Isabelle Brigden

Russell Cameron

Benjamin Christian

Chloë Dalglish

Pete Davis  

Jon Evans

Joel J. Muyau

Ollie Neale

Maria Smithies

Ceri Williams

Figure 1: Maps of Cornwall to show the location of Falmouth and the Fal estuary.

Together the Fal and Helford estuaries have been designated as a Special Area of Conservation (SAC) due to their support of rare, endangered or vulnerable habitats and/or species, as listed in the Annexes I and II of the Habitats directive (Fal and Helford Management Forum, 2006). These areas are labelled as ‘interest features’ and in terms of the Fal and Helford SAC include inter alia. mudflats, maerl beds (Phymatolithon calcareum), Eelgrass (Zostera marina), shore dock (Rumex rupestris), subtidal sandbanks and saltmarshes. In total the SAC covers an area of 6387.8 ha (JNCC)

However the species and habitats protected by the SAC designation are under threat from properties of the Fal estuary and catchment. The catchment area is predominantly rural and supports intensive mixed arable and dairy farming leading to eutrophication in some areas of the estuary. Although the majority of the nutrient inputs into the SAC may be due to diffuse sources from the agricultural runoff, sewage treatment works such as the Truro Newham Sewage Treatment Works are significant in the more enclosed areas of the estuary where the chronic contamination and high nutrient loading has led to toxic algal blooms. An example is the 1995-1996 bloom of the dinoflagellate Alexandrium tamarense.

Boat Information

CTD

Plankton net

Secchi disc

Sidescan sonar

This allows the seabed topography to be mapped including contrasting sediments. A transducer in the tow fish emits a wedge-shaped sound pulse which is reflected off the seafloor. The returning intensity is measured and displayed for the operator to analyse. A strong reflection indicates a slope facing the towfish whilst low backscatter values indicate softer sediments. Areas of no return, or 'shadows' are slopes facing away from the sonar which are steeper than the angle of the sonar beam. A 2-dimensional picture of the seabed is produced in black and white with saturation intensity giving an indication of roughness, slope and shadowing showing bedforms.

Van Veen Grab

This is used to obtain a sediment sample of 0.25m³ using a claw mechanism which is released when the weight is taken of the chain as it hits the seafloor. A sharp edge allows the claws to easily cut through soft sediment but is not as effective on coarse sediment. The sample collected is normal greatly disturbed and some of the sample can be lost on retrieval. Mainly used to study the benthic community at grab sites.

A CCD video camera

This was lowered over the deck at each grab site to assess whether the sidescan was accurate and if it was an area that needed to be sampled. It provided an undisturbed overview of the benthic habitat on the seabed, as well as preventing disturbance of protected seagrass beds Zostera sp.

Applied Microsystems Smart CTD

This is used to measure salinity, temperature and depth. Six Niskin bottles in a rosette system are attached, along with a transmissometer to measure suspended sediment concentration, a fluorometer as a proxy for chlorophyll concentration, a sensor for light (Li Cor sensor) and a sensor for dissolved oxygen (this sensor was un-calibrated and therefore its data were unreliable). The CTD is linked to a computer via Smart Capture, which captures the data as it is measured so graphs can be plotted onboard.  The Niskin bottles can be triggered individually which enables samples to be collected at depths determined from the Smart Capture data.

ADCP (Acoustic Doppler Current Profiler)

The ADCP was used to measure the current flow velocites (speed and direction) across the estuary, and can also show particulate matter in the water column by backscatter. It works by sending out sound pulse that get doppler shifted by moving water parcels to higher or lower frequencies depending on whether the water parcel is moving towards or away from the ADCP respectively. Three sound beams in different planes are used to build up a full profile of current magnitude and direction.

Secchi Disk

A black and white disk which is used to measure the attenuation of light in the water column. When the secchi disk can no longer be seen the depth is measured from graduations on the lowering rope. This depth 'the secchi depth' is multiplied by three to give the depth of the euphotic zone.

T/S Probe

This contains a thermistor and set of conductive plates which measure temperature and salinity respectively. It is used to measure the temperature and salinity every 5 minutes to produce a temperature and salinity profile for the estuary.

Plankton Net

A 200um mesh size plankton net with a 500ml bottle was used to collect samples of zooplankton to be quantified in the lab. A closing plankton net is used to collect samples from specified depth ranges.

Sidescan sonar towfish

Van Veen grab

CCD video camera

Geophysics

Weather

Tides

A benthic sea survey was completed on the east side of the Fal estuary on the 1st July 2009 between 0800-1300 GMT using SV Xplorer as the survey platform. Four lines, each about 1.5km in length and approximately parallel to each other were surveyed and mapped using the sidescan sonar at a frequency of 100 kHz and a speed of 4 knots. Time, position, depth and other observations, including wakes of boats that might affect the sidescan sonar readings were recorded.

Several possible grab sites were identified from the sidescan sonar trace as it was being produced. The underwater video camera was lowered first to check grab suitability, then the Van Veen grab was lowered to take benthic marine samples. Samples were transferred into a tray, from which the sediment type was identified, as well as marine benthic species using Guide to the Sea & Shore Life of British and North-West Europe. The sample was then wet sieved using a 2.5mm sieve on top of a 1mm sieve to remove the smaller sediments and identify the major substrate/habitat present.

On returning to the lab, the continuous sidescan sonar trace was separated into each transect and interpreted. Calculations were done on the features observed to determine their position and characteristics such as height. After identifying the features and their positions, the data was transferred onto a smaller track plot which was produced using Surfer software.

Figure 2: Track plot produced on Surfer, then annotated by hand.

Figure 18: Grab 3 sample.

Figure 19: Maerl, Phymatolithon calcareun - this is a calcareous algae found in the littoral and sublittoral zones.

Figure 20: Xantho incisus - this crustacean is found in the lower shore and shallow sub littoral to about 40m.

Figure 21: Galathea squamifera - this species is the most commonly found squat lobster. It is found beneath rocks on the lower shore and shallow sublittoral. This crustacean feeds on suspended detritus.

Summary

Overall the results of the geophysical survey and benthic habitat map concur with previous work done in this area. Analysis of the sidescan mosaic showed the area to be primarily homogenous maerl beds interspersed with inter alia. anthropogenic markings, the deep channel and rocky outcrops. The location of the maerl beds agree with the World Beneath the Waves benthic habitat map seen in Figure 25.

Overall the grab samples showed a general increase in maerl density with distance from the channel and finer sediments, closer to Saint Mawes Bank (grab 3). The distribution according to sediment size is opposite to what would be expected from previous studies, so other factors such as pollution may be exerting a greater control on the maerl distribution than sediment size.

Figure 25: Benthic habitat map of the Fal and Helford estuaries.

Estuarine

Aim

Introduction

Weather

Methods

Analytical Procedures

Nitrate
The nitrate concentration in the samples was analysed through flow injection analysis as described by Johnson and Petty (1983).

Phosphate and Silicon
The phosphate and silicon concentrations in the samples were analysed through spectrophotometry analysis as described by Parsons et al. (1984).

Dissolved oxygen
Dissolved oxygen percentage in the samples was analysed using the Winkler method as described by Grasshoff et al. (1999).

Chlorophyll
Chlorophyll concentration was determined by the using acetone extract method as described by Parsons et al. (1984).

Detection limits for nutrient analysis.

Any concentrations below these values has been set to zero.

Results

Figure 26 shows a range of graphs made from the YSI Multiprobe data collected on the Ocean Adveture RIB, and CTD data collected on the SV Xplorer. RIB Station 1 (see Table 1 for locations of RIB stations) was collected at Malpas, with four subsequent stations taken down the river towards the mouth of the estuary.  The CTD data was collected at three separate sites, the first at the head of the Carrick Roads, and finishing at the mouth by St Anthony Head. 

The RIB data generally showed that surface salinity increases down the river towards the estuary mouth, from roughly 17 to 32. This is due to the increase in the seawater component in the estuary. The figure also shows that salinity increases with depth due to the fresher river water flowing over denser salty water.

Temperature decreases with depth, with the greatest change recorded at Station 4, which decreases from 19.9ºC to 18.1ºC. Heating of the surface waters by the sun, produces a temperature gradient with depth due to the low heat conductivity of water. There appears to be a weak thermocline at Stations 2 and 5. Conversely, at Station 1 the temperature increases although only by 0.4ºC. However, unlike salinity the surface temperature does not vary down the river, with all stations maintaining a temperature of roughly 19ºC.

At Stations 1, 4 & 5 dissolved oxygen decreased with depth, for example Station 4 it decreases from 94.1% to 86.9%. It is thought that this is due to high levels of primary production at the surface with decreased levels at depth due to light attenuation. Station 2 has an increasing oxygen level with depth from 78.2% to 83.6% and Station 3 shows some variation in the top 2m, but the levels then remain constant below this depth at around 83.6%. The figure shows dissolved oxygen to also increase down the river, with Station 1 recording 73.8% and Station 5 97.7% possibly as a result of eutrophication in the upper estuary increase biochemical oxygen demand.

The final parameter recorded at each RIB station was pH. All stations display very little change and range from 7.8 to 8.3, therefore suggesting that there is no significant difference in pH with depth.

The first CTD Station (Table 2) was at the head of Carrick Roads. Here salinity increased with depth from 28.6 to 33.9. However the next two stations show very little change with depth as can be seen in Figure 27.

Temperature decreased with depth at all stations, similar to the RIB Stations. Each station appeared to have two thermoclines, one near the surface (0 – 10 m) and one at depth (10 – 20m) representing a stratified water column.

The fluorometer readings (which are a proxy for chlorophyll concentrations) increased with depth to between 4 and 8 metres, before decreasing. This increase is due to increased primary production. The decrease is caused by a decreased irradiance causing a drop in primary production.

Transmissometer readings display a slight decrease at the surface corresponding to the increasing fluorometer readings at Station 1, yet in the other stations there seems to be no correlation between turbidity and the fluorometer readings.

Figure 26: graphs from the RIB data

Table 1: RIB station locations

Figure 27: graphs from the CTD data
 

CTD Stations
Station	Latitude	Longitude
1	50º 12.148' N	05º 02.468' W
2	50º 10.321' N	05º 02.099' W
3	50º 08.588' N	05º 01.488' W

Table 2: CTD station locations

Figure 30: ADCP Track 1

Figure 31: ADCP Track 2

Figure 32: ADCP Track 3

The Fal estuary can be classified as partially/well mixed depending on the spring neap cycle. This conclusion is supported by the position of the temperature and salinity contours in figures 28 and 29 which can be seen to slant diagonally downwards towards the bed rather than vertically downwards which would be typical of a well mixed estuary. A partially mixed estuary is formed when a river discharges into an estuary with a moderate tidal regime. The result is that over the tidal cycle the entire water mass is moved up and down the estuary as a whole. The null point associated with a partially mixed estuary is likely to cause a turbidity maximum to exist somewhere within the estuary

Three ADCP tracks were taken to enable analysis of the estuaries circulation. The tracks were taken between:

Track 1, taken between Pill Point and Turnaware Point clearly shows asymmetrical circulation (Figure 30). The velocity magnitude contour shows the greatest flow speed on the east side of the channel (line A) with an average current around 0.4ms^-1. By examining the velocity direction contour we can see that this flow is in a northwards direction. Either side of this point (line B) the current is both slower and in the opposite direction creating significant shear within the water column. The average velocity stick ship track shows a distinct average current direction in the centre of the transect with a much more random direction to either side which will result in significant vorticity.

Track 2 (figure 31) is similar to Track 1 although the velocity magnitude is much more homogenous throughout the entire transect. There is still significant shear within the water column (lines C and D) as currents with opposing directions flow past each other within the water column. The average stick ship track shows the only significant variability in the average current direction throughout the transect except on the west side of the estuary in the deep channel.

The final track (Figure 32) was taken at the mouth of the estuary between Pendennis Point and St. Anthony Head. Current magnitude is highest again in the main channel and reduces westward towards Pendennis Point (Line E). Current direction within the channel is northwards into the estuary whilst the current along the surface is in the opposite direction out of the estuary (Line F). The two layer flow represents the ebbing tide flowing out of the estuary over a most likely denser incoming flow.    

Figure 28: contour plot of temperature

Figure 29: contour plot of salinity.

(Table 3: Table showing turbidity found in the estuary at three different sites)

Euphotic Depth

Table 3 shows the lowest transmissometer average was found at Station 1, this is because it is closest to the riverine input which brings in sediment particulate matter causing a high light attenuation.  The Secchi disc data agrees with the transmissometer data.

The furthest station from the riverine input, Station 3, shows the highest transmissometer average as well as the deepest Secchi disk depth. This is because in this area most particulate matter has been mixed deeper, been scavenged or settled out of the water column so light has a lower attenuation. This allows light to penetrate deeper, increasing the euphotic zone so phytoplankton can take advantage of nutrients at greater depths where nutrients are not as depleted.

Figure 33: Time series on the pontoon of temperature and salinity.

Figure 34: Time series on the pontoon of pH and oxygen concentration.

Figure 33 shows an overall increase in temperature throughout the time series. This is due to the heating effect of solar radiation. This effect is reduced at depth due to the attenuation of the radiation by the water column, the low heat conductivity of water, and a lack of mixing.

Variation in the top 1.5 metres is due to atmospheric effects such as mixing by the wind and cloud cover. For example, the large dip from the surface to 2.5 metres at 10:30 GMT is possibly due to a prolonged period of cloud cover. The spike at 2.5 and 3.5 metres at 11:30 GMT is probably due to increased mixing of warmer surface water to depth, possibly caused by increased wind mixing. This increase is mirrored by a decrease in surface temperatures.

There is an initial decrease in salinity (Figure 33) due to the effect of the ebbing tide, increasing the influence of the riverine component of the estuarine water body. However, there is an overall increase in salinity due to the effect of the flooding tide, after low water at 09:21 GMT, which carries saline water further up the estuary. This effect is delayed due to the distance of the sampling point up the estuary.

The salinity at depth is higher due the high density salt water flowing under the overlying freshwater. This variation decreases over time due to the flooding tide causing increased mixing which results from turbulence caused by shear between the headward flow of saline water and the mouthward flow of river water.

There is an overall increase in dissolved oxygen saturation (DOS) over time (Figure 34) from 0.5 metres to 3.5 metres due to the increase in the photosynthetic activity of phytoplankton, as confirmed by the chlorophyll time series in Figure 35. DOS at the surface is very variable due to the mixing effect of the wind and diffusion over the surface layer.

DOS is lower at depth as the inputs from the atmosphere are not fully mixed to these depths and also possibly due to reduced photosynthetic activity resulting from a shallow euphotic depth caused by re-suspension of sediments resulting in high turbidity. However, it must be noted that this is hypothetical as the euphotic depth was not measured.

pH (Figure) 34 increases over time due to carbon dioxide usage by phytoplankton which results in a decrease in carbonic acid as determined by the equilibrium characteristics of the reversible reaction:

H₂CO₃ ↔ CO₂ + H₂O

pH is also lower at depth due to hypothesized reduced photosynthetic activity.

Figure 35 shows an increase in surface nitrate concentration between 9:00 and 10:00 GMT before decreasing between 10:00 and 12:00 GMT.  At 4m depth the concentration decreases constantly from 9:00 -12:00 GMT. Between 9:00GMT and 12:00GMT there would be an increase in irradiance, causing an increase in phytoplankton production and therefore a decrease in nitrates from 9:00 – 12:00GMT.

At both 0m and 4m depth dissolved silicon stays constant between 9:00-10:00GMT and then decreases in concentration though to 12:00GMT.  This again could be due to the increase in irradiance though the morning resulting in increased removal from primary production.

Phosphate results differ from nitrate and dissolved silicon in that they increase at 0 metres from 9:00-11:00 GMT and then decreases until 12:00 GMT. At 4m depth phosphate increases between 9:00-10:00GMT and then decreases until 12:00GMT.  This difference is possible due to anthropogenic inputs such as the sewage outflow demonstrated in the mixing diagram (Figure 38).

At both 0m and 4m depth chlorophyll concentrations increase from 9:00-12:00GMT indicating the increase in phytoplankton production.  This can also be seen from the dissolved oxygen data in figure 34 showing that photosynthesis increases from 9:00-12:00GMT so more chlorophyll is generated.

Figure 35: Time series of nutrients from the pontoon.

Figure 36: Nitrate estuarine mixing diagram.

Figure 37: Dissolved silicon estuarine mixing diagram.

Figure 38: Phosphate estuarine mixing diagram

The nitrate mixing diagram (Figure 36) shows a higher concentration of nitrate at the riverine end member as expected due to nitrates being added to the estuary mainly by riverine and anthropogenic inputs such as agricultural effluents.  The samples plot below the theoretical dilution line (TDL) indicating that nitrate is behaving non conservatively and is being lost from the system.  This is due to biological removal. So even though there is a large anthropogenic input of nitrate the biological activity at this time of year is high enough to cause a net removal from the system.  There are relatively large gaps in the data towards the lower salinity areas which may be hiding different nutrient behaviour.

The dissolved silicon mixing diagram (Figure 37) shows that silicon is at a higher concentration in river water than sea water and that it behaves conservatively within the estuary as all the samples plot on or close to the TDL. This means the concentration of dissolved silicon depends entirely on the degree of mixing between fresh and salt water. However dissolved silicon is a biologically essential nutrient and may only be behaving pseudo-conservatively if the in situ rate of removal is slower than the rate of transportation through the estuary or if any addition or removal is exactly matched by corresponding removal or addition. However, it must be noted that the riverine end member replicas are not accurate and therefore the behaviour may be more conservative or become non conservative respectively depending on whether the lower or higher concentration is more accurate. Comparisons with data from other groups proved inconclusive.

Phosphate (Figure 38) shows a different pattern than seen for nitrate and dissolved silicon.  It still shows a decrease in concentration toward the seaward end member but the samples plot above the line showing it is behaving non-conservatively as it is being added to the system.  This is because there is a large phosphate input from the sewage treatment plants which are located at Truro and the head of the Tresillian river.

The riverine input value for all of these nutrients may be particularly high do to the effect of the previous days precipitation.

A plot of nitrate concentration against phosphate concentration was constructed using the corresponding concentrations from each of the 21 water samples collected.  From the graph it can be seen that once nitrate has been fully depleted there is still some phosphate available. This shows that nitrate is depleted by phytoplankton before phosphate, suggesting that nitrate is the limiting nutrient within the estuary. This may be a result of the addition of phosphate to the estuary from sewage input as seen in the mixing diagram (Figure 39).

Figure 39: phosphate vs. nitrate graph

CTD Depth Profiles (Figure 40)

Dissolved silicate concentrations decrease throughout the estuary from station 1-3. Dissolved silicate concentrations at Station 1 are detectable throughout the entire water column with the highest concentration (2.1 μM) at 1.5 metres, decreasing to 1.8 μM at 13 metres. At Stations 2 and 3, dissolved silicon is below detection limit or at a higher concentration at depth respectively. At Station 2 concentrations are only detectable at 22 metres where phytoplankton population and irradiance are low.

Phosphate concentrations are highest at the shallow depths at all stations and are lowest at the mid sample depths. This is due to sampling taking place just after the spring bloom, so phytoplankton have depleted the nutrient concentrations. Phosphate concentrations are highest at Station 1, 0.4 μM due to high inputs of phosphate from sewage outfalls at Truro.

Nitrate concentration at the surface decreased from Station 1 at the top of Carrick Roads to the mouth of the estuary at Station 3, near Black Rock, where the concentration was 0.0μM compared to 3.5μM at Station 1.

Station 1 shows an overall decrease in nitrate from the surface, 3.5 μM, to 1.0 μM at 12 metres which is mimicked by the chlorophyll data. Stations 2 and 3, have a lower amount of nitrate at the surface, with increases with depth until 6-8 metres, before decreasing again.

Chlorophyll trends follow the nutrient trends. It decreases from Station 1 down to Station 3, which follows the decrease in nutrients. It also shows an increase towards the middle of the water column and a decrease at depth due which follows nutrient concentrations. Station 1 is an exception with highest values at the surface. The fluorometer data from the CTD profile only shows a strong relationship with the chlorophyll values collect from water samples at Station 1.

Figure 40: nutrient depth profiles

	Secchi Depth (metres)	Actual Depth (metres)
Station 1	2	6
Station 2	3.25	9.75
Station 3	6.25	18.75

Secchi depth vs. actual depth.

Two sets of phytoplankton samples were taken, one as a time series from the King Harry Pontoon (Figure 42) and another set taken on the RIB as it moved down the estuary (Figure 41).

During the time series, there is an overall decrease in phytoplankton which is the opposite to all other indicators of phytoplankton activity including chlorophyll which increases and nutrient concentrations which decreases (Figure 35). This anomaly maybe due to different people, with varying experience, judging differently between plankton and debris and using different characteristics to identify the phytoplankton, skewing the results. Figure 41 from the RIB samples had an improved correlation for phytoplankton indicating that the amount of phytoplankton present increases down the estuary.

Figure 41 and 42 show that diatoms are dominating over dinoflagellates throughout the head of Fal Estuary and Truro River. 90% of the diatoms are made up of diatom chains, such as Chaetoceros sp, with a very small amount of centric diatoms, Coscinodiscos sp., and an even smaller amount of pennate diatoms, Rhizosolenia sp, in all the samples. Dinoflagellates were absent at lower salinity samples taken higher up the river.

The sample taken at Malpas, on the Truro River has the highest concentration of chlorophyll, phosphate (P), nitrate (N) and silicate (Si) which could be due to the close proximity to the sewage outfalls at Truro and on the Tresillian River. It has the lowest salinity of any sample taken and is dominated by diatoms. Diatoms thrive in  water with high nutrient concentrations possibly explaining why they can outcompete the dinoflagellates.

At RIB Station 6 there is an increase in the amount of phytoplankton overall, especially in the dinoflagellate Alexandrium sp., which can cause toxic algal blooms. This increase may be due to a counting error or the presence of a different nutrient environment.

Figure 41: To show phytoplankton from the RIB river samples

Figure 42: To show phytoplankton from the pontoon river samples

Zooplankton trawl data for both sites

Zooplankton samples were collected from two sites in the estuary using a 200µm plankton net with a 54cm diameter opening.  Site F was located just off Turnaware Point, at the head of Carrick Roads and site D was just off St Anthony Head, at the mouth of Carrick Roads. 

The sites show differences in zooplankton abundance mainly due to differences in nutrients.  A total of 43 organisms were found in the first sample and 18 organisms found in the second sample at site F (Figure 43). This is less than the total number of organisms found in each sample at site D (Figure 44), which was 114 and 111.  Site F shows a higher concentration of nutrients (Table 4) than site D because it is closer to the river input.

The most dominant zooplankton found at site F were hydromedusae, cirripedia larvae and cladocera.  These three are all filter feeders and therefore need a high abundance of food resources in the water column to thrive.  At this site there are high nutrient concentrations and lots of detritus in the water column inflowing from the river providing a perfect environment for these species.

Site D shows the most dominant zooplankton being echinoderm larvae and hydromedusae.  Here cladocera and cirripedia larvae abundance is low, this could be due to the drop in nutrients in the water column. 

Hydromedusae populations are high at both sites, this is because in their polyp form they are filter feeders so thrive in high nutrient environments but in their medusae form they are predators so are able to survive in relatively lower nutrient environments.

Table 4: Nutrient concentrations at both zooplankton trawl sites.

Figure 43: pie chart to show zooplankton distributions at site F.	Figure 44: pie chart to show zooplankton distributions at site D.

Summary

Offshore

Introduction

Weather

Figure 45: ADCP data from Black Rock station.

This transect (Figure 45) was taken at Black Rock. Starting at 50^o 08.777 N, 05^o 01.484 W, at the centre of the mouth of the estuary (approximate depth of 31 metres), the vessel moved SSE towards the shore (approximate depth of 17 metres).

The current is strongest near the surface with velocities up to 0.3ms^-1. Between 8 and 10m the current slows to approximately 0.1ms^-1 which will result in significant shear and is then constant with depth. In the deeper water below 20m the current direction is variable between 180^o and 300^o resulting in some shear. Otherwise the current direction is fairly homogeneous.

There is significant backscatter at 10m which indicates a large amount of zooplankton is present.

This station follows the nutrient depth profiles seen in the estuary and shows a similar pattern to Plot 3 from the estuarine nutrient depth profiles. All nutrients peak below the surface at 12m (Figure 46) just above a weak thermocline. Chlorophyll corresponds to this peak, and nitrate decreases constantly down the water column. This graph shows the water column is well mixed with only small change in temperature with depth.

Temperature 

The temperature (Figure 48) remains relatively constant at 13.9°C down to 10 metres. There is then a thermocline where the temperature drops down to 13.2°C at 24 metres. The temperature then remains constant again down to the bottom at 31 metres. The thermocline is weaker than that recorded offshore as the waters are less stratified.

Salinity

The salinity (Figure 48) also shows two distinct, homogenous layers. The upper layer has a salinity of 31.1 and is present down to 11 metres. There is then a halocline where the salinity changes to 35.2 in the lower layer, which starts at 19 metres. This is due to the presence of the thermocline, which prevents inputs of fresh water (for example from rain) from mixing into the lower layer.

Fluorometer

The fluorescence (Figure 47) at the surface is 1.4 volts it then increases to a maximum of 1.8 volts at 15 metres. This is due to the depth of the euphotic zone being 17 metres (Table 6) and so below this there is insufficient light for photosynthesis. However, the fluorescence doesn’t return to the surface level of 1.4 until 26 metres. This is due to the phytoplankton being mixed down below the euphotic zone but not beyond the thermocline.

Transmission

The percentage transmission (Figure 47) is relatively low at the surface  (84.8%) before increasing to 85.8% at 12 metres, suggesting that the surface has quite a high turbidity as this increase cannot be explained by the fluorescence levels. The transmission then dips down to 85.1% at 15 metres, matching the peak in the fluorescence data. It then increases again to a maximum of 86.8% at 26 metres, again matching the fluorescence data. Below 26 metres, the transmission stays relatively constant.

Figure 46: nutrient depth profile at Black Rock

Figure 47: Transmissometer and fluorometer depth profile

Figure 48: temperature and salinity profile

Figure 49: phytoplantkton profile for Black Rock.

Black rock phytoplankton profile (Figure 49) follows on from the estuary data and shows a increase in the amount of dinoflagellates present. There is the highest amount of plankton at 11m which correlates with the thermocline and chlorophyll maxima shown in figures 47 and 48. There are more diatoms than dinoflagellates as silicate concentrations are still high and they can outcompete dinoflagellates. The main species of dinoflagellates present are Karinia Mitimotoi which are characteristic of themoclines, compared to the Alexandium sp. found in the estuary. After the 11m the nutrients are rapidly depleted.

Time Series

Figure 50: time series of temperature

Temperature: Figure 50

All depth profiles show a well defined thermocline at around 17 metres. For example, at 10:30 GMT the temperature drops from 15.8°C at 13 metres to 11.5°C at 19 metres. Later in the day the thermocline becomes less abrupt and migrates downward through the water column. For example, at 15:00 GMT the water temperature dropped from 16.4°C at 12 metres to 11.5°C at 27 metres.

Salinity: Figure 51

Salinity is relatively homogenous throughout the water column, although it is separated into a two layer system by a slight halocline around the same depth as the thermocline (18 metres). For example, at 09:37 GMT it is 35.1 at 15 metres and 35.2 at 22 metres. This is too significant a change to be caused just by recent rainfall and is due to a build up of the freshwater input over a prolonged period of time caused by the prevention of mixing of the fresher water into the lower layer by the thermocline.

The salinities of the upper and lower layer stay constant throughout the day.

Salinity spikes occur around the depth of the thermocline. This is due to the difference in the refresh rates of the temperature and conductivity instruments which causes salinity to be miscalculated.

Figure 51: time series of salinity

Figure 52: time series of fluorescence

Fluorescence: Figure 52

The fluorescence shows a significant increase from around 1.4 volts at the surface to a maximum of between 2.2 and 4.4 volts. This maximum is present between 13 metres at 13:00 GMT and 16 metres at 15:00 GMT throughout the day. This represents a high chlorophyll concentration which in turn demonstrates a large phytoplankton population just above the level of the thermocline. This is because the nutrient levels in the surface waters are depleted after the spring bloom but there are nutrients being mixed up from below the thermocline, allowing phytoplankton growth. The reason this growth isn’t deeper is because the thermocline prevents mixing of phytoplankton into the lower layer, even though for most of the day the euphotic zone is deeper and so photosynthesis could occur below this depth (as shown in Table 6).

Up until 14:30 GMT fluorescence drops back to the surface value of 1.4 below 21 metres. At 14:30 and 15:00 GMT this deepens to 26 metres suggesting chlorophyll is being mixed downwards.

Transmission: Figure 53

The percentage transmission is relatively low in the upper layer of the water column and drops to a minimum between 11 and 15 metres. For example, at 14:00 GMT the percentage transmission is only 81.1% at 12 metres. This is due to phytoplankton in the water column causing greater light attenuation, reaching a maximum at the chlorophyll maximum around 15 metres, as shown by the fluorometer. The transmission then increases again until around 21 metres throughout most of the day, for example at 10:30 GMT, although this also deepens to 26 metres at 14:30 and 15:00 GMT, corresponding with the deepening chlorophyll maximum.

Figure 53: time series of tranmission

Figure 54: time series of surface temperature

Surface Layer Temperature: Figure 54

The surface layer temperature increases steadily throughout the day from 15.9°C at 9:30 GMT to 16.5°C at 15:00 GMT. There is a slight dip at 12:30 GMT where the temperature drops down to 15.9°C from 16.1°C at 12:00 GMT. This is either due to the instrument misreading slightly or atmospheric effects, such as increased wind speed or prolonged cloud cover.

Figure 55: nutrient depth profiles

Figure 56: nutrient depth profiles, part 2.

Figure 57: nutrient time series.

The plots show 5 different CTD casts (Figures 55 and 56), the first at 9.13 is at Black rock outside the Fal Estuary and is a continuation of the estuarine plots. The rest are a time series at a offshore point 50’03.424N 04’49.903W. The graph for 11.37 has only two points as bottles failed to close at 67m. Figure 57 shows a time series at offshore station for each depth.

Time Series 9:37 – 14:30: Figures 55 and 56

Nitrate concentrations are below detection at nearly every depth sampled at every station; this is because it is constantly being utilized by phytoplankton and is limiting growth. There are small peaks at the surface at CTD casts 12.25 and 14.30, but shows no pattern.

Phosphate shows a constant pattern of increasing from the surface to the thermocline and then decreasing down to depth. This is because nutrients are concentrated at the thermocline by being mixed up from deeper water.

Silicate profiles generally increase at the thermocline from 0.0 µM to above 2µM throughout the time series but show no definite pattern to greater depth. The last time series is the only exception with high concentrations at the surface and lower concentrations at the thermocline. This could be due to different types of plankton migrating to the thermocline at different times of the day which use silicate e.g. diatoms.

The chlorophyll data correlates with the fluorometer accurately and in one case follows it exactly. The chlorophyll maximum always peaks when there is a high concentration of at least one or more main nutrients. The chlorophyll maxima and thermocline always appear above 20m with the chlorophyll maxima getting higher throughout the day and the thermocline migrating slightly deeper as it is thermally heated.

The nutrient concentrations are far lower offshore than in the estuary this is due to the low input of nutrients to the water.

Depth Time Series (09:37- 14.30)

Figure 57 shows a time series for each nutrient at each depth to show clearly how each nutrient changes over time at each depth. Nitrate concentration is below detection at most stations as mentioned for figure 55 and 56 above. Nutrient concentrations at each depth don’t show any general patterns or trends, some decrease throughout the day while some increase. Between 13.14 and 14.30, at every depth, there is a decrease in chlorophyll which maybe caused by a vertical migration of plankton. Phosphate and silicate are usually depleted by this point as well, except at 5m where there is large increase in phosphate. There are also peaks in nitrate at 5m and 20m depth.

An ADCP profile (Figure 58) was taken throughout the duration of the time series. The velocity magnitude and direction is dominated by the channel tidal regime throughout the profile. At the start of the profile the current (approx. 0.3ms^-1) is flowing out of the channel with the ebbing tide on a bearing of 245^o. As low tide is approached (12:20 GMT at Falmouth) the current speeds up and begin to swing round in a clockwise direction, passing though north until eventually at the end of the profile the current direction is now into the channel with the flooding tide on an angle of 90^o. The highest current velocity is found around the time of low tide (approx 0.45ms^-1) before then decreasing with the flooding tide to its lowest value in the profile around 0.1 ms^-1.

There is a constant high backscatter at 30m at the start of the day which slowly rises throughout the day to 15m indication a possible upward advection of zooplankton. The high backscatter between 10 and 20 meters indicates high abundances of zooplankton and subsequently it was this depth range that was sampled with the closing net. The very backscatter at the very surface is likely to be noise caused by for example the anchor chain or surface bubbles.

Figure 58: ADCP data.

Table 5: Richardson's number and Brunt-Vaisala frequency

The Richardson Number and Brunt-Vaisala Frequency (Table 5) were used to quantify the extent of density stratification and mixing for each CTD profile collected in the time series. A Richardson Number below 0.25 indicates the presence of shear instabilities such as Kelvin – Helmoltz billows whilst a value greater than 1 indicates laminar flow (no mixing). Values for each parameter were calculated over one metre intervals in the upper mixed layer (5 to 6 m), over the thermocline (varying depths) and in the lower mixed layer below the thermocline (45 to 46 m). All the values except one follow the expected pattern with low Brunt-Vaisala Frequencies and Richardson Numbers less than 0.25 above and below the thermocline, with high values within the thermocline itself. The low values found above and below the thermocline are indicative of the weak density stratification and high shear found in these areas. This results in turbulent mixing and hence the straight lines of temperature and salinity as seen on the CTD depth profiles (Figures 50 and 51). 

Within the thermocline the values are larger and all Richardson Numbers are above 1. This is caused by the increase in density stratification stabilising the water column overcoming the effects of shear and therefore reducing the extent of turbulent mixing. The high Brunt-Vaisala Frequency is a result of the strong density stratification causing a high natural oscillation rate of internal waves.

At station 2e the lower mixed layer Richardson Number is particularly high with a value of 6.69. This is most likely not a representative value for the entire region and only arose because the difference in velocity magnitude between 45 and 46 metres was 0.001 ms^-1 resulting in very little shear.

Figure 59 shows a phytoplankton profile at the first CTD cast (Station 2, 9:13 GMT) and the last CTD cast (Station 9, 14:30 GMT). There is a greater amount of growth in the surface layer at station two with a large percentage of diatoms over dinoflagellates as diatoms prefer the higher irradiance at the surface. At depth the dinoflagellates show dominance as they prefer the lower irradiance at the thermocline. By 67 metres there are low amounts of phytoplankton due to no light and low levels of nutrients except for an accumulation of phosphate.

Station 9 shows a greater dominance of dinoflagellates due to the low levels of silicate and nitrate limiting diatom growth, there is also a far greater concentration of plankton per ml rising from 340/ml at 20 metres (Station 2) to 840/ml at 20 metres (Station 9) and 2040/ml at 13 metres. Also, there was an increase in nitrate concentration compared to the depleted source at Station 2.

The main difference between the offshore and estuary samples are the ratios between diatoms and dinoflagellates and the species present. In the estuary diatoms were 90% diatom chains, where as offshore they are mainly centric e.g. Coscodiscus. The main dinoflagellates present were Alexandrium sp in the estuary where as the main species present offshore had a variety including Karina mikimotoi, Dinophysis and Polykrikos. Diatoms dominate in the estuary due to the high turbulence, nutrient levels and cool water. Dinoflagellates thrive offshore with low nutrients, and less turbulence as they can outcompete the diatoms once silicate is depleted.

Figure 59: phytoplankton time series.

 Zooplankton trawls data

Table 7 Nutrient data from all sites where zooplankton trawls took place

Conclusion

References