Climate Change, Hydrologic Responses and Impacts on Carbon Cycling as Inferred by Changes in Fluvial Dissolved Organic Carbon Fluxes

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This project investigates the links between terrestrial and marine carbon cycling and fluvial transport of freshwater and dissolved organic carbon (DOC) to the near-coastal ocean. The project analyzes DOC export that integrates complex interacting processes in natural and human-impacted terrestrial and aquatic environments. Changes in DOC export may indicate changes in terrestrial ecosystem carbon cycling that are difficult to quantify directly but that may affect exchanges between the biosphere, geosphere, and atmosphere. Detecting changes in DOC export can provide insight into ongoing changes in terrestrial carbon cycling. In northern temperate forested ecosystems changes in carbon export may reflect a shift from carbon sink to carbon source or vice versa. Changes in freshwater and carbon export can influence aquatic and marine biogeochemical processes including eutrophication, hypoxia, carbon sequestration or release, ocean acidification, and ocean net primary productivity. This project examines the controls on the fluvial export of terrestrially-derived DOC including climatic variability and landscape variables.

Statement of Problem: This project is studying how ongoing and projected changes in climate are, or will likely, affect the export of freshwater and dissolved organic carbon (DOC) to the near-coastal ocean.  This project is also examining the fate of DOC in rivers to address questions of whether DOC is transported conservatively or whether a substantial fraction of DOC is lost through biotic or abiotic processes during transport in rivers.  Additionally, it is investigating whether concentration discharge (C-Q) relations for DOC may have changed over time that could indicate that the fundamental C-Q relation, that is generally assumed to be stationary, has changed.

 

Why this Research is Important: The evidence suggests that the global hydrologic cycle is intensifying (Huntington, 2006, 2010). The intensity of the water cycle increased over most of the conterminous United States between the periods 1945 to 1974 and 1985 to 2014 (Figure 1, Huntington and others, 2018). In the northeastern U.S. this hydrologic response to climate change includes an increase in precipitation and runoff and is projected to result in substantial changes in the seasonal delivery of water and organic carbon to the near-coastal ocean, and to an increase in summer drought severity. These hydrologic changes are altering the timing and magnitude of the delivery of DOC to the Gulf of Maine. Evidence suggests that increasing discharge is affecting nutrient inputs to the Gulf of Maine and that increasing export of DOC may be reducing light absorption by phytoplankton reducing primary productivity that could ultimately affect fisheries in the region. There are also concerns that increasing export of freshwater and DOC may be exacerbating ongoing ocean acidification and the harmful algal blooms (Figure 2).

If it is shown that the concentration discharge relations for DOC has changed over time this would indicate fundamental changes in carbon cycle processes including rates of microbial decomposition of soil organic carbon (SOC), primary production, or the partitioning of SOC decomposition into DOC or CO2.

Figure 1:

Map of continental U.S. depicting changes in water cycle intensity

Map showing the difference in annual average water cycle intensity between the averages of 1985 to 2014 minus 1945 to 1974. (Huntington, T. G., Weiskel, P. K., Wolock, D. M., and McCabe, G. J. 2018. A new indicator framework for quantifying changes in intensity of the water cycle. Journal of Hydrologyv. 559, p. 361-372.) 

(Public domain.)

 

Objective(s): The project objectives include investigation of the links between terrestrial and marine carbon cycling by studying the fluvial transport of freshwater and dissolved organic carbon. The primary goal is to improve our understanding of how changes in climate are affecting the export of freshwater and dissolved organic carbon to the near-coastal ocean.

 

Methods: The project uses non-parametric time-series analysis to investigate historical and future projected trends in precipitation, runoff, and evapotranspiration over large watershed at the regional and continental scale. Through a partnership with other USGS researchers (Greg McCabe, Dave Wolock, and Peter Weiskel) this project employs a water-balance model in conjunction with climate and soils data to investigate historical changes in water cycle intensity defined as precipitation plus actual evapotranspiration.  

This project employs the LOADEST regression model to estimate riverine dissolved organic carbon (DOC) fluxes (loads) exported to the near coastal ocean. Regression modeling is also used to study the relative importance of climate and landscape variables in controlling the carbon yield from individual rivers. In partnership with research scientists at the University of Massachusetts, Boston this project is studying DOC export from the Penobscot River using the Regional Hydro-Ecologic Simulation System (RHESSys) model.

 

Figure 2:

VIIRS Suomi NPP satellite image of northwestern Atlantic Ocean

Data from the Visible Infrared Imaging Radiometer Suite (VIIRS) sensor on the Suomi NPP satellite was used to create this enhanced color composite image of the Gulf of Maine and the northwestern Atlantic Ocean on May 14, 2015. Light green colors indicate areas where water conditions and nutrients near the surface (including dissolved organic carbon) create phytoplankton blooms. (credit: Norman Kuring, NASA Ocean Color Group)

Huntington et al. (2016) reported increasing export of dissolved organic carbon (DOC) to the Gulf of Maine in the latter half of the 20th and early 21st centuries and discussed projections for increasing DOC export in the 21st century based on climate projections.  Huntington et al. (2016) suggested that increasing DOC export to the near coastal ocean may influence marine biogeochemistry including the development of nuisance and harmful algal blooms and carbon sequestration. In the near‐coastal ocean, colored dissolved organic matter (CDOM), which is a component of terrestrially derived DOC, can lower the accuracy of satellite‐derived chlorophyll concentrations. CDOM strongly absorbs in the blue wavelengths, thus lowering the water‐leaving radiance in those wavelengths that the satellite sensor's chlorophyll algorithm mistakenly interprets as chlorophyll absorption. Terrestrially derived DOC exported to the marine environment could also decrease phytoplankton productivity through increases in light attenuation (Balch et al. 2012, Balch et al. 2016).

Cited References

(Credit: Norman Kuring, NASA Ocean Color Group)

 

 

Below are publications associated with this project not in the USGS pubs warehouse:

Huntington, T.G., 2010, Climate warming-induced intensification of the hydrologic cycle: A review of the published record and assessment of the potential impacts on agriculture: Advances in Agronomy, v. 109, p. 1-53.

Huntington, T.G., Weiskel, P.K., Wolock, D.M., and McCabe, G.J., 2018, A new indicator framework for quantifying changes in intensity of the water cycle: Journal of Hydrology, v. 559, p. 361-372.

Huntington, T.G. Roesler, C.S., and Aiken, G.R., 2019, Evidence for conservative transport of dissolved organic carbon in river systems in the Gulf of Maine watershed: Journal of Hydrology, v. 573, p. 755-767.

Contosta, A.R., Casson, N.J., Garlick, S., Nelson, S.J., Ayers, M.P., Burakowski, E.A., Campbell, J., Creed, I., Eimers, C., Evans, C., Fernandez, I., Fuss, C., Huntingtong, T.G., Patel, K., Sanders-DeMott, R., Son, K., Templer, P., and Thornbrugh, C., 2019, Northern forest winters have lost cold, snowy conditions that are important for ecosystems and human communities: Ecological Applications, published online 7/16/2019, doi:10.1002/eap.1974.