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. 2016 Jan 5;113(1):58-63.
doi: 10.1073/pnas.1512651112. Epub 2015 Dec 22.

Aquatic carbon cycling in the conterminous United States and implications for terrestrial carbon accounting

David Butman  1 , Sarah Stackpoole  2 , Edward Stets  3 , Cory P McDonald  4 , David W Clow  2 , Robert G Striegl  3
Affiliations

Aquatic carbon cycling in the conterminous United States and implications for terrestrial carbon accounting

David Butman et al. Proc Natl Acad Sci U S A. .

Abstract

Inland water ecosystems dynamically process, transport, and sequester carbon. However, the transport of carbon through aquatic environments has not been quantitatively integrated in the context of terrestrial ecosystems. Here, we present the first integrated assessment, to our knowledge, of freshwater carbon fluxes for the conterminous United States, where 106 (range: 71-149) teragrams of carbon per year (TgC⋅y(-1)) is exported downstream or emitted to the atmosphere and sedimentation stores 21 (range: 9-65) TgC⋅y(-1) in lakes and reservoirs. We show that there is significant regional variation in aquatic carbon flux, but verify that emission across stream and river surfaces represents the dominant flux at 69 (range: 36-110) TgC⋅y(-1) or 65% of the total aquatic carbon flux for the conterminous United States. Comparing our results with the output of a suite of terrestrial biosphere models (TBMs), we suggest that within the current modeling framework, calculations of net ecosystem production (NEP) defined as terrestrial only may be overestimated by as much as 27%. However, the internal production and mineralization of carbon in freshwaters remain to be quantified and would reduce the effect of including aquatic carbon fluxes within calculations of terrestrial NEP. Reconciliation of carbon mass-flux interactions between terrestrial and aquatic carbon sources and sinks will require significant additional research and modeling capacity.

Keywords: aquatic ecosystems; carbon; carbon flux; inland waters; terrestrial ecosystems.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. S1.
Fig. S1.
USGS two-digit HUC regions used for analysis of both aquatic carbon and terrestrial carbon balance. HUCs 17 and 18 have been modified as identified in SI Materials and Methods to represent differences in precipitation regimes across the Coastal and Cascade mountain ranges.
Fig. 1.
Fig. 1.
(A) Total aquatic carbon flux measured in TgC per year by modified two-digit HUCs. (B) Normalized aquatic carbon yields in gC per square meter per year per area of each HUC using Eq. 2. (C) Average NEP gC per square meter per year for the years 1990–2010 derived from the MsTMIP model outputs. (D) Maximum potential fraction of NEP accounted for by aquatic carbon if all aquatic carbon is derived from terrestrial sources [does not include (fa) from Eq. 2] represented as in B and C. Uncertainty ranges are presented in Table S5.
Fig. S2.
Fig. S2.
Box plots by two-digit HUCs for the MsTMIP model ensemble terrestrial carbon stocks and flux estimates. Boxes represent the 25th and 75th percentiles, black lines represent the median, red lines represent the mean, and whiskers represent the 10th and 90th percentiles.
Fig. S3.
Fig. S3.
Nonlinear correlation between the emission of CO2 from lakes and the burial of OC across two-digit HUCs in the conterminous United States. Red lines indication the 95% prediction interval, and blue lines represent the 95% confidence interval. Error bars represent the SD about the mean.
Fig. 2.
Fig. 2.
Correlation between the summed aquatic carbon yield (Eq. 2) and average annual precipitation (PRISM Climate Group, Oregon State University; prism.oregonstate.edu) (A) and NEP (MsTMIP model ensemble mean) (B) by HUC. Points are labeled by HUC. Red lines indicate the 95% prediction interval, and the blue lines represent the 95% confidence interval. Error bars represent the SD about the mean.
See this image and copyright information in PMC

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