Estimates of atmospheric dry deposition and associated input of nutrients to Gulf of Aqaba seawater

Abstract

Dry deposition rates and associated inputs of soluble inorganic nitrogen (N) phosphorus (P) and iron (Fe) were calculated for the north coast of the Gulf of Aqaba in Eilat, Israel, between August 2003 and September 2005. The main inorganic N compounds in the water soluble fraction of aerosol particles were nitrate (60%) and ammonium (38%), with mean concentrations of 35 and 22 nmol m−3 of air, respectively. Soluble phosphate ranged between 0.09 and 2 nmol m−3 of air with a mean value of 0.4 nmol m−3 of air. The soluble inorganic nitrogen and soluble phosphate account for approximately 86% and 69% of total soluble nitrogen and total soluble phosphorus, respectively; the difference is assigned to organic N and P compounds. The mean concentration of soluble aerosol Fe was 0.3 nmol m−3 of air. Our measurements of the soluble nutrient concentrations are comparable to data previously reported for the eastern Mediterranean area. Dry deposition fluxes of nutrients were estimated for each sampling date using a size‐dependent deposition model. The estimated fluxes were highly variable over the sampling period with the mean fluxes of 38, 0.2 and 0.02 μmol m−2 d−1 for seawater soluble inorganic N, P and Fe, respectively. The soluble phosphate flux shows a seasonal pattern with higher input during the winter (September to December) than in other seasons. The inorganic N/P molar ratios in the seawater‐soluble fraction of the dry deposition (ranging from 32 to 541) were well above the Redfield ratio (N/P = 16), suggesting that atmospheric inputs of nutrients increase the likelihood for P limitation in the Gulf. Atmospheric deposition could contribute a substantial fraction (35%) of dissolved inorganic N to the euphotic zone during the stratification period (April to October), and the N flux from this source could support over 10% of surface primary production and possibly all of the new production during the summer; however, these estimates have a relatively large uncertainty due to error associated with deposition flux calculation and the temporal variability in dust flux. Atmospheric input of seawater‐soluble Fe is in large excess compared to that required for the phytoplankton growth driven by the N deposition.