Date of Award
Doctor of Philosophy in Oceanography
In an estuary, characterized by biogeochemical reactions that affect the input, cycling, and removal of river-borne components, an understanding of the factors that regulate biomass and production of the estuarine ecosystem requires insight into the controls on phosphorus dynamics.
Phosphate, the principal dissolved phosphorus species in natural waters, interacts rapidly with an assortment of surfaces, being taken up by and released from inert particles through a complex series of sorption reactions. Phosphate is especially susceptible to adsorption on freshly precipitated or coagulated iron particles. A number of studies have documented the colloidal flocculation of filtrable iron in the low salinity waters of dozens of estuaries. During flocculation, filtrable iron and phosphate are removed from solution in estuarine waters as particles composed of iron oxyhydroxide, organic matter, and adsorbed phosphate. However, in most of these estuaries, total iron appears to be conservative within the water column and evidence, in the estuarine sediments, of iron removal from the water column has been reported by only one investigator (Mayer, 1982). The conservative nature of iron and the lack of sediment deposit evidence has been attributed to long settling time for particles coupled with relatively short residence time of waters; i.e., the particles remain in the water column and are transported out of the estuary before settling to the bottom.
My approach utilized two techniques which are different than those used by previous investigators: (1) analyzing the sediments for the oxyhydroxide fractions of phosphorus and iron, instead of total iron and phosphorus, and (2) subjecting intact sediment cores to reducing conditions and measuring the flux of both phosphorus and iron across the sediment-water interface. The hypothesis is that during anoxic conditions, any Fe (Ill) oxyhydroxide present in the surface sediment will have dissolved, releasing quantities of both iron and adsorbed phosphate to the overlying waters in proportion with the quantity of those substances present in the sediment.
Removal of filtrable iron in the low salinity waters of the Parker River estuary has been demonstrated by other investigators (Boyle et al., 1977) and my analysis shows that oxyhydroxide fractions of both P and Fe appear to be preferentially deposited within the sediments of the upper and the middle reaches of the estuary. In addition, the association of P-oxyhydroxide to Fe-oxyhydroxide was significant at the 99% confidence level. However, the anoxic benthic flux pattern of P and Fe along the estuary were different from one another, which may be due to the formation of insoluble Fe minerals in high salinity, high sulphate sediments of the lower estuary. In comparing benthic flux to sediment contents, I found the levels of P and Fe anoxic benthic flux to be coupled to the solid phase oxyhydroxide levels in the surface sediments of the estuary.
Anoxic conditions in the sediment and overlying water triggered substantial benthic release of both iron and phosphorus. However, the subsequent onset of oxic conditions in the water column resulted in a disproportionate removal of iron with respect to phosphorus. That is, Fe (II) in the water column was oxidized to insoluble Fe (III), but the amount of phosphorus originally released along with the Fe (II) did not re-adsorb to the Fe (III) as it precipitated.
This study indicated the importance of adsorption processes on estuarine phosphorus cycling and a possible mechanism for phosphorus recycling into the water column during temporary anoxic events. Phosphorus cycling was closely related to the physical and chemical processes acting on inert particles in the estuarine environment.
Chinman, Richard, "Phosphorus Dynamics in the Parker River Estuary" (1991). Open Access Dissertations. Paper 1348.