Aim:

The Pontoon located at Saltash in the midpoint of the Estuary Transect was the location of a time series, the aim of which was to investigate the changes in the composition of the estuary over time due to tidal differences.

Method:

Measurements were taken every 30 mins at both the surface and at 4m depth using the XO2 multiprobe. Using this we were able to sample the oxygen concentration, turbidity, chlorophyll, temperature and salinity. Those on the pontoon also measured the flow of the estuary, as well as using a secchi disk to assess light penetration.

Results:

Figure 1 shows the current speed throughout the water layer every half an hour from Saltash pontoon. This indicates the flow of the tide as well as the change in speed throughout the change leading to high tide and returning to low tide over the day. For this graph the negative direction is flowing up the estuary and positive current speed back out to sea. High tide being at 11:47UTC and Low tide 17:40UTC on the 6/7/18 can be seen on the graph as speeds switch from flowing into the estuary between the 10:30 and 11:15UTC lines. On average the fastest speeds are found between the 1-2m depth, with the fastest speed flowing out of the estuary at 13:30UTC (~0.4m/s). Surface speeds are typically small likely due to wind stress, the anomaly being at 8:30UTC with that lines fastest speed being found in the surface layer. This could be due to a matching of current speed direction with the direction of the winds frictional forcing boosting surface speed.

Figure 1: A map of the current speed and direction at various times and according to depth at Saltash pontoon

Figure 2: A time series of temperature plotted against salinity at Saltash Pontoon (50°40.981’N 004°20.611’W) 06/07/18.

Figure 2: Each graph in this figure takes the temperature and salinity data from the pontoon every half an hour on 06/07/18 showing each layer of the water column individually on a time series.

There is a clear inverse relationship between the increase in temperature and the decrease in salinity. Throughout the day, salinity content decreases and temperature increases with a noticeable spike after 13:00 in the surface layer. The highest temperature reached was 27°C.

The temperature decreases throughout the water column, which also matches the diffuse attenuation gradient profile.

In the bottom two layers between 11:00 and 12:00 there is a very sharp salinity change, it can be seen as a slight lag factor just after high tide (11:47)

Figure 3 shows the chlorophyll data over time and a variety of depths. Typically, at around 2 metres depth there is a chlorophyll maximum throughout the day. Furthermore, there is a relationship between light attenuation and chlorophyll concentrations whereby with decreasing light attenuation there is a decrease in chlorophyll apart from at the surface, this is due to frictional stress from wind and the air – surface interface.

There was a light sensor set up on the pontoon and also one on the multiprobe – both instruments were recording at the same time at half an hour intervals to give us the data for Ez/E0 ratio. In order to get the diffuse attenuation coefficient (Kd) we divided the surface PAR values by the PAR values throughout the water column and natural logged the ratio.

Throughout the day, all variables measured increased in almost all of the water column depths. This can be due to more exposure to light and the changing in tide. From the turbidity data, as it reaches high tide the turbidity values decrease due to a lack of movement in the slack. There’s a consistent dip in turbidity throughout the water column as we reach high tide.

There is an obvious positive relationship between turbidity and oxygen concentration, this is due to the increase in mixing which causes the oxygen to mix rather than settle. Oxygen also increases as chlorophyll increases which makes sense as chlorophyll is a photosynthesis indicator where oxygen is produced.

At the surface layer and at the 4th layer, the chlorophyll concentrations are lower than the surrounding layers which could be explained by shear stress by air-water interface and at the 4th layer the stress could be because of freshwater and marine water moving past each other.

Figure 3: A time series of turbidity, chlorophyll a, and oxygen at Saltash Pontoon (50°40.981’N 004°20.611’W) 06/07/18.

On 06/07/18 at 50°40.981’N 004°20.611’W (Saltash Pontoon) a time series was carried out throughout the day. A general decrease in temperature with depth occurred over the day, the temperature increases through the day as does diffuse attenuation coefficient. Over the course of the day, salinity decreases. This corresponds to patterns we would expect from the tidal cycle.

Figure 4: Time series contour plot of chlorophyll a concentration variation with depth at pontoon station. High tide 10:47UTC (4.57m) and low tide at 14:00UTC (2.07m).

Chlorophyll concentration is low as the tide floods a minimum of 1.8 µg/L at 4.5m depth at 8:30 and 5m depth at 10:30. Surface chlorophyll concentration is also very low at this time. Chlorophyll concentration is higher at 1 – 3.5m as the tide floods and around high tide at 10:47UTC. As soon as the tide begins to ebb chlorophyll concentration increases rapidly throughout the water column as the tide brings phytoplankton into the estuary and the chlorophyll concentration gradient throughout the water column disappears and the becomes more homogenous. Chlorophyll concentration reaches a maximum of ~7µg/L from 13:00UTC onwards. From 12:00UTC the chlorophyll gradient returns with lower concentrations at the surface.

Figure 5: Time series contour plot of oxygen variation with depth at pontoon station. High tide 10:47UTC (4.57m) and low tide at 14:00UTC (2.07m).

Oxygen concentration is low as the tide floods a minimum of 102% at 5m depth between 8:30 and 10:30. Oxygen concentrations at the surface are higher between these times (~110%). As the tide ebbs after high water at 10:47UTC oxygen concentration throughout the water column increases as phytoplankton are transported down the estuary and produce oxygen through photosynthesis. The oxygen gradient in the water column also disappears and the becomes more homogenous. Oxygen concentrations are highest throughout most of the water column between 12:30 and 14:30 reaching 122%. Surface oxygen concentrations between these time are lower (~110%) possibly due to the slightly lower chlorophyll concentration at the surface at this time, and therefore less oxygen being produced through photosynthesis.