Monthly Year-Round Characteristics and Ocean Export of Riverine Organic Matter: Relationship with Microplastics

Rivers play a crucial role in transporting carbon from terrestrial watersheds to oceans. Understanding the quantity and characteristics of riverine organic matter discharged into the ocean is essential for predicting changes in marine and global organic carbon cycles. Riverine organic matter, comprising both allochthonous and autochthonous fractions, is influenced by shifts in watershed sources driven by climate change, as well as socio-economic transformations that affect its production and characteristics. Plastics, a significant source of allochthonous organic carbon, could contribute substantially to rivers, and microplastics (MPs) generated from plastic degradation may alter carbon cycling within river systems through interactions with other organic materials. Despite their importance, MP exports from rivers to oceans remain poorly quantified and rarely measured in terms of carbon mass, with even less understanding of their interactions with other forms of riverine organic matter. To address this gap, we have investigated the five major rivers in South Korea, accounting for 90% of the freshwater discharge. Here, we present the preliminary results for three major rivers, representing Korean fluvial system connected to the Yellow Sea. Both particulate and dissolved organic matters were characterized in quantitative and qualitative terms by monthly sampling at each river-mouth station, including particulate organic carbon (POC), chlorophyll-a, transparent exopolymer particles (TEP), Coomassie stainable particles (CSP), and MPs for particulate forms and dissolved organic carbon (DOC) and dissolved organic matter (CDOM and FDOM) of dissolved forms. Considering the spatiotemporal variability of organic matters and MPs, river samples were collected three times a day at 2-3 hour intervals and in each sampling by compositing the samples taken from horizontally three cross-sectioned sites and vertically 3–5 water column layers per site. This study aims to quantify the monthly loads of total organic carbon (POC and DOC) entering the ocean from these rivers, assess the relationships between various forms of organic matter, and determine the relative contribution of MP-derived organic carbon to total organic carbon. Our results are expected to provide valuable insights into the ocean load and their inter-relationships of various organic matter forms originated from fluvial system, and their potential impact on the marine carbon cycle.

Acknowledgement: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. RS-2024-00356940).