Date: 03/07/17
Time: 14:30 - 17:00 BST
Location: 50° 012.570’N, 005° 001.390’W
Depth: 5-6m
Low Tide: 06:45 UTC (1.55m)
High Tide: 12:57 UTC (4.07m)
Low Tide: 19:10 UTC (1.66m)
The biological, chemical, and physical processes in the Fal estuary are tide dominated and so being able to assess the changes at one location in the estuary can tell us a lot about the estuary’s mixing throughout the day.
The aim was to take temperature, salinity, turbidity and chlorophyll measurements at regular intervals. Unfortunately the T/S probe at the pontoon was malfunctioning, so we were unable to attain temperature and salinity data over the time the data was collected on the pontoon. Therefore, the creation of depth profiles and analysis on the change in temperature and salinity down the water column over the course of the afternoon were not possible. We took our measurements during an ebb tide over a period of 2.5hours starting at 13:30 and ending at 16:00 UTC. Measurements were taken every hour in order to determine a time series over the day. Weather: 7/8 Cloud cover at 13:30 UTC, 8/8 cloud cover from 14:00 UTC onwards, very calm/flat sea state, 4m/s Southerly wind.
Light Sensor: A light sensor was used to collect data from the surface to the bottom of the water column at intervals of every 1 metre. Every 30 minutes two light sensors were used to measure the irradiance, one on the pontoon and one deployed over the side. This was to account for the changes in surface irradiance due to factors such as clouds. The irradiance data can then be used to indicate the turbidity of the water column.
Current Meter: An analog current meter was also deployed every 30 minutes. The current meter measured both the direction (degrees) and the speed (m/s) of the current. Measurements were taken from subsurface to the bottom of the water column to compare the changes in tidal flow throughout the day.
Niskin Bottle: Chlorophyll samples were collected using a Niskin bottle at subsurface and 4m every hour. A syringe was used to filter 50ml of the water sample through a GFF filter. The filter paper was then folded and stored into a test tube of 90% acetone and stored in the freezer to be analysed in labs.
Figure 3 shows that irradiance was highest at surface waters compared to depth, which is supported by the contour plot Figure 5. The irradiance quickly declined between 0 and 2 m, then decreased more slowly deeper in the water column. The waters below 2 m depth have consistently low irradiance across the time series, which is expected, as light does not generally penetrate far into the water column. It is also after midday,when the light energy is strongest due to the angle of the sun.
Given that the time series was recorded from only 13:30 to 16:00, irradiance is expected to decrease in the surface waters over time. At 15:30UTC, however there is a peak in irradience at the surface. This can partly be attributed to the 8/8 cloud cover which occurred until 13:30 UTC, with a brief break of ⅞ cloud cover around 15:00 UTC. At around 15:00 UTC, however, there is an abrupt decrease in light attenuation at the surface. This could be due to an increase in turbidity at the time, as there was a relatively fast current of water moving in the different direction to the ebbing tide as shown by the flow contour plot. The appearance of this fast-moving parcel of water could have been river input, which will have contained lots of particles and may lead to a decrease in irradiance at the surface at the time, however the direction of the flow appears to be upstream so this seems unlikely. We have also considered the impact of the frequent crossings by the King Harry Ferry to the pontoon, which may have disrupted flow during sampling at these times. After 15:30 UTC, the high level of irradiance disappears, and irradiance levels at the surface return to lower levels, possibly due to increased cloud cover. This inverse relationship between surface irradiance and light attenuation is well demonstrated by the contour plots for these two variables.
To calculate the attenuation coefficient we divided 1 by the gradient of the line of best fit of the logged Irradiance data, shown in Figure 4. Therefore it can be seen that the attenuation coefficient is similar for all of the measurements with values within a small range of 0.35 and 0.45 except at 15:00 UTC which had an attenuation coefficient of 0.55, which had a less steep slope of best fit line. The higher the attenuation coefficient, the more light that is attenuated at depth, which reduces the 1% light depth. At 15:00 UTC the 1% light depth was recorded to be 8.38m, compared to 13.11m at 14:30 UTC which had a lower attenuation coefficient (k) of 0.35m-1. Figure 6 supports this, as at 15:00 despite low surface irradiance, the irradiance decreases more rapidly with depth than at other times, suggesting that the attenuation coefficient is greater at 15:00 UTC.
A contour plot of chlorophyll from the pontoon site also seems to begin to change suddenly at around 15:00. It seems unlikely that this would be due to natural phytoplankton growth as it was such a sudden change at depths receiving low light. It may be due to the changes in flow observed at this time, it is possible that the strong upstream flow observed brought in chlorophyll from another area leading to the increase seen at the bottom of the water column.
Overall the unexpected changes in flow and chlorophyll seen have presented a challenge when interpreting results. Interpretation may have been made easier if a T-S probe was available. Looking ahead it may be helpful to combine this data with other groups to determine an overall pattern in the area.
