Organic matter and grain size comparisons in river sediments: Final thoughts


Acidifying sediment samples to remove inorganic carbon

I cannot believe the REU program is almost over and we leave on Friday! I’m really going to miss all of the REUs and the other people I have met here at UTMSI, as well as living right on the channel full of turtles and dolphins. My project on sediments from tidally-influenced freshwater rivers has wrapped up nicely, and I am almost ready for the symposium on Wednesday, when all of the REUs will present our research. I investigated the grain size, percent water, and organic carbon and nitrogen content of sediments (0-10 cm) from the Mission and Aransas Rivers in order to help understand organic matter and nitrogen cycling. My results reveal that sediments from the Mission River are generally composed of finer grains, are more porous, contain more organic matter, and have higher C:N ratios than sediments from the Aransas River. Clay and silt are the dominant fraction in the Mission River, while the average Aransas River sediments are fine sand. Average sediment organic carbon content is 0.995% and 0.350% in the Mission and Aransas Rivers, respectively. Both rivers display a downstream decrease in grain size and increase in organic matter, suggesting the existence of a significant depocenter as the river widens and deepens and the water slows down. Percent water and organic content generally decrease with sediment depth but vary due to differences in grain size through the sediment column. Aransas River sediments are less variable with depth, which may indicate recent deposition and/or resuspension. These data reveal important differences between the sediments of the Mission and Aransas Rivers that will likely influence decomposition and lead to differences in nitrogen cycling. Further investigation is needed in order to fully understand whether these differences in river sediments are caused by geomorphIMG_2544 (1)ology, hydrodynamics, anthropogenic influences, or something else. After I return home, Amber and Jim will continue analyzing nitrogen cycling and will study the grain size, porosity, and organic matter of the sediments again in the winter. I look forward to following the progression of the Oscillic Freshwater Zone project!


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