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.