The fate of terrestrially derived phosphorus in estuaries
In this study the phase distribution of phosphorus (P) is examined in 7 Scottish East Coast rivers. Rivers with contrasting catchment properties were selected, including pristine highland rivers, lowland rivers with agricultural land use and a range of intermediate rivers. Suspended particulate matter (SPM) was rich in organic P and had total P contents in the range of 0.1 to 1%. Highest total P contents of particles were observed for the river richest in dissolved P. The pristine highland systems showed the lowest total P contents. Large increases in exchangeable P in summer were partly attributable to the presence of phytoplankton. Phytoplankton accounted for less than half and more typically around 20% of organic carbon in SPM. The remainder is likely to consist largely of amorphous, humic substances. Sorption properties (e.g. EPC0, equilibrium phosphate concentration at zero adsorption) of SPM were examined using adsorption isotherms. Particle concentrations for sorption experiments were kept low (ca. 200 mg/l) to allow extrapolations to natural conditions. SPM from highland rivers showed the largest affinity for P. Lowland rivers displayed a comparatively low affinity for P. EPC0 values of SPM closely reflected soluble reactive phosphorus concentrations in river water. SPM from highland and lowland rivers showed consistent differences in organic carbon content. Organic carbon content of SPM was correlated to sorption parameters of SPM. The sorption parameters of intermediate rivers could be explained by mixing particles of different affinity, using organic carbon contents to trace the proportions of particle mixing. Under estuarine conditions the affinity of fluvial SPM for P decreases. The magnitude of the change is insufficient to cause desorption in the pristine highland rivers. Up to 80-fold increases in EPC0 were observed for the remaining rivers suggesting desorption takes place in the estuary.