Intense biological phosphate uptake onto particles in subeuphotic continental margin waters

Elucidating the processes that affect particulate phosphorus (P) export from the euphotic zone and burial in sediments is important for models of global phosphorus, nitrogen, and carbon cycling. We investigated dissolved inorganic P-i incorporation into particles (>0.2 mu m) in the subeuphotic zone and benthic boundary layer of high-productivity Mauritanian and Namibian shelf waters, using (PO43-)-P-33 tracer experiments combined with a sequential chemical extraction analysis. P-i uptake (5.4 to 19.9nmolPL(-1)d(-1)) by particulate matter was biologically mediated (similar to 50% into the organic fraction) and similar to estimated rates of heterotrophic growth. Thus, a substantial fraction of P-i must be recycled through a particle-associated microbial pool. Rapid adsorption of P-33 in the anoxic waters of Namibia indicated the additional existence of a large pool of surface exchangeable P. Particle-associated P-i recycling and adsorption may influence the export flux and ultimate fate of particle bound P in continental shelf waters