On Monday 29th June 2015, a survey of the Fal River was carried out on the King Harry Pontoon (50o12.57N, 005o01.39W) between 11:30 UTC and 15:00 UTC. Low tide was at 09:13 UTC, high tide was at 15:10. The aim of this was to create a full day time series of several of the physical properties. This was done by putting our data collected during this time period together with another group who surveyed the area in the morning. This was to enable comparisons to surveys done up the estuary on the RV Bill Conway and the offshore measurements. The weather was calm with no precipitation, a sea state 0, 6/8 cloud cover, swell 0, slight wind and good visibility. A mussel farm is situated approximately 100m downstream. With the incoming tides, the ammonia produced by the mussels may have an effect on the water quality.
A light meter was used to measure light, a YSI EXO2 multisensor probe and a current meter were all used to measure the physical properties at different depths. Surface samples were taken to measure chlorophyll.
Figure 1.1: Image of current meter deployed off of the pier.
Figure 1.2: Image showing how YSI EXO2 multisensor probe and light meter were deployed off of the pier.
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King Harry Pontoon
As can be seen from this graph, there is a clear pattern of a drop in temperature with depth. This is to be expected as surface water layers were heated by the sun and the water was calm leading to little vertical mixing. Furthermore, water temperatures start low around 8:30 GMT and increase, reaching a peak at 11:30 GMT. From this point onwards temperatures decrease, reaching a minimum between 14:00 GMT to 15:00 GMT. It should be noted that at this time the weather was relatively cloudy and cold.
Surface salinity measurements were low in the morning at the station, staying below readings of 31.5 PSU. This may be due to the fact that in the morning the tide was going out, leading to an influx of low salinity water from further up the estuary. In the afternoon the tide would have been coming in, leading to a mixed water column of high salinity close to that of seawater, shown by the fact that the readings for this time between 13:00 GMT and 15:00 GMT fell between 34 and 35 PSU.
Due to the tide going out in the morning there is an obvious influx of freshwater,
low values between 40,000v and 42,000v were recorded due to a lack of conductive
electrolytes in the water (Na+ and Cl-
Measurements for dissolved oxygen levels exhibited similar patterns to those for oxygen saturation. The highest levels were measured with increasing values throughout the morning, peaking at 11:30 GMT. In the afternoon, dissolved oxygen values decreased to fall somewhere between 10 and 9.5 mg/L. There was little variation at the station in dissolved oxygen concentration with depth apart from the measurements taken at 12:30 GMT to 13:00 GMT which fell rapidly with depth. As mentioned above oxygen is less soluble in seawater than freshwater, meaning the incoming afternoon tide could be responsible for the dissolved oxygen decrease.
This time series contour graph shows that turbidity levels were relatively low and
stable in the afternoon from 11:30 UTC onwards, with the majority of readings being
under 2FTU. In the morning however, turbidity was more variable. Readings taken from
9:30 to 11:00 UTC exhibit increasing turbidity with depth, with the reading for 9:30
reaching a maximum of 9.5FTU. This could be attributed to the fact that in the morning
the tide was going out, bringing a higher river influx and thus a higher influx of
suspended particulate matter. Studies have shown that turbidity increases at the
seawater-
Progressively throughout the morning, pH increased, reaching a maximum at 11:00 GMT. From this point onwards, pH starts to decrease, reaching a minimum towards the afternoon. There is a general pattern of decreasing pH with increasing depth, as seen most clearly with the measurements taken for 12:30 GMT to 13:30 GMT. The fact that pH was higher in the morning when the tide was outgoing could be due to river water influx, maybe causing the estuarine water mass to become more alkaline due to some kind of upstream pollution, (Begum, 2009).
It is evident that chlorophyll levels are highest in the morning, peaking between 9:00 GMT to 11:00 GMT being highest at a depth of around 2 metres. Towards the afternoon, chlorophyll levels drop between 13:00 GMT and 15:00 GMT, an average of 5ug/L is present throughout the whole water column. This could be attributed to the fact that the tide was coming in, bringing with it high salinity water unfavourable for phytoplankton.
As depth increases there is a decline in irradiance levels. There is a greater decrease
in the surface 2m than the lower depths where it was almost zero, this is as expected
as more light is attenuated at greater depths. As the afternoon progressed the initial
levels of irradiance increased until 12:30 GMT and the dropped again until our final
measurement at 15:00 GMT, in line with the timings of strongest sunlight during the
day. The highest level of irradiance was about 700 µmol m-
Evident is the fact that flow speed is higher in the morning and early afternoon and decreases throughout the afternoon. A flow speed of over 0.2 metres per second was observed at 11:30 GMT. Towards the end of the afternoon current speed reduces with a minimum speed observed for the water column at 14:30 GMT. These measurements correlate with the fact that high tide was at 15:10 GMT, bringing with it reduced flow speeds in the late afternoon, compared to the relatively high flow speeds in the morning from the outgoing tide and increased river flow.
The tide rose over the course of our time series with high tide occurring just after sampling ended. As a result, all of the parameters that were measured exhibited a large change as time progressed.
During the morning, the water column was stratified with less saline river water towards the surface lying above denser, more saline water from further down the estuary.
The incoming tide also served to mix the estuary. As the day progressed, the water column became more homogenous with the stratification observed in the morning being reduced.
References
Begum, A. et al.. (2009). Heavy Metal Pollution and Chemical Profile of Cauvery River
Water . E-
Cloern, J.E. 1987. Turbidity as a control on phytoplankton biomass and productivity
in estuaries. Continental Shelf Research 7(11/12), 1367-
Environmental Protection Agency (2006) Volunteer Estuary Monitoring: A methods manual.
Washington, D.C. 9. pp. 9-
New South Wales Government (2010) Waterwatch estuary guide. Sydney. 2. Pp. 2-
New South Wales Government (2010) Waterwatch estuary guide. Sydney. 2. Pp. 2-
Relatively higher values for oxygen saturation were found in the morning, between
130 and 145% with little variation between readings for specific depths. In the afternoon
however, values fall to around 125% at the surface and 120% at depths of 2-
Figure 2.2: Salinity contour plot
Figure 2.1: Temperature contour plot
Figure 2.4: Turbidity contour plot
Figure 2.5: Oxygen saturation contour plot
Figure 2.6: Dissolved oxygen contour plot
Figure 2.3: Conductivity contour plot
Figure 2.7: pH contour plot
Figure 2.8: Oxygen saturation contour plot
Figure 2.9: Irradiance against depth time series
Figure 2.10: Current flow speed against depth time series