Adsorption of dissolved phosphate onto loess particles in surface and deepEastern Mediterranean water

The objective of this study was to examine whether dissolved inorganic phos phate (DIP) is removed from the Eastern Mediterranean Sea by adhering to at mospherically deposited loess particles sinking through the water column. I n a series of radiolable experiments, loess from the Negev Desert, treated in various ways, was added to surface (SSW) and deep (DSW) seawater spiked with (PO4-3)-P-32. It was shown that when fresh loess reaches the Mediterra nean SSW approximately 1.3 mu mol P/g are released (similar to 11% of the t otal P concentration). Biological activity and inorganic particles removed similar amounts of the tracer (30-40%) from SSW. It was estimated that abou t 0.2 mu mol P/g of 'aged loess' (proxy of particles sinking into DSW), wer e removed from poisoned SSW and DSW, while there was minor adsorption when either nothing or quartz powder was added. The adsorbed DIP accounts for ap proximately 15% of the released P and is equivalent to about 2% of the rema ining P (11.17 mu mol P/g loess). Therefore, the process of DIP removal by atmospherically derived particles exists, but due to the higher release of P, the result is a net addition of dissolved atmospheric phosphorus in seaw ater. It is postulated that in actual SSW where dust concentrations are muc h lower, biological uptake out-competes inorganic adsorption, although it w as demonstrated that the decrease in loess particle concentrations tends to increase their adsorption capacity. As the loess descends into the DSW it continues to remove DTP and thus transports phosphate from the water column to the sediments. A preliminary quantitative estimate suggests that the pr ocess of DIP removal by loess particles cannot explain the phosphate 'defic it' in the Eastern Mediterranean DSW characterized by unusually high DIN/DI P ratios (similar to 27) reported by others. (C) 1999 Elsevier Science B.V. All rights reserved.