similar to the multiyear average promoted intensive phytoplankton production and
a significant increase in the C SOM concentration in comparison to previous years.
Two temporal intervals can be distinguished for the summer periods: 1979–1985
and 1985–1992 (Kukushkin 2013a), which are characterized by a high variability of
C SOM and C Chl concentrations and their components. The first period (1979–1985)
is characterized by an uneven decrease in the concentrations of these compounds
observed for all stations situated on the northwestern shelf. In 1979–1981, when the
summer was moderately warm (Titov 2003), the C SOM and C Chl concentrations
were nearly equal (except for higher in situ concentrations of C SOM in 1980). The
second part of this period, when the summers of 1982–1984 were relatively cold,
and in 1985, cold, the C SOM and C Chl concentrations in the northwestern area were
lower compared to the multiyear summer average in 1985; the same pattern was
observed for C SOM for the entire period of 1982–1985. During the period of
1982–1985, an uneven increase in the concentrations of both C SOM and C Chl was
observed in the northwestern area, when the maximal concentrations were recorded
in the extremely warm year of 1992 after a cold winter. In the central area, high
C SOM and C Chl concentrations were recorded in 1986 (warm summer) and in
1989–1990 (average summer). We assume that the anticyclonic circulation
observed in the summers of 1989–1990 may be a possible reason for these events,
since it may have fostered the transport of SOM and nutrient-rich riverine water
masses to the central shelf area (Kukushkin et al. 2006). In 1993, when the summer
was moderately cold, the C SOM and C Chl concentrations decreased in the northwestern area; the opposite pattern was observed for the central area. In addition, the
absolute concentrations of these parameters were close to the values obtained in
1979–1981.
Two temporal intervals may be also defined for the autumn periods, 1980–1987
and 1989–1992, that are characterized by high interannual variability of C SOM and
C Chl concentrations, and different patterns of such variability (Kukushkin 2013a).
In 1980–1987, the variability was relatively high. The low concentrations of SOM
components were recorded by both onboard (in situ) measurements (1980, 1987)
and satellite observations (1979, 1982, 1984), and high concentrations, in 1981,
1983, and 1985. We assume that the intensive phytoplankton production in October
of 1985 might have fostered high concentrations of measured C Chl and calculated
C SOM ; this event was favored by the intrusion of transformed riverine water masses
to the central shelf area (Kukushkin et al. 2008). In addition, the cold winter and
summer suppressed active phytoplankton production and, therefore, helped keep
the micronutrient stock at a high level. The low C SOM and C Chl concentrations in
early December 1987 can be explained by the low temperatures of both water and
air (Kukushkin et al. 2008). After 1989, the concentrations of SOM components
began to increase, when the maximal concentrations were recorded in the extremely
warm year of 1992.
10 Seasonal Changes of Hydrobiological and Bio-Optical Parameters in the. . .
173
a significant increase in the C SOM concentration in comparison to previous years.
Two temporal intervals can be distinguished for the summer periods: 1979–1985
and 1985–1992 (Kukushkin 2013a), which are characterized by a high variability of
C SOM and C Chl concentrations and their components. The first period (1979–1985)
is characterized by an uneven decrease in the concentrations of these compounds
observed for all stations situated on the northwestern shelf. In 1979–1981, when the
summer was moderately warm (Titov 2003), the C SOM and C Chl concentrations
were nearly equal (except for higher in situ concentrations of C SOM in 1980). The
second part of this period, when the summers of 1982–1984 were relatively cold,
and in 1985, cold, the C SOM and C Chl concentrations in the northwestern area were
lower compared to the multiyear summer average in 1985; the same pattern was
observed for C SOM for the entire period of 1982–1985. During the period of
1982–1985, an uneven increase in the concentrations of both C SOM and C Chl was
observed in the northwestern area, when the maximal concentrations were recorded
in the extremely warm year of 1992 after a cold winter. In the central area, high
C SOM and C Chl concentrations were recorded in 1986 (warm summer) and in
1989–1990 (average summer). We assume that the anticyclonic circulation
observed in the summers of 1989–1990 may be a possible reason for these events,
since it may have fostered the transport of SOM and nutrient-rich riverine water
masses to the central shelf area (Kukushkin et al. 2006). In 1993, when the summer
was moderately cold, the C SOM and C Chl concentrations decreased in the northwestern area; the opposite pattern was observed for the central area. In addition, the
absolute concentrations of these parameters were close to the values obtained in
1979–1981.
Two temporal intervals may be also defined for the autumn periods, 1980–1987
and 1989–1992, that are characterized by high interannual variability of C SOM and
C Chl concentrations, and different patterns of such variability (Kukushkin 2013a).
In 1980–1987, the variability was relatively high. The low concentrations of SOM
components were recorded by both onboard (in situ) measurements (1980, 1987)
and satellite observations (1979, 1982, 1984), and high concentrations, in 1981,
1983, and 1985. We assume that the intensive phytoplankton production in October
of 1985 might have fostered high concentrations of measured C Chl and calculated
C SOM ; this event was favored by the intrusion of transformed riverine water masses
to the central shelf area (Kukushkin et al. 2008). In addition, the cold winter and
summer suppressed active phytoplankton production and, therefore, helped keep
the micronutrient stock at a high level. The low C SOM and C Chl concentrations in
early December 1987 can be explained by the low temperatures of both water and
air (Kukushkin et al. 2008). After 1989, the concentrations of SOM components
began to increase, when the maximal concentrations were recorded in the extremely
warm year of 1992.
10 Seasonal Changes of Hydrobiological and Bio-Optical Parameters in the. . .
173