Human impact on long‐term organic carbon export to rivers

Abstract

Anthropogenic landscape alterations have increased global carbon transported by rivers to oceans since preindustrial times. Few suitable observational data sets exist to distinguish different drivers of carbon increase, given that alterations only reveal their impact on fluvial dissolved organic carbon (DOC) over long time periods. We use the world's longest record of DOC concentrations (130 years) to identify key drivers of DOC change in the Thames basin (UK). We show that 90% of the long‐term rise in fluvial DOC is explained by increased urbanization, which released to the river 671 kt C over the entire period. This source of carbon is linked to rising population, due to increased sewage effluent. Soil disturbance from land use change explained shorter‐term fluvial responses. The largest land use disturbance was during the Second World War, when almost half the grassland area in the catchment was converted into arable land, which released 45 kt C from soils to the river. Carbon that had built up in soils over decades was released to the river in only a few years. Our work suggests that widespread population growth may have a greater influence on fluvial DOC trends than previously thought.

The importance of sewage effluent discharge in the export of dissolved organic carbon from U.K. rivers

“…Given the population connected to main sewerage reported in this study, this would give a per capita figure of contribution of carbon from sewage effluent discharge of 4.1 kg C/ca/year, that is, considerably larger than the value found in this study. Noacco et al () studied a 130‐year record of DOC concentration and flux from the River Thames and showed that, although there were short‐term increases in DOC concentration due to land‐use change, the most important driver of change was increasing discharge from STWs as the population of the catchment increased. Even with advances in sewage treatment, the increased amount of water coming into the catchment via STWs caused increased DOC concentration in the river.…”

“…Even with advances in sewage treatment, the increased amount of water coming into the catchment via STWs caused increased DOC concentration in the river. In the study period considered by Noacco et al (), transient DOC concentrations from increasing population rose from 1.5 mg/L in 1884 to 4.1 mg/L in 2005, with 90% of the long‐term trend in increased DOC concentrations being explained by the rise in C inputs from population growth and urban expansion ( p < 0.001). Over the same period, the population of the Thames rose from 911,000 and 3.7 million, which indicates a change in per capita export from 3.46 to 2.28 kg organic C per person, per year; that is, there was an increased rate of removal, but this was not weighed by the increase in population using STWs.…”

“…As shown by the previous studies of Finlay et al (), Noacco et al (), and Worrall et al (), the extensive monitoring records maintained in the United Kingdom over time and space mean that the United Kingdom is the ideal location for considering STWs as a source of DOC to rivers.…”

“…Recently, Noacco, Wagener, Worrall, Burt, and Howden () have ascribed a long‐term trend (130 years, 1884 to 2005) in DOC concentrations for the River Thames to increases in sewage discharges as the population of the catchment rose from 900,000 to 3,200,000. Further, Worrall, Howden, Burt, and Bartlett () have shown that there have been recent declines (the period 2003 to 2015) in the DOC concentration of U.K. rivers at their tidal limits.…”

Riverine carbon export in the arid-semiarid Wuding River catchment on the Chinese Loess Plateau

“…Carbon export from terrestrial ecosystems into drainage networks is controlled by hydrological 295 regime, geomorphological landscape, biogeochemical processes, and human impact within the catchment of concern (Noacco et al, 2017;Stimson et al, 2017). Affected by sparse vegetation coverage, both DOC and POC contents in the Wuding catchment were relatively low compared with most rivers in the world.…”