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

We examined forms of solid phosphorus fractions in intertidal marsh sedimen ts along a salinity (0-22 parts per thousand) 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 is consistent with a trend toward P-limitation of primary pr oduction in freshwater and N-limitation in salt marshes. However, total P c oncentration, 24.7 +/- 11.1 mu mol P g dw(-1) (+/-1 SD) averaged over the u pper meter of sediment, was greatest in the freshwater marsh where bioavail ablity of P is apparently limited. In the freshwater marsh the greatest fra ction of total P (24-51%) was associated with humic acids, while the import ance of humic-P decreased with increasing salinity to 1-23% in the salt mar shes. Inorganic P contributed considerably less to total sediment P in the freshwater marsh (15-40%) than in the salt marshes (33-85%). In reduced sed iments at all sites, phosphate bound to aluminum oxides and clays was an im portant inorganic P pool irrespective of salinity. Inorganic P associated w ith ferric iron [Fe(III)] phases was most abundant in surface sediments of freshwater and brackish marshes, while Ca-bound P dominated inorganic P poo ls in the salt marshes. Thus, our results showed that particle-bound P in m arsh sediments exhibited changes in chemical association along the salinity gradient of an estuarine system, which is a likely consequence of changes in ionic strength and the availability of iron and calcium.