## 4 Discussion  
The previous studies reported that the deployment of ARs resulted in increases in primary production in the surrounding environment, enhancing the feeding environment for marine organisms induced by the promotion of settling suspended particles, thus contributing to the augmentation of fishery productivity [10; 29; 30]. The primary productivity increment is often explained as the upwelling generated by ARs bringing nutrient-rich water, which enhances primary production in the overlying water [9; 10]. Diatoms, the major marine primary producer [31], are classified into two types, namely centric and pennate [23], and the key ecophysiological characteristic of the most centric planktonic diatom species is their dormant stages [13]. These dormant stages can be of two types: resting cells [32] or resting spores [33]. Once produced, they settle down to the bottom, serving as seed populations for a future diatom bloom [18]. Thus, if enhancement of primary production by ARs is contemplated, it is essential to also integrate information on the distribution of diatom dormant cells and the physical effect of ARs such as the wake region, known to accumulate biodegrads [19; 20]. The present study has demonstrated that the concentration of the diatom resting stages at A2 (north end of the cube reef area) reached 8920 cells g\({}^{-1}\) wet sediment with the direct microscopic observation method, mainly composed of _Chaetoceros_, the dominant phytoplankton group distributed worldwide [34], and the value was more than ten times the concentration at the non-reef station (A5). On the other hand, interestingly, the cell concentration at A1, also a station near ARs but located at the south end, was the same level as that at the non-reef station. From the point of view of the environmental variables observed in this study, there were no distinct differences in the vertical environmental profiles (Figure 2) and temporal changes in water temperature at the seafloor. On the west coast of Hokkaido, where the study area is located, the Tsushima Warm Current flows northward in the surface layer, and northward flows were also observed at a depth of 100 m [35]. The dominant flow direction observed near the seafloor at A5 during the study period was also northerly (Figure 3). A2 was located approximately 55 m north of A1, and multiple cubic reefs were densely distributed to the south of A2, while only tower reefs were distantly located to the south of A1 (Figure 1c,d). In general, TOC concentration is inversely proportional to the particle size of the sediment; however, both the TOC concentration and LOI at A2 showed the highest values, even though the particle size at A2 was much larger than at A5 (Table 2). In general, soft sediments such as clay and silt are not suitable substrates for AR placement because ARs may sink into these sediments; thus, hard substrates are recommended as a deployment site for ARs [36]. The original substratum of the AR system in this study had mainly been composed of gravel before its deployment. However, although the proportion of gravel (>2.0 mm) was still high, accounting for 58.4% and 44.4% at A1 and A2, respectively, the proportion of clay and silt smaller than 0.075 mm was also relatively high (A1: 20.4%, A2: 28.5%), and the particle size at A2 was also smaller than at A1. It is therefore assumed that A2, which is positioned on the downstream side of A1, was within the range of the wake region that spreads out northward. Assuming the same amount of supply, the highest concentration of diatom resting stages at A2 can reasonably be explained by the positional relationship between the stations and the dominant flow direction. In other words, our results suggest that ARs do not necessarily accumulate diatom resting stages in their vicinity, but the wake region generated by ARs traps settling diatom resting stages and accumulates in their vicinity.