Inorganic chemistry of phosphorus cycling in fresh and marine waters

Sammanfattning: In the present Thesis, new results are reported on critical chemical mechanisms controlling the cycling of phosphorus in aquatic environments. These findings have implications for several fields of earth system sciences, including large-scale global change, both Recent and in older geological periods.Two chemical mechanisms of phosphate immobilisation are highlighted, incorporation into Fe(III) oxyhydroxide particles and formation of apatite. The findings presented are based on laboratory experiments with natural water solutions, sediment-water microcosms and synthetic water solutions. Also, results from field studies including water column depth profiles and from colloidal iron-rich particles formed in situ, are considered. The field stations were situated in the freshwater Lake Mälaren and the adjoining brackish Baltic Sea proper.During the hydrolysis of Fe(III) induced by oxidation of Fe(II), phosphate is incorporated into the iron-rich particles formed. Studies on both fresh and marine systems show that this phase has a lower limiting Fe/P stoichiometry of two. Accordingly, it was found that when the dissolved initial Fe/P ratio was >2, virtually all phosphate was bound into particulate form, whereas at Fe/P <2, iron was in too short supply, and some phosphate stayed dissolved. Concentration depth profiles recorded in the stratified Hallsfjärden Bay, Baltic Sea proper, also conform to this trend, with phosphate escaping precipitation at the oxic/anoxic boundary due to a dissolved Fe/P ratio <2 in the anoxic water. In contrast to phosphorus, the uptake of calcium by the iron-rich particles seemed to be controlled by surface adsorption processes. It was more efficient in freshwater than in seawater, suggesting that salinity per se restrains the calcium uptake.A compilation of data from a wide range of marine and lacustrine environments revealed a systematic difference in the dissolved Fe/P ratio of anoxic waters. In general, marine systems had low dissolved Fe/P ratios (<2), whereas freshwater systems had Fe/P ratios >2. Combined with our experimental findings, this systematic trend in dissolved Fe/P ratio provides a chemical basis explaining why phosphate can be expected to be more available in temperate coastal marine waters than in freshwater systems, resulting in a difference in biomass-limiting nutrient in freshwater lakes (generally P limited) and coastal marine areas (primarily N limited).Long-term incubation experiments (700 days) demonstrate that apatite precipitates from natural brackish seawater of the Baltic only when the solution is supersaturated with respect to octacalcium phosphate (OCP). Apatite failed to precipitate from solutions undersaturated with respect to the nucleation of OCP, despite a supersaturation with respect to hydroxyapatite (HAP) and fluorapatite (FAP). However, in seawater supersaturated with respect to OCP, formation of apatite took place. These are findings in favour for formation of authigenic apatite via a precursor pathway in the marine environment in general, rather than through direct nucleation of apatite. Based on experiments of varying Ca/Mg ratios, it is inferred that magnesium in seawater inhibits the precipitation of apatite at low to moderate degrees of supersaturation with respect to this mineral.