Estimating organic carbon burial in freshwater impoundments with a rapid-assessment model and geospatial analysis

Abstract

Freshwater ecosystems transport a tremendous amount of organic carbon (OC) from terrestrial landscapes; however, freshwater systems have been poorly integrated into the global carbon budget which focuses largely on the atmospheric, oceanic, and terrestrial pools. Dams constructed along riverine systems greatly alter the hydrology leading to increased sedimentation and therefore increased carbon burial rates. GIS provides the generation of vast amounts of data on the watersheds and parameters linked to sediment and OC sources. Land-use, climate, standing biomass, topography, and soil type are correlated with allochthonous carbon inputs to freshwater ecosystems. Integrating these geospatial data with the analysis of sediment samples has the potential to produce more accurate models of OC burial rates and stock in freshwater impoundments. In this study, two approaches were used to estimate OC burial in freshwater impoundments. The first approach was a rapid assessment based on existing data from the reservoir sedimentation database (RESSED) and generalizations about the OC content of sediments in impoundments. This rapid assessment approach estimates OC accumulation rates in freshwater impoundments to be 140.5 g OC/m2

/y (Adj. R2 = 0.7875, n = 1085). The second was a geospatial approach which considered 24 independent variables including data on land-cover, watershed morphology, soils, and above-ground biomass. Independent variables were tabulated from various publicly available databases. Sediment samples were collected across 4 level III ecoregions from 24 impoundments and analyzed for OC content, the dependent variable in the model. The model estimated the mean OC content of 5.88% (w/w) in impoundment sediments (Adj. R2 = 0.4955, n = 24). An additional model was created from the geospatial data to estimate the stock of OC in freshwater impoundments. The mean predicted OC stock in the top 5 cm of impoundment sediment is 783.4 g OC/m2 (Adj. R2 = 0.7122, n = 24). Extrapolations were made for both assessments based on reported estimates of global impoundment surface area. The sediment OC content predictions from the geospatial model were paired with sedimentation rates and suggest a mean accumulation rate of 1533.4 g OC/m2 /y, comparable to previous studies. Global extrapolations place estimates for the accumulation of OC in freshwater impoundments at 36.3 and 396.5 Tg OC/y for the rapid and geospatial assessments, respectively. Incorporating geospatial data proves to be plausible when comparing to previous estimates.