Terrestrial carbon inputs to inland waters: A current synthesis of estimates and uncertainty

Abstract

Globally, inland waters receive a significant but ill-defined quantity of terrestrial carbon (C). When summed, the contemporary estimates for the three possible fates of C in inland waters (storage, outgassing, and export) highlight that terrestrial landscapes may deliver upward of 5.1 Pg of C annually. This review of flux estimates over the last decade has revealed an average increase of 0.3 Pg C yr 21, indicating a historical underestimation of the amount of terrestrial-C exported to inland waters. The continual increase in the estimates also underscores large data gaps and uncertainty. As research continues to refine these aquatic fluxes, especially C outgassed from the humid tropics and other understudied regions, we expect the global estimate of terrestrial-C transferred to inland waters to rise. An important implication of this upward refinement is that terrestrial net ecosystem production may be overestimated with ramifications for modeling of the global C cycle.In the last decade, the model of inland waters has evolved from simple "pipes" that transport terrestrial carbon (C) from land to ocean to complex "processors" that transform, store, and outgas significantly more C than they export (Cole et al. 2007). New research under this framework has revealed that fluxes of C within and through aquatic ecosystems are much larger than previously thought, especially the flux of carbon dioxide (CO 2 ) from surface waters to the atmosphere Raymond et al. 2013). These fluxes are primarily sustained by the supply of organic carbon (OC) and inorganic carbon (IC) from terrestrial landscapes (Prairie and Cole 2009).Most carbon enters inland waters through two primary pathways. The first is through the export of OC from *Correspondence: draketw@gmail.com Author Contribution Statement: TWD and RGMS conceived the manuscript. TWD generated new estimates and wrote the manuscript with significant contributions from PAR and RGMS.Data Availability Statement: All data in the manuscript can be found in cited publications and in Table 1. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.This article is part of the Special Issue: Carbon cycling in inland waters Edited by: Emily Stanley and Paul del Giorgio Scientific Significance StatementA number of large-scale studies on carbon (C) fluxes through and from inland waters have been conducted over the last decade and have not been synthesized into the contemporary global estimate. This review updates our global estimate for C fluxes in inland waters and identifies remaining knowledge gaps and sources of uncertainty. Our current estimate of terrestrial C inputs to inland waters is 5.1 Pg C yr21 , yet we expect this number to change given the high uncertainty, ongoing anthropogenic impacts, and continual refinement. 132Limnology and Oceanography Letters 3, 2018, 132-142

Litter movement pathways across terrestrial–aquatic ecosystem boundaries affect litter colonization and decomposition in streams

“…Freshwater ecosystems and their riparian zones constitute one of the best examples of meta‐ecosystems in that they are connected by spatial fluxes of energy, materials and organisms across ecosystem boundaries (Loreau, Mouquet, & Holt, ). Secondary productivity is supported by fluxes of organic matter from terrestrial origin in lakes (Drake, Raymond, & Spencer, ; Tanentzap et al, ), forest streams (Cummins, Wilzbach, Gates, Perry, & Taliaferro, ; Naiman & Décamps, ) and wetlands, where the bulk of the emergent plants enters the detrital pool following senescence and death of standing plant parts above the water surface (Gulis, Kuehn, & Suberkropp, ). In low‐order forest streams, riparian vegetation also shades the water, preventing temperature oscillations and inhibiting in‐stream photosynthesis, thus shaping ecosystem functioning (Vannote, Minshall, Cummins, Sedell, & Cushing, ).…”

Emissions from thaw ponds largely offset the carbon sink of northern permafrost wetlands

“…However, few in-depth pond studies exist, especially so regarding their quantitative importance for the C exchange between land and atmosphere. Freshwater lakes are well known to be significant emitters of CO 2 and CH 4 on a landscape scale 10–12 , but previous studies do not include empirical measurements from small thaw ponds. Here, we examined the importance of small ponds for C fluxes in permafrost wetlands in northern Sweden.…”

Pulse of dissolved organic matter alters reciprocal carbon subsidies between autotrophs and bacteria in stream food webs

“…, Drake et al. ). This may partly stem from an evolving understanding of the nature and structure of organic matter delivered to aquatic ecosystems.…”

“…The fate of this organic matter in riverine systems remains poorly understood at the global scale, notably CO 2 emissions resulting from the mineralization of the organic matter (Drake et al. ). The role of terrestrial organic matter is also highly debated regarding its contribution to aquatic food webs (e.g., Cole , Brett et al.…”