FALMOUTH FIELDCOURSE

Group 5

The views expressed are those of the individuals concerned and do not express the views of the University of Southampton or those of the National Oceanography Centre Southampton.

Nutrients

Silicon concentration was calculated using a calibration curve and the absorbance measured by a spectrometer.

The technique for measuring the nitrate concentration uses the conventional azo dye chemistry of Morris and Riley (1963). This produces a pink fluid that absorbs at 543nm. This is implemented as a flow injection method. For this the nitrate must be reduced to nitrite, which is done with the use of a cadmium column that causing the reduction reaction. The flow injection method involved the flow of a sample along narrow tubes that allow mixing to occur. This mixing is highly reproducible (Ruzika and Hansen, 1988). Reagents are added to the sample producing a pink colour. The pink colour is then measured in a flow through 1cm path length cell in a spectrophotometer. The signals produced is plotted onto a chart recorder.

Phosphate was measured by putting 30ml of seawater into 3 ml of reducing agent. This was left for 2 hours to allow the reaction to occur. The absorbance was measured using a spectrometer.

Oxygen samples were taken from every bottle at each station, ensuring the sample was first to be extracted, minimising contamination and change to the sample. The water was collected into a glass flask, making sure no bubbles were present. Two chemicals were added to the sample collected, Manganous chloride (1ml) and alkaline iodine (1ml). This precipitated out the oxygen and neutralised the reagents. Once this was completed the flasks were placed into water to preserve them further.

In the lab the sample was mixed using a magnetic spinner. It was then titrated against thiosulfate until the sample the sample became clear, this was made more accurate by using a chart reader to aid observations of clarity.

Click below to see our chemical data from offshore

Click below for more data from the estuary

Nitrates

In general, the nitrate concentration decreases with depth. However, there are several outliers, station 38 appears to have a large increase to a maximum point before decreasing to a lower concentration. This also occurs at station 37 but here it is less extreme. These peaks in the nitrate concentration could be caused by a nutricline being present here.

Another outlier is station 44 where the concentration increases sharply with depth. This could be caused by an error with the equipment or a misreading.

The nitrate concentration seems to remain similar in all surface samples except for station 37, this station has a higher concentration at the surface. This could be because this station was located the furthest from the estuary, so the water may be more well mixed, increasing the nutrients.

Phosphates

The phosphate concentration varies greatly with each station. Stations 40 and 37 show a peak concentration occurring at around 10 meters. Whereas stations 38 and 39 show a decrease with as the depth increases, with station 39, 41 and 44 showing a linear decrease. This may be due to varying plankton levels in the water. In general, the data shows an increase in the phosphate levels the closer to the estuary you are with station 41 having the highest concentrations (around 0.38 µm/L). There are two extreme measurements taken at stations 40 and 42. These could be explained by the thermocline affecting the plankton growth and therefore, altering the phosphate concentration. However, as both points go in opposite directions it is more likely due to error in the readings or equipment

Dissolved Oxygen

All stations have high oxygen saturations near the surface due to the atmosphere. Oxygen decreases with depth as organisms respire. The rate of this change is affected by deep chlorophyll maxima being present at all three stations at around 20m.

The majority of stations show a decrease in silicon concentration with an increasing depth, however, station 37 shows an additional increase occurring at 10 meters. The decrease in silicon may be caused by planktonic activity, consuming the silicon for respiration and use in cell walls. The increase shown at station 37 is most likely to be error in the data, however it could be caused by a nutricline brining nutrients up from deeper water.  

Silicates

R.V. Bill Conway

Winnie the Pooh

Nitrates

There is a high level of nitrate present at station N2 (98 µmol/L), this may be due to an input from a nearby tributary or it may be because this station is the furthest down the estuary. Stations O2 and P2 show similar concentrations (around 40 µmol/L). Station Q2 shows an increase (64 µmol/L), this may be because this station was located close to a sewage treatment and dump site.

Phosphates

Phosphate concertation shows an increase at each station. This shows that the phosphate concentration increases the further the distance up the estuary. Station Q2 shows a large increase (1.9 µmol/L), potentially caused by the sewage treatment plant and dump site near this location.

The Silicon shows the same pattern as the nitrate, with a high concentration (17 µmol/L) located further from the estuary (N2). There is then a decrease in the concentration (by 10µmol/L) as you travel up the estuary, before another increase. The increase at station Q2, to 15µmol/L, is potentially caused by the sewage treatment and dump site at this station.  

Silicates

Mixing Diagrams

Silicate, phosphate and nitrate concentrations, calculated from the bottle samples, were plotted against salinity to determine if the Fal estuary is conservative or non-conservative in terms of these nutrients. An end member was collected further up the river with a salinity of 0 on the same day the estuary was sampled and the mouth of the estuary was also sampled, which would have been almost pure seawater, to give the ability to draw a TDL.

The salinity did not drop below 28 whilst sampling was in progress which has therefore lead to diagrams that are difficult to interpret or determine the type of mixing that occurs. The tide was dropping to a neap tide when sampling started in the morning and then was at its lowest at 12:21 UTC when the upper estuary was starting to be sampled. The tide was therefore starting to turn leading to a higher concentration of seawater. Also, there has been little precipitation in the surrounding area which may also mean there is a reduced fresh water input into the estuary.

For silicate and nitrate there are some point that are both below and above the TDL and therefore without further samples at lower salinities it is very difficult to determine if these nutrients are either conservative or not.

The phosphate is a lot easier to interpret, assuming no errors were made with sampling or analysis, as there is a very large increase in concentration in a small salinity range which suggests that there is a large addition of phosphate containing compounds further up the river than was possible to sample in the Winnie The Pooh.