Back to top
Methods
Silicate, phosphate, nitrate and dissolved oxygen were all sampled, preserved and then analysed in the onshore labs following the same procedures as the offshore chemistry here.
S
ilicate
The highest silicate concentrations are found at the riverine end of the estuary. As a result of rock weathering silicate is released into the Truro river, causing a flux of silicate into the river. This explains the high silicate concentrations at station 17D, peaking at a depth of 1 m with a concentration of 7.1 µmol/L. Silicate is then removed from the water as it is incorporated into the skeletal structures of diatoms. The lowest concentrations can be found at station 14G, reaching a minimum of 0.4 µmol/L at 10 m. This station is the furthest point downstream so receives the least amount of silicate from the river. However, the silicate concentration increases by 1.7 µmol/L between 10 and 25 m – this is probably due to the remineralisation of silicate at depth which releases the silicate from diatom frustules.
The silicate mixing diagram displays that some silicate is being removed from the estuary – shown by some of the points lying below the TDL. However, the plots are quite close to the TDL, therefore, silicate behaves fairly conservatively in the Fal estuary. The concentration of silicate decreases with increasing salinity. The main source of silicate to the estuary is from the Truro river, as rocks are weathered releasing their silicate into the water.
Nitrate
Stations 14G – 17D are progressively further upstream. Station 17D has the lowest
salinity (averaging with depth at 33.6) as it is furthest upstream. As it is further
upstream the nitrate concentration increases more quickly with depth, reaching a
maximum of 10.5 µmol/L at a depth of 10.4 m. This is because nitrate is supplied
to the estuary from the river, therefore the further upstream the greater the input
of nitrate. Nitrate enters the river through sewage, industrial buildings, run off
from farming and other anthropogenic inputs. Moving down the estuary the concentration
of nitrate between stations 14G-
The nitrate mixing diagram displays that nitrate
in the Fal estuary has a fairly
conservative behaviour, shown by the nitrate sample plots lying close to the theoretical
dilution line (TDL). The sample nitrate concentration does sit slightly below the
TDL, this is most likely due to the removal of nitrate by plankton, as it is required
for protein synthesis.
Phosphate
Similarly to the nitrate samples, the phosphate concentration increases as you progress
further upstream – with the highest phosphate (0.67 µmol/L) concentration at a depth
of 10.4m at station 17. This is because the river supplies the estuary with phosphate
and so further upstream the phosphate concentration is higher. At Stations 14, 15
and 16 the phosphate concentration is noticeably lower than at station 17, as phosphate
is removed from the water by plankton. This is because phosphate is required for
protein synthesis which allows the plankton to grow. The lowest phosphate concentration
(0.17 µmol/L) is at station 15 at 6.0 m. As a result of tidal mixing, stations 14,
15 and 16 have much more similar concentration profiles.
The phosphate mixing diagram shows that salinity and phosphate concentration has
a non-
Dissolved Oxygen
The Figure shows oxygen saturation values taken from a variety of different stations and depths within the estuary Fal estuary. Station 17D was the only site which showed super saturation (104%) at 10.4m depth, which could be due to a large amount of photosynthesis. Minimal error is shown between repeats for each depth apart from at station 16E where there is larger variation. This random error could have occurred either during sampling or the lab analysis.
The views expressed here are not representative of the University of Southampton.