PONTOON RESULTS
AND DISCUSSION
Results:
The profile associating temperature and depth displays two main trends (Fig. 1). Firstly, temperature is seen to decrease with depth (except at 1000 UTC, where at 3 metres, the temperature increases from 15.39 °C to 15.43 °C). For example, at 0900 UTC the temperature decreases from 15.34 °C at the surface to 15.30 °C by 1 metre, further decreasing down to 4 metres. Time 1100 UTC shows the largest decrease in temperature (from 15.80 °C at the surface, down to 15.55 °C by 2 metres; the deepest depth). The other main trend seen is that of an increase in surface water temperature over the duration of the time series from 0900 UTC to 1100 UTC (from 15.34 °C to 15.85 °C).
Discussion:
The decrease in surface water temperature between 0830 UTC to 0900 UTC (from 15.61 °C to 15.34 °C) is thought to result from the sampling equipment being moved from the estuarine side to the landward side of the pontoon which is significantly more sheltered and will show inherent differences in the physicality. This change in sampling site occurred after the first depth profile was undertaken at 0830 UTC. The two main trends can be explained by separate oceanographic factors. Firstly, surface water temperature increased between 0830 UTC and 1100 UTC. This is due to solar heating; in the morning there is very little solar heating which increases as the morning progresses. However, the increase in surface water temperature is minimal – only 0.51 °C over two and a half hours. There may be a further and more significant increase as the day progressed past 1100 UTC however sampling finished before this could be detected. Next, the decline in temperature with depth can be explained by two main factors. One of which being that cooler water is denser and so sinks below the warmer surface water [TERC, 2016] and the other, light penetration (thus heating) decreases exponentially with depth and so temperature will naturally be higher at the surface (Fig 1) [TERC, 2016].
Figure 1. Variations in water column temperature with increasing depth, shown as a temperature depth profile. Recordings were taken using an EXO probe at the King Harry’s Ferry pontoon (fixed location).
Results:
Salinity also displays two main trends (Fig. 3). First, salinity increases with depth (except at 1000 UTC, where there is a slight decrease in salinity from 2.8 to 3 metres depth; 28.56 PSU to 27.90 PSU). For example, the 0830 UTC time station salinity begins at 27 PSU at the surface, increasing by 0.5 PSU by 1 metre depth, which continues to increase to 28.10 PSU by 4 metres depth. Not all increases are as dramatic with depth however; time station 0900 UTC begins at 28.15 PSU at the surface, only increasing to 28.20 PSU. The second trend is that surface salinity values increase with time from 0830 UTC to 1000 UTC (from 27 PSU to 28.51 PSU) afterwards, value decreases down to 27.95 PSU by 1030 UTC, further decreasing to 27 PSU by 1100 UTC.
Discussion :
At 1000 UTC there is a minor decrease in salinity between 2.8 metres and 3 metres, completely inconsistent with the rest of the data. This is thought to be an outlier due to an instrumental error during sampling at this time and location. There are two trends seen in figure 3, whereby salinity increasing with depth, and the variability within the surface salinity samples. Firstly, the increasing salinity with depth can be reinforced by the data in figure 3. Temperature decrease with depth shows the colder water sitting below the warmer surface water; temperature being the other contribution to density other than salinity. Therefore, the cooler, more saline water is denser and thus sinks and sits below the warmer, less saline and less dense surface water. Secondly, the Fal estuary is partially mixed, and within partially mixed estuaries tidal currents create turbulence which stimulates vertical mixing within the water column. However, these tidal currents are deficient in strength to fully mix the water, thus salinity varies vertically and horizontally causing this salinity stratification [Webster et al, 2016].
Figure 3. Water column salinity shown on a salinity depth profile of the Fal estuary at a fixed location.
Figure 5. The variation of light transmission with depth for the 8 sampling stations on the Fal Estuary, measured using a CTD.
Results:
The first notable trend seen in figure 5 is at time station 0830 UTC the percentage of light penetration starts off at the surface at ~ 0.1 %, staying constant down to 1m where it suddenly increases up to 11 % penetration, then decreasing once more down to 3 % by 5 metres. The trends, from 0900 UTC onwards changes dramatically. Secondly, surface light penetration increases from 0900 UTC to 1000 UTC (from 18 % up to 57 % penetration). From 1000 UTC to 1030 UTC, surface penetration decreases from 57 % to 44 %. However, from 1030 UTC to 1100 UTC, surface water light penetration increases from 44 % to 55%. Lastly, the other notable trend is that with depth, % of light penetration decreases exponentially. For example, at 1000 UTC at the surface, light penetration is at 58 %, whereas by 2 metres depth, this had more than halved (down to 13 %), continuing to decrease down to 8 % by 3 metres.
Discussion:
The data for time 0830 UTC is slightly random and does not follow the trend seen in the rest of the data set. This can be partially attributed to the fact that after 0830 UTC sampling had to be moved to the other side of the pontoon, where the values would be fundamentally different from the side facing into the estuary. The other reason for the random dataset at 0830 UTC could be credited to instrument error. From 0900 UTC onward, the trends are much more regular and normal. Light penetration is not 100% at the surface due to the fact that an amount of light is reflected when it reaches the surface of the water, especially if the water is more turbulent [Water Encyclopaedia, 2016]. The other main point to consider is why the light penetration decreases exponentially with depth. Light penetration depends on reflection at the surface, refraction in the water column and the properties of the light itself (wavelength and intensity). Larger amounts of solid particles in the water column will reduce the penetration depth of light. Therefore, water in estuaries which is more turbid (due to various particles brought in by fluvial systems and marine based inputs, plus the biological content of the water) display an exponential decrease in light penetration with depth (Fig. 5). The slight anomaly between 1000 UTC to 1030 UTC where surface penetration decreases from 57 % to 44 % could be due to a slight increase in surface turbidity at this time where a boat caused an abnormally large wake increasing the level of reflection at the surface (water encyclopedia, 2016) for this single sample (by 1100 UTC the penetration increases back up to 55% when the surface was less turbulent).
Figure 6. Variations in water column flow speed (velocity) at various depths , at a fixed location in the Fal estuary. Measurement period between 0830 UTC to 1100 UTC, recording periods were separated by 30 minute intervals.
Results:
The trends in figure 6 are difficult to deduce. There is one major trend in surface flow, which is that from 0830/0900 UTC, current speed increases from 0 m/s to ~ 0.14 m/s by 1000 UTC. This then decreases to 0.02 m/s by 1030 UTC, once again increasing to 0.03 m/s by 1100 UTC. Depth is even more variable than the surface flow trends; at 0830 UTC the surface flow was shown to be 0 m/s (increasing to 0.14 m/s by 2 metres depth). However, deeper than 2 metres, the current speed decreases majorly to ~ 0.04 m/s at 3 metres depth (staying fairly constant down to 4 metres depth). The depth profile at 0900 UTC is in a pattern of increasing from 0 m/s at the surface to 0.03 m/s by 1 meter. This increase and decrease in current flow continues down to 4 metres, where it stops at 0.02 m/s. At 0930 UTC the depth profile is a little more regular, such that flow increases 5 times from 0.01 m/s at the surface to ~ 0.044 m/s at 2 metres depth. However current flow decreases again at 3 metres depth down to 0.03 m/s. At time 1030 UTC the flow becomes more regular again, tripling from 0.02 m/s to 0.062 m/s down at 1 metre depth. The flow decreases slightly down to 0.06 m/s by 2 metres depth. Lastly, at 1100 UTC, flow increases by just over 5 times from 0.03 to 0.16 m/s from the surface to 1 metre depth.
Discussion:
Figure 6 shows minimal patterns of current speed with temporal/depth variation. However, some trends can be identified; at 0930 UTC, the depth profile of flow measured throughout the water column is curved. There is a peak at 2 metres depth of ~0.044 m/s. This is due to the fact that at the surface there is friction slowing down the current speed down to 0.016 m/s and at the estuary bed there is friction slowing the current down to ~0.02 m/s. This theory can also explain the general peak in current speed at 2 metres depth for 0830 UTC, 1000 UTC and 1030 UTC time stations. The only other main pattern is that with increasing time of day there is generally a slowing down of current speed. This is because the tide changes from high tide at 0630, progressing to low tide by 1300; from slack to ebb tide. The remaining of the graph has very little emergence of patterns. This can be attributed to the mixing of the Fal estuary; it is a partially mixed estuary and therefore there is little current stratification. Lastly, the flow meter was not precise enough to measure the very low current speeds, attributed to the tide progressing from slack to ebb; the current was too low to turn the propeller measuring flow properly.
- Temperature decreases with depth – surface water temperature increases with increasing solar irradiance (percentage light penetration at the surface).
- Salinity increases with depth.
- Partially mixed estuary, only some mixing within the water column.
- Light penetration consistently decreases exponentially with depth.
- Current speed peaks at ~ 2 metres depth generally, decreasing at the surface and bottom of the water column.
References:
TERC, (2016). ‘About Water Temperature’. Available at: https://staff.concord.org/~btinker/GL/web/water/water_temperatures.html [Accessed: 29th June 2016].
Water Encyclopaedia, (2016). ‘Light Transmission in the Ocean -
Webster, I., Atkinson, I. and Radke, L. (2016). ‘OzCoasts Coastal indicators: Salinity’. Available at: http://www.ozcoasts.gov.au/indicators/salinity.jsp [Accessed: 29th June 2016].
Figure 2. The temporal variation in water column temperature, shown on a contour plot. Measures were taken at a fixed location on the King Harry’s Ferry pontoon.
Figure 4. Changes in water column salinity, measured at a fixed location in the Fal estuary. Salinity is shown to increase with depth.
The views and opinions expressed on this website are those of individuals within group 8 and are not associated with The National Oceanography Centre, University of Southampton or Falmouth Marine School.