Comparison between pure-water- and seawater-soluble nutrient concentrations of aerosols from the Gulf of Aqaba

Abstract

Seawater-soluble nutrient fractions of aerosols better represent the contribution of aerosol dry deposition to the nutrient load from this source to the ocean than any estimates based on aerosol nutrients leached in pure water or acidic solutions. To understand the solubility difference between seawater and pure water, 31 pairs of aerosol samples collected from the Gulf of Aqaba were extracted in Sargasso seawater and pure water under consistent experimental conditions and procedures. Major inorganic N species (NO 3 − and NH 4 +) in the aerosols show similar solubilities in Sargasso seawater (average 44 and 23 nmol m − 3 ) and pure water (average 41 and 23 nmol m − 3 ). Seawater-soluble PO 4 3− concentrations (average 0.4 nmol m − 3 ) are slightly lower than the purewater concentrations (average 0.5 nmol m − 3 ). Total soluble N and P, which include dissolved organic compounds, extracted from the aerosols into the seawater (average 65 and 0.4 nmol m − 3 ) are significantly lower than those extracted by pure water (average 75 and 0.7 nmol m − 3 ). It was found that the dissolution of crustal-dominated trace metals (e.g. Fe and Al) strongly decrease in the seawater compared to that in pure water, while similar amounts of aerosol Zn are leached in both seawater and pure water. The percentage solubilities of non-crustal trace metals (Cu, Ni and Zn) are about one or two orders of magnitude higher than those of Fe and Al in the seawater. Our comparison experiments suggest that some previous reports may have overestimated the dry deposition inputs of aerosol P, Fe, Al, Cu, and Ni to the ocean as a result of the use of solubility estimates obtained from pure water extractions. The estimated dry deposition fluxes of soluble nutrients showed that the atmospheric nutrient input could increase the possibility of P limitation in the Gulf of Aqaba and also contribute a significant fraction of dissolved nutrients to the euphotic zone during stratification period (April to October).