Flow Injection Chemistries for the In Situ Monitoring of Nutrients in Sea Water

Trevor McCormack

May 1996

Department of Environmental Sciences, University of Plymouth

in collaboration with

Plymouth Marine Laboratory, Plymouth, U.K.

In order to investigate the biogeochemistry of aquatic ecosystems, a quantitative knowledge of nutrient status i.e. spatial and temporal distribution, and primary production is necessary. This thesis describes chemistries for the determination of the major nutrient species, nitrate (as total oxidised nitrogen minus nitrite) and phosphate (as orthophosphate) using Flow Injection (FI) analysis. Chapter One describes the role of nitrogen and phosphorus in the aquatic environment and overviews laboratory and in situ methods for their determination. Chapter Two describes the development and optimisation of an FI method for the determination of nitrate in natural waters of varying salinity (0-35 ). The final manifold can be used for the direct determination of nitrate in riverine, estuarine and sea water without the need for sample pre-treatment or salinity correction. The limit of detection is 0.1 mmol l-1 NO3-N. The method is validated by participation in two interlaboratory exercises (EU Community Bureau of Reference (BCR) and International Council for the Exploration of the Sea) for nitrate in river and sea water respectively.

The analytical performance of reverse FI and FI molybdenum blue methods for the determination of orthophosphate in riverine, coastal and open ocean waters is described in Chapter Three. The high phosphate blank and salt error of the methods using tin(II) chloride and ascorbic acid as the reducing agents are investigated, in order to achieve a lower limit of detection (0.1 mmol l-1 PO4-P) for the determination of environmental levels of orthophosphate.

The shipboard operation and performance of the FI methods for the determination of nitrate and orthophosphate are described in Chapters Four and Five. Field data from the Tamar Estuary, Sutton Harbour and the coastal waters of the North Sea are presented and related to biogeochemical processes. TON concentration showed an inverse correlation with salinity at all three locations as well as seasonal variation in the Tamar Estuary with levels varying between 0.01-0.39 and 1.2-3.3 mgl-1 N at the high and low salinity endmembers respectively. Buffering of orthophosphate by Fe phase in the sediments was also evident in the Tamar Estuary as well as anthropogenic inputs into the lower 10 km of the estuary. A comparison between FI and the traditional AutoAnalyzer method is also made in Chapter Five for the shipboard deployment in the North Sea along the NE English coastline and into the high nutrient Humber Plume.

The final chapter investigates the coupling of FI with a charge coupled device detector using fibre optic connections for the spectrophotometric determination of nitrate in water. Comparisons are made with the traditional UV/Visible spectrophotometric detection method as described in Chapter Two in terms of LOD and suitability for shipboard use.

Copyright © 1996 Trevor McCormack. All rights reserved.

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