Decadal scale variation of the lower trophic level ecosystem in the Japan
Sea: a 30-yr retrospective study
Sanae Chiba
Marine Biological Process Model Group
The Japan Sea is so-called as a “miniature ocean” because it has some characteristics of a
major ocean despite its small size and semi-closed boundaries. Its average depth is over
1000 m and it includes large-scale gyres driven by wind stress and thermohaline circulation.
The goals of this study are to detect the pattern of long-time scale variation of the lower
trophic level ecosystem in the Japan Sea and elucidate mechanisms of the observed changes.
By studying this smaller scale system, I expect to gain good insights on the climate-ecosystem
interaction in the subarctic and subtropical gyres in the North Pacific. Two approaches have
been made based on the observational data sets collected after the mid 1960s: 1) analysis of
change in the plankton community structure offshore Tsushima Current region using the PM
line data sets of the Japan Meteorological Agency and 2) regional comparison of the
mechanisms between northern, subarctic region and southern, Tsushima Current region using
a zooplankton biomass date sets, the Hirota Collection (Hirota & Hasegawa, 1999, Fish.
Oceanogr. 8: 274-283) .
1) Plankton community structure along the PM line: did summer come early during the1980s?
A decadal scale variation in diatom community structure was detected in relation to change in
the upper water environment. I observed a distinct change in the community structure in
spring during the 1980s, the period after the 1976/77 and climate regime shift which has been
reported to occur in the vast areas of the North Pacific. Chlorophyll a (chl a) concentration
were markedly low (Fig. 1a), and summer species including Pseudonitzschia spp. dominated
the diatom community in spring during the 1980s. Mixed layer phosphate concentrations
during the 1980s were lower in spring compared to the1970s and 1990s, suggesting that
nutrient depletion to levels limiting diatom growth might occur early (Fig. 1a). This change
seemed subsequently to cause a shift in the dominant diatom species from those adapted to
eutrophic conditions to those adapted to oligotrophic conditions. Density profiles between
the surface and 300 m showed the thickness of the surface Tsushima Current water and the
cold subsurface water decreasing and increasing, respectively, from the late 1970s to the late
1980s (Fig. 1c). In addition, spring solar radiation increased during the 1980s. These
conditions indicate intensified stratification of the upper water column. Increases in the
phosphate gradient between the surface and subsurface layers suggested that the intensified
stratification reduced nutrient supply to the surface and may be responsible for early
formation of the summer-like oligotrophic conditions. Based on these results, I propose the
‘early summer hypothesis’ as the cause of the apparent decline of the spring phytoplankton
biomass in the1980s.
Fig. 1a Time series
anomaly of integrated Chl a
(mg m-2) and phosphate
concentration (µM) within a
mixed layer of offshore
Tsushima Current region in
spring.
0
100
200
300
Fig. 1b Time series density
profile of offshore
Tsushima Current region in
spring. Blue area indicates
the cold subsurface water
mass.
1) North-South comparison of variation in the lower trophic level ecosystem: light-limited vs.
nutrients-limited
I found a decadal scale variation in phytoplankton and zooplankton abundances both in the
northern, subarctic region, and southern, Tsushima current region. However, mechanisms and
timings of the variations differed between the north and south. Springtime mixed layer
shoaled and stratification increased due to surface water warming after the late 1970s, the
regime shift years, in the north, and after the early 1980s in the south (Fig. 2). Responding
the stratification increase, springtime abundances of phytoplankton and zooplankton increased
in the north while these decreased in the south (Fig. 2). In the north, spring phytoplankton
production might be enhanced by increase in light availability due to the mixed layer
stabilization. In the south, on the other hand, stratification increase was likely to limit
nutrients supply to the surface layer, resulting in decrease in phytoplankton production.
These results were consistent to the theories that light availability is the most important
limiting factor for primary production in the subarctic region where rich nutrients are derived
by wintertime deep mixing, and that nutrient availability determines spring primary
productivity in the subtropical regions where the surface water weakly stratified even during
wintertime.
As there was a significant positive correlation between variations of
phytoplankton and zooplankton abundances, bottom-up control was likely in the spring lower
trophic levels both in the north and south. This study demonstrates that regional comparison
of mechanisms of long-term scale variation is indispensable to understand how a common
climatic forcing could differently alter regional ecosystems.
North
regime shift year
1.5
1.0
Phytoplankton
0.5
0.0
-0.5
Fig. 2 Time series ofdensity
gradient (Δσt) between the surface
and 100 m, andplankton
abundances in spring in the northern
and southern Japan Sea.All values
are normalized anomaly. The
regime shift year os indicated with
thick yellow line.
-1.0
-1.5
Zooplankton
year