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Introduction
On the 6th of July 2017, over a 4-hour period from 12:00 UTC to 16:00 UTC, we sampled the water column at a single location (The King Harry Pontoon). The weather was sunny, with a cloud cover of 2 and a sea state of 2 on the Beaufort scale (this varied as sea state was affected by passing vessels). The tide varied throughout the 4 hours and so the number of measurements being taken increased throughout the day. As the equipment was deployed by hand the depth of the measurements recorded would have some error and would change whilst readings were being taken. The maximum depth varied from 3.8m at 12:16 UTC to 6.0m at 16:00 UTC, with high tide at 15:35 UTC.


Method

The exoprobe measured temperature, salinity, dissolved oxygen, pH and depth at the surface and then at 1 meter intervals through the water column. The probe was partially submerged when between samples to make sure that the sensors were calibrated and worked efficiently. We sampled the water column with the exoprobe every half hour (a total of 9 times throughout the investigation).

The flow meter was deployed once an hour and measured the speed and direction of flow at the surface and then at 1 meter intervals through the water column. Flow was measured past the meter by recording the number and speed of rotations of the impellor.

To obtain the light attenuation, two light sensors were needed; one which was lowered (at one meter intervals) through the water column and the other stayed stationary at the surface. Two sensors are needed to account for temporal variation, creating a correction ratio. Light attenuation was measured hourly.

Every hour a horizontal Niskin bottle was deployed obtaining samples at 1 meter and 4 meter depths. The water samples collected were filtered and placed in test tubes with 6ml of 90% acetone to obtain chlorophyll concentrations in µg/L (which was analysed at the onshore labs using a Turner 10-AU Fluorometer). Three repeats were done per depth each time.


Results

Chlorophyll

The plot shows the changes in Chlorophyll concentration varying with depth across the time series taken from the pontoon. The pattern in Chlorophyll concentration and distribution is very varied. At around 12:00 when the time series began there is the highest Chlorophyll concentration. Low water was at 09:41 UTC on the day the study was taken and so across the time series the tide is flooding. The higher concentration could be at the beginning as water depth is less and o the chlorophyll is concentrated into a smaller space. As the time series moves on there is a more consistent pattern to the chlorophyll concentration, with the peaks being in the middle of the water column, between 5m and just before the surface.



Tidal Time Series


Velocity Profiles

Velocity measurements show that as the tide progressed towards high water at 15:36 UTC, there was a consistent low velocity at the surface, and patches of higher velocities at depths of 3 to 5m. Usually there is a higher velocity at the surface during ebbing tide when less dense river water is flowing down an estuary on top of seawater, and a higher velocity at depth during a flooding tide as denser seawater travels into the estuary with the rising tide. These measurements were only collected on a flooding tide; however, they do not exhibit the expected pattern consistently. This may be due to the many passing vessels that were noted.


When plotting the log of the ratio of surface irradiance to irradiance at depth, it can be observed that there is an increase in the gradient of the slope with time and tide level. An increase in the gradient of the lines of best fit with time was observed, showing that as the tide came in and the level of the water got higher, the attenuation of light increased. The attenuation coefficient (k, m⁻¹) also increased with tide time.

The views expressed here are not representative of the University of Southampton.



Turbidity

At the beginning of the time series the flow seems to be greatest especially at the surface. At 12:00 the tidal flow would be at its peak as it is half way through the 6-hour flow cycle. After this the flow rate drops down consistently with a slight increase at lower depths at around 14:30 UTC. Some of the data was filtered out as there was a pleasure boat that came into the pontoon interrupting the normal flow in the river, this came in at 16:00 so could be the cause of the increase at that time.


Light Attenuation

The attenuation of light with depth increases from 12:40 to 15:36 UTC with increasing tide level. A change in cloud cover from 2 to 4/8ths was noted at 14:37 UTC and from 4 to 5/8ths at 15:36 UTC – this may have been influencing the readings shown by the sensors. Data collected at 12:40 was overlooked as it displays an increase then decrease in irradiance down the water column which is unlikely. Results also may have been influenced by passing vessels and/or cloud cover. Additionally, an increase in 1% light depth with time and tide level was seen.