Fluvial transport of carbon along the river-to-ocean continuum and its potential impacts on a brackish water food web in the Iwaki River watershed, northern Japan

Riverine transport of dissolved inorganic carbon (DIC) from land to the ocean is an important carbon flux that influences the carbon budget at the watershed scale. However, the dynamics of DIC in an entire river network has remained unknown, especially in mountainous Japanese watersheds. We examined the effects of watershed land use and geology on the transports of inorganic carbon as well as weathered silica (Si) and calcium (Ca) in the Iwaki River system where agricultural and residential areas have developed in the middle and lower parts of the watershed. The concentration and stable carbon isotope ratios (δ¹³C) of DIC showed the longitudinal increase of ¹³C-depleted inorganic carbon along the river. As a result, most streams and rivers were supersaturated in dissolved CO₂ that will eventually be emitted to the atmosphere. The possible origin of ¹³C-depleted carbon is CO₂ derived from the decomposition of organic matter in agricultural and urban landscapes, as well as from in-stream respiration. In addition, agricultural and urban areas, respectively, exported the large amount of dissolved Si and Ca to the rivers, suggesting that CO₂ increased by respiration accelerates the chemical weathering of silicate and carbonate materials in soils, river sediments, and/or urban infrastructure. Furthermore, riverine bicarbonate flux is likely to enter shell carbonates of Corbicula japonica, an aragonitic bivalve, in the downstream brackish lake (Lake Jusan). These results revealed that the flux of DIC from the human-dominated watersheds is a key to understanding the carbon dynamics and food-web structure along the land-to-river-to-ocean continuum.