Anthropogenic perturbation of the carbon fluxes from land to ocean

Abstract

A substantial amount of the atmospheric carbon taken up on land through photosynthesis and chemical weathering is transported laterally along the aquatic continuum from upland terrestrial ecosystems to the ocean. So far, global carbon budget estimates have implicitly assumed that the transformation and lateral transport of carbon along this aquatic continuum has remained unchanged since pre-industrial times. A synthesis of published work reveals the magnitude of present-day lateral carbon fluxes from land to ocean, and the extent to which human activities have altered these fluxes. We show that anthropogenic perturbation may have increased the flux of carbon to inland waters by as much as 1.0 Pg C yr(-1) since pre-industrial times, mainly owing to enhanced carbon export from soils. Most of this additional carbon input to upstream rivers is either emitted back to the atmosphere as carbon dioxide (similar to 0.4 Pg C yr(-1)) or sequestered in sediments (similar to 0.5 Pg C yr(-1)) along the continuum of freshwater bodies, estuaries and coastal waters, leaving only a perturbation carbon input of similar to 0.1 Pg C yr(-1) to the open ocean. According to our analysis, terrestrial ecosystems store similar to 0.9 Pg C yr(-1) at present, which is in agreement with results from forest inventories but significantly differs from the figure of 1.5 Pg C yr(-1) previously estimated when ignoring changes in lateral carbon fluxes. We suggest that carbon fluxes along the land-ocean aquatic continuum need to be included in global carbon dioxide budgets

Human-induced nitrogen–phosphorus imbalances alter natural and managed ecosystems across the globe

“…With rapid recycling of carbon and nutrients, the ocean carbonclimate feedback seems weak at the century timescale, but a remaining question is whether the changes in the C:N:P ratios in both the particulate and dissolved organic matter pools may provide a mechanism for biological processes to change the amount of carbon sequestered by the ocean 66 . A recent study 67 suggests that coastal oceans are an anthropogenic sink of CO 2 of 0.2 Pg C per year and that it is mainly due to phosphorus and nitrogen increasing accompanied by usually also increasing N:P ratios, biological activity and burial of carbon in organic shelfsediments, but not in open ocean.…”

Spatial variability in surface-water <i>p</i>CO<sub>2</sub> and gas exchange in the world's largest semi-enclosed estuarine system: St. Lawrence Estuary (Canada)

“…With an estimated global efflux of 0.10-0.15 Pg C yr −1 (Chen et al, 2013;Laruelle et al, 2013), estuarine CO 2 degassing is thought to counterbalance CO 2 uptake on the continental shelves (Chen and Borges, 2009;Laruelle et al, 2010;Cai, 2011). Almost every estuary on Earth, for which data are available, is generally supersaturated with CO 2 with respect to the atmosphere (Cai and Wang, 1998;Frankignoulle et al, 1998;Borges, 2005;Borges et al, 2005Borges et al, , 2006Chen and Borges, 2009;Laruelle et al, 2010;Cai, 2011;Chen et al, 2012;Bauer et al, 2013;Chen et al, 2013;Regnier et al, 2013), with CO 2 partial pressures (pCO 2 ) ranging from 400 to 10 000 µatm (in contrast, the atmospheric pCO 2 in coastal zones was approximately 360-385 µatm in the year 2000) (Cai, 2011). Although estuaries are generally net sources of CO 2 , there is considerable variability and uncertainty in estimates of their CO 2 emissions, reflecting the limited spatial and temporal coverage of pCO 2 measurements in estuaries as well as their heterogeneous nature (hydrological and geomorphological differences, differences in magnitude and stoichiometry of 3222 A.…”

“…These estimates are subject to large uncertainties due to data paucity. A recent synthesis by Regnier et al (2013) highlighted the meagre spatial coverage of estuarine pCO 2 measurements, particularly along the eastern Canadian seaboard. Ironically, the Estuary and Gulf of St. Lawrence (EGSL) in eastern Canada is the largest semi-enclosed estuarine system in the world and is among the world's most intensively studied estuaries (El-Sabh and , but was left unmentioned in recent global (Cai, 2011;Chen et al, 2012Chen et al, , 2013 and regional (Laruelle et al, 2015) data compilations.…”

Isotope‐based Fluvial Organic Carbon (ISOFLOC) Model: Model formulation, sensitivity, and evaluation

“…Quantifying fluvial carbon budgets at the watershed-scale remains difficult due to a lack of mechanistic understanding of how physical and biogeochemical processes alter the composition of carbon species during transit from source to sink [Cole et al, 2007;Battin et al, 2009;Regnier et al, 2013]. One area of particular interest is streams where high nutrient loads such as from agriculture and urban land uses and low canopy cover promote autochthonous benthic carbon production and turnover.…”

“…Recent findings suggest that autochthonous carbon production and turnover can be on the same order of magnitude or greater than allocthonous carbon inputs in some agriculture and urban streams [e.g., Tank et al, 2010]. The significance of understanding autotrophic carbon dynamics has been recently highlighted through a contemporary macro scale study that estimates stream and river derived autochthonous carbon fluxes from land to ocean equal 0.3 PgC y 21 , yet the spatial distribution and fate of autochthonous carbon via burial, turnover, or transport remains an open question [Regnier et al, 2013]. The traditional assumption that stream generated carbon is turned over and can be neglected in energy utilization studies has been refuted recently by studies that highlight burial and long-term storage as a prominent fate mechanism of autochthonous carbon [Ford and Fox, 2014;Hotchkiss and Hall, 2015].…”