discharge and air temperature during this period (Voskresenskaya et al. 2011) to
analyze the interannual variability of SOM in the winter–spring period (Fig. 10.9).
Analysis allowed us to reach several conclusions.
The period of 1979–1982 was characterized by the absence of ENSO, relatively
mild winters (except winter 1980) and positive anomalies in Danube River discharge (Fig. 10.9). Such conditions favored relatively intensive phytoplankton
growth in areas affected by riverine discharge in the winter–spring period. The
C SOM (Kukushkin 2013a) and chlorophyll a concentrations (except winter 1980)
exceeded the annual average. In 1984–1988, ENSO was also absent. High C SOM
and C Chl concentrations during the warm winter of 1986 may be explained by the
intensive vegetation of phytoplankton during that period (Yunev 1989). In spring,
the C SOM (Kukushkin 2013a) and C Chl concentrations decreased significantly in the
central area. This may be linked with intrusion of open sea water masses with a low
productive potential, which did not compensate the balance of micronutrients
assimilated by phytoplankton during the winter period. In 1984 and 1986, when
similar meteorological conditions were observed (in “average” winters, the Danube
River discharge in winter was lower, and in spring, higher, compared to the
multiyear average), the C SOM and C Chl concentrations differed significantly in the
winter and spring periods. A probable explanation may be found in the meteorological conditions that affect the phytoplankton community in previous periods. In
particular, the warm autumn of 1983 might have favored the state of the phytoplankton community (the C Chl concentration exceeded the average values
(Kukushkin et al. 2008)), which resulted in exhaustion of the micronutrient stock
by the end of the year. In addition, the low Danube River discharge in 1984 did not
promote intensive phytoplankton growth in winter. However, the cold year conditions of 1987 were also unfavorable (low Danube River discharge again). That is
why the micronutrient stock was not limiting factor for phytoplankton in the winter
period of 1988, and the C SOM concentration significantly exceeded the values
observed in 1984.
During ENSO years (1983, 1987, 1991–1995), the pattern differed. In 1983, the
winter was relatively mild and the Danube River discharge was similar to the
multiyear average. In the winter period, the C Chl (satellite observations) and
C SOM (Kukushkin 2013a) concentrations were lower than average in areas affected
by riverine discharge. In 1987, the winter was cold and the Danube River discharge
was low, so water transparency on the northwestern shelf was also high
(Voskresenskaya et al. 2004, Kukushkin et al. 2004). That is why we conclude
that the C SOM and C Chl concentrations were low also in the winter–spring period
compared to the average year, even taking into account the absence of in situ
measurements for these parameters. During the period of 1991–1995, which was
characterized by a long-lasting ENSO with several development and fading phases,
the cold winters of 1992 and 1993 can be cited when the Danube River discharge
was low. The measurements performed in winter 1992 in the central area testified to
low concentrations of C SOM (Kukushkin 2013a) and chlorophyll a. In spring 1993,
the concentrations of these components increased but remained relatively low. The
relatively mild winters of 1994 and 1995 and a Danube River discharge volume
172
A.S. Kukushkin
analyze the interannual variability of SOM in the winter–spring period (Fig. 10.9).
Analysis allowed us to reach several conclusions.
The period of 1979–1982 was characterized by the absence of ENSO, relatively
mild winters (except winter 1980) and positive anomalies in Danube River discharge (Fig. 10.9). Such conditions favored relatively intensive phytoplankton
growth in areas affected by riverine discharge in the winter–spring period. The
C SOM (Kukushkin 2013a) and chlorophyll a concentrations (except winter 1980)
exceeded the annual average. In 1984–1988, ENSO was also absent. High C SOM
and C Chl concentrations during the warm winter of 1986 may be explained by the
intensive vegetation of phytoplankton during that period (Yunev 1989). In spring,
the C SOM (Kukushkin 2013a) and C Chl concentrations decreased significantly in the
central area. This may be linked with intrusion of open sea water masses with a low
productive potential, which did not compensate the balance of micronutrients
assimilated by phytoplankton during the winter period. In 1984 and 1986, when
similar meteorological conditions were observed (in “average” winters, the Danube
River discharge in winter was lower, and in spring, higher, compared to the
multiyear average), the C SOM and C Chl concentrations differed significantly in the
winter and spring periods. A probable explanation may be found in the meteorological conditions that affect the phytoplankton community in previous periods. In
particular, the warm autumn of 1983 might have favored the state of the phytoplankton community (the C Chl concentration exceeded the average values
(Kukushkin et al. 2008)), which resulted in exhaustion of the micronutrient stock
by the end of the year. In addition, the low Danube River discharge in 1984 did not
promote intensive phytoplankton growth in winter. However, the cold year conditions of 1987 were also unfavorable (low Danube River discharge again). That is
why the micronutrient stock was not limiting factor for phytoplankton in the winter
period of 1988, and the C SOM concentration significantly exceeded the values
observed in 1984.
During ENSO years (1983, 1987, 1991–1995), the pattern differed. In 1983, the
winter was relatively mild and the Danube River discharge was similar to the
multiyear average. In the winter period, the C Chl (satellite observations) and
C SOM (Kukushkin 2013a) concentrations were lower than average in areas affected
by riverine discharge. In 1987, the winter was cold and the Danube River discharge
was low, so water transparency on the northwestern shelf was also high
(Voskresenskaya et al. 2004, Kukushkin et al. 2004). That is why we conclude
that the C SOM and C Chl concentrations were low also in the winter–spring period
compared to the average year, even taking into account the absence of in situ
measurements for these parameters. During the period of 1991–1995, which was
characterized by a long-lasting ENSO with several development and fading phases,
the cold winters of 1992 and 1993 can be cited when the Danube River discharge
was low. The measurements performed in winter 1992 in the central area testified to
low concentrations of C SOM (Kukushkin 2013a) and chlorophyll a. In spring 1993,
the concentrations of these components increased but remained relatively low. The
relatively mild winters of 1994 and 1995 and a Danube River discharge volume
172
A.S. Kukushkin