This thesis describes the use of in situ and laboratory techniques for monitoring nitrate, phosphate and master variables in the environment. Chapter One presents a general overview of nutrients, how they are essential for aquatic biota, their general characteristics and the importance of their behaviour in estuaries. Commonly used analytical methods for nutrient determinations in natural waters are also evaluated. Chapter Two reports the use of chemometrics to determine trends in historical time-series physicochemical datasets {e.g. nitrate+nitrite, phosphate, river flow, precipitation, suspended solids, water temperature and chlorophyll a) from the Tamar catchment, UK. The results provided useful information about correlations between the studied variables. The influence of time delay to correlate variables, such as rainfall and river flow, rainfall and nitrate+nitrite, river flow and nitrate+nitrite, rainfall and phosphate, river flow and phosphate, demonstrated the interaction between variables over time, and increased their correlation coefficients. Seasonal trends were separated using PCA. Models for nitrate +nitrite and phosphate were built to predict their behaviour based on the other five physicochemical variables, and explained 91.3 and 72.9 % of the variance for nitrate+nitrite and phosphate, respectively. In Chapter Three evaluation of different natural waters sample storage techniques is reported. Results indicate that it is generally not possible to develop a single storage protocol for different kinds of natural waters. During sample storage the preservation of nutrients was found to be extremely matrix dependent, e.g. phosphate co-precipitated when calcium rich samples were frozen, and chemical treatment of samples rich in dissolved organic carbon resulted in a slight increase in phosphate concentration. In order to overcome these problems, practical guidelines on how to elaborate a site-specific protocol for sample storage are presented
