The trace-metal composition of size-fractionated plankton in the South China Sea: Biotic versus abiotic sources

We report the elemental composition (P, Si, Al, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) in the size-fractionated plankton and suspended particulate matter (SPM) in the surface waters of the South China Sea. The size-fractionated plankton were effectively and gently separated by gravity through a novel trace-metal-clean filtration device, equipped with 150-, 60-, and 10-μm aperture nets in sequence to concentrate the plankton. P-normalized metal quotas in the largest fraction, mostly composed of copepods, were close to or slightly lower than the average metal quotas obtained from previous field studies. However, the total metal quotas in the two smaller fractions were 8- to 40-fold higher than the largest one, and they increased with decreasing sizes. The striking correlation between some essential metals and chlorophyll (Chl) in the SPM at the offshore site indicates that the majority of the metals were associated with algae. Nevertheless, the P-normalized metal quotas also showed positive correlations with abiogenic Al and Mn, indicating that most of the metals associated with phytoplankton were from extracellular inorganic particles. Preliminary evidence suggests that the extracellular metals were originally derived from anthropogenic aerosols, which contain abundant dissolvable trace metals. The metal quotas of the zooplankton assemblages have a fairly consistent value as compared to observations from other regions, but algae larger than 10 μm carry overwhelming amounts of extracellular metals even in offshore areas. The stoichiometry concept for trace-metal composition in marine plankton assemblages is unrealistic in the field.