Discussion
Abiotic variables and diatom abundance in Korean coastal
waters
In the sea, abiotic environmental factors such as salinity, water
temperature, light, nutrients, and tidal currents—together with the
physical properties of substrates—have been reported as being the main
influencers of diatom distribution in coastal waters (Desianti, et al.
2019, Trobajo and Sullivan 2010). We sampled during winter, and the
water temperature did not exceed the 13℃ seasonal average for Korean
waters. Although the water temperatures in YS were low, the diatom
species found were not determined by water temperature, but rather by
the relatively low pH (< 8.1), which was related to the high
conductivity exhibited by the well-developed mudflat. Rather than water
temperature, pH and salinity were the main influencers of diatom species
occurrence in YS, as here, the dominant species such asActinoptychus senarius , Paralia sulcata ,Cyclotella littoralis frequently occur year-round.
Although we did not analyze inorganic nutrients in the present study,
previous research reported that rivers supply inorganic nutrients to
both coastal upwelling and tidal fronts in YS and SS, and the nutrient
concentrations are associated with increased diatom community abundance
(Jung, et al. 2012, Yeo and Kang 1998). Warm currents from Tsushima (a
branch of the Kuroshio warm currents) continuously flow into SS which
also experiences coastal upwelling and front structures. Fronts in both
of these seas occur at the boundary between turbid coastal waters and
stratified offshore waters leading to high levels of phytoplankton
abundance and primary production (Choi 1991, Seung, et al. 1990). Jung,
et al. (2013) reported that in YS and SS, inorganic nutrient
concentrations (dissolved nitrogen, dissolved phosphorus, and dissolved
silica) were three times higher than those measured for ES, and such
differences probably played a major role in increasing diatom abundance.
Overall, the data indicated that differences in the geographic,
physical, and environmental factors affected diatom abundances in each
coastal area. In particular, the low diatom abundance for ES, which has
the characteristics of the open sea, may be related to a low inorganic
nutrient flux caused by the area’s deep water and lack of river mixing
inputs (Kang, et al. 2004, Kim, Kim, Min, Volkov, Yoon and Takematsu
2001, Yun, et al. 2004).
Indicator species in each diatom-based
ecoregion
We performed diatom indicator species analyses to identify the
meaningful species from four diatom-based ecoregions in the South Korean
coastal waters. This technique was used to identify diatom species that
reflected the geographic and seasonal characteristics of each ecoregion.
In YS, the water column in winter is well-mixed vertically by tidal
currents and winds; the NW monsoon cools the surface water and the water
temperature vertical gradient becomes more uniform (Gebuehr, et al.
2009, Hobson and McQuoid 1997, Roelofs 1984). Vertical mixing in the
water column provides nutrient enrichment from the sediment to the
surface and gives benthic and tychoplanktonic diatoms a chance to access
both nutrients and light. The Han and Keum rivers (two of the five
largest rivers in South Korea) drain into the YS region providing
additional nutrient enrichment (Koh and Khim 2014, Wang, Wang and Zhan
2003).
In the YS ecoregion, the indicator species included tychoplanktonic
diatoms known to be typically benthic and brackish water species (Table
2). Paralia sulcata, Actinoptychus senarius, Pleurosigma
angulatum , Cyclotella littoralis, and Asteroplanus
karianus were selected as indicator species here. Paralia
sulcata is an environmental indicator for vertically well-mixed water
due to its tychoplanktonic nature ((McQuoid and Nordberg 2003); this
species is typically found in regions with frequent upwelling, and its
abundance is correlated with high nutrient concentrations (Abrantes
1988). Pleurosigma angulatum is a typical epipelic
intertidal diatom which overcomes its severe environment by undergoing
periodic vertical migrations coincident with the tides and light
(Happey-Wood and Jones 1988). Cyclotella littoralis is a
euryhaline diatom frequently found in YS estuaries (Park, et al. 2013),
while Asteroplanus karianus can rapidly uptake nutrients
and form very large blooms (Yamaguchi, et al. 2014), and so nutrient
inputs from sediments and rivers draining into YS provide a suitable
habitat for this species. In summary, the characteristics of the YS
indicator species have straightforward explanations including a liking
for areas exhibiting well-established vertical mixing of the water
column, freshwater inputs, and nutrient enrichment.
In the SS ecoregion, the indicator species were colony and chain-forming
diatoms, such as Asterionellopsis glacialis ,Chaetoceros spp., and Eucampia zodiacus, and
Thalassiosirales such as Detonula pumila ,Skeletonema dohrnii -marinoii complex,Thalassiosira curviseriata, and T. nordenskioeldii .
The SS coastline of Korea is geologically a ria with many bays and
islands, and is physically influenced by SW, wind-driven currents, tidal
currents, and the Tsushima Warm Current. These physical factors
encourage SS sediment re-suspension and the geologically complex
coastline emphasizes tidal effects in bays (Bae and Kim 2012). Several
theories have been advanced to explain the success of chain-forming
diatoms with the advantages of chain formation being reported as
including beneficial responses to physical, chemical, and biological
constraints (Bjaerke, et al. 2015, Musielak, et al. 2009, Peters, et al.
2006). Recently, turbulence shear has been reported as enhancing
nutrient uptake in chain-forming diatoms (Bergkvist, et al. 2018), and
although the type of turbulence in SS was not studied, its physical and
geological characteristics cause continuous turbulence, which may well
be increasing the nutrient availability for chain-forming diatoms.
The ES has a coastal terrace with a simple, linear coastline, where
diatom distribution is mainly affected by two major currents—the
S-trending Liman Current, and the N-trending Tsushima Current. These
currents meet and form the subpolar fronts and mesoscale eddies that
influence phytoplankton community structure and distribution in this
area (Choi, et al. 2016). In our study, the ES diatom-based ecoregions
were divided into distinct southern and northern groups. In the SES, the
indicator species were estuarine, stalk-forming diatoms, such asLicmophora grandis , L. paradoxa , Achnanthesspp., and Odontella aurita . Licmophora is a
stalk-forming diatom that is usually found submerged in rock pools
throughout the littoral zone (Honeywill 1998). Its species are known to
survive on various substrates, including sediments, rocks, microalgae,
vertebrates, and ice. The stalk attachment has to be strong to survive
intertidal forces and wave action, as well as being pounded against
rocks and macroalgae, although in sub-optimal conditions cells can
easily become dislodged (Honeywill 1998). In a recent study,Licmophora grew well and successfully formed colonies under
experimental high light intensities and low turbulence, and their growth
rate showed no relationship to nutrients (Ravizza and Hallegraeff 2015).
Currently, no clear explanation exists for the significant presence of
attached diatoms such as Licmophora species, including L.
grandis and L. paradoxa , in the water column. The occurrence ofLicmophora species and Odontella aurita as
indicator species in SES might be related to the extensive presence of
the massive macroalgae habitats which are preferred by stalk-forming
diatoms (Jeong, et al. 2014). It is also likely that the continuous
effect of the Tsushima current may cause diatoms to become detached from
substrates and suspended in the water column (Kooistra, et al. 2009).
The selected indicator species in the NES ecoregion included
psychrophilic diatoms such as Corethron pennatum ,Coscinodiscus centralis, Porosira glacialis , andThalassiosira pacifica . Coscinodiscuscentralis is a large, centric diatom known to be cosmopolitan
(Hasle and Syvertsen 1996) even occurring in the Arctic region
(Duerksen, et al. 2014, Lovejoy, et al. 2002). Porosiraglacialis is a typical polar diatom (Villareal and Fryxell 1983),
while Thalassiosira pacifica has been reported from cold
to tropical regions (Park, et al. 2016), although most frequently
occurring in the Arctic (Joo, et al. 2012). In the NES ecoregion, the
Liman Cold Current has a more undiluted impact, while cold water and
warm water probably differ in their effects. In winter, the Liman Cold
Current flows strongly southward, and the indicator species are those
related to the temperature transitions associated with the Liman Cold
Current (Yun, Magaard, Kim, Shin, Kim and Byun 2004). Specifically, our
work indicated that the NES ecoregion was probably characterized by the
action of the Liman Cold Current weakening the Tsushima Warm Current
(Kim and Min 2008) suggesting that this ecoregion’s diatom community
assemblage may be most influenced by current, rather than by geography.