GROUP 11
FALMOUTH 2017
Estuary Time Series
To generate a time series at King Harry Pontoon by sampling to create a depth profile of different parameters. By measuring current flow direction and velocity, identify how the water column structure changes with the tidal state and how this affects phytoplankton in the water column.
Location: The King Harry Pontoon (50º 12’ 58’’ N 005º 01’ 59.5’’ W)
Date: 07/07/17
Time: 12:0 0- 16:00 UTC
Weather: Initially 4/8 cloud cover, cleared to 2/8 cloud cover with almost no wind
Sea state: Smooth with respect to the Douglas Sea Scale
Tide times: Low tide at 10:27 UTC, high tide at 16:17 UTC (flood tide)
Once every hour a XO2YSI multiprobe was inserted into the water just below the surface and at 1m intervals, as deep as it would go which was initially only 4m but as these pontoon surveys were undertaken during flood tide, this increased to 6m. The YSI multiprobe allowed for the recording of measurements of the following parameters: salinity; turbidity; chlorophyll a; temperature; dissolved oxygen. Light attenuation was also measured using a light sensor each hour, again at 1m depth intervals. Water samples were also taken in order to compare plankton species abundance between stations sampled on the vessels. Current flow speed and direction was measured in order to determine the tidal state.
Image shows location of King Harry’s Pontoon, the location used to create a time series of depth profiles.
Falmouth
Truro
King Harry’s Pontoon
Figure 1 shows the highest concentration of chlorophyll was at 13:30 around 2m deep, at this time the chlorophyll is low below 4m. From around 15:00 the chlorophyll concentration is relatively constant with depth.
Figure 2 shows Oxygen saturation is highest around 14:00 between 0.5 and 3m. It is relatively constant with depth at 13:00 and from 15:00, however from 13:30 to 15:00 the maximum depth of high chlorophyll concentration decreses.
Figure 3 shows salinity is highest at the surface around 16:30. It is lowest at the surface around 14:00, with a sharp gradient in salinity between these two times at the surface. At depth the gradient of change in salinity is much lower, changing from about 32 to 35 over the whole time scale (13:00 to 16:30).
Figure 4 shows temperature is highest at the surface around 14:00 but decreases to around 16.5°C at 15:00. It is lowest at depth around 15:30, but remains relatively low at depth throughout the time series (max 16.5°C).
pH is high (around 8.28-8.3) at 13:00, and highest at 3m. It decreases at both the surface and at depth around 14:30 till it relatively constant (8.24) throughout the water column at 15:30. At 16:00 pH increased below 3m and decreased at depths above 2m.
Figure 6 shows turbidity is highest (~3 NTU) at 6m and 13:00) it increases with decreasing depth (to about 1 NTU at 3m) and with time (to about ~3 at 15:00)
Temperature
Between 13:00 UTC and 14:30 UTC, the water column appears to be stratified. Generally, temperature is highest at the surface and then decreases with depth, which is what we would expect to observe. Around 14:00 UTC the heat penetrates deeper to around about 1.5m where the temperature is still above 18˚C. This could be explained by the sun being highest around 11:00 UTC and then over time the heat in the surface layer accumulating to slowly heat up the deeper waters. This is further aided by mixing of the water column by the wind. After 15:30 UTC, the surface is as cool as at depth. This could be explained by the incoming tide which will flush the warm surface layer of water away from the location on the pontoon up the estuary, replacing this water with much cooler water from off-shore water. (Montani et al., 1998).
Salinity
In general, salinity is lowest at the surface and highest in deeper waters, as expected. After 14:00 UTC, the salinity begins increasing at the surface as the tide begins to come in with the more saline water. Tidal input gradually increases above riverine input. Around 16:00 UTC, which is around high tide time, the salinity at the surface is as high as at depth (around 35).
Oxygen saturation
Oxygen is always super saturated this could be explained by the increase in photosynthetic organisms’ present. This could be supported by the phytoplankton abundances observed. Generally, Oxygen saturation is higher at the surface than at depth as would be expected. As the high tide comes the oxygen saturation decreases from 14:00 UTC at the surface waters because the tide flushes the more oxygen rich waters up the estuary.
pH
Between 13:00 UTC and 14:30 UTC, the water is more alkali near the surface than below 4m. This could be due to a combination of two factors; the first is that high phytoplankton abundance at the surface, leads to high uptake of carbon dioxide, resulting in higher pH. Secondly, the lack of rainfall in the past few days means that there would have been less weathering of rocks, hence less dissolved carbon dioxide and higher pH. After 14:45 as the tides starts to come in, this body of water is flushed further up the estuary and is replaced by sea water, which has a uniform pH of around 8.22-8.24.
Turbidity
Between 13:00 UTC and 14:45 UTC, turbidity near the seabed is high, ranging from 2 to 2.5 NTU between 5 and 6 m. Above 4 m turbidity is much lower at about 1 NTU. This is due to the suspended load in the river water being higher near the river bed. After 14:45 UTC, this signal of higher turbidity disappears which is likely due to the tidal input outweighing the riverine input as the tide rises up the estuary.
Chlorophyll
Before 14:45 UTC chlorophyll concentration is higher at the surface than at depth, due to a higher abundance of phytoplankton at the surface where light intensity is greatest. The area of low chlorophyll concentration below 5 m between 13:00 UTC and 14:30 UTC is likely due to the area of high turbidity limiting light penetration and hence phytoplankton growth. As the tide comes in after 14:45 UTC, the water is flushed and well mixed, even at depth. This homogenises the chlorophyll concentration across the water column.
Figure 1: Shows chlorophyll highest concentration at 13:30, 2m. Lowest at 6m at 13:00
Figure 2: Shows Oxygen saturation highest at 1m, 14:00 and lowest at 6m at 15:00
Figure 3: Shows salinity lowest at the surface at 14:00 and highest at the surface at 16:00
Figure 4: Shows temperature highest at the surface at 14:00 and remains low at depth throughout the time series
Figure 5: Shows pH high at the start of the time series (13:00) and decreases throughout the water column at around 15:00
Figure 6: Shows turbidy highest at 6m at the beginning of the time series, and decreases with time and increasing depth
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