Tanya Churilova 1 , Vyacheslav Suslin 2
1 Institute of Biology of the Southern Seas of National Academy of Sciences of Ukraine, 2 Nakhimov Ave.
Sevastopol, 99011 Ukraine; e-mail: tanya.churilova@gmail.com
2 Marine Hydrophysical Institute of National Academy of Sciences of Ukraine, 2 Kapitanskaya Str., Sevastopol,
Ukraine, 99011; e-mail: slava.suslin@gmail.com
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
(K d
), euphotic zone depth (PAR
1%
) 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 a
CDM

spectral destribution (a
CDM
(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 ], (1) where  (z), Mol C ( Mol quanta) -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:
PUR(z) =
700

E
400 d
( z ,

)
 a ph
( z ,

) d

, (2) where a ph
(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
= k psp
 PUR*, (3)
Where k psp
= [ (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 (k d
), 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 a
CDM
(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 Z eu
was related with summer values, while the winter Z eu
(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 k d
reached maximum value for summer (0.13 m -1 ). The maximal summer Z eu
values (~ 45 m) were in 2003, 2004, 2005, because of the minimal values of k d
(~
0.1 m -1 ). This interannual variability in summer values of Z eu
and k d
resulted from year-to-year variation of surface Tchl and absorption by NAP and CDOM (a
CDM
(490)). It is evident that k d summers values are in a good agreement with the a
CDM
(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 a
CDM
(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 a
CDM
(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 a
CDM
(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 a
CDM
(490) could be used as a marker of river discharge penetration close to river delta. Near the Danube delta (fig 3) values of a
CDM
(490) varied from ~ 0.1 to ~0.2 m-1 and exceeded in ~2 and ~ 3 times a
CDM
(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.1-
3). 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 (k d
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 a
CDM
(490) coefficient (fig. 5). The relationship between a
CDM
(490) and k d
is described by: kd = 0.99*[a
CDM
(490)] + 0.089, r 2 =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 510-
550 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. A s a marker of river waters propagation the a
CDM
(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) Nitrogen- and 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. 6 th - 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

0.3
0.2
0.1
Deep-western 15.6
North-western shelf 15.2
near Danube delta 15.2
160 180 240
15.4
160
14.8
220
14.8
140
15.2
140
14.4
200
14.4
120
15
120
14 180
14
100
14.8
160
13.6
80
100
13.6
14.6
140
13.2
60
1998 2000 2002 2004 2006 2008
14.4
80
1998 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
k d
, m -1
0.4
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 (a
CDM
(490) and light attenuation coefficient averaged for euphotic zone (k d
): 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 (a
CDM
(490)) in

0.006
0.004
0.002
different regions: deep-western waters (blue symbols); north-western shelf (green symbols); near
Danube delta (brown symbols).
E d
(

), E m -2 d -1
0.01
1% PAR
0.008
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).