Back Row: Liam Goodes, Danny Barnes,  Krissy Reeve, Richard Hemingway, Chris Harrison, Annemarie Cubbin

Front Row: Charlotte Cooke, Francesca Crickmere, Nicola Lukes, Jo Souter

Introduction

The Fal estuary is a Ria (drowned river valley) situated on the South-West coast of England. It is an area of particular scientific and oceanographic interest having suffered heavy metal pollution due to acid mine drainage from abandoned mining sites, which have impacted the numerous shellfish beds of the area. The Fal drains a large area and draws from 6 major tributaries, and 28 smaller sources, with significant run-off from agricultural and industrial drainage. The Fal is the largest estuary in the UK, and is largely dominated by its marine end-member, and can be considered macro-tidal at Falmouth, although is classified only meso-tidal at Truro.

The aim of this field course, which took place over a 12 day period from the 3^rd July 2007 to the 14^th July 2007, is to develop an understanding of the physical, biological, chemical and sedimentary processes in the estuary and the practical aspects of fieldwork. The role of the estuary as a transition zone between marine and fluvial regimes will be studied in a chemical and biological context. The boat work took place in three stages – estuarine and offshore locations, and a geophysical survey of the bed.

All locations are quoted relative to the WGS 1984 grid and all times are GMT

Mouse over areas of interest on map

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RV Bill Conway (Equipment Profiles)

• CTD Frame with:

  • Core Temperature/Salinity/Pressure Unit

  • Transmissometer

  • Fluorometer

  • Rosette with 6 Electronically-Fired Niskin Bottles

• Acoustic Doppler Current Profiler

• Plankton Tow Net

• Materials and chemicals for onboard chemistry

• Secchi Disk

RV Ocean Adventure (Equipment Profiles)

• YSI Multi-Probe

• Secchi Disk

• 50cm Diameter Plankton Tow Net

• Hand-Held Niskin Bottle

• Materials and chemicals for onboard chemistry

Estuarine Sampling Strategy

Route Chart (Above): Blue dashed lines represent ADCP Transects, labelled in order of sampling (T1-T4). Red stars indicate CTD profiles, C1-C7 are profiles from the RV Bill Conway, R1-R5 indicate profiles from RV Ocean Adventure (RIB). The two phytoplankton tows from RV Bill Conway were taken as reverse trawls along transect lines T1 and T3.

The estuarine data was collected on Thursday 5th July 2007, between 08.30GMT and 15.17GMT. The workload was shared between the Rigid Inflatable Boat (RIB) "RV Ocean Adventure", and mono-hull vessel "RV Bill Conway".

The RIB started at Malpas (R1) and worked down to the King Harry Ferry (R5). Five stations were sampled, comprising the "upper estuary". Although this involved travelling with the tide (against the normal practice of sampling against the tide), this was necessary due to the timings and availability of boats, which meant the RIB needed to travel directly to the upper end of the sample area before the tide fell sufficiently to prevent access. At each station the RIB was moored to a pontoon for stability, which allowed a more accurate vertical profile to be collected. At each station the YSI probe was deployed to a safe depth (usually around 1m from the bottom). The Secchi disk was also deployed but forgotten at the first 2 stations. Surface samples were taken for nitrate, phosphate, silica and chlorophyll a. A Niskin bottle was deployed to sample the chlorophyll maxima (as determined by the YSI multi-probe) for dissolved oxygen samples. A zooplankton trawl was performed at station 3, towing for 5 minutes. Weather conditions throughout the trip were severely wet and windy, with 8 octants cloud cover all day..

RV Bill Conway started at the mouth of the Fal estuary, conducting an ADCP transect from Shag Rock to Pendennis Point after taking a CTD profile from the deep water channel. The vessel then travelled north, conducting CTD profiles periodically in the deep water of the main channel (Carrick Roads). Second and third ADCP transects were conducted near Mylor Harbour and Restronguet Creek respectively, before entering the narrower channel of Truro River, conducting the fourth ADCP transect north of the King Harry Ferry, near Smuggler's Cottage at 50° 13.262N; 005° 01.577W.

At CTD stations 1, 3, 4, 5, 7 water samples were taken using the Niskin bottles mounted on the CTD frame's rosette attachment. Six bottles were carried, fired electronically from deck.

Weather conditions:

• Wind speed and direction – range of 7-14.8 knots in a south to south westerly direction.

• Cloud cover – 8/8

• Precipitation – constant medium to heavy precipitation, mist

• Water conditions – wave height varied from 20cm to 0.5metres

• High tide – 0826 GMT 4.8metres at Falmouth

• Low tide – 1450 GMT

Results of Estuarine Sampling

Nutrient Analysis

By plotting the dissolved phosphate, nitrate and silica concentrations (from the samples collected on the RV Bill Conway and Ocean Adventurer) against salinity and comparing them to the theoretical dilution line (representing conservative behaviour) (Figs. 1,2 + 3 respectively), the chemical behaviour of the nutrients within the Fal estuary system can be examined.

Phosphate (Fig. 1) shows the greatest divergence from the Theoretical Dilution Line (TDL) and displays evidence of non-conservative behaviour in the form of addition at high salinities. The addition of phosphate in the Fal estuary has been mentioned in previous publications, and has been found to be particularly high in the upper estuary/Truro area in previous years reaching values of up to 1.8mg/L (Langston et al, 2003). Dissolved silica and nitrate (Figs. 2 & 3) show little divergence from the TDL and generally show conservative behaviour at high salinities.

Fig. 4 shows a general increase of all nutrient concentrations upstream and, with the exception of phosphate, the highest concentrations are at the riverine end member. Also, all three of the major nutrients show a rapid, significant increase in concentration close to the centre of the sample area, just east of Lamouth Creek. At this location, high dissolved silica and nitrate concentrations can be seen in samples from the Ocean Adventure (RIB) and high phosphate in samples from the RV Bill Conway. Also, nutrient concentration at one point in the estuary will vary with freshwater flow and salinity (which will change almost on a hourly basis with tides, etc) (Langston et al., 2003) and as only a single sample was taken at a single time at each point, results are not particularly representative of such a dynamic environment.

In Fig. 4 values of all nutrients appear considerably diluted closer to the mouth due to the dominant marine input. Low values can also coincide with and influence phytoplankton species composition. In the study of the Fal this shows a progression from diatom dominance in the lower reaches to dinoflagellates dominance in the upper reaches at Malpas (Fig. 5). Diatoms require greater quantities of dissolved silica than dinoflagellates in order to build their external frustules, and due to higher abundance, this may help contribute to the low values of dissolved silica at Weir Point (50˚ 11.395N; Figs. 4 & 5).

However, there are many factors and complex interactions which contribute to the behaviour and availability of nutrients in the estuary and influence the uptake of these nutrients by phytoplankton groups, so these types of relationships cannot be determined in detail in this study.  

The shift seen in species composition is likely to be due to a range of factors including temperature, freshwater flow and anthropogenic inputs that can considerably vary both diurnally and annually along the estuary, becoming more favourable to a particular group at different locations and times. For example, dinoflagellates such as Alexandrium tend to prefer the higher temperatures currently found in the higher reaches at Malpas during the study and this may help them to gain a competitive advantage over their diatom rivals (Langston et al, 2003).

CTD Results

These profiles were obtained by lowering the CTD to the desired depth in the water column. The CTD provides an overview of the physical structure of the water column by collating data on temperature and salinity. In addition, the CTD probe is fitted with a Fluorometer which provides a proxy for chlorophyll analysis, and Niskin bottles are attached which allow us to take samples of water at given depths, enabling nutrient analysis in the lab. Thus the physical data can be compared with this biological and chemical data.

A selection of the CTD profiles are shown in figures 6, 7 and 8. The first CTD profile, C1 (Fig. 6) was obtained at the mouth of the estuary near Black Rock. The second CTD profile, C4 (Fig. 7), was obtained at an intermediate station in the Carrick Roads channel. The third profile, C7 (Fig. 8), was obtained in the Truro River, in the narrowing interior part of the estuary. We started at the mouth of the estuary as the tide began to ebb, and moved into the estuary as far as possible in order to sample a wide range of salinities as the fresh water mixes with the marine.

The C1 profile (Fig. 6) demonstrates a homogenous salinity profile that shows no freshwater ‘spike’ at the surface. The Fal estuary is marine-dominated and the tidal intrusion into the estuary penetrates a great distance. The temperature decreases with depth, probably due to heat transfer from the atmosphere overnight. The Fluorometer reading is low, indicating a low concentration of chlorophyll, and therefore a poorly established phytoplankton community. Nutrient concentrations are low (PO₄ <0.25µM; NO₃ < 5µM and dissolved silica <6 µM) as this station is far from the main nutrient sources and sampling took place just after the high tide. Slight depletion of nitrate and phosphate is observed in the surface due to some uptake by phytoplankton.

The C4 profile (Fig. 7) was obtained further into the estuary. Here, a layer of freshwater from the river is clearly visible in the surface above the thermocline and halocline. Mixing of the two separate water masses occurs across this layer. The nutrient concentrations are significantly higher in the surface in the rich, riverine water. The Fluorometer reading is of the same order as at Black Rock; the uptake by phytoplankton is not enough to deplete the inward flux of nutrients.

At the C7 profile (Fig. 8) in the Truro river, a very steep halocline is observed with a change in salinity of about 4 over 10m. In addition, the temperature decreases with depth as expected as the warmer fresh water flows over the salty layer due to the density gradient. The chlorophyll concentration is again low despite the high availability of nutrients, particularly in the surface layer. All three major nutrients increase in concentration up the water column. This seems likely to be due to input from the more nutrient-rich freshwater than to addition from other sources in the surrounding area, as the TDLs for nitrate and dissolved silica do not show addition of nutrients. However some phosphate could be added by re-suspension of sediments in times of increased rainfall as runoff and current speeds tend to increase during wet periods such as those experienced in the weeks prior to the fieldcourse.

Acoustic Doppler Current Profiler 

Transect 1

This transect was taken 25 minutes after high water, at the beginning of the ebb tide. Fig. 9 shows a contour plot of the direction of flow of the water across the transect.  It shows water moving in a southerly direction on half of the body of water towards the west. At the centre of the transect, the flow starts to change towards the east.  On the upper part of the eastern water column, the water flows in a south easterly direction. However, from a depth of ~7m down, the flow completely reverses and flows towards the North. This can also be seen on Fig. 10.  Fig. 11 shows velocity magnitude. The water is flowing fastest (~0.3m/s) at the surface to ~8m on the western side of the transect, and decreases with depth. The slowest body of water is moving at ~0.0m/s and coincides with the location of the body of water moving northwards in Fig. 9. Therefore these two plots show the fastest moving water to the west, flowing southwards towards the sea, which coincides with the tidal movements. The magnitude of the north flowing body of water is slow as it is flowing against the tide. This could be due to the topography of the land, where the northwards flow occurs around a deep channel which could show a delay in terms of the tide conditions. It may also show the effect of the Coriolis force on the estuary in the fact that the water is flowing in a gradual anticlockwise direction. 

Fig. 12 shows a plot of backscatter in the water column.  At the surface there is a lot of backscatter due to turbulence, which means data at the surface is unreliable and can be disregarded.  From ~5m downwards, there is minimum backscatter, showing a small amount of activity at this depth.  However, from the surface down to ~5m there is a higher amount of backscatter, which means that there is increased activity in the upper part of the column.  Comparing this with the CTD profile (Station 1) that was taken at a location in the middle of the transect, the chlorophyll maximum was at ~2.5m.  Therefore this increased backscatter may represent phytoplankton in the water column.

Discussion of Estuarine Results

Conclusions from Nutrient Analyses

Possible sources of the additional phosphate found in the samples taken on the RV Bill Conway and Ocean Adventure could include point sources, such as the sewage treatment works at Malpas, the fish farm just south of King Harry Passage and the disused quarry at Cove Wood, and diffuse sources such as runoff from the surrounding farmland, discharge from vessels entering both Falmouth Docks and small quays such as Roundwood Quay at Lamouth Creek. However, from the data collected, the exact sources involved and the extent to which each source is contributing to the addition of phosphate cannot be distinguished and, therefore, definitive conclusions cannot be made.

It was noted that dissolved silica and nitrate (Fig 2 & 3) show little differentiation from the TDL and generally show conservative behaviour at high salinities with very slight indications of removal. This is unusual considering that phytoplankton populations have been found to peak in summer (Langston at al., 2003), and would be expected to be utilising (and removing) nitrate and dissolved silica in growth (Miller, 2003). It should be noted though that chlorophyll at all sites was found to be lower than expected.

One of the major set backs in the data collected is the lack of data from lower salinities. The lowest salinity sampled for all three nutrients was only s=24, and as a result there is no information about chemical behaviour of nitrate, phosphate or dissolved silica near the riverine end-member and, as a result, the majority of the estuarine mixing diagram remains largely unknown. The riverine end-member itself was collected by hand rather than from a boat due to it's prohibitive distance upstream.

The reason for the extreme values in the nutrient data shown in Fig. 5 is unclear. One possibility could be that the samples were contaminated during filtration, though both nitrate and dissolved silica samples were stored in separate bottles with separate filters being used for each. Another suggestion for the possible high values could be nearby addition by anthropogenic or natural sources into the system such as those mentioned in relation to phosphate addition in Fig. 1, with silica originating from natural sources such as weathering and nitrate and phosphate linked to anthropogenic sources such as fertilizers from agricultural runoff, the discharges of sewage from small vessels moored near Roundwood Quay and disturbance of nutrient rich benthic sediments.

In reality the potential sources, or sources of error, of the extreme values can only be speculated and there is no solid evidence in the data to favour one over another.

Conclusion from CTD Profiles

In all three profiles the Fluorometer reading varies little and is apparently independent of nutrient availability. Except for possibly phosphate and nitrate in the mouth of the estuary, no depletion is observed in the surface that could be associated with uptake by phytoplankton. This leads us to believe that the photosynthetic activity of the phytoplankton is limited by light availability. This is supported by our observations in the log book that cloud cover on this day was 8 octants throughout the whole day. Furthermore, the data from the ‘Offshore’ day presents almost universally higher Fluorometer readings and much lower nutrient concentrations. This ‘Offshore’ campaign was preceded by several days of improved weather conditions that could have permitted the phytoplankton to bloom. The average chlorophyll concentration from all observations from the ‘Estuaries’ practical was 4.3 µg/L which corresponds to a Fluorometer reading of 0.14.

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RV Callista (Equipment Profiles)

• CTD Frame with:
  • Core Temperature/Salinity/Pressure Unit
  • Transmissometer
  • Fluorometer
  • Rosette with 6 Electronically-Fired Niskin Bottles
• Acoustic Doppler Current Profiler
• Plankton Tow Net
• Materials and chemicals for onboard chemistry
• Secchi Disk

Offshore Sampling Strategy

The offshore data was collected on Monday 9th July 2007, utilising the offshore capabilities of the RV Callista. The intention was to cruise South - South -East to around 12 nautical miles offshore to the main front between deep stratified Channel waters and mixed coastal waters.

On the day it was decided that the original plan to study this frontal system was unfeasible due to an unfavourable weather forecast. To adapt to this we decided to remain inshore around the mouth of the estuary and build up a time series over half the tidal cycle, between 8.45GMT and 16.05GMT, which included high water at 11.56GMT. The aim was to take ADCP transects between Pendennis Point and Shag Rock with CTD profiles, water samples, vertical zooplankton profiles and Secchi disk drops in the deepest part of the channel at roughly 50º 08.700N 005º 01.500W. We then decided to develop the series by increasing the number of transects over a larger area. 

By the end of the day, 17 transects had been completed which formed a quadrat linking Pendennis Point, Trefusis Point, Castle Point and Shag Rock. The CTD was deployed and water samples were collected for nitrate, phosphate, silica, chlorophyll, oxygen and phytoplankton using attached Niskin bottles at roughly        50º 08.700N  005º 01.500W (Black Rock) and 50º 09.500N  005º 02.000W (Castle Point) on each circuit. We also took Secchi disk readings and conducted a vertical profile of zooplankton using a 200µm plankton net at the same positions as the CTD profiles. The depth which the net was deployed to was determined from the observed chlorophyll maxima, measured by the Fluorometer and shown on the previous CTD profile as fluorescence. The collected water and plankton samples were later analysed in the laboratory.

Results of Offshore Sampling

CTD Analysis

The profile of salinity, temperature and fluorescence  in the water column is plotted against depth (Fig. 13, 14 and 15), with separate profiles showing the progression of the halocline as the tidal cycle continues. We begin with the red line, which is the profile as determined by a CTD cast at 09.13GMT (before HT). The yellow, green, blue and purple plots show the profile at the same point at 11.03, 12.34, 14.01 and 15.28GMT respectively. High tide is at 11.56GMT so the majority of the tidal cycle is covered.

The 09.13GMT (red) profile demonstrates a slight stratification in the stable water column with slightly lower density, with slightly warmer water in the surface layer. A chlorophyll maximum is observable at 14m, around the level of the thermocline. This is in agreement with the Euphotic depth estimated from the Secchi disk data (estimate of 14.8m).

The 11.03 GMT (yellow) profile was taken shortly before high tide and the discharge from riverine input is minimal at this time. The homogeneity of the vertical profile and the high salinity are indicative of an influx of marine water. The reduction in temperature compared to the last profile is due to the colder seawater entering into the mouth of the estuary. The chlorophyll profile is similar to that of the red line, although the plankton are better distributed throughout the water column rather than having a very obvious maximum at one particular depth.

The green profile (12.34 GMT) was obtained after high tide, so the tide is ebbing at this time. The salinity in the surface ten metres decreases slightly compared to the last profile, perhaps due to the riverine water which should be beginning to penetrate the lower reaches of the estuary, although the mixing of fresh and saline water along the gradient of the estuary reduces the signal of the freshwater ‘spike’ to a fraction of a salinity unit. A corresponding increase in temperature supports this observation as freshwater is generally expected to be warmer than seawater. A notable chlorophyll maximum is present at the base of the Euphotic zone (~11-12m) again corresponding to the observation made with Secchi disk, in this case 13.5m.

The blue profile (sampled at 14.01 GMT) shows a continuation of the out flux of freshwater in the surface. An increase in the rate of out-flowing water compared to the last is to be expected because the tidal cycle is moving towards low tide. Again, this freshwater signal is accompanied by a significant increase in temperature. Due to the combined salinity and temperature difference, the water column is very stable in our sample zone at this time. The phytoplankton find themselves at the thermo/halocline with a chlorophyll maximum at the same depth (8-10m), which is supported by the Secchi Euphotic depth (10.5m).

The last profile (in purple) that we obtained was at 15.28 GMT. Once again, a spike of warm, freshwater is highly visible in the surface layer, where there is also a high concentration of chlorophyll. The high nutrient concentration in the freshwater permits a large bloom of plankton in the surface layer, at around 4 metres. However, the Euphotic depth predicted from the Secchi disk was this time at about 12m depth. An operational error could be to blame for this difference.

In addition to the points collected at Black Rock, a regular CTD cast was performed at the Narrows (roughly 50° 09.450N, 005° 02.100W) with 4 profiles. The first cast (red) was performed at 10.19 GMT. The second cast (yellow) was performed at 12.14 GMT, after HT at 11:58 GMT. The third and fourth (green and blue) casts were performed at 13.42 GMT and 15.01 GMT.  Fig. 16 shows the Salinity Time-Series, Fig. 17 shows the Temperature time-series, and Fig. 18 shows the Fluorescence time-series.

These profiles show similar patterns to those collected at Black Rock. After the high tide, the freshwater layer in the surface becomes more prominent and the minimum salinity decreases. As we were closer to the freshwater end member for this point, the overall salinity is lower in the surface layer than that which was observed at Black Rock. Once again, the freshwater layer flowing over the denser saline layer is warmer than the seawater. There is a chlorophyll maximum on the thermo/halocline in each profile, which is generally at about 5-8m depth. However, the depths of the Euphotic zone obtained from estimations from the Secchi disk overestimated the chlorophyll maximum.

Nutrient concentrations across the time series at Black Rock

Plotting surface level nutrient data against the time of sampling demonstrates no particular relation with the fresh water signal, which had been the case with the estuarine data set.  However this graph allows us to see that the nutrient concentrations are extremely low even compared to the data set obtained from the ’Estuarine’ practical. This suggests increased uptake of nutrients by phytoplankton. Comparing the two Fluorometer readings would suggest that this is the case. Bearing in mind that the two Fluorometers are different, they cannot be directly compared. However the average chlorophyll concentration of all readings taken on this practical was 6.14µg/L, compared to 4.3µg/L for the ‘Estuarine’ practical, which could explain the apparent increase in nutrient uptake by photoautotrophs. The much improved weather seems to have allowed the phytoplankton community to bloom and it is now limited by availability of nutrients, especially phosphate.

Phytoplankton Levels

Dinoflagellates numbers show very little variation with time. Looking at the raw data their numbers seem to increase with depth. This may be due to the presence of higher nutrients and lower predation and so they may migrate to the surface at night. Diatoms and ciliate numbers appear to peak shortly before high tide. This may be due to an influx with incoming coastal waters on the tide. Numbers drop once more after high tide, strengthening the suggestion that they are carried on the tide.

ADCP Transects

Fig.20. Velocity direction contour plot – This shows that the net flow of water is generally northeast, fitting with the tide flooding into the estuary due to high tide at 11.56GMT. However, nearer to the right shore there is no general direction, indicating that an eddy may be present.

Fig. 21. Velocity magnitude contour plot – This shows the velocity magnitude of the flow at transect 1.  On the western side the flow is slightly slower, by ~0.25m/s. Closer to the eastern shore the magnitude increases slightly, splitting the estuary into faster moving north flowing water on the left, with a slow moving eddy to the right.

Fig.22. Velocity direction contour plot – This shows that the main body of water towards the West is flowing in an easterly direction.  On the eastern side of the estuary the flow is in a north-east direction.  On the far eastern side, the flow is southwards. The tidal forces are at a minimum as we are just before high tide so there is no particular general direction or ‘main flow’.

Fig. 23. Average Backscatter contour plot – An ADCP works by sending out acoustic signals that reflect back to the transmitting receiver at different frequencies thus recording a time delay that is converted into velocity.  Backscatter shows the amount of “activity” in the water column that can affect the quality of the data.  At the surface, there is a high amount of backscatter which is due to surface turbulence, from wind induced waves.  There is a region of low backscatter in the channel, with fairly low backscatter in the main body of water.  In the top ~8 metres of the water column on the western side of the transect there is a high amount of backscatter which could be the location of a phytoplankton bloom.

Fig.24. Velocity direction contour plot – This shows that the net flow of water is generally south, fitting with the falling tide that occurred at 11.56GMT.   The layer of fresh water in the surface is clearly visible as being a separate water body, as shown in the CTD profiles. However, nearer to the left shore there is no general direction, indicating that an eddy may be present, although this could be due to the curve taken at the beginning of the transect.

Fig. 25. Velocity magnitude contour plot – This shows the velocity magnitude of the flow at transect 10.  The current velocity in the surface, in the fresh water layer, is much higher than the sea water intrusion. This stratification is also shown on the CTD profiles. Although some currents are shown to be present in the denser bottom layer, they are much smaller in magnitude than the surface layer.

Fig. 26. Average Backscatter contour plot – This is a measure of the amount of ‘activity’ within the water column. At the surface, there is a high amount of backscatter that is due to surface turbulence, from wind-induced waves, due to fairly heavy south westerly winds. However, the backscatter towards the left shore of the estuary is significantly higher, and could be attributed to the high level of chlorophyll present in this fresh water layer, as shown in the CTD profile.

Fig. 28. Velocity magnitude contour plot – This shows the velocity magnitude of the flow at transect 14. Again the higher current velocity in the fresher surface layer is clearly visible as this layer flows out over the bottom sea water layer due to its lower density.

Fig. 29. Average Backscatter contour plot – At the surface, there is again a high amount of backscatter that is due to surface turbulence, from wind-induced waves, due to fairly heavy southwesterly winds. There is a layer of reasonably isolated backscatter in the middle of the water column, which is probably caused by phytoplankton. This corresponds to the chlorophyll maximum at 8-10m as shown on the CTD profiles from Black Rock.

Richardson Number

The Richardson Number is a scale that describes the physical conditions in terms of mixing and stability.  If the number is less than 0.25, then the system is well mixed.  If the number is more than 1, then the system is statically stable.  A plot of the Richardson Number against depth can be compared to the CTD profiles of the corresponding locations to understand the physical behaviour of the water column.  This has been done for the time series of the transect from Pendennis Point to Shag Rock.

Transect 14 – profile 10 – 1401 GMT (125 minutes after high water at Falmouth)

There is a well mixed layer from the surface to ~4m, shown by 3 Richardson Numbers that are all less than 0.25.  There is a sharp increase in the Richardson number from 0.042 at 5.1m, to 1.21 at 7.1m.  This shows stability in the water column that can also be seen in the CTD profiles, as a halocline from 35.0 at ~4m, to 35.3 at ~7m.  The change in salinity overall is still very low.  A thermocline is also present, from 13.7^oC at ~4m, to 13.0^oC at ~9m.  However, if the whole of the water column is well mixed, this stability may be temporary, and may also show two well mixed bodies of water that are stable in comparison to each other.  This cannot be determined for sure without more practical data.  These two bodies of water allow nutrients to pass from the deep to the upper water column at ~8.44m, which is coherent with the chlorophyll maximum as shown on the Fluorescence profile.  This is where the Richardson Number decreases to 0.0001 just below the chlorophyll maximum depth. 

Discussion of Offshore Results

• The effects of the tide on the surface layer of the water column show both the marine-dominated nature of the Fal Ria and the freshwater out flow from its many tributaries.
• The change from light-limited to nutrient-limited phytoplankton in the space of a few days demonstrates the importance of these conditions to phytoplankton communities.
• The chlorophyll maximum varies in response to changing temperature and salinity conditions as the density of the water column changes.

Geophysics Data was collected on Thursday 12th July 2007 from the vessel "MV Grey Bear"

MV Grey Bear (Equipment Profiles)

• Side-Scan Sonar
• Van Veen Sediment Grab

Geophysics Sampling Strategy

Fig.31

On Thursday the 12^th of July the Geophysics boat practical took place from 08.27GMT until 12.10 GMT just outside Falmouth harbour.  Three transects were followed while towing the side scan sonar fish to investigate an area of topographical interest. The structure and biology of the surface sediments were studied using grab samples collected via a Van Veen Grab. The original plan was to perform 5 profile lines, 100m apart with a 25m overlap, and 3 grab samples. Due to time restrictions only 3 transects and 2 grab samples were achieved.  

• Line 1 started at 10.05AST and finished at 10.19AST from 50º09.594N   005º03.231W.
• Line 2 started at 10.24AST and finished at 10.42AST from 50º10.352N   005º02.028W.
• Line 3 started at 10.49AST and finished at 11.04AST from 50º09.442N   005º03.089W.
• Grab 1 was taken at 11.21GMT from 50º09.710N 005º03.009W.
• Grab 2 was taken at 11.52GMT from 50º09.523N 005º03.122W.

Towards the end of some tow lines the fish height had to be readjusted to account for sudden reduction in depth but this did not appear to significantly affect results.

Back in the lab the trace from the side scan sonar was analysed and any features and zones of differing sediment/bedforms were identified. Various measurements and calculations were used to transfer these observations onto a plot of our transects produced by Surfer. This creates a generalised map of the bathymetry of the area surveyed.

Results of Geophysics Sampling

As you move north west from the south eastern to the middle transect the channel shallows and narrows, vanishing completely from the north western transect. This indicates that we sampled over the top curving corner of the channel on its north western edge, as shown in Fig. 31.

Ripples: Several areas of well defined bedforms from the side scan sonar were observed. There are large ripples amongst the coarse grained sediment area which have a wavelength of 20m and an average height of 10cm. The ripples found in the finer grained sediment are generally more variable with heights averaging around 13cm and a wavelength of around 10m.

Anchor Trails: A large area of disturbed sediment was found across the top of our three transects as shown the Surfer map. These were identified as anchor trails, the largest of which measured 69m long and 1m deep.

An unusual sedimentary structure was also noticed in the south-west part of the transect. It appears that a spike of fine-grained sediment is overlaying or draping the ripple beds below. There is little obvious difference in height in the two layers, and some ripples can be seen through the fine layer. This could have been recently deposited in a packet of fine material falling out of suspension.

SS Stanwood: The remains from the SS Stanwood, a ship sunk in December 1939, were identified at the end of the last transect. All that remains is a debris field after the wreck was destroyed by dynamite to reduce the risk of damage to other waterborne traffic in the estuary. The two most prominent points of the wreckage on the trace are two boilers, one 3.4m high and the other 1.8m.

The side-scan showed consistency of fine sediments across the bed, with patchy areas of ripples in the south west, formed by current movement of the fine sediments. The bed was also littered with anchor drags, with a concentration in the north east.

Grabs showed a slight fining in sediments from grab position 1 to grab position 2. Grab 1 was taken within the channel and so finer sediment would have been suspended in the faster flowing waters. Biodiversity was found to be low in both sites with only 2% of maerl present being alive. This was possibly due to a lack of stable substrate. Many of the maerl beds in Falmouth are still recovering since the end of maerl dredging in the area.

The channel profiles show a shallowing and narrowing of the channel to the NE. The disintegrated steamship was found in the NE with a debris field 75m long and 15m wide, with two major structures of height, 3.4m and 3.8m which may have been the boilers. Although the wreck is a well-used dive site, it is not well mapped or photographed, other than the location as published on Admiralty charts.

Equipment Profiles

CTD rosette:

This piece of multi-purpose equipment is used to measure temperature and salinity with changing depth through the water column. It can be deployed from the RV Bill Conway and RV Callista using an A-frame and winch. The instrument is manufactured by Falmouth Scientific Instruments (I.D. code F7a) and can have a variety of other devices attached.  The equipment is mounted on a rosette frame, and can include a Fluorometer, used to measure in situ chlorophyll, and a Transmissometer, used to measure turbidity.

Niskin bottles: 

Niskin bottles can be used singularly, deployed on a line down to the desired depth or as part of the CTD rosette (e.g. image for CTD Rosette) on which twelve bottles can be mounted at a time, although only six were used for this investigation on the RV Bill Conway, and 5 bottles on RV Callista. The CTD rosette is used on the larger vessels (RV Bill Conway and RV Callista). The single Niskin bottle can be used on most boats including the smaller RIB. An electric pulse or mechanical messenger (weight) is sent down the line which causes the top and bottom lids to snap shut capturing a water sample, the six bottles on the rosette can be used to collect multiple samples from the same depth or samples from up to six different depths. These water samples can then be used for chemical analysis of oxygen and/or nutrient levels.  The manufacturer of the Niskin bottles for this investigation was General Oceanics.

Plankton net:

This apparatus is towed behind the boat during motion usually for a time period of 3-5minutes, the longer the tow is, the better the representation of zooplankton species and quantities in the sampled water. A counter can be attached which revolves as water passes through the net. The number of revolutions is then used to calculate the volume of water that has passed through the net and ultimately the number of zooplankton per unit volume. The mesh size and diameter of plankton nets can vary depending on the size of organism that needs to be filtered from the water. On this investigation, a mesh of 200 microns pore size, with a diameter of 50cm was used. The manufacturer was Duncan Associates.

YSI probe:

The YSI is a hand-held multi-parameter probe, deployed over the side of the boat by a cable which attaches to a handheld digital monitor, displaying multiple parameters in real time. These include depth, temperature, salinity, pH, turbidity, chlorophyll and oxygen concentration (%) amongst others. This is a very simple and useful piece of equipment which can be deployed over the side of the Callista, Bill Conway and RIB, but is used primarily on the RIB as it's portability makes it a good replacement for a CTD, which is too large to use on a RIB. 

Secchi disk:

A Secchi disk is a low-tech instrument which allows observers to measure light penetration through the water column without the need for computers, calculators or even batteries! Essentially, a disk painted with alternating white and black segments is lowered down by hand over the side of the vessel. When it disappears completely from sight it is raised slowly back up until it can be seen again. Distance marks on the line from which the disk is suspended allow a depth to be recorded. Multiple measurements by several operators can provide an average, as eyesight varies from individual to individual. A note is also taken of light levels which can be affected by changes in cloud cover as this will again affect the readings. The Secchi depth is generally accepted to be around one third of the Euphotic zone or "1% depth" below which photosynthesis cannot take place.

T/S probe:

This simple instrument uses conductivity to measure temperature and salinity and display it on a digital readout. It can be used independently to aid calibration and is also mounted on the hull of the Callista for continuous temperature and salinity surface profiles. This is especially useful for use in estuarine studies where varying salinity between the mouth and head can be recorded and used to determine sampling stations.

Van Veen Grab:

Side Scan Sonar:

A “torpedo-like” fish is towed behind the boat, sending out acoustic pulses and receiving the reflections off the sea bed. The sound pulses are usually on a frequency between 100 and 500 KHz. For the geophysics practical, the frequency used was 500 KHz as this gave better resolution at the towing depth we used. As the fish is towed, an image is displayed, showing areas of high returns and areas of shadow (which represent ripples, bed forms and other objects/bathymetric features on the sea floor) on a monitor and printed out on paper plot, which are saved for later analysis. The fish itself can be towed behind most sized vessels, including RIBs. However, the attendant computer equipment necessitates a larger vessel with mains power supplies, such as RV Bill Conway or larger. During this course, MV Grey Bear was used for this duty.

Materials for onboard chemistry:

To preserve collected samples until they can be returned to the lab for storage and analysis, certain procedures need to followed on the vessel. Lugols bottles are used to hold phytoplankton samples and samples for analysing silica content is stored in plastic bottles to avoid contamination by glass. Nitrate, phosphate and oxygen samples are stored in glass bottles. Before storage, the nitrate, phosphate and oxygen samples are filtered for phytoplankton and the filter paper is stored in test tubes of acetone. The acetone denatures the phytoplankton cells and releases the chlorophyll for later lab work. The oxygen samples have Winkler reagents added before storage which causes precipitation of the oxygen out of solution. This helps to preserve the concentration of oxygen present at the time of sample collection.

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2. Miller, C.B. (2003). Biological Oceanography. Blackwell Publishing. 416pp.

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