[A-CG39] Coastal Ecosystems - 1. Water Cycle and Land-Ocean Interactions

[ACG39-09] Long-term benthic and planktonic primary production on estuarine tidal flats in relation to tidal cycle

*Tatsuya Ozaki¹, Tomohiro Komorita¹, Nobuaki Nakatsu¹, Takumi Magome¹, Nana Yamashita¹, Rikuto Honda¹, Akira Tai², Katsumasa Yamada³ (1.Prefectural University of Kumamoto, 2.Fukuoka Institute of Technology, 3.Kumamoto University)

Keywords:Tidal flat, Primary production, Mooring system, Tidal cycle

・Introduction
The regulating function of microalgae is highly valued as an ecosystem service of tidal flats. However, the area of tidal flats is decreasing by 0.55% per year and the ecosystem services of tidal flats are threatened by human activities. Understanding the dynamics of primary producers underlie the multifaceted functions of tidal flats is an important insight for their conservation.
Tidal flats are exposed to intense light on the sediment surface during ebb tides. In addition, nutrients are supplied from rivers, and floating mud is formed by cohesion and resuspension, resulting in high turbidity. Therefore, due to the high nutrient concentrations and high turbidity of tidal flats, photosynthesis by microalgae is limited by the photosynthetically active radiation (PAR). Since the attenuation rate of light in water varies with water quality, the PAR reaching the bottom vary greatly due to wave action. Therefore, the primary production change with the tide. However, few studies have simultaneously quantified the primary production of phytoplankton and microphytobenthos on tidal flats, and none of the quantification methods take tides into account. In this study, we conduct laboratory culture and mooring system in the Midorikawa River tidal flat facing the Ariake Sea from October 2021 to November 2022, and estimating the primary production in situ over a long period (-1 year) with a high temporal resolution. The purpose is to examine the factors that cause fluctuations in the primary production.

・Materials and Methods
Water quality was measured once a month from October 2021 to November 2022 at two sites on the sandy bottom at the Midorikawa River tidal flat using a multiple water quality meter(AAQ), surface water and surface sediment (1 cm) samples were collected. Water samples were measured for chlorophyll-a (Chl-a) concentration and dissolved inorganic carbon, sediment samples were measured for water content and Chl-a content. The mooring system was installed in the field to measure PAR, Chl-a concentration, water depth, and water temperature just above the bottom. The PAR at the sea surface was measured on the rooftop of Prefectural University of Kumamoto and corrected by the AAQ. Each parameter of the mooring system was calibrated with CTD data obtained at the same time by the AAQ. The laboratory culture for primary production was conducted under five light conditions, with the maximum PAR at the ebb tide(approximately 2000 µmol m^-2 s^-1). Water samples were added to the culture bottles for phytoplankton and sediment samples for microphythobenthos, and the bottles were filled with filtered seawater and NaH¹³CO₃ solution was added. A random forest (RF) regression model was constructed using the primary production calculated from the ¹³C concentration in the samples as the output variable and PAR, water temperature, salinity, Chl-a concentration and content, and sampling location at the time of incubation as input variables. The primary production of the water column and sediments at the site was calculated.
・Results and Discussion
From October 2021 to early February 2022, the primary production of both species decreased, and the average of the primary production in January 2022 was the lowest of the year (water column: 58 mgC m^-2 d^-1, sediments: 48 mgC m^-2 d^-1). The minimum water temperature in the water column was 6°C, but the surface temperature of the sediments decreased to -1°C during ebb tides, suggesting the primary production of the sediments was affected by the low temperature. The average of the primary production of the water column in February and March reached 236 mgC m^-2 d^-1 and 184 mgC m^-2 d^-1, respectively. On the other hand, the primary production of sediments remained relatively flat until April, but increased from April to May, replacing the primary production of the water column, and the average of the primary production of sediments in April reached 223 mgC m^-2 d^-1, the highest value in the annual primary production of sediments. The increase in sediment primary production occurred after a rapid increase in water column primary production, suggesting sediment primary production was limited by water column primary production.