Introduction
The importance of atmospheric input of nutrients to marine environments has
been revealed and demonstrated for both oligotrophic (Guerzoni et al., 1999)
and even eutrophic systems (Langner et al., 2009). For the Black Sea, this
source of nutrients has been scarcely accounted (Medinets and Medinets,
2012), and neither its overall significance, nor its spatial and seasonal
variations have ever been studied. Such data are very important for
understanding, modelling and assessing negative impacts of anthropogenic
eutrophication as well as other effects in the Black Sea ecosystem.
The major considered sources of nutrients for marine environments are usually
limited to riverine and coastal inputs, as well as recycling of organic
matter in the water column and upper layer of sediments. It has been found
that dry and wet atmospheric precipitations can be an important source of
nutrients to open offshore areas (Donaghay et al., 1991; Duce et al., 2008).
When it comes to relative inputs, atmospheric precipitation has been
expectedly found important for oligotrophic Mediterranean Sea (Guerzoni
et al., 1999), but this input has appeared important even for highly
eutrophic marine systems, like the Baltic Sea (Rolff et al., 2008; Langner et al.,
2009). However, in case of the Black Sea, only riverine input and respiration
of organic matter have been practically considered (Cociasu et al., 1996;
Konovalov et al., 1999; Konovalov and Murray, 2001). Though scarce published
data (Kubilay et al., 1995; Chaykina et al., 2006; Medinets and Medinets,
2012) demonstrate an input of inorganic fixed nitrogen (IFN) with atmospheric
precipitations, neither basin-wide magnitude, nor after-effects of this input
to the Black Sea have been ever evaluated.
The Black Sea is extremely important for the regional economy. This is
primarily due to the possibility to use its mineral and biological resources
and recreational and transport potentials. Where it comes to biological
resources and ecological problems of any marine system, the knowledge on the
flows of carbon and nutrients is of primary importance. The Black Sea has
been documented for various environmental problems (Mee, 1992; Konovalov
et al., 1999) due to heavy loads of nutrients. Though the riverine input of
nutrients has decreased several-fold (Mee et al., 2005), the system remains
suppressed or reveals only minor positive changes (Friedrich et al., 2014),
assuming the presence of unaccounted sources of nutrients.
Jickells (1995) reviewed published data to show a dramatic increase in the
atmospheric IFN fluxes in the 20th century and to demonstrate that
atmospheric inputs could contribute 5–25 % of the nitrogen
requirement for new production. A number of studies have been completed over
the last 2 decades to show that this source can account for 60 %
of the total continental supply of IFN to the oligotrophic Mediterranean
(Guerzoni et al., 1999), but it may also reach 8 % for the highly
eutrophic Baltic Sea (Langner et al., 2009). It can level riverine inputs and
support a large short-term increase in phytoplankton growth under certain
meteorological conditions (Spokes and Jickells, 2005).
Observational data on the deposition of IFN with atmospheric precipitations
at the surface of the Black Sea have been limited until recently to very few
publications (Kubilay et al., 1995; Chaykina et al., 2006; Medinets and
Medinets, 2012) and scarce information in cruise reports and unpublished data
(Fig. 1). Kubilay et al. (1995) published the first known data on the content
of oxidized IFN (nitrate and nitrite) in the Black Sea aerosols. Applying
several assumptions on deposition velocities, annual rainfall and scavenging
ratios, Kubilay et al. (1995) estimated the atmospheric deposition of
oxidized IFN to reach 44 100 tNyr-1, mostly in wet form
(77 %), which is up to 13 % of the total IFN input of the
Danube River, the major considered source of nutrients to the Black Sea
(Cociasu et al., 1996). Medinets and Medinets (2012) have published their
estimates of the IFN atmospheric input based on samples collected near the
mouth of the Danube River in 2004–2010. They estimated the annual IFN input
of 0.69 ± 0.045 tNkm-2, mostly in dry form (up to
80 %). Im et al. (2013) have used the WRF/CMAQ modelling system and
estimated the IFN atmospheric deposition in dry and wet form for the year of
2008 by the value of 0.82 kgNkm-2. Im et al. (2013) have also
concluded that dry deposition dominates over wet deposition in line with data
published by Medinets and Medinets (2012), but against all other
observational data for the Mediterranean (Guerzoni et al., 1999), Black Sea
(Kubilay et al., 1995), and North Sea (de Leeuw et al., 2003). However, neither
published data on spatial variations in the atmospheric input of IFN, nor
information on effects of this deposition is available.
Monitoring sites in Sevastopol and Katsiveli (*) and off-shore
sampling locations (•). Data from off-shore locations and from Odessa
have been used to verify the multiple regression equation.
The chemical composition of wet atmospheric precipitations and the input on
IFN depend on both long-range atmospheric transport and scavenging below
clouds during atmospheric precipitation (Bertrand et al., 2008). Atmospheric
transport in the Black Sea region is dominated by trajectories from Eastern
Europe (38 %), Russia (33 %), local region (19 %),
and North Africa (10 %) (Kubilay et al., 1995). Three major long-range transport trajectories account for 90 % of precipitation
events, and these trajectories cross areas with rather equal, yet seasonally
variable, magnitude of anthropogenic IFN emission (Im et al., 2013). This
means that long-range transport is important for seasonal variations in the
content of IFN in the atmosphere and its input to the Black Sea, but local
processes of scavenging below clouds on time of precipitation are important
for regional spatial and short-term temporal variations.
This paper is aimed (i) to present multi-annual observational data set of IFN
(ammonium, nitrate and nitrite) with wet atmospheric deposition in an urban
(Sevastopol) and rural (Katsiveli) sites of the Crimean coast of the Black
Sea; (ii) to analyse interannual, seasonal, and mesoscale variations in IFN
deposition; (iii) to parameterize this deposition; and (iv) to evaluate
impact of IFN deposition to the Black Sea on various timescales.
Data and methods
Sampling sites and observational data
Monitoring of wet atmospheric precipitations and analysis of IFN (nitrate,
nitrite, ammonium) has been organized in Katsiveli (southern coast of Crimea)
and Sevastopol (western coast of Crimea) from 2003 to 2008 (Fig. 1). While
samples from Katsiveli represent rural conditions, those from Sevastopol are
representative of urban conditions.
Katsiveli (http://en.wikipedia.org/wiki/Katsiveli) is a settlement of
about 650 permanent residents. It is located right at the sea coast and at
a distance of 100 km from the nearest largest cities (Sevastopol,
Simferopol, and Yalta). Besides, this site is protected by
600–1100 m high cliffs from northern and north-western winds, to
additionally block urban air pollution.
Sevastopol (http://en.wikipedia.org/wiki/Sevastopol) is one of the
largest and industrially developed cities of Crimea. There are about 385 000
permanent residents, but this population can easily triple during summer time.
Wet atmospheric precipitations of every single rain have been collected.
Sample collection procedure conformed to requirements of EMEP (Berg et al., 2001). Over the period of 2003–2008, 228 and 217
samples of atmospheric precipitations were collected to carry out 684 and 651 chemical
analyses in Sevastopol and Katsiveli, respectively. The number of samples per
year varied from 25 to 54, except for Sevastopol in 2003, when the number of
samples was 10 (Fig. 1).
The sampler was open for collection of wet atmospheric precipitations and
closed on other time in Sevastopol. The sampler collected dry and wet
atmospheric deposition in Katsiveli, because expected dry precipitations of
IFN were 23–33 % of wet deposition (Kubilay et al., 1995; Guerzoni
et al., 1999). To verify and compare data, 17 samples were collected in
parallel with a permanently open sampler for dry and wet precipitations in
Sevastopol. Though an elevated fraction of dry forms of atmospheric
precipitations was expected for Sevastopol, an urban site, the concentration
was only 0 to 30 % (average 14 %) higher for samples of bulk
deposition, as compared to samples of only wet precipitations. This is
exactly in line with published data for the Mediterranean (Guerzoni et al.,
1999), Black (Kubilay et al., 1995), and North (de Leeuw et al., 2003) seas. But
Medinets and Medinets (2012) reported dry deposition to reach 40 to
80 % of bulk deposition of IFN for the north-western part of the
Black Sea.
The collected samples were analysed for nitrate, nitrite, and ammonium
concentrations following standard analytical procedures (Berg et al., 2001).
The operational reproducibility was 2.9 % and accuracy was
2.0 % for nitrite, 12.5 and 20.0 % for nitrate, and 3.1 and
9.0 % for ammonium. Primary data were quality verified and
statistically filtered to eliminate potentially erroneous and/or abnormal
results applying the 3σ rule. Thus, three values were discarded for
Sevastopol, but no data were discarded for Katsiveli.
The maximum concentrations of nitrate in Sevastopol and Katsiveli (3.41 and
2.94 mgNL-1) were detected in December 2006 and April 2005,
respectively. The minimum values (0.10 and 0.21 mgNL-1) were
determined in rainwater in June 2008 and October 2006. The maximum (3.95 and
3.85 mgNL-1) and minimum (0.08 and 0.01 mgNL-1)
concentrations of ammonium were revealed in February and December 2004 in
Sevastopol and in July 2007 and May 2004 in Katsiveli. There were 7 cases in
Sevastopol and 10 cases in Katsiveli of the nitrite concentration below its
detection limit. But maximum concentrations were detected in December 2006
(0.24 mgNL-1) in Sevastopol and in November 2005
(0.09 mgNL-1) in Katsiveli. The sum of IFN species reached its
minimum (0.36 and 0.26 mgNL-1) in February 2004 and February
2007 in Sevastopol and Katsiveli accordingly. The maximum values (5.94 and
2.96 mgNL-1) were detected in July 2006 and November 2005.
Of all collected samples, 65 % were collected from October to March
and 45 % – from April to September making it possible to trace
intra-annual variations (Fig. 2.).
Monthly volume-weighted mean IFN concentrations in atmospheric
precipitations in Sevastopol (a) and Katsiveli (b).
Meteorological data were also recorded for the rate of precipitation, wind
speed and direction, air temperature, pressure and relative humidity making
possible statistical and regression analyses.
Multiple regression analysis
In order to analyse spatial and temporal variations in the input of IFN to
the Black Sea with atmospheric precipitations and to scale the effect of this
input, we have followed Vautz et al. (2003) to parameterize the concentration
of IFN as a function of meteorological variables. A multiple regression
equation is used in this model to bind meteorological parameters (daily data
of the precipitations amount, wind direction, season and preceding dry
period) with the flow of contaminants from the atmosphere.
Many studies have been published assessing the influence of
meteorological parameters on the concentration of IFN in atmospheric
precipitations. One of the most important parameters is the rate of
precipitation (Sweet et al., 1999). Beverland (1998) and later Garban (2004)
assessed this dependence by the power law in Eq. (1).
C=a⋅xb,
where C is the concentration of IFN in atmospheric precipitations; x is
the rate of precipitations; a and b are specific coefficients, which are
individual for sampling sites.
Jenkins et al. (2006) demonstrated that nitrate concentrations in rainwater
depend on local weather patterns. Padgett et al. (2008) studied the effect of
storm duration and wind direction. The influence of the wind speed on
mineralization of atmospheric depositions could be approximated by the third- or
fourth-order power law (Abesalashvili et al., 2001).
In order to reconstruct a multiple nonlinear regression equation for the IFN
concentration as a function of meteorological parameters, we have followed
the approach discussed by Vautz et al. (2003) and suggested by Brandon (1959).
The method is based on (i) identification of regressions between the
concentration of IFN and individual statistically significant meteorological
variables and (ii) successive introduction of these individual regressions to
the multiple regression equation.
Four meteorological variables (precipitation rate, wind speed, wind
direction, and relative humidity) have been identified of having relevant and
statistically significant influence on the concentration of IFN in samples of
rainwater. They have been successively introduced to the multiple non-linear
regression equation (Eq. 2), and their contributions have been evaluated. The
influence of the rate of atmospheric precipitation is expectedly the highest
one (Sweet et al., 1999), and it reaches 68 %. The contributions of
other components are comparable, and they equal 12, 10 and 10 % for
the wind speed, wind direction and relative humidity, respectively.
C=1.0826⋅exp-0.0496⋅Ri⋅(0.0012⋅Vx3-0.008⋅Vx2-0.0221⋅Vx+1.1071)×(0.0004⋅Vy+0.9535)⋅(-0.0006⋅f+0.96),
where Ri is the daily precipitation amount, mm; Vx and Vy
are latitudinal and longitudinal wind components; f is relative humidity,
%.
This equation has been verified against observational data from Sevastopol,
Katsiveli and Odessa, other published data (Medinets at Medinets, 2012) and
unpublished data from scientific cruises (Fig. 1). It has been found that the
difference between calculated and measured values is under 14 % and
does not exceed analytical errors.
Intra-annual variations in monthly volume-weighted mean IFN
concentrations (observed, corrected for variations in the rate of
precipitation, and approximated by a Cosine function) in Sevastopol
(a) and Katsiveli (b); the difference between the observed
and calculated IFN concentrations in atmospheric depositions for individual
months in Sevastopol (c) and in Katsiveli (d).
Meteorological data, satellite images, and surface seawater chemistry
Information on precipitation, wind, and relative humidity has been taken from
http://meteocenter.net, http://www.wetterzentrale.de and
http://rp5.ua to quantify the input of IFN to the Black Sea with wet
precipitations.
To assess the effect of atmospheric deposition for the distribution of IFN in
the surface layer of the Black Sea, data from the National Oceanographic Data
Center (http://ocean.nodc.org.ua/) for the period of 1990–2000 have
been used to plot the average distributions of ammonium, nitrate and total
IFN in the upper active layer of the sea in cold (November–March) and warm
(April–September) periods of the year.
Satellite data
(http://gdata1.sci.gsfc.nasa.gov/daac-bin/G3/gui.cgi?instance_id=ocean_8day)
on the distribution of pigments have been used to evaluate the influence of
atmospheric IFN deposition on the concentration of chlorophyll a in the
surface waters. And satellite data on the sea surface temperature
(http://podaac.jpl.nasa.gov/datasetlist?search=Pathfinder) have been
used to separate and compare the effects of atmospheric deposition and
upwelling.
Results and discussion
IFN speciation, average concentrations and temporal variations
Atmospheric depositions of IFN in Sevastopol and Katsiveli were mainly presented
in 2004–2008 by nitrate (52–53 %) and ammonium (44–45 %).
Nitrite was in the range of 2–4 % presenting, most probably, intermediate
products of oxidation of ammonium to nitrate. The difference in these values between
Sevastopol (urban site) and Katsiveli (rural site) is statistically insignificant.
These values are typical for both the Black (Medinets and Medinets, 2012) and
Mediterranean (Guerzoni et al., 1999) seas. For the Black Sea, Medinets and
Medinets (2012) report the average contribution of 64.4 ± 9.2 %
of ammonium, 35.3 ± 9.1 % of nitrate, and only 1.7 ± 0.1 %
of nitrite to the content of IFN in atmospheric bulk depositions, but
46.0 ± 15.3 % of nitrate, 53.9 ± 15.3 % for ammonium,
and < 0.2 % for nitrite in wet depositions in 2004–2010. Thus, our
data fit exactly the range of regional values reported for wet depositions.
Our data are also in good agreement with those reported for the North Sea
(Jickells, 1995), the eastern Mediterranean (Cretan) Sea (Guerzoni et al., 1999), and the northern Levantine Basin (Koçak et al.,
2010).
The calculated average annual input of IFN in 2003–2008 for rural areas
(Katsiveli) was about 0.36 tNkm-2yr-1 and for urban areas
(Sevastopol) – about 0.56 tNkm-2yr-1.
Interannual variations in monthly volume weighted mean concentrations of IFN
in atmospheric precipitations are expected and have been traced both in
Sevastopol and Katsiveli (Fig. 2). They depend on seasonal variations in
meteorological conditions, but they are also related to seasonal oscillations
in anthropogenic emission from remote and local sources. The range of
oscillations in the volume-weighted mean concentrations exceeds
4.5 mgNL-1 for Sevastopol, but it is about
1.8 mgNL-1 for Katsiveli. This suggests an important influence
of local sources related to seasonal variations in fuel combustion, which is
expectedly more important for urban areas. Besides, a slight (but
statistically insignificant) interannual trend has been also traced in
Sevastopol, but Katsiveli, from 2003 to 2008. This could be an evidence of
increasing anthropogenic emission from local sources in Sevastopol, as
compared to rural conditions in Katsiveli.
Intra-annual variations and local sources
The monthly volume weighted mean concentrations of IFN in atmospheric
precipitations reveal the presence of similar seasonal oscillations in
Sevastopol and Katsiveli (Fig. 3). Significantly higher concentrations are
revealed in Sevastopol from November to March, as compared to summer
(Fig. 3a). Though it is expected because the magnitude of anthropogenic
emission (Martin et al., 2008) varies seasonally due to seasonal variations
in fuel combustion, this feature has never been reported for the Black Sea.
Whereas similar seasonal oscillations are revealed in Katsiveli, they are
statistically insignificant (Fig. 3b). The absence of powerful local sources
of air pollution is the very obvious explanation. Neither industrial, nor
agricultural local sources are known for this rural site.
The monthly volume weighted mean concentrations of IFN increases from summer
to winter (Fig. 3a and b), though the rate of wet precipitation also
increases (Ivanov and Belokopytov, 2013). For both locations, the annual
cycle has been approximated by a Cosine equation as a function of
intra-annual period in radians. For Sevastopol, C=0.84cos(X-0.31)+2.23,
R=0.67. For Katsiveli, C=0.16cos(X-0.57)+1.28, R=0.37. The maximum of
approximated IFN concentration is in February.
If the magnitude of anthropogenic emission and other conditions do not vary,
seasonal changes in the rate of precipitation would result in lower
concentrations of IFN in wet precipitations in winter. The observed
variations are opposite suggesting that the magnitude of anthropogenic
emission dramatically increases from warm to cold seasons overwhelming all
other factors. Despite the fact that the total amount of wet precipitation
increases in winter, their intensity increases in summer. Indeed, if the
average monthly concentrations, calculated for days with atmospheric
precipitation, in Fig. 3 are normalized for the amount of summer
precipitation, following the dependence of the concentration on the rate of
precipitation (Eq. 2), the winter values increase 5–17 % in
Sevastopol and 5 % in Katsiveli (“corrected” values in Fig. 3a and
b).
The summer volume-weighted mean concentration of IFN is equal to
1.2 ± 0.20 mgNL-1 in Katsiveli and
1.7 ± 0.57 mgNL-1 in Sevastopol. The ratio of nitrate to
ammonium is equal to 1.40. From summer to winter, the volume-weighted mean
concentration of total IFN increases to 2.8 ± 0.25 mgNL-1 in
Sevastopol, but it is 2-fold of the value in Katsiveli
(1.3 ± 0.11 mgNL-1). The ratio of nitrate to ammonium
remains unchanged in Katsiveli, but it insignificantly increases to 1.50 in
Sevastopol.
If the multiple regression (Eq. 2) is used to quantify the concentration of
total IFN in wet precipitations (Fig. 3), the difference between the observed
and calculated values is neither systematic nor in excess of analytical
errors for Katsiveli (Fig. 3d). For Sevastopol, the observed and calculated
values coincide within the range of analytical errors for summer, but the
calculated values are systematically and significantly below the observed
concentrations in winter (Fig. 3c). It fits the period and intensity of
seasonal heating. When combined with data of the increased nitrate to
ammonium ratio, it has revealed local sources of IFN to atmospheric wet
precipitations in Sevastopol. Thus, the multiple regression (Eq. 2) provides
correct estimates if significant local anthropogenic sources are absent. It
is true for the off-shore part of the Black Sea and rural sites, but coastal
areas can be under additional anthropogenic influence from large cities and
ports.
There are several large industrial cities and sites of the scale of
Sevastopol at or near the coastline of the Black Sea (Fig. 1): Istanbul,
Varna, Constantsa, Odessa, Kerch, Novorossiisk, etc. To assess the extent of
their influence, we have followed Izrael et al. (1987).
Cj(x)=Cj⋅exp(λjx),
where Cj is the maximum concentration of ingredient near the source (mg L-1);
λj is the coefficient characterizing the rate of
changing concentration (km-1).
The value of λj depends on the aerosol composition, wind speed and
wind direction. For an average wind speed of about 5 ms-1,
values of λj can be calculated:
λj=k⋅exp(-0.025⋅η),
where k is the coefficient characterizing the aerosol composition (for
example, k=0.35 for nitrate and ammonium); η is the wind direction
frequency in %.
We have found that the effect of local sources associated with large cities
for typical conditions of Sevastopol is limited to coastal zone within
25 km distance. Despite the fact that local sources have no
significant direct effect on off-shore areas of the sea, monitoring of IFN
deposition remains important to correctly evaluate the budget of nitrogen in
coastal waters near industrial sites. It is specifically true for winter,
when these sources are most significant. Thus, the input of IFN to the major
off-shore part of the Black Sea can be correctly estimated applying the
multiple regression (Eq. 2). This is specifically true and important for open
off-shore areas, where direct observations for rain events and sampling are
hardly possible.
The annual IFN deposition (tNkm-2yr-1) with
atmospheric precipitation at the Black Sea surface: the average values from
2004 to 2008 (a), the highest annual values in 2005 (b), the
lowest annual values in 2008 (c).
Input of IFN, its spatial and temporal variations, and importance for the nitrogen budget
Applying Eq. (2) to calculate the concentration of IFN in wet atmospheric
precipitations for regional meteorological conditions and multiplying this
concentration by the rate of precipitation, we have quantified inputs of IFN
for individual rain events. These results have been integrated over time or
the sea to investigate spatial and temporal variations from the local to
basin scale and from individual event to annual deposition.
The total amount of IFN deposited at the surface of the Black Sea over the
period of observations (2004–2008) is about
1.55 × 106 tn of nitrogen, which is on average about
0.75 tNkm-2yr-1. It does vary in space and time (Fig. 4).
Spatial variations mainly depend on the distribution of precipitations over
the sea. The rate of atmospheric precipitations (Ivanov and Belokopytov,
2013) increases from the coast of Romania and Crimea to the coast of Turkey,
but it reaches the highest values in the south-eastern part of the sea near
Batumi. Similar spatial variations have been revealed for the magnitude of
IFN deposition.
The deposition of IFN at the Black Sea surface.
Year
IFN deposition, t N yr-1
% of the annual input
2004
0.33 × 106(0.79 tNkm-2yr-1)
29–49
2005
0.38 × 106(0.91 tNkm-2yr-1)
34–57
2006
0.28 × 106(0.67 tNkm-2yr-1)
25–42
2007
0.31 × 106(0.73 tNkm-2yr-1)
28–46
2008
0.27 × 106(0.65 tNkm-2yr-1)
24–40
Average
0.31 × 106(0.75 tNkm-2yr-1)
28–46
When it comes to individual years (Fig. 4), the overall pattern of spatial
variations in the deposition of IFN remains similar, but interannual
variations in meteorological conditions result in substantial spatial
variations and even in 1.5-fold variations in the total deposition from
0.27 × 106 tNyr-1
(0.65 tNkm-2yr-1) in 2008 to
0.38 × 106 tNyr-1
(0.91 tNkm-2yr-1) in 2005 (Table 1). For comparison, the
value of 1.04 tNkm-2yr-1 has been reported for the North
Sea (de Leeuw et al., 2003), 0.40 tNkm-2yr-1 for the Baltic
Sea (Rolff et al., 2008), 1.6–2.6 tNkm-2yr-1 for the
Mediterranean Sea (Guerzoni et al., 1999). This makes the observed values for
the Black Sea similar to those for other seas in the region.
The input with riverine waters is usually considered the major and sometimes
the only source of IFN for the Black Sea. However, data in Table 1 reveal
that the atmospheric input of IFN can account for 24–57 % (average
39 %) of the input with riverine waters. This is definitely an
important contribution to the total budget of nitrogen.
The annual input of IFN to the Black Sea with the riverine waters is
0.67 × 106 tNyr-1 (Gregoire et al., 2003) to 1.12 × 106 tNyr-1
(Ludwig et al, 2009), with industrial
and domestic sewage is 0.14 × 106 tNyr-1
(http://www.blacksea-commission.org/).
It has been reported that the input of IFN from the deep sea reaches
0.28 × 106 tNyr-1 (Konovalov et al., 2000) to
0.51 × 106 tNyr-1 (Gregoire et al, 2003). Thus,
considering the average annual atmospheric input of
0.31 × 106 tNyr-1 (Table 1), 41–60 % of
the total input of IFN to the Black Sea is with the riverine waters,
7–10 % with industrial and domestic wastewater, 15–31 %
due to physical exchange processes, and 15–22 % with the
atmospheric precipitations.
One of the consequences and evidence of this large atmospheric input is
a slow response of the Black Sea ecosystem (Konovalov and Eremeev, 2012;
Friedrich et al., 2014) to the reported decrease in the riverine load of IFN
(Cociasu et al., 1996). The maximum concentration of nitrate in the Black Sea
waters remains 1.5- to 2-fold of typical values in the 1960s before the
beginning of intensive eutrophication (data from the 33rd cruise of RV Maria S. Merian)
and the ecosystem remains highly eutrophic (Yunev et al., 2002)
and oxygen deprived (Friedrich et al., 2014), which is the reason for
questions on sources of nutrients. Indeed, even an immediate and complete
elimination of coastal inputs of IFN would result in less than 50 %
of the total input. The reported 2- to 3-fold decrease in the riverine input
(Cociasu, 1996) has resulted in only 15–25 % decrease in the total
input of IFN.
Another issue is related to spatial variations in the relative contribution
of the atmospheric input of IFN (Spokes and Jickells, 2005). Riverine,
industrial and domestic sources deliver IFN to coastal and estuarine areas.
Only about 2.2 % from the total riverine input (i.e. about
0.02 × 106 tNyr-1) reaches the deep sea area
(Gregoire et al., 2003) via lateral mixing and advection, yet it takes a few
months. The rest is distributed in the Black Sea waters through processes of
recycling and physical exchange at the timescale of years. Unlike these
coastal sources, the atmospheric input provides IFN directly to the place of
atmospheric precipitation and to the surface layer of active primary
biological processes. Up to 0.08 × 106 tNyr-1 of
IFN is deposited to the surface waters of the open sea providing over 4-fold
of the riverine contribution of IFN to these waters. Intra-annual variations
in the rate of atmospheric precipitations (Ivanov and Belokopytov, 2013) with
its maximum in autumn to winter and minimum in spring to summer result in
similar seasonal variations in the atmospheric IFN deposition (Table 2).
Seasonal variations in deposition of IFN at the Black Sea surface.
Season
IFN deposition,
% of the seasonal
tNkm-2season-1(min–max)
riverine input
Spring
0.11–0.15
16–22
Summer
0.06–0.09
13–19
Autumn
0.21–0.24
70–80
Winter
0.17–0.19
40–44
While the average annual (Table 1) and seasonal (Table 2) atmospheric IFN
deposition appear to level other major sources of nutrients to the Black Sea,
a possibility of individual rain events to change the content of nutrients in
the surface waters over short time periods is an open question. They have
demonstrated that episodic atmospheric inputs can be up to
12 mgNm-2event-1 of IFN or up to
0.012 mgNkg-1 if mixed to 1 m (Donaghay et al., 1991).
The average for April–September content of IFN in the upper layer of the
Black Sea varies from 100–200 mgNm-2 in the open sea to
1500–3000 mgNm-2 in coastal areas near the Danube and Dnieper
rivers (Fig. 5a). The average values for November–March vary from
200 mgNm-2 in the open sea to 1200 mgNm-2 in
coastal areas near the Danube and Dnieper rivers (Fig. 5b). Two individual
rain events supporting similar IFN fluxes of up to
50–60 mgNm-2 (Fig. 5c and d) can increase the concentration
of IFN in the surface waters by up to 5 % of its content at the
north-western shelf of the Black Sea (Fig. 5e and f). This influence does not
substantially vary from season-to-season because the background content of
nutrients in coastal and shelf waters is high (Spokes and Jickells, 2005). In
the central areas of the sea, where the amount of IFN especially during
summer is low, the contribution of individual rainfall can reach
35 % of the background content (Fig. 5e and f). This contribution is
expectedly higher for summer time because the ambient content of nutrients is
depleted in primary production processes from spring to summer.
The average IFN content in the upper 10 m layer of the sea
in warm (a) and cold part of the year (b)
(mgNm-2), the deposition of IFN with atmospheric precipitations
at the surface of the Black Sea in 12 August 2004 (c) and in
29 January 2007 (d) (mgNm-2) the ratio of the
deposited amount of IFN to its average content in the upper 10 m
layer of water (e, f). Oval figures represent regions of significant
contributions to the IFN content in seawater.
Deposition of IFN and mesoscale processes
The revealed input of IFN with atmospheric precipitations should result in
a substantial increase in the export production. Krishnamurthy et al. (2010)
have reported > 25 % of the export production due to atmospheric IFN
input. De Leeuw et al. (2003) have reported the potential to promote primary
production of 5.3 mmolCm-2day-1 for an average deposition
of 11.2 mgNm-2day-1. Applying the Redfield C : N ratio
of 106 : 16 for newly generated organic matter in the Black Sea (Burlakova
et al., 2003), an input of 50–60 mgNm-2 in one rain event may
result in additional primary production of
330–400 mgCm-2day-1 increasing 2-fold the average
primary production 310 mgCm-2day-1 (Stelmakh et al.,
1998). This increase should be several times higher for the central part of
the sea during summertime, where and when the rate of primary production is
dramatically decreased due to the lack of nutrients.
The influence of atmospheric deposition of IFN at the concentration of
chlorophyll a in the surface waters in the Black Sea has never been studied.
The previously published data have attributed spatial anomalies in the
distribution of chlorophyll a to coastal sources of nutrients (Vasiliu et.
al., 2012), physical flux of nutrients (Yunev et al., 2002), and dust
deposition (Katara et al., 2008). Yet, an episodic input of up to
50–60 mgNm-2 should also result in detectable mesoscale
anomalies in the distribution of chlorophyll a (Varenik et al., 2010). As an
example, mesoscale variations in the distribution of the chlorophyll a
concentration after atmospheric precipitation in 31 May 2004 are shown on
Fig. 6. The concentration of chlorophyll a in the central part of the sea is
about 0.4 mgm-3 and it increases towards the coastal area
(Fig. 6a). This is typical for the Black Sea in summer. It reflects the major
coastal sources of nutrients, their active recycling in shallow parts of the
sea, limited lateral exchange between coastal and open parts of the sea. As
the result of rain on 31 May, the input of IFN with atmospheric
precipitations at the surface of the Black Sea reached
1.85 mgm-3 for the central southern region (Fig. 6b), where
typical concentrations of IFN (Fig. 5) and phytoplankton (Fig. 6) were low.
It resulted in more intensive primary production. The concentration of
chlorophyll a in this area increased in several days (Fig. 6c). Then, the
concentration of chlorophyll a decreased to background values because fixed
inorganic and organic nitrogen was gradually removed from the euphotic zone
of this region (Fig. 6d) due to vertical export of particulate organic matter
and lateral physical exchange.
The atmospheric IFN input, mgNm-3, to the surface
layer of seawater in 31 May 2004 (b) and averaged chlorophyll a
concentrations, mgm-3, in 24–31 May 2004 (a), in
1–8 June 2004 (c), 17–24 June (d). Square figures
represent the area of the maximum effect of the atmospheric IFN input.
In order to identify and separate the input of IFN with atmospheric
precipitations from results of local upwelling, which may also lead to an
increase of the IFN concentration in the surface waters, both the
concentration of chlorophyll a and temperature in the region have been traced.
The temperature does not reveal any decrease, while the concentration of
chlorophyll a has increased by about 50 % (Fig. 6c). The IFN input
of 1.85 mgm-3 should result in 0.14 mgm-3 of
chlorophyll a, as the input of 1 mmol of IFN results in production of
1.05 mg of chlorophyll a (Gowen et al., 1992). It proves that the
observed change in the chlorophyll a concentration is the result of
atmospheric precipitations of IFN. Thus, the input of IFN with atmospheric
precipitations does support active mesoscale processes and spatial patchiness
of biological processes.
Conclusions
Atmospheric deposition of IFN in urban and rural locations at
the Crimean coast of the Black Sea was mainly presented in 2004–2008 by
nitrate (52–53 %) and ammonium (44–45 %). Nitrite was in
the range of 2–4 %. The difference in these values between urban
and rural sites was statistically insignificant. The average total content of
IFN in urban wet precipitations (2.51 mgNL-1) was about
2-fold of that content in rural samples (1.16 mgNL-1). The
difference was mainly due to local sources, which were most active on winter
time (Fig. 3).
The average annual input of IFN was 0.36 tNkm-2yr-1 for
rural areas and 0.56 tNkm-2yr-1 for urban areas in
2003–2008. Seasonal variations in average monthly concentrations have been
revealed with maximum concentrations in winter and minimum values in summer,
but intra-annual variations are statistically significant for only urban
samples.
The input of IFN with atmospheric precipitations has been commonly neglected
for the Black Sea because of the lack of regular observational data,
significant riverine input and high eutrophic level of this marine system.
Yet, it has been found that the annual deposition of IFN with atmospheric
precipitations on the surface of the Black Sea is on average about
0.31 × 106 tNyr-1
(0.75 tNkm-2yr-1), which is 39 % of the riverine
input (Table 1). It does vary in space and time (Fig. 5). The magnitude of
seasonal deposition of IFN with atmospheric precipitations varies from
0.06–0.09 tNkm-2 for summer to
0.21–0.24 tNkm-2 for autumn (Table 2).
In case of individual rain events supporting up to
50–60 mgNm-2, the influence of IFN deposition is up to
5 % of its background content in the upper 10 m layer of
water at the north-western shelf of the Black Sea. In the central areas of
the sea, where the amount of IFN during summer is low, the contribution of
individual rainfall can reach 35 %. The input of IFN to the Black
Sea has the potential to enhance 2-fold the level of primary production. The
traced increase in the concentration of chlorophyll a (Fig. 6) has reached
1.5-fold for mesoscale processes.
Thus, the results of this work demonstrate that the atmospheric sources of
IFN are of great importance even for the highly eutrophic marine system of
the Black Sea. This source of nutrients has to be accounted and monitored to
correctly explain and forecast the Black Sea state and evolution.