Introduction
RV Bill Conway
MV Grey Bear
Small Boats
RV Callista

 

Group 3's Webpage

Meet the Group!

From left to right: Emma Collins, Lucinda Bufton, Jonathan 'Skwirrel' Yirrell, Dominic Cooke , Mark Evans, Michael Thompson, Lydia Gibson, Beverly Oh, Emily Kearns.

RV Bill Conway
MV Grey Bear
Small Boats
RV Callista

Group

Date July 2006

 

1

2

3

4

5

6

7

8

9

12

4

Callista

Grey Bear

Bill Conway

Small Boats

 

 

 

 

 

 

5

 

 

 

 

Callista

Bill Conway

Grey Bear

Small Boats

 

 

6

 

 

 

 

 

 

 

 

Callista

Grey Bear

7

Small Boats

Bill Conway

Grey Bear

Callista

 

 

 

 

 

 

8

 

 

 

 

Small Boats

Grey Bear

Callista

Bill Conway

 

 

9

 

 

 

 

 

 

 

 

Small Boats

Bill Conway

11

Grey Bear

Callista

Small Boats

Bill Conway

 

 

 

 

 

 

12

 

 

 

 

Grey Bear

Small Boats

Bill Conway

Callista

 

 

13

 

 

 

 

 

 

 

 

Grey Bear

Callista

14

Bill Conway

Small Boats

Callista

Grey Bear

 

 

 

 

 

 

15

 

 

 

 

Bill Conway

Callista

Grey Bear

Small Boats

 

 

16

 

 

 

 

 

 

 

 

Bill Conway

Small Boats

Introduction
RV Bill Conway
MV Grey Bear
Small Boats
RV Callista

Introduction to the Fal Estuary

The Fal is a drowned river valley which formed as ice retreated from the valley at the end of the last ice age and the sea level rose. The Fal River stretches for 29km and has its source at Goss Moor near St. Dennis in mid-Cornwall. The Fal and the Helford River which runs into the estuary are protected by the JNCC (Joint Nature Conservation Committee) and is also a candidate SAC (Special Area of Conservation) site.

The Helford River is the main freshwater input to the system, other tributaries include Restronguet Creek and Carnon River. Each freshwater input brings its own characteristics to the estuary with varying physical and chemical properties.

The catchment of the Helford is comprised of Devonian carboniferous rocks to the south and west. To the North of the Helford the Devonian rocks are metamorphosed and contain small amounts of mineralisation.

The Fal Estuary has distinctly different animals living in it compared to other south west estuaries. This is due to the mining industry which first started around 600 B.C. Industry has had a large impact on the water quality of the Fal. Mine waste was a large factor in the pollution of the estuary. Acid mine drainage - water discharged from local mines which may carry metals in solution plays a large part in polluting the estuary. Arsenic, copper, tin and zinc are the main metals contaminating the Fal estuary.
TBT (tributyl tin) was used as an antifouling paint but was proved to have devastating effects on marine life resulting in the side effect of imposex in dog whelks. Despite the ban in 1987 on vessels below 25m, there are still areas which are contaminated.
Sewage is also a problem in the Fal with discharge from each of the main centers of population as well as waterside properties and watercraft discharging directly into the estuary. The main problem associated with this is bacterial and viral contamination of bivalve shellfish.
One major contamination incident occurred at Restronguet Creek in January 1992. The Wheal Jane Tin mine mainly produced cassiterite, the main ore mineral for tin. On the 4th of January there was an accidental release of 50 million litres of acidic (pH 3.1) metal laden water which may have played a part in the loss of mussel (Mytilus edulis) farms which have been decreasing recently due to toxins in the water.


Other characteristics of the area are the large Maerl beds which are rare in the UK and seagrass beds which have high biodiversity.


 

Mussel farming in the Fal estuary

Studies determining the environmental effects of mussel farming on its surrounding area are very limited however some investigations have shown that they can have a significant effect in the nutrient concentrations in the water column. Studies carried out at a local mussel farm in 1984 showed that the concentration of organic nitrogen was dramatically higher in the area of the mussel farm compared to a reference site which was not exposed to direct effects of marine farming. The sediments beneath the mussel farm showed signs of denitrification, which was 21% higher than at the reference site. The infauna that was present at the farming site consisted of mainly polycheate worms where as the reference site also contained bivalve molluscs, brittle stars and crustaceans.

Southampton University Survey

From 4th to the 17th July 2006 students from the University of Southampton carried out the first in depth oceanographic survey of the Fal Estuary in Falmouth, England. The aim of the study was to determine the variation in biological, chemical, physical and geological parameters and the interactions between these parameters within the upper and lower reaches of the Fal estuarine system and offshore from the estuary.  To accomplish this five research vessels were used, each adapted for particular sampling requirements to facilitate a survey of the entire estuarine area  from the head of the Fal to offshore in the Western English Channel.  The following results are based on the data collected by group 3. 


 

Introduction
RV Bill Conway
MV Grey Bear
Small Boats
RV Callista
Grey Bear 07/07/06
Weather Tides (GMT) Equipment Location
Temp: 19-20ºC
Humidity: 95%(am) dropped to 69%(pm)
Heavy Fog in the morning clearing to 3/8 cloud coverin afternoon
Pressure: 1019 mbar
Wind 0 ms-1 to 1.5ms-1 in the afternoon
0451 1.8m LW
1050 4.2m HW
1713 2.0m LW
2300 4.3m HW
Map Chart- Navifisher
Secchi disk
Plankton net:
Counter-Hydro bioskiel
Duncan & Associates
CTD-flurorometer,transmissometer, niskin bottle FSI (Falmouth Scientific Instruments)
ADCP-Winriver
Oxygen- MnCl2 first; alkaline iodide second
T/S probe-Profi lab
Cool box
Filter -Whatman glass microfibre filter
Transmissometer
Niskin -GO (General Oceanics),
GPS locator systems- Trimble, Simrad electronics navigation system
Winch- Spencer Carter

 

The RV Bill Conway along with the small boats has been an essential part of looking at both the estuarine chemistry, biological processes and physical mixing of the Fal Estuary this would allowing the group to ascertain the dynamic nature of the estuary.

Aims

We aimed to use the Bill Conway vessel to collect water samples for oxygen, nutrient and chlorophyll analysis from as far north as King Harry Passage to as far south as Black Rock (estuary mouth). In addition to water samples the group took CTD casts and completed 6 ADCP transects as well as 2 plankton nets at the head and the mouth of the estuary.
 

Methods

We performed cross-estuary transects using the ADCP system and took CTD casts at pre-planned locations. Water samples were also collected from pre-planned locations for later analysis of nutrients, chlorophyll and oxygen. We aimed to sample from the top of the estuary to the bottom of the estuary against the tide so that we weren’t following the same body of water all day. Samples for nutrient and chlorophyll analysis were stored in a dark, cool box to prevent any further photosynthesis and photo degradation.

Nutrients: Calibrated against standard solutions and colorimetry used to determine concentration. For nitrate flow injection colorimetry used.

Chlorophyll: Left in fridge over night and sample placed in a spectrophotometer.

Oxygen: On the boat oxygen is scrubbed from the water by adding Manganese chloride followed by alkaline iodide. In the lab 5 standard solutions of known concentration prepared from 10ml iodate, 1ml sulphuric acid and 1ml iodide. Standards and samples then titrated against starch to determine oxygen concentration (Winkler method)

Plankton: Two trawls done with 200um net for zooplankton, formalin added to prevent further grazing and preserve sample . Lugol's iodine added to 100ml filtered samples taken from niskin bottles to stain phytoplankton and preserve sample. Both phytoplankton and zooplankton samples analysed under a microscope in the lab later on.

Salinity: 1 water sample taken in marina as DAILY calibration for CTD/salinity probes. CTD used on station.

Current: ADCP used, nutrient fluxes can be determined using combination of lab nutrient data and ADCP data.
 
Results

CTD Analysis

The main thermocline for all the stations and sharpest increase in temperature occurred within the first 3m which is expected due to the upward movement of the thermocline in response to summer stratification. The largest difference in temperature, on average, was 1oC below 5m for stations 2 and 5. These two stations also showed the largest difference in salinity of just over 1 below 5m. This was expected as station 2 was our station furthest up the estuary so would have been fresher water which holds heat less efficiently than sea water which has a larger surface area. There was also the highest oxygen concentration at this station due to the lower temperature; it was the only station where oxygen increased with depth which may be due to the sun heating the top layer of water reducing the concentration of oxygen held in solution. This may also be explained by the corresponding increase in chlorophyll biomass at this site. The high fluorescence at this station can in turn be explained by a higher nutrient flux, as seen from the nutrient mixing diagrams. Within the first meter station 5, situated furthest seaward, had the sharpest increase in salinity from 34.5 to 35.3 which may be due to dilution from rainwater. Station 9 also had a similar profile to station 2 which may be due to an inflow of fresh water from Saint Just Creek. Station 11 was situated midway between the estuary mouth and the top of the estuary. This was reflected in the midway temperature and salinity profiles in contrast to the other profiles which shows a degree of freshwater and seawater mixing. There also appears to be a salt wedge of varying strengths at all stations bar 5 and 9. Fluorescence shows a sharp decrease at stations 2, 3 and 9 with depth which may be in response to high riverine sediment load which reduces light availability. The sharpest decrease in oxygen concentration at station 3, situated adjacent to the mussel farm, is most likely explained by increased heterotrophic respiration. Towards the mouth of the estuary mixing velocity decreases enabling sediment to settle decreasing turbidity. This is also seen from the shallowing euphotic zone further up the estuary towards the river, as the secchi depth decreases up the estuary. All of the oxygen saturations were above 100% therefore indicating super saturation showing that the estuary is very productive with a large nutrient supply.

Chemical Analysis

Silicate

At the head of the estuary the points fall mostly very close to the line indicating conservative behaviour of silicate with salinity.  There are a few points that are above the line at ~7 salinity which may indicate a small addition of silicate.  This sometimes occurs when there are areas of sewage effluent input as the freshwater entering the system ay have higher silicate content.  At around salinity 14 the points start to fall below the TDL, indicating non-conservative behaviour of silicate within the lower regions of the estuary.  Although we cannot say for sure, it is likely that silicate is being biologically removed by diatoms as they use dissolved silicate to form tests or frustules.  It is also worth noting that station 3 has higher silicate concentrations than expected which maybe due to the presence of a mussel farm found close to this station. 

Nitrate

In the lower reaches of the Fal, nitrate shows non-conservative behaviour as points fall below the TDL.  In the upper reaches there is possible addition of nitrate, however more sampling stations are needed as the points are scattered quite far from the line.  The reasons for addition of nitrate can be found by examining the land area around the upper regions of the estuary.  The area has a large number of farms, in particular livestock farms which create high volumes of manure, very rich in nitrate.  This is a major input of nitrate to the upper estuary through direct run-off into the Fal, Truro and Tressilian Rivers and via leeching through soil.  Also, upon examination of phytoplankton samples, lower numbers of phytoplankton were counted in the upper estuary (most probably due to the fact that most species are marine and cannot cope with freshwater conditions).  This is an indication that phytoplankton are utilising nitrate in the lower regions of the Fal, therefore showing removal of nitrate on the mixing diagram.  Another reason for lower nitrate concentrations may be due to the increasing distance from sources of nitrates. 

Phosphate

Analysis of the phosphate estuarine mixing diagram shows non-conservative behaviour with removal of phosphate from the water column.  There is a large proportion of uptake in the lower salinity region, although the actual concentration being removed is small in comparison to nitrate, showing agreement with the Redfield Ratio.  Inputs of phosphate originate from sewage, agriculture and urban run-off from the extensive farmland and towns around the Fal estuary.  Phytoplankton

Phytoplankton

Growth of phytoplankton is affected by the available nutrients in the water column.  This availability varies throughout the estuary depending on inputs and uptake by phytoplankton.  At mid-salinities there is the highest proportion of removal of phosphate and nitrate, which corresponds with the high chlorophyll concentration at around 15-20 salinity.  Phytoplankton abundance is also high at low salinities, near the mouth of the estuary and ciliates and dinoflagellates were found only at this point.  At all other stations where samples were taken, diatoms are the dominant species.  It is important to note that the phytoplankton samples were taken over a small salinity range, with station 6, the most northerly station, having salinity 31.39 and station 5/6 having salinity 35.4.  The reasons for the low phytoplankton numbers at station 5/6 may be due to the tidal state which was on an ebb tide.  The ADCP data shows a reasonably strong flow out of the estuary which may have given the low phytoplankton numbers and corresponding low chlorophyll concentration.  This is an anomaly however, and cannot only be explained by the tidal flow.  It is possible that due to the nature of patchiness of phytoplankton that the sample was taken from an area of low biological importance. 

Zooplankton

Net 2 and net S1 were collected in the King Harry Ferry area and show similar structure of zooplankton with the main difference being the large number of eggs that were found.  The reason for the high abundance of eggs in this trawl may be due to a number of reasons.  Firstly the mussel farm in the area may act as a source of eggs which have a degree of buoyancy and therefore enter the plankton net.  Secondly the estuary was being sampled at spring tides which is often the period when fish choose to release their eggs as it gives the best chance eggs reaching the lower estuary.  Net 1 was collected from around Malpas point at salinity 32.7 and was the most northern trawl.  Estuarine species must be physiologically flexible and pressures in the upper estuary are often greatest as these areas experience large salinity fluxes.   Therefore as expected, zooplankton numbers and diversity are very low as most species cannot adapt to the stressful environment.  In the southern most station diversity is high and the greatest range of species can be found.  This also fits the expected theory that where conditions are easier to cope with, a higher number of species can co-exist.  There is a lower numbers of species however which may be due to competition.  Further up the estuary where net 2 and net S1 were trawled the species that are able to thrive do not suffer such great competition and are therefore found in higher numbers. 

 

ADCP Analysis

An Acoustic Current Doppler Profiler (ADCP) was used to look at current velocity and direction over transects.  This instrument works by sending out sounds waves at a frequency of 1200 kHz and measuring the Doppler shift of the sound wave to calculate the speed and direction of water movement. 

 

Transect 1 –Between P6 and P7 North to South bend after Lamouth Creek

Start – 50 13.470N  005 01.438W  10.24.30 GMT
Finish – 50 13.342 N  005 01.437W  10.27.32 GMT

click to enlarge

The current velocity is relatively slow varying between 0.05 and 0.20 ms-1 over the whole transect.  However there is an area of higher velocity at the surface in the East and West of the transect which is likely to be due to the fact that the Conway passed through the wake of another boat causing turbid water and also causing the sections of the missing data.  There appears to be a slightly faster flow of water in the Western part of the transect which is due to water flowing faster around the outside of the bend in comparison to the inside of the bend. 

click to enlarge

The velocity direction plot shows the main flow of the water is at 090° indicating that the water was flowing into the estuary as the transect was taken around 10 minutes before high water.  There is some southern movement in the eastern part of the transect which is likely caused by passing through the wake of another boat. 

 

ransect 3 - Shag Rock to Black Rock

Start – 50 08.436N  005 01.097W  12.28.50 GMT
Finish – 50 08.705N  005 01.991W  12.40.17 GMT

Velocity of the water column varies from 0 to 0.3 ms-1.  The movement of the water is southwards which correlates with the tidal state as it was an ebb tide at this time. 

 

Transect 5 - Eastern Breakwater to Trefusis Point

Start – 50 09.337N  005 002.921W  13.28.29 GMT
Finish – 50 09.612N  005 03.386W  13.35.15 GMT

click to enlarge

The current velocity is relatively low over the entire transect varying from 0.05 to 0.30 ms-1.  There is faster flow at the surface compared to at the seabed.  When combined with the velocity direction plot and knowledge of the tidal state, it can be determined that the freshwater flow from Penryn River is flowing out over the top in a south-easterly direction and the ebbing estuarine water on the bottom is flowing in more southerly direction. 

 

Transect 6 - Mylor Harbour to Messack Point

Start – 50 10.555N  005 02.569 W   13.59.30 GMT
Finish – 50 10.885N  005 01.572 W  14.13.07 GMT

click to enlarge

This transect was taken in an easterly direction across the middle of the estuary across the deep channel.  From the velocity magnitude transect it was determined that water at the surface is flowing slightly faster than at depth.  The velocity direction profile showed that all of the water was flowing in a southerly direction.

 

Introduction
RV Bill Conway
MV Grey Bear
Small Boats
RV Callista

Grey Bear 07/07/06

Weather Tides (GMT) Equipment Location
Temp: 19-20ºC
3/8 cloud cover
 
0230 4.31m HW
0900 1.94m LW
1500 4.38m HW
2130 1.96m LW
Navigation System - Hydropro
Van Veen grab
Sidescan sonar - Geoacoustics Ltd. model 1590
 

 

  

Aims

The main aim of this investigation is to perform a benthic habitat survey of a selected area of the Fal estuary and characterize the sediment structure as well as biological composition. Sidescan sonar was used to survey the seafloor along the Inner Falmouth Bay aboard the RV Grey Bear. 6 transect lines 100m apart and 1km long, were taken parallel to Gyllyngvase Beach starting at 50008.7N 05’03.1W. This is an interesting site as it is a known dumping ground for dredged material. Whilst surveying, particular attention was paid to identifying bedforms and different sediment types, and spotting important anthropogenic features such as wrecks, artificial reefs, pipelines, buoys, sewage outfalls and dredged channels. 5 grab sites were then selected based on the information obtained from the side-scan sonar output. A geological map of the seafloor was then produced ( see Figure x).

Results

Sidescan Sonar

The main types of bedform in Gyllyngvase Bay (inner Falmouth Bay) are megaripples. These are non-cohesive, flow-transverse bedforms and are characteristic of medium to coarse sediment which was found over the entire survey area.  There were distinct differences in the ripples, although all ripples were classified as megaripples (0.06-1.5m height, 0.6-20m length). The data suggests that the crests are not sharp as there is a subtle contrast between the dark and light areas, which may mean that these are 3-D megaripples, however this is difficult to infer from the data as this may have more to do with data quality than to do with the actual shape of the ripples. The megaripples occur in one large ripple field which suggests an abundant supply of sediment.  Ripples closest to the shore in the northern part of the first transect were at the lowest end of the scale, whilst ripples over the rest of the survey area were significantly larger (table 1). There was also variation within the larger ripples at the survey site.  Near the centre of the site the megaripples were larger than to the eastern and western edges of the area. The small megaripples are likely to be storm generated due to evidence of bifurcation of the ripple beds.The larger megaripples may be generated by current and are therefore indicative of the deeper water flow in the area. 

Area Average wave height (m) Average wave length (m) Wave orientation (°) D50 Ripple pattern classification
Profile of the seafloor between track times of 10:16:19 and 10:23:49. 
1 0.185 2.06 080°/260° 0.00206 Sinuous almost perfectly in phase
2 0.260 2.60 109°/289° 0.00260 Sinuous almost perfectly in phase
3 0.190 2.30 093°/273° 0.00190 Sinuous almost perfectly in phase
4 0.083 0.82 156°/336° 0.0820 Sinuous out of phase
 
Characteristics of megaripples in four areas that were measured within the survey area.

 

The profile is vertically exaggerated 86x and shows a depression in the seafloor that corresponds to the change from small megaripples (green) to large mega ripples (dark blue) and back to small mega ripples. This along with the change in orientation of bedform at area 4 adds weight to the theory that the smaller mega ripples are storm generated and the larger ones are current generated.

Northings

Eastings

Grab Sites

Site 1 (50° 08.6N, 5° 03.3W)
Depth: 8.7 m
Time survey taken: 12:09:43 AST, 07/07/06
Sea state: ¾ cloud cover, light breeze, sea state 2
Side scan sonar: large homogeneous area

Biological composition: -

  • starfish Asteria reubens was caught between grab, 23cm
  • juvenile eel, juvenile pipefish
  • 3 hermit crabs, 1 small crab, 5 bivalves(10cm across): Venus fasciata 2cm and Donax vittatus 2.5cm
  • 3 pink polychaetes
  • Chiton: Tonicella rubra attached to shells 1.25cm
  • Chondus crispus and Chorda filum
  • Oyster: Ostrea edulis

Sediment composition: -

  • Sediment was predominatly dead maerl (90%) pieces with sizes ranging from 0.5 to 1.5 mm
  • 10% empty shells
  • Sediment had high shear strength due to highly angular maerl pieces
  • Many rounded pebbles were found on average 50mm diameter

  

Site 2 (50° 08.3, 5° 03.9)
Time of survey: 12:39:52 AST
Depth: 12.5m
Side scan sonar: small localized flat area
 

 

Biological composition: -

  • Red worms (nematodes)
  • Large bivalves of sizes 45mm
  • Sea urchin Psammechinus miliaris, 3cm
  • Live maerl
  • One Hermit crab

Sediment composition: -

  • Smaller grain size 1-2mm in size
  • 20% dead maerl and 75% broken shells, 5% small gravel
  • Higher proportion of sediment to biota found

 

 

Site 3 (50°08.4 N, 5°03.4W)
Time of survey: 12:57:12 AST
Depth: 14.8m
Sea state: 7/8 cloud cover, light ripples, slight wind, strong current
Side scan sonar: raised area

 

 

Biological composition -

  • Small red polychaetes were found
  • European cowry: Trivia monacha, 1.2cm
  • Many tube worms were found Pomatoceros triqueter
  • Fragment of razor shell

Sediment composition -

  • Sediment similar in composition and size to site 2.
  • 30% empty shells of large sizes 25-45mm
  • large proportion of live maerl (40%)
  • 5% silt

 

Site 4 (50° 08.4 N, 5° 02.8 W)
Time survey taken: 01:15:36 AST
Depth: 14.5m
Sidescan sonar: strange lumps
A kelp bed, usually expected in sheltered areas, may have caused grab to close early.

 

 

Biological composition: -

      • Many kelp pieces (laminaria saccharina) attached to grab up to 2.76m long.
      • Holdfasts attached to slate and shell
      • blue rayed limpet: Helicon pellucidum 1mm and bryozoan mat
      • Tube worms Pornatoleros triqueter on the slate pieces and shell
      • Very small crab was found
      • Chiton: Tonicella rubra on kelp 1.25cm

Sediment composition: -

      • predominantly slate pieces of 35-55mm pieces
      • Live maerl was attached to kelp holdfasts

 

 

Site 5 (50° 08.3 N, 5° 03.5 W)
Depth: 15.8 m
Time survey taken: 01:39:57 AST 07/07/06
Sidescan sonar: homogenous flat area

 

 

Biological composition: -

      • Many calcerous sepulid Pomatoceros triqueter worm tubes
      • Brittle stars: Ophiothnx fragillis 3cm, Ophiocomina nigra 2.5cm
      • Small crab, 4mm across
      • Evidence of predatory gastropod from spherical holes in clam
      • Worms (polycheates)

 

Sediment composition: -

      • Small grain size
      • 20% Many live maerl (average 30mm diameter)
      • Small proportion of silt
      • 80% shell fragments and empty shells
      • Small fragment of welsh coal
 

Conclusion

Sediment:

The sediment analysis agreed with the results from a similar survey undertaken by MESH (Mapping European Seabed Habitats) in 1994 in the same area that also categorized the seabed into faunal turf (Maerl) and kelp beds.

Biological:

There was a high species diversity found on the dead maerl which was also typical of other studies such as Hall-Spencer in 1998, who recorded a rich fauna in a dead Maerl bed in the Clyde Sea.
 

References

 

  • Campbell, A. Nicholls, J. (Illustrations) 1996 Hamlyn Guide: Seashores and shallow seas of Britain and Europe. Octopus Publishing Group 
  • Davies, J.Southerton, I. 1995 Mapping the distributions of benthic biotopes in Falmouth bay and the lower Fal Ruan Estuary. English nature research
  • Fish, J.P. Carr, H.A. 1990 Sound Underwater Images A Guide to the Generation and Interpretation of Side Scan Sonar Data Lower Cape Publishing
  • Langston, W.J., Chesman, B.S, Burt, G.R, Hawkins, S.J., Readman, J. and Worsford, P. 2003. The Fal and Helford.  Marine Biological Association
Introduction
RV Bill Conway
MV Grey Bear
Small Boats
RV Callista

Grey Bear 07/07/06

Weather Tides (GMT) Equipment Location
Cloud cover- 2/8-3/8
Sea State- 1
warm sunny
wind:-
Station 2: 4ms-1 270°
Station 4: 0ms-1
Station 6: 5.4ms-1 30°
Station 8: 3ms-1 0 °
Station 10: 6ms-1 340°
Station 12: 2ms-1 180°
Station 14: 5ms-1 330°
Station 16: 0ms -1
wind direction seemed to have followed the direction of the valley
0020 1.10m LW
0600 4.92m HW
1230 1.04m LW
1820 5.31m HW
Equipment
Boats - Ocean adventure RIB and Coastal Research Bathtub
YSI salinity probe - F98d condition meter F76a
phytoplankton net
Secchi disk
lugols iodide
map chart
niskin bottle - GO (general oceanics), USA
cool box
filter - Whatman glass microfibre filter
GPS locator system
 
  

Aims

Our aims were to use the small boats (ocean adventure and coastal research) to access the shallower parts of the estuary up the Truro river to measure salinity, temperature, pH and dissolved oxygen using a YSI multiprobe. Secchi disc depth was also measured and water samples were taken to analyse for oxygen and nutrients later on in the lab. The data can be used in connection with data from the Bill Conway on the same day to give an overview of the biological, physical and chemical interaction within the Fal estuary.

Method

In order to sample the upper reaches of the Fal estuary (i.e. Fal and Truro rivers) most effectively, we employed a “leap-frog” method of sampling. The RV Coastal Research and RV Ocean adventure sampled at alternating stations up the river to just North of Malpus point. At each station, a YSI multiprobe was deployed to give vertical profile measurements for temperature, salinity, pH and dissolved oxygen concentration. Turbidity measurements were made from the bow of the ribs using a secchi disk. In addition, surface water samples were collected and filtered at every station for nutrient analysis back in the lab. Water samples for silicate analysis were stored in plastic bottles, while samples for nitrate/phosphate analysis were stored in brown glass bottles, and bottle numbers recorded. The glass-fibre filters used for filtrating water samples were extracted and stored in test-tubes filled with acetone. These were in turn preserved in a cooling box for chlorophyll analysis in the lab. Oxygen samples and replicates were collected using Niskin bottles at only 2 stations (start and end station). On collection, the concentration of dissolved oxygen was fixed using 1ml each of Maganese Chloride and Alkaline Iodide. The glass bottles were stored under water to maintain air-tight conditions until the time of lab analysis. 100ml of seawater was collected at 4 stations (station 1, 8, 9 & 16) and preserved in bottles containing Lugols Iodine for phytoplankton lab analysis. Two zooplankton net trawls were carried out over 3 minutes

Results

Analysis of estuarine mixing diagrams.

The nutrients sampled include nitrate, phosphate and silicate and are found in high concentrations in riverine water compared to seawater.  To analyse the abundance of these nutrients the Theoretical Dilution Line (TDL) was plotted using riverine and marine end-members. Very low salinities were unable to be collected as the RIBs were unable to reach zero salinity as the river became too shallow.  The assumptions upon which the TDL is based are:

-that the estuarine system is in a steady state (not true as estuarine environments are dynamic and as such are continually changing)

-that end-member concentrations remain constant

-that there are no additional sources to the water column e.g. pore water from sediments. 

Nitrate

Nitrate was found to be lower in concentration in the lower reaches of the Fal estuary. This was shown to increase as the salinity of the estuary decreased. The behavior of the nitrate in the estuary was shown to act in a non conservative manner as the majority of the points fell below the TDL. To gain a better understanding of the nitrate distribution within the estuary, it would be necessary to collect more samples as there were none collected at salinities lower than 20 psu. A possible reason for the decrease in the nitrate concentration with in the Fal may be due to the increasing distance from the sources of nitrates. The removal of the nitrate in the estuary is also likely to be caused by the increase in phytoplankton numbers as the estuary becomes more saline (due to most of them being marine species). This would indicate that the phytoplankton are utilizing the nitrate in the lower reaches of the Fal and would explain the non conservative behavior shown on the mixing diagram.

Phosphate

The behavior of phosphate within the Fal estuary and Truro River show that it acts in a non conservative manner and both addition and removal occur throughout the estuary. The additions of phosphate are likely to originate from inputs from sewage, agriculture and urban run-off from the extensive farmland and towns around the Fal estuary. These additions are shown to occur throughout the estuary. The phosphate is likely to have been utilized by the phytoplankton in the water column and so is likely to be the explanation for the removal shown on the graph. It would also explain why the removal occurs in greater concentrations in the lower reaches of the estuary.

Silicate

The concentration of silicate in the estuary is a demonstration of non conservative mixing with both addition and removal occurring throughout the estuary. The addition of silicate to the water column is likely to have been caused by areas of sewage effluent input. The removal of the silicate within the estuary is likely to be caused by the biological removal of silicate by diatoms, used to form tests and frustules. This is a likely cause as the water samples collected showed abundance in diatoms throughout the estuary.

The presence of the mussel farm may also have an effect on the concentrations of nutrients found within the estuary.

 
The Secchi disc depth decreased moving north up the estuary indicating increased turbidity. However when the euphotic zone was calculated, it was deeper than the total water depth and therefore phytoplankton was not light limited as they could not be mixed below the critical depth.

Introduction
RV Bill Conway
MV Grey Bear
Small Boats
RV Callista
Grey Bear 07/07/06
Weather Tides (GMT) Equipment Location
0240 0.59m LW
0820 5.19m HW
1450 0.71m LW
2040 5.54m HW
Map Chart- Navifisher
Secchi disk
Plankton closing net with depth gauge
CTD-flurorometer,transmissometer, niskin bottle FSI (Falmouth Scientific Instruments)
ADCP-Winriver
Oxygen- MnCl2 first; alkaline iodide second
T/S probe-Profi lab
Cool box
Filter -Whatman glass microfibre filter
Transmissometer
Niskin,
GPS locator systems- Simrad electronics navigation system
Winch- Spencer Carter

 
  

Aims

To assess how the physical and chemical properties in offshore waters off Falmouth affect the structure and functional properties of plankton communities, paying particular attention to the structure of the water column and the development of the thermocline and front in the Western English Channel. 

Methods

The position of the furthest station was pre-planned. Other stations were sampled using the ADCP, CTD with attached flurometer, light sensor and transmissometer, as well as zooplankton nets. Water samples were collected in Niskin bottles attached to the CTD rosette.  Calibration of the CTD was carried out at Mylor dock before departure and station 1 was taken at Black rock at the mouth of the estuary for an equipment check and for sampling continuity. 

 

CTD Analysis

Station 1 depth profile Station 2 depth profile Station 3 depth profile

Station 1 at the mouth of the estuary (black rock), showed a gentle thermocline between 6-8m with temperature constant at 15.5°C above and 14.3°C below this depth. Flurometer readings reveal a chlorophyll maximum of about 2.52 volts just below the thermocline. As light is not a limiting factor due to the relatively shallow water column, phytoplankton species are able to establish themselves lower down where there is greater availability of nutrients. This is supported by the similar trend in fluorescence at station 2 and 3. Salinity is fairly constant throughout the shallow water column as there are few freshwater inputs at this point.

Station 2 had a greater maximum depth of 40m, so a thermally mixed layer on top of a tidally mixed layer is expected. Salinity remains fairly constant with small fluctuations between 35.27 and 35.37. There is a shallow but clear thermocline present showing a temperature change of about 3°C between 6-12m. Vertical mixing of the water column due to storms the previous night and gale force 6 winds on the day of sampling could have accounted for this shallow thermocline. A clear chlorophyll maximum of 2.9 volts, occurs just below the thermocline at 12m. A smaller, second peak of chlorophyll (2.7 volts) occurs at about 27m, which could be explained by phytoplankton drifting in from an adjacent water body.

At station 3 there is no distinct thermocline or fluorescence peak due to the shallower depth of the water column compared to station 2, where there was a direct relationship between the change in temperature and the fluorescence. The reason for decreasing fluorescence is likely due to the effect of photoinhibition of phytoplankton at the surface resulting in lower chlorophyll content (and therefore lower fluorescence values). Salinity shows little fluctuation suggesting well-mixed waters, because the water column is shallow and so could be completely mixed by the wind which was strong at gale force 6. Fluorescence increases with depth to 2.0 volts at 8m, for the same reasons as at station 1. There is lower chlorophyll biomass at Station 3 in Gerrans bay, which is likely to have slightly higher suspended material and is more sheltered, thus reducing light availability.
 

 

ADCP Analysis

Station 1-time series transect from Black Rock

Position: 50 08.323N  05 01.675W 

From: 09:16 GMT  To: 09:24 GMT

Unfortunately due to rough water the data from this station was badly corrupted.  However it was possible to tell the general direction and speed of the water movement.  The speed of the water was between 0.1 and 0.3 ms־¹ and was flowing in a south-easterly direction out of the estuary with the outgoing tide. 

 

Station 2-time series transect offshore from St Anthony Head

Position: 50 09.917N  04 54.863W 

From: 10:16 GMT To: 10:54 GMT

The offshore ADCP data was also corrupted with a high proportion of missing data due to rough seas.  It is possible to determine a velocity magnitude difference between the layers with the top layer flowing at ~0.1ms־¹ and the lower layer flowing at ~0.2ms־¹ suggesting that a shear between layers is occurring although the data is not definitive. 

 

Station 3-time series transect from Gerran’s Bay

Position: 50 11.924N  04 55.949W 

From: 11:30 GMT To: 11:53 GMT

The velocity direction plot shows that the top layer which is approximately 10m deep is flowing in a northerly direction whilst the lower layer is flowing south.  This clear pattern shows the shear between layers.  The velocity difference between layers also varies with the top layer flowing at around 0.2 to 0.3ms־¹ and the bottom layer flowing slightly slower at around 0.1ms־¹. 

click to enlarge

 

Transect 1- Offshore from St Anthony Head to Gerran’s Bay

Start – 50 09.830N  04 54.863W  10:54 GMT
Finish – 50 12.081N  04 56.639W  11:22 GMT

click to enlarge

The shear between layers is very clear in this plot.  As we moved into the bay there was another body of water lying at the surface which is wind-mixed lying over the deeper tidally affected water.  The surface water is flowing at 0.1ms־¹ west in comparison to the deeper water which is flowing south at around 0.25-0.3ms־¹.  There is an area of transition between the two layers as the seabed rises into the bay.

 

Richardson Numbers

Station 1 – Black Rock
50’ 08.294N  05’ 01.511W

 At station 1 the Richardson number (Ri) indicates that above and below the thermocline the water column is well mixed and over the thermocline we see several stratified and stable areas where there is likely to be a large amount of biological activity due to the density change giving ideal conditions.

Station 3 – Gerrans Bay
50’ 11.924N  04’ 55.953W

This station shows a sharp increase Ri at just over 6m metres despite there being no clear thermocline. This means that there is still a density change where Ri increases but is likely due to a combination of factors such as salinity and surface and deep water flowing in opposing directions. Opposing flow leads to Kelvin-Helmholtz disturbances at the interface which are directly related to the Ri values.

Course Conclusion 

ESTUARINE

  • Estuarine mixing diagrams showed non-conservative behaviour of all 3 nutrients down the estuary indicating utilization by phytoplankton.
  • Silicate removal illustrates the presence of high numbers of diatoms, as seen from the cell counts.
  • The mussel farm did have an effect on the Dissolved Oxygen concentration.
  • High diversity of zooplankton occurred further seaward
  • 2 distinct habitats, namely kelp and maerl beds, were identified from the geophysical survey of the seafloor of Falmouth Bay.

 OFFSHORE

  • Distinct stratification occurred in the deeper water offshore where the water column was too deep to be completely mixed
  • This stratification determined the chemical and thus biological structure of the water body studied.

Over the 14 days we have learnt and improved upon many new skills and scientific techniques, such as team building skills, scientific write ups and using new software, for example WinRiver. Fortunately we found we got on well together as a group, and managed to overcome many obstacles, such as losing data, hangovers and staying on board the boats!

We liked exploring the setting of the Fal estuary, and we recorded new findings in a previously undiscovered area. Although the course was hard work and with early starts we enjoyed ourselves.

We would like to thank Brian Dickie, our tutor, for his guidance and assistance in labs, Simon Boxall for coordinating the course, the boat crew for all their help with boat practicals, and the rest of the staff for all their hard work at making our work possible.

 

All views and opinions that are expressed on this website are entirely that of group not necessarily representative of the views and opinions of the university. Any anomalous data is a representation of the dom factor.

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