[A-HW27] Biodiversity, nutrients and other materials in ecosystems from headwaters to coasts

[AHW27-11] Assessing Nutrient Cycling in a Highly Urbanized Semi-Enclosed Bay through Sediment Core Analysis

*Muhammad Shahinur Rahman^1,2, Mitsuyo Saito¹, Shin-ichi Onodera¹, Takuya Ishida¹, Kunyang Wang¹, Takahiro Hosono³, Yu Umezawa⁴, Atsuko Amano⁵ (1.Graduate School of Advanced Science and Engineering, Hiroshima University, 2.Physical and Space Oceanography Division, Bangladesh Oceanographic Research Institute, 3.Faculty of Advanced Science and Technology, Kumamoto University, 4.Department of Environmental Science on Biosphere, Tokyo University of Agriculture and Technology, 5.National Institute of Advanced Industrial Science and Technology)

Keywords:Anthropogenic inputs, Nutrient reduction, Ecosystem functioning, Nutrient management strategies, Long-term sustainability, Eutrophication

Urbanized semi-enclosed bays experience complex nutrient cycling driven by anthropogenic inputs, hydrodynamic conditions, and sediment biogeochemistry. Osaka Bay has undergone significant changes in nutrient loading over the past decades, with external phosphorus (P) reduction implemented in the 1980s and nitrogen (N) reduction in the 1990s. Despite these efforts, the bay remains highly eutrophic, suggesting that internal phosphorus recycling may be delaying recovery from eutrophication. This phenomenon raises critical questions about the bay's long-term nutrient dynamics and resilience.
This study aims to investigate the interplay between external nutrient reductions, sedimentary processes, and internal nutrient cycling in Osaka Bay. By integrating sediment core data, long-term water quality records, and numerical simulations, we will assess the biogeochemical role of nutrient cycling. Special attention will be given to the role of sediments in regulating nutrient fluxes and their implications for eutrophication trends. Additionally, the study will evaluate the combined effects of historical nitrogen and phosphorus reductions on biogeochemical cycling and carbon accumulation.
The findings will provide insights into the effectiveness of past nutrient management strategies and the potential risks associated with internal nutrient regeneration in delaying ecosystem recovery. This research will support future coastal management policies aimed at improving water quality and ensuring the long-term sustainability of urbanized coastal systems.

Acknowledgment
This study is supported by Asia-Pacific Network for Global Change Research Project (CRRP2019-09MY-Onodera) and Grant for Environmental Rehabilitation and Creation of the Osaka Bay Area (PI: Mitsuyo Saito, 2022-2024) and funding by the Japan International Cooperation Agency (JICA) Agriculture Studies Networks for Food Security (Agri-Net) program.