University of Southampton OES Undergraduate Falmouth Field Course 2016 - Group 3 databank and initial findings.
Disclaimer: The views and opinions expressed are solely those of the contributors, they do not reflect the views and opinions of the University of Southampton.
References
Atkinson, C. Jolley, D. Simpson, S. (2007). Effect of overlying water pH, dissolved oxygen, salinity and sediment disturbances on metal release and sequestration from metal contaminated marine sediments. Chemosphere, 69(9), pp. 1428-1437.
Cole, B. and Cloern, J. (1987). An empirical model for estimating phytoplankton productivity in estuaries. Marine Ecology Progress Series, 36, pp.299-305.
Davies-Colley, R.J. and Smith, D.G. (2007) Turbidity Suspended Sediment, and Water Clarity: A Review. Journal of the American Water Resources Association. Vol. 37, Issue 5, Pages 1085-1101.
Geyer, W. and Farmer, D. (1989). Tide-Induced Variation of the Dynamics of a Salt Wedge Estuary. J. Phys. Oceanogr., 19(8), pp.1068-1072.
Simpson, J. Brown, J. Mattews, J. and Allen, G. (1990). Tidal Straining, Density Currents, and Stirring in the Control of Estuarine Stratification. Estuaries, 13(2), p. 125.
Soulsby, R. and Clarke, S. (2005). Bed shear-stresses under combined waves and currents on smooth and rough beds. Wallingford: HR Wallingford.
Zappa, C. Raymond, P. Terray. E and McGillis, W. (2003). Variation in surface turbulence and the gas transfer velocity over a tidal cycle in a macro-tidal estuary. Estuaries, 26(6), pp.1401-1415.
pH
Over the course of the time series from 12:00 to 15:00 UTC the pH increases, this means that the estuary is getting more basic. From 12:00-13:00 UTC the water column has a uniform pH. From 13:00- 15:00 UTC the upper three meters of the water gets more basic uniformly as time passes. From 3-4 meters, the water changes more quickly than the upper 3 meters and so at a specific time the upper water has a lower pH and so is more basic than the lower water column.
Salinity
Over the course of our time series measurements, salinity is shown to decrease from ~ 33 PSU to ~29 PSU within the surface layer (0-2m). At the start of the time series, the salinity data shows a slight increase with depth but then remained constant at ~33psu throughout the water column between 12:00 UTC and 13:30 UTC.
The results of the time series data collected 25th June 2016, between 12:07 and 15:01 UTC show the progression of light attenuation, temperature, salinity, flow speed, dissolved O2 and chlorophyll over time and through the water column. The amount of light attenuation through the water column over the time the data was collected remained constant at 50%. The time of day meant the irradiance from the Sun was relatively constant. At 14:10 UTC the St. Mawes ferry that docks on this pontoon had just left. There is a strong possibility that this could be the reason for the sudden decrease in light attenuation at the surface (from 50% to ~20% at 14:10 UTC) because the wake from the ferry causes increased scattering of light through the water column [Cole and Cloern, 1989]. The ferry will also increase turbidity due the disturbance of the water by the propellers [Davies-Colley & Smith, 2007]. At low tide, the water depth is shallower and therefore the water heats up quicker due to a smaller volume of water being heated [Simpson et al. 1990]. This is why the temperature in the surface waters around the pontoon is shown to increase towards low tide and be highest at this point in the time series data.
As time progresses towards low tide, the volume of river water input to the estuary is greater than the input from the sea [Geyer and Farmer, 1989]. The water input from the River Fal and the River Truro is fresh, therefore when the river input outweighs the sea water input salinity in the estuary will decrease. This explains the decrease in salinity observed in the surface layer (0-2m).
The presence of chlorophyll in the water indicates the presence of phytoplankton, algae and other photosynthesising organisms. The amount of chlorophyll is greatest from 1m to 2m depth because light attenuation is high (~50%) [Cole and Cloern, 1987]. Dissolved oxygen increases as time progresses towards low tide because of the increase in surface water temperature also observed [Zappa et al. 2003]. The percentage of dissolved oxygen is highest in the surface waters due to the presence of photosynthesising organisms. The percentage then decreases as depth increases as fewer phytoplankton are present at depth where less light is available [Cole and Cloern, 1989]. The smaller amount of dissolved oxygen present at depth could also be due to bacteria in the sediments that are decomposing detritus and respiring oxygen.
The minor oscillations in flow speed, in the surface layer over the time series profile, are expected due to small variations in water flow. As depth increases, speed of water flow decreases due to friction between water layers and shear stress at the river bed [Soulsby and Clarke, 2015]. The peak at 14:10 UTC could be due to the St. Mawes ferry leaving the pontoon which, as it left, caused an increase in flow speed as water was increased by the propellers. The propellers extended down into the water and also caused an increase in flow speed at depth.
There is an increase in pH from 13:00-15:00 UTC, this is may be due to the tide being on ebb, this means that there is a decrease in the seaward input and so the river input has a larger effect on the water. Due to mining and agricultural runoff into the river, the river input is more basic than the seaward input and so the estuary becomes more basic. As the tide gets closer to low tide there is less water in the estuary and so more sediment is suspended. This sediment contains heavy, basic metals released by mining which causes the water to become more basic. [Atkinson, Jolley and Simpson, 2007]
Pontoon Observations - Discussion
Dissolved oxygen
Within the surface layer (0-2m) dissolved oxygen % increases in value from ~120 to ~126% during the time period of 12:00-15:00pm. Dissolved oxygen declines with depth to a value of ~108 %. Near the end of the time-series (~14:10pm) the dissolved oxygen gradient is greatest at ~3.5m deep.
Pontoon Observations - Contour Plots
Flow speed
Speed of water flow oscillates within the surface layer (0-2m) throughout the time period (12:00-15:00 UTC) between the values of 0.35- 0.10 m/s. The water column below 2m depth, from 13:07pm onwards, has a flow speed that remains relatively low at ~0.10 m/s. At 14:10 UTC, the flow increased to about 0.35m/s and was high down to a meter above the river bed. The direction data indicated that the water was flowing South, out towards sea, as expected on ebb tide.
Light attenuation
As we sampled towards and through low tide at 14:41 UTC, the graph shows quite clearly how the tides affect the light attenuation. While the water depth decreased steadily towards low tide (from 12:00 UTC to 15:00 UTC), the light attenuation stayed fairly constant at about 50% at the surface and through the first two meters of the water column. Then the amount of light rapidly decreased at about 14:10 UTC to around 20% in the surface layer. The light was rapidly absorbed within the first meter of water, but the surface attenuation decreased again towards the end of the measurements.
Temperature
From 12:00 UTC to 15:00 UTC, temperature increases in value from ~16.0 °C to ~17.0 °C within the water column down to a depth of 3m. Temperature is highest in the surface waters around low tide. Below 3m depth, temperature remains low at a value of 15.5°C.
Chlorophyll
The concentration of chlorophyll is highest within the 1-2m depth layer at ~ 14:10pm with a value of ~180 µg/L. Chlorophyll concentration increases from 60 µg/L at the start of the time-series (12:00pm) to 180 µg/L at the end of the time period.
