[A-OS16] Coastal ocean circulation and material cycle

[AOS16-04] Changes in the size composition of phytoplankton in the Harima Nada, Seto Inland Sea, Japan

*Manato Iwasa¹, Yuya Hamazaki⁴, Masatoshi Nakakuni², Hitomi Yamaguchi², Kuninao Tada^2,3, Kazuhiko Ichimi^2,3 (1.Graduate school of Agriculture, Kagawa University, 2.Faculty of Agriculture, Kagawa University, 3.Seto Inland Sea Regional Research Center, Kagawa University, 4.The United Graduate school of Agricultural Science)

Keywords:Seto Inland Sea , Harima Nada, picophytoplankton, size fraction, miniturization

In Seto Inland Sea, fish catches have been decreasing since the late 1980s, the main causes of which are still unidentified. Decreasing in DIN concentrations has been observed since about the same period (Nishikawa et al. 2010), which may have led to a decrease in phytoplankton, the primary producer, which may have propagated to the subsequent decrease in zooplankton and fish. However, decreasing trend in Chl a standing stock has not been observed in the regular observation data of the Harima Nada conducted by Kagawa University since 1992. On the other hand, observations conducted since 2019 have shown that the contribution of picophytoplankton (0.2–2 µm) and nanophytoplankton (2–20 µm) to the total Chl a concentration is higher than in the 1990s and 2000s. Therefore, the purpose of this study was to conduct a similar size composition survey in the Harima Nada and to clarify the relationship between phytoplankton size composition and nutrient concentrations from nutrient addition growth tests.
Monthly observations were conducted in the Harima Nada (Stn. NH) from January 2022 to December 2023, and samples water at depths of 0, 5 and 10 m. The DIN concentration of the sample water was measured and filtered through filters of different pore sizes (0.2, 2 and 20 µm) to determine the Chl a concentration by size. From September to December 2022 observations, treatments were prepared by adding 0.2, 2 and 20 µmol/L of NaNO3 and Si(OH)4 and 0.02, 0.2 and 2 µmol of K2HPO4 to 10 m layer seawater and incubated at water temperature in situ, 100 µmol/m²/s (14:10 L:D). Chl a concentrations were measured by size after a certain time after the start of incubation. From July to November 2023 observations, seawater obtained in the 10 m layer was size fractionated to <0.2 µm, 2-20 µm and >20 µm. Then, the filtrate added 0, 1, 2.5, 10 and 25 µ mol/L of NaNO3 and constant amounts of K2HPO4 and Si(OH)4 (respectively, 2.5 µmol/L and 25 µmol/L) were created five treatments. These were incubated at water temperature in situ, 150 µmol/m²/s (14:10 L:D) in July and 150 µmol/m²/s (12:12 L:D) in September to November. Chlorophyll fluorescence values were measured daily from the start of incubation and phytoplankton growth curves were drawn up by size.
In Stn. NH, DIN concentrations remained low (≦1 µM) from March to September, but started to increase (0.82–6.21 µM) after October. Total Chl a concentrations were low in March to June, with a high proportion of picophytoplankton, but the proportion of microphytoplankton (>20 µm) increased after October, when DIN concentrations were found to increase. Comparing the results of this study with the 1990s and 2000s, DIN concentrations were reduced by half and the proportion of microphytoplankton decreased. The results of the incubation experiments showed no differences in the size composition of the phytoplankton community at different nutrient concentrations. On the other hand, when growth curves were drawn up, differences were not observed in the growth rates of the different size fractions, but it was observed that the difference in the maximum chlorophyll fluorescence values exhibited by micro, nanophytoplankton and picophytoplankton increased with increasing nutrient concentrations.