Rapid accumulation of organic carbon, nitrogen and phosphorous in karst river sediments: Climatic and ecological implications

Assessing the role of sediment organic carbon (OC) and nutrients (N and P) in controlling ecological functions of rivers requires evaluation of their sources and accumulation rates. However, river systems, particularly in karst areas, have long been overlooked in models of terrestrial carbon and nutrient cycling. Recent studies have shown that spring-fed rivers, with clear water and elevated primary productivity, could play an important role in regulating regional carbon cycles as well as the global carbon cycle by producing and burying large amount of OC. Here, we estimate sediment accretion through 210Pb to determine OC, total nitrogen (TN), and total phosphorous (TP) accumulation rates over the last 150 years in spring-fed Silver River in north-central Florida. The excess 210Pb measured in sediment cores indicate high and constant sedimentation rates that range from 1.6 to 2.2 mm yr-1. Despite limited surface run-off sources to the river, rapid sediment deposition could reflect reworking of sediments as river meander bends migrate downstream. We estimate OC accumulation rates to be 170-238 g OC m-2 yr-1. These rates are similar to or greater than those found in mangrove forests, an environment that provides some of the fastest known organic carbon accumulations that average 163 g m-2 yr-1 globally. TN and TP accumulation rates (13-16 and 2-8 g OC m-2 yr-1, respectively) are also comparable with accumulation rates (around 15 and 2 g OC m-2 yr-1, respectively) estimated in some mangrove forests. OC and TN stable isotope ratios (δ13C and δ15N) reflect both allochthonous and autochthonous origin of organic matter. Organic matter was mostly allochthonous near the spring and was derived from surrounding terrestrial C3 plants as suggested by their relatively heavier δ13C and lighter δ15N values. In contrast, much of the organic matter was derived from autochthonous sources mid- and down-stream, including algal and planktonic contributions. High accumulation rates of OC, TN and TP from autochthonous sources reflect a labile substrate that could enhance the release of ecologically important solutes to the overlying water column through microbial-mediated biogeochemical processes.