Rebecca Ansorge

Elena Bollati

Ben Coppin

Sara Eisler

Luke Kelly-Granger

Falmouth 2011

Group 10

June 26-July 9, 2011

Alex Griffths

 Lexi Mackee

 William Passfield

 Rosalyn Putland

 Jordan Thomas

 Phillip Turner

Introduction

An investigation into the chemical, biological and physical properties of the Fal Estuary was carried out in June and July 2011 by Marine Biology and Oceanography students from University of Southampton.

THE FAL ESTUARY

The Fal Estuary is situated on the South coast of Cornwall in the Southwest of the United Kingdom (Figure 1), it is a ria system (drowned river valley), which was developed in response to Holocene sea-level rise. The estuary extends 18 km inland from its mouth to the northern tidal limit at Tresillian and has a total shoreline length of 127 km. It can be divided into two regions: the inner tidal tributaries and the outer tidal basin, termed Carrick Roads. (Pirrie et al. 2003). Carrick Roads is a deep meandering channel reaching 34m depth at its maximum and holds 80% of the main water body of the estuary, making it an important natural harbour (Pirrie et al, Cambourne School of Mines.).

The estuary is macrotidal with a maximum spring tide of 5·3 m at Falmouth, but mesotidal at Truro with a spring tide of 3·5 m. The estuary covers 24.82 km² with 17.36 km² covered by the subtidal area, 6.53 km²  of intertidal mudflats and 0.93 km² of saltmarsh (Stapleton & Pethick, 1996).

SPECIAL AREA OF CONSERVATION (SAC)

The Fal and Helford have been selected as a SAC due to the variety of habitats causing a diverse range of marine and coastal communities, such as; rocky shores, kelp forests, mearl (at St. Mawes bank) and eelgrass beds, saltmarshes and intertidal mudflats.

ACTIVITIES

Within the Fal Estuary many anthropogenic activities occur that may result in significant changes to the results, especially in terms of the nutrient levels, therefore the Fal, despite its SAC status, has been labelled as one of the most polluted estuaries in the area.

Mining has taken place in the area since the Bronze age (2500-600BC), however has become much more extensive since the industrial revolution which used methods that did not necessarily protect the estuary from contamination. (Pirrie et al, Cambourne School of Mines) A notable source of pollution was after the closure of the Wheal Jane mine in January 1992, when the withdrawal of dewatering pumps lead to the extensive flooding of the estuary with 50 million tonnes of acid water carrying metal ions. (Bowen et al, 1998)

Another cause of pollution is the leeching of tributyl tin (TBT) from ships hulls that enter Falmouth Harbour continuously throughout the year, causing imposex in Nucella spp. along with suggested effects in other species.

Equipment

A rosette is a metal frame which other apparatus can be attached to. It is attached to the crane on the back of a scientific research vessel with a connecting wire and winch and is also linked to the onboard computer via a conducting data cable. Rosettes are very versatile as specific parameters can be sampled with a range of different equipment, and large volumes of water can be collected by closing multiple bottles individually with signals from the ship. CTD stands for Conductivity, Temperature, and Depth. It is lowered via a cable through the water column and measures these three parameters, enabling the production of depth profiles. It can be attached to a rosette.

An Acoustic Doppler Current Profiler is used to measure the current speed around the boat. A transducer fires a sound wave into the water column. This is then returned as an echo, after being scattered by the water column. It will have been Doppler shifted by velocity of the water column relative to the ADCP; experiencing a frequency change. This change allows the current velocity to be calculated. Multiple transducers are used, each firing a sound wave in a different, known direction. The backscatter for each of these echoes can be used to locate zooplankton. The velocity of the boat holding the ADCP is also taken into account, to correct for the true Doppler shift.

A probe used to measure temperature and salinity. Temperature is measured using a thermometer, while salinity is measured in the form of conductivity of the water column, which is a function of salinity and temperature. It is easy to deploy manually on the end of a rope or hydroline, and is a simple but effective piece of equipment.






               

The Van Veen grab is a mechanical claw which is lowered from a mechanical crane on board. Once the grab reaches and penetrates the sediment it is closed, obtaining a sample of 0.5m³ capacity. It is an excellent tool to obtain sediment and organisms from the sea floor. However, the grab can only be used as a “blind sampling” technique unless it is used in conjunction with other analysis techniques. In addition, coarse sediment and rocks have a tendency to get stuck in the jaws of the grab, resulting in a sediment loss upon recovery, restricting its use to fine/coarse sediments.
  

The device emits two acoustic pulses simultaneously, with a swath of 75m that is reflected back off the seabed. The degree of reflection depends on the density of the material and the slope of the sea floor. Darker lines represent areas of higher reflectance e.g. due to more dense or rough substrate, enabling the identification of elevated or lowered bed forms. The system used in the geophysics practical was of a low frequency (110kHz), limiting the detail that can be seen on the sidescan print-out.

                   

Sieves were used to separate sediment samples collected during the geophysics survey. Three sieves were stacked in decreasing mesh size from >10mm, 2-10mm, and 1-2mm, enabling the sample to be analysed more easily and a picture of the sediment general composition to be obtained.
 

YSI 6600 Multiprobe

The instrument provides data on a range of variables at one second intervals. Variables include water temperature, salinity, depth, percent saturation of dissolved oxygen, chlorophyll (via fluorometer) and turbidity (via nephelometer). The sensor is connected to a hand-held computer and the data is recorded from the screen as it is lowered through the water column.

Transmissometer

Contains a light source and a receiver to measure transmission, according to how much scattering occurs due to how turbid the water column is. Light is shone towards the receiver, but will be scattered and absorbed. The more turbid the water column is, the less light that will reach the receiver. 100% and 0% transmission also need to be measured for calibration, by using the instrument out of the water and blocking the light off respectively.

Fluorometer

Used to measure fluorescence of the water column. This is due to the amount of chlorophyll present, which is an indicator of phytoplankton levels. It emits blue energy, which is absorbed by the chlorophyll in the phytoplankton. The cells then release red light, the levels of which are detected by the fluorometer, indicating the chlorophyll levels.

Niskin Bottles

These are simple and reliable plastic tubes which open and close at the top and bottom to let water in. Once primed, they are attached to a steel cable (a hydroline) and lowered into the water. Both ends are held open with an internal rubber band held under stress to exchange water with the surroundings. When the bottle is at the desired depth, a ‘messenger’ is sent down the hydroline, closing the bottle. Alternately, an electrical impulse sent from the monitor on board the vessel (when used with a CTD) can be used. The end caps are sealed with an O-ring, minimizing contamination of the water samples to enable accurate measurements of variables. Multiple samples are collected from depth upwards, to prevent bottles collected at the surface from imploding at depths with higher pressures.

Winch

These are used with a hydraulic crane to lower heavy equipment into the water, such as rosettes that hold a large amount of apparatus. They are situated on the backs of boats, and are operated electronically.






               

Spectrophotometer

A spectrophotometer is a device that can be used to calculate a range of chemical concentrations in a solution by passing a light at a set wavelength through a cuvette containing a sometimes coloured solution. A photometer, in the device, measures how much light is absorbed by the solution. Unknown samples need to be compared to predetermined standards to calculate the concentration.

Video Sled

A waterproof camera, mounted on a frame to stabilise the footage, is connected to a monitor on board the vessel via an electrical cable, with a graduated rope attached to allow an estimation of depth. The video produced gives an illustration of the benthos along the tract of the boat from which species may be identified and an idea of the general community and sediment structure obtained. However, this system works to the assumption that the picture following the boat track is representative of the entire sample area and should therefore be used in conjunction with other techniques.
 

Secchi Disk

Used to measure water transparency. It is a circular disk with a black and white pattern, which is lowered through the water using a rope. The depth at which the pattern disappears is the Secchi depth, Zs, giving an idea of how transparent the water column is due to plankton. It can be assumed that the depth of the euphotic zone, Ze is 3x the Secchi depth. Secchi disks are cheap and very easy to use, though there can be errors due to human interpretation of where the disk disappears, and in exact measurement of the rope length.

             

Plankton Net

Plankton Nets are used to collect samples of both zoo- and phytoplankton within the size range determined by the mesh size of the net (most commonly >200µm). The cone-shaped net is dragged along behind the vessel in a trawl, capturing plankton that are in the way of the net. The sample is collected into a bottle attached to the bottom of the net. When recovered onboard the boat, the nets are washed down with a hose, to ensure that the plankton is in the bottle, rather than being caught around the net.
                        

Vessels

A twin hulled research vessel owned by the University of Southampton and first dispatched in August 2005. With a passenger capacity of 30, large working deck and onboard wet and dry lab facilities to analyse samples the Callista is an ideal vessel for research off the South Coast.

Overall length: 19.75m,
Max Breadth: 7.40 metres
Draught: 1.8 metres
Speed: 15 knots
A Frame: 4 tonnes

Used in the Offshore Survey


 

Figure 4: Image of the R.V Callista

A Lochin 38 research vessel owned by the University of Southampton and built by Lochin Marine Ltd in 1991; which is licensed by MCA (Workboat Certificate category 2), enabling the vessel to operate up to 60 miles from security. With a passenger capacity of 12 and a limitation of single day sampling trips, the Bill Conway is ideal for smaller scale research projects.

Overall Length: 11.74m
Max Breadth: 3.96m
Draught: 1.3m
Speed: 10 knots
A-Frame: 750Kg

Used in Estuarine Survey
 

Figure 5:  Image of the Bill Conway during estuary survey in the Fal  Estuary

                                      Grey Bear

A Shallow draft workboat that is owned by FD Marine Ltd. It is highly stable and manoeuvrable, making it ideal for surveying and sampling with a license to carry 12 passengers and a three tonne winch for sample collection.

Hull: Steel with a double bottomed cargo well deck.
Engines: Twin Screw 2 x Perkins-Sabre 135hp engines,
Speed: 7.5 knots.
Working Deck: 10m x 6m

Used in the geophysical survey
 

Figure 6: Image of the Grey Bear docked at the Prince of Whales pier before the geophysics survey

Offshore

The principle aims of the offshore practical were to locate and analyse the characteristics of the frontal system and its surrounding waters. The front, located just off the mouth of the Fal estuary, is a result of the interactions between stratified and well mixed water mass. From this, fronts can be said to form at boundaries of both temperature and salinity and in coastal regions are greatly influenced by the topography and tidal cycles of the area, impacting on the level of mixing within the system (Fogg et al., 1985).

With the presence of three different water masses (see figure) the area surrounding the frontal region is subject to great variation in physical characteristics such as salinity, temperature and nutrient concentration. Nutrient concentration is often high in the well mixed water mass, with the surface waters replenished by the cold, nutrient rich bottom waters. In the surface stratified water body, nutrient depletion is a common characteristic which follows large phytoplankton blooms (Beardall et al., 1982). With this in mind, it is important to investigate the physical, chemical and biological aspects surrounding the frontal region and adjacent waters in order to understand this dynamic oceanographic feature.

CTD DATA

The CTD measures temperature, salinity, fluorescence, light intensity and turbidity within the vertical water profile (Figure 8). The calibrated fluorescence data show the chlorophyll concentration.

Temperature:

At station 1 the surface water temperature of 13.5°C decreases gradually down to 12.1°C at 30m. A similar trend is observed at station 2 where a relatively linear decrease in temperature within the upper 20m occurs between approximately the same temperatures as at station 1. The beginning of a very weak thermocline is seen at station 3 at 15m depth (from 12.3-11.9, and clear temperature stratification is observed at station 4 at around 20m, where the temperature decreases rapidly by 2°C from 13.5°C to 11.5°C. Both station 3 and 4 show higher surface temperatures and lower deep water temperatures than at stations 1 and 2.

Fluorometer calibrated (chlorophyll):

The chlorophyll concentration at station 1 remains relatively constant with depth, at station 2 the chlorophyll increases with depth, the highest concentration being between 10 and 20 meters. The chlorophyll concentration at station 3 shows a peak of 1µg/L at 15m depth. A similar chlorophyll peak of a concentration of 1µg/L is also observed at station 4 around 20m depth.

Light intensity:

Between stations 1 and 4 the depth of the 1% light penetration calculated from the CTD data increases from 20-21m at station 1 and 2 down to 29m at station 4. The values calculated from the CTD data do not correlate well with those calculated form the secchi depth, especially at stations 3 and 4 showed in Table O1.

Salinity:

The surface salinity between the stations increases moving away from the shore (from station 1 to station 4). The salinity at each station increases gradually with depth and then remains constant after a certain value. At station 1 salinity increases to 10m and then remains constant at around 35.27PSU. At station 2 and 3 salinity increases to approximately 35.3PSU at 20m and at station 4 it increases to 35.34 at about 30m.

Turbidity:

The turbidity increases with depth at each of the stations.

Density (calculated from the CTD data):

At stations 1, 3 and 4 the water density increases logarithmically with depth, they show a rapid increase in density in the surface waters which becomes a gradual increase at depth. The graph at station 2 shows approximately a linear increase of density with depth.  At station 3 and 4 the point at with the increase in density lessens is approximately at the base of the thermocline (15 and 25m respectively).


CHEMISTRY

Oxygen:

At station 1 there are insufficient data available to analyse the oxygen profile.

Within the first 20m of station 2 there is little difference in the oxygen saturation which remains approximately 100%, in comparison, station 3 shows a high surface oxygen saturation of approximately  125% which decreases down to 105% at 35m. The oxygen saturation at station 4 shows a very different distribution with an oxygen minimum layer of about 85% observed at 20m (Figure 9).

Phosphate:

Comparing the phosphate concentration (Figure8) of the four stations a development of different situations can be observed. The phosphate concentration at station 1 is low in the surface waters with a weak increase down to a concentration of 0.026µg/L at 10m, which remains relatively constant in deeper water. At station 2 the first 10m also have a low phosphate concentration but this only increases slightly with depth from 0.017µg/L to 0.022µg/L at 17m. A lower phosphate concentration is seen within the upper 15m of station 3 with a steep increase to a maximum of 0.04µg/L at 35m depth. A stratification of phosphate concentration is observed at station 4; low concentrations exist in the surface 20m of the water column. The concentrations increase at approximately 30m and remain at a higher value of about 0.04µM in deeper waters.

Silicon:

The silicon concentration in the water column (Figure 8) at station 1 decreases weakly from 2 to 3µg/L over the water column.  At station 2 a low concentration of 1µg/L exists within the upper 10m of the water column, the concentration then increases by 2.7µg/L down to 15m. At station 3 and 4 a linear increase of the silicon concentration from approximately 1 to 4µg/L is seen over the depth of the profiles. But while at station 3 a high concentration of 4µM is reached at 35m the same value is not reached until 60m at station 4 and by 35m at station 4 the silicon concentration is only 3µg/L. This shown the general trend of the silicon concentration is a decrease in the upper water column between the stations moving away from the shore.

PLANKTON

Phytoplankton:

The number of phytoplankton individuals (Figure 10) analysed by cell counts at station 1 shows little difference within the top 30m with all values being about 1000 cells ml^-1. Comparatively, at station 2 the surface phytoplankton number is very large, between 5000-6000 cells ml^-1 at 5m, at 10m and below the number of cells decreases to 1000-2000 cells ml^-1. Within the upper 15m of station 3 1000-2000 cells ml^-1 were counted a decrease is observed to 35-40m. The data for station 4 shows a minimum in phytoplankton numbers at 20m depth. Either side of this at both 5m and 30m depth the cell numbers were between 1000-2000 cells ml^-1. At 60m water depth very few phytoplankton organisms were recorded.

Zooplankton and ADCP backscatter:

The ADCP backscatter data are compared to the zooplankton individual counts and identification, in theory the higher the backscatter in the ADCP the higher the zooplankton number (Figure 11).

At station 1 the most abundant zooplankton group is copepoda of which 270 individuals m^-3 were recorded. Larvae of several different organisms were also recorded at approximately 100 individuals m^-3.A very high abundance and species diversity of zooplankton is observed at station 2 where hydromedusae and appendicularia dominate with abundances of 500 individuals m^-3. Copepoda are also found in abundance (400 individuals m^-3), as are decapoda larvae (300 individuals m^-3). In contrast the ADCP backscatter at this station is not high relative to other stations. At station 3 the zooplankton numbers vary between upper 20 meters and whole water column, copepoda and decapod larvae are most abundant within the first 20m, in the whole water column copepods and hydromedusae dominate. At station 3 the ADCP backscatter is highest compared to all other stations, however the counted zooplankton numbers were highest at station 2. At station 4 two bands of high backscatter are seen, one within the thermocline at 20m and the other one below at 30m. Zooplankton counts at this station are quite low and correlate well with the relatively weak backscatter bands. The dominant groups are hydromedusae, polychaeta larvae and copepoda.
 

RICHARDSON NUMBER

The Richardson number (Ri) is a dimensionless number that expresses the ratio of potential energy to kinetic energy in the water column. It is an indication of turbulence. For turbulence to be maintained, the rate of kinetic energy input must exceed the potential energy demand. Ri is given by the following equation:

Where g is gravity (9.81 ms^-2), r is the density, and U is the mean flow. Typically when Ri < 1, turbulence can be maintained, when Ri > 1 turbulence will either die out or be absent, and if there is no turbulent flow, the condition for it to become turbulent is when Ri < 0.25.

At station 1 the Richardson number shows a mainly turbulent water flow except of one single data point at 3.5m depth which is laminar. The diagram of station 2 shows a laminar flow between 12 and 14m and a turbulent flow at most of the depth. A turbulent flow dominates station 3 with a single data point of laminar flow at 4.5m. Station 4 shows a laminar flow at around 18m which is in the thermocline. Everywhere else in the water column a turbulent or intermediate flow can be observed. The deeper layer below the thermocline is more turbulent.
 

POTENTIAL ENERGY ANOMALY

The Potential Energy Anomaly (f) is a measurement of the amount of energy needed to mix the water column. Therefore it is also an indication of how stratified the water column is. When f > 0 the water column is stratified and when f = 0 the water column is completely vertically mixed. The potential energy anomaly is given by the following equation:

where h is the depth of the water column and r(z) is the density at point z and  is the mean density of the water column. Simplified, the equation is the difference between the potential energy of the theoretically mixed water column and the actual stratified water column.

We calculated the potential energy anomaly at each station (Table O1) using the density profile data to estimate the average density of each stratification layer within the water column. From this we could determine the potential energy of the water column with the following equation:

where h_c is the height of the centre of gravity and dz is the thickness of the layer of water.

The values obtained for the potential energy anomaly at each station indicates that each station had stratified water columns. As the distance away from shore increases with, as does the potential energy anomaly as to be seen in Figure 12. This indicates that the water is more stratified in the open waters as opposed to at the mouth of the estuary.

Figure 8: Depth profiles for each of the four stations showing temperature, chlorophyll (from CTD), phosphate and nitrate concentration (from discrete samples)
 

Figure 9: dissolved oxygen depth profiles for each of the four stations, obtained by titration of discrete samples.

Figure 10: depth profiles for total phytoplankton cell counts at each station.
 

Figure 11: ADCP backscatter profiles for each station lined up with bar charts of zooplankton counts from plankton net samples.

Figure 12: Richardson number vs depth for each station, compared with CTD temperature plots. Vertical control lines show turbulent and laminar flow thresholds.

Analysing the calibrated data of the fluorometer from the CTD, which gives an indication of the chlorophyll concentration, the vertical profile at station 1 and 2 was seen to be relatively invariable with depth. Phytoplankton need light and nutrients to carry out primary production by photosynthesis. As chlorophyll was found relatively evenly distributed throughout the whole water column at station 1 and 2 it is likely that the organisms are being mixed into deeper water layers where light intensity is insufficient for photosynthetic processes. The temperature profiles at these stations show a decrease with depth but no clear stratification, also indicating that the whole water column is well-mixed (Sharples and Simpson, 2009). The nutrient concentrations also support this idea because nutrients are available in the entire water column, although at station 2 the nutrient concentration is lower at surface. The chlorophyll concentrations at stations 3 and 4 both peak within the thermocline. The temperature drop of 0.5°C within the slight thermocline at station 3 is very small but suggests the presence of a front nearby. A well-developed stratification with a clear thermocline occurs at station 4 and probably indicates the presence of a front. This is also supported by the density data at station 4 which shows a more stratified profile than station 1 or 2, and the Richardson numbers which show laminar flow within the thermocline (indicating stratification) and turbulent areas above and below it. Consequently the nutrient concentrations in the surface mixed layer are very low and higher below the thermocline where nutrient exchange by mixing activities can occur. The silicon in the surface waters is depleted as it is used by diatoms to build their frustules (Martin-Jézéquel et al., 2003). The phytoplankton is therefore found within the thermocline where both nutrients and light are available. At station 4 the oxygen minimum layer at 20m within the thermocline indicates high respiration rates by heterotrophic organisms feeding directly and indirectly on the high number of phytoplankton.

The 1% light depth calculated from the CTD data does not correlate well with the depths calculated form the Secchi disc. This can be explained by the inaccuracies of the measurement method of the Secchi depth which can be influenced by cloud cover and boat shadow. Settling out of suspended material input from the estuary causes the 1% light depth to increase with each station moving away from the shore.

The increase of the salinity with each station can be explained by increasing distance from the fresh water input of the estuary. Moving away from the estuary the fresh water is mixed into the seawater as the profile at station 4 shows while at station 1 the water is less mixed and a layer of less saline water exists at the surface.

Comparing the phytoplankton data to the fluorometer data of the CTD should show good correlation. At station 1 the two measurements do correlate well and both show little change with depth. However the large increase in cell counts at the surface of station 2 is not observed in the CTD data. At station 3 and 4 the fluorometer data show maximum values at the thermocline which should also be shown in the cell counts. At station 3 the phytoplankton numbers do not increase around the thermocline and at station 4 the cell counts actually decrease. Therefore the phytoplankton cell counts are inaccurate and cannot be analysed.

Zooplankton found in the samples are mainly phytoplanktivores, therefore their abundance should correlate with chlorophyll concentration. The strength of the ADCP backscatter can also be used to determine the amount of zooplankton in the water column. A high zooplankton abundance observed at station 2 is not reflected in the backscatter and does not correlate with the chlorophyll data. The backscatter intensity at station 3 and 4 is relatively high around the thermocline which is to be expected because of high chlorophyll. The backscatter at station 4 shows an interesting picture of two bands, probably caused by zooplankton feeding strategies. Some species are feeding within the thermocline and some are feeding below on the rain of organic matter (e.g. faecal pellets and dead phytoplankton). The backscatter at station 3 and 4 do not correlate well with the zooplankton counts, suggesting the counts are inaccurate.

 

Errors encountered during the offshore sampling work include problems with the CTD data. As it was the first time the CTD had been used on this field course, there were a number of unrealistic data points, especially for the chlorophyll and light attenuation. At station 4, while the CTD was stationary at depth, the reading increased and then on the ascent the readings were much higher.

The light sensor also behaved strangely with huge peaks on both the down cast and up cast, possibly caused by the light levels being so low that the machine malfunctioned.

During the vertical plankton trawl, readings on the flow meter appeared inaccurate. Therefore, the flow had to be calculated from diameter and mesh size. At station 3, for an unknown reason, the plankton net trawl returned purely clear water; indicating very little plankton. With this, the sample was discarded and another trawl carried out. A further issue with the plankton net was it did not close with the aid of a messenger.  Due to this, instead of taking samples between certain depths, the samples were of the entire water column.

During transit, the stopper of bottle 82 came out resulting in possible contamination by the water used to keep it cool. In addition to this, the phosphate samples were not filtered before being stored, this meant that the phytoplankton was not removed and some of the phosphate may have been utilised whilst being stored.

During the laboratory work there were a few possible errors. The phosphate reagents appeared to have a contaminant floating in the bottles whilst measuring cylinders were used to measure the reagents volumes, which are less accurate than pipettes.

There were no silicon standards to work the concentration of the samples from on the day of analysis. Some standards were received several days later, however, these would not take account of small scale variations in the spectrometer.

During the dissolved oxygen analysis there were bubbles present in bottles 64, 82 and 83 bubbles, which would have affected readings.

During the chlorophyll analysis there were some differences between repeats of the same station at the same depth. In this case, the highest value was assumed to be correct as the fluorometer only measures chlorophyll, which would not give a high reading unless chlorophyll is present. A further potential error in plankton biomass estimates is within the phytoplankton and zooplankton cell counts; which, with each team member analysing separate bottles could make the counts relatively subjective as each person may see things differently and misidentify the species.

Conclusion

The aim of the offshore survey was to find and characterize the frontal system between the mixed water of the estuary and the stratified conditions further offshore. The front was identified at approximately station 3 where very slight stratification is found and mixing processes still occur. Furthermore the offshore stratification is clear but not as strong as it was in the previous 2 years probably caused by this years’ colder weather (weatheronline.co.uk). The phytoplankton and zooplankton distribution and abundance differes between the mixed waters, the frontal system and the stratified habitat due to water movement and nutrient and light availability. Within the mixed area nutrients and phytoplankton are relatively evenly distributed throughout the water column and no real thermocline exists. At the frontal region different conditions are seen above and below the weak thermocline in which the majority of phytoplankton and zooplankton live, however mixing processes still occur. In the stratified water at station 4 the two water layers (surface and deep water) are separated by the thermocline which is also where a high chlorophyll concentration is found. The zooplankton are feeding both within and below the thermocline.

Geophysics

ABOUT FALMOUTH BAY:

Falmouth Bay is an area of coastal water found to the south of Falmouth town in Cornwall UK. The bay is an area which has populations of maerl and macroalgae which may imply a high diversity of benthic fauna. It has also been a site for dumping of material dredged from shipping nearby channels, a process which will have significant affected local fauna and flora, and also the characteristics of the sediment and bedforms throughout the area.
A sidescan survey was carried out to identify any significant changes in bedforms and sediments across the bay. Van Veen grabs were the used to determine the flora and fauna of the area and how they change with bedforms. A video sledge was also used to gain a broad picture of the benthic habitat.

THE SPOIL GROUNDS OF FALMOUTH BAY:

Spoil grounds are specific areas where dredged material is permitted to be dumped. This environment can be diverse and unstable due to heavy anthropogenic influences. The spoil grounds in Falmouth Bay are designated for the dumping of maintenance material from the dredging in the Fal estuary. As a consequence of this activity, Premier Marinas issued a Falmouth Marina Dredging- Environmental Impact Assessment in order to regulate the silt build-up and Tributyltin contamination of the sediment. In 2008, the assessment suggested that 0.2m to 3.0m of the top layer of sediment be dredged and removed from the estuary because of the contamination (Wathen, 2010). The lower clean sediment was suggested to be dumped in the Falmouth Bay spoil grounds.      

As a result of the frequent dumping of disposed sediment in this area, the benthic community in this region is often changing. This can be seen through video observations of the sea floor.                     

AIMS:
The principal aims of this geophysical survey were to analyse the sedimentology, bathymetric features and benthic community; whilst determining how any changes in bedforms affect the biology within the area. Such effects include whether the presence of maerland macroalgae populations allow a high diversity of other species to inhabit the bay, and also to determine the effects of the anthropogenic input of dredged material on the habitat.
 

The sidescan was carried out using a towfish for the physical part of the survey. It was attached to the boat via a winch and a data cable and lowered into the water using a crane off the starboard side of the boat. The depth of the fish in the water column was dependant on the velocity of the boat and the length of the cable. It is controlled by and monitored from a computer on the boat. Throughout the scan, the operators communicated with the boat skipper in order to keep the crew informed of important modifications, such as course changes.

The data was printed on a sidescan plot showing the bedform morphology and the features of the survey area. Two people were assigned to analysing the sidescan; recording times, fix numbers and interesting features. It is important to note that the output from the printer was four minutes behind the scan. Times between each transect were also recorded to allow for later editing.

After scanning had finished, areas of interest were noted as possibilities for sediment grabs for the biological part of the survey. At these areas, the grab was deployed and samples were collected. Some repeats were necessary due to rocks being caught in the grab and holding it open so contents were lost. Once a grab was successful, the contents were put into a tray and sorted for larger rocks and shells. The rest were then passed through the three sieves, flushed through with water to help sort the sediment.
 

GENERAL INFORMATION:

Start: 13:00 GMT
Finish: 16:46 GMT

 

Transect	Start Position	End Position
1	50 08.30N 005 02.96W	50° 08.24N 005° 03.94W
2	50° 08.26N 005° 04.04W	50° 08.58N 005° 02.92W
3	50° 08.59N 005° 03.06W	50 08.29N 005 04.12W
4	50 08.35N 005° 04.11W	50° 08.54N 005°  03.43W

TIDE DATA:
 

	Time (GMT)	Height (m)
High	04:28	4.40
Low	10:52	1.20
High	16:51	4.70
Low	23:19	1.20

WEATHER:

Wind: Westerly direction, force 3 (~16-17mph)
Sea State: 2/3 – small waves but none breaking, some tops caught by the wind

Cloud cover:
Transect 1 – 6 octants at start and end
Transect 2 – 6 octants at start and 3 octants at end
Transect 3 – 3 octants at start and 2 octants at end
Transect 4 – 2 octants at start and 7 octants at end

                Figure 14: Pea crab (Pinnotheres pisum), grey topshell (Gibbula cineraria), venus bivalve (family: Veneroidea), alive fragment of maerl, keelworm on pebble (Pomatoceros triqueter)

            Figure 15: needle whelks (Bittium reticulatum)

                                 Click Images to Enlarge

                        Figure 16: bivalves and alive maerl from the grab

                      Figure 17: alive fragments of maerl from the grab sample

By analysing the side scan of the Falmouth Bay two features were detected on the sea floor. One of the features (1) in Figure 18 can be identified as a rock while the identity of the other feature (2) is to be discussed. The idea of feature (2) being a scour mark is not reasonable due to the fact it does not appear on the scan of the following return transect, even though the areas should overlap in the scan prints. Another suggestion is the feature could be a diver swimming in the same direction as the sampling boat. The velocity of the sampling boat in addition to the velocity of the diver would cause the potential diver to appear longer on the side scan than in reality. The idea can be supported with a made observation of a diver flag within that particular region.

The rest of the sea ground shows a very similar structure on the first glance. However, by looking at the detailed ripple structure, different patches can be found. There is a patch with in-continuous ripples (3) where the structure is interrupted frequently. In addition to that two areas of continuous ripples were found which differ in their ripple sizes from each other (number (4) contains bigger ripples that number (5)).

                                                Click Image to Enlarge

       (1)                             (2)                       (3)                           (4)                            (5)

Figure 18: This image shows 5 selected features from the recorded side scan.

A video sled was used to gather real time footage of the surveyed area. The video started within the surveyed area closest to the shore and moved across the transects towards the sea-side. The footage showed a succession of finer sandy, calcareous sediment with small patches of seaweed and echinoderms to a more coarse calcareous sediment dominated with large kelp populations and crustaceans. When sediment is dumped in the spoil ground it smothers the life beneath it and may lead to patchy ecosystems and an ecological succession.

Figure 18: A snap shot from the video sled recording                    Figure 20: A snap shot from the end of the sled recording

                   showing an edible crab (Cancer pagurus) on                                  showing a common sea star (Asterias rubens).

                   the right hand side.                                                                                              

TRANSECT ANALYSIS

Upon analysis of the side scan imagery from the surveyed area, different areas were identified in terms of seabed roughness and bedform morphology.

The blue area covering the largest proportion of the survey (31000N, 181000E) and (31200, 182200E) showed no indication of bedforms, with a uniform shade and no distinguishable shadows illustrating the presence of ripples or any other features. In the centre of the surveyed area, ripples were present in both a non continuous (wavelength 1.07m, wave height 0.15m and target depth 0.89m) and continuous form (wavelength 0.89m, wave height 0.14m and target depth 0.81m) (31200N, 181400E). The two types of ripples had very similar measurements, only to be distinguished between each other by the breaks in the banding within the non continuous area (green) to the east (31200N, 181700E). A further section of ripples is highlighted in orange (31400N, 181800E); characterised by the largest ripple morphology found in the survey area with a wavelength of 1.19m, wave height 0.21m and target depth of 1.24m. The final bedform section identified was in the north west of the survey (31400N, 182200E) (coloured red). This area shows a rougher sea bed with darker shading on the original imagery than the surrounding areas.

Finally, two individual features were found within the survey area. The first (31200N, 181200E) clearly lifted above the seafloor with a measured height of 1.75m and covered an area 10.44m wide and had an irregular angular shape.

The second was much smaller and only appeared on one swath despite being at the outer boundary of the print out. The second feature (31200N, 181800E) had a straight rectangular shape area with a width of 2.53m and height 0.43m.

CONTOUR PLOT ANALYSIS

The geophysical contour plot of the seafloor was produced using co-ordinate measurements of Eastings and Northings coupled with corresponding depth soundings at each location. From these measurements a three-dimensional perspective was constructed using surfer software, allowing the seafloor morphology to be visualized and analysed alongside the side-scan data.  The sidescan transects overlapped by approximately 10 metres in each trajectory, ensuring that the images display an accurate representation of the seafloor.
                                                                                                                                                                     Click Images to Enlarge

                                      Figure 22: Survey contour map showing the depths of the surveyed area                                                          Figure 23: 3D map depicting the surveyed area

 Depth measurements were adjusted to give depth (m) relative to admiralty chart datum (ACD) ^*1. The preliminary adjustment was for the distance between the transducer and sea level height (draft  height :1.14m )^*2 , the secondary adjustment calculated the difference between the recorded water level height and ACD using interpolated corresponding tidal heights for each location.

4 transects were taken, each line being roughly 2km in length, aiming to be as parallel as possible.

Transect  1 (31357N-182314E)- (30893N, 180845E) shows a 7m variation in depth (-5- -12m) with three -7m peaks and three noticeable troughs down to -12m troughs.

Transect  2 (30893N- 180844E)- (31421N-182230E) shows a 2m variation in depth (-7-  -9m) with three slight peaks (-7/-8m)and two troughs at -9m .

Transect  3 (31493N -182197E)- (30986N-180872E) shows a 2m variation in depth (-7-  -9m) with three -7m peaks and four -8m troughs.

Transect 4 (31087N- 180793E)-(   31580N-182160E)  shows a 2m variation in depth with one peak at     -7m and one trough of -9m. 

When the four transects are combined and interpolated using the surfer software package it can be seen that Falmouth bay has significant dune structures of up to 5m in height (-7m - -12m), the gradient and quantity of dune structures decreases landwards, with the maximum difference on transect 4 (most landward) being only 2m with only 1 peak. This could be showing that the large dunes are attenuated closer to the surf zone, possibly by the increased interaction and influence of tidal and wave energy observed in shallower waters. The sidescan data showed medium grey reflection from the seafloor; this is normally an indication of medium sized sediment such as gravel/shells, and is grey due to slight adsorption of the acoustic pulse by loose sediments . This observation was confirmed by the sediment samples collected using the van veer grabs. The grab samples were sieved, with the majority of sediment being between 2-10mm in diameter. The grabs samples also contained a significant amount of biological artefacts, which were not detected using the sidescan sonar. Following these observations; the deployment of an underwater video system enabled a visual survey of the sampled area, confirming large amounts of kelp and, crabs and seastars.

In coastal and estuarine environments, the hydrodynamic conditions of shallow waters control sediment distribution patterns and characteristics. Environments close to the shore have a wide range of hydrographic settings due to riverine input, currents and most importantly, tidal waves (producing unsteady and non-uniform flow). Such significant changes in hydraulic conditions ultimately result in high levels of sedimentation and regular reworking of seafloor sediments.
When analysing seafloor topography, two key factors need to be considered; flow strength and sediment type (Blondel, 2009). Increased flow strength (flow velocity or boundary shear stress) increases the ability of seawater to erode coarser material and increased fluid velocity results in larger bedforms. Furthermore, bedforms are a function of grain size, i.e. the greater the bedform size, the coarser the sediments that form the structures.

The high proportion of coarse grained sediments (1.00mm) coupled with low hydrodynamics (low riverine input) causes a lack of distinct morphological features. Irrespective of this, the region of small continuous ripples and coarse lithology in the centre of the grid suggests higher current flows and increased flow strength towards the centre of the bay. However, the intermittent “patches” of non-continuous ripples (31200N, 181600E) and no ripples (31050N, 181250E) enclosed within the continuous ripple zone suggests no particular hydrodynamic pattern (Langston et al.,2003). Towards the north of the transect (31400N, 181400E) an area of larger ripples indicates an increase in sediment flow patterns towards the coast. The distinctive bedforms identified in this side-scan image are characteristic of tidal current transport and are in a dynamic equilibrium with their hydraulic environment and sediment particle size.

The dumping of dredging material into Falmouth Bay creates a smothering effect for the flora and fauna that is living in the benthos. Due to the relatively high disturbance to the benthic habitat, living flora and fauna retrieved in our grab samples was limited. However, the video footage recorded a number of echinoderms and one large crustacean as well as seaweed masses and large kelp bed. Further research on the change of benthic habitat in relation to the types of sediment dumped in Falmouth Bay and frequency of dumping should be conducted to document long term change of that area.

During the course of the geophysical survey, some minor errors occurred which prevented as much data being collected in the allocated time. At the beginning of the survey the side scan printer did not function properly and so the first transect was delayed. Due to the failure of the printer, the side scan was adjusted to a lower resolution to prevent conflicting software, resulting in a lower contrast side scan and a more difficult interpretation of the bedforms. This problem also meant there was less time overall throughout the day to carry out grabs at more than two different sites and so the ground truthing of certain areas became limited.
Another error was the failure of grabs to collect a sample of the seabed. Due to the nature of the substrate, rocks became trapped in the grab causing any collected material to fall back into the water column upon return to the surface. In order to overcome this error, the grab had to be repeated until a significant sample size was brought to the surface. Furthermore, the grab type used did not have teeth around its closure; therefore the majority of the fine sediment was also washed out during the return to the surface.

 

From the geophysical survey of the Falmouth Bay area, it can be seen that there is homogeneity in both sediment type and bedforms across the majority of the area surveyed. It is speculated that the reasoning behind this is due to the dumping of dredged material into this area from the Fal estuary channel.

The video scan of the seafloor used towards the end of survey showed a clear difference between the area within the spoil grounds and that outside of its boundaries. This difference indicates a succession of marine communities, with the earlier imagery showing a large coverage of bare sediment and limited biota and the latter a seemingly more diverse system. This succession is not surprising, especially considering the constant refilling of material into the area preventing a well established community developing.

With this in mind, if the group could investigate the area further, a priority would be to undertake transects which span from inside to ouside the spoil grounds. This would help distinguish between the two apparent areas and expand on our previous conclusions from this study. In addition, more grabs should be used to more accurately define areas on the side scan imagery as well as those of the expanded survey area.
 

Estuary

The main aim of the estuary survey was to analyze the physical, chemical and biological characteristics of the Fal estuary. An estuarine system is a semi-enclosed coastal area with freshwater input in the form of rivers which dilutes the seawater and leads to a lower salinity. Terrestrial runoff into the estuary contains suspended particles of nutrient rich material. Tides and waves are also an important factor influencing the daily estuary conditions (Uncles, 2002). Eutrophication can be an issue due to the high nutrient amount in the river water.

Half of the day was occupied by taking samples from the pontoon at 50° 12.970N, 005° 01.659W to document nutrient levels during tidal change. The later half of the day was on the Bill Conway recording transects and taking samples to document spatial change of nutrient levels.

Group 10 sampled at the pontoon on the morning of 01/07/2011 and then in the afternoon onboard Bill Conway sampled the mouth of the river. Group 2 sampled the head of the river in the morning prior to low tide and  then sampled at the pontoon in the afternoon. The two sample sets were merged for analysis.

 Samples were taken on the pontoon from 09:00GMT to 11:26GMT. A horizontal Niskin bottle water sample was taken from the “riverside” of the pontoon on every hour, as well as at low water - 11:26GMT.

When the Niskin bottle was brought to deck, a dissolved oxygen sample was immediately taken. Each 50ml water sample was filtered twice, with the two filter papers fixed in acetone to collect chlorophyll samples. The filtered water was used to collect a phosphate sample in a glass bottle, whereas silica was sampled in plastic bottle due to the nature of glass possibly interacting with the sample to be measured. Furthermore, for each filtered sample approximately five to ten milliliters were flushed through the glass fiber filter in order to clear it of any loose particulates that may contaminate the silica sample.

Following this, 100 ml of unfiltered water was placed in a glass bottle containing Lugol's iodine to collect a preserved phytoplankton culture.

In total four samples were taken at 09:00GMT, 10:00GMT, 11:00GMT, and 11:26GMT. However, at 11:00GMT oxygen and phytoplankton samples were not taken due to lack of sample bottles.

In addition to the water collection, every half hour a YSI probe (6600 V2) was used on both the riverside and landside of the pontoon to record salinity, depth, pH, turbidity and temperature.

Aboard Bill Conway, 9 transects in total were recorded with an Acoustic Doppler Current Profiler (ADCP). (Note: Transect 7** was not a full transect because the boat had to veer off course due to a ship blocking the course). The first four transects were carried out in between key stations of the river, whilst the latter five where carried out across the estuary at each key station. At the five stations a CTD was used to carry out a vertical profile of the water column. On the CTDs up-cast Niskin bottles were fired at three to four depths and from each of these, silica, phosphate, chlorophyll and dissolved oxygen samples were taken in the same way as on the pontoon. Additionally, at each station a secchi disk was used to gauge the light attenuation.  Finally, 3 plankton net trawls were taken in total, 2 in the upper section of the river and the last at station 10 near the mouth. Each plankton net was thrown over the back of Conway for five minutes and a 500ml plankton sample taken and combined with formalin to fix the plankton sample.

PONTOON DATA

Nutrients
Both the phosphate and the silicon concentrations show an increase following low tide at 11:26 GMT. Phosphate increases from less than 0.7 to approximately 1µMol/L between 11:00 and 13:00. Silicon also increases from 4 to 9µMol/L between 12:00 and 13:00. Both nutrients then decrease in concentration at 14:00. An earlier increase in phosphate and silicon at 10:00 and 11:26 respectively is also observed.

Oxygen
The oxygen concentrations decrease following low tide from almost 100% at 11:26 to only slightly more than 60% at 13:00. After this the concentrations increase again to 90% saturation.

Phytoplankton and zooplankton
Phytoplankton cell counts are relatively uniform at less than 2000cells/ml apart from a large increase at 12:00. At this time the cell counts increased to almost 9000cell/ml, and decreased soon after at 13:00.

Salinity
Salinity on both sides of the pontoon is low at all depths between 10:00 and 14:00 and relatively high before and after this time period.

Chlorophyll
Chlorophyll concentrations were found to increase substantially in the surface waters between 12:30 and 14:00 and are general low before and after this time period.

Temperature
Temperature profiles show higher temperature values between 11:00 and 14:00 than are observed before and after this period. The maximum values are approximately 16oC compared to lowest values of approximately 14.5oC
 

ADCP SHIP TRACT DATA

Transect 1 was the highest site upstream, recorded at the earliest time in the day; before the change in tide. This change in tide was at approximately 12:26. There was an ebb tide in the morning, showing similar velocity throughout the full water column. The sticks in the ship stick track show a fairly uniform, moderate flow in the southern direction (indicating the ebb) from the river to the mouth. There is, however, some variation in the stick directions in the west side of the channel. This could possibly be due to the fact that the water is shallower and apart from the main channel, increasing turbulence and resulting in more random flow directions. The average backscatter generally varies between 68-78dB, with some higher amounts in the top 2-3m as high as 107db. These high amounts of backscatter indicate increased levels of suspended plankton and material.

For transect 2, velocity magnitude is fairly uniform throughout all depths, with a range of up to 0.282ms^-1. The ship track shows a uniform southern flow with the ebb tide, which has decreased in the surface waters according to the approaching low tide at midday.

The ship track on transect 3 shows a further decreasing surface flow, though it is still southern mean velocity. There are some much larger flows in the west of the estuary where the water is very shallow, within about 2m, which causes turbulence. Water velocity is also fairly uniform with depth. Backscatter is high in the surface waters, extending down to 6m, indicating high levels of suspended particulate and biological matter. Below this, there is very little variation.

Transect 4 ship tracks show that the direction of flow has changed. It is now north, though surface velocity is very low. This shows the changing tide direction with the dominant flow pushing up into the estuary with the flooding tide. Both backscatter and water velocity are quite uniform with depth, showing little variation.

Following the tidal pattern, the ship track for transects 5 and 6 show an increasing northerly flow in surface current. This is uniform in direction and with depth, though higher velocities are noted in shallower waters, up to 0.565ms^-1, as opposed to 0.282ms^-1 in lower waters. Backscatter is mostly constant with depth, though there are some areas in shallower waters showing small areas with significantly higher values.

Some sticks observed in transects 7 and 8 are pointing in the opposite directions to the other sticks, indicating return flow along the sides of the estuary. Aside from this, high flow velocities are seen in the centre of the channel for transect 7, while the higher velocities are in the left for transect 8. Again, though water column velocity varies laterally across the channel, both transects show a moderately uniform vertical change.

Transect 9 is at the bottom of the estuary near the mouth. Results show a moderate southern flow, in contrast to the northern flow in the past few transects. This is indicative of a change in tidal flow, as high tide has passed. There is now an ebb tide again, which coincides with the tidal height data for the evening.  Once again, velocity is relatively constant with depth, with some lateral variation. However, the average velocity is the highest recorded for all stations, with the possible exception of transect 1. The data for this station is however not helped by a lack of detail in the plots. Backscatter changes with distance along the transect, being highest in the west, as high as 105ms^-1, then reaching 70ms^-1 in the east.
 

CHEMISTRY DATA

Mixing Diagrams

In the Fal Estuary, the hydrological conditions (low riverine input) in conjunction with a macrotidal system produce a narrow range of high salinity values. All the samples taken were greater than 30 and ranged between 30.23 and 34.32. Consequently, all the data points which were sampled on the mixing diagrams are restricted to the higher salinity region and this ultimately creates issues when interpreting the behaviour of the chemical constituents within the estuary. Nevertheless, when the data was plotted with the riverine end members and with the addition of a theoretical dilution line (TDL- linking the two data points with the highest and lowest salinities), phosphate and silicate exhibited contrasting behaviour.

PHOSPHATE- The phosphate estuarine mixing diagram illustrates a relatively low range of phosphate concentrations at high salinity values. The diagram highlights extreme non-conservative behaviour, indicating a possible input of phosphate to the estuary, possibly of anthropogenic origin and agriculture runoff.

SILICON- The silicon concentration shows a linear relationship with respect to the salinity; an increase in salinity causes a decrease in silicon. The riverine end member has a salinity of 0, has a silicon concentration of 113.3µmol/L, the highest value in the estuary. Conversely, the seawater end member, salinity 34.32, has a concentration of 1.3µmol/L. Silicon behaves conservatively along the estuary following the theoretical dilution line precisely, suggesting that the chemical is equally mixed throughout the Fal and is not involved in (bio)chemical reactions.

N.B. The initial readings on the CTD at station 1 and 2 for salinity were incorrect. This was caused by overnight crystalisation of salt on the conductivity sensor resulting in inaccurate measurements. Consequently, the salinities for some bottles of silicon and phosphate were measured using the thermosalinograph (readings indicated by red marks on the mixing diagrams). Therefore, the salinity values do not correspond to the macronutrient values sampled at depths below the surface

Oxygen

Surface oxygen saturation increased dramatically from the head of the estuary (station 1- 92.9%) to the mouth (station 10-99.4%). These surface concentrations must be in equilibrium with the atmosphere, increasing down the estuary due to an increase in oxygen exchange rate across the sea-surface interface due to greater tidal and wave action and thus surface turbulence. The fact that Station 1, at the head of the estuary has severely undersaturated oxygen surface waters suggests a possible influx of deoxygenated water and/or sewage waste, with a high BOD. All stations experienced an initial increase within the first 10m. This is attributable to photosynthetic activity where the photosynthetically active radiation is available and mass oxygen is being produced. The mid estuary (station 5) exhibits the highest oxygen saturation (>102%) at 5m depth indicating the area of most primary production. Station 5 and 10 then continue to decrease with depth which may be a result of zooplankton respiration and decomposition and lack of available light for photosynthesis. However, at station 5 around 10 metres depth, the oxygen concentration begins to increase again. This area of rapid decrease of oxygen saturation (5-10m depth) coincides with a large increase in phosphate (0.6 µmol/L). This is an indication of pollution causing an elevation of phytoplankton activity , an increase of organic material that has a high biological oxygen demand and consequently a reduction in oxygen concentration.  
 

PLANKTON DATA

 The dominant taxa recorded include copepods (>25%) and copepod nauplii with a relatively high proportion of gastropod larvae (Figure 31).

LIGHT ATTENUATION

The 1% light depth graph (Figure 32 ) illustrates an increase with an increase down the estuary.  Station 1 showed rapid attenuation of light, which may be due to riverine input and wind stress resulting in mixing, thereby increasing turbidity of the surface layer.
 

VERTICAL PROFILES
 

Figure 33 is a vertical profile of stations 1, 4, 9 and 10, illustrating the relationship between silicon, phosphate, temperature and chlorophyll concentration. Temperature remains uniform with depth changing by 0.2 ^oC within a 4.4 metre change in depth. Naturally, silicon has a higher concentration than phosphate. However, at the surface they follow a similar trend, increasing from 6.3 and 0.9 to 6.9 and 0.91 respectively. The nutrients then appear to decrease in concentration which coincides with a peak in chlorophyll at 3.6 m.

Further down the estuary at station 4, there is a greater temperature gradient ( from 14.3 ^oC at the surface to 13.6 ^oC at 13m). At the surface both nutrients behave oppositely, with phosphate decreasing with depth and silicon increasing with depth. This trend continues down to 4m depth where the trend reverses, i.e. phosphate increases with depth and silicon decreases with depth. This coincides with a sharp increase in chlorophyll from 0.156 to 0.1583µg/L which, from this point, is relatively constant with depth.

Station 9 and station 10 represent the transition between the Estuarine environment and the marine environment. Significantly, chlorophyll and phosphate exhibit the same trends at each station; phosphate decreases from the surface to ~10m depth and increases at depths below 10m. Chlorophyll increases gradually with an increase in depth and is fairly homogenous below 10m. Importantly, there was a noticeable thermocline at station 10 that did not exist at the previous station. This is the onset of water column stratification indicating the beginning of the offshore structure. Silicon on the other hand increases with depth, which is associated with a large decrease in salinity (35.52 to 34.5 salinity), supporting the conservative behaviour shown in fig the estuarine mixing diagram for silicon. 

Click Images to Enlarge

            Figure 25: Phosphate concentration (light blue) and tidal         height (dark blue) at Fal River Pontoon on July 1, 2011.

Figure 26 : Silicon Concentration (red) and tidal height (blue) at the Fal River Pontoon on July 1, 2011

Figure 27: Dissolved Oxygen (purple) and tidal height (blue) at the Fal River Pontoon on July 1, 2011

Figure 28: Phytoplankton (light blue) and tidal height (dark blue) at the Fal River Pontoon on July 1, 2011

Figure 29: Ship track images of Transects 1, 4, 7, and 9.

Figure 30: Estuary Mixing Diagrams for Phosphorus and Silicon

Figure 31: Breakdown of zooplankton cell count from the estuary trawl

Figure 32: 1% Light attenuation depth from Secchi Disk data

Figure 33: Vertical Depth Profiles for Stations 1, 4, 9 and 10 measuring Temperature, Chlorophyll, Silicon, and Phosphorus

The decrease in chlorophyll concentration from the head to the mouth of the estuary can be accounted for by the assumption that zooplankton populations increase down the estuary resulting in an increase in grazing pressure upon the phytoplankton and microbial component of the estuary (Miller, 2004). (The data does not support the assumption that there is an increase in zooplankton down the estuary, but the methods for zooplankton data collection are too subjective to be reliable). However, the fact that there is an overall decrease in silicon and phosphate concentrations down the estuary allowing phytoplankton to exist in the upper estuary (uptake of nutrients in the upper estuary by primary producers) underpins the inverse zooplankton-phytoplankton relationship down the Fal estuary.

There is an apparent relationship between silicon concentration and salinity which cannot be accounted for by physical mixing processes shown in the mixing diagram. This suggests some component of biological uptake of silica by pelagic/benthic phytoplankton (i.e. diatoms). This suggests that the silicon is behaving as a pseudo-conservative element, but this is difficult to say without the flow rates of the estuary.
 

The changes in phytoplankton cell counts and chlorophyll correlate well and can be explained by artificially high concentrations of nutrients moving up the estuary from the mussel farm with the rising tide. The lower oxygen saturation following this is believed to be directly caused by higher amount of phytoplankton and zooplankton these are presumed to support consuming oxygen in respiration.

The salinity and temperature time series show the expected variation with time as colder, more saline seawater moves in and out of the estuary with the tide.

The Fal estuary can certainly be categorised as a well mixed, tidally dominated estuary. The physical profiles (temperature and salinity) are relatively uniform throughout. The upper part of the estuary represents the region of most uniform physical conditions due to the low riverine freshwater input coupled with the relatively strong wind stress. Progression down the estuary shows a very gradual stratification in the water column but only becomes apparent at station 10, when the offshore environment becomes prominent.

Overall Conclusions

During the three surveys within one week on the Fal estuary, the Falmouth Bay and offshore region the biological, chemical and physical conditions have been measured and analysed. The weather during the sampling period was dominated by a mix of clouds and sun with a little rain on the first day.

The offshore region showed a characteristic frontal system with a mixed region on the estuary side and stratification on the offshore side. However, due to a long cold weather period in June the stratification is relatively weakly developed. The coastal front was localised near 50° 06.07N, 5° 00.02 W.

The Fal estuary is influenced by the strong daily tides, whereas the freshwater input barely influences the estuary conditions. Differences from last years’ findings occur in the silicon concentration which is highly conservative, in contrast to the removal of silicon last year.  A phosphate input by anthropogenic activities (for example the mussel farm and agriculture) causes eutrophication of the estuary.

For both the estuarine and offshore system, it is important to consider the interactions between all physical, chemical and biological factors.

References

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Reference list for Equipment Images:

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