Aim

Aims to collect a time series data set to compare with other data collected up and downstream of that day.

Objectives of this data collection was to investigate tidal / diurnal influences on the chemical, physical and biological aspects of the estuary at one fixed station and to investigate how temperature and salinity varied with depth in the water column.

Results - Physical

Although the contours show a large variation in colour, there is very little change in pH across the time series, over time and depth. There is a slightly lower pH during the beginning of the time series between 08:35 and 09:35 UTC at both surface and depth and the pH increases very slightly as the time progresses, this could be due to the addition of Seawater with a higher pH as the tide floods in. Overall, there is little significance in the change in pH.

The O2 content is higher on the surface than at depth which can be expected due to air-sea interaction and breaking waves incorporating oxygen from the air. The minimum seen during 0-80 minutes after 08:35 UTC is due to the flooding tide. The High Tide was at a time of 15:45 UTC and so throughout the day the tide was flooding in; at this earlier part of the day the depth of the water column was only about 3m and this increased to about 5m throughout the day, therefore this beginning part Is extrapolated data which was not actually measured.  The contour of 9mg/L slopes off throughout the time series, this is because the depth was increasing and so at 3m the YSI was no longer hitting the seabed but was still immersed in the water column.

Figure 2. Contour Plot of Temperature (oC)/Time (UTC)/ Depth (m) at the King Harry Pontoon. Temperature consistently decreases from surface to depth, with a change of 6oC over the 5m change. There is a clear trend of warmer surface waters and cooler waters at depth. From 3m to the seabed, the water is fairly homogeneous however a there is a section of warmer water (20oC) occurring in the upper 2m between 09:35 UTC and 10:35 UTC.

The contour plot shows that this location is well stratified; with Salinity remaining constant along the time series but increasing over depth. There is a change in Salinity of 6 over the 5m water column. As the time progresses, the tide is flooding in. This is evident in the higher Salinity deeper waters pushing up and mixing with waters at around 2.5m depth.

Figure 8. Depth profiles (m) of temperature at time stations each with 30 minute intervals between each time station. The samples were taken from a pontoon on the river Truro. All stations show a negative correlation of temperature with depth. The time station 10:01-10:07 (Light blue) shows an outlier from this pattern. Over the time series notice an increase in surface temperature in the 3 metres below the surface.

Contour Plots

Vertical Profiles

Figure 9. Depth profiles (m) of pH at time stations each with 30 minute intervals between each time station. The samples were taken from a pontoon on the river Truro. All time stations show variation on the figure however the variation is very small e.g. 0.02 at 09:07-09:12 (yellow). There is no noticeable trend in pH throughout the time series.

Results: Chemistry

Table 2. Metadata collected on the pontoon near King Harry Chain Ferry on 05/07/17

Figure 10. Depth profiles (m) of Salinity at time stations each with 30 minute intervals between each time station. The samples were taken from a pontoon on the river Truro. All time stations show a positive correlation of increasing salinity with depth. Minimal variation of salinity profile with time throughout the time series.

Figure 11. Depth profiles (m) of turbidity at time stations each with 30 minute intervals between each time station. The samples were taken from a pontoon on the river Truro. All time stations have the same surface turbidity of 0 and show no variation of turbidity with depth. Time stations starting at 08:35 and 09:37 show outliers as both show turbidity spikes past 3 metres depth.

Figure 12. Depth profiles (m) of O2 saturation at time stations each with 30 minute intervals between each time station. The samples were taken from a pontoon on the river Truro. All stations show a similar trend of little variation of O2 saturation with depth. All time statins have a surface O2 saturation of 120% and show minimal increase with depth to 4 metres. Time station starting at 11:45 shows an outlier from this trend as it has a spike in O2 saturation at 4 metres up to 200% O2 saturation.  

Figure 13. Depth profile (m) of Chlorophyll at time stations each with 30 minute intervals between each time station. The samples were taken from a pontoon on the river Truro. All time stations show a positive correlation of increasing chlorophyll with depth. Time stations past 9:30 show a decrease at depth e.g.  the time station that started at 11:14 had a increasing chlorophyll concentration with depth up to 3 metres, past this point it rapidly decreased until it reached the floor.

Regarding the contour plots any data displayed over the true depth throughout the time series must be disregarded as the tide was ebbing and the data has been extrapolated.

Figure 1. Maximum Depth over time for pontoon station over 4 hour period.

Chlorophyll - Although Chlorophyll concentrations are variable over time and depth, generally there is a decrease in chlorophyll across the entire time series; with a more severe decrease in the upper 2metres over the first 1 hour, between 08:35 UTC and 09:35 UTC.

The little change in flow speed in surface layers is explained by the constant flow rate from the riverine system. The more vertical contours in the bottom 2metres can be explained by the increasing marine influence on flow rate of the deeper marine water mass during the flooding tide up to high tide at 15:20. The intermediate layers show mixing as the increasing flow rate seems to resonate from deeper waters.

The flow direction may be misleading because of the way direction has been measured; the difference between 50o and 350o gives the impression of a large change in direction, however there is only 60o between the two. The flow in the surface layers at the beginning of the time series is an area of interest. Low water occurred at 09:50 UTC, with the flow direction time series beginning just 20minutes after Slack water. In the upper 1m we see flow travelling southward at 200o on this turn from Slack water at low water to travelling at 50o on the flooding tide.

Figure 14. Percentage irradiance over a time starting from 08:48 UTC (Time Series 1) through to 11:15 UTC (Time Series 4). Exponential decrease in irradiance as depth increases with small variation between different time series all reaching around 15 % irradiance at 3 m. Time series 3 has the lowest percentages of the data.

Figure 15. Natural log of percentage irradiance time series between 08:48 UTC (Time series 1) and 11:15 UTC (Time Series 4). Lines are near straight however still differences in line gradients with depth changes. Time series 1 has inverse line at from 2-4 m which returns to negative relationship from 2 to surface.

Disclaimer: All the opinions expressed in this site are that of group 14 and not necessarily the University of Southampton or the National Oceanography Centre, Southampton.

The warm section seen between 0m and 2m could be due to a passing vessel. During the 4hour period in which measurements were taken on the Pontoon, there were numerous passing vessels which might have emptied water at this location; both of these factors could contribute to the slightly increased surface temperatures. The temperature contours slope down over time; this indicates the flooding tide increasing the depth of the water column, mixing warmer surface layers with cooler lower layers.

Figure 7. Contour Plot of Flow Direction (o from North)/Time (UTC) /Depth (m) at the King Harry Pontoon. Flow direction is measured in degrees from North, with red being 350o or 10o from north. The flow direction in the upper layers in travelling Southwards.

Figure 6. Contour Plot of Flow speed (m/s)/ Time (UTC) / Depth (m) at the King Harry Pontoon. The flow rate oscillates within the top 2 metres throughout the time period 10:20-11:20 between 0-0.2 m/s. The flow speed contours are more horizontal in the surface layers indicating little change in flow speed in the surface layer up to 1 metre depth. In the bottom 2 metres the flow rate contours are more vertical representing a more rapid change and steeper gradients within the water column. All speeds can be regarded as fairly low speeds.

Figure 4. Contour Plot of O2 (mg/L)/Time (UTC)/ Depth (m) at the King Harry Pontoon. Oxygen content varies from 8.0mg/L to 10.0mg/L through the time series. The O2 content is higher on the surface than at depth. During 0-80 minutes after 08:35 UTC the oxygen content reaches a minimum of 8.0mg/L at depth.

Figure 3. Contour Plot of pH/Time (UTC)/ Depth (m) at the King Harry Pontoon. The pH measurements remain relatively constant through time and time, with pH concentrations thetime, increasing from 8.12 at 08:35 UTC to 8.26 at 11:35UTC, however overall there is no significant pH change.

Figure 5. Contour Plot of Salinity/Time (UTC)/ Depth (m) at the King Harry Pontoon. Surface salinity remains constant across the time series at a value of 31, showing a stratified estuary. Salinity increases with depth; increasing from 31 at 0m to 37 at 5m between 08:35 UTC and 08:55 UTC.

Temperature

pH

Dissolved Oxygen

Salinity

Flow Speed

Flow Direction

The negative correlation with depth observed is due to solar radiation on a sunny hot day (conditions that occurred on the day of sampling). The solar radiation would warm the surface layers more as the day progresses due to increased time of exposure and increasing solar intensity. This explains the second overall trend observed in figure 8.

The variation of pH is so minimal that the variation observed maybe explained by the accuracy of a YSI probe’s ability to measure pH of a solution up to 0.01 precision. This would suggest that pH is more constant with depth and time than visualised on figure 9 .

The positive correlation observed on this figure is explained by the mixing processes that occur in an estuary. The first time station is more stratified as it has lower temperature and stronger riverine input than time stations later on in the time series. Later time series e.g. time station beginning at 11:45 shows a more mixed water column which is expected as it has a higher temperature and stronger tidal influence due to the presence of a flood tide.

The increase in surface turbidity is explained by an increasing chlorophyll concentration which can be observed in figure 6. The increased phytoplankton abundance associated with this observation would cause the turbidity of the water to increase. However, as the pontoon has a high level of boat traffic the influence of these vessels on the surrounding water column must be considered when surveying the turbidity. The spikes observed at the deepest point of each profile is irrelevant to analysis as this is due to the sensor making contact with the bottom of the estuary producing faulty readings.

The high oxygen saturation observed at all stations was constant with depth this is due to the shallow nature of this water column and the low turbidity observed in figure 4. This increased the sunlight penetration which facilitated a high level of photosynthesis at all depths. The outlier time station 11:45-11:49 was an anomalous result.

The positive correlation observed in this figure seems to show a chlorophyll maximum at depth. This was determined as past a peak at depth all time stations show a decrease in chlorophyll toward the bottom of the estuary. This chlorophyll maximum may have occurred due to an influx of marine water associated with the flooding tide. This influx may cause an increase in nutrients at depth causing the chlorophyll maximum to occur at depth.

Temperature

pH

Salinity

Turbidity

Dissolved Oxygen

Chlorophyll

Discussion

This follows the general trend of light attenuation in water as light is absorbed (at different wavelengths) gradually, decreasing the amount of available light at depth. This “half-life” of light, seen in Figure 14, will influence many of the biological factors in the system such as the chlorophyll concentration. Light attenuation directly influences the availability of light in the water column which will mediate the ability of phytoplankton to photosynthesise.

Figure 2 is a natural log of the exponential graph to provide a more linear plot. However, as the plot is not linear there is variation in the rate of reduction of light with depth. This would suggest that the water column is not homologous in regards to turbidity.

Precipitates form in an estuary due to the process of flocculation when the riverine influx comes into contact with the marine system. This increases the suspended solids within the estuary. When combined with high chlorophyll concentrations (see above) present in an estuary, the result in a lower clarity of water. Organics, nutrients and sediment particles will refract light in the water column reducing the penetration of light. This would reduce the depth that light is able to penetrate.   

The variation of light attenuation throughout the time series is minimal. The intensity of light varies throughout the day due to the suns position in the sky. To calibrate to this variation, we used a sensor that was exposed to light constantly throughout the day above the surface of the water. When the tide had come in by time series 4 (11:15) it was possible to go deeper down to 4 m which some gave an extra insight what happens with depth.

Map 1. Locations of Pontoon near King Harry Ferry on 05/07/17