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Chemical analysis

Silicon

Nitrate

Phosphate

Oxygen Concentration

Chlorophyll

Across all six of the sampling sites, there was little noticeable difference in the % Oxygen Saturation at two meters depth (Fig. 4). All of the values sit between 60-70%, with the highest value seen at site C9 and the lowest at F11.


Silicon was found to follow conservative behaviour along the Fal Estuary (Fig. 1). The plotted point follows the theoretical dilution very well meaning conservative. The main source of silicon to estuaries is from erosion of silicate rocks and transport by rivers. The fact it is conservative means that silicon is not being removed from the estuary by any processes other than dilution due to mixing. Silicate could behave non-conservatively if there is a diatom bloom in the water as diatoms remove silicate from the water to incorporate into their frustules. The mixing diagram therefore suggests there was no diatom bloom at the time of measurement.


Nitrate was found to follow non-conservative behaviour along the Fal Estuary (Fig. 2). Between salinities of 32 and 35, concentration of nitrate fell below the TDL, suggesting that at along the course of the estuary there is a net removal of nitrate. It would be expected to see a greater amount of nitrate from leaching fertilisers into the Fal. However, this increased nitrate concentration could lead to a phytoplankton bloom, as they need nitrate for growth, which in turn decreases the dissolved nitrate concentration in the water column, significantly reducing nitrate levels in the water column.


A steep decrease in phosphate concentration from 0.5 µmol/l to 0.2µmol/l was observed between salinity of 32 to 35 (Fig. 3). Phosphate followed non-conservative behaviour within the Fal Estuary and values lay about the TDL indicating that additional sources of phosphate are entering the estuary. One of the main additional sources could be from agricultural runoff. Fertilisers often contain phosphate which can leach into the water column resulting in the unnatural levels of phosphate which will build up if there is not enough biological uptake. Several rivers run into the Fal which could increase the amount of agricultural run off.



Fig. 1 Silicon estuarine mixing diagram with associated theoretical dilution line connecting the river and marine  endmembers within the Fal Estuary

Fig. 2 Nitrate estuarine mixing diagram with associated theoretical dilution line connecting the river and marine  endmembers  within the Fal Estuary

Fig. 3 Phosphate estuarine mixing diagram with associated theoretical dilution line connecting the river and marine endmembers within the Fal Estuary

Fig. 4 Surface oxygen saturation along six sample stations within the Fal Estuary

Fig. 5 Chlorophyll concentration with depth along seven sample stations within the Fal Estuary

The overall trend in the chlorophyll data shows an increase in chlorophyll concentration with depth (Fig. 5). This pattern is seen in 5 of the seven sites that were sampled (8b, 10d, 11e, 12f & 13g). At site 7a, there was a value of approximately 0.2μg l-1 which then increased to around 0.7 μg l-1 at around 3m depth then decreases to 0.4 μg l-1 at 5m. The only other sample that didn’t follow the overall trend was Site 9c, where there was a decrease in chlorophyll concentration from the surface to approximately 5m from 1.4 μg l-1 to 1.1 μg l-1, then between 5m and 15m where there is a significant increase to approximately 2.5 μg l-1. The lowest surface chlorophyll value (0.2 μg l-1) were seen at 7a, the furthest point upstream and the highest surface chlorophyll value (2.75 μg l-1) was seen at 13g.

The overall trend suggest that there is a greater amount of phytoplankton in surface waters.