Phytoplanktons are important organisms responsible for primary production in marine ecosystems. As planktonic protists, diatoms, dinoflagellates, and euglenoids are the dominant taxa in the oceans. Diatoms are the main constituents of micro-sized phytoplankton communities, and their growth rates vary depending on temperature and nutrient concentrations (Sergeeva et al., 2010, Giesbrecht et al., 2019). Species-specific differences are present in physiological activity; half-saturation constants for the nitrate uptake rate are high for large species, such as Thalassiosira spp., and low for small species, such as Chaetoceros spp. (Eppley et al., 1969, Turpin and Harrison, 1980). In addition, the preferred water temperature differs among species; for example, Thalassiosira spp. prefer low water temperatures (Durbin, 1974). Diatoms exhibit a unique physiology with many species responding to environments unsuitable for growth by forming resting cells and settling on the sea floor (Hargraves and French, 1983, McQuoid and Hobson, 1996). These resting cells are highly durable, germinate mainly through light irradiation, and proliferate in suitable environments (Itakura, 2000). Diatoms are classified as pelagic or benthic based on their habitat. The habitats of benthic diatoms are limited to substrates in the photic layer; therefore, they are often found in shallow coastal areas (Cadée and Hegeman, 1974).
Dinoflagellates are major constituents of phytoplankton communities and are classified into three types depending on their nutritional modes: autotrophic, mixotrophic, and heterotrophic (Elbrachter, 1991, Sanders, 1991). Autotrophic and mixotrophic dinoflagellates are the second most important producers in the classical grazing food chain (Raymont, 1980), and heterotrophic dinoflagellates act as microzooplankton, increasing in particle size from pico-nano to micro in the microbial food chain (Sherr and Sherr, 1988). The seasonal pattern of dinoflagellates differs from that of diatoms; in Funka Bay, Hokkaido, autotrophic and mixotrophic species occurred at high densities in an environment depleted of inorganic nutrients after the spring diatom bloom (Miyazono and Shimada, 2000). In addition, some dinoflagellates form cysts during their life cycle, which play an important role in expanding the distribution of the species, initiating and ending blooms, and surviving in unsuitable environments (Wall, 1971, Anderson, 1984, Steidinger, 1993). As a notable aspect of this taxon, some dinoflagellate species are classified as harmful algal blooms and cause damage to fisheries by forming red tides, which is a problem not only in Japan but also worldwide (Oh et al., 2005).
The warm and high-salinity Tsushima Warm Current flows into the southern coastal area of Hokkaido on the coast of the Sea of Japan. The flow volume changes seasonally, being low from March to May and peaking from August to November (Sato, 2001, Yasui et al., 2022). The Tsugaru Warm Current, which is a branch of the Tsushima Warm Current, also has high temperature and salinity. It flows into the Tsugaru Strait from the west entrance throughout the year and spreads across the Hakodate Bay at the southern end of Hokkaido throughout most of the year (Nishida et al., 2003). In contrast, from autumn to winter, the cold, low-salinity coastal Oyashio originates from the east entrance of the strait along the Hokkaido coast and occasionally reaches the Hakodate Bay (Ohtani, 1987). In recent years, changes have been observed in these ocean currents, and the flow volumes of the Tsushima Warm Current and Tsugaru Warm Water are reported to be increasing annually (Kida et al., 2021).
Because responses to environmental changes such as water temperature, salinity, and nutrient concentrations differ among protist species (Olli et al., 1996, Li et al., 2011, Liu et al., 2011, Karthik et al., 2017), protist community composition is considered an indicator of environmental change (e.g., Tadokoro et al., 2008). For example, in Japan, the dominant species has shifted from Skeletonema spp. to Chaetoceros spp., which was associated with decreased dissolved inorganic nitrogen (DIN) in Harimanada, Japan (Nishikawa et al., 2010). Red tides caused by Karenia mikimotoi occurred in the Hakodate Bay for the first time in 2015, and the mortality of fish and shellfish, such as salmon, Japanese common squid, and Ezo abalone, was recorded (Shimada et al., 2016, Kakumu et al., 2018, Shimada, 2021). The harmful species are believed to be transported by the Tsushima Warm Current with warming and increasing volume (Shimada et al., 2016, Kakumu et al., 2018). The dinoflagellate Alexandrium pacificum, which cause paralytic shellfish poisoning, and raphidophyte Heterosigma akashiwo are also found in the bay (Natsuike et al., 2019, Natsuike et al., 2021). On the other hand, several phytoplankton studies have been reported for the southern coastal area of Hokkaido, including species composition during spring blooms in the Iwanai Bay (Odate et al., 1993) and long-term changes in phytoplankton communities using net sampling in the Oshoro Bay (Fukui et al., 2010). However, information on the relationship between environmental variables and the entire protist community or the mechanisms causing harmful blooms in the Hakodate Bay is lacking.
As another aspect of phytoplankton response to environmental changes, stress conditions and photosynthesis activity in cells are useful parameters (Schreiber et al., 1995). To measure these, a pulse amplitude modulation (PAM) fluorescence method (Schreiber et al., 1986) is broadly used in field investigations because this method has the advantage that measurements can be made in a short time and non-destructively. The most commonly used parameter is the maximum quantum yield (Fv/Fm), which indicates the photochemical reaction per unit of light absorbed by chlorophyll in photosystem II (Schreiber et al., 1986). Fv/Fm has a maximum value of 0.8–0.83 in healthy cells and is used as an index of photoinhibition and temperature or nutrient stress tolerance (Björkman and Demmig, 1987, Goto et al., 2008). For example, under low-temperature conditions, the viscosity of the cell membrane decreases, suppressing photochemical reactions in the electron transport chain and resulting in a decrease in Fv/Fm (Morgan-Kiss et al., 2006). Additionally, using a light-curve function in PAM, the electron transport rate (ETR) can be estimated (Ohara et al., 2020). However, as well as the response in community composition, seasonality in the photosynthetic activity is not investigated in the Hakodate Bay.
In the present study, we aimed to investigate seasonal changes in the protist community and photosynthetic activity based on weekly sampling, and to reveal the response in the protist community and its activity by environmental variables in the Hakodate Bay.
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