The R.V. Callista was taken offshore on 27^th June 2012 into deeper coastal waters off Falmouth, Cornwall, UK, to investigate:

Figure 4.1 - R.V. Callista.

How vertical mixing processes in the waters off Falmouth affect, directly and indirectly, the structure and functional properties of plankton communities?

Surrounding Falmouth, in the western English Channel, waters show thermal stratification in the summer months due to an increase in solar radiation. This stratification is controlled by water depth and tidal strength, which thus determine the shear and flow characteristics in the water column.

In the western English Channel there is a prominent seasonal cycle of phytoplankton growth, where blooms can be observed in the spring and autumn. These have been investigated primarily at the L4 and E1 stations for a number of years (Western Channel Observatory, 2007). These blooms are largely controlled by vertical mixing of the surrounding waters, which in turn influence the physical and chemical properties of the surface waters.

The predictable spring bloom is a response to increased solar radiation and the resultant stratification, this ensures the phytoplankton remain in the nutrient rich surface waters which they rapidly colonise. However in the summer months nutrient levels deplete, the population collapses and the phytoplankton cells die off, being grazed upon and sinking to deeper waters (Winder & Cloern, 2010).

The stratification in the summer months controls the rate at which primary nutrients are mixed upwards across the thermocline. The rate of this process determines the variations in biological productivity at this time of year.

The changing levels of three primary nutrients: phosphate, dissolved silicon and nitrate, determine the composition of the phytoplankton community (Officer & Ryther., 1980) and vice versa.  As stipulated by Pingree et al. (1977), the fluctuating nutrient levels mirror the biological and chemical processes occurring within the water. This therefore controls the distribution, abundance and variety of organisms within a community.

Offshore an ADCP (Acoustic Doppler Current Profiler) was used to measure Eulerian flow at a series of points. This provided a cross-sectional velocity profile within the water column. These demonstrate how the seabed affect the flow.

A CTD (Conductivity Temperature Depth) probe was used to record temperature and salinity with depth, allowing  density within the water column to be calculated. From this data it can be determined whether the estuary is undergoing turbulent or laminar mixing. The Richardson number (R_i) is a dimensionless number used to show whether turbulent or laminar flow is present in the vertical water column.

Calculating the Richardson Number

Figure 4.2 - Formula for Richardson Number (R_i) calculation.

If the Richardson number (R_i) calculated is less than 0.25 then the water column is turbulent and there may be a strong shear present for which a lot of energy will be required to overturn the water column (weak radiance). If the number is greater than 1.00, flow is laminar, and no vertical mixing is occurring. This suggests that there is a stable flow within an area or a strong density gradient between the water layers. Thus indicating overturning will not occur under this conditions. The area between these two values (know as a 'grey area'�) portrays real world conditions whereby there is a transition between the two water states of turbulent and laminar flow (Stewart R, 2005).

Below maps the sampled stations and the transects taken within an embedded Google Map. Station longitudes and latitudes are listed within the key beneath the map.

Figure 4.3 - Offshore locations of sampled stations and linear transects (between stations).

CTD

(a)    (b)    (c) 

(d)    (e)    (f) 

Figure 4.4 - CTD Offshore Data collected from Station 1 to Station 4 - Falmouth, UK - (a) Temperature, (b) Salinity, (c) Density, (d) Fluorescence, (e) Irradiance, and (f) ln(irradiance). Sampled on 27 June 2012. All graphs can be enlarged in a new window upon clicking on the relevant image.

Temperature [Figure 4.4a]

The vertical profiles at all stations offshore show a decrease in temperature from the surface waters to the bottom waters [Figure 4.4a]. At Station 1 the difference between surface (14.3 °C) and bottom (12.5 °C) waters was approximately 1.8 °C. The profile at this station showed a rapid decrease from the surface to 3.5 m, after which the temperature reduction was observed as more gradual. The rapid change is likely to be a result of the introduction of warmer, fresher river water flowing through the estuary to the mouth (where Station 1 was sampled). This warmer water is at the surface because it has lower density and so is lighter and floats at the surface. This warmer fresher water can also be seen in the salinity and density profiles for Station 1 [Figure 4.4b and Figure 4.4c].

At Station 2 and Station 3 two thermoclines were observed. One from the surface to approximately 10 m in depth and the other from 47 m to approximately 60 m in depth. The surface thermocline is likely to be a seasonal thermocline produced due to solar heating of the surface water during the summer resulting in an increase in the temperature. The difference in temperature is 0.3 °C at Station 2 and 0.6 °C at Station 3. Usually for this time of year the seasonal thermoclines are stronger with temperature differences of the order of 2 °C to 4 °C (Western Channel Observatory L4 and E1 buoy data). On the day of sampling, and during previous weeks of June 2012, the weather had been cloudy and of high precipitation, thus reducing the amount of incoming short wave radiation and a reduction in the solar heating of the surface water and so the thermocline present was weaker. Despite this the presence of this thermocline indicates that the water column is stratified. The thermocline present near the bottom of the water column has a temperature difference of 0.5 °C at Station 2 and 0.8 °C at Station 3, and at this depth could signify the convergence of two water masses at Station 2 that have different water properties such that one water mass is downwelling under the other. This idea is further supported by the presence of a high fluorescence peak at Station 2 at a depth of 49 m [Figure 4.4d]. Also witnessed at Station 3 was a temperature inversion at 25 m. This means that the water temperature has increased, in this case by 0.1 °C, and could result from mixing between the converging two water masses seen at Station 2, therefore producing changes to the properties of the water. This temperature inversion was balanced by an increase in salinity of 0.05 psu between 25 m and 30 m [Figure 4.4b] meaning the density at this depth was not significantly changed. Station 4 also shows a thermocline near to the surface decreasing to approximately 14 m with a temperature difference of 1.0 °C from 13.4 °C to 12.4 °C. The bottom thermocline is still present at Station 4 but is less prominent compared to Station 2 and Station 3.

Salinity [Figure 4.4b]

All four profiles show an increase in the salinity with depth at Station 1 showing a significant increase and Station 4 a very small increase [Figure 4.4b]. The stations are positioned at increasing distances from the shore meaning that the salinity is also increasing from the coast to offshore as the influence of the higher salinity seawater becomes more dominant. The lower salinity seen in Station 1 is the result of the input of warmer and fresher riverine water into the estuary as well as the high precipitation input experienced in the month of June 2012. This input of fresher water dilutes the water incoming from the sea thereby lowering the salinity value of the water and combined with the warmer temperature mentioned above, the resulting water density is lower (1026.3 kgm^-³).

As mentioned before there is a more rapid increase in salinity at Station 3 at 25 m depth coinciding with the temperature inversion at this depth. This balance prevents the density of the water from changing significantly and therefore means that no overturning of the water column occurs.

There are several sharp peaks seen in the vertical salinity profiles in particular near the surface at Station 1, 3 m at Station 3 and 13 m at Station 4. These peaks are known as salinity spikes and occur in all CTD profiles as a result of the time response difference between the conductivity and temperature sensors on the CTD. They commonly occur in thermocline areas where the temperature changes are rapid and thus the faster conductivity response results in a spike in the data.

Density [Figure 4.4c]

At all four stations the density of the water increases with depth [Figure 4.4c]. This is because density is controlled by the temperature and salinity characteristics of the water. Water is denser when it is colder and more saline and from the temperature and salinity graphs the coldest and most saline water is found at the bottom of the water column and therefore this is where it has the greatest density. These density profiles show that the water column at all four stations is statically stable as the lower density water is on top of the higher density water and no overturning of the water column is occurring.

Fluorescence [Figure 4.4d]

The vertical profiles for fluorescence vary with depth [Figure 4.4d]. At Station 1 surface fluorescence values were 0.10 mgm^-3 and increased to a peak value of 0.17 mgm^-3 at 4.2 m depth. This peak occurred due to the chlorophyll pigments present in the sample fluorescing, signifying the presence of phytoplankton at this depth. This correlates to the recorded irradiance data, where the euphotic zone at this station reaches 11 m. This is the level where enough light is available for phytoplankton to photosynthesise. At Station 2 there were two fluorescence peaks of 0.2 mgm^-3 at 7 m and 0.11 mgm^-3 at 49 m. The 7 m peak was due to a chlorophyll maxima resulting from phytoplankton in the euphotic zone. The peak at 49 m could be a deep chlorophyll maximum due to the convergence of the two water masses one of which contained a high chlorophyll concentration/large phytoplankton numbers and so this downwelling water has subducted the chlorophyll/plankton also. As the light level is low, it is unlikely that the phytoplankton are able to photosynthesise at this depth. There are also fluorescence peaks at Station 3 at depths of 9 m to 11 m (0.26 mgm^-3) and at Station 4 at depths of 12 m (0.28 mgm^-3)  and 22 m (0.23 mgm^-3) respectively. The peak at 22 m at Station 4 may result from sinking phytoplankton rather than actively photosynthesising plankton due to irradiance levels of only 10 µmolm^-²s^-1 which is too low for photosynthesis. The presence of a high number of zooplankton at this depth supports the theory of a chlorophyll maxima at this depth since zooplankton feed on phytoplankton. So if a high number of zooplankton are present then logically one would assume there to be a food source present to sustain them.

The graph of chlorophyll against depth derived from acetone extraction [Figure 4.11] shows peaks of chlorophyll at a similar concentration to that derived from the fluorometer [Figure 4.4d]. At Station 1, the acetone derived chlorophyll graph has a peak at 27 m with a value of 5 µgL^-1. However the main chlorophyll fluorescence peak from the CTD at 4.2 m is not seen in the acetone extracted chlorophyll values. At Station 2 the deeper chlorophyll maxima perceived on the CTD is also seen in the acetone extracted chlorophyll data where values are 4.4 µgL^-1 thereby signifying that this is not just an unexpected result from the CTD only. Station 3 was only a CTD cast so no water bottle samples were taken and therefore there is no acetone extracted chlorophyll values for comparison at this station. At Station 4 there is a peak in chlorophyll extracted from the acetone at 13 m with a value of 5.6 µgL^-1 which once again coincides with the peak seen by the fluorometer from the CTD data. This supports the idea that there is likely to by a chlorophyll maxima at this depth.

When compared to the chlorophyll data measured by the fluorometer on the CTD the chlorophyll values extracted from acetone have higher values. The highest acetone derived chlorophyll value is 5.6 µgL^-1 whereas the highest chlorophyll concentration from the fluorometer is 0.28 mgm^-3. The reason for this difference is due to the chlorophyll extracted by the acetone measuring all the chlorophyll present in the water column at that particular depth whereas the fluorometer only measures the fluorescence by the chlorophyll that have been excited by the blue light emitted and therefore any other photosynthetic pigments present that are not excited by the blue light will not register on the fluorometer. This means that the acetone extracted chlorophyll values are more representative of the amount of chlorophyll and therefore more indicative of the concentration of phytoplankton present in the water column and each of the stations (chlorophyll concentration is regularly used as a proxy for phytoplankton concentration, but gives no indication on population size, as numbers, biomass, and size are all factors).

Irradiance [Figure 4.4e and Figure 4.4f]

For each station the irradiance profiles show an exponential decrease with depth, with rapid attenuation of light occurring at shallower depths (between 0 m and 10 m) and slower attenuation occurring below 10 m [Figure 4.4e]. Irradiance decreases exponentially as a result of the attenuation of light from both absorbance and scattering. Light is absorbed by the photosynthetic pigments of phytoplankton in the water column as well as by other particles in the water and the water molecules themselves. Light can also be scattered by particles in water particularly if high suspended particulate matter is present. Consequently, the amount of light reaching greater depths decreases due to these factors. The surface irradiance values for each station varied from 410 µmolm^-2s^-1 (at Station 1) to 1600   µmolm^-2s^-1 at Station 3. This variation is likely to be as a result of the weather conditions present (in particular the cloud cover affecting the irradiance reaching the surface waters). Other factors that affect the amount of radiation reaching the surface waters are the time of day which affects the height of zenith, zenith angle, absorption and scattering in the atmosphere. The 1% value of the surface irradiance can be used to give an estimate of the depth of the euphotic zone (and so the depth to which photosynthesis is occurring). At Station 1 the 1% irradiance value occurs at 11.5 m. At Station 3 the 1% irradiance value occurs at 8.7 m depth.

When the natural log of irradiance is taken the graphs produced closely resemble straight line relationships (particularly from Station 1). The plots for the other stations are however not perfect straight lines since other factors  affect light attenuation in the water column. The natural log of irradiance can be used to calculate the attenuation coefficient which helps to gain a quantitive value of how much light radiation is absorbed per metre. The vertical lines at the bottom of the plots are due to the fact the irradiance values remain consistently low from a depth of approximately 50 m.

ADCP

(a)   (b)    (c)    (d) 

(e)    (f)    (g)    (h) 

 Figure 4.5 - ADCP Offshore Data collected from Station 1 to Station 4 - Falmouth, UK - (a) Velocity magnitude - Station 1, (b) Average backscatter - Station 1, (c) Velocity magnitude - Station 2, (d) Average backscatter - Station 2, (e) Velocity magnitude - Station 3, (f) Average backscatter - Station 3, (g) Velocity magnitude - Station 4, (h) Average backscatter - Station 4. Sampled on 27 June 2012. All graphs can be enlarged in a new window upon clicking on the relevant image.

Richardson Number Graphs:

(a)    (b) 

Figure 4.6 - ADCP Offshore Data of Richardson Numbers (R_i) calculated over a 1 metre layer (a) Station 1 and Station 2 and (b) Station 3 and Station 4. Sampled on 27 June 2012. All graphs can be enlarged in a new window upon clicking on the relevant image.

Station 1 [Figure 4.5a and Figure 4.5b] [Figure 4.6a]

The ADCP profile of velocity magnitude shows variable velocity in flow with values from slow velocities of 0.011 ms^-1 to 0.379 ms^-1 with a greater proportion of the flow flowing at lower velocities at this station. The ship track profile shows that there is a minimum amount of shear in terms of flow in different directions but there is shear occurring due to flow at different velocities. The backscatter data from Station 1 shows very high average backscatter between 2 m and 10 m depths with values of greater than 80 db. The highest backscatter value recorded at this station was 93 db. These high values could be produced by zooplankton in the surface water column but a high backscattering signal can also be the result of turbulence in the water column.

Between 5 m and 8 m the Richardson number (R_i) is below 0.25 and so the water column at this point is unstable and there is turbulent flow. This is likely to be a result of the strong winds causing stress and waves on the surface and below the surface of the water. High water was at 10:20 UTC on this day, so the turbulent flow may also have been as a result of the ebbing tide. From 8 m to 11 m and 16 m to 19 m the Richardson number is above 0.25 and therefore flow is stable and laminar so the high backscatter values of 76 db to 78 db between 10 m and 20 m is more likely to be produced by zooplankton with some backscatter resulting from the turbulent flow at 11 m to 16 m. Except for 25 m to 27 m and 30 m to 32 m the rest of the water column exhibits turbulent flow [Figure 4.6a].

Station 2 [Figure 4.5c and Figure 4.5d] [Figure 4.6a]

The ADCP profile of velocity magnitude also shows variable velocity in flow at Station 2 with values from slow velocities of 0.005 ms^-1 to high velocities of 0.614 ms^-1 with a greater proportion of the flow flowing at lower velocities at this station. The ship track profiles show that at the shallower depths (2 m to 5 m) and the deeper depths (39 m to 48 m) of the water column there is significant shear occurring with flow travelling in different directions and at different velocities relative to one another.

The backscatter data from Station 2 also shows very high backscatter between 2 m and 10 m depths with values of greater than 80 db and high backscatter (values above 70 db) up to 20 m depths [Figure 4.5d]. The highest backscatter value recorded at this station is 106 db. Due to the waves created by the weather the very high backscatter is likely to be created by these whereas the high values lower down is more likely to be created by zooplankton. The surface waters of Station 2 have values above 0.25 with a Richardson number high of 8.5. This shows that the waters are laminar which coincides with the presence of the thermocline at the surface creating stratification and the laminar flow. Below 34 m to 44 m the Richardson number is lower than 0.25 which may be to do with the mixing occurring between the two converging water masses creating small scale turbulence. The second thermocline seen at Station 2 has formed a small laminar flow layer at 45 m where the Richardson number is above 0.25 (R_i = 1.17) [Figure 4.6a].

Station 3 [Figure 4.5e and Figure 4.5f] [Figure 4.6b]

The ADCP profile of velocity magnitude at Station 3 shows higher velocity flow than at Stations 1 and 2 with values from 0.195 ms^-1 to 0.673 ms^-1. The ship track profiles show that at the shallower depths (2 m to 10 m) of the water column there is significant shear occurring with flow travelling in different directions and at different velocities relative to one another but below 15 m the direction of flow is relatively constant (approximately 215°) [Figure 4.5e].

For a large proportion of the water column at Station 3 the backscatter is low with values of 63 db to 68 db with only a thin layer of water at the surface exhibiting large backscatter. Once again due to the weather this is likely to be the result of wind and wave mixing causing scattering of the sound emitted from the ADCP. This weather affect is seen in the Richardson number graph where the surface waters have a Richardson number below 0.25 representing turbulent flow at which point the water column has instability [Figure 4.6b]. The Richardson numbers with depth at Station 3 are below 0.25 for the majority of the water column except at 7.5 m to 13.7 m, 21 m to 27 m and 40 m to 46 m where the Richardson numbers are 2.67, 2.29 and 12.2 respectively, indicating that at these depths the flow is stable and laminar.

Station 4 [Figure 4.5g and Figure 4.5h] [Figure 4.6a]

The ADCP profile of velocity magnitude at Station 4 also shows higher velocity flow than at Station 1 and Station 2 with values from 0.087 ms^-1 to 0.946 ms^-1. The ship track profiles show that at the shallower depths (2 m to 10 m) and the deeper depths (43 m to 50 m) of the water column there is significant shear occurring with flow travelling in different directions and at different velocities relative to one another but below 12 m the direction of flow is relatively constant (approximately 235°) [Figure 4.5g].

Similar to Station 3 the average backscatter for a large proportion of the water column at Station 4 is low with values of 64 db to 70 db. There is an area at the surface with very high backscatter (115 db) which is likely caused by turbulence which is shown from the Richardson number graph with values below 0.25. The relatively high backscatter (76 db) in the top left of the backscatter contour could be related to high zooplankton numbers in this layer scattering the sound. This explanation is more likely particularly at depths of 13 m to 18 m where the Richardson number is above 0.25 and so the flow is laminar and the water column stable - thus not likely to be scattering the sound.

Dissolved Oxygen

Figure 4.7 - Offshore dissolved oxygen saturation. Sampled on 27 June 2012. Graph can be enlarged in a new window upon clicking on the relevant image.

The samples from Station 1 show percentage of dissolved oxygen (%O₂) saturation increased from a depth of 1.16 m where saturation was 100.0% to 6.30 m where saturation was at 103.6%. Station 2 and Station 4 generally showed a decrease in %O₂ saturation with depth. The decrease in %O₂ saturation at Station 2 between the depths 0.80 m and 6.98 m with 3.6% with %O₂ saturation values of 114.0% and 110.4% respectively (decreased at a rate of 0.58% m^-1 depth). There was also a 3.6% decrease between the depths 6.98 m and 26.98 m with %O₂ saturation values of 110.4% and 94.5% respectively (decreased at a rate of 0.79% m^-1 depth), which was slightly more gradual than at Station 4. At depths of 1.34m and 13.12m Station 4 showed higher %O₂ saturation values of 115.96% and 103.53% respectively (decreased at a rate 1.06% m^-1 depth). Between the depths of 13.12m and  54.99m, Station 4 showed a more gradual rate of decrease in %O₂ saturation with depth (0.32% m^-1 depth) when compared with Station 2 (0.79% m^-1 depth). Station 2 showed a slight spike in %O₂ saturation between a depth of 26.983m and 47.897m from 94.5% to 99.9% (5.37% increase) after which %O₂ saturation gradually decreased again with depth. Station 2 and Station 4 showed much higher surface %O₂ saturation (114.0% and 115.96% respectively) than that of Station 1 (100.00%).

Phosphate

Figure 4.8 - Offshore dissolved phosphate concentration. Sampled on 27 June 2012. Graph can be enlarged in a new window upon clicking on the relevant image.

During the offshore investigation into phosphate concentrations Niskin Bottles were used to collect water samples at three stations. The samples were places in a spectrophotometer to analyse, this piece of equipment had a detection limit of 0.03 µmolL^-1 for phosphate. From this data it was found that Station 1 had a high surface phosphate concentration of 0.333 μmolL^-1 which decreased to 0.169 μmolL^-1 by 6 m depth; this is the minimum which then rises again at 25 m.

These lower values correspond to the fluorescence peak observed on the CTD data which could be an indication of a chlorophyll maximum. The increased chlorophyll levels may be linked to an abundance of phytoplankton at a depth of 6 m, whereas they are not present in the same abundance at depth so phosphate concentrations increase again.

At Station 2 two thermoclines were observed along with a chlorophyll maximum at these depths. The phosphate data corresponds with this maximum, with a high value at the surface which again decreases at 6 m the same as Station 1; however the lowest value observed here is 0.075 μmolL^-1 compared with 0.169 μmolL^-1  at the previous site showing it has been removed more in this area than at Station 1 sampling site.

By 27 m this concentration has dropped to 0.157 μmolL^-1  but increases to 0.274 μmolL^-1 by 47.9 m as it is being used less at this depth compared to nearer to the surface where there is more biological activity occurring. At 60 m depth two bottle samples were taken showing both a decrease to 0.204 μmolL^-1  and an increase to 0.298 μmolL^-1  at the same site which could be due to them sitting on slightly different positions along the CTD rosette.

Station 4 also showed the thermocline was still present, here five bottles were fired on the CTD showing very low concentrations of phosphate just under the surface waters at only 0.110 μmolL^-1, and this lowered even further by 13 m. No further readings were taken until 55 m depth where the concentration was found to be 0.298 μmolL^-1 and it remained around this figure at the next sample depth of 68 m. This drop in concentration may again link to the maximum reading shown by the fluorometer during the investigation at this station.

Nitrate

Figure 4.9 - Offshore dissolved phosphate concentration. Sampled on 27 June 2012. Graph can be enlarged in a new window upon clicking on the relevant image.

The nitrate data shows a vertical profile for Station 1 and Station 2. Nitrate is an important macronutrient in phytoplankton growth; it is generally considered to be the limiting nutrient for plant growth in the sea. Nitrate is taken up by phytoplankton and reduced to ammonium so it can be incorporated into carbon skeletons. Therefore, it was hypothesised that when there was a low nitrate concentration, this may correlate with a peak in fluorescence on the CTD data as fluorescence is indicative of the presence of phytoplankton.

At Station 1, the nitrate concentration was 0.417 µmolL^-1 in the surface sample, then decreases to a level that was undetectable by the flow injection analysis, due to removal by phytoplankton. The detection limit for the equipment used was 0.1 µmolL^-1, meaning that if the concentration was  less than 0.1 µmolL^-1, then this was plotted on the graph as 0.09 to show that it was below this value. The maximum nitrate concentration was measured at 27 m, and here the fluorescence levels were low, indicating low phytoplankton presence to utilise the nitrate.

At Station 2, the nitrate levels were undetectable in approximately the top 6 m of water. This minima correlates with the fluorescence peak in the CTD data of nitrate concentration was greatest, where small peaks can be seen at approximately 4 m, 6 m and 8 m. The maximum nitrate concentration is seen when the fluorescence drops to 0.10 mgm^-3, as there are fewer phytoplankton to cause removal of nitrate from the water column.

The two main sources of nitrogen in the euphotic zone are either regenerated e.g. bacterial oxidation to nitrate by the benthic decomposition of organic matter (Kemp et al., 1988) below the thermocline replenishes surface waters with nitrate, or new nitrogen e.g. imported from the deep ocean or the atmosphere in the form of ammonium or urea.  Phytoplankton can only take up regenerated forms of nitrogen such as nitrate, which is why removal is seen in correlation with peaks in chlorophyll.

Other possible influences on nitrate levels could be from terrestrial origins that have washed into the estuary and entered the area offshore. The rainy weather conditions recently could have increased the amount of runoff occurring and therefore raised the nitrate levels. This would have particularly affected Station 1 which was located at the mouth of the estuary as this is closest to any terrestrial sources, and has a higher surface concentration of nitrate than Station 2 which was further offshore.

In reality there would be much more variation in nitrate concentration through the water column, but as the number of water samples that could be collected and processed was limited, the data only shows a snapshot of the vertical variations and not a continuous profile.

Dissolved Silicon

Figure 4.10 - Offshore dissolved silicon concentration. Sampled on 27 June 2012. Graph can be enlarged in a new window upon clicking on the relevant image.

Dissolved silicon is present in seawater within silicate, SiO₄^4-. It is utilised within the frustules of diatoms as a structural component, with diatoms accounting for 90% of suspended dissolved silicon in the world's oceans (Harper., 1975). It is also important in other siliceous phytoplankton, such as dinoflagellates. As such, dissolved silicon concentration is an indicator for phytoplankton presence, and vice versa.

At Station 1, the dissolved silicon concentration was almost constant with depth. Over the 20 m sample depth range, there was a decrease of only 0.1 µmolL¯¹, from 1.1 µmolL¯¹ at 1.2 m to 1.0 µmolL¯¹ at 27.0 m. The replicate analyses of the 6.3 m had a difference of 0.1 µmolL¯¹ between them, suggesting that the method of analysis was precise. At Station 2, the dissolved silicon concentration increased with depth. There was an overall difference in dissolved silicon concentration of 1.9 µmolL¯¹ over the 60 m depth range. The highest concentration was 2.4 µmolL¯¹ at 60.4 m. The two sets of replicates analysed from water sample taken at 27.0 m and 47.9 m both had a difference of 0.1 µmolL¯¹ between them, suggesting that the method of analysis was precise.

At Station 4, the dissolved silicon concentration increased with depth. There was an overall difference in dissolved silicon concentration of 2.7 µmolL¯¹ over the 67 m depth range. The highest concentration was 3.1 µmolL¯¹ at 68.2 m.

In general, there was an exponential increase in the dissolved silicon concentration with increased depth. The nearest sample taken to the surface at each station decreased in dissolved silicon concentration as the stations moved further offshore.

Chlorophyll

Figure 4.11 - Offshore chlorophyll concentration (Niskin bottle). Sampled on 27 June 2012. Graph can be enlarged in a new window upon clicking on the relevant image.

For surface waters the highest chlorophyll concentrations are found to be at Station 2 (> 5 µgL^-1). The lowest surface chlorophyll concentrations can be found at Station 1 (Black Rock) (< 3 µgL^-1) and intermediate chlorophyll concentration values are located at Station 4 (approximately 3.6 µgL^-1). Down to a depth of 28 m chlorophyll concentration at Station 1 continues to increase but at this depth no more samples are taken and it peaks at approximately 5 µgL^-1. Chlorophyll concentration at Station 4 peaks at 15 m depth where it reaches concentrations > 5.5 µgL^-1. Further down in the water column at approximately 55 m depth the concentration had dropped to approximately 1. 5 µgL^-1 which was the joint lowest concentration measured in the offshore practical. Between 55 m and 70 m the chlorophyll concentration increased again to above 3 µgL^-1 and sampling did not continue at any greater depths. At Station 2 the highest chlorophyll concentrations are found at the surface and they generally decrease with depth until the lowest concentration is found at 60 m (approximately 1.5 µgL^-1).

Figure 4.13 - (a) CTD, (b) Water collection from a filled Niskin bottle on deck, (c) Wet chemical lab on R.V. Callista, and (d) Offshore chemical analysis. Sampled on 27 June 2012. All pictures can be enlarged in a new window upon clicking on the relevant image.

(a)    (b)    (c)    (d)    (e) 

Figure 5.3 - YSI Estuary Data collected from Station 1 to Station 3 - Falmouth, UK - (a) Temperature, (b) Salinity, (c) Chlorophyll, (d) Dissolved O₂, and (e) pH. Sampled on 30 June 2012. All graphs can be enlarged in a new window upon clicking on the relevant image.

Temperature [Figure 5.3a]

The temperature at each station decreases with increased depth. Station 1 experiences the highest temperatures overall, showing temperatures of 16.0 °C in the surface waters, and then dropping to 15.2 °C at 2 m. This station may have experienced warmer waters due to this area of the estuary being shallower than other areas. Station 2 shows the largest difference in temperature from surface waters showing temperatures of 15.7 °C to 12 m witnessing 14.0 °C. This could be caused by thermal stratification in the estuarine waters. Station 3 shows the smallest change in temperature with depth: 1°C change over 8 m and may represent the coastal waters being well mixed.

Salinity [Figure 5.3b]

The salinity profiles at all stations show a halocline present in the surface waters, about 1m down. Station 1 and 2 show a vast increase in salinity; Station 1 increases in salinity from 24 to 32 and Station 2 increases from above 29 psu to 33 psu. Station 1 shows the lowest salinity overall, this corresponds to its position in the upper part of the estuary which would experience lower salinity values due to the higher freshwater input. Station 3 also shows the higher salinity values, corresponding to its position closer to coastal waters. The lower salinity values experienced at Station 1 could also be caused by the increased freshwater input from higher levels of precipitation experienced this month.

Chlorophyll [Figure 5.3c]

Chlorophyll was also measured using a fluorometer on the YSI probe. Station 1 and Station 3 show that chlorophyll is constant throughout the water column at approximately 5 ugL^-1, from the surface waters to depth. This would suggest that the waters at these stations are well mixed throughout in the vertical. Station 2 however, shows a sharp decrease in concentration from 25 ugL^-1 in the surface waters, to under 5 ugL^-1 at depths between 1 m and 12 m. This may represent a high proportion of phytoplankton in the surface water masses above the thermocline.

Dissolved Oxygen [Figure 5.3d]

The vertical profile derived for dissolved oxygen at each station shows the variation with depth and a longitudinal change down the estuary. Station 1 shows a lower percentage of oxygen when compared to Station 2 and Station 3. In the surface waters, Station 1 shows a 97% content of dissolved O₂; where as Station 2 and Station 3 both show dissolved oxygen content of above 100%. This high percentage of O₂ in the surface waters may be caused by a high number of marine phytoplankton present, which subsequently photosynthesise in the photic zone of the surface waters producing oxygen. When a large numbers of phytoplankton are present oxygen does not leave the water quick enough and can produce a dissolved oxygen percentage higher than 100%. For all stations the percentage of dissolved oxygen decreases down the water column, this is caused by phytoplankton not being able to photosynthesise at deeper depths due to a lack of irradiance. However Station 3, situated closest to coastal waters, continuously shows a higher percentage of dissolved O₂ in total. This may be caused by a lack of zooplankton present which would utilise the O₂ in respiration.

Density

All 3 stations show an increase in density with depth. This was expected as the temperature from the surface to the bottom decreases with depth, thus increasing the density of the water. This combined with the increasing salinity with depth, forms a layer of water of greatest density at the bottom and a less dense water layer at the surface. Overall the water column at all 3 sampled estuarine stations can be described as being statically stable since the lower density water is above the lower salinity water. The lowest density calculated (at the surface of Station 1) was 1015 kgm^-³ and would have resulted from the large amount of freshwater flowing down the estuary from the River Allen and the River Kenwyn, coupled with the freshwater surface runoff due to high rainfall over the preceding weeks. The highest density recorded in the estuary was 1023.5 kgm^-³. This was calculated at Station 2 (at a depth of 12 m). The high salinity influence from the sea and the colder water at depth are explanations for this value. Station 2 shows a significant pycnocline between the surface and depth compared with the other sampled stations This is likely due to its position in the estuary where the water is influenced by both riverine and marine water. Combined inputs results in the stratified water column observed.

ADCP

Transect 1  [Figure 5.4a and Figure 5.4b]

Figure 5.4 - ADCP Estuary Data collected for Transect 1 - Falmouth, UK - (a) Velocity magnitude -Transect 1, (b) Average backscatter - Transect 1. Data collected on 30 June 2012. All graphs can be enlarged in a new window upon clicking on the relevant image.

As with the offshore data the estuarine stations also show variability in the velocity of the flow. Velocity values along Transect 1 range from 0.058 ms^-1 on the edge of the estuary to 0.497 ms^-1 in the central channel. Velocities are faster in the central channel as the water here is deeper with a maximum calculated depth from the ADCP of 7.39 m. This greater depth means that less water is in contact with the estuary bed and so is less affected by bottom friction which would slow the velocity of the water. This slowing by friction both by the bottom and the sides of the estuary result in decreased velocities on the outer estuary.

Average backscatter for this transect shows a large area within the central channel from a depth of 2 m to 6 m with a low backscatter of 68 db to70 db. This coincides with the Richardson number for depths 2 m to 3 m which was much greater than 0.25 and therefore flow is laminar and stable and so is not causing disruption to the water column and resulting in backscattering. Further evidence is from the zooplankton counts which were low for this station so low numbers of phytoplankton means there are low number of particles in the water column to scatter the sound back to the transceiver.

Transect 2 [Figure 5.5a and Figure 5.5b]

Figure 5.5 - ADCP Estuary Data collected for Transect 2 - Falmouth, UK - (a) Velocity magnitude -Transect 2, (b) Average backscatter - Transect 2. Data collected on 30 June 2012. All graphs can be enlarged in a new window upon clicking on the relevant image.

Transect 2 shows an area of lower velocity to the left side of the central channel and an area of higher velocity at the surface on the right side. The lower velocities have values of 0.005 ms^-1 to 0.256 ms^-1 and the higher velocities values of 0.286 to 0.619 ms^-1. The reason for the higher velocities is due to the incoming flood tide (CISCAG, 2011). In the Fal estuary it has been measured that the flood tide is stronger than the ebb tide. On the sampling day of the 30^th June high water was at 13:51 UTC and so when this transect was taken at 11:26 UTC the tide would have been flooding into the estuary towards high tide therefore increasing the velocity of the water.

Similar to the backscatter data collected from Transect 1, Transect 2 also has a very large area in the central channel with low backscatter values of 67 db to 70 db. It is difficult to see from the Richardson number data for this transect because it varies from below 0.25 to above 0.25 across the whole depth range at which the backscatter is low. This then means that the low backscatter is a result of low zooplankton numbers or low suspended particulate matter in the water column at this point in the estuary and so fewer particles to backscatter the sound produced by the ADCP.

Transect 3 [Figure 5.6a and Figure 5.6b]

Figure 5.6 - ADCP Estuary Data collected for Transect 3 - Falmouth, UK - (a) Velocity magnitude -Transect 3, (b) Average backscatter - Transect 3. Data collected on 30 June 2012. All graphs can be enlarged in a new window upon clicking on the relevant image.

The velocity data for Transect 3 shows variability in the velocity across the entire transect. The lowest velocity value is 0.008 ms^-1 and the highest velocity value is 0.343 ms^-1. This variability in speed will result in a large amount of shear throughout the water column, both vertically and horizontally, and is possible able to form small scale eddies in the water. This transect was taken 30 minutes before high tide and therefore the tide is likely to still be flooding in some areas leading to higher velocities. In others it may have already reached high tide and be at slack water and so the velocities are slower therefore resulting in variable velocities across the transect.

Once again backscatter data for Transect 3 is relatively low across the transect with values ranging from 63 db to 71 db. This low coincides with the Richardson number data which for depths 8 m to 9 m is greatly above 0.25 meaning the flow is laminar and stable. Similar to the other two stations zooplankton numbers collected at this point were also low so there are fewer particles in the water backscattering the sound so producing a smaller signal.

Transect 4 [Figure 5.7a and Figure 5.7b]

(a)    (b) 

Figure 5.7 - ADCP Estuary Data collected for Transect 4 - Falmouth, UK - (a) Velocity magnitude -Transect 4, (b) Average backscatter - Transect 4. Data collected on 30 June 2012. All graphs can be enlarged in a new window upon clicking on the relevant image.

From the velocity magnitude figure it can be seen that the velocity either side of the main channel is slower with velocities of 0.012 ms^-1 to 0.235 ms^-1 whereas directly in the middle of the channel there is an area of very high velocities from the surface down to 11 m depth. These high velocities range from 0.409 ms^-1 to 2.185 ms^-1 in this middle channel section.  The reason for this higher velocity is because the central channel is deeper there is a greater volume of water here which results in higher water velocities.

Average backscatter for Transect 4 is high at the surface with values of 90 db to 98 db and is low from 3 m down to the bottom of the central channel with values of 68 db to 70 db. Because we did not do a net sample for this transect we cannot see whether the backscatter signifies high zooplankton numbers so the other explanation for high backscatter at the surface is due to turbulence created by the wind and the waves resulting in the sound scattered in all directions.

Richardson Number (R_i) [Figure 5.8]

Figure 5.8 - ADCP Estuary Data with Richardson numbers (R_i) calculated for Station 1, Station 2, and Station 3. Data collected on 30 June 2012. Graph can be enlarged in a new window upon clicking on the relevant image.

All 3 stations in the estuary show variable Richardson numbers with depth, indicating that the flow changes from being laminar to turbulent, and vice versa at different depths and at different positions in the estuary. The Richardson number for Station 1 is above 0.25 at all depths, indicating a laminar flow at this station. This is supported by the low velocity magnitude seen on the ADCP data. The majority of the flow at Station 2 resulted in a Richardson number above 0.25, also signifying laminar flow. However, for the depths of 3.2 m to 3.8 m, 5.1 m to 5.8 m, 8.8 m to 9.2 m and below 11.4 m the Richardson number is below 0.25. This corresponds to turbulent flow, which is due to the incoming flood tide. This increases the velocity as the water flows from the sea and outer estuary into the upper estuary towards high water.

Between 1.3 m and 2.7 m at Station 3, the Richardson number was below 0.25, which indicates turbulent and unstable flow. Whilst at this station, surface wind rows and Langmuir cells were observed. These occurred as a result of the surface wind stresses present, creating surface shear at this atmospheric-water boundary. This observation correlates with the turbulent flow calculated by the Richardson number, as the wind stress creates turbulence within the water column. Between depths of 8.0 m and 8.7 m at Station 3, the Richardson number was greater than 0.25, thus the water flow at this location was stable and laminar. This correlates to the timing of the tidal cycle at which the station was sampled. The station was sampled 30 minutes before high water, at which point flood tidal currents were beginning to reduce. The tidal cycle becomes a slack tide, reducing the velocity of the water and therefore the turbulence experienced in the water column.

Throughout the estuary survey, temperature and salinity values were recorded for point latitudes and longitudes against a known time. Surfer graphing software has been used to map temperature and salinity contour plots, with a post map overlay created to demonstrate a change in these variables with time (labels in red). The latitude and longitude is in decimal degrees as opposed to Northings and Eastings. Times are labelled in red and in UTC.

(a)    (b) 

Figure 5.9 - Thermosalinograph estuary data collected in Falmouth, UK - (a) Temperature and time contour plot of latitude against longitude and (b) Temperature and time contour plot of latitude against longitude. Recorded on 30 June 2012. All graphs can be enlarged in a new window upon clicking on the relevant image.

The initial track taken up the estuary was followed by a slow track down, with intermittent stops for sampling against a flood tide. As can be seen from the 08:35 UTC time stamp, temperatures of 15.6 °C and salinities of 25.7 psu were recorded. Upon reaching further up the estuary (09:25 UTC), temperatures and salinities had decreased to 16.0 °C and 22.5 psu respectively. Following the track down the estuary seawards, temperatures and salinities increased as the incoming flood tide arrived. Significant changes in these variables occurred in the time period of 1 to 2 hours (12:00 to 13:30 UTC) preceding high tide at 13:51 UTC, when the tidal flow up the estuary was at its greatest.

Dissolved Oxygen

Figure 5.10 - Estuary dissolved oxygen saturation. Sampled on 30 June 2012. Graph can be enlarged in a new window upon clicking on the relevant image.

For Station 1, which is located at the beginning of estuary only has a depth of 2.9 m. Oxygen saturation increases towards bottom water column from 99.8%, which is the lowest oxygen saturation measured of water samples from all stations, to 101.8%. Samples from Station 2 show that oxygen saturation has an increase of 1.30% from surface to 5 m, and then decreases towards the bottom with a value of 1.18%. Overall, the oxygen saturation does not change greatly from the surface to the bottom. Oxygen saturation at Station 3 decreased by 6.62% from the surface to the bottom. Station 3 showed a much higher oxygen saturation than Station 1 and Station 2. The surface water sample had a dissolved oxygen percentage of 111.8%, the highest of the water samples from all the stations. Compared to the decrease through the vertical water column, the decrease of 0.37% from 2 m to 4 m is much more gradual. The overall trend of surface oxygen saturation showed an increase from the upper estuary at Station 1 to the lower Station 3, which coincides with the increase of phytoplankton abundance. The primary source of the dissolution of oxygen is the air-sea exchange with oxygen in the atmosphere. However, factors which affect the oxygen saturation level are mainly are biological processes, including photosynthesis, respiration and biochemical oxygen demand (BOD). The decrease of oxygen saturation from the surface to the bottom may be due to less surface interaction between water and air in deep sections and increased respiration from aquatic vegetation, microorganisms and algae and decreased photosynthesis. Increased phytoplankton abundance from Station 1 to Station 3 brings higher phytoplankton primary production, which causes the increase of oxygen saturation. Station 3 is much higher than Station 1 and Station 2. This may be due to the phytoplankton community at Station 3 being in the period of growth and reproduction, and the phytoplankton community at Station 1 and Station 2 being in the period of decay, which has a higher BOD.

Phosphate [Figure 5.11a and Figure 5.11b]

Figure 5.11 - (a) Estuary dissolved phosphate concentration and (b) Phosphate estuary mixing diagram. Sampled on 30 June 2012. All graphs can be enlarged in a new window upon clicking on the relevant image.

Station 2 shows the opposite to Station 1, with a decrease in concentration with depth from 0.371 µmolL^-1 at the surface to 0.217 µmolL^-1 at 4 m, and then to 0.189 µmolL^-1 at 6 m. From the processed chlorophyll data it can be seen that, at 4 m depth, a value of 2.0 µgL^-1 was recorded, so there is low abundance of phytoplankton at this depth. The surface waters registered a higher value of 3.3 µgL^-1, the opposite to phosphate concentration which was higher at the surface than at depth.

At Station 3, phosphate shows almost homogenous behaviour with a surface concentration of 0.189 µmolL^-1, which remains fairly constant throughout the data series. This low concentration could be linked to a higher number of phytoplankton present in the estuary at this station; however, from the processed chlorophyll data it is shown that the values do not exceed 3.3 µgL^-1. This shows that there was a low phytoplankton abundance, so the phosphate concentration was not affected.

As well as phosphate concentration being affected by the abundance of phytoplankton, sediments can play a role in controlling the amount of phosphate present in estuaries (Sundby, 1992) via the decomposition or dissolving of organic/inorganic material falling to the sediment surface. Although the main controlling factor of phosphate concentration in the estuary will be phytoplankton abundance, there are other factors that also have to be taken into account when studying data sets. From studying the phosphate concentration alongside the chlorophyll data, however, there does not appear to be much correlation between the amount of phosphate and the abundance of phytoplankton.

Phosphate - Estuarine Mixing Diagram [Figure 5.11b]

When plotting the estuarine mixing diagram for phosphate concentration against salinity in the Fal Estuary, a riverine end member and marine end member (Latitude: 50’ 12.553 N. Longitude: 005’ 01.673 W) were used to create the Theoretical Dilution Line (TDL). From this line, it is evident that the phosphate is acting non-conservatively and has many data points positioned well above the TDL, meaning excess phosphate is being added into the estuary and there are processes other than dilution occurring.

Nitrate [Figure 5.12a and Figure 5.12b]

Figure 5.12 - (a) Estuary nitrate concentration and (b) Nitrate estuary mixing diagram. Sampled on 30 June 2012. All graphs can be enlarged in a new window upon clicking on the relevant image.

The estuarine mixing diagram plots dissolved nitrate against salinity in the estuary to determine whether nitrate in the estuary is mixing conservatively or non-conservatively (Loder & Reichard, 1981). The theoretical dilution line (TDL) is created using the riverine end member (salinity = 0 psu, nitrate concentration = 36.2  µmolL^-1) and marine end member (salinity = 33.1 psu, nitrate concentration = 0.848 µmolL^-1). If the data points plot on the TDL it is said that the nitrate is mixing conservatively and if it plots off the TDL then mixing is non-conservative and addition (above the line) or removal (below the line) has taken place.

The graph [Figure 5.12b] indicates that, through the majority of the estuary, the nitrate was mixing conservatively, with nitrate concentration decreasing as salinity increases.  Three points deviated sufficiently from the TDL to indicate there may be some non-conservative behaviour. This could have been caused by several water masses with differing nitrate concentrations or an external source (Loder & Reichard, 1981), which would cause addition of nitrate e.g. introduced through river water via surface run off. Higher precipitation levels usually cause a higher nutrient input. The concentration of nitrate in one of the river end member samples taken at the surface and at 1 m at Station 1 (the furthest point reached up the estuary) was below the TDL enough to indicate removal. The cause of this removal is likely to be phytoplankton, which incorporates it into their skeleton through biological fixation.

Figure 5.13 - Estuary nitrate and chlorophyll concentration. Sampled on 30 June 2012. All graphs can be enlarged in a new window upon clicking on the relevant image.

The vertical profile [Figure 5.13] compares the nitrate concentration (µmolL^-1) and chlorophyll concentration (µgL^-1) changes with depth at each station. The samples from Station 1, located furthest upstream with the lowest salinity values, contained the highest nitrate and chlorophyll concentrations overall.  The lowest nitrate concentrations were found at 1 m, which coincides with the peak in chlorophyll concentration of 5.59 µgL^-1, also at 1 m. As chlorophyll is a proxy for phytoplankton presence, this indicates that the phytoplankton are removing the nitrate from the water column. A similar pattern can be seen at Station 3, although it is not as pronounced. At Station 2, however, the chlorophyll concentration profile in the top 4 m of water matches the nitrate profile, indicating that there is an alternative factor that is limiting phytoplankton growth.

The concentration of nitrate is not affected by depth, but is affected by the presence of chlorophyll. The overall nitrate concentration decreases at each station, as the salinity increases from Station 1 to Station 3.

Dissolved Silicon [Figure 5.14a and Figure 5.14b]

(a)    (b) 

Figure 5.14 - (a) Estuary dissolved silicon concentration and (b) dissolved silicon estuary mixing diagram. Sampled on 30 June 2012. All graphs can be enlarged in a new window upon clicking on the relevant image.

Calibration Curve

All data points of the calibration curve plot show a very strong linear relationship with an R² value of 0.9996.

Theoretical Dilution Line (TDL)

Generally, the concentration of dissolved silicon decreased from the riverine end member to the sea end member as salinity increased. The difference between the two end members was 92.37 µmolL^-1. The dissolved silicon concentrations conformed well with the theoretical dilution line, although further up the estuary the concentration was slightly lower by 7.7 µmolL^-1. The dissolved silicon in the estuary varied fairly proportionally with salinity, demonstrating conservative behaviour.

Depth Profile

At Station 1 (furthest up the estuary) and Station 2, dissolved silicon concentrations decreased rapidly with depth between 0 m and 4 m. Between the depths of 4 m and 6 m, Station 2 showed a more gradual decrease and at 6 m dissolved silicon became uniform with increasing depth. At Station 3 (furthest down the estuary) dissolved silicon varied very little with depth. There was a slight increase in concentration of  0.9 µmolL^-1 between the depths of 2 m and 4 m, beyond which there was a slight decrease. Dissolved silicon was much higher at the surface at Station 1, with a value of 28.80 µmolL^-1. Although dissolved silicon concentrations at Station 1 were higher at each depth, Station 1 showed the greatest decrease with depth, with the lowest value of 15.94 µmolL^-1 at 5 m. Station 2 had a lower surface concentration (13.81 µmolL^-1) than Station 1 but showed a less drastic decrease in concentration with depth (over the same depth range) of 9.68 µmolL^-1. Station 3 had the lowest surface concentration of dissolved silicon (6.50 µmolL^-1). However the dissolved silicon concentration was slightly higher at Station 1 (6.91 µmolL^-1) than Station 2 (4.76 µmolL^-1) at a depth of 6 m and remains higher at depth beyond 6 m.

Chlorophyll [Figure 5.15]

Figure 5.15 - Estuary chlorophyll concentration. Sampled on 30 June 2012. All graphs can be enlarged in a new window upon clicking on the relevant image.

For the surface water, chemical chlorophyll analysis conducted from water samples in the lab shows a trend from highest chlorophyll concentrations (> 5.5 µgL^-1) in the uppermost estuary at Station 1, to  lower concentration at Station 2 (< 3.5 µgL^-1) [Figure 5.13]). The concentrations continue to decrease to a minimum at Station 3 (< 3 µgL^-1). Overall, Station 1 has higher chlorophyll concentrations than other stations from the surface to the depth of 2.5 m. Station 2 shows a clear decrease of chlorophyll concentration from the surface to the bottom (< 1.5 µgL^-1). Station 3 has lower concentrations than Station 2 in the upper water column (0 m to 2.4 m) and higher concentrations than Station 2 in the lower water column (2.4 m to 8 m). All samples are taken from low tide at the top of the estuary. 

(a)    (b)    (c) 

Figure 5.16 - (a) Estuary R.V. Conway, (b) Chemical sampling aboard R.V. Conway and (c) Chemical sampling aboard R.V. Conway. Sampled on 30 June 2012. All pictures can be enlarged in a new window upon clicking on the relevant image.

Geophysics using vessel Xplorer

The geophysics vessel Xplorer was used for this practical.

Tow Fish Sidescan

GeoAcoustics limited (159D) 100 KHz model. Sends beams of sonar  out and the backscatter forms an image of the sea bed showing any present bedforms or artefacts. The returning data is printed off and the colour of the image shows whether there is coarse or fine sediment in a particular area being surveyed.

Underwater Video Camera (Drop Camera)

(a)    (b) 

Figure 7.6 - Geophysics equipment (a) Sidescan tow fish and (b) Drop camera.