Abstract. We analyzed net-tow samples of natural assemblages of
plankton, and associated particulate matter, from the Exclusive Economic
Zone (EEZ) of Qatar in the Arabian Gulf. Size-fractionated suspended particles
were collected using net tows with mesh sizes of 50 and 200 µm to examine the composition of small- and large-size plankton
populations. Samples were collected in two different years (11 offshore
sites in October 2012 and 6 nearshore sites in April 2014) to examine
temporal and spatial variabilities. We calculated the excess metal
concentrations by correcting the bulk composition for inputs from
atmospheric dust using aluminum (Al) as a lithogenic tracer and the metal∕Al
ratios for average Qatari dust. Atmospheric dust in Qatar is depleted in Al
and enriched in calcium (Ca), in the form of calcium carbonate (CaCO3),
relative to the global average Upper Continental Crust (UCC). To evaluate the
fate of this carbonate fraction when dust particles enter seawater, we
leached a subset of dust samples using an acetic acid–hydroxylamine
hydrochloride (HAc–HyHCl) procedure that should solubilize CaCO3
minerals and associated elements. As expected, we found that Ca was removed
in Qatari dust; however, the concentrations (ppm) for most elements actually
increased after leaching because the reduction in sample mass resulting
from the removal of CaCO3 by the leach was more important than the loss
of metals solubilized by the leach. Because surface seawater is
supersaturated with respect to CaCO3 and acid-soluble Ca is abundant in
the particulate matter, we only used unleached dust for the lithogenic
correction. Statistical analysis showed that for many elements the excess
concentrations were indistinguishable from zero. This suggested that the
concentrations of these elements in net-tow plankton samples were mostly of
lithogenic (dust) origin. These elements include Al, Fe, Cr, Co, Mn, Ni, Pb,
and Li. For several other elements (Cd, Cu, Mo, Zn, and Ca) the excess
concentrations present after lithogenic correction are most likely of
biogenic/anthropogenic origin. The excess concentrations, relative to
average dust, for most elements (except Cd) decreased with distance from
the shore, which may be due to differences in biology, currents, proximity to
the coast, or interannual processes.
Spatiotemporal Variability of Surface Phytoplankton Carbon and Carbon-to-Chlorophyll a Ratio in the South China Sea Based on Satellite Data