## 4 Discussion
### The Distribution and Density of Microalgae  
Toxic dinoflagellate _A. catenella_ (Group I) cells were frequently detected from the surface water at Stn. 3 offshore (Figure 4) but were not confirmed at Stn. 1 or Stn. 2 during the investigation period. The distribution of phytoplankton was previously investigated in Akkeshi-ko Estuary [29], and it is consistent with the present findings that pennate diatoms predominated and few centric diatoms were observed, dinoflagellates were few, and no _A. catenella_ cells were detected. It was reported that the cysts of _A. catenella_ decreased from the offshore of Akkeshi Bay toward the inner bay, and very few were at points close to the strait in Akkeshi-ko Estuary; no cysts were detected at the other three stations in Akkeshi-ko Estuary [52]. It is thus presumed that the survival and completion of the life cycle of _A. catenella_ are difficult in Akkeshi-ko Estuary after the invasion from Akkeshi Bay due to the existence of growth-inhibiting bacteria. The absence of paralytic shellfish poisoning (PSP) in Akkeshi-ko Estuary is presumably attributable to the relatively abundant existence of the growth-inhibiting bacteria provided by the large-scale seagrass beds.  
Pennate diatoms were dense in the seawater at Stn. 1 with the large-scale seagrass beds. Abundant pennate diatom cells on the surface of seagrass leaves are thought to detach to float in the surrounding seawater. In the seagrass beds in Dongdae Bay, Korea, the average density of phytoplankton is reported to be 2.1 \(\times\) 10\({}^{4}\) cells L\({}^{-1}\) and the minimum is 6.0 \(\times\) 10\({}^{2}\) cells L\({}^{-1}\)[53]. These values appear to be very low and showed the same tendency as the present study's data. The culture experiments [54] were conducted to investigate the interaction of _Zostera_ spp. and phytoplankton, and the obtained result was a significant suppression of the growth of the phytoplankton.  
The densities of phytoplankton were reported to be low at seagrass beds and high at a no-seagrass area in southern Australia [55]. It is recently reported that the eelgrass habitat studied is consistently associated with a marked reduction in dinoflagellate abundance both within the beds and for \(\leq\)15 m contiguously outside the beds, relative to nearby sites without eelgrass [56]. At an offshore area of Osaka Bay, Seto Inland Sea, the densities of the centric diatoms in the seawater showed higher values about 10-1000 times than those in the seawater from the seagrass bed [6]. In the Akkeshi-ko Estuary, the chlorophyll \(a\) amounts were found to be low in seawater in the Akkeshi-ko Estuary despite sufficient nutrient conditions, due to the supply of nutrients through the inflow of Bekanbeushi River, and an intense spring bloom was confirmed in Akkeshi Bay before the growth of \(Z\). _marina_[57].  
In the present study, the survey was conducted 1\(\times\)/week, and it was revealed that the phytoplankton densities in seawater were clearly lower in the seagrass beds than in the outer area of Akkeshi Bay. We thus propose the following hypothesis. Growth-inhibiting bacteria on the surface of seagrasses in Akkeshi-ko Estuary are released into the seawater, and these bacteria suppress the growth of phytoplankton, leading to the decrease of plankton abundance. It is reasonable to speculate that planktonic-centric diatoms predominated in seawater at Stn. 1 in Akkeshi-ko Estuary at the first observation in April when the seagrass had not started to grow (Figure 3). The fluctuations of bacteria that affect the growth of diatoms should be examined in future studies.