Trace metal composition of size-fractionated plankton in the Western Philippine Sea: The impact of anthropogenic aerosol deposition

The surface water of the Western Philippine Sea (WPS) receives a significant amount of anthropogenic aerosols from East Asia, serving as an ideal location to investigate the impact of anthropogenic aerosol deposition on trace metal composition and cycling in the Northwestern Pacific Ocean (NWPO). As part of the Taiwan GEOTRACES process study, we have collected size-fractionated plankton from surface water to investigate their metal composition in the open ocean. Elemental ratios in plankton, P-and Al-normalized, are used to evaluate the sources of trace metals and the relative contribution of different metal components in plankton assemblages. Most of the trace metal quotas in plankton are one to two orders of magnitude higher than their intracellular plankton quota, indicating that the majority of the metals are most likely to be extracellularly adsorbed or aggregated on plankton. The metal to Al ratios for most of the trace metals in plankton are also one to two orders of magnitude higher than their lithogenic composition, but are relatively close to metal composition in aerosols collected in situ. This supports the notion that particulate trace metals associated with plankton mainly originate from anthropogenic aerosols, not lithogenic particles. Compared to plankton metal quotas obtained in other oceanic regions, trace metal quotas observed in the WPS rank among the highest for Fe, Mn, Zn, and Cu globally. Our study demonstrates that anthropogenic aerosol deposition has significantly elevated trace metal concentrations in the size-fractionated plankton in the surface water of the NWPO relative to the biological requirements.