In order to investigate the biogeochemistry of aquatic ecosystems, a quantitative understanding of primary production and the temporal and spatial distribution of nutrients is necessary. This thesis describes the development of a submersible FI based nutrient sensor for the in situ determination of nitrate in estuarine and coastal waters. Chapter One describes the role of nitrogen in the global and marine nitrogen cycles and provides an overview of laboratory and in situ methods for its determination. Chapter Two describes the key parameters for a field instrument and culminates with the overall design specification for the system. Chapter Three describes in detail the design, build and optimisation of the key individual components of the system, e.g. sample delivery system, injection valve, reduction column, reaction column, flowcell, on-board control system and the housing of the complete integrated system. Chapter Four describes the optimisation and analytical performance of the FI instalment prior to field trials. The key operational parameters such as flowcell path length, injection volume and detector response were investigated. LOD, reproducibility and linear range were determined and the control programme for the onboard computer is reported. For example, a LOD of 0.01 Nitrate-N, a linear range of 0-140 | iM Nitrate-N and a reproducibility of ± 5 % were achieved. Chapter Five describes the field experiments where the FI system was initially used as a bench instrument and compared to a laboratory FI method which had been validated by participation in two interlaboratory exercises and for nitrate in river and sea water. The first submersed deployments involved the optimisation of the system operational characteristics and developing the field techniques. The final part of this chapter describes the weekly field studies of Barn Pool in Plymouth Sound using the submersed nutrient sensor for a period of two months. The operation and performance of the submersed sensor was assessed against an air segmented continuous flow analyser during a Land Ocean Interaction Study (LOIS) North Sea cruise aboard the RVS Challenger. The results from this exercise and the relevant salinity and turbidity measurements are presented in Chapter Six.

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