10:45 AM - 12:15 PM
[AOS17-P08] Dynamics of microphytobenthos in sublittoral sediments of Shido Bay, Japan
Keywords:Subtidal zone, Microphytobenthos, Photon flux on the seafloor, Phytoplankton, Primary Producer
Nearshore seafloor is often illuminated by sunlight. Microphytobenthic assemblages, which is generally consisted mainly of benthic diatoms, is ubiquitously observed therein. Microphytobenthos contribute as a main food source for benthic invertebrates and is expected to be a significant primary producer in wide coastal regions. However, in contrast to intertidal areas, quantitative evaluation of microphytobenthos in sublittoral areas is less examined. In this study, we investigated abundance and biomass of microphytobenthos, together with temperature, light, and nutrient status, during a 5-year period at a 6 m depth site in Shido Bay, Japan. The aim is to evaluate the importance of microphytobenthos as a primary producer in this shallow embayment and to reveal what factors affects their seasonal variability.
Sampling was conducted once or twice a month between January 2018 and December 2022. At each sampling, seawater temperature and photosynthetically available radiation (PAR) was measured vertically. Subsequently, seawater and sediment samples were collected. The seawater was used for the analysis of chlorophyll-a (Chl-a) as phytoplankton biomass. The sediment sample was used for the analysis of pennate diatom abundance and benthic Chl-a. The former and latter was regarded as a proxy for benthic diatom and hence microphytobenthos abundance, and microphytobenthos biomass, respectively. The sediment sample was also used for the determination of nutrient in the pore water. Calculation of PAR on the seafloor was based on the sea surface PAR and attenuation coefficient of PAR in the seawater.
Benthic diatom abundance in the upper 0.5 cm of the sediment ranged from 0.5×10^8 to 23.4×10^8 cells/m^2. There was a significant positive relationship between the abundance and the benthic Chl-a. These two variables typically showed annual peaks in late winter to early spring (Jan. to Apr.), while the minimum values were apt to record in autumn. Such a seasonal variation pattern of the two variables was opposite to those of seawater temperature and nutrient in pore water, which recorded minimum during late winter to early spring. On the other hand, the seafloor PAR often showed peaks in late winter to spring due to low levels of PAR attenuation in the seawater, and positively correlated with both of the benthic diatom abundance and benthic Chl-a. Thus, light seems to be the prime resource to affect the variation of the microphytobenthic abundance and biomass in the study area. The benthic Chl-a frequently exceeded integrated Chl-a in the overlying water during late winter to early spring, suggesting phytoplankton as well as microphytobenthos is a major primary producer in this shallow environment in that season.
Sampling was conducted once or twice a month between January 2018 and December 2022. At each sampling, seawater temperature and photosynthetically available radiation (PAR) was measured vertically. Subsequently, seawater and sediment samples were collected. The seawater was used for the analysis of chlorophyll-a (Chl-a) as phytoplankton biomass. The sediment sample was used for the analysis of pennate diatom abundance and benthic Chl-a. The former and latter was regarded as a proxy for benthic diatom and hence microphytobenthos abundance, and microphytobenthos biomass, respectively. The sediment sample was also used for the determination of nutrient in the pore water. Calculation of PAR on the seafloor was based on the sea surface PAR and attenuation coefficient of PAR in the seawater.
Benthic diatom abundance in the upper 0.5 cm of the sediment ranged from 0.5×10^8 to 23.4×10^8 cells/m^2. There was a significant positive relationship between the abundance and the benthic Chl-a. These two variables typically showed annual peaks in late winter to early spring (Jan. to Apr.), while the minimum values were apt to record in autumn. Such a seasonal variation pattern of the two variables was opposite to those of seawater temperature and nutrient in pore water, which recorded minimum during late winter to early spring. On the other hand, the seafloor PAR often showed peaks in late winter to spring due to low levels of PAR attenuation in the seawater, and positively correlated with both of the benthic diatom abundance and benthic Chl-a. Thus, light seems to be the prime resource to affect the variation of the microphytobenthic abundance and biomass in the study area. The benthic Chl-a frequently exceeded integrated Chl-a in the overlying water during late winter to early spring, suggesting phytoplankton as well as microphytobenthos is a major primary producer in this shallow environment in that season.