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Introduction

The aim of the practical was to collect a time series data set, showing the changes in water column structure at a single location within an estuary over a period of 3 hours. The site chosen was the Trelissick Pontoon: 50˚12.57'N, 005° 01.39' W in the Fal estuary. To obtain an accurate structure of the water column the site was sampled with a Secchi disk, light meter and YSI probe. The survey plan originally included the use of current velocity meter; however, since this was damaged during previous measurements we were forced to work without the physical information it provides.

Results, hover over images to enlarge:

Table 1 shows us the results gathered by the Secchi disk and the light probe. One way of representing water clarity is by calculating the attenuation coefficient (k) of the water column; this gives the rate of attenuation of light with depth and is expressed per meter (m-1). For the light probe method, attenuation coefficients were calculated by plotting the natural log of the ratio of deep to surface light readings against depth. The coefficient was then calculated by taking 1/slope of the line of best fit. In comparison, the secchi disk values were calculated by taking the average of the depths at which the disc disappears and at which is reappears; a constant (1.44) divided by this depth gave the coefficient value.

Discussion

High water was at 11:20 and thus all of our survey was conducted during the ebb of the more marine water from the estuary. This justifies the trend showed in Figure 2 were salinity is decreasing over time. The ebbing tidal flow also explains the results presented in Figure 3, as the saltier and colder water will have a higher density than the warmer and fresher water and thus will tend to sink to the bottom. As we can see from the contour plot the area coloured in blue (low density area) increases with time and this is simply due to the fact that the marine water is on the ebb. There is also a blank area on Figure 3 on the last measurement merely because as the tide ebbs the depth of the survey site reduces with time (tidal range: 2.8 m). The other important feature visible on this figure is the advection of denser bottom water to the surface layer around the 13:45 measurement. This upward movement of bottom water is also shown in Figure 1 (temperature contour plot) where a blue area (cold water) is clearly visible. This abnormal advection of water was caused by the approach of small vessel that passed over the survey area and berthed for about 10 to 15 min nearby.

In the remainder of Figure 1 it can, as anticipated, be seen that the warmer water is found at the surface (red-yellow area) and the colder water at the bottom (blue-purple area). Figure 1  also shows how water layers just below surface warm up with time until 14:45 and then get slightly colder again as irradiance reduces.

This feature explains the rise in oxygen saturation (a proxy for photosynthetic activity) of the water column since temperature of water is a direct indicator of the amount of sunlight being absorbed by the area. The amount of light absorbed by the water column is then enhancing the activity of photosynthetic organisms which will result in higher production of oxygen (Figure. 4). There is also a decrease in oxygen that follows the increase at 13:45 caused by the boat movement (oxygenating the water by mixing the water column), the decrease is simply the re-establishment of the Oxygen saturation to normal levels.

As stated by Steel and Neuhausser (2002), visual water clarity is a primary regulator of biological and ecological functions in aquatic systems; having a significant impact on hunting, predator avoidance and primary production for pelagic phytoplankton. Table 1 displays how the light probe readings are consistently higher than the Secchi disk readings, with values being 0.207m-1 greater on average. This is primarily due to the Secchi disk method being less precise than the light probe method, measuring apparent rather than inherent optical properties of water (Steel and Neuhausser, 2002). Sources of inherent variation associated with Secchi disk measurements include; height of the observer, sun altitude, shadows, diameter of the disc and optical state of the sea surface. From the light metre the attenuation coefficient increases until 1415 and until 1445 on the Secchi disk, which is likely to be a reflection of both phytoplankton utilising the high solar irradiance and disturbance of sediment by the passing boat.

To summarise, the general features of the survey site are: the gradual decrease in salinity of all the water layers due to the ebbing tide, the overall warming of the water column, the steady increase in oxygen saturation and thus in phytoplankton productivity. An increase in phytoplankton biomass in the area could be a reason for the increase in the attenuation coefficient between 1245 and 1415. Moreover, the variations in temperature and salinity can be easily interpreted in the density variations of the water column.

Figure 2: Salinity and Temperature variation over time

Figure 3: Density Contour Plot

Figure 4: Dissolved Oxygen saturation % variation with time and depth

Table 1. Attenuation Coefficient for  the 6  light measurements

Figure 1: temperature contour plot