Seasonal fluctuations in riverine nutrients entering the East Coast

Abstract

In the East Sea, the phytoplankton rely on nitrogen as a major energy source for their primary production. The average ratio of nitrogen-to-phosphorus (N:P) for phytoplankton in the East Sea is approximately 13:1, lower than the Redfield ratio (N:P=16:1), indicating the significance of nitrogen as a primary factor for influencing phytoplankton primary production in this region. Because the Korean Peninsula is high in elevation in the east and low in topography in the west, there are no large rivers connecting with the East Coast and it was expected that lots of nutrients could not flow from rivers into the East Coast. However, there are small rivers connecting to the East Coast including Namdaecheon (25 km) and Wangpicheon (60 km). Normally, during dry seasons, the rivers located on the East Coast are blocked by dunes between the river and the ocean. However, during high precipitation season, the flow rate of river currents significantly increases and flows through the dunes to the East Coast, increasing the amount of nutrient input from the river to the coast. Thus, the seasonal distribution of nutrients from rivers connecting to the East Coast is important for estimating the total amount of nutrient input from the rivers to the East Coast. We investigated the river currents and nutrient contents in the Namdaecheon and Wangpicheon connecting to the East Coast during dry and wet seasons. Sampling points were designated up from the headwaters to the coast of rivers in the East Sea, and the water samples were filtered with 0.45-μm filters to analyzed nutrients (NO3–, SiO2, NH4+, PO4–). It was observed that nitrate, a controlling factor for primary productivity, decreased in concentration as it moved towards the coast due to dilution effects from seawater with lower concentrations than upstream. During the high precipitation months of August (250 mm) and September (275 mm), the concentrations were lower due to dilution, whereas in October (12 mm), with lower rainfall, the concentrations were highest. For both phosphate and silicate, except at the Wangpicheon site, concentrations decreased towards the coast. Phosphate exhibited a similar trend to nitrate, decreasing in concentration during high precipitation months, whereas silicate did not show significant differences. Considering the flow rate and the concentrations of nutrients, nitrate was discharged from Namdaecheon and Wangpicheon to the ocean from 1.0 to 6.0 × 106 moles month–1, phosphate 0.01 to 0.04 × 106 moles month-1, and silicate 0.6 to 14.7 × 106 moles month-1 with the high amount in high precipitation months, which was insignificant compared to Geum Estuary that is located in the west coast of Korea and discharged a large amount of nitrogen to the Yellow Sea via rivers (47 to 162 × 106 moles TN month–1). Therefore, in the case of the East Sea, river discharges were expected to have a low contribution to primary productivity due to limited nutrient inputs, and thus the nutrient inputs to the East Sea through the atmosphere are expected to have a more significant impact on primary production. Thus, we will investigate quantitative analysis of nitrogen deposition into the East Sea via atmospheric deposition and compare the nitrogen input from rivers to the atmosphere and their respective contributions to primary productivity in the East Sea. We will also compare the sources of nitrogen entering the East Sea using an isotope analysis of atmospheric and riverine nitrate. Through these comparisons, we hope to assess the relative contribution of different pathways and sources to primary productivity in the East Sea.