Marine Chemistry 120 (2010) 100–107
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Marine Chemistry
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a r c h e m
Iron speciation, solubility and temporal variability in wet and dry deposition in the
Eastern Mediterranean
C. Theodosi, Z. Markaki, N. Mihalopoulos ⁎
Environmental Chemistry Processes Laboratory, Department of Chemistry, University of Crete, P.O. Box 2208, 71003 Heraklion, Greece
a r t i c l e
i n f o
Article history:
Received 19 March 2008
Received in revised form 16 May 2008
Accepted 19 May 2008
Available online 9 July 2008
Keywords:
Iron speciation
Solubility
Wet and dry deposition
Eastern Mediterranean
Implications
a b s t r a c t
Iron speciation was studied in wet and dry deposition samples collected at two locations on the island of Crete
(Greece), in the Levantine Basin, Eastern Mediterranean, from November 2004 to February 2007. Iron
solubility ranged from 27.2% for pH between 4 and 5 (polluted rainwater) to 0.5% for pH close to 8 (Sahara
dust episodes), indicating that Fe solubility, and therefore Fe bioavailability to ecosystems, is enhanced in the
presence of acidic species.
During the studied period, Dissolved Reactive Iron (DSRFe) levels deposited in the Eastern Mediterranean Sea
were sufficient to account for the dissolved iron levels in seawater reported by Statham and Hart [Statham, P.J.,
Hart, V., 2005. Dissolved iron in the Cretan Sea (Mediterranean), Limnolology Oceanography, 118–124.].
Therefore dissolved iron in the Mediterranean Sea could be exclusively attributed to atmospheric deposition.
The biogeochemical implications of atmospheric dissolved iron on phytoplankton growth and nitrogen fixation
were also investigated. During summer and autumn less than 5% of the deposited dissolved Fe is required for
phytoplankton growth (i.e., when water stratification is at its maximum). The calculated nitrogen fixation
potential induced by the measured deposition dissolved iron, was found to be at least 1.5 to 3 times smaller than
the atmospheric nitrogen deposition, indicating the significant role of atmospheric deposition in the
biogeochemical N cycle in the Mediterranean.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Iron is a critical micronutrient that limits the phytoplankton growth
in HNLC (high nutrient low chlorophyll) regions (e.g. Martin and
Fitzwater, 1988; Martin et al., 1990). Furthermore, iron is a critical
nutrient co-factor for the nitrogenase enzyme in diazotrophic microorganisms and therefore may influence nitrogen fixation in oligotrophic
oceans, and hence may influence phytoplankton community structure
(Falkowski, 1997; Gruber and Sarmiento, 1997; Baker et al., 2006).
Iron is present in environmental samples (rain, dry deposition samples
and aerosols) in particulate and dissolved forms and includes both Fe(II)
and Fe(III) species. Model calculations and correlation analysis indicate
that in rainwater Fe(II)(aq) occurs almost exclusively as the free ion,
whereas Fe(III)(aq) occurs as both iron oxalate and Fe(OH)+
2 (aq)
depending on the pH range from 4.0 to 5.0 (Willey et al. 2000).
⁎ Corresponding author.
E-mail address: [email protected] (N. Mihalopoulos).
0304-4203/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.marchem.2008.05.004
The atmosphere is recognized to be an important pathway for the
transport of trace elements of continental origin, including Iron, to
oceanic systems (Duce et al. 1991; Jickells, 1995; Prospero et al. 1996,
Guerzoni et al. 1999). Especially at oligotrophic oceanic areas, such as
the Mediterranean Basin, aeolian input could be the most effective
external source of nutrients and trace elements (Martin et al. 1989;
Guerzoni et al. 1999; Markaki et al., 2003; Krom et al., 2004; Kocak et
al., 2005). Riverine inputs to the Eastern Mediterranean and to the
Mediterranean as a whole are small (Guieu et al. 1997) and localized
and will not affect offshore sites such as the Cretan Sea (Statham and
Hart, 2005). Thus the effect of atmospheric inputs of macro (N and P)
and micro (Fe) nutrients on biological cycles could be particularly
important in the Mediterranean. Indeed Krom et al. (2004) have shown
that atmospheric inputs represent 60% of the N and 30% of the P input to
the Eastern Mediterranean Basin. During summer and autumn, water
column stratification will prevail (to a typical depth of 20–30 m) which
will maximize the impact of aeolian inputs into the surface layers.
Furthermore recent studies have shown the significance and
influence of atmospheric deposition of desert dust on the biogeochemistry of the Mediterranean Sea (Ridame and Guieu, 2002;
Saydam and Senyuva, 2002; Carbo et al., 2005; Herut et al., 2005).
C. Theodosi et al. / Marine Chemistry 120 (2010) 100–107
101
Fig. 1. Location of the two sampling sites at the Levantine basin, Eastern Mediterranean (Heraklion and Finokalia).
Despite its potential role, the majority of studies on the role of the
atmosphere as a source of iron in the Mediterranean have been
performed in the western basin (Bonnet and Guieu, 2004, 2006;
Guieu et al., 2002a,b; 2005). For the eastern basin there is only one
study (Özsoy and Saydam, 2001), examining the role of wet
deposition on the iron cycle. Wet deposition is limited to the winter
months and dry deposition is the main mechanism of atmospheric
input during summer and autumn (Markaki et al., 2003; Kouvarakis et
al., 2001).
The current work presents data on the atmospheric deposition of
iron (wet and dry) to the Eastern Mediterranean collected over a
2 year period (2005–2006). Moreover the factors controlling
dissolved and total iron speciation and deposition will be considered
such as the pH and dust content. Finally, the significance of Fe
deposition on seawater productivity in the Eastern Mediterranean will
be discussed.
2. Experimental
2.1. Sampling site
Rainwater and dry deposition samples were collected on the island
of Crete (Greece), located at a central position in the Mediterranean
Basin, relatively far from Saharan and anthropogenic emissions.
Consequently, the data from this site could be considered representative of the open Eastern Mediterranean Sea.
Apart from the main sampling site, which was situated at Finokalia
(35°20′N, 25°40′E), a coastal site in northern Crete, there was a
secondary station at the University of Crete, 6 km south of Heraklion
(35°20′N, 25°07′E) to examine the spatial variability in Fe concentration detected in wet deposition. Characterization of the sites and the
prevailing meteorology can be found in Arsene et al. (2007) (Fig. 1).
affected by wet deposition. The glass beads were positioned on a
funnel installed 3 m above the ground as described by Kouvarakis et al.
(2001). The glass beads system was exposed to the atmosphere for 1
to 2 weeks at a time. After which the system was washed with
ultrapure water (300 ml). More details on sample collection can be
found in Kouvarakis et al. (2001).
Wet and bulk/dry deposition samples have been filtered immediately after collection, through a pre-weighed 0.45 μm cellulose filter
and the pH of the rinse solution was determined. Then the samples
were stored in the freezer until analysis, which was performed within
a month. The crustal mass was estimated by weighing the cellulose
filters before and after the filtration.
Dissolved Reactive Iron (DSRFe: Fe(II) and Fe(III)) was determined
spectrometrically using the Ferrozine colometric method developed
by Stookey (1970). Fe(II) was quantified using the same procedure
without any addition of hydroxylamine hydrochloride (reducing
reagent), while Fe(III) was calculated indirectly as the difference
between DSRFe and Fe(II). The absorbance was measured at 562 nm
using a 5 cm cell. The detection limit was 0.010 μM with a
corresponding precision of 4% RSD at typical rainwater concentrations.
It should be noted that the detection limit was estimated as the mean
of the blank sample plus three times the standard deviation obtained
on the blank value. To check for possible interferences from matrix,
standard addition tests were performed. The recovery was about
98.3% for both DSRFe and Fe(II) (n = 6; both polluted and Saharainfluenced rainwater samples).
Particulate Iron (PFe) was also determined, after acid digestion
with HNO3, using Inductively Coupled Plasma Mass Spectrometry
(ICP-MS). Iron recovery obtained using three certified marine
sediments reference materials (MESS-3, GBW 07313 and BCSS-1,
about 20 mg of each reference material) were 91.2 ± 3.4%, 94.2 ± 2.3%
and 91.5 ± 0.2% respectively. Finally Total Iron (TFe) was calculated by
adding both dissolved and particulate iron.
2.2. Sampling and analytical techniques
3. Results
Rainwater was collected at both locations, on an event basis,
using wet-only collectors (Van Essen) with a lid activated by the rain
sensor. Since iron is biologically labile and can be photo reduced,
samples were immediately filtered and frozen at the cessation of the
event (see below).
At Finokalia station, bulk deposition samples have been collected
in parallel to the wet deposition samples. Bulk deposition was
estimated based on the collection of particles on a flat surface covered
by multiple layers of glass beads which can trap larger particles, thus
avoiding resuspension. Dry deposition refers to bulk samples not
3.1. Iron speciation
Due to the oxidizing environment in rain water samples, Fe(III) has been assumed
to be the dominant species present in atmospheric samples (Moore et al., 1984,
Zhuang et al., 1990). However the present study reveals that a significant fraction of
DSRFe was in the form of Fe(II). This trend has been observed also in other studies
(Behra and Sigg, 1990, Pehkonen et al., 1992; Zhuang et al., 1995; Siefert et al., 1998;
Willey et al., 2000; Özsoy and Saydam, 2001). More specifically Fe(II) accounts for on
average 82 ± 49% (median value = 78%, slope = 0.74, R 2 = 0.91) and 74 ± 27%
(median = 73%, slope = 0.80, R2 = 0.97) of the dissolved reactive iron in rainwater
samples (n = 125) collected at Heraklion and Finokalia respectively, while the
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C. Theodosi et al. / Marine Chemistry 120 (2010) 100–107
percentage in bulk deposition at Finokalia (n=61) was 73±23% (median=71%, slope=0.75,
R2 =0.94) (Fig. 2).
Although Fe(II) undergoes rapid transformation when it enters the surface
seawater, it is clear that atmospheric input by both wet and dry deposition is a very
important direct source of soluble Fe(II) in the surface seawater.
3.2. Iron atmospheric fluxes
3.2.1. Wet deposition
During the sampling period (November 2004–February 2007), 76 and 49 rainwater
samples were collected at the two sampling sites Heraklion and Finokalia, respectively.
The monthly volume weighted mean (VWM) of various Fe species in wet deposition
samples from both stations are presented in Fig. 3a,b.
The levels of Fe(III), Fe(II) and Total Fe observed in this work (average of 0.09 μM,
0.27 μM and 15.87 μM, respectively) compare very well with those reported by Özsoy
and Saydam (2001) in their study conducted at Erdemli (Turkey) (average values of
0.03 μM, 0.11 μM and 24.31 μM respectively).
To examine the factors controlling total and dissolved iron concentrations in wet
deposition, back trajectories were calculated for each rainwater sample using the
HYSPLIT 4 model (Hybrid Single Particle Langrangian Integrated Trajectory) (http://
www.arl.noaa.gov/ready/hysplit4.html). On the basis of air mass backtrajectory
analysis, our data have been classified into 4 categories corresponding to the 4 main
wind sectors in the Eastern Mediterranean (N/NW; E/NE; W; S/SW/SE). Table 1 reports
the percentage of air masses originating from each wind sector and associates it with
the frequency of the rain events, the percentage of precipitation and the volume
weighted mean (VWM) average concentration of DSRFe and TFe.
Both the occurrence of the trajectories associated with rain events and the
percentage precipitation, were quite uniformly distributed within each sector. TFe
concentrations associated with Southern derived air masses (i.e. Saharan) were
higher (by up to 10 times) than those associated with air masses associated with
the European (NE or NW) sectors. Of particular interest is that the variation of
the DSRFe/TFe ratio as a function of the air mass origin and the variation of rainwater pH (Table 1). Lowest values of the DSRFe/TFe ratio (4.7) are associated with
the S–SE–SW sector, in contrast to the highest values being associated with the W and
E–NE sectors (29.8 and 24.2 respectively). Therefore, despite the fact that dust events
transport large amounts of iron to the Eastern Mediterranean owing to the low
solubility of Fe in dust, such event may be of a lower importance as a source for DSRFe
to the sea surface during a wet deposition event than expected (see further discussion
in Section 3.4).
3.2.2. Dry deposition of iron
In total 61 bulk deposition samples have been collected at Finokalia station, out of
which 24 can be considered as dry deposition samples as they have not affected by wet
deposition. During the months of May, June and October with a single rain event, the
dry deposition has been calculated as the difference between bulk and wet deposition.
An estimation of both wet and dry deposition fluxes of dissolved and total iron can be
thus obtained for the Eastern Mediterranean and the results are depicted in Fig. 4a, b
and Table 2.
At Finokalia, TFe is removed from the atmosphere almost equally by wet (49%)
and by dry (51%) deposition, DSRFe on the opposite site is mainly (64%) removed via
wet deposition. Annual wet deposition fluxes of TFe at Heraklion and Finokalia equal
124.6 and 188.5 mg m− 2, respectively, whereas the corresponding figures for DSRFe
are 6.5 mg m− 2 at Heraklion and 5.8 mg m− 2 at Finokalia. Annual dry deposition
fluxes of TFe and DSRFe are estimated to be 193.8 and 3.2 mg m− 2, respectively. The
measured dry deposition fluxes of TFe are in good agreement with the calculated dry
deposition fluxes reported by Kocak et al., 2005 for the Eastern Mediterranean (230 mg
m− 2 year− 1 and 420 mg m− 2 year− 1 at Erdemli (Turkey) and Tel-Shikmona (Israel),
respectively). During the studied period 5.94–12.25 mg of dissolved Fe m− 2 year− 1
have been deposited in the Eastern Mediterranean via wet and dry depositions, similar
to fluxes reported by Guieu et al., 1997 for the Western Mediterranean (5.03–10.05 mg
Fe m− 2 year− 1).
Total iron deposition presents a clear seasonal trend (Fig. 4). From November to
April rain is the main pathway of atmospheric input in seawater. In contrast from May
to October iron is mainly deposited to the sea surface via dry deposition. This last
process is characterized by a high interannual variability depending on the frequency
and the magnitude of the Saharan dust events (Table 2).
Indeed our data set highlights the importance of one extreme Saharan dust event
on 24 February 2006, which affected the annual Fe fluxes at both sampling sites
for the year 2006. This event leads to higher mass input and consequently higher
TFe. Such large dust amounts are associated with low DSRFe and thus reduce the
dissolved iron annual fluxes. The specific event was more efficient at Finokalia
adding 166.58 mg Fe m− 2 to the TFe flux during Nov 05–Oct 06, in contrast to almost
half this contribution (85.37 mg Fe m− 2) measured at Heraklion station (55% and
62% of the total annual iron input, respectively). Soluble iron at Heraklion was found
to be almost 2 times higher than observed at Finokalia during this specific period
(Nov 05–Oct 06). The above observation can be explained, to some extent, by the 3
times lower dust content in Heraklion samples, mean value of 4.1 mg, compared to
the 14.1 mg for the Finokalia samples (see further discussion in Section 3.4).
3.3. Origin of total and dissolved iron in precipitation
To understand the origin of iron in deposition samples, total iron has been
compared with insoluble dust estimated by pre- and post-weighing of the filter.
A significant correlation has been observed with a slope (TFe/insoluble dust mass)
equal to 3.8 ± 0.6% (median = 4.0%, slope = 3.8, R2 = 0.97) (n = 68, Fig. 5a). This
value is in good agreement with the crustal iron content quoted in the literature
(3.5%; Meskhidze et al., 2005) and/or in western Mediterranean samples (4.3%;
Guieu et al., 2002b).
In deposition samples dissolved iron was found to correlate significantly with
dissolved aluminium with a (DSRFe/DSAl) ratio equal to 72 ± 50% (median = 81%,
slope = 0.69, R2 = 0.80) (n = 90, Fig. 5b). A similar ratio has been reported for earth's
crust, as well as for western Mediterranean dust samples (Guieu et al., 2002b) and
demonstrates similar solubility for both elements in deposition samples in the Eastern
Mediterranean.
3.4. Factors controlling the solubility of iron
The present section examines the behavior of the different forms of iron as a
function of pH, as well as the solubility of Fe. Solubility is defined as the percentage
of the dissolved iron concentration divided by the total (dissolved plus particulate)
iron concentration (Βaker et al., 2006).
kSolubility ¼ 1004Fedissolved =Fetotal Z
kSolubility ¼ 1004DSRFe=TFe
Fig. 2. The correlation between Fe(II) and DSRFe ratio for (a) wet deposition samples
(Heraklion and Finokalia) and (b) dry deposition samples (only at Finokalia).
Chemical, biotic and physical processes such as pH, presence of dissolved organic
complexing ligands, particle size and concentration, bacteria, phytoplankton, and
temperature, can influence the extent of metal dissolution (Chester et al., 1990, 1993;
Lim et al., 1994; Bonnet and Guieu, 2004; Biscombe et al., 2004).
Fig. 6a,b reports on the variability of DSRFe as a function of pH, distinguishing
between wet and dry deposition samples. DSRFe decreases with pH increase. A similar
pattern is also valid for each of the soluble forms of Fe (Fe(II) and Fe(III) separately;
not shown) for both wet and dry deposition samples, indicating a clear influence of
pH on iron solubility. Indeed iron solubility (Fig. 6c,d) reaches a maximum value of
27.2% for pH between 4 and 5, characteristic of polluted rainwater (Loye-Pilot and
Morelli, 1988) and a minimum value of 0.5% for pH close to 8, characteristic of Saharaninfluenced rain (Mahowald et al., 2005). Mean iron solubility in wet and dry deposition
samples was estimated to be 4% and 1.7%, respectively, similar to recent estimations
of iron solubility in rain water and aerosol samples, respectively (Jickells and Spokes,
2001). Thus the presence of acidic species enhances iron solubility and therefore the
capacity to induce soluble, bioavailable iron to the marine ecosystem.
On the other hand TFe shows an opposite trend compared to DSRFe, as it clearly
increases as pH increases (Fig. 6e,f). The above observation can be attributed to the
presence of high dust loads, particularly when pH ranges from 7 to 8, characteristic of
Sahara-influenced rain. Sahara dust is considered to be a rich source of iron, especially
iron oxides such as hematite. In addition the presence of the higher dust load will
influence the iron solubility as progressively higher particle concentrations would be
present in solution. Decreased Fe solubility with particle concentration in aqueous
C. Theodosi et al. / Marine Chemistry 120 (2010) 100–107
103
Fig. 3. Variability of the monthly volume weighted mean (VWM) of TFe, DSRFe and Fe(II) in rain water collected at (a) Heraklion and (b) Finokalia (in μΜ).
solutions has been observed in our samples (Fig. 6g,h), but such behavior is not
unexpected, given the opposite trend between total and dissolved iron and the increase
of total iron with dust load (see Section 3.3).
4. Discussion—Biochemical implications for the Eastern
Mediterranean
4.1. Role of atmospheric deposition in dissolved and total iron levels in
the water column
Statham and Hart (2005) reported concentrations of dissolved iron
in the water column of the Cretan sea in the Eastern Mediterranean
during March and September.
The concentration of DSRFe in the water column (Cx) derived from
atmospheric deposition of dissolved iron (Fx) can be estimated from
the following equation:
Fx ¼
Cx H
;
tx
ð1Þ
where (Η): the water column depth and tx: the lifetime of dissolved Fe
in the water.
Based on the dissolved atmospheric iron inputs in the Eastern
Mediterranean of 5.94 to 12.25 mg Fe m− 2 year− 1 as deduced from
this work, and taking into account an average mixed layer depth of
80 m and a lifetime of 1 year (Statham and Hart, 2005), the potential
DSRFe concentration in surface seawater can be estimated from Eq.
(1). The DSRFe thus calculated in seawater as a result of atmospheric
input ranges from 1.33 to 2.74 nmol Fe L− 1.
The above estimation is in good agreement with the values of 1.44 and
1.95 nmol L− 1 in March and September respectively, reported for the
Eastern Mediterranean, by Statham and Hart (2005). Even though the
atmospheric dissolved iron is expected to be lower when it enters the sea,
since a conversion to insoluble iron will occur, the estimation presented
above indicates that the dissolved iron concentration in the water column
of the Mediterranean could be exclusively attributed to atmospheric
deposition. Note also that during the dry period the mean solubility of
1.5% estimated for this work during the dry period is in excellent
agreement with the mean dissolution of 2% reported by Chen et al., 2007
to explain the dissolved iron levels in the Gulf of Aqaba, Red Sea.
The measured atmospheric deposition of total iron of 0.2 to 0.6 g
Fe m− 2 year− 1 (average 0.4 g Fe m− 2 year− 1) (Table 2) during 2005–
2006 have been compared to sediment traps deployed in the Cretan Sea
during a seven year survey (1999–2005; Markaki, 2007). The
mean value of atmospheric iron deposition of 0.4 g Fe m− 2 year− 1
compares very well to the value of 0.5 g Fe m− 2 year− 1 derived from
sediment trap deployment and confirms the significant role of atmospheric iron input.
Table 1
Four main wind sectors responsible for air masses to Heraklion during the rainy period.
Sector
Event
%
Precipitation
%
VWM
DSRFe (μM)
VWM
TFe (μM)
Average
solubility
pH
N–NW
(n = 33)
E–NE
(n = 11)
W (n = 4)
S–SW–SE
(n = 24)
45.8
(39.6)
0.15
1.31
17.0 ± 8.7
5.2
15.3
(27.5)
0.23
1.25
24.2 ± 13.5
5.3
5.6
33.3
(7.9)
(25.0)
0.27
0.17
0.84
9.61
29.8 ± 13.1
4.7 ± 4.2
5.1
6.6
The numbers indicate for each wind sector (i) the percentage of air masses originating
within each wind sector and associated with rain events, (ii) the percentage of precipitation
within each sector, (iii) the per sector VWM (μM) of DSRFe and (iv) VWM (μM) of TFe, (v)
the average solubility and (vi) the pH value.
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C. Theodosi et al. / Marine Chemistry 120 (2010) 100–107
Fig. 4. Temporal variability of monthly averaged Fe flux in wet (a) and dry deposition (b) samples collected at Finokalia during the sampling period November 2004–February 2007.
4.2. Dissolved iron and phytoplankton growth
Atmospheric input of Fe can control phytoplankton growth in high
nitrate low chlorophyll (HNLC) regions (Behrenfeld et al., 1996; Boyd et
al., 2000). Fe can also be a rate-limiting nutrient to diazotrophic
microorganisms and therefore may influence nitrogen fixation in
oligotrophic areas, such as the Eastern Mediterranean (Falkowski, 1997;
Gruber and Sarmiento, 1997). An estimation of the role of atmospheric
input of dissolved Fe on marine phytoplankton (non-diazotrophic)
development (FeR) can be obtained using the following equation:
FeR ¼ Naero ðFe=NÞphyrto
During the dry period, summer and autumn, when the water
column is stratified and the contribution of nutrients from deeper
layers is reduced, the atmospheric influence on new production is
maximized.
ð2Þ
where Naero is the atmospheric nitrogen deposition flux and ðFe=NÞphyrto
is the cellular Fe:N ratio of non-diazotrophic phytoplankton, which equals
to 86 μmol/mol (Baker et al., 2003).
Table 2
Annual Fe deposition fluxes (mg m− 2 year− 1) for the two sampling sites Heraklion and
Finokalia.
Rain Heraklion
Rain Finokalia
Dry Finokalia
Νοv 04–Οct 05
Νοv 05–Οct 06
Νοv 04–Οct 05
Νοv 05–Οct 06
May 05–Οct 05
Μay 06–Οct 06
ΤFe
(mg m− 2 year− 1)
DSRFe
(mg m− 2 year− 1)
111.06
138.17
71.88
305.19
131.50
256.05
6.16
6.91
8.11
3.58
4.14
2.36
Fig. 5. Relation between (a) Total Fe and dust and (b) dissolved Fe and Al.
C. Theodosi et al. / Marine Chemistry 120 (2010) 100–107
105
Fig. 6. Variation of DSRFe (a, b), solubility (c, d) and total Iron (e, f) as a function of pH. Variation of solubility as a function of dust load (g, h).
Considering the dissolved N and Fe aeolian deposition during the
dry period of 30.0 mmol m− 2 (Markaki et al., 2003 and Markaki,
2007) and 58.2 μmol m− 2 (this work), respectively, our data
indicate that the fraction of dissolved atmospheric Fe removed
through N-stimulated growth (FeR/Faero) was less than 5%. This
percentage confirms that dry atmospheric deposition, is a major
source of Fe inputs to the Eastern Mediterranean, providing over
95% of iron in excess.
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C. Theodosi et al. / Marine Chemistry 120 (2010) 100–107
The dissolved Fe concentration was found to be sufficient for
supporting phytoplankton growth in the Eastern Mediterranean Sea,
thus it cannot be considered as a limiting factor for phytoplankton
growth in the area.
4.3. Nitrogen fixation potential induced by dissolved iron
Fe can also be a rate-limiting nutrient to diazotrophic microorganisms as it controls the N2 fixation in the oligotrophic ocean and the
growth of natural and cultured populations of Trichodesmium spp. (Paerl
et al., 1994). Howard and Rees (1996) indicated that Fe is a critical
nutrient co-factor for the nitrogenase enzyme. In general, there is
indication that the aeolian deposition of Fe to the oceans may ultimately
control the rate of N2 fixation on regional and global scales (Michaels
et al., 1996; Falkowski, 1997).
Diazotrophs and especially Trichodesmium, the most prominent
planktonic marine nitrogen fixer, is present throughout the open
waters of oligotrophic tropical and subtropical oceans (Capone et
al., 1997). Trichodesmium spp. are capable of contemporaneously
fixing N2 and CO2 at relatively high rates and are considered a major
contributor to oceanic primary production C and N cycling (Paerl et
al., 1994). This cyanobacterium supplies up to half of new nitrogen
used for primary production in oligotrophic waters (Karl et al.,
1997), and thereby has a critical role in the biogeochemical cycles of
C and N.
Increased diazotrophic N2 fixation (Nfix) by marine phytoplankton
has been proposed as an alternative mechanism to explain the high
nitrogen to phosphorus ratio in the Mediterranean (from 24 to 29)
compared to the standard N/P Redfield ratio of 16 (Bethoux et al.
1998).
The nitrogen fixation potential (Nfix) induced by dissolved iron can
be calculated by the following equation:
fix
N
i−1
h
¼ Feaero −FeR ðFe=NÞfixers
For the period 2005–2006, 12.25 mg m− 2 and 5.94 mg m− 2 DSRFe
are estimated to be deposited annually in the Eastern Mediterranean
Sea by wet and dry deposition, respectively. The estimated DSRFe
concentration in the water column derived from the above numbers is
in very good agreement with the values reported by Statham and Hart
(2005). They also indicate that the dissolved iron concentration in the
water column of the Mediterranean could be attributed exclusively to
atmospheric deposition.
Finally, the biogeochemical implications of atmospheric dissolved
iron on phytoplankton growth and the nitrogen fixation have been
investigated. During the summer and autumn period (i.e., when water
stratification is at its maximum) less than 5% of the total dissolved Fe
input is required for phytoplankton growth, confirming that atmospheric deposition is a major source of Fe input to the Eastern
Mediterranean and Fe is not a limited factor for phytoplankton
growth in the Eastern Mediterranean Sea.
Furthermore the calculated nitrogen fixation potential induced
by the deposited dissolved iron, of 11.3–19.7 mmol N m− 2, is 1.5
to 3 times smaller than the measured total atmospheric nitrogen
deposition in the same area, indicating the significant role of
atmospheric deposition in the biogeochemical N cycle in the
Mediterranean.
Acknowledgements
Zambia Markaki is supported by the Greek Ministry of Education
(Irakleitos Grant). We would like to thank Dr M. Nimmo for his
helpful comments and Dr S. A. Pergantis for his comments and
assistance making the ICP-MS measurements. Finally we would like to
thank the two anonymous reviewers for their helpful comments.
References
ð3Þ
where Feaero is the iron atmospheric deposition flux, FeR is the
quantity of Fe removed by non-diazotrophic production stimulated by
N atmospheric input as discussed above and ðFe=NÞfixers is the cellular
Fe:N ratio of N fixing organisms, which varies between 2.8 and
4.9 mmol/mol (Baker et al., 2003; Berman-Frank et al., 2001).
Thus, for the dry period, the maximum potential for N2 fixation by
Trichodesmium sp. ranges between 11.3 and 19.7 mmol N m− 2, values
significantly lower (by a factor of 1.5–3) to the observed atmospheric
N deposition fluxes. These factors could be considered as lower limits
as disolution of iron could be decreased when it enters the sea.
Therefore N fixation induced by dissolved iron deposition is much
lower compared to atmospheric deposition of reactive nitrogen. This
result highlights the significant role of atmospheric deposition of
nitrogen in the Eastern Mediterranean productivity.
5. Conclusion
This work reports the first complete data of atmospheric deposition
of iron in the Eastern Mediterranean, covering a 2 year period. This data
set confirms that atmospheric deposition provides a sufficient amount
of iron, in both particulate and dissolved forms, to the seawater of the
Eastern Mediterranean. Iron deposition is found to be of comparable
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