Dissolved organic matter (DOM) helps regulate aquatic ecosystem structure and function. In small streams, DOM concentrations are controlled by transport of terrestrial materials to waterways, and are thus highly variable. As rivers become larger, the River Continuum Concept hypothesizes that internal primary production is an increasingly important DOM source, but direct evidence is limited. Recently, the Pulse-Shunt Concept postulated that terrestrial DOM concentrations in larger rivers increase with flow and temperature, which seemingly contradicts previously reported DOM chemostasis in large rivers. This study estimates daily gross primary production (GPP) in 13 streams and rivers across the Connecticut River watershed (watershed areas 0.4–25,019 km²) from 2015 through 2017. Chemostasis of DOM concentrations is maintained by a switch from autochthonous sources of DOM at low flows to terrestrial sources of DOM at high flows in a large temperate river and to a lesser degree in smaller tributaries. At low flow, autochthonous DOM linked to aquatic GPP is the dominant fraction of the DOM pool in large rivers. This autochthonous DOM maintains chemostasis in the main stem and to a lesser extent upstream. Thus, in larger rivers, low-flow autochthonous production stabilizes DOM concentrations during the summer, a critical time for riverine ecology. Consistent with the Pulse-Shunt Concept, terrigenous DOM is the dominant fraction of DOM during higher flow periods and about 70% of annual DOM fluxes to the coast are terrestrial. This pattern of DOM switching is potentially widespread in temperate watersheds with implications to both inland waters and coastal ecosystems.

溶解有机物（DOM）有助于调节水生生态系统的结构和功能。在小溪流中，DOM浓度是由陆地物质向水路的运输控制的，因此变化很大。随着河流变大，“河流连续体概念”假设内部初级生产已成为越来越重要的DOM来源，但直接证据有限。最近，Pulse-Shunt概念假定大河流中的陆地DOM浓度随流量和温度的升高而增加，这似乎与先前报道的大河流中的DOM化学稳定性相矛盾。这项研究估计了康涅狄格河流域（流域面积0.4–25,019 km ²）从2015年到2017年。在温带大河中，从低流量的DOM的本地来源向高流量的DOM的陆源（在较小的支流中，程度较小）的转换可保持DOM的化学稳定性。在低流量下，链接到水生GPP的本地DOM是大河中DOM库的主要部分。这种自发的DOM在主茎和上游的较小程度上维持化学稳定。因此，在较大的河流中，夏季的低流量本地化生产稳定了DOM的浓度，这是河流生态的关键时期。与脉冲分流概念相一致，陆源DOM是较高流量期间DOM的主要组成部分，每年约有70％的DOM海岸通量是陆地的。