Spatial Analysis of Organic Carbon and Nitrogen Sedimentation in Four Northern Missouri Reservoirs: Implications for Optimal Sampling

Abstract

Recent research suggests that the rate of sediment carbon storage of small agricultural impoundments alone may be of the same order-of-magnitude as that of the world's oceans. This estimate, and the fact that reservoirs continue to steadily increase in number, affirm the possibility that the role of such waters in the carbon budget has not received adequate attention. However, scaling up the impact of these small size-class ([less than or equal to] 1 km2 ) reservoirs requires reliable estimates of OC burial rates in individual water bodies. Accomplishing this necessitates understanding the nature of spatial distribution of organic carbon (OC) sedimentation. This study used approximately 30-40 sediment cores per reservoir (n=4), collected in a uniform random distribution; an average of 2.7 samples per hectare. Sediment OC values ranged from 0.7 to 7.9% by dry weight in the four reservoirs, and sediment nitrogen (N) from 0.08 to 0.88% (n =136). Universal kriging analysis was performed using the samples from the surfical (top 5 cm) sediment using ArcGIS's Geostatistical Analyst. Analysis revealed a gradient of increasing sediment OC and N concentrations from in-flow to dam. A similar analysis of OC/N ratios in the surficial sediment showed in the majority of cases that the influence of allochthonous inputs became less important the further the sample location was from primary and secondary inflows. Furthermore, the analysis showed that the majority of the predicted sediment surface area had a ratio of less than 10, signaling significant autochonous influence.