Abstract
The dissolved organic carbon (DOC) export from land to ocean via rivers is a significant term in the global C cycle, and has been modified in many areas by human activity. DOC exports from large global rivers are fairly well quantified, but those from smaller river systems, including those draining oceanic regions, are generally under-represented in global syntheses. Given that these regions typically have high runoff and high peat cover, they may exert a disproportionate influence on the global land–ocean DOC export. Here we describe a comprehensive new assessment of the annual riverine DOC export to estuaries across the island of Great Britain (GB), which spans the latitude range 50–60° N with strong spatial gradients of topography, soils, rainfall, land use and population density. DOC yields (export per unit area) were positively related to and best predicted by rainfall, peat extent and forest cover, but relatively insensitive to population density or agricultural development. Based on an empirical relationship with land use and rainfall we estimate that the DOC export from the GB land area to the freshwater-seawater interface was 1.15 Tg C year−1 in 2017. The average yield for GB rivers is 5.04 g C m−2 year−1, higher than most of the world’s major rivers, including those of the humid tropics and Arctic, supporting the conclusion that under-representation of smaller river systems draining peat-rich areas could lead to under-estimation of the global land–ocean DOC export. The main anthropogenic factor influencing the spatial distribution of GB DOC exports appears to be upland conifer plantation forestry, which is estimated to have raised the overall DOC export by 0.168 Tg C year−1. This is equivalent to 15% of the estimated current rate of net CO2 uptake by British forests. With the UK and many other countries seeking to expand plantation forest cover for climate change mitigation, this ‘leak in the ecosystem’ should be incorporated in future assessments of the CO2 sequestration potential of forest planting strategies.
Highlights
The Intergovernmental Panel on Climate Change (IPCC) estimates that the global export flux of carbon (C) from the terrestrial biosphere to rivers is 1.7 Pg CG
Our analysis suggests that the continuation of this policy could have implications for the magnitude of CO2 uptake that can be achieved; the 0.17 Tg C year-1 of forest-related dissolved organic carbon (DOC) export from GB rivers equates to 15% of the estimated net C uptake by growing trees in the UK after accounting for reemission via Harvested Wood Products (Brown et al 2019)
To the extent that Great Britain can be considered representative of the relatively peat-rich, high-rainfall oceanic temperate zones of the Northern and Southern Hemisphere, our results suggest that the contribution of these areas to the global estimated land–ocean DOC flux may have been somewhat under-estimated
Summary
Forty rivers draining 36% of the GB land mass (Fig. 1) were sampled in the third week of each month during 2017. They were sampled close to the tidal limit, and where possible at long-term national hydrometric and water quality monitoring sites. Where existing discharge and water quality monitoring sites did not coincide, sampling was conducted at the long-term water quality monitoring sites unless it was unsafe to access this site. Water samples were filtered immediately through 0.45 lm cellulose acetate filters using rubber free syringes into HDPE bottles for DOC analysis and amber glass bottles for UV absorbance measurements. Samples were posted to the laboratory under cool conditions within 24 h of collection.
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