Falmouth Field Trip 2014- Group 3

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Produced by: Alice Duff, Philippa Fitch, Joanna Gordon, William Harris, Thomas Jefferson, Eirian Kettle, Jesse Marshall, Dominique Mole, Emma-Jo Pereira, Joshua Walton

Results - Figure 1 shows plots of current speed and direction through the water column over a 16 hour period. The arrows show the orientation of the water flow (left=south, right=north), with the length of each arrow relating to speed. Some plots display little or no arrows due to an instrument malfunction where speed was not recorded and therefore could not be plotted. Grey boxes on each graph represent the sea-bed. Inconsistencies within data may have been caused by disturbance by boats and larger ferries during sampling.

Current speed generally increased at one to two meters depth, and then decreased downwards, mostly having the lowest speeds at the sea-bed. A clear change in direction can be seen through the hour period of 13:30 UTC to 14:30 UTC sample, going from south to north. Previously to this, nearly all data showed an orientation to the south (anomalies possibly due to boat disturbance) and after 14:30 UTC all directions were north. A large difference in depth was seen throughout the day, ranging from 1.6m to 4.8m. The shallowest depth was at 11:00 UTC, and the largest depth of 4.8m was seen at 07:30 UTC and 15:00 UTC.

Discussion - Current speed changed through the water column as expected due to the increased friction at the sea-bed which reduces speed. Flow direction is dependent upon the tides (NGA), which is clearly shown within the plots as a change in orientation from south to north is seen just after the time of low water (12:25 UTC, UKHO) due to the transition between an ebb and flood tidal flow. A time lag is present which can be explained by the tidal information relating to the estuary mouth whilst the pontoon is situated further north, shown by the map.

Pontoon

Currents

Temperature

Salinity

pH

Dissolved Oxygen

Turbidity

Chlorophyll

As the tide begins to ebb at 09:00 UTC, the cooler seawater being flushed out of the estuary was replaced with warmer riverine water. During low tide (11:00-12:00 UTC) the shallow water begins to heat up. When flood tide begins there is still a lighter warm body of water above the more dense cold seawater that was heated by the midday sun.

During high tide there is a greater amount of saline seawater in the estuary. As the tide begins to ebb this saline water is flushed out of the estuary and replaced by less saline riverine water. At low water the lowest salinity reading were recorded, 31.2. At flood tide the denser seawater begins to flow back into the estuary pushing the lighter riverine water up creating stratification at 14:00 to 15:00 UTC

Seawater has a higher acidity due to a higher concentration of dissolved CO2 (Hofmann et al., 2009). During high tide (07:30 to 09:00 UTC) the data reading from the YSI probe show a low pH. As the tide begins to ebb around 09:00 UTC there is a distinct change in pH as the more saline water is flushed out of the estuary and replaced by more alkaline riverine water. As the tide begins to flood, the lighter riverine water floats on top of the denser acidic seawater that is beginning to flow back into the estuary.

The oxygen saturation is linked to the temperature. O2 saturation tends to be higher when temperature is higher. A relatively low ODO (%) is seen at high tide (07:30- 09:00 UTC). As the tide begins to ebb, the O2 saturated seawater is flushed out of the estuary past the YSI probe. At low tide (11:00 to 12:00 UTC), the ODO is at approximately 120%. As the tide begins to flood again, the ODO level remains high which may be due to photosynthetic activity of organisms such as phytoplankton during the midday sun.  

As the tide ebbs from 09:00 to 12:00 UTC, the water shallows and friction between the riverbed and the overlying water column increases. This increase in friction results in an increase in re-suspension and thus turbidity. The highest turbidity recordings were taken at low tide, 11:00- 12:00 UTC. During the flood tide, turbidity drops, but is still higher than during the ebb tide. Figure 1 may suggest this is due to the flood tide having a higher speed than the ebb tide – this however, is difficult to conclude confidently as some measurements for the flood tide are missing due to the current meter malfunctions.

At high tide, the chlorophyll levels were relatively high. As the tide begins to ebb, data shows the chlorophyll levels dropping. This may be due to organisms such as phytoplankton begin flushed out past the sample station with the ebb tide (Trigueros and Orive, 2000). Chlorophyll levels would be expected to return to a higher level, this was not reflected in the data. This could be due to the increase riverine input from a few days of heavy rainfall.

Click to figure enlarge