Recent studies indicate an increase in atmospheric turbulence in the Chukchi
Sea due to the recent drastic sea-ice reduction during summer months. The
importance of the effects of this atmospheric turbulence on the marine
ecosystem in this region, however, is not fully understood. To evaluate the
effects of atmospheric turbulence on the marine ecosystem, high-frequency
sampling (daily) from five layers of the microplankton community between 0
and 30 m at a fixed station in the Chukchi Sea from 10 through 25 September
2013 was conducted. During the study period, a strong wind event (SWE) was
observed on 18 and 19 September. The abundance of microplankton was 2.6 to
17.6 cells mL
In the marine ecosystem of the western Arctic Ocean, microplankton,
including diatoms, dinoflagellates and ciliates, play several roles, such as
primary producers, consumers and food resources for mesozooplankton (Sherr
and Sherr, 1988, Sherr et al., 1997; Olson and Strom, 2002). The
microplankton community in the western Arctic Ocean is divided into three
groups – shelf, continental slope and basin (Sukhanova et al., 2009; Matsuno
et al., 2014). As a special characteristic, during the summer, the
development of pycnocline prevents the supply of nutrients to the surface
layer, and phytoplankton (as determined by chlorophyll
In recent years, a drastic decrease in sea ice has been reported for the western Arctic Ocean during the summer months, and even greater related changes in sea surface temperatures have been reported (Stroeve et al., 2007; Steele et al., 2008). The changes in sea surface temperatures, increases in the frequency and intensity of cyclones, and northward shifts from their tracks during the summer months as well as during other seasons have also been reported (Serreze et al., 2000; McCabe et al., 2001; Sepp and Jaagus, 2011). While these changes are important, little is known about the effects of atmospheric and oceanic changes on the marine ecosystem in the western Arctic Ocean. During the period from 10 to 25 September 2013, high-frequency (daily) sampling and observations were conducted at a fixed station in the western Arctic Ocean, and the occurrence of strong wind events (SWEs), vertical flux of nutrients and changes in primary production were reported (Nishino et al., 2015). However, it is not clear how the microplankton assemblages – diatoms, dinoflagellates, ciliates – respond to the SWEs and the changes in nutrient supply and primary production.
In the present study, we evaluate short-term changes in the microplankton community in the Chukchi Sea during the autumn months by quantification of both autotrophic and heterotrophic microplankton assemblages – diatoms, dinoflagellates, ciliates – based on the samples collected during the same time frame as Nishino et al. (2015). Note that we only observed microplankton and did not quantify nano- and pico-plankton in this paper. We conducted a cluster analysis based on microplankton abundance and evaluated the effect of SWEs on microplankton assemblages under weak stratification in the Chukchi Sea due to atmospheric cooling during the autumn months.
Location of the sampling station in the Chukchi Sea. Depth contours at 50, 100 and 1000 m are superimposed.
Water samples were collected from a fixed station in the Chukchi Sea
(72
In a land laboratory, 1 L preserved samples were concentrated to 18 mL with
the settlement of microplankton cells at the bottom of the bottle, and a
syphon was used to drain the clear water from the top. To obtain cell counts
of diatoms and ciliates, subsamples (0.1 to 0.2 mL) were mounted on a glass
slide and counted under an inverted microscope. For species identification,
we referenced Hasle and Syvertsen (1997) and Hoppenrath et al. (2009) for
diatoms and Maeda (1997) and Taniguchi (1997) for ciliates. To distinguish
thecate and athecate forms for cell counts of dinoflagellates, after staining
subsamples with calcofluor (1 mg ml
For cluster analysis, the abundance (
Temporal and vertical changes in temperature
(
Temporal changes in cell density and species composition of total
diatoms
Temporal changes in cell density and species composition of total
dinoflagellates
Through the sampling period, temperatures ranged from
In the present study, diatoms belonging to 7 genera and 35 species, dinoflagellates belonging to 7 genera and 25 species, and ciliates belonging to 7 genera and 8 species were identified (Table 1). Within the microplankton species, 11 species increased in abundance after the SWE, while no species decreased in abundance after the SWE (Table 1).
The mean abundance of diatoms (0 to 30 m) ranged from 1.6 to
14.1 cells mL
List of microplankton species and their mean cell densities
(
The mean abundance of dinoflagellates (0 to 30 m) ranged from 0.5 to
2.4 cells mL
Mean abundance of ciliates (0 to 30 m) ranged from 0.1 to
2.8 cells mL
Temporal changes in cell density and species composition of
total ciliates
Mean cell densities (
As a feature of microplankton assemblages in this study, diatoms were the
dominant taxa (comprising 68.0 % of mean abundance). For dinoflagellates,
the proportion of the autotrophic species (such as
A cluster analysis based on diatom abundance classified their community into
five groups (A to E) at 46.1, 65.9 and 78.7 % dissimilarity levels
(Fig. 6a). Each group contained between 7 and 24 samples. The highest
abundance was observed for group C, followed by groups B, E, A and D. Group A
exhibited no distinct dominant species, while groups B and C were dominated
by
Comparisons between and among groups indicated that there were eight species
with significantly different numbers from one group to another according to a
one-way ANOVA,
With respect to temporal and vertical distribution of each group, group D dominated the water column on 10 September (Fig. 6c). From 12 to 18 September, group B dominated at the 0 to 20 m level in the water column, while other groups were observed to dominate on various occasions. For example, from 19 to 23 September, after the SWE, group C dominated the water column group. After that, group E was found to be dominant on 24 and 25 September. At the greatest depth, 30 m, group A was dominant throughout most of the study period.
To obtain information on the microplankton community in the Chukchi Sea,
geographical changes in community structure during the summer months (Joo et
al., 2012; Matsuno et al., 2014; Yang et al., 2015) as well as seasonal and
horizontal changes in diatoms (Sukhanova et al., 2009) were recorded. Because
the study region and season were comparable to those in Matsuno et
al. (2014), we compared the characteristics of the microplankton community in
this study. Matsuno et al. (2014) classified the microplankton community into
five groups (A to E) based on abundance and concluded that the grouping was
strongly correlated with the environmental parameters, which varied by water
mass. When comparing the findings of this study with the environmental
parameters of Matsuno et al. (2014), ranges of surface salinity (31.0–32.7)
and chl
In the biomass base, Yang et al. (2015) divided the microplankton community
in this region into three groups – the diatom-dominated eutrophic Chukchi
Sea Shelf, the picoplankton-dominated oligotrophic Northwind Abyssal Plain
and the picoplankton- and diatom-dominated Northwind Ridge. Comparing the
classifications, the diatom-dominated microplankton community of this study
may correspond to Yang et al.'s (2015) Chukchi Sea Shelf group. The dominant
species of this study – the pennate diatom
With respect to seasonal changes, diatoms (> 5
Compared with these seasonal patterns, the subsurface chl
The hydrographic condition of the Arctic Ocean means that low salinity occurs at the surface layer due to the melting of sea ice during the summer months, while high salinity is the result of brine, which occurs during the formation of sea ice during winter months (Macdonald et al., 2002; Nishino et al., 2011). Throughout the study period, sea surface temperatures decreased while salinity gradually increased (Fig. 2). These environmental changes may result from the following processes – atmospheric cooling during autumn induces high-density sea surface water and weakens the density of the pycnocline layer, which then promotes the mixing of the cold and the saline deep water. From 10 to 14 September, less saline ice-melt water was found in the surface layer, the pycnocline layer formed at approximately 25 m, and nutrient-rich and saline Pacific summer water was found beneath the ice-melt water (Fig. 2c) (Nishino et al., 2015). It was noted that the SWE, which was observed from 18 to 19 September, temporally weakened the pycnocline layer, thus causing vertical mixing to occur, which then resulted in the supply of rich nutrients to the surface layer (Nishino et al., 2015).
A schematic diagram of short-term changes in the microplankton community and
the dominant species during the study (10 to 25 September) is presented in
Fig. 7. Based on the dominant species and community structure during the
study period, the microplankton community was classified into five phases,
each of which occurred at 2- to 5-day intervals. Thus, from 10 to 14
September, the abundance of most species as well as the levels of chl
On 17 and 18 September, diatom abundance decreased, while there was an
increase in the thecate dinoflagellate
In phase 3, 18 and 19 September, the SWE occurred (Nishino et al., 2015).
Effects of the SWE included a decrease in temperature and an increase in
salinity and chl
Schematic diagram of temporal changes in environmental parameters (upper panel), diatom community (middle bar) and abundance of dominant microplanktonic species (lower panel) at a water column of a single station in the Chukchi Sea from 10 to 25 September 2013. Values of the upper panel indicate integrated mean data. The solid bar indicates the timing of a strong wind event. Black, grey and white in the lower panel indicate relative abundance – high, middle and low, respectively – of each species in a 0 to 30 m column of water. Based on a dominant community and species, temporal changes in a microprotist community were divided into five phases, which are indicated by the circled numbers (1 to 5) and dashed lines in the upper panel. For details, see text.
From 22 to 23 September, most of the microplankton species increased in
abundance and formed a small bloom (phase 4). As a characteristic of this
small bloom, the abundance of the pennate diatom
From 24 to 25 September, the dominant microplankton group shifted to group E,
which is characterized by a high abundance of
Throughout this study, it was revealed that atmospheric turbulence, such as
SWE, may supply sufficient nutrients to the surface layer, which
subsequently enhances a small bloom under the weak stratification of the
Chukchi Sea Shelf during the autumn months. After the bloom, the dominant
diatom community shifts from centric diatoms to pennate diatoms, thus
suggesting that a SWE accelerates the seasonal succession of the
microplankton community from summer to winter. Such a SWE-enhanced small
bloom in autumn may be fed upon by copepods (
S. Nishino, J. Inoue and T. Kikuchi designed and coordinated this research project. S. Nishino and
J. Inoue were chief scientists during the MR13-06 cruise of RV
We are grateful to the captain, officers and crew of the RV