Submitted to: Inland Waters: Journal of the International Society of Limnology988
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/1/2012
Publication Date: 1/28/2013
Citation: Pittman, B., Jones, J.R., Millspaugh, J.J., Kremer, R.J., Downing, J.A. 2013. Sediment organic carbon distribution in 4 small northern Missouri impoundments: implications for sampling and carbon sequestration. Inland Waters: Journal of the International Society of Limnology. 3(1):39-46 DOI: 10.5268/IW-3.1.507. Interpretive Summary: Small man-made impoundments including farm ponds, drinking water reservoirs, and recreational lakes ranging in size from 0.5 to 250 acres occupy over five million acres of the land surface in the United States. Sediments at the bottom of these impoundments accumulate annually from entry of runoff water that may carry suspended soil from erosion within the watershed during rainfall or irrigation. Organic materials also enter the water and can become part of the sediment. The organic carbon (C) buried in the sediments is an important component of the global C cycle and its measurement is critical to understand how the buried C can help alleviate carbon dioxide levels in the atmosphere. Although we have good measures of sediment C in the world’s large natural lakes, no standard methods are available for sediment sampling and calculation of the sediment C contents in small lakes and ponds. Estimates of sediment C in lakes and ponds less than 10 acres are extremely limited. The objective of our study was to quantify, with intensive sampling (30 to 40 samples per lake), the concentration and distribution of sediment organic C in four small man-made lakes in northern Missouri. Sediment core samples were taken from the lake bottoms using a specialized sampling probe. Sampling followed a grid pattern imposed upon the surface water area, which allowed collection of about 1 sample per acre. Samples were dried and analyzed for organic C contents. The distribution of sediment C increased from the inflow (entry point) to the outflow (at dam site). We found that sampling along a line that bisects the lake (the mid-line) at a density of one sample for every three acres of water surface gave the best and least variable estimates of organic C content in the lake sediments. The sampling method developed in this study allows accurate calculation of organic C burial in small lakes and should improve our understanding of small lake contributions to global C accumulation. Results of this study are important to scientists and policy makers involved in climate change and carbon cycling research and interpretation because a portion of the C cycle represented by sediments in numerous small lakes in a given region can be more accurately quantified, thereby aiding in research planning and decision-making.
Technical Abstract: Recent research suggests that the rate of sediment carbon storage of small agricultural impoundments may be of similar magnitude on a per unit area as that of the world’s oceans. However, the role of small impoundments in the carbon budget has not received adequate attention; indeed availability of carbon budget values for impoundments < 5 ha is limited to nonexistent. Reliable estimates of organic carbon (OC) in individual water bodies are needed to characterize spatial distribution of sediment OC. Four impoundments (5 to 25 ha) in the Glacial Plains of the Midwestern USA were sampled to determine sediment OC. We collected 30-40 sediment cores per reservoir following a uniform grid pattern, yielding an average of 2.7 samples per hectare. Sediment OC values ranged from 20.3 to 54.8 g kg-1 by dry weight (n =136) in the four reservoirs. Universal kriging analysis was performed using the samples from the surficial (top 5 cm) sediment using ArcGIS Geostatistical Analyst. This analysis revealed a distinctive pattern of increasing sediment OC concentration from in-flow to dam. Results from kriging analyses were used to inform selection of preferred sampling techniques. We examined use of a center-point approach, which involved taking one sample at the average depth at the mid-point of the lake. The OC values obtained using this approach approximated the median value of all sampled reservoirs within 2.5%. Therefore, if only one sample can be taken, the center-point approach should be used. In irregularly shaped impoundments, this sample should be taken along the longitudinal center-line and at the average depth as figured on a volume/area basis. Using sediment core data from all impoundments, we used a power analysis to identify OC sampling requirements for impoundments in the region. We assumed a simple random sampling method and pooled data from all impoundments for the regional assessment. Using the mean and variance of OC, and assuming an alpha = 0.05, we determined it would require an average of 10 core samples of OC per reservoir, or an average of 0.8 samples per hectare, to obtain regional estimates of OC that had a relative precision of 26%. This value is comparable to the precision of data used as the basis for global estimates for larger water bodies. Additional sampling beyond 10 cores per reservoir provided only marginal gains in precision. Methodologies evaluated in this study yielded accurate estimates relative to previous studies where OC burial rates were measured, but did not use preferred sampling techniques. This study is a first step toward accurate global calculation of OC burial in sediments of small impoundments.