Distribution and speciation of phosphorus along a salinity gradient in intertidal marsh sediments

摘要

Forms of solid phosphorus fractions were examined in intertidal marsh sediments along a salinity (0-22%) gradient in a river-dominated estuary and in a marine-dominated salt marsh with insignificant freshwater input. Freshwater marsh sediments had the highest ratio of organic N:P of between 28:1 and 47:1 mol:mol, compared to 21:1 to 31:1 mol:mol in the salt marshes, which was consistent with a trend toward P-limitation of primary production in freshwater and N-limitation in salt marshes. However, total P concentration, 24.7 ±11.1μmol P/g (±1 SD) averaged over the upper metre of sediment, was highest in the freshwater marsh where bioavailability of P was apparently limited. In the freshwater marsh the greatest fraction of total P (24-51%) was associated with humic acids, while the importance of humic-P decreased with increasing salinity to 1-23% in the salt marshes. Inorganic P contributed considerably less to total sediment P in the freshwater marsh (15-40%) than in the salt marshes (33-85%). Inreduced sediments, phosphate bound to aluminium oxides and clays was an important inorganic P pool, irrespective of salinity. Inorganic P associated with ferric iron [Fe(III)] phases was most abundant in surface sediments of freshwater and brackish marshes, while Ca-bound P dominated inorganic P pools in the salt marshes. The results showed that particle-bound P in marsh sediments exhibited changes in chemical associations along the salinity gradient of an estuarine system, which was a consequence of changes in ionic strength and the availability of Fe and Ca.