Biogenic silica, eutrophication risk and different forms of phosphorus in surface sediments of Anzali wetland, Caspian Sea.

This investigation aimed to determine the contents of biogenic SiO and different phosphorus forms (P) and to evaluate phosphorus ecological risk in surface sediment of Anzali wetland. According to the results, biogenic SiO ranged from 0.29 to 3.04%. Also, the average biogenic SiO at all studied stations was 1.36 ± 0.83%. Results indicated that total P (TP) was between 493 and 771 ppm, averaged 637.20 ± 79.41 ppm. Moreover, inorganic P (INTP) ranged from 256.63 to 376.89 ppm and composed 51.46 ± 4.68% of total P. The percentage of P-forms was in descending order: residual-P > Fe-P > Ca-P > Al-P > labile-P. Phosphorus pollution index (PPI) ranged from 0.82 to 1.29, with an average of 1.06 in the sediment of the Anzali wetland. The Sediment P saturation (SPS) values varied considerably from 40.96 to 83.57, with an average SPS value of 49.1. Based on the eutrophication risk index, all stations except one had a low eutrophication risk index.