Seasonal and inter-annual variability in waters transparency, chlorophyll a
content and primary production in the Black Sea simulated by spectral biooptical models based on satellite data (SeaWiFS)
Tanya Churilova1, Vyacheslav Suslin2
1
Institute of Biology of the Southern Seas of National Academy of Sciences of Ukraine, 2 Nakhimov Ave.
Sevastopol, 99011 Ukraine; e-mail: [email protected]
2
Marine Hydrophysical Institute of National Academy of Sciences of Ukraine, 2 Kapitanskaya Str., Sevastopol,
Ukraine, 99011; e-mail: [email protected]
The standard SeaWiFS algorithm generally overestimates summer chlorophyll
concentration (Tchl) and underestimates pigment content during spring phytoplankton bloom
in comparison with in situ measurements. It requires development of regional algorithms
which are based on biooptical characteristics typical for the Black Sea and consequently could
be used for correct transformation of spectral features of water-leaving radiance to Tchl, light
absorption features of detritus-like matter (CDM), downwelling light attenuation coefficient
(Kd), euphotic zone depth (PAR1%) and primary production (PP).
Taking into account regional peculiarities of the biooptical parameters, their difference
between seasons, shallow and deep-waters, their depth-dependent variability within
photosynthetic zone regional spectral models for estimation of Tchl concentration (Chl Model),
CDM absorption (CDM Model), downwelling irradiance (PAR SM) and primary production (PP
SM) have been developed based on satellite data (SeaWiFS and MODIS).
The aim of this paper to do a comparative analysis of seasonal and inter-annual
variability of chlorophyll a concentration, CDM light absorption, waters transparency and PP
simulated by regional spectral models based on SeaWiFS data (Suslin et al., 2008, Churilova et
al., 2008).
Methods
The developed regional spectral model of primary production estimation (PP-SM) is based on
several regional models: a) chlorophyll model (Chl–M), which allows to retrieve surface
chlorophyll a concentration (Tchl) and light absorption coefficient of colored dissolved organic
matter (CDOM) in sum with non-algal particles (NAP) at 490 nm and slope coefficient of aCDM
spectral destribution (aCDM(490)) (Suslin et al., 2008); b) Tchl profile was retrieved from surface
Tchl value following approach ( Finenko et al., 2005);
b) spectral modeling of downwelling
irradiance (PAR-SM) (Churilova et al., 2008).
Primary production (Gross) at different depths (PP(z)) was estimated:
PP(z) = 12000 (z) PUR(z) [mgC day -1 m-3],
where (z), Mol C ( Mol quanta)
-1
(1)
- the quantum yield of photosynthesis; 12000 – constant,
which converts moles of carbon to milligrams of carbon; PUR(z), E day-1 m-3 – photosynthetic
usable radiation:
700
PUR(z) =
 E ( z,  )  a
d
ph
( z,  )d ,
(2)
400
where aph(zwascalculated based on parameterization of link between phytoplankton
absorption and Tchl a concentration, which was obtained for different seasons and layers of
euphotic zone (Churilova et al., 2007). To compute quantum yield (value the approach
(Woznyak et al., 2007) was adapted using the relationship between quantum yield ( (z)) and
amount quanta absorbed by photosynthetic active pigments normalized per chlorophyll a
concentration (PUR*psp), which was obtained based on the Black Sea bio-optical data set.
PUR*psp = kpspPUR*,
(3)
Where kpsp = [(1 - NPP) /(1+NPP)],
(4)
NPP - coefficient of relative content of non-photosynthetic pigments, which was calculated as
described in ( Babin et al., 1996).
Input data used in this modeling: 1) PAR at the sea surface (SeaWiFS data,
http://oceancolor.gsfc.nasa.gov/cgi/level3.pl); 2) sea surface temperature – SST (MODIS-Aqua
data, http://oceancolor.gsfc.nasa.gov/cgi/level3.pl); 3) normalized water leaving radiance at
490,
510
and
555
nm
-
nlw(490),
nlw(510),
nlw(555)
(SeaWiFS
data,
http://oceancolor.gsfc.nasa.gov/cgi/level3.pl). Special software was developed to perform
spatial (2.5 × 3.5 km) and temporal (two weeks) averaging data. As result of this modelling
maps of surface concentration of Tchl, averaged for euphotic zone defuse attenuation
coefficient (kd), depth of euphotic zone (1%PAR), light absorption coefficient aCDM(490) and
gross primary production at surface and totally for euphotic zone have been calculated for ten
years
from
1998
to
2007
(most
maps
are
presented
on
site
http://blackseacolor.com/browser3.html).
Results and discussions
For analysis of seasonal and intra-annual variability of pigment concentration, light absorption
coefficient aCDM(490), water transparency, primary production three fixed stations were
chosen: in western deep-waters region (30.8 - 31.6E; 42.9 - 43.5N); north-western shelf (30.8 31.6E; 44.5 - 45.1N) and near Danube delta (29.6 - 30.0 E; 44.7 - 45.1 N).
Dynamics of the parameters are presented on figures 1- 3. Strong seasonal variability of all
parameters is evident, but the type of seasonal cycle differs between stations due to different
mechanism of nutrient supply of upper photosynthetic layer of deep-waters region and northwestern shelf in the Black Sea. In the western deep-waters region U-shaped surface Tchl
seasonal dynamics is typical with low Tchl values in summer ( ~0.2 mg m-3 ) and relatively
higher Tchl values (> ~1 mg m-3) in cold period of year with sharp maximum in March –April
appeared in particular years. The differences in the spring blooming of phytoplankton between
years are related to meteorological conditions during the winter, because intensity of
convective mixing, which result in erosion of picnocline and supply of upper layer with nutrients
from deeper layer, depends on degree of surface waters cooling. Consequently cold winter (in
1998, 2003, 2004 years) is accompanied stronger spring phytoplankton bloom, when Tchl could
reach 2-3,5 mg m-3. In western deep-waters region water transparency varied ~ twice between
seasons. It result in that euphotic zone depth values ( Zeu) are in a range from minimum (~ 25
- 30 m) in October - December to maximum ( ~ 50 m) in July-August. In this region interannual
variability of Zeu was related with summer values, while the winter Zeu (minimum in annual
oscillations) varied slightly being 27 ± 2 m on averaged. The minimum summer Z eu value (36 m)
was obtained in 2001, when kd reached maximum value for summer (0.13 m-1). The maximal
summer Zeu values (~ 45 m) were in 2003, 2004, 2005, because of the minimal values of kd (~
0.1 m-1). This interannual variability in summer values of Zeu and kd resulted from year-to-year
variation of surface Tchl and absorption by NAP and CDOM (aCDM(490)). It is evident that kd
summers values are in a good agreement with the aCDM(490) (fig. 1). In fact CDOM absorption
mainly effects on light attenuation, because CDOM relative contribution to summary
absorption is much higher, than NAP contribution (Churilova et al., 2008). In addition to the
light absorption coefficient the particles back scattering coefficient could cause a short-term
pick in light attenuation coefficient in the beginning of summer. It is likely to be related with
coccolithophores (Emiliania huxleyi) bloom mainly in June, when the contribution of E.huxleyi
to the total phytoplankton biomass could reach ~ 90% (Churilova et al., 2004). About twice
seasonal variation was obtained for aCDM(490)with minimum in summer (~0.03 m-1) and
maximum in cold period (~0.06 m-1). It should be noted that seasonal dynamics of aCDM(490)
were similar to those of Tchl. In deep-waters region primary production in the layer of
euphotic zone (PPtot) varied from minimum in winter (~110 mgC m-2 day-1 in averaged) to
maximum in summer (~ 400 mgC m-2 day-1 in averaged). Annual averaged PP values (PPav)
were in a range from 75 to 153 gC m-2 y-1 being in inverse relationship with SST averaged for
year ( fig 4), because of the intensive spring bloom following the cold winters.
Unlike to deep-waters region in the shelf waters seasonal dynamics of productivity is related
with inflow of nutrients from river runoff, which is maximal during the spring - beginning of
summer. As result of this in the shelf waters seasonal dynamics of Tchl is characterized winter
and summer picks and the later is more pronounced. In summer of particular year surface Tchl
reached ~3 ( 1999) and exceeded 5 mg m-3 (2000, 2005, 2007) (fig. 2). Similar to Tchl in PPtot
dynamics more pronounced picks were obtained in summer of 2000, 2005 and 2007 years,
when PP tot was in a range 1200 – 1600 mgC m-2 day-1. The described above maximum in Tchl
and PPtot dynamics correspond well with aCDM(490) dynamics. River discharge includes of not
only inorganic matter (nutrients) but dissolved organic matter (DOM) also. Although CDOM is
not relatively constant part of DOM, nonetheless dynamics of aCDM(490) could be used as a
marker of river discharge penetration close to river delta. Near the Danube delta (fig 3) values
of aCDM(490) varied from ~ 0.1 to ~0.2 m-1 and exceeded in ~2 and ~ 3 times aCDM(490) values
in north-western shelf and deep-waters region, correspondently (fig. 1-3). The waters near
Danube delta differs higher Tchl and PPtot in comparison with the other two stations.
In coastal waters near Danube delta summer Tchl (0m) values were more than ten times
higher compared with deep-waters region, while PP values differed less (2-3 times only) (fig.13). It is resulted from the markedly decreased photosynthesis efficiency integrated for water
column – The later is related mainly with low phytoplankton light absorption efficiency
due to both high Tchl concentration in waters and relatively higher pigment concentration in
phytoplankton cells due to low waters transparency in comparison with deep-waters region.
The waters transparency decreased markedly (kd increased) from deep-waters region to northwestern shelf and to near Danube delta waters due to increasing of main optically active
components – CDOM concentration, which result in a rise aCDM(490) coefficient (fig. 5). The
relationship between aCDM(490) and kd is described by: kd = 0.99*[aCDM(490)] + 0.089, r2=0.86.
The relative high absorption of light by CDM (mainly by CDOM) results not only in different
waters transparency (depth of euphotic zone) in deep-waters and shelf waters (especially near
to Danube delta) but also in spectral features of irradiance downwelling to the bottom of
euphotic zone (fig. 6, 7). In deep-waters region is evident, that irradiance at 510 - 550 nm is
most penetrating (max) to the bottom of euphotic zone, while in shelf waters max is shifted to
the longer wavelengths 550 - 570 nm due to relative high absorbance of short wavelengths by
CDM. It was shown that in deep waters in warm seasons when waters is seasonally stratified,
deep phytoplankton population chromatically adapted to the penetrating irradiance at 510550 nm (Churilova et al., 2011) These blue-green wavelengths are suitable for absorption by
the phycobiliprotains – pigment marker of cyanobacteria. The advantage of the cyanobacteria
in light absorption and consequently in growth rate leads to altering of phytoplankton
taxonomic structure – namely, to dominating of cyanobacteria. Maximum in vertical profile of
picoplankton abundance was observed at depths of 1 – 0.1% PAR, where their contribution to
total phytoplankton biomass reached to ~60%.
Conclusions
The developed regional models allow using remote sensing data for estimation of different
indicators of the Black Sea productivity with high spatial and temporal resolution.
The comparative analysis showed that seasonal variability of pigment concentration, water
transparence and primary production is more significant than their year to-year changes.
The feature of seasonal dynamics and sources of inter annual variability of productivity
indicators in deep-waters differ from those in shelf regions due to different mechanisms of
nutrient supply into the upper productive layer of these regions.
In the deep- waters region inter annual variability in productivity is related to winter
temperature, which causes different degree of water convective mixing and consequently
different enrichment of upper layer with nutrients. The later defines an intensity of spring
phytoplankton blooming.
In the shelf waters the productivity (seasonal and inter annual variability) depends on river flow
abundance, which enrich the sea waters with nutrients and wind direction driving a propagation of
water mass enriched with nutrients from estuary. As a marker of river waters propagation the
aCDM(490) could be used.
References
1. SeaWiFS data, http://oceancolor.gsfc.nasa.gov/cgi/level3.pl
2. (MODIS-Aqua data, http://oceancolor.gsfc.nasa.gov/cgi/level3.pl)
3. Suslin V., Sosik H., Churilova T., Korolev S. Remote Sensing of Chlorophyll a
Concentration and Color Detrital Matter Absorption in the Black Sea: A Semi-Empirical
Approach for the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) // Ocean Optics
conference proceedings., Ocean Optics XIX, Il Ciocco-Barga, Italy, 6-10 October 2008,
6pp (CD-ROM).
4. Churilova T., Suslin V., Sosik H. Bio-optical spectral modelling of underwater irradiance
and primary production in the Black Sea // Ocean Optics conference proceedings.,
Ocean Optics XIX, Il Ciocco-Barga, Italy, 6-10 October 2008, 6pp (CD-ROM).
5. Churilova, T., V. Suslin, G. Berseneva, S. Pryahina (2007). Parameterization of light
absorption by phytoplankton, nonalgal particles and coloured dissolved organic matter
in the Black Sea - ONW2007, Proceeding of IV International conference “Current Optical
Problems of Natural Waters, Nizhny Novgorod, 70-74 (English).
6. Finenko, Z., T. Churilova, R. Lee (2005). Dynamics of the Vertical Distributions of
Chlorophyll and Phytoplankton Biomass in the Black Sea, Oceanology, Vol. 45, Suppl. 1,
S112–S126 (English).
7. Babin M, Morel A, Claustre H, Bricaud A, Kolber Z, Falkowslu. PG (1996) Nitrogenand irradiance-dependent variations of the maximum quantum yield of carbon
fixation in eutrophc, mesotrophic and oligotrophic marine systems. //Deep-Sea Res
41:1241-1272
8. B.Woźniak, D. Ficek, M. Ostrowska, R. Majchrowski, J.Dera Quantum yield of
photosynthesis in the Baltic: a new mathematical expression for remote sensing
applications // Oceanologia, 49 (4), 2007. pp. 527–542.
9. Churilova T., Suslin V., Rylkova O., Dzhulay A. Spectral features of downwelling radiance
and chromatic adaptation of phytoplankton in the Black Sea // Current Problems in
Optics of Natural Waters: Proc. 6th - Int. Conf. (St. Petersburg). 2011. P. 117-121.
Acknowledgement
The authors are grateful to Ocean Biology Processing Group, NASA Goddard Flight Center,
for the production and distribution of SeaWiFS and MODIS data. This work was supported by projects
MyOcean (FP7/2007-2013 grand agreement no218812 ), ODEMM (FP7 2010-2013 grand agreement
#244273), PERSEUS and DEVOTES FP 7 projects.
Fig. 1 Dynamics of gross primary production in the euphotic layer (PPtot), surface chlorophyll a
concentration ( Tchl), diffuse attenuation coefficient averaged euphotic zone (kd), light absorption
coefficient of colored dissolved organic matter in sum with non-algal particles absorption (aCDM(490)),
assimilation number at surface (AN), efficiency of photosynthesis in euphotic layer ( in western deepwaters region
Fig. 2 Dynamics of parameters ( the same as in fig.1) in north-western shelf waters
Fig. 3 Dynamics of parameters ( the same as in fig.1) near Danube delta
Deep-western
160
15.6
North-western shelf
15.2
240
160
14.8
220
14.8
140
14.4
200
14.4
120
14
180
14
100
13.6
160
13.6
13.2
140
15.4
140
near Danube delta
15.2
180
SSTav
PPav, gC m-2y-1
100
SSTav
PPav, gC m-2y-1
15
SSTav
PPav, gC m-2y-1
15.2
120
14.8
80
14.6
14.4
60
1998
2000
2002
2004
2006
80
1998
2008
2000
2002
2004
2006
2008
13.2
1998
2000
2002
2004
2006
2008
a
b
c
Fig 4 Dynamics of annual-averaged primary production (PPav, gC m-2 y-1) and surface temperature (SST
av) in western deep-waters (a), north-western shelf (b) and near Danube delta (c)
0.5
kd, m-1
0.4
0.3
0.2
0.1
0
0
0.05
0.1
0.15
aCDM(490), m-1
0.2
0.25
Fig. 5 Relationship between light absorption coefficient of colored dissolved organic matter in sum with
non-algal particles (aCDM(490) and light attenuation coefficient averaged for euphotic zone (kd): western
deep-waters ( blue symbols), north-western shelf (green symbols), near Danube delta ( brown symbols).
Fig. 6 Dependence of spectral features of downwelling irradiance at 1%PAR depth (max) on values of
absorption coefficient of colored dissolved organic matter in sum with non-algal particles (aCDM(490)) in
different regions: deep-western waters (blue symbols); north-western shelf (green symbols); near
Danube delta (brown symbols).
Ed(), E m-2 d-1
0.01
1% PAR
0.008
0.006
0.004
0.002
0
400
500
, nm
600
700
Fig. 7. Spectra of downwelling irradiance at depth 1%PAR in July 1998: deep-western waters (blue line);
north-western shelf (green line); near Danube delta (brown line).