Asymmetric springtime responses of carbon, nitrogen, and phosphorus export to hydrological variability across South Korean estuaries

Rivers are major pathways transferring terrestrial nutrients to coastal waters, where both the magnitude and stoichiometry of exported nutrients play a critical role in regulating primary production and coastal ecosystem functioning. Nutrient export is strongly controlled by hydrological processes, yet large-scale understanding of how seasonal hydrological variability shapes multi-nutrient export and stoichiometry remains limited, particularly when hydrological circulation and biological activity re-initiate during spring. Here we investigated springtime (March–May) export dynamics of total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP) across 19 major estuaries discharging into the West (Yellow Sea), South, and East coastal regions of South Korea over the period 2012–2023. We compiled a paired concentration–discharge–precipitation (C–Q–P) dataset by spatially matching river water quality, streamflow, and precipitation monitoring stations. Nutrient loads, long-term trends, and responses to different hydrological conditions (e.g., dry, normal, and wet years) were analyzed with spatial patterns in C:N:P stoichiometry. Springtime nutrient export was highly uneven across coastal regions, with 73–80% of total loads delivered to West Sea, reflecting geomorphological and hydrological controls. Over the study period, springtime TOC, TN, and TP loads exhibited declining trends, with mean decreases of 511, 668, and 53 ton/yr, respectively. However, load dynamics differed markedly among nutrient species. While TOC and TN exports were predominantly discharge-driven, showing symmetric responses to hydrological variability, with 34–36% reductions in dry years and 26–39% increases in wet years, TP export displayed a pronounced asymmetric response, decreasing by 57% in dry years but increasing by only 4% in wet years. This result suggested strong regulation by concentration dynamics, soil retention processes, and phosphorus management regulations. Stoichiometric analysis revealed widespread nitrogen-enriched conditions along the Korean Peninsula coastal lines except for the southern estuary, a distinct hotspot with near-Redfield C:N:P ratios. Along the southern coastal lines, the probability to balanced stoichiometry increased during dry years with co-occurrence of higher chlorophyll-a concentrations, indicating coupled hydrological and biological controls. Our results demonstrated that spring hydrological variability induces nutrient-specific and asymmetric export responses, highlighting the need to account for both magnitude and stoichiometry in management for not only in-land catchments but also coastal ecosystems.