10.2.2.2 Variability of the Bio-Optical Parameters
In 1998–2015 in the winter-spring period the inter-annual variations of the chlorophyll a concentration (the MODIS scanner data) in the NWBS western (the
Danube) area (Fig. 10.11) are less dependent on the changes in the Danube
discharge volume (Fig. 10.10) as compared to the previous time interval
(1978–1995). Especially in spring, these variations were conditioned to a large
extent by the water temperature changes. Thus, the chlorophyll a concentration
grew in spring, 1998–2001 and 2005–2009, accompanying increase of the water
temperature (T w ) during these periods (Fig. 10.11). Its sharp decrease in 2011 and
increase in 2013–2014 was followed by the same variations of С Chl . In the summerautumn period in 1997–2003 С Chl and Т w varied synchronously (Fig. 10.11). At the
same time, in summer, 2004–2012, the water temperature increased, and the
chlorophyll a concentration decreased. Note a good agreement between the variations of the Danube discharge and С Chl in summer, 2000–2010. In the autumn
period such an agreement was not observed.
In the central NWBS the С Chl variations in winter practically did not depend on
the air temperature (Т а ) and the Danube discharge volume. In spring, the impact of
the С Chl variations was insignificant. The Danube discharge effect was more
significant. Thus, rather high chlorophyll a concentrations were in 1999, 2000,
2005, 2009 and 2010 when the Danube discharge was heightened. In summer,
2005–2015, the air temperature increase and С Chl decrease were observed similarly
to that in the Danube region.
The annual variability pattern of the chlorophyll a concentrations is almost the
same both for the Romanian and the Bulgarian coastal zones. The correlation
coefficients of С Chl series for the spring-autumn period were rather high
(0.7–0.8). The low correlation coefficient, 0.35, was obtained only for the winter
period. Some discrepancies in the С Chl variations in winter, 2006–2010 and those in
autumn, 2003–2008 were probably related to different hydro-chemical and hydrometeorological conditions in these regions that influenced phytoplankton. On the
whole the effect of the air temperature changes upon the С Chl variations was weakly
manifested. Perhaps this was due to the fact that such an effect was assessed through
the air temperature defined for NWBS, but not for the areas under study.
Comparison of the seasonal variability patterns of the Danube discharge volume
and the chlorophyll a concentration in the Romanian and the Bulgaria areas showed
no good agreement. However such an effect actually exists. For its assessment the
correlation coefficients of the seasonal chlorophyll a concentrations in the Danube
and in these two areas were calculated. The correlation coefficient of the С Chl
values in the Danube and the Romanian areas in the spring-summer period was
equal to 0.7, and in autumn – 0.6. It testifies to the fact that the phytoplankton bloom
and variability in these areas were influenced by the same factors, one of which was
the riverine discharge. The correlation coefficient between the values of С Chl in the
Danube and the Bulgarian areas was smaller. In spring it was equal to 0.37, and in
summer, when the transformed river waters propagated maximally to the south, it
174
A.S. Kukushkin
In 1998–2015 in the winter-spring period the inter-annual variations of the chlorophyll a concentration (the MODIS scanner data) in the NWBS western (the
Danube) area (Fig. 10.11) are less dependent on the changes in the Danube
discharge volume (Fig. 10.10) as compared to the previous time interval
(1978–1995). Especially in spring, these variations were conditioned to a large
extent by the water temperature changes. Thus, the chlorophyll a concentration
grew in spring, 1998–2001 and 2005–2009, accompanying increase of the water
temperature (T w ) during these periods (Fig. 10.11). Its sharp decrease in 2011 and
increase in 2013–2014 was followed by the same variations of С Chl . In the summerautumn period in 1997–2003 С Chl and Т w varied synchronously (Fig. 10.11). At the
same time, in summer, 2004–2012, the water temperature increased, and the
chlorophyll a concentration decreased. Note a good agreement between the variations of the Danube discharge and С Chl in summer, 2000–2010. In the autumn
period such an agreement was not observed.
In the central NWBS the С Chl variations in winter practically did not depend on
the air temperature (Т а ) and the Danube discharge volume. In spring, the impact of
the С Chl variations was insignificant. The Danube discharge effect was more
significant. Thus, rather high chlorophyll a concentrations were in 1999, 2000,
2005, 2009 and 2010 when the Danube discharge was heightened. In summer,
2005–2015, the air temperature increase and С Chl decrease were observed similarly
to that in the Danube region.
The annual variability pattern of the chlorophyll a concentrations is almost the
same both for the Romanian and the Bulgarian coastal zones. The correlation
coefficients of С Chl series for the spring-autumn period were rather high
(0.7–0.8). The low correlation coefficient, 0.35, was obtained only for the winter
period. Some discrepancies in the С Chl variations in winter, 2006–2010 and those in
autumn, 2003–2008 were probably related to different hydro-chemical and hydrometeorological conditions in these regions that influenced phytoplankton. On the
whole the effect of the air temperature changes upon the С Chl variations was weakly
manifested. Perhaps this was due to the fact that such an effect was assessed through
the air temperature defined for NWBS, but not for the areas under study.
Comparison of the seasonal variability patterns of the Danube discharge volume
and the chlorophyll a concentration in the Romanian and the Bulgaria areas showed
no good agreement. However such an effect actually exists. For its assessment the
correlation coefficients of the seasonal chlorophyll a concentrations in the Danube
and in these two areas were calculated. The correlation coefficient of the С Chl
values in the Danube and the Romanian areas in the spring-summer period was
equal to 0.7, and in autumn – 0.6. It testifies to the fact that the phytoplankton bloom
and variability in these areas were influenced by the same factors, one of which was
the riverine discharge. The correlation coefficient between the values of С Chl in the
Danube and the Bulgarian areas was smaller. In spring it was equal to 0.37, and in
summer, when the transformed river waters propagated maximally to the south, it
174
A.S. Kukushkin