## 3 Results
### Diatom Resting Stages  
#### 3.2.1 Abundances and Composition Based on Direct Microscopic Observation  
The concentration of viable cells at A1, A2, and A5 was 560, 8820, and 840 cells g\({}^{-1}\) wet sediment, respectively, and the proportion of _Chaetoceros_ was more than 50% at all the stations, in particular, accounting for 93.4% at A2 (Figure 4a). The concentration and composition of the dominant _Chaetoceros_ resting stages are displayed in Figure 4b, and seven different representative resting stage types morphologically identified as _Chaetoceros_ in the present study are also shown in Figure 4c. However, the resting stages of C. _affinis_ and _C. vanheurckii_ were counted as _\"Chaetoceros affinis_ or _vanheurckii_\", owing to inadequate information on morphological characteristics to distinguish between the two species. There were also multiple resting stages of _Chaetoceros_ that could not be identified at the species level in this study. The species composition of _Chaetoceros_ exhibited a general resemblance among the stations, in which _C. contortus_ and _C. radicans_ were consistently dominant. Putative resting stages of _Actinocyclus_ sp., _Actinotypchus senarius_, and _Odontella aurita_, and _Thalassiovista_ sp., together with resting spores of _Leptocylindrus danicus_, were also observed (Figure 5). It is noteworthy that several motile penate diatoms with chloroplasts were constantly observed in all the sediment samples, reaching the highest concentration (260 cells g\({}^{-1}\) wet sediment) at A2 (Figure 6 and Table 3). The concentration of diatom resting stages at A2 located on the northernmost side of the AR area was about 10-fold that \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{3}{*}{**Station**} & \multirow{3}{*}{**Clay**} & \multirow{3}{*}{**Silt**} & \multicolumn{4}{c}{**Sand**} & \multicolumn{4}{c}{**Gravel**} & \multirow{3}{*}{**D50**} \\ \cline{3-3} \cline{5-10}  & & & **Fine** & & **Medium** & & & **Coarse** & **Fine** & **Medium** & **Coarse** \\ \cline{2-10}  & **\textless{}0.005** & **0.005–0.075** & **0.075–0.25** & **0.25–0.85** & **0.85–2.0** & **2.0–4.75** & **4.75–19** & **19–75** \\  & **mm** & **mm** & **mm** & **mm** & **mm** & **mm** & **mm** & **mm** & **mm** \\ \hline A1 & 10.5 & 9.9 & 7.1 & 8.3 & 5.8 & 14.0 & 44.4 & 0.0 & 3.70 \\ A2 & 16.6 & 11.9 & 8.8 & 12.4 & 5.9 & 14.9 & 29.5 & 0.0 & 0.89 \\ A5 & 28.3 & 18.1 & 13.8 & 20.9 & 3.8 & 5.7 & 9.4 & 0.0 & 0.12 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The particle size distribution and D50 (median diameter) of the bottom sediment at A1, A2, and A5. Figure 3: Frequency distribution of flow speed and direction at 3 m above the bottom of A5 (non-reef station) from 25 July to 7 August 2018. Flow speeds and directions were measured every one second for two minutes at an interval of twenty minutes through the period. at A5 (non-reef station), although the concentration at A1, the south end station of the AR area, was lower than at A5. Figure 4: Cell concentrations and species compositions of (**a**) diatoms and (**b**) _Chatoceros_ species in the bottom sediment at A1, A2, and A5, determined by direct microscopic observation (an open circle represents cell concentration), and (**c**) photomicrographs of the resting spores/cells of _Chatoceros_ spp. ; (**i**) _C. contortus_, (**ii**) _C. debilis_, (**iii**) _C. radians_, (**iv**) _C. affinis_ or _vanheurckii_, (**v**) _C. didymus_, (**vi**) _C. coronatus_, (**vii**) _C. diadema_. Scale bars are 20 μm. Figure 5: Photomicrographs of the resting stages of diatoms other than _Chaetoceros_ spp. by direct microscopic observation; **(I)**_Actincoyclus_ sp., **(II)**_Actincoyclus_ _senaris_, **(III)**_Thalassiosira_ sp., **(IV)**_Odontella aurita_, and **(V)**_Leptocylindrus danicus_.