RV
Callista |
MV Xplorer |
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Ocean adventure RIB |
CTD
Rosette |
Van Veen Grab |
Sidescan
sonar
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YSI multi probe
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Zooplankton closing nets
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TS
probe
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●Tides and Weather
●Equipment
●Station Location
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Date: 01/02/09
Departure: 7.50GMT
Return: 16.30 GMT
PSO: Vicky Leader ●Background: Planktonic community structure and abundance is primarily controlled by the physical and chemical conditions in the surface waters of the ocean. These properties are affected by vertical mixing in the water column which can be subjected to thermal stratification if conditions (depending on water depth, tidal mixing and surface insolation) permit. ●Aims: To collect water samples at pre-determined stations and perform chemical, physical and biological analysis in order to locate the position of the tidal front. To determine how vertical mixing processes in the waters off Falmouth affect, directly or indirectly, the structural and functional properties of plankton communities. ● Sampling Route: The route taken (shown on the left) was chosen with knowledge of the previous group's data in the effort to locate the tidal front, whilst expanding the area over which the sampling was taken place. This would aim in the completion of the offshore dataset when other groups' data had been collated. Starting at Black Rock, which will be sampled by every group and therefore act as a control and a view as to changes in the water column over the two weeks, the decision was made not to travel so far offshore as the front had been discovered relatively close to shore the day before. The sampling strategy included staying near the coast east to Falmouth in the effort to ascertain the spatial position of the front. The aim was to cross the front several times from station 5 onwards in order to gain several vertical profiles demonstrating the changes in stratification either side of the front. ● Sampling Procedures
Dissolved Oxygen Sampling Samples were taken for calculating dissolved oxygen as soon as the Niskin Bottles were recovered from the rosette. Small samples were decanted from the Niskin bottles via plastic tubing and then transferred to the wet lab where they were fixed with 1ml of Manganese chloride solution and 1ml of Alkaline iodide solution. Storage in a cool box submerged in water to prevent atmospheric oxygen contaminating the samples. Nutrient Sampling
● Experimental Procedures
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Results |
Figure 1 - Phytoplankton abundance at all stations |
Phytoplankton analysis |
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CTD and Nutrient Analysis |
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Figure 3 - Vertical profile at station 1. Figure 4 - Vertical profile at station 2. Figure 5 - Vertical profile at station 3. Figure 6 - Vertical profile at station 7. |
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ADCP Figure 7 (taken after station 7) shows the presence of a eddy. These eddies are formed from contrasting water currents at the Oodman point, along the 50m contour line. There was a westerly surface current, influenced by the outgoing tide, which circled the headland from the East. The magnitude of this Westerly current was 0.15 m/s, whereas the deeper easterly current, at approximately 25m, was propagating with a magnitude of 0.015 m/s. It was also noticed that the top 15m of the water column was stratified minimising the eddies influence of the surface water. Due to its size, tidal influence and proximity to the coastline it is hard to predict the time scale and overall influence of the eddy on the biota. Larger eddies, such as those found in the Gulf Stream, can be modelled and accurately predicted as to their behaviour and temporal variability. The backscatter image produced from the ADCP at station 7 shows distinct
patchiness throughout the entire water column (Fig 8). The backscatter
can be seen in three layers, 10m, 25m, and 45m suggesting three
different layers of plankton.
Richardson's number
If the Richardson’s number is greater than 0.25 then the water column is stratified, if below the velocity shear enhances mixing. The Richardson’s number was calculated for the vertical profile of the following stations 1-7. Due to the small boundary between stratified and mixed water masses it is hard to determine the magnitude and accurate use of the Richardson’s number. The Richardson’s number is also an indication of the differing magnitudes and directions of the currents throughout the water column. The vertical profile of station 1 was mostly mixed except at three depths, 4, 21, and 30m (Fig 9). As mentioned in the CTD analysis, there is a prominent thermocline present at 20m at station 2. The Richardson number shows that there is mixing taking place just above this thermocline (Circa -12) with some variation above and below this point. At station 3, according to the analysis of the temperature/salinity data we concluded that station three was the approximate position of the tidal front (Fig 10). Station 7 exhibited water column mixing in the first 20m of water whereas just beneath the thermocline there are stratified waters (approximately 20-30m). Below this there is variation between stratified and turbulent waters. |
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Figure 11 - Velocity versus depth at station 2 |
Velocity
with Depth |
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CTD and Secci disk light
data comparison
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Figure 13 - Light comparisons |
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Conclusions | |||
Fronts are areas of larger than average horizontal gradients of water parameters for example, temperature and salinity. Differences in stratified and turbulent water aid in identifying these fronts. Stratification and mixing depend on the tidal stream velocity and the depth of the water column. Using the CTD data we were then able to differentiate, in situ, the likely position of the tidal front on the day (see Richardson graphs). The tidal front influences the abundance of certain species of phytoplankton and zooplankton. This is used as an indicator of the relative position of the front throughout the year. From the CTD data we expected station 3 to be the location of the front. Station 1 showed mixed water (see graph) displaying less vertical variation. Station 2, ten nautical miles offshore, displayed stratified waters which were reflected from the thermocline on the CTD data. We therefore expected to find an intermediate between the two extremes, station 3. As evidenced from the CTD data and the Richardson numbers there is no clear indication of the existence of a distinctive tidal front. The CTD data suggested the region of a front was present at Station 3, due to neither distinct stratified water, nor definite mixed water, and so was decided as the correct area to sample. However, when attempting to locate the coastal side of the front, we were not successful and ended up in estuarine waters. This would suggest, along with the erroneous Richardson data, that a distinctive front was not found. |
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Date: 04/07/09
Departure: 07.30GMT
Return: 12.15GMT
PSO: Keiron Roberts
Introduction Aims
Sampling Route |
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● Tides and Weather
● Equipment
● Track Locations
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Sidescan Trace |
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A benthic map was produced using the sidescan trace
with areas of interest mapped as well as bedforms and sediment
properties. Geometric corrections and calculations were used to measure
certain bedforms such as sandwaves and outcrops of bedrock. Below are
artefacts and interesting points from the trace:
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Grab Samples |
PURPOSE |
Grab 1 |
Table 1 - Biota in sample from Grab 1
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Location 20m NE Eastern Narrows buoy Time: 10:27 GMT Latitude: 50°09.4433N Longitude: 005°01.8896W Depth: 12.4m
Findings: |
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Grab 2 |
Table 2 - Biota in
sample from Grab 2
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Figure 25 - Grab 2 Figure 26 - Amphiouxus |
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Location |
Grab 3 |
Table 3 -
Biota in sample from Grab 3
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Figure 27 - Grab 3 Figure 28 - Live Maerl
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Location |
Grab 4 |
Table 4 -
Biota in sample from Grab 3
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Figure 29 - Heart urchin Figure 30 - Isopod |
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Time: 11:44 GMT
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Summary |
The sidescan data
shows that the benthos has predominately coarser sediment to the west in
the estuary gradually becoming finer to the east near the shore. Down
the transect there is a channel with exposed metamorphic mudstone slate
and a slope consisting of finer sediment. On the eastern shore there is
a presence of seaweed showing a reduced flow rate. Seagrass is present
in Saint Mawes harbour with a finer sediment present either side. The grabs revealed a varied benthic community with all grabs having maerl present in varying conditions. Grabs 1, 2 and 4 taken at shallow depths have a larger proportion of living organisms in comparison to grab site 3 taken at below 30m depth. This is due to the increased velocity currents moving the dead organisms and sediment out to sea. |
Date: 08/07/09
Departure: 07.45 GMT
Return: 17.30 GMT PSO: Charlee
Bennett Introduction Aims
Sampling Route Sampling Procedures Experimental
Procedures
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Results |
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Figure 31 - Vertical CTD profile at station 3 Figure 32 - Vertical CTD profile at station 4 Figure 33 - Vertical CTD profile at station 5 |
Vertical Depth profile from the CTD and YSI data Xplorer stations Station 3 - There is a thermocline present at 13m with a chlorophyll maximum at 12m (Fig 31). However, according to the constant salinity, the water column is well mixed suggesting that the thermocline is seasonal. As expected the dissolved oxygen concentration decreases with depth. Station 4 - Both the temperature at salinity remains homogeneous with depth suggesting a well mixed water column (Fig 32). The chlorophyll fluctuates between 0.16µg/L and 0.32µg/L throughout the water column. Station 5 - Measurements were made at station 5 to quantify the effect of a sewage output in the surrounding area (Fig 33). Compared with other stations, the chlorophyll levels are relatively high and increase with depth, 25-30µg/L. This suggests high levels of phytoplankton explaining the low levels of nutrients. Unfortunately this prevents a correlation to be found between the sewage input and nutrient levels. Station 6 - The salinity fluctuates in the top 14m of the water column which is representative of the flooding tide (Fig 34). Station 6 is located at the most seaward end of the estuary exhibiting typical well mixed conditions. Rib stations The salinity profile for station one shows a well mixed homogeneous water column which is also seen with the temperature and oxygen profile (Fig 35). Salinity Increases by 0.6 and temperature decreases by 0.4ºC. The Oxygen saturation declines by 8% from surface to depth showing a well mixed water column from depth up to the surface/atmosphere. A chlorophyll maximum exists at 2m depth, which is possibly due to high irradiance levels in the first metre of water. Station 3 has a well mixed surface layer with some saline ocean waters being seen at 6m – 7m depth with a drop in temperature of 0.8ºC and an increase of salinity of 0.5 (Fig 36). Oxygen saturation declines with depth and chlorophyll concentrations are high throughout the column in comparison to Rib station 1, which is possibly due to the higher concentrations of phosphate and nitrate in the surface waters compared to Station 1. |
Figure 34- Vertical CTD profile at station 6 Figure 35 - Vertical YSI profile at RIB station 1 Figure 36 - Vertical YSI profile at RIB station 3. |
Turbidity Station 3 (lat: 50°10.850, long: 5°01.732) and Station 4 (lat: 50°08.919, long: 5°01.648) both had depths of 28-29 m within the channel, but different locations, with Station 3 further upstream and Station 4 at Black Rock (see Site Plot). A difference of 3 meters in the euphotic depth suggests more turbidity upstream. Station 1 (lat: 50°12.183, long: 5°02.491) and Station 5 (lat: 50°09.027, long: 5°02.280) at depths of ~11m, again display increased turbidity upstream. Station 1 has a euphotic zone four meters above the seafloor, whereas the euphotic zone reaches the bottom depth at Station 5. As the only sites with similar depths, there is not the satisfactory amount of data as needed to present a firm conclusion. However, for the region surveyed with the Xplorer vessel it can be assumed that there is greater turbidity upstream. This could be explained by the high energy input from the river and tributaries carrying suspended particulate matter into the estuary. This is diluted downstream by the larger volume of the estuary and also by tidal influence. Attenuation coefficient is highest at RIB 3 (lat: 50°12.183, long 5°01.732) this shows maximum turbidity and therefore is the possible null point of the estuary. |
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Residence Time of The Fal Estuary |
Pictures from the Boat work |
Figure 37 - Estuarine mixing diagram for Phosphate Figure 38 - Estuarine mixing diagram for Silicate Figure 39 - Estuarine mixing diagram for Nitrate |
TDL Analysis It is already clear
that as there are several tributaries entering the Fal estuary including
2 large inputs (The Percuil and the Penryn rivers) close to the mouth,
this will affect the TDL and must be taken into account when analysing
data. There is a large gap in concentrations as the riverine end member
was taken high up the Truro River at salinities ranging from 0-5,
however samples could not be taken very far up the river in the rib and
as the tide was relatively high at the times of sampling the salinity
generally did not get lower than 30. Therefore for all the TDL lines
there is a gap in the data ranging from salinities of 5-30 and resulting
in an area of unknown concentrations which cannot be commented on.
Silicate Estuarine Mixing Diagram
Nitrate Estuarine Mixing Diagram |
Phytoplankton and Zooplankton Analysis |
Figure 40 - Google image of zooplankton trawls. Figure 41 - Phytoplankton abundance Figure 42 - Zooplankton abundance |
Phytoplankton analysis
Zooplankton analysis |
Figure 43 - ADCP data for transect 1 Figure 44 - ADCP data for transect 2 Figure 45 - ADCP data for transect 3 |
ADCP Analysis |
Figure 46 - ADCP data for transect transect 4 Figure 47 - Equation used to calculate the coriolis displacement. |
Analysis
Conclusion |
Figure 48 - Vertical salinity profile |
The Salinity profile of
the Fal Estuary
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Figure 49 - Horizontal salinity profile |
Conclusions |
Using the data recorded, the Fal estuary exhibits large variation depending on the distance from the coastline. According to the CTD and YSI data the estuary exhibited a well-mixed structure with little to no stratification. The salinity also remained uniform with depth with little variation throughout the tidal cycle. This then affected the phytoplankton and zooplankton abundance resulting in an increase in diversity at the river end member and an increase in abundance of species at the mouth. It was difficult to measure the behaviour of the nutrients due to the influence of more than one contributing river. However, both the phosphate and nitrate appeared to behave conservatively and the silicate behaved non-conservatively. |
The Fal estuary is a well mixed and tidally dominated area of water affected by physical parameters such as tidal, local weather and anthropogenic inputs. Large riverine inputs result in high concentrations of nutrients which are reflected in the planktonic abundance and biodiversity. The anthropogenic inputs were hard to quantify as no direct inputs were found. Clear stratification was seen offshore due to the presence of a seasonal thermocline. In the adjacent coastal areas, however, mixing was not as evident as the thermocline, potentially due to the calm weather. Nutrient profiles were typical for this time of year and the chlorophyll maxima reflected this. The bathymetric survey found a higher abundance of rare, protected species such as Maerl and seagrass than expected, suggesting the Fal estuary is more productive than previously thought. In order to gain a full overview of the physical, chemical and biological conditions of the Fal estuary, a long term study covering different tidal states and seasonal conditions should be conducted, as well as surveying a wide area. |
Brown,
E., Park, D., Phillips, J., Rothery, D., and Wright, J., 1999, “Waves,
Tides and Shallow Water Processes” Second Edition, Buterworth Heinmann,
Chapter 6 |
Disclaimer: All the information contained within this project is entirely the work of the members of group 4 (Falmouth field trip 2009) , the ideas and results contained within are independent of the National Oceanography Centre of Southampton.
We would all like to thank our project
tutor, Eric Achterburg, for his insightful yet sarcastic inputs. They were
invaluable: