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Introduction & Methods

AIM: To observe how the physical and chemical properties of the River Fal at the King Harry pontoon changed through a shifting tidal state over time.

All equipment was deployed at the same location - from the pontoon, facing the opposite bank.

Methods

Multiprobe recordings were taken every half hour from 08:30 UTC – 11:30 UTC. Recordings of depth, temperature, oxygen concentration (%), oxygen concentration (mg/L), salinity, pH, turbidity and chlorophyll a measurements were taken and recorded into a premade table. Measurements were taken below the surface and then at each meter until reaching the bottom of the estuary (3 or 4 m depending on time of day with the incoming tide).

A flow meter was used to measure the direction and speed of flow through the water column every hour from 08:30 UTC – 11:30 UTC. Recordings were taken below the surface and then at each meter until the bottom of the estuary (again 3 or 4 m). Measurements were recorded into the logbook.

The irradiance meter was used every hour to measure the irradiance at the surface and at increasing depths from below the surface and ending at the bottom of the estuary (3 or 4 m). 2 measurements were taken for each recording; the irradiance at the surface and the irradiance at the current depth. Measurements were taken every hour from 08:30 UTC – 11:30 UTC.

Water samples were taken to measure chlorophyll a in the water (water sample analysis for chlorophyll a tends to be more accurate than the multiprobe chlorophyll a measurements). A horizontal Niskin bottle was used as it is easier to deploy in shallower waters such as those by the pontoon. The Niskin bottle was used to collect water samples from 1 m and at 3 m, every hour from 08:30 UTC – 11:30 UTC. After collection, water samples were filtered and stored in numbered bottles containing 90% acetone. Their bottle numbers were recorded in the log book to be used in further chemical analysis.

Physics & Chemistry

Chlorophyll a

Figure 1 was created using the data collected every half hour between 8:30 UTC and 11:30 UTC on 6/7/2017, from the pontoon on the estuary. It shows how chlorophyll a concentrations vary with depth as the tides change. Low tide occurred at 9:41 UTC on this date, which can be seen in the contour plot by the steep loss of chlorophyll above 3m depth at this time. The turning of the tide is evident by the rapid increase in phytoplankton levels in the higher depth waters, caused by the increase of turbidity as the tide direction changes, causing mixing of sediments and dispersion into the water above. This explains why chl a concentrations are higher immediately after low tide.

Flow direction

Figure 2 is important when considering the effect of the tides on other characteristics. It was created using 4 depth profiles which took place every hour, using a flow meter. The graph demonstrates the direction of flow with depth, between 8:30 UTC and 11:30 UTC on 6th July 2017. This plot shows the exact point where the pontoon experienced the changing of the tide, at around 11:00, where the direction of flow changed dramatically, experiencing a 180o change between 10am and 11:30am. This is likely to impact most other variables within the later column. Plotting this as a contour graph makes it easier to see exactly when low tide is experienced here, as looking up tide times online in not necessarily accurate, since the low tide, which was said to be at 9:41 UTC is actually the tide expected in Falmouth Marina. Since the samples were taken much further upstream, there is a lag between the effects of the low tide being experienced between the mouth of the estuary and the pontoon. The graphs therefore show the actual time of low tide at the pontoon at the King Harry Passage.

The surface meter of the water does not change direction during this time period, as the fresh, warm and less dense riverine water flows downstream, lying on the surface of the more saline waters below. When the tide comes in, it moves up the estuarine basin as deep water due to its high density, and as the flood tide moves upstream at depth, it slows the flow velocity of the water in the layer above, and eventually reverses the direction of it, so that eventually, all the layers will change direction and flow upstream, however, the surface layer direction is reversed a lot later than the deeper layers.

Flow speed

Figure 3 shows how flow velocity changes with depth over a three-hour period. The graph is further proof of where the low tide occurs. This is show by the change in velocity to almost zero ms-1 between 9:30 UTC and 10:30 UTC matching the plot above, showing how tidal direction is changing at approximately 10:00am. Between 8:30 UTC and 10:00 UTC, the tide appears to be ebbing. Flow speed then increases again after 10am, and at around 11:00 UTC, where the direction of flow has rotated 180o, and the tide begins to flood.

Temperature & salinity

Temperature and salinity react inversely to each other during a low tide. As the tide ebbs, the main source of water supplied to the estuary is from riverine inputs, rather than tidal inputs. As river water is fresher and warmer than sea water, the closer to a low tide, the fresher and warmer the water within the estuary is, particularly at the surface, as river water is less dense than sea water. At 10:30 UTC, the fresh, warm water penetrates deeper into the surface layer than earlier in the morning, this corresponds with the low tide experienced at the pontoon on this day.

As the tide begins to flood, the deep water begins to become more saline and cold again as the water from the sea begins to move back up the estuary. This has a larger impact at depth because saline water is denser than fresh river water.

Turbidity

Turbidity is expected to increase around high tide as increased turbulence and mixing occurs, and while this is partially shown by the pale blue colour shown around 10:30 UTC, it is not the main cause of turbidity in this location. Turbidity is at its highest at 1m depth at 9:00 UTC, however, there is no clear reason for this to be the case in terms of tidal movement, but instead it is important to note than regular chain ferry movement was occurring close by, with ferries crossing the Fal River approximately every 20 minutes. Furthermore, there was irregular disturbance from other people using the pontoon, including canoeists and boats mooring up on the pontoon while these measurements were being taken. The turbidity in this location is likely to be due to increased sediment suspension in the water rather than phytoplankton distribution.

Light attenuation

Light Attenuation is seen to be increasing with time. This is expected as the weather improved throughout the morning. The little cloud cover that was present cleared throughout the morning as well.

Disclaimer: All the opinions expressed in this site are that of Group 2 and not necessarily the University of Southampton or the National Oceanography Centre.

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