Results

Species composition, variable characteristics, and diatom-based ecoregions

Based on the cophenetic correlation coefficient values (r = 0.73), the UPGMA clustering algorithm provided the best dissimilarity approximation compared to the other seven methods, and so we adopted the groupings achieved by this method to establish diatom-based regions. The UPGMA clustering, based on the Bray-Curtis similarity matrices, clustered the data into four major groups and three additional unique groups (Fig. 2). Group 1 consisted of 21 sites, mostly in the YS ecoregion but including one in the SS (site 64) and two in the ES (sites 75 and 86). Group 2 ecoregion included 51 sites located in the SS with four sites from the YS (3, 10, 12, and 13) also included in this ecoregion. Group 3 ecoregion included 25 sites in the SES, while Group 4 ecoregion had 11 sites from the NES, as well as solitary site (82) from the SES region. Three unique groups with one to two sites were separated from these four major groups: Unique Group 1 and Unique Group 2 included two sites each: 47 and 48, and 102 and 104, respectively, while Unique Group 3 included just site 105.
Abiotic variability for water temperature, salinity, and pH, was relatively low in the YS, high in the SS, and moderate in the ES (Table 1, Fig. 1 (a)–(c)). Shannon diversity indices were variable (0.25–2.82) site-by-site with low values in the western SS (Fig. 1 (e)), dominated by Thalassiosira nordenskioeldii(0.42–0.79 in sites 25–30), and Eucampia zodiacus (0.42 at site 31). Diatom richness was 7–36 with low values in the SES (7–9 at sites 89–91, 93, 100, and 101) and reduced levels of abundance (12,105–34,792 cells·l-1) (Fig. 1 (f)).

Indicator species

The indicator species for the four major ecoregions were identified using calculated indicator values (p < 0.05), and 7, 17, 6, and 7 indicator species were found for the four ecoregions, respectively (Table 2, Fig. 3). Among the 37 indicator species, Actinoptychus senarius (Fig. 4 i), Asteroplanus kariana, Cyclotella littoralis(Fig. 4 ii), Melosira nummuloides (Fig. 4 iii), Paralia sulcata (Fig. 4 iv), Thalassiosira eccentrica (Fig. 4 v) andPleurosigma angulatum (Fig. 4 vi) were identified as being significant for YS, while Asterionellopsis glacialis (Fig. 4 vii–viii), Chaetoceros affinis, C. brevis, C. constrictus (Fig. 4 xii), C. contortus (Fig. 4 ix), C. curvisetus (Fig. 4 x), C. debilis (Fig. 4 xi), C. laciniosus, Chaetocerosspp., Detonula pumila (Fig. 4 xiii), Skeletonema dohrnii-marinoii complex (Fig. 4 xiv), Eucampia zodiacus (Fig. 4 xvii), Pseudo-nitzschia pungens (Fig. 4 xviii), Thalassionema nitzschioides, Thalassiosira curviseriata (Fig. 4 xv), and T. nordenskioeldii (Fig. 4 xvi) were identified as being significant for SS. Achnanthes spp. (Fig. 4 xix–xx), Entomoneis paludosa (Fig. 4 xxiv), Licmophora grandis (Fig. 4 xxi), L. paradoxa (Fig. 4 xxii), Naviculaspp. and Odontella aurita (Fig. 4 xxiii) were identified as being significant for SES, with Chaetoceros radicans (Fig. 4 xxv), Corethron pennatum (Fig. 4 xxvi), Coscinodiscus centralis (Fig. 4 xxvii), Licmophora ehrenbergii (Fig. 4 xxx), Porosira glacialis (Fig. 4 xxiv), Rhabdonemaspp. and Thalassiosira pacifica (Fig. 4 xxviii) identified as being significant for NES.
The three unique groups also had indicator species: Amphora spp.,Bacillaria paxillifera , Grammatophora marina ,Lauderia annulata , Licmophora flabellata , Navicula elegans , and Tabularia fasciculata species, were identified for Unique Group 1and Licmophora debilis and Licmophora spp., representing Unique Group 2. An indicator value was not calculated for Unique Group 3 as only one sample (105 site) was included, but although an indicator was not calculated here, we noted that the nearby site 105 was strongly dominated by Chaetoceros curviseriata .