In Situ Monitoring and Biogeochemical Cycling of Nutrients in Estuarine Waters

Paulo Cesar Ferreira da Costa Gardolinski

September 2002

Department of Environmental Sciences, University of Plymouth, Plymouth, U.K.

in collaboration with

CNPq-Brazil

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 physico-chemical 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 physico-chemical 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.

Chapter Four reports the development and deployment of a submersible flow injection spectrophotometric (submersible FI) analyser for determination of nitrate in estuarine waters. Key features of the analyser are its portability and ease of deployment owing to its small size, weight and low buoyancy. The analyser was optimised to perform accurately in the laboratory, on board ship (during the Impact Cruise) and in situ during field surveys. The detection limit achieved was 2.8 mg L-1 N whilst the linear range could be varied from 2.8 - 100 mg L-1 N up to 100 - 2000 mg L-1 N over a salinity range of 0 - 35 requiring only small changes in the manifold. The good instrument performance was assured during an intercomparison exercise (32 laboratories) for the determination of nitrate in seawater (assigned tolerance 327.6 ± 33.6 mg L-1, submersible FI 351.4 ± 28.0 mg L-1) and comparison with the University of Plymouth reference method.

The results from eight surveys within the Tamar Estuary for the monitoring of nitrate, phosphate, pH, dissolved oxygen, salinity, conductivity and temperature, are presented in Chapter Five. These results were comparable with previous literature datasets, e.g. nitrate+nitrite and phosphate concentrations. This suggests that estuarine conditions have remained fairly consistent over the last 20 years.

Laboratory experiments undertaken to help understand some of the environmental processes previously described are presented in Chapter Six. The aim of the investigation was to monitor dissolved (<0.45 mm) organic and inorganic phosphorus (Po, Pi) forms that could be released into the estuarine mixing zone. Results indicate that a measurable and significant release of Po, from suspended sediments, occurred when salinity was increased to 5 and 10 prior to the release of Pi. The analytical method proved to be very sensitive and reproducible (RSD < 3%) for this type of matrix, achieving limits of detection ranging from 1.8 - 3.0 mg L-1 P-PO4 with recovery of the organic phosphorus model compound typically between 76 - 87 %.

© 2002 by Paulo Cesar Ferreira da Costa Gardolinski. All Rights Reserved

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