This thesis describes the development of robust flow injection (FI) techniques for the determination of nutrients, with a focus on phosphorus species, in natural waters. Chapter one presents a general overview of nutrients (P, N and Si) and their aquatic biogeochemistry and analytical methods for nutrient determinations in natural waters. Chapter Two reports a four channel Fl manifold incorporating a thiosulphate stream for on-line masking of arsenate, which is potentially a major interferent in the determination of low concentrations of phosphate in natural waters. The method is suitable for the determination of filterable reactive phosphorus (FRP) in anthropogenically impacted waters such as the Tamar Estuary. The effects of [H+]:[molybdate] ratio, temperature and matrix interferences were systematically investigated. Typical figures of merit were; typical RSDs of 1.5%, a practical limit of detection of 2 µg P Lˉ¹ and a linear range of 2 - 100 µg P Lˉ¹ at 30 °C. This was the optimum temperature to minimise interference from silicate. The method was applied to the determination of FRP in the Tamar Estuary and the results were in good agreement (paired t-test; P = 0.05) with those obtained using a segmented flow analyser reference method. In Chapter Three, a four channel Fl manifold, incorporating a thiosulphate stream and two micro-columns (an iminodiacetate resin to mask trace metals and a strong anion exchange resin to mask phosphate) is described. The figures of merit at 60 °C were a practical limit of detection of 1 0 µg Si Lˉ¹ , a linear range of 10 - 1000 µg Si Lˉ¹ and typical RSD of 1.5%. The method showed good agreement (P = 0.05) with a segmented flow analyser reference for Tamar Estuary samples. Chapter Four reports the effect of pH on the recovery of dissolved organic phosphorus (DOP) from fresh waters after batch autoclave digestion using acidic peroxydisulphate. A final pH of 1.5 - 2.0 after digestion and an acid to molybdate mole ratio of 62 - 65 in the Fl manifold gave the best recoveries, especially at low DOP concentrations (10- 50 µg P Lˉ¹ ). Peroxydisulphate in the absence of acid gave the best results for the determination of DOP in high salinity waters. Chapter Five extends the applicability of Fl to investigate phosphorus biogeochemistry by incorporating a micro-column containing immobilized phytase in the Fl manifold to determine a single species (phytic acid) within the DOP pool. The optimised method was applied to the determination of phytase hydrolysable phosphorus in the Tamar Estuary and results showed that this fraction constituted 20 - 25% of the DOP pool. The results from six surveys (five transects and one tidal cycle) of the Tamar Estuary are reported in chapter six and highlight the temporal and spatial variability of SRP, silicate and nitrate. Other chemical (As, Cu, Ni, Co, AI, Fe, Mn, Cr) and physical (pH, dissolved oxygen, salinity, conductivity and suspended particulate matter) parameters are also presented. Results were generally in agreement with historical data for nutrient concentrations in the Tamar Estuary.

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