Any opinions expressed on this website do not reflect the opinions of the University of Southampton or the National Oceanography Centre

Chemistry

Figure 3.2.4

Figure 3.2.5

Samples were taken from Stations 1 to 8 on RV Conway on 27/06/2014. Water samples were collected from various depths of interest and then chemically analysed for Nitrate, Phosphate and Dissolved Silicon concentrations ([], [ ] and  []). Station 1 was located at the top of the Fal estuary, i.e. closest to freshwater. Subsequent stations were situated further downstream at chosen sites, with station 8 being closest to the mouth of the estuary.

Due to the estuarine location we would expect nutrient concentrations to be greater, at least in the surface water, the further up-river the sampling station is located. This will be due to dilution via physical mixing processes and due to nutrient extraction via phytoplankton growth.

Nitrate Concentration vs. Depth

Stations 1, 2, 3 and 6 all show a continual decrease in [] with depth. Stations 1, 2 and 3 start with values near 20 µmol.L-1 and decrease down to ~7 µmol.L-1. Station 6 independently shows a lower start and end concentration with 6 µmol.L-1 to 2 µmol.L-1. Stations 4, 7 and 8 show a decrease in [] from start to finish with some deviation in the mid-depths. Station 7 and 8 initially increase, then decrease in []. Station 4 decreases but then has a slight increase to a level less than that of the surface concentration. Station 3 had the largest concentration of any station with around 18 µmol.L-1 at the surface.

Figure 3.2.1

Phosphate Concentration vs. Depth

Relatively conservative behaviour is illustrated at stations 3, 4, 7 and 8 below 5m from the surface. Station 7 shows a decrease in [] from the surface to 5m depth.

Station 1 shows a decrease in [] from 0 – 5m with [] values of 0.4 - 0.2 µmol.L-1. Station 2 shows an initial decrease to 5m then increase to 10m. Station 6 shows an initial increase in [] to ≈2.5m with a value of 1.2 µmol.L-1, then a decline to 0.1 µmol.L-1 at ≈12.5m.

Figure 3.2.2

Dissolved Silicon Concentration vs. Depth

Station 1, 6 and 8 show a relatively conservative distribution with depth but at differing concentrations; around 6 µmol.L-1, 3 µmol.L-1 and 1 µmol.L-1 respectively.

Stations 2, 3 and 4 show a near continuous decrease of dissolved silicon concentration with depth, with station 2 having the largest range of 16 to 0 µmol.L-1 of over around 9m depth.

Station 6 shows an increase with depth of around 1 µmol.L-1 to 8 µmol.L-1 from the surface to 14m. Dissolved silicon concentration then decreases at 14-19m from 8 µmol.L-1 to 1 µmol.L-1.

Figure 3.2.3

Station Number vs. Silicon and Diatom Abundance

A silicon concentration of 16 µmol.L-1 peaks at station 2 which then decreases exponentially until station 7 with 1 µmol.L-1. The concentration then plateaus with station 8, which has the same concentration of dissolved silicon.

Diatom abundance decreases marginally from station 1 to station 3, 7e5 L-1 to 6.5e5 L-1 respectively. Diatom abundance then increases to 8e5 at station 4. This is followed by a large decline to station 6 at 3e5 and station 7 at 2e5. Abundance increases slightly at station 8 to 2.5e5 L-1.

It seems evident that as diatom abundance increases, the dissolved silicon concentration decreases and, when dissolved silicon concentration hits a certain low level, the diatom abundance rapidly depletes. Factors such as turbidity and salinity could also create anomalies shown in this graphical data.

Station Number vs. Nitrate and Dinoflagellate Abundance

[] undulates initially, peaking at station 3 with around 18 µmol.L-1 then decreasing suddenly to 6µmol.L-1 at station 4. A further decrease is observed with a subsequent plateau for stations 7 and 8 greater than 0 µmol.L-1.

The Dinoflagellate population is initially low, with little undulation at stations 1 to 6. Station 7 illustrates a large peak (60 000.L-1) followed by an equally sudden drop to just greater than 0.L-1 at station 8. Dinoflagellate abundance appears to correlate inversely with - concentration, as diatoms did with the dissolved silicon concentration. Dinoflagellates are also favoured in stratified waters as we found at stations 7 but not 8, hence explaining the peak and fall at station 7 and 8 respectively.

Introduction

Mixing Diagrams

Mixing diagrams have been created for Phosphate, Nitrate and Dissolved Silicon. These diagrams show the Theoretical Dilution Line (TDL) joining the riverine and sea end-members. The TDL demonstrates how the nutrients would behave if the only process affecting mixing is physical dilution down the estuary. Silicon lacks a true riverine end member in this case, consequently the lowest salinity has been substituted.

Dissolved silicon appears to follow the TDL quite uniformly with no real variation until greater salinities (>30) are reached. This is due to the utilisation of dissolved silicon by diatoms for the formation of frustules.

has anomalies in the data that suggests nitrate is being removed from the estuary.

again has some indication of removal, however there are some outlier data points above and below the TDL. Due to insufficient numbers of outliers, they cannot be directly interpreted.

Figure 3.2.6

Figure 3.2.7

Figure 3.2.8