) to depths of 4-5m.  This may be due to zooplankton populations, vessels in Falmouth Docks or wave action.

Vertical Profile Data

The data explained here groups different CTD stations in similar locations. Although the ADCP does not indicate a front, data from other groups has indicated a front present in the region shown in the ADCP data and the vertical profile data works on the basis that a front may have been present although rough weather makes this difficult to verify. The analysis looks at both temporal and spatial scales.

Stations 1-3 (Figures 12-14)

Physics
A loosely defined elongated front was located approximately 2 nm offshore at Station 2.  On the landward side of the “front” (Station 1) a slight thermocline was found at 8 metres with a temperature change of 0.3°C change.  Due to the shallowness of the estuary at this point the water column is more mixed than further out in deeper water due to shear stress.  This is shown with the Richardson number which at many points is below 0.25.  (A Richardson number of below 0.25 means the water column is more likely to overturn and vertical mixing is greater.) 
Further offshore the thermocline becomes more pronounced.  At Station 2, which is within the front, there is a thermocline at 25 m depth with a change in temperature of a degree.  The Richardson number has a value of 2860.27 at the thermocline showing very laminar flow and stable conditions at this depth.
Station 3 has what appears to be two thermoclines and is situated in what is typically considered to be stratified waters.  However, due to the high winds and low irradiance over the days preceding our survey this stratification was weakened.  This is shown in the Richardson number which is lower than Station 2.
Salinity is essentially constant with depth at all 3 stations with only minor fluctuations due to salt spiking when there is a rapid change in temperature.

Chemistry

Nutrients are heavily affected by the amount of stratification in the water column.  Stratification prevents mixing and so can lead to depleted nutrient levels in surface waters as the supply is not replenished by nutrient rich water being mixed up from below.  Both Station 1 and 2 show lower concentrations of nitrate, phosphate and silicate in the surface waters compared to the concentrations found below the thermocline. However the change at station 1 is negligible with depth which is consistent with its well mixed profile. 

Station 3 situated 3 nm offshore, displays an erratic plot for both silicate and phosphate which, without risking data aliasing, is difficult to analyse. For phosphate this profile may reflect lab processing problems. Silicate shows an association with chlorophyll and is most depleted at 8 m where concentrations are highest and most abundant at 54 m. For dissolved oxygen the concentration declines with depth, with a faster decline occurring through the chlorophyll peak between 9 and 14 m. Nitrate shows a low concentration of 0.3 µMolL^-1 which increases at the base of the thermocline/ chlorophyll maximum. At the base of the water column Nitrate reaches 3.5 µMolL^-1 at 54 metres.

Biology

The fluorescence and discreet chlorophyll values correlate well.
At all three stations the biology is closely related to the physical conditions. Station 1 shows a bulge in fluorescence at 6 metres with high values throughout due to the well mixed nature of the water column (see at the front > physics). Station 2 shows a sharp fluorescence peak at the base of the thermocline just below the maximum Ri number (2860 at 19.6 m). This is a product of nutrients being unable to mix into the surface layer due to lamination and is typical of offshore and frontal summer production patterns. Station 3 shows what could be considered an atypical offshore profile.

Black Rock (Figure 15)

Physics
Between station 1 and 4 there is a 2.5 hour difference in time. The tide during this period was a spring ebb with a southern heading. A subtle decline in salinity is seen, as is a rise in temperature. This is consistent with the ebbing tide as the conditions within the estuary are warmer and fresher (see estuary section). The Ri data shows that between station 1 and station 4 the number of points above 1, indicating that the water column is more stable, increases. This is probably due to the slower tidal flow with progression of the tidal cycle

Chemistry
The nitrate levels at station 1 where higher throughout the water column compared to station 4. The Phosphate levels earlier in the ebb flow where at low but detectable concentrations whereas when station 4 was sampled the Phosphate was below detection levels, this is likely due to dissolved phosphates acting as a limiting nutrient in freshwater. Silicate is higher at station 4 when there is a higher freshwater fraction because silicate is highest in riverine waters. The phytoplankton data (see Figure 23) also suggests that at station 1 a high number of euryhaline diatom species maybe responsible for utilising the silicate.  Dissolved oxygen changes where negligible, remaining within 10 µMol L^-1 of 270 µMol L^-1 at both depth and surface.

Biology
The biology earlier in the ebb flow was distributed unevenly, with a broad peak of around 2.66 volts at 7 m and depletion at the surface. The peak coincides with a high Ri number suggesting that the chlorophyll build up at this depth is associated with its relative stablility. Later in the ebb the chlorophyll level becomes uniform with depth.  Discrete chlorophyll increases with the ebb with higher levels at surface

Carrick Roads (Figures 16-18)

Physics
Station 5, 6 and 7 are taken half an hour apart starting half an hour after low tide.  Salinity increases as the tide floods as the influence of salt water increases.  Haloclines coincide with the thermoclines on all 3 profiles.  Though the profiles were all taken after low tide, the Richardson data shows turbulent flow above the thermocline at Station 5, more stable conditions at Station 6 and turbulent flow at Station 7.  This suggests that Station 6 shows slack water conditions and Station 5 and 7 are ebbing and flooding respectively.

Chemistry
The phosphate concentration shows 0 µMol L^-1 though this may mean that the concentrations are below detection levels or due to method errors in the lab.  Nitrate concentrations decrease with depth and also are lower at Station 7 than Station 5.  Silicate concentrations are higher in surface waters which coincides with higher fluorescence and chlorophyll concentrations.  Due to method errors in the lab, the surface dissolved oxygen concentration is missing though the concentrations decrease with depth at Station 5.

Biology
The highest fluorescence levels are found at the thermocline where nutrients are being mixed up from below.  Chlorophyll is at higher concentrations at the surface and the concentrations decrease by 0.5 µg L^-1 between Station 5 and Station 7.

Secchi disk data (Table 2) shows that for the samples taken in the estuary, Sample Numbers 5-7, there is a deeper euphotic zone that at some of the offshore sites. This may be due to the rough offshore conditions which would have impacted the mixing and as such increased the turbulence in the water. Also, when sampling the estuary, the tide was flooding meaning that large amounts of river sediment were not flowing seaward possibly reducing turbidity in our sampling area.

Table 2: Secchi Disk results showing depth of euphotic zone as 3x secchi disk depth and the attenuation coefficient (k)

                               
These two images illustrate the time series of sea surface temperature for the days surrounding the sampling of Callista, these are included to help explain the data analysed above.

                                             
                                                         01/07/08                                                 05/07/08

The satellite image on the left from the 1^st July displays a front very close to the southwest coast of Cornwall caused by heating of the surface layer causing thermal stratification. By the 5^th July, when we collected offshore data, high winds and cloud cover from the southwest had broken down the stratification and produced a more mixed water column. On the right hand plot you can clearly observe the reduced sea surface temperature between the cloud covered areas.

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In order to fully examine the patterns shown by the Biology, both the data from the Estuarine sampling on Bill Conway and the Offshore/Estuary sampling on Callista have been collaborated to provide an overall picture of the water column.
 

Phytoplankton

13 phytoplankton samples were collected from Malpas to 3 miles offshore. Up the estuary, the dominant phytoplankton found were dinoflagellates (Figure 20), indicating the water was stratified and nutrient poor conditions were present. Near the mouth of the estuary diatoms became dominant, suggesting mixing, and the total number of phytoplankton increased. Then as samples were taken in the offshore region, dinoflagellates were found to be the abundant group of phytoplankton showing the water became stratified again (Figure 23).

The water surrounding Black Rock had the highest abundance of phytoplankton as it was estimated to be 600 phytoplankton cells/ml. The lowest value (25 cells/ml) was found at the riverine end of the survey, Malpas. In the vertical profile the surface samples of every station had the most phytoplankton as they received the most light.

Looking at phytoplankton species in detail, the dinoflagellate Alexandrium, a common, toxic coastal species, is dominant at most stations that are dominated by Dinoflagellates (Figures 19 and 24). Where diatoms dominate, the dominant species is often Rhizosolenia setigera.

Zooplankton

Zooplankton samples were collected at 7 stations ranging from Smuggler’s Cottage to 3 miles offshore. The zooplankton abundance is correlated to the phytoplankton abundance as the zooplankton feed on phytoplankton. Therefore, the zooplankton abundance follows the same trend as the phytoplankton abundance. A total of 19 different classes of zooplankton were found in the entire survey range (Figures 21 and 22). As expected, Copepods were the most abundant at the majority of the stations. Only one station had another dominant zooplankton and that was the offshore station 5 where gastropod larvae were found to be the dominant zooplankton. This could be due to breeding of gastropods occurring in that area.

The copepods are expected to be abundant as they are the largest source of protein in the oceans. The most copepods (98,730 copepods/m³) were found in the surface waters (7m deep) surrounding Black Rock. This correlates with the chlorophyll concentration as that too was highest in the Black Rock waters. Thus confirming the zooplankton is most abundant at this location because its food source, in the form of phytoplankton, is most abundant. Offshore, the hydromedusae were more abundant than inshore (Figure 22), whereas the opposite trend was seen for Siphonophores and Cirrepedia larvae.

Figure 17 illustrates examples of some of the main zooplankton classes found from our survey.

Figure 17: Zooplankton Samples (A) Fish Larvae, (B) Hydromedusae, (C) Decapod Larvae

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Figure 19: Estuarine Phytoplankton Species Data (RIB and Conway)

Figure 20: Estuarine Phytoplankton Taxa Data (RIB)

Figure 21: Estuarine Zooplankton Group Data


Figure 22: Offshore Zooplankton Group Data


Figure 23: Offshore Phytoplankton


Figure 24: Offshore Phytoplankton Species Data

On 8th July 2008, Group 3 used RV Xplorer to undertake a geophysics survey of the Helford River area using Side-Scan Sonar and Grab samples.

The aims of this investigation are:

-         To survey the benthic habitat around the Helford River area to show main seabed types and geophysical features

-         To determine how these features are influencing the benthic biology via grab samples at certain locations

-         Take video footage of transect area to compare scan features to virtual images
 

Method

RV Explorer was used in conjunction with a towed side-scan sonar to produce a benthic habitat map of the lower Helford River area from five transects. After the side-scan sonar was conducted areas of interest were examined in more detail by taking grab samples using a Van Veen grab. These were further investigated for biological interest on deck via sieving using 2mm and 1mm mesh sieves. Photos were taken of each grab sample for later identification and key species noted. Video footage was initially taken across the five side-scan transects and more detailed footage was taken at each grab site.

                                 
                    Figure 25: Printed Side-Scan sonar plot        Figure 26: Map of transect line with grab sites and video transect

Transect Data for Side-Scan Sonar

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Results/Discussion

From the side-scan sonar profiles several areas of different bed types were identified. These areas are shown in Figure 27 and help to indicate the different benthic habitats, data which is then supported by the biological evidence from the grab
samples.

                                                              
                                                          Figure 27: Seabed classification map from side-scan data

Our work on board Xplorer within the Helford estuary allowed us to produce a seabed classification map, displaying clear boundaries between different sediment types. The dominant sediment types are gravel, shingle and coarse sand, with bars of fine sand and silt running through them, predominantly at the mouth.  These bars are most likely formed by the process of sediment transport by river flow. 
The dynamics of the river flow are causing deposition in this zone, which is most likely to be due to a combination of tidal influences and recent storm activity. A long term ADCP and surveying stations would be needed in order to completely determine the causes of the location and orientation of the sand bars.
There is also a large sand bar by Porthallack, which is a slightly sheltered location.  This may cause a reduction in current velocities, which would result in deposition of the suspended sediments and the formation of the sand bar.
The sidescan sonar showed a large dark area across much of the sample area.  When the video transects and grabs were employed, it was revealed that this dark area is an area of graded boundaries containing more than one sediment type.
The bedrock on the right bank is at the edge of the headland ‘The Gew’.  This is a very shallow location, which is why the sidescan sonar was able to pick it up.  On the sidescan trace, this is displayed as a very dark area, as it is a hard substance.

Video Analysis

Most of the grab samples focused to the South East of the studied area. A video transect was taken in the North West as well as each station to compare the side-scan profiles and the grabs with what the actual sediment type is like. The video recorded shingle sediment and dense seagrass at transect 1. Greater diversity of biology including large sea stars, invertebrates and juvenile fish.  Midway between transect 1 and 2 seagrass becomes less dominant on the sea floor and shingle interspersed with bedrock forms the main sediment type. The sediment type under transect 2 is mainly shingle and sand with sparsely distributed shell fragments. There is evidence of burrowing by infauna. Midway between transect 3 and 4, the seafloor becomes very dense with shell fragments, interspersed with bedrock and associated kelp. Transect 4 onwards begins to become a less dense distribution of shell fragments. The evidence from the video correlated well with those from the side-san and grab samples; isolating areas where sand, gravel and shingle were dominant as well as shell fragments. The species present did not show a great deal of correlation between sampling methods, though this is due to a lack of live samples retained in the grabs as well as seen on video.

As seen on Figure 29, a wreck was found on transect 2, the wreck of The Rock Island Bridge.
The wreck measures approx 12m in length and was 10-12m deep.
This steamship sunk in 1920, and came to rest on the bottom. After salvage attempts were abandoned, the ship was flattened by explosives to maintain the channel.
Heavy deposition rates from the river have led to silt covering the majority of the wreck. By measuring the lighter shadow down the long axis of the wreck we find this feature to be 11.1m long and 0.5m high. This correlates with photos and evidence from previous commercial and recreational dives, which show metal hull ribs protruding from the sand (Figure 28 files).

                                     
          Figure 28: Photograph of The Rock Island Bridge      Figure 29: Side-scan sonar image of The Rock Island Bridge

Grab 1
The sediment here was determined to be Bioclastic, made up of mainly broken shell fragments that led to the sediments being coarse and gravel like. This has allowed sessile organisms to dominate as was seen from the biology obtained from the grab sample (see below).

Grab 2
Video footage showed evidence of Arenicola burrows, however no live samples were found in the grab. The sediment type was fine sand with an average grain size of 187µm or 2.5 phi, there were no obvious geophysical features. This type of sediment would support burrowing infauna as opposed to sessile fauna.


Grab 3
The video footage showed dense kelp populations which would provide shelter and protection for juvenile fish and other larvae. A sand eel was found in the grab supporting this area as a protective site for fish. Many different seaweed species were seen here, including Ulva lactuca and the species Marthasterias glascialis. The sediment type showed coarse sand and shingle with broken shell fragment, the average grain size was 2000µm or -1 phi.

Grab 4
Video footage again showed dense kelp populations as for Grab 3. The sediment was also of a similar type, coarse sand and shingle with broken shell fragments, the average grain size here was 750µm or 0.5 phi.

Grab 5
As in Grab 2, Arenicola burrows were identified from the video footage. The sediment was seen to be well sorted medium sand with an average grain size of 375µm. Only infauna were found here as the sediment type indicates an area unsuited to epifaunal species.

Video Grab 2

Video Grab 3

Video Grab 4

Video Grab 5

1) 50° 05.815 N, 005° 06.459 W

Class Bivalvia

Aequipecten opercularis

Queen Scallop

       ·        Shell up to 9cm long with ~20 radiating ridges and corrugated
                grooves.

 ·        Young attached to substrate by byssus threads, adults can swim
     by ‘flapping’ shell

 ·        Occurs commonly off British coasts on coarse gravel and
    sandy sediments.
 

2) 50° 05.721 N, 005° 05.966 W

Class Echinoidea

Echinocardium cordatum

·        Thin heart shaped test with dense covering of yellow spines.

·        Lives buried in sand, 10-20 cm deep. Feeds by extending long
   tube feet of abulacral plate to the surface through a hole.

·        Common around all coasts of Britain.
 

Class Osteichthyes

Ammodytes tobianus

Sand Eel

·        Long, slender eel like fish, silvery/sandy coloured

·        Belly scales arranged in chevrons, simple lateral

·        Very common around all British coastline. Occurs from intertidal
   down to about 30m

3) 50° 05.524 N, 005° 05.814 W

Class Gastropoda

Hinia reticulate

Netted dog-whelk

      ·        Brown shell, up to 3cm tall, fat and conical with about 7 poorly
               defined whorls.

·        Predator of other small invertebrates

·        Widespread around Britain. Found in muddy sand or gravely  
    sediments from the lower shore and shallow litoral zones.
 

Class Polychaeta

Lanice conchilega

Sand mason

(tubes only, in sediment)

·        Up to 30cm long, head with numerous white or cream tentacles,
    three pairs of blood red, branched gills behind head.

·        Tube constructed from coarse sand grains, shell and stone
   fragments.

·        Found mid shore downwards around all of Britain.

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We completed biological, chemical, physical and geophysical surveys of the Fal estuary and surrounding offshore areas between 1^st July and 8^th July 2008. 
During the summer period, one would expect to see an increase in thermal stratification of the water column coupled with the occurrence of an offshore frontal system, where warmer more stratified seawater meets a cooler estuarine water body. The ADCP data from our offshore survey did not indicate the presence of a significant frontal system (Figure 11) although this has been put down to rough weather conditions. The vertical profiles further indicated the presence of stratification, with Figures 12-14 showing how the patterns changed across the theoretical position of the front. As expected we found more stratified waters where the front had previously been found, with a strong thermocline typified by chlorophyll maximum and increasing numbers of dinoflagellates (Figure 23). The estuary itself was seen to be more well mixed, as would be expected (Figures 1-6) and there was a clear transition from well mixed to partially mixed with progression landward as would be expected in an estuarine environment.

During our sampling time, issues such as bad weather conditions, especially in the offshore environment, arose.  During the seven days that we were sampling, the Fal area received more than the monthly average of precipitation for the Cornwall area.  This obviously led to increased surface runoff and river input, along with strong winds that broke down the frontal system found earlier in the week. The increased precipitation may explain the addition of phosphate within the estuarine system (Figure 10), other nutrient values within the estuary were found to be conservative (Figures 8 and 9) indicating a balance between addition and removal, most likely through biological activity (Figure 19).
The data from estuary and offshore was complemented by that of the geophysics survey which indicated the seabed types and enabled us to look more closely at the benthic ecology and their associated habitats (see Working with Geophysics).
The data collected over the survey period enabled our group to gain an understanding of the change in physical, chemical and biological processes and activity with progression from the head of the estuary to offshore. We have been able to correlate results to show how alterations to physical and chemical parameters have affected the biology of the water column from nutrient and current pattern changes to the affect of various geophysical bed forms.

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