Date of Award
Doctor of Philosophy in Oceanography
Michael Q. Pilson
Mineral equilibria and sorption reactions are often invoked as mechanisms that regulate the concentration of nutrient phosphorus (P) in estuaries, and are sometimes termed collectively as the "phosphate buffer". However, the estuarine chemistry of P is only partly known and from theory alone it is not possible to predict the reactions that occur in a particular estuary. The objective of the present study is to quantify the sorption of P on particles transported through an estuarine salinity gradient, and to evaluate the importance of sorption processes in the seasonal nutrient chemistry of the estuary.
Regionally important but largely unstudied, the Taunton River estuary (MA) is the study area for this work. The Taunton drains urban and rural watershed and supplies one third of the freshwater drainage into the Narragansett Bay (RI) system. Total, dissolved, and particulate nutrients, oxygen, pH, salinity, alkalinity, suspended load, current speed, and particle mineralogy were sampled monthly in the estuary from July 1988 to December 1989. Sorption experiments were done with particles collected from the estuary, and the results are presented in the context of the chemical surveys. The survey data indicate high nutrient levels in the Taunton and should be useful in future studies of Narragansett Bay. The data also underline the importance of flow conditions in the estuarine nutrient cycles, as well as the influence of various biogeochemical processes. Many relationships between the variables were found.
Sorption experiments simulated in-situ conditions and mixing along a salinity gradient from freshwater to S = 20 at constant temperatures, taking into account pH, solid to solution ratio, mineralogy and biological activity. Natural particles were manipulated at solid to solution ratios and phosphorus concentrations approaching in-situ values. This empirical approach was adopted because calculations indicate that there are several solid phases with which dissolved Pin the Taunton could be in equilibrium.
Experimental results show that freshwater particles collected in different seasons are able rapidly and reversibly to exchange P; exchange persists on time scales of minutes to days, involving both uptake and release. Flocculation was shown not to be a factor. Adsorption isotherms for particles in solutions spiked with Pare described by the Langmuir model. At apparent equilibrium, when there is no net exchange of sorbed P on particles, dissolved P was in the range of 3 to 7 μM and the amount of exchangeable P was in the range of 2 to 9 μmol per gram of solids, in the salinity range from 2 to 20 and at 20 C. At lower temperatures (2 C) the values may be higher. The concentration at which no net sorption occurs, as well as the amount of P sorbed onto particles, probably varies along the salinity gradient of the estuary. Under typical conditions in the Taunton, P should desorb from particles as salinity increases, and sometimes at low salinity, P may be adsorbed. Amounts of exchangeable P on suspended sediments comprise several percent of the total P present in the estuary, based on the sorption experiments and monthly surveys.
Boucher, Joceline M., "Nutrient and Phosphorus Geochemistry in the Taunton River Estuary, Massachusetts" (1991). Open Access Dissertations. Paper 1337.