University of Southampton OES Undergraduate Falmouth Field Course 2016 - Group 3 databank and initial findings.
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Estuarine Chemical Data
Silicon - Depth Profile & Mixing Diagram
The Silicon concentration decreased from 4.96 µmol/L at the surface of station A22 furthest from the mouth of the river to 0.43 µmol/L at the surface of station G30. Stations towards the mouth of the estuary showed very similar concentration throughout the water column. Stations A22 to D28 show a decrease with depth whereas stations F29 and G30 show slight increases
with depth.
Results collected are concentrated between salinities of 32 and 36. As a result,
interpretation of behaviours is difficult and interpretations may lack accuracy.
Despite this, using the data present, a theoretical dilution line (TDL) has been
plotted, shown in red, using minimum and maximum salinity values. If conservative
behaviour was present, it is reasonable to assume that data points would follow the
TDL. As can be observed, silicon fails to demonstrate this property. This suggests
non conservative behaviour and are depleted between salinities of 0 and 36. This
theory however is only based on the data present at high salinities. Without data
from medium-low salinities, this theory cannot be fully supported.
The non-conservative depleted behaviour in the silicon mixing diagram is most likely due to biological uptake by centric diatoms (see Biological Data). These diatoms uptake silicon in order to create frustules, and will place a demand on silicon within the estuary, resulting in depletion of silicon concentrations. Strong decreases in silicon concentrations with depth at sites further up the estuary can be assumed to be caused by silicon rich riverine waters overlying higher salinity estuarine waters, with little mixing between the two at this point.
Nitrate - Depth Profile & Mixing Diagram
Surface nitrate concentration decreases as the stations move towards the mouth of the estuary. The highest concentration was 7.63uM/L found at the surface of station A22. The lowest concentration was 0.18uM/L found at the surface of station G30. Station B26 shows a distinct minimum at 9.5m depth. The other stations have almost linear profiles.
Like Silicon, nitrate also appears to show a non-conservative depleted behaviour
over the course of the estuary. The uptake of dissolved nitrate is often due to denitrification
by heterotrophic bacteria in sub oxic sediments. A further uptake of nitrate is oxidation
by Anammox bacteria
(Graham 2008). Both of these process have been identified in estuarine conditions, and thus may explain the depleted behaviour of nitrate theorised within the estuary.
Phosphate - Depth Profile & Mixing Diagram
The Phosphate concentration showed different behaviours up the river (Station A22) compared to the stations towards the sea. Phosphate showed a minimum at 5m and 10m at most stations. The highest concentration was 0.41uM/L at the surface of station A22. The minimum was 0.09uM/L at the surface of station G30. There was a general decrease in concentration as the stations moved towards the mouth of the estuary.
Despite poorly correlated data displayed on the phosphate mixing diagram for the
Fal estuary , it was theorised that phosphate showed non-conservative behaviour.
However, due to the non-linear results, it is difficult to draw any definitive conclusions
from the data collected. In comparison to the other nutrient mixing diagrams, which
show good linearity, it may be necessary to repeat sampling of phosphate in pursuit
of an accurate representation of phosphate mixing along the Fal.
The creation of the Theoretical Dilution Lines used freshwater end-members from the River Allen, and rejected freshwater end-members from the River Kenwyn. This decision was based on further data collected during day 8 of the field course, with salinities of 0.4 and 7.0, which suggested that the River Allen was the dominant control on water parameters downstream.
Oxygen Saturation
Dissolved oxygen samples were taken at the surface of every station. They have decreasing values with increasing distance from the head of the estuary. Increasing oxygen saturation towards the mouth is to be expected as primary production increases from the higher populations of marine phytoplankton.
Navigate to: Physical Data - Biological Data