University of Southampton OES Undergraduate Falmouth Field Course 2016 - Group 3 databank and initial findings.
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ADCP data from the Fal estuary identified strongest, more turbulent flow further from the estuary mouth, with flow rate and turbulence decreasing with movement towards the estuary mouth. This observation will be as a result of widening of the estuary channel, and a resulting increase in cross sectional area reducing flow rate. A reduced flow will further decrease the Reynolds number, resulting in decreased turbulence and greater uniformity. Strongest flows were consistently recorded in the central zones of the estuary, which is explainable by the effects of Ebb tide flow at the time of data collection.
Transect 001
Start: 09:03 UTC
Lat 50° 14.373460’ N
Long 005° .969713’ W
End: 09:05 UTC
Lat 50° 14.344111’ N
Long 005° .828342’ W
Transect 001 was taken at our first station nearest the head of the river. It shows a stronger current at the surface, but is not very uniform anywhere along the transect.
Transect 005
Start: 10:48 UTC
Lat 50° 12.370179’ N
Long 005° 1.806295’W
End: 10:50 UTC
Lat 50° 12.503240’ N
Long 005° 1.868008’ W
This transect shows a higher velocity of water in the middle of the river at the surface. This is probably due to the flow of the ebb tide.
Transect 011
Start:13:18 UTC
Lat 50° 8.761334’ N
Long 005° 1.084015’ W
End: 13:37 UTC
Lat 50° 8.804533’ N
Long 005° 2.512042’ W
Transect 011 was taken at the mouth of the estuary. The area on the left at about 28m deep is the main channel which is located on the Eastern side of the estuary. It is fairly uniform in water velocity and is mostly between 0.0 and 0.2m/s. The mouth of the estuary is much wider than further up the estuary and is also deeper which accounts for the slower current speed. The transect was also taken near the end of the ebb tide and beginning of the flood tide so not much movement was occurring. There is a white patch located in the channel which is where the ADCP was interrupted by an unknown disturbance.
Estuarine Physical Data - ADCP
Salinity
Generally, higher salinity values are found at s
tations closer to the mouth of the
estuary. The increase in salinity with depth at stations A22, C27 and D28 indicated
stratification and there is a notable halocline around a depth of 2m at A22, 5m at
C26 and 4m at D28. Stations F29 and G30 were well-mixed with almost constant salinity
at all depths (i.e. 33.1-33.4 for F29 and 33.4-33.5 for G30). Although the measurements
at station B were taken over the period of an hour at the peak of low tide, the graph
does not show very large variation between the different deployments. The salinity
was mostly the same at the surface (i.e. around 31.7), but decreased with time at
approximately 9m depth (from 32 at B23 to 31.5 at B26). Additionally, the weak halocline
visible at about 5m at station B23 dropped to approximately 9m at station B26.
Sites further from the river mouth showed vertical salinity gradients and haloclines at 2m-4m, with highest salinity values present at greatest depths. These gradients were not visible at sites F29 and G30, closest to the estuary mouth. This suggests Fal estuary waters were not fully mixed at stations further from the estuary mouth, and low salinity river water was overlying higher salinity estuarine water, causing a halocline. Closer to the estuary mouth waters appear to be well mixed, as this gradient ceased.
Temperatu
re
Water temperature overall was higher and less variable towards the river end of the estuary. While the stations up the River Truro (A-B) display a mixed situation, all stations out in the Fal (C27-G30) show a clear temperature decrease with depth and a thermocline. At station G30, there are even two thermoclines at 3 and 8m which break up the water column into 3 very well-mixed layers. C27 and F29 also indicate a well-mixed surface layer with a thermocline at 4 and 6m respectively. The temperature structure varied very little between the different deployments at station B.
Temperature values collected were in general at their highest furthest from the estuary mouth, showing that river water running across the land was of a greater temperature than oceanic water entering the estuary. All sites bar A and B displayed small thermoclines and negative correlations between temperature and depth. The suggestion interpreted from this is surface waters were being warmed by irradiance, causing temperatures to be warmest at surface and the development of small thermoclines as overturning was limited.
Turbidity
The transmissometer readings out in the estuary were higher and more uniform throughout the water column out in the estuary (F29, G30) compared to the river stations (A22, B23-26). Station A22, C27 and D28 show an increase of turbidity with depth, while the water was of a uniform turbidity of about 4.2 FNU at station B and did not change much over the course of the different deployments.
Highest turbidity values were identified closest to the mouth of the estuary. Sites closest to the estuary also showed greatest uniformity of turbidity values across depths. These trends can be related to the input of suspended material to the estuary by surrounding rivers, resulting in the accumulation of suspended matter within the estuary across all depths and higher turbidity values as a result.
Fluorescence
The depth profile shows fluorescence to, overall, be uniform with depth. Peaks in fluorescence are shown to occur in the water column as depth increases at certain stations.
Station A is positioned in the estuary furthest upstream, closest to where to river input. The Stations then get progressively further downstream, closer to the mouth of the Fal Estuary, from B to G. Stations A22 to B26 show similar fluorescence values. Surface waters have values of 0.30-0.40abs and all stations follow the trend of fluorescence increasing with depth. From ~0.2m, all stations show a gradual increase in fluorescence which then decreases slightly by 0.7m. At Station A22, a peak in fluorescence was measured at the surface, with a value of ~0.70abs, and another peak of over 1.0 was found at the maximum depth sampled. Peaks were also measured at the maximum depth sampled at Stations B23, B24 and B26 with values of 0.85, 0.75 and 0.70abs respectively. The fluorescence measured at Stations C27 and D28 was also around 0.30abs in the surface waters but the trend at these two Stations shows the amount of fluorescence measured to decrease with depth. Both Stations also show peaks in fluorescence at the maximum depth sampled. Stations F29 and G30 are shown to have fluorescence values of 0.20abs in the surface waters, less than in the surface waters of the other stations further upstream. They too show a slight peak from ~0.3m to 0.7m depth, with a maximum value of 0.35abs. From 10m to maximum depth sampled the fluorescence then becomes, overall, uniform with depth.
Average fluorescence across the water column was found to decrease with movement towards the mouth of the Fal estuary. This may be relatable to turbidity correlations analysed above, which showed the opposite trend. An increase in turbidity would result in an increase in light attenuation, which would in turn limit photosynthesis. The limiting of photosynthesis would result in reduced chlorophyll concentrations, and lower fluorescence across the water column. Spikes occur in data from sites A22 and B23. These are most likely errors caused by mechanical inaccuracies.
Estuarine Physical Data - CTD
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