[A-OS22] Ocean circulation and material cycle in coastal seas

[AOS22-05] Material cycle in lower trophic ecosystem in the Harima-Nada from 1970’s to 2020’s

*Siraporn Tong-u-dom¹, Akihiko Morimoto¹, Naoki Yoshie¹, Kuninao Tada², Kazuhiko Ichimi², Hitomi Yamaguchi², Masatoshi Nakakuni² (1.Ehime university, 2.Kagawa university)

Keywords:Material cycle, standing stock, lower trophic ecosystem , box model, Harima-Nada

Since the Interim Law for Conservation of the Environment of the Seto Inland Sea was enacted in 1973, and it was revised in 1978 and renamed the Law Concerning Special Measures for Conservation of the Environment of the SIS (LCSMCE) to control and limit the total allowable pollutant loads into the environment. The LCSMCE resulted in the reduction of nutrients, dissolved inorganic nitrogen (DIN) and phosphorus (DIP), loads into the SIS and turned to the oligotrophic sea. Changes in nutrient concentrations are associated with primary production and fluctuation of the ecosystem in the coastal area. Therefore, we examined the standing stock and fluxes of the lower trophic ecosystem in the Harima-Nada from 1970’s to 2020’s (LCSMCE is still ongoing) to represent the high, medium and low concentration of nutrients based on the box model approach. The ecosystem box model well reproduced seasonal variations of nutrients and phytoplankton observed in Harima-Nada in 2020’s. Standing stock of DIN (DIP) was 5.04 (0.61) μM during eutrophication and decreased to 4.24 (0.50) and 2.55 (0.25) μM in oligotrophication. The DIN/DIP ratio increased from about 8.19 during eutrophication to about 10.37 in oligotrophication. The DIN/DIP ratio was smaller than the Redfield ratio (16:1) suggesting that DIN is the limiting factor for photosynthesis in this lower trophic ecosystem. The standing stock biomass of phytoplankton (zooplankton) decreased from eutrophication about 0.86 (0.2) μM to oligotrophication about 0.35 (0.11) μM. The major phytoplankton component was microphytoplankton (chain-forming diatoms) about 47% during the eutrophication and it became nanophytoplankton (Dinoflagellate) about 51% during nutrients reduction. This change may be related to the increase in the DIN/DIP ratio during oligotrophication. The zooplankton was always dominated by mesozooplankton (Copepod) around 80 – 85%, however, the composition of zooplankton also changed. Mesozooplankton decreased from 85% during eutrophication to 80% in oligotrophication, in contrast, the nanozooplankton increased.