Falmouth Field Course
2007 Group 4 - 'The Deep Sea Karma Junkies' |
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The Fal is macrotidal at the lower parts of the estuary around Falmouth with a maximum Spring tidal range of 5.3m, but becomes mesotidal in the more upper regions near Truro. It is the 3rd largest natural harbour in the world, and has recently been granted status as an SAC- Special Area of Conservation, due to the extensive Maerl beds and fisheries present within the estuary. Large amounts of mining discharge in the past have led to the Fal being heavily polluted, especially during the Wheal Jane incident in January 1992 where large amounts of highly acidic metal laden water was released into the estuary. Evidence of this pollution is still seen in the sub surface sediments of the estuary, especially around Restronguet Creek. During our time in Falmouth, our main aim was to measure the biological, chemical, physical and geophysical parameters within the Fal estuarine system and the immediate offshore area. Therefore allowing us to integrate, compare and contrast the data gained from all the research surveys undertaken to discover how the four main parameters change throughout the estuary.
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RV Callista |
RV Bill Conway |
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The RV Callista is primarily used for offshore research. Having a large Stern deck with 3 deployment points and dry and wet labs on the vessel allows instantaneous data interpretation. |
The RV Bill Conway is better suited for river and estuarine surveying, having an ‘A’ frame and side winches for various equipment deployment |
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Specification: |
Deck: |
Specification:
Length -
11.74m |
Deck:
‘A’ frame -
750kg max |
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MV Grey Bear |
Ocean Adventure RIB |
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The MV Grey Bear is used for
offshore Geophysics studies using Side scan sonar and Grabs to obtain
bathymetric data. |
Ocean Adventure RIB is a fast and versatile Boat with a low Draft, therefore allowing it to reach into the upper most regions of the estuary to take samples where the other vessels cannot. |
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Specification:
Length -
15.00m |
Equipment:
Winches -
3 in
total |
Specification:
Length -
7.00m |
Cruising Speed -
25
knots |
Niskin
Bottles:
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Acoustic Doppler
Current Profiler:
Secchi Disk: |
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YSI Probe: |
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Below are outlines of the various methods used in the laboratory for chemical and biological analysis of samples collected on the offshore and estuarine boat practicals.
Nitrate Analysis Method:
Phosphate
Analysis
Method:
Silicon Analysis
Method:
Chlorophyll Analysis Method:
Phytoplankton Taxonomy
Analysis Method: |
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Introduction:
Aims & Objectives:
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Action Plan & Equipment: Collect data from: Callista (incorporating fully equipped wet & dry labs; ADCP). Using: CTD with Niskin bottle rosette system; Plankton net; Secchi disk. |
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Logistics: Date: The surveys were conducted on the 4th July 2007. Location: The survey stretched from Mylor (50°09.111N, 005°03.546W) to 50°05.236N, 004°59.343W (see route map). Time: 0815GMT-1400GMT Tide: HW 0744GMT 4.8m; LW 1408GMT 1.1m. Environmental conditions: westerly force 6 rising 8, cloud cover ranging 3/8 to 8/8, scattered showers, sea state moderate. |
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The Team: PSO: Rob General Dogsbody: Alex Scribe: Dafydd Wet lab team: Sarah, Katie, Joana, Harriet Dry lab: Beth Stern deck team: Elly, Martyn, Kris |
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Offshore Survey Method:
The survey commenced at
Mylor (station 1), headed south to Black Rock (station 2), then east to
stations 3&4. (figure 4.1). |
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2. The CTD was deployed for a down-cast, establishing the
vertical profile.
Primarily from each Niskin
bottle, samples were collected for dissolved oxygen analysis, fixed in
preparation for analysis by the Winkler method and subsequently stored
within a water bucket. Secondarily, bulk samples were obtained and
divided into the following sub-samples:
The Callista cruised
between stations at a maximum speed of 5kts ensuring appropriate
conditions for ADCP use. The electronic data obtained from the ADCP &
CTD was monitored and recorded from the “dry” lab. Vertical profiles
were constructed and henceforth the sampling strategy was determined in
accordance with the position and strength of the thermocline or the
chlorophyll maxima. |
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CTD Data Analysis: |
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Station 1: |
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Station 2: |
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Station 3: |
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Station 4: |
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Chlorophyll: |
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ADCP Data Analysis: |
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ADCP data of the average backscatter within the water column (Figure 4.6 ), shows a small section of weak inflowing salt water close to the basin floor contrasting to the rest of the estuary where the outflow of less dense fresh water is predominant. This would be expected as a high water of 4.8m occurred at 07:44 and the transect began at 09:37, suggesting that the estuary was in the initial stages of ebbing. A southerly ebbing flow of surface waters towards the boundaries of the estuary with values reaching up to 0.3m/s and directional average of between 150° and 220° when nearing the shoreline. |
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Black Rock Transect: Figure 4.7, The velocity direction plot, indicates a southerly flow of surface water during an ebbing tide, supported by the data in table 4.2 showing a net surface water flow of -742.39[m³/s]. There appears to be a small wedge of denser salt water still flowing into or remaining stationary at the base of the water column just about the estuary bed. This can be seen in the average backscatter plot by the increase in returning signals from this point. The ship track plot correlates closely with the previous data confirming an ebbing tide and southerly surface water flow at the mouth of the estuary.
Transect between Stations 3 and 4:
Transect after station 4: |
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Nitrate: |
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Silicate: Silicate concentrations at stations 1 and 2 (figure 4.15) remain relatively constant with depth at around 3.5μmol/L. Stations 3 and 4 showed a much greater range in silicate concentration, varying from around 2μmol/L at the surface and increasing with depth to greater than 4μmol/L. The increase in silicate with depth coincides with the thermocline suggesting utilisation by phytoplankton such as diatoms above the thermocline and remineralisation below the thermocline. Silica is in highest concentration in fresh water, and therefore it may be expected to be found in higher concentrations closest to the estuary and freshwater inputs as seen stations 1 and 2. |
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Discussion:
The
chlorophyll concentrations measured by the lab fluorometer supports the
data recorded by the fluorometer on the CTD. Each show greater
chlorophyll concentrations at station 3, with a large chlorophyll
maximum between 5 and 15m, suggesting greater primary production in this
region. At station 4 there is a chlorophyll maximum at 10m, below which
the chlorophyll concentration falls, as also shown by the lab analysis
of the chlorophyll data. The CTD fluorometer shows conservative
behaviour by chlorophyll in the water column at stations 1 and 2, as did
the laboratory analysis.
An
increase in nitrate with depth at station 3 would tend to suggest a
depletion of nutrients in the surface waters by phytoplankton growth,
most notably immotile diatom species. At station 3 the depth of the
euphotic zone is as deep as the depth of the mixed layer, creating very
favourable conditions where nutrients mixed across the thermocline can
be utilised for photosynthesis. Within the mixed layer silicate
concentrations are low (~1.5μmol/L) suggesting utilisation by diatom
species, shown to be present by high chlorophyll levels (>5μmol/L) in
the top 20m and phytoplankton taxonomic analysis. |
Introduction:
Aims &
Objectives:
Logistics:
2. RIB:
The Team:
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Estuarine
Survey Method:
2.
Rib: |
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Nutrient Analysis: |
Nitrate: |
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Silicate: Ribs - Silicate concentrations at the river end (lowest salinity) were very high, with a concentration of over 100 μmol/L. In comparison the river samples taken from the rib were much lower at below 20 μmol/L. Stations 2, 4 and 5 all decrease in concentration with depth with Station 2 showing the largest range of just below 20 μmol/L to about 4 μmol/L within the surface 6m of the water column. Station 3 however shows a constant distribution through the water column with a concentration of around 22 μmol/L between 4m and 14m. Bill Conway - Silicate concentrations at stations 1a b and c (all sampled at 10m depth) vary between 25 and 61μmol/L despite being sampled at sites very close to each other. Station 2 showed a decrease in silicate concentration, varying from around 23μmol/L at the surface and decreasing with depth to a value of 8μmol/L. Stations 3, 4 and 5 (those closest to the sea) have very consistently small Silicate concentrations throughout the water column suggesting high mixing in this area. |
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Discussion:
The high nutrient
concentrations found at Bill Conway station 1 can be attributed to the
proximity of the station to the Fal River. This inputs fresh water and
high concentrations of nutrients from sources such as agriculture to the
Fal system. The surface nutrient values at Bill Conway station 3 are
high whilst the deeper water displays low nutrient levels suggesting a
flow of nutrient rich river water supplied by the Lamouth and Cowlands
Creek, above nutrient poor sea water. The relatively low nutrient levels in the middle of the estuary could be attributed to the high concentration of phytoplankton in this region. This is visible on the theoretical dilution lines of nitrate and silicate, which shows removal. The large numbers of diatoms present in the bloom would remove nitrate for photosynthesis and use silicate to produce their casts. The succession of this bloom would have reduced the level of nutrients, until eventually they became limiting and the bloom would cease. The bloom may have been stimulated a large nutrient inputs as a result of the heavy rain that occurred for two weeks prior to the study. This would increase the land runoff of nutrients such as nitrate into the estuarine system. The bloom may have initially been stimulated by the high levels of sun light that occurred during the weekend before the study. On the day of the survey, conditions were warm and sunny which may have stimulated the formation of the diurnal thermocline in the upper estuarine stations sampled by the rib. The low salinities sampled by Bill Conway at Stn 1 can be attributed to the influx of FW from the river Fal. The rest of the Bill Conway sites were found to have salinity values of 34-35 psu, this can be explained by tidal flooding of the estuary at the time of the survey. Although weak thermoclines appear to be present at most stations, the sunny conditions of the day of survey may have induced a temporary state of stratification, hence the apparent presence of a diurnal thermocline. However, the greatest proportion of the data and observations are indicative of a parially mixed body of water with a slight two layer flow observed with the ADCP. The dynamic nature of the partially mixed estuarine environment such as the Fal system and the variability caused by factors such as the weather would create a spectrum of possible conditions effecting the vertical stratification of the estuary, and hence greatly effect the distribution of nutrients and phytoplankton. |
Introduction:
Aims & Objectives:
Action Plan & Equipment:
Logistics:
The Team:
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Geophysics Survey Method: The survey was started at the co-ordinates 50° 05.9554N 005°03.2732W. At this point the “fish” was towed behind the boat and then calibrated. At 9.27 GMT the first line was started. In total 7 lines were surveyed running parallel to each other creating a rectangular grid. Each line was 2km in length and the swath width covered 75m each side of the tow fish. Each line slightly overlapped the previous one by approximately 10m ensuring no seafloor features were missed. Spotters on deck were used to record the position of buoys and boat traffic in our range which could have interfered with the side scan and affected our results. The table below shows the times and locations for the start and end of each line:
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Using the images produced by the sidescan transects, 4 sites of different sediment type were selected for sampling using a Van-Veen grab. For each site the grabs were analysed in relation to the abiotic (sedimentary) and biotic (biological) characteristics.
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The Wreck of the
Volnay: The possible location of the Volnay Wreck, positioned off Porthallow bay, using the side scan sonar was confirmed through analysis of the sea floor print outs and further background reading. The Volnay sank on the 14th December 1917 after striking a German mine off the Manacles and being towed into Porthallow bay. The vessel lies in around 20m of water, the bow section facing NNE after being extensively broken up due to storm events and planned explosions. 2 large and 1 small “donkey” boiler (1), foreword and to the port side of the wreck, are still in tact and can be seen clearly on the side scan printout. Calculations used have given boiler height estimates of 4.88m, 4.03m and 2.9 m respectively which correspond accordingly to recorded heights documented in literature reports. The rest of the wreck lies relatively flat against the sea bed, with only minor perturbations height recorded using the side scan equipment.
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Main Findings of the
Side Scan Sonar: |
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Scour Marks:
Formed from anchors
dredging against the sea floor |
Bedforms:
Megaripples with an
average height of 0.16m |
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Wreckage:
Remnants of the Volnay |
Sediments and Rocks:
Majority of the area
covered was rock, |
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1 & 2. The horseshoe
shape displayed above is caused by a ship’s anchor failing to bite the
underlying substratum and henceforth being dragged across the seabed
3.
This anchor drag also shows a unique extra
drag line and it has been suggested that this has been caused by the
ship being moved with the changing tide |
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Discussion: The survey undertaken has allowed us to identify the major type of sediment as sand, with rocky outcrops present, especially in the first 3 survey lines. This was known from the side scan sonar and then backed up using the Van Veen Grabs. The grabs also allowed us to identify the different types of fauna present, ranging from bivalves and polychaete’s to shore crabs. The reason for the major differences in fauna are due to the changes in the sediment. The organisms are adapted to different conditions so that they can only survive in the sediment best suited to their niche. The identification of the wreck, The Volnay, made for interesting analysis as it contrasted to the normal bedforms found. |
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Vertical mixing processes in the waters off Falmouth affect the structure and functional properties of plankton communities directly and indirectly. The extent of vertical mixing in the offshore environment near Falmouth created a lot of variability among the plankton communities. The depth of thermocline in this offshore environment gave evidence to the amount of vertical mixing of these plankton communities. For example, lower phytoplankton populations were observed in a region with a deeper thermocline, so that plankton were mixed below the euphotic zone, so that light limited primary production in this region. The evidence of a frontal system observed using the ADCP would have brought about more variability in the observed results, possibly showing more Dinoflagellates which are indicative of the frontal systems in this region. Estuarine Study Conclusion:
Nitrate and phosphate show
non-conservative behaviour whereas silicate generally show conservative
behaviour with some removal. The source of phosphate could be caused by
agricultural input whilst the removal of nitrate and some silicate maybe
a result of high primary production caused by the diatom bloom (Nitszchia
sp.) in the estuary which may have been stimulated by the high rain fall
in the weeks preceding the study. There is a mussel farm located at
Northwood that may have affected nutrient levels; however there is no
evidence of this in our results. Geophysics Study Conclusion:
The data gained from the side scan survey
lines identified many features, including a wreck site, which has been
identified as The Volnay. The main sediment type identified was sand
with bedrock outcrops. Four areas where the sediment types changed from
mud through to gravel were found. At sites of interest and significant
variation in sediment type observed from the side scan data, Van Veen
Grabs were taken. The grabs allowed us to confirm the sediment types and
to identify the different faunas in the samples. The fauna ranged from
large amounts of Polychaetes in the mud sediment to shore crabs in the
gravel and rocky samples. In addition, live Maerl was found in some
sample, representing an important discovery for the local environment. |
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Johnson K. and Petty R.L. (1983) “Determination of nitrate and nitrite in seawater by flow injection analysis”. Limnology and Oceanography 28 1260-1266. Parsons T. R. Maita Y. and Lalli C. (1984) “A manual of chemical and biological methods for seawater analysis” 173 p. Pergamon. Grasshoff, K. K. Kremling, and M. Ehrhardt. (1999). Methods of seawater analysis. 3rd ed. Wiley-VCH. Langston, W. J. et al. (2003) Fal-Helford. Marine Biological Association Occasional Publication 8 Bernardi Aubry, F. et al. 2004. Phytoplankton succession in a coastal area of the NW Adriatic, over a 10-year sampling period (1990-1999), Continental Shelf Research, 24, 97-115 Rodriguez, F. et al. 2000. Temporal variability of viruses, bacteria, phytoplankton and zooplankton in the western English Channel off Plymouth, Journal of the Marine Biological Association of the United Kingdom, 80, 575-586. Leveridge, B.E. Holder, M.T and Goode, A. J. J. 1990. Geology of the Country around Falmouth. British Geological Survey
Websites: |
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