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Chemical Analyses of the Tamar

Mixing diagrams & TDL

Mixing Diagrams and the theoretical dilution line (TDL) show how solutes act in the estuarine environments. The TDL is created by drawing a line through the points of lowest salinity (river water) and the point of highest salinity (seawater).  The first graph shows silicon and how it behaves in the Tamar estuary. The points drop below the TDL which shows a removal of dissolved silicate from the water column, the proposed explanation for this decrease in silicon is the blooming of phytoplankton such as diatoms which use to silicon to build their silicate frustules (armour). Due to irradiance and thermal stratification, phytoplankton bloom during spring. Silica concentration would be expected to show the most pronounced draw down during spring and summer as sown by the findings of Peterson et al (1975). Given the sampling time in the year, this biological removal is most likely to explain the removal.

Phosphate is a crucial nutrient for protein and DNA production. Phosphate shows different mixing to silicon. Phosphate shows removal higher in the estuary and addition in the lower estuary. This change in mixing could be caused organisms taking up phosphate initially then later in the estuary when it reaches more industrialised zones, outflows may introduce further phosphate causing the addition of phosphate to the estuary.

Nitrate, the final nutrient studied by us in the Tamar shows conservative mixing. None of the nitrate leaves or enters the system through processes in the water. Depth profiles for phosphate show that phosphate levels do not rise above 0.5 umol/L regardless of depth. It also shows that there are is a large variety of values at similar depths. Possibly relating to the differences in physical parameters in the stations up and down the estuary. Silicon values have a much larger variability in comparison to the phosphate numbers. This higher variability could be attributed to the fact that some of the phosphate is removed while some of it is added as evidenced in the mixing diagrams. As a result, this will accentuate the minimums and maximums shown on the graph.