Group 2 Plymouth Field Course

THE PONTOON

Time Series from Probe

TURBIDITY

TEMPERATURE

SALINITY

O2 SATURATION

CHLOROPHYL A

O2 CONCENTRATION

On the 7th July 2018 we collected data for a time series at the pontoon at Saltash. This allows us to collect and analyse data which changes across a time scale, affected by the tide but not by changes in space such as that collected in the estuary.

Every half an hour, at intervals of every 1m in the 4m water column, a YSI multiprobe was used to measure temperature, oxygen saturation, dissolved oxygen, salinity, turbidity and chlorophyll a. A current meter recorded current speed and direction at the same intervals every half an hour, and a light meter measured the surface and sub-surface irradiance levels.

Every hour a Niskin bottle was deployed to the surface to collect a water sample from which 3 50ml samples were filtered and added to acetone to take to the lab for later analysis of chlorophyll. This was repeated at depth.

Irradiance Metre

Current Meter

Fig 1 - Time series contour of turbidity with depth at the Pontoon with a line showing high tide (11:44)

Fig 2 - Time series contour of temperature with depth at the Pontoon with a line showing high tide (11:44)

Fig 3 - Time series contour of Dissolved oxygen concentration with depth at the Pontoon with a line showing high tide (11:44)

Fig 4 - Time series contour of Dissolved Oxygen saturation with depth at the Pontoon with a line showing high tide (11:44)

Fig 5 - Time series contour of chlorophyll with depth at the Pontoon with a line showing high tide (11:44)

Fig 6 - Time series contour of salinity with depth at the Pontoon with a line showing high tide (11:44)

Fig 7 A - Irradiance with depth over six hours at the Pontoon

Fig 8 - Current speed time series from the pontoon over five depths

Fig 8 - Current direction contour over-laid on current speed depth profile; a=Before high water, b=At high water and c=After high water

a

b

c

Fig 7 B - Attenuation Coefficient Time Series from the Pontoon with the attenuation coefficient calculated from the PAR data and calculated from the Secchi Depth

Table 1 - Showing the Data used to calculate the attenuation coefficients for each time station at the Pontoon

The attenuation coefficient has been calculated for all times using two methods. The first method uses the Secchi depth and the empirical relationship KdxZs≈1.5 (where Kd is the attenuation coefficient and Zs is the Secchi depth). The second method is the Lambert-Beer Law which gives an estimate of the attenuation coefficient using the surface irradiance (Ed(0)) and the irradiance at depth z (Ed(z)): . The Lambert-Beer Law is a more accurate method as the Secchi depth is calculated by eye, allowing more room for human error. Holmes R.W, 1970, suggests that the Secchi disk is only acceptable for some work as it has large standard errors. The graph in figure _ shows that the attenuation coefficient calculated from the two methods follow a very similar pattern, with the lowest values occurring around 11:00 UTC time and the highest values at either end of the data set. This is expected as the smaller the attenuation coefficient, the less the light is attenuated with depth and the further the light reaches through the water column and the strongest light occurs at midday. The offset in time is due to the difference between BTS (British Summer Time) which we are currently in and UTC (Universal Time Clock) which is what the times are recorded in. The fluctuations seen on the graph at 09:00, 11:30 and 13:30 UTC may be explained by an increase in turbidity; light attenuation is increased by suspended sediments in the water column, which has a large impact on phytoplankton activity in the estuary (Cloern, J.E, 1987). The Tamar Estuary has a relatively high turbidity this year due to the warmer weather meaning the agricultural runoff is high.

Before high water, all flow is between the directions of 310°N and 360°N. At the surface, flow is all in the same direction regardless of velocity. However, at depth flow direction is more variable with velocity, as can be seen by the curvature of the contour lines. Halfway down the water column, there is flow in a slightly different direction compared to the water above and below it. This may possibly be due to there being less friction with the estuary bed and with the surface. The direction shown in this plot is expected, as northward flow (up the estuary) is expected during the flood tide leading up to high water.

Current at 11:47UTC, just after high water at 11:44UTC, is - as expected - much lower due to slack water being reached as the tide turns from flood to ebb. Direction of flow is more southward as ebb tide begins. Direction, covering a wide range between 92°N and 212°N, shows this change in direction. The direction is also more variable with current velocity – shown by the vertical contour lines

After high water the direction of flow changes to southward, between 120°N and 200°N. This is due to the ebb tide causing current to flow out of the estuary as the water level drops. At mid depths the current is most strongly in the southward direction with slightly more lateral movement at the bottom and top of the water column – possibly due to friction. Direction is much more variable with velocity at mid-depths.

The time series shows that at the surface current velocity weakens as high tide approaches, and beings to increase again during the ebb tide. This seems to be the general trend for all depths, with low velocities at slack water. Just before 10:00UTC velocity is higher at all depths, which is probably due to the maximum flood tide being reached. As well as before high water there is also a peak in velocity after 14:00UTC, a few hours after high water due to maximum ebb tide being reached. The velocities at the top and bottom of the water column seem to be, generally speaking, lower than mid water column. This be due to frictional forces with the wind at the surface or estuary bed at depth causing flow to slow down.

References

Holmes, R.W, 1970, The Secchi Disk in Turbid Coastal Waters, Limnology and Oceanography, Volume 15, Issue 5, 688694

Cloern, J.E, 1987, Turbidity as a control on phytoplankton biomass and productivity in estuaries, Continental Shelf Research, Volume 7, Issues 11-12, p1367-1381

The views and opinions expressed are those of the individual and not representative of the University of Southampton or the National Oceanography Centre.

The contour graphs are overlaid by the actual data points as black dots. This shows where the data supports the contour colour value and where the values have been assumed by the contouring software.

High tide was at 11:44. Samples collected before this were therefore on a flood tide, and afterwards on an ebb

The highest chlorophyll concentrations were recorded at the beginning of sampling, at 08:08 UTC, when the chlorophyll concentration at 3m was 4.7 micrograms/L. The presence of a high chlorophyll pocket around this time at 2-4m was further confirmed by other 08:05 and 08:35 measurements. Chlorophyll then reduced to around 3micrograms/L across the depth profile until a temporal increase to ~3micrograms/L, from 10:45-11:05 at depth 2-4m. Just after the high tide, a consistent, broad chlorophyll peak passed through at 3m to 5m, from 12:45 until the end of the transect.

The chlorophyll peaks observed imply the presence of photosynthetic organisms. The observed peaks show a mini ‘bloom’ of these organisms, passing by the stationary pontoon, moved by the tide. The presence of a large ‘pocket’ of phytoplankton at the start of the ebb tide is surprising, as this bloom was not seen passing by on the flood tide. It could have been related to the smaller increase in chlorophyll seen at 11:00, or possibly the greater pocket at 08:05, although it is unlikely this pocket will have made it back down the river in this time.

The initial 08:05 readings for dissolved O2 were consistently low (below 8micrograms/L). The rest of the transect showed some low oxygen pockets at the surface, such as at 09:30 and around the ebb tide at 11:30. Some higher oxygenated pockets were seen, such at 11:00 and after 13:00, which correlated with the higher chlorophyll concentrations identified at these times and locations

Aligning with the dissolved O2 Mg/L, the initial values were low, with surface pockets of low saturation identified around the turn of the tide also. Higher saturation was identified at 11:00 and after 13:00.

As expected, the salinity values increased as the tide came in and oceanic water intruded into the estuary, before starting to decrease after the tide turned. The deeper water was saltier, as the denser oceanic incoming tide tends to ‘slide’ underneath the riverine outflow of fresher, less dense water. Interestingly, the pocket of low oxygen on the turn of the tide is also seen here to be a pocket of low salinity

The saline oceanic water was cooler on the incoming tide, and at depth. After the turn of the tide, the pocket of fresh water just after the turn of the tide was seen with a higher temperature. The outgoing tide was then warmer, possibly as water from the shallows which had been heated began to outflow

The majority of data points showed low turbidity. This implies the water was relatively clearer, with less particulates present in the water column. This increased slightly towards 5m in the first and third recordings, possibly due to greater mixing with the sediment layer. The turbidity also increased in the pocket at 11:40-12:40