The transect distributions of temperature and salinity in the NSCS area are shown in Fig. 2. The temperature ranged from 13.8°C to 30.9°C, and the salinity ranged from 33.4 to 35.1. In general, temperature decreased with depth while salinity increased with depth. The stratification was obvious in the survey region. From Fig. 2a, we observed a significant difference in the thermocline between Stations DC6–C3 and Stations E1–E3. In addition, we observed a significant hypersaline intrusion under 100 m of Stations E1–F2. In brief, all the hydrologic results suggested that Kuroshio resulted in profound influence on the study area. Water temperature and salinity data were analyzed to draw T-S (temperature and salinity) scatter diagram as shown in Fig. 2c. The survey area is divided into two distinct areas, KC and SCS.
A total of 287 species (79 genera and 5 phyla) of phytoplankton were identified in the NSCS. Among all the species, Bacillariophyta (129 species) and Pyrrophyta (150 species) were the most abundant, accounting for 44.94% and 52.26% of the total species, respectively. However, Cyanophyta, Chrysophyta and Haptophyta were recorded sporadically, and they only accounted for about 2.8% of the total species. Among Bacillariophyta, Chaetoceros (32 species) and Rhizosolenia (13 species) had the maximum species numbers, which accounted for 24.80% and 10.07% of the total diatom species. The most common species of Pyrrhophyta was Ceratium (26 species) Propolydina (23 species), accounting for 32.91% and 29.11% of the total Pyrrophyta species respectively. In this investigation, cyanobacteria belong to 2 genera and 4 species. Trichodesmium is mainly Trichodesmium thiebautii, Trichodesmium erythraeum and Trichodesmium hildebrandtii. Trichodesmium thiebautii is the most dominant species. Additionally, Richelia intracellularis appeared in two lifestyles, i.e., free living and extracellular endosymbiosis with Rhizosolenia styliformis.
Phytoplankton abundance in the NSCS ranged from 50×104 to 2.04×104 cells/L, with an average of 2.14×103 cells/L. Cyanobacteria had the largest abundance of 2.02×104 cells/L, with an average of 1.85×103 cells/L, accounting for 86.84% of the total phytoplankton abundance. Diatom abundance varied between 0.04×103 and 1.41×103 cells/L, with an average of 0.22×103 cells/L. Compared with Cyanobacteria and Diatom, Dinoflagellates presented to low abundance, with an average of 0.03×103 cells/L. Overall, Cyanobacteria accounted for a large proportion of phytoplankton abundance, indicating the phytoplankton abundance was dominated by Cyanobacteria. Cyanobacteria abundance decreased gradually from Station F2 to Station DC6, however, the number of species did not change dramatically.
The transect distributions of phytoplankton, Cyanobacteria are shown in Fig. 3. The transect distribution of phytoplankton varies greatly, especially Cyanobacteria. The distribution of phytoplankton was mainly determined by Cyanobacteria, and Trichodesmium thiebaultii were dominant species in Cyanobacteria. The diatom was the second dominate species in phytoplankton community. The maximum cell abundance (2.56×103 cells/L) appeared at 75 m depth of Station DC6, which may be mainly affected by the topography of seamounts. The maximum cell abundance of Dinoflagellate (0.20×103 cells/L) was found at 25 m depth of Station F2. The vertical variation of total phytoplankton was similar to that of Cyanobacteria, showing the maximum abundance at the subsurface (0–50 m). Cyanobacteria abundance was mainly determined by Trichodesmium thiebautii in this survey (Fig. 3). Diatom was not homogeneous distributed in the water column, with the maximum abundance near the DCM layer (~75 m). Dinoflagellates were mainly distributed in the upper layer (0–75 m).
The first 16 most dominant species were belonged to Bacillariophyta (11 species), Pyrrophyta (2 species), Cyanophyta (2 species), and Chrysophyceae (1 species) (Table 1). Trichodesmium thiebautii was the dominant species during the period of investigation. It is mainly distributed in the water layer above 50 m. The cell abundance of Trichodesmium thiebautii was between not detected (ND) and 19.88×103 cells/L with an average cell abundance at 1.82×103 cells/L. The maximum cell abundance of Trichodesmium thiebautii were observed in 5 m depth at Station F2.
Dominant species Abundance ratio/% Frequency (fi) Dominance index (Y) Trichodesmium thiebautii 85.11 0.607 0.51675 Leptocylindrus mediterraneus 1.62 0.714 0.01160 Thalassionema nitzschioides 1.45 0.589 0.00853 Synedra spp. 0.45 0.786 0.00355 Scrippsiella trochoidea 0.38 0.643 0.00243 Chaetoceros pelagicus 0.56 0.429 0.00242 Coscinodiscus subtilis 0.28 0.804 0.00228 Rhizosolenia styliformis 0.27 0.768 0.00209 Dictyocha fibula 0.47 0.357 0.00168 Prorocentrum compressum 0.24 0.625 0.00151 Hemiaulus hauckii 0.26 0.571 0.00150 Eucampia zodiacus 0.30 0.482 0.00146 Richelia intracellularis 0.60 0.232 0.00140 Navicula spp. 0.18 0.643 0.00114 Chaetoceros messanense 0.23 0.500 0.00113 Pseudosolenia calcar-avis 0.18 0.536 0.00097
Table 1. Dominant species
The main ecological types of phytoplankton species are tropical coastal species and alongshore warm-water species, which are consistent with the characteristics of subtropical climate in this sea area. The distribution of dominant species is shown in Fig. 4. Hemiaulus hauckii (5.63 cells/L in average), Leptocylindrus mediterraneus (34.78 cells/L in average) and Pseudosolenia calcar-avis (3.89 cells/L in average) were the main representative species of warm water species. Rhizosolenia styliformis (5.83 cells/L in average), Thalassionema nitzschiides (31.01 cells/L in average), Chaetoceros pelagicus (12.09 cells/L in average) were the main representative species of the eurythermal species and wide-distribution types. Scrippsiella trochoidea (8.08 cells/L in average) and Prorocentrum compressum (5.16 cells/L in average) were dominated in Dinoflagellates, and they presented to highest abundance at Stations F2 and E1 where the temperature and salinity were optimal for the Dinoflagellate growth. Synedra spp. (9.67 cells/L in average), Coscinodiscus subtilis (6.08 cells/L in average), Dictyocha fibula (10.06 cells/L in average), Eucampia Zodiacus (6.49 cells/L in average), Richelia intracellularis (12.88 cells/L in average) and Navicula spp. (3.81 cells/L in average) were all commonly detected in the study area. The distribution of Richelia intracellularis in Cyanophyta is similar to that of its symbiont Rhizosolenia styliformis.
Figure 4. Distribution of the dominant species. a. Eucampia zodiacus, b. Hemiaulus hauckii, c. Chaetoceros pelagicus, d. Synedra spp., e. Thalassionema nitzschioides, f. Leptocylindrus mediterraneus, g. Richelia intracellularis, h. Dictyocha fibula, i. Rhizosolenia styliformis, j. Navicula spp, k. Coscinodiscus subtilis, l. Chaetoceros messanense, m. Trichodesmium thiebautii, n. Scrippsiella trochoidea, o. Prorocentrum compressum, and p. Pseudosolenia calcar-avis. Unit: cells/L.
Diversity indexes are important tools to measure the stability of community structure (Sun and Liu, 2005), but it is easy to cause deviation by using only one diversity index to illustrate the diversity of the whole phytoplankton community. Therefore, Shannon-Wiener (S-W) diversity index and Pielou evenness index (J) were used in the present study to conduct a comprehensive analysis of community species diversity. From Fig. 5, the distribution trend of S-W diversity index was similar to Pielou evenness index. The S-W diversity index of phytoplankton ranged from 0.14 to 4.68 (mean=2.65±1.40) and the highest value was found in 100 m depth at Station E1. Pielou evenness index ranged from 0.03 to 0.97 (mean=0.56±0.31). The distribution pattern of Pielou evenness index of phytoplankton was consistent with that of the S-W diversity index, presenting lower values in the upper layer of Stations F2 and E1.
Cluster analysis was carried out on account of the cell abundance value and species number of each station (Fig. 6). According to Bray-Curtis similarity, eight stations were divided into two groups. Stations F2 and E1 were divided into the first group (KC), and the other six stations (C3, B3, A9, SEATS, DC2 and DC6) were divided into the second group (SCS).
In order to further study phytoplankton community structure, we divided these stations into two groups according to cluster analysis. The dominant species of each group were calculated separately, and four of the top ten dominant species of the two groups were the same (Table 2).
Dominant species of Kuroshio Dominance index (Y) Dominant species of South China Sea Dominance index (Y) Trichodesmium thiebautii 0.87482 Trichodesmium thiebautii 0.32714 Scrippsiella trochoidea 0.00185 Leptocylindrus mediterraneus 0.03676 Trichodesmium erythraeum 0.00117 Thalassionema nitzschioides 0.02515 Rhizosolenia styliformis 0.00118 Synedra spp. 0.00784 Pseudosolenia calcar-avis 0.00109 Chaetoceros pelagicus 0.00731 Synedra spp. 0.00093 Coscinodiscus subtilis 0.00475 Prorocentrum compressum 0.00083 Hemiaulus hauckii 0.00422 Richelia intracellularis 0.00072 Eucampia zodiacus 0.00404 Coscinodiscus subtilis 0.00072 Dictyocha fibula 0.00382 Ceratium teres 0.00070 Scrippsiella trochoidea 0.00368
Table 2. Dominant species of Kuroshio and South China Sea
In the present study, CCA was used to analyze the relationship between environmental parameters and dominant species. According to the results of 999 times Monte Carlo permutations, temperature (p<0.01) and salinity (p<0.01) contributed significantly to the total variance in the survey area (Fig. 7), species in the survey area are mainly divided into three groups in the two region, i.e., groups suitable for living at high temperature, groups with high salinity tolerance, and groups with wide temperature distribution. Scrippsiella trochoidea, Trichodesmium erythraeum and Prorocentrum compressum showed a significant positive correlation with temperature. Nutrient is a key factor affecting phytoplankton community structure in the South China Sea (Fig. 7). Leptocylindrus mediterraneus, Thalassionema nitzschioides, Synedra spp., Chaetoceros pelagicus, Coscinodiscus subtilis, and Hemiaulus hauckii were positively correlated with silicate, nitrate, phosphate and nitrate levels.
Figure 7. CCA ordination of dominant species. a. Eucampia zodiacus, b. Hemiaulus hauckii, c. Chaetoceros pelagicus, d. Synedra spp., e. Thalassionema nitzschioides, f. Leptocylindrus mediterraneus, g. Richelia intracellularis, h. Dictyocha fibula, i. Rhizosolenia styliformis, j. Navicula spp., k. Coscinodiscus subtilis, l. Chaetoceros messanense, m. Trichodesmium thiebautii, n. Scrippsiella trochoidea, o. Prorocentrum compressum, and p. Pseudosolenia calcar-avis.
Compared with the historical data of phytoplankton community in water sample of similar regions and same season (Table 3), the survey area of this study included the outer Luzon Strait, where the total and Trichodesmium cell abundance were higher than those of other stations in SCS. The higher cell abundance may be related to high temperature and salinity and low nutrient contents (Ding, 2009). In Tan et al.’s (2013) study in 2008, the research area was mainly concentrated in the Luzon Strait. There are more species of Diatoms and Dinoflagellates and lower the cell abundance in this survey compared with that in Tan’s study. This may be due to different investigation methods. In the present study, we filtered the water sample by 20 µm mesh, in which only the phytoplankton with diameter larger than 20 µm would be retained. However, more rare species would be found in larger volumes of seawater.
Time Research area Depth Number of species Cell abundance/103 cells·L–1 References Jul.–Aug. 2017 14°–22°N, 114°–124°E 0–200 287 2.141 this study Aug. 2014 18°–22°N, 114°-116°E 0–200 229 14.653 Xue et al. (2016) Jul.–Aug. 2009 18°–23.5°N, 109°–120°E 0–200 150 26.490 Ma and Sun (2014) Aug. 2009 18°–22°N, 110°–117°E 0–200 109 8.197 Li et al. (2012) Aug.–Sep. 2008 18°–23°N, 120°–122.5°E 0–200 169 1.448 Tan et al. (2013) Aug. 2007 18°–23°N, 110°–120°E 0–200 216 11.220 Ke et al. (2011) Aug. 2004 18°–22°N, 110°–117°E 0–200 159 115.050 Le et al. (2006)
Table 3. Comparing with the historical data
Previous studies revealed that the composition of phytoplankton community was closely related to the variable circulations and water masses (Liu et al., 2004). Yang et al. (2011) declared that the Kuroshio branches were significant drivers of the biogeochemical cycles in the East China Sea and South China Sea. The T–S diagram analysis suggested that the relatively high temperature and salinity appeared in the KC. The results of cluster analysis divide the survey area into two parts. Somewarm-water species such as Leptocylindrus mediterraneus, Rhizosolenia styliformis, Thalassionema nitzschiides, and temperate coastal species such as Thalassionema nitzschiides are abundant in the first group. Some eurythermal species and coastal species occur mainly at the site of the second group, such as Chaetoceros pelagicus and Coscinodiscus subtilis. In the second group, the cell abundance of Trichodesmium in the first subgroup was much higher than that in the second subgroup. Three species of Dinoflagellates (Scrippsiella trochoidea, Prorocentrum compressum and Ceratium teres) were found with high dominance in Kuroshio area. This may be caused by higher ecological adaptability of Dinoflagellates than that of Diatoms under the conditions of high temperature and low nutrients (Li et al., 2003; Wang et al., 2006).
The dominant species in the two regions were analyzed, and there was a significant correlation between the distribution of dominant species and the water mass. For example, the distributions of Trichodesmium thiebautii, Scrippsiella trochoidea, Prorocentrum compressum and Pseudosolenia calcar-avis are determined by their different tolerance to temperature and salinity (Figs 4m-p) (Sun et al., 2001). Diatoms, such as Eucampia doracus, Hemiaulus hauckii, Chaetoceros pelagicus, Synedra spp., Thalassionema nitzschioides and Richelia intracellularis, were dominant in the SCS (Figs 4a-e, g), which further demonstrated the response of phytoplankton to the environment. Given the above discussion, we believe that Cyanobacteria (Trichodesmium thiebautii) and Diatom (Thalassionema nitzschioides) can be the indicator species in the KC and the SCS, respectively.
The ratio of Diatom and Dinoflagellate is a fundamental factor to measure the stability of community structure in the ocean. In general, the higher proportion of Dinoflagellate may lead to the occurrence of harmful algal bloom which is toxic to water mass (Sun and Liu, 2005). In Station F2 the proportion of diatom species number was 31.97% and cell abundance was 41.30%. Overall, the proportion of diatom from Station F2 to Station DC6 was gradually increasing and reached more than 50% in SCS (Fig. 8) because Dinoflagellates have higher ecological adaptability than Diatoms under the conditions of high temperature and low nutrients (Li et al., 2003; Wang et al., 2006)
Figure 8. The histogram of ratio of Diatom and Dinoflagellate species number and cell abundance. a. The histogram of ratio of species number and b. the histogram of ratio of cell abundance.
The Kuroshio had very high species richness (nearly 500 species) and phytoplankton diversity (Sun et al., 2001). When the Kuroshio invades the South China Sea, phytoplankton may also move with the water mass. There are 23 common species in the survey area, including 15 Diatoms, 6 Dinoflagellates and 2 Cyanobacteria. Comparing species at Station F2 with those at other stations, there were great differences in phytoplankton species in the investigated area. Each station was compared with Station F2 with Jaccard similarity (Table 4). The similarity in Station E1 and Station C3 was higher (0.414 and 0.368) while the similarity at other stations was lower and the difference was not significant. The main group of Station F2 is Dinoflagellates, which is related to the fact that Dinoflagellates prefer higher water temperature.
Station DC6 DC2 SEATS A9 B3 C3 E1 F2 Same species with F2 59 40 52 55 50 74 87 154 Similarity index value 0.289 0.206 0.275 0.286 0.255 0.368 0.414 1.000
Table 4. The Jaccard similarity index
The physical factors (temperature, salinity), chemical factors (nutrients) and biological factors (competition among phytoplankton) were the main variables affecting the phytoplankton community distribution (Sun et al., 2007). The relative positions of Diatom and Dinoflagellate in CCA biplot reflected the dependence of the species on environmental factors. CCA was used to analyze dominant species in the Kuroshio and South China Sea areas (Fig. 7). Combined with temperature and salinity distribution and transect distribution of phytoplankton community structure, we found that the Kuroshio area was greatly affected by temperature. There were three species of Cyanobacteria and three species of Dinoflagellates in the top ten dominant species. The reason may be that Cyanobacteria and Dinoflagellates were more tolerant to temperature (Fig. 7), and the distribution of Diatoms in the two regions was mainly affected by nutrients. The salinity of the sea area near the Luzon Strait was high due to the Kuroshio intrusion. The inshore diatoms decreased in the phytoplankton composition. The species and cell abundance of Dinoflagellates and Cyanobacteria increased obviously (Tan et al., 2013). Due to the higher water temperature in the area affected by the Kuroshio, Diatoms were rare, but warm-water Dinoflagellates and Cyanobacteria predominated. Higher water temperatures (>30°C) will limit the phytoplankton growth. The surface layer was affected by tropical high temperature and strong radiation, what was more, the nutrient concentration was low and could not meet the needs of phytoplankton growth (Zhu et al., 2003). It indicated that temperature had the greatest influence on the phytoplankton community structure during the Kuroshio invasion to the SCS.
The profound influence of Kuroshio intrusion on microphytoplankton community in the northeastern South China Sea
- Received Date: 2019-04-02
- Accepted Date: 2019-05-30
- Available Online: 2020-12-28
- Publish Date: 2020-08-25
Abstract: To further understand the effect of Kuroshio intrusion on phytoplankton community structure in the northeastern South China Sea (NSCS, 14°–23°N, 114°–124°E), one targeted cruise was carried out from July to August, 2017. A total of 79 genera and 287 species were identified, mainly including Bacillariophyta (129 species), Pyrrophyta (150 species), Cyanophyta (4 species), Chrysophyta (3 species) and Haptophyta (1 species). The average abundance of phytoplankton was 2.14×103 cells/L, and Cyanobacterium was dominant species accounting for 86.84% of total phytoplankton abundance. The abundance and distribution of dominant Cyanobacterium were obviously various along the flow of the Kuroshio, indicating the Cyanobacterium was profoundly influenced by the physical process of the Kuroshio. Therefore, Cyanobacterium could be used to indicate the influence of Kuroshio intrusion. In addition, the key controlling factors of the phytoplankton community were nitrogen, silicate, phosphate and temperature, according to Canonical Correspondence Analysis. However, the variability of these chemical parameters in the study water was similarly induced by the physical process of circulations. Based on the cluster analysis, the similarity of phytoplankton community is surprisingly divided by the regional influence of the Kuroshio intrusion, which indicated Kuroshio intrusion regulates phytoplankton community in the NSCS.
|Citation:||Xingzhou Wang, Yuqiu Wei, Chao Wu, Congcong Guo, Jun Sun. The profound influence of Kuroshio intrusion on microphytoplankton community in the northeastern South China Sea[J]. Acta Oceanologica Sinica, 2020, 39(8): 79-87. doi: 10.1007/s13131-020-1608-y|