Bioavailability and geochemical speciation of phosphorus in surface sediments of the Southern Caspian Sea

The purpose of this study was to quantify phosphorus bioavailability and various forms of this element in surface sediments of the southern Caspian Sea. To do, sediment samples were collected from different depths in autumn 2015 using a Van Veen Grab. Phosphorus forms were recognized by a sequential extraction procedure (SEDEX). Results revealed that the range of total phosphorus (TP) was between 431 ppm and 594 ppm with a mean value of 535.25 ± 45.05 ppm. Organic phosphorus (OP) ranged from 62 to 99 ppm and contributed 14.85 ± 2.21% of total phosphorus in average. The contents of loosely phosphorus, iron-bound phosphorus, authigenic phosphorus and detrital phosphorus varied 35-55 ppm, 50-94 ppm, 125-189 ppm and 152-217 ppm, respectively. Generally, inorganic phosphorus (IP) was between 365 and 522 ppm which comprised 81.9-88.53% of total phosphorus. Findings showed levels of different phosphorus forms in a descending order: detrital P > Authigenic P > Organic P > Fe-P > Loosely P. Bioavailable phosphorus concentration was between 153 ppm and 240 ppm and consisted 37.21 ± 3% of total phosphorus. Based on molar ratio of TOC/OP, organic matter obtained from the studied stations suggested a terrestrial origin.