Barak Herut

Global distribution of atmospheric phosphorus sources, concentrations and deposition rates, and anthropogenic impacts

A worldwide compilation of atmospheric total phosphorus (TP) and phosphate (PO4) concentration and deposition flux observations are combined with transport model simulations to derive the global distribution of concentrations and deposition fluxes of TP and PO4. Our results suggest that mineral aerosols are the dominant source of TP on a global scale (82%), with primary biogenic particles (12%) and combustion sources (5%) important in nondusty regions. Globally averaged anthropogenic inputs are estimated to be similar to 5 and 15% for TP and PO4, respectively, and may contribute as much as 50% to the deposition over the oligotrophic ocean where productivity may be phosphorus-limited. There is a net loss of TP from many (but not all) land ecosystems and a net gain of TP by the oceans (560 Gg P a(-1)). More measurements of atmospheric TP and PO4 will assist in reducing uncertainties in our understanding of the role that atmospheric phosphorus may play in global biogeochemistry.

The role of atmospheric deposition in the biogeochemistry of the Mediterranean Sea

Abstract Estimates of atmospheric inputs to the Mediterranean and some coastal areas are reviewed, and uncertainities in these estimates considered. Both the magnitude and the mineralogical composition of atmospheric dust inputs indicate that eolian deposition is an important (50%) or even dominant (>80%) contribution to sediments in the offshore waters of the entire Mediterranean basin. Model data for trace metals and nutrients indicate that the atmosphere delivers more than half the lead and nitrogen, one-third of total phosphorus, and 10% of the zinc entering the entire basin. Measured data in sub-basins, such as the north-western Mediterranean and northern Adriatic indicate an even greater proportions of atmospheric versus riverine inputs. When dissolved fluxes are compared (the form most likely to impinge on surface water biogeochemical cycles), the atmosphere is found to be 5 to 50 times more important than rivers for dissolved zinc and 15 to 30 times more important for lead fluxes. Neglecting co-limitation by other nutrients, new production supported by atmospheric nitrogen deposition ranges from 2–4 g C m−2 yr−1, whereas atmospheric phosphorus deposition appears to support less than 1 g C m−2 yr−1. In spite of the apparently small contribution of atmospheric deposition to overall production in the basin it has been suggested that certain episodic phytoplankton blooms are triggered by atmospheric deposition of N, P or Fe. Future studies are needed to clarify the extent and causal links between these episodic blooms and atmospheric/oceanographic forcing functions. A scientific program aimed at elucidating the possible biogeochemical effects of Saharan outbreaks in the Mediterranean through direct sampling of the ocean and atmosphere before and after such events is therefore highly recommended.

Dry atmospheric inputs of trace metals at the Mediterranean coast of Israel (SE Mediterranean) : sources and fluxes

Abstract This study presents the first detailed data on aerosol concentrations of trace metals (Cd, Pb, Cu, Zn, Cr, Mn, Fe and Al) at the SE Mediterranean coast of Israel, and assesses their sources and fluxes. Aerosol samples were collected at two sampling stations (Tel-Shikmona and Maagan Michael) along the coast between 1994 and 1997. Two broad categories of aerosol trace metal sources were defined; anthropogenic (Cd, Cu, Pb and Zn) and naturally derived elements (Al, Fe, Mn and Cr). The extent of the anthropogenic contribution was estimated by the degree of enrichment of these elements compared to the average crustal composition (EFcrust). High values (median >100) were calculated for Cd, Pb and Zn, minor values for Cu and relatively low values (

The characterisation of Saharan dusts and Nile particulate matter in surface sediments from the Levantine basin using Sr isotopes

Abstract The provenance of sediments within the Levantine basin of the eastern Mediterranean was studied using 87 Sr/ 86 Sr isotopic ratios, together with major elements. Measurements were made on the detrital fraction of surface sediments, and the two most important sources of detrital matter to the region. Saharan dust was characterised by an 87 Sr/ 86 Sr isotopic ratio in the range 0.7160–0.7192. There was a small systematic decrease in 87 Sr/ 86 Sr from west to east which is interpreted as due to a change in the balance of aeolian source material. The Nile particulate matter had a 87 Sr/ 86 Sr isotopic ratio in the range 0.7057–0.7071. 87 Sr/ 86 Sr isotopic signatures together with Sr concentration were used to calculate the contributions made by Saharan dust and Nile particulate matter to the surface sediments of the Levantine basin. It was shown that Nile-derived sediment was dispersed widely across the Levantine basin enabling the area affected by retention of sediment behind the Aswan dam to be determined.

Trace metals in shallow sediments from the Mediterranean coastal region of Israel

Abstract Concentrations of mercury, lead, copper, zinc, cadmium, iron and aluminium were recorded in coastal and river mouths surficial sediments, collected along the Mediterranean coast of Israel, during 1988–1991. The levels of trace metal concentrations when normalized against Fe concentration (trace metal/iron ratios) indicated no contamination at most of the stations along the coast. Enrichment of Hg, Cu, Zn and Cd attributed to land-based point sources of pollution, was found in the estuaries of the Kishon and Yarkon rivers. The normalized Pb, Cu and Zn data showed a trend of increasing values from the south toward the north. This S-N trend could be a result of three processes which may be occurring simultaneously: progressive mixing between sediment with a low trace metal/iron ratio in the south, which probably represents a single source derived from the Nile region with (a) non-point source input of pollutants by atmospheric transport, or (b) with point sources of pollutants introduced by rivers and streams, or (c) a natural change in the mineralogy of the sediments along the coast with a relative increase in the trace metals or a decrease in iron as one progresses further north. The relative importance of these three possible processes remains to be established.

The significance of the episodic nature of atmospheric deposition to Low Nutrient Low Chlorophyll regions

In the vast Low Nutrient Low-Chlorophyll (LNLC) Ocean, the vertical nutrient supply from the subsurface to the sunlit surface waters is low, and atmospheric contribution of nutrients may be one order of magnitude greater over short timescales. The short turnover time of atmospheric Fe and N supply (<1 month for nitrate) further supports deposition being an important source of nutrients in LNLC regions. Yet, the extent to which atmospheric inputs are impacting biological activity and modifying the carbon balance in oligotrophic environments has not been constrained. Here, we quantify and compare the biogeochemical impacts of atmospheric deposition in LNLC regions using both a compilation of experimental data and model outputs. A metadata-analysis of recently conducted field and laboratory bioassay experiments reveals complex responses, and the overall impact is not a simple “fertilization effect of increasing phytoplankton biomass” as observed in HNLC regions. Although phytoplankton growth may be enhanced, increases in bacterial activity and respiration result in weakening of biological carbon sequestration. The application of models using climatological or time-averaged non-synoptic deposition rates produced responses that were generally much lower than observed in the bioassay experiments. We demonstrate that experimental data and model outputs show better agreement on short timescale (days to weeks) when strong synoptic pulse of aerosols deposition, similar in magnitude to those observed in the field and introduced in bioassay experiments, is superimposed over the mean atmospheric deposition fields. These results suggest that atmospheric impacts in LNLC regions have been underestimated by models, at least at daily to weekly timescales, as they typically overlook large synoptic variations in atmospheric deposition and associated nutrient and particle inputs. Inclusion of the large synoptic variability of atmospheric input, and improved representation and parameterization of key processes that respond to atmospheric deposition, is required to better constrain impacts in ocean biogeochemical models. This is critical for understanding and prediction of current and future functioning of LNLC regions and their contribution to the global carbon cycle.

Seasonal dynamics of the endosymbiotic, nitrogen-fixing cyanobacterium Richelia intracellularis in the eastern Mediterranean Sea.

Biological nitrogen fixation has been suggested as an important source of nitrogen for the ultra-oligotrophic waters of the Levantine Basin of the Mediterranean Sea. In this study, we identify and characterize the spatial and temporal distribution of the N-fixing (diazotrophic) cyanobacterium Richelia intracellularis. R. intracellularis is usually found as an endosymbiont within diatoms such as Rhizosolenia spp and Hemiaulus spp. and is an important diazotroph in marine tropical oceans. In this study, two stations off the Mediterranean coast of Israel were sampled monthly during 2005–2007. R. intracellularis was identified by microscopy and by reverse transcribed-PCR which confirmed a 98.8% identity with known nifH sequences of R. intracellularis from around the world. The diatom–diazotroph associations were found throughout the year peaking during autumn (October–November) at both stations. Abundance of R. intracellularis ranged from 10 to 55 heterocysts l−1 and correlated positively with the dissolved Si(OH)4/(NO3+NO2) ratio in surface waters. Although the rates of nitrogen fixation were very low, averaging ∼1.1 nmol N l−1 day−1 for the R. intracellularis size fraction (>10 μm) from surface waters, they correlated positively with heterocyst counts during thermal stratification. The lack of large-scale diatom–diazotroph blooms and the low rates of nitrogen fixation by these diazotrophs may result from the P-starved conditions affecting the Levantine basin.

Hydrogen Peroxide Production Rates in Clean and Polluted Coastal Marine Waters of the Mediterranean, Red and Baltic Seas

Abstract The main aim of this study was to assess the hydrogen peroxide (HP) production rates (HPPR) related to anthropogenic pollution in coastal waters by laboratory and field experiments. HPPRs were assessed by simultaneous measurements of HP concentrations, cumulative solar UV irradiation and dissolved organic matter (DOM) fluorescence in the seawater samples at clean and polluted sites in the Mediterranean, Red and the Baltic Seas. The natural HP concentrations at all sites (8–100 nM) fall within the normal range recorded elsewhere, and follows a diurnal pattern. The polluted stations in the Mediterranean and Baltic Seas showed higher HPPR (3.2–16.6 nM m 2 W −1 h −1 ) than the clean stations, while in the Red Sea no significant differences were found because the station that was considered a priori polluted was actually rather clean (2–3 nM m 2 W −1 h −1 ). Laboratory experiments demonstrated that HPPR was positively linearly correlated to DOM fluorescence, however, this relationship was not found in the natural seawater samples examined in the field. The lack of relationship between HPPR and DOM in the field was attributed to enzymatic breakdown of HP as observed in dark decay experiments. HP dark decay rates were highest in polluted stations, probably due to larger bacterioplankton populations in these samples. Moreover, the HP dark decay rates were much lower in filtered than in non-filtered samples. Sun incubation of filtered (0.2 μm) seawater samples were performed to assess whether elimination of particles would yield higher HPPRs than in non-filtered samples. In the Baltic Sea (low UV irradiation) HPPR was higher in the filtered samples, while at sites with relatively high UV irradiation (Mediterranean and Red Seas), filtration of seawater did not stimulate HPPRs, probably due to UV-damage to planktonic microorganisms.

Atmospheric Input of Nutrients and Dust to the SE Mediterranean

Concentrations of dissolved nutrients (PO4, NO3, NH4 and Si(OH)4) and pH have been monitored in 84 rain water samples collected during 3 winters at the northern Mediterranean Israeli coast (Haifa). The main dissolved inorganic nitrogen (DIN) compound was nitrate, with concentrations between 4.2 and 300 μM. Ammonium was dominant only in 12 out of 60 events, and its concentrations ranged between 2.2 and 96.9 μM. Phosphate showed a range of concentrations between 0.09 and 8.8 μM. High silicic acid concentrations (>2 μM) were detected only during relatively basic rains (pH>6), probably due to greater dissolution of quartz and clay particles into rain water under more basic conditions. Although about 30% of the rains are considered to be acid rains, their main H+ donor is non-seaspray sulfate and not nitrate. Nitrate and phosphate showed similar temporal variability, suggesting their common origin for most of the rain events.

Dinitrogen Fixation in Aphotic Oxygenated Marine Environments

We measured N2 fixation rates from oceanic zones that have traditionally been ignored as sources of biological N2 fixation; the aphotic, fully oxygenated, nitrate (NO−3)-rich, waters of the oligotrophic Levantine Basin (LB) and the Gulf of Aqaba (GA). N2 fixation rates measured from pelagic aphotic waters to depths up to 720 m, during the mixed and stratified periods, ranged from 0.01 nmol N L−1 d−1 to 0.38 nmol N L−1 d−1. N2 fixation rates correlated significantly with bacterial productivity and heterotrophic diazotrophs were identified from aphotic as well as photic depths. Dissolved free amino acid amendments to whole water from the GA enhanced bacterial productivity by 2–3.5 fold and N2 fixation rates by ~2-fold in samples collected from aphotic depths while in amendments to water from photic depths bacterial productivity increased 2–6 fold while N2 fixation rates increased by a factor of 2 to 4 illustrating that both BP and heterotrophic N2 fixation were carbon limited. Experimental manipulations of aphotic waters from the LB demonstrated a significant positive correlation between transparent exopolymeric particle (TEP) concentrations and N2 fixation rates. This suggests that sinking organic material and high carbon (C): nitrogen (N) micro-environments (such as TEP-based aggregates or marine snow) could support high heterotrophic N2 fixation rates in oxygenated surface waters and in the aphotic zones. Indeed, our calculations show that aphotic N2 fixation accounted for 37 to 75% of the total daily integrated N2 fixation rates at both locations in the Mediterranean and Red Seas with rates equal or greater to those measured from the photic layers. Moreover, our results indicate that that while N2 fixation may be limited in the surface waters, aphotic, pelagic N2 fixation may contribute significantly to new N inputs in other oligotrophic basins, yet it is currently not included in regional or global N budgets.