Landscape controls on riverine export of dissolved organic carbon from Great Britain

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.

Contrasting Estuarine Processing of Dissolved Organic Matter Derived From Natural and Human‐Impacted Landscapes

“…Our results indicate a clear influence of land use on the area-specific estuarine DOC efflux. They are also consistent with the conclusions of a parallel study in British rivers (Williamson et al, 2021) and resemble the spatial distribution of the DOC fluxes modeled from catchment properties, such as land use, soil type and evaporation rates (Worrall et al, 2012). Our area-specific DOC export fluxes from arable and (sub)urban dominated estuaries (<2.1 g C m −2 yr −1 ) are comparable to other large European and world estuaries (i.e., 0.9 g C m −2 yr −1 for the Seine estuary, 3 g C m −2 yr −1 for the Po estuary and 2.3 g C m −2 yr −1 for the Lena estuary Worrall et al, 2012 and references therein).…”

“…The variability of DOC was significantly and positively correlated with annual average river flow ( r = 0.65, p = 0.02, n = 13). A comparison of these annual estuarine DOC effluxes to the annual riverine DOC influx for each estuary presented in a parallel river study (Williamson et al., 2021) (Table 1) suggests that three estuaries show no net addition or removal of DOC within the estuary, nine estuaries had additional sources of DOC within them and one estuaries showed DOC losses (Table 1). These results agree with the patterns observed in the DOC mixing dynamics (Figure 2b), as those estuaries presenting mainly conservative mixing trends (Groups 1–2) showed estuarine DOC export to riverine DOC export ratios of ∼1 (ratio between 0.9 and 1.2, Table 1).…”

“…Therefore, we adopted the “effective” concentration at S = 32 for our calculations of DOC efflux in order to give consistency between estuaries. Annual estuarine DOC effluxes were calculated as the average proportional change in DOC within the estuary (ratio of “effective” over “measured” DOC concentration at the lowest salinity) multiplied by the annual average riverine DOC export: where DOCeff i is the “effective” DOC concentration at the sampling time i , DOCo i is the “measured” DOC concentration at the lowest salinity waters at the sampling time i , n is the number of sampling surveys taken, and DOC r represents the annual average riverine DOC influx calculated for the rivers draining to our estuaries during a parallel study (Williamson et al., 2021). Areal‐estuarine DOC effluxes were calculated by dividing the annual DOC effluxes by the estuarine catchment area.…”

Effects of peatland management on aquatic carbon concentrations and fluxes

“…Although the Gaffney et al (2020) study did not measure CO 2 and CH 4 , this did not lead to large differences in carbon export between the studies, as DOC was the dominant flux term in both overall budgets. This region of Scotland has been identified as an important contributor to the total carbon flux from land to sea on the GB scale (Williamson et al, 2021), and as such, it is important that the effects of land management on fluvial carbon exports are considered, as this may have disproportionately larger impacts than in other areas of the country. As to the end fate of this exported carbon, specifically DOC, the short residence time of the Halladale River into which the streams feed suggests that much of this carbon is delivered to the estuarine environment, which, for this particular system, has been shown to display conservative mixing behaviour (García-Martín et al, 2021).…”

“…This results in a larger pool of soluble carbon species that can be transported via soil throughflow to the surface drainage system, where increases in DOC concentrations are subsequently detected (Evans et al, 2016;Menberu et al, 2017;Strack et al, 2008;Worrall et al, 2004). Notably in Great Britain, upland conifer plantations including those on drained deep peat are estimated to have raised the overall DOC export by as much as 0.168 Tg C yr −1 (Williamson et al, 2021).…”