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

Abstract

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 sizefractionated plankton were effectively and gently separated by gravity through a novel trace-metal-clean filtration device, equipped with 150-, 60-, and 10-mm 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 mm 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.Particles, including both biogenic and abiogenic, play a vital role in controlling trace-metal distribution and cycling in the ocean (Turekian 1977;Whitfield and Turner 1987;Li 1991). In particular, the cycling of many biologically essential trace metals is driven and transformed by biogenic particles generated in the euphotic zones (Collier and Edmond 1984;Bruland et al. 1994). Biogenic particles in oceanic surface waters, mainly composed of diverse phytoplankton and zooplankton, interact with dissolved and particulate trace metals in ambient seawater through various processes, including active biological uptake, adsorption-desorption, zooplankton grazing and repackaging, particle aggregation, microbial decomposition, and so on. In terms of vertical cycling, large biogenic particles, mostly composed of sizable plankton, such as large diatom, zooplankton, and their hard parts and detritus (Alldredge and Silver 1988), serve as the predominant components of sinking particles and thus are principal agents in transporting both major and trace elements from oceanic surface waters to the deep waters (Bruland 1983;Fowler and Knauer 1986;Alldredge and Jackson 1995). The trace-metal composition in large-sized marine plankton assemblages may thus provide fundamental information for further studies and help models of the cycling processes of trace metals in marine water columns (e.g., Martin an...

The Trace Metal Composition of Marine Phytoplankton

“…This concept was advanced by Bruland et al (1991) through a compilation of three studies of marine biogenic particulate matter, and more recently has been linked to the metal stoichiometries of various phytoplankton in laboratory cultures . These studies emphasize the similarities between metal stoichiometries in ocean plankton and their environment, and indeed there is great utility for a unified ratio to convert between biogenic metals and C, N, and P (e.g., Gordon et al 1997) or to estimate the composition of a particle assemblage according to its elemental stoichiometry (e.g., Ho et al 2007). However, valuable information is lost when biogenic metal data are aggregated into a single stoichiometry, and there is much to be learned about metal biogeochemistry from examining variations in phytoplankton metal composition across environmental, taxonomic, and spatial gradients.…”

Comparison of heavy metal accumulation by a bloom-forming cyanobacterium, Microcystis aeruginosa

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Dynamics of phytoplankton community structure in the South China Sea in response to the East Asian aerosol input

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