Total Duration 8 minutes 14 seconds

03:35 - Descent into water column, particulate matter visible

03:49 - Estuary bed becomes visible

03:55 - Video transect begins. Bed is sandy, no visible benthic organisms apart from few pieces (less than 1 meter length) of detritus. No noticable bedforms

04:07 - Isolated non-anchored kelp fronds, scale ~1 meter. Isolated shell fragments, small stones.

04:39 - Large kelp frond, isolated, non-anchored

04:48 - Detritus becomes more numerous. Pieces on scale between ~10cm to ~1 m. Small shell fragments become more numerous

05:11 - Touchdown on estuary bed for several seconds. Range of sediment size, estimated range  ~2mm - ~1cm. Well sorted.

05:15 - Motile marine organism sighted on estuary bed buried in sand (centre bottom on image, pointing up image). Scale ~20 cm. Well camouflaged, fast, suspected to be fish/shrimp. Unidentifiable due to video and organism speed (fig.15)

05:20 - Kelp fronds become more numerous, varying sizes. Detritus becomes more numerous

05:29 - Ripple bedforms identified (fig.16)

05:40 - Detritus concentration decreases, bed becomes relatively clear

05:56 - Crab, species unknown (fig.17)

06:52 - Detritus concentration increases. Shell fragments and sediment size range increases. Brittle star identified, species unknown. Pale colouration, 5 arms (fig.18)

07:14 - Dense amounts of detritus, large range of sediment size, shell fragments, bivalves (fig.19)

07:31 - Grab Site 1 hole (fig.20)

07:44 - Ascent

RV Bill Conway

• Phytoplankton net

• Secchi disk

• ADCP

• CTD rosette system with 4 Niskin bottles

• Chemical analysis

• Underwater pump system

• Microscopes (Lab)

• Spectrophotometer (Lab)

• Titration Detector (Lab)

• Fluorometer (Lab)

• Date: 29/06/12

• Weather: sunny with cloud, moderate wind

• Air temperature: 14°C

• Wave height:  upper estuary 5cm, lower estuary 100 cm

• Cloud cover: 7/8

• Tides: High water 12:40 GMT, 4.4m

• Sea surface: moderate

Methodology

It was decided in the briefing to produce 4 transect profiles at 4 predetermined points situated from the top of the estuary at Malpus Reach down to the bottom of the estuary at Black Rock. At each transect an ADCP cross section was to be carried out, giving an indication of changing flow patterns, and a CTD profile showing the changes in chlorophyll, turbidity, salinity and temperature. Each CTD transect undertaken would collect samples via the Niskin bottles at 4 different depths for further chemical analysis, including observing silicon, nitrate, phosphate, chlorophyll and oxygen concentration changes.  Deploying a secchi disk at each of the 4 locations would give an idea of the changing light attenuation in the estuary, and 2 plankton trawls would be undertaken - one at the top of the estuary and one at the bottom, to try and identify different plankton species in different areas of the estuary. Between each transect, 3 surface water samples were taken from the underwater pump system to give continuous surface data for silicon, nitrate and phosphate throughout the estuary.

The RV Bill Conway departed at 08:00 GMT and headed straight up to the first sampling station, located at Malpus Reach which was as far up the estuary as  safely possible in order to attain the riverine end member to produce an estuarine mixing diagram . At this station a CTD was used at 4 sampling depths of  6m, 4m, 2m and 0m and the samples then prepared for chemical analysis. A plankton net was also used at this station and towed behind the vessel just below the water surface for 5 minutes in order to collect samples for later analysis in the lab. The diameter, length and mesh size of the net were recorded along with the initial and final flow meter readings recorded in order to later calculate the volume of water that passed through the net. A secchi disk reading was also taken and the distance from the disk to surface of the water marked and then measure on deck using a tape measure. A further two repeats were then taken to ensure accuracy. Finally the last procedure carried out was an ADCP transect, which was taken in a straight line across the estuary, with the start and end locations and times recorded.

Station 2 was located between Turnaware and Pill Point. On the way to this station 3 surface samples were taken from the underwater pump system at intervals of equal distances between the two stations. The procedures at station 2 were the same as those carried out at station 1, however as this station was an area of particular interest two CTD profiles were taken. As the CTD system that we were using on the Conway only had  4 Niskin bottles, two CTD’s were carried out in the same area, the first sampling the depths 14m, 12m, 10m and 8m and the second sampling the depths 6m, 4m, 2m and 0m.

Station 3 was located between Penarrow Point and Carclase Point. While underway between Station 2 and 3, more surface samples from the underwater pump were taken, however this time only 2 were taken. Due to the adverse weather conditions at station 3, sampling as this station and station 4 was not deemed to be able to be carried out safely and therefore the stations were relocated to further up the estuary. At the new Station 4 the CTD was launched and sampling was carried at the depths 18m, 12m, 6m and 0m. The data for the first 3 samples were collected successfully however on the way to collecting the final surface sample the CTD became caught underneath the vessel. At 12.03 GMT the signal to the CTD was lost. Luckily it was recovered by the crew, however the powerline had been severed and, out of the four niskin bottles attached, two were lost and the other two were broken. This meant that the CTD profile was unable to be completed and no water samples could be collected for this station. However the rest of the procedures were still carried out and so an ADCP transect, phytoplankton trawl and Secchi disk reading were all taken.

Station 1

Station 1 had a deepest point of 6.89m. The transect measured 151.50m in length but as seen in the ship track the boat deviated by about 22m from the originally planned, straight transect. We cannot directly link the data collected from our CTD profile at station 1 to the ADCP transect as the profile was taken about 60m away from transect. We can use it to help assume some parameters in the water column such as chlorophyll concentrations but we cannot rely wholly on the data to explain the patterns seen in the ADCP cross section.

The backscatter plot shows large amounts of backscatter near the surface, between 83 and 92dB. The most likely cause of this backscatter is phytoplankton within the surface waters, although some backscatter may be caused by turbulence from passing boats. It is unlikely that the vessel performing the ADCP transect caused turbulence as it was traveling at low speed and the ADCP device is mounted at about 1.5m into the water column below the vessel.  CTD chlorophyll data shows levels of chlorophyll between 5 and 6μgl^-1 in the surface waters, this confirms that phytoplankton causing the backscatter is an appropriate conclusion. Another area of high backscatter is 5.2m in from the right hand bank, where there is a relatively large disturbance on the bed. The disturbance measures around 0.5m in height and 2m in length, this may be an anthropogenic object related to anchoring on the moored jetty, this can be seen in the Google map image of station 1. This disturbance causes backscatter of up to 100dB. The backscatter shown represents eddy formation, backward flow and turbulence caused by the friction between the flow and the anthropogenic object. The rest of the transect shows low backscatter with some higher patches in the water column to the right hand side. These patches may be due to increased turbidity or higher plankton concentrations.

The velocity cross section shows that the flow in the river is highly variable throughout the water column. The velocities vary from 0.013ms^-1 to 1.091ms^-1. There was high winds throughout the night before and up to when our cross section was taken , this may have caused mixing throughout the water column creating turbulence that could cause the variety of velocity that we can see in the cross section.

The stick ship track shows a general flow upstream throughout the water column. There is an area near the left bank that has a small flow downstream which may be caused by shear with the bank. The highest magnitudes of flow are just under the surface layer at about 1.5m, although the magnitudes throughout the water column are similar. There is a reduction of velocity at the bed due the frictional forces with the sediment on the bed.

Station 2

The average flow of water from looking at the ship track can be seen to be moving upstream. This is due to the tide coming in, as high water was at 1140GMT. The average flow is 0.124m/s. There is obvious eddying on the West side of the transect where there is no uniform flow direction.

The velocity magnitude contour clearly shows differences in velocity throughout the transect. Between 10-15m depth, in the main channel, there is a clear increase in velocity. This may be due to the tidal flow being stronger than the riverine flow. The tidal flow is found in the bottom layer of the estuary as it is denser than the overlying river water. The West side of the transect has a smaller velocity magnitude than the rest of the transect. A reason for the reduction in velocity may be the two tributaries coming down the estuary converging near this location, and cancelling out the tidal flow.

The average backscatter contour shows high backscatter at the surface. This is highest on the East side of the transect where there is shallower water (5m) compared with the main channel (16m) where the backscatter is relatively low.  There may be more turbid water in the shallower regions due to friction which causes bubbles that affect the backscatter reading. From the CTD and bottle data of chlorophyll and fluorometer measurements we can see there is a maximum around 2m which corresponds to the higher backscatter at the surface, suggesting zooplankton is the cause, although chlorophyll doesn’t directly indicate the presence of zooplankton it does indicate the presence of phytoplankton; zooplanktons main food source. At mid depth in the main channel we can see least amount of backscatter but increases near the bed, which again could be cause from suspended particulate matter increased due to frictional flow.

Station 3

This transect was taken not long after high water. This is apparent from the ship track data where there is no uniform direction of flow. The flow on the West side of the transect shows the water is beginning to flow out of the estuary, whereas the East side is much more sporadic, suggesting high amounts of shear. In the middle of the transect there is a dramatic change in flow, it is unidirectional towards the East and of a strong magnitude. This can be explained by the input of the Restronguet creek tributary into the Fal estuary. By looking at a Google map plot of our transect it can be seen that we cross the path of the outflowing water.

The velocity magnitude is uniform varying between 0.10m/s and 0.25m/s. The area in the centre of the transect had bad values as the water was deeper than 20m (greater than the ADCP’s capacity).

The average backscatter map of our transect, shows the levels of zooplankton in the water column. The backscatter is generally high across the surface of the transect but is at its highest on the western side of the estuary.

Station 4 

This transect was taken at high water, but the average flow of water along this transect is upstream. As the location of the transect is close to the mouth of the estuary the flow is clearly dominated by tidal influence. The average flow speed at this location is 0.536m/s. At 1080.1mE, -17.5mN along the transect there is a different flow pattern. The flow changes from a bearing between 25° and 140° to 348°. From the Google Earth image we can see there is a tributary entering the estuary close to this location, Mylor Creek. The flow from this tributary could be affecting the transect and would explain the drastic change in flow.

The velocity magnitude profile shows a uniform flow throughout the cross-section, not ranging more than 0.1m/s to 0.25m/s. There was a large section of the transect with no data as the depth went below 20m (greater than the ADCP capacity).

Across this cross-section there is a high backscatter at the surface to a depth of around 2.5m. From the trawl data there are high amounts of zooplankton and phytoplankton which would explain the high backscatter.

Figure 25 shows the CTD data from transect 1 including temperature, salinity, turbidity and fluorescence. There appears to be much fluctuation in each of the profiles.

There is large amounts of fluctuation of temperature within the top surface 2 meters of between 15.4⁰C and 15.8 °C, this may be due to weather inputs as there was moderate wind that day; this may have caused mixing in the top layer of the water column affecting the possibly usual stratified structure of the surface layer. The temperature then steadily decreases from around 15.4 at 2 metres to around 14.3 at the sea bed. There is evidence of a small potential thermocline at just above 2 metres where there is a sharp decrease in temperature from around 15.8°c to 15.0°c.

Salinity shows a similar band of fluctuation within the top surface 2 meters varying between 28 and 30. It then steadily increases throughout the water column reaching a maximum of just over 34 at the sea bed. Again this fluctuation may be due to the continuous mixing caused by the wind action.

Turbidity appears to remain fairly constant and relatively high within the top surface 2 metres remaining around 4 NTU. There is then a sharp decrease in turbidity between 2 and 3 metres to between 0.1 and 0.5 NTU, indicating a much clearer and less turbid area within the water column. Turbidity then increases between 3 and 4 metres and then remains fairly constant at a value of around 4 NTU again from 4 metres to the sea floor. The relatively high turbidity in the surface waters may be due to the wind action mixing the upper surface layer.

Fluorescence is the most fluctuating variable in transect 1. It was one of two means used to calculate the concentration of chlorophyll in the water column although is considered less accurate than the other method used in this study of the estuary (acetone extracted chlorophyll samples) often because a fluorometer can be incorrectly calibrated; this may explain why the profile appears to be so erratic. There is a small amount of fluctuation within the surface two layers around 0. Fluorescence then appears to remain fairly constant between 2 and 3 metres at around 0.2 volts and then there is a sharp increase in fluorescence at around 3 metre to around 0.6 volts. This may suggest there is a bloom of phytoplankton in this area at this particular depth. After this sharp increase, fluorescence then returns to a constant value of around 0.2 volts between 4 and 6 metres. Just below 6 metres, there is a very sharp increase to just under 1.0 volts again suggesting the presence of a large phytoplankton bloom.

Figure 26 shows the CTD data from transect 2 including temperature, salinity, turbidity and fluorescence. There appears to be much fluctuation in each of the profiles as in transect 1 however, the patterns within in the profile for transect 2 seem to be slightly more well defined with fluorescence being an exception.

There is much less fluctuation in temperature of the surface layer of transect 2. The general profile shows a steady decrease in temperature from just under 14.6°c at the surface to around 13.2°c at the sea bed. There is another small patch of fluctuation between 13 and 14 metres and there is evidence of a  very small potential thermocline at around 7 metres depth where there is a small but sharp decrease in temperature from 14.3°c to just over 14.0°c.

Salinity appears to fluctuate in the top surface 2 metres between around 34.75 and 34.20. There is then a steady increase in salinity from 2 metres to 10 metres. At 10-11 metres, there appears to be some more fluctuation between 35.5 and 36.0. After this patch, salinity appears to steadily increase to just over 36.5 from around 12 metres to the sea bed.

Comparing the temperature and salinity depth profiles of transects 1 and 2 may suggest that the water column in transect 2 is perhaps more stable although not particularly well mixed whereas the water column of transect 1 appears less stable although more well mixed in the lower layers.

Turbidity remains relatively high and constant at around 4.2 NTU throughout most of the water column from the surface to around 10 metres. There are then a few sharp decreases in turbidity, the first at around 10.5 metres to 3.5 NTU from just over 4 NTU. The second is then at around 11 metres to just over 1 NTU, indicating a there are two very thin layers of the water column that are very clear and of low turbidity. After these two spikes, the turbidity then increases to a constant and relatively high value of around 4.5 NTU between 11 and 15 metres.

Fluorescence appears the most varied profile in the transect and is considerably more varied than that of transect 1. There is large amounts of variation in fluorescence throughout the entire water column; values generally vary between 0.2 and 0.6 volts although there is a significant peak at around 9 metres of just under 1.6 volts. This would suggest a large bloom of phytoplankton is present at this depth in the water column. The large amounts of fluctuation may be due to other smaller blooms of phytoplankton in the water column causing some peaks or the fluorometer may have been poorly calibrated causing the erratic profile of fluorescence in the water column. This is why another method of chlorophyll measurement was used to provide more accurate measurements for concentration of chlorophyll in the water column.

A measure of the length of time a water molecule spends in a reservoir. It is expressed as a ratio of the amount of the reservoir over the total outflow of the estuary, assuming that the estuary is in a steady state. It is of paramount importance when estimating the time a pollutant or contaminant will spend in an estuary following an accident.

During our investigation we monitored salinity values throughout the whole length of the estuary except from the lower estuary (no data from Black Rock, data were obtained from the Offshore survey of Group4/29.06.12/ 50^o08’.876 N 005^o 01^’.595 W). The salinity value for S_sea was 35.0 (Group4/29.06.12). Due to the adverse weather conditions we only managed to reach until Penarrow Point (50^o 10’.387 N 005^o 01’.992 W) and undertake our last CTD measurements. From the CTD data recorded during the estuarine investigation the S_mean was taken as an average of all the salinity values measured at station 1, 2a and 2b and was found to be 33.98.

The river discharge of the Fal estuary was obtained as an average of all the values from 1978-2010 (station 48003/ Fal at Tregony/National River Flow Archive, Centre of Ecology and Hydrology) for June and was found to be 1.04 m³ s^-1.

The total volume of the Fal estuary was calculated from the Admiralty Charts 5602.3 and 5602.5 and was found to be approximately 1.35*10^-7 m³.

Chlorophyll Concentrations

The graph shown in figure 28 the change in chlorophyll concentration with depth for both transects 1 and 2. Chlorophyll concentrations were measured by both acetone extraction of chlorophyll and also by using a fluorometer on a CTD rosette system. Figure 28 shows the data obtained from the acetone extraction sampling; which is considered a more accurate method of chlorophyll measurement. The two transects seem to show differing patterns in their depth profiles.

Transect 1 shows a steady decrease in concentration of chlorophyll as depth increases from just under 6µg/l at the surface to around 0.5µg/l at 6 metres. This would be expected as photosynthetic production would decrease with depth as light penetration decreases.

The depth profile for transect 2 shows a more varied pattern with a chlorophyll maximum of around 5µg/l occurring at around 2 metres depth. Chlorophyll concentration then decreases again between 2 and 4 metres but then there is another sharp increase in chlorophyll concentration at 6 metres of just under 5µg/l. This may be due to a temporary bloom of phytoplankton slightly deeper in the water column than the chlorophyll maximum. There is then a steady decrease in chlorophyll concentration with depth throughout the rest of the water column with a very small increase between 10 and 14 meters at 12 meters.

Fig. 29 -  Phytoplankton Transect 1	Fig. 30 - Phytoplankton Transect 2
Fig. 31 - Phytoplankton Transect 4	Fig. 32 - Zooplankton Transect 1
Fig. 33 - Zooplankton Transect 2	Fig. 34 - Zooplankton Transect 3

Phytoplankton samples taken at station 2 were most abundant with an estimated 240000 phytoplankton samples found within 1m³ however it was not the most diverse. Samples taken at station 4 showed 6 different genus compared to the 4 and 5 found at the other stations yet this had the lowest abundance of total species with 110000 per m³. At station 1 Alexandrium and Karenia were the two most dominant genus found. Karenia was also the most dominant genus at station2 along with Guinardia flaccida. At station 4 the most dominant genus was Guinardia flaccida only.

The highest abundance of zooplankton was found at station 4 with an estimated 24000 individuals per m³. This transect was also the most diverse with 9  different orders present. The least abundant and diverse samples were found at transect 1 which was further up the estuary, it was estimated with just 2600 individuals per m³ with only 4 different orders present. Station 4 had an estimated 8600 of individuals per m³ with 8 different orders found. At all stations the most dominant order was copepod. At station 4 numbers of Copepoda nauplii were also very high.

The k coefficient at station 1 was highest with an average of 1.14. This indicates a high turbidity at the top of the estuary. However, the turbidity form the tranmissometer shows contradictory results, the lowest turbidity was found at the station 1 with 3.981 NTU at surface. This may be due to human error when deploying the secchi disc or the transmissometer may not have been calibrated properly. The 1% light attenuations were lowest here at an average of just 3.14m. The average K coefficient at station 2 was 0.78 and the turbidity was 4.267 NTU on the tranmissometer. 1% light attenuation was at an average depth of 5.76 m. The lowest K coefficient was at station 4 with an average of 0.56 indictating clearer waters. This station however, had the lowest turbidity measured on the tranmissometer at 2.981 NTU. The average 1% light attenuation was deepest at 13.15m.

• Current meter

• YSI probe

• Light sensor

• 6^th July 2012

• Weather was sunny spells with heavy showers

• Air temperature- 15⁰C

• Tides HW 0655 GMT, LW 1335 GMT

• Sea surface- calm

• Longitude 50 deg.12.967N

• Latitude 5 deg.1.657W

The aim of the practical was to gather a time series during the ebbing tide on the estuary

Methodology

Measurements started at 0845GMT and were taken every 15 minutes to get a clear view of changes during the outgoing tide. A light sensor was used within air and water to calculate light attenuation. It was lowered to between 5 and 4m depth depending on the time and measurements recorded.  A YSI probe was used to measure depth, temperature, salinity, pH, chlorophyll and dissolved oxygen content. Readings were taken both going up and down. Current measurements were to be taken but we were unable to as it was faulty.

Results

The contour plot for light intensity shows high percentage light attenuation in the surface depths above 1m. After 1m depth it decreases rapidly to less than 10% light attenuation. At 0945 GMT there was a heavy rain shower, this is clearly visible at this time as the percentage light attenuation at the surface is greatly reduced to less than 20%. This is due to increased turbidity of the surface water due to rain.

Figure 2 shows a decrease in percentage light attenuation as the ebbing tide progresses to low tide.  When the water level decreased during ebbing tide the light sensor could only be deployed to 4m because the sea floor was reached. As the ebbing tides reached low water the velocity of the outgoing tides increase which may have increased turbidity of the water column as you move towards low tide. However, as the current meter was not working on the day this is not definitive.

Between 0845-1115GMT it is clear that the surface waters get gradually warmer. When the temperature was first measured at 0845GMT the surface waters were 14.6°C. By 1115GMT the water temperature had increased to 15.2°C. There is also a temperature gradient which intensifies with time between the surface waters and the bottom waters. The surface waters are warmer. At 0845GMT the surface waters are 0.2°C warmer than the water at 5m, however by 1145GMT the surface waters are 1.2°C warmer than at 5m.

The salinity at the fixed point in the estuary appears to change in layers. At the beginning of the time series the salinity throughout the water column is around 2 salinity units less than the water at the end of the time series. At 0845GMT the halocline is much more intense with a salinity difference of around 2.5 units whereas at 1145GMT the difference is around 1.5 units.

As the tidal cycle continued it appeared that the water column became more alkaline. This could also be because there was a period of rain at 1015GMT, diluting the water and therefore reducing the pH. The pH was higher and therefore more acidic in the surface layers of the water column. However, throughout the period of time of sampling the pH only varied by 0.05.

At the beginning of the times series the levels of phytoplankton (chlorophyll a) were greater at the bottom of the water column. However as the day continued and the temperature and dissolved oxygen levels began to increase, the levels of chlorophyll a began to rise as well, especially in the surface layers. This could possibly be due to migratory patterns of the phytoplankton, for example diel migrations. 

Figure 7 – The levels of dissolved oxygen increased throughout the time series. At 0845GMT the levels of dissolved oxygen were around 95%, whereas they increased to 97.5% in the surface of the water by 1115GMT. At the beginning of the time series the levels of dissolved oxygen were greater in the lower part of the water column. However, by the end of the time series the percentage of dissolved oxygen was greatest in the surface  layer  and lowest at the bottom of the water column. This is most likely due to changing of the tides aerating the water column and increased phytoplankton activity (refer to figure 4), photosynthesising and adding oxygen to the surrounding water.

• Vertical plankton net

• Secchi disk

• ADCP

• CTD

• Chemical analysis

• Date: 3/7/2012

• Weather: Light continuous rain

• Wave height: 1m to 1.5m

• Cloud cover: 8/8

• Air temperature: am-15.0 °C, pm-14.6°C

• Tides: Low water 12.10 GMT

• Wind speed: am- 19.5 knots  direction: 172°, pm- 21.2 knots direction: 169.9°

• Sea surface: big swell

Methodology

In order to achieve our aim, it was decided to choose a sheltered position and anchor there in order to carry out measurements. Although an idea of the position was discussed prior to the boat work, the final position was not determined until just before departure after discussion with the skipper. The objective was to take an ADCP transect on the way towards our position and another one on the way back in order to try and look at the flow before and after low water. Upon reaching our station a CTD profile was to be taken every half an hour in order to see changes in chlorophyll, turbidity, salinity and temperature and to take water samples from the CTD every hour for chemical analysis of silicon, nitrate, phosphate, chlorophyll and oxygen.  A secchi disk reading was also to be taken every hour to give an idea of changing light attenuation and a vertical plankton net was also to be  taken hourly at different depths depending on the backscatter.

The R.V Callista departed at 07.45 GMT, and headed towards Black Rock to perform a CTD at the location 50 °10.111 N, 005° 02.488 W where 5 bottles were taken at depths 20m, 14m, 8m, 4m and 1m. This was in order for cross calibration with the group on the R.V Conway. A secchi disk and plankton net between 20m depth and the surface were also taken at this station. We then moved on to the predetermined position chosen for our time series station, located about 1 mile of Porthouse stock at 50° 03.845 N, 005° 02.782 W. On the way to the station an ADCP transect was performed in order to look at changes in flow and the backscatter. The transect began at  08.51 GMT at 50° 09.313 N, 005° 01.919 W and ended at  09.29 GMT at position 50° 08.629 N, 005° 01.403 W.

 Upon reaching our chosen location, the Callista dropped anchor to secure itself in position. The first CTD was taken at 11.17 GMT and 5 sample bottles were taken. Chemical analysis was then carried out on each bottle and a sample for nitrate, silicon, dissolved oxygen, lupus iodine and two chlorophyll were taken. The CTD was then lowered and data recorded every half an hour at 11.17 GMT, 11.55 GMT, 12.20 GMT, 12.38 GMT, 13.19 GMT and 13.43 GMT with water bottle samples being taken hourly at 11.17 GMT where 3 bottles were taken, 12.20 GMT where 3 bottle samples were collected and at 13.19 GMT where 4 bottles were taken and chemical analysis was carried out for each.

A secchi disk reading was also taken every hour at 11.17 GMT, 12.20 GMT, 13.19 GMT and the distance from the disk to surface of the water measured using markings on the rope. A further two repeats were then taken to ensure accuracy.  A vertical plankton net was also taken hourly. The first net was taken at 11.35 GMT where two samples were taken  between depths 25m and 15m and 15m and the surface, the next at 12.34 GMT where again two samples were taken between depths 30m and 20m and 20m and the surface and finally at 13.28 where one samples was taken between 20m and the surface. The plankton net had a diameter of 60.3µm and a mesh size of 200µm.

The final CTD reading was recorded at 13.55 GMT and the anchor brought up. We began to return back to the harbour on the way taking the second ADCP transect which began at 14.18 at 50° 03.806 N and 005° 02.675 W and ended at 15.01 GMT at 50°03.806 N and 005°01.588 W.

The Station 3a time series was taken the period before during and the hour after low water low water. In the backscatter contour there is an area of error from the ADCP where there were bad values(Fig.44.). But the values stay fairly constant showing zooplankton sat in the top 10m. There is an increase with depth also through time, suggesting that the zooplankton are migrating. There are a few anomalies in the backscatter, for example 3700s into the series there is a high amount of backscatter at 20m depth. This may be due to bubbles in the water, but as nothing suggests this from the velocity magnitude contour, it could also be down to a collection of zooplankton or a shoal of fish. There is a slight decrease in velocity with time over the entire water column, but is most apparent in the top 10m of the water column from 0.350m/s to 0.120m/s.

The Station 3c time series was taken 2hours after low water. The backscatter contour indicates again that zooplankton is sat in the top 10m of the water column(Fig.46). After 2800s into the series there is an increase in backscatter between 12m and 20m. Again this may be due to migration of zooplankton. There is little change in velocity over the time period, with it mainly being 0.1m/s.

The first profile was done at 11:17, followed by 12:20 and then 13:19. Knowing that low tide was 12:10, we decided that a time series over the low tide time would provide an interesting profile.

Looking at the graph, we can see that as time advances, the depths of both the thermocline and the halocline becomes shallower, rising from approximately 15m to 10m. The thermocline becomes shallower due to solar radiation, with a difference of about 0.6^oC. This increases stratification, thus reducing the depth of the thermocline.

The graph also shows a distinct halocline with a difference of about 0.2 PSU. This is a result of both fresh water input from the rivers Fal and Helford, and from the large amount of rainfall that fell around the time of our data collection. Less dense relatively fresh water is trapped between the surface and the thermocline.

Figure 51 shows the fluorescence profiles through a time series at Station 3.

Fluorescence is a function of chlorophyll, and it is possible to determine the location of the chlorophyll maxima by using the fluorescence data.  There is a very sharp gradient that rises from 26m to 10m over the course of the time series. This is a result of the movement of the thermocline, as discussed above.

The chlorophyll maxima will follow the thermocline as the phytoplankton containing chlorophyll  cannot mix below the thermocline. Interestingly, we also see an increase in fluorescence over the time period also. This is a result of the concentration of chlorophyll increasing.

As the thermocline becomes shallower, the phytoplankton are trapped in a smaller and smaller volume, and so the concentration of chlorophyll increases, thus increasing fluorescence.

The table in figure 52 shows the Richardson Numbers for each station, where the whole water column has been treated as a single layer. These values show an increasing stability from 1117GMT to 1319GMT and then a drop in stability at 1343GMT. Low water was at 1110GMT. This meant that the tide was in flood at all the stations, which caused a layer of lower salinity water at the top of the water column, as seen in the salinity profile graph.  This lower salinity layer causes salinity and therefore density stratification in the water column with increasing stabilisation through time as the flooding layer increases.  

Station 3a has a lot of negative values due to the densities of the top layers being slightly larger than the bottom layers. Station 3a was taken about 10 minutes after low water, therefore the water column density appears homogeneous with depth due to the change in tide direction during low water. This is the reason for the potential mixing due to the denser water being above the less dense water. This homogenous water column causes very similar densities in the layers within the water column. This causes some of the Richardson Numbers to be negative as they have slightly larger densities in the top of the layer, these differences in density are insignificant as they are ±0.001kgm^-3. Station 3a has a vorticity of 3.72x10^-3s^-1 and a period of 268.74s.

Station 3c shows more stability than Station 3a. This increase in stability is due to the continuous flooding of the tides and increasing stability. This station has a vorticity of 4.50x10^-3 and a period of 221.91s.

Station 3e was taken two hours after low tide. It has the highest Richardson Number of all the stations - 8.99. This suggests the water column was very stable compared to the other stations, which is again due to the flooding tide increasing the density stratification. There is a stability peak within the layer at 27m, this is most probably due to a reduction in velocity values recorded by the ADCP.  Station 3e has a vorticity of 6.38x10^-3 and a period of 156.65s.

Chlorophyll

Our offshore study took place over a period of 3 hours covering the low water period. This gave us a detailed data set on how the water column changed with time over a period of low water.

Figure 52 displays the depth profile for chlorophyll at Black Rock. It shows both profiles for acetone extracted chlorophyll samples and fluorometer calculated concentrations. Both profiles appear to correlate well and follow a similar pattern of decreasing chlorophyll concentration with depth. The data shows a chlorophyll maximum at the surface of the water column at 1m suggesting a large population of phytoplankton occupy this particular depth. As phytoplankton are photosynthetic organisms, it would be expected that they reside in the upper most layers of the water column where light penetration is at its maximum. As the depth of the water increases, the light level decreases; therefore lowering the potential for photosynthetic output and so the concentration of chlorophyll is lower due to a lack of phytoplankton. Figure 53 shows the calibration plot for station 2. Although the two profiles seem to follow a similar pattern and the calibration plot appears to have some correlation, a Pearson Product Moment Correlation test revealed the P value to be >0.050 and so there is no significant relationship between the two profiles. Acetone extracted chlorophyll samples are considered more accurate as there can often be calibration problems with fluorometers.

Station 3a (1117GMT) shown in figure 54 represents the chlorophyll depth profile for our station approaching low water. Acetone extracted chlorophyll sampling is considered a more accurate method of sampling chlorophyll concentration and so these are the measurements used in this analysis.  There appears to be a chlorophyll maximum at 6 metres of around 3.3ug/l. This would suggest the presence of a large bloom of phytoplankton. Phytoplankton occupy shallower depths in the water column due to their photosynthetic nature. Populations of phytoplankton are rarely found in deeper waters as there is not sufficient light for photosynthesis to occur. This then explains why chlorophyll concentration then decreases as depth increases; reaching a minimum of around 0.8 ug/l at 40m. This profile is also reflected in the fluorometer profile however when looking at the calibration plot shown in figure 55, there appears to be a considerable amount of variation within the two profiles and further analysis by the use of a Pearson Product Moment Correlation test. This revealed that there was no correlation between the two profiles as P > 0.050. This may be due to poor calibration of the fluorometer which is a common problem; this is why acetone extraction is considered a more reliable and accurate method.

Station 3c (1220GMT) shown in figure 56 displays the chlorophyll depth profile just 10 minutes after low water (1210GMT). Looking at the acetone extracted chlorophyll profile, it appears that the chlorophyll maximum has increased in depth from 6m to 13m. The chlorophyll maximum concentration decreased from 3.3 ug/l to around 2.8 ug/l. This evidence suggests that the phytoplankton may be migrating within the water column with the tidal cycle as a stimulus and are possibly epipelic algae; phytoplankton that are free living within the water column and migrate with tidal cycles. The fluorometer profile appears to contradict the acetone extracted profile. The calibration plot for station 3c shown in figure 57 displays no correlation between the two profiles which is further supported by a Pearson Product Moment Correlation test. This test revealed there was no correlation between the two profiles as P > 0.050.

Station 3e (1319GMT) shown in figure 58 represents the chlorophyll depth profile approximately 1 hour and 9 minutes after low water (1210GMT).  It appears that the chlorophyll maximum has decreased in depth from 13m to 1m, remaining at a value of 2.8ug/l. This may be due to phytoplankton migration towards the surface as the tidal cycle moves from low water to high water. The fluorometer profile shows a chlorophyll maximum of similar magnitude at the same depth and appears to follow the same pattern as the acetone extracted profile just slightly more exaggerated; chlorophyll concentration decreases as depth increases after the chlorophyll maximum.  Figure 59 shows the calibration plot for this station and although the plots seem to have an obvious positive correlation, a Pearson Product Moment Correlation test reveals that P > 0.050 and so there is no significant relationship between the two profiles.

In conclusion it appears that the chlorophyll maximum seems to increase in depth whilst approaching high water and then appears to move closer to the surface after low water and towards high water. The chlorophyll concentration appears to decrease towards low water. Based on these profiles I would predict that chlorophyll concentration may begin to increase closer to high water although a full tidal cycle study would be needed to confirm this.

At depth 6m at station 3a the highest phytoplankton numbers were  found, with 200,000 per M³. The majority of the phytoplankton species were domintated by the phytoplankton Guinardia flaccidia. There was also large numbers of Rhizosolenia stolerterfothii found.

In contrast, 15m depth there was little numbers of phytoplankton found, just 10% of the numbers found 11m closer to the surface. At this depth, different phytoplankton such as Rhizosolenia imbricate are more dominant. However, Guinardia flaccidia, the more dominant species from surface waters is still present. The fact that Rhizosolenia imbricate dominate, indicates that they are more adapted to lower light levels.

At 6m during station 3c, a large amount of phytoplankton were present with 316,000 present per m³. Once again, the dominant species at this depth is Guinardia flaccida with around 2/3 of the species found being this.

Station 3c at a depth of 13m has a relatively low amount of phytoplankton present compared to the waters above. The main species that do dominate at these depths, as seen in 3a, is Rhizosolenia imbricata. Guinardia flaccidia, Rhizosolenia alata and Rhizosolenia stolerterfothii were all found in the same concentrations, half of imbricate.

Although 3e, 4m, showed the highest abundance of species per m³, it still showed a lower total population than other stations (such as 3a at 6m and 3c at 6m). The dominant species once again is Guinardia flaccidia, with most Rhizosolenia species present, apart from Rhizosolenia imbricate.

The dominant species at this depth is Guinardia flaccidia which is surprising as the dominant species found below the main phytoplankton band is Rhizosolenia in all of our other stations. Rhizosolenia alata is also present in large numbers, around 12000 per m³.