Falmouth Field Course 2017

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Meta data

Date: 04/07/17

Time: 08:30-12:00 UTC

High water: 01:18, 13:55

Environmental conditions:

Cloud cover = 8/8

- No rain

- Temperature (°C) 17-18

Tidal State: Neap flood tide

Max Depth: 3.7m

Results

Light Attenuation:

The ratio between the irradiance at the surface and at depth can be used to obtain an indication of turbidity, the greater the ratio, the greater the attenuation at depth. Irradiance is predicted to decrease with depth which is  clear in Figure 2. Between 08:45 - 09.00 irradiance at the surface is very low with irradiance increasing at the surface throughout the day related to the angle of the sun above the water.

Flow Velocity:

As shown in Figure 3, the current flows at highest velocities at intermediate depths. For example, the max velocity, 0.258m/s, was recorded at 2m at 10:41. In comparison, values at the surface and deepest regions ranged between 0.01m/s and 0.1m/s. A flow speed of 0m/s was recorded at 08:45 at 3.5m, this seems unlikely and perhaps a result of lowering the probe onto the seabed. The data supports the prediction that flow velocity is reduced at the bottom of the water column due to increased friction at the water-seabed interface. Under normal circumstances it could be expected that velocities would be fastest at the surface due to drag by the wind which then dissipates with depth, however during our time series there was little/no strong winds. Furthermore, faster currents between 1.5 - 3.5m can be explained by stronger tidal currents compared to minimal wind driven currents.

Temperature:

Figure 4  shows that temperature increased throughout the day and decreases with depth. On average, water temperatures were 16.28oC, with a maximum of 17.29oC at 0m at 10:00. Lowest recorded temperature was 15.2oC at 4m at 09:00. As expected, the contour plot displays a strong correlation between temperature and depth. This is especially prominent later in the day, in which most profiles showed a decrease in temperature by almost 2oC. However, the vertical profile with the largest gradient was at 09:00 which displayed a decrease of 1.83oC, from 17.03oC to 15.2oC. This difference is likely a result of; incoming solar radiation warming the upper layers of the water body and stratification, limiting mixing between the colder, deep water and warmer surface water.

Salinity:

In Figure 5, salinity increased throughout the day, increasing from a surface minimum of 29.9 at 08:30, to a maximum of 34.05 at 5.5m at 11:30. This increase with time correlates to the increasing depths and incoming flood tide into the estuary. Since estuarine water is freshest upstream, with the incoming tide, salinity increased with time. As well as increasing temporally, the readings show salinity increasing with depth. This is due to water of higher salinity having a greater density.

Oxygen Saturation:

Oxygen saturation is expected to be greatest at the surface due to diffusion across the water-atmospheric interface. In addition, oxygen saturation will decline throughout the water column due to respiration of organisms. In Figure 6, there is no strong decline in oxygen with depth.  

Chlorophyll:

The contour plot for the chlorophyll in Figure 7 showed no clear correlation between either chlorophyll with time or chlorophyll with depth. Chlorophyll appears mostly homogenous with little variation, ranging from the highest value of 5.56 at 0m at 11:30 , to the lowest value of 1.65 at 0m at 08:31. This is not as  expected as chlorophyll is expected to be highest at the surface where phytoplankton concentrations are highest.  

pH:

The contour plot in Figure 8 shows little variation with ~8 pH throughout the day. Although the range in pH is small, a slight increase in pH can be seen from 8.20 to 8.26 between 08:30 - 11:00.This could be due to the influence of the incoming tide, which brings water with different constitutes and ions into the estuary.  

Summary

The incoming tide was a big factor in the changes to the chemical properties to the waters surrounding the King Harry pontoon. As the estuary is  flooded with more saline water, combined with warmer temperatures  due to the progression of solar radiation throughout the day, the strength of the pycnocline had increased. Furthermore, stratification increased leading to the formation of two water bodies in the estuary; the surface water, which was warm and fresh in nature, and the deep water, which was colder and more saline.

Limitations

Logistical issues arose as a result of frequent use of the pontoon by King Harry Ferries and small vessel users. Irregular time periods between samples reduced reliability due to increased turbulence caused by approaching vessels. Additionally, due to the flooding tide, at various depths  along the time series for each parameter data has been extrapolated. See Figure 1 for the extent of real data.

Biology

Niskin bottle samples were taken once every hour resulting in the creation of a chlorophyll time series. Samples were taken from the surface and at a depth of 3m. These samples were preserved by filtering 50ml and adding to a 5% acetone solution. The samples were then frozen over night for laboratory analysis.

Physical

Light intensity and flow readings were taken at 1m intervals every hour using an irradiance probe and a flow meter between the surface and estuary bed (µmol photon.m2.s-1). This produced the data for a time series depth profile which demonstrated the changes in water column mixing, with increased light attenuation and and water velocity at higher tides indicating increased water column mixing in the afternoon and at higher tides.

Chemical

A YSI multiprobe was used to examine the relationship between a  number of chemical

parameters  and the tidal cycle, within the estuary. The temperature, salinity,

dissolved O2 and pH where measures every metre at 30 minute intervals.

Methodology

On the 04/07/17, analysis of the water column structure at King Harry Pontoon (50o 12. 96’N, 005o 01.67’W) was conducted to develop a time series for a range of parameters resulting in the production of vertical profiles. Over a  four hour period, regular in situ measurements  were taken in order to obtain chlorophyll, pH, dissolved oxygen, temperature, salinity, light attenuation and flow rates.

Introduction

Figure 2. Irradiance Contour Plot

Figure 3. Flow Velocity Contour Plot

Figure 4. Temperature Contour Plot

Figure 6. Dissolved Oxygen Contour Plot

Figure 5. Salinity Contour Plot

Figure 7. Chlorophll Concentration Contour Plot

Figure 8. pH Contour Plot

Figure 1. Depth-time series, all data below the black line has been extrapolated.