Impact of harbour, industry and sewage on the phosphorus geochemistry of a subtropical estuary in Brazil

The distribution of different forms of phosphorus in surface sediment from 17 sites were investigated by SEDEX method. The sites were divided into three sectors: Santos Channel (SC - influenced by harbour, fertilizers plants and phosphogypsum mountains), São Vicente Channel (SVC- domestic waste) and Santos Bay (SB - sewage outfall). The average percentage of each P fraction of the surface sediments in this region followed the sequence P-Fe (38%) > Porg (27%) > Pexch (13%) > Detrital - P (12%) > Auth - P (10%). Ptotal varied from 3.57 to 74.11 μmol g−1 in both seasons. In SVC, Pexch ranged from 13% to 27% and Porg varied from 12% to 56%. These high percentages of Pexch/Ptotal (greater than 20%) may be related to low oxygen resulting from oxygen consumed by intensive organic matter decomposition as well as the salty water that leads to cation and anion flocculation. Also, the possibility of an influence related to the industrial source of Pexch is not ruled out. No significant seasonal differences were found among sites, except for sewage outfall, with changing in the grain size and hence, the P geochemistry. During the summer in the sewage outfall station, Porg represented 37% of Ptotal, which decreased to 13% in the winter. These results suggest that high percentages of organic phosphorus cannot be attributed only to autochthonous and allochthonous organic matter, but also to detergents and/or domestic waste. In contrast, spatial differences among sectors were observed, with the highest values of each fraction associated with sites near industrial and domestic waste activities.