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Estuarine - Chemistry

Winnie the Pooh time series

All of the samples taken were at 0.3m depth:

Silicate

Sites N to Q are taken from the highest point up the estuary to the pontoon. The silicate concentration is very high at the highest point up the estuary at around 16.0 µg/l and then decreases rapidly at the next station further down the estuary at around 5.38 µg/l. The concentration then increases at the next station to 9.02 µg/l before decreasing at the station nearest the pontoon to around 5.59 µg/l again. Sites N to Q were tested while the tide was ebbing and testing was finished as the tide was at its lowest point.

Sites N1 to Q1 are taken from the pontoon up to the highest point up the estuary. The silicate concentration is at its highest point of the day near the pontoon at site N1 at around 17.11 µg/l. It then decreases rapidly to site O1 to 7.96 µg/l and only decreases very slightly between O1 and P1. At the final site at the upper part of the estuary the silicate concentration rises again to around 15.21 µg/l. Sites N1 to Q1 were tested while the tide was flowing, testing beginning at low tide.

Procedure

A CTD was used to take water samples at the five different stations within the Sampling Site. One sample was taken at the chlorophyll maximum and one at surface for every station, by firing a Niskin bottle.

Lab Analysis

Water samples were then collected into bottles to be analysed in the ashore lab. Samples for phosphate, nitrate and silicate analysis were pipetted into collection bottles through a filter, in order to remove larger molecules. Samples for chlorophyll were pipetted through a filter at 50ml volume and the filter paper was then placed in. Samples for oxygen were placed in glass bottles ensuring no air bubbles were present and then 1ml manganese chloride solution was added, followed by 1ml of alkaline iodide solution to precipitate the oxygen.

The samples were then taken to the in shore lab for further chemical analysis.

The high silicate concentrations at sites N and Q1 can be explained by the presence of a sewage works that will lead to high anthropogenic inputs of nutrients. These inputs cause a bloom of phytoplankton and bacteria that re-mineralise the silicate leading to higher values. These anthropogenic inputs have a far lesser effect at the next station tested (O and P1) and therefore at these stations the silicate concentration is far lower. The spike at site P may be explained by a phytoplankton bloom that would lead to remineralisation of nutrients. There may also be an input of silicate from rock erosion near the site P. The very high value seen at site N1 is unusual as it is far down the estuary where silicate concentration should be decreasing. One of the possible explanations for this is the presence of a chain ferry across the river, the chain on the river bed will kick up a lot of sediment, in hand leading to an increase in silicate concentration.

Nitrate

The nitrate concentration follows a similar pattern to silicate, with peaks at sites N, N1 and Q1. The nitrate peak at N is 95 µg/l and it then troughs at site P at 30 µg/l. The peak is then 105 µg/l at site N1 nearest the pontoon before it troughs at 40 µg/l at site P1. It finally peaks at the upper part of the estuary at 65 µg/l.

At sites N and Q1 the peaks can be explained by anthropogenic inputs caused by the sewage works present there that released nitrate into the water column. The troughs are due to a lack of anthropogenic or natural inputs and so the nitrate concentration decreases as it is diluted by seawater and flocculates as it meets higher salinities.  The peak at site N1 is unexpected much like the silicate value. The same possible explanation can be applied, with the chain ferry kicking up sediment from the chain on the river bed leading to nitrate being released into the water column.

Phosphate

The phosphate concentration is highest at sites N and Q1, at 1.61 µg/l and 1.87 µg/l respectively. This is explained by the presence of a sewage works at these sites that will release nutrients into the water column. The phosphate concentration decreases down the estuary towards the sea as expected for the morning sites (N-Q) that were taken as the tide was ebbing, reaching 0.39 µg/l.at site Q. For the afternoon sites (N1-Q1) the concentration decreases to site O1 as expected but then has a slight increase to the final N1 at 0.65 µg/l site near the pontoon. This peak may be due to the chain ferry. The chain on the river bed will kick up sediment, leading to nutrients such as phosphate being released into the water column.

Figure 1: Time series of nutrient data recoreded from each station on ‘Winnie-the-Pooh’

Estuarine Mixing Diagrams

Bill Conway

Nitrate

The nitrate concentration is fairly constant at around 1µg/l at all sites. Site 44 remains fairly constant between the surface and 30m depth with only a very slight decrease in concentration. Sites 41, 42, 43, 38 and 40 all have a more significant decrease, all of which having a concentration above 1µg/l at the surface and decrease to below 1µg/l as depth increases. The nitrate concentration decreases between the furthest up the estuary and the sea end member sites excluding site 39

Site 39 has a rapid increase in concentration between the surface and 5m depth rising from 0.6 to 2.2 µg/l and then decreases again to 0.63 at 16.6m depth.

Site 37 is very high compared to the other values measured. At the surface it has a nitrate concentration of 3 µg/l and this increases to 4.6 µg/l at 26m depth. This very high value is unexpected, as seawater has a lower concentration of nutrients compared to freshwater and therefore the value should be lower rather than higher than the other sites. This indicates that there was some sort of error in either sample collection or testing in the lab, most likely contamination of the sample.

Silicate

The silicate concentration is highest at the upper sites of the estuary and decreases as the sites get closer to the sea end member at the surface. This is due to dilution of high concentration freshwater with low concentration seawater as well as due to flocculation of the Si molecules when they come in to contact with the higher salinity seawater. All of the sites, beside site 37, have a decrease in concentration with depth. This is due to the dilution of the high concentration freshwater with lower concentration seawater, which underlies the freshwater. It may also be due to the use of Si by plankton at upper to mid-range depths meaning very little Si can fall to the greater depths.

Site 37 could be explained by the boat drifting while the samples were being taken as there is a rapid increase in Si concentration with depth after 10m depth, before which it decreases at a similar rate to the other samples. Human error in the lab could also be the cause, however to be certain of this more samples would have to be taken at the same location.

Figure 1: Nitrate concentration depth profiles for each station

Phosphate

The phosphate concentration decreases towards the sea end member as the high concentration freshwater is diluted by the low concentration seawater. The PO4 molecules will be caused to flocculate as they come into contact with the higher salinity seawater further reducing the PO4 concentration.

For all of the sites, apart from sites 37 and 43, the phosphate concentration decreases with depth it is diluted and flocculates when it meets the underlying higher salinity seawater.

For sites 37 and 43 there is a very slight increase in phosphate concentration, 0.01 µg/l for site 37 and 0.1 µg/l for site 43. This may be due to the boat drifting during sample collection, as the boat was small and was susceptible to being carried by the tide/river current. It could also be explained by there being a very low change in salinity with depth due to the very well mixed estuarine waters, meaning that it is not being diluted and is not flocculating. The changes in the phosphate concentrations at these sites may be explicable by the thermocline that will change as the tides change.

Figure 3: Silicate concentration depth profiles for each station

This means that remineralisation by phytoplankton can be the dominating factor and therefore a very slight increase in concentration can be seen.

Sites 42, 39 and 40 all fluctuate between the surface and depth concentrations. Sites 39 and 42 both have a rapid decrease in concentrations, site 42 decreasing from 0.29 to 0.16 µg/l between 1.0 and 4.2 m depth and site 39 decreasing from 0.27 to 0.16 µg/l between 1.0 and 13.2 m depth. The concentrations both then increase rapidly to 0.20 µg/l for site 42 and 0.22 µg/l for site 39. This leaves a slight net decrease in concentrations for both sites.

Site 40 has a rapid increase in phosphate concentration from 0.25 to 0.34 µg/l between 1.0 and 8.2 m depth before then rapidly decreasing to 0.23 µg/l at 13.73 m depth.The changes in the phosphate concentrations at these sites may be explicable by the thermocline that will change as the tides change. At the thermocline chlorophyll concentration will rapidly change, and this will lead to a change in remineralisation and therefore the amount of phosphate present in the water column. This is, however, unlikely as sites 42 and 39 show opposite patterns to site 40. Therefore, it is more likely that error in the testing of the samples has occurred, such as contamination. To be certain of this the sites would need to be tested again to see if they follow a pattern similar to the other sites tested.

Figure 4: Phosphate concentration depth profiles for each station

Nitrate

Nitrate concentration shows a positive skew on the estuarine mixing diagram, meaning there is nitrate input in the estuary. The input occurs in the upper part of the estuary, where salinities are lowest. The nitrate is eventually depleted and mixed by the time it reaches the sea end member. A sewage works at the upper part of the estuary can help explain the input of nitrite into the water column.

Silicate

Si concentration shows a positive skew which indicates that there is addition of Si taking place. The diagram becomes more conservative at higher salinities nearer the ocean end member, indicating that addition is taking place higher up the estuary. Si addition is not usually explicable by anthropogenic inputs and so could be explained by high levels of rock erosion caused by the river in the upper parts of the estuary. It could also be explained by the bacterial degradation of organic matter and SiO2 as this is likely to occur at its maximum in the summer months (Meybeck. et al 1988)

Phosphate

PO4 concentration shows a very positive skew in the estuarine mixing diagram. This indicates there is a large amount of PO4 input into the estuary at the upper locations in the estuary. The skew then returns towards the TDL at the lower part of the estuary where salinity increases, meaning the input is only found at the upper locations. The presence of a sewage works at the upper locations of the estuary can explain this.

Figure 5: Nitrate mixing diagram

Figure 6: Silicate mixing diagram

Figure 7: Phosphate mixing diagram

Chlorophyll and Fluorometer Analysis

When plotting chlorophyll α values against the converted CTD fluorometer data at the same depths we can see that the correlation is linear, but it is also very scattered implying errors present in the method of measurement. This shows that that data from the CTD is therefore overestimating the chlorophyll concentrations, implying that there are interfering compounds within the water column which the fluorometer picks up along with chlorophyll measurement.

Figure *: Fluorometer data against Chlorophyll

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